Irreversible Made Reversible: Increasing the Electrochemical Capacity by Understanding the Structural Transformations of $\mathrm{Na_{x}Co_{0.5}Ti_{0.5}O_{2}}$

Two new structural forms of Na$_{x}$Co$_{0.5}$Ti$_{0.5}$O$_{2}$, the layered O3- and P3-forms, were synthesized and comprehensively characterized. Both materials show electrochemical activity as electrodes in Na-ion batteries. During cell charging (desodiation of the NaxCo0.5Ti0.5O2 cathode), we observed a structural phase transformation of O3-Na$_{0.95}$Co$_{0.5}$Ti$_{0.5}$O$_2$ into P3-NaxCo$_{0.5}$Ti$_{0.5}$O$_2$, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-Na$_{0.65}$Co$_{0.5}$Ti$_{0.5}$O$_2$. During Na insertion (cell discharging), the reconversion of the P3-form into O3-NaxCo$_{0.5}$Ti$_{0.5}$O$_2$ was impeded for both materials and occurs well below 1 V versus Na$^+$/Na only. The reconversion is hindered by the charge and spin transfers of Co (LS-Co$^{3+} →$ HS-Co$^{2+}$) and by a significant unit cell volume expansion at the P3 $→$ O3 transformation, as revealed from the magnetization, crystallographic, and spectroscopic studies. As the kinetics of such transformations depend on numerous parameters such as time, temperature, and particle size, a large cell overpotential ensues. An extended cutoff voltage at 0.2 V versus Na$^+$/Na during discharging allows to complete the P3 $→$ O3 transformation and increases the specific discharging capacity to 200 mA h g$^{−1}$. Moreover, a quasisymmetrical full cell, based on the O3- and P3-forms, was designed, eliminating safety concerns associated with sodium anodes and delivering a discharge capacity of 130 mA h g$^{−1}$