Improving the electrochemical performance of hard carbon|| NaxNi1= 3Mn1= 3Co1= 3O2 full-cell by presodiation methods

Introduction: We study the electrochemical behaviors of sol-gel NaxNi1=3Mn1=3Co1=3O2 (NaNMC) material assembled in sodium half-cell and full-cell configuration, focusing on the cycling performance.

Method: The crystalline structure and the lattice parameters of synthesized NaNMC were characterized by using X-ray diffraction (XRD). The electrochemical characterization of NaNMC sample was investigated in sodium-ion half-cell by cyclic voltammogram (CV) and galvanostatic cycling with potential limitation (GCPL). To improve the reversible capacity of sodium-ion full-cell, we suggested two presodiation methods to reduce the irreversible capacity of the hard carbon before assembling full-cell: i) the chemical method and ii) the electrochemical method.

Results: In the half-cell, the specific capacity of NaNMC is about 106 mAh.g-1 and well maintained 88 mAh.g-1 after 100 cycles in a potential range of 2.0 – 4.0 V. When assembling the full-cell with a pristine hard-carbon, the cell capacity of only 30 mAh.g-1 but quickly decreased after several cycles due to large irreversible capacity of hard carbon in the initial cycle. As a result, both methods significantly increased ICE (Initial Coulombic Efficiency) of hard carbon by over 90% and enhanced the capacity of the full-cell. In particular, the reversible capacities of full-cell based on presodiated hard carbon (HC) were twice higher than those with pristine HC (75 mAh.g-1 and 65 mAh.g-1, respectively, for the electrochemical method and the chemical method).

Conclusion: These encouraging results indicate an accessible solution to overcome the low ICEs of hard carbons, leading to improving the sodium-ion full-cell's reversible capacity.