An electrolyte for SiOx/LiNi0.5Mn1.5O4 batteries

doi:10.21203/rs.3.rs-1759356/v1

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An electrolyte for SiO_x/LiNi_0.5Mn_1.5O₄ batteries

https://doi.org/10.21203/rs.3.rs-1759356/v1

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The Li-ion batteries composed of high-capacity SiO_x anode and high-potential LiNi_0.5Mn_1.5O₄ cathode is the most realistic options to meet the increasing demands for higher-energy-density storage systems. However, the absence of electrolytes covering the multifaceted issues from highly reducing and oxidizing conditions at the SiO_x anode and LiNi_0.5Mn_1.5O₄ cathode, respectively, has limited its applications. Herein, we present an electrolyte that can solve these issues simultaneously. A concentrated LiN(SO₂F)₂/methyl (2,2,2-trifluoroethyl) carbonate electrolyte upshifts the reaction potentials of the SiO_x anode and enables an earlier formation of a robust anion-derived passivation film upon charge to diminish the reductive degradation of the electrolyte. The weak solvating ability of the electrolyte provides a high oxidation tolerance against Al corrosion and transition metal dissolution from the cathode material. Excellent long-term cycling of 5 V-class SiO_x|LiNi_0.5Mn_1.5O₄ full cells (96% capacity retention after 500 cycles at a low constant current of 0.5 C-rate) was achieved.

Li-ion batteries

high-voltage electrolyte

wide potential window

concentrated electrolyte

silicon oxide anode

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 SiO_x anode (theoretical value C_T = 2680 mAh g^-1 with x=1 at ≤0.4 V vs. Li/Li⁺) and a high-potential (5 V-class) spinel LiNi_0.5Mn_1.5O₄ cathode (C_T = 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 SiO_x (≤0.4 V vs. Li/Li⁺) and LiNi_0.5Mn_1.5O₄ (≥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 SiO_x and LiNi_0.5Mn_1.5O₄, respectively. Nevertheless, to the best of our knowledge, a stable SiO_x|LiNi_0.5Mn_1.5O₄ 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 SiO_x, 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 SiO_x 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 SiO_x surface is the formation timing of SEI upon lithiation.^18,19 The lithiation/delithiation of SiO_x accompanies huge volume change up to 200%.⁶ When the SEI is belatedly formed on the expanded SiO_x 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 SiO_x 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 SiO_x|LiNi_0.5Mn_1.5O₄ 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 SiO_x, 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 SiO_x surface, allowing earlier-stage (at higher potential) formation of an anion-derived SEI before the huge expansion of SiO_x. This phenomenon effectively suppresses further electrolyte degradation at the SiO_x surface (Figure 1). Finally, several technical issues encountered at the LiNi_0.5Mn_1.5O₄ 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|SiO_x 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 SiO_x), 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 SiO_x) 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 SiO_x) 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 SiO_x surface.

In addition to the thermodynamic upshift of the electrode potential, kinetic hindrance of the electrolyte decomposition, including the formation timing^18,19 and the composition of SEI film^33,34 on the SiO_x surface, should be considered. This is because the reaction potential of the SiO_x 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 SiO_x electrode set at approximately 0.4 V vs. Li/Li⁺ upon the initial lithiation process because a significant volume expansion of SiO_x commenced at that potential.^19,35,36 From the XPS profiles of Si2p, the peaks of SiO_x and lithiated SiO_x (Li_xSiO_y) were detected at 0.4 V in 1.0 M LiFSI/EMC, suggesting that the SiO_x surface was directly exposed to the electrolyte without sufficient passivation above 0.4 V vs. Li/Li⁺(Figure 4a). In contrast, the SiO_x 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 SiO_x 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 SiO_x 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|SiO_x 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 SiO_x(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 Li_xSiO_y peak in 1.0 M LiFSI/EMC after 30 cycles was observed from the XPS profiles of Si2p with cycled SiO_x electrodes (Figure S8). Furthermore, the electrochemical impedance spectroscopy (EIS) measurement revealed that the interfacial resistance of SiO_x electrodes gradually increased as the cycling number in 1.0 M LiFSI/EMC increased (Figure S9). All these results indicate that the SiO_x surface was exposed to the electrolyte with partial destruction of SEI, resulting in severe electrolyte decomposition. Conversely, the SiO_x 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 SiO_x 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 SiO_x 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|LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄ cathode and high-capacity and low-potential SiO_x 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 LiNi_0.5Mn_1.5O₄ 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 SiO_x|LiNi_0.5Mn_1.5O₄ batteries

Before assembling full cells, a pre-activation process was applied to SiO_x 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 SiO_x|LiNi_0.5Mn_1.5O₄ 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 SiO_x anode and LiNi_0.5Mn_1.5O₄ cathode electrodes, which are incomparable to those of the other electrolytes proposed to date (Figure 6b).

