Revealing the Formation Mechanism of Inactive Sodium in Anode‐Free Sodium Batteries: Crystal Mismatch and Weak Lattice Force

Abstract The formation of inactive sodium on the substrate is considered to be a critical cause of capacity decay in anode‐free sodium batteries (AFSBs), but its formation mechanism has been insufficiently understood. Herein, for the first time, it is revealed that the inability of the substrate to provide an efficient electron transport pathway for sodium stripping due to crystal mismatch and weak lattice forces between the substrate and sodium is an intrinsic cause of inactive sodium formation in AFSBs. Therefore, an indium coating strategy that offers favorable crystal matching and lattice force with sodium is proposed. Sodium can uniformly grow on indium‐modified substrates with low nucleation barriers, without shedding, and this modulating effect can be sustained over extended cycling periods. Ultimately, the pouch‐type AFSBs assembled with a Na 4 Fe 2.91 (PO 4 ) 2 (P 2 O 7 ) (NFPP) cathode (≈18 mg cm −2 mass loading) and indium‐modified copper foil exhibit a capacity retention of 78.3% after 320 cycles at 2C rate, whereas that with bare copper foil only cycles less than 60 times. Most importantly, the mechanism of inactive sodium formation proposed in this work can offer lattice‐ and atomic‐scale insights for the design of advanced substrates in AFSBs.