The Preparation And Energy Storage Properties Of Mg/Sn-based Intermetallic Compound Nanoparticles

With the growing shortage of fossil fuels and acceleration of global warming, energy economy has to be restructured in the future. Energy storage, an important role in the whole chain, deserves more and more attention from worldwide governments. Hydrogen storage has been one of hot research topics chiefly because of its relevance to the energy economy of the future. For utility applications, such as vehicle propulsion, existing hydrogen storage technology (gas cylinders) will have to be expanded. It is currently thought that metal hydride alloys in the powder form will provide a solution to this problem because of their gravimetric and volumetric storage capacities and safe operating pressures. In addition, Li-ion batteries are currently used as portable power sources in the consumer electronic market for its excellent performances among the chemical power sources. However, the materials (LiCoO₂ for the anode and carbon-based materials for the cathode) is claimed to have already achieved an energy density close to the theoretical limit. The only way to meet demand for even more power is to find a new electrode material.Mg-based and Sn-based compound nanoparticles were fabricated by a physical vapor condensation method (DC arc discharge). The compressed targets based micrometer powders were used as materials, then the targets were evaporated in a mixture of active gases (H₂ or CH₄) and inert gas (Ar). After condensation and passivation process, compound nanoparticles were obtained. It partly overcomed the difficulties in melting master alloys because of the large difference between Mg/Sn and the other element. Additionally, compound nanoparticles can be prepared in situ by this method, which may be accompanied by a reduction of cost and the improvement of practicality. In view of the difficulties in evaluating the corrosion properties of Mg nanoparticles, a indirect method was adopted in the present work, ie the binding material (pure conductive tin powders) was used to form various Mg/Sn composites and the corrosion properties of nanoparticles were evaluated indirectly by comparing those composite samples each other.Based on Miedema semi-empirical model and an empirical specific heat equation, the effective heat of formation and its temperature dependence were calculated to explain phase formation in binary Sn-Fe, Sn-Ni, Mg-Ni and Ni-Cu nanoparticles systems. It is shown that predictions of primary phase based on the thermodynamic model have good agreements with the experimental observations. Various Mg-based nanoparticles were prepared by evaporating bulk magnesium. It is shown that the Mg-based nanoparticles produced in inert atmospheres have hexangular crystal habits with particles' sizes ranging from 50 to 400 nm, while the nanoparticles produced in CH₄ atmosphere have amorphous carbon out layers with particles' sizes among 20-100 nm. Two-steps oxidation process can be confirmed for all samples, which may be attributed to the oxidations of out layer and core of nanoparticle. Potentiodynamic polarization results indicate that Mg/C nanoparticles prepared in CH₄ atmosphere exhibit better corrosion resistance properties due to its peculiar carbon doping. For Mg-based hydrogen storage, Mg-Ni, Mg-Cu, and Mg-Sn-Ni nanoparticles were synthesized by arc discharge in a mixture of argon and hydrogen atmosphere and gas hydrogen properties were explored by by a volumetric method. The intermetallic compounds of Mg₂Ni and MgNi₂ formed with existence of Mg, Ni, and MgO in Mg-Ni nanoparticles. After one cycle of hydrogen absorption/desorption process (activation treatment), Mg-Ni nanoparticles exhibited excellent hydrogen absorption kinetic properties. Mg₂Ni phase became the main phase by a phase transformation during the hydrogen treatments. The phenomenon of refinement of grain size in the nanoparticle was also observed after the hydrogen absorption/desorption processes, which was attributed the break of nanoparticles caused by volume expansion/shrinkage during cycling. Maximum hydrogen absorption contents are 1.75, 2.21 and 2.77 wt. % at 523, 573 and 623 K, respectively. As to Mg-Cu system, four phases (Mg₂Cu, MgCu₂, Mg and MgO) were detected in the as-prepared Mg-Cu nanoparticles. It was found that the sizes of nanoparticles were diminished after several cycles of hydrogen absorption-desorption. Measured Pressure-Composition-Isotherms (P-C-I) curves demonstrated that the hydrogen absorption contents were 1.92, 1.98 and 2.05 wt. % at 573, 598 and 623 K, respectively. In order to explore the effect of Sn-doping, Mg_2-xSn_xNi (x =0, 0.1, 0.2) nanoparticles were synthesized by the same method. It is found that the as-prepared nanoparticles exhibit multiphase complex structure. In the first hydriding process, the kinetics properties were improved with the increase of Sn content. After one hydriding/dehydriding cycle, the samples display fast hydrogen absorption kinetics. Based on measured pressure-composition isotherms (P-C-I) curves, the formation enthalpy of hydrides obtained from the equilibrium plateau pressures decreases with increasing Sn content, implying Sn-doping can weaken the stability of hydrides.Four system nanoparticles, i.e. Sn-Fe, Sn-Ni, Sn-Mg and C-coated Sn-Fe nanoparticles, were synthesized by the same method and lithium storage properties were studied by model cell. As to Sn-Fe system, the electrochemical properties of electrodes based on Sn-Fe nanoparticles and its corresponding carbon-coated nanoparticles were studied. For carbon-coated nanoparticles, it is found that Sn-Fe nanoparticles (the "active" phase) dispersed in an electrically conductive matrix of nanometer-sized carbon with poor crystallization (the "inactive" phase). The electrochemical testing of the electrodes suggests that the initial capacity is 562.1 mAh/g of electrode based on Sn-Fe nanoparticles, while it is 385.3 mAh/g for its corresponding carbon-coated nanoparticles. The electrode based carbon-coated nanoparticles exhibits better cycling stability compared to its corresponding carbon-free sample, which is tightly associated with the appropriate active/inactive microstructure. According to XRD analysis, FeSn₂ decomposes to Fe and Sn completely for carbon-free sample after 30 cycling, and it is adverse in the carbon-coated sample. Compared with Fe-Sn carbon-free electrode, Sn-Ni electrode displays low initial capacity (186.6 mAh/g), but better cycling stability is obtained. After 30 cycles, there is no clear evidence to support the decomposition of Ni₃Sn₄ in Sn-Ni electrode. Compound Mg₂Sn was generated and coexisted with residual phases of Mg and Sn in nanoparticles. The initial capacity of Mg₂Sn electrode reaches 430 mAh/g. Two visible plateaus at 0.2-0.3 and 0.5-0.75 V were observed in the discharge/charge curves, which can be attributed to alloying/dealloying reactions between Li and Mg₂Sn, respectively.