Towards a fundamental understanding of the improved electrochemical performance of silicon-carbon composites

Silicon-carbon composites consisting of Si particles embedded in a dense and non-porous carbon matrix are prepared by the pyrolysis of intimate mixtures of poly(vinyl chloride) (PVC) and Si powder at 900 degrees C under a flow of N-2. In contrast to bare micrometer-sized (1-10 mu m) and nanometer-sized (10-100 nm) Si powders, which show poor cycling behavior with almost no capacity remaining after 15 cycles, the texture of the composite is seen to greatly enhance the reversibility of the alloying reaction of Si with Li. For instance, a capacity of ca. 1000 mA h g(-1) is achieved for 20 cycles (0-2.0 V vs. Li+/Li) for a silicon-carbon composite containing nanometer-sized Si particles. We also demonstrate that a mild manual grinding treatment degrades the cycling performance of the composites to levels as low as the parent Si, even though free Si is not released. The electrochemical measurements in conjuction with Raman spectroscopy data indicate that a huge stress is exerted on the Si domains by the in situ formed carbon. This carbon-induced stress is found to disappear during the milling of the composites, indicating that the carbon-induced pressure, along with the accompanying improvement in electrical connectivity, are the key parameters for the improved cycling behavior of Si versus Li.