Lithium Bis(oxalate)borate Reinforces the Interphase on Li-Metal

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Lithium Bis(oxalate)borate Reinforces Interphase on Li Metal Anode Qiankui Zhang, Kang Wang, Xianshu Wang, Yaotang Zhong, Mingzhu Liu, Xiang Liu, Kang Xu, Weizhen Fan, Le Yu, and Weishan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 22, 2019

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ACS Applied Materials & Interfaces

Lithium Bis(oxalate)borate Reinforces Interphase on Li Metal Anode

Qiankui Zhanga, Kang Wanga, Xianshu Wanga, Yaotang Zhonga, Mingzhu Liua, Xiang Liua,b,* Kang Xuc,* , Weizhen Fand, Le Yud, and Weishan Lia,b,*

a

School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China

b

National and Local Joint Engineering Research Center of MPTES in High Energy and Safety LIBs,

Engineering Research Center of MTEES (Ministry of Education), and Key Lab. of ETESPG(GHEI), South China Normal University, Guangzhou 510006, China c

Electrochemistry Branch, Sensor and Electron Devices Directorate, Power and Energy Division, U.S.

Army Research Laboratory, Adelphi, MD 20783, USA. d

Guangzhou Tinci Material Technology Co., Ltd, Guangzhou 510760, China.

ABSTRACT Li metal provides an ideal anode for the highest energy density batteries, but its reactivity with electrolytes brings poor cycling stability. Electrolyte additives have been employed to effectively improve the cycling stability, often with underlying mechanism poorly understood. In this work, applying lithium bis(oxalate)borate (LiBOB) as a chemical source for dense and protective interphase, we investigate this issue with combined techniques of electrochemical/physical characterizations and theoretical calculations. It was revealed that the solid electrolyte interphase (SEI) formed by Li and carbonate electrolyte is unstable and responsible for the fast cell deterioration of Li anode. When LiBOB is present in the electrolyte, a reinforced SEI was formed enabling significant improvement in cycling stability, due to the preferential reduction of BOB-anion over the carbonate molecules and the 1

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strong combination of its reduction products with the species from the electrolyte reduction. The effectiveness of such new SEI chemistry on Li anode supports excellent performance of a Li/LiFePO4 cell. This approach provides a pathway to rationally design interphase on Li anode so that high energy density batteries could be realized.

KEYWORDS: Lithium metal batteries, electrolyte additives, LiBOB, strong combination, reinforces interphase

1. INTRODUCTION Lithium ion batteries (LIBs) based on graphitic anode materials have made a great contribution to the fast development of information technology, but their energy density cannot keep the pace with the increasing energy needs from applications such as electric vehicles with further driving ranges and electronic devices with longer working time.1-7 Lithium metal batteries (LMBs) are considered the most promising alternative to LIBs because Li metal anode can deliver a high theoretical specific capacity ten times of graphite (3860 mA h g-1) at low potential (-3.04 V vs. standard hydrogen electrode).3,8,9 When coupled with new cathode chemistries such as sulfur or oxygen, a remarkably enhanced energy density can be achieved.10-13 However, the challenge that always faces Li metal is its high reactivity. It reacts with all known electrolyte materials, resulting in sustained growth of a solid electrolyte interphase (SEI) that fails to prevent consumptions of both Li and electrolyte.14,15 Meanwhile, Li dendrites and isolated Li tend to be formed due to such incessant side reactions, resulting in rapid loss of active materials and even safety concerns.16,17 Extensive researches have been conducted to improve the stability of Li/electrolyte interface, with 2


