Study On Synthesis And Electrochemical Properties Of Li[Li_0.23Ni_0.15Mn_0.62]O₂ Lithium-rich Cathode Material

Recently, the Li-rich layered oxides are becoming appealing as the cathode materials of energy storage batteries and power Li-ion batteries. The reason is that they possess high specific capacity?>200 mAh/g?, wide voltage range, lower cost and low toxicity. However, this type of material also has inherent drawbacks such as high first cycle irreversibility, capacity fade upon prolonged cycling. In this paper, we mainly studied the electrochemical behavior and the reaction mechanism of Lithium-rich layered oxide Li[Li_0.23Ni_0.15Mn_0.62]O₂ as the cathode material in lithium-ion batteries. The effect of Co doping in Li[Li_0.23Ni_0.15Mn_0.62]O₂ material on the electrochemical properties has also been investigated. Specific contents are as follows:Lithium-rich layered oxide Li[Li_0.23Ni_0.15Mn_0.62]O₂, which also can be written as 0.6Li₂MnO₃·0.4LiNi_0.5Mn_0.5O₂ or 0.9Li[Li_1/3Mn_2/3]O₂·0.4LiNi_0.5Mn_0.5O₂, is synthesized using hydrothermal method following with high temperature solid state synthesis method. Its crystal structure and electrochemical behavior as the cathode material in lithium-ion batteries are studied. A reaction mechanism is proposed to interpret its unique electrochemical behavior shown in the first charge–discharge cycle. It includes four reactions:?1? LiNi_0.5Mn_0.5O₂ ? Li⁺ + Ni_0.5Mn_0.5O₂ + e^-,?2? Li[Li_1/3Mn_2/3]O₂ ? Li⁺ + [Li_1/3Mn_2/3]O₂ + e^-,?3? [Li_1/3Mn_2/3]O₂ ? 1/3 Li⁺ + 2/3 MnO₂ + 2/3 O· + e^-, and?4? Li⁺ + Ni_0.2Mn_0.8O₂ + e^- ? LiNi_0.2Mn_0.8O₂. The extraction of oxygen atoms?O·? in the reaction?3? results in the crystal structure rearrangement. Based on this hypothesis, it is found that the expected capacity of activated lithium-rich layered oxide xLi₂MnO₃·?1-x?LiNi_0.5Mn_0.5O₂?0 x 1? increases from 230 to 280 mAh/g with increasing x value. Li[Li_0.23Ni_0.15Mn_0.62]O₂ has an expected total first charge capacity of 396 mAh/g, but its expected capacity is only 247 mAh/g due to an initial capacity loss caused by the oxygen loss. Experimentally, within a charge–discharge voltage window from 2.0 to 4.8 V, Li[Li_0.23Ni_0.15Mn_0.62]O₂ delivers a charge capacity of 310 mAh/g and a discharge capacity of 215 mAh/g respectively at 40 mAh/g during the first cycle. It retains a discharge capacity of 185 mAh g-1 after the 50 th cycle.The EIS results shows that the electrochemical kinetic behavior of Li[Li_0.23Ni_0.15Mn_0.62]O₂ is controlled by the charge^-transfer process rather than by Li⁺ diffusion or blockage of solid-electrolyte interphase?SEI? layers at the end of Li⁺ extraction in the first charge. The apparent Li⁺ diffusion coefficient calculated by CV is 3.38×10^-10 cm²/s for a fresh cathode material, and decreased to 6.28×10^-11 cm²/s after 50 cycles.Lithium-rich layered oxide Li[Li_0.23Ni_0.15Mn_0.54Co_0.08]O₂ was also synthesized successfully by doping Co in the Li[Li_0.23Ni_0.15Mn_0.62]O₂. It also keeps well layered structure. Compared with Li[Li_0.23Ni_0.15Mn_0.62]O₂, it's initial discharge capacity at 20 mA g-1 is up to 238.5 mAh/g from 224.7 mAh/g in the voltage range of 2.0⁴.8 V. And the initial coulombic efficiency also increases to 76.6% from 72.0%. Li[Li_0.23Ni_0.15Mn_0.54Co_0.08]O₂ exhibits better rate capacity. It's apparent DLi⁺ calculated by CV remains almost the same even upon several continuous cycling more than Li[Li_0.23Ni_0.15Mn_0.62]O₂ would. This result shows that Li[Li_0.23Ni_0.15Mn_0.54Co_0.08]O₂ is more stable.