An ever-growing demand for Li-ion batteries (LIBs) with larger energy density has recently been directed to their widespread adoption in electric vehicles and renewable energy technologies.1-4 The energy density of a battery is defined as the product of capacity and working voltage. In this regard, a combination of a high-capacity SiOx anode (theoretical value CT = 2680 mAh g-1 with x=1 at ≤0.4 V vs. Li/Li+) and a high-potential (5 V-class) spinel LiNi0.5Mn1.5O4 cathode (CT = 147 mAh g-1 with an average operating potential of 4.7 vs. Li/Li+) has been considered as a realistic battery system. This battery system offers a higher theoretical energy density (~710 Wh kg-1) than that of commercial LIBs (~290 Wh kg-1) with a graphite anode (theoretically 372 mAh g-1) and 4 V-class LiCoO2-based cathode.5–7 However, its realization has been limited by severe electrolyte decomposition at anode and cathode surfaces because the lithiation/delithiation reaction potentials of SiOx (≤0.4 V vs. Li/Li+) and LiNi0.5Mn1.5O4 (≥4.7 V vs. Li/Li+) are located outside the operating potential window of existing electrolytes.8,9
To address this issue, functional electrolytes and electrolyte additives have been developed over the last decades. These materials provide a wide potential window and form a passivation film (solid electrolyte interphase, SEI) on the anode surface, retarding the electrolyte degradation kinetically by blocking direct contact between the electrode and electrolyte. For instance, ether-based electrolytes (such as 1,2-dimethoxyethane (DME) and tetrahydrofuran)3,10 and fluorinated solvent-based electrolytes (such as fluoroethylene carbonate (FEC))11,12 were applied to improve the reversibility of SiOx and LiNi0.5Mn1.5O4, respectively. Nevertheless, to the best of our knowledge, a stable SiOx|LiNi0.5Mn1.5O4 battery has not been achieved due to the absence of electrolytes guaranteeing high reduction and oxidation stabilities.
For establishing the design strategy, the thermodynamic lithiation/delithiation potential of SiOx, which is dominated by the chemical potential of Li+ in the electrolyte, should be highlighted as a critical factor in the reduction stability of the electrolyte.13–16 In particular, the reductive decomposition of the electrolyte at the SiOx anode can be significantly suppressed by upshifting its inherent lithiation/delithiation potential close to the potential window (reduction limit) of the electrolyte, thereby disburdening the kinetic support of SEI. However, this simple strategy is yet to be applied for high-voltage batteries because the mechanisms behind the potential shift are still unclear, although it is well known that the redox potential of an electrode depends on the electrolyte.13–16 Recently, our group reported that the chemical potential of Li+ increases in an ion-crowded (Li+–Li+ and Li+–anion) environment.17 The study indicated that the progressive formation of ion-pair aggregates contributes to an upward shift of the redox potential of an electrode, thus thermodynamically mitigating the electrolyte decomposition at the electrode surface.
Another critical factor in protecting the SiOx surface is the formation timing of SEI upon lithiation.18,19 The lithiation/delithiation of SiOx accompanies huge volume change up to 200%.6 When the SEI is belatedly formed on the expanded SiOx surface during lithiation, it is easily damaged with large volume contraction during delithiation. This situation accelerates electrolyte degradation by continuously exposing the electrode surface to the electrolyte. Therefore, the SiOx surface should be fully covered with a robust SEI at an earlier lithiation stage before its dramatic expansion to ensure stable cycling with minimum electrolyte decomposition.
Considering these two emerging but important factors (Li/Li+ upshift and early-stage SEI formation), we herein report a unique electrolyte strategy. This strategy enables unprecedentedly stable SiOx|LiNi0.5Mn1.5O4 full cells (96 % capacity retention after 500 cycles with an upper cut-off voltage of 4.9 V at a low constant current of 0.5 C-rate), adopting thermodynamic and kinetic mechanisms to hinder several degradation modes. Details of the electrolyte design, mechanism of improved reduction and oxidation stabilities, and battery performance are also presented.
Concept of the electrolyte
With multiple considerations toward total functionalization, we designed a concentrated LiN(SO2F)2 (LiFSI)/methyl (2,2,2-trifluoroethyl) carbonate (FEMC) electrolyte. The LiFSI salt was selected owing to its high solubility and ability to form robust anion-derived SEI.20,21 FEMC was selected as a solvent because the fluorine moiety lowers its highest occupied molecular orbital (HOMO), increasing the oxidation stability.11,22 In addition, it reduces the negative partial charge on an oxygen atom in the carbonate,23,24 weakening Li+(solvent)n solvation and helping to form more Li+-anion ion pairs.25,26 Increasing the salt concentration is essential to achieve a peculiar solution structure, wherein Li+ and FSI- ions are strongly coordinated with each other and form a congested ion-pair network, realizing several advantageous features that are mentioned below.
