Stabilizing NaCrO2 By Sodium Site Doping with Calcium

Abstract

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 [Na1-2xCax]CrO2 materials were synthesized using a solid state method. Figure 1(a) shows the XRD pattern and Rietveld refinement of [Na0.9Ca0.05]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 [Na0.9Ca0.05]CrO2 when cycled in the voltage range of 2 V – 3.6 V. Both materials have similar initial capacity, However the capacity retention of Na0.9Ca0.05CrO2 is significantly improved, compared to NaCrO2. The coulombic efficiency of Na0.9Ca0.05CrO2 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. Figure 1