Beneficial vs. Inhibiting Passivation by the Native Lithium Solid Electrolyte Interphase Revealed by Electrochemical Li + Exchange
Despite being a leading candidate to meet stringent energy targets of Li-ion batteries, the lithium (Li) metal anode has yet to achieve Coulombic efficiency (CE) requirements for long cycle life (>99.9%), particularly at high rates (>1 C). These limitations derive from the native solid electrolyte interphase (SEI) which, among multiple functions, stabilizes and protects deposited Li.The SEI also plays a critical role in regulating Li + exchange between the electrolyte and the electrode, but quantification of this effect has been non-straightforward, and a general relationship between Li + exchange and CE has not been clearly elucidated to date. Using electrochemical impedance and voltammetry, we report self-consistent Li + exchange values of native SEIs over a range of relevant electrolytes with CE spanning 78.0% to >99%. CE and its retention at high rates are found to be positively correlated with the rate of SEI Li + exchange. Additionally, Li + exchange rates increased during cycling in high-CE electrolytes, in some cases by an order of magnitude to exceed 10 mA/cm 2 , whereas for low-CE electrolytes they remained low (<1 mA/cm 2 ), revealing a chemistry-dependent picture of SEI evolution with often-complex dynamics. The evolution in Li + exchange unique to high-CE electrolytes also provides insights into the role and effectiveness of the formation cycle on Cu current collectors upon the first plating step. Altogether, these findings indicate that Li + exchange governs several key processes related to Li deposition and cycling efficiency. Consequently, its quantification can help to guide future high-CE electrolyte design, particularly targeting high rates (> 1 mA/cm 2 ).
Introduction<!>4/27<!>Measuring Li + exchange through impedance spectroscopy<!>S1, ESI<!>8/27<!>Determining Li + exchange by cyclic voltammetry<!>14/27<!>𝒑<!>Implications of 𝒋 𝟎 𝐩 on Li rate capability<!>Coulombic efficiency<!>Electrochemical impedance spectroscopy<!>Cyclic voltammetry
<p>The lithium (Li) metal anode is the most compelling alternative to today's graphite anodes for meeting stringent gravimetric energy targets (>500 Wh/kg) in Li-ion batteries, as it allows a theoretical 10-fold upgrade in capacity (3860 mAh/g vs. 372 mAh/g) at comparable electrode potential. Unlike graphite, Li reversibility in liquid electrolytes, as quantified by Coulombic efficiency (CE)the ratio between the stripping and plating capacity onto a non-Li substrate such as Custill falls below the >99.9% required for >1000 cycles, although electrolytes exceeding 99% CE for at least a portion of cycle life are increasingly common. 1,2 The Li anode also displays rate capability well below the requirements for fast charge (>2 C). 3 These gaps have motivated extensive efforts in increasing CE through electrolyte and interface design. 4 Cycling inefficiencies associated with the Li anode reflect a loss of cyclable Li 0 inventory from the cell. Hence, these inefficiencies can manifest in two ways: electrolyte reduction with concerted consumption of Li 0 to form the solid electrolyte interphase (SEI), and as disconnected metallic Li 0 that becomes electronically stranded during cycling. [5][6][7] Typically, capacity loss due to SEI formation, rather than to metallic Li 0 , predominates at higher CE where deposition is more uniform and less porous. 5,8 Once formed, an imperfect SEI may invite further non-uniform Li/plating stripping 2 and SEI breakage, which necessitates its sustained formation on subsequent cycles.</p><p>Critically, the SEI also regulates transport of Li + ions between electrolyte and electrode.</p><p>As such, the presence of an SEI can have profound effects on transport and kinetics at the interface.</p><p>For instance, cyclic voltammetry of Li plating/stripping on microelectrodes at ultrafast scan rates (e.g., > 10 V/s, Fig. 1, left), which are fast enough to disrupt the native SEI, have found that the intrinsic kinetics of Li plating/stripping, as parametrized by a formal exchange current j0, 9 can be as high as ~40 mA/cm 2 in carbonate-based electrolytes. 