Influence of carbon distribution on the electrochemical performance and stability of lithium titanate based energy storage devices

Abstract

We explore different electrode microstructures and the associated implications on the electrochemical stability of activated carbon/lithium titanate (Li4Ti5O12, LTO) composite electrodes by incrementally increasing the LTO content. At low LTO concentrations, the electrochemical stability is progressively improved with respect to neat activated carbon based electrodes. This trend is abruptly changed for high LTO concentrations (72 mass%) as the electrolyte starts to decompose unexpectedly far below the electrochemical stability boundaries of the single materials. We attribute this to a loss of electrical percolation and local degradation spots caused by peculiarities of the carbon distribution: Initially the sub-micrometer-sized LTO solely occupies spaces between the large, micrometer-sized activated carbon. With increasing LTO content the activated carbon particles get separated in an insulating LTO matrix. Electrochemical stability can be reestablished with electronic conduction paths of well distributed sub-micrometer-sized carbon black particles. By this way, cell degradation can be reduced and the cycle life of cells with high LTO concentration is prolonged from 10 to >36,000 cycles. Finally, we propose a simple method to distinguish cell fading caused by electrolyte decomposition from cell fading caused by poor electrical percolation.