Stable long-term cycling of high-capacity SiO_x and high-potential LiNi_0.5Mn_1.5O₄ full cells (96% capacity retention after 500 cycles at 0.5 C-rate) was obtained with the 3.4 M LiFSI/FEMC electrolyte via combined thermodynamic and kinetic mechanisms. In particular, a distinct solution structure of 3.4 M LiFSI/FEMC, in which most Li⁺ and FSI^- ions are intensively coordinated together to form a crowded ion-pair network, increases the chemical potential of Li⁺ and allows approaching large amounts of anion at a negatively charged anode. This situation contributes to stable cycling of the SiO_x anode by upshifting the thermodynamic reaction potential of SiO_x and suppressing the electrolyte decomposition kinetically with an anion-derived SEI formation before a huge expansion of SiO_x upon lithiation. Moreover, several high-potential issues of the cathode can be resolved because of the thermodynamically enhanced oxidation stability with solvent fluorination and kinetically prevented Al corrosion and transition metal dissolution from the cathode material based on the weak solvating power of the FEMC solvent and high salt concentration. We believe that the proposed electrolyte strategy will facilitate the realization of high-energy-density batteries with high-capacity anodes and high-potential cathodes.

Preparation of electrolytes and electrodes

Electrolyte: The electrolytes were prepared using a lithium bis(fluorosulfonyl)imide (LiFSI, Nippon Shokubai) salt and solvents (EMC (Kishida Chemical Co., Ltd.), FEMC (Halocarbon), and diglyme (Kishida Chemical Co., Ltd.)) in an Ar-filled glove box. The commercial 1.0 M LiPF₆/EC:DMC (1:1, v/v) electrolyte was purchased from Kishida Chemical Co., Ltd. and used as received.

SiO_x electrode: SiO_x (BTR New Energy Material Ltd.), acetylene blacks (ABs, Denka Black, Denka Company Limited), and polyamide-imide (PAI, Torlon-4000T, Solvay) binder were dissolved in N-methylpyrrolidone (NMP, FUJIFILM Wako Pure Chemical Corporation) with a weight ratio of 70:15:15. The prepared slurry was coated onto a Cu foil (Fchikawa Rare Metal) and dried at 80°C in an oven. The resulting electrode was further heated at 400°C for 2 h in an Ar atmosphere to strengthen the mechanical stability of the PAI binder.⁴²

LiNi_0.5Mn_1.5O₄ electrode: LiNi_0.5Mn_1.5O₄ (Samsung SDI, Japan), ABs, and polyvinylidene fluoride binder or lithiated poly(acrylic) acid (LiPAA) binder were mixed with NMP at a weight ratio of 80:10:10 or 85:10:5. The obtained viscous slurry was cast onto an Al (Fchikawa Rare Metal) or carbon-coated Al foil (SDX-PM, Showa Denko) and then dried at 80°C in an oven. LiPAA was prepared by titrating the aqueous solution of PAA (FUJIFILM Wako Pure Chemical Corporation) with LiOH (FUJIFILM Wako Pure Chemical Corporation) until the pH of the solution was 7.⁴³

Electrochemical study

The 2032 coin-type cell parts were purchased from Hoshen Corporation. All cell parts, SiO_x electrodes, and LiNi_0.5Mn_1.5O₄ electrodes were dried at 200 or 120°C under a vacuum before use.⁴⁴ The coin-type half-cells and full-cells were assembled in an Ar-filled glove box under the supplied conditions. The cellulose membrane (Nippon Kodoshi Corporation), polypropylene membrane (Celgard), and glass fiber filter (Advantech) were used as a separator. The galvanostatic charge and discharge tests were performed at various temperature conditions using a charge–discharge machine (TOSCAT-3100, Toyo System).

For full cells, the pre-activation of the SiO_x electrode was conducted to compensate for Li consumption on its initial activation.⁶ Initially, a Li|SiO_x half-cell was fabricated and cycled (lithiated and delithiated) once in the range of 0.01–1.5 V. Subsequently, it was re-lithiated for 30 to 40 min at a constant current of 150 mA g^-1 in the prepared electrolyte (1.0 M LiFSI/EMC, 1.0 M LiFSI/FEMC, 3.4 M LiFSI/FEMC, or 1.0 M LiPF₆/EC:DMC (1:1,v/v)) to prevent the corrosion of Cu current collector. The lithiated capacity ratio of the pre-activated SiO_x electrode was approximately 5%. Finally, the pre-activated SiO_x electrode was carefully collected from the cell and reassembled with the fresh electrolyte.

The pre-activated SiO_x|LiNi_0.5Mn_1.5O₄ full cell, as shown in Fig. 6, was fabricated with LiNi_0.5Mn_1.5O₄ electrodes (loading level of 5 mg cm^-2), pre-activated SiO_x, and 3.4 M LiFSI/FEMC electrolyte. The glass fiber filter was used as a separator. The negative/positive (N/P) ratio was controlled to 1.4. The N/P ratio and C-rate were calculated using the capacities of LiNi_0.5Mn_1.5O₄ (147 mAh g^-1) and SiO_x (1500 mAh g^-1). The cell was cycled at 0.2 C-rate 10 times before applying 0.5 C-rate to stabilize the SEI. The electrode and cell information of other full-cells and half-cells are provided in the figure captions.

