He Liu, Tao Li, Xiangqun Xu, Peng Shi, Xueqiang Zhang, Rui Xu, Xinbing Cheng, Jiaqi Huang

1 School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China

2 Advanced Research Institute of Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, China

3 School of Resource Environment and Safety Engineering, University of South China, Hengyang 421001, China

4 Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

Keywords:Lithium metal anode Dendrite Solid electrolyte interphase Ester electrolyte Highly concentrated ether electrolyte

ABSTRACT Lithium metal batteries(LMBs)are highly considered as promising candidates for next-generation energy storage systems.However, routine electrolytes cannot tolerate the high potential at cathodes and low potential at anodes simultaneously,leading to severe interfacial reactions.Herein,a highly concentrated electrolyte (HCE) region trapped in porous carbon coating layer is adopted to form a stable and highly conductive solid electrolyte interphase(SEI)on Li metal surface.The protected Li metal anode can potentially match the high-voltage cathode in ester electrolytes.Synergistically,this ingenious design promises high-voltage-resistant interfaces at cathodes and stable SEI with abundance of inorganic components at anodes simultaneously in high-voltage LMBs.The feasibility of this interface-regulation strategy is demonstrated in Li |LiFePO4 batteries,realizing a lifespan twice as long as the routine cells, with a huge capacity retention enhancement from 46.4% to 88.7% after 100 cycles.This contribution proof-ofconcepts the emerging principles on the formation and regulation of stable electrode/electrolyte interfaces in the cathode and anode simultaneously towards the next-generation high-energy-density batteries.

1.Introduction

Lithium (Li)-ion batteries (LIBs) have achieved great success in the smartphones,laptops,and most recently,electric vehicles since 1990s.Relative to the conventional graphite anode, innovations in the electrode materials are required to further boost the energy density of advanced batteries and satisfy the top-end requirements[1-3].Li metal with a high specific capacity (3860 mA·h·g-1) and low electrochemical potential(-3.04 V vs.standard hydrogen electrode)is usually considered as the‘‘Holy Grail”anode in the energy storage systems [4-7].A high energy density can be potentially obtained when Li metal anode matches high-voltage cathodes,such as LiFePO4(LFP),LiNi1-x-yCoxMnyO2(NCM),and LiNi0.5Mn1.5-O4[8-11].

Li metal anode is currently subject to the problems of poor safety performance, low utilization efficiency, and short lifespan due to its highly reactive nature and dendrite growth preference[12-15].Various strategies have been proposed to handle these issues during repeated cycles, including electrolyte additives [16-21], artificial SEI [22-30], highly concentrated electrolytes[31,32], solid-state electrolytes [33-37], and three-dimensional Li hosts[38-43].The electrolyte regulation of the non-aqueous electrolytes is highly expected due to their convenient compatibility with the current battery technology [44-47].

Ester- and ether-based electrolytes are widely applied in Li metal batteries.Ester-based electrolyte has a high tolerance against electrochemical oxidation, which can be adopted in the batteries with high-voltage cathodes, such as NCM cathodes [48].However, ester electrolyte has a poor stability against Li metal anodes, leading to serious interfacial reactions and dendrite growth issues(Fig.1a)[49-52].In contrast,ether-based electrolyte(such as the highly concentrated ester electrolyte) [32,53]has a high tolerance against electrochemical reduction,which can match well with Li metal anode.However, ether electrolyte is easily oxidized and decomposed at high voltages (Fig.1b).It is of great significance to explore a strategy to realize stable anode/electrolyte interface by the ether electrolyte and cathode/electrolyte interface by the ester electrolyte [54-58].

Fig.1.Schematic illustration of the interface-regulation strategy.(a) The routine ester electrolyte with a wide voltage window cannot form a stable interface on Li metal anodes, resulting in dendrite growth and low utilization of Li metal anodes.(b) The routine ether electrolyte is able to form a relatively stable interface on Li metal anodes,while its narrow voltage window leads to severe oxidation on cathodes and poor cycling performance.(c)A highly concentrated electrolyte region trapped in porous carbon coating layer is adopted to form a stable interface on Li metal anode,and the protected Li metal anode can potentially match the high-voltage cathode in the ester electrolyte.

