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655ecdfecf8b3c3cd7e9e56c | 0 | Solid fast-ion conductors represent a distinct class of ion-conducting materials characterized by the diffusion of a specific type of atom through the interstitial spaces within a solid lattice while other atoms remain stationary in their crystal sites. This unique behavior allows liquid-like conduction of the particular atom within a solid framework. Notably, solid fast ion conductors exhibit comparable ionic conduction to liquids, particularly organic solvents, such as 25 mS cm -1 for Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 , and 70 mS cm -1 for Na 2 (CB 9 H 10 )(CB 11 H 12 ) . Additionally, these materials have extended operating temperature range compared to their liquid counterparts , higher energy density, longer life cycles, and enhanced safety due to the absence of organic liquid or polymer electrolytes . The technological applications of solid fast-ion conduction are of significant interest, particularly in the development of all-solid-state batteries (ASSBs), fuel cells, and gas sensors . |
655ecdfecf8b3c3cd7e9e56c | 1 | ASSBs, in particular, are considered promising candidates for high-energy-density batteries, addressing the growing demand in the electric vehicle market as conventional lithium-ion batteries (LIBs) approach the theoretical limit . Solid oxide fuel cells (SOFCs), another important application, can also benefit from the utilization of solid-fast ion conductors. Operating at elevated temperatures, SOFCs require stable and efficient materials to ensure optimal performance. By replacing liquid or polymer electrolytes with solid-fast ion conductors, the limitations associated with lower stability and efficiency can be overcome. |
655ecdfecf8b3c3cd7e9e56c | 2 | Conducting ions in a solid phase faces various challenges due to the dense packing of atoms within the crystal structure, which limits the availability of space for ionic diffusion. As a result, the ionic conductivity in solids is generally low. However, achieving ionic conductivities above 1 mS cm -1 is typically crucial for battery applications, with even higher conductivities exceeding 10 mS cm -1 required for high-power density batteries . |
655ecdfecf8b3c3cd7e9e56c | 3 | Additionally, maintaining effective connectivity between the electrode and electrolyte poses another significant challenge. The development of solid-fast ion conductors has witnessed notable progress in synthesizing new materials (Fig. ) and addressing these challenges. Researchers have explored a diverse range of mobile ionic species, including but not limited to Li + , Na + , H + , Ag + , Cu + , F -, O 2-, and H -, in their efforts to enhance the performance of solid fast ion conductors. Recent reviews have extensively documented and summarized this progress . In addition to materials that conduct monovalent ions, there is growing interest in the potential of divalent-cation-conducting materials, such as Mg 2+ , Ca 2+ , and Zn 2+ , for various energy-storage applications . These materials are garnering attention due to their significant promise in energy-storage devices, primarily attributed to their higher energy densities when compared to all-solid-state batteries (ASSBs) constructed using monovalent cations. Remarkably, ions such as O 2-and Mg 2+ have demonstrated notable mobility, with certain recently discovered Mg 2+ ion conductors achieving an impressive ionic conductivity of up to 0.1 mS cm -1 at room temperature (RT) . Among the various types of ionic conductors, particular emphasis has been placed on Li-ion and Na-ion conductors since the year 2000. These materials have been extensively studied due to their potential applications in energy storage and other technologies requiring efficient ion transport within solids. |
655ecdfecf8b3c3cd7e9e56c | 4 | For the purpose of comparison, Fig. provides an overview of the ionic conductivities (low to high conductivity range in (b)) and the activation energy of key ionic conductors along with their structures. The chronological order of their discovery is also displayed in Fig. . Notably, Yttria (Y 2 O 3 ) stabilized zirconia (ZrO 2 ) and AgI have been recognized as fast ion conductors since the early twentieth century . In more recent times, the breakthrough discoveries of M Ag 4 I 5 (where M = Rb, K, and NH 4 ) , and Na β-alumina have significantly contributed to the field. Hong and Goodenough et al. conducted a comprehensive study in 1976, reporting high alkali ion conduction in a series of skeleton structural solids such as KSbO 3 , RbMgAlF 6 , NaAlSiO 4 , and Na 1+x Zr 2 P 3-x Si x O 12 (0 ≤ x ≤ 3). Inorganic solid electrolytes can be further classified into oxide, sulfide, halide, and hydride-based categories . Among oxide-based materials, Lithium Super Ionic Conductors (LISICONs), Na superionic conductors (NASICONs), garnets, perovskites, and anti-perovskites are the most widely studied . Sulfide-based electrolytes are thio-LISICONs, Li 10 GeP 2 S 12 (LGPS) and its analogs, argyrodites (Li 6 PS 5 X , where X = Cl, Br, or I). In halide class, Li 3 M X 6 (M = Y, Er; X = Cl, Br, I) find attractive, owing to the high ionic conductivity (≈ 1 mS cm -1 ) and a higher electrochemical stability comparable to known oxides . Complex metal hydrides fall under the hydride category. Each group of materials has distinct advantages: oxide-based electrolytes generally exhibit lower ionic conductivity and high hardness but superior stability in atmospheric and electrode environments. Sulfide and hydride-based electrolytes, on the other hand, tend to demonstrate higher ionic conductivity, though they are quite sensitive to ambient conditions. Notably, the mixture of two different complex hydrides (NaCB 9 H 10 and NaCB 11 H 12 ) has achieved the highest known ionic conductivity thus far, based on our current knowledge . |
655ecdfecf8b3c3cd7e9e56c | 5 | Over the past few years, researchers have made significant progress in synthesizing high ion-conducting materials for solid-state devices. Despite decades of research, only a limited number of materials demonstrate both high RT conductivity (σ RT > 100 mS cm -1 ) and a low activation energy (E a close to 0.1 eV). In addition to these electrical properties, practical materials must meet other essential criteria, including good chemical, mechanical, and thermal stability and cost-effectiveness . Unfortunately, as of now, no solid-state electrolytes have managed to fulfill all these requirements. Therefore, it becomes crucial to gain a systematic and fundamental understanding along with high-conducting materials synthesis. However, various challenges hinder this understanding due to the intricate interplay of multiple factors. Several design principles have been proposed to achieve fast alkali conductivity, such as 3D interstitial networks with suitable bottlenecks (BN) , alkali ion disorder , and polarizable anion frameworks with body-centered cubic or hexagonal close-packed anion frameworks . These principles are, however, specific to certain chemistries or structure types and cannot be universally applied. Consequently, the search for new frameworks that exhibit these characteristics has proven to be a challenging task. |
655ecdfecf8b3c3cd7e9e56c | 6 | X-ray and neutron diffraction are particularly useful for determining the crystal structure, which are crucial for understanding ionic conduction. Quasielastic neutron scattering (QENS) measurements help in understanding atomic orientation, particularly in cases where isolated anions exhibit significant reorientation. NMR spectroscopy allows for the measurement of self-diffusion behavior, providing insights into ion-ion correlations in ionic diffusion. Theoretical techniques, including MD simulations, density functional theory (DFT) calculations, genetic algorithms for determining unique structures, and machine learning force-fields, nudge elastic band (NEB) method, are indispensable for gaining atomistic energy barrier between two hops into ion conduction following minimum energy path. It is worth emphasizing that while the NEB method is a widely used tool in this context, its accuracy relies heavily on the availability of precise initial and final sites for ion hopping. In many real-world cases, these sites may not be readily accessible or well-defined. Moreover, the NEB method does not inherently account for the influence of materials characterized by rotating frameworks on the ion diffusion process or entropy effects. Therefore, while it is a valuable tool, it is crucial to be mindful of its limitations in certain situations. |
655ecdfecf8b3c3cd7e9e56c | 7 | This review is structured as follows: We briefly highlight a few fundamental equations related to ionic conductivity and the major factors responsible for controlling ionic conductivity in section II. Then, we categorized the materials based on the framework structures in section III. In each class of materials, we analyze how the framework anions, carrier concentration, and diffusion mechanism affect ionic conduction and how to optimize them. In the section IV, we discuss the general trends and common factors that contribute to fast ion conduction. Then, we demonstrate a systematic approach for achieving an ideal ionic conductor in section V. Finally, there is a conclusion and future directions in section VI. |
655ecdfecf8b3c3cd7e9e56c | 8 | In liquid electrolytes, the transport of lithium ions involves their movement within the solvent medium. In contrast, in crystalline solids, the conduction of lithium ions requires passing through periodic BN points, creating an energy barrier between two local energy minima. The conductivity of ions in liquid electrolytes can be improved in two ways: first, by increasing the dissociation of salts or ions in solvents of higher dielectric constants. This promotes the increase of the solvated ion. Second, reducing the viscosity of solvents facilitates the movement of solvated ions. |
655ecdfecf8b3c3cd7e9e56c | 9 | Fast molecular or ion exchange is necessary for fast ion conduction in liquid electrolytes. In contrast, in crystalline solids, the diffusion of mobile species, such as lithium ions, encounters periodic BN points offered by immobile framework species. These points define energetic barriers that separate two local minima, typically represented by crystallographic sites for the ions. The energy barrier, also known ). (b) Overview of various classes of fast ion conductors, including their conductivity ranges, activation energy ranges, and structural characteristics ( ). Some of the high conductivity in each class of materials is also indicated, such as 25 mS cm -1 for Li9.54Si1.74P1.44S11.7Cl0.3 in LISICON, 1.1 mS cm -1 for Na2-xZn2-xGaxTeO 6 in Honeycomb Layered Oxide, 1.3 mS cm -1 for Li6.55La3Zr2Ga0.15□0.3O12 in Garnet, 26 mS cm -1 for Li5.3PS4.3Cl0.7Br in Argyrodites, 1.3 mS cm -1 for Li1+xAlxTi2-x(PO4)3 in NASICON, 41 mS cm -1 for Na2.9Sb0.9W0.1S4 in Na 3 P nCh 4 , and 70 mS cm -1 Na 2 (CB 9 H 10 )(CB 11 H 12 ) in Closo-Borane. Notably, the closo-borane class material exhibits the highest conductivity among them, as per our knowledge. |
