O3-type NaCrO₂ is a promising cathode material as it offers decent energy density and is easy to synthesize from abundant elements. A capacity fade of ~20% in 50 cycles is usually observed for NaCrO₂ in organic solvents [1–3]. Research regarding improving the capacity retention of NaCrO₂ includes coating the NaCrO₂ particle with carbon [1,2], or increasing grain boundary content by nanosizing the grains [3]. In this study, NaCrO₂ was Na-site doped with calcium to improve cycling retention in Na cells.

A series of [Na_1-2xCa_x]CrO₂ materials were synthesized using a solid state method. Figure 1(a) shows the XRD pattern and Rietveld refinement of [Na_0.9Ca_0.05]CrO₂. A single O3 phase was obtained with no observable impurity. Figure 1(b) shows the capacity and coulombic efficiency versus cycle number for NaCrO₂ and [Na_0.9Ca_0.05]CrO₂ when cycled in the voltage range of 2 V – 3.6 V. Both materials have similar initial capacity, However the capacity retention of Na_0.9Ca_0.05CrO₂ is significantly improved, compared to NaCrO₂. The coulombic efficiency of Na_0.9Ca_0.05CrO₂ is also constantly higher than NaCrO₂. It is speculated that calcium substitution results in a more stable structure during sodium (de)intercalation. The effect of calcium substitution on structural changes during cycling and air-stability will be discussed.

[1] J.J. Ding, Y.N. Zhou, Q. Sun, Z.W. Fu, Cycle Performance Improvement of NaCrO₂ Cathode by Carbon Coating for Sodium Ion Batteries, Electrochem. Commun. 22 (2012) 85–88.

[2] C.Y. Yu, J.S. Park, H.G. Jung, K.Y. Chung, D. Aurbach, Y.K. Sun, S.T. Myung, NaCrO₂ Cathode for High-Rate Sodium-Ion Batteries, Energy Environ. Sci. 8 (2015) 2019–2026.

[3] Y. Tsuchiya, A.M. Glushenkov, N. Yabuuchi, Effect of Nanosizing on Reversible Sodium Storage in a NaCrO₂ Electrode, ACS Appl. Nano Mater. 1 (2018) 364–370.