First, extensive formation of ion-pair network destabilizes Li+ in the electrolyte (increases the chemical potential of Li+) and upshifts the reaction potential of SiOx, narrowing the gap to the thermodynamic potential window of the electrolyte and hence disburdening the kinetic support of SEI (Figure 1).17 Second, high salt concentration and ion pair-predominant solution structure increases anion concentration at the negatively charged SiOx surface, allowing earlier-stage (at higher potential) formation of an anion-derived SEI before the huge expansion of SiOx. This phenomenon effectively suppresses further electrolyte degradation at the SiOx surface (Figure 1). Finally, several technical issues encountered at the LiNi0.5Mn1.5O4 positive electrode under a high potential, such as electrolyte oxidation, Al corrosion, and transition metal dissolution, are highly repressed in concentrated LiFSI/FEMC by lowered HOMO and weak solvating ability of the electrolyte.11,22,23,27,28 Overall, high-level reduction and oxidation stabilities are expected to be guaranteed.
The design strategy of the electrolyte structure was verified by molecular dynamics (MD) simulation. The calculated solution structures of 1.0 mol L-1 (L) LiFSI/ethyl methyl carbonate (EMC), 1.0 M LiFSI/FEMC, and nearly saturated 3.4 M LiFSI/FEMC are shown in Figure 2a. The solution structure of the electrolytes dramatically changes with the introduction of FEMC and high salt concentration. For instance, Li+ is surrounded by three or four EMC solvent molecules and one FSI- anion in 1.0 M LiFSI/EMC. In contrast, multiple Li+ and FSI- ions are coordinated together while forming a closely packed ion-pair network in 3.4 M LiFSI/FEMC. The position of primary peaks in the radial distribution function plots of Li+–Li+ and Li+–FSI- shifts toward closer distances, and their intensities are significantly increased in 3.4 M LiFSI/FEMC (Figure S1). The solution structure of the prepared electrolytes was also evaluated via Raman spectroscopy (Figures 2b and S2). The S–N–S stretching vibrational mode of FSI- was largely upshifted from 728 to 740 cm-1 by replacing EMC to FEMC, indicating that the coordination state of FSI- changed from the solvent-separated ion pairs (SSIP; bare FSI- and/or FSI- solvating to solvent molecules) and contact ion pairs (CIPs; FSI- coordinating to one Li+) to ion-pair aggregates (AGG-I and AGG-II, where more than two FSI- and Li+ ions coordinated together while forming an ion-pair network).29 The peak position further upshifted to 752 cm-1 with an increase in the salt concentration. Thus, the computational and experimental studies suggest that the 3.4 M LiFSI/FEMC electrolyte has an ion-pair aggregate-predominant solution structure.
Suppressed reductive degradation
Figure 3a illustrates the charge–discharge curves and cycling stabilities of Li|SiOx half-cells in three electrolytes, that is, 1.0 M LiFSI/EMC, 1.0 M LiFSI/FEMC, and 3.4 M LiFSI/FEMC. After 80 cycles, 93 % of the capacity was retained in 3.4 M LiFSI/FEMC, which was much higher than that in 1.0 M LiFSI/EMC (19% after 80 cycles) and 1.0 M LiFSI/FEMC (85% after 80 cycles). A similar trend was observed in the galvanostatic Li plating/stripping test wherein the 3.4 M LiFSI/FEMC electrolyte presented significantly higher Coulombic efficiency (~97 %) than that of 1.0 M LiFSI/EMC (≤60 %) (Figure S4). The best performance was achieved using the 3.4 M LiFSI/FEMC electrolyte designed in this study.
As an important factor dominating the reversibility, we focus on the location of the redox potential of Li/Li+ (the lowest possible value of reaction potential of SiOx), which should have a close relationship with the degree of reductive decomposition of the electrolyte.17 Cyclic voltammetry (CV) was performed with an IUPAC-recommended electrolyte-independent redox system (ferrocene, Fc/Fc+) as a reference electrode to estimate the redox potential of Li/Li+ (and hence the reaction potential of SiOx) in various electrolytes.30,31 As shown in Figure S5, the redox potentials of ferrocene were 3.25, 3.07, and 2.91 V vs. Li/Li+ in 1.0 M LiFSI/EMC, 1.0 M LiFSI/FEMC, and 3.4 M LiFSI/FEMC, respectively. Correspondingly, the redox potential of Li/Li+ (V vs. Fc/Fc+) greatly upshifted by 0.34 V in 3.4 M LiFSI/FEMC relative to that in 1.0 M LiFSI/EMC (Figures 3b and S5).