10 However, chemical Li + exchange rates, as determined by nuclear magnetic resonance (NMR) measurements on a pristine SEI soaked in similar carbonate electrolytes, have been reported to be substantially lower at ~0.5-3 mA/cm 2 . 11 These lower values correspond to charge-transfer resistances up to ~100-fold 10 higher compared to an SEI-less interface imposed by the ultrafast scan rates, indicating that the SEI can substantially bottleneck Li + exchange.</p><p>Earlier attempts to quantify j0 under more-realistic operating conditions with an SEI present focused largely on carbonate-based electrolytes (Table S1). In those studies, numerical values of 3/27 j0 were variable, e.g. reported values of j0 varied by over one order of magnitude for 1 M LiClO4 in propylene carbonate (PC) across different studies, 12,13 which may relate in part to how the Li electrode was prepared and/or aged, as well as which experimental technique was used. Despite some scatter in the data, however, meaningful differences across electrolytes can begin to be discerned. For instance, j0 was reported to range between 0.1-0.3 mA/cm 2 (0.1 M) 14,15 or 0.95-1.8 mA/cm 2 (1 M) 14,15 for LiClO4 in PC, indicating that j0 can be sensitive to salt concentration.</p><p>Similarly, j0 was found to have a lower range in 1 M LiPF6 PC (0.3-2.2 mA/cm 2 ) 16,17 than 1 M LiBF4 PC (0.5-7.5 mA/cm 2 ) 16,17 , suggesting that the anion also influences j0. Exchange current values have also been reported to a more limited extent in ethers. In those solvents, j0 again exhibited sensitivity to the salt anion, ranging between 1.4-3.6 mA/cm 2 for 1 M LiPF6, LiClO4, LiBF4 and LiAsF6 in 2-methyl-tetrahydrofuran, 2-Me-THF, in increasing order. 17 Similarly, the relative volume fraction of ether blends can affect j0, reported to range between 2.5-7 mA/cm 2 for 1 M LiAsF6 in blends of diethyl ether, DEE, and tetrahydrofuran, THF. 18 Whether any clear and universal relationship between j0 and CE exists has remained less clear, however. This is largely because comprehensive data for j0, as well as concurrently measured CE, are missing across a wide range of electrolytes. Moreover, prior reports have presented conflicting interpretations of whether higher j0 is beneficial [19][20][21][22] (hence desired) or detrimental 17,[23][24][25] (hence to be avoided) for promoting Li reversibility, sometimes based on relatively limited data sets. For example, experiments in tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes led to a conclusion that higher j0 leads to lower CE. 24 In contrast, other studies have reported that incorporation of additives such as FEC 19 or SiCl4 21 into a carbonate electrolyte increases both j0 and CE. As noted, one challenge in generalizing these observations arises, in part, from variable measurement and analysis protocols, e.g., microelectrode 17,23,25 vs. coin cells [19][20][21][22] , and/or slow (< 100 mV/s) 17,19,21,22 vs. fast (> 100 mV/s) 23,25 scan rates (Table S1) used by different groups. Experiments under such conditions report on an apparent or "pseudo"-exchange current density regulated by an SEI, herein denoted 𝑗 0 𝑝 . The above schematic assumes negligible electronic conductivity throughout the SEI and makes no assumptions about its microstructure or chargetransport mechanisms, other than being dominated by Li + as charge carrier.</p><!><p>In this context, there is a significant need for quantification of Li + exchange under practical battery conditions and elucidation of its dependence on electrolyte chemistry, testing protocol, and rate. In this study, Li + exchange at the Li anode is systematically quantified using cyclic voltammetry (CV) at slow scan rates (1 mV/s) and electrochemical impedance spectroscopy (EIS), both of which intentionally allow an SEI to develop natively (Fig. 1, right). In order to span a diverse set of SEI chemistries of interest, we employ a selection of historically-relevant and more modern, high-performance electrolytes. Given that Li + exchange is both electrode-history-and surface-area-dependent, we interpret the exchange rates in the framework of a "pseudo"-exchange 5/27 current, 𝑗 0 𝑝 , that represents the total rate of Li + exchange on the electrode. 