Simulation-Based Defect Engineering in “α-Spodumene”

Naturally occurring lithium-rich α-spodumene (α-LiAlSi2O6) is a technologically important mineral that has attracted considerable attention in ceramics, polymer industries, and rechargeable lithium ion batteries (LIBs). The defect chemistry and dopant properties of this material are studied using a well-established atomistic simulation technique based on classical pair-potentials. The most favorable intrinsic defect process is the Al-Si anti-site defect cluster (1.08 eV/defect). The second most favorable defect process is the Li-Al anti-site defect cluster (1.17 eV/defect). The Li-Frenkel is higher in energy by 0.33 eV than the Al-Si anti-site defect cluster. This process would ensure the formation of Li vacancies required for the Li diffusion via the vacancy-assisted mechanism. The Li-ion diffusion in this material is slow, with an activation energy of 2.62 eV. The most promising isovalent dopants on the Li, Al, and Si sites are found to be Na, Ga, and Ge, respectively. The formation of both Li interstitials and oxygen vacancies can be facilitated by doping of Ga on the Si site. The incorporation of lithium is studied using density functional theory simulations and the electronic structures of resultant complexes are discussed.

Defects and Calcium Diffusion in Wollastonite

Wollastonite (CaSiO3) is an important mineral that is widely used in ceramics and polymer industries. Defect energetics, diffusion of Ca ions and a solution of dopants are studied using atomistic-scale simulation based on the classical pair potentials. The energetically favourable defect process is calculated to be the Ca-Si anti-site defect cluster in which both Ca and Si swap their atomic positions simultaneously. It is calculated that the Ca ion migrates in the ab plane with an activation energy of 1.59 eV, inferring its slow diffusion. Favourable isovalent dopants on the Ca and Si sites are Sr2+ and Ge4+, respectively. Subvalent doping by Al on the Si site is a favourable process to incorporate additional Ca in the form of interstitials in CaSiO3. This engineering strategy would increase the capacity of this material.

Defect, Diffusion and Dopant Properties of NaNiO2: Atomistic Simulation Study

Sodium nickelate, NaNiO2, is a candidate cathode material for sodium ion batteries due to its high volumetric and gravimetric energy density. The use of atomistic simulation techniques allows the examination of the defect energetics, Na-ion diffusion and dopant properties within the crystal. Here, we show that the lowest energy intrinsic defect process is the Na-Ni anti-site. The Na Frenkel, which introduces Na vacancies in the lattice, is found to be the second most favourable defect process and this process is higher in energy only by 0.16 eV than the anti-site defect. Favourable Na-ion diffusion barrier of 0.67 eV in the ab plane indicates that the Na-ion diffusion in this material is relatively fast. Favourable divalent dopant on the Ni site is Co2+ that increases additional Na, leading to high capacity. The formation of Na vacancies can be facilitated by doping Ti4+ on the Ni site. The promising isovalent dopant on the Ni site is Ga3+.

Defect Process, Dopant Behaviour and Li Ion Mobility in the Li2MnO3 Cathode Material

Lithium manganite, Li2MnO3, is an attractive cathode material for rechargeable lithium ion batteries due to its large capacity, low cost and low toxicity. We employed well-established atomistic simulation techniques to examine defect processes, favourable dopants on the Mn site and lithium ion diffusion pathways in Li2MnO3. The Li Frenkel, which is necessary for the formation of Li vacancies in vacancy-assisted Li ion diffusion, is calculated to be the most favourable intrinsic defect (1.21 eV/defect). The cation intermixing is calculated to be the second most favourable defect process. High lithium ionic conductivity with a low activation energy of 0.44 eV indicates that a Li ion can be extracted easily in this material. To increase the capacity, trivalent dopants (Al3+, Co3+, Ga3+, Sc3+, In3+, Y3+, Gd3+ and La3+) were considered to create extra Li in Li2MnO3. The present calculations show that Al3+ is an ideal dopant for this strategy and that this is in agreement with the experiential study of Al-doped Li2MnO3. The favourable isovalent dopants are found to be the Si4+ and the Ge4+ on the Mn site.