The Use of Cognate Cosolvent to Mediate Localized High-Concentration Electrolytes for High-Voltage and Long-Cycling Lithium-Metal Batteries

Abstract

Localized high-concentration electrolytes (LHCEs) are promising systems for improving the high-voltage performance and interfacial stability of lithium-metal batteries (LMBs). Unfortunately, they are always challenged by liquid–liquid phase separation during solution preparation. Further investigation is always required when the prepared electrolyte has encountered liquid–liquid phase separation previously. Here, we propose a “cognate cosolvent” strategy to mediate phase-separated LiBF₄/fluoroethylene carbonate (FEC)|ethyl trifluoroacetate (TFAE) mixtures with ethyl acetate (EA), forming effective LiBF₄/FEC/EA/TFAE-based LHCEs (B-LHCEs). Because of their unique solvation structure, the B-LHCEs exhibit high oxidative stability, facilitating Li⁺ transport. The optimized B-LHCEs help single-crystal LiMn_0.8Mn_0.1Co_0.1O₂/Li batteries form robust interphases, improving interfacial stability. As a result, distinct performance can be obtained (4.5 V, 500 cycles, ~90%, 1400, ~70%; 25 C, 128 mAh g⁻¹, 4.7 V, 500, 82.5%). This work turns the “impossible” into an “effective” high-voltage electrolyte design, transcending the previous paradigms of electrolyte investigation and enriching LHCE preparation research.

1. Introduction

Improving the energy density and longevity of lithium-ion batteries has become an urgent issue in academia and industry [1,2,3,4,5]. However, the mainstream-utilized lithium-ion battery system with a graphite anode is limited by gravimetric energy density (~300 Wh kg⁻¹). For further improvement (>500 Wh kg⁻¹), replacing the graphite anode with a lithium-metal anode coupled with a high-voltage cathode, such as a Ni-rich layered cathode (LiNi_xMnyCo_1−x−yO₂, x ≥ 0.8, Ni-rich NMC), is considered effective for achieving this target [1,2]. On the Ni-rich NMC cathode side, increasing the charging cutoff is regarded as a productive approach to enhancing capacity and energy density (e.g., enabling an about 15–35% capacity enhancement from 4.3 to 4.7 V) [1,2]. However, Ni-rich NMC cathodes with a high charging voltage suffer from severe interfacial incompatibility when combined with commercial carbonate-based electrolytes, resulting in an unstable generated interphase and parasitic reactions. These interfacial instabilities can cause particle pulverization, phase transition, gas evolution, and transition metal dissolution, which inevitably result in poor cycling performance. Although the use of single-crystal NMC (S-NMC) to replace polycrystal NMC (P-NMC) cathodes can partially suppress particle pulverization, there are still the challenges of limited long-term cycling stability, poor rate performance, and, even worse, structural collapse under a high-voltage cut-off (>4.3 V) [6,7,8,9]. For lithium-metal anodes, due to critical interfacial incompatibility with carbonate-based electrolytes, the formation of an unstable interphase, dendrite growth, and parasitic reactions lead to the degradation of anode performance, resulting in fast capacity fading [1,2].

