Insights from Computational Studies on the Anisotropic Volume Change of Li_xNiO₂ at High States of Charge (x < 0.25)

Abstract

The need for high-capacity Li-ion battery cathodes has favored the increase of Ni content in commercial battery cells. However, at high states of charge (SOCs), Ni-rich materials undergo a phase transition and volume collapse with deleterious effects on battery performance. It is uncertain whether this drastic volume change is caused by the phase transition or not. To provide more insight into the volume-phase transition relationship in the high Ni cathode Li x NiO 2 , we performed density functional theory calculations, along with molecular dynamics simulations using machine learning potentials to calculate the temperature-and composition-dependent free energy differences between the suspected phases at high SOCs (x < 0.25). We find that the calculated free energy difference between the suspected phases containing different oxygen stacking sequences is small at room temperature. Furthermore, we find that the collapse of the layered LiNiO 2 c-lattice parameter at high SOCs is mainly due to the electronic depletion of the oxygen sublattice and the lack of screening from positive Li ions. The interactions between adjacent oxygen ions across an empty Li layer (NiO 2 ) are largely controlled by van der Waals interactions and are in fact similar regardless of the oxygen stacking, which explains the negligible free energy differences between O1 and O3 stacking in NiO 2 .

Stepwise Dopant Selection Process for High‐Nickel Layered Oxide Cathodes

“…This maximum c ‐lattice parameter drops to 13.56 Å with further delithiation up to x = 0.16, resulting in c ‐lattice contraction of ≈5.1%. [ 47 ] In contrast, the maximum a ‐lattice parameter change was less than 0.01% (from 5.62 to 5.60 Å) for the same degree of delithiation. These non‐symmetrical lattice parameter changes imply anisotropic strain generation in the secondary particles that are typically comprised of randomly oriented primary particles.…”

Does trapped O2 form in the bulk of LiNiO2 during charging?

“…S3 and Table S1. LiNiO 2 undergoes a series of well-studied phase transitions upon delithation (H1 -M1 -H2 -H3), between hexagonal phases, H, which differ slightly in their lattice parameters and a monoclinic phase, M. 5,38,[40][41][42][43][44][45] Upon reaching the onset of the voltage plateau at 4.1 V vs. Li + /Li, LiNiO 2 has transitioned to the H2 phase with an expanded unit cell along the crystallographic c axis, ESI † Fig. S4 and Table S2.…”

Constant-potential molecular dynamics simulation and its application in rechargeable batteries

“…Thirdly, constant-potential MD modeling of battery interfaces can be combined with machine learning potentials (MLPs), which would hopefully lead to fast computation as in CMD and high electronic accuracy as in AIMD at the same time. 6,7,[88][89][90][91] Currently, MLPs have already been extensively utilized in the study of bulk liquid 92,93 and solid battery materials, [94][95][96] especially to investigate their transport properties. 91,[97][98][99] Some studies have demonstrated the possibility to model interfaces with MLPs, [100][101][102][103][104][105] even though these interfaces are not held at constant potentials and may not be related to batteries.…”