655ecdfecf8b3c3cd7e9e56c | 10 | The diffusion process is examined by calculating cations and anions' mean square displacement (MSD). In a real system, the local energy landscape usually determines the hop of mobile atoms. However, for simplicity, we start with a random-walk model , where the cross term becomes zero and the self-diffusion coefficient, D r , the diffusion coefficient under the theoretical framework of random walk, can be expressed as |
655ecdfecf8b3c3cd7e9e56c | 11 | where R n is the total displacement of a moving ion in n steps, t n is the duration to complete n steps of ion displacement, a is the hopping distance between two neighboring sites or free path, ν is the jump frequency of successful jumps, and b is a geometry factor of 2, 4, or 6 for one-, two-, or three-dimensional diffusion, respectively. A general form of R n is as follows: |
655ecdfecf8b3c3cd7e9e56c | 12 | where N m denotes the number of target ions in the system for diffusion calculation, ∆⃗ r i = ⃗ r i (t + t ′ ) -⃗ r i (t) is the displacement of the i th ion; t is the time origin, while t ′ is the time difference, and the angular bracket, the average over several points in time, for statistical equal weight at each point, typically half of the total time origins are chosen. The jump frequency can also be described as |
655ecdfecf8b3c3cd7e9e56c | 13 | where ν 0 is the attempt frequency, or the number of attempted jumps within unit time, and includes both successful jumps that can lead to macroscopic diffusion and unsuccessful ones. ∆G m is the Gibbs free energy of activation, k B is the Boltzmann constant, T is the absolute temperature, ∆S m is the activation entropy, and ∆H m is the activation enthalpy (this is also called the activation energy or activation barrier and denoted E a ). Substituting ν in eq. 1, we obtain. |
655ecdfecf8b3c3cd7e9e56c | 14 | Once the system is in the diffusive region, the MSD graph follows a straight-line behavior, and the diffusion coefficient, D r , can be calculated from the slope of the straight line against the time difference. It is worth noting that if there is a correlated diffusion, the self-diffusion differs from the collective diffusion. The self-and collective diffusion coefficients are succinctly linked by Haven's ratio (H R ), given by, |
655ecdfecf8b3c3cd7e9e56c | 15 | Essentially, the H R is a ratio between the average MSD of all the mobile atoms and the MSD of the center of mass of all the mobile atoms (denominator ). As the center of mass of the target ions is equivalent to a single particle behavior, it is a fluctuating quantity. Thus, in practice, several independent runs are counted for better statistics. The ionic conductivity, σ, can be linked to the diffusion coefficient, D r , according to the Nernst-Einstein equation, |
655ecdfecf8b3c3cd7e9e56c | 16 | The pre-exponential component encompasses multiple factors, including the exponential dependence of entropy. Even a slight alteration in any of these factors, particularly within the exponential part, can substantially impact ion conduction. Consequently, this phenomenon explains why materials with similar structures do not necessarily exhibit similar ionic conductivity. Such complexities in understanding and generalizing materials' behaviors pose significant challenges. |
655ecdfecf8b3c3cd7e9e56c | 17 | Anionic Rotation: Anion rotation is a phenomenon that occurs in some ionic compounds, where the anions can rotate around their own axes, resulting in reorientation. It can be a partial or full rotation, depending on the materials. This can affect the ionic conductivity of the material, as the rotation can create or disrupt pathways for the migration of ions. This behavior makes the material unique and attractive. The anionic reorientational motion is directly identified by calculating the angular autocorrelation function (ζ) of anions: |
655ecdfecf8b3c3cd7e9e56c | 18 | where rj (t), and rj (t + t ′ ) denote the unit vectors connecting from the center of mass of anions to an edge atom in the same anion at time t and t + t ′ , respectively. N e is the number of edge atoms in the anion. This function typically shows an exponential decay (≈ e -λt ′ ) within a short time with a decay rate λ. The anion reorientational activation energy (E r a ) also can be calculated using the Arrhenius equation: |
655ecdfecf8b3c3cd7e9e56c | 19 | Layered oxide materials are a class of solid fast-ion conductors that have a distinctive structure consisting of mobile cations sandwiched between rigid framework layers. These materials exhibit high ionic conductivity, especially for alkali metal ions, and have potential applications in energy storage and electrochemical devices. Two important sub-classes of layered oxide materials are β-alumina and honeycomb layered oxide. |
655ecdfecf8b3c3cd7e9e56c | 20 | Notably, β-alumina also demonstrates fast ion conduction for divalent and even trivalent cation species . This characteristic expands its potential applications and highlights its versatility as a solid-state ionic conductor. The ability of β-alumina to facilitate the rapid conduction of alkali metal ions is attributed to its unique crystal structure. This structure comprises interconnected channels or tunnels that enable the efficient movement of ions, enhancing their conductivity. |
655ecdfecf8b3c3cd7e9e56c | 21 | Moreover, synthesizing these materials is straightforward, allowing researchers to obtain high-quality samples for study and characterization. The general formula of β-alumina is M 2 O.xAl 2 O 3 (where M = Li, Na, Ag, K, and Rb) . This material consists of two distinct subgroups: β ′ and β ′′ , each characterized by a unique structure (Fig. )). The β ′′ type exhibits higher ionic conductivity due to its distinctive framework structure, accommodating a larger number of vacant cationic sites. The crystal structure of β-alumina comprises compact layers of spinel blocks formed by Al 3+ tetrahedra and octahedra coordinated with oxygens. These spinel blocks are sandwiched between mobile cations, with oxygen bridges connecting them to form a 3D hexagonal structure. In the case of β ′ -alumina, the space group is P6 3 /mcm, while for β ′′ -alumina, it is R3m. The layer between the rigid spinel blocks is often called the conduction layer (Fig. )). |
655ecdfecf8b3c3cd7e9e56c | 22 | Within the conduction layer, three different cationic sites exist: anti-Beevers-Ross (aBR), Beevers-Ross (BR), and mid-Oxygen (mO) (Fig. )) . The sites are different based on the local oxygen environment, such as BR is formed by three top and three bottom oxygens, and aBR is formed by two oxygens (one top and one bottom). These sites are crucial in facilitating cationic diffusion and ion hopping within the material, as presented in Fig. ). Occupancy at these sites and the diffusion mechanism have been subjects of intense debate and investigation . Kummer conducted systematic experimental studies on various monovalent counterparts (Li + , Na + , Ag + , K + , Rb + ) of β-alumina. The studies revealed that the ionic radius, particularly between Na + and Ag + , was suitable for ion diffusion within the β-alumina framework (Fig. )) . This observation suggests the existence of an optimal cation size that maximizes diffusion within the material. |
655ecdfecf8b3c3cd7e9e56c | 23 | These MD simulations revealed the importance of non-stoichiometric composition in enhancing cationic diffusion and observed a transition from low-temperature hopping transport to a higher-temperature liquid-like transport . Further MD studies by Thomas and Zendejas focused on Na-β ′′ -alumina and provided additional atomistic insights into the diffusion of Na + ions, including correlation effects and the presence of mobile and immobile Na + ions in the diffusion mechanism. |
655ecdfecf8b3c3cd7e9e56c | 24 | In β ′ -alumina, the dominant conduction mechanism involves the migration of oxygen ions (O 2-) through the crystal lattice, utilizing oxygen vacancies as diffusion paths. In β ′′ -alumina, the conduction mechanism revolves around the migration of sodium ions (Na + ) through the crystal lattice, facilitated by the presence of oxygen vacancies. In addition to the oxygen, repulsive forces between Na ions were found to play a crucial role in reducing the activation energy required for ion diffusion, as observed by Sato and Kikuchi and further emphasized by Wang and Pickett . |
655ecdfecf8b3c3cd7e9e56c | 25 | Although empirical fitting potentials by Walker and Catlow have been successful in MD studies, accurately describing lattice vibrations has remained a challenge. Kamishima et al. developed a set of potential parameters by fitting phonon vibrational modes, providing a more accurate representation of lattice vibrations in Ag β-alumina. Their studies showed that mobile Ag + ions in the conducting plane are distributed over BR and aBR sites, exhibiting asymmetric large thermal amplitudes. The repulsion between Ag ions influences the dynamics, transitioning the diffusion mechanism from random hopping to cooperative motion. |
655ecdfecf8b3c3cd7e9e56c | 26 | Layered oxide solids have attracted considerable attention as potential materials for Sodium-ion batteries since Na x CoO 2 was first introduced as a cathode material. Typical layered honeycomb oxides consist of alkali or coinage metal atoms sandwiched between slabs made exclusively of transition metal and chalcogen (or pnictogen) atoms arranged in edge-shared octahedra in a honeycomb fashion (Fig. ). These materials offer several advantages, such as wider interstitial spaces between two-dimensional polyhedral framework layers that facilitate cation migration, enabling high ionic conductivity at RT, and excellent chemical and ambient stability . However, a rigid octahedral framework layer completely hinders diffusion along the crystalline c-direction. The materials are classified based on the local cationic environment, with oxygen (tetrahedral, octahedral, or prismatic) sharing rectangular faces, thereby creating a wide BN for facile cation transport . In this context, layered honeycomb oxides differ from the β-alumina structure, where oxygen atoms are located in the conduction layer. Van der Waals forces weakly connect the framework layers in these honeycomb oxides, and the inter-layer distance can be adjusted by framework modifications or a larger size cationic substitution. Alkali ion conductivity is significantly influenced by the inter-slab distance, cationic concentration, and their different octahedral arrangements in the framework layer. Several experimental and theoretical studies, including MD simulations and topological analysis, have been conducted to gain insights into alkaline ion conduction and material stability mechanisms . |
