Design Principle, Optimization Strategies, and Future Perspectives of Anode-Free Configurations for High-Energy Rechargeable Metal Batteries

Metal anodes (e.g., lithium, sodium and zinc metal anodes) based on a unique plating/stripping mechanism have been well recognized as the most promising anodes for next-generation high-energy metal batteries owing to their superior theoretical specific capacities and low redox potentials. However, realizing full utilization and the theoretical capacity of metal anodes remains challenging because of their high reactivity, poor reversibility, and nonplanar metal evolution patterns, which lead to irreversible loss of active metals and the electrolyte. To minimize the above issues, excess metal sources and flooded electrolytes are generally used for laboratory-based studies. Despite the superior cycling performance achieved for these cells, the metal-anode-excess design deviates from practical applications due to the low anode utilization, highly inflated coulombic efficiency, and undesirable volumetric capacity. In contrast, anode-free configurations can overcome these drawbacks while reducing fabrication costs and improving cell safety. In this review, the significance of anode-free configurations is elaborated, and different types of anode-free cells are introduced, including reported designs and proposed feasible yet unexplored concepts. The optimization strategies for anode-free lithium, sodium, zinc, and aluminum metal batteries are summarized. Most importantly, the remaining challenges for extending the cycle life of anode-free cells are discussed, and the requirements for anode-free cells to reach practical applications are highlighted. This comprehensive review is expected to draw more attention to anode-free configurations and bring new inspiration to the design of high-energy metal batteries. Anode-free metal batteries can deliver higher energy densities than traditional anode-excess metal batteries and metal-ion batteries. Yet the cycle life of anode-free cells is limited by the non-planar growth and low coulombic efficiency of the metal anodes. In this review, we not only systematically elaborate the working/failure mechanisms and achieved progress for the reported anode-free Li/Na/Zn/Al battery systems, but also propose a series of conceptually-feasible yet unexplored anode-free systems.