Heterovalent Chromium-Doped Na3Fe2(PO4)P2O7 Cathode Material with Superior Rate and Stability Performance for Sodium-Ion Storage

Sodium-ion batteries have great potential for large-scale energy storage applications. The cathode material makes an important contribution to the energy density of sodium-ion batteries. A sodium iron phosphate (Na3Fe2(PO4)P2O7), which is a mixed polyanionic cathode material, has good structural stability and a large theoretical capacity. However, the material's poor electrical conductivity prevents adequate capacity release. Heterovalent ion-doping is considered to be an important means of improving the intrinsic electrical conductivity of materials. Here, Cr3+-doped mixed sodium ferric pyrophosphate was synthesized (specific molecular formula after doping: Na3–xFe2–xCrx(PO4)P2O7). The introduction of Cr3+ can change the electron cloud distribution of the material, thus improving the electronic and ionic conductivity of the material. At the same time, due to the charge balance effect, the crystal structure of the material creates sodium-ion vacancies, which can effectively facilitate the transport of sodium ions during the charging and discharging process. The modified material has an initial capacity of 113.6 mA h g–1 at 0.1C (theoretical capacity: 119 mA h g–1), while offering excellent stability and excellent rate (a retention of capacity of 92.1% for 4000 cycles at 20C rate). This work has been insightful and provides several options for further applications of sodium-ion batteries.