Stabilizing Cathode Materials of Lithium-Ion Batteries by Controlling Interstitial Sites on the Surface
A surface-vacant-site-occupation (SVSO) strategy is reported to be very effective at stabilizing the high-voltage cathode. The precise control of the interstitial lattice sites on the surface is able to modulate the movement of metal ions by suppressing the diffusion and dissolution of transition metal ions while maintaining the mobility of Li + , resulting in a very stable cathode with excellent cyclability and rate capability.
INTRODUCTION<!>The Bigger Picture<!>RESULTS<!>DISCUSSION<!>EXPERIMENTAL PROCEDURES<!>SUPPLEMENTAL INFORMATION
<p>Lithium ion batteries (LIBs) have recently been identified as promising power sources for applications such as electric vehicles and smart grids, which are essential to reduce the dependency on nonrenewable energy sources. [1][2][3][4][5][6] Therefore, extensive research efforts have been devoted to the development of high-energy electrode materials for LIBs. However, an inherent problem for the high-energy cathode materials is their reduced stability in the LIB system. 2 The increase in energy density, either by the charge/discharge capacity or by the working voltage, is commonly accompanied by increased chemical, electrochemical, and thermal instabilities of the cathode materials. 2,7 Notably, the parasitic side reactions at the electrode/electrolyte interface and the structural degradation caused by the transition-metal (TM) ion dissolution have been well studied for LIBs. [8][9][10][11] When an LIB is charged to a high voltage, the cathode may either evolve oxygen to cap the voltage, as in the Li 1Àx CoO 2 cathode, or it may lose TM cations into the electrolyte and form unwanted phases at the electrolyte/electrode interface, as occurs in the spinel Li 1Àx [Ni 0.5 Mn 1.5 ]O 4 . 2,12 It becomes critically important to develop cathode materials with significantly improved stabilities for the next generation of high-energy-density LIBs.</p><p>Two strategies have been used to solve the instability problems for high-energydensity cathode materials. First, surface coating of the active particles can provide a chemically inert layer for their protection. [12][13][14][15][16] Although a detailed understanding</p><!><p>Enhancing the energy density of lithium-ion batteries (LIBs) is becoming increasingly important to the ever-growing demand for energy storage. The major challenge comes from the paradox between the high energy density and the intrinsic instability of the electrode materials, causing concerns about the feasibility of the practical use of the LIBs with high energy density. Cathode materials are considered as the key component for LIBs and their degradation is proposed to originate from the unwilling electrode/electrolyte interface issues. Therefore, surface modification is believed to be critical for the stabilization of highenergy-density battery systems. This work provides a unique perspective on the modification of cathode materials by directly modulating the vacant lattice sites on the particle surface, opening up great opportunities for both the practical application of highenergy-density LIBs and fundamental studies in the surface modification of functional materials.</p><p>of the stabilization mechanism is still lacking, an optimized coating has been reported to contribute to an alleviation of the surface degradation and to reducing electrolyte decomposition. 13 Second, cationic substitution has also been well practiced as a means to tune the crystal chemistry toward more stable cathode materials. 17,18 Interestingly, recent progress revealed that it is possible to suppress the structure degradation of the spinel cathode materials by introducing Ti 4+ on the surface instead of the bulk phase. 19,20 This observation represents a unique combination of two approaches for improved cell performance.</p><p>In this contribution, a surface-vacant-site-occupation (SVSO) strategy is reported to be very effective to stabilize the high-voltage cathode. Although the importance of the vacant lattice sites for cation diffusion has been well validated, 21,22 their manipulation to control ionic transport across the electrode/electrolyte interface has not been studied systematically. This lack is intriguing given that the movement of cations across the interface is involved in phase transitions and the structural degradation of cathode materials. 