Researchers from Institute of Solid State Chemistry and Mechanochemistry assessed the effect of mixed Li+/Na+-ion electrolyte on the electrochemical performance of Na4Fe3(PO4)2P2O7 in hybrid batteries
Sodium-ion batteries (SIBs) are a type of rechargeable metal-ion battery that uses sodium ions as charge carriers. Factors such as high abundance, high voltage characteristics, low price, and safety of sodium have led to increasing research and development activities regarding SIBs. However, sodium ions have a higher ionic radius, which results in less number of sodium intercalation electrodes. Polyanionic-type cathode materials offer excellent thermal stability to improve safety characteristics of the batteries. Now, a team of researchers from Institute of Solid State Chemistry and Mechanochemistry studied the effect of mixed Li+/Na+-ion electrolyte on the electrochemical performance of Na4Fe3(PO4)2P2O7 in hybrid batteries.
The team also assessed the contribution of lithium (Li+) and sodium (Na+) ions in charge-discharge cathode reactions. The team assembled three types of electrochemical cells: de-sodiated Na4Fe3 (PO4)2P2O7 with the Li electrolyte (ed-Na4Fe3 (PO4)2P2O7/Li), the pristine Na4Fe3 (PO4)2P2O7 in a Li cell with the Li electrolyte (Na4Fe3 (PO4)2P2O7/Li), and the de-sodiated Na4Fe3 (PO4)2P2O7 with the mixed 0.9Li-0.1Na electrolyte (ed- Na4Fe3 (PO4)2P2O7/Li-Na). The team found that none of the three assemblies demonstrated full Na/Li electrochemical exchange. This can be attributed to the fact that although the hosting sites are suitable for the Na intercalation, they are not appropriate for the Li+ ions. The team noticed no significant structural change for Na4Fe3 (PO4)2P2O7 when Li+ ions were inserted. The team simultaneously inserted Li+ and Na+ ions in the Na4Fe3 (PO4)2P2O7 structure at high voltage.
Furthermore, Na+-ions were predominantly inserted with fewer coordination numbers at lower voltage. The team found that the electrochemical performance of Na4Fe3 (PO4)2P2O7 is significantly improved when the Na concentration in the mixed Li-Na electrolyte increases to 0.9/0.1 mol·L−1. Moreover, the Li intercalation/de-intercalation dominates at high voltage and the sufficient amount of the Na+ ions offer stability to the crystal structure of Na4Fe3 (PO4)2P2O7. Therefore, changes in the concentration of the Na+ ions in the mixed Li+/Na+-ion electrolyte and current density can help to regulate the electrochemical performance of Na4Fe3 (PO4)2P2O7. The team recorded high-rate capability, the highest specific capacity, and the best capacity retention for the preliminarily de-sodiated ed-Na4Fe3 (PO4)2P2O7/Li-Na cycled in the mixed Li-Na electrolyte. The research was published in the journal MDPI Batteries on April 11, 2019.