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numerous strategies developed up to date, such as reducing the overpotential for Li striping/depositing,18 substituting solid-state electrolytes for liquid ones,19-21 artificially constructing protective coating on Li anode,22 increasing the concentration of the salt in the electrolytes to extremes,23,24 and forming a protective SEI by applying electrolyte additives.25-33 Among these, forming a protective in situ SEI using electrolyte additives remains the most convenient as well as economically viable.34-37 Additives of varying structures have been identified to form protective SEIs,38-45 especially those in Li salt form that are believed to help reducing the impedance of the SEIs.44 Unfortunately, the underlying mechanism on the contribution of these additives remains unclear. One prominent example is lithium bis(oxalate)borate, which is well known for its protection of graphite anode in LIBs, with the assumption that a protective SEI containing B-O species was formed from the reduction of BOBanion.39 However, little is known about how BOB-anion breaks down, and why B-O species ensures better cycling stability? In this work, we investigated the effect of LiBOB on the interphase formed on Li anode by combining techniques of electrochemical, physical characterizations as well as theoretical calculations. It is found that SEI derived from the decomposition products of the baseline electrolyte is unstable and fails to prevent sustained electrolyte decomposition during prolonged cycling. In presence of LiBOB in the electrolyte, a reinforced SEI is built thanks to the preferential reduction of BOB-anion. The B-O species provided by the reduction of LiBOB not only introduced more inorganic nature into the SEIs, but also exhibited a strong combination with the reduction products of carbonate molecules, resulting in much better chemical and electrochemical stability of SEI that was responsible for the significantly improved cycling stability of Li metal anode.

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2. EXPERIMENTAL SECTION 2.1. Density Functional Theory Calculation. All calculations were performed on Gaussian 09 program package.46 The geometry of molecules were optimized using the B3LYP level of theory in conjunction with the 6-311++G (d) basis set.47 To investigate the role of the environment, the acetone dielectric constant (20.5) was used to represent the solvent for polarized continuum models (PCM) calculations. 48,49 The electron affinity is calculated by changing charge number and spin multiplicity. 2.2. Electrode and Electrolyte Preparation. Lithium bis(oxalato)borate (≥99.0%) was purchased from Aladdin. Battery-grade carbonate solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and lithium hexafluorophosphate (LiPF6) were provided by Guangzhou Tinci Materials Technology Co. Ltd, China. The base electrolyte was 1.0 M solution of LiPF6 in Ternary solvent (EC/EMC/DEC 3:5:2 by weight). Various amounts (by weight) of additive were added into the base electrolyte to obtain additive-containing electrolytes. The LiFePO4 material was provided by Shenzhen Optimum Nano Energy Co., Ltd. The cathodes was prepared by coating the slurry of LiFePO4, and polyvinylidene fluoride (PVDF), and Super-P binder (8:1:1, weight ratio) in N-methylpyrrolidone (NMP) solvent onto Al foils collector. The cast electrodes were then dried at 120 oC

for 12 h before used. The areal mass loading of the LiFePO4 was 7 mg cm−2. The battery-grade Li

foil (diameter = 12.5 mm, thickness = 550 μm) was purchased from China Energy Lithium Co., Ltd. The CR2025-type coin cells (Li/Li, and Li/LiFePO4) with different types of electrodes and Celgard 2400 separators were assembled in an argon-filled glove box (H2O and O2 < 1 ppm). The amount of electrolyte is 60 μL in Li/Li symmetric cells and Li/LFP cells. 2.3. Electrochemical Tests. The charge/discharge tests were performed on LAND cell test system (Land CT 2001A, China). The Li/Li cells were tested at various current densities with time controlled 4


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charge and discharge cycles. In each cycle, the Li/Li cells were charged for 1.0 h and then discharged for 1.0 h. The Li/LiFePO4 cells were cycled in the potential range of 2.2~4.1 V (vs. Li/Li+) at 1 C (1 C = 170 mA h g-1) in the cycling performance test. Cyclic voltammetry of the Li/Cu cell was performed on Solartron-1470 instrument (U.K.) in the potential range of -0.5-2.5 V (vs. Li/Li+) at a scanning rate of 0.01mV s-1. Electrochemical impedance spectroscopy was performed in frequency ranging from 200 kHz to 100 mHz under amplitude of 10 mV using electrochemical station (PGSTAT-30, Autolab Metrohm, Netherlands). 2.4. Physical Characterization. The cycled Li/Li cells were disassembled in the argon-filled glove box and Li electrode was washed three times with DMC solvent to remove away the residual electrolyte. The morphologies of Li metal anode were observed with field emission scanning electron microscope (FEI-SEM, FEI-Quanta-250, America). The surface compositions of Li metal anode before and after cycling were detected by the X-ray photoelectron spectroscopy (XPS) using X-ray photoelectron spectra (ESCALAB 250, Thermo Fisher Scientific, America) and FTIR (Bruker Tensor 27, Germany).

