Binary Transition Metal Oxides Surface Modifications?Morphology Control And Lithium Storage Properties

Li-ion batteries?LIBs?have been used as power sources for the portable electronic devices due to their high energy density,long cycle life,no memory effect and environmental friendliness.To reduce the dependence on the oils and protect the environment,the large-scale applications of the LIBs with higher energy densities for the electric vehicles,hybrid electric vehicles and smart grids are expected.Although the commercial graphite,has been widely used as the anode for the LIBs,its applications in high energy density LIBs have however been hindered by the low theoretical capacity.It is therefore important to develop the alternative anodes with high theoretical capacity for the high energy density LIBs.Transition metal oxides have attracted significant attention because of their high theoretical capacities?600-1000 mAh/g?.In particular,binary transition metal oxides shows enhanced performance for the Li ion storage in comparison to their single metal counterparts due to the complex chemical composition and synergic effects of multiple metal species.However,the use of binary transition metal oxides still faces challenges:first,the volumetric changes of the materials during charge and discharge processes result in the capacity degradation and bad cycling performance;second,the severe decomposition of the electrolyte resulting from the transition metal ionsconsumes the extra active Li ion and thus decrease the reversible capacity;third,the electric conductivity of the materials has to be improved to meet the high energy density/power density requirements.To solve these problems,the following aspects of research have been carried out:?1?Preparation of the Zn_0.33Mn_0.67CO₃ microspheres through a hydrothermal method and the subsequent calcination to obtain the ZnMn₂O₄ microspheres with a hierarchical porous morphology consisting of ZnMn₂O₄ sub-nanoparticles.The obtaioned Zn_0.33Mn_0.67CO₃microspheres show good lithium storage properties.A stable capacity of 723.7 mAh/g at a current density of 400mA/g can be obtained after 200 cycles.The improved electrochemical performance can be ascribed to the hierarchical porous structure,which can increase the accessibility of the electrode materials to the Li⁺storage,leading to a better utilization of the materials.Besides,the porous structure can also reduce the volumetric expansion during cycling.?2?Synthesis of ZnCo-carbonate precursors with various morphologies,the porous micro/nano ZnCo₂O₄ with various morphologies can be obtained after the calcination process.It is found that the structure of the precursors can be influenced by the concentration of NH₄HCO₃.The porous ZnCo₂O₄ microspheres composed of numerous sub-nanocubes shows improved electrochemical performance,which can deliver a capacity of 629.5 mAh/g after1000 cycles at a current density of 2.0 A/g,even at 3.2 A/g,a capacity of 643.6 mAh/g can be obtained.The 3D microspheres framework composed of numerous porous sub-nanocubes can reduce the diffusion length of the Li ion and increase the structural stability.?3?Influence of fluoroethlene carbonate?FEC?on the Li/ZnCo₂O₄ cell has been investigated.It is found that the use of FEC can promote the generation of a stable and uniform surface layer on the ZnCo₂O₄ electrode,resulting in low resistance and better diffusivity of the Li ion,as well as the improved electrochemical performance.The surface layer with less LiF can reduce the resistance and provide better rate performance.Therefore,the addition of FEC can be promising method to improve the electrochemical performance of the transition metal oxides with the formation of a stable surface layer,which is very economic and helpful to promote the development of the LIBs technology.?4?A core-shell structured composite consisting of the porous NiCO₂O₄ microspheres wrapped with graphene oxide is prepared by a facile electrostatic assembly process?NiCo₂O₄@GO?.The obtained NiCo₂O₄@GO shows an initial capacity of 1046.6 mAh/g when used as anode for LIBs,a capacity retention of 77%can be still obtained after 100cycles.The core-shell structure can suppress the aggregation of GO,facilitating the diffusion of Li ion,and the GO layer with intrinsic excellent flexibility can reduce the volumetric changes of the NiCo₂O₄ materials.Additionally,NiCo₂O₄ with GO coating can restrict the decomposition of the electrolyte with the formation of a stable surface layer.?5?A template-free Ostwald ripening method is used for the synthesis of rGO support splode-like NiCo₂O₄?SNHM/rGO?.Graphene oxide is found to play an important role in the formation of the hollow structure.Due to its unique structure,the SNHM/rGO exhibits excellent electrochemical performance.Moreover,full cell coupling SNHM/rGO and commercial LiCoO₂ also shows good performance,suggesting the possibility of the practical use of SNHM/rGO in LIBs.