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

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 20854−20863

www.acsami.org

Lithium Bis(oxalate)borate Reinforces the Interphase on Li-Metal Anodes Qiankui Zhang,† Kang Wang,† Xianshu Wang,† Yaotang Zhong,† Mingzhu Liu,† Xiang Liu,*,†,‡ Kang Xu,*,§ Weizhen Fan,∥ Le Yu,∥ and Weishan Li*,†,‡

Downloaded via UNIV OF SOUTHERN INDIANA on July 18, 2019 at 02:35:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

School of Chemistry and Environment and ‡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 Laboratory of ETESPG (GHEI), South China Normal University, Guangzhou 510006, China § Electrochemistry Branch, Sensor and Electron Devices Directorate, Power and Energy Division, U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States ∥ Guangzhou Tinci Material Technology Co., Ltd, Guangzhou 510760, China S Supporting Information *

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 the underlying mechanism poorly understood. In this work, applying lithium bis(oxalate)borate (LiBOB) as a chemical source for a 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 the carbonate electrolyte is unstable and responsible for the fast deterioration of the 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 the BOB anion over the carbonate molecules and the strong combination of its reduction products with the species from the electrolyte reduction. The effectiveness of such new SEI chemistry on the Li anode supports excellent performance of a Li/LiFePO4 cell. This approach provides a pathway to rationally design an interphase on the Li anode so that high energy density batteries could be realized. KEYWORDS: lithium-metal batteries, electrolyte additives, LiBOB, strong combination, reinforces interphase

1. INTRODUCTION

incessant side reactions, resulting in rapid loss of active materials and even safety concerns.16,17 Extensive research have been conducted to improve the stability of the Li/electrolyte interface, with numerous strategies being developed to date, such as reducing the overpotential for Li striping/depositing,18 substituting solidstate electrolytes for liquid ones,19−21 artificially constructing protective coating on the 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 method.34−37 Additives of varying structures have been identified to form protective SEIs,38−45 especially those in the Li salt form that are believed to help reduce the impedance of the SEIs.44 Unfortunately, the

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 pace with the increasing energy needs from applications such as electric vehicles with further driving ranges and electronic devices with longer working times.1−7 Lithium-metal batteries are considered the most promising alternative to LIBs because the Li-metal anode can deliver a high theoretical specific capacity 10 times that of graphite (3860 mA h g−1) at a 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 the electrolyte.14,15 Meanwhile, Li dendrites and isolated Li tend to be formed due to such © 2019 American Chemical Society

Received: March 19, 2019 Accepted: May 20, 2019 Published: May 22, 2019 20854

DOI: 10.1021/acsami.9b04898 ACS Appl. Mater. Interfaces 2019, 11, 20854−20863

Research Article

ACS Applied Materials & Interfaces

Figure 1. Optimized structures and electron affinity energies (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. (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 were prepared by coating the slurry of LiFePO4, poly(vinylidene fluoride), and Super-P binder (8:1:1, weight ratio) in N-methylpyrrolidone solvent onto an Al foil collector. The cast electrodes were then dried at 120 °C for 12 h before use. 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 glovebox (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 a LAND cell test system (Land CT 2001A, China). The Li/Li cells were tested at various current densities with timecontrolled 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 1C (1C = 170 mA h g−1) in the cycling performance test. Cyclic voltammetry of the Li/Cu cell was performed on a Solartron-1470 instrument (U.K.) in the potential range of −0.5−2.5 V (vs Li/Li+) at a scanning rate of 0.01 mV s−1. Electrochemical impedance spectroscopy was performed in frequency ranging from 200 kHz to 100 mHz under amplitude of 10 mV using an electrochemical station (PGSTAT-30, Autolab Metrohm, The Netherlands). 2.4. Physical Characterization. The cycled Li/Li cells were disassembled in the argon-filled glovebox, and the Li electrode was washed three times with dimethyl carbonate solvent to remove away the residual electrolyte. The morphologies of the Li-metal anode were observed with a field emission scanning electron microscope (FEISEM, FEI-Quanta-250). The surface compositions of the Li-metal anode before and after cycling were detected by X-ray photoelectron spectroscopy (XPS) using X-ray photoelectron spectra (ESCALAB 250, Thermo Fisher Scientific) and Fourier transform infrared (FTIR, Bruker Tensor 27, Germany).

