First Principles Study on the Corelation between Oxygen Evolution and Electrochemical Behavior of Manganese-based High Capacity Cathode Materials

Author:Chen Qing

Supervisor:zhen liang song yan


Degree Year:2018





Manganese-based high capcity cathode materials,for example,Li2MnSi O4 and x Li2Mn O3·(1-x)Li MO2,have much higher energy density than other species,making them to be the most promising cathode materials for Li-ion batteries.However,intrinsic drawbacks such as the irreversible structure change of Li2MnSi O4,the serious capacity and voltage decay of x Li2Mn O3·(1-x)Li MO2 during long-term cycling blocked their commercial applications.To address those issues,several theoretical interpretations and modification methods were proposed.Structure-collapsing model was raised to explain the irreversible phase transition of Li2Mn Si O4,and the voltage decay in x Li2MnO3·(1-x)Li MO2 was found out to be originated from the migration of Mn ions during the cycling.However,the recent experiments did not support this model and the results indicated that the migration of Mn ions in x Li2Mn O3·(1-x)Li MO2 was found out to be hardly avoided.Thus,the complement and perfection of the mechanisms of irreversible phase transition in Li2MnSi O4 and an in-depth understanding of the essentials of Mn ion migration in x Li2Mn O3·(1-x)Li MO2 are of great significance to the experimental investigations.First-principles calculations based on the Density Functional Theory(DFT)are utilized to investigate the structure evolutions during the extraction of Li ions from Li2MnSi O4.Based on the calculations,it was found theoretically that the structure collapsing could be a reversible process,and the structure has a seldom effect to the Li migration on kinetic and thermodynamic aspects.The ex-situ XRD measurements verified such results,when the amount of extracted Li is less than 1.1 per formula,the XRD peaks gradually disappeared during the charging process but recovered after fully discharged.Further investigations revealed that the irreversible phase transition of Li2Mn Si O4 may be traced to the oxygen evolution.The influence of oxygen evolution to the de/lithiation was also explored.It was found out that the full lithiation in the oxygen-released structure,could only be achieved at extremely low potentials(<0.4 V)which is unavailable for cathode materials.In that way,oxygen release would not only lead to the irreversible structure change but also hurt the reversible capacity.The Minimum Energy Path(MEP)and transition states of oxygen releasing in the electrochemical active facets on Li2Mn Si O4 were calculated.The oxygen release was revealed to be both kinetic and thermodynamic favorable,and the most difficult step during that process was the rotation of oxygen ions from forming Mn O6ligands to O-O peroxides.Al and P doping were also investigated,and the relationship between charge transfer among surface atoms and the reaction kinetic was establish.The oxygen release was found out to be closely related to the amount of removed electrons,the more electrons were removed from oxygen peroxides,the slower the reaction kinetic of oxygen release.DFT calculations were performed on Li2MnO3,to figure out that the(001),(010)and(110)facets of Li2Mn O3 are all electrochemical active.The extracting of Li ions from all these three facets was investigated.Based on these calculations,the structures of different delithiation states were identified,the MEP and transition states of oxygen release and Mn migration were clarified as well.The calculations indicated that the essential step of oxygen release was the desorption of O-O peroxides,the migration of Mn which leads to the phase transition from Li2Mn O3 to spinel phase was induced by the releasing of nearby oxygen.The changes of Bader charges of surface atoms during the delithiation process were analyzed,revealing that the electric neutrality of the delithiated system was maintained by the removal of electrons from oxygen ions.Base on that recognition,deposition of sulfide,which has higher redox reaction activity than oxygen,may be an effective way to suppress the irreversible oxygen release.DFT calculations revealed that when proper amounts of sulfide are deposited on the surface of Li2Mn O3,SO32-would formed on the(001)facet while SO2x-would formed on the(010)and(110)facets.The SOx species were found out to be stably absorbed on the surface of Li2Mn O3 and had little influence on the extraction of Li ions.Extraction of Li from different facets would change the form of SOx species,SO4 and spinel structure were found to be coexist on the delithiated(001)facet,while typical SO4 tetrahedral model would formed on the delithiated(010)and(110)facets.In all the above conditions,oxygen were found out to be retained during the delithiation,and the Bader charge analysis indicated that sulfide ions were involved in the charge transfer process which stabilize the electronic and geometric structures of oxygen.Modifications through sulfide deposition were carried out on Li-rich layered oxides(LLO)experimentally.Sulfide deposition did not change the morphology and phase constitution of LLO,and SOx species were found on the surface of LLO.The irreversible capacity were significantly reduced on the surface treated samples,besides,the cyclic and rate performance were also enhanced by the sulfide deposition.The ex-situ XPS measurements carried out on both bare LLO and sulfide deposited LLO at different charge/discharge states revealing that oxygen ions on the surface did not participated in the redox reaction in the sulfide deposited samples.