High-energy rechargeable lithium (Li) metal batteries (LMBs) with Li metal anode (LMA) were first developed in the 1970s, but their practical applications have been hindered by the safety and low-efficiency concerns related to LMA. Recently, a worldwide effort on LMA-based rechargeable LMBs has been revived to replace graphite-based, Li-ion batteries because of the much higher energy density that can be achieved with LMBs. This review focuses on the recent progress on the stabilization of LMA with nonaqueous electrolytes and reveals the fundamental mechanisms behind this improved stability. Various strategies that can enhance the stability of LMA in practical conditions and perspectives on the future development of LMA are also discussed. These strategies include the use of novel electrolytes such as superconcentrated electrolytes, localized high-concentration electrolytes, and highly fluorinated electrolytes, surface coatings that can form a solid electrolyte interphase with a high interfacial energy and self-healing capabilities, development of "anode-free" Li batteries to minimize the interaction between LMA and electrolyte, approaches to enable operation of LMA in practical conditions, etc. Combination of these strategies ultimately will lead us closer to the large-scale application of LMBs which often is called the "Holy Grail" of energy storage systems.

Lithium Metal Anodes with Nonaqueous Electrolytes

The solid electrolyte interphase (SEI) layer is pivotal for stable lithium (Li) metal batteries especially under a high rate. However, the mechanism of Li+ transport through the SEI has not been clearly elucidated to build robust Li anodes for practical Li metal batteries. Herein, an artificial hybrid SEI layer consisting of lithium-antimony (Li3Sb) alloy and lithium fluoride (LiF) is constructed to explore the ion diffusion behaviors within the SEI. As evidenced theoretically and experimentally, Li3Sb is identified as a superionic conductor for Li+ transport and as an interfacial stabilizer for the SEI layer while the LiF component with superior electron-blocking capability reduces the electron tunneling from the Li anode into the SEI, resulting in uniform dendrite-free Li deposition at the SEI/Li interface and stable Li plating/stripping behaviors at an ultrahigh rate of 20 mA cm−2. A practical 325.28 W h kg−1 pouch cell is well demonstrated under a high sulfur loading of 6 mg cm−2 and a low electrolyte/sulfur ratio of 3 μl mg−1. This work uncovers the internal mechanism of Li+ transport within the SEI component, and provides an avenue to stabilize the Li anode under practical high-rate conditions.

An artificial hybrid interphase for an ultrahigh-rate and practical lithium metal anode

The structures and components of solid electrolyte interphase (SEI) are extremely important to influence the performance of full cells, which is determined by the formulation of electrolyte used. However, it is still challenging to control the formation of high-quality SEI from structures to components. Herein, we designed bisfluoroacetamide (BFA) as the electrolyte additive for the construction of a gradient solid electrolyte interphase (SEI) structure that consists of a lithophilic surface with C-F bonds to uniformly capture Li ions and a LiF-rich bottom layer to guide the rapid transportation of Li ions, endowing the homogeneous deposition of Li ions. Moreover, the BFA molecule changes the Li+ solvation structure by reducing free solvents in electrolyte to improve the antioxidant properties of electrolyte and prevent the extensive degradation of electrolyte on the cathode surface, which can make a superior cathode electrolyte interphase (CEI) with high-content LiF.

Gradient Solid Electrolyte Interphase and Lithium-Ion Solvation Regulated by Bisfluoroacetamide for Stable Lithium Metal Batteries.

In carbonate electrolytes, the organic-inorganic solid electrolyte interphase (SEI) formed on the Li-metal anode surface is strongly bonded to Li and experiences the same volume change as Li, thus it undergoes continuous cracking/reformation during plating/stripping cycles. Here, an inorganic-rich SEI is designed on a Li-metal surface to reduce its bonding energy with Li metal by dissolving 4m concentrated LiNO3 in dimethyl sulfoxide (DMSO) as an additive for a fluoroethylene-carbonate (FEC)-based electrolyte. Due to the aggregate structure of NO3 - ions and their participation in the primary Li+ solvation sheath, abundant Li2 O, Li3 N, and LiNx Oy grains are formed in the resulting SEI, in addition to the uniform LiF distribution from the reduction of PF6 - ions. The weak bonding of the SEI (high interface energy) to Li can effectively promote Li diffusion along the SEI/Li interface and prevent Li dendrite penetration into the SEI. As a result, our designed carbonate electrolyte enables a Li anode to achieve a high Li plating/stripping Coulombic efficiency of 99.55 % (1 mA cm-2 , 1.0 mAh cm-2 ) and the electrolyte also enables a Li||LiNi0.8 Co0.1 Mn0.1 O2 (NMC811) full cell (2.5 mAh cm-2 ) to retain 75 % of its initial capacity after 200 cycles with an outstanding CE of 99.83 %.

