Understanding charge transfer dynamics in blended positive electrodes for Li-ion batteries

This paper investigates the electrochemical behavior of binary blend electrodes comprising equivalent amounts of lithium-ion battery active materials, namely LiNi0.5Mn0.3Co0.2O2 (NMC), LiMn2O4 (LMO), LiFe0.35Mn0.65PO4 (LFMP) and LiFePO4 (LFP)), with a focus on decoupled electrochemical testing and operando X-ray diffraction (XRD). All possible 50:50 blend combinations were studied and the distribution of current between blend components was followed during continuous and pulsed charge and discharge processes. The results demonstrate the significant impact of the voltage profiles of individual materials on the current distribution, with the effective C-rate of each component varying throughout the state of charge (SoC). Pulsed decoupled electrochemical testing reveals the exchange of charge between blend components during relaxation, showcasing the "buffer effect", which has also been captured through time-resolved operando XRD experiments in real blends carefully considering beam-induced effects. The directionality and magnitude of the charge transfer were found to depend on the nature of the components and the cell State of Charge (SoC), being also influenced by temperature. These dependencies can be rationalized considering both thermodynamics (voltage profile) and kinetic properties of the blend constituents. These findings contribute to advancing the understanding of internal dynamics in blended electrodes, offering valuable insights for the rational design of blends to meet the diverse operational demands of lithium-ion batteries.