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

Two new structural forms of Na _xCo_0.5Ti_0.5O₂, 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 Na _xCo_0.5Ti_0.5O₂ cathode), we observed a structural phase transformation of O3-Na_0.95Co_0.5Ti_0.5O₂ into P3-Na _xCo_0.5Ti_0.5O₂, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-Na_0.65Co_0.5Ti_0.5O₂. During Na insertion (cell discharging), the reconversion of the P3-form into O3-Na _xCo_0.5Ti_0.5O₂ 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.