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 anodes in LIBs, with the assumption that a protective SEI containing B−O species was formed from the reduction of the BOB− anion.39 However, little is known about how the 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 the Li anode by combining techniques of electrochemical and 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 the presence of LiBOB in the electrolyte, a reinforced SEI is built thanks to the preferential reduction of the 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 the Li-metal anode.

2. EXPERIMENTAL SECTION 2.1. Density Functional Theory (DFT) Calculation. All calculations were performed on the Gaussian 09 program package.46 The geometry of molecules was 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 the polarized continuum model calculations.48,49 The electron affinity is calculated by changing the 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 a ternary solvent (EC/EMC/DEC 3:5:2 by weight). Various amounts

3. RESULTS AND DISCUSSION The optimized structures 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 20855


Research Article


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.

reduction activity of the investigated solvent molecules or anions (Figure 1b). However, BOB− remains the species that is the most willing to accept an electron. On closer examination of Figure 1a,b, it becomes apparent that the presence of Li+ significantly increases the reductive activity of molecules, as expected from previous studies on the Li+-solvation sheath and its activation of the inner members for reaction. Since the reduction activity of solvents is affected by both anions and other solvents in the electrolyte,50 EAE of LiBOB under the influence of LiPF6 and solvent molecules is also obtained (Figure S2a,b). Compared with other solvent molecules and anions, the lowest EAE of LiBOB predicts its preferential reduction in the electrolyte system. It has been well established that LiBOB reductively decomposes on the graphite surface at a potential of 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 the Li-metal surface, cyclic voltammetry was

Scheme 1. Mechanism of SEI Formation in a LiBOBContaining Electrolyte

Information), where the electron affinity energy (EAE) is defined as the amount of energy after the molecule/compound gained one electron (being reduced). Hence, a lower EAE indicates relative ease of reductive activity. The calculated EAE (Figure 1a) apparently suggests that such ease decreases on the order of BOB− > EC > PF6− > EMC > DEC > Li+. This order changes a little upon the addition of Li+, which affects the 20856


Research Article


Figure 3. Cycling performances at current densities of (a) 1.0 mA cm−2 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.

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 the graphitic anode where the formation of SEI is stepwise as the anode potential is gradually polarized, the formation of SEI on the Li-metal anode occurs almost instantaneously. Such an 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 (Figure S3a), the length of the C−O bond increases dramatically compared with that of the other bonds in BOB−, which suggests that the reaction should

Scheme 2. Mechanism of SEI Formation in the Base Electrolyte

conducted using a Li/Cu cell. A small peak at about 0.75 V was observed for the cell using the LiPF6-based electrolyte (Figure 1c), which should be contributed by the reduction of ethylene carbonate (EC). This peak can also be observed for electrolytes containing LiBOB (Figure 1d). In the presence 20857


Research Article


Figure 5. Optimized structures and binding energies (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 the Li-metal interface.

pronounced contribution of this anion in SEI, which displays a significantly higher content of semicarbonate (R-CH2OCO2Li) at ∼289 eV. This result is in good agreement with previous reports for SEIs formed on the graphite surface, where the predominant chemical species deposited as the main building material of SEI are likely the semicarbonates.53 With the progress of 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

proceed with the cleavage of this bond. There are two possible pathways for BOB− reduction through the C−O bond cleavage (Figure S3b,c). Comparing the Gibbs free energy change (ΔG) for reactions 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 Xray photoelectron spectroscopy (XPS). There are two prominent C 1s peaks in the SEI from the base electrolyte (Figure 2a): C−C bonds at 284.7 eV and R-CH2OCO2Li at 289.3 eV.53,54 With LiBOB in the electrolyte, C 1s reveals the 20858


Research Article


Figure 7. Charge/discharge profiles of Li/LFP cells at the 30th, 60th, and 90th cycles in (a) base and (b) 5% LiBOB-containing electrolytes. (c) Cycling stability of Li/LFP cells at 0.2C for the initial three cycles and at 1C 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.

