Facet Degradation And Building Stable Surface Structure For Lithium-rich Layered Cathode Materials

Lithium-rich Layered oxider cathode materials（LLOs）are highly potential cathode naterials for high-energy,high-safety,and low-cost batteries due to their high specific capacity,relatively low cost,and high thermal stability.However,LLOs suffer from low nitial coulombic effeicicency,capacity decay and voltage decay,which hinders its practical application in large scale.Paritical degradation is the main reason for the performance decay,which is likely to be closely related to the facet orientation of the naterial.Therefore,in this thesis,we first systematically study the facet degradation in hermal and electrochemical.Inspired by the facet degradation mechanism,we proposes to milding a multifunctional surface layer structure by a composite gas-solid surface reatment method to suppress the facet degradation,thereby improving the performance stability.For building stable surface structure,two different composite gas-solid surface reatment methods were applied to treat nickel-rich and manganese-rich LLO,respectively.The details are as follows:（1）About the facet degradation.We prepared a flake-like LLO and using scanning electron microscope,aberration-corrected scanning transmission electron microscopy（STEM）,In-situ high temperature XRD and soft X-ray absorption spectrum to explore its crystal-facet degradation behavior in both thermal and electrochemical processes.We found that void-induced degradation behavior of flake-like LLO in different facet is strongly dependent on crystal facets,and more importantly,depends on voltage.When voltage is greater than 4.5 V,Li2MnO3 is activated,void degradation originates from side facets,such as the（010）facet,and oxygen loss,while the close（003）facet is stable;when voltage is less than 4.4 V,the lattice oxygen activity is not active,the void-induced degradation shows isotropy.These results are further understood through ab initio molecular dynamics calculations,which shown that oxygen atoms escpased from the（010）facet.More interestingly,we aslo found that the electrochemical degradation behavior is fully similar with the thermal degradation.By comparing materials with different（003）facet ratios,it was found that although the high stable face ratio help in improvement electrochemical stability,the degree of improvement was still limited.Therefore,new solutions need to be search.（2）Building integrated spinel/rock salt phase and fast ion conductivity surface layer structure.Inspired by the facet degradation,surface/interfacial engineering is critical for preventing the particle degradation of LLOs,especially facet degradation to improve the electrochemical performance.Here,P2S5 was used to conduct composite gas-solid surface treatment of nickel-rich LLO with high Ni/Mn to build a stable material surface interface.After surface modification,the surface layer of LLO is transformed into a spinel/rock-salt composite structure（in which the spinel phase is derived from the Li2MnO3 C2/m phase,and the rock-salt phase is derived from the Ni-rich R-3m phase）,and the amorphous fast lithium ion conductor is formed on the surface.This reconstructed surface has a highly stable surface layer structure,which significantly improves the capacity retention and suppresses voltage decay.the surface modified LLO cathode exhibited extremely capacity retention of 80%after 1000 cycles,and even a high capacity retention of 69.6%and a low discharge medium voltage with a decay rate of 0.44 mV cycle-1 after 2,000 cycles.The stability of the surface layer was confirmed from the structural and morphological changes after prolonged cycling by the TEM analysis,which shows that the significant improvement in electrochemical performance origin from improvement in the stability of（010）active facet.（3）Building N dopping amorphous carbon and a spinel layer surface structure.The the manganese-rich LLO with low Ni/Mn ratio was treated by thiourea composite gas-solid surface treatment method.The composite gas produced in the thermal decomposition process of thiourea interacts with the surface of the material to form N dopping amorphous carbon layer and a surface spinel layer.After surface modification,this composite surface layer effectively improves the electrical conductivity/lithium ion conductivity of the material and surface facet.Therefore,the capacity and cycling performance of the modified materials have been greatly improved.The cycle stability at 1C was improved significantly.The unmodified sample,the discharge capacity is 177.3 mAh g-1,which decrease to 40.4 mAh g-1 after 500 cycles,and the capacity retention rate is only 22.3%.However,after modification,the capacity increased to 207.7 mAh g-1,and there was still 180.3 mAh g-1 after 500 cycles,and capacity retention was also as high as 87%.The voltage decay performance is also somewhat improved,from 2.87 mV cycle-1 decay to 1.26 mV cycle-1.And through the comparative study,it is found that the lithium-rich Li4Mn5O12 spinel is formed,which inhibits the formation of the electrochemically inactive Mn3O4.It shows that after the protection of the surface,the formation of surface porosity is inhibited,which helps to form an ordered lithium-rich Li4Mn5O12 type spinel,thereby ensuring that the capacity does not decay,but it cannot prevent voltage decay.In addition,it was found that after the surface treatment was modified by the high potential greater than method,a new redox peak appeared at the higher voltage,which was continuously enhanced with the cycling.