Irreversible Made Reversible: Increasing the Electrochemical Capacity by Understanding the Structural Transformations of NaxCo0.5Ti0.5O2

Two new structural forms of NaxCo0.5Ti0.5O2, 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-Na0.95Co0.5Ti0.5O2 into P3-NaxCo0.5Ti0.5O2, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-Na0.65Co0.5Ti0.5O2. During Na insertion (cell discharging), the reconversion of the P3-form into O3-NaxCo0.5Ti0.5O2 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-Co3+ → HS-Co2+) 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 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–1.