Resistance Growth in Lithium-Ion Pouch Cells with LiNi0.80Co0.15Al0.05O2 Positive Electrodes and Proposed Mechanism for Voltage Dependent Charge-Transfer Resistance

High nickel content positive electrode materials, like LiNi0.80Co0.15Al0.05O2 (NCA), have been observed to lose capacity due to impedance growth when cycled above 4.1 V vs. Li/Li+. In this paper, we tested pouch cells with NCA positive electrodes and graphite, graphite-SiO, or graphite-SiC negative electrodes. In each case the pouch cells show high resistance at high voltage for every cycle, and resistance growth during cycling that is in part responsible for capacity loss. Symmetric cell experiments were used to show that the high resistance does, in fact, come from the positive electrode charge transfer resistance which is attributed to a thickening rock salt layer on the positive electrode (seen by other researchers). We propose a mechanism to explain why the charge transfer resistance depends on voltage for the positive electrode.