23 Here, by using the high-voltage Li[Ni 0.5 Mn 1.5 ]O 4 (LNMO) as an example, we show that it is possible to introduce extra metal ions, typically Al 3+ , into some of the unoccupied octahedral sites on the particle surface to form a stable 10 nm epitaxial layer with desired surface chemistry to combat the parasitic side reactions. A combined effort of experimental data and theoretical calculations shows that the modified surface is able to modulate the movement of metal ions by suppressing the diffusion and dissolution of TM ions while maintaining the mobility of Li + , resulting in a very stable cathode with excellent cyclability and rate capability. This SVSO strategy is conceptually different from the conventional strategies for stabilizing cathode materials. Although the control on Li sites has been known to be beneficial for improved structural stability, 20,23,24 our work on the interstitial lattice sites provides a highly efficient way to directly tackle the key problem of structural degradation related to TM migration. Moreover, the capability of constructing a precisely tunable nanoshell makes it possible to build a model system for research on the interfacial chemistry of the surface modified high-energy-density cathode materials. This contribution not only provides an explicit answer to the complicate chemistry of the solid-state reactions on the material surface but also identifies an interesting SVSO route for the stabilization of cathode materials by forming an epitaxial surface layer with its interstitial lattice sites well controlled.</p><!><p>The selection of LNMO as a model system in our research is not only driven by its higher energy density than that of insertion cathodes but also initiated by its wellknown unstable surface at a high working voltage of 4.7 V, 12 whose control can provide essential information for the stabilization and practical application in the nextgeneration LIBs. In brief, the LNMO was first coated with a AlPO 4 layer and then treated at a suitable temperature to initiate the solid surface reaction. Figure 1A showed a representative transmission electron microscopy (TEM) image of the pristine LNMO sample we used, which had a free surface borderline. Our continuous efforts in growing uniform coatings endow us with a capability to prepare a continuous and dense AlPO 4 shell whose thickness is precisely controlled at nanometer accuracy. [25][26][27][28] As shown in Figure 1B, a uniform surface layer formed on each LNMO particle, which is amorphous in nature with a thickness around 4 nm. Elemental mapping (Figures 1C-1F) analysis confirmed the homogeneous dispersion of AlPO 4 all across the LNMO particle, showing a typical core-shell structure with a conformal coating. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) revealed an AlPO 4 content of around 0.99% in weight (Table S1). Systematic control of the synthesis conditions, typically the concentrations of the reactants and the reaction time, enabled us to control the shell thickness (Figures S1A and S1B) by different AlPO 4 loadings. Notably, our efforts in growth control provided a unique capability to form a continuous and dense AlPO 4 nanoshell on the surface of LNMO particles. Meanwhile, precise control of its thickness was achieved in nanometer accuracy. For the Al-LNMO sample we prepared, solid reaction was clearly observed on the surface and the uniformity of the coating layer guaranteed the homogeneous diffusion of Al 3+ into the surface layer of Al-LNMO, ensuring reliable identification of the surface structure at the atomic level, which made it possible for us to discover that directly modulating the interstitial lattice sites on the surface was very effective for the stabilization of cathode materials. The surface AlPO 4 layer was susceptible to further heat treatment. When heated to 700 C for 2 hr, the sample turned smooth on the surface with a total disappearance of the amorphous surface layer (sample denoted as Al-LNMO, Figure 1G). Characterization by high-resolution TEM (inset in Figure 1G) and X-ray diffraction (XRD; Figure S2A) showed no discernible effect on the crystalline structure after heating. However, X-ray photoelectron spectroscopy (XPS) showed the emergence of weak signals for Li 3 PO 4 together with obvious peak shifts for both Al 2p (Figure S2B) and P 2p (Figure S2C), indicating the change of their local bonding environments after the vanishing of the AlPO 4 shell. 16,29 To further understand the effect of the surface reaction, we managed to increase the amount of AlPO 4 to a high level by depositing a much thicker shell for the surface transformation (Figure S3). As shown in Figure S3C, when a larger amount of Al 3+ was introduced, a left shift of the XRD peaks could be observed, indicating a small expansion of the surface lattices after Al 3+ insertion. Moreover, tiny peaks of Li 3 PO 4 could finally be identified in the XRD patterns and the intensity of these peaks increased for the one with a larger AlPO 4 surface layer, confirming the possibility to form Li 3 PO 4 during the heat treatment. Considering the low melting point of Li 3 PO 4 , it was inferred that the Li 3 PO 4 would easily sinter together so that the Al-LNMO showed a clean surface under TEM.