Study On The Application Of High Specific Energy And Composite Cathode Materials

Developing power batteries with high specific energy is an urgent need to equip electric vehicles with extended driving ranges for better application.However,it is difficult for one single cathode material to meet all requirements of power batteries,like high specific energy,reliable safety,stable cycle life,low cost,etc.To this end,strategies of blending different materials as cathode mixtures are adopted to balance the electrochemical performances and all other requirements.Presenting an olivine structure with high stability,Li Mn PO₄(LMP)can significantly improve the poor safety of Li Ni_1-x-yCo_xMn_yO₂(NCM)materials when blended together.Also,the operation potential of Li Mn PO₄,4.1V(vs.Li⁺/Li),falls in the stable voltage operating window of current commercial electrolytes.These in theory make LMP a good partner with NCM.Meanwhile,the cheap price of LMP also further reduce the overall cathode material costs.Otherwise,further developments of LMP are restricted by its low electronic conductivity and low Li⁺diffusion coefficient,which fortunately can be enhanced by modification methods,such as controlling nanoparticles granularity,coating carbon,and doping Fe²⁺.In this thesis,the modified Li Mn_0.75Fe_0.25PO₄(LMFP)cathode material was coupled with graphite(C)anode to prepare LMFP-C full batteries,then the performance of LMFP battery was compared with that of LFP battery.Results show that LMFP battery is advantageous in energy density,voltage platform and low temperature performance,but less competitive in high temperature performance and cycle life.To improve the safety of high energy batteries,this thesis has also made efforts to combine Li Mn_0.75Fe_0.25PO₄(LMFP)and the Li Ni_0.6Co_0.2Mn_0.2O₂(NCM)materials together,aiming for synergistic effects from each individual component therefore enhanced overall performances,which has been confirmed by experimental results.The composite battery can effectively reduce the short-circuit current and temperature rise of the battery during the nail test,thus reducing the risk of thermal runaway.In addition,when the composite ratio of LMFP is increased to above 30%,it is found significant rise of the battery direct-current resistance(DCR),reduced overcharge time,the state of charge(SOC),temperature rise as well as the consequent risk of thermal runaway once overcharging occur.In terms of electrochemical performance,the discharge curve of the composite battery is basically the same as that of the NCM battery when the proportion of LMFP to the composite battery is below 30%.At the same time,the low temperature performance and rate discharge performance of the composite battery are significantly improved compared with the NCM battery,and the optimal composite ratio(30%LMFP in this thesis)can significantly improve the cycling performance of the original NCM battery.Furthermore,blending Li Mn_0.75Fe_0.25PO₄ with Li_1.44Mn_0.544Ni_0.136Co_0.136O₂ has also been performed,and the resultant mixture performances were investigated with three types of high-voltage electrolytes.The charge and discharge curve of the composite battery is an approximate oblique line in the voltage window of 2.7-4.6V.Although the general electrolyte without any additives has better comprehensive performance,it has no discharge performance at-20?.Relatively the fluorinated electrolyte containing flame retardant additives has the lower rise of temperature and the more reliable safety,but the performance of high temperature and cycle life are poor.In a word,the composite battery is still with some problems,such as significant gas expansion.Therefore,it is necessary to develop a high-voltage electrolyte to exploit the full potential of the mixture cathode.The results of this thesis have been verified in practical vehicle test,and demonstrated valuable for commercialization.The results also offer additional cathode opportunities,i.e.blended cathode mixtures,It can be used as a reference for the battery industry in the selection of cathode material system,which is a kind of cathode material of LMFP composite NCM,in addition to the conventional single cathode material.With both high energy density and high safety,the composite cathode is a competitive material system,well balanced in many aspects,therefore promising to meet different needs by adjusting the proportion of compounding.