Atomic Level Investigations of Lithium Ion Battery Cathode Materials

Abstract

The defect energetics, ion migration processes and surface structures of lithium ion battery cathode materials LiCoO 2 and LiMPO 4 (M = Mn, Fe, Co or Ni), probed using a Born model description of atomic interactions, are reported. The lowest energy intrinsic disorder types comprise lithium deficiency in the case of LiCoO 2 and cation antisite defects in the case of the orthophosphates. Lithium diffusion in LiCoO 2 is confirmed to be two dimensional, with a calculated activation barrier of 0.45 eV, whereas in LiMPO 4 compounds diffusion is one dimensional only, with a barrier decreasing from 0.62 to 0.44 eV across the transition metal series. Unlike the linear path calculated for LiCoO 2 , in orthophosphates the Li ion follows a curved path between vacancies. Examination of low index surfaces in LiCoO 2 and LiFePO 4 further illustrates the utility of these methods for probing materials systems on the atomic level.

Thermal expansion, diffusion and melting of Li2O using a compact forcefield derived fromab initiomolecular dynamics

“…Using LAMMPS [1] as integrated in the MedeA ® environment [18], the density was obtained from NPT molecular dynamics simulations using a 6×6×6 Li 2 O supercell containing 2592 atoms. The simulation time was 100 ps for each temperature leading to statistical error bars which are smaller than the line width in figure 6.…”

“…Nevertheless, even in such systems, the use of the simple interatomic potential of the form of a Buckingham potential has been successful in the simulation of oxides [6], enabling a realistic description of the structure and dynamics of systems ranging from transition metal oxides to silica glasses [7]. In recent years, this type of potential has also been used in simulations of the diffusion of Li ions in lithium oxide [8][9][10][11][12][13][14][15][16][17] and battery materials [18]. In view of the known shortcomings of simple pair potentials, for example, in predicting the Cauchy violation or in reproducing phonon dispersions, it is not completely clear how well simple ionic pair potentials capture diffusion processes.…”

Strain effects on lithium ion diffusion in various crystal structures

“…These E a values are given in Table S4 for comparison with the previously reported results. [34][35][36][37] In the spinel LiM 2 O 4 (M = Ti, Mn, Co) structures, the Li diffusion path is along 8a-16c-8a as shown in Fig. 3(f).…”

Pushing the boundaries of lithium battery research with atomistic modelling on different scales

“…Oxide systems are widely described using a Buckingham potential form, as given in equation ( 16), and for layered structures, including NMC and its ternary systems, variations of the Buckingham potentials are presented. Some use rigid ion models [147][148][149][150], others use core-shell models [147,[151][152][153][154][155][156], and a mixture of formal and partial charges have been implemented. With literature in disagreement over which variation of the Buckingham potential is the most accurate for representing the system, a code capable of fitting different permutations of the Buckingham potential is needed.…”

“…There are predominately two types of core-shell models: the relaxed (massless shells) model [157] and the dynamic (adiabatic shells) model [158]. The adiabatic shell model is more widely used in literature, including all core-shell related cited works in this section [147,[151][152][153][154][155][156], for calculating long trajectories, as it is less computationally taxing. In the adiabatic shell model, a fraction of the atomic mass is assigned to the shell.…”