This remarkable thermodynamic variation of the redox potential originates from the chemical potential of Li+ (the stability of Li+) in the electrolyte.17 As shown in Figures 2 and S1, the 3.4 M LiFSI/FEMC electrolyte exhibits a unique solution structure, wherein most Li+ and FSI- are intensively coordinated to form a crowded ion-pair network. This configuration drastically increases the electrostatic potential at the Li+ site in the electrolyte (Figure S6), upshifting the redox potential of Li/Li+. Notably, the redox potential of Li/Li+ in the ion-pair aggregate-predominant 3.4 M LiFSI/FEMC (2.91 V vs. Fc/Fc+) was approximately 0.6 V higher than that in the solvated Li+-predominant 1.0 M LiFSI/diglyme (3.48 V vs. Fc/Fc+) (Figure S5),32 which exhibited much poorer stability at the anode (Figure S7). Consequently, the largely upshifted redox potential of Li/Li+ (and thus, simultaneously upshifted reaction potential of SiOx) in 3.4 M LiFSI/FEMC disburdens the kinetic support of SEI, contributing to the decrease in the reductive decomposition of the electrolyte at the SiOx surface.
In addition to the thermodynamic upshift of the electrode potential, kinetic hindrance of the electrolyte decomposition, including the formation timing18,19 and the composition of SEI film33,34 on the SiOx surface, should be considered. This is because the reaction potential of the SiOx anode was still outside the thermodynamic potential window of the electrolyte, although the burden of the SEI kinetic support was decreased by upshifting the electrode potential in 3.4 M LiFSI/FEMC. In this regard, X-ray photoelectron spectroscopy (XPS) was performed for the SiOx electrode set at approximately 0.4 V vs. Li/Li+ upon the initial lithiation process because a significant volume expansion of SiOx commenced at that potential.19,35,36 From the XPS profiles of Si2p, the peaks of SiOx and lithiated SiOx (LixSiOy) were detected at 0.4 V in 1.0 M LiFSI/EMC, suggesting that the SiOx surface was directly exposed to the electrolyte without sufficient passivation above 0.4 V vs. Li/Li+ (Figure 4a). In contrast, the SiOx surface was fully covered with the SEI in 3.4 M LiFSI/FEMC before its significant expansion below 0.4 V vs. Li/Li+, evidenced by the absence of Si2p-derived peaks at 0.4 V (Figure 4b). Moreover, from the F1s, N1s, and S2p spectra, large amounts of anion-derived components (S–O–F, Li–F, N–S, Li–N–O, S=O, and S–S) were detected on the SiOx surface in 3.4 M LiFSI/FEMC. It should be noted that these SEI components possess high mechanical/chemical stability and high ionic conductivity,34,37,38 thus alleviating stress from a huge volume change of SiOx during its lithiation/delithiation.
As mentioned previously, a highly salt-concentrated 3.4 M LiFSI/FEMC electrolyte has a unique solution structure wherein multiple FSI- anions are coordinated with a strong Lewis acid Li+. This phenomenon promotes the complete formation of anion-derived SEI at a higher potential by increasing the anion concentration at the negatively charged anode surface.20,39,40 The initial capacity of the Li|SiOx half-cell, including the apparent capacity observed by the additional consumption to form SEI, was larger in 3.4 M LiFSI/FEMC down to 0.4 V vs. Li/Li+ (~90 mAh g-1) than that in 1.0 M LiFSI/EMC (~55 mAh g-1). Thus, the SEI was progressively formed above 0.4 V vs. Li/Li+ with sacrificial anion reduction in 3.4 M LiFSI/FEMC before the expansion of SiOx (Figures 4a and 4b).
The earlier formation of the anion-derived SEI upon the initial lithiation process achieves an essential effect to stabilizing the structural integrity of the SEI layer (Figure 4c). For example, the LixSiOy peak in 1.0 M LiFSI/EMC after 30 cycles was observed from the XPS profiles of Si2p with cycled SiOx electrodes (Figure S8). Furthermore, the electrochemical impedance spectroscopy (EIS) measurement revealed that the interfacial resistance of SiOx electrodes gradually increased as the cycling number in 1.0 M LiFSI/EMC increased (Figure S9). All these results indicate that the SiOx surface was exposed to the electrolyte with partial destruction of SEI, resulting in severe electrolyte decomposition. Conversely, the SiOx surface was robustly protected by the anion-derived SEI, while maintaining low interfacial resistance in 3.4 M LiFSI/FEMC even after 50 cycles (Figures S8 and S9). Overall, stable cycling of the SiOx anode was thermodynamically (upshifted electrode potential) and kinetically (early formation of the anion-derived SEI) achieved with 3.4 M LiFSI/FEMC electrolyte, which originated from its distinct solution structure.
