Understanding steric hindrance effect of solvent molecule in localized high-concentration electrolyte for lithium metal batteries

High energy density lithium metal batteries (LMBs) have garnered significant research interests in the past decades. However, the growth of lithium dendrites and the low Coulombic efficiency (CE) of Li metal anode pose significant challenges for the development of LMBs. Herein, we report a triethyl orthoformate (TEOF)-based localized high-concentration electrolyte (LHCE) that facilitates a highly reversible Li metal anode with dendrite-free deposition morphologies and an average Coulombic efficiency of 99.1% for 450 cycles. Mechanistic study reveal that the steric hindrance caused by the terminal ethyl groups in the TEOF solvent molecule results in a weak solvating ability, leading to the formation of anion-dominant solvation structures. The anion-dominant solvation sheaths play an essential role in the formation of a LiF-rich solid-electrolyte interphase (SEI), which effectively suppresses the growth of Li dendrites. Furthermore, the TEOF-based electrolyte demonstrates the stable cycling of high-voltage Li||NMC811 cells. These results provide insights into understanding of steric hindrance effect on electrolyte solvation structure and offer valuable guidance for the design of electrolyte solvents in the development of lithium metal batteries.


Introduction
Lithium metal batteries (LMBs) hold significant promise as next-generation rechargeable storage devices capable of delivering high energy densities > 500 Wh kg −1 [1][2][3].However, the development of LMBs is greatly hindered by the low Coulombic efficiency (CE) and the serious dendrite growth of lithium metal anode [4][5][6][7].During the repeated Li plating/stripping processes, the vigorous and uncontrollable chemical/ electrochemical reactions between Li metal anode and electrolytes lead to the constant growth of solid-electrolyte interphase (SEI) [8][9][10][11][12].The resulting unstable SEI not only continuously consumes the active Li and electrolytes, but also results in low CE and the growth of dendritic Li, which accelerates the failures of Li metal anode [13][14][15][16].Also, the uncontrollable Li dendrite may penetrate through the separators, leading to short circuits and serious safety issues.
Optimizing electrolytes plays an essential role in improving the cycling stability of Li metal anode [17][18][19].While traditional carbonate-based electrolytes match well with the graphite anode [20], they exhibit high reactivity with Li metal anode [21][22][23], leading to poor reversibility and a shorten lifespan of Li metal anode.In contrast, ether-based electrolytes exhibit higher compatibility with Li metal anode, which can greatly improve the CE and suppress the dendrite growth [24,25].Unfortunately, the ether-based electrolytes show incompatibility with the high-voltage cathode due to their poor anodic ability [26].Recently, many electrolyte strategies have been developed to further improve the compatibility of ether-based electrolytes with Li metal anode and high-voltage cathode, including solvent fluorination [27][28][29], weakly solvating electrolytes (WSEs) [30][31][32][33][34] and localized high-concentration electrolytes (LHCEs) [35][36][37].Among these, LHCEs are currently one of the most advanced electrolytes for high-voltage LMBs [38][39][40].In LHCEs, the non-solvating diluents can further enhance the aniondominant solvation structures [41,42], which facilitate the formation of inorganic-rich SEI to improve the cycling stability of Li metal anode [43].However, most efforts regarding LHCEs focus on the development of new diluents [37,44], with little attention given to the design of new ether molecules [45].Therefore, there is an urgent need to explore the solvent structure effect in LHCEs for high-voltage LMBs.
In this work, we present the steric hindrance effect of solvent molecules in LHCEs by systematically comparing triethyl orthoformate (TEOF) and trimethyl orthoformate (TMOF) as weakly solvating solvents for high-voltage LMBs.We found that the TEOF molecules with ethyl groups can significantly increase steric hindrance and weaken the solvation ability, thus facilitating the formation of suitable SEI on Li metal anode and also enhancing the anodic stability on high-voltage NCM811 cathode.Raman spectra and theoretical simulations reveal that the weakly solvating TEOF solvent contributes to an anion-dominant solvation structure.Such solvation structures facilitate the formation of LiF-rich SEI, which enables a high reversible Li metal anode with dendrite-free Li deposition morphologies and a high CE of ~ 99.1% for 450 cycles.In addition, the TEOF-based electrolyte enables stable cycling of high-voltage Li||NMC811 cells, with a high capacity retention of ~ 95.6% for 150 cycles.More importantly, our work provides new sights for the rational design of high-performance electrolytes for high energy density LMBs.