Due to the strong correlation between high-voltage issues and carbonate-based electrolytes, it is essential to investigate new electrolytes that are compatible with both cathodic and anodic interfaces and produce stable interphases. Therefore, high-concentration electrolytes [8,10], localized high-concentration electrolytes (LHCEs) [2,4,11,12,13], fluorinated carbonate-based electrolytes [14], and cyclic phosphate-based electrolytes [15,16] have been developed. Among the developed electrolytes, LHCEs retain the local solution structure of concentrated electrolytes while reducing viscosity [12]. Built on hydrofluoroethers (HFEs), such as 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) [17,18,19,20] and bis(2,2,2-trifluoroethyl) ether (BTFE) [4,21], LHCEs can significantly improve high-voltage performance and slow down the growth of lithium dendrites, thereby enhancing interfacial stability and cycling performance. However, these HFEs have high density (>1.4 g cm⁻³) and are expensive, reducing the energy density of the corresponding batteries and increasing costs, thus inhibiting their further development [2]. In addition, the long-term cycling performance of HFE-based LHCEs needs to be further improved. LiBF₄ is a salt with great promise for battery applications and has been widely investigated to improve the high-voltage performance of NMC/Li batteries [10,14,22]. Fluoroethylene carbonate (FEC) is regarded as a valuable compound for constructing a passivated interphase for lithium-metal anodes, and it is often selected as the solvent for LHCEs [23]. Ethyl trifluoroacetate (TFAE) possesses high oxidation stability, is inexpensive (being about one-fifth the cost of HFEs), and has low density (1.19 cm⁻³). It could be an excellent dilute solvent for forming LHCEs. Due to their unique properties, LiBF₄/fluorinated ethyl acetate-based systems have been widely discussed. Unfortunately, TFAE is absolutely excluded in this regard since electrolytes with TFAE always encounter liquid–liquid separation or are claimed to be “insoluble” [10,24].

It is well known that it is highly undesirable to have to abandon further investigations when the prepared electrolyte has previously encountered liquid–liquid phase separation. In fact, liquid–liquid-phase-separated mixtures are heterogeneous, meaning that they are not helpful in battery applications. Even worse, the existence of a liquid–liquid interface results in almost no ion transport taking place. Therefore, liquid–liquid phase separation may prevent a large number of solvents/systems with excellent properties from being used as electrolytes for batteries. If a third part (solvent or system) that is miscible with both incompatible parts is added as a phase-bridging agent, this severe problem can be addressed. As shown in Figure 1a, the LiBF₄/FEC|TFAE system exhibits immiscible and liquid–liquid phase separation, is quite inhomogeneous, and is nearly ionically insulated [10,14,23,25]. In such a situation, further exploration should be ceased.

To solve this dilemma, we employed the cognate–cosolvent strategy for electrolyte mediation. Ethyl acetate (EA), a cognate solvent with TFAE, is highly miscible with TFAE. It also can be miscible with FEC and dissolve LiBF₄ (Figure 1a and Figure S1). In this work, we used EA to mediate a phase-separated mixture of LiBF₄/FEC|TFAE, forming the homogeneous LiBF₄/FEC/EA/TFAE-based LHCE (Figure 1a), which is easily designable and tailorable. Surprisingly, the mediated LHCEs (B-LHCEs) exhibit high oxidative stability, relatively low viscosities, and high ionic conductivities. Furthermore, the optimized B-LHCE helps both the cathode and anode of the S-NM811/Li batteries to develop robust interphases, thus enhancing interfacial stability under high voltages. Hence, remarkable electrochemical performance of the S-NMC811/Li batteries was observed (4.5 V, 500 cycles, ~90%, 1400 cycles, ~70%; 25 C, 128 mAh g⁻¹, and 4.7 V, 500 cycles, 82.5%). Additionally, due to the unique solvation structure, the B-LHCE facilitates Li⁺-transport, significantly improving the rate capacity. Our work not only provides a potential high-voltage electrolyte system for high-performance Li-metal batteries but also introduces a design strategy that expands the family of LHCEs. More importantly, since liquid–liquid separation is hardly considered in former electrolyte investigations, our work provides a case for making the “impossible” “effective”, revealing new horizons for the paradigm of electrolyte exploration.

2. Materials and Methods

2.1. Electrolyte Preparation

1 M LiPF₆ ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (commercial carbonate-based electrolyte, BE) and lithium tetrafluoroborate (LiBF₄, 99.9%) were purchased from DodoChem (Suzhou, China). Ethyl acetate (EA, 99.5%, Aladdin, Shanghai, China), ethyl trifluoroacetate (TFAE, 99%, Aladdin), and fluoroethylene carbonate (FEC, 98%, Aladdin) were dried with 4 Å molecular sieves before use. B-LHCEs were prepared by mixing 3 m LiBF₄ molar ratio with FEC/EA/TFAE by volume, similar to what was performed for the reference B-LHCE-0 (3 m LiBF₄ in FEC/EA/TFAE = 1/9/0 v/v/v), B-LHCE-1 (3 m LiBF₄ in FEC/EA/TFAE = 1/7/2 v/v/v), and B-LHCE-2 (3 m LiBF₄ in FEC/EA/TFAE =1/4/5 v/v/v). The 5 m LiBF₄/FEC and 6 m LiBF₄/EA were prepared by mixing the molar ratio salts with FEC and EA, respectively.