655ecdfecf8b3c3cd7e9e56c | 27 | Despite several improvements in materials synthesis, the fundamental understanding was lacking. Taking a broader perspective, MD simulations of Na 2 Ni 2 TeO 6 aim to understand the physics of sodium ion transport in honeycomb layered oxide materials, which have potential applications in energy storage and electrochemical devices. In this context, Sau and Kumar shed light on it by proposing an interatomic potential for performing force-field-based MD simulations on the same series of honeycomb layered oxides (Na 2 M 2 TeO 6 ). |
655ecdfecf8b3c3cd7e9e56c | 28 | The presence of cationic vacancies and configurational entropy was found to play a crucial role in the ion migration mechanism. The conductivity can be tuned by varying the cationic concentrations among the inter-layer on the conduction layer, as shown in Fig. ) . A general characteristic of honeycomb layered material can be defined in a radar plot, as shown in Fig. ). Sau et al. expanded their MD study to several other honeycomb layered oxide materials, such as A 2 Ni 2 TeO 6 , where A = Li, Na, and K, and Na 2 LiFeTeO 6 . Fig. (j) indicates the goodness of the structural fitting with the XRD structure, whereas a comparative study in A 2 Ni 2 TeO 6 for different cation sizes can provide insights into how different factors, such as ion-ion repulsion , pressure, temperature, and structural stability, affect the diffusion, conductivity, and occupancy of cations at different cationic sites. The study revealed an exponential relationship between cationic diffusion, inter-layer distances (Fig. ), inverse relationship with activation energy (Fig. |
655ecdfecf8b3c3cd7e9e56c | 29 | Conventionally, materials in this category are characterized by a single conduction layer. However, their work has yielded a particularly intriguing material, namely Ag 6 M 2 TeO 6 (where M can represent Ni, Mg, or other transition metal atoms), which exhibits a distinctive structure featuring bi-layers of Ag atoms within Ag-rich crystalline domains, as depicted in Fig. (i) . This bi-layer-type honeycomb layered oxide configuration represents a unique and novel structural motif. |
655ecdfecf8b3c3cd7e9e56c | 30 | In order to explore the distinctive properties of these bilayer-type honeycomb layered oxides, a preliminary MD simulation was conducted by Sau using a previously reported force field, with interaction parameters similar to those of Na 2 Ni 2 TeO 6 . During this simulation, the polyhedral layers were held fixed at their centers to prevent any bending. Interestingly, the theoretical simulation successfully confirmed the formation of these bilayer structures using the predefined force field. |
655ecdfecf8b3c3cd7e9e56c | 31 | In the case of Ag 6 M 2 TeO 6 , it was observed that the inter-layer distance is relatively substantial, approximately 6 Å, in contrast to Na 2 Ni 2 TeO 6 , which exhibits an inter-layer distance of approximately 3 Å. Consequently, mobile cations within the bilayer have a tendency to approach the oxygen layer, resulting in a splitting of the Ag + conduction layer. However, it is essential to emphasize that a more comprehensive and detailed investigation is needed to gain a thorough understanding of the cationic diffusional pathways and the underlying diffusion mechanisms that govern these intriguing materials. |
655ecdfecf8b3c3cd7e9e56c | 32 | These materials face several formidable obstacles, including but not limited to low ionic conductivity under ambient conditions, restricted diffusion along the c-direction owing to non-penetrable rigid framework layers, structural instability stemming from larger inter-layer separations, and volumetric alterations during ion migration. Conversely, a weakly connecting framework affords facile adjustments of the inter-layer spacing, a parameter that exhibits exponential dependence on ionic diffusion. |
655ecdfecf8b3c3cd7e9e56c | 33 | Polyhedral Connecting Framework Structures are another class of materials that exhibit a three-dimensional network of polyhedra. These polyhedra, which can be of various shapes, such as tetrahedra and octahedra, are connected at their corners or edges to form a continuous framework. This structure creates channels or cages where mobile ions can reside. The connectivity and geometry of the polyhedra can greatly influence the properties of the material, including its thermal stability, electronic structure, and ion conductivity. |
655ecdfecf8b3c3cd7e9e56c | 34 | Solid electrolytes with covalently bonded, interconnected polyhedral structures form another class of materials. These materials usually exhibit high Debye temperatures, thermal and chemical stability, and melting points, offering high operating temperatures for practical applications. Lithium Garnet type materials, with the general chemical formula A 3 B 2 (XO 4 ) 3 (A = Ca, Mg, Y, La or rare earth; B = Al, Fe, Ga, Ge, Mn, Ni or V; X = Si, Ge, Al), promise high thermal stability, high electrochemical window, as well as high ionic conductivity . They generally stabilize in the cubic phase with the space group Ia3d. The A, B, and X sites are respectively 8-fold (anti-prismatic), 6-fold (octahedral), and 4-fold (tetrahedral) coordinated to oxygen (Fig. ). |
655ecdfecf8b3c3cd7e9e56c | 35 | Lithium conducting garnets, Li 3 Ln 3 Te 2 O 12 (Ln = Y, Pr, Nd, Sm-Lu), typically contain 5-7 formula units where lithium ions occupy at tetrahedral and octahedral sites, as depicted in Fig. . Since the original work by Murugan et al. , it has attracted numerous experimental and theoretical studies. It is observed that at low lithium content, Li 3 Ln 3 Te 2 O 12 , where Li + ions are tetrahedrally coordinated, does not show significant ionic conductivity (σ ≈ 10 -5 S cm -1 in Fig. )) . However, higher ionic conductivity has been achieved for Li-rich, "stuffed garnets," such as Li 5 La 3 M 2 O 12 (M = Nb, Ta, and Sb), Li 6 ALa 2 M 2 O 12 (A = Mg, Ca, Sr, and Ba; M = Nb and Ta), and Li 7 La 3 M 2 O 12 (M = Zr and Sn) where Li ions occupy the tetrahedral as well as octahedral interstitial sites partially, increasing migrational entropy, ∆S m (eq. 8). Generally, the conductivity value increases with higher Li content. The conductivity is enhanced by 9 orders of magnitude from Li 3 Ln 3 Te 2 O 12 to Li 7 La 3 M 2 O 12 due to significant Li + occupancy switches from tetrahedral to octahedral sites . |
655ecdfecf8b3c3cd7e9e56c | 36 | The high conducting cubic garnet Li 7 La 3 Zr 2 O 12 (LLZO) was reported by Murgun et al. , and later on, the tetragonal phase was detected at a lower temperature by Awaka et al. . The cubic phase shows two orders of magnitude higher conductivity than the tetragonal phase . Several computational studies have been conducted to explain such behavior . |
655ecdfecf8b3c3cd7e9e56c | 37 | The low-temperature tetragonal phase exhibits synchronous collective Li + ion hopping, causing a high activation energy barrier. In cubic LLZO, the Li + finds unoccupied sites, resulting in uncorrelated Li + motion that costs less activation energy. Hu et al. , and others reported that higher valent ion (Al 3+ ) substitution at Li sublattice improves the stability of cubic garnet of nominal composition in Li 7 La 3 Zr 2 O 12 . The Al 3+ substitution generates more Li sites, leading to more entropy and reducing the free energy. Furthermore, (e) Li + ion diffusion path indicated by iso-surface Li + ion density plot . This figure is taken from ref. (Copyright 2016 Springer Nature). (f) Various LISICON type materials and their conductivities . |
655ecdfecf8b3c3cd7e9e56c | 38 | the aliovalent substitutions (e.g., Ta 5+ ) at Zr sites also enhance the ionic conductivity . An order of magnitude of conductivity is enhanced from Li 5 La 3 Ta 2 O 12 to Li 6 BaLa 2 Ta 2 O 12 where the Li ions are occupied in tetrahedral as well as octahedral sites . Thus, garnet-type materials are found to be promising fast ion conductors owing to their wide range of substitutions and the stability of their skeleton structure. |
655ecdfecf8b3c3cd7e9e56c | 39 | Significant research efforts have been dedicated to enhancing the ionic conductivity in Lithium Super Ionic Conductors (LISICONs). In 1978, Hong identified poor ionic conductivity (10 -7 S cm -1 at ambient temperature) in Li 14 Zn(GeO 4 ) 4 and related materials such as Li 4 GeO 4 -LiVO 4 , Li 2+2x Zn 1-x GeO4, Li 2 O-V 2 O 5 -SiO 2 , and Li 4 SiO 4 -Li 3 PO 4 , which were synthesized around the same period and exhibited slightly improved ionic conductivity . However, a significant breakthrough in lithium-ion (Li + ) conductivity was achieved when Kamaya et al. discovered a material with ionic conductivity reaching 10 -2 S cm -1 at RT, surpassing that of liquid organic electrolytes. |
655ecdfecf8b3c3cd7e9e56c | 40 | LGPS has since garnered substantial attention, both for elucidating its underlying mechanisms and addressing cost-related concerns. The LGPS material features (Ge 0.5 P 0.5 )S 4 tetrahedra and PS 4 tetrahedra with space group P4 2 /nmc, as illustrated in Fig. (d). Li + ions occupy tetrahedral and octahedral sites, allowing for one-dimensional Li + diffusion along the crystallographic c-axis , as depicted in Fig. . |
655ecdfecf8b3c3cd7e9e56c | 41 | Despite its high Li + ion conductivity, a portion of Li + ions in LGPS remains localized at octahedral sites, not contributing to Li + ion diffusion. This material exhibits highly anisotropic Li + conduction through partially occupied tetrahedral Li-sites with a low activation energy of 0.24 eV. However, the atomic origins of these characteristics remained unclear until Mo et al. shed light on the topic by conducting ab initio MD simulations, supporting the hypothesis of Li + ion diffusion along the c-direction . To gain deeper insights, Adams and Rao performed MD simulations based on derived force fields, revealing the atomistic dynamics of Li ions. They found that Li + ions are distributed along the c-direction among several interstitial Li sites. The study also indicated the possibility of PS 4 3-/GeS 4 2-rotation, which facilitates Li + dynamics through a paddle-wheel mechanism . In an effort to replace the expensive germanium (Ge) component, . A conductivity comparison Arrhenius plot is displayed in Fig. (f). |
