Research On The Low Temperature Properties Of Ternary Li-rich Cathode Materials

Recently,lithium-ion batteries have been widely used due to their high energy density,long cycle life,no memory effect.With the expansion of application areas,especially to the field of aerospace and new energy vehicles,lithium-ion batteries with great low-temperature performance are demanded.However,lots of researches have demonstrated that at low temperature,the degradation of electrochemical performances and lithium plating of electrodes are serious,which greatly limits the further application of lithium-ion batteries.Being the Li source,cathode materials almost determine the energy and power density of lithium-ion batteries,and the cost of cathode materials accounted for more than 1/3 of the battery,so it is of practical significance to improve the low-temperature performance of cathode materials.Li-rich oxides are one of the most promising cathode materials with high working voltage and specific capacity,But,there also exists some problems like discharge voltage attenuation and capacity decay at low temperature.However,to our best knowledge,there are few researches on low-temperature fade mechanisms of li-rich cathode materials and lack of systematic.So extending this work is pressing and valuable.In order to solve these problems,the synthesis process,electrochemical properties,low-temperature fade mechanisms and performance modification of Li_1.55Ni_0.15Co_0.15Mn_0.70O_2.55 are studied in this paper.Effects of technological parameters such as stirring speed and metal ion concentration on the composition and morphology of Ni_0.15Co_0.15Mn_0.70CO₃ precursor are investigated.Effects of the mixing method and lithium salt on the dispersion and lithium content of sintered particles are also analyzed.The results show that when the content of sodium carbonate and ammonia is 120%and 20%of the metal ions,the stoichiometric ratio of carbonate precursors can be well guaranteed if first complexation before precipitation.And the best technological parameter for carbonate precursors with great sphericity and dispersion is using a three-junction bottle as reactor,the stirring speed being 750 rpm,the metal ion solution being 0.3 mol L^-1,1.8 mL min^-11 of the feeding rate.Cathode particles obtained sintering after Ni_0.15Co_0.15Mn_0.70CO₃ mixing Li₂CO₃ at a liquid-phase method show better clean surface,crystallinity,dispersion and consistency.The physical and electrochemical properties of Li_1.55Ni_0.15Co_0.15Mn_0.70O_2.55 materials are tested by XRD,XPS,SEM,ICP,CV,EIS,and galvanostatic method.Effects of low temperature on their electrochemical performance are emphatically investigated.Results show that the as-prepared cathode materials exhibit much dependence on temperature and their electrochemical performances are greatly attenuated with the decrease of temperature.Further analyses indicate there are five limited factors corresponding to such capacity decay at low temperature:the decreased capacity contributed by Li₂MnO₃because of deactivation;more difficulty for Li⁺intercalation into MnO₂;the limited kinetic of Li⁺diffusion because of lattice contraction;the increased electrochemical impendences,especially for R_ct;the decreased D_Li+by 1² orders of magnitude from25? to 0?.As for Li₂MnO₃ deactivation,pre-activation at room temperature is found to be quite effective on the modification of low temperature performance.The results show that after five cycles of pre-cycling at room temperature,the reversible discharge capacity of Li_1.55Ni_0.15Co_0.15Mn_0.70O_2.55 cathodes at 0? increased from 75 mAh g^-1 to135 mAh g^-1.