Preparration and Electrochemical Characteristics of the LiMn₂O₄/Marimo Carbon Composite

1. INTRODUCTION Electrode of a lithium ion battery is produced of the mixture of an active material, a conductive agent and a binder. The dispersal state of these components significantly affects battery performance. Generally, carbon black is used to improve conductivity, but it tends to aggregate because of its fine particles, and homogeneous dispersion requires a complicated process. The heterogeneous dispersal state in the electrode increases the internal resistance of the battery, which causes a reduction in battery performance. In this research, we used a new carbon material called “Marimo carbon”(MC) as a conductivity enhancer to achieve an ideal dispersal state. MC has an oxidized diamond core, and a spherical structure in which nanocarbon fibers are radially intertwined with each other in a complicated fashion. (1) Between carbon fibers of MC, there exist vacant spaces of a few hundred nanometers. By depositing an active material into the vacant spaces of the MC fiber network, good contact between the active material and carbon and uniform distribution in the electrode are expected to be achieved. Furthermore, vacant spaces are sufficiently large to allow water molecules to pass unlike conventional carbon materials, and it is easy to handle in solution even in an untreated state. LiMn2O4 (LMO) is used as the active material in this research, in which a composite of LMO and MC formed by hydrothermal treatment was examined. 2. EXPERIMENTAL MC synthesized by chemical vapor deposition and grown by heating a Ni/oxidized diamond catalyst at 550 °C for 3 h in methane gas, which acted as a carbon source. LMO/MC composites were synthesized using a hydrothermal method. In the synthesis, MC was dispersed in 100 cm3 deionized water by ultrasonic vibration for 2 h. Then, 1.58 g KMnO4 was added to the above mentioned suspension, and the solution was mixed using a magnetic stirrer for 1h. Next 0.63 g LiOH・H2O and 5 mL ethanol were added to the solution before stirring for additional 1 h. Finally, the stirred solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed and placed into an electric oven at 180 °C for 5 h. (2)After hydrothermal treatment, the produced LMO/MC composite was collected and dried in vacuo. For comparision, pure LMO was also prepared by the hydrothermal method without the incorporation of MC. Electrochemical measurements of the composite were performed using CR2032 type cells. In the preparation of working electrodes, a mixture of either a LMO/MC composite or LMO + acetylene black(AB) and LMO/MC + AB was pasted onto an aluminum foil with polyvinylidene difluoride (9:1 wt.%). A lithium foil was used as the counter electrode, and a solution of 1 M LiPF6 in a mixture ethylene carbonate and diethyl carbonate (v/v= 1:1) was used as the electrolyte. 3. RESULTS AND DISCUSSION The results of constant galvanostatic charge/discharge tests in the range of 0.1 C to 2 C are presented below. The discharge capacity of the LMO/50 wt.% MC composite electrode at 0.1 C was 113 mAh g-1, it was not obtained capacity at 2 C. The discharge capacity of the LMO + 10 wt.%AB electrode at 0.1 C was 112 mAh g-1, and that at 2 C was 74 mAh g-1. The discharge capacity of the LMO/5 wt.%MC + 5 wt.%AB electrode at 0.1 C was 123 mAh g-1 , and that at 2 C was 83 mAh g-1. Assuming that the discharge capacity at 0.1 C is the full capacity of the electrode, the LMO/50 wt.%MC composite electrode retains 0 %, LMO + 10 wt.%AB electrode retains 67 %, and the LMO/5 wt.%MC + 5 wt.%AB electrode retains 68 % of its full discharge capacity at 2 C. In addition, the LMO/5 wt.% MC + 5 wt.%AB electrode exhibited a higher capacity than other electrodes at all rates ; thus, it can be considered to have the best rate characteristics. (1) K. Nakagawa et al., J.Mater Sci. 44, 221-226 ( 2009) (2) B. Lin, et al., J.Solid State Chem. 209, 23–28 (2014)