A Novel Si/C Composite Anode Material for High Performance Lithium Ion Battery Using a Cost-Saving Approach
Authors: Moon Jong-Seok , Kim Se-Won , Shim Kyu-Eun , Jo Sung-Nim , Yu Tae-Hwan , Seo Jong-Hyun , Ahn Jae-Pyoung , Han Sungsoo , Doo Seok-Kwang 
Published: February 2020

To response the dramatically 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 strongly desired. With respect to the anode if LIB, silicon have received a great deal of interest as fully lithiated (ca. Li22Si5) alloy has the highest theoretical capacity of any known material at 4140 mAh/g (3579 mAh/g for Li15Si4 at room temperature). Although it is about 10 times higher than that of conventional graphite anode, 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.

 At first, this study presents a novel cost-saving Si-base anode material with high coulombic efficiency and prolonged cyclability of excellent electrochemical performances using a low-cost synthesis of 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 Si particles (ASP), the by-product, is derived homogeneously from the nucleation of gas dust in silane-based gas phase reactor. Our pioneering attempt is advantageously characterized by follows: 1) obtained sub-micron sized Si particle is superior to nano-sized Si in battery performances and business value and 2) the production cost is expected to be significantly lower than commercially available nano-Si by utilizing the by-product from an already matured mass production system.

 Further improvement in consideration of commercial feasibility and processing cost is achieved by employing ASP into spherical Si/graphite composite systems. In the optimized system, the mechanical stress of Si particles induced by huge volume expansion on Li insertion is further 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 materials which is a critical element in the composites. In such a way, the achieved advantageous characteristics are summarized as follows: 1) well-distributed Si particles in the composite system preventing agglomeration and 2) a strong bonding between Si and graphite maintaining electrical contact under repeated charge-discharge, 3) a ductile matrix to buffer the huge stress caused by the volume expansions and 4) buffer spaces inside the composite to accommodate the volume swings. It is worth noting that such an extraordinary ASP/graphite composite is manufactured in inexpensive way as the process is consists of only a few steps.

 The electrochemical characterization demonstrates remarkable performances as the coulombic efficiency at the first cycle reaches up to 91% and the capacity retention after 100 cycles is over 95% at 620 mAh/g. Material characterization is performed through several different analytical methods including HRTEM, in-situ solid state lithiation, in-situ dilatometer to elucidate and discuss the unique property of ASP and the superior performance of our spherical Si/graphite composite systems.