Understanding pillar chemistry in potassium-containing polyanion materials for long-lasting sodium-ion batteries.

K-containing polyanion compounds hold great potential as anodes for sodium-ion batteries considering their large ion transport channels and stable open frameworks; however, sodium storage behavior has rarely been studied, and the mechanism remains unclear. Here, using a noninterference KTiOPO₄ thin-film model, the Na⁺ storage mechanism is comprehensively revealed by in situ/operando spectroscopy, aberration-corrected electron microscopy and density functional theory calculations. We find that incomplete K⁺/Na⁺ ion exchange occurs and eventually 0.15 K⁺ remains as a pillar to stabilize the tunnel structure. The pillar effect substantially maintains the volume change within 3.9%, much smaller than that of K⁺(Na⁺) insertion into KTiOPO₄(NaTiOPO₄) (9.5%; 5%), thus enabling 10,000 cycles. The powder electrode demonstrates comparable capacity and can work efficiently at commercial-level areal capacity of 2.47 mAh cm^-2. The quasi-solid-state pouch cell with high safety under extreme abuse also manifests long-term cycling stability. This pillar chemistry will inspire alkali metal ion storage in hosts containing heterogeneous cations.