Having high energy densities, lithium ion rechargeable batteries are widely used for energy storage and still are the subject of intensive investigation. As anode materials, tin-based materials have long been considered as a promising candidate due to their twice larger theoretical specific capacity compared to commercially used graphite carbon material. Another anode candidates with extremely high electrochemical performances have been developed steadily resulting from intensive researches, but the interests on tin-based anode materials recently have been increased again since they also show great performance as an anode for next-generation sodium-ion batteries. Most of the various tin-based materials researched for high energy battery anodes can be subdivided into two groups, pure tin (Sn) metal based and tin oxide (SnO_x) based compounds. Both anode materials are based on reversible alloying reaction mechanism forming Li_xSn alloys in lithium-ion batteries, but tin oxide undergoes additional reaction, i.e. conversion reaction, before alloying reaction occurs in the first charging process. Even though the conversion reaction, which indicates the reaction forming reduced tin metal and lithium oxide from tin oxide, is known as almost irreversible and it consumes plenty of lithium ions, it is normally considered as an advantage because the resulting lithium oxide is believed to act as mechanical buffer layers preventing from drastic volume expansion/contraction and therefore contributes capacity retention.

However, there is still a lack of understanding on the reaction mechanism of tin oxide. In particular, the SnO₂ anode usually shows additional specific capacity that greatly exceeds the theoretical value. On the basis of our experimental data, the extra capacity reaches 130% of its theoretical value on the first charging process. The extra capacity commonly found in transition metal oxide (TMO) anodes has also been a subject of great interest, and Hu et al. explained that reaction of lithium hydroxide from electrolyte decomposition mainly contributes to additional capacity in RuO₂ anode (Nature Materials, 2013.12.). But it still does not give an explanation of SnO₂ anode because they have different fundamental electrochemical reaction mechanisms that enable lithium-ion battery to be rechargeable; TMO anodes such as the RuO₂ anode are operated by the reversible conversion reaction, whereas the SnO₂ anode is operated by the reversible alloying reaction. Furthermore, the partial reversibility of conversion reaction has long been in debates on SnO₂ anode materials. The partial reverse conversion reaction of the SnO₂ anode has been commonly accepted and considered to contribute the extra capacity, but previous reports are mostly based on indirect experimental evidences.

Here, we elucidated the origin of additional capacities and studied the possibility of partial reversibility of conversion reaction through ex situ transmission electron microscopy (TEM) research. We improved and optimized an ex situ TEM analysis technique to investigate electrochemical reactions inside the actual battery-operating system. Direct (dis)charging of SnO₂ particles on TEM grid in a coin cell enabled us to track the same locations of nano-areas throughout the full reaction cycle and, therefore, all of the phase evolutions including solid electrolyte interphase (SEI) layers were identified. We additionally performed same experiments with pure Sn particles to draw an exact conclusion. Eliminating the conversion reaction step, pure Sn particles give us better understanding on the reaction mechanism of SnO₂. Combining all of the experimental results as well as the aid of computational works, we found out that reactions of the Li₂O phase contribute to the extra capacity and the reverse conversion reaction of SnO₂ hardly occurs in the real battery system. We expect that this study will provide fundamental knowledge on behaviors of various tin-based materials in lithium-ion batteries.