All the electrolyte preparation procedures were carried out in a Mikrouna universal glovebox (O₂ < 0.1 ppm, H₂O < 0.1 ppm). The viscosity tests were carried out with LC-NDJ-5S (Shanghai, China) in the glovebox. Ionic conductivity was evaluated using electrochemical impedance spectroscopy (EIS) via a Solartron 1260 + 1287 (10 mV,1 MHz to 0.01 Hz).

2.2. Symmetric Li/Li Cell and Li/Cu Cell

2032-type symmetric Li/separator-electrolyte/Li (φ = 14 mm) and Li/separator-electrolyte/Cu (φ = 12 mm) cells were assembled. The long-term plating/stripping test was conducted using a Land CT2001A (Wuhan, China).

2.3. Preparation of NMC811 Cathodes

S-NMC811 (Beijing Easpring Material Technology, Beijing, China), conductive carbon, Super-p, VGCF, and PVDF (80/10/10% in weight) were thoroughly mixed and then stirred in N-Methyl-2-pyrrolidone (NMP) to create a uniform slurry. After that, the slurry was coated on C-coated aluminum foil (Showa Denko Corp., Tokyo, Japan) and then dried at 80 °C for 24 h in a vacuum. The 1C-rate was set as 180 mAh g⁻¹. Then, the dried foil was cut into the resulting coin-shaped electrodes, with an active mass loading of 2–3 mg cm⁻². The 2032-shaped coin cell was assembled with the cathode, separator (Celgard 2400, Celgard, NC, USA), and Li metal in the glovebox. The cycled cathode and lithium metal were carefully dissembled and rinsed with Dimethyl carbonate (DMC) and dried in a glovebox. For high-mass-loading cathodes, the ratio of the S-NMC811 (Beijing Easpring Material Technology) to conductive carbon, Super-p, and VGCF and PVDF was 91/4/5% by weight, with a high active material mass loading of 10–11 mg/cm². The cells were assembled with a thin piece of lithium foil measuring 50 μm. Electrochemical performance was measured using a Land CT2001A operating within 3.0–4.5 V and 3.0–4.7 V, respectively. The EIS was tested using a Solartron 1260 + 1287 (10 mV, 1 MHz to 0.01 Hz) after the various cycles.

