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Suppression of Dendritic Lithium Growth by In-situ Formation of a Chemically Stable and Mechanically Strong Solid Electrolyte Interphase Guojia Wan, Feihu Guo, Hui Li, Yuliang Cao, Xinping Ai, Jiangfeng Qian, Yangxing Li, and Hanxi Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14662 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017
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ACS Applied Materials & Interfaces
Suppression of Dendritic Lithium Growth by In-situ Formation of a Chemically Stable and Mechanically Strong Solid Electrolyte Interphase ‡
‡
Guojia Wan†, Feihu Guo†, Hui Li , Yuliang Cao†, Xinping Ai†, Jiangfeng Qian†*, Yangxing Li *, Hanxi Yang†* † Hubei Key Laboratory of Electrochemical Power Sources, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, China. ‡ Watt Laboratory, Central Research Institute, Huawei Technologies Co., Ltd., Bantian, Longgang District, Shenzhen 518129, China. KEYWORDS: Lithium metal anode, solid electrolyte interphases, dendrite growth, sulfate additives, Li metal batteries
ABSTRACT: The growth and proliferation of Li dendrites during repeated Li cycling has long been a crucial issue that hinders the development of secondary Li metal batteries. Building a stable and robust solid state electrolyte interphases (SEI) on Li anode surface is regarded as a promising strategy to overcome the dendrites issues. In this work, we report a simple strategy to engineer the interface chemistry of Li metal anode by using tiny amount of dimethyl sulfate
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(DMS, C2H6SO4) as SEI-formation additive. With the preferential reduction of DMS, a SEI layer composed of Li2S/Li2O forms on the Li surface. This inorganic SEI layer features high structural modulus and low interfacial resistant, enabling a dense and dendrite-free Li deposition as evidenced by SEM, AFM and in-situ optical images. In addition, this SEI layer can prevent the deposited Li from direct contact with corrosive electrolyte, thus rendering an improved cycling stability of Li anodes with average Coulombic efficiency of 97 % for up to 150 cycles. When DMS additive is introduced into a Li/NCM full cell, the cycle life of Li metal batteries can be also improved significantly. This work demonstrates a feasible route to suppress Li dendrite growth by designing appropriate film-forming additives to regulate the interfacial properties of SEI layer and also the sulfonyl-based derivatives revealed in this work represent a large variety of new film-forming molecules, providing a broad selectivity for constructing high efficiency and cycle-stable Li anode to address the intrinsic problems of rechargeable Li metal batteries.
Introduction The demand for high energy batteries for emerging energy storage applications, such as electric vehicles, renewable power stations and smart grids, have fueled active research on “beyond Liion technology” in order to surpass the capacity limits of conventional intercalation anodes.1,2 In this background, rechargeable Li batteries based on metal Li anode have attracted worldwide attentions in recent years with focus on extremely high theoretical energy density Li–O2 (3580 Wh kg-1) and Li–S (2600 Wh kg-1),3 which are benefited from the highest specific capacity (3860 mAh g-1) and lowest negative potential (−3.04 V) of metal Li anode. However, despite these attractive merits of Li metal, its practical application for rechargeable batteries has suffered from the low Coulombic efficiency (CE) and poor cycle life due to Li dendrite formation during
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charge/discharge cycles, which not only consume large amount of Li+ ions and electrolyte to form “dead lithium”, but also cause internal short circuit,4 adding a potential safety issue. During the past decades, extensive studies have been conducted to understand the dendrite formation mechanism and therefore to develop effective strategies to establish dendrite-free Li anodes.5,6,7 It is now well recognized that the Li dendrite growth originated from the diffusioncontrolled electrochemical deposition of Li+ ions due to its much higher deposition rate (exchange current density io > 30 mA cm-2)8 than its diffusion rate (D =3.0×10-6 cm2 s-1)9 in electrolyte, which leads to a rapid depletion of interfacial Li+ concentration and thereafter a preferential deposition of Li onto surface protrusions to form dendrites.10 To suppress the Li