Tin-based material is an attractive choice for next generation high energy density lithium-ion battery (LIB) because of a good balance between electrochemical and mechanical properties, such as specific capacity, reaction potential, amount of volume change with lithiation and material density. Discharge capacity of more than 700 mAh g^-1 can be achieved readily. Even though the capacity is less than that of silicon, the volume expansion of Sn-based materials is smaller than that of Si-based materials, and is therefore easier to manage in a coated electrode. Among the different Sn-based materials, metallic tin, with a bulk density of more than 7 g cm^-3, has a theoretical capacity of 994 mAh g^-1 with high volumetric energy density. However, it suffers from poor cycle performance, partly due to the large number of phases transitions that the material undergoes with lithiation.¹ In addition, melting point of tin is only about 230°C, limiting the processibility of the material and also increasing the chance of cold welding during charge-discharge. Tin oxides such as SnO and SnO₂ have also been highly researched as anode materials for LIB. The presence of oxygen suppresses the phase transitions of Sn-Li, raises the melting temperature of the material, reduces the amount of volume expansion during lithiation and increases the cycling stability of the material.² During initial discharging, the materials undergo a conversion reaction to form Li₂O (step 1),

SnO₂ + 4 Li⁺ + 4 e^- → Sn + 2 Li₂O  (~711 mAh g^-1)

followed by alloying of Sn and Li (step 2).^2,3

Sn + 4.4 Li⁺ + 4.4 e^- → Li_4.4Sn  (~783 mAh g^-1)

Most research works limit the charge potential to about 1V (utilization of only the Sn-Li alloy reaction), but the first cycle efficiency is less than 40%,^2,4 meaning that in a full cell configuration, excess cathode has to be used to overcome the lithium lost during the first cycle. First cycle efficiency and energy density can be further improved by raising the charge potential to utilize the Li-O reaction, but it often results in fast capacity drop.^3,5-7

In this study, we overcome the issue by designing a stretchable polyimide (PI) coating to stabilize both the conversion and alloy reactions of commercial nano-SnO₂. The coating layer can protect the surface of the active materials and prevent materials loss during charge and discharge. Furthermore, volume change can be suppressed and the Sn atoms can be kept from fusing together during cycling. Even with PI coating and sodium carboxylmethyl cellulose as the binder, a capacity of more than 900 mAh g^-1 can be delivered at a current of 100 mA g^-1 between 0.01 and 2.5 V. The capacity is about 720 mAh g^-1 at 250 mA g^-1. After 80 cycles, capacity retention of 98% can be remained for the PI coated SnO₂ samples, as opposed to 80% without PI coating (Fig. 1). Further work to study the effect of the amount of PI and conducting additives in coating layer on electrochemical performance is underway and the charge-discharge mechanism will be discussed at the meeting. By utilizing both the conversion and alloy reactions of SnO₂, first cycle efficiency is improved to about 60% and the capacity is increased by 51.8 % compared to just using the Sn-Li alloy reaction between 0 and 1V.

References:

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4. Y. J. Hong, M. Y. Son & Y. C. Kang, Adv. Mater. 25, 2279 (2013).
5. H-J. Ahn, H-C. Choi, K-W. Park, S-B. Kim & Y-E. Sung, J. Phys. Chem.B 108, 9815 (2004).
6. X. W. Lou, Y. Wang, C. Yuan, J. Y. Lee & L. A. Archer, Adv. Mater. 18, 2325 (2006).
7. X. Zhu, Y. Zhu, S. Murali, M. D. Stoller & R. S. Ruoff, J. Power Sources 196, 6473 (2011).