Exploring Charge Distribution and Rate Variation in LiNi0.8Mn0.1Co0.1O2/LiMn0.7Fe0.3PO4 Blends for High Rate Electrodes

Abstract Composite electrodes containing physical mixtures (“blends”) of multiple active materials have generated commercial interest for lithium battery applications due to the possibility of enhanced electrochemical performance, which are not fully characterised. This investigation uses mixtures of well-known LiMn0.7Fe0.3PO4 (LMFP) and LiNi0.8Mn0.1Co0.1O2 (NMC) materials as a model blend system to investigate rate- and composition-dependent effects on performance. A combination of electrochemical and operando techniques reveals that deviations in performance with respect to the linear combination of the component materials originate from uneven current distribution, leading to different effective rates between components and spontaneous lithium-ion redistribution, commonly termed the “buffer effect”. Results of experiments using a specially adapted three-electrode setup show that the effective rates of the components vary considerably depending on the cell state of charge, with much higher effective rates being reached than the applied cell rate. The same setup allows the direction of lithium redistribution to be visualised when the system relaxes following a current pulse, and this directionality also varies with cell state of charge. Electrochemical investigations and Raman spectra reveal that blending reduces chemical sensitivity to ambient storage conditions and leads to reduced degradation by forming a protective LMFP layer around the more reactive NMC particles. Operando data shows that the blend components evolve largely as expected when taking into account their specific effective rates, in line with the materials retaining their individual electrochemical properties. This study highlights the importance of considering relative windows of electrochemical activity when selecting blend components and paves the way for future optimisation of electrode blends for high rate lithium-ion batteries.