Chi Hoon Song

The effects of sulfur doping on the performance of O3-Li0.7[Li1/12Ni1/12Mn5/6]O2 powder

Li0.7[Li1/12Ni1/12Mn5/6]O2 and Li0.7[Li1/12Ni1/12Mn5/6]O2-ySy (y=0.1, 0.2, 0.3) powders were synthesized by using a sol-gel method. As-prepared samples showed typical rhombohedral O3 layered structure. The shape of the initial discharge curve for the samples was almost equal to that of the layered structure. However, the electrode materials were transferred from layered to spinel structures with cycling. At the first cycle, Li0.7[Li1/12Ni1/122Mn25/6]O2 and Li0.7[Li1/12Ni1/12 Mn5/6]O1.9S0.1, Li0.7[Li1/12Ni1/12Mn5/6]O1.8S0.2, and Li0.7[Li1/12Ni1/12Mn5/6]O1.7S0.3 delivered the discharge capacities of 238, 230,224, and 226 mAh/g, respectively, with their capacity fading rates of 0.34, 0.21, 0.12, 0.25%/cycle, respectively. The partial substitutions of Ni and S for Mn and O in Li0.7[Li1/12Ni1/12Mn1/12]O2 significantly enhanced the electrochemical properties of the lithium manganese oxide materials.

Structural and Electrochemical Characteristics of Li0.7(Li1/6Mn5/6)O2 Synthesized using Sol-Gel Method

Layered O2-lithium manganese oxide (O2-Li0.7[Li1/6Mn5/6]O2) was prepared by ion-exchange of P2-sodium manganese oxide (P2-Na0.7[Li1/6Mn5/6]O2)· P2-Na0.7[Li1/6Mn5/6]O2 precursor was first synthesized by using a sol-gel method, and then O2-Li0.7[Li1/6Mn5/6]O2 was produced by an ion exchange of Li for Na in the P2-Na0.7[Li1/6Mn5/6]O2 precursor. Structural and electrochemical analyses suggested that good quality O2-Li0.7[Li1/6Mn5/6]O2 was prepared from P2-Na0.7[Li1/6Mn5/6]O2 synthesized at 800 °C for 10 h using glycolic acid as a chelating agent. During the cycle, the discharge profile of the synthesized samples showed two plateaus at around 4 and 3 V, respectively, with a steep slope between the two plateaus. The discharge curve at 3 V escalated with an increase in the cycle number, presenting a phase transition from a layered to a spinel like structure. The sample prepared at 800 ‡C for 10 h using glycolic acid delivered a discharge capacity of 187 mAh/g with small capacity fading.

The Effects of Ni Doping on the Performance of O3-Lithium Manganese Oxide Material

Li0.7[Li1/6Mn5/6]O2 and Li0.7[Li1/12Ni1/12Mn5/6]O2 powders were synthesized by a sol-gel method. The powders had a typically rhombohedral layered O3 structure. Both the samples were nanometer-sized powders and the size of Li0.7[Li1/12Ni1/12Mn5/6]O2 was smaller than that of Li0.7[Li1/6Mn5/6]O2. The discharge curve shape of both the sample electrodes was almost equal to that of the layered structure. However, the electrode materials were transferred from layered to spinel structures with increasing the cycle number. Li/Li0.7[Li1/6Mn5/6]O2 and Li0.7[Li1/12Ni1/12Mn5/6]O2 cells initially delivered a discharge capacity of 261 and 238 mAh/g, respectively. The capacities of Li/Li0.7[Li1/6Mn5/6]O2 and Li0.7 [Li1/12Ni1/12Mn5/6]O2 after the 45th cycle were 174 and 221 mAh/g, respectively, corresponding to the retentions of 67% and 93%. The nanostructure of the synthesized powders seems to result in high initial discharge capacity as well as in the suppression of the discharge capacity fading by providing high surface area needed for Li ion reaction. In Ni doped-Li0.7[Li1/12Ni1/12Mn5/6]O2, the capacity fading was reduced by suppressing the oxidation state of Mn from 4+ to 3+ due to the role of Ni ion doped.