Coaxial-Nanocable Structured MnFe₂O₄ Based on Polydopamine-Modified MWCNT for Anode Materials of Lithium-Ion Batteries

Energy storage technology for lithium-ion batteries (LIBs) has made rapid progress, and has been applied in a wide range of applications such as portable electric devices, hybrid electric vehicles (HEVs) or electric vehicles (EVs) even in large-scale energy storage system (ESS). Nevertheless, the electrode materials still require the enhancement on its energy or power densities for satisfying the needs of global market. Meanwhile, research on high-capacity anode materials beyond the low theoretical capacity limit of 372 mA h g-1 of commercialized graphite materials is performed actively in numerous research groups. As one of the alternative candidates, transition metal oxides have strong advantages, which can substitute the graphite materials, such as high theoretical capacity (≥500 mA h g-1), earth abundant and simple preparation process. However, the charge/discharge process of “conversion reaction”, which is typically shown in oxide-based anode materials, causes lower cyclability resulted by repetitive volume change in the reaction process. Also, oxide materials generally exhibit lower electrical conductivity than carbon or metal-based anode materials. Recently, metal ferrites (MFe2O4, M = Mn, Co, Ni, Zn) show higher level of electric conductivity and chemical stability from the modified electronic structure which is resulted by doping of foreign metal atoms. Among the various metal ferrites, MnFe2O4 is attractive anode material, because it shows high theoretical capacity of 917 mA h g-1, and it consists of low cost Mn, Fe elements. In our work, to compensate for the limitations of oxide materials using MnFe2O4 nanoparticles, we have introduced two representative technologies of nano-structuring and carbon composite to improve the rate capability and cycle performance of the ferrite system. In particular, we also have introduced polydopramine (PDA) coating technique using versatile properties of the polydopamine such as high hydrophilicity and adhesive ability from rich functional groups (e.g. –OH catechol, –NH2 amine). Mussel-inspired PDA is well known as its strong adhesion to organic/inorganic surfaces, good environmental stability, biocompatibility, and excellent dispersibility in water. In this regard, we applied the PDA coating layer to carbon nanotubes (CNTs) to mitigate the strong inner-tube van der Waals interactions, and it helps to fabricate coaxial nanocable structures with increased dispersibility in the precursor solution of the MnFe2O4. In our process, metal ions in the solution are attracted by –OH catechol groups in PDA layer, and MnFe2O4 nanoparticles are co-precipitated at low temperature (98 oC) around CNT surfaces. Eventually, the nanoparticles are attached on the CNT by PDA layer. As a result, well defined coaxial nanocable structures, which is composed of MnFe2O4 nanoparticles and CNT core, were successfully prepared without a particle aggregation after our synthesis process. These coaxial nanocable anode materials effectively supply electrons through the CNT core with well contacted MnFe2O4 nanoparticles on the surface by the aid of the strong adhesive ability of the polydopamine coating layer. Besides, increased surface area for a facile electrolyte penetration provides better Li+ ion kinetics with an enhanced ionic conductivity. Furthermore, homogeneous CNT matrix can accommodate volume changes in the repetitive conversion reaction, and improves the cyclic stability of anode materials.