Correlation between battery material performance and cooperative electron-phonon interaction in LiCo$_y$Mn$_{2-y}$O$_{4}$

Understanding the basic physics related to archetypal lithium battery material (such as LiCo$_y$Mn$_{2-y}$O$_{4}$) is of considerable interest and is expected to aid designing of cathodes of high capacity. The relation between electrochemical performance, activated-transport parameters, thermal expansion, and cooperativity of electron-phonon-interaction distortions in LiCo$_y$Mn$_{2-y}$O$_{4}$ is investigated. The first order cooperative-normal-mode transition, detected through coefficient of thermal expansion, is found to disappear at a critical doping ($y \sim 0.16$); interestingly, for $y \gtrsim 0.16$ the resistivity does not change much with doping and the electrochemical capacity becomes constant over repeated cycling. The critical doping $y \sim 0.16$ results in breakdown of the network of cooperative/coherent normal-mode distortions; this leads to vanishing of the first-order transition, establishment of hopping channels with lower resistance, and enhancing lithiation and delithiation of the battery, thereby minimizing electrochemical capacity fading.