Superlubricating electrical contact between graphite layers

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.

Because of their high energy density, Li-ion batteries have attracted attention as large storage systems over their traditional use for portable and electric vehicle applications. However, there are some concerns about the scarcity, uneven global distribution and high cost of lithium resourced. Compared with Li, Na has the advantages of natural abundance and low cost. Furthermore, Na and Li are in the same main group, exhibiting similar chemical properties, which translates to a resemblance between operational characteristics in Li-ion batteries and Na-ion batteries. Therefore, Na-ion batteries have been suggested for the alternatives to Li-ion batteries for large-scale systems. For the practical use of Na-ion batteries, studies for finding suitable electrode materials with high energy density, good cyclability and rate performance for Na-ion batteries have been actively conducted. Although several cathode materials have been suggested, the anode choices are severely limited because of the unique characteristic of Na. Na-ion cannot be stored much in commercial layered graphite because of its large radius. Si based materials are expected to be the most promising anode for Li-ion batteries, but it is electrochemically inactive with Na. On the contrary, Sn can store 3.75 Na/host-atom (Na15Sn4) resulting a maximum storage capacity of 847 mAh g-1 as an anode for Na-ion battery. Besides the high theoretical capacities, Sn also has advantages as an anode material for Na-ion battery such as low reaction potential and non-toxicity. However, pure Sn anode exhibits poor cyclability due to a loss of electrical contact with the current collector, resulting from a severe volume change (up to approximately 420%) during sodiation/desodiation. To improve the cyclability of Sn electrode, several strategies have been suggested such as introducing carbon or designing Sn-M intermetallic compounds for providing an active or inactive phase that acts as buffer matrix, and synthesizing a micro/nanostructured electrode for facilitating the relaxation of material stress. Among these strategies, synthesizing a nanostructured pure Sn, especially using electrodeposition, should be ideal solution. The material fracture is presumably due to the repeated non-uniform expansion and contraction during alloying and de-alloying. Because of a sluggish kinetic of sodium ion, the diffusion time is large compared to the charge/discharge time for the large sized particles which results uneven sodiation and desodiation states (different phases with different mechanical strengths) inside one particle. These variations in phase lead to stress, which can lead to crack propagation and pulverization. For this reason, nanostructured Sn could greatly reduce the stress induced during sodiation/desodiation by reducing the diffusion time of sodium ions in the electrode. Furthermore, using electrodeposition process, theoretical specific capacity of Sn can be fully used because the Sn anode are deposited directly on the current collector without mixing and drying processes for binder and conductive agent. Recently, we synthesized Sn nanodendrites with nano-sized branches. The Sn nanodendrites exhibited a high reversible capacities and excellent cycle performances. The maximum capacity was measured to be 783.88 mAh g-1 and the charge capacity at 100th cycle was measured to be 759.27 mAh g-1 that correspond to 96.86% of the maximum value. After cycling, the Sn nanodendrites exhibits no loss of active materials and it was attributed to the nano-sized branches greatly reducing the internal stress during sodiation/desodiation.