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

Abstract

Density functional theory has been used to investigate the electrochemical and mechanical properties of the well-known cathode material Li x Ni 0.833 Mn 0.083 Co 0.083 O 2 (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. • DFT is used to examine the electrochemical and mechanical properties of Li-NMC-811. • The cathode experiences a transition from ductile to brittle. • The ductile/brittle transition corresponds to jumps in the voltage profile.