Capacity fading of Li_xMn₂O₄ spinel electrodes studied by XRD and electroanalytical techniques

Li_xMn₂O₄ spinels were synthesized in different ways, leading to different particle morphologies and different electrochemical behavior. Two types of Li_xMn₂O₄ electrodes comprised of active mass synthesized in two different ways were investigated in a standard solution (ethylene carbonate-dimethyl carbonate 1:3/LiAsF₆ 1 M) using X-ray diffraction technique (XRD) in conjunction with a variety of electroanalytical techniques. These included slow scan rate cyclic voltammetry, chronopotentiometry, impedance spectroscopy and potentiostatic intermittent titration. We discovered two types of capacity fading mechanisms. One involves the formation of a new, less symmetric and more disordered phase (compared with the pristine Li_xMn₂O₄ materials) during the first Li deinsertion reaction of a pristine electrode in the 3.5-4.2 V (Li/Li⁺) potential range. This new phase, although inactive, has no detrimental effect on the kinetics of the remaining active mass. Another capacity fading mechanism occurs at > 4.4 V (Li/Li⁺) potential and involves dissolution of Mn into the solution, and a pronounced increase in the electrode's impedance. It appears that dissolution of Mn at elevated potentials is connected with degradation of the solution, which also occurs at these potentials at low rates.