Salt Template Assisted Controllable Preparation Of Tin-based Lithium/sodium Ion Battery Anodes And Their Energy Storage Mechanisms

With the development of electric vehicles,the requirements of lithium ion battery for electrode material is getting higher,in which graphite anode has been difficult to meet the needs of high energy and power density of power batteries due to the low theoretical capacity.In addition,due to the growing shortage of lithium salt resources,scientific researchers are also committed to the development of sodium-ion batteries as a substitute for lithium-ion batteries.Whether for lithium-ion battery or sodium ion battery,the development of excellent anode material is the current central issue.Among the new generation of anode materials,tin based materials have attracted much attention due to their high theoretical specific capacity,good conductivity,and suitable working voltage.However,the large volume expansion of tin based materials during lithium/sodium insertion limits their development.The combination of tin based materials with high conductivity carbon materials（such as porous carbon,carbon nanotubes,graphene and so on）is an effective way to improve the electrochemical properties of tin based anodes.However,there are still many problems in the Sn/C composites,such as complex preparation process,high cost and the performance of lithium/sodium storage still need to be further improved.Therefore,with the assistance of low-cost soluble salt template,we designed and synthesized a series of tin/carbon micro-nano composites by in situ chemical vapor deposition（in-situ CVD）and explored the relationship between structure and energy storage properties for anode materials.The main research contents and results are as follows:1.Using the surface of water-soluble NaCl as template,nickel salt as catalyst,two dimensional carbon coated metal composites,i.e.,carbon encapsulated Ni₃Sn₂nanoparticles uniformly embedded into two-dimensional porous graphitized carbon nanosheets（Ni₃Sn₂@C@PGC）are synthesized by the combination of chemical vapor deposition（CVD）and chemical vapor transformation（CVT）method.The influences of calcination temperature,tin-salt content in CVT process and the amount of template in the precursor on the phase and structure of Ni₃Sn₂@C@PGC are investigated,the electrochemical performance of Ni₃Sn₂@C@PGC as anode for LIB is explored.It is revealed that Ni₃Sn₂ is the most stable phase during the transformation of Ni to Sn,which is in accordance with the most stable alloy proportion in Ni-Sn phase diagram;The addition of NaCl can not only promote the formation of two-dimensional nanosheets,but also reduce the particle size of Ni₃Sn₂.Meanwhile,the thickness of two-dimensional carbon nanosheets can be controlled by adjusting the ratio of NaCl to carbon source,when the ratio of NaCl to carbon source is 14.7 g:2.5 g,carbon coated Ni₃Sn₂ nanoparticles（²0 nm）can be embedded into30 nm porous graphitized carbon nanosheets.Since Ni₃Sn₂ nanoparticles are protected by carbon coating and two-dimensional carbon nanosheet encapsulating,the Ni₃Sn₂@C@PGC has excellent conductivity,structural stability and mechanical strength thereby significantly improved the cycling stability and rate capability of active material:the capacity can be maintained at 585.3 mAh g^-1 after 100 cycles at0.2 C(¹14 mA g^-1)and a high capacity of 314 mAh g^-1 can be delivered at 2 C for the Ni₃Sn₂@C@PGC electrode.2.Using 3D NaCl assembly as template,SnCl₂ as metal source,citric acid as carbon source,3D porous graphene networks anchored with Sn nanoparticles are synthesized by one-step in situ chemical vapor deposition（in-situ CVD）method.The influences of calcination temperature,atmosphere and other CVD parameters on the phase and particle size of tin materials are investigated.The synthesis mechanism and electrochemical property of 3D Sn@G-PGNWs as anode for LIB are explored.It is revealed that the calcination temperature and atmosphere have a great influence on the phase composition and morphology of Sn@G-PGNWs.When the calcination temperature is increased to 750℃and hydrogen is used as the reducing atmosphere,the carbon source can be converted into three-dimensional graphene structure through the catalyzing of Sn nanoparticles.The confinement effect between the NaCl assembly leads to the in situ synthesis of Sn@graphene nanoparticles and