First principles investigation into the interwoven nature of voltage and mechanical properties of the Li_xNMC-811 cathode

Density functional theory has been used to investigate the electrochemical and mechanical properties of the well-known cathode material Li_xNi_0.833Mn_0.083Co_0.083O₂ (Li_x-NMC-811) as a function of Li concentration. We used the PBE functional with dispersion corrections to calculate a voltage in good agreement with experiment, which gradually changes from 3.3 to 4.2 V during the charging process. A similar profile trend can be found without dispersion, though voltages were lower by ∼0.4 V and the known NMC lattice collapse was not captured as it was with dispersion. We compare calculated quantities such as Young's modulus (E) to better understand the mechanical response of the material to outside forces with varying Li concentration. We also compare Poisson's ratio (ν) and the Pugh ratio (γ), which are used as metrics of ductility, for each of these lithiation concentrations. We have shown that as the concentration of Li reaches x=0.25, the material transitions from ductile to brittle based on both ductility metrics. It is near this concentration that we also see the largest change in voltage from our calculations. We propose that this transition between ductile and brittle provides additional insight and understanding into structural failure and, consequently, capacity loss during normal cycling for cathode materials. Furthermore, it demonstrates a mechanism for reversibly controlling mechanical properties by electrochemical means.