Electrochemical and Chemical Na+/Li+ Ion Exchange in Na-Based Cathode Materials

Na2FeP2O7 sodium iron pyrophosphate with a triclinic symmetry (the space group P-1) has been prepared by mechanochemically assisted solid state synthesis. Electrochemical performance of the pristine Na2FeP2O7, NaFeP2O7 prepared by chemical oxidation with NO2BF4 and NaLiFeP2O7 obtained by non-oxidative Na+/Li+ ion exchange with LiBr were studied by galvanostatic cycling in a Li cell with Li as an anode and LiPF6 dissolved in EC+DMC as an electrolyte. XRD, EDX, Mossbauer and NMR spectroscopy studies showed that all the samples were active upon cycling in Li cells which resulted in exchange of one similar to Na+ for one Li+ per f.u. to produce a new mixed phase NaLiFeP2O7 with the same triclinic structure as Na2FeP2O7 and good electrochemical performance.