𝑗 0 𝑝 is found to vary considerably across electrolytes, with an unambiguous positive correlation with CE consistent across different measurement protocols. In addition, tracking of 𝑗 0 𝑝 evolution upon cycling reveals unexpected dynamic behaviors that allow for new insights into the evolution of electrochemicallyactive surface area and SEI quality upon cycling. These findings provide a new approach for future efforts to develop high-rate and high-CE electrolytes by considering how to maximize the SEI rates of Li + exchange.</p><!><p>Representative electrolytes were selected for analysis to span a range of reported CE values based on a previous comprehensive literature analysis. 4 The electrolytes (Fig. 2a) included 1. (99.0% and 99.3%, respectively). All CE values were measured using a pre-formation/reservoir cycling protocol following Adams et al. 26 as described further in the Experimental methods (Fig.</p><!><p>). These values generally agree well with prior literature reports (Table S2 and Fig. S2, ESI), with small variations likely explained by the difference in CE protocols between the original reports and the methodology used herein.</p><p>Before examining the proposition that SEI can be a relevant factor that limits, and thus determines, electrochemical measures of Li + exchange, we first examined possible relationships between CE and two key bulk electrolyte properties. First, Li + diffusivity in the bulk electrolyte (D), which governs mass transport upon Li + plating, 2, 9, 27 was measured using diffusion-ordered spectroscopy (DOSY) NMR. Values of D ranged from 8.90 × 10 -8 cm 2 /s for 7 M LiFSI FEC to 3.42 × 10 -6 cm 2 /s for 1 M LiTFSI DOL/DME + 3wt% LiNO3 (Fig. S3, ESI). No clear relationship with CE was observed. Additionally, we also examined the 7 Li chemical shift in liquid NMR, which has been shown to reflect the degree of contact-ion pairing when compared between electrolytes of alike solvent (e.g., varying anion chemistry and/or concentration in the same 6/27 solvent). 28 Under these circumstances, 7 Li chemical shift has been shown to correlate with CE 29 and intrinsic j0 (i.e., absent an SEI). 10 Here, however, when compared over electrolytes of dissimilar chemistry (e.g., different salt and solvent chemistries), 7 Li chemical shift did not reveal a correlation with CE (Fig. S3, ESI). These observations indicate that intrinsic properties measured at the bulk electrolyte level are not directly predictive of CE measured under conditions that permit an SEI to form. Now considering the presence of the SEI, Li + exchange currents were first measured using EIS analysis in symmetric Li/Li cells. Fig. 2b shows a typical protocol. Following cell assembly, an initial impedance spectrum, denoted by an open blue circle, was conducted in the pristine state ("Pre-cycle EIS") after a 5 h rest at OCV, sufficient to establish a native, intact SEI. This interphase includes artifacts from the preparation and handling of the electrodes. 30 Next, a complete plating/stripping cycle was conducted (0.5 mA/cm 2 , 1 mAh/cm 2 ). Following a 5 min. rest at OCV, which is sufficient to re-form a native SEI 31 but short enough to avoid exacerbated aging effects, 30,32 a second impedance spectrum was collected. This process repeated subsequently for additional plating/stripping+EIS cycles.</p><!><p>For the studied electrolytes, the pre-cycle EIS spectrum was typically largest in magnitude.</p><p>Subsequent plating/stripping+EIS iterations consistently yielded smaller semi-circles, attributed to breakage of the original SEI and formation of a thinner native interface. Notably, after 5 cycles the RSEI magnitudes varied significantly across electrolytes (Fig. 2d), being highest for 1.5 M LiAsF6 in 2-Me-THF (512.6 Ω cm 2 on cycle 5), followed by 1 M LiPF6 in EC/DMC (47.6 Ω cm 2 ) and 1 M LiTFSI DOL/DME + 3% LiNO3 (18.3 Ω cm 2 ). Correspondingly, the 𝑗 0,𝐸𝐼𝑆 𝑝 values across these three electrolytes showed an increasing trend of 0.03, 0.30 and 0.79 mA/cm 2 , respectively.  