Research On The Electrochemical Lithiuate/delithiuate Performance Of Intermetallic Compounds

One of the hotspots in the research of lithium-ion batteries is the development for the high-capacity and high-cycling-behavior anode materials to replace the conventional carbon-based materials. In the present work, some novel intermetallic anode materials with high capacity are explored and their cycling stability is improved by using nano-technology and composite-technology.In the present work, single-phase nano-Ni₃Sn₂ intermetallic compound has been prepared by a solvothermal method using SnCl₂·2H2_O, NiCl₂-6H₂O as the starting materials and NaBH₄ as the reducing agent. The solvothermal reactions were carried out at 150℃ for 24 h with Sn as the intermediate product during the formation of Ni₃Sn₂ as detected by XRD characterization. Single-phase nano-Ni₃Sn₂ can be successfully prepared by the solvothermal method at 240℃ for 24 h. SEM observation shows that the particle size of Ni₃Sn₂ prepared by the solvothermal method is 30-50 nm. The first lithiated and delithiated capacities for nano-Ni₃Sn₂ are 386 and 136 mA h g^-1 respectively, and rapid capacity fade occurs during the following cycles. The reasons for its low reversible capacity and poor cyclic performance have been explored by ex-situ XRD experiments. It was found that nano-Ni₃Sn₂ decomposes into Ni and Sn atoms during the lithiated process, then Sn atoms react with Li to formation Li_xSn. However, Ni atoms may form an impenetrable "skin" on the surface of particles or grains, and this skin may block the complete reactions of the intermetallic compound with Li. During the delithiated process, the Li_xSn decomposes into Li and Sn atoms, then Sn atoms react with Ni to form the original phase Ni₃Sn₂. The effect of annealing on the electrochemical performance of nano-Ni₃Sn₂ has been discussed. It was found that the nano-Ni₃Sn₂ after annealing shows improved crystallization and better cycling stability than the unannealed annealed one.Single-phase FeSb₂ nanorods have been prepared by the solvothermal method using FeCl3·6H₂O, SbCl₃ as the starting materials and NaBH₄ as the reducing agent. It can be successfully prepared by the solvothermal method at 250℃ for 72 h as detected by XRD characterization. TEM observation shows that the FeSb₂ nanorods are 20 40 nm in diameter and 0.2 ¹ μm in length. The first charge and discharge capacities for FeSb₂ nanorods are 543 and 854 mA h g^-1, respectively. A reversible capacity of 353 mA h g^-1 is still maintained after 10 cycles with a retention rate of 65%. Although the first coulomb efficiency of this materialis only 64%, it reaches 90% after 10 cylces. However, the ex-situ TEM results show that the FeSb2 nanorods may undergo pulverization and cracking during cycling.Single-phase nano-Bi2Te3 and Bi2Te3/MWNTs composite materials have been prepared by the solvothermal method using B1CI3, Te as the starting materials and NaBFLj as the reducing agent. It was found that nano-Bi2Te3 and Bi2Te3/MWNTs can be successfully synthesized by the solvothermal method at 150eC for 24 h. SEM observation shows that nano-Bi2Te3 particles are sheet-like composed of fine particles, and Bi2Te3 and MWNTs wrap together in Bi2Te3/MWNTs composite material. The capacity for nano-Bi2Te3 is relatively low, but the cycling stability can be improved by changing the range of the cut-off voltage to control correlative electrochemical reactions. The Bi2Te3/MWNTs nano-composite exhibits superior cycling stability than nano-Bi2Te3.Furthermore, a simple low temperature aqueous chemical route using SnCl2-2H2O, SbCb and BiCb as the starting materials and NaBRj as the reducing agent has synthesized Sn, Sb, Bi-based composite materials such as Sn/MWNTs, Sb/MWNTs and Bi/MWNTs. XRD results show that the products have been successfully synthesized at 65QC for 12 h. SEM observation shows that the metal and MWNTs wrap together in composite materials. The sharp difference between the Sb and Sb/MWNTs shows that composite-technology can increase the capacity obviously and improve cycling stability effectively. Ex-situ TEM experiments also show that the capacity fade for the intermetallic compounds occurs in the initial cycles especially in the first lithiated process. This is due to the fact that the largest volume changes occur in the initial cycles causing the severe pulverization and exfoliation of active material.