3. Results and Discussion

3.1. Physical Properties and Solvation Structures

Figure 1a shows a typical LiBF₄/FEC-EA-TFAE-based LHCE (1/1/1, v/v/v), which is bright and uniform, revealing the miscibility of this system. Because of the high melting point and high viscosity of FEC, we selected 10% FEC by volume, blending it with 90% of various EA/TFAE ratios by volume and 3 m LiBF₄ to form the LHCEs [23]. The B-LHCEs exhibited relatively low viscosities (Figure 1b) and high ionic conductivities (Figure 1c), similar to those of commercial carbonate electrolyte (BE). Although the B-LHCEs possess considerably high concentrations, they still present low viscosities, which can be attributed to the low viscosities of EA/TFAE. The HOMO and LUMO were calculated to assess the electrochemical stability of the solvents and salts, and the results are presented in Figure 1d. Preferentially, electrochemical reduction should be evaluated using the calculated LUMO levels. The low LUMO level of TFAE (−2.331 eV) illustrates that it is easily reduced by lithium, generating the LiF-based SEI. The oxidation priority can be determined by the HOMO level [2]. The low values of the HOMO level for the TFAE (−7.041 eV), LiBF₄ (−8.512 eV), and FEC (−7.625 eV), as well as the localized high concentration, suggest the LiBF₄/FEC-EA-TFAE-based LHCEs have high anti-oxidative capacity. Indeed, these results are further confirmed by the linear sweep voltammetry (LSV) results in Figure 1e. The baseline electrolyte (BE) and the reference electrolyte (B-LHCE-0) decomposed at 4.4 V and 4.5 V, respectively, while the oxidative voltage for the B-LHCEs was significantly increased to 4.7–5.0 V (Figure 1e). The high-decomposition voltage of the B-LHCE is proof of coupling with a high-voltage cathode. Since B-LHCE-2 presented the highest oxidation voltage stability among the B-LHCEs and approached that of the saturated solution (in Figure S1g, LiBF₄|FEC/EA/TFAE = 1/3/6 v/v/v indicates partial dissolution), B-LHCE-2 was selected as the optimized electrolyte for further electrochemical performance investigation and compared to the carbonate-based electrolyte BE. Also, we tested the Li⁺ transference (t_Li+) numbers for BE and B-LHCE-2 (Figure S1i,j, respectively). B-LHCE-2 possessed a higher t_Li+ (0.58) compared to that of BE (0.47), which could compensate for the lower ionic conductivity of B-LHCE-2, facilitating ion transport.

We also employed ab initio molecular dynamics (AIMD) simulations to investigate the solvation structures. Figure 2a–c show simulation snapshots of BE, B-LHCE-0, and B-LHCE-2. Obviously, the LiPF₆ and LiBF₄ salts are homogenous and distributed throughout the solvents in the BE and B-LHCE-0 electrolytes, respectively. In stark contrast, close-packed ion clusters (aggregations) and TFAE as the diluent can be observed in B-LHCE-2, revealing the formation of LHCE [26]. The profiles of the radial distribution function (RDF) at 25 °C are exhibited in Figure 2b,c. The RDFs reveal that the solvation sheath of the comparative sample B-LHCEs-0 (without TFAE) is dominated by EA in its first layer, while B-LHCE-2 (With TFAE) is significantly occupied by FEC in the first solvation shell [27,28,29]. These results indicate that the addition of TFAE can considerably alter solvation structure, changing the dominant form of the first solvation shell from EA, with a strong solvation power (1.17), to a weak FEC (0.63). Also, the RDF results show that the BE presents a much more powerful solvation EC (1.14) as the dominant coordinated solvent in the first solvation shell.

The RDF profiles also illustrate that the Li⁺ solvation structure in the BE is Li⁺(DMC)_1.095(EC)_1.66(EMC)_1.016 (Figure 2d), and the average coordination distances are 1.975 Å (Li−DMC), 2.025 Å (Li−EC), and 1.975 Å (Li−EMC), respectively, while the B-LHCEs-0 (without the TFAE) present Li⁺(FEC)_0.25 (EA)_2.26 (Figure 2e) values of 1.975 Å (Li−FEC) and 1.975 Å (Li−EA). Because of the high anti-solvation capability of TFAE, the coordination numbers (CNs) are significantly reduced in B-LHCE-2. In sharp contrast (Figure 2f), B-LHCE-2 displays a Li⁺ solvation structure of Li⁺(FEC)_0.35(EA)_0.87(TFAE)_0.40, with coordination distances of 1.975 Å (Li−FEC), 1.975 Å (Li−EA), and 2.025 Å (Li−TFAE). Enhancing the intensity of Li⁺-anion coordination is considered an alternative effective approach for fabricating weakly solvating electrolytes (WSEs) and generating the anion-derived interphases [22]. The BE presents a coordinated number of 1.09 for Li⁺-PF₆⁻, while those for B-LHCEs-0 (Li⁺-BF₄⁻) and B-LHCEs-2 (Li⁺-BF₄⁻) were 2.16 and 2.85, respectively. The increased coordination number of the Li⁺-anion suggests the generation of a WSE for B-LHCEs-2, which may benefit the formation of anion-derived interphases and promote interfacial stability. Therefore, the generation of a weak solvation structure can improve Li⁺ transport and thus improve kinetic capacity [2,22,30,31]. Also, the unique solvation structure can enhance high-voltage stability [11,12,13]. The solvation structure is further verified by the Raman spectra (Figure 2g). The peak at 766 cm⁻¹ was assigned to free BF₄⁻. Blue shifts of the peaks can be observed in the B-LHCEs, revealing the formation of coordinated BF₄⁻, contact ion pairs (CIPs), and aggregates (AGGs). The extent of the peak blue shift increases with the increase in TFAE content, indicating that the increase in the TFAE component leads to an increase in concentration. In addition, a broad peak can be detected at 775 cm⁻¹ in B-LHCE-2, which is similar to what was observed for the HCEs (6 m LiBF₄/EA and 5 m LiBF₄/FEC), demonstrating that the concentrated solvation structure was retained after dilution with ‘inert’ TFAE [10,14,22].

