Electrochemical Phase Transition Of Li_xCoO₂ From Machine-Learning-based Global Optimization

The structural changes of the cathode material of lithium-ion battery during the（dis）charging process will significantly affect its electrochemical performance.As a dominant cathode material,lithium cobalt oxide undergoes a very complex phase transition during the delithiation process,especially the irreversible phase transition at high voltage,which seriously damages the cycle stability of the battery.The structure-activity related phenomena of lithium-ion battery materials have received extensive attention from industry and science.In this paper,the structure of Li_xCoO₂（x=n Li/n Co,where n Li=1,2,3....16）was calculated under different proportions using the recently developed machine learning method（SSW-NN）.The structural changes and corresponding electrochemical properties of Li_xCoO₂ electrode materials during charge and discharge were studied,and the diffusion energy barrier of lithium ions under different voltage conditions was further explored.The main conclusions are as follows:（1）Since the decrease in the Li/Coratio creates lithium vacancies in the bulk structure,at x>0.25,the lithium vacancies and the remaining lithium ions in the lithium layer tend to be evenly distributed,at x<0.25,the distribution of lithium vacancies is more concentrated,often occupying an entire lithium layer,and lithium ions are more inclined to be distributed in lithium vacancies occupying adjacent layers of the lithium layer,so the lithium-rich layer and lithium-poor layer are appeared in the structure.The distribution of lithium vacancies leads to different stable configurations of lithium cobalt oxide structures with different lithium content,in Li_xCoO₂,when x>0.5,the stable phase is the O3 structure of the hexagonal configuration,and when x=0.5,the O3 structure of the monoclinic configuration is thermodynamically stable.At x<0.25,the structure is more inclined to exist in the H1-3 configuration.The decrease in the Li/Coratio of the warrior will lead to an increase in the height of the lithium layer,a decrease in the height of the cobalt layer,an increase in the length of the Li-O bond,a decrease in the length of the Co-O bond,and an increase in the volume of the unit cell.（2）The voltage of Li_xCoO₂ gradually increases with the decrease of Li/Coratio,this is due to the phase transition of the structure at low lithium content.There are roughly four plateaus in the voltage-Liratio curve.Each plateau is associated with a characteristic Listructure.At high Licontent（x>0.85）,Liion delithiation requires a relatively lower voltage（<4.0 V）.As the Licontent decreases（0.6<x<0.75）,the voltage gradually increases to 4.07 V.When the Licontent drops below 0.5,the voltage curve rises to 4.26 V,indicating a phase transition from hexagonal to monoclinic layer.For Licontent lower than 0.25,a steep increase in the voltage curve is observed,indicating an irreversible phase transition.it is observed that Li_xCoO₂ is particularly prone to O₂ evolution with low Licontent（x<0.20）,indicating that stability of（de）lithiated Li_xCoO₂ decreases with decreasing Licontent.The results show that segregated structure at low Liratios leads to degradation and reduced stability.The migration barriers of Li_xCoO₂ also show that the migration of Li⁺at low lithium content is more difficult,where the two oxygen atoms of octahedral apexes and its coplanar oxygen tetrahedron must be lengthened to allow Lito pass through.（3）The cation mixed Li_xCoO₂（cm-Li_xCoO₂）has been further computed.Compared to the Li_xCoO₂ structure,the cation mixed Li_xCoO₂（cm-Li_xCoO₂）has a larger average voltage and more stable formation energy in the entire delithium range.It is explained that the cation mixing phenomenon increases the migration difficulty of lithium ions and reduces the possibility of structural phase separation.As the lithium content decreases,oxygen is more easily removed from the structure,accelerating the phase transition towards the spinel structure of Co₃O₄.