Surface/Interface Structure Design And Performance Regulation Of Tin-Based Anode Materials For Lithium Storage

The commercial graphite anode in lithium-ion batteries has issues of limited specific capacity,low lithiation potential and slow kinetics,leading to challenges such as difficulty in improving the energy density,inadequate safety,inferior fast charging capability and poor electrochemical performance in low temperature.Therefore,it is imperative to explore a new generation of anode material system to address the above issues.Tin-based anodes,storing lithium through conversion and alloying reactions,have high specific capacities and moderate lithiation potentials,and thus have been highly promising candidates for lithium-ion batteries.However,to realize the practical application of tin-based anodes,the following four key surface/interface challenges need to be addressed:（1）low Coulombic efficiencies and safety concern due to the poor stability of electrode-electrolyte interface;（2）inferior reaction reversibility and cycling stability caused by the electrode structural instability;（3）large voltage hysteresis and low energy efficiency due to the electrochemical polarization and the electrochemical reactions;（4）inferior fast charging ability and unstable electrochemical performance at low temperature due to the slow Li⁺diffusion kinetics at the electrode-electrolyte interface and in the electrode bulk phases.To address the above issues,this thesis firstly achieved a breakthrough in the high reversible and stable capacity of SnO₂-based anode through surface modification and interfacial structure modulation.Furthermore,minor voltage hysteresis and high energy efficiency of SnO₂and other metal oxides are obtained by destablizing the delithiation reactions and boosting the kinetics.On this basis,the Sn₂P₂O₇-based anodes with stable surface/interface structure are developed,which have great potential for high rate and low-temperature lithium storage.The main research contents and conclusions of this thesis are summarized as follows:Firstly,for the unclear features of solid electrolyte interphase（SEI）of tin-based anodes,the SEI composition and its structural evolution of the SnO₂film anode were visualized via multiple scales characterization methods.By time of flight secondary ion mass spectrometry and transmission electron microscope,it is determined that the SEI film has an organic-inorganic bilayer structure.And the relationship between the structural evolution of SEI film and the cycling performance of SnO₂anode is investigated.The rupture SEI film would expose the inner inorganic SEI layer directly to the electrolyte,causing the failure of SEI film.Continuous rupture and accumulation of the SEI film leads to low Coulombic efficiency of97.5%and rapid capacity decay of SnO₂ anodes.Accordingly,as an inorganic layer（Li F or Li₂CO₃ layer）is pre-deposited on the surface of SnO₂anode,the uniform and stable SEI film can be induced,which effectively enhances the Coulombic efficiency to 99.5%and improves the cycling stability.Secondly,to solve the issues of Sncoarsening and volume effect within the bulk tin-based anodes,the SnO₂-Mo multilayer film anodes with stable interface structure was prepared.And the reaction mechanism of oxygen redistribution at the SnO₂/Mo interface is innovatively proposed and demonstrated.It is elucidated that the SnO₂/Mo amorphous interface induced by oxygen redistribution not only promotes the highly reversible and fast conversion reaction in the lithiated SnO₂,but also enhances the Li⁺kinetics in the bulk SnO₂anode by inducing a built-in electric field,which solves the issue of sluggish ion conduction rate in highly dense electrodes.Furthermore,the regulation of the SnO₂/Mo interfacial density on the stability and reversibility of the conversion reaction is established by adjusting the number of layers of SnO₂ and Mo in the SnO₂-Mo anodes.The Mo/SnO₂/Mo with symmetric structure has an outstanding initial Coulombic efficiency of 92.6%,high reversible capacity of1067 m A h g^-1 and 100%capacity retention after 700 cycles,achieving a breakthrough in the comprehensive performance of SnO₂-based anodes.More importantly,the established interface-performance relationship in the SnO₂-Mo film anodes is demonstrated to be extensible to the SnO₂-Mo powder anodes.Afterward,on the basis of achieving stable and high reversible capacity of SnO₂-based anodes,the huge voltage hysteresis between the conversion reaction and the inverse conversion reaction steps,and the resulted low energy efficiency in full cells become the primary obstacle of SnO₂-based anodes.In the SnO₂-Mo-P anode prepared by compounding SnO₂-Mo with phosphorus（P）via ball milling,the voltage hysteresis can be reduced to 0.3 V along with high reversibility and cycling stability,which is much lower than the 0.9 V of SnO₂ and SnO₂-Mo anodes.The energy efficiency of the SnO₂-Mo-P anode in full cell reaches up to 88.5%,which is comparable to the tested graphite anode.The origins of voltage hysteresis are clarified from both thermodynamic and kinetic perspectives,and it is proposed that the thermodynamically established phosphorylation reaction（Sn+P→SnP_xO_y）,in synergy with enhanced kinetics,is an effective strategy to reduce the voltage hysteresis.More importantly,this strategy can also reduce the voltage hysteresis of other metal oxides by0.6～0.7 V and increase their energy efficiency in the full cells by～23%,which enhances the application value of low-cost metal oxides represented by SnO₂ in the high-energy-density lithium-ion batteries.Finally,inspired by the positive effect of SnP_xO_y generated from the above phosphorylation reaction in reducing voltage hysteresis,and combined with the advantages of Sn₂P₂O₇ anode with high theoretical specific capacity and moderate lithiation potential,the Sn₂P₂O₇-P-Mo composite with stable surface/interface structure is developed.The reaction reversibility,specific capacity,and cycling stability of the Sn₂P₂O₇-P-Mo anode are investigated at different testing rates and temperatures.The Sn₂P₂O₇-P-Mo anode has a high reversible capacity of 746 m A h g^-1 at 10.0 A g^-1,with a capacity retention of 84%after 200cycles.In addition,Sn₂P₂O₇-P-Mo anode achieves a high capacity retention of 89%after 100cycles at-30°C.With Li Co O₂（LCO）as the cathode,the LCO||Sn₂P₂O₇-P-Mo full cell can also obtain high reversible capacity and cycling stability under high rates and low temperatures.The superior fast-charging and low-temperature lithium storage performance of Sn₂P₂O₇-P-Mo anode is attributed to its superior structural stability and kinetic properties.The P phase in Sn₂P₂O₇-P-Mo provides fast Li⁺transport channels,and the uniform and dense SEI film can be formed on the surface of Sn₂P₂O₇-P-Mo.The developed Sn₂P₂O₇-P-Mo anode with superior kinetic properties provides important guidance for the further development and optimization of tin-based anodes for fast-charging and low-temperature lithium storage.