The compatibility of a lithium-metal anode/electrolyte is a critical factor in long-term cycling. Figure 3a compares the lithium deposition/stripping profiles for the BE and typical B-LHCE-2 based Li/Li symmetric cells at 0.25 mA cm⁻². Superior reversibility with smaller polarization was observed for the B-LHCE-2-based cell compared to that of the BE-based cell, demonstrating uniform and smooth lithium-plating/stripping in the B-LHCE-2-based cell. Moreover, the deposition/stripping process for the BE-based cell terminated at 400 h, while that for the B-LHCE-2-based cell lasted more than 700 h. After 200 h, the cycled lithium anodes were disassembled for further analysis. The Aurbach CE Protocol (Li/Cu cell, Figure 3b,c) was also applied in the evaluation. Similar to the carbonate-based electrolytes [32], the BE-based cell spent most of the Li inventory during several cycles exhibiting abnormal curves and a low CE of 74%. In stark contrast, the B-LHCE-2 based cell exhibited a higher CE of 95.1%. The SEM images (Figure 3d and Figure S2a) reveal that the lithium-anode cycled by BE is significantly deteriorating, with cracking and moss appearance, indicating the highly incompatible interface of the lithium anode/BE. In stark contrast, the anode cycled by B-LHCE-2 exhibits a smooth and dense image (Figure 3e and Figure S2b), corresponding to the Li/Li cycling results, which suggests the generation of a stable interface. The TOF-SIMS (Figure S2c,d and Figure 3f) was also used to investigate the composition and structure of the interphases. The 3D-mapping of typical species, LiF₂⁻, C₂H₂O⁻, and LiBOF₃⁻, which originate from LiF; organic species; and the decomposition of LiBF₄ are indicated in Figure 3f, respectively. The LiF-based species and dense layer structure are exhibited in the anode cycled by B-LHCE-2; these features can effectively mitigate the attacks of interfacial parasitic reactions and dendrite growth. Also, the Li-B-O-F layer generated in the SEI can protect the anode. For comparison, the anode cycled by BE possesses few and scattered pieces of LiF with an inhomogeneous distributed interphase, which makes it difficult to prevent dendrite growth and plays a shielding role on the anode [22,33].

3.2. Battery Performance

Because of its high-voltage stability and excellent compatibility with Li-anodes, B-LHCE was expected to deliver outstanding performance in S-NMC811/Li batteries. Figure 4a and Figure S3a,b compare the rate-performance of the S-NMC811/Li with BE and B-LHCE-2 within 3.0–4.5 V. The battery with B-LHCE-2 presents distinct and low polarized charge–discharge curves even at a high rate of 25 C, demonstrating superb rate capacity. In sharp contrast, the BE shows almost ‘gaping’ curves with an undefined plateau above 3C, indicating severe polarization at high rates and poor rate performance. Also, the rate-cycling curves indicated in Figure 4a highlight the excellent rate capacity of the cell using B-LHC-2. The cell integrated with B-LHCE-2 displayed a capacity as high as 128.6 mAh g⁻¹ even at the ultrahigh rate of 25 C (64.2% for 25 C/0.1 C), showing fast electrochemical kinetics, while the cell cycled with BE barely ran at 25 C, only attaining a capacity retention of 15.4% for 25 C/0.1 C. It is impressive that the B-LHCE-2 possesses relatively lower ionic conductivity and higher viscosity compared to the BE but can deliver such outstanding rate-performance. Thus, the solvation structure plays a critical role in kinetics, aiding in fast-Li⁺ desolvation and greatly improving the rate capability [27,28].

