Synthesis And Lithium Storage Properties Of Carbon-based Metal Oxide Nanocrystalline Anode Materials

Lithium ion batteries?LIBs?are considered as a new type of green rechargeable battery due to their high energy density,long cycling life and no memory effect.The graphite anode material has been widely used in the commercial LIBs,but its application is inhibited because of the low theoretical capacity(372 mAh g^-1),which cannot meet the world's growing demand for batteries with high energy density and high power density.Germanium dioxide?GeO₂?and tin dioxide?SnO₂?have high theoretical capacities of 1125 mAh g^-1 and 782 mAh g^-1,respectively,both of which have been used as anode materials for LIBs.However,the practical application of both anode materials is prevented by the large volume change during lithiation/delithiation process,which leads to electron isolation from current collector and aggregation of active materials,and finally results in quick capacity fade.Additionally,both materials show poor electron transport,as a result,significantly impact on their rate performance.The theoretical capacity of titanium dioxide?TiO₂?is 335 mAh g^-1,and it can remain structural stability during charging and discharging,which is vital for the safe operation of LIBs.However,the rate performance of TiO₂ electrode is also poor owing to the poor electron transport.Herein we have synthesized a series of metal oxide composites through simultaneous polymerization method or twin polymerization method.The synthesized composites are designed into nanostructure and coated by carbon materials.Nanocrystallization of the metal oxide can shorten the transmission distance of ions and increase the contact area between the electrode and electrolyte.Meanwhile,the three-dimensional carbon network coating of the metal oxide nanoparticles not only improves the electrical conductivity of the composite,but also effectively alleviates the volume expansion of the metal oxide during lithiation/delithiation process,and also inhibits the agglomeration of the active material.The research works are mianly divided into the following three parts:?1?Four kinds of GeO₂/S-C nanocomposite?GSC₀,GSC₁,GSC₂ and GSC₃?consisted of nano-GeO2 coated with three-dimensional sulfur doped carbon network were synthesized by simultaneous polymerization method.Among them,GSC₃ shows the smallest average particle size?16 nm?and the highest carbon content?30.8%?.The electrochemical property of four composites?GSC₀,GSC₁,GSC₂ and GSC₃?as anode materials for LIBs were investigated.It was found that the GSC3 electrode had the highest capacity retention rate?46.6%?when the current density increased 20 times(50-1000 mA g^-1).When the current density returns back to 50 mA g^-1,a highly stable capacity of 634.8 mAh g^-1 can be resumed,and the capacity retention rate is as high as 90.6%.The cycling performances of four composites were measured at a current density of 200 mA g^-1.For GSC₃,the discharge capacity starts at 606.3 mAh g^-1 and still maintains 289 mAh g^-1 after 200 cycles,the capacity retention rate is 47.7%.Even at a higher current density of 1000 mA g^-1,a capacity of 179mAh g^-1 is retained after 500 cycles.?2?Four kinds of TiO₂/S-C nanocomposite?TSC₀,TSC₁,TSC₂ and TSC₃?consisted of naono-TiO₂ coated with three-dimensional sulfur doped carbon network were fabricated by simultaneous polymerization method.Among them,TSC3 sample shows the smallest average particle size?7.5 nm?and the highest carbon content?60.7%?.The electrochemical properties of the four composite anodes?TSC0,TSC1,TSC2 and TSC3?were tested.TSC3 delivers the highest discharge capacity of 386 mAh g^-1 at 50 mAh g^-1,when the current density to 500 mA g^-1,a reversible capacity of 213 mAh g^-1 can be retained,and when the current density turns back to 50 mA g^-1,a reversible capacity of 343 mAh g^-1 can be resumed.Furthermore,the reversible capacity maintains 232 mAh g^-1 after 200 cycles,and the capacity loss is only 4.1%.TSC₃ exhibits a reversible capacity of 156 mAh g^-1 at a high rate of 1000 mA g^-1 after 500stable cycles,and retains 96.8%of its initial capacity(162 mAh g^-1).?3?SnO₂/C nanocomposites were successfully synthesized by twin polymerization method?TP-SnO2/C?and simultaneous polymerization method?SP-SnO2/C?with the nano-SnO₂ evenly coated by continuous phase carbon matrix,respectively.The TP-SnO₂/C nanocomposite synthesized by twin polymerization method has the smallest average particle size?4.8 nm?and the highest carbon content?32%?.The electrochemical properties of these composites as anode materials for LIBs were investigated.TP-SnO₂/C exhibits discharge capacities of 706.1 mAh g^-1 and 341.5 mAh g^-1 at current densities of 100 mA g^-1 and 1000mA g^-1,respectively,and the capacity retention rate is 48.4%.In addition,a reversible capacity of 542.6 mAh g^-1 is obtained when the current density returns to 100 mA g^-1,and the capacity retention rate is 76.9%.Furthermore,the reversible capacity of TP-SnO₂/C electrode remains at 280.3 mAh g^-1 after 200 cycles at a current density of 200 mA g^-1 with a capacity retention rate of 52.8%.