</p><p>To better understand the structure of the Al-LNMO sample, we studied it under the aberration-corrected scanning TEM (STEM), which allows the direct imaging of the atomic scale structure, 30 in a high-angle angular dark-field (HAADF) mode. Figure 2B shows the STEM-HAADF result for a randomly picked particle. The image was recorded along the [110] crystallographic direction so that the positions of the heavy atomic columns could be clearly identified (Figure 2A). 23 The STEM-HAADF analysis confirmed that the spinel framework would survive after the introduction of Al 3+ . Surprisingly, a distinct difference in atomic configuration between the surface and the interior of the particle was observed (Figure 2B). The inner part (labeled with a blue rectangle in Figure 2B) showed a characteristic diamond shape (Figure 2C) with empty 16c octahedral sites in the center, which were clearly occupied for the parts located on the particle surface (marked with a yellow rectangle in Figure 2B), forming a center-filled diamond as shown in the magnified image in Figure 2D. Such an occupation of the 16c sites showed a depth of around 10 nm (indicated by the white dashed line), forming an epitaxial structure on the surface. The whole process is shown in Figure 2A, in which the unoccupied 16c sites are pictured as green and hollow circles and blue and solid once they are occupied. Detailed analysis by electron energy loss spectroscopy (EELS) was used to identify the atoms in the 16c sites (demonstrated in Supplemental Information). The periodical variation of the EELS signal intention clearly showed that Al ions were mainly distributed in the 16c sites (Figure S4).</p><p>Inspired by the importance of surface chemistry for the electrochemical properties of cathode materials, we performed a systematic investigation of the battery performance on these LNMO samples. Figure 3A compares two representative samples of the LNMO we prepared and the Al-LNMO after its surface modification. The plateaus at around 4.7 V are characteristic of the Ni 2+/3+ and Ni 3+/4+ redox couples, and the one at 4.0 V corresponds to the trace amount of Mn 3+ . 12,31 Interestingly, after 150 cycles, the pristine LNMO sample showed not only obvious decay in the discharge capacity but a substantial change in the voltage plateau as well, indicating a severe polarization of the electrode. 32 In contrast, Al-LNMO had an almost unchanged discharging profile, revealing an extraordinary stability of the cathode materials during the electrochemical process. Notably, significant improvement in capacity retention (from 85.4% to 97.6%) had been realized after the Al 3+ insertion. The discharge capacities and the coulombic efficiencies at different cycling stages (Figure further confirmed the advantage of Al-LMNO. The battery test at an elevated temperature (55 C) showed that the Al-LNMO sample had a more stable output (Figure S5).</p><p>It is surprising to see that the insertion of Al 3+ into the empty 16c sites showed a boost in the rate capability of the cathode materials (Figure 3C). This was outwith our expectations considering the fact that an exotic atom in the 16c site, which was known to be the key component of the diffusion path for metal ions, 21,22  probably block the diffusion path of Li + . The alternating current (AC) impedances of the coin cell from different samples (Figure 3D) showed that the surface treatment of the LNMO sample did not have an obvious effect on the impedance of the LNMO sample. The most significant feature was the change of the impedance after cycling (Figure 3D). A much increased interface resistance was observed on the LNMO sample, which is probably a result of the formation of thick solid-electrolyte interface films or structure degradations. 33,34 To understand the surface chemistry of the cycled electrode better, we characterized the samples after long cycles with TEM and XPS. The TEM image revealed the emergence of an amorphous layer on the surface of those untreated LNMO particles after cycling (Figure S6A), which was probably related to the accumulated products resulting from the electrolyte decomposition on the surface. 