Delithiation-induced oxygen vacancy formation increases microcracking of LiCoO2 cathodes

Cracking of cathode materials during cycling is a main cause of capacity fading in Li-ion batteries. In this work, by performing atomistic and microscale simulations, we study the possible reason behind the cracking of LixCoO2 (LxCO) microstructures. It is shown that tensile uniaxial lattice strains larger than 2% along the epsilon-direction (epsilon(c)) can cause displacement of Li ions and a yield drop in the stress-strain sigma(c) (epsilon(c)) plot in LxCO. By modelling a typical microstructure consisting of packed microparticles and performing continuum mechanical analysis on the mesoscale we found that the electrochemically-induced (L1.00CO -> L0.50CO) mechanical epsilon(c) in the microstructure is, however, only -2.5% <= epsilon(c) <= +0.5%. Moreover, we found that even a sharp space charge region cannot cause any significant local tensile strain. However, a small amount of oxygen vacancy (V-O(x)) introduces a large local strain of epsilon(c) = 3% leading to the displacements of Li ions. Furthermore, we found that the formation of V-O(x) becomes more favourable with delithiation (L1.00CO -> L0.50CO). The results of this work, thus, indicate that the delithiation-induced formation of V-O(x), which is a well-known phenomenon observed experimentally in operating cathode materials, can be a reason of microcracking of Li-based layered cathodes.