KVOPO4: A New High Capacity Multielectron Na‐Ion Battery Cathode. Issue 21 (9th May 2018) - Ding, Jia
Abstract: 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 V 3+ ↔ V 4+ ↔ V 5+ 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. Abstract : K y VOPO4, is a new intercalation host for sodium batteries. The V 3+ ↔ V 4+ ↔ V 5+ redox couple is for the first time fully activated by sodium ions in a vanadyl phosphate phase, delivering a high specific capacity of 235 Ah kg −1, and an energy density of over 600 Wh kg −1 .