Li-ion batteries (LIBs) have been widely employed in a myriad of portable electronics and gained great success. To realize their successful applications to power electric vehicles, it is essential to improve the electrochemical performance of the electrode materials, especially the power/energy densities and the cyclic life of electrodes. This thesis focuses mainly on developing new nanocarbon composites containing metal oxides, e.g., Li₄Ti₅O₁₂ (LTO) and SnO_x, in an effort to formulate anode materials having high capacities at high discharge currents with excellent capacity retention after long cycles. The objective also includes establishing the relationship between the electric conductivity, Li-ion transfer coefficient, surface area and micro-structure of the anode materials and thei...[ Read more ]

Li-ion batteries (LIBs) have been widely employed in a myriad of portable electronics and gained great success. To realize their successful applications to power electric vehicles, it is essential to improve the electrochemical performance of the electrode materials, especially the power/energy densities and the cyclic life of electrodes. This thesis focuses mainly on developing new nanocarbon composites containing metal oxides, e.g., Li₄Ti₅O₁₂ (LTO) and SnO_x, in an effort to formulate anode materials having high capacities at high discharge currents with excellent capacity retention after long cycles. The objective also includes establishing the relationship between the electric conductivity, Li-ion transfer coefficient, surface area and micro-structure of the anode materials and their electrochemical performance.

LTO possesses an excellent cyclic life due to negligible volume change during cycles, but suffers from a very low conductivity which limits its application at high discharge currents. LTO powders are doped with Sn ions and carbon nanofibers (CNFs) are incorporated to achieve a satisfactory power density. Doping with Sn results in the formation of Li_3.9Sn_0.1Ti₅O₁₂ powders with concomitant enhancement in conductivity by about six times and the creation of numerous pores and a thin carbon coating layer on the surface of particles, which greatly improve the electrochemical performance. The incorporation of CNFs establishes an extensive conductive network both in and out of LTO secondary particles to form an urchin-like structure, which gives rise to an exceptional capacity of 122 mAh g^-1 after 500 cycles charge/discharged at 10C. Graphene is also investigated as potential conductive additives in LTO to replace the traditional additives, carbon black (CB). Graphene’s extremely large aspect ratio results in a low percolation threshold of about 1.8 wt.% among LTO particles. With only 5 wt.% graphene, the LTO/graphene composite anode delivers a much better rate capability than those containing 15 wt.% CB.

SnO_x anodes have a theoretical capacity approximately twice that of the traditional graphite anode. To improve the poor cyclic performance arising from the huge volume change of Sn during cycles, nanosized SnO₂ particles are uniformly dispersed on the surface of carbon nanotubes (CNTs) and graphene. Hybrid composites containing SnO₂, graphene and CNTs in the form of both power and paper are fabricated to achieve much higher capacities at high currents than SnO_x anodes acting alone. Electrospun CNF films containing sub-nanosized SnO_x are prepared as freestanding electrodes. The amorphous, sub-nanosized morphology facilitates the reaction, Sn+xLi₂O→SnO_x +2xLi⁺ + 2xe^-1, highly reversible, resulting in an extraordinary capacity of 674 mAh g^-1 at 0.5 A g^-1 after 100 cycles.