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 vibration56 (Figure S4). Elemental mapping reveals that B, F, and P are evenly distributed throughout the 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 tricoordinated boron centers (Scheme 1). These reduction substances combine the reduction products from the solvent (R-CH2OCO2Li) to construct an SEI that is more ductile, uniform, and dense. Li 1s (Figure 2e,f) and F 1s spectra (Figure 2g,h) reveal LiF peaks at 55.7 and 685.2 eV.26,57−59 In the base electrolyte, the peak intensities of LiF and LixPOyFz significantly increase with cycling. Since these compounds are from the decomposition of LiPF6, we can conclude that the SEI film obtained with an additive is more conducive to suppress decomposition of Li salt.

chemical species in SEI. The organic components (RCH2OCO2Li) 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 on 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 the LiBOB electrolyte is more compact as a 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 Li−B−O (193.4 eV)54,55 species detected is consistent with the assumption that LiBOB participates in the SEI chemistry. With further cycling, the intensity of the 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 20859


Research Article


generated from the equivalent circuit shows high consistency with the electrochemical impedance spectra (Figure S9), which deconvolute the contribution from each component 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 LiBOBoriginated SEI maintains the stable interface with the highly reactive Li metal, providing the cell with constant Li + conduction. Under a scanning electron microscope (SEM), the Li deposited from the base electrolyte is revealed to contain “islands” that consist of needlelike dendritic crystallization (Figure S10a,b). In the 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). Lidepositing morphologies in these electrolytes were characterized using SEM (Figure 4). The surface and cross-sectional views 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 needlelike Li was formed after cycling (Figure 4b). The dendritic nature renders large area for the Li metal to interact with the electrolyte, where the electrolyte decomposition is further catalyzed. In contrast, the Li electrode recovered from the LiBOB-containing electrolyte shows no significant cracking (Figure 4d) along with a corrosion depth of about 46.4 μm (Figure 4f). Consequently, the Li surface consists of round-edged Li depositions enclosed by dense SEI (Figure 4e). The reactivity of these round-edged Li depositions with the 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. Due to the strong reactivity of Li metal, the loose SEI formed from the side reaction products between the 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 produce Li2BOx, which was inferred based on theoretical calculations (Figure S3). These Li2BOx species with a higher inorganic nature are less soluble compared with semicarbonates 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 (lithium ethylene dicarbonate (LEDC), lithium ethyl carbonate (LEC), lithium methyl carbonate (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

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 denser and more protective SEI built from the coreduction of LiBOB and EC than that from the reduction of only EC.60−63 The electrochemical impedance spectra below confirm 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 that 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 the 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%. To exclude the interference of higher Li+ concentration brought by LiBOB addition, we also used 1.2 M LiPF6 electrolyte as the 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 the 1.2 M LiPF6 electrolyte is still inferior to that of the 5% LiBOB electrolyte but slightly better than that of the base electrolyte. This comparison thus establishes that the improved performance in Li/Li symmetric cells should mainly come from BOB− contribution to the SEI. It can be noted from Figures 3 and S7 that the polarization in the 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 the initial 50 cycles (Figure S7a). The stability of the SEI resulting from the LiBOB-containing electrolyte can also be demonstrated by cycling Li/Cu cells (Figure S8). Electrochemical impedance spectroscopy was employed to demonstrate the interfacial resistivity of SEIs in Li/Li symmetrical cells. After the initial three cycles, the interfacial resistance of the LiBOB-containing Li/Li cell was higher than that of the base (Figure 3c, 375.4 vs 120.8 Ω), which is due to the more resistive initially BOB−-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 coreduction 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 the 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 the electrolyte. The fitting line 20860


Research Article


tribution of LiBOB is helpful for the rational designing of an interphase on the Li anode.

