Gradient Designs for Efficient Sodium Batteries

Sodium batteries have received extensive attention as low-cost and high-performance devices for next-generation energy storage. There are, however, significant challenges toward practical deployment such as poor electrode integrity and durability. These are primarily caused by severe stress and strain upon electrochemical (de)sodiation, as a result of large Na ions. Gradient designs, either chemically or structurally, have proven uniquely capable of mitigating these issues. Despite the early stage of development, gradient designs have enabled significant progress in sodium batteries. In this Focus Review, we highlight the principles and features of gradient designs and their successful applications in sodium batteries. A particular focus is placed on the understanding of how the gradient idea could address some critical issues such as stress dissipation, structure stabilization, charge and mass transport, and dendrite suppression. The remaining challenges and future research directions related to gradients in sodium batteries are also discussed.