Ionic conductive polymers as artificial solid electrolyte interphase films in Li metal batteries – A review

Abstract

Lithium (Li) metal has been considered as the ultimate anode material for next-generation rechargeable batteries due to its ultra-high theoretical specific capacity (3860 mAh g⁻¹) and the lowest reduction voltage (−3.04 V vs the standard hydrogen electrode). However, the dendritic Li formation, uncontrolled interfacial reactions, and huge volume variations lead to unstable solid electrolyte interphase (SEI) layer, low Coulombic efficiency and hence short cycling lifetime. Designing artificial solid electrolyte interphase (artificial SEI) films on the Li metal electrode exhibits great potential to solve the aforementioned problems and enable Li–metal batteries with prolonged lifetime. Polymer materials with good ionic conductivity, superior processability and high flexibility are considered as ideal artificial SEI film materials. In this review, according to the ionic conductive groups, recent advances in polymeric artificial SEI films are summarized to afford a deep understanding of Li ion plating/stripping behavior and present design principles of high-performance artificial SEI films in achieving stable Li metal electrodes. Perspectives regarding to the future research directions of polymeric artificial SEI films for Li–metal electrode are also discussed. The insights and design principles of polymeric artificial SEI films gained in the current review will be definitely useful in achieving the Li–metal batteries with improved energy density, high safety and long cycling lifetime toward next-generation energy storage devices.

Introduction

With the ever-increasing demand of portable electronics, electric vehicles and grid-scale storage, rechargeable batteries with high energy density are highly desired [1], [2], [3], [4], [5], [6]. The energy density of rechargeable batteries is related with different parameters, and the specific capacities and operating voltages of cathodes and anodes are the key parameters in determining their ultimate values [7], [8]. The cell level energy densities of current commercial lithium-ion batteries (LIBs) are ∼240 Wh kg⁻¹ or ∼640 Wh L⁻¹ based on the total weight or volume, respectively [9], [10]. With specific capacity and delithiation voltage of the commercial graphite anode being 372 mAh g⁻¹ and 0–0.2 V, respectively, anode materials with high specific capacity and low lithiation/delithiation voltage are always preferred [11], [12], [13], [14], [15]. Lithium (Li) metal having ultra-high theoretical specific capacity (3860 mAh g⁻¹) and lowest reduction voltage (−3.04 V vs the standard hydrogen electrode) has been considered as the “Holy Grail” anode material in advanced rechargeable batteries [16], [17], [18], [19], [20], [21], [22]. Generally speaking, Li metal batteries (LMBs) refer to the rechargeable batteries with Li metal as anode, and based on the types of cathode materials, LMBs can be divided into several categories, such as Li–sulfur (Li–S) batteries, Li–air (Li–O₂) batteries and Li–lithium metallic oxide (Li–LMO) batteries [23]. The ultrahigh theoretical energy densities of the LMBs (∼3500 Wh kg⁻¹ in Li–O₂ battery, ∼2600 Wh kg⁻¹ in Li–S battery and 1000 ∼ 1500 Wh kg⁻¹ in Li–LMO battery) make them highly promising for next-generation energy storage systems [24], [25], [26], [27].

However, LMBs have been considered as the “unsafe” energy storage devices since its invention in 1970s [28], [29]. Li metal has high reactivity: (1) it only exists as the compounds in nature. (2) The commercial metallic Li is normally produced by electrolysis. (3) Even without moisture, slow deterioration observed at ambient environment still bring numerous technological issues in processing, characterization and commercial applications [30]. When used as the electrode material in batteries, the lowest reduction voltage of Li metal can increase the operating voltage. On a different perspective, the lowest reduction voltage suggests a high electrochemical reactivity, making it unstable interacting with electrolyte leading to two main challenges, i.e., Li dendrite growth and unstable solid electrolyte interface (SEI) [31], [32], [33]. Firstly, the Li dendrites, which is caused by the inhomogeneous Li ion (Li⁺) plating during the cycling process, could penetrate the separator, cause short circuit and ultimately render catastrophe failure of batteries [34], [35], [36]. The morphologies of Li dendrites can be mainly divided into three categories: needle-like, tree-like, and moss-like, which strongly depend on the electrolyte composition and current density [37], [38], [39]. Various theories/models have been developed to unravel the mechanism of Li dendrites growth, and one of the popular theories claims that the protrusions with large curvature possess a higher electric field on tips sites, which could attract more Li⁺ for deposition [40], [41]. The Li dendrites may even be isolated from the bulk Li metal and become “dead” Li in the subsequent plating/stripping process, which will result the increased resistance and battery degradation and failure [42], [43]. Secondly, the unstable SEI layer will make the scenarios even worse. The uncontrolled interfacial reactions between Li surface and electrolyte along with the huge volume variation of Li metal electrode during the Li⁺ plating/stripping process are the major causes for the unstable SEI layer [44]. The newly deposited Li continuously reacts at the interface and consumes electrolyte and Li metal, which will result in continuous increase of interfacial resistance and decrease of Coulombic efficiency (CE) [45], [46], [47]. Till now, tremendous efforts have been devoted in understanding the problems of Li metal electrode to establish an efficient approach to maintain its long-term performance. Zhang and co-workers revealed that under low current density, the major cause for Li metal electrode failure are the powdering and induced polarization, while under higher current density, the short-circuit due to the penetration of Li dendrite leads to the safety concerns [48]. Cui and co-workers found that the nucleation and growth of Li dendrites are temperature-dependent processes, and a dendrites-free electrode can be obtained by elevating temperature from 25 °C to 60 °C [49]. Song et al. studied the dynamic distribution of Li⁺ by using the operando neutron radiography and static tomography [50]. They concluded that the short circuit of battery is initiated by the Li dendrites growth, which will partially penetrate the separator from anode to cathode, leading to the fluctuation of voltage profiles.

