Electrochemical properties of high rate bias sputtered LiCoO2 thin films in liquid electrolyte

LiCoO2 thin films have been prepared on aluminium substrate with RF sputtering using a bias effect combined with a moderate post-annealing treatment at 500 °C and their electrochemical properties investigated in liquid electrolyte by cyclic voltammetry, galvanostatic experiments in various voltage windows, and ac impedance spectroscopy. The positive effect of bias is shown allowing to minimize the presence of the cubic LiCoO2 phase. The lithium extraction process is highly reversible in the biased LiCoO2 films with charge-discharge curves which really superimpose at C/10 rate. A 450 nm thin film is shown to exhibit an initial charge capacity of ≈60 μAh cm−2 μm−1 at C/10 and a high discharge capacity of ≈50 μAh cm−2 μm−1. The rate capability study demonstrates the film can easily sustain high charge-discharge rates and exhibits good cycling properties with capacity retention of 90% after 50 cycles up to a 2C rate and still 73% of the maximum capacity is recovered at 8C in the 4.2 V-3 V potential window. This constitutes remarkable performances both in terms of rate capability and cycling stability. The charge-discharge behaviour as a function of the film thickness in the range 0.25-3.6 μm is investigated and indicates an efficient and homogeneous electrochemical activity with a linear increase of the capacity with thickness. As a consequence an optimization of the capacity is proposed with the use of the thickest deposit of 3.6 μm leading to the value of 200 μAh cm−2, the best one available for a LiCoO2 film in liquid electrolyte.