A Novel Silicon/Graphite/Carbon Composite Anode for High Performance Lithium Ion Batteris

To response the multiply increased demand in portable electronics, hybrid electric vehicles and large scale energy storage over the last decade, Li-ion batteries (LIBs) with higher energy density and power capability have been desired. With respect to the anode of LIB, Silicon have received a great deal of interest as the fully lithiated (ca. Li₂₂Si₅) alloy has the highest theoretical capacity of any known material at 4140 mAh/g (3579 mAh/g for Li₁₅Si₄at room temperature). Although it is about 10 times higher than that of a conventional graphite anode, a Si-based electrode has yet to find use in commercial application because of its low coulombic efficiency (C.E.) and poor capacity retention. Si is typically known to undergo over 300% of volume expansion on Li insertion, which leads to cracking and crumbling of active particles and the resultant loss of electrical contact between electro-active materials causes rapid capacity fading. Although extensive researches have been reported to alleviate the problems, most of approaches are commercially unviable as they usually involve complicated and prohibitively high cost processes. Hence, a Si anode with sufficient performances and low cost production is highly desired.

  This study presents a novel cost-saving Si-based anode material with high coulombic efficiency and prolonged cyclability of excellent electrochemical performances using a low-cost synthesis though the attractive route. As a part of our strategy, we attempted to employ a by-product derived from a granular poly-Si mass production system for the feedstock of solar cell application. In such a way, a certain amount of amorphous silicon particles (ASP), the by-product, is derived homogeneously from the nucleation of gas dust in a silane-based gas phase reactor. (Fig. 1 shows HRTEM image and SAD pattern for ASP.) Our pioneering attempt is advantageously characterized by follows: 1) obtained sub-micron sized Si particles are superior to nano-sized Si particles in the battery performance and business value and 2) the production cost is expected to be many times lower than nano-sized Si by utilizing the by-product from an already matured mass production.

  Further improvement in consideration of commercial feasibility and processing cost is achieved by employing ASP into spherical Si/graphite/carbon composite systems. In Fig. 2, it is seen that spherical ASP/graphite/carbon composite is obtained with the particle size around 7-11 μm. Representative cross-section view demonstrates carbon coated ASP are enclosed by graphite layers; though vacant areas exist sparsely. In the optimized system, the mechanical stress on Si particles induced by huge volume expansion on Li insertion is alleviated and the direct contact between Si and electrolyte is prevented, by which irreversible reaction leading to deterioration of coulombic efficiency is reduced. In this context, we have performed a systematic study on the influence of choice and application method of carbon material which is a critical element in the composites. In such a way, ASP are carbon coated before or during the fabrication process of spherical composites and the achieved advantageous characteristics are summarized as follows: 1) separation of each silicon particles, preventing agglomeration and 2) a strong bonding between Si and graphite for maintaining the electrical contact on charge-discharge, 3) a ductile matrix to buffer the huge stress caused by volume expansion and 4) vacant space inside the composite to accommodate the huge volume changes. It is worth noting that such extraordinary ASP/graphite/carbon composites are manufactured in an inexpensive way as the process consists of only a few steps.

  The electrochemical characterization demonstrates remarkable performances as the coulombic efficiency at the first cycle reaches up to 90% and the capacity retention after 100cycle is over 92% at 620 mAh/g. Material characterization is performed through several different analytical methods including HRTEM, in-situ solid state lithiation, in-situ dilatometer to support and discuss the unique properties of ASP and the superior performances of our spherical Si/graphite/carbon composite systems.