3. RESULTS AND DISCUSSION The optimized structure of EC, DEC, EMC, BOB-, and PF6-, before and after one electron reduction, and in the absence and presence of Li+, are given in Figure S1 (supporting information), where the electron affinity energy (EAE) is defined as the amount of energy after the molecule/compound gaining one electron (being reduced). Hence, a lower EAE indicates relative easiness for reductive activity. The calculated EAE (Figure 1a) apparently suggests that such easiness decreases in the order of BOB- > EC > PF6- > EMC > DEC > Li+. This order changes a little upon the 5



addition of Li+, which affects the reduction activity of the investigated solvent molecules or anions (Figure 1b). However, BOB- remains the species that will be most willing to accept electron. In closer examination of Figure 1a and 1b, it becomes apparent that the presence of Li+ significantly increases the reductive activity of molecule, as expected from previous studies on Li+-solvation sheath and its activation of the inner members for reaction. Since the reduction activity of solvents is affected by both anions or other solvents in the electrolyte,50 EAE of LiBOB under the influence of LiPF6 and solvent molecules is also obtained (Figure S2a and b). Compared with other solvent molecules and anion, the lowest EAE of LiBOB predicts its preferential reduction in the electrolyte system. It has been well established that LiBOB reductively decomposes on graphite surface at a potential around 1.70 V (vs. Li/Li+),51 which is higher by nearly 1.0 V than the reduction of carbonates (EC).52 To understand how LiBOB reacts on Li metal surface, the cyclic voltammetry was conducted using a Li/Cu cell. A small peak at about 0.75 V was observed for the cell using LiPF6-based electrolyte (Figure 1c), which should be contributed by the reduction of ethylene carbonate (EC). This peak can also be observed for electrolyte containing LiBOB (Figure 1d). In presence of LiBOB, however, a new peak was observed at around 1.70 V, which arises from the LiBOB-reduction. This reaction should precede the EC reduction in accordance with calculations, and an interphase consisting of ingredients from both LiBOB and EC should ensue. It should be pointed out here that, differing from graphitic anode where the formation of SEI is stepwise as the anode potential is gradually polarized, the formation of SEI on Li metal anode occurs almost instantaneously. Such indiscriminating reduction manner would not exclude the EC reduction products from the eventual SEI. Instead, a mixed chemistry from both EC and LiBOB should constitute the interphase. Based on the optimized geometric structure of BOB- before and after one electron reduction 6


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(Figure S3a), the length of the C-O bond increases dramatically compared to the other bonds in BOB-, which suggests that the reaction should proceed with the cleavage of this bond. There are two possible pathways for BOB- reduction through the C-O bond cleavage (Figure S3b and c). Comparing the Gibbs free energy change (△G) for the reaction 1 (BOB-) and 2 (LiBOB), one would expect the former to be more probable due to its lower value (165.4 vs. 178.5 kJ mol-1). The interphase constructed by LiBOB was examined by X-ray photoelectron spectroscopy (XPS). There are two prominent C 1s peaks in the SEI from base electrolyte (Figure 2a): C-C bounds at 284.7 eV and R-CH2OCO2Li at 289.3 eV.53,54 With LiBOB in the electrolyte, C 1s reveals the pronounced contribution of this anion in SEI, which displays significantly higher content of semi-carbonate (R-CH2OCO2Li) at ~289 eV. This result is in good agreement with previous reports for SEIs formed on graphite surface, where the predominant chemical species deposited as the main building material of SEI are likely the semicarbonates.53 With the progress of the cycling, in which Li is repeatedly deposited and stripped with newly exposed surfaces generated, the LiBOB participation resulted in an SEI of a very distinct chemical nature, where the content of semicarbonate-like compounds increased significantly and most probably became the major chemical species in SEI. The organic components (R-CH2OCO2Li) can afford the flexibility and accommodate the volume changes during Li plating/stripping. Such volume change is negligible in graphitic anode materials (~10%), but would become significant for Li metal depending the excess fold of Li metal used. Generally, the C-C peak intensities (284.7 eV) for the Li electrode from the LiBOB electrolyte are much weaker than those from the base electrolyte, suggesting that the SEI built by LiBOB electrolyte is more compact as result of the minimized electrolyte consumption. Meanwhile, significant increase in B intensity is observed in the interphase formed in the presence of LiBOB (Figure 2d). The high abundance of B-O (192.2 eV) and 7



