Converting Residual Alkali into Sodium Compensation Additive for High-Energy Na-Ion Batteries

Sodium-ion batteries (SIBs) have attracted widespread attention in large-scale electrical energy storage. However, the dissolution of the solid-electrolyte interphase (SEI) and the abundant defect sites in hard carbon (HC) lead to serious Na⁺ loss in sodium-ion full cells, limiting the energy density and cycle life of SIBs. Here, we introduce acetic acid (AC) in layered cathode materials to neutralize the residual alkali species and form sodium acetate (AC-Na). AC-Na possesses a high specific capacity of ∼300 mAh g^-1 and serves as the Na compensation additive with ∼92% capacity utilization and an appropriate oxidation potential (∼4.1 V). On the basis of sodium compensation, the 2.0 Ah P2-Na_0.85Li_0.12Ni_0.22Mn_0.66O₂@AC||HC pouch cell affords a capacity retention of 95.1% over 120 cycles, with the energy density improved from 112 to 130 Wh kg^-1 (based on the mass of the cell core). The high-efficiency sodium compensation strategy opens up a new route to enable high-energy SIBs for practical application.