Spatially anion-confined electrolyte enables high-rate and durable anode-free sodium batteries

Anode-free sodium batteries (AFSBs) with near-theoretical energy density hold great promise for next-generation sustainable energy storage systems. However, their practical implementation is impeded by the low operating rate threshold (<1 milliampere per square centimeter) and poor cycling stability, owing to dendritic sodium (Na) growth. Here, a high-rate and durable AFSB is successfully realized via a spatially anion-confined electrolyte strategy. Specifically, positively charged nanoparticles are introduced into the electrolyte to selectively anchor anions, generating localized contact ion pair–dominated solvation to facilitate rapid Na + desolvation at electrode interface and form an anion-derived solid electrolyte interphase. Meanwhile, rapid ion transport in the bulk electrolyte is maintained by the solvent-separated ion pair solvation structure in the nanoparticle periphery. These factors conjointly enable flat and dense Na deposition at high current densities. Consequently, an energy-type Na(Ni 1/3 Fe 1/3 Mn 1/3 )O 2 ||Al full cell exhibits an energy density of 415.6 watt-hour per kilogram cathode+anode even at 1 C (2.1 milliamperes per square centimeter) with 70.2% capacity retention over 400 cycles. A power-type Na 3 V 2 (PO 4 ) 3 ||Al cell achieves a trebled operation current density compared to the state-of-the-art AFSBs, exhibiting an unprecedented 5-C rate (3.8 milliamperes per square centimeter) with 70.0% capacity retention over 1400 cycles. This strategy presents a potentially universal approach for high-rate alkali metal batteries.