components (Figure 5b,c). 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 cycling is performed on Limetal anode in the base electrolyte, 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 RCH2OCO2Li leads to the continuous reactions of Li metal and electrolytes, and then the 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. During the cycle process, LiBOB as additives leads to dense and robust SEI on the Limetal surface because of its preferential reduction, and the reduction product (Li2BOx) of LiBOB can bind to such organic components (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 the cathode material due to its excellent cyclic stability. Selected charging/discharging profiles of these cells were shown at a rate of 1C (Figure 7a,b). Apparently, the lithiation/delithiation platform of LFP constantly fades in base electrolytes. Given the robust LFP lattice structure and the mild redox potential, the fading definitely comes from the degradation of the Li-metal anode (Figure S13), with the corrosion of bulk Li metal and sustained electrolyte decomposition as the major causes. Such degradation is effectively eliminated in the presence of LiBOB, as evidenced by the much-improved cycling stability (Figure 7c). The effect of the presence of BOB− becomes rather pronounced when compared with that of the base electrolyte after 45 cycles. Correspondingly, the Coulombic efficiency of the LiBOB-containing electrolyte is constantly retained. Postmortem analysis was performed on Li/LFP full cells cycled in these electrolytes. A crude Li electrode surface with a great amount of mossy Li is obtained from the base electrolyte (Figure 7d,e), with a significant amount of electrolyte decomposition products accumulated. LiBOB presence in the electrolyte obviously suppressed such decompositions (Figure 7f,g).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04898.

■

Optimized structures of EC, DEC, EMC, BOB−, and PF6− before and after one electron reduction without and with Li+; optimized structure and NBO charge distributions of BOB− before and after one electron reduction; FTIR spectra of the Li-metal electrode after 10 cycles in base electrolyte, LiBOB-containing electrolyte, and pure LiBOB (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.L.). *E-mail: [email protected] (K.X.). *E-mail: [email protected] (W.L.). ORCID

Weishan Li: 0000-0002-1495-4441 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was 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).

■

REFERENCES

(1) Peng, H.-J.; Huang, J.; Zhang, Q. A Review of Flexible LithiumSulfur and Analogous Alkali Metal-Chalcogen Rechargeable Batteries. Chem. Soc. Rev. 2017, 46, 5237−5288. (2) Tarascon, J.-M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2010, 414, 171−179. (3) Guo, Y.; Li, H.; Zhai, T. Reviving Lithium-Metal Anodes for Next-Generation High-Energy Batteries. Adv. Mater. 2017, 29, No. 1700007. (4) Cheng, X.; Zhang, R.; Zhao, C.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117, 10403−10473. (5) Larcher, D.; Tarascon, J. Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2015, 7, 19−29. (6) Jurng, S.; Brown, Z.; Kim, J.; Lucht, B. Effect of Electrolyte on the Nanostructure of the Solid Electrolyte Interphase (SEI) and Performance of Lithium Metal Anodes. Energy Environ. Sci. 2018, 11, 2600−2608. (7) Zhong, Y.; Li, B.; Li, S.; Xu, S.; Pan, Z.; Huang, Q.; Xing, L.; Wang, C.; Li, W. Bi Nanoparticles Anchored in N-Doped Porous Carbon as Anode of High Energy Density Lithium Ion Battery. NanoMicro Lett. 2018, 10, 56. (8) Lin, D.; Liu, Y.; Cui, Y. Reviving the Lithium Metal Anode for High-Energy Batteries. Nat. Nanotechnol. 2017, 12, 194−206. (9) Xin, S.; Chang, Z.; Zhang, X.; Guo, Y. Progress of Rechargeable Lithium Metal Batteries Based on Conversion Reactions. Natl. Sci. Rev. 2017, 4, 54−70. (10) Bruce, P. G.; Freunberger, S.; Hardwick, L.; Tarascon, J. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29.

4. CONCLUSIONS LiBOB as additives leads to dense and robust SEI on the Limetal 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 electron affinity energy calculations and electrochemical evaluations using symmetric and full Li-metal cells. The DFT calculations 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 (RCH2OCO2Li) of solvents. The eventual SEI with hybrid organic and inorganic natures constructs a dense and robust SEI that ensures nondendritic deposition of Li. The application of LiBOB provides a facile solution to the issue that highenergy lithium-metal cells exhibit severe electrolyte decomposition, whereas the underlying mechanism on the con20861


