Synthesis And Electrochemical Performances Of Tin-based Composites As Anode Materials For Lithium-ion Batteries

Lithium-ion batteries (LIBs) serve widely as the power source for many portable electronic devices due to their outstanding advantages such as high energy density, long cycle life, environmental benignity, etc. LIB is also a promising solution for clean vehicles. However, their power and energy densities need to be further increased particularly for the application in electric vehicles. Compared to the commercial graphite anode materials (372mAh/g, LiC6), Sn and its compounds are very attractive because of their high theoretical specific capacity (990mAh/g for Li4.4Sn). Nevertheless, the practical use of Sn anodes in lithium-ion batteries is generally hindered by the severe volume change during the Li alloying and dealloying process, resulting in serious structure damage of the active material and the electrode consequently rapid loss of the electrode capacity. In our work, we compared several strategies proposed to improve the electrochemical performance of the tin-based negative electrode materials, including (1) forming active/active composites and (2) active/inactive composites;(3) constructing porous metal anodes;(4) doping with Sb for nano-SnO2with improved electronic conductivities and (5) assembling graphene clamped nano-SnO2. We are pursuing innovations in design and preparation of both active materials and electrodes, trying the develop Sn based anodes with both high capacity and cycling stability and put forward their practice application in LIBs. The main results and new findings of this work are summarized as follows:(1) Sn-Sb/C composite with different chemical composition have been prepared by a simple hydrothermal polymerization reaction followed by hydrogenthermal reduction method. It was found that the Sn/Sb atomic ratio and the reduction temperature have important influence on the morphologies and particle sizes subsequently the electrochemical performances of the resultant Sn-Sb/C. By feeding a Sn/Sb atomic ratio of1, nanometer SnSb/C materials (-200nm in particle sizes) were obtained at a hydrogenthermal treatment temperature of550℃. Lowering the reduction temperature or increasing the Sn/Sb atomic ratio have resulted in micometer (2-5μm in particle sizes) SnSb/C and Sn/SnSb/C respectively. Among all the prepared samples, the SnSb/C obtained at550℃showed the highest electrochemical performances, which delivered a reversible lithium-ion storage capacity of576mAh/g at the first cycle, and436mAh/g after20cycles.(2) Sn/MgO/C composite has been synthesized by a mechanochemical reduction of SnO with Mg in the presence of acetylene black. Although the nanometer dispersion among Sn, Mg and acetylene black has been realized, and the delivered initial capacity of about605mAh/g reached the theoretical capacity of the composite, it exhibited a very poor cycling stability. Only a capacity of150mAh/g was found at the fiftieth cycle, suggesting the Sn/MgO/C structure is not a good design for Sn based anode metarials.(3) Considering the inevitable volume expansion during the lithium alloying of Sn based materials, we proposed a new strategy for metal based electrodes with in-situ pores by a direct electrochemical removal of O from solid oxide electrodes. Porous Sn/SnSb electrodes were prepared by electrolysis of mixed SnO2-Sb2O3(molar ratio=4:1) solid electrodes in1mol/L aqueous H2SO4solution. After the electroreduction, the macrometer oxide particles were converted to porous aggregations of nanoparticles of Sn/SnSb. Such electrode exhibited lithium-ion storage capacity (800mAh/g) and good cycling stability (70%of capacity retention at the40th cycle) between0.02and1.50V (vs. Li+/Li) at a current density of100mA/g. In comparison, although the oxide precursor electrode showed considerable initial capacity as well, most of its capacity lost after40cycles. The porosity of the Sn/SnSb electrode can be controllable by pore-forming the SnO2-Sb2O3electrodes using NH4HCO3. With an addition of15vol.%NH4HCO3in the precursor, more porous Sn/SnSb electrode with enhanced electrochemical performances was achieved. It delivered an initial capacity of880mAh/g and a40th cycle capacity of620mAh/g at a charge/discharge current of100mA/g. Particularly, the more porous electrode showed very high rate performance, which delivered520mAh/g at1A/g at the40th cycle.(4) Sb-doped SnO2(ATO) nanocomposites were synthesized by a simple and rapid sucrose assisted hydrothermal method using SnC14˙5H2O and Sbc13as the starting materials aiming for improved electronic conduction. The particle sizes of the Sb-doped SnO2decreased with the Sb content increased, and Sb-doped SnO2with particle sizes as fine as5nm has been prepared for a Sb doping content of10at.%. Which also showed the best perfornances used as anode materials for lithium-ion batteries in a potential range of0.02-2.OOV. Its lithium storage capacity was789mAh/g at initial and348mAh/g at100th cycle. Sb-doped SnO2with lower or doping content showed much worse performances. Compared to pure SnO2, the10at.%Sb-doped SnO2showed much smaller electrochemical polarization, and can alleviate the volume change greatly.(5) Considering the high cost, processing difficulties of the SnO2/graphene composie materials, we proposed a new strategy based on sulfuric acid intercalated graphene oxide (SIGO) for the preparation of graphene clamped nano-SnO2materials (GCNSnO2) with high performance for anode materials of LIBs. SIGO is the direct product of graphite oxidation in sulfuric acid, which is easily available in comparison with the general GO, but its properties have not been studied. One of the most important characteristic is that it can be expanded and exfoliated to graphene at a very low temperature (just above100℃). We then carried out dydrolysis reaction of SnCl4˙5H2O at the presence of SIGO to coat nano-SnO2on SIGO, which was then exfoliated to GCNSnO2via the low temperature expansion. Compared to other existing methods, the SIGO method can proceed in air, and has the advantages of reductant free, easy operation, reductant free, low-energy and environmental friendly, and is readily scalable to industrial levels. The initial charge specific capacity of the SnO2-graphene composite at a current density of200mAh/g was858mAh/g. After270th discharge-charge test, it still delivers572mAh/g, corresponding to a capacity attenuation rate of only0.11%per cycle. In the GCNSnO2nanostructures, all the theoretical capacity of SnO2relating to Sn can be fully and stablely utilized. Furthermore, this composite show a good rate performance with a reversible capacity of360mAh/g at2A/g, which delivers a reversible capacity of360mAh/g at2A/g during the studied forty cycles. These findings suggest that both the SIGO process and the resultant GCNSnO2are promising for practical applications.