The converged values obtained in other carbonate electrolytes (for instance, 0.30 mA/cm 2 for 1 M LiClO4 PC) were also within the same order of magnitude (typically < 1 mA/cm 2 ) as values noted in earlier works by EIS (Table S1). Significantly, when averaging across all cycles 1-50, a clear and positive correlation is observed between 𝑗 0,𝐸𝐼𝑆 𝑝 and CE across electrolytes (Fig. 3c). This trend also held true after a single cycle, when the magnitudes of 𝑗 0,𝐸𝐼𝑆 𝑝 were more similar across  after just a single cycle, 34 leading to higher porosity as well as formation of inactive Li 0 and SEI Li + . 5 Curiously, the absence of any decreasing 𝑗 0,𝐸𝐼𝑆 𝑝 values in Fig. 3a suggests that the accumulation of these inactive materials does not suppress total Li + exchange rates, even in low-CE electrolytes. Additionally, a qualitative comparison between scanning electron microscopy (SEM) images of plated Li electrodes after 5 cycles in 1 M LiPF6 EC/DMC and 1 M LiTFSI DOL/DME + 3wt% LiNO3 confirms that these electrolytes are, as expected, porous after cycling, and that the lower-CE electrolyte shows higher surface area and more irregular deposits (Fig. S6, ESI). 35 Thus, the higher degree of porosity and surface area in the low-CE electrolytes do not translate into an increase in the rate of Li + exchange upon cycling in these electrolytes. This observation stands in contrast to high-CE electrolyte which, despite developing less SEI over a comparable number of cycles, displays significant increases in total Li + exchange rates. In other words, unlike in low-CE electrolytes, the additional surface created in high-CE electrolytes is active, as it allows Li + exchange between the electrolyte and the underlying Li metal, even if it forms to a comparatively lesser extent.</p><p>We more-carefully explored whether Li + exchange continued to increase unabatedly in high-CE electrolytes. Interestingly, 𝑗 0,𝐸𝐼𝑆 𝑝 for the DOL/DME-based electrolytes (exhibiting Type 2 behavior) further increased for approximately ten additional cycles, but did eventually stabilize around 60 cycles, e.g., 8.3 mA/cm 2 for 2 M LiFSI 1 M LiTFSI DOL/DME + 3wt% LiNO3 and 5.7 mA/cm 2 for 1 M LiTFSI DOL/DME + 3wt% LiNO3 (Fig. 4a). This stable value was, however, much higher compared to low-CE electrolytes (0.39 mA/cm 2 for 1 M LiClO4 PC, Fig. 4a).</p><p>Although the underlying reasons for this behavior are not fully clear, a possible explanation is as follows (Fig. 4b). In low-CE electrolytes, the residual SEI has low Li + exchange values, thus provides little advantage for preferential nucleation of freshly-deposited Li in subsequent cycles.</p><p>Because the SEI is not effectively re-utilized, subsequent Li plating requires re-establishment of the entirety of the SEI on each cycle, leading to lower CE. This regrowth process forms Li deposits 11/27 and SEI to a similar extent cycle-to-cycle for a fixed plating/stripping capacity. In contrast, in high-CE electrolytes, SEI residue from previous cycles may remain active for Li plating, and can be built-upon; hence, each cycle is history-dependent. These observations are consistent with cryo-EM evidence that demonstrate substantial necking and loss of active deposits upon stripping in low-CE electrolytes, whereas high-CE electrolytes show "reusable" SEI scaffolds that can be refilled on cycling. 36 We suggest that the eventual stabilization of Li + exchange in the high-CE electrolytes reflects a point at which the growth of new SEI scaffolds has completed, possibly due to their percolation throughout the electrode, although this point remains speculative at present.</p><p>Overall, these findings indicate that "passivation", a commonly used term to describe the function of an SEI, 6,32,37,38 has substantially different meaning depending on the system: while all SEIs at least minimally passivate Li against chemical reactivity with the electrolyte, some SEIs are inhibiting with respect to Li + exchange specifically, whereas others permit it.</p><!><p>Li + exchange can also be determined from CV measurements during a plating/stripping step by evaluating the current-voltage slope around the Li 0 /Li + equilibrium potential. 