Effect of 3D Metal on Electrochemical Properties of Sodium Intercalation Cathode P2-Na[sub.x]Me[sub.1/3]Mn[sub.2/3]O[sub.2]

This research aims to evaluate the influence of different 3D metals (Fe, Co, and Ni) substituted to Mn on the electrochemical performance of P2-Na[sub.x]Me[sub.1/3]Mn[sub.2/3]O[sub.2] material, which was synthesized by the coprecipitation process followed by calcination at high temperature. X-ray diffraction (XRD) results revealed that the synthesized Mn-rich materials possessed a P2-type structure with a negligible amount of oxide impurities. The materials possessed their typical cyclic voltammogram and charge-discharge profiles; indeed, a high reversible redox reaction was obtained by Na[sub.x]Co[sub.1/3]Mn[sub.2/3]O[sub.2] sample. Both Na[sub.x]Co[sub.1/3]Mn[sub.2/3]O[sub.2] and Na[sub.x]Fe[sub.1/3]Mn[sub.2/3]O[sub.2] provided a high specific capacity of above 140mAh·g[sup.-1]; however, the former showed better cycling performance with 83% capacity retention after 50 cycles at C/10 and high rate capability. Meanwhile, the Ni-sub Na[sub.x]Ni[sub.1/3]Mn[sub.2/3]O[sub.2] exhibited excellent cycling stability but a low specific capacity of 110mAh·g[sup.-1] and inferior rate capability. The diffusion coefficient of Na[sup.+] ions into the structure tended to decrease with a depth of discharge; those values were in the range of 10[sup.-10]-10[sup.-9]cm[sup.2]·s[sup.-1] and 10[sup.-11]-10[sup.-10]cm[sup.2]·s[sup.-1] in the solid solution region and biphasic region, respectively.