The Synthesis And Electrochemical Perfromance Study Of Sn-based Anode Materials

Lithium ion batteries（LIBs）have attracted wide attention for their advantages of high specific capacities,long circular life,splendid rate performance,environmentally friendly,and so on.And benefiting from these virtues,they have become the dominating mobile energy storage equipment and have broad applications.However,as the demand for lithium ion batteries with higher performance grows,particularly in order to meet the need of fast-growing electric vehicles,the current mainstream commercial graphite anode has been confronted with a bottleneck because of its low theoretical capacity(372 mA h g^-1).Sn has virtues including high theoretical electrochemical specific capacities(994 mA h g^-1 for LIBs and 847 mA h g^-1 for SIBs),moderate lithium storage potential（0.8-0.3 V vs.Li/Li⁺）,and high conductivity(9.17×10⁶ S m^-1),hence is regarded as one of the most promising candidates to replace graphite and becomes the next generation high-performance anode.However,the drastic volume change during cycling seriously damages the structure of the Sn anode materials,and leads the superfluous formation of SEI film,which results in the rapid decay of capacities.Concerning these issues,researchers have developed many modification strategies for Sn-based anodes,including nanocrystallization,recombining with cushioning materials,microstructure design and so on.Based on the background discussed above,we have synthesized high performance Sn-based anode materials through size control,oxidation treatment,and combination modification,and evaluated their electrochemical properties as LIBs and SIBs anodes.The cycling stability was significantly improved and the modification mechanism was also investigated.The main conclusions are listed as follows:（1）Sn particle samples with size ranging from 100-500 nm,30-50 nm,to¹0 nm were synthesized through a wet chemistry method.Their cycling stability of LIBs indicated that size control of Sn only improved the cyalability to some extent,the 10nm Sn sample still suffered severe capacity attenuation problem caused by the particle aggregation and irreversible reaction of oxidation layer.To inhibit particle agglomeration and enhance kinetics,we further modified 10 nm Sn with reduced graphene oxide（RGO）and prepared multideck 3D nano-Sn/RGO composite.Because of the advanced structure,this composite showed superior cycling stability and excellent rate performance in LIBs.After initial 50 cycles,it could cycle steadily in the subsequent 800 cycles with an average capacity of 590 mA h g^-1.The composite delivered a reversible capacity of 361 mA h g^-1 at 2 C(1 C≈1 A g^-1),and 241 mA h g^-1 at a high rate of 5 C.（2）We investigated the modification mechanism of 3D nano-Sn/RGO.It showed from the ex-situ TEM observation that the composite undergoes structural evolution from a sandwich-like layered structure to a curly Sn@C structure.The curly Sn@C structure generated after about 50 cycles and kept intact after 200 cycles,which effectively suppressed reunion of Sn particles and enhanced the stability of electrode.It revealed that the Li₂O exhibited reversible lithium storage ability when the kinetics of anode materials was improved sufficiently.The amorphous SnO_x particles with sub-5 nm in size and the SnO_x/TiO₂@C composite were prepared to study the reversibility of Li₂O.The constitutionally stable TiO₂ buffered the expansion of SnO_x particles and the coated carbon improved conductivity.We studied the effect of conductive additives to the electrochemical properties,and the result indicated that the decrease of capacity in the initial 50 cycles primarily resulted from the decay of the Li₂O reversibility.（3）Combining the advantages of size control and carbon coating,we synthesized the Sn/N-doped carbon microcage composites（Sn/N-dCMC）through a spray drying method.The Sn/N-dCMC had novel structural characteristics:ultrasmall Sn particles uniformly embedded in the N-doped carbon framework.The N-doping supplied abundant nucleation sites for Sn particles,resulting in the compact distribution of Sn particles in carbon,and the surface chemical bonding suppressed the particle growth and aggregation.Benefiting from the novel structural properties,Sn/N-dCMC showed excellent electrochemical performance.As anode for LIBs,Sn/N-dCMC delivered780 mA h g^-1 at 200 mA g^-1 and maintained 472 mA h g^-1 after 500 cycles;323 mA h g^-1 was sill obtained even at a high current density of 10 A g^-1;it also exhibited extra long cycling stability at 1 A g^-1.As anode for SIBs,Sn/N-dCMC delivered 439 mA h g^-1 at 50 mA g^-1 and maintained 332 mA h g^-1 after 300 cycles;149 mA h g^-1 was sill obtained at a high current density of 5 A g^-1.