three-dimensional graphene networks as well as the strong interfacial bonding between them.When the NaCl,SnCl₂ and citric acid are mixed at ratio of 14.7 g:0.384 g:2.5 g,the Sn nanoparticles with uniform particle size in the as-produced composite are completely coated with graphene shell（¹ nm）,meanwhile,the Sn@graphene nanoparticles are tightly and uniformly riveted by the three-dimensional graphene network.As anode for LIB,the 3D Sn@G-PGNWs demonstrate excellent lithium storage performance including superior rate capability(a capacity of 270 mAh g^-11 is delivered at 10 A g^-1)and long cycle life at high rate(657 mAh g^-1 is maintained after 1000 cycles at 2 A g^-1,capacity retention is 96.3%).The important factors influencing the lithium storage performance of 3D Sn@G-PGNWs composites are as follows:the uniform distribution of small tin nanoparticles facilitates the fast transport of lithium ions and electrons;core-shelled Sn@graphene nanostructures can effectively alleviate the volume change of Sn nanoparticles;the high electrical conductivity and mechanical property of 3D graphene as well as the strong interface bonding between graphene network and Sn@graphene nanoparticles can greatly enhance the mechanical properties,structural stability and conductivity of Sn@G-PGNWs composites.3.Three dimensional continuous hollow sandwiched carbon-tin dioxide-carbon composites（C@SnO₂@C HNSs）are prepared by one-step CVD method using NaCl as template.The influences of CVD temperature,carbon source and other parameters on the phase composition and structure of C@SnO₂@C HNSs are investigated.The electrochemical properties of the composite are explored.It is revealed that CVD parameters have important influences on the phase and microstructure of C@SnO₂@C HNSs,When the CVD temperature is 700℃,the acetylene content is5%,and the calcination time is 0.5 h,the resulting C@SnO₂@C HNSs is composed of three-dimensional interconnected hollow carbon nanoboxes,and the size of a single nanobox is about 1μm,the walls of the nanoboxes are sandwich structure,i.e.,⁶ nm SnO₂ layer which consist of ultrasmall SnO₂ nanocrystals（2⁵ nm）are tightly confined between the citric acid pyrolytic carbon core（4 nm）and the acetylene pyrolysis carbon shell（5 nm）.The C@SnO₂@C HNSs composites have excellent electrochemical performance as anode for LIB:the capacities of 900 mAh g^-1 and 600mAh g^-11 are obtained at 0.1 and 5 A g^-1,respectively.As anode for sodium ion batteries,a capacity of about 400 mAh g^-1 is obtained by the C@SnO₂@C HNSs electrode at 0.1 A g^-1,and the capacity can be maintained at about 200 mAh g^-1 after3000 cycles at 4.6 A g^-1,which has exceeded the best level of SnO₂-based anode materials for SIB.4.SnSb in-plane nanoconfined 3D N-doped porous graphene networks（3D SnSb@N-PG）are prepared by industrial spray-drying method combined with in-situ CVD process using salt assembly as template.The influences of different salt templates（sodium chloride,sodium carbonate and sodium silicate）and CVD temperature on the morphology and microstructure of 3D SnSb@N-PG are studied,the synthesis mechanism and energy storage performance of 3D SnSb@N-PG are explored.It is revealed that the salt assembly template provides confined-space for the process of SnSb nanocrystals in-situ catalyzing three-dimensional graphene;The diameter of SnSb nanocrystals in 3D SnSb@N-PG can be reduced to about 13 nm by in-situ catalysis,and the SnSb nanocrystals are in-plane embedded into graphene（2nm）,whose surface can be completely coated with graphene.The 3D SnSb@N-PG composite（SnSb content:⁴5 wt%）has excellent performance as anode for sodium ion batteries:a capacity of about 400 mAh g^-1 is obtained at 0.1 A g^-1,and the capacity can be maintained at about 190 mAh g^-1 after 4000 cycles at 10 A g^-1.In-situ TEM observation of 3D SnSb@N-PG shows that the SnSb nanocrystals turn into hollow structure after repeated cycles,further confirms that the SnSb particles are in-plane embedded in graphene（completely encapsulated by graphene）;DFT simulation verifies that the interface of SnSb/graphene has the highest adsorption energy to Na atom,which proves that SnSb@graphene have interfacial sodium storage.The calculation of ion diffusion coefficient verifies the advantage of the 3D porous open-frame structure in shortening the solid-phase diffusion path of ion and increasing the ion diffusion rate.