Ionic liquids for high performance lithium metal batteries

Abstract

The pursuit of high energy density has promoted the development of high-performance lithium metal batteries. However, it faces a serious security problem. Ionic liquids have attracted great attention due to their high ionic conductivity, non-flammability, and the properties of promoting the formation of stable SEI films. Deeply understanding the problems existing in lithium metal batteries and the role of ionic liquids in them is of great significance for improving the performance of lithium metal batteries. This article reviews the effects of the molecular structure of ionic liquids on ionic conductivity, Li⁺ ion transference number, electrochemical stability window, and lithium metal anode/electrolyte interface, as well as the application of ionic liquids in Li-high voltage cathode batteries, Li-O₂ batteries and Li-S batteries. The molecular design, composition and polymerization will be the main strategies for the future development of ionic liquid-based electrolytes for high performance lithium metal battery.

Introduction

With the widespread use of lithium ion batteries in portable electronic devices, electric vehicles, grid energy storage systems, aerospace and other fields, lithium ion batteries (LIB) will also move towards higher energy density, higher safety and longer life [1], [2], [3]. The commercialized lithium ion battery using carbon anode is almost close to its theoretical capacity, which is difficult to meet the increasing energy density requirements of portable electronic devices, electric vehicles and large-scale energy storage. Lithium metal anode has a theoretical capacity ten times higher than that of traditional graphite cathode (3860 mAh/g, graphite anode 372 mAh/g) and the most negative potential (-3.045 V vs NHE), becoming the best choice for the anode materials of next-generation high energy density lithium batteries such as Li-S and Li-O₂ batteries [4], [5], [6].

Although lithium metal anodes have shown great potential in high energy density battery applications, they have not been applied in practical applications since they appeared in the 1970 s due to many challenges. For the successful application of lithium metal anodes, two main issues need to be solved. The most important issue is the formation of lithium dendrites due to high current density, irregular Li⁺ ion flux or low Li⁺ ion transference number, which can penetrate the separator and directly contact the cathode, resulting in short circuit and thermal runaway of the battery. Secondly, the formation of uneven SEI film on lithium metal anode is difficult, which depends mostly on the composition of the electrolyte. The uniform, relatively thin, compact, SEI film with high ion conductivity and high elastic strength can prevent the parasitic reactions consuming the lithium metal material and the electrolyte, and leading to reduced coulombic efficiency [7], [8]. Based on the above problems, scientists have proposed a variety of strategies.

The anodes with 3D structure are designed and fabricated to regulate the deposition of lithium. For example, the 3D Cu/Li composite electrode can reduce the local current density, which is beneficial to reduce the growth rate of dendrites and control the surface charge distribution, making the deposition of lithium more uniformly [9]. The carbon nanotube paper with high electronic conductivity also favors the homogeneously deposition of lithium metal to form a stable 3D lithium metal anode which reduces the local current density, and inhibits the formation of lithium dendrites [10]. Good lithiophilicity is a necessary factor for the host material. However, most materials cannot be wetted by molten lithium. In this case, the surface modification will be helpful. For example, the ZnO layer-modified 3D porous lithiophilic Zn framework shows excellent Li⁺ diffusion and electron conductivity on copper foil, which favors the uniform deposition of lithium and good cycle stability [11].

As the surface area of the 3D lithium metal anode structure increases, the contact area between the electrolyte and the lithium metal also increases. Therefore, the problem of SEI film becomes more prominent. The stability of SEI has a direct impact on the plating/stripping behavior and cycling life of lithium metal batteries, so the regulation of SEI is an important direction to solve the challenges of lithium metal [6], [12]. The most commonly used method of stabilizing lithium surface with a protective layer, i.e. artificial SEI (ASEI), which should have good chemical stability and reasonable lithium ion conductivity. ASEI is generally obtained by reacting lithium metal with specific chemicals [13]. Recently, Kozen et al. prepared an organic elastic layer (thickness 800 nm) by electrochemical polymerization to provide mechanical flexibility for ASEI. Furthermore, the lithium phosphorus oxynitride (LiPON) (thickness ~ 15 nm) was prepared by atomic-layer deposition (ALD). The organic/inorganic hybrid protective layer on lithium metal was found to have no failures during 110 plating/stripping cycles of 2 mA/cm² [14]. Another method is to use chemically stable and mechanically strong brackets to strengthen the SEI [15], [16]. Cui et al found that growing two-dimensional hexadecimal boron nitride (H-BN) on copper collector was beneficial to the smooth deposition of lithium due to its good chemical stability and mechanical strength [17].

Based on above analysis, the electrolyte plays an important role on the stability lithium metal anode. The lithium metal anode is thermodynamic instability in the carbonate-based liquid electrolyte, and the irreversible consumption of electrolyte accompanies with the growth of lithium dendrite and the formation of dead lithium, which eventually leads to serious safety problems and capacity decline [12]. Solid-state electrolytes may relieve the safety issues caused by the leakage and flammability of liquid electrolytes, and the poor stability of lithium metal anodes in liquid electrolyte. High-concentrated electrolytes have also been confirmed to show great potential in suppressing the growth of lithium dendrite and enhancing the cycling stability of lithium metal anodes [18]. Moreover, modifying the electrolyte with additives is one of the most convenient and widely used methods to stabilize lithium metal anodes by constructing a stable SEI film or affecting Li⁺ ion plating/stripping behavior [7].

Ionic liquids (ILs) have shown exciting potential for applications in lithium metal batteries and played versatile roles due to their completely different physicochemical properties from molecular solvents and inorganic salts. They can be used as electrolyte solvents to replace carbonate solvents to effectively improve the safety of batteries because of their non-volatile and non-flammable characteristics [19], [20]. They also can be used as additives in conventional electrolytes with low viscosity to obtain composite electrolytes with both excellent conductivity and safety [21]. They can be filled in polymer frameworks or ionic gels to obtain solid electrolytes with excellent electrical conductivity and mechanical properties [22], [23]. They can participate in the reaction with lithium metal electrodes as an electrolyte component to form a functional SEI film containing specific elements such as F, N [23], [24]. The ionic liquid-derived polymer can directly form a film on the electrode surface to protect lithium metal anode [25]. The ionic liquids can work as the wetting agents to stabilize the interfacial properties between solid electrolytes and solid electrodes [26].

Briefly, solving the stability problem of lithium metal anode is of great significance to realize the safe practical application of metal lithium battery. Ionic liquids composed of organic cations and inorganic/organic anions have extremely low volatility, high ionic conductivity, good thermal stability, low flammability and electrochemical stability. Some poly(ionic liquids), ionic gels and other related derivatives also retain the similar properties of ionic liquids. These properties make them show great potential in protecting lithium anode and attract tremendous attention. A deep insight into the role of ionic liquids in protecting lithium anode is of great significance for expanding the practical application of ionic liquids in lithium metal batteries. In this review, we systematically summarize the current issues of lithium metal anodes and efforts to overcome them by using ionic liquid, and also give some proposal for the future perspectives of ionic liquids in lithium metal batteries with the aim to provide some practical guidance for the researchers in relative areas.