Li-B-O (193.4 eV)54,55 species detected is consistent with the assumption that LiBOB participates the SEI chemistry. With further cycling, intensity of B 1s spectrum gradually grew stronger, indicating that LiBOB is constantly repairing SEIs during the cycling, which breaks old SEI and exposes new Li metal surfaces that would induce continuous reduction of LiBOB. Meanwhile, Fourier transform infrared (FTIR) spectroscopy was applied to analyze the interface composition of the Li anode. In the reflection FTIR spectrum of the SEI, some new peaks appeared at 1355.8, 1057.3, 1024.2, and 609.1 cm−1, which are assigned to the B-O, B-O-B bending, or O-B-O stretching vibrations56 (Figure S4). Elemental mapping reveals that B, F, P are evenly distributed throughout interphase (Figure S5). Upon electrochemical reduction, the B-O bond in BOB- is cleaved via a ring opening mechanism, and the resultant species that eventually constitute the main composition of SEI consist of carbonyl moieties and tri-coordinated boron centers (Scheme 1). These reduction substances combine the reduction products from solvent (R-CH2OCO2Li) to construct an SEI that is more ductile, uniform and dense. Li 1s (Figure 2e and f) and F 1s spectra (Figure 2g and h) reveal LiF peaks at 55.7 and 685.2 eV.26,57-59. In the base electrolyte, the peak intensity of LiF and LixPOyFz significantly increases with cycling. Since these compounds are from the decomposition of LiPF6, we can conclude that the SEI film obtained with additive is more conducive to suppress decomposition of Li salt.

Scheme 1. Mechanism of SEI-formation in LiBOB-containing electrolyte. 8


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The Li/Li symmetric cell cyclic tests were conducted to assess the effectiveness of interphases in stabilizing Li metal anodes during Li plating/stripping. In the first 50 cycles at a current density of 1 mA cm−2, the polarization in Li/Li cells containing LiBOB is higher than the cell containing base electrolytes (Figure 3a), which is the result of a more dense and protective SEI built from the co-reduction of LiBOB and EC than that from the reduction of only EC.60-63 The electrochemical impedance spectra below confirms this explanation. At the 100th cycle, the Li/Li cell without LiBOB starts to show a relatively higher overpotential (340 mV) than its LiBOB-counterpart (100 mV) at a current density of 1 mA cm−2. This crossover behavior reflects the effectiveness of a dense, robust but more resistive SEI can suppress continuous decomposition of the electrolyte, and eventually outperform the less resistive but more porous SEI. At a current density of 0.5 mA cm−2, the cell with base electrolyte shows that the overpotential increases rapidly with the end of life at 760 h (Figure 3b). In contrast, LiBOB enables a lower polarization and excellent stability without shorting in the course of 1500 h. Even at the large current density (2.0 mA cm-2) and with the high capacity (4.0 mA h cm-2), the Li/Li cell containing LiBOB maintains a good cycling stability (Figure S6). The effectiveness of SEI on the concentration of LiBOB was also examined. The comparison in polarization voltages showed that 5% LiBOB outperforms 1% and 3% (Figure S7a). Due to the poor solubility of LiBOB, white insolubles will appear after more than 5%. In order to exclude the interference of higher Li+ concentration brought by LiBOB addition, we also used 1.2 M LiPF6 electrolyte as reference, whose additional 0.2 M LiPF6 would provide the equivalent amount of Li+ to 5% LiBOB (Figure S7b). Experimental results show that the polarization voltage of 1.2 M LiPF6 electrolyte is still inferior to 5% LiBOB electrolyte but slightly better than base electrolyte. This comparison thus establishes that the improved performance in Li/Li symmetric cells should mainly come from BOB- contribution to the 9



