Structure and Electrochemistry of Na_xFe_xTi_1-xO₂(1.0 ≥ x ≥ 0.75) for Na-Ion Battery Positive Electrodes

Fe containing Na-ion cathode materials are desirable due to their low cost and use of earth abundant raw materials. Here, the electrochemistry of layered Na x Fe 3+ x Ti 4+ 1-x O 2 is described. It was found that Mössbauer spectra can accurately detect Na vacancies in Na x Fe 3+ x Ti 4+ 1-x O 2 material from their effects on neighboring Fe 3+ ions and this model can be used to analyse materials at different stages of charge or discharge. This Na vacancy model potentially has broader implications and may be useful as a probe for Na-vacancies in iron containing Na-ion cathode materials in general. The oxidation of Fe 3+ to Fe 4+ during electrochemical cycling is shown to be a major source of hysteresis and capacity fade, while the activation of the Fe 2+ /Fe 3+ and Ti 3+ /Ti 4+ redox couples results in stable cycling performance. Possible reasons for capacity fade and hysteresis are discussed in terms of ex-situ Mössbauer and XRD studies.