Cell assembly and electrochemical tests
All electrochemical tests in this work were carried out using 2032-type coin cells, which were assembled in the Ar-filled glove box (< 0.1 ppm H 2 O and < 0.1 ppm O 2 ).Li||Cu cells were assembled with one piece of lithium metal foil (450 μm, Φ = 14 mm) and one piece of copper foil (25 μm, Φ = 19.1 mm).Li||Li cells were assembled with two pieces of lithium metal foils.For the long-term cycling of Li||Cu cells, Li metal was firstly deposited on the Cu substrate at a current density of 0.5 mA cm −2 or 1 mA cm −2 for 1 mAh cm −2 , then stripped to 1 V.For the preparation of NMC811 electrode, NMC811 cathode materials, super P and poly(vinylidene fluoride) (PVDF) were mixed with a mass ratio of 85:7.5:7.5 using N-methyl-2-pyrrolidone (NMP) as a solvent, the as-prepared mixture was stirred to get a slurry.The slurry was further coated on the carbon-coated Al foil and dried at 60 ℃ overnight under vacuum, then cut into pieces (Φ = 14 mm).Before use, the NMC811 cathode was dried at 110 ℃ overnight and transferred into the glovebox.The loading of cathode material in this work was ~ 3 mAh cm −2 .In Li||NMC811 cells, one piece of thin lithium metal foil (50 μm, Φ = 16 mm) and one piece of NMC811 electrode were used.In Li||LCO cells, one piece of thin lithium metal foil (450 μm, Φ = 15.6 mm) and one piece of LCO electrode were used.In each cell, Celgard 2325 (25 μm, Φ = 19.1 mm) was used as separator and 80 µL of electrolyte was applied.The cells were tested on a Neware battery testing system (CT-3000n).Linear sweep voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were tested in Li||Carbon paper cells, Li||Cu cells, and Li||Li cells, respectively.Both LSV, CV, and EIS measurements were conducted on a CHI760e electrochemical station.

Characterizations
The Li deposition morphologies were observed using scanning electron microscopy (SEM, FEI Apreo) operated with a working voltage of 2 kV.The modulus of deposited Li was tested by atomic force microscopy (AFM, Bruker Dimension Icon).The modulus measurements were operated in the peak-force quantitative nano-mechanical mapping (QNM) mode.Before SEM and AFM measurements, the samples were washed with DME for several times to clean the residues and dried naturally.The surface chemistry of the Li deposits was characterized by X-ray photoelectron spectroscopy (XPS, KRATOS AXIS SUPRA + ).The samples were washed with DME beforehand, and loaded on the XPS transfer holder to avoid exposure to air environment.The depth profiling sputtering was carried out with an Ar-ion gun (5 kV, 2 mm × 2 mm).The sputtering time was 4 min for Li metal samples and 2 min for NMC811 samples, respectively.Raman microscope (Raman, Renishaw inVia) and nuclear magnetic resonance spectrometer (NMR, JNM-ECZ600R/S1) were used to characterize the solvation behaviors in the two electrolytes.

Theoretical simulations
Density function theory (DFT) calculations were performed to optimize molecular geometries of TEOF and TMOF molecules using the Jaguar 8.8 software package at the level of B3LYP level with the 6-311 + G(d,p) basis sets [46].Dipole moment and electrostatic potential (ESP) were calculated using at B3LYP/6-311 + G(d) level, which has been reported to well predict the formation charge.The binding energy (△E binding ) was calculated as follows: (1) Where E(sol) and E(sol-Li + ) are the potential energies of the solvent and Li + together with solvent, respectively.E(Li + ) represents the energy of the Li + .
The dielectric constant of electrolytes, solvation structures and radial distribution function (RDF) were obtained from classical molecular dynamics (cMD) simulations.All cMD simulations were conducted with the GROMACS 2020 program [47].The simulation of TMOF consists of 100 LiFSI, 120 TTE, and 360 TMOF molecules.The size of the simulation cell of the TMOF system is 109.39nm 3 .The simulation of TEOF consists of 100 LiFSI, 120 TTE, and 360 TEOF molecules.The size of the simulation cell of the TEOF system is 142.00 nm 3 .The electrolyte concentrations were simulated in consistent with experiments.The systems were firstly equilibrated under the isothermal-isobaric (NPT) ensemble.In 2 ns NPT simulations, the temperature was kept at 300 K using a V-rescale thermostat with a relaxation time of 0.1 ps and the pressure was kept at 1 bar using a Berendsen barostat with a relaxation time of 0.5 ps.Thereafter, the systems were simulated in the canonical (NVT) ensemble for 1 ns.In 1 ns NVT simulations, the temperature was kept at 300 K using a V-rescale thermostat with a relaxation time of 0.1 ps.

