P2-a2= 3Mn2= 3M1= 3O2 (M= Fe, Co, Ni) cathode materials in localized high concentration electrolyte for the long-cycling performance of sodium-ion batteries

Introduction: Localized high concentration electrolytes (LHCE) have been intensively studied due to their unique properties, especially in suppressing the Na dendrite formation and long-term cycling. Therefore, the low electrochemical performance of the P2-type cathode can be overcome by using LHCE.

Methods: P2-type sodium layered oxides Na2=3Mn2=3M1=3O2 (M = Fe, Co, Ni) cathode materials were synthesized via a simple co-precipitation following a supported solid-state reaction. XRD and Rietveld method analyzed the phase composition and lattice parameters. SEM images observed the morphology of materials. The half-cell of three cathode were performed in LHCE consisting of 2.1 M sodium bis(fluorosulfonyl)imide (NaFSI) dissolved in 1,2-dimethoxyethane (DME) and bis(2,2,2-trifluoroethyl) ether (BTFE) (solvent molar ratio 1:2). The galvanostatic charge-discharge, striping-plating, and linear sweep voltage tests were carried out to investigate the electrochemical behaviors.

Results: As-prepared electrode materials exhibited discharge capacities of 94.5, 147.1, and 142.9 mAh/g at C/10 in the potential range of 1.5-4.2 V for Na2=3Mn2=3Fe1=3O2 (MFO), Na2=3Mn2=3Co1=3O2 (MCO) and Na2=3Mn2=3Ni1=3O2 (MNO), respectively. Interestingly, the MNO cathode material has a superior cycling performance with 86.5% capacity retention after 100 cycles than MCO and MFO.

Conclusion: Such superior electrochemical performance of synthesized MNO could be ascribed to the combined synergistic effects between the nickel partially substituted MNO cathode structure and using LHCE 2.1 M NaFSI/DME-BTFE (1:2). Nickel substituted MNO cathode exhibited the enhancement of discharge capacity and the long cycling stability in LHCE due to the mitigation of dendrite formation on sodium metal anode.