Enhancing reversible Na-ion intercalation by introducing K-ions into layered vanadyl phosphate for sodium-ion battery cathodes

Vanadium-based phosphates are being extensively studied as an important family of sodium-ion battery (SIB) cathodes. Among many compositions, NaVOPO₄ is considered because of various polymorphs and the high redox potential of V^4+/5+. However, due to relatively poor intrinsic kinetics and electronic conductivity, approaches such as nanostructuring and carbon composites are commonly used to avoid fast performance degradation. Being different from mainstream approaches, this work utilizes the knowledge gained from potassium-ion batteries (PIBs) and applies layered KVOPO₄, a PIB cathode material, as a SIB cathode material. The results demonstrate that KVOPO₄ experiences an electrochemical K⁺-Na⁺ exchange during the initial cycle and a Na-dominated (de)intercalation process in the following cycles. The initial exchange results in a small amount of K⁺ (∼0.1 K per formula) remaining in the interlayer space and owing to the larger size of K⁺ than Na⁺, the residual K⁺ effectively acts as ‘pillars’ to expand interlayer spacing and facilitates the Na (de)intercalation, leading to enhanced reversible Na storage and diffusion kinetics of KVOPO₄ compared to its Na counterpart NaVOPO₄. KVOPO₄ delivers an initial discharge capacity of 120 mAh g⁻¹ (90% of the theoretical capacity) at 10 mA g⁻¹ and retains 88% capacity after 150 cycles. It also delivers 52 mAh g⁻¹ at 1 A g⁻¹ and 91% capacity retention after 1000 cycles at 100 mA g⁻¹, completely outperforming NaVOPO₄.