Moessbauer study of FePO4 cathode for lithium- and sodium-ion batteries

LiFePO4 of olivine type has attracted much interest as a rare-metal free cathode for lithium-ion battery. The present authors' group has found a low-cost preparation method for new cathode material FePO4 (P321) from an aqueous solution of metallic iron and P2O5. A cathode pellet was prepared by mixing FePO4, acetylene black (AB) and polytetrafluoroethylene (PTFE) binder at a mass ratio of 70:25:5. Electrochemical cathode performance was investigated under a constant current density of 0.1 mA/cm2 with a coin-type cell with an anode of metallic Li and Na for Li- and Na-ion batteries, respectively. Moessbauer spectra were measured by a constant accele- ration method with a Moessbauer source of 57Co(Pd) and an α-Fe foil as a reference of the isomer shift. Moessbauer spectra of the original cathode pellet top left and right) composed of amorphous FePO4 indicate that all the FeIII atoms have a tetrahedral symmetry. After discharging the Li-(second left) and Na-batteries (second right), all the FeIII were reduced to octahedral FeII. The reduction of FeIII to FeII indicates penetration of Li+ and Na+ into the FePO4 cathode which accompanies an electron capture; Li+ (or Na+) + FeIIIPO4 + e- = LiFeIIPO4 (or NaFeIIPO4) After charging the Li-cell (bottom left), all the Fen atoms were oxidized to tetrahedral FeIII, while 69 % of FeII was oxidized to FeIII in the Na-ion cell (bottom right) due to a lower cell voltage: 4.0 V. Oxidation of FeII to FeIII reflects a release of Li+ or Na+ ions from the FePO4 cathode to the electrolyte, i.e., an inverse reaction of eq.