The galvanostatic Li plating/stripping test was conducted with half-cells (Li|Cu) in various electrolytes at a constant current density of 0.5 mA cm^-2 and an areal capacity of 0.5 mAh cm^-2 with a cut-off voltage of 0.5 V. The deposited diameter of Li on the Cu foil was 1.2 cm.

The interfacial resistance between the electrode and electrolyte with cycled half-cells (Li|SiO_x) at a fully discharged state was analyzed using EIS (VMP3 potentiostat, BioLogic), with an amplitude of 10 mV over a frequency range of 10 mHz–1 MHz.

The oxidation stability of the electrolytes and the Al corrosion in various electrolytes were investigated via LSV measurement in a three-electrode cell with a Pt plate or an Al foil as a working electrode and Li metal as counter and reference electrodes. The measurement was performed with the VMP3 potentiostat from open circuit potential to 6 V (vs. Li/Li⁺) at a scan rate of 0.1 mV s^-1.

Characterization

The basic physicochemical properties of the prepared electrolytes are listed in Table S1. The density and viscosity of the electrolytes were measured with an oscillator-type densitometer (DMA35, Anton Paar) and a viscometer (Lovis 2000 M, Anton Paar), respectively. The ionic conductivity of electrolytes was evaluated in a two-electrode glass cell (Pt|electrolyte|Pt) using AC impedance spectroscopy (Solartron 147055BEC, Solartron Analytical).

The solution structure of the electrolytes was analyzed via Raman spectroscopy (NRS-5100, JASCO) at a laser excitation of 532 nm. The electrolytes were placed in a quartz cell and sealed with a parafilm in an Ar-filled glove box to avoid air contamination.

The surface chemistry of the cycled SiO_x electrodes was studied through XPS (PHI 5000 VersaProbe II, ULVAC-PHI), equipped with a monochromatic Al Kα X-ray source. The samples were prepared by disassembling the cycled cells and then rinsing them several time with DME in an Ar-filled glove box. The samples were then transferred into the XPS chamber without exposure to air using a specially designed transfer vessel.

Energy-dispersive X-ray spectroscopy of SiO_x electrodes was performed for estimating the amount of dissolved transition metal from the cathode materials. The SiO_x electrodes were carefully collected from the cycled SiO_x|LiNi_0.5Mn_1.5O₄ full cells under the provided conditions and then washed several times with DME before conducting the measurement.

Computational study

The MD simulations were conducted to estimate the electrolyte structures and electrostatic potential at the Li⁺ site in the prepared electrolytes using the AMBER16 package. The details of the calculation model are shown in Table S2, where the numbers of the solvents/ions are provided based on the experimental composition. The generalized AMBER force field⁴⁵ was used for all the solvents/ions in the simulations. The atomic point charges were obtained by gas-phase density functional theory calculations at B3LYP/cc-pvdz level using the Gaussian 16 package. The time step was set to 1 fs using the SHAKE method⁴⁶, constraining the hydrogen–heavy atom bond distances. The simulation cells were adjusted using NPT-MD simulations at 1 bar and 298 K, followed by NVT-MD simulations (298 K) to equilibrate the system for 1 ns and then sampled for 10 ns. The calculated solution structure was in agreement with the experimentally obtained data via Raman spectroscopy (Fig. 2). The electrostatic potential was evaluated by summing all the electrostatic interactions from the electrolyte solvents/ions to each Li⁺ ion using the particle mesh Ewald method under periodic boundary conditions and then averaging the obtained values.

Acknowledgement

This work is supported by Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the ’Elements Strategy Initiative for Catalysts and Batteries’ (ESICB Project, Grant Number JPMXP0112101003), Japan Society for the Promotion of Science (JSPS) Grant-in Aid for Scientific Research (S) (Grant Number 20H05673), and Japan Science and Technology Agancy (JST), the Program on Open Innovation Platform for Industry-Academia Co-Creation (COI-NEXT, Grant Number JPMJPF1234). Y.Y. and S.K were supported by KAKENHI No. 16H06368. S.K. also acknowledges the financial support from Murata Science Foundation.

Author contribution

A.Y. conceived and directed the projects. S.K. designed the electrolyte based on his thermodynamic and kinetic considerations. H.X. and S.K. conducted the experiments and analyzed the data. T.S. and N.T. performed computational simulations. S.K., N.T., Y.Y., and A.Y. supervised the research and wrote the manuscript.

Declaration of interests

The authors declare no competing interests.

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An electrolyte for SiO_x/LiNi_0.5Mn_1.5O₄ batteries

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