Lately, sodium ion batteries (SIB) have been studied very intensively as a potential alternative to Li ion batteries (LIB). As sodium is 6^th most abundant element in Earth which makes it very cheap compared to lithium. Also, the cell chemistry is similar to LIB system. In addition, cheap and lighter aluminum can be used as a current collector at the anode side instead of copper foil. Among the studied cathodes, P2-type Na_0.7MnO₂ cathode is more attractive as it can deliver high capacity and high energy density, moreover Mn is one of the cheap elements. However, the long cyclability of the composite has been an issue due to the Jahn-Teller distortion which is associated with the existence of Mn³⁺. In this study, our approach was to suppress the Jahn-Teller distortion by Ni doping by varying the dopant amount from 0%, 10% and 20%.

The composites were synthesized by combustion method. The aqueous solution of stoichiometric values of sodium nitrate (98%), manganese (II) nitrate hexahydrate (97%), and nickel (II) nitrate hexahydrate (98%) were added to aqueous citric acid solution (nitrates : citric acid, 1 : 0.5 in weight). The solution was heated on a hot plate at 100 °C for overnight under constant stirring to evaporate the solvent. Then dried powder was further heated to 200 °C for auto combustion of citric acid. Burnt powder further heated at 500 °C for 3 h to decompose the nitrates and yield a homogeneously mixed amorphous powder containing carbon residues. The obtained decomposition product was pelletized and heated in a tube furnace at 900 °C (heating rate – 5 °C / min) for 10 h in air atmosphere and then slowly cooled to room temperature. The obtained powder was transferred to Ar-filled glove box to avoid the contact with moisture in the air.

X-ray diffraction (XRD, Xpert, PANalitical) using Cu-Kα radiation was employed to characterize the crystal structure of the synthesized powders. XRD measurement was carried out in the 2θ range of 10−80° with a step size of 0.03°. The FULLPROF Rietveld program was used to analyze the observed powder diffraction patterns. Structural studies during cycle were examined by means of in-situ synchrotron X-ray diffraction (XRD) and ex-situ X-ray Absorption Spectroscopy (XAS). In-situ XRD and ex-situ XAS measurement was respectively carried out at 9B beamline and 8C beamline of Pohang Accelerator Laboratory (PAL), Pohang, South Korea.

Electrochemical properties were studied in an half-cell configuration assembling a R2032 coin-type cell using sodium metal as the negative electrode in an Ar-filled glove box. The electrolyte solution comprised 0.5 M NaPF₆ in propylene carbonate and fluorinated ethylene carbonate (PC:FEC, 98:2 in volume). The cells were charged and discharged between 2.0 V and 4.3 V at a rate of 0.1C at 25 °C.

Among the successfully synthesized samples Na_0.7Mn_0.8Ni_0.2O₂ composite displayed better electrochemical performance. As shown in Figure 1 the composite delivered initial discharge capacity of 160 mAh g^-1 at 0.1C between voltage of 2.0 V and 4.3 V and the capacity retention is remarkably improved compared to bare composite. Data on structural change during cycling and other detailed studies will be presented at the meeting.

Figure 1. Initial charge-discharge profiles of P2-Na_0.7Mn_1-xNi_xO₂ composites at 0.1C.