Ex-Situ TEM Analysis on Electrochemical Reactions of Tin Based Anode Materials in Lithium Ion Batteries

Having high capacities and voltages, lithium ion rechargeable batteries are widely used and still are the subject of intensive investigation. In comparison with commercially used graphite carbon, tin-based materials have emerged as prospective anode materials because of their much higher theoretical specific capacity. Most of the various tin-based materials having been researched for inventing high energy anode materials can be subdivided into two groups, pure tin(Sn) metal based and tin oxide(SnO_x) based compounds. Tin metal and its oxides are both based on reversible alloying reaction forming Li_xSn alloys. But they are different in that tin oxide additionally undergoes conversion reaction, known as almost irreversible, forming lithium oxide and reduced tin metal before alloying reaction occurs in the first charging process. Although the lithium oxide formed by the conversion reaction consumes plenty of lithium ions, it is normally considered as an advantage because it acts as mechanical buffer layers and therefore it contributes the capacity retention.

 However, there is still a lack of understanding on the reaction mechanism of tin oxide. Particularly, there has been a debate on the partial reversibility of the conversion reaction of tin oxide. The unexpected experimental peak of differential capacity curves and the additional capacity over the theoretical prediction are considered as the evidences of partial reversibility of the tin oxide conversion reaction. Recently, Hu et al. revealed the origin of extra capacities in transition metal oxides using RuO₂ material. But, tin oxide should be distinguished from the transition metal oxides which are mostly operated by reversible conversion reaction in the lithium ion batteries. Therefore, we have traced single SnO₂ particle by ex-situ transmission electron microscopy(TEM) research. We dispersed SnO₂ particles on a carbon film deposited copper mesh TEM grid and the grid was used as a working electrode in a coin cell. The coin cell including the TEM grid was dis/charged toward specific voltages and it was disassembled to analyze the sample which is directly dis/charged on the TEM grid. Repeating this process with the same sample grid enabled us to observe the transformation of the single tin oxide particle and its surroundings. We observed the lithium hydroxide on the specific voltage state, which contributes the additional capacities of tin oxide electrode. The detail explanations on the contribution of lithium hydroxide and the reversibility of the tin oxide conversion reaction will be presented.

 Furthermore, we performed the same experimental process with pure tin particles. Eliminating the conversion reaction step, pure tin particles give us better understanding on the reaction mechanism of tin oxide. By comparing with corresponding reaction steps between them, we can figure out where the lithium hydroxide originated from; electrolyte decomposition or lithium oxide formed by the tin oxide conversion reaction. In addition, we can ensure whether the reversible conversion reaction of tin oxide is possible. Including the above experimental results, the reaction mechanisms of tin and tin oxide will be discussed. We expect that these ex-situ TEM studies will provide fundamental knowledge of various tin-based materials.