Steric hindrance effect on solvation structures
We selected the commercially available solvent, trimethyl orthoformate (TMOF), as a foundational solvent, owing to its distinct open-chain structure, as depicted in Fig. 1a.However, the multiple coordination sites result in strong solvation ability of TMOF, which may generate solvent-dominated solvation structures that have poor compatibility with Li metal anode.We expect that increasing the steric hindrance by lengthening the terminal alkyl group could decrease the solvation ability of TMOF.Under this design principle, we introduced triethyl orthoformate (TEOF) as a weakly solvating solvent.Both TMOF and TEOF molecules have three O atoms that can serve as the Li + binding sites, leading to the highly centralized negative charge density center indicated from the DFT-derived electrostatic potentials (ESPs) (Fig. 1b, c).Compared with TMOF, the larger ethoxy groups on TEOF provide a stronger steric hindrance.The steric effects of terminal alkyl groups could hinder the interactions between TEOF and Li + , resulting in a lower solvation ability of TEOF than TMOF [30,33].In order to substantiate this hypothesis, we have constructed the LHCEs utilizing TMOF and TEOF as equivalent solvents, selecting LiFSI as the salt, and employing non-solvation solvent TTE as the diluent.
Molecular dynamics (MD) simulations and Raman spectroscopy were conducted to elucidate the Li + solvation structures in both electrolytes.Diverse solvation structures in the electrolytes can be identified based on the differing coordination states of FSI − anions.These structures can be categorized into separated solvated ion pairs (SSIP), contact ion pairs (CIP), and aggregates (AGG), as illustrated in Fig. S1.In both electrolytes, a typical AGG structure consists of two anions and one solvent molecule directly coordinating with one Li + .As depicted in Fig. 2a, b, the increased steric hindrance affected the interactions between Li + and FSI − anions in two electrolytes, which aligns well with our electrolyte design principles.The Raman signals detected in the 1200-1250 cm −1 range correspond to the vibrations of the S = O stretching mode of FSI − anions [48].Notably, both LiFSI-1.2TMOF-3TTE and LiFSI-1.2TEOF-3TTEelectrolytes manifest an overlapping peak within this range, which can be further vested into three characteristic peaks of SSIP, CIP, and AGG (Fig. S2).A noteworthy contrast in the solvation structures is evident when comparing the LiFSI-1.2TMOF-3TTE and LiFSI-1.2TEOF-3TTEelectrolytes.The latter exhibits an increased intensity of AGG and a simultaneous decrease in SSIP intensity (Fig. 2c), suggesting that more FSI − anions are incorporated into the solvation sheaths.Statistical calculations further corroborate this observation, indicating that the LiFSI-1.2TEOF-3TTEelectrolyte (~ 78%) has a broader distribution of coordinated anions compared to the LiFSI-1.2TMOF-3TTE(~ 73%) (Fig. 2d).This divergence in solvation structures between the two electrolytes can be attributed to the molecular differences between TMOF and TEOF.MD simulations are carried out for a more profound understanding of these solvation structures (Fig. 2e-f ).For the LiFSI-1.2TMOF-3TTEelectrolyte, the TMOF solvent primarily resides in the first solvation sheath of Li + (Fig. 2g, Fig. S3).Conversely, the LiFSI-1.2TEOF-3TTEelectrolyte shows an enhanced signal of Li + -O(FSI − ) pairs in the first solvation sheaths, indicative of increased interactions between Li + and anions.These anion-dominant solvation structures in LiFSI-1.2TEOF-3TTE may promote a prior decomposition of anions, potentially leading to the formation of an inorganic-rich SEI.

