Stabilization of Silicon Anode for Li-Ion Batteries Jie Xiao, Wu Xu, * Deyu Wang, ** Daiwon Choi, Wei Wang, Xiaolin Li, Gordon L. Graff, Jun Liu, and Ji-Guang Zhang * ,z Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA Micrometer-sized Si particles with nanopore structures were investigated as anode material for Li-ion batteries. The porous structure of Si helps accommodate the large volume variations that occur during the Li insertion/extraction processes. To improve the electronic integrity of the Si-based anode, a two-step process was utilized. First, chemical vapor deposition CVD was used to enhance the electronic conductivity of individual Si particles by depositing a uniform carbon coating on both the exterior surfaces and the pores. Next, the electronic contact among silicon particles was improved by adding Ketjenblack KB carbon, which exhibits an elastic, chainlike structure that maintains a stable electronic contact among silicon particles during cycling. Using this approach, an anode with a reversible capacity of more than 1600 mAh/g after 30 cycles was obtained. The combination of the nanopore structure, CVD-coated carbon on the Si surface, and the elastic carbon KB among the silicon particles provides a cost-effective approach to utilize the large micrometer-sized Si particles in Li-ion batteries. © 2010 The Electrochemical Society. DOI: 10.1149/1.3464767 All rights reserved. Manuscript submitted April 13, 2010; revised manuscript received June 22, 2010. Published August 2, 2010. Si-based anodes could significantly increase the energy density of Li-ion batteries. Compared with the capacity of conventional graphite anode material 372 mAh /g, Si exhibits a theoretical capacity of 4200 mAh /g, 1,2 which is more than 10 times that of graphite. However, the large volume changes that occur during the lithiation and delithiation processes cause severe cracking and pul- verizing of the electrode, which in turn leads to a significant capac- ity fade during cycling. Many factors need to be investigated before Si anodes can be used in Li-ion batteries. The morphology of Si plays an important role in determining the capacity and cyclability. Nanosized Si par- ticles exhibit better electrochemical properties than micrometer- sized Si particles because of the reduced mechanical stress. 3-6 The Si nanowires maintain an excellent cyclability because the large num- ber of pores between the wires accommodate the expansion that occurs during cycling. 7 However, the weight ratio of Si nanowires to the substrate needs to be increased significantly for the practical applications of Si nanowires. Si nanofibers prepared by electrospin- ning can be used directly as an anode material without adding bind- ers or carbon additives. 8 The initial capacity obtained from the use of these fibers is greater than 1000 mAh/g; however, fast fading, probably because of the organic residual left in the Si fibers, is still observed. Recently reported porous Si particles coated with carbon show superior cyclability because of the three-dimensional structure, 9 which facilitates Li transport and alleviates the large vol- ume changes associated with Si anodes. Coating carbon on the Si surface by mechanical mixing, 10,11 sometimes combined with py- rolysis reactions, 12 has effectively improved the conductivity of pure Si and alleviated the effects of the volume changes. Si-based com- posite materials can easily be obtained by high energy mechanical milling 13,14 or by depositing Si particles in carbon fibers. 15-17 How- ever, to maintain good cyclability, 18 a large amount of carbon needs to be used in these composites; therefore, the energy densities of the composite anode are still low. The interactions between the inactive components and Si also play important roles in stabilizing the Si anode during extensive cycling. For example, the morphology and content of the carbon- based conductive additive have profound effects on the cycle life and irreversible capacity of the Si anode. 19,20 The binder system consisting of sodium carboxymethylcellulose Na–CMC 21-25 is found to be more effective than polyvinylidene fluoride in improv- ing the cycling performance of the Si anode. The formation of a covalent chemical bond between the Na–CMC binder and Si to sta- bilize the long-term cycling was clarified by Hochgatterer and co-workers. 26 A recent paper published by Magasinki and co-workers 27 on CVD synthesis for Si nanoparticles on the surface of annealed carbon-black dendritic particles reports very stable cy- cling of a Si-based anode. However, the processes used in both of their Si-deposition and C-particle preparations are expensive. A less expensive approach that uses commercially available Si and carbon sources is needed for the preparation of Si-based electrodes. In this paper, we report the improved cycling stability of a Si-based anode using several approaches. First, nanostructured porous Si was used to accommodate the volume changes of the Si particles and to main- tain mechanical stability during the cycling process. Next, deposi- tion of intimate carbon coatings and addition of KB carbon with its elastic chainlike structure were used to improve the electrical con- ductivity at both the particle and electrode levels. The mechanism that leads to the stabilization effect is also described in this paper. Experimental The schematic setup for CVD-carbon coating is shown in Fig. 1. Spongelike Si with porous structures Vesta ceramics, Monmouth Junction, NJ was used as received particle size 4 m. The Si powder was loaded in a ceramic boat and placed at the center of a quartz tube furnace, as shown in Fig. 1. The furnace was pumped at room temperature overnight to a vacuum level of less than 1 mTorr. Next, the furnace temperature was increased from room temperature to 600°C at a rate of 10°C /min. Then, the precursor gas  argon:acetylene = 9:1 was introduced into the furnace. The tem- perature then was increased from 600 to 690°C over a 10 min period and kept at 690°C for 30 min. The furnace was cooled slowly in pure argon to room temperature. At high temperature, the acetylene decomposed quickly and deposited onto the surface and within the pores of the spongelike Si particles. The carbon content in the as- coated Si was controlled at 6%. The crystalline structures of the as-received porous Si and CVD- carbon-coated porous Si were determined by X-ray diffraction XRD using a Philips X’Pert X-ray diffractometer in -2 scan mode and a Cu K sealed tube   = 1.54178 Å at 0.5°/min. High resolution transmission electron microscopic TEM analysis was carried out on a JEOL JEM 2010 microscope fitted with a LaB 6 filament and an acceleration voltage of 200 kV. The Si electrodes before and after cycling were characterized by a JEOL 5900 scan- ning electron microscope SEM equipped with a Robinson Series 8.6 backscattered electron detector. An EDAX Genesis energy- dispersive spectroscopy EDS system with a Si Li EDS detector was used to investigate the particle morphology. The Si-based electrode was prepared by casting a slurry of the carbon-coated Si or the original Si powder, Super P from Timcal, and Na–CMC from Nippon Paper Chemicals, Japan in deionized * Electrochemical Society Active Member. ** Electrochemical Society Student Member. z E-mail: jiguang.zhang@pnl.gov Journal of The Electrochemical Society, 157 10 A1047-A1051 2010 0013-4651/2010/15710/A1047/5/$28.00 © The Electrochemical Society A1047 ecsdl.org/site/terms_use address. Redistribution subject to ECS license or copyright; see 130.20.65.226 Downloaded on 2013-06-26 to IP