Effect of lithium salt concentration on the capacity retention of Lithium rich NMC cathodes

Abstract

Layered lithium-rich cathode materials xLi2MnO3.(1-x)LiMO2 (M=Mn, Co, Ni) also named High energy (HE-NMC) cathodes have attracted considerable attention as a promising material for LIBs thanks to their high operating voltage (4.8V) and remarkable specific capacity up to 280mAhg−1. In such materials, the lithium extraction from HE-NMC in the activation step and during subsequent cycles, involves reversible redox reactions that consume a portion of the lithium from the electrolyte and generate at the same time oxygenated actives species and molecular oxygen which is soluble in the alkyl carbonates. In order to understand the relationship between the amount of lithium contained in the electrolyte [LiPF6], oxygen extracted and cyclability, we explore in this work, the effect of LiPF6 concentration from (1.00 to 2.00) molL−1 on the capacity retention at different C-rates of full (graphite//HE-NMC) cells. In order to further understand the reaction mechanism during charging involving simultaneous removal of lithium and oxygen, which is matter of considerable scientific debate, the pressure change due to the generated gases in the cell during cycling was monitored in-situ allowing us to calculate the relative contribution from two electrochemical reactions. The pressure variation provides evidence that the reaction above 4.4V contributes about 56% to the total charge capacity while transition metal oxidation accounts for the remaining 44% during the first charge. These results confirm the observations made by galvanostatic measurements showing that the electrolyte containing more than 1.25molL−1 of LiPF6 exhibits stable cycling and much higher capacity retention as compared to cells with 1.00molL−1 LiPF6. Finally, the modification of the voltage profile of charge/discharge during galvanostatic cycling in the temperature range (0 to 40) °C demonstrates that the reaction kinetics of low voltage discharge peak (∼2.6V) attributed to the manganese and oxygen species redox system has much higher sensitivity to the temperature while others metal transition redox systems are relatively less affected.