Phase Control And Reaction Mechanism Of 0.5li₂Mn O₃·0.5LiMn_0.5Ni_0.5O₂-based Cathode Materials For Li-ion Batteries

Benefited by the much higher absolute storage capacity than nickel-cadmium battery and lead acid battery,Li-ion batteries are now used as the most popular secondary batteries.In the past 20 years,Li-ion batteries have been successfully applied to small-scale mobile electricals such as laptops and smart phones.At this stage,considering the increasing pressure on the environment and energy issues,people starts to conceive Li-ion batteries with much higher energy density which can satisfy the requirement of large-scale mobile electricals e.g.all-electric vehicles(EVs or hybrid electric vehicles（HEVs）.However,restricted by the energy density and cost of cathode materials,this conceive is still under great challenges,commercialized EVs are still high-cost and with relative short cruising distances.Recently,a new kind of cathode materials,named as lithium-rich transition-metal oxides（LLO）have caught much attentions.LLO,with the formula of x Li₂Mn O₃·（1-x）Li MO₂（M=Mn,Ni,Co）,have an energy density of about 900 Wh kg^-1when cycled between 2.0-4.8 V;the fabrication cost is also much lower than that of nick rich layered cathode materials,which have been currently considered as the best commercialized cathode materials in EVs.However,drawbacks such as the low cyclic stability,low rate property and significant voltage decay during long-term cycles have yet restricted the application of LLO.A novel polyols method have been utilized into the synthesizing of LLO/spinel composites.Compared with traditional methods,this method is much sample and the phase constitution can be controlled by temperature adjusting.Detailed analyses on the phase constitution,element ratio and valence of transition metals of samples obtained at different temperatures revealed that LLO is formed by the decomposing of Li-rich spinel phase.Thus,a“Li-rich spinel phase decomposition”phase transition mechanism was proposed to explain the formation of such composites.After comparing the electrochemical properties of LLO/spinel composites obtained at different temperatures were found to be affected by the ratio of spinel to layer phases,the leach out of rock salt phase,and the change of crystallinity and particle size.Product with improved cyclic and rate performance was achieved by annealing at 700℃ for 12 h,with a discharge capacity retention of 70%at 0.2 C after 60 cycles and discharge capacity of about 170 m A h g^-1at 2 C.The in/ex-situ XRD measurements were combined to have an in-depth understanding of the above phase transition mechanism.Three phase transitions during the decomposition reaction of spinel phase have been revealed,namely,Li-rich spinel（S_L）to LLO（L）,normal spinel（S_N）to rock salt（R）and rock salt to LLO.Density functional theory calculations have revealed spontaneous structure evolutions in the oxygen released Li rich spinel and normal spinel.Li ion near the oxygen vacancy would migrates from the 8a tetrahedral site to the interstitial 16 c octahedral site,forming quasi-Li₂Mn O₃ and quasi-rock salt crystals,respectively.Such structure evolution may be the essence of the above three phase transitions.The thermodynamic and dynamic driving forces of each reaction have been determined through experiments and DFT calculations.The phase transition from S_L to L have higher thermodynamic and dynamic driving forces than the phase transition from S_N to R,and the phase transition from R to L is energetically favorable once excess Li is provided.The reaction priority has been utilized to design LLO/spinel composites with different phase constitutions,the capacity,rate property and cyclic stability of the obtained LLO/spinel composites are changed along with the phase constitution.Synthesize a modified LLO,which is coated with a non-stoichiometric layer（AM-LLO）to suppress the voltage decay.The valence and content of elements on the surface were analyzed,to find out the non-stoichiometric layer is rich in Ni and Li,along with enormous oxygen vacancy.The electrochemical behaviors of AM-LLO and LLO synthesized by traditional co-precipitation method（LLO）were compared,to reveal that the non-stoichiometric layer effectively suppressed the irreversible oxygen release during the first cycle of charging,and the subsequent phase transition from LLO to spinel phase was consequently restrained.The capacity of AM-LLO is lower than that of LLO,however,the cyclic stability is remarkably improved and the voltage decay has been significantly suppressed in AM-LLO.