Research Article


Healing Electrostatic Shield Mechanism. J. Am. Chem. Soc. 2013, 135, 4450−4456. (29) Kanamura, K.; Shiraish, S.; Takeharo, Z. Electrochemical Deposition of Very Smooth Lithium Using. J. Electrochem. Soc. 1996, 143, 2187−2197. (30) Qian, J.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Henderson, W. A.; Zhang, Y.; Zhang, J.-G. Dendrite-Free Li Deposition Using Trace-Amounts of Water as an Electrolyte Additive. Nano Energy 2015, 15, 135−144. (31) Cheng, X.; Yan, C.; Chen, X.; Guan, C.; Huang, J.; Peng, H.; Zhang, R.; Yang, S.; Zhang, Q. Implantable Solid Electrolyte Interphase in Lithium-Metal Batteries. Chem 2017, 2, 258−270. (32) Yuan, Y.; Wu, F.; Bai, Y.; Li, Y.; Chen, G.; Wang, Z.; Wu, C. Regulating Li Deposition by Constructing LiF-rich Host for DendriteFree Lithium Metal Anode. Energy Storage Mater. 2019, 16, 411−418. (33) Zhang, X. Q.; Chen, X.; Cheng, X.; Li, B.; Shen, X.; Yan, C.; Huang, J.; Zhang, Q. Highly Stable Lithium Metal Batteries Enabled by Regulating the Solvation of Lithium Ions in Nonaqueous Electrolytes. Angew. Chem., Int. Ed. 2018, 57, 5301−5305. (34) Ye, C.; Tu, W.; Yin, L.; Zheng, Q.; Wang, C.; Zhong, Y.; Zhang, Y.; Huang, Q.; Xu, K.; Li, W. Converting Detrimental HF in Electrolytes into a Highly Fluorinated Interphase on Cathodes. J. Mater. Chem. A 2018, 6, 17642−17652. (35) Yang, X.; Chen, J.; Zheng, Q.; Tu, W.; Xing, L.; Liao, Y.; Xu, M.; Huang, Q.; Cao, G.; Li, W. Mechanism of Cycling Degradation and Strategy to Stabilize a Nickel-Rich Cathode. J. Mater. Chem. A 2018, 6, 16149−16163. (36) Zhu, Y.; Luo, X.; Zhi, H.; Liao, Y.; Xing, L.; Xu, M.; Liu, X.; Xu, K.; Li, W. Diethyl(thiophen-2-ylmethyl)phosphonate: a Novel Multifunctional Electrolyte Additive for High Voltage Batteries. J. Mater. Chem. A 2018, 6, 10990−11004. (37) Wang, B.; Xia, Q.; Zhang, P.; Li, G.; Wu, Y.; Luo, H.; Zhao, S.; van Ree, T. N-Phenylmaleimide as a New Polymerizable Additive for Overcharge Protection of Lithium-ion Batteries. Electrochem. Commun. 