Reversibility of Li metal anode
The Li deposition morphologies in these two electrolytes were observed by scanning electron microscopy (SEM).At a deposition capacity of 0.5 mAh cm −2 , the LiFSI-1.2TMOF-3TTEelectrolyte exhibits the growth of dendritic Li (Fig. 3a, Fig. S4a-c).With a deposition capacity of 1 mAh cm −2 , dendritic Li with lengths ranging from 5 to 10 μm can be observed (Fig. 3b, Fig. S4d-i).As shown in the cross-sectional SEM images, the Li deposits exhibit loose structures, with a thickness of ~ 12 μm (Fig. 3c).The dendritic Li and loose structure with high specific surface areas may cause serious parasitic reactions, accelerating the consumption of electrolytes and decreasing the cycling stability of Li metal anode.In contrast, the Li deposits in LiFSI-1.2TEOF-3TTEshow closely packed and dendrite-free morphologies (Fig. 3d, e, Fig. S5).With a deposition of 1 mAh cm −2 , the deposited Li in LiFSI-1.2TEOF-3TTEexhibits a thickness of ~ 7.2 μm (Fig. 3f ), indicating a better compatibility between the electrolyte and Li metal anode.Atomic force microscopy (AFM) characterizations were further conducted to investigate the mechanical properties of SEI formed on the Li metal surface.As shown by the quantitative nanomechanical mapping-AFM images, the modulus distribution of the SEI formed in LiFSI-1.2TMOF-3TTE is inhomogeneous (Fig. 3g), which potentially leads to Li dendrite growth.By contrast, the SEI formed in LiFSI-1.2TEOF-3TTEshow a smooth and uniform distribution of modulus (Fig. 3h), leading to an average modulus of ~ 6.28 GPa, which is much higher than that formed in LiFSI-1.2TMOF-3TTE(~ 4.39 GPa, Fig. 3i).The higher modulus of SEI formed in LiFSI-1.2TEOF-3TTEverifies its inorganic-rich feature, which is beneficial in suppressing the Li dendrite growth and improving the reversibility of Li metal anode [49].
To evaluate the compatibility of two electrolytes with Li metal anode, Li||Cu cells were assembled to determine the Coulombic efficiencies (CEs) of Li plating/ stripping processes.As shown in Fig. 4a, the LiFSI-1.2TMOF-3TTEelectrolyte exhibits an average CE of 99.1% for 380 cycles at 0.5 mA cm −2 and 1 mAh cm −2 .However, the CEs after 380 cycles show significant fluctuations (Fig. S6a).In contrast, the Li||Cu cell cycled in LiFSI-1.2TEOF-3TTEshows improved cycling stability, with an average CE of 99.1% for 450 cycles without short circuits (Fig. S6b).Furthermore, when cycled at 1 mA cm −2 and 1 mAh cm −2 , the LiFSI-1.2TEOF-3TTEelectrolyte also exhibits an enhanced cycling stability of Li||Cu cells than LiFSI-1.2TMOF-3TTE(Fig. 4b, Fig. S7).The former can enable stable cycling of Li||Cu cells for 450 cycles while only 320 cycles for the latter.Although the two electrolytes show similar values of CEs (Fig. S8), the improved kinetics of Li plating/stripping processes Fig. 4 Coulombic efficiencies of Li||Cu cells cycled at a 0.5 mA cm −2 and b 1 mA cm −2 with a deposition capacity of 1 mAh cm −2 in two electrolytes.Electrochemical impedance spectra of Li||Li cells cycled in two electrolytes after c 1 cycle and d 10 cycles and the enhanced lifespan of long-term cycling both verify the higher compatibility of LiFSI-1.2TEOF-3TTEelectrolyte with Li metal anode (Fig. S9).Furthermore, electrochemical impedance spectra (EIS) measurements were conducted in Li||Li symmetric cells to compare the electrochemical kinetics of the Li metal anode in two electrolytes.The EIS results of Li||Li cells after 1 cycle and 10 cycles were shown in Fig. 4c and d, respectively.Two semicircles can be observed from the EIS spectra, where the semicircle at high frequency can be assigned to the SEI resistance (R SEI ) and the semicircle at middle frequency represents the charge transfer resistance (R ct ).After 1 cycle, the R SEI and R ct of the Li||Li cell cycled in LiFSI-1.2TEOF-3TTE(8 Ω and 18 Ω) were lower than that cycled in LiFSI-1.2TMOF-3TTE(12 Ω and 20 Ω).The R SEI of Li||Li cell cycled in LiFSI-1.2TEOF-3TTEdecreased to only 3 Ω after 10 cycles, while the LiFSI-1.2TMOF-3TTEshows a much higher value of R SEI (18 Ω), which suggests that the former has better electrochemical kinetics [50].

