| Although metal oxides possess relative high theoretical capacities as the electrode materials of lithium-ion battery(~ 1000 m Ah g- 1), there are two major obstacles ahead of their further practical applications: one is the poor cycle stability during charge and discharge processes, Li-ions insertion/extraction in the electrode materials could lead to large volume change, structure collapse and agglomeration, which bring serious decay in reversible capacity during repeated charge-discharge cycles;Another is poor rate capability, generally,metal oxides have low conductivity which is undesirable for electrode materials, there is also ahigh capacity loss when charge and discharge current densities increase. In order to address the issues, intensive works focusing on improvementof electrical conductivity and inhibition of volume change during Li-ions insertion and extraction have been conducted to avoid structure collapse and agglomeration. Here, we selected two typical metal oxides(Mo O3 and Sn O2) as electrode materials, with an aim of improving their electrochemical properties by decreasing the material size dimension and compositing with graphene. The resulting materials demonstrate excellent electrical conductivity and characteristics of Li+/Na+ storage. The detailed research works include:(1) Fabrication of ultra-thin two-dimensional Mo O3 nanosheets by exfoliationin mixed solvents. The typical synthesis protocol involves using α-Mo O3 powder as the precursor and low-boiling point solvents(isopropanol(IPA)/water) as the dispersion media. By choosing solvents with appropriate composition(IPA: water =1:1 v %), Mo O3 nanosheets with high aspect ratios(80~800), thin thickness(2~6 nm) and single-crystalline structure can be achieved. The results show that the exfoliated Mo O3 nanosheets exhibit a very high reversible capacity of 1110 m Ah g-1, almost close to theoretical capacity of α-Mo O3, good rate capability and cycling stability, which could be attributed to the ultrathin two-dimensional morphology which can effectively shorten diffusion path length and give rise to ultra-fast solid-state diffusion of Li+ ions in the nanosheets. The results indicate that such a 2D Mo O3 nanosheet holds a promise as anode material for high-capacity LIBs.(2) Molybdenum oxide(Mo Ox) nanoplatelets/graphene nanocomposites were synthesized by a simple template-free hydrothermal reaction followed by controlled thermal treatment.Mo Oxnanoplatelets are uniform vertically decorated on graphene surface with average thickness of 20 nm and a width of 50-150 nm. The r GO framework provides high-conductive electron pathways between Mo Ox nanoplatelets, and additionally it alleviates the nanoplatelets degradation during cycling as r GO relieves stress during Li-ion intercalation and deintercalation. Moreover, as Mo Ox nanoplatelets directly grew on the r GO surface, binders and additives that typically used in traditional electrode fabrication processes are eliminated in the anode, leading to lower resistance, shorter ions diffusion distance and better electrochemical impedance of the electrode. Meanwhile, those Mo O3 nanoplatelets could increase specific surface area thus provide much more Li+ diffusion pathways and facilitate electrolyte wetting to the electrode materials during electrochemical charge-discharge processes. The volume expansion during electrochemical processes and mechanical stress could also be relieved by numerous ravines between nanoplatelets.The reversible capacity of Mo Ox-p-G with the Mo Ox content of 79% as anode of Li-ion and Na-ion batteries at the 30 th cycle are 835 and 200 m Ah g-1, respectively.(3) Based on two dimensional properties of graphene template, hollow Sn O2 nanopartices coated graphene sheet composite(Sn O2-h-G) were fabriacted. The prepared composites retain the 2D morphology of graphene, and Sn O2 nanopartices with size distribution of 3-10 nm and specific surface area of 216.6 m2 g-1(measured by BET method) were synthesized. Microporous, mesoporous and macroporous structures co-exsist in the sheet composites,mainly in the form of mesoporous. This kind of hollow sheet structure could largely take advantage of graphene surface area, and the composite were obtained with uniform distribution and minimum size of Sn O2 nano-particles.Hollow structure could provide buffer space for volume expansion during Li-ion insertion/extraction. As an anode material of Li-ion battery, the Sn O2-h-Gdelivered 200 m Ah g-1 higher reversible capacity than that of pure Sn O2 electrode. It demonstrates that graphene can actually improve the electrochemical performance of Sn O2 due to its excellent electric conductivity.(4)Sn O2–graphene oxide nanocomposite has been synthesized by a pot process of hydrothermal reaction without any surfactant. The influence of p H value on the electrochemical properties of the product was investigated, the results showed that the sample(Sn O2-G-400) with p H = 1.0 and heat treatment under 400 ℃ exhibited the highest capacity and best cycle stability. This nanocomposite has ultrafine Sn O2 nanoparticles(2-5nm) decorated on an r GO framework. When used as an anode of Li-ion battery, the Sn O2-G-400 delivered a reversible Li-storage capacity of 1037 m Ah g-1 with an outstanding capacity retention of 91% over 80 cycles. Moreover, it also demonstrates a good rate capability, displaying a capacity of 940 and 320 m Ah g-1 at high rates(200 and 800 m A g-1). The high performance of Sn O2/r GO anode can be explained by the lamellar structure which effectively provideshigh specific surface area, and make full use of Sn O2 active substances, the nanoscaled Sn O2 can also shorten the Li-ion diffusion path.With combined advantages of easy operation, low cost and environmental benignity, the Sn O2/r GO nanocomposite would be a promising anode for Li-ion batteries. |
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