655ecdfecf8b3c3cd7e9e56c | 42 | After the discovery of β-alumina, there was a strong belief in existing similar types of materials with three-dimensional ion conduction . The expectation became a reality when Hong and Goodenough et al. synthesized NASICON (an acronym for Na SuperIonic CONductor), marking a significant breakthrough in the pursuit of fast ion conductors. |
655ecdfecf8b3c3cd7e9e56c | 43 | NASICON exhibits promising characteristics, including high three-dimensional ionic conductivity, as well as thermal and chemical stability. It is a class of 3D materials with remarkable RT ionic conductivity (≈ 10 -3 S cm -1 ), often exceeding that of liquid electrolytes at high temperatures. Furthermore, NASICONs offer adaptability through a wide range of substitutional windows, providing potential for further improvement. |
655ecdfecf8b3c3cd7e9e56c | 44 | Later on, several new sites (A2, A3, or mid-A, etc., where A = Li or Na) were introduced by modifying the framework. Detailed site environments are here. The A1 is situated between two TiO 6 octahedra along the crystallographic c-axis, while A2 connects two A1 sites. The A1 is surrounded by oxygen octahedra (A 1 O 6 ) and exhibits a coordination number of six, while eight oxygens irregularly coordinate A2 . Structural modifications of the framework involve partial replacement of P with Si, resulting in the generation of an additional site known as mid-A (36f). These mid-A sites form a circular arrangement parallel to the ab-plane and are surrounded by each A1 site. The presence of such three-dimensional distributed sites facilitates the formation of a 3D migration path for Na ions. The composition Na 1+x Zr 2 Si x P 3-x O 12 (NZPO) with x = 2.0 exhibits the highest ionic conductivity, as shown in Fig. (c) . Consequently, theoretical studies, particularly those involving MD simulations, had been dedicated to comprehending the dynamics of ionic motion within NASICONs and elucidating the factors responsible for high ionic conductivity at this specific composition . |
655ecdfecf8b3c3cd7e9e56c | 45 | Kumar et al. developed a comprehensive interionic potential that accurately predicts the lattice constants and ionic conductivity of NZPO as a function of composition (x = 2) , as displayed in Fig. ). The study revealed that, at this composition, the number of Na + ions and available vacancies is exactly doubled, indicating that a carrier concentration of 50% yields the highest ionic conductivity, as discussed in eq. 7 . Such a balance of mobile carriers and vacancies offers the highest attainable configurational entropy, denoted as ∆S m in equation 4 . Additionally, their study elucidated the free energy landscape along the Na + ion diffusion path. Several subsequent studies have explored the influence of Si/P ordering and compositions on the conductivity and its nature, whether three-dimensional (3D) or two-dimensional (2D). Notably, the Li counterpart in this material exhibits significantly lower Li + ion diffusion, expecting the existence of an unfavorable pathway for Li + ion conduction. Two types of conduction channels have been investigated: one connecting A1 and neighboring A2 sites and the other connecting A2 to A2. In the Na 4 Zr 2 Si 3 O 12 system, a BN in the form of a trapezoid, with oxygen atoms at the corners, has been reported by Tran Qui et al. . This trapezoidal BN is wider than the triangular BN. Kumar et al. found that the triangular moderate BN radius produces the best ionic conductivity in a specific NASICON structure. Despite numerous computational studies, the identification of a saddle point at the BN remains elusive, necessitating further investigation. Furthermore, Roy and Kumar conducted a few more MD studies for the understanding of P 5+ /Si 4+ ordering in the framework anion for the Na 1+x Zr 2 Si x P 3-x O 12 (NZPO) with x = 2.0 and 1.0, where a particular an ordering in case of x = 2.0 produce order of magnitude higher diffusivity than random ordering (Fig. ), and such increasing diffusivity was reflected in the Na+ ion distribution (Fig. )) . They also found that a particular ordering of P 5+ /Si 4+ can change the type of Na conduction (2D or 3D) . . They observed a significant enhancement in Li + ion conductivity within the Ti 4+ framework (Fig. |
655ecdfecf8b3c3cd7e9e56c | 46 | 5(h)), which has a smaller size compared to Zr, resulting in a smaller BN for the smaller cation (Li + ). Further studies have been conducted to understand the correlation between the framework skeleton and cationic conductivity in this series. In general, ionic diffusion increases with a larger BN formed by the NASICON framework skeleton. Martínez-Juárez et al. reported similar behaviors in the LiM 2 (PO 4 ) 3 system, where M = Ge, Ti, Sn, Hf, and Zr, with respective ionic radii σ Ge < σ T i < σ Sn < σ Hf < σ Zr . Consequently, the BN radius formed by the skeleton framework systematically increases with larger cationic substitutions, resulting in higher diffusivity of mobile Li + ions. |
655ecdfecf8b3c3cd7e9e56c | 47 | The activation energy also decreases systematically with an increasing BN radius (Fig. ). Pramanik et al. performed a systematic MD study to understand atomistic insights into such systematic behavior . An interesting dynamic behavior of framework BN (the BN opens up while passing the cation through its center) was revealed (Fig. ). |
655ecdfecf8b3c3cd7e9e56c | 48 | Mouahid et al. ( ) conducted a comprehensive analysis of the structure and electrical properties of Na 1+x Ti 2-x Al x (PO 4 ) 3 materials within the range of 0.6< x <0.9, which belong to the NASICON type. This investigation employed a combination of X-ray diffraction, Rietveld analysis, and impedance spectroscopy techniques. The results of their research unveiled that the ion exchange processes were not fully completed. Specifically, they observed that sodium ions predominantly occupied the Na1 site within the NASICON structure, while Li + ions preferentially resided at the Na2 site. This behavior is schematically illustrated in Fig. |
655ecdfecf8b3c3cd7e9e56c | 49 | 5 , where it becomes evident that larger-sized atoms tend to occupy the more stable Na1 site. Furthermore, the authors proposed that this phenomenon leads to a mixed alkali effect, which in turn reduces the ionic conductivity and elevates the activation energy of the (Li/Na) 1+x series when compared to pure sodium analogs. They postulated that lithium diffusion could potentially occur through Na2-Na2 pathways, whereas sodium ions might obstruct the Na1-Na2 pathways. However, a deeper understanding of the atomistic mechanisms underlying these behaviors necessitates further investigations. |
655ecdfecf8b3c3cd7e9e56c | 50 | In the pursuit of enhancing ionic conductivity, researchers have explored a fascinating approach involving the substitution of divalent elements within the NASICON framework, leading to the creation of cationic vacancies. A compound of particular interest in this regard is Ba x/2 Li 1-x Ti 2 (PO 4 ) 3 , where the range of substitution was limited to 0.0 < x < 0.83. Intriguingly, the presence of immobile Ba 2+ ions within the framework was found to impede the conduction of cations along specific pathways. However, an optimal composition of x = 0.67, achieved through Ba 2+ /Li + substitutions, exhibited the highest level of conductivity . |
655ecdfecf8b3c3cd7e9e56c | 51 | Motivated by this phenomenon, Sau et al. conducted MD (MD) study on Ba x/2 Li 1-x Ti 2 (PO 4 ) 3 , exploring compositions ranging from 0.0 < x < 0.83 to unravel the atomistic origins of enhanced cationic diffusion . Their investigations revealed that below the composition of x = 0.67, an abundance of cationic vacancies dominated, leading to a significant enhancement in cationic conduction. However, as the composition exceeded the critical value of x = 0.67, the blocking effect of cation channels became more pronounced, thereby diminishing overall cationic conduction. They also suggested a specific type of ordered arrangement of Ba atoms to avoid Li + ion channel blocking that enhances conductivity significantly. Such knowledge of Ba atom ordering and underlying mechanisms is crucial for improved conductivity within the same framework. Moreover, Duan et al. studied the influence of Ga 3+ substitution in Li 1-3x Ga x Zr 2 (PO 4 ) 3 solid-state electrolytes on their structure and ionic conductivity . Their findings indicated that Ga 3+ doping increased both the activation energy and the prefactor of ionic conductivity, with the prefactor dominating due to increased migration entropy. The study employed various techniques such as electrochemical strain microscopy, speed of sound measurements, Raman spectroscopy, and NMR spectroscopy to explore the structure-property relationships and the influence of lattice dynamics on ionic transport. The researchers suggested that optimizing both the activation energy and the prefactor is necessary to maximize the ionic conductivity of solid-state electrolytes. |
655ecdfecf8b3c3cd7e9e56c | 52 | This structural feature facilitates facile anion rotation, associated with large cell volume change (Fig. )), making these materials unique. The ordered (O)-disordered (D) phase transition in these materials can be qualitatively elucidated by considering the Gibbs free energy difference between the high-temperature (high-T) D and low-temperature (low-T) O phases, denoted as ∆G ≡ G D -G O , as depicted in Fig. (c) . |
655ecdfecf8b3c3cd7e9e56c | 53 | This free energy difference comprises three key terms: internal energy (∆E), entropy (-T ∆S), and volume (p∆V ). Notably, the internal energy remains relatively constant throughout the phase transition, while the volume term opposes the stabilization of the D phase due to ∆V > 0. Consequently, within the domain of cluster anion-type materials, the O-D phase transition appears to be predominantly governed by the change in entropy, denoted as ∆S. Importantly, this change in entropy assumes heightened significance in the context of ion conductivity and molecular reorientational frequencies, particularly above the transition temperature (T tran ), where it is expected to be relatively substantial. |
