Understanding operando electronic resistance variation in Li-ion battery cathode materials using in-plane analysis

Insertion-type lithium-ion battery cathode materials undergo a series of Li + extraction and insertion reactions during the charge and discharge cycles (respectively), accompanied by electron transfer to/from the external circuit. This process requires an optimized ionic and electronic percolation network, as well as minimal structural variation, for the sake of a long-term lifespan of the batteries. Moreover, both the volume and density of state changes are expected to occur upon cycling, which means that understanding the changes in electrode electronic and ionic conductivity at different states of charge is of high relevance. LiNiₓMn y Co z O 2 (NMC) layered cathodes, developed from the parent phase LiCoO 2 (LCO), are among the most crucial commercial cathode active materials of the current date. These systems undergo several structural changes upon cycling, resulting in significant changes to the crystal cell lattice parameters. Our in-plane operando resistance measurement technique enables the deconvolution of resistance variation and its subsequent correlation with structural evolution. Our findings show, from the electronic percolation point of view, why LCO’s limiting operating potential must be restricted to 4.2 V vs. Li/Li + , as it comes with the detrimental effect on O1–3 phase formation when cycled beyond 4.2 V vs. Li/Li + , along with CEI degradation. Similarly, the in-plane EIS method shows the compromised effect of high Ni content in NMC materials, where increased Ni content at a high state of charge leads to structural collapse. Although both these materials have similar issues, the high degree of delithiation was found to be more detrimental for LCO materials, leading to a high irreversible resistance compared to NMC.