High-performance micro-sized Si-C composite for Li-ion anodes offers high tap density for high volumetric capacity
|Cycling performance of the Si-C composite and porous Si at 400 mA/g. Source: Yi et al. Click to enlarge.|
A team at Penn State University has synthesized a micro-sized silicon-carbon (Si-C) composite consisting of interconnected Si and carbon nanoscale building blocks as anode materials for Li-ion batteries (LIBs). The Si-C composite, produced by a low-cost and large-scale approach, exhibits a reversible capacity of 1459 mAh/g after 200 cycles at 1 A/g with a capacity retention of 97.8%, with high-rate performance of 1100 and 700 mAh/g at current densities of 6.4 A/g and 12.8 A/g.
The material, reported in the journal Advanced Energy Materials, also features the highest tap density (0.78 g/cm3) of Si-based materials yet reported in literature and thus achieves a high volumetric capacity, the researchers said. (Tap density is bulk density of a powder after a compaction process.)
Silicon has been intensively pursued as the most promising anode material for high-energy-density LIBs because of its high specific capacity (>3500 mAh/g) and abundance. Despite its high capacity, Si suffers from fast capacity fading caused by its large volume change (>300%) during lithiation/delithiation and the serious issues stemming from this volume change, e.g., unstable solid electrolyte interphase (SEI) and disintegration (cracking and crumbling) of the electrode structure. The development of Si-C nanocomposites (e.g., nanowires, nano- tubes, or nanoparticles) has been widely studied.
These nanocomposites proved to be an effective method of improving capacity and cycling stability, since nano-sized Si can alleviate fracture during volume changes and the contact between Si and carbon can maintain electrical contact and improve conductivity of the nanocomposites. However, practical application of nano-sized Si materials in LIBs is difficult. First, achieving a high tap density is important for fabrication of high-energy LIBs for EVs and PHEVs, because it offers a high volumetric energy density. Unfortunately, the tap density of nano-sized materials is generally low, which in turn holds down their volumetric capacity. Furthermore, preparation of nano-sized Si either requires chemical/physical vapor deposition or involves complicated processes, leading to costly, low-yield synthesis that is difficult to scale up to practical levels. To date, the abundance of Si has not been fully capitalized upon due to lack of a low-cost strategy for large-scale synthesis of Si anode materials with superior performance.—Yi et al.
While micro-sized silicon materials enable higher tap density than nanosized materials, they are more likely to undergo disintegration upon volume change during lithiation/delithiation compared with nanosized materials, resulting in severe capacity fading and also have long ion/electron transport paths that adversely affect high rate capability. A solution that some research teams have suggested would be to develop new materials that combine the advantages of both micro-sized and nano-sized Si materials to improve the cycling performance, rate capability, and energy density of Si anodes.
The Penn State team proposed that micro-sized Si-C composites should have the following features to obtain excellent electrochemical performance:
The size of primary Si building blocks should be small enough to avoid building block fracture induced by volume changes during the lithiation/delithiation process.
Si and C should be uniformly mixed at the nanoscale to improve the conductivity but also be highly packed to improve the tap density of the micro-sized particles.
The carbon and Si building blocks should maintain intimate contact even after the micro-sized Si-C particles are pulverized to prevent loss of electrical contact.
To produce the material, the team started with commercially available micro-sized SiO as the Si source at a gram scale. They heated the SiO to form an Si/SiO2 composite composed of interconnected Si nanoparticles embedded in an SiO2 matrix due to the disproportionation of SiO. This was followed by removal of the SiO2 via an etching route, transforming the Si/SiO2 composite into porous Si particles. Finally, thermal decomposition of acetylene fills a large portion of the original pores with carbon—creating a micro-sized Si-C composite in which Si and carbon are three-dimensionally interconnected at the nanoscale.
|Preparation process from SiO precursor to the Si-C composite. Source: Yi et al. Click to enlarge.|
They characterized the electrochemical performance of the Si-C composite as well as the porous Si without the carbon filling at different charge/discharge cycles.
The excellent performance [of the micro-sized Si-C composite] is attributed to the nanoscale size of primary particles and interconnected carbon and Si networks which can maintain internal electrical contact and sustain cycling stability. The synthesis method is low-cost and easy to scale up, and is thus believed to have great potential in practical production of high-performance Si materials for Li-ion batteries.—Yi et al.
Yi, R., Dai, F., Gordin, M. L., Chen, S. and Wang, D. (2012), Micro-sized Si-C Composite with Interconnected Nanoscale Building Blocks as High-Performance Anodes for Practical Application in Lithium-Ion Batteries. Adv. Energy Mater. doi: 10.1002/aenm.201200857