655ecdfecf8b3c3cd7e9e56c | 54 | In the field of solid electrolytes for all-solid-state batteries (ASSBs), the comparison between β-alumina and NASICONs has been a subject of scientific exploration. However, it wasn't until 2007 that hydride materials, which have been used for hydrogen storage since 1969, were recognized for their potential as fast ion conductors . A pivotal discovery was made when it was found that Li + ions exhibited significant conduction behavior in LiBH 4 during a structural transition from orthorhombic to hexagonal at 380 K, with an approximate conductivity of 10 -3 S cm -1 . This breakthrough propelled the research focus towards hydrides and their derivatives as solid electrolytes for ASSBs, owing to their remarkable ionic conductivity, high thermal and chemical stability, lightweight nature, and favorable mechanical properties, making them an ideal interface between the electrode and electrolyte. |
655ecdfecf8b3c3cd7e9e56c | 55 | In light of the promising characteristics exhibited by hydrides, it is imperative to address several formidable challenges before realizing their practical utility, with a notable concern being their relatively deficient ionic conductivity at RT . Taking lithium borohydride (LiBH 4 ) as an illustrative example, it is evident that its RT ionic conductivity is significantly limited, being three to four orders of magnitude lower than desired . However, a transformative phase transition occurs at elevated temperatures, specifically at 388 K, leading to a remarkable augmentation in lithium-ion mobility, culminating in an impressive conductivity value of 5×10 -3 S cm -1 at 423 K . |
655ecdfecf8b3c3cd7e9e56c | 56 | This underscores the pivotal role played by phase transitions within this class of materials. Consequently, it is paramount to gain a comprehensive understanding of the intricacies surrounding phase transition phenomena, encompassing the underlying factors and origins. This improved conductivity in LiBH 4 arises due to the rotational disorder of the [BH 4 ] -units in the high-T phase, a unique characteristic of such materials. The cationic sites' lattice disorder contributes to the high ionic conductivity, and the rotational disorder of BH 4 units has been investigated through techniques such as synchrotron X-ray powder diffraction, quasi-elastic neutron scattering (QENS), and ab initio MD simulations . In the pursuit of maximizing ionic conductivity, various efforts have been dedicated to stabilizing the highly conductive phase at or near RT. One approach involves doping halides into LiBH 4 , resulting in compounds such as Li(BH 4 ) 1-x X x (X = Cl, Br, and I), where a RT high conducting phase is observed for x > 0.25, particularly in the case of iodine doping . Another popular strategy involves incorporating neutral molecules, such as LiBH 4 •1/2NH 3 , which also exhibit notable ionic conductivity (σ(Li + ) = 7 × 10 -4 Scm -1 at 40 • C) . A more in-depth discussion regarding the role of these neutral molecules will be presented subsequently in this section on materials. |
655ecdfecf8b3c3cd7e9e56c | 57 | By tuning the size and shape of the anions and cations, one can manipulate the diffusion pathways and barriers for cationic diffusion. This leads to the formation of novel anions, such as M 2 B n H n , where (M = Li, Na, and n = 10, 12), that have wider interstitial spaces and thus facilitate cation diffusion. These anions also exhibit remarkable stability due to the strong B-B bonding within the cages . These materials also exhibit the phase transition from an O to a D structure, accompanied by a dramatic increase in ionic conductivity (Fig. ). This transition is triggered by the thermal motion of the anions, which causes them to adopt different orientations and positions in the lattice. For example, Na 2 B 12 H 12 undergoes a transition from an O monoclinic structure to a fully D bcc structure at 545K, passing through an intermediate pseudo-bcc structure at 529K . This results in three orders of magnitude enhancement in Na + ion conductivity, reaching a high value of 0.1 S cm -1 at 540K . A similar but more pronounced transition occurs in Na 2 B 10 H 10 at a much lower temperature of 373 K, leading to a remarkable conductivity above 0.01 Scm -1 at 383K . Unlike the sodium compounds, both Li 2 B 10 H 10 and Li 2 B 12 H 12 require much higher temperatures to change their phase. In the case of Li 2 B 12 H 12 , the anions are arranged in a face-centered position. A possible factor that influences the conductivity of (Li/Na) 2 B 12 H 12 is the size of the anions, which creates a bottleneck effect for the cations. The larger Na ions can pass through the anions more easily than the smaller Li ions, resulting in a higher Na + ion conductivity than Li + . Another factor that affects the T tran of (Li/Na)B 12 H 12 and (Li/Na)B 10 H 10 is the geometrical shape of the anions: |
655ecdfecf8b3c3cd7e9e56c | 58 | The story of ionic conductivity in Li 2 B 12 H 12 and its analogs is a tale of O and D, of cations and anions, of rotations and vibrations . However, the fast and complex motion of the anions also poses a challenge in determining their lattice sites and rotational modes, resulting, in most cases, undetermined structure at the D phase. |
655ecdfecf8b3c3cd7e9e56c | 59 | The study argued that the facile anion reorientations and other low-frequency thermal vibrations lead to fluctuations in the local potential that enhance cation mobility by creating a local driving force for hopping. Sau et al. also shed light on Li 2 B 12 H 12 using force-field-based MD simulations. They developed a force-field that accurately described the structural and dynamical properties of Li 2 B 12 H 12 and captured the O and D phases and their atomistic insights (cationic site occupancy, energy barrier of migration, and hopping mechanism). |
655ecdfecf8b3c3cd7e9e56c | 60 | The intriguing behavior of LiCB 11 H 12 as a solid electrolyte was further explored by Sau et al. , using force-field-based MD simulations. The study revealed underlying mechanisms such as cation-cation and cation-anion correlations. From the comparative study with Li 2 B 12 H 12 , a sharp phase transition from O to D structures at different temperatures was revealed, consistent with experimental observations. The key factors that determined the T tran and the cationic diffusion were the strength of the anion-cation interaction and the symmetry of the anion. LiCB 11 H 12 has a weaker interaction and a lower symmetric anion than Li 2 B 12 H 12 , enabling a lower T tran and a higher diffusion coefficient. Moreover, the symmetry of the anion also affected the sharpness of the transition, as LiCB 11 H 12 exhibited a less abrupt change than Li 2 B 12 H 12 . A further improvement was made by Tang et al. reporting (Li/Na)CB 9 H 10 , which exhibited even lower T tran , close to RT. These transitions are accompanied by a dramatic increase in conductivity, reaching values of 0.03 S cm -1 for LiCB 9 H 10 at 354K and 0.05 S cm -1 for NaCB 9 H 10 at 323K. |
655ecdfecf8b3c3cd7e9e56c | 61 | These vacancies increased the number of carriers available for ion transport, resulting in a remarkable improvement of lithium-ion conductivity by three orders of magnitude (2.0 × 10 -5 S cm -1 at 30 • C). Interestingly, the activation energy of the ion conduction remained almost unchanged, suggesting that the atom deficiencies did not alter the intrinsic mechanism of the process. |
655ecdfecf8b3c3cd7e9e56c | 62 | Tang et al. Through their comprehensive investigation, it was found that the optimal composition occurred at x = 0.7, showcasing the highest ionic conductivity. It was attributed to the structural frustration caused by the mismatched sizes of the anions, which flattened the energy landscape for anion rotation, resulting absence of an O-phase. These studies demonstrate the power of alloying as a strategy to design novel solid-state ion conductors with exceptional properties and potential applications in energy storage devices. |
655ecdfecf8b3c3cd7e9e56c | 63 | The intricate interplay between anions and cations in solid electrolytes plays a vital role in determining their functional properties. In the case of (Li/Na)CB 11 H 12 , an intriguing solid electrolyte, the interaction between anions and cations can be further manipulated by introducing an additional carbon atom, resulting in the compound C 2 B 10 H 12 . |
655ecdfecf8b3c3cd7e9e56c | 64 | As previously highlighted, a comprehensive understanding of the factors influencing the diminishment of rotational activation energy (E r a ) is imperative for stabilizing the high-conductivity phase in complex hydrides at RT. In this regard, we have identified cell expansion as a pivotal factor in modulating anion-cation interactions. Notably, at the T tran , a sharp transition in the unit cell volume (V ) takes place, enabling the anionic and cationic species to move farther apart from each other. This phenomenon leads to a weakening of their mutual interactions or the release of the trapping constraints imposed by the cations, thus affording greater spatial freedom for enhanced anion rotation, as visually depicted in Fig. ). Furthermore, it is worth noting that the strength of cationic trapping can also be mitigated through deliberate modifications. |
655ecdfecf8b3c3cd7e9e56c | 65 | This includes reducing the cation density or decreasing the valency of the anionic species. Such strategic adjustments yield a discernible reduction in the rotational activation energy (E r a ), as illustrated in Fig. ). This multifaceted approach ultimately leads to a significant decrease in the critical T tran , as exemplified by the sequence T tran (C 2 B 10 H 12 )< T tran (LiCB 11 H 12 )< T tran (Li 2 B 12 H 12 ). These observations underscore the intricate interplay of structural and compositional factors in governing the phase transition behavior of complex hydrides. |
655ecdfecf8b3c3cd7e9e56c | 66 | This scientific journey begins with the pioneering work of Higashi et al., reporting remarkable Mg 2+ ion conductivity properties of Mg(BH 4 )(NH 2 )(demonstrated a conductivity of 10 -6 S cm -1 at 150 • C) . The conduction mechanism involved the movement of magnesium ions within tetrahedral structures formed by complex ions. |
655ecdfecf8b3c3cd7e9e56c | 67 | Importantly, Mg(BH 4 )(NH 2 ) was the first magnesium ion conductor where Mg stripping and plating behavior were confirmed. Building on this breakthrough, subsequent investigations explored the electrochemical properties of all-solid-state batteries utilizing Mg(BH 4 )(NH 2 ) as a solid electrolyte, sulfide-based positive electrode materials, and metallic magnesium negative electrodes, revealing promising initial discharge behavior. |