SEI. It can be noted from Figure 3, S7 that the polarization in initial 100 cycles for the Li/Li cells containing LiBOB fluctuates compared to the incessant increase of the cells without LiBOB. This phenomenon can be explained by their SEI components that consist of the reduction products of LiBOB and EC, which vary during initial cycles and are related to the LiBOB concentrations that are responsible for larger polarization in 3% than in 5% during initial 50 cycles (Figure S7a). The stability of the SEI resulting from LiBOB-containing electrolyte can also be demonstrated by cycling Li/Cu cell (Figure S8). Electrochemical impedance spectroscopy was employed to demonstrate the interfacial resistivity of SEIs in Li/Li symmetrical cells. After initial 3 cycles the interfacial resistance of LiBOB-containing Li/Li cell was higher than the base (Figure 3c, 375.4 vs. 120.8 Ω), which is due to the more resistive BOB- initially originated SEI, as the polarization observed in Figure 3a,b. Over time, the interfacial resistance of the LiBOB-containing Li/Li cell decreases due to the change in SEI components, resulting from the co-reduction of LiBOB and EC. However, its SEI maintains a stable and low resistance (33.4 Ω after 50 cycles and 45.7 Ω after 130 cycles, Figure 3d,e), in sharp contrast to base cell, which significantly increases with cycling (34.1 Ω after 50 cycles and 242.7 Ω after 130 cycles), reflecting the accumulation of decomposition products from electrolyte. The fitting line generated from the equivalent circuit shows high consistency with the electrochemical impedance spectra (Figure S9), which deconvolute the contribution from each components to the overall cell impedance. The interfacial resistance of the Li/Li symmetric cell, especially RSEI, increases significantly during cycling in the base electrolyte. On the other hand, a LiBOB-originated SEI maintains the stable interface with highly reactive Li metal, providing the cell with constant Li+-conduction. Under scanning electron microscope (SEM) the Li deposited from base electrolyte is revealed to 10


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contain “islands” that consist of needle-like dendritic crystallization (Figure S10a,b). In presence of LiBOB, the Li deposition starts with the formation of nuclei uniformly distributed on the Li surface (Figure S10c), followed by the continuous Li deposition around the nuclei (Figure S10d). Consequently, the nucleation of Li metal can be regulated by the chemical composition of the SEI film.64 Similarly, from the morphology of Li dissolution, LiBOB reduction is more conducive to form a dense and compact Li surface layer (Figure S11). Li depositing morphologies in these electrolytes were characterized using SEM (Figure 4). The surface and cross-sectional view of a fresh Li foil are used as reference (Figure S12). It can be noted that, after 50 cycles, significant cracking occurs on the Li surface recovered from the base electrolyte (Figure 4a) along with significant corrosion (133.9 μm) of the bulk electrode (Figure 4c), which should be attributed to the porous SEI formed on the Li electrode surface by the base electrolyte. More needle-like Li was formed after cycling (Figure 4b). The dendritic nature renders large area for Li metal to interact with electrolyte, where the electrolyte decomposition is further catalyzed. In contrast, Li electrode recovered from LiBOB-containing electrolyte shows no significant (Figure 4d) along with a corrosion depth of about 46.4 μm (Figure 4f). Consequently, Li surface consists of round-edged Li depositions enclosed by dense SEI (Figure 4e). The reactivity of these round-edged Li depositions with electrolyte is much lower. This dense SEI can better cover the Li metal surface, and prevent the unwanted reactions while adhering more tightly to the Li surface.

Scheme 2. Mechanism of SEI-formation in base electrolyte.