Figure 4b,c and Figure S3c display the cycling performance of S-NMC811/Li with the BE and B-LHCE-2 within 3.0–4.5 V. The cell with B-LHCE-2 had an initial discharge capacity of 193.5 mAh g⁻¹. It showed excellent cycling performance, with a capacity retention of 89.2% after 500 cycles and 70.4% after 1400 cycles. Unfortunately, the BE-based cell underwent a rapid capacity decline within 200 cycles, only delivering a capacity retention of 58.9%.

Also, the charge–discharge profiles of the B-LHCE-2-based cell maintained well-defined shapes even at 1400th cycle, while the profile of the BE-based cell was already highly distorted at the 200th cycle. Furthermore, the S-NMC811/Li with B-LHCE-2 also exhibited an aggressive cutoff of 3.0–4.7 V (Figure 4d and Figure S3d,e). The initial discharge capacity was 201.1 mAh g⁻¹. Capacity retentions of 82.5% and 53% after 500 and 1000 cycles were obtained, respectively, suggesting remarkable high-voltage interfacial stability. The supreme cycling capability of the B-LHCE-2-based cell was attributed to the high voltage stability of the electrolyte and the formation of a durable interphase.

To further investigate the practical application potential, the cells assembled with a high-loading cathode (10.6 mg cm⁻², 50 μm Li) were evaluated (Figure 4e and Figure S3f,g). The B-LHCE-2 cell with an initial discharge capacity of 188.4 mAh g⁻¹ exhibited high capacity retention of 90.8% after 120 cycles, while the BE-based cell suffered from rapid capacity attenuation after 40 cycles, with a low-capacity retention of only 25% after 120 cycles. This result highlights the superb performance of the B-LHCE-2 electrolyte in high-voltage applications. Figure 4f and Table S1 show the recently reported high-voltage cycling performance of S-NMC/Li cells [34,35,36,37,38,39]. It is clear to see that our results are competitive when compared to the former reports, especially in terms of long-term cycling. Furthermore, based on the prices of the agent suppliers, we compared the cost of our B-LHCE-2 electrolyte with that of previously reported high-voltage electrolytes in Table S2. Owing to the large production of the solvents (EA and TFAE), our B-LHCE-2 electrolyte is still competitive in terms of cost. The electrochemical impedance spectra (EIS) were applied for verification, as shown in Figure S4. The cell with B-LHCE-2 presents a small alteration in reaction resistance after the 1st, 100th, and 200th cycles, revealing the generation of stable interphases and interfaces [2]. In contrast, significant variation can be observed for the cell with BE after the 1st, 100th, and 200th cycles, suggesting the formation of unstable interfaces.

Owing to the cathode/electrolyte interface playing essential roles in performance, we further used XRD, SEM, XPS, and TOF-SIMs for investigation. SEM images of the 200 time-cycled S-NMC811 cathodes (3.0–4.5 V) with various electrolytes are depicted in Figure 5a,b. As indicated in Figure 5a, the particles cycled with BE were severely damaged by cracks, and a rough surface can be spotted, which reveals the poorly generated CEI and interface, which hardly prevented the attack on the electrolyte. By comparison, the cathode cycled with B-LHCE-2 exhibits intact particles with a smooth surface (Figure 5b), suggesting that the formed CEI protected the electrode well.

