Contact resistance of carbon-Li𝑥(Ni,Mn,Co)O2 interfaces

Electronic resistance in lithium-ion battery positive electrodes is typically attributed to the bulk resistance of the active material and the network resistance of the carbon additive. Expected overpotentials from these bulk components are minimal relative to that from charge-transfer resistance. However, literature reports show that cell overpotentials are often much more sensitive to conductive additives than the expected level from bulk or percolating-network transport. This discrepancy motivates a detailed examination of the contact resistance between the active material and conductive additive. We simultaneously measure contact and bulk resistances using dense bar samples of lithium layered oxides (LixNi1/3Mn1/3Co1/3O2 and LixNi0.5Mn0.3Co0.2O2) in contact with carbon black. We find that the contact resistance dominates the overall electronic resistance when the length scale is smaller than millimeters; after correcting for contact effects, bulk conductivity of layered oxides is determined to be orders-of-magnitude higher than previously reported. In porous electrodes, we find from three-electrode electrochemical impedance spectroscopy that the carbon content most heavily influences the low-frequency regime (around 0.01 Hz), as opposed to the high frequency (>10^3 Hz) regime expected from electronic percolating properties. We identify constriction effects within the layered oxide as the dominant mechanism for contact resistance and investigate its implication for porous electrodes.