Investigating Disordered Positive Electrode Materials for Lithium-Ion Batteries Using Li(1+x)Ti2xFe(1-3x)O2
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Typical positive electrode materials for lithium-ion batteries have ordered structures, containing well defined lithium percolation networks through the entire structure. Another type of structure which has until recently received little attention is the disordered rock-salt structure. These materials have in the past exhibited poor capacity and cycling performance due to the lack of structure-spanning lithium networks. Recent theoretical works have modelled the available lithium content in favourable sites for lithium percolation in various disordered structures. This work compared the recent model to experimental results obtained from structural and electrochemical studies on Li(1+x)Ti2xFe(1-3x)O2 (0.00 ≤ x ≤ 0.28) with various degrees of disorder, synthesised using solid state synthesis methods. Charge-discharge cycling studies of coin cells were used to confirm the model as well as to explore various cycling conditions and electrochemical mechanisms responsible for trends in performance. x-ray diffraction was used to characterize structural parameters and the degree of disorder at various temperatures and compositions. Fe and Ti K-edge x-ray absorption spectroscopy (XAS) of pristine materials and oxygen K-edge XAS of cycled electrodes were used to analyze structural trends and electrochemical processes such as oxygen charge compensation.