9 For the voltammetry analysis presented herein, coin cells were used to best capture conditions in a typical Li battery 12/27 with limited electrolyte volume (100 µL) and also to provide direct correspondence with the conditions used for EIS. In the adopted protocol (Fig. 5a), 11 continuous CV cycles were conducted in Cu/Li coin cells at 1 mV/s between -0.2 V and 1 V, with full stripping of Cu occurring on the end of each sweep. A single CV scan provides one data point for pseudo-Li + exchange current, which we here term 𝑗 0,𝐶𝑉 𝑝 (ESI Note 2). To further compare values on a freshly deposited Li reservoir, 1 mAh/cm 2 of Li was galvanostatically plated at 0.5 mA/cm 2 , followed by another set of 11 CV cycles from -0.2 V to 0.2 V. Finally, a full galvanostatic stripping step was conducted to 1 V to yield back a formed Cu current collector, followed by a final set of 11 CVs.</p><p>The current-voltage curves obtained on pristine Cu (Fig. 5b) in an exemplar 1 M LiPF6 EC/DEC electrolyte show a typical behavior of metal deposition onto an inert substrate. On the forward scan, in which Li is first plated on Cu, an overpotential of ~100 mV is required to initiate</p><p>Li plating on all cycles. On the reverse scan, Li continued to be plated on Cu until voltage exceeded the equilibrium potential (E 0 , the voltage at which j = 0), after which Li was stripped from Cu. At low plating/stripping overpotential windows of this reverse scan (typically <20 mV, see Fig. S7, ESI), the magnitudes of the anodic and cathodic currents are small (< 0.2 mA/cm 2 ), and the current exhibits a linear response to overpotential. The slope of this response is proportional to 𝑗 0,𝐶𝑉 𝑝 by j = 𝑗 0,𝐶𝑉 𝑝 (RT/F)Ew,corrected, 9 where Ew,corrected is the measured potential corrected for ohmic losses and E 0 (Experimental methods, ESI Note 2). Fitting 𝑗 0,𝐶𝑉 𝑝 at the linear limit avoids reliance on more complex kinetic models such as Tafel extrapolation, Butler-Volmer or Marcus-Hush formalisms that may be unphysical for interphase-dominated processes. 9 Li + exchange for the pristine, Li-plated and fully-stripped Cu current collectors determined by this method are shown in Fig. 5c. In 1 M LiPF6 EC/DEC, 𝑗 0,𝐶𝑉 𝑝 is low for the pristine Cu electrode (0.02 mA/cm 2 ), but increases substantially and stabilizes to an average of 0.26 mA/cm 2 after Li is plated on the electrode. After Li is stripped away, 𝑗 0,𝐶𝑉 𝑝 remains more similar to that of the Li-plated electrode than the pristine Cu electrode, i.e. is initially 0.40 mA/cm 2 in the 1 st CV cycle before stabilizing around 0.19 mA/cm 2 in the 11 th cycle. Altogether, these results confirm that Li plating behaviors are strongly dependent on surface conditioning and its history. Thus, in the following, we first examine the cross-electrolyte results on plated Li (middle column of Fig. 5b) for direct comparison with the EIS data, and later return to compare the differences between uncycled and cycled Cu.</p><p>13/27</p><!><p>Comparable voltammetry analysis on plated Li was performed in all other electrolytes. Fig. 6a shows Tafel plots for the three electrolytes highlighted in Fig. 2, where a steeper j-Ew,corrected capacities (Fig. 6c), typically 1-9 mAh/cm 2 depending on the electrolyte (ESI Note 3). This agreement emphasizes that the obtained values of Li + exchange hold physical meaning, as they emerge to govern Li plating/stripping consistently regardless of measurement technique. Indeed, the same positive 𝑗 0,𝐶𝑉 𝑝 -CE correlation was observed on plated Li from CV measurements (Fig. 6d). We also observe that, for all electrolytes, the j-Ew,corrected relationship shows typical Tafel behavior at high overpotentials, i.e., linear Ew,corrected-log j behavior as seen in Fig. 6a. This behavior could be fitted well by Butler-Volmer 9 and Marcus-Hush-Chidsey 39 models at potentials below 100 mV (Fig. S8, ESI), even though such models do not consider an SEI. However, it is unclear at present whether this agreement carries any physical meaning.</p><p>15/27</p><!><p>We now return to compare 𝑗 0 𝑝 on the Cu substrate before and after significant amounts of Li have been cycled (Fig. 5c). Fig. 7a shows 𝑗 0,𝐶𝑉 𝑝 over the first 11 CV scans on pristine Cu for all electrolytes. Given that unformed Cu surfaces typically exhibit low current densities, the total amount of charge plated/stripped was small (e.g., ~0.03-0.3 mAh/cm 2 per cycle). In general, 𝑗 0,𝐶𝑉 𝑝 values exhibited some scatter depending on the electrolyte but were typically low (<1 mA/cm 2 ).