Aligned carbon nanotubes@ manganese oxide coaxial arrays for lithium ion batteries

Abstract

Lithium ion batteries (LIB) have attracted widespread interests due to their higher specific energy compared with nickel–cadmium batteries and lead–acid batteries. However, their specific power remains too low to power hybrid and full electric vehicles. The specific energy needs to be improved further to meet the requirement for the future applications. Three dimensional (3D) nanomaterials could be a new approach to enhance the specific power by tuning the pore structure for fast mass transfer of the electrolyte and reducing electron and lithium ion diffusion distance, and improve the specific energy by fully utilizing the electrochemically active materials.

In this thesis, aligned carbon nanotubes (ACNTs) were grown directly over different metal foils as 3D current collectors with high specific surface area, high electrical conductivity and regular mesopores for fast electrolyte transfer. Subsequently, manganese oxide (MnOx) was coated over ACNTs through spontaneous reduction of potassium permanganate as extra active materials to increase the specific capacity. The unique structure provides remarkable advantages such as large surface area of the 3D thin MnOx layer, very low electric resistance of the electrode, excellent lithium ion diffusion in the MnOx layer and in the channel between the tubes, and excellent elastic properties to tolerate the large volume changes during charge and discharge.

At the beginning, Al foil was selected as substrate for the growth of aligned carbon nanotubes. The obtained 3D aligned carbon nanotubes@manganese oxide coaxial arrays on Al foils (ACNTs@MnOx/Al) were studied as cathodes for LIB. A remarkable improvement of specific capacity, rate capability and stability of the LIB were achieved. The obtained electrodes can deliver a specific capacity of 308 mAh g-1 at 0.1 C based on the mass of MnOx, which corresponds to the incorporation of one lithium ion per MnOx unit, 95 mAh g-1 at 20 C and maintain a capacity of 133 mAh g-1 after 100 cycles at 1 C. The continuous growth of a solid electrolyte interface layer due to trace amounts of water remaining in the MnOx layers has been identified as the main cause of the capacity fading. The stability was improved due to the elimination of water, but the initial capacity decreased with increasing annealing temperature owning to the decrease of manganese oxidation state.

Subsequently, copper foil was selected as substrate for the growth of carbon nanofiber yarns. The obtained carbon nanofibers@manganese oxide over copper foil (CNFs@MnOx/Cu) was tested as anodes for LIB. It shows superior electrochemical performance. The initial specific capacity reaches up to around 998 mAh g-1 at 0.05 C based on the mass of carbon nanofibers and manganese oxide. The electrodes could deliver a capacity of 630 mAh g-1 at the beginning and maintain a capacity of 440 mAh g-1 after 105 cycles at 0.5 C. The high initial capacity can be attributed to the presence of porous carbon nanofiber yarns which have good electrical conductivity and the MnOx thin film which make the entire materials electrochemically active. And the high cyclic stability of CNFs@MnOx can be ascribed to the MnOx thin film which can accommodate the volume expansion and shrinking during charge and discharge and the good contact of carbon nanofibers with MnOx and copper foil.

Then, stainless steel foil was selected as substrate for the growth of aligned carbon nanotubes followed by MnOx coating before carbon coating. The carbon coated aligned carbon nanotubes@manganese oxide arrays over stainless steel foil (ACNTs@MnOy@C/SS) can deliver an initial capacity of 738 mAh g-1 at 0.5 C based on the mass of carbon and MnOy, with 100% capacity retention up to 100 cycles, and a capacity of 374 mAh g-1 at 5 C. The external carbon layer was recognized as a key component for the excellent performances and the mechanisms were investigated via electrochemical impedance spectroscopy, electron microscopy and X-ray diffraction. The external carbon layer increases rate capability by enhancing electrical conductivity and maintaining the low mass transfer resistance, improves cyclic stability by avoiding metal oxide aggregation and stabilizing the solid electrolyte interface. These coaxial nanotube arrays present a promising strategy for rational design of high performance binder free anodes for LIB.

Finally, ACNTs@LiMn2O4/SS is prepared through hydrothermal method. A high initial discharge capacity of 122.1 mAh g-1 has been achieved. Discharge capacities of 103.4 and 88.1 mAh g-1 are retained after 1000 and 2000 cycles, respectively, indicating excellent cyclic stability.