Electrochemical performances of Li||NMC811 cells
To compare the high-voltage tolerance of these two electrolytes, linear sweep voltammetry (LSV) measurements were performed in Li||carbon paper cells to investigate the oxidation stability.Compared with LiFSI-1.2TMOF-3TTE, the LiFSI-1.2TEOF-3TTEelectrolyte exhibits a broadened electrochemical stability window (> 4.5 V vs. Li/Li + ) (Fig. S10), indicating its enhanced anti-oxidation ability.Furthermore, Li (50 μm)||NMC811 (3 mAh cm −2 ) full cells were assembled to evaluate the cycling performances under high-voltage conditions.The cells were tested within the voltage range between 2.6 and 4.3 V, with a charge rate of 0.2 C and a discharge rate of 0.33 C after three formation cycles of 0.1 C. As shown in Fig. 5, the Li||NMC811 cell cycled in LiFSI-1.2TMOF-3TTEreleased an initial discharge capacity of ~ 175 mAh g −1 , which rapidly decreased to 150 mAh g −1 after 70 cycles, followed by a drop almost to no capacity release after 120 cycles (Fig. 5c).By contrast, the LiFSI-1.2TEOF-3TTEelectrolyte exhibits improved cycling stability of Li||NMC811 cells.With a high initial discharge capacity of ~ 182 mAh g −1 , the cell remains a stable discharge capacity release of ~ 180 mAh g −1 in the following cycles, with a capacity retention of ~ 95.6% after 150 cycles (Fig. 5b).More importantly, the cell cycled in LiFSI-1.2TMOF-3TTEexhibits an obvious reduction of the CE after 90 cycles, which indicates serious side reactions, while the CE of cell cycled in LiFSI-1.2TEOF-3TTEmaintains stable during cycling.In addition, the LiFSI-1.2TEOF-3TTEelectrolyte also shows improved cycling stability of Li (450 μm)||LCO (1.4 mAh cm −2 ) cells cycled within 2.7-4.4V (Fig. S11).These results suggest that the anion-dominant solvation structures in the LiFSI-1.2TEOF-3TTEelectrolyte show improved compatibility with the high-voltage cathodes compared to LiFSI-1.2TMOF-3TTE.

Interfacial chemistry and understanding
SEM and X-ray photoelectron spectroscopy (XPS) characterizations were conducted to investigate the morphology evolution and interfacial chemistry of the Li metal anode cycled in Li||NMC811 cells after 50 cycles.The cycled Li metal in the LiFSI-1.2TMOF-3TTEelectrolyte presents extensive cracks (Fig. 6a, Fig. S12), which results in the continuous penetration of electrolyte to occur in continuous side reactions with fresh Li metal.By contrast, the Li metal anode cycled in LiFSI-1.2TEOF-3TTEexhibits a uniform and smooth surface without any cracks (Fig. 6b).The distinct morphologies of the cycled Li metal origin from the different properties of SEI, which were regulated by the different solvation structures in two electrolytes [51][52][53].XPS characterizations were further applied to investigate the compositions of SEI.As shown in Fig. 6c, although the similar atomic ratio of C element can be observed in the outer layer of SEI formed in two electrolytes (30% for LiFSI-1.2TMOF-3TTE,31% for LiFSI-1.2TEOF-3TTE), after argon ion cluster sputtering for 4 min, more content of C can be detected in the inner layer of SEI formed in LiFSI-1.2TMOF-3TTE(21%) than LiFSI-1.2TEOF-3TTE(13%).(~ 288.5 eV), and CO 3 2− (~ 289.8 eV) can be observed in the SEI formed in two electrolytes (Fig. 6d h).The increased content of C-containing organic species in the SEI formed in LiFSI-1.2TMOF-3TTEindicates the major contribution of solvent molecules on interfacial reactions, which matches well with the results of solvation structures.More importantly, the Li metal anode cycled in the LiFSI-1.2TEOF-3TTE(7% and 14%) electrolyte shows an increase in the atomic ratio of F element in the outer and inner layer of SEI than that cycled in LiFSI-1.2TMOF-3TTE(4% and 8%).In the F 1s spectra, higher intensity of LiF signal (~ 684.7 eV) and S-F species (~ 687.8 eV) can be detected in the SEI formed in LiFSI-1.2TEOF-3TTE (Fig. 6e and i), which matches with the trend of LiF signal in the Li 1s spectra (Fig. 6f and j).After 4 min sputtering, the Li 2 O (~ 528.2 eV) peak intensity in the O 1s spectra in LiFSI-1.2TEFO-3TTE is slightly higher than that in LiFSI-1.2TMOF-3TTE (Fig. 6g and k).The enrichments of LiF and Li 2 O are critical to improving the compatibility of LiFSI-1.2TEOF-3TTEtowards the Li metal anode.In addition, the interfacial chemistry of cathode cycled in Li||NMC811 cells after 50 cycles were also characterized (Figs.S13, S14).The cathode cycled in LiFSI-1.2TEOF-3TTEshows an increased peak intensity of the LiF signal, which improves the cycling stability of the high-voltage NMC cathode.Combined with the steric hindrance effect, we found that the anion-dominant solvation structures in the LiFSI-1.2TEOF-3TTEelectrolyte contribute to the formation of inorganic-rich SEI and CEI, which are beneficial in improving the cycling stability of high-voltage Li||NMC811 cells.