dendrite growth, many different approaches have been suggested, including: (1) Physically blockage of the dendrite propagation by use of solid electrolytes such as inorganic ceramic electrolytes ( Li10GeP2S12, Li7La3Zr2O12)11,12 and solid polymer electrolytes (PEO-LiTFSI, PEALiTFSI);13,14,15 (2) Electrochemically regulating the lithium deposition kinetics by tuning the electrolyte composition, such as the use of Cs+ additive to create a self-healing electrostatic shield for slowdown of electrochemical reduction of Li+ ions16,17 and highly-concentrated electrolyte to increase Li+ flux;18,19,20 (3) Constructing 3D host framework to store Li metal, such as ‘lithiophilic’ layered reduced graphene oxide21,22,23 and hollow carbon nanocapsules.24 Although these approaches have gained some success in improving the cycle life of Li anode, they are still far from practical application due to the low Li+ conductivity of solid electrolytes and the breakdown of the nano-engineered surface architecture during cycles. Constructing a stable and compact solid electrolyte interphase (SEI) seems to be a most effective and viable strategy to inhibit the dendrite growth and therefore to enhance the cycling performance of Li anode.25,26 Early studies of Li anodes have already revealed that metallic Li
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can easily react with almost all the organic reagents (such as carbonates, ethers, furan),27 to form a SEI film, which protect the bulk Li from being further attacked by the chemicals. However, such formed SEI films in the conventional Li-ion battery electrolytes are mainly composed of organic ROCO2Li and ROLi components,28 which is structurally porous and fragile, partially soluble, dimensionally unstable, and easy to crack during volume changes of the Li layer underneath, therefore failing to prevent Li dendrite formation. To solve this problem, various artificial SEI films are constructed to dope inorganic components such as Li3N,29 LiF,30,31,32 Li3PO4,33 Li2S34 and Li2O into the interfacial layer for enhancing their mechanical strengths. For example, Lu et al.30 reported LiF nanoparticles as electrolytes additive to reinforce SEI, which led to a large improvement in the cycling stability of Li anode. Later research by Qian et al.31 have revealed that small amounts of HF and H2O in carbonate electrolytes promote the formation of a dense and uniform LiF layer on the Li surface, leading to smooth and hemispherical Li deposition. Cui et al.34 reported a stable SEI based on Li2S/LiNO3 formed by a synergetic action of both lithium polysulfide and lithium nitrate in ether-based electrolyte, which can prevent Li dendrite growth in Li/S cells. Recently, Guo et al.33 reported an artificial SEI film mainly consisting of Li3PO4 via in-situ reaction between Li metal and H3PO4 and demonstrated its effectiveness for suppressing the growth of lithium dendrites. These results demonstrate that the physico-chemical properties of the SEI film can be modified significantly by use of film-forming additives, through which the deposition morphology and cycling efficiency of Li anode can be tuned considerably to achieve a better cyclability. Selecting a suitable film-forming additive to build a stable SEI film is a difficult challenge because several chemical, electrochemical and mechanical requirements for the SEI should be taken into considerations.35,36 Firstly, a stable SEI must have the mechanical strength strong
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enough to withstand the huge volumetric change of Li anode during cycling and also must be compact to inhibit the Li dendrite growth from the void spaces. Secondly, the SEI must have sufficiently high Li+ conductivity, facilitating fast transport of Li+ ions across the interface. Moreover, the SEI film must be insoluble chemically and electrochemically inert in electrolyte, thus ensuring structural and morphological stability during long term cycles. All these requirements give a clue for selection of a well-qualified SEI-forming additive that its decomposition products as a main chemical composition must have high Young’s modulus, strong film-forming ability and sufficient Li+ conductivity. In search for such a film-forming molecule with all the above-mentioned properties, we paid our attention to organic sulfates/sulfites37 because their decomposition products Li2S/Li2O have much larger Young’s modulus (82.6/169.0 Gpa) than conventional SEI components (ROCO2Li/ROLi,