Figure 5c compares the XRD spectra for the electrodes cycled with BE and B-LHCE-2. In the XRD spectra of layered NMC811, the ratio of I₍₀₀₃₎/I₍₁₀₄₎ is always used as an indicator for cation mixing (Figure 5c). The higher the value of the ratio, the lower the degree of cation mixing. In the spectra, the I₍₀₀₃₎/I₍₁₀₄₎ ratio for the cathode cycled with B-LHCE-2 200 times is 5.688, which is significantly larger than that of 3.073 for the BE-based cathode. Also, distinct resolved peak splitting of the (108)/(110) doublets can be detected in cathode cycled with B-LHCE-2, highlighting the protective effect of B-LHCE-2 [16,40]. Therefore, the employment of B-LHCE-2 can maintain bulk structure stability during high-voltage cycling. XPS spectra (Figure S5 and Figure 5d) were applied for further examination. The C1s spectra for both BE and B-LHCE-2-based cathodes can be divided into C-C, C-O, and O-C = O species. Compared to the cathode cycled with BE, weaker peak intensities of C-O can be detected in the cathode cycled with B-LHCE-2, revealing that degeneration can be suppressed [16,40]. In Figure 5d, the intensity of the M–O species (transition metal lattice O²⁻) in O 1s is associated with the thickness of the formed CEI. The intensity of the M–O species is difficult to observe in the BE-based cathode, suggesting that the generated thick CEI makes the M–O signal (lattice O²⁻) hard to scrutinize [33,41]. This result can be ascribed to the generation of incompatible interfaces, leading to continuous CEI growth. As a comparison, the M–O species are quite obvious in the cathode cycled with B-LHCE-2, demonstrating the formation of a thin and robust CEI. And in the F1s, the cathode cycled with B-LHCE-2 exhibited a higher quantity of LiF species, which can mitigate the deterioration of the electrolyte. Also, in Figure S5e, the B 1s of the cathode cycled with B-LHCE-2 shows the presence of the B_xO_y species, conforming the contribution of BF₄⁻ to the formation of CEI [40]. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling is a powerful tool for analyzing the chemical composition of the CEI. Figure 5e and Figure S6 compare the depth profiling of typical species of the interphase (CEI) for the cathodes cycled with BE and B-LHCE-2 200 times, respectively. Both interphases contain organic and inorganic substances, and there is no obvious phase separation. The 3D-depth distribution of the species (Figure 5e) reveals that the CEI with B-LHCE-2 has a higher content of LiF-based species and LiBOF₃⁻ species with a dense layer structure, allowing it to effectively avoid interfacial parasitic reactions. In stark contrast, the CEI created by the BE electrolyte contains more organic species (e.g., C₂H₂O⁻), which are sparse and have a weaker protective effect on the cathode [22,33].

4. Conclusions

In this work, we provide a ‘cognate-cosolvent’ strategy for mediating the phase-separated mixture to form the uniform B-LHCE. B-LHCE presents relatively low viscosities, high ionic conductivity, high oxidative stability, and a unique weak solvation structure, making it promising for application in high-voltage S-NMC811/Li batteries. Due to the unique solvation structure, B-LHCEs facilitate Li⁺ transport and a high rate capacity. F-containing FEC, TFAE, and BF₄^- also form robust SEI and CEI, enhancing the interfacial stability. As a result, excellent electrochemical performance of the S-NMC811/Li batteries was obtained (4.5 V, 500 cycles, ~90%, 1400 cycles, ~70%; 25 C, 128 mAh g⁻¹, and 4.7 V, 500 cycles, 82.5%). This work provides a design strategy for high-performance Li-metal batteries. The simple “cognate solvent” can be applied as a guideline when encountering liquid–liquid phase separation. More significantly, our work also provides a case for “turning waste into treasure”. Because previous electrolyte studies directly ignored liquid–liquid separated mixtures and thus may have excluded many excellent candidates, our work opens a new avenue for revisiting neglected electrolyte systems and breaks new ground in electrolyte exploration.