In this contribution, a stable interface on the Li metal anode is constructed by the highly concentrated ether electrolyte and the protected Li metal anode can potentially match the high-voltage cathode in the ester electrolyte (Fig.1c).The highly concentrated ether electrolyte (4.0 mol·L-1lithium bis(fluorosulfonyl)imide(LiFSI) in 1,2-dimethoxyethane (DME), 2.5 μl) was added and trapped by the porous carbon layer coated on the Li metal anode to form stable interfaces due to the high stability of highly concentrated electrolyte (HCE) against Li metal anode [59,60].The ester electrolyte (1.0 mol·L-1lithium hexafluorophosphate (LiPF6) in ethylene carbonate/diethyl carbonate (EC/DEC, the volume ratio is 1:1),60 μl)was then added as the working electrolyte to promise a stable high-voltage cathode.Thereby, stable interfaces can be constructed at the cathode and anode, which sheds new lights on the rational design of high-voltage LMBs.

2.Experimental

2.1.Raw materials

Reagents including DME, LiFSI, EC, DEC, and LiPF6were purchased from Alfa Aesar Chemical Co., Ltd.Aluminum(Al)foil,copper (Cu) foil, Celgard 2400, poly(vinylidenefluoride) (PVDF), and lithium iron phosphate (LiFePO4, LFP) powders were purchased from Shenzhen Kejing Star Technology Co., Ltd.Ultrathin Li metal foils with a thickness of 50 μm were purchased from China Energy Lithium Co., Ltd.

2.2.Synthesis of Li-HCE-KB electrode

Ketjen black(ECP600JD)was blade-coated on Cu foil with a carbon/binder weight ratio of 95:5 by using PVDF as the binder and Nmethyl pyrrolidinone (NMP) as the solvent.The carbon coated Cu foil was dried in a vacuum oven at 110 °C overnight.The carbon layer on Cu foil was grafted onto the thin Li belt (50 μm in thickness, China Energy Lithium Co., Ltd.) via a continuous roll to roll calendering method.The carbon layer can be easily peeled off from the Cu foil and grafted to the Li belt surface by calendering Li belt with the carbon coated Cu foil.The calendering process was performed in a dry room at a dew point of - 40 °C.Highly concentrated electrolyte (HCE) composed of 2.5 μl LiFSI (4.0 mol·L-1)-DME was added onto the Li-KB electrode to obtain the Li-HCE-KB electrode.The bare Li, Li-KB, and Li-HCE (dipping 2.5 μl HCE onto Li surface) were adopted as the control samples.

2.3.Material characterization

The morphology evolution of Li anode was investigated using a scanning electron microscope (SEM, JEOL JSM-7401F) operated at 3.0 kV.The chemical environment of samples was analyzed by an X-ray photoelectric spectroscopy (XPS, ESCALAB 250Xi,Thermal-Fisher).All the spectra were calibrated with the C 1s core level (284.6 eV) peak.Standard CR2025 coin cells with separator(Celgard 2400) were assembled in an argon-filled glove box with O2and H2O content below 0.0001% (atom).

2.4.Electrochemical measurements

All the electrochemical measurements with the electrolyte of LiPF6(1.0 mol·L-1) in EC/DEC were conducted on a Neware multichannel battery cycler.The assembled Li | Cu half cells were used to investigate the Coulombic efficiency of Li metal anode, while Li | Li symmetrical cells were adopted to probe the polarization,deposition morphology of cycled Li metal anode.In the full-cell test, LFP cathodes (6.0 mg·cm-2, 1.0 mA·h·cm-2) were assembled against ultrathin Li foil(50-μm thick,10.0 mA·h·cm-2)anode,corresponding to a N/P ratio of 10.The diameter of cathode and anode is 13 mm,while the separator is with a diameter of 16 mm to avoid the contact between cathode and anode.The dosage of electrolyte in each cell is 60 μl.

3.Results and Discussion

Fig.2.Morphology characterization of Li foil coating with the interfacial KB layer.(a, b) Top-view, and (c, d) cross-sectional SEM images of Li-KB sample.

