Building a Beyond Concentrated Electrolyte for High‐Voltage Anode‐Free Rechargeable Sodium Batteries

Low-cost and scalable sodium ion (Na-ion) batteries serve as an ideal alternative to the current lithium-ion batteries. To compensate for the shortage of energy density, the most accessible solution is developing a high-voltage anode-free configuration comprising a lightweight Al current collector on the anode and a high-voltage sodiumized cathode. However, it imposes stringent Na reversibility and high-voltage stability requirements on the electrolyte. A 3A zeolite molecular sieve film is rationally designed, and a highly aggregated solvation structure is constructed through the size effect. It suppresses the trace but continuous oxidative decomposition and extends the oxidative stability to 4.5 V without sacrificing the Na reversibility of the anode (99.91 %). Thus, we can make anode-free cells with high energy density of 369 and 372 W h kg^-1 for 4.0 and 4.25 V class cells, respectively. Furthermore, this strategy enables a long lifespan (250 cycles) for 4.0 V-class anode-free cells.