Electrochemical Performance of LiMnPO[subscript 4] Synthesized with Off-Stoichiometry

Abstract

LiMnPO[subscript 4] was synthesized from an off-stoichiometric mix of starting materials with nominal composition LiMn[subscript 0.9]P[subscript 0.95]O[subscript 4−δ]. Stoichiometric LiMnPO[subscript 4] with particle size <50 nm was found with X-ray diffraction even with the large overall deviation from stoichiometry in the sample, indicating that other noncrystalline compounds are present. The off-stoichiometric sample had a discharge capacity of 145 mAh/g at C/10 and ∼100mAh/g at 2C after a constant current constant voltage charge. Capacity retention was excellent without a significant capacity loss at 1C. The performance of LiMn[subscript 0.9]P[subscript 0.95]O[subscript 4−δ] could not be significantly improved by diluting the electrode with more carbon, indicating a more intrinsic kinetic limitation of the material for LiMnPO[subscript 4] in contrast to LiFePO[subscript 4] [Nature (London), 458, 190 (2009)]. This is further corroborated by the difference in the overpotential between the two materials. Although LiMnPO[subscript 4] has a sloping voltage profile, LiFePO[subscript 4] tends to have a constant overpotential at high rate.