It can be seen that, in LMBs, the electronically insulating and ionically conductive SEI layer plays a crucial role in mitigating the growth of dendritic Li and protecting Li metal from reacting with electrolytes [51], [52]. Therefore, tremendous efforts have been devoted in achieving a stable SEI layer, such as electrolyte modification [53], [54], separator modification [55], [56], [57], novel design of electrode structure [58] and construction of artificial solid electrolyte interphase (artificial SEI) films [16], [24]. Among all the methods, constructing a stable artificial SEI film to protect Li metal anode is an emerging and effective approach. In some reports, the terms like “protecting/protection layer/film” are also used to describe these films. For convenience, the “artificial solid electrolyte interphase” or “protecting/protection layer/film” are hereinafter uniformly referred to as “artificial SEI” in the current paper. The artificial SEI films, which can accommodate the volume variation of Li–metal electrode and suppress the Li–dendrite growth, can be mainly divided into two categories: inorganic artificial SEI films like Al₂O₃ [59], garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ [60], Li₃N [61], LiF [62], Li₂S [63] and carbon materials; organic artificial SEI films such as polyethylene oxide [64], Nafion [65], and polyvinylidene fluoride [66]. Different types of artificial SEI films have their own advantages and disadvantages in regulating the electrochemical performance of Li–metal electrodes. The inorganic artificial SEI films show good electrochemical stability, high mechanical modulus and more efficient Li⁺ diffusion pathways (for some electrolyte components, such as LiF, LiNO₃). While the brittle nature of inorganic artificial SEI films make them prone to be ruptured during a huge volume variation of Li metal, leading to the failure of the LMBs, especially under high current density. For organic artificial SEI films, although the mechanical modulus is lower than that of inorganic materials, their superior processability and high flexibility can achieve intimate contact with electrode and allow effective suppression of the dendritic Li growth, thus significantly improving the electrochemical performance [41], [67], [68].

Recently, several corresponding review articles have summarized the research progress of artificial SEI films from different perspectives. For example, Zhang and co-workers have summarized the research advances on the interfaces between Li metal electrode and liquid/solid electrolyte, including the both native formed SEI films and artificial SEI films [18]. Xu et al. reviewed different fabrication approaches on artificial protecting films of the Li metal electrode that interacting with liquid or solid electrolyte [69]. According to the protection mechanisms of Li–metal electrodes, Qi et al. summarized existing methods for constructing artificial SEI layers on the Li metal anode [70]. Yang and co-workers focus on summarizing the recent key progress of multi-functional interlayer systems for high-performance LMBs, including tackling the shuttling of Li–S battery and the Li–dendrite issue of Li metal anode [71]. Bao and co-workers discussed the polymer designs for advanced battery chemistry, which also covers some polymeric artificial SEI films [72]. With a few reviews involved the artificial SEI films, till now, there is no review focusing on the ionic conducting polymeric artificial SEI films. Considering the significant research interest on ionic conducting polymers and Li–metal electrode, it is a good momentum to review the achievement and present future perspective of ionic conductive polymers as artificial SEI films in LMBs. In this article, as shown in Fig. 1, recent progress of constructing ionic conductive polymers as artificial SEI films, along with their systematic classifications and applied research techniques, will be comprehensively reviewed and summarized. At the end, the future research directions regarding to the rational design of polymeric artificial SEI films in advanced LMBs will also be presented. The deep understandings and design principles of artificial SEI films obtained from the advances of these polymeric protecting films are of significant importance to achieve high-performance LMBs with prolonged lifetime.