Synthesis of Si/Marimo Carbon Composite and Its Charge-Discharge Performance for a Lithium Ion Battery
Authors: Hasegawa Kota , Gunji Hiroyuki , Eguchi Mika , Ando Toshihiro 
Published: December 2020

1. Introduction
The theoretical capacity of graphite as a lithium ion battery (LIB) anode (372 mAh g-1) is not enough sufficient for electric vehicles (EVs). Si can include Li ions into its crystal lattice with a charge voltage as low as that of graphite. The use of Si for LIB applications has attracted considerable attention because of its high capacity (3579 mAh/g). However, insertion of Li ions into Si crystal lattice causes a large volume change (approximately about 300 %), which produces many cracks in the Si crystal during charge-discharge processes. Although carbon black is used to improve the conductivity of Si, a large volume change could break the electron conduction networks. This phenomenon makes Si anodes not viable for LIBs, particularly due to the shortening of battery lifetime. In this research, we applied a new carbon material, Marimo carbon (MC), as a conductivity enhancer. MC has a spherical structure comprising carbon nanofilaments (CNFs) growing radially from an oxidized diamond core. The CNFs are intertwined with each other in a complicated fashion. Moreover, there are vacant space volumes of a few hundred nanometers between the CNFs (1). We attempted to synthesize Si/MC composites in which Si particles existed in the space volumes between the CNFs. These space volumes can minimize the influence of volume changes of Si particles. Si/MC composites can maintain a uniform electron network during charge-discharge processes (2).
2. Experimental
Oxidized diamond supported Ni catalyst (Ni-O-diamond, containing 5 wt% Ni metal) was prepared by impregnating an aqueous solution of Ni(NO3)2 · 6H2O into the suspended O-diamond, followed by evaporation, and calcination at 400 °C 3 h in air. Si powder and Ni-O-diamond were mixed to prepare Si+Ni-O-diamond by dry or wet processes. In the dry process, 500 mg of Si+Ni-O-diamond was prepared by mixing 450, 400, and 250 mg Ni-O-diamond and 50, 100, and 250 mg of Si powder, respectively, a rotary device for 1 h at room temperature. In the wet process, Si+Ni-O-diamond was prepared by dispersing Ni-O-diamond and Si powder in the same aforementioned proportions in 30 mL hexane, followed by drying at room temperature for a few days. Si/MC composites were synthesized in a chemical vapor deposition rotary fluidized bed flow-reactor. CNFs were grown by heating Si+Ni-O-diamond at 550 °C for 3h in methane gas. The charge-discharge tests of the composites were performed between 0.05 to 1.5 V using a three-electrode cell assembled in Ar-filled globe box. A lithium foil was used as counter electrode, and a solution of 1 M LiPF6 in a mixture of ethylene carbonate, ethyl methyl carbonate (v/v = 1 : 2) and 1 % vinylene carbonate was used as the electrolyte.
3. Results and discussion
We succeeded in synthesizing Si/MC composites without the separation of Si and MC regardless of the process or the proportions of the samples. We conducted charge-discharge tests on three samples to determine which might have more CNFs based on the SEM images. These samples were synthesized using Si+Ni-O-diamond prepared in the following proportions and processes : (A) Si : Ni-O-diamond = 1 : 1 in the dry process, (B) Si : Ni-O-diamond = 4 : 1 in the wet process, (C) Si : Ni-O-diamond = 9 : 1 in the dry process. Figure 1 shows the results of the charge-discharge tests at 0.05 mA cm-2 between 0.05 and 1.5 V. We concluded that the Si/MC composites were viable anodes as all samples exhibited charge-discharge curves. Sample C gave the best specific discharge capacity (968 mAh g-1). We intend to improve the synthesis process to attain the target capacity for EVs (2830 mAh g-1). Sample C, which had the highest proportion of Si, exhibited charge-discharge plateaus at a voltage bellow 0.4 V. All samples showed gaps between the charge capacity and discharge capacity. This irreversible capacity loss is caused by the decomposition of the electrolyte on the surface of the electrode to form SEI. In future studies, we aim to reduce this gap for increasing the initial coulombic efficiency. Nonetheless, we expect Si/MC composites to increase the anode capacity based on our success in synthesizing a Si/MC composite which contained a high proportion of Si (79.1 %) using Si+Ni-O-diamond prepared in the proportion of 9 : 1 (Si : Ni-O-diamond) in a dry process.
(1) K. Nakagawa et al., J.Mater Sci. 44, 221-226 (2009), (2) T. ANDO and Y. Kanda, JP Patent No.6106870 (2013)

Figure 1