Abstract
Owing to the emergence of energy storage and electric vehicles, the desire for safe high-energy-density energy storage devices has increased research interest in anode-free lithium metal batteries (AFLMBs). Unlike general LMBs, in which excess Li exists to compensate for the irreversible loss of Li, only the current collector is employed as an anode and paired with a lithiated cathode in the fabrication of AFLMBs. Owing to their unique cell configuration, AFLMBs have attractive characteristics, including the highest energy density, safety, and cost-effectiveness. However, developing AFLMBs with extended cyclability remains an issue for practical applications because the high reactivity of Li with limited inventory causes severely low Coulombic efficiency, poor cyclability, and dendrite growth. To address these issues, tremendous effort has been devoted to stabilize Li-metal anodes for AFLMBs. In this review, we highlight the importance and challenges of AFLMBs. Then, we thoroughly review diverse strategies, such as modifying current collectors, the formation of robust interfaces by engineering advanced electrolytes, and operation protocols. Finally, a future perspective on the strategy is provided to insight into the basis of future research. We hope that this review provides a comprehensive understanding by reviewing previous research and arousing more interest in this field.
1. Introduction
Since the first commercialization of lithium-ion batteries (LIBs) by Sony Corp. in 1991, LIBs have been successfully used in applications ranging from small portable devices to grid energy storage systems.[1,2] In the 21st century, global environmental issues have driven the development of electric vehicles (EVs) and renewable energy, which require greater energy storage density. However, state-of-the-art LIBs have almost reached the theoretical limit of energy density (~ 300 Wh kg-1) because of the low theoretical capacity of intercalation-type electrode materials (e.g., graphite and lithium metal oxides).[3,4,5]Therefore, next-generation anodes such as alkali metals (Li, Na,[6,7,8,9,10]K,[11,12,13,14]), alkaline earth metals (Mg,[15,16,17,18,19]Ca[20,21,22,23,24]), and multivalent metals (Zn,[25,26,27,28,29]Al[30,31]) are in the spotlight to go beyond LIBs. Among them, Li metal has gained the most attention as a next-generation anode material to overcome the theoretical limitations of intercalation-type anodes.[4,32] Li metal has very promising characteristics for high energy density, namely the lowest reduction potential (-3.04 V vs. standard hydrogen electrode, SHE), high theoretical capacity (3,860 mA h g-1), and being lightweight (0.53 g cm-3). However, thick Li metal anodes (>250 μm) are frequently used in lithium metal batteries (LMBs)[33], [34] to compensate for the irreversible loss of Li formed by electrical isolation[35,36] and solid-electrolyte interphase (SEI) layer generation.[37,38,39] This configuration extends the cyclability of the cells but significantly lowers the energy density, which is often lower than that of LIBs.[4,39] To overcome the above issue, thin Li metal (20 ~ 50 μm) electrodes have been employed. Recently, an anode composed of sole CCs without Li metal was used by pairing with a lithiated cathode, where the negative-to-positive (N/P) ratio was 0. With respect to energy density, this extreme system, called anode-free lithium metal batteries (AFLMBs), is an optimum circumstance.[40,41] Despite the considerable advantages of the anode-less configuration, the cycle stability of AFLMBs is too poor because the absence of a Li reservoir in the anode causes rapid degradation of the cycle.[42,43]Nevertheless, AFLMBs have received significant attention since a breakthrough by Quian et al. in 2016,[44] who initiated an anode-less boom by proving the feasibility of AFLMBs. Subsequently, various efforts have been made to enhance the cyclability of AFLMBs.[43,45] In this review, we categorize the various strategies for high performance AFLMBs into three types. The first is the rational design of CCs to enhance the reversibility of Li metal. The construction of elaborate 3D structures,[46] diverse lithiophilic sites[47], artificial layers on CCs,[48,49] and carbon hosts[50] has been explored to reduce the nucleation energy barrier, lead to uniform deposition of Li, and suppress dendrites. The next one is electrolyte engineering. The SEI layer formed by electrolyte decomposition has the greatest effect on the reversibility of Li metal.[43,45,51, 52] Various studies have fabricated high-quality SEI layers, including by adjusting salt concentration,[44] changing solvent characteristics,[45,53] and introducing additives.[52,54] The last category is operating protocols. Various parameters such as temperature, external pressure, current density, and cut-off voltages dramatically affect the cycle stability.[55,56,57]
The initial cell configuration of AFLMBs is the same as that of general LMBs, except that the anode is composed of sole CCs without Li metal (Figure 1A). Therefore, lithiated cathode materials are necessary in AFLMBs. Generally, intercalation materials such as lithium cobalt nickel manganese oxide (NMC),[43] lithium iron phosphate (LFP),[44,58] lithium nickel cobalt aluminum oxide (NCA), or conversion-type (e.g., Li2S) cathodes[59,60] are utilized as cathodes for AFLMBs. During the initial charging process, Li ions are delithiated from the cathode and reduced on the surface of the CCs in the anode. The subsequent discharge and charge processes progressed similarly to the LMBs.