The porous carbon layer herein was realized by coating Ketjen black(KB)to Li metal surface(Fig.S1,see Supplementary Material).The KB layer is uniformly coated onto Li metal surface(Fig.2a and c) and its particle size is ～50 nm (Fig.2b).Generally, HCE cannot wet the Li metal surface due to its high viscosity and therefore form a liquid drop on the surface (Fig.S2a).On the contrary, HCE can be absorbed and trapped inside the porous KB layer with a thickness of ～2 μm through the capillary force (Fig.2d) once HCE gets in touch with the porous carbon layer (Fig.S2b).The size of HCE drops can be reduced to nanometers due to the nanosized pore of KB layer.This largely increases the contacting points between HCE and Li metal,which effectively improves the wetting behavior of HCE on Li metal surface compared to the situation without KB layer.DME molecules in the HCE are mostly solvated and even FSI-anions can participate in the solvation process.A highly compact and conductive SEI layer with high LiF content can be formed on the anode side with HCE due to the decomposition of FSI-anions [32].LiF is a significant component in SEI in terms of stabilizing SEI layer,considering its high mechanical modulus, high chemical stability against Li metal and organic electrolytes, and excellent electronic insulation [61,62].The interfaces between LiF particles can be important channels to transfer Li ions and enhance the ionic conductivity in the LiFbased SEI[63].Therefore,Li metal anode with the porous HCE layer is expected to render a high utilization with a dendrite-free morphology.

The utilization of Li metal anodes with various interfaceregulation strategies was investigated in Li|Cu cells(Fig.3a).Conveniently,routine Li foil,Li with HCE,Li with a KB coating layer,Li with a KB layer storing HCE are noted as Li, Li-HCE, Li-KB, Li-HCEKB,respectively.Routine Li foil without any protection exhibits the lowest initial Coulombic efficiency (CE) and rapid decay after 30 cycles with an average CE of 91.2%at 0.5 mA·cm-2and 2.0 mA·h·c m-2.The low CE at initial cycles can be assigned to the formation of SEI on anode surface,while the rapid decay is attributed by the corrosive nature of ester electrolyte to Li metal anode.The former issue can be remedied by dipping HCE, which is verified by the enhanced initial CE in Li-HCE sample.HCE and coating KB layer are both beneficial in prolonging the cycling stability of Li metal anodes considering KB layer with nanometer particle size can decrease the local current density of electrodes.However, these methods cannot sustain their capability during long cycles.HCE cannot efficiently wet Li metal surface,and KB coating layer cannot render stable SEI on anode surface,especially at high current rates(1.0 mA·cm-2, 1.0 mA·h·cm-2).In contrast, Li-HCE-KB integrates the advantages of two strategies, rendering a stable and high CE for long cycles (Fig.S3).This benefits from the sustainable highquality SEI and reduced local current density of electrodes.Consequently,the CE of Li-HCE-KB is improved from 91.2%(30 cycles)to 95.7% (60 cycles) at 0.5 mA·cm-2and 2.0 mA·h·cm-2, and from 88.9% (45 cycles) to 92.7% (65 cycles) at 1.0 mA· cm-2and 1.0 mA·h·cm-2.

Li|Li symmetrical cells with the ester electrolyte were also conducted to probe the role of HCE-KB layer on Li metal protection(Fig.3b).The oscillatory charge curves in Li foil anode during the initial 20 h are caused by the unstable SEI formation process.The inferior SEI leads to a rapid rise in polarization from 50 to 90 mV after 120 h, and even higher than 150 mV after 150 h.A sudden voltage drop at 155 h is due to the short-circuit resulted from dendrites piercing, which can be confirmed from the deposition morphology (Fig.3c).There are large amounts of dendrites on the routine Li metal anode due to its fragile interface.Instead, Li-HCE-KB electrode displays a low and stable polarization during the whole cycling.This indicates that superior SEI is constructed on Li metal surface before the cell works, and Li metal can be well-protected during the whole cycling, further demonstrating the sustainability of this HCE-KB layer.The superior SEI layer also renders dense and uniform Li deposition without generation of Li dendrites(Fig.3d).Therefore,a high Li utilization and long cycling stability are achieved.

Fig.3.Half-cell performance of Li metal anodes with different interface-regulation methods.(a)Coulombic efficiency of Li|Cu cells;(b)Cyclability of Li|Li symmetrical cells;Li deposition morphologies of (c) Li and (d) Li-HCE-KB electrodes from Li | Li cells operated at 1.0 mA·cm-2/1.0 mA·h·cm-2 after 8 cycles.

