Verification of electrolyte decomposition in lithium-ion batteries: Based on the unique bowling-like Sn@C/EG-S composite

The practical application of alloy materials has been limited due to the huge volume expansion during the lithiation/delithiation process. Designing reasonable structure is an effective method to alleviate volume expansion. Therefore, various structures are synthesized, for example, 1D nanorods, 2D nanosheets, 3D nanospheres, and so on. However, the synthesis of special structures usually has to go through a complex process and expensive, which is not conducive to large-scale application. Herein, the S-doped expanded graphite, Sn and carbon composite anode material with bowling-like structure for lithium ion battery is synthesized by a simple chemical vapor deposition (CVD) method. The unique cavity provides usable space for the volume expansion. The Sn particles are avoided contacting with the electrolyte directly under the protection of the carbon shell so that the side reactions are suppressed. Expanded graphite also improves the conductivity and provides secondary protection for the agglomeration of crushed Sn particles. Benefiting from the unique structure, the sample exhibits excellent electrochemical performance. A high specific capacity of 733 mAh g−1 is remained after 200 cycles at 500 mA g−1. Especially, at a high current density of 5000 mA g−1, the specific capacity of 428 mAh g−1 is obtained after 400 cycles. In addition, the contribution of electrolyte decomposition was also analysed by accurately controlling the volume of electrolyte. With the volume increasing of electrolyte to a certain extent, the trend of capacity upward is more distinct until the electrolyte is ran out. The phenomenon is intuitively verified that the electrolyte is decomposed continually under the condition of excessive, which result in the capacity increment. Therefore, the influence of electrolyte decomposition on cycle performance is verified quantitatively.