Improved oxidation stability
After confirming that the unique coordination environment of Li+ and FSI- in 3.4 M LiFSI/FEMC enables a highly reversible SiOx anode, we evaluated the oxidation stability of the electrolyte. Most electrolytes developed for silicon-based anodes do not utilize high-potential cathodes because of their poor oxidation stability.6,8 However, the 3.4 M LiFSI/FEMC electrolyte allows a stable operation of the Li|LiNi0.5Mn1.5O4 half-cell (≥90% capacity retention after 100 cycles with an average Coulombic efficiency of ~99 %) with an upper cut-off voltage of 4.9 V at a low constant current of 0.2 C-rate, which is unachievable in 1.0 M LiFSI/EMC (continuous oxidative decomposition at 4.5 V vs. Li/Li+) and 1.0 M LiFSI/FEMC (78% capacity retention after 100 cycles with poor Coulombic efficiency of < 90%) (Figures 5a and S10). Therefore, the 3.4 M LiFSI/FEMC electrolyte can offer a wide operating potential window covering high-potential LiNi0.5Mn1.5O4 cathode and high-capacity and low-potential SiOx anode based on a combination of thermodynamic and kinetic effects, which is demonstrated herein.
Linear sweep voltammetry (LSV) was performed to investigate the anodic limit of the electrolytes and their tolerance against Al corrosion, using Pt or Al as a working electrode. The anodic current flow on Pt was commenced at over 5.3 V (vs. Li/Li+) in 1.0 M and 3.4 M LiFSI/FEMC electrolytes, which was higher than that in 1.0 M LiFSI/EMC (4.7 V vs. Li/Li+), therefore embracing the upper cut-off potential of the LiNi0.5Mn1.5O4 cathode (4.9 V vs. Li/Li+) (Figures 5a and 5b). It is widely accepted that introducing electron-withdrawing fluorine groups to a solvent lowers the HOMO level, increasing the oxidation stability of electrolyte thermodynamically.11,22 However, the Al corrosion was not observed up to 5.8 V (vs. Li/Li+) in 3.4 M LiFSI/FEMC, although it was started below 4.5 V (vs. Li/Li+) in 1.0 M LiFSI/EMC and 1.0 M LiFSI/FEMC electrolytes (Figure 5c). The Al corrosion occurs with the formation of Al3+(solvent)n solvates and/or Al(anion)n solid complexes, followed by their diffusion into the bulk electrolyte.27 However, such solvates are hardly formed in 3.4 M LiFSI/FEMC owing to the weak solvating power of FEMC.23 Moreover, the diffusion of Al3+ complexes into the bulk electrolyte is kinetically retarded because of the poor dissolution ability of highly concentrated electrolyte.28 Briefly, the solvent fluorination and high salt concentration improved the oxidation stability of the electrolyte and prevented Al corrosion in a thermodynamic (lowered HOMO level) and kinetic (impeded generation and diffusion of Al3+ solvates and complexes) manner.
Highly stable SiOx|LiNi0.5Mn1.5O4 batteries
Before assembling full cells, a pre-activation process was applied to SiOx electrodes to reach its inherently large irreversible capacity at the first cycle (Figure S11).6 It should be noted that the pre-activation does not indicate the pre-full-lithiation that provides a large amount of additional Li source to the cell; the details on the pre-activation process are mentioned under the Method section. The cycling stability of the pre-activated SiOx|LiNi0.5Mn1.5O4 full cells with an upper cut-off potential of 4.9 V at a low constant current of 0.2 C-rate in various electrolytes is illustrated in Figure S12. Notably, 85% of the maximum capacity was maintained after 300 cycles in 3.4 M LiFSI/FEMC, whereas the cell performance drastically dropped in 1.0 M LiFSI/EMC (continuous oxidative decomposition upon charge process) and 1.0 M LiFSI/FEMC (56 % after 100 cycles). A negligible capacity decay was observed in 3.4 M LiFSI/FEMC at a constant current of 0.5 C-rate over 500 cycles, with the Coulombic efficiency near to 100% (Figure 6a). The high reduction and oxidation stabilities with 3.4 M LiFSI/FEMC were further proved under a tough cycling condition at 55 °C (Figures S13 and S14), demonstrating largely improved cycling stability (72% capacity retention after 300 cycles) with highly suppressed transition metal dissolution from the cathode material, which aggravated the battery performance significantly.41 This result was in contrast to the state-of-the-art commercial electrolyte, 1.0 M LiPF6/ethylene carbonate (EC):dimethyl carbonate (DMC) (1:1, v:v), which exhibited poor cycling stability (52% after 100 cycles) with severe transition metal dissolution. In conclusion, the 3.4 M LiFSI/FEMC electrolyte provides unusual stabilities at the SiOx anode and LiNi0.5Mn1.5O4 cathode electrodes, which are incomparable to those of the other electrolytes proposed to date (Figure 6b).