Microstructure and Composition Tuning: Enhancing Reversibility of SnO2 Based Anodes and Reducing Their Initial Capacity Loss

The applications of LIBs are being extended to electric vehicles and the smart grid because of their potential for high power and high energy density. SnO₂-based materials are widely studied to replace the commercial graphite anode with low theoretical capacity (372 mAh/g), owing to their high total theoretical capacity (1494 mAh/g), low cost, low toxicity, and widespread availability. In a SnO₂-based anode, it has been generally accepted that two principal electrochemical processes occur: (1) SnO₂+4Li⁺→Sn+2Li₂O; (2) Sn +xLi⁺←→Li_xSn (0≤x≤4.4). The first reaction was regarded as an irreversible reaction because Li₂O is thermodynamically stable, and leading to very large initial irreversible capacity (712mAh/g) in the first charge–discharge cycle, which was different from the reversible transition metal oxide(such as CuO, MnO₂) with relative lower initial capacity loss. The second process is reversible, with a high reversible lithium storage capacity of 782mAh/g, a value more than twice the theoretical capacity of graphite anodes.

    However, It should be pointed out that the initial irreversible capacity means irreversible consumption of materials (Li and electrolyte), which have to be minimized because they are detrimental to both the specific energy and energy density of the cell. Moreover, this increases the expense of materials due to the necessary excess of costly Li source cathode materials after cell assembly. In addition, another great challenge for using SnO₂as an anode material is also the capacity fading due to fracture and disintegration of the electrode, which is induced by the extremely large volume change of Sn in reaction (2) during cycling.

      After long-term studies, nanopainting of SnO₂ with various carbonaceous materials, with high-dispersion state of SnO₂ in carbon matrix, has been found effective for improving the cycleability of SnO₂-based anodes.However, generally, there were very large initial irreversible capacities (200-1000 mAh/g) in these SnO₂-C composite electrodes due to unavoidable Li trapping in the amorphous carbon and SEI layer formation as well as other Li-consuming irreversible reactions on the carbon matrix surface, in addition to the irreversible of Li₂O. And thus, in order to promote their practicalities as anodes in the high energy density LIBs, the three aspects below should deserve much more attention on research for the SnO₂/carbon nanocomposites, which are the targets we effort in this presentation.

(1)  Complete reversibility of SnO₂ reacting with Li and keeping the SnO₂ crystals during lithiation/delithiation cycling, by combining with the transition metals (M: Cu, Fe, Mn, etc.) in which the reversible reaction of M+Li₂O←→MO was expected to generated, to increase the reversible specific capacity and cycleability of electrodes and cells.

(2)  Design unique structure that ultrafine SnO₂ nanocrystals are supported in a high-dispersion state on the nanosize carbon particles, to make sure the good structure integrity and thus enhanced cycle performance. Meanwhile, Li-consuming irreversible reactions on these nanosize carbon matrix were tried to reduce by pre-lithiation treatment or others, to lower down the irreversible capacity loss.

(3)  It is still desirable to develop facile strategies, which are cost-effective and have good potential for large scale applications, to synthesize SnO₂-based carbonaceous nanocomposite anodes for LIBs.