655ecdfecf8b3c3cd7e9e56c | 68 | In recently, the introduction of neutral molecules has emerged as a promising strategy to enhance conductivity in complex hydride-based magnesium ion conductors. Such as introducing ether can also improve ionic conductivity for closo-type magnesium boron cluster salt (10 -3 mS cm -1 at 45 • C), reported by Mizuno et al. , a much higher conductivity than previous solid magnesium electrolytes. Kisu et al. further investigated the divalent conduction of zinc (Zn 2+ ) and magnesium (Mg 2+ ) in hydrated closo-type complex hydrides M B 12 H 12 -nH 2 O (M = Zn, Mg). Among the anhydrous and hydrated MB 12 H 12 -nH 2 O compounds (where n = 0-12 and M = Zn, Mg), ZnB 12 H 12 -12 H 2 O and MgB 12 H 12 -12 H 2 O exhibited exceptional ionic conductivities of 3.8 × 10 -5 and 6.1 × 10 -5 S cm -1 at 50 • C, respectively, surpassing their less-hydrated counterparts. NMR measurements provided insights into the rapid water exchange between crystal water molecules and cation solvation shell promotes Zn 2+ conduction in ZnB 12 H 12 -12 H 2 O. This was further confirmed by Campos et. |
655ecdfecf8b3c3cd7e9e56c | 69 | [165] using metadynamics simulation and correlation analysis between activation energy and other physical properties. The metadynamics simulations study found an increase in the activation energy of cation migration with a lower coordination number of H 2 O. In addition, in the correlation analysis, the activation energies of all divalent ion migrations show similar changes with respect to the number of H 2 O in the structure, suggesting that these ion migrations are governed by hydration. A similar behavior was also observed in Li 2 Sn 2 S 5 -nH 2 O . Moreover, electrochemical cells utilizing ZnB 12 H 12 -12 H 2 O as a solid electrolyte demonstrated reversible Zn 2+ migration, highlighting its potential as a Zn 2+ conductor in all-solid-state Zn batteries. Campos et. al. also identified the key factors that determine the ionic conductivity in this system, such as the cation valencies, the water content, the cation-water coordination number, and the cation-anion distance. |
655ecdfecf8b3c3cd7e9e56c | 70 | In pursuit of developing efficient solid electrolytes for advanced battery technologies, significant research has been dedicated to thiophosphate-based materials. Among these, lithium argyrodites have emerged as a highly researched class, with promising conductive properties reaching up to 24 mS cm -1 . However, it is important to note that these materials often exhibit varying degrees of disorder, particularly in the form of anion exchange between halides and sulfur sites. This anion exchange phenomenon has a significant influence on the cationic transport properties, prompting numerous studies aimed at understanding the impact of iso-and aliovalent substitutions on this exchange process. Nevertheless, altering the chemical composition introduces complexities in comprehending the resultant transport properties, thereby making it challenging to isolate individual influences. |
655ecdfecf8b3c3cd7e9e56c | 71 | Arising from the general formula Li 7 PS 6 , argyrodite materials display a transition from a normal to a superionic phase from orthorhombic to a cubic phase. However, their RT ionic conductivity remains relatively low, with values around 1.6 ×10 -6 S cm -1 . The high-conducting phase can be stabilized by introducing compositions such as Li 6 PS 5 X (where X represents a halogen atom like Cl, Br, I, or a vacancy). The crystal structure of argyrodite materials at the superionic phase is cubic, featuring the space group F 43m. Within this structure, Li ions occupy 24g and 48h sites, S atoms are found in 4a and 4c sites, P atoms are situated in 4b sites, and either halide atoms (X) or vacancies occupy 4a sites. Specifically, the S atoms in 4a sites form PS 4 tetrahedra with P atoms in 4b sites, while the S atoms in 4c sites bridge between PS 4 tetrahedra (as depicted in Fig. )) . The Li ions are distributed in octahedral coordination around the S atoms. The presence of various interstitial sites surrounding the halide ions facilitates high ionic conductivity (Fig. (f)), typically within the range of 10 -2 to 10 -3 S cm -1 at RT, with low activation energies typically around 0.2 to 0.3 eV . |
655ecdfecf8b3c3cd7e9e56c | 72 | Among the single halide argyrodites, Li 6 PS 5 Cl exhibits the highest ionic conductivity at ambient temperature, approximately 10 -3 S cm -1 , followed by Li 6 PS 5 Br with around 10 -4 S cm -1 , while Li 6 PS 5 I displays the lowest ionic conductivity, approximately 10 -6 S cm -1 , as illustrated in Fig. ). The origin of such high ionic conductivity in Li 6 PS 5 Cl and Li 6 PS 5 Br has been the focus of several investigations. It has been observed that disorder involving S 2-/X -site occupancy plays a significant role, resulting in ionic conductivity that is three orders of magnitude lower in fully ordered Li 6 PS 5 I. However, some studies argue that an increase in Li-ion concentration and Li-site disorder is the primary contributing factor . |
655ecdfecf8b3c3cd7e9e56c | 73 | A comprehensive investigation into bulk conductivity within the argyrodite material Li 6 PS 5 Cl has been conducted, employing a combination of 7 Li nuclear magnetic resonance (NMR) experiments and DFT-based MD simulations by Yu et al. . This study has shed light on two distinct lithium-ion diffusion mechanisms within this material: (1) Local Transitions: The study has revealed that lithium ions within Li 6 PS 5 Cl undergo local transitions within the cage-like structures formed by their positions around the sulfur (S) or chlorine (Cl) atoms. These local transitions represent a crucial aspect of lithium-ion mobility within the material. (2) Inter-Cage Transitions: In addition to local transitions, the study has identified inter-cage transitions as a limiting step for macroscopic lithium-ion diffusion within the crystalline bulk structure of Li 6 PS 5 Cl. This implies that the movement of lithium ions between different cage-like structures is a critical factor influencing the overall lithium-ion mobility in the material. |
655ecdfecf8b3c3cd7e9e56c | 74 | Notably, Hanghofer et al. have put forth an alternative perspective based on their research, offering new insights into the diffusion mechanisms within argyrodite materials, especially in cases where X represents Cl, Br, or I. Their investigation employed 31 P magic angle spinning (MAS) NMR to probe the rotational movements of the PS 4 3-units within these materials, yielding the following observations: In cases where X corresponds to Cl or Br, they detected a coupling between the rotational motion of PS 4 3-units and the diffusion of lithium ions, as illustrated in Fig. . This coupling promotes inter-cage jumps of lithium ions, facilitating long-range diffusion within the material. However, in the instance where X represents I, they noted that, despite the presence of local lithium-ion motion, lithium ions do not exhibit long-range inter-cage diffusion. This is attributed to the decoupling of PS 4 3-unit motion from lithium-ion diffusion, primarily due to the soft lattice nature of the material. |
655ecdfecf8b3c3cd7e9e56c | 75 | These complementary studies provide valuable insights into the intricate mechanisms governing ion diffusion within argyrodite materials. They underscore the significance of considering both local and long-range diffusion processes in comprehending the ionic conductivity properties of these materials. Continuing efforts have been dedicated to enhancing ionic conductivity further. For instance, Zhang et al. |
655ecdfecf8b3c3cd7e9e56c | 76 | reported the partial substitution of lithium (Li) sites with Al 3+ /B 3+ in Li 6 PS 5 X (X = Cl and Br), and Li 5.4 Al 0.2 PS 5 Br exhibited an increased ionic conductivity of 2.4 × 10 -3 S cm -1 at RT . In another study, tellurium (Te) doping in Li 6.25 PTe 0.125 S 5.125 Cl 0.75 resulted in a relatively high ionic conductivity of 4.5 × 10 -3 S cm -1 at RT . Additionally, various aliovalent substitutions have been explored to enhance ionic conductivity . |
655ecdfecf8b3c3cd7e9e56c | 77 | In the pursuit of high-performance solid-state batteries, the presence of an electrochemically stable solid electrolyte possessing an exceptional ionic conductivity exceeding 10 mS cm -1 is paramount. This remarkable attribute can be attained through the utilization of sulfide-based Na + solid ionic conductors . The class of materials denoted as Na 3 P nCh 4 represents another category of fast ion-conducting materials. These materials are characterized by the presence of isolated P nCh 4 tetrahedra arranged in a cubic structure. |
655ecdfecf8b3c3cd7e9e56c | 78 | Mobile M -ions occupy the tetrahedral voids within this cubic structure, which belongs to the space group I43m, as illustrated in Fig. . They typically undergo a transition from a normal to a superionic phase, specifically from a tetragonal to a cubic structure. Interestingly, the cubic phase remains stable at RT for compounds like M 3 PO 4 (M = Li, Na, and K). The conductivity of M 3 PO 4 materials is on the order of 10 -3 S cm -1 at 873 K . In contrast, Na 3 PS 4 and Na 3 PSe 4 exhibit lower conductivity values of 0.46 × 10 -3 and 0.10 × 10 -3 S cm -1 , respectively, at RT . It is worth noting that Famprikis et al. made a significant discovery regarding a new highly conductive γ phase in Na 3 PS 4 with an activation energy of 0.11 eV. They proposed that the exceptional Na + ion conductivity in this phase is linked to the rotation of PS 4 tetrahedra. Their findings were corroborated by ab initio MD simulations, which provided additional insights into this phenomenon. |
655ecdfecf8b3c3cd7e9e56c | 79 | A distinct shift in the pair distribution function, as depicted in Fig. , indicated a phase transition. In the high-conductivity phase, a ring-like local density of Na + ions emerged, and their strong connectivity reflects long-range diffusion. The distribution of sulfur (S) atoms, as shown in Fig. , provided further evidence of PS 4 rotation. This rotational behavior was reaffirmed by the rapid decay of the angular autocorrelation function of PS 4 tetrahedra, as illustrated in Fig. . These findings collectively shed light on the intricate mechanisms governing the high ionic conductivity observed in the γ phase of Na 3 PS 4 . |
