Controllable Synthesis Of Sn-Based Materials For High Performance Sodium-Ion Storage

Lithium-ion batteries（LIBs）have been widely used in consumer electronic devices and electric vehicle owing to the high energy density,long cycle life,and portability,However,the limited reserves and uneven distribution of lithium resources make LIBs costly and low sustainablely.Compared with LIBs,sodium-ion batteries（SIBs）show the similar energy storage mechanism,which are considered as the promising substitutes or supplements for LIBs for large sacle energy storage because of the abundant resources and low price of sodium.However,the commercial graphite anodes in LIBs generally demonstrate the poor sodium storage activity because of the larger dimater of Na⁺compared to Li⁺.Therefore,it is urgent to develop novel high-performance anode materials for advaenced SIBs.Sn-based anode materials with high specific capacities have attracted great attention in recent years.However,the large volume expansion during cycling and low conductivity lead to poor cycling and rate performance.Therefore,suitable structural controlling is essential to improve their sodium storage performance.In this thesis,we study the Sn metal anodes firstly and then expand to the Sn-based compound anodes.Through the reasonable micro-nano structure designing,the sodium storage performances of the Sn-based anode materials are improved.The main conclusions and innovations are summarized as follows:（1）Sn metal anodes suffer from serious volume expansion and increased resistance as a result of the in-situ formed large resistance phase a-NaSn during sodiation.To solve these problems,we design and synthesize the Co₂P/Sn@NC core-shell nanobox comprising the plum pudding-like Co₂P/Sn core and the nitrogen-doped carbon shell（NC）.The metallic Co₂P particles with the size of 8-10 nm are uniformly dispersed in the Sn matrix,and the Co₂P can be converted into uniformly dispersed Co particles with better conductivity.The homogeniously dispersed conductive particles can built a fast electron transport channel in the a-NaSn phase by electric field-assisted tunneling effect.In addition,the porous core can buffer the volume expansion of Sn.The carbon shell not only improves the electrode conductivity,but also suppresses the volume expansion of the core Co₂P/Sn.Therefore,the Co₂P/Sn@NC demonstrates 394 mA h g^-1 at 0.1 A g^-1,high rate capability of 168 mA h g^-1 at 5 A g^-1,and excellent cyclability with 87%capacity retention after 10000 cycles.（2）Layered SnSe has a high Na-ion storage capacity,large interlayer space and narrow bandgap,which is a promising anode material for SIBs.We prepare SnSe nanoplates vertically grown on nitrogen-doped carbon nanobelts（SnSe/NC）with strong interfacial Sn-C bonding by ion-exchange strategy.The Sn-C bonding can be verified by the XPS and DFT techniques and the N atoms were demonstrated to play a key influence on the formation of Sn-C bonding.The Sn-C bonding at the interface can not only promote the rapid electrons transfer between SnSe and NC,but also inhibit the agglomeration of SnSe nanosheets.In addition,DFT calculations show that SnSe delivers a low interlayer Na⁺diffusion barrier（0.1 eV）,beneficial to the rapid transfer of Na⁺between SnSe layers.As expected,SnSe/NC exhibits large capacity and high rate performance with a specific capacity of 570 mA h g^-1at 0.1 A g^-1 and 88 mA h g^-1 at 20 A g^-1.（3）Thick electrodes with high mass loading of active materials are important to achieve large areal capacity and energy density.However,the long Na⁺transport paths and large interface resistance in the thick electrode result in their lower electrochemical utilization ratio and capacity.SnSe is a promising anode material for thick electrode in SIBs due to its large interlayer space enabling fast electrochemical kinetics.Herein,few-layer SnSe nanosheets are prepared by a simple liquid-phase exfoliation method and then free-standing film of SnSe/carbon nanotubes（SnSe/CNTs）are obtained by vacuum-filtering the dispersion of the SnSe nanosheets and CNTs.The as-synthesized electrode film of SnSe/CNTs has abuandant pore which is favorable for the penetration of electrolyte and high areal mass loading of SnSe(20 mg cm^-2),resulting in large areal capacity.Combing the merits of high conductive CNTs skeleton with rapid Na⁺transport in few-layer SnSe nanosheets,the SnSe/CNTs electrodes with a thickness of 196μm and 20 mg cm^-2 loading has a reversible initial specific capacity of 321 mA h g^-1 at 0.2 mA cm^-2,and high areal capacity of 6.3 mA h cm^-2,suggesting good sodium storage performance.（4）Sn-based and P-based anode materials have large capacities and Se has a higher conductivity compared to sulfur.Therfore,we,for the first time,investigate the Na-ion storage properties of the Sn-based ternary compound SnPSe₃.We prepare the SnPSe₃@graphene（SnPSe₃@G）nanocomposite using a facile secondary ball milling method.The ex-situ Raman,HRTEM and SAED techniques confirm the reversible phase transition mechanism of SnPSe₃@G for sodium storage.Owing to the good intrinsic conductivity induced by the high Se content,intimately coating of conductive graphene and optimal choice of electrolyte,SnPSe₃@G shows excellent rate performance and cycle life.It can reach 823 mA h g^-11 at 0.1 A g^-1 with the high first coulombic efficiency of 95%.A high specific capacity of 362 mA h g^-1 can be achieved at 10 A g^-11 and 298 mA h g^-1 can be retained after 4000 cycles.When the SnPSe₃@G anode was tested in a full cell of Na₃V₂（PO₄）₃/C//SnPSe₃@G,a high energy density of 126.2 Wh kg^-1 can be reached,demonstrating the great application potential of SnPSe₃ for SIBs.