Mechanical activation enhanced solid-state synthesis of NaCrO₂ cathode material

NaCrO₂ has been studied lately as a promising cathode material for Na-ion batteries. Consequently, we have conducted the first investigation on how high-energy ball milling before the high temperature reaction influences the synthesis reaction of NaCrO₂ derived from the typical Na₂CO₃ and Cr₂O₃ reactants. In-situ synchrotron X-ray diffractometry is employed for the first time to provide a comprehensive understanding of the critical reaction temperatures and reaction pathway. It is found that high-energy ball milling at room temperature can result in significant changes in the synthesis reaction of NaCrO₂ when compared to reactants without high-energy ball milling. These changes include a decreased onset temperature for the formation of O₃–NaCrO₂, an increased reaction kinetics, an alternation of the reaction pathway, and a complete reaction at 900 °C to form thermally-stable O₃–NaCrO₂ phase. These phenomena have been ascribed to the mechanical activation induced by high-energy ball milling before high temperature reaction. In contrast, without high-energy ball milling the reaction product at 900 °C is a highly impure NaCrO₂ with a poor thermal stability. The thermally-stable O₃–NaCrO₂ powder produced with mechanical activation of the reactants at RT has much higher specific capacity (∼115 mAh/g) than the Na-deficient NaCrO₂ powder with unreacted Cr₂O₃ generated without mechanical activation of the reactants at RT. Furthermore, the thermally-stable O₃–NaCrO₂ powder exhibits the best capacity retention among all the NaCrO₂ without coatings reported so far in the literature.