Na3.12Fe2.44(P2O7)2/multi-walled carbon nanotube composite as a cathode material for sodium-ion batteries

Abstract

Na_3.12Fe_2.44(P₂O₇)₂/multi-walled carbon nanotube (MWCNT) composite was fabricated by a solid state reaction and was further used to fabricate a cathode for sodium-ion batteries. The electrochemical behaviors were thoroughly investigated in assembled non-aqueous Na_3.12Fe_2.44(P₂O₇)₂/MWCNT//Na cells, showing higher specific capacity (over 100 mA h g⁻¹ at a rate of 0.15C) and better stable cycle performance than those of the pristine Na_3.12Fe_2.44(P₂O₇)₂-based one. It is noted that with increased charge–discharge cycles, the specific capacity of Na_3.12Fe_2.44(P₂O₇)₂/MWCNT gets close to the theoretical capacity (ca. 117.4 mA h g⁻¹). These good performances could be attributed to the incorporated MWCNTs, which improve the conductivity for lower charge transfer resistance and shorten the diffusion length for faster Na⁺ diffusion to access the reaction sites. Through systematic studies of EIS at different states of charge and discharge, it is discovered that R_ct decreases with the increase of voltage and reaches a minimum value at redox sites, but R_e and D_Na⁺ show the opposite trend. Moreover, a full cell test using a carbon black negative electrode also demonstrates good capacity retention up to 50 cycles and a reversible capacity of 145 mA h g⁻¹ with the average operation voltage of 2.8 V.