33 In contrast, the cycled Al-LNMO sample was much cleaner on the surface, such that no coating layer was observed (Figure S6B). Meanwhile, XPS analysis on the untreated LNMO sample showed a much lower Mn 2p XPS signal than the Al-LNMO one after cycling (Figure S6C), which was in good agreement with the TEM observation on the LNMO surface considering that a covered surface of LNMO would produce a reduced signal of Mn. Therefore, it was expected that the surface treatment possessed a capability to effectively suppress the side reactions on the surface, forming a much more stable cathode/electrolyte interface for improved electrochemical performance. Besides, a contrast sample with the same amount of Li 3 PO 4 as Al-LNMO showed an almost identical electrochemical performance to the pristine LNMO sample (Figure S7), indicating that the existence of Li 3 PO 4 did not have a substantial effect on the battery performance. Moreover, similar improvements on battery performance could also be achieved on different LNMO samples (Figure S8), confirming the general applicability of the SVSO strategy.</p><p>We found that the existence of Al 3+ in the empty 16c octahedral site could sustain a long cycle life. Figure S9 shows the surface atomic structure of a typical Al-LNMO particle after 100 cycles. No discernible change in the positions of Al, Mn, or Ni was observed, showing a very stable surface of Al-LNMO. Importantly, the ICP-AES results showed that the SVSO strategy could effectively alleviate the dissolution of Mn during cycling, which had been known as a major problem for the structural failure of this kind of Mn-based cathode. 35,36 As shown in Table S2, the level of dissolved Mn in the electrolyte after cycling was much lower for Al-LNMO than for LNMO. Further experiments showed that it was possible to reduce Mn dissolution as we introduced more Al 3+ into the sample (Table S2). The one with five times the Al 3+ content showed almost no detectable Mn loss after 100 cycles. Unfortunately, the excess Al 3+ also jammed the Li + transport, as revealed by the increased impedance and a much lower capacity. Accordingly, a balanced effect was necessary to ensure sufficient Li + mobility with suppressed Mn loss.</p><p>Our XRD characterization on the LNMO sample after 100 cycles showed the emergence of a tetragonal phase (Figure 4A). 20,37,38 It had been proposed that this phase transition was induced by the migration of TM ions from octahedral sites to adjacent tetrahedral sites in the spinel framework, 37 which seems reasonable considering that the neighboring tetrahedral sites were found to be energetically more favorable for the hop-in of TMs. 22 Such a structural degradation was obviously suppressed in Al-LNMO, as revealed by the absence of these extra peaks. Considering the fact that the migration and dissolution of Mn species were usually initiated on the surface of the particles, [39][40][41] the SVSO strategy showed a good capability to combat this problem right from its source, revealing itself as an efficient way to stabilize the cathode. The synchrotron-based soft X-ray absorption spectroscopy (sXAS) analysis is also used to study these two comparative systems after extended cycles. Figure 4B compares the Mn L-edge sXAS spectra of the LNMO (solid) and Al-LNMO (dashed) electrodes. Significant differences if the discharged states could be observed after 200 cycles (200D). The Mn sXAS signal of the LNMO electrode after 200 cycles was greatly reduced and severely distorted, suggesting that the LNMO electrode surface is covered by a thick layer of the electrolyte decomposition products after extended cycles. 42 In sharp contrast, for the Al-LNMO electrode, the Mn sXAS spectrum retains the same line shape as that of the first discharge state. Meanwhile, after 200 cycles, a strong fingerprinting peak of carbonate species is observed in the O K-edge sXAS spectrum on the LNMO electrode (Figure 4C). However, for the Al-LNMO electrode, the O K-edge sXAS spectrum resembles that of the first cycle discharge state. These sXAS results show clearly and consistently that the electrolyte decomposition with extended electrochemical cycling is significantly suppressed for the Al-LNMO sample. Although such surface analysis is not direct evidence of reduced Mn dissolution in Al-LNMO, we note that the remarkable resistance to Mn dissolution also helps maintain a highly stable surface by avoiding severe structural and stoichiometric phase transformation on the Al-LNMO electrode surface.