Irreversible Made Reversible: Increasing the Electrochemical Capacity by Understanding the Structural Transformations of Na CoTiO.

Two new structural forms of Na CoTiO, 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 Na CoTiO cathode), we observed a structural phase transformation of O3-NaCoTiO into P3-Na CoTiO, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-NaCoTiO. During Na insertion (cell discharging), the reconversion of the P3-form into O3-Na CoTiO 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 → HS-Co) 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. Moreover, a quasi-symmetrical 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.