Regulating Sodium Deposition Kinetics to Decouple the Electrochemo‐Mechanical Effects in Anode‐Free Sodium Batteries

Abstract

Sodium (Na) metal batteries with an anode‐free configuration exhibit a high energy density comparable to that of practical lithium‐ion batteries. However, the intricate electrochemo‐mechanical effects arising from the inherent softness of Na during deposition, in conjunction with the problem of soft short circuits at high current densities, have impeded the practical application of anode‐free Na batteries. Herein, the critical factor for decoupling the electrochemo‐mechanical effects is revealed to lie in controlling the kinetically rate‐determining step during Na deposition. Specifically, the charge‐transfer‐dominated Na deposition exhibits low risk of dendrites growth, whereas the diffusion‐controlled Na deposition requires external pressure to maintain uniform deposition and is prone to generating detrimental internal stresses in the cell. It is shown that appropriately increasing the salt concentration in the electrolyte can facilitate sufficient Na+ availability at the electrode surface, thereby ensuring that Na deposition is controlled by charge transfer. As a result, the critical current density of Na deposition can be significantly boosted to >20 mA cm−2. Furthermore, fast‐charging anode‐free Na batteries with a Na3V2(PO4)3 cathode can be realized with high current rate >10 C (≈10.7 mA cm−2). This contribution offers valuable insights into the design, implementation, and operation of anode‐free Na batteries.