The surface chemistry of Li metal anodes with porous HCE-KB layer and without protection after cycling in ester electrolyte was compared with X-ray photoelectron spectroscopy (XPS).LiF is observed in both Li foil and Li-HCE-KB electrodes(Fig.S4),which is derived from the decomposition of PF6-.However, the atomic concentrations of F in the SEI of Li-HCE-KB (10.1%) is one order of magnitude higher than that in Li foil electrode (1.0%) (Fig.4a,Table S1).The abundance of LiF in Li-HCE-KB is owing to the additional reduction of FSI-,demonstrated that HCE trapped in KB layer participates in constructing SEI.Additionally, the SEI layer of Li-HCE-KB electrode is composed of more inorganic components,such as Li3N, Li2S, and other sulfides (Fig.4a-c, Fig.S5), which is also contributed by the decomposition of HCE.Notably, it can be considered that almost all HCE has been consumed during SEI formation due to its tiny dosage (2.5 μl) compared to the working ester electrolyte(60 μl).The abundance of these inorganic components in the SEI contributes to a low resistance and rapid kinetics for Li ion transport [32,64-68].This highly conductive feature of the SEI layer renders a uniform and dense Li deposition morphology as well as a high Li utilization and a low polarization at high rates.In addition, the highly compact SEI formed by the HCE-KB layer also relieves further corrosion of Li metal anode and leads to better cycling stability.

Fig.4.(a)Atomic concentration of F,N,and S in Li and Li-HCE-KB electrodes calculated from XPS analysis.XPS spectra of(b)N 1s and(c)S 2p of Li-HCE-KB electrode in Li|Li cells after working in ester electrolyte for 2 cycles.

Full cells with LFP cathode were assembled to confirm the role of HCE-KB layer in practical cells.Though ether-based HCE can effectively protect Li metal by forming a stable interface and suppressing dendrite growth, its narrow voltage window limits the applications in the relatively-high-voltage batteries, such as LFP and NCM, etc.Considering ether electrolyte is usually adopted in lithium sulfur batteries with a voltage platform of 2.1 V,LFP herein could be regarded as a high-voltage cathode relative to the sulfur cathode.Compared to the ether electrolyte, the ester electrolyte affords a wide voltage window and stays relatively stable against the high-voltage cathode.Therefore, the ester electrolyte with a pre-protected Li metal anode is hopeful to render a stable highvoltage LMB.At 1.0 C, Li-HCE-KB cells have a high initial capacity of 135.2 mA·h·g-1and maintain 88.7% and 82.1% of its initial capacity at 100th and 120th cycle,while the other three cells decay sharply after 60 cycles(Fig.5a).Li-HCE cell exhibits the same decay tendency with routine Li foil cell with a capacity retention of 58.2%at 86th cycle,indicating the failure of HCE drop with the ester electrolyte.This also proves the indispensable role of KB coating layer in trapping HCE and generating a HCE region on Li metal surface.A slight performance enhancement in Li-KB cell(a capacity retention from 46.4%to 54.7%at 100th cycle)is originated from the reduced local current density in the anode with KB coating layer.The rapid decay in capacity has a strong relation with voltage polarizations(Fig.5b).This can be clearly characterized by the mid-capacity voltage (Fig.5c), which is defined as the voltage at the half of charging or discharging capacity.During the long-term cycles, Li-HCE-KB cell has a higher mid-capacity discharging voltage and lower mid-capacity charging voltage than the other three cells due to its stable Li plating/stripping behavior.The high electrode utilization and small polarization of Li-HCE-KB also render a superior energy efficiency (Fig.5d).The energy efficiency herein is defined as the ratio between discharge and charge energy, which is a critically important parameter for working cells in practical applications.

Fig.5.Full-cell applications of Li metal anodes matching LFP cathodes.(a)Specific capacity,(b)charge-discharge curves,(c)mid-capacity voltage,and(d)energy efficiency.

4.Conclusions

In conclusion,a stable interface at the anode side is constructed by the porous HCE layer and the protected lithium metal anode can potentially realize high-voltage LMBs in the ester electrolytes.This strategy is capable of forming a highly stable and conductive SEI layer with abundance of inorganic components (LiF, Li3N, Li2S,etc.)on Li metal surface.Therefore,uniform and compact Li deposits without dendrite formation are achieved, which renders a high Li utilization, low polarization, and better cycling stability in half cells.Notably,this strategy is also proved to be effective in protecting Li metal anode in high-voltage batteries with LFP cathode.The capacity retention of a full cell with routine Li foil anode is largely improved from 46.4% to 88.7% after 100 cycles with this HCE-KB layer.Considering its easy scalability and accessibility, the interface-regulation strategy is promising in the formation and regulation of stable electrode/electrolyte interfaces when matching high-voltage cathodes towards the next-generation highenergy-density batteries.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Beijing Natural Science Foundation(JQ20004), National Natural Science Foundation of China(21805161, 21808121, and U1932220), China Post-Doctoral Science Foundation (2020M670155 and 2020T130054), Scientific and Technological Key Project of Shanxi Province (20191102003).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.03.021.