Enhanced electrochemical performance of Na2/3[Mn0.55Ni0.30Co0.15]O2 positive electrode in sodium-ion batteries by functionalized multi-walled carbon nanotubes

Nax[MnyNi1-y-zCoz]O2 (NMNC), with a layered oxide structure and a high theoretical capacity (240 mAh g−1), is a promising cathode material for sodium-ion batteries (SIBs). NMNC has a low electronic conductivity, which severely affects the cell performance at high current densities. In this study, we have synthesized P2-phase Na2/3[Mn0.55Ni0.30Co0.15]O2 by a solvothermal process, and used multi-walled carbon nanotubes (MWCNTs) functionalized with carboxylic acid groups to form a nanoscale conductive network in the material. Our results indicate that the electrode kinetics are governed by the mass transport of the reactants, and MWCNTs facilitate better flow of electrons and Na+, especially at high C rates (> 2C). We observed a lower charge-transfer resistance in the composite electrode than in pristine NMNC. We also correlate the difference in the Na+ diffusion coefficients of the cathodes to their electrochemical performance. The incorporation of MWCNT enhanced high rate performance of NMNC up to 10C rate and resulted in a specific capacity of 150 mAh g−1 at 0.1C rate. Such high rate capability and specific capacity of the composite electrode confirm that the MWCNTs could enhance the electrochemical performance of the P2-NMNC and make it a viable positive electrode material for SIBs.