An Inorganic‐Rich Solid Electrolyte Interphase for Advanced Lithium‐Metal Batteries in Carbonate Electrolytes

In situ construction of a multifunctional solid electrolyte interphase (SEI) for Zn anodes is promising to address the dendrite growth and side reactions (corrosion and hydrogen evolution) in aqueous Zn-ion batteries. However, there is a lack of constructive methods for choosing suitable SEI compounds and feasible implementation routes. Here, inspired by the SEI-design for Li-metal batteries, we identified that Zn3(PO4)2 with high interface energy could suppress Zn dendrite growth effectively and ZnF2 could accelerate the kinetics of Zn2+ transference and deposition, and thus constructing a composite SEI mainly composed of Zn3(PO4)2 and ZnF2 (ZCS) is likely to improve interface deposition and electrode kinetics comprehensively. However, the high redox potential of Zn/Zn2+ and H2/H+ makes it difficult to develop an in situ SEI for Zn anodes in aqueous electrolytes via traditional electrochemical routes. Considering this dilemma, we take advantage of the instability of KPF6 in an aqueous environment and build in situ ZCS on the Zn anode through the PF6− anion-induced chemical strategy. Surprisingly, ZCS-Zn exhibits enhanced reversibility with a smooth and compact structure during long-term cycling. Both cumulative capacity (2020 mA h cm−2) and the product of the largest current density and areal capacity (10 mA cm−2 × 20 mA h cm−2) applied to ZCS-Zn reach the highest levels compared with those reported in recent reports under mildly acidic conditions. This work paves a new way for designing a desirable SEI on the Zn anode and may also guide the interface engineering of other systems to overcome the intrinsic defects in constructing favorable interphases.

In situ built interphase with high interface energy and fast kinetics for high performance Zn metal anodes

Engineering a stable solid electrolyte interphase (SEI) is critical for suppression of lithium dendrites. However, the formation of a desired SEI by formulating electrolyte composition is very difficult due to complex electrochemical reduction reactions. Here, instead of trial-and-error of electrolyte composition, we design a Li-11 wt % Sr alloy anode to form a SrF2-rich SEI in fluorinated electrolytes. Density functional theory (DFT) calculation and experimental characterization demonstrate that a SrF2-rich SEI has a large interfacial energy with Li metal and a high mechanical strength, which can effectively suppress the Li dendrite growth by simultaneously promoting the lateral growth of deposited Li metal and the SEI stability. The Li-Sr/Cu cells in 2 M LiFSI-DME show an outstanding Li plating/stripping Coulombic efficiency of 99.42% at 1 mA cm-2 with a capacity of 1 mAh cm-2 and 98.95% at 3 mA cm-2 with a capacity of 2 mAh cm-2, respectively. The symmetric Li-Sr/Li-Sr cells also achieve a stable electrochemical performance of 180 cycles at an extremely high current density of 30 mA cm-2 with a capacity of 1 mAh cm-2. When paired with LiFePO4 (LFP) and LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes, Li-Sr/LFP cells in 2 M LiFSI-DME electrolytes and Li-Sr/NMC811 cells in 1 M LiPF6 in FEC:FEMC:HFE electrolytes also maintain excellent capacity retention. Designing SEIs by regulating Li-metal anode composition opens up a new and rational avenue to suppress Li dendrites.

High Interfacial-Energy Interphase Promoting Safe Lithium Metal Batteries.

An elastic and safe electrolyte is demanded for flexible batteries. Herein, a stretchable solid electrolyte comprised of crosslinked elastic polymer matrix, poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), and flameproof triethyl phosphate (TEP) is fabricated, which exhibits ultrahigh elongation of 450%, nonflammability and ionic conductivity above 1 mS cm-1. In addition, in order to improve the interface compatibility between the electrolyte and Li anode and stabilize the solid-electrolyte interphase (SEI), a protecting layer containing poly(ethylene oxide) (PEO) is designed to effectively prevent the anode from reacting with TEP and optimize the chemical composition in SEI, leading to a tougher and more stable SEI on the anode. The LiFePO4/Li cells employing this double-layer electrolyte exhibit an 85.0% capacity retention after 300 cycles at 1 C. Moreover, a flexible battery based on this solid electrolyte is fabricated, which can work in stretched, folded, and twisted conditions. This design of a stretchable double-layer solid electrolyte provides a new concept for safe and flexible solid-state batteries.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202003241

A Stretchable and Safe Polymer Electrolyte with a Protecting-Layer Strategy for Solid-State Lithium Metal Batteries.

Stabilizing the interphase between Li and Argyrodite electrolyte through synergistic phosphating process for all-solid-state lithium batteries

Uncontrollable dendrite growth and low Coulombic efficiency are the two main obstacles that hinder the application of rechargeable Li metal batteries. Here, an optimized amount of potassium hexafluorophosphate (KPF6, 0.01 M) has been added into the 2 M LiTFSI/ether-based electrolyte to improve the cycling stability of lithium-sulfur (Li-S) batteries. Due to the synergistic effect of self-healing electrostatic shield effect from K+ cations and the LiF-rich solid electrolyte interphases derived from PF6- anions, the KPF6 additive enables a high Li Coulombic efficiency of 98.8% (1 mA cm-2 of 1 mAh cm-2). The symmetrical Li cell can achieve a stable cycling performance for over 200 cycles under a high Li utilization up to 33.3%. Meanwhile, the polysulfide shuttle has been restrained due to the higher concentration of the LiTFSI in the electrolyte. As a result, the assembled Li-S full cell displays excellent capacity retention with only 0.25% decay per cycle in the final electrolyte. Our work offers a smart approach to improve both the anode and cathode performance by the electrolyte modification of rechargeable Li-S batteries.

Jingru Li

Papers

An Inorganic‐Rich Solid Electrolyte Interphase for Advanced Lithium‐Metal Batteries in Carbonate Electrolytes

High Interfacial-Energy Interphase Promoting Safe Lithium Metal Batteries.

A Stretchable and Safe Polymer Electrolyte with a Protecting-Layer Strategy for Solid-State Lithium Metal Batteries.

Stabilizing the interphase between Li and Argyrodite electrolyte through synergistic phosphating process for all-solid-state lithium batteries

Potassium Hexafluorophosphate Additive Enables Stable Lithium-Sulfur Batteries.