655ecdfecf8b3c3cd7e9e56c | 80 | Recent research by Bo et al. has focused on enhancing sodium ion conductivity in Na 3 PS 4 and Na 3 PSe 4 by conducting combined experimental and ab initio MD studies . They found nearly identical energy barriers for Na + diffusion in both Se and S substituted systems. Their study proposed that introducing a 2.1% Na ion vacancy could reduce the activation energy to 0.11 eV, resulting in a nearly two-fold increase in conductivity to approximately 28.9 × 10 -3 S cm -1 at RT. Additionally, Zhu et al. |
655ecdfecf8b3c3cd7e9e56c | 81 | investigated Na 3 PS 4 using ab initio MD simulations and discovered that Na + ions primarily migrate through interstitial Na1 (6b) and Na2 (12d) sites, with a preference for the Na1 sites. Aliovalent substitution, such as Na 3+x M x P 1-x S 4 with M = Si, Ge, and Sn, significantly enhances sodium ion conductivity at RT. The excess Na + ions introduced by substitution create a shallow energy landscape, promoting cooperative Na + ion motion and ultimately resulting in high conductivity. |
655ecdfecf8b3c3cd7e9e56c | 82 | Maximum Na + conductivity of 1.66 × 10 -3 S cm -1 is achieved with a Si/P ratio of 6:94, closely matching reported experimental conductivity values . Aliovalent substitution is a commonly employed strategy for enhancing ionic conductivity, as demonstrated by Maus et al. . Their work resulted in remarkable Na + ion conductivity, reaching 41 mS cm -1 in the case of Na 2.9 Sb 0.9 W 0.1 S 4 . It is noteworthy that while pristine Na 3 SbS 4 crystallizes in a tetragonal structure, the aliovalent-substituted Na 2.9 Sb 0.9 W 0.1 S 4 adopts a cubic phase at RT, as indicated by its X-ray diffractogram. |
655ecdfecf8b3c3cd7e9e56c | 83 | 3-ions and exhibits pseudo-rotation at RT. The study also found the existence of pseudo-rotation in other complex transition metal hydrides containing OsH within this class of materials, Li 6 NbH 11 deserves attention, as it features Li + ions exhibiting several meta-stable sites, which implies a lower hopping barrier for Li + and a facile Li + ion conduction (79 mS cm -1 RT ). The high anion rotation may cause structural instability, while the immobile H - ions can increase the structural stability. However, a comprehensive investigation is needed to validate the theoretically calculated ionic conductivity and associated speculation. |
655ecdfecf8b3c3cd7e9e56c | 84 | These materials encounter challenges such as low ionic conductivity at RT and high sensitivity to ambient conditions. The substantial anionic rotational disorder represents a formidable barrier to gaining structural insights during the high-conductivity phase. Elucidating the intricacies of structure-property relationships and comprehending the impact of anion exchange and substitution on ion transport mechanisms within cluster anion-type materials necessitates the application of sophisticated experimental and theoretical methodologies. Additionally, a notable deficiency exists in the establishment of systematic and universally applicable design principles for augmenting ionic conductivity. However, anion rotation is a key feature for designing high-conducting materials. |
655ecdfecf8b3c3cd7e9e56c | 85 | The development of solid ion conductors has primarily relied on the ion conduction mechanism described by Equation , where ion movement occurs through hopping within a crystalline lattice . Numerous structural families have demonstrated reasonably high Li-ion conductivity at RT, with the ability to vary Li-ion conductivity by several orders of magnitude within the same crystal structures . |
655ecdfecf8b3c3cd7e9e56c | 86 | Coordination Number: Early studies found that the conditions that are responsible for high ionic conductivity are realized with monovalent cation in a four-(tetrahedral) or three-coordinated configuration with the anions . Recent studies have expanded this idea to link the high ionic conductivity observed in lithium sulfides (e.g., LGPS and Li 7 P 3 S 11 ) to body-centered cubic (bcc) sulfur-sublattice, which can lead to energetically-equivalent tetrahedral (four-coordinated) sites of lithium and large transport channels . Collective motion: Subsequent investigations have revealed that the collective transport of lithium ions can flatten the migration energy barrier, thereby facilitating rapid ion conduction in materials like LGPS, cubic LLZO, and LATP. In these materials, it has been observed that, on average, two to three Li-ions migrate simultaneously . For instance, Burbano et al. ]. An interesting feature of these ionic conductors is the so-called paddle-wheel effect characterized by a strong correlation between the translational motion of the cation and the rotational one of the cluster , as depicted in Fig. ). Under thermal excitation, non-spherical clusters can lead to a rotationally disordered phase, creating low-energy pathways for ionic migrations and greatly enhancing ionic conductivity . This is exemplified by the high Na-ion conductivity of 30 mS cm -1 of NaCB 9 H 10 as it enters a rotationally disordered phase at RT. Therefore, fully exploiting the rotational degrees of freedom and utilizing the paddle-wheel effect could be a viable way to achieve high cation conductions. However, other than NaCB 9 H 10 , most of the known cluster-containing ionic conductors can only exhibit a transition to the rotational disordered phase at high temperatures (higher than RT) . It has been found that an amorphous structure with low density and a relatively large volume to accommodate the cluster can support the paddle-wheel effect even at low temperatures (e.g., RT) . Anion Valencies: Yet, the rotational freedom of the clusters in such a system seems to be limited, as it is still not comparable to that of the high-temperature phase; for instance Li 2 B 12 H 12 . On the other hand, one notices that the high charge states of the clusters (e.g., '-3' of PS 3 - moment of inertia may hinder their rotations under thermal excitation. Therefore, a combination of light mono-anion clusters with a chemically/structurally accommodating lattice framework could be the key to achieve exceptional high conductors that enjoy great rotational degrees of freedom of the cluster at RT. Introducing Anharmonicity: Anharmonicity is the deviation of a system from being a harmonic oscillator, which means that the restoring force is not proportional to the displacement of the system (Fig. |
655ecdfecf8b3c3cd7e9e56c | 87 | Several theoretical and experimental studies (DFT calculations and neutron scattering experiments) have been performed recently to understand the phonon modes that are coupled most strongly with ionic conductivity and assess the role of anharmonicity. It has been ascertained that the anharmonicity inherent in the phonon vibrational modes of the host lattice exhibits substantial coupling with cationic diffusion. This phenomenon has been notably observed in AgI . The recognized strong coupling between mobile ions and the host lattice holds pivotal importance for the diffusion process. Furthermore, investigations extended to materials like AgCrSe 2 , CuCrSe 2 , Li 7 P 3 S 11 , Li 10 GeP 2 S 12 , β-Li 3 PS 4 , Li 3 YBr 6 , and Li 3 ErCl 6 have revealed analogous coupling phenomena (Niedziela et al., ; Xu et al., ). |
655ecdfecf8b3c3cd7e9e56c | 88 | Dimensionality: Three-dimensional (3D) diffusion pathways are crucial for achieving high σ, as they provide a lower possibility of blocking the ion conduction path. Structures that possess 3D diffusion pathways often exhibit a correlation with high σ, while lower dimensionalities often result in reduced ionic transport due to the potential obstruction of diffusion channels by defects . Even though the conductivity eq. 8 tells a different story, as the dimension term is in the denominator. Optimal bottleneck radius: Ionic conductors with intrinsically low activation barriers are highly desirable in the field of materials science. These materials exhibit the ability to facilitate ionic jumps through polyhedral faces, resulting in a change in the coordination environment surrounding the moving ion along its diffusion pathway. |
655ecdfecf8b3c3cd7e9e56c | 89 | This change in the coordination environment creates a structural "bottleneck" for the diffusion process. It has been established that this BN is closely linked to the activation barriers and conductivities observed in ionic conductors . Structural modifications arising from substitutions and strains can significantly impact the enthalpy change (∆H m ) by altering the interaction between charges , the migration mechanism , frustrated energy landscapes , and the geometry of conduction pathways , among other factors. Of particular importance are materials possessing a percolating, three-dimensionally extended network composed of connected polyhedra with similar coordination environments. This structural characteristic can be achieved, for instance, by exclusively incorporating face-sharing polyhedra of the mobile species (refer to Fig. , ). Such an arrangement minimizes the change in the coordination environment and naturally results in low activation barriers. Framework Modification: Aliovalent substitutions not only alter the lattice size but also the charge carrier density . Since most superionic conductors feature numerous empty or partially filled crystallographic sites , these substitutions have an impact on ionic conductivity. These approaches are primarily concerned with the static crystallographic aspects, focusing on the actual crystallographic structure and stoichiometries. The fourth strategy involves utilizing a softer and more polarizable anion sublattice. The idea behind this approach is that modifying bonding interactions, such as reducing the degree of bond polarity, weakens the bonding between the mobile cations and surrounding anions . Consequently, the lowered activation barrier facilitates the departure of a cation from a stably coordinated lattice site, as less energy is required. This decrease in bond strength, or the vibrational amplitudes around a saddle point, results in a flatter energy landscape for the ion jump . |
655ecdfecf8b3c3cd7e9e56c | 90 | This rationale explains why sulfide ionic conductors often exhibit low activation barriers and higher ionic conductivity compared to more ionic oxides . Several notable examples exist where the modification of the anion framework and the alteration of the strength of local bonding interactions exert a discernible influence on the ionic conductivity of specific compounds. These instances include LISICONs such as Li 3.2 Ge 0.25 P 0.75 (O/S) 4 , NASICONs represented by (Li/Na)Ti 2 (PS 4 ) 3 , argyrodites exemplified by Li 6 P(O/S) 5 Cl) and Li 6 P(S/Se) 5 I, as well as rare-earth halides, denoted as Li 3 Er(Cl/I) 6 , and sodium thiophosphates characterized as Na 3 P(S/Se) 4 . |
