Strain Effects On Ion Migration In LiCoO₂

LiCoO2 is one of the most commonly used cathode materials in lithium-ion batteries because of its ease of synthesis,high-quality density and high-volume energy density,excellent cycle stability and high-voltage stability.Although LiCoO2 cathode materials have been well developed,there is still room for improving rate capability.Material treatment methods such as doping or surface coating have been used to improve the conductivity of LiCoO2.Applying strain is another effective method.The main work content and innovations of this paper are as follows:(1)Based on the atomic simulation technology,the experimental structure parameters and crystal properties of LiCoO2 are accurately reproduced.The defect energy in LiCoO2 under unstrained conditions is calculated,and an optimized calculation program is designed to determine the position of oxygen interstitial atoms.We have identified two main migration mechanisms in layered LiCoO2—oxygen dumbbell hop(ODH)and tetrahedral site hop(TSH);(2)The defect energies of LiCoO2 and the migration energy barriers of two lithium-ion transport mechanisms under different hydrostatic pressures are calculated.The study found that under the mechanism of ODH and TSH,the defect energy of ions and the energy barrier of lithium-ion migration increase with the increase of pressure;(3)In a certain range(Δa/a(Δb/b=Δa/a)and Δc/c between-3%～3%),the influences of strain on the Li-ion migration energy barrier of lithium-ions in layered LiCoO2 have been systemically studied using lattice dynamics simulations.Our work provides the complete strain-map for enhancing the diffusivity of Li-ion in LiCoO2.The focus is on the two typically strain conditions that can be achieved in the LiCoO2 battery film:c-axis strain and in-plane strain.The results show that it is possible to further reduce Li-ion migration energy barrier with applied tensile uniaxial c-axis strain for ODH and TSH cases or compressive in-plane strain for TSH case.We first proposed the relationship between the TSH migration mechanism and strain;(4)The database obtained from the calculation of lattice dynamics is analyzed with analytical function and back propagation neural network(BPNN)respectively,and a method to predict the energy barrier of lithium-ion migration under arbitrary strain conditions is proposed.