Conclusion
In summary, by regulating the steric hindrance of the TMOF molecule, we developed a novel LHCE that greatly enhanced the cycling stability of high-voltage LMBs.The TEOF molecule, with a lengthened terminal alkyl group than TMOF, has a larger steric hindrance and, therefore, a weakened solvation ability.Raman characterizations and theoretical simulations reveal that the weak solvation ability of TEOF contributes to an anion-dominant solvation structure, which facilitates the formation of a LiFrich SEI.Therefore, the LiFSI-1.2TEOF-3TTEelectrolyte enables dendrite-free Li deposition morphologies and high reversible Li metal anode with a high CE of 99.1% for 450 cycles.In addition, the electrolyte delivers stable cycling of high-voltage Li||NMC811 full cells with a high capacity retention of 95.6% after 150 cycles.The research demonstrates the potential of rational molecular design of electrolytes and provides important implications for the development of high-performance LMBs.

Fig. 1 a
Fig. 1 a Schematic illustration of the molecular design using steric hindrance effect.The calculated electrostatic potentials (ESPs) of b TMOF and c TEOF solvent

Fig. 2
Fig. 2 DFT optimized AGG solvation structure in the a LiFSI-1.2TMOF-3TTE and b LiFSI-1.2TEOF-3TTEelectrolytes.c Raman spectra of these two electrolytes.d The corresponding statistics of disassociated and solvated FSI − anions based on the Raman results.Snapshots of the MD simulations boxes in e LiFSI-1.2TMOF-3TTE and f LiFSI-1.2TEOF-3TTE.Radical distribution function g(r) from the MD results of g LiFSI-1.2TMOF-3TTE and h LiFSI-1.2TEOF-3TTE

Fig. 3
Fig. 3 SEM images of the deposited Li with a capacity of a, d 0.5 mAh cm −2 and b, e 1 mAh cm −2 in the LiFSI-1.2TMOF-3TTEandLiFSI-1.2TEOF-3TTEelectrolyte at 0.5 mA cm −2 .c, f Cross-sectional SEM images of the deposited Li with a capacity of 1 mAh cm −2 .AFM observations of the deposited Li in g LiFSI-1.2TMOF-3TTE and h LiFSI-1.2TEOF-3TTE.i The corresponding values of the modulus of SEI formed in two electrolytes

Fig. 5 a
Fig. 5 a Cycling stability of the Li||NMC811 cells.The corresponding capacity-voltage profiles of the cells cycled in the b LiFSI-1.2TEOF-3TTE and c LiFSI-1.2TMOF-3TTE electrolytes

Fig. 6
Fig. 6 SEM images of the Li metal anode cycled in Li||NMC811 full cells for 50 cycles using the a LiFSI-1.2TMOF-3TTE and b LiFSI-1.2TEOF-3TTEelectrolytes.c Elemental ratios of the SEI on Li metal surface.XPS characterizations of the Li metal anode cycled in d-g LiFSI-1.2TMOF-3TTE and h-k LiFSI-1.2TEOF-3TTE