Synthesis And Characterization Of XLi₂Mno₃·（1-x）LiMo₂（M=Ni,Fe,Al） As Cathode Material For Li-Ion Batteries

With further application of Li-ion batteries, the new cathode materials withhigh capacity, low cost, good cycling performance and safety are beinginvestigated thoroughly. This work aims at developing new Li-rich cathodematerials with no cobalt, low cost and high capacity. In this work, thesynthesis and characterization of xLi₂MnO₃（1-x）LiMO₂（M=Ni,Fe,Al） cathodematerials were studied.A new process of “Sol-spray drying-two step annealing” for preparingLi-rich layered composite materials xLi₂MnO₃（1-x）LiMO₂was put forward.By the process, xLi₂MnO₃yLiFeO₂（1-x-y）LiNiO₂（x≥0.5,y≤0.5） cathodematerials were synthesized. The relation of composition-structure-property forxLi₂MnO₃yLiFeO₂（1-x-y）LiNiO₂was investigated. The results show thatall materials with y≤0.25prepared at800～950℃present pure-NaFeO₂structure. However, LiFeO₂content y greater than0.30can make the suitableannealing temperature range shrink to800～850℃, and impure cubic LiFeO₂appears while annealed at900～950℃due to decreased reduction from bothlayered composition of Li[Li1/3Mn2/3]O₂and LiNiO₂. The optimized annealingtemperature decrease with the content of Li[Li1/3Mn2/3]O₂down and the content of LiNiO₂up. Increasing LiFeO₂content leads to decrease ofdischarge capacity of the composite materials, which may be attributed to itsworse electrochemical active than that of Li[Li1/3Mn2/3]O₂and LiNiO₂. In thecase of electrochemical performance, the suitable composition ofxLi[Li1/3Mn2/3]O₂·yLiFeO₂·zLiNiO₂are optimized to be0.7≤x≤0.8，0.1≤y≤0.25，0.1≤z≤0.2，x+y+z=1.Based on both the electrochemical performance and cost ofxLi₂MnO₃yLiFeO₂（1-x-y）LiNiO₂, Li（Li0.23Mn0.47Fe0.2Ni0.1）O₂（0.7Li（Li1/3Mn2/3）O₂-0.2LiFeO₂-0.1LiNiO₂） was selected and further investigatedthoroughly. The effects of annealing conditions on the structure and propertiesof the products were studied. The results reveal that in the process of“Sol-spray drying-two step annealing” to prepare Li（Li0.23Mn0.47Fe0.2Ni0.1）O₂,both high-temperature annealing and low-temperature retreatment exhibitimportant effects on the structure and properties of products. Increasing theannealing temperature from800℃to900℃gradually results in wellcrystallization, improved2D cation-ordering, decreased interface impedanceand remarkable increased capacity. However, annealing at950℃make the2Dcation-ordering decrease and the crystal particles aggregate seriously, leadingto increased difficulty of Li+diffusion and decreased capacity.Low-temperature retreatment at700℃in O₂after annealing at900℃isbeneficial to improve the layered structure and increase the capacity. Theannealing condition for preparing Li（Li0.23Mn0.47Fe0.2Ni0.1）O₂is optimized to be annealing at900℃for12and followed by retreatment at700℃in O₂for10h. The sample prepared under the optimized process presents an initialcapacity of181.3mAh/g and capacity retention of90.4%after100cycleswhile charge-discharged at10mA/g in the voltage range of2.5⁴.7V. Thesample also exhibits better electrochemical performance at elevatedtemperature and high rate.New Li-rich material Li（Li₀.25Mn₀.5Al₀.05Ni₀.2）O₂was synthesized by using“Sol-spray drying-two step annealing” process, and the effects of annealingconditions on the structure and properties of the products were studied. Theresults indicate that the process can be used to prepareLi（Li₀.25Mn₀.5Al₀.05Ni₀.2）O₂with well structure, morphology and betterelectrochemical performance. Increasing the annealing temperature from750℃to900℃gradually can improve the layered structure and crystalgrowth, decrease interface impedance, and increase the discharge capacity ofthe product. Annealing at950℃leads to decrease of2D cation-ordering,formation of bigger particles from aggregation of crystal particles, whichcontributes to the increased interface impedance and decreased capacity. Thesample prepared at the optimized annealing temperature of900℃presents aninitial capacity of285.4mAh/g when charge-discharged at10mA/g in thevoltage range of2.5～4.7V. It also shows better cycling performance, and thecapacity still remains279.4mAh/g after20cycles. Compared withLi（Li0.23Mn0.47Fe0.2Ni0.1）O₂, however, Li（Li₀.25Mn₀.5Al₀.05Ni₀.2）O₂exhibits larger interface impedance, leading to more capacity loss at high rate.