2008, 10, 727−730. (38) Yoo, D.; Kim, K. J.; Choi, J. The Synergistic Effect of Cation and Anion of an Ionic Liquid Additive for Lithium Metal Anodes. Adv. Energy Mater. 2018, 8, No. 1702744. (39) Zhao, W.; Zou, L.; Zheng, J.; Jia, H.; Song, J.; Engelhard, M.; Wang, C.; Xu, W.; Yang, Y.; Zhang, J.-G. Simultaneous Stabilization of LiNi0.76 Mn0.14 Co0.10 O2 Cathode and Lithium Metal Anode by Lithium Bis(oxalato)borate as Additive. ChemSusChem 2018, 11, 2211−2220. (40) Shi, P.; Zhang, L.; Xiang, H.; Liang, X.; Sun, Y.; Xu, W. Lithium Difluorophosphate as a Dendrite-Suppressing Additive for Lithium Metal Batteries. ACS Appl. Mater. Interfaces 2018, 10, 22201−22209. (41) Wood, S.; Pham, C. H.; Rodriguez, R.; Nathan, S.; Dolocan, A.; Celio, H.; Souza, J.; Klavetter, K.; Heller, A.; Mullins, C. K+ Reduces Lithium Dendrite Growth by Forming a Thin, Less-Resistive Solid Electrolyte Interphase. ACS Energy Lett. 2016, 1, 414−419. (42) Yan, C.; Yao, Y.; Chen, X.; Cheng, X.; Zhang, X.; Huang, J.; Zhang, Q. Lithium Nitrate Solvation Chemistry in Carbonate Electrolyte Sustains High-Voltage Lithium Metal Batteries. Angew. Chem., Int. Ed. 2018, 57, 14055−14059. (43) Wan, G.; Guo, F.; Li, H.; Cao, Y.; Ai, X.; Qian, J.; Li, Y.; Yang, H. Suppression of Dendritic Lithium Growth by in Situ Formation of a Chemically Stable and Mechanically Strong Solid Electrolyte Interphase. ACS Appl. Mater. Interfaces 2018, 10, 593−601. (44) Liao, B.; Li, H.; Xu, M.; Xing, L.; Liao, Y.; Ren, X.; Fan, W.; Yu, L.; Xu, K.; Li, W. Designing Low Impedance Interface Films Simultaneously on Anode and Cathode for High Energy Batteries. Adv. Energy Mater. 2018, 8, No. 1800802. (45) Huang, Z.; Ren, J.; Zhang, W.; Xie, M.; Li, Y.; Sun, D.; Shen, Y.; Huang, Y. Protecting the Li-Metal Anode in a Li-O2 Battery by Using Boric Acid as an SEI-Forming Additive. Adv. Mater. 2018, 30, No. 1803270. (46) Xing, L.; Borodin, O. Oxidation Induced Decomposition of Ethylene Carbonate from DFT Calculations–Importance of Explicitly

