Molecular dynamics (MD) simulations of the charging of Li2MnO3 reveal that the reason nanocrystalline-Li2MnO3 is electrochemically active, in contrast to the parent bulk-Li2MnO3, is because in the nanomaterial the tunnels, in which the Li ions reside, are held apart by Mn ions, which act as a pseudo 'point defect scaffold'. The Li ions are then able to diffuse, via a vacancy driven mechanism, throughout the nanomaterial in all spatial dimensions while the 'Mn defect scaffold' maintains the structural integrity of the layered structure during charging. Our findings reveal that oxides, which comprise cation disorder, can be potential candidates for electrodes in rechargeable Li-ion batteries. Moreover, we propose that the concept of a 'point defect scaffold' might manifest as a more general phenomenon, which can be exploited to engineer, for example, two or three-dimensional strain within a host material and can be fine-tuned to optimize properties, such as ionic conductivity.

Akin to Le Chatalier s principle we show that a mesoporous material can mitigate the effect of stress by ex panding or contracting elastically into the pore space we simulate this breathing-crystal phenomenon using MD simulation. In particular, our simulations reveal that mesoporous Li-MnO2 is electrochemically active because the stress, associated with charge cycling, does not influence the structure or dimensions of the (unlithiated) 1x1 tunnels in which the lithium ions intercalate and reside. Conversely, the parent bulk material suffers structural collapse and blockage of the 1x1 tunnels under stress. The mechanism associated with Li deintercalation is presented together with the activation energy barriers, which are calculated to be 0.4eV -irrespective of whether the mesoporous host is unstrained or under considerable (1.6 GPa) tensile or compressive stress.

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Layered transition metal oxides LiMO2 (M= Mn, Ni, Co), exhibit good electrochemical performances and are considered as the prototype materials in the first commercial lithium-ion products. However, their performance as individual cathode has shown drawbacks and ignited interest in transition metal doping to form highly efficient cathodes. Such interest has driven efforts towards development of interatomic potentials to help provide information pertinent to the fundamental aspects of the interaction between atoms and allow accurate modelling of structures. Developing force fields is a tedious process as such cost functions often feature several competing minima. This work aims to obtain interatomic interactions (Ni-Ni, Ni-O, Co-Co and Co-Co) suitable for large scale simulations. The potentials are fitted from the cross-platform, streaming task runner (code-based) GULP. The procedure fits the ionic size (Aij), dispersion parameter (Cij), and the hardness of ions (ρij), according to the Buckingham potentials. The fitted interactions produced structures with lattice constants with a difference of less than 1% in NiO and 8.75% in CoO in comparison to experimental data. Furthermore, they yielded elastic constants with a difference of 0.35% in NiO and 2.01% in CoO. The high temperature molecular dynamic calculations validated the potentials through the melting temperatures. The nanostructures and their radial distribution curves confirmed melting temperatures of 2250K and 2000K in NiO and CoO, respectively. These are in good agreement accord with the experimental melting temperatures of 2206K and 2228K for NiO and CoO, respectively. Moreover, the derived interatomic potential accurately simulates the structural properties and behavior of and LiCoO2. The findings of the current study will enable the implementation of these potentials into LiMO2 (M: Ni, Co and Mn) structures for incorporation as dopants into the LiMnO2 cathode material.

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