</p><p>After the 1 mAh/cm 2 galvanostatic cycle (Fig. 7b), an evident upgrade in Li + exchange was observed in certain, but not all, electrolytes. This is further depicted in Fig. 7c-d, which summarize 𝑗 0,𝐶𝑉 𝑝 on cycle 11 on the pre-and post-conditioned Cu, but now organized by CE. First, we observe that no correlation was found on Cu prior to the 1 mAh/cm 2 plating step (Fig. 7c), which is contrasted with post-plated Cu, where a strong monotonic trend was observed linking 𝑗 0,𝐶𝑉 𝑝 and CE. This finding shows that, similar to the trends on Li, higher-CE electrolytes are also more effective at modifying the Cu interface, an effect that persists even after full galvanostatic stripping of the underlying Li. In contrast, low-CE electrolytes display minimal difference in Li + exchange between pre-plated and stripped Cu, indicating that SEI formation on Cu is ineffective in these electrolytes even after a 1 mAh/cm 2 cycle. The quantitative CE-𝑗 0,𝐶𝑉 𝑝 monotonic relationship specific to post-stripped Cu is fully summarized in Fig. 8. Even when subjected to repeated 1 mAh/cm 2 plating/stripping cycles, Type 1 electrolytes could not reach Li + exchange rates comparable to those achieved after just a single cycle in Type 2 electrolytes (Fig. S9, ESI). 17/27 The protocol in Fig. 5 was used to collect all data. Color legend in (d) as in (a-c).</p><p>18/27 exchange and CE as the SEI is formed on Cu, we exploited the fact that CV measurements enable the calculation of CE and Li + exchange concomitantly on each scan. This 𝑗 0,𝐶𝑉 𝑝 -CE relationship is shown in Fig. 9a for 1 M LiPF6 EC/DEC (Type 1 electrolyte) and Fig. 9b for 2 M LiFSI 1 M LiTFSI DOL/DME + 3wt% LiNO3 (Type 2 electrolyte); these data were obtained from the same CV scans underlying Fig. 7a-b. Focusing first on the Type 1 electrolyte (Fig. 9a), a monotonic increase in per-cycle CE was observed with increasing 𝑗 0,𝐶𝑉 𝑝 from cycles 1-10. Following a 1 mAh/cm 2 galvanostatic cycle, 𝑗 0,𝐶𝑉 𝑝 exhibited a step-change increase to higher values, with accompanying improvements in CE. A similar behavior was observed in the Type 2 electrolyte (Fig. 9b), though expectedly with much higher CE and 𝑗 0,𝐶𝑉 𝑝 . These results agree with our previous findings on the relationship between these two parameters, but it also highlights a subtle new insight: CE is also coupled to Li + exchange on a per-cycle basis. In this context, the lower firstcycle CE typically found in anode-free batteries 26,40  LiNO3.</p><!><p>Finally, we examined the implications of Li + exchange on the rate capability (here defined as CE vs. cycling current density) attainable in different electrolytes. As extensively discussed, Li + exchange quantifies the facility by which an SEI exchanges Li + from the electrolyte to the electrode. Thus, in addition to its relationship to CE, we hypothesized that the rate of SEI Li + exchange defines the maximum cycling current density j at which Li can be cycled without substantial decline in CE. For this analysis, we focused on the Li + exchange obtained by EIS (𝑗 0,EIS 𝑝 ) which was measured between galvanostatic cycles, a protocol that is most relevant to how CE is typically measured. The Type 1 and Type 2 electrolytes tested herein exhibit two distinct regimes:</p><p>Type 1 electrolytes show 𝑗 0,EIS 𝑝 < 1 mA/cm 2 , a value that is in the range of typical cycling currents used in Li anode studies (0.1-2 mA/cm 2 ). Type 2 electrolytes, however, can sustain 𝑗 0,EIS 𝑝 exceeding 10 mA/cm 2 , a value well-beyond typical cycling currents.</p><p>In this context, Fig. 10 shows CE as a function of current density (see Fig. S1 factor that may enhance their ability to cycle well at high rate compared to Type 1 electrolytes.</p><p>Because growth of SEI surface area increases total Li + exchange, it is possible that non-uniform Li deposition is not necessarily detrimental and may even be beneficial for high cycling currents in these electrolytes, as long as such deposits remain electronically-percolated. We further note that, when 𝑗 0 𝑝 is very high, other rate-limiting factors may become relevant before the SEI becomes the major bottleneck. For instance, 𝑗 0 𝑝 can reach values >5 mA/cm 2 in Type 2 electrolytes; under these conditions, mass transport at the bulk electrolyte may become limiting prior to SEI Li + exchange. 