Irreversible Electrochemical Reaction at High Voltage Induced by Distortion of Mn and V Structural Environments in Na4MnV(PO4)3

As the importance of developing low-cost and high-capacity materials is emerging, Mn-based Na4MnV(PO4)3 positive electrode materials that allow multiredox reactions are in the spotlight for Na-ion batteries. The structure that gives highly reversible electrochemical reactions, when two Na+ (out of four) are de-inserted, deteriorates rapidly when the third Na+ is extracted at high voltage, resulting in poor cyclability. In this work, using synchrotron-based operando techniques, we perform long-range and local structural analyses to determine the origins of the rapid structural decay of Na4MnV(PO4)3 when the third Na+ is extracted. Operando XRD shows a significant change in the crystal structure (c parameter increases rapidly) as the occupancy of the Na (1) site decreases at high voltage. The local environments of Mn and V, monitored by operando XAS, remain rather symmetrical up to the extraction of two Na+, while both Mn and V show drastic local distortions when the third Na+ is extracted. These structural degradations are found to further progress when cycling to high voltage. This study presents important aspects of how local and long-range structure modifications can affect the electrochemical performance in multiredox NASICON materials.