The composite iron and sodium phosphate, sodium iron pyrophosphate, is regarded as a highly promising positive electrode material for sodium ion batteries due to its low cost and excellent cycling performance. A team led by Zhao Junmei, a researcher at the Institute of Process Engineering, Chinese Academy of Sciences, has boosted the reversible capacity and energy density of iron-based pyrophosphate positive electrode materials by exciting inert sodium iron phosphate. Their findings were published on March 28th in the Journal of the American Chemical Society (DOI: 10.1021/jacs.3c14452).
Sodium iron phosphate (NaFePO4, NFP), sodium pyrophosphate (Na2FeP2O7), and sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP-4.0) are commonly used iron-based phosphate positive electrodes for sodium ion batteries. However, NaFePO4 lacks effective ion channels and thus lacks electrochemical activity; Na2FeP2O7 possesses three-dimensional ion channels but has a low theoretical capacity and is unstable in air due to its low potential; sodium iron pyrophosphate NFPP-4.0, with a theoretical capacity of up to 130 mAh/g and a voltage close to 3.1V, is stable in air and has been widely studied in recent years, even leading in industrialization. However, due to structural defects, its actual capacity (110~120 mAh/g) is lower than its theoretical capacity, making it a focus of attention and a challenging problem to further enhance the actual capacity of NFPP-4.0.
The research found that adding a certain amount of NFP during the preparation of NFPP-4.0 can effectively excite the inert NFP. The optimized new type of sodium iron pyrophosphate is Na4.5Fe3.5(PO4)2.5P2O7 (NFPP-4.5), with a ratio of NFP/NFPP-4.0 of 0.5:1. Experimental results show that NFPP-4.5 has a reversible capacity of up to 130 mAh/g and an energy density of 400 Wh/kg. X-ray diffraction (XRD) confirmed that NFPP-4.5 consists of both NFPP-4.0 and NFP phases, with NFP nanocrystals continuously distributed in the NFPP-4.0 crystal domain and wrapped by NFPP-4.0, and a large number of transition grain boundary regions conducive to cross-transport of sodium ions, effectively activating the electrochemical activity of inert NFP. During charge and discharge processes, it was found that NFP undergoes an amorphization process in the first charge cycle and then cycles in the amorphous state, fully confirming that the activation of NFP is the key to the high capacity of NFPP-4.5. Subsequently, researchers scaled up this material to kilograms and assembled pouch batteries with excellent fast charging and cycling performance. Even under 5C fast charging conditions (12min), the pouch battery can still obtain over 80% reversible capacity relative to 0.1C. Cycling 2000 times at 3C, the capacity retention rate exceeds 88%, demonstrating enormous application prospects.
This work was supported by the Beijing Natural Science Foundation (2222078), the National Natural Science Foundation of China (52072370, 22309203, 51672069, 22138012), the Inner Mongolia Science and Technology Department project (2021GG0162), and the China Postdoctoral Science Foundation (2023M733718).