41 Further studies on this interplay, and transition between limiting regimes, 4 are expected to be fruitful.  along with a total of 50 µL of the desired electrolyte. The assembled CR2032 was closed with a wave spring, crimped and taken outside the glovebox for testing.</p><!><p>Coulombic efficiency (CE) measurements were conducted as described by Adams et al. 26 in Cu/Li cells. All measurements include a single formation cycle, plating 4 mAh/cm 2 Li on Cu and then stripping until all reversible Li is depleted to 1 V at 0.5 mA/cm 2 . A Li reservoir of 4 mAh/cm 2 is then plated onto Cu, and 1 mAh/cm 2 from the reservoir is cycled 10 times, after which the reservoir is completely stripped to 1 Between each half-cycle, the cell rests at open-circuit voltage (OCV) for 1 min. CE is determined as the ratio of the total amount of Li stripped to the total amount plated, excluding the formation cycle. For all measurements, the formation cycle was performed at 0.5 mA/cm 2 but current density for reservoir cycling was performed at the indicated current density. Fig. S1, ESI, demonstrates the CE measurement protocol. All CE measurements were conducted on a battery cycler (BTS, MTI).</p><!><p>Electrochemical impedance spectroscopy (EIS) measurements were conducted in Li/Li cells that were taken outside of the glovebox immediately after cell assembly. EIS spectra were collected on Biologic MPG or VSP-3 potentiostats with a sine-wave excitation amplitude of 5 mV around OCV, with a typical frequency range of 10 mHz to 20 kHz, with 10 points collected per decade. Before the initial EIS measurement, the cell rested at OCV for 5 h.</p><p>After the spectra were collected, the single semi-circle (typically corresponding to frequencies >20 Hz) was fit to an equivalent resistor-capacitance-Warburg circuit, where the RC impedance represents the Li 0 /Li + redox charge-transfer resistance and the Warburg impedance represents diffusional transport across the interphase. 33 The circuit was fit to data using a custom Python script that employs a least-square method. The total resistance to interface exchange (RSEI) was taken as the sum of the Warburg coefficient (W1) and the charge-transfer resistance Rct to capture all SEI-relevant processes including charge transfer (kinetics) and transport, which are physically coupled in a thin but finite SEI. The equivalent EIS exchange current (𝑗 0,EIS p</p><p>) is then inversely proportional to the total SEI resistance (RSEI) 9  where 𝑘 is the Boltzmann constant, 𝑇 is the temperature, 𝑛 is the number of electrons transferred per reaction (𝑛 = 1 for Li + /Li redox) and 𝑒 is the electron charge. See ESI Note 1 for more information on the fitting process.</p><!><p>Cyclic voltammetry (CV) data were acquired in two-electrode coin cells using Biologic MPG or VSP-3 potentiostats, with a typical scan rate of 1 mV/s unless otherwise mentioned. For experiments where Cu was the working electrode (i.e., pristine or fully-stripped Cu), the measurement started at OCV (typically ~3 V for pristine Cu and ~0 V for Cu) and voltage was scanned from OCV to -0.2 V, and then from -0.2 V to 1 V, cycling within the voltage window for the desired number of cycles. For experiments where Li was the working electrode (i.e., plated Li), the measurement starts at OCV (~0 V) and voltage was scanned from OCV to -0.2 V, and then between -0.2 V to 0.2 V, for the desired number of cycles. All CV cycles were conducted continuously. In order to obtain 𝑗 0,𝐶𝑉 𝑝 , the CV data was processed such that, first, the cell voltage was iR-compensated and corrected such that the potential at zero current was set to zero (i.e., Ew,corrected(j = 0) = 0 V). Then, data in the low-overpotential regime, where the current response is linear with overpotential, was used to obtain a representative exchange current 𝑗 0,𝐶𝑉 𝑝 such that j = 𝑗 0,𝐶𝑉 𝑝 (F/RT) Ew,corrected, thus avoiding the use of more complex kinetic models or Tafel interpolation at high overpotentials that may yield unphysical results for interphase-dominated processes. 9 More information on the CV analysis is presented in ESI Note 2.</p>