Engineering Li Metal Anode for Garnet-Based Solid-State Batteries

The past 30 years have witnessed the great achievements of Li-ion batteries (LIBs) based on a graphite anode and liquid organic electrolytes. Yet the limited energy density of a graphite anode and unavoidable safety risks caused by flammable liquid organic electrolytes hinder further developments of LIBs. To reach higher energy density, Li metal anodes (LMAs) with high capacity and low electrode potential are a promising choice. However, LMAs suffer from more serious safety concerns than the graphite anode in liquid LIBs. The dilemma of safety and energy density remains an inevitable obstacle in the way of LIBs.Solid-state batteries (SSBs) offer new opportunities to simultaneously achieve intrinsic safety and high energy density. Among all types of SSBs that are based on oxides, polymers, sulfides, or halides, garnet-type SSBs seem to be one of the most attractive choices due to garnet's merits in high ionic conductivities (10(-4)-10(-3) S/cm at room temperature), wide electrochemical windows (0-6 V), and intrinsically high safety. However, garnet-type SSBs are faced with large interfacial impedance and short-circuit problems caused by Li dendrites. Recently, engineered Li metal anodes (ELMAs) have shown unique advantages in tackling interface issues and attracted extensive research interest.In this Account, we focus on fundamental understandings and provide an in-depth review of ELMAs in garnet-based SSBs. Considering the limited space, we mainly discuss the recent progress made in our groups. First, we introduce the design guidelines for ELMAs and emphasize the unique role of theoretical calculation in predicting and optimizing ELMAs. Then we discuss the interface compatibility of ELMAs with garnet SSEs in details. Specifically, we have demonstrated the advantages of ELMAs in enhancing interface contact and suppressing Li dendrite growth. Next, we attentively analyze the gaps between laboratory and practical applications. We strongly recommend establishing a unified testing standard, with a practically desired areal capacity per cycle (> 3.0 mAh/cm(2)) and a precisely controlled Li capacity excess. Finally, novel chances to enhance ELMAs' processability and fabricate thin Li foils are highlighted. We believe that this Account will offer an insightful analysis of ELMAs' recent advancements and push forward their practical applications.