Changing the carbon framework to produce low-expansion silicon-carbon composites for high-performance lithium-ion batteries
Silicon-carbon (Si/C) composites are promising high-capacity anode materials for next-generation lithium-ion batteries, but their commercialization is hindered by severe volume expansion during cycling. We report a chemical vapor deposition method using the pyrolysis of silane, in which ultrafine nano-Si enters a porous carbon framework to produce kilogram-scale Si/C composites. The carbon framework with abundant micropores (~1.9 nm) confines the amorphous silicon and accommodates the volume changes of nano-Si during both lithiation and de-lithiation. The resulting Si/C composites have a 56.76% Si content and have a specific capacity of 2179 mAh g–1, a high initial Coulombic efficiency (ICE) of 93.5%, and a low specific surface area (1.32 m2 g–1). In addition to the nanoconfinement effect, the median particle size (D50, 7.3-13.0 μm) of the carbon framework was shown to control the mechanical strength, coating uniformity and Li+ transport. A D50 of 8.2 μm endows the Si/C composites with outstanding comprehensive properties. They have an excellent rate performance with a 97.0% retention at 3 C relative to 0.1 C, show only minor variations in ICE difference at 60 ℃/-20 ℃ compared with room temperature, and have a low expansion of 35.8% from the delithiated to the lithiated state. The composite was then mixed with graphite to prepare the anode, which was then paired with an NCM523 cathode to assemble pouch cells. The pouch cell retained 87.46% of its initial capacity after 1000 cycles at 1 C. Because of the low expansion of the electrode, the material avoids structural degradation during cycling and thus has an excellent long-term stability.