(11) Cao, R.; Xu, W.; Lv, D.; Xiao, J.; Zhang, J.-G. Anodes for Rechargeable Lithium-Sulfur Batteries. Adv. Energy Mater. 2015, 5, No. 1402273. (12) Li, X.; Rao, M.; Lin, H.; Chen, D.; Liu, Y.; Liu, S.; Liao, Y.; Xing, L.; Xu, M.; Li, W. Sulfur Loaded in Curved Graphene and Coated with Conductive Polyaniline: Preparation and Performance as Cathode of Lithium-Sulfur Battery. J. Mater. Chem. A 2015, 3, 18098−18104. (13) Park, M.; Sun, H.; Lee, H.; Lee, J.; Cho, J. Lithium-Air Batteries: Survey on the Current Status and Perspectives Towards Automotive Applications from a Battery Industry Standpoint. Adv. Energy Mater. 2012, 2, 780−800. (14) Lu, D.; Shao, Y.; Lozano, T.; Bennett, W.; Graff, G.; Polzin, B.; Zhang, J.; Engelhard, M.; Saenz, N.; Henderson, W.; Bhattacharya, P.; Liu, J.; Xiao, J. Failure Mechanism for Fast-Charged Lithium Metal Batteries with Liquid Electrolytes. Adv. Energy Mater. 2015, 5, No. 1400993. (15) Xing, L.; Zheng, X.; Schroeder, M.; Alvarado, J.; Cresce, A.; Xu, K.; Li, Q.; Li, W. Deciphering the Ethylene Carbonate-Propylene Carbonate Mystery in Li-Ion Batteries. Acc. Chem. Res. 2018, 51, 282−289. (16) Yan, C.; Cheng, X.; Tian, Y.; Chen, X.; Zhang, X.; Li, W.; Huang, J.; Zhang, Q. Dual-Layered Film Protected Lithium Metal Anode to Enable Dendrite-Free Lithium Deposition. Adv. Mater. 2018, 30, No. 1707629. (17) Wang, X.; Pan, Z.; Wu, Y.; Xu, G.; Zheng, X.; Qiu, Y.; Liu, M.; Zhang, Y.; Li, W. Reducing Lithium Deposition Overpotential with Silver Nanocrystals Anchored on Graphene Aerogel. Nanoscale 2018, 10, 16562−16567. (18) Wang, X.; Pan, Z.; Wu, Y.; Ding, X.; Hong, X.; Xu, G.; Liu, M.; Zhang, Y.; Li, W. Infiltrating Lithium into Carbon Cloth Decorated with Zinc Oxide Arrays for Dendrite-free Lithium Metal Anode. Nano Res. 2019, 12, 525−529. (19) Porz, L.; Swamy, T.; Sheldon, B.; Rettenwander, D.; Frömling, T.; Thaman, H.; Berendts, S.; Uecker, R.; Carter, W.; Chiang, Y.-M. Mechanism of Lithium Metal Penetration through Inorganic Solid Electrolytes. Adv. Energy Mater. 2017, 7, No. 1701003. (20) Xin, S.; You, Y.; Wang, S.; Gao, H.; Yin, Y.; Guo, Y. Solid-State Lithium Metal Batteries Promoted by Nanotechnology: Progress and Prospects. ACS. Energy Lett. 2017, 2, 1385−1394. (21) Yang, L.; Wang, Z.; Feng, Y.; Tan, R.; Zuo, Y.; Gao, R.; Zhao, Y.; Han, L.; Wang, Z.; Pan, F. Flexible Composite Solid Electrolyte Facilitating Highly Stable “Soft Contacting” Li-Electrolyte Interface for Solid State Lithium-Ion Batteries. Adv. Energy Mater. 2017, 7, No. 1701437. (22) Li, N.; Yin, Y.; Yang, C.; Guo, Y. An Artificial Solid Electrolyte Interphase Layer for Stable Lithium Metal Anodes. Adv. Mater. 2016, 28, 1853−1858. (23) Qian, J.; Henderson, W.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Borodin, O.; Zhang, J.-G. High Rate and Stable Cycling of Lithium Metal Anode. Nat. Commun. 2015, 6, No. 6362. (24) Fan, X.; Chen, L.; Ji, X.; Deng, T.; Hou, S.; Chen, J.; Zheng, J.; Wang, F.; Jiang, J.; Xu, K.; Wang, C. Highly Fluorinated Interphases Enable High-Voltage Li-Metal Batteries. Chem 2018, 4, 174−185. (25) Ma, Y.; Zhou, Z.; Li, C.; Wang, L.; Wang, Y.; Cheng, X.; Zuo, P.; Du, C.; Huo, H.; Gao, Y.; Yin, G. Enabling Reliable Lithium Metal Batteries by a Bifunctional Anionic Electrolyte Additive. Energy Storage Mater. 2018, 11, 197−204. (26) Zheng, J.; Engelhard, M.; Mei, D.; Jiao, S.; Polzin, B.; Zhang, J.; Xu, W. Electrolyte Additive Enabled Fast Charging and Stable Cycling Lithium Metal Batteries. Nat. Energy 2017, 2, No. 17012. (27) Shimizu, M.; Umeki, M.; Arai, S. Suppressing the Effect of Lithium Dendritic Growth by the Addition of Magnesium Bis(Trifluoromethanesulfonyl)Amide. Phys. Chem. Chem. Phys. 2018, 20, 1127−1133. (28) Ding, F.; Xu, W.; Graff, G.; Zhang, J.; Sushko, M.; Chen, X.; Shao, Y.; Engelhard, M. H.; Nie, Z.; Xiao, J.; Liu, X.; Sushko, P. V.; Liu, J.; Zhang, J.-G. Dendrite-free Lithium Deposition Via Self20862


