Improved interfacial resistance and crystal-structure stability in a low-cobalt P2-type sodium-ion battery cathode material

TL;DR

This study introduces a low-cobalt P2-type sodium-ion cathode with high capacity, enhanced structural stability, and reduced interfacial resistance, making it a promising candidate for advanced sodium-ion batteries.

Contribution

It presents a novel low-cobalt sodium-ion cathode with improved electrochemical performance and stability, supported by impedance modeling and structural analysis.

Findings

Discharge capacity close to 190 mAhg-1

Specific energy density over 500 mWhg-1

Reduced polarization and interfacial resistance

Abstract

We describe Na0.67Mn0.625Fe0.25Co0.125O2 (NMFCO), a P2-type sodium-ion battery cathode. Our composition, with significantly less Co than in an earlier study, shows discharge capacity close to 190 mAhg-1 and specific energy density exceeding 500 mWhg-1 in the 1.5 to 4.3 V range. The material also shows an improved structural stability over similar materials. Such changes, between the pristine phase (P63/mmc, P63 (OP4), or orthorhombic Cmcm) and the so-called Z phase, are endemic to other P2-type cathodes such as Na0.67Mn0.65Fe0.35O2 (NMFO). We propose two equivalent circuit models of impedance spectroscopy to understand electrochemical processes in our cells with a sodium metal anode. Our equivalent circuit modeling, combined with an analysis of the initial galvanostatic slope, reveals a significant reduction in the polarization and interfacial charge-transfer resistance at the solid electrolyte interface. We reveal that the combined effects of crystal structure stability, lower internal resistance, relatively high specific energy density, and improved battery health make this low-cobalt P2-type cathode composition a very promising candidate for new sodium-ion batteries.