| Under the dual pressure from energy crisis and environmental pollution,it is extremely urgent to develop new energy and improve energy utilization efficiency.As an efficient energy storage device,Li-ion battery has attracted people’s attention.It has been widely used in portable devices and gradually expanded into the field of power batteries.Positive electrode material,which affects the key technical parameters of Li-ion battery(LIB),is under developing toward higher energy density direction.Facing with such an opportunity and challenge,layered Li-rich Mn-based oxide(LRO)material is a good candidate for the next generation of cathode material due to the high capacity and low cost despite some drawbacks.To boost the practical process of LRO in LIB,this thesis has probed the relevant strategy to suppress its voltage decay.From the viewpoint of kinetics related factors,we put forward a new understanding of the voltage decay mechanism.We also attempt to decrease the charging voltage of LRO.And at last,this paper explored new Li-rich materials with low cost.Using synchrotron radiation X-ray absorption spectroscopy(XAS)as an effective tool,relationship between structure and properties of these new materials were studied.The content of this thesis includes the following four parts:1.The Re/B co-modified layered LRO cathode material was prepared successfully by sol-gel method,to improve the retention of capacity and mitigate the voltage decay.The voltage retention rate of Re doped samples after 300 cycles under a current density of 2C was 81%,while that of the original samples was only 62%.Through detailed structural characterizations,it can be concluded that Re is successfully introduced into the Mn site in the transition metal layer.By using the XAS and the first principle calculations,the improved performance especially the suppression of voltage decay,is attributed to structural variation around doping Re that the introduction of Re modulate the local electronic structure of the nearby O.This relaxation of the local structure can stabilize the oxygen in the lattice,rather than release it as O2 gas which leads a structural damage.Therefore,the use of rhenium to modulate the electronic structure of LRO is of great reference significance for the design of other cathode material with high specific capacity and high stability.2.It is proposed that the kinetics deterioration is the key factor affecting the voltage decay of LRO,based on the structural characterizations and electrochemical tests.Furthermore,by combining the raw material with metal halide such as MgCl2,the simulation material with high discharge specific capacity but serious voltage decay was prepared.The results show that cubic rock-salt phase with no lithium generated on the particle surface after cycling.The cubic rock-salt phase with poor electron/ion conduction,significantly increased the barriers of lithium ion migration at the particle surface,leading to worse kinetics performance on the surface and severe voltage hysteresis.As the cycling going on,it gradually got worse and developed to voltage decay.Thus inhibiting the formation of rock-salt structure on the surface of particles is a new effective strategy to suppress the voltage decay of high-capacity and high-stability materials,which is a key to promote their commercialization.3.In order to reduce the charging voltage of LRO,the anionic doping has been studied by conventional preparation methods.It has been found that it is hard to substitute the lattice O2-with Cl-ions under normal temperature and pressure.The preparation at high temperature and high or non-equilibrium state that exceeds the thermodynamics energy barrier may be the breakthrough to realize the anionic doping of LRO.It provides an important reference for the exploration of anionic doping of LRO.4.Lithium-rich cathode material of Li1.23Mo0.47Fe0.3O2was prepared by solid phase method.The initial discharge capacity of Lii.23Mo0.47Fe0.3O2was 176.69 mAh/g,and the capacity retention was up to 74.66%.To explore the influence of cut-off voltage on its electrochemical performance,ex-situ XRD and XAS were used to characterize the charging and discharging process of the material.The results show that the low voltage platform has a certain modulation effect on the platform of 2.5 V.As a new cathode material with low cost,simple preparation and good performance,it has made a meaningful exploration of the cathode material for the next generation high-performance LIBs. |