</p><p>To elucidate the contribution from Al 3+ and exclude the interference from the other elements, here we compared the surface property of two representative samples with compositions defined as Li 0.25 Ni 0.5 Mn 1.5 Al 0.25 O 4 , the value derived from a simplified calculation based on the Al 3+ content on the surface of Al-LMNO, and Li 0.25 Ni 0.5 Mn 1.5 O 4 as its counterpart without Al 3+ . To investigate the migration of metal cations, we did some preliminary calculations by using the combination of density functional theory (DFT) and bond valence (BV) methods, which are effective tools for extracting possible diffusion pathways for different ions. The calculation revealed that it was energetically favorable for Al 3+ to stand in the 16c sites (Table S3). Figures 5A-5D show the migration pathways of Mn and Ni ions on the basis of the BV calculation. Besides hopping between two neighboring 16d sites through the center of an oxygen tetrahedron, pathways for TM ions among 16c sites were also formed after Li + removal. As shown in Figure 5E, Al 3+ stationed in the 16c sites interrupted the connectivity of the pathways of TM ions and thus reduced the migration probability of TM ions in Li 0.25 Ni 0.5 Mn 1.5 Al 0.25 O 4 compared with those in Li 0.25 Ni 0.5 Mn 1.5 O 4 , which finally diminished the structural degradation during cycling.</p><p>Because of technical challenges, an accurate E a was not possible in a reasonable time for such a complex system with a multitude of possible diffusion paths. Therefore, we tried a semi-quantitative interpretation of the diffusion energy barriers by using the BV method because E a from the BV calculation has been proved to be qualitatively consistent with the activation energy given by a DFT-based calculation. 43,44 The BV calculation on E a of Li + diffusion showed no obvious change at a relatively low Al content (x < 0.25, Table S4). However, if excessive 16c sites were occupied, Li + movement would also be a problem because of an increase in the activation energy (x > 0.25, Table S4), eventually deteriorating the performance of LNMO. Therefore, the synthetic conditions, particularly the AlPO 4 content and the heating treatment, must be under proper control so as to guarantee sufficient activity for Li + by optimizing the Al 3+ content on the surface. It was reasonable to expect that, compared with TM ions, Li + was more tolerable to the jammed traffic caused by Al 3+ because the former had a much stronger interaction with the oxygen anions. 45 Our experimental data and theoretical calculations showed that the TM mobility was more susceptible to the existence of Al 3+ in the diffusion path. Accordingly, a favorable effect was achieved with a much reduced structural degradation without affecting the Li + mobility.</p><!><p>In summary, we have demonstrated a conceptually different approach to stabilizing cathode materials through the manipulation of the vacant lattice sites of a spinel at the particle surface. By using the high-energy 5 V spinel cathode, LiNi 0.5 Mn 1.5 O 4 , as an example, we showed that the empty 16c octahedral sites could be controllably occupied by Al 3+ through an on-surface solid reaction process, resulting in a 10 nm epitaxial surface layer with tunable surface chemistry. A combined effort of experimental approaches and theoretical calculations revealed that such controlled occupation at the 16c sites can achieve optimized effects including highly suppressed TM-ion dissolution, well retained Li + mobility, and greatly reduced undesired side reactions with the electrolyte, resulting in a much more stable 5 V cathode with good cyclability and rate capability. Notably, our efforts in growth control have provided us with a unique capability to form a uniform and continuous AlPO 4 nanoshell on the surface of LNMO particles. Meanwhile, precise control of its thickness was achieved with nanometer accuracy. For the Al-LNMO sample we prepared, solid reaction was clearly observed on the surface and the uniformity of the coating layer guaranteed the homogeneous diffusion of Al 3+ into the surface layer of Al-LNMO, ensuring reliable identification of the surface structure at atomic level, which made it possible for us to discover that directly modulating the interstitial lattice sites on the surface was very effective for the stabilization of cathode materials. Such an SVSO strategy not only provides an effective way to stabilize cathode materials with high-energy density but also sheds light on the mechanism of improved surface chemistry related to lattice site control in a wide range of research fields, such as catalysts, ceramics, and solid-state ionics, and can provide guidance for the development of advanced materials.</p><!><p>All the experimental and setup details, as well the EELS line scan measurements and theoretical calculations, are provided in the Supplemental Experimental Procedures.</p><!><p>Supplemental Information includes Supplemental Experimental Procedures, nine figures, and four tables and can be found with this article online at https://doi. org/10.1016/j.chempr.2018.04.020.</p>