High-resolution electron microscopy investigation of capacity fade in SnO2 electrodes for lithium-ion batteries

Nanocrystalline SnO2 thin films have been cycled electrochemically vs. a lithium electrode. They have shown a reversible capacity of about 400-500 mAh/g over more than 100 cycles. However, a capacity fade usually occurs after a few hundred cycles. A high-resolution electron microscopy (HREM) investigation has shown the decomposition of SnO2 crystallites into 10 to 50 nm wide tin grains during the first cycle as previously reported. An amorphous phase containing carbon and oxygen has also been detected in the cycled samples. Furthermore, the tin particles are surrounded by an amorphous 5 to 10 nm wide ring made of Sn and O. The size of the tin crystallites formed during the first cycle increases from 40 nm to an average value of 110 nm after 500 cycles. In addition, the structure of the amorphous compound made of Sn and O surrounding the tin particles changes after 500 cycles, suggesting that a beginning of crystallization has occurred. We assume that either particle expansion or the formation of this semicrystalline layer is responsible for the capacity fade observed in SnO2 negative electrodes. (C) 1999 The Electrochemical Society. S0013-4651(98)11-004-2. All rights reserved.