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Due to the strong reactivity of Li metal, the loose SEI formed from the side reaction products between Li metal and electrolytes fails to completely protect the lithium metal anode interface. However, LiBOB is expected to easily undergo preferential reduction (Scheme 1) on the surface of Li metal and produces Li2BOx, which was inferred based on theoretical calculations (Figure S3). These Li2BOx species with higher inorganic nature are less soluble as compared with semi-carbonates from the solvent reduction (Scheme 2), hence providing better protection to Li metal. Density functional theory (DFT) calculations were performed in acetone to understand the interaction between R-CH2OCO2Li (LEDC, LEC, LMC) and inorganic components (Li2BOx, Li2CO3, LiF, Li2O) in SEI films. As shown in Figure 5a, LEDC-Li2BOx displays a more negative binding energy (-49.78 kJ mol-1) than LEDC-Li2CO3 (-42.83 kJ mol-1), LEDC-Li2O (-32.76 kJ mol-1), and LEDC-LiF(-37.22 kJ mol-1), suggesting that Li2BOx would win over other inorganic components in the competitive coordination with LEDC. Similarly, when the species are changed to LEC and LMC, Li2BOx still shows better coordination than other inorganic components (Figure 5b and 5c). Obviously, the calculation results show that Li2BOx can better combine the R-CH2OCO2Li to make the SEI film more dense and compact. Based on the results and discussions above, the construction mechanism of the protective interphase can be illustrated schematically in Figure 6. When Li metal anode is performed with cycling in the base electrolyte, the loose SEI formed from the decomposition products of LiPF6 and solvents, which mainly consist of LiF and R-CH2OCO2Li. The weak interaction between the inorganic component (LiF, Li2O, Li2CO3) and R-CH2OCO2Li leads to the continuous reactions of Li metal and electrolytes, and then Li metal anode suffers from structural collapse during long-term cycling. Most importantly, LiBOB exhibits the strongest reduction activity due to its lowest electron affinity energy. 12


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During the cycle process, LiBOB as additives leads to dense and robust SEI on Li metal surface because of its preferential reduction, the reduction product (Li2BOx) of LiBOB can bind to such organic component (R-CH2OCO2Li) and reinforce the interphase. Full Li metal cells were assembled to verify the effectiveness of BOB- originated SEI under practical conditions, where lithium iron phosphate (LiFePO4, LFP) was used as cathode material due to its excellent cyclic stability. Selected charging/discharging profiles of these cells were shown at a rate of 1 C. (Figure 7a and b). Apparently the lithiation/delithiation platform of LFP constantly fades in base electrolytes. Given the robust LFP lattice structure and mild redox potential, the fading definitely comes from the degradation of Li metal anode (Figure S13), with the corrosion of bulk Li metal and sustained electrolyte decomposition as the major cause. Such degradation is effectively eliminated in the presence of LiBOB, as evidenced by the much improved cycling stability (Figure 7c). The effect of BOB- presence becomes rather pronounced when compared with base electrolyte after 45 cycles. Correspondingly, The Coulombic efficiency of the LiBOB-containing constantly remains. Post-mortem analysis was performed on Li/LFP full cells cycled in these electrolytes. Crude Li electrode surface with great amount of mossy Li is obtained from base electrolyte (Figure 7d and e), with significant amount of electrolyte decomposition products accumulated. LiBOB presence in electrolyte obviously suppressed such decompositions (Figure 7f and g).

4. CONCLUSION LiBOB as additives leads to dense and robust SEI on Li metal surface because of its preferential reduction, which significantly suppresses side reactions between lithium metal and electrolytes. This benefit from the new interphasial chemistry originated from BOB- is confirmed by both calculations of 13



electron affinity energy as well as electrochemical evaluations using both half, symmetric and full Li metal cells. The DFT calculation of binding energy and possible decomposition paths along with XPS analysis on the recovered Li metal electrodes suggest that reduction products (Li2BOx) of LiBOB are formed in combination with the decomposition product (R-CH2OCO2Li) of solvents. The eventual SEI with hybrid organic and inorganic natures constructs a dense and robust SEI that ensures non-dendritic deposition of Li. The application of LiBOB provides a facile solution to the issue that high energy lithium metal cells exhibit severely electrolytes decomposition, while the underling mechanism on the contribution of LiBOB is helpful for the rational designing interphase on Li anode.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected]

