KVOPO₄: A New High Capacity Multielectron Na-Ion Battery Cathode

Sodium ion batteries have attracted much attention in recent years, due to the higher abundance and lower cost of sodium, as an alternative to lithium ion batteries. However, a major challenge is their lower energy density. In this work, we report a novel multi-electron cathode material, KVOPO₄, for sodium ion batteries. Due to the unique polyhedral framework, the V³⁺ ↔ V⁴⁺ ↔ V⁵⁺ redox couple was for the first time fully activated by sodium ions in a vanadyl phosphate phase. The KVOPO₄ based cathode delivered reversible multiple sodium (i.e. maximum 1.66 Na⁺ per formula unit) storage capability, which leads to a high specific capacity of 235 Ah kg⁻¹. Combining an average voltage of 2.56 V vs. Na/Na⁺, a high practical energy density of over 600 Wh kg⁻¹ was achieved, the highest yet reported for any sodium cathode material. The cathode exhibits a very small volume change upon cycling (1.4% for 0.64 sodium and 8.0% for 1.66 sodium ions). Density functional theory (DFT) calculations indicate that the KVOPO₄ framework is a 3D ionic conductor with a reasonably, low Na⁺ migration energy barrier of ≈450 meV, in line with the good rate capability obtained.