Research Of Li-rich Cathode Materials For Lithium-ion Batteries

The improvement of mobile electronics and electric vechicles demand better lithiumion batteries. Present commercial cathode materials cannot meet their further requirementon capacity and cost. Li-rich cathode materials attract much attention, because they havehigh energy density and stable cycling performance.A series of layered0.6Li[Li_1/3Mn_2/3]O₂-0.4LiNi_0.5-xMn_0.5-xCo_2xO₂（0≤x≤0.5） materialswere prepared by carbonate coprecipitation. The variation of the structural, morphologicaland electrochemical characteristics of these materials with Co content （x） was investigated.Each sample is single phase with layered structure in space group R-3M. The latticeparameters of a, c and V decrease, while the c/a ratio increases with the increase of Cocontent. This result suggests that higher cobalt content facilitates the formation of betterhexagonal structure, which is beneficial to the electrochemical properties. However, theincrease of Co content is always accompanied by the decrease of Ni and Mn content, whichleads to a decrease in Ni²⁺/Ni⁴⁺redox couple and an increase in structure instability. Thus itis detrimental to the charge/discharge capacity and cycling stability. The former positivefactor manifests itself when the cobalt content is relative low （x≤0.12）, while the latternegative factor becomes dominant when the cobalt content becomes higher （x≥0.25）.Therefore, only the materials with low cobalt content of0.03≤x≤0.12have high capacity,good rate capacity and excellent cycling stability. The0.6Li[Li_1/3Mn_2/3]O₂-0.4LiNi_0.38Mn_0.38Co_0.24O₂（x=0.12） sample shows the highest initial capacity of218mAh·g-1and100%capacity retention over fifty cycles, which is the optimum cobalt content in the series.0.6Li[Li_1/3Mn_2/3]O₂-0.4LiNi_0.38Mn_0.38Co_0.24O₂samples were prepared by sintering at800℃、850℃、900℃, respectively. The highest capacity and best cycle performance wereobtained by the sample sintered at850℃, thus850℃was selected as the optimum sinteringtemperature. The variation of the structural, electrochemical characteristics of LiyNi_0.552Mn-_0.152Co_0.096O₂with Li content（y） was also investigated. The sample with y=1.3showed theminimum Li/Ni disorder, the highest capacity, excellent cycle performance and good ratecapability.A series of zLi[Li_1/3Mn_2/3]O₂-（1-z）LiNi_0.38Mn_0.38Co_0.24O₂（z=0.4、0.5、0.6）weresynthesized by co-precipitation. The change of the structural, morphological and electrochemical characteristics of these materials with Li[Li_1/3Mn_2/3]O₂content wasinvestigated. SEM observation indicates that the size of primary particles increases with anincrease in Li[Li（1/3）Mn_2/3]O₂content. Electrochemical testing shows that the firstcharge/discharge capacity, irreversible capacity and cycle performance increases, while ratecapability decreases with an increase in Li[Li（1/3）Mn_2/3]O₂content. Taking both the cyclingperformance and the rate capability into account,0.5Li[Li_1/3Mn_2/3]O₂-0.5Li Ni_0.38Mn_0.38Co-_0.24O₂has the best electrochemical performance.