NOTES The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 21872058), the Key Project of Science and Technology in Guangdong Province (2017A010106006) and the Innovation Project of Graduate School of South China Normal University (2018LKXM004). 14


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Figure captions Figure 1. Optimized structures and electron affinity energy (EAE, kJ mol-1) of (a) X (X = EC, DEC, EMC, and PF6-, BOB-) and (b) X-Li+. Cyclic voltammograms of Li/Cu cells cycling in (c) base and (d) 5% LiBOB-containing electrolytes. Figure 2. XPS patterns of Li metal electrodes before and after cycling in base and LiBOB-containing electrolytes: (a, b) C 1s, (c, d) B 1s, (e, f) Li 1s, and (g, h) F 1s. Figure 3. Cycling performances at a current density of (a) 1.0 and (b) 0.5 mA cm-2 and electrochemical impedance spectra at 1.0 mA cm-2 after (c) 3, (d) 50, and (e) 130 cycles for Li/Li cells using base and 5% LiBOB-containing electrolytes. Figure 4. Top-view and cross-sectional view SEM images of Li electrodes after 50 cycles using (a-c) base and (d-f) 5% LiBOB-containing electrolytes at 1.0 mA cm-2. Figure 5. Optimized structures and binding energy (Eb, kJ mol-1) of (a) X-LEDC, (b) X-LEC, and (c) X-LMC, (X= Li2CO3, Li2O, LiF, Li2BOx). Figure 6. Schematic illustration of the effect of LiBOB on the stability of Li metal interface. Figure 7. The charge/discharge profiles of Li/LFP cells at the 30th, 60th and 90th cycles in (a) base and (b) 5% LiBOB-containing electrolytes, (c) The cycling stability of Li/LFP cells at 0.2 C for the initial three cycles and at 1 C for the subsequent cycles between 2.2 and 4.1 V (vs. Li/Li+ ). SEM images of Li electrodes in Li/LFP cells after 50 cycles using (d, e) base and (f, g) 5% LiBOB-containing electrolytes.

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Figure 1. Optimized structures and electron affinity energy (EAE, kJ mol-1) of (a) X (X = EC, DEC, EMC, and PF6-, BOB-) and (b) X-Li+. Cyclic voltammograms of Li/Cu cells cycling in (c) base and (d) 5% LiBOB-containing electrolytes.

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Figure 2. XPS patterns of Li metal electrodes before and after cycling in base and LiBOB-containing electrolytes: (a, b) C 1s, (c, d) B 1s, (e, f) Li 1s, and (g, h) F 1s.

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Figure 3. Cycling performances at a current density of (a) 1.0 and (b) 0.5 mA cm-2 and electrochemical impedance spectra at 1.0 mA cm-2 after (c) 3, (d) 50, and (e) 130 cycles for Li/Li cells using base and 5% LiBOB-containing electrolytes.

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Figure 4. Top-view and cross-sectional view SEM images of Li electrodes after 50 cycles using (a-c) base and (d-f) 5% LiBOB-containing electrolytes at 1.0 mA cm-2.

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Figure 5. Optimized structures and binding energy (Eb, kJ mol-1) of (a) X-LEDC, (b) X-LEC, and (c) X-LMC, (X= Li2CO3, Li2O, LiF, Li2BOx).

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Figure 6. Schematic illustration of the effect of LiBOB on the stability of Li metal interface.

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Figure 7. The charge/discharge profiles of Li/LFP cells at the 30th, 60th and 90th cycles in (a) base and (b) 5% LiBOB-containing electrolytes, (c) The cycling stability of Li/LFP cells at 0.2 C for the initial three cycles and at 1 C for the subsequent cycles between 2.2 and 4.1 V (vs. Li/Li+ ). SEM images of Li electrodes in Li/LFP cells after 50 cycles using (d, e) base and (f, g) 5% LiBOB-containing electrolytes. 30


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Graphical abstract

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Lithium Bis(oxalate)borate Reinforces the Interphase on Li-Metal

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