655ecdfecf8b3c3cd7e9e56c | 91 | where σ 0 = H R q 2 a 2 N c(1-c)ν0 exp( ∆Sm For cations, the key attributes include a uniform coordination environment, the presence of three-dimensional ion-conducting channels, a high concentration of mobile ions, and a vacancy-rich structure. Additionally, for anions, the ideal features encompass a highly frustrated structure, anion modifications generating new conduction sites, and rotational anions facilitating enhanced cationic diffusion through a paddle-wheel mechanism. These properties collectively contribute to efficient and rapid ion conduction within the material. |
655ecdfecf8b3c3cd7e9e56c | 92 | (σ) is directly proportional to the square of the carrier charge. Consequently, augmenting the charge, as seen in the case of divalent conducting carriers, is more advantageous than relying on monovalent carriers. However, it is essential to acknowledge that higher valency is concurrently associated with stronger interactions with the host framework, thereby increasing the activation energy (E a ) required to impede facile cationic diffusion. |
655ecdfecf8b3c3cd7e9e56c | 93 | Another pivotal factor to consider is the jump distance, denoted as a. A larger value of a is conducive to achieving fast ion conduction; nevertheless, it can potentially lead to structural instability. Similarly, a high cationic concentration is directly proportional to enhanced ionic conductivity, but an excessive number of cations can result in cation-cation repulsion, obstructing the diffusional pathways for cations. Intriguingly, increasing the number of vacancies is also conducive to facilitating rapid cationic conduction. However, this must be balanced against the potential drawbacks of reducing carrier concentration and introducing structural instability. In this context, maintaining a vacancy concentration of around 50% is deemed optimal for achieving the highest ion conduction rates. |
655ecdfecf8b3c3cd7e9e56c | 94 | Additionally, a higher attempt frequency favors fast ion conduction. It is important to note that the dimension factor is found in the denominator of the conductivity equation. Therefore, lower dimensions are favorable for achieving higher conductivity values. However, this preference for lower dimensions must be carefully considered, as it can result in a reduced number of available ion channels, which can have implications for overall ion conduction performance. |
655ecdfecf8b3c3cd7e9e56c | 95 | In this section, we put forth a set of potential characteristics for an ideal fast ion conductor based on our comprehensive understanding of various material classes and the underlying theoretical advancements in fast ion conduction. In order to explore these features, we have classified the materials from both the cationic and anionic perspectives, recognizing the pivotal role played by the immobile framework in facilitating mobile ion conduction. Firstly, it is generally observed that stable lattice sites exhibit higher coordination environments. Consequently, these sites foster stronger interactions, resulting in deeper energy minima, as depicted in Fig. . However, it is important to note that such deeper energy minima can also give rise to larger migration energy barriers, impeding cationic diffusion. Conversely, if we design a crystal structure wherein the coordination number along the ion migration path remains relatively constant, we can achieve a flatter energy landscape that facilitates faster ionic diffusion. Secondly, the presence of a three-dimensional ion migration path is crucial for achieving fast ion conduction. This architectural arrangement minimizes the probability of ion transport blockages due to the presence of grain boundaries. Thirdly, it has been established that the concentration of mobile ions directly affects ion conduction, as evident in the conduction equation. However, it is worth highlighting that both mobile ion concentration and vacancies play significant roles in this context. Therefore, the optimal concentration of mobile ions can provide the ideal vacancy concentration, ultimately leading to enhanced ionic conductivity. Fourthly, ion-ion correlation can also significantly reduce the activation energy barrier. Another important factor is anion rotation, which can introduce structural frustration, enabling a flatter energy landscape for faster cationic diffusion. |
655ecdfecf8b3c3cd7e9e56c | 96 | In this review, we focused on the major classes of fast ion conductors, such as layered oxides, polyhedral connecting frameworks, and cluster anion-type materials, and a comprehensive exploration of their structural and dynamical properties, mechanisms governing ion transportation, and strategies for enhancing ionic conductivity has been undertaken. The pivotal roles played by experimental and theoretical approaches in unraveling the atomistic insights of these materials have been highlighted. This review concluded that fast ion conduction is a complex phenomenon that depends on various factors, such as structural bottlenecks, framework connectivity, carrier concentration, anion rotation, and cation-cation correlation. Based on a systematic understanding of these factors, we suggest some general principles and guidelines for designing and optimizing fast ion conductors. |
655ecdfecf8b3c3cd7e9e56c | 97 | VII. Acknowledgments K.S. thanks the JSPS International Fellowship. K.S. also thanks Prof. Padma Kumar Padmanabhan for his useful discussions. This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research on Innovative Areas "Hydrogenomics", No. JP18H05513 and JSPS Fellowship grant (21F21345). This work was also supported by JSPS KAKENHI Grant-in-Aid for Early-Career Scientists, No. JP23K13542. |
657552c45bc9fcb5c97b17c9 | 0 | First synthesized in 1963 by van Tamelen, Dewar benzene consists of two fused strained cyclobutenes. Suitably substituted derivatives are promising as energy storage materials due to the reversibility of the Dewar benzene formation. For example, hexafluorobenzene is selectively and in high yield photoisomerized to its high energy Dewar isomer, whereas the Dewar isomer of hexamethylbenzene is sufficiently stable to release thermal energy only on demand (Figure ). Dewar benzene derivatives have also been utilized in holographic 3D-information storage, taking advantage of quantum-amplification effects of the photoisomerization, and they have been embedded into polymers to achieve new reconfigurable materials that undergo main-chain structural transformations via valence isomerization. The materials discussed above consist solely of carbon atoms in their backbone, and heteroaromatic analogs of Dewar benzene remain exceedingly rare. Heteroarenes with B-N units embedded in the aromatic framework provoke ever increasing interest due to the extensive applications that are emerging in biochemistry and pharmacology, materials science, and catalysis. The isosteric replacement of C=C for B-N units in benzene to furnish 1,2-azaborinines has proved to be a particularly effective approach. Importantly, azaborinines exhibit significant differences in the aromatic delocalization from benzene. As a consequence, they present distinct reactivity at different ring positions that allows for selective functionalization. In a remarkable recent development, Bettinger and Liu discovered that 1,2-dihydro-1-tert-butyldimethylsilyl-2-mesityl-1,2-azaborinine (A) undergoes photoconversion into the corresponding Dewar valence isomer (B) upon irradiation with UV light (> 280 nm) (Figure ). The kinetically stable isomer B can be converted back to A by a thermal electrocyclic ring-opening reaction that requires an activation energy of (27.0 ± 1.2) kcal mol -1 (half-life of 25 min at 100 ºC). In the presence of Wilkinson's catalyst, the ring-opening occurs rapidly and exothermically even at room temperature. Pursuing new synthetic pathways that take advantage of the facile formation of highly functional BN-Dewar benzene derivatives, Liu and coworkers also developed a strategy to 1,2-substituted cyclobutane derivatives via hydrogenation and subsequent ring-opening of the 4-membered B-N heterocycle. Inspired by these results, we hypothesized that the presence of a strained cyclobutene ring system in BN Dewar |
657552c45bc9fcb5c97b17c9 | 1 | isomers may provide an avenue to new classes of highly functionalized polyolefins via ring-opening metathesis polymerization (ROMP). Although relatively less studied, ROMP of strained cyclobutenes is well established and typically accomplished using Grubbs 2 nd (G2), Grubbs 3 rd (G3), or Hoveyda-Grubbs 2 nd (HG2) generation catalysts. 5b, 9 For instance, Bielawski and coworkers reported the ROMP of a dibromo derivative of Dewar benzene, which upon MeLitriggered elimination rapidly converted into transpoly(acetylene) (Figure ). More recently, Xia and coworkers demonstrated the ROMP synthesis of poly(ladderane)s and poly(benzoladderene)s that could be mechanochemically transformed into polyacetylene derivatives. Different from these polymeric materials, in which rearrangements are triggered by ring-opening of cyclobutene or unzipping of multiple fused rings, the ROMP of pyridinones has recently been shown to result in functional polymers that incorporate 4-membered lactam heterocycles (Figure ). 5b Similarly, we anticipated that ROMP of BN-Dewar isomers could offer access to a novel class of polyolefins that retain the four-membered heterocycle containing an amine and a borane moiety. Here we report the first synthesis of poly(BN-Dewar benzene)s via Ru-catalyzed ROMP of a bicyclic azaboretidine and their structural corroboration by multinuclear and two-dimensional NMR, GPC, FTIR and Raman experiments. The new approach to highly functionalized polyolefins that is presented here is very attractive as the B-N heterocycles can potentially be further elaborated into amine, hydroxyl and other polar functional groups through postmodification approaches that involve B-C and/or B-N bond cleavage. [a] Analyzed by gel permeation chromatography with refractive index (GPC-RI) detection relative to narrow polystyrene standards; Đ = Mw / Mn. [b] Conversion estimated for the crude product based on 1 H NMR integration of the t-butyl H NMR signal of the residual monomer relative to the t-butyl H signal of the polymer; GPC analysis of crude product in THF. [c] Conversion estimated for the crude product based on 1 H NMR integration of the olefinic signals of the residual monomer relative to anisole as a reference; GPC analysis of isolated product in THF. |
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