Research Article

ACS Applied Materials & Interfaces Treating Surrounding Solvent. Phys. Chem. Chem. Phys. 2012, 14, 12838−12843. (47) Wang, Y.; Xing, L.; Borodin, O.; Huang, W.; Xu, M.; Li, X.; Li, W. Quantum Chemistry Study of the Oxidation-Induced Stability and Decomposition of Propylene Carbonate-Containing Complexes. Phys. Chem. Chem. Phys. 2014, 16, 6560−6567. (48) Xing, L.; Borodin, O.; Smith, G.; Li, W. Density Functional Theory Study of the Role of Anions on the Oxidative Decomposition Reaction of Propylene Carbonate. J. Phys. Chem. A 2011, 115, 13896−13905. (49) Xing, L.; Wang, Z.; Li, W.; Xu, M.; Meng, X.; Zhao, S. Theoretical Insight into Oxidative Decomposition of Propylene Carbonate in the Lithium Ion Battery. J. Phys. Chem. B 2009, 113, 5181−5187. (50) Vatamanu, J.; Borodin, O.; Smith, G. Molecular Dynamics Simulation Studies of the Structure of a Mixed Carbonate/LiPF6 Electrolyte Near Graphite Surface as a Function of Electrode Potential. J. Phys. Chem. C 2012, 116, 1114−1121. (51) Panitz, J.; Wietelmann, U.; Wachtler, M.; Strö bele, S.; Wohlfahrt, M. Film Formation in LiBOB-Containing Electrolytes. J. Power Sources 2006, 153, 396−401. (52) Chen, Z.; Liu, J.; Amine, K. Lithium Difluoro(oxalato)borate as Salt for Lithium-Ion Batteries. Electrochem. Solid-State Lett. 2007, 10, A45−A47. (53) Xu, K.; Lee, U.; Zhang, S.; Wood, M.; Jow, T. Chemical Analysis of Graphite/Electrolyte Interface Formed in LiBOB-Based Electrolytes. Electrochem. Solid-State Lett. 2003, 6, A144−A148. (54) Xu, K.; Lee, U.; Zhang, S.; Jow, T. Graphite/Electrolyte Interface Formed in LiBOB-Based Electrolytes. J. Electrochem. Soc. 2004, 151, A2106−A2112. (55) Qiao, L.; Cui, Z.; Chen, B.; Xu, G.; Zhang, Z.; Ma, J.; Du, H.; Liu, X.; Huang, S.; Tang, K.; Dong, S.; Zhou, X.; Cui, G. A Promising Bulky Anion Based Lithium Borate Salt for Lithium Metal Batteries. Chem. Sci. 2018, 9, 3451−3458. (56) Zhao, W.; Zheng, J.; Zou, L.; Jia, H.; Liu, B.; Wang, H.; Engelhard, M. H.; Wang, C.; Xu, W.; Yang, Y.; Zhang, J.-G. High Voltage Operation of Ni-Rich NMC Cathodes Enabled by Stable Electrode/Electrolyte Interphases. Adv. Energy Mater. 2018, 8, No. 1800297. (57) Neumair, S.; Vanicek, S.; Kaindl, R.; Többens, D.; Wurst, K.; Huppertz, H. High-Pressure Synthesis and Crystal Structure of the Lithium Borate HP-LiB3O5. J. Solid State Chem. 2011, 184, 2490− 2497. (58) Zhang, X.; Cheng, X.; Chen, X.; Yan, C.; Zhang, Q. Fluoroethylene Carbonate Additives to Render Uniform Li Deposits in Lithium Metal Batteries. Adv. Funct. Mater. 2017, 27, No. 1605989. (59) Zhu, Y.; Luo, X.; Zhi, H.; Yang, X.; Xing, L.; Liao, Y.; Xu, M.; Li, W. Structural Exfoliation of Layered Cathode under High Voltage and Its Suppression by Interface Film Derived from Electrolyte Additive. ACS Appl. Mater. Interfaces 2017, 9, 12021−12034. (60) Xu, M.; Lu, D.; Garsuch, A.; Lucht, B. Improved Performance of LiNi0.5Mn1.5O4 Cathodes with Electrolytes Containing Dimethylmethylphosphonate (DMMP). J. Electrochem. Soc. 2012, 159, A2130− A2134. (61) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (62) Dalavi, S.; Xu, M.; Knight, B.; Lucht, B. Effect of Added LiBOB on High Voltage (LiNi0.5Mn1.5O4) Spinel Cathodes. Electrochem. Solid-State Lett. 2012, 15, A28−A31. (63) Mun, J.; Lee, J.; Hwang, T.; Lee, J.; Noh, H.; Choi, W. Lithium Difluoro(oxalate)borate for Robust Passivation of LiNi0.5Mn1.5O4 in Lithium-Ion Batteries. J. Electroanal. Chem. 2015, 745, 8−13. (64) Tang, W.; Yin, X.; Chen, Z.; Fu, W.; Loh, K. P.; Zheng, G. Chemically Polished Lithium Metal Anode for High Energy Lithium Metal Batteries. Energy Storage Mater. 2018, 14, 289−296.

20863


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

Recommend Documents