Electrochemical Behavior of Layered Solid Solution Li2MnO3-LiMO2 (M = Ni, Mn, Co) Li-Ion Cathodes with and without Alumina Coatings

Thin films of oxides, phosphates, fluorides and other analogous materials on lithium-ion cathode particles are well known to improve cathode performance in terms of cycle life and rate performance. Explanations for this phenomenon abound, but the underlying mechanisms that dictate the nature of these effects are still in question, which motivates the work herein. We have carried out systematic PITT, EIS, Tafel, and cycling experiments as a function of temperature for Al2O3-coated and uncoated layered solid solution Li2MnO3-LiMO2 (M Mn, Co, Ni) cathode materials and shown that we can reproduce the well-documented improvement in performance with surface coatings. In particular the effects are most pronounced at reduced temperatures and after temperature cycling (23 to 0 degrees C to 30 to 0 degrees C). Interestingly, we find the activation energies for the diffusion coefficients estimated from PITT data are nearly identical to the activation energy for exchange current measured from Tafel polarization data. This finding may provide some insight into the relative control of the mass transfer and the charge transfer processes on the overall cathode reaction. Alternately, it may be the due to inadequate correction for the mass transfer effects in the Tafel and PITT analyses. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3597319] All rights reserved.