The Reversibility Of Conversion Reaction And The Structure Of SnO₂-based Film Anode For Lithium Ion Battery

Replacing the commercial graphite with tin dioxide?SnO₂?is one of the promising strategies on account of SnO₂ anode's higher theoretical capacity and moderate operating potential.For the SnO₂-based anode,the most challenging problem is the inferior reversibility of the conversion reaction rather than the structure stability.Aiming at impoving the reversibility of the conversion reaction thus obtaining the anode with high initial coulombic effeiciency?ICE?,high reversible capacity and great rate performance,this dissertation addresses the preparation and characterizations of various SnO₂-based thin films with multilayer structures,emphasizing the fundamental origins for the poor reversibility of conversion reaction and the influence of microstructure on the electrochemical performance.Firstly,a pure SnO₂ film prepared by magnetron sputtering is researched.In the initial 20cycles,the discharge capacity of the SnO₂ film anode decreases from 1270mAh/g to780mAh/g,corresponding to a capacity retention of 61.4%.This capacity decaying should be attributed to the gradually coarsening of Sn grains among the Li₂O matrix.The Sn coarsening would reduce the fraction of the interface between the Sn and Li₂O,which decrease the interface diffusing between Sn and Li₂O and ultimately reduces the reversibility of the conversion reaction.After 100 cycles,the capacity would decline again due to the volume change of the electrode.The grain size of coarsening Sn and the degree of irreversibility monotonically increase with cycling,which is quantitatively expressed with a linear equation of y=0.0236x-0.266.It is extrapolated that if the Sn grains remain with diameters<11.3nm,the conversion reactions would be complete reversible,thus avoiding the irreversible capacity.Secondly,in order to suppresses the Sn coarsening and verify the relationship between the size of Sn and the reversibility of the conversion reaction,a SnO₂/Al₂O₃ ultrathin film electrode was prepared.The discharge capacities of the SnO₂/Al₂O₃ film at the first cycle and the 200^th cycle are 0.0317mAh/cm² and 0.0283mAh/cm²,respectively.This designed structure has a dramatically enhanced reversibility of conversion reaction and cycle ability with a high ICE?88.1%?,superior reversible capacity retention?89.3%?after 200 cycles while those of the pure SnO₂ anode were 80.3%and 30.6%,respectively.The Al₂O₃ coating could support the active SnO₂ layer and prevent the SnO₂ particles peeling off from the film layers.The SEI formed on the surface of Al₂O₃ layer is compact and stable.More importantly,the counter diffusion path is obstructed by the inside filling Al₂O₃,which acts as barriers to prevent Sn coarsening.Finally,on the basis of the theory obtain before,various SnO₂/Mo multilayer films were designed,which aims at acquiring the film anode with high ICE,high reversible capacity and excellent rate performance.Among the different films,the sandwich structured Mo/SnO₂/Mo-1 thin film performed greatest electrochemical performance with a high ICE of86.5%.The discharge capacity of the Mo-SnO₂-Mo-1 film electrode at the first cycle and the150^th cycle are 0.153mAh/cm² and 0.138mAh/cm²,respectively.Among the cycling of 2 to150 times,the reversible capacity retention almost keeps 100%.As for the rate performance,the Mo/SnO₂/Mo-1 anode delivered a stable capacity of 0.094mAh/cm² at 5mA/cm².In addition,its original capacity could obtain again when the current density get back to200?A/cm².The Mo layer could improve the electron transfer and Li⁺diffusion kinetics.Furthermore,the pinning effect of Mo on Sn particles would suppress the coarsening of Sn and maintain the Sn/Li₂O nanostructure,thus enhances the reversibility and stability of the conversion reaction.