Preparation And Interfacial Stability Of High Energy Density Lithium Cobaltate Materials Of Lithium Ion Batteries

It is one of the most promising strategies to broaden the lithium-ion batteries commercial process by improving their energy density.In this paper,we focus on the development of high voltage performance of lithium cobalt oxide(LCO)in order to achieve higher energy density.The coating/doping modification,electrolytes additives of LCO are systematically studied as well as the rational design of battery system.The structural stability,interfacial stability and the optimal design of commercial lithium cobalt oxide batteries are explored.The main research contents are listed as follows:ABSTRACT(1)Firstly,the influences of doping/coating modification on the electrochemical performance of LCO are systematically studied.The results showed that the cut-off voltage of LCO can be largely improved by doping/coating methods.In this way,the severe phase transition and the instable interfaces caused by the enlarged voltage were suppressed and thus LCO with higher energy density was achieved.Besides that,the side reaction between LCO and electrolyte can be also greatly weakened.Moreover,lanthanum(La),as a new doping element,was introduced into LCO cooperated with traditional Ti,Mg and Al doping strategy so that the high voltage performance of LCO can be further improved.It showed that multicomponent co-doping could effectively inhibit the phase transition,enhance the structural stability and the thermal stability.The specific capacity retention of the LCO full cell with Ti,Mg,Al and La maintained at 80.9% after 300 cycles at 1C/1C(45℃).As a contrast,the conventional commercial LCO materials was only 68.2%.(2)Secondly,we studied the effective methods to improve the interfacial stability of high-voltage LCO and thus different electrolyte additives were filtrated.The results showed that a certain amount of dihydro-1,3,2-dioxazolethiophene-2,2,5,5-tetraoxide(DDDT)or trimethylsilane dimethylphosphonoacetate(DMTMSP)and(2,3,4,5,6-pentafluorophenyl)methanesulfonic acid(PPMS)can facilitate the formation of a stable solid electrolytes interfaces(SEI)film on the cathodic electrode surface.Therefore,the direct contact between the cathode and the electrolyte was avoided.Moreover,the oxidation gas generation of the solvent was largely suppressed during the cycling process,and thus the cycling performance of the full battery can be successfully improved.Compared to the fresh electrolyte system,the modified electrolyte with a certain electrolyte addition delivered much better electrochemical performance at room temperature.In specific,the capacity retention of the coin cell with 2 wt.% DDDT addition increased from 62.1% to 84.5% after 250 cycles.The capacity retention of the full cell with 0.5 wt.% DMTMSP additive was 90.5% after 400 cycles.The capacity retention of the full cell containing 1 wt.% PPMS electrolyte additive was as high as 81.5% after 500 cycles.(3)Thirdly,we also explored the bulk phase and electrode/electrolyte interface structure of lithium cobalt oxide by TEM and XPS,focusing on the evolution of morphology and structure.The working mechanisms of electrolyte additives were revealed by XPS test in qualitatively/semi-quantitatively analysis.The structural evolution of anode/cathode-electrolyte interface was also investigated.The results showed that the interfacial films formed on the surface of high-voltage LCO were different with different electrolyte additives and exhibited different effects on the electrochemical performance.The compositions and the contents of the interface film were analyzed as well.It was found that the cycle failure was maily caused by the variation of cathodic structure and the increased surface impedance.(4)Finally,the graphite anode(FG-1)with high capacity and C-rate was prepared by the surface modification of NFG.When matched with the modified LCO cathode and employed high-voltage electrolyte additives,the cell with a volumetric energy density as high as 800 Wh/L was successfully prepared under the help of Highpower Technology Co.,Ltd.,batteries with.No lithium deposition in the negative electrode can be found for the full cells after 10 cycles at 0.3 C/0.5 C at 0℃.There is also no gas production after continuous charge-discharge 12 days at 60 ℃.The capacity retention of the optimized full cells was 87.3% after 1000 cycles at 1 C/1 C current density.The as-prepared battery showed excellent high and low temperature performance,long cycling stability and high safety,demonstrating the promising application prospects for industrialization.