Dual Stabilization and Sacrificial Effect of Na₂CO₃ for Increasing Capacities of Na-Ion Cells Based on P2-Na_<i>x</i>MO₂ Electrodes

Sodium ion battery technology is gradually advancing and can be viewed as a viable alternative to lithium ion batteries in niche applications. One of the promising positive electrode candidates is P2 type layered sodium transition metal oxide, which offers attractive sodium ion conductivity. However, the reversible capacity of P2 phases is limited by the inability to directly synthesize stoichiometric compounds with a sodium to transition metal ratio equal to 1. To alleviate this issue, we report herein the <i>in situ</i> synthesis of P2-Na_<i>x</i>MO₂ (<i>x</i> ≤ 0.7, M = transition metal ions)-Na₂CO₃ composites. We find that sodium carbonate acts as a sacrificial salt, providing Na⁺ ion to increase the reversible capacity of the P2 phase in sodium ion full cells, and also as a useful additive that stabilizes the formation of P2 over competing P3 phases. We offer a new phase diagram for tuning the synthesis of the P2 phase under various experimental conditions and demonstrate, by <i>in situ</i> XRD analysis, the role of Na₂CO₃ as a sodium reservoir in full sodium ion cells. These results provide insights into the practical use of P2 layered materials and can be extended to a variety of other layered phases