Novel Pn Polymorph for Li2MnSiO4 and Its Electrochemical Activity As a Cathode Material in Li-Ion Batteries

We report on the synthesis and characterization of a new metastable polymorph of Li2MnSiO4 adopting the Pn space group., prepared by ion-exchange from Na2MnSiO4. Density-functional theory methods were used to predict the lattice parameters and atom positions of the new polyrnorph material and those of Na2MnSiO4 and LiNaMnSiO4, allowing their identification by X-ray diffraction profiles, as well as the comparison of the measured and calculated cell parameters. The electrochemical activity of this new polymorph as a cathode material for lithium ion batteries was evaluated in coin cells and compared to that of the thermodynamically stable Pmn2i polymorph of Li2MnSiO4, as well as LiNaMnSiO4 and Na2MnSiO4. Carbon coating, very vital to the electrochemical activity of the material, was added in situ to the material before ion-exchange because the metastable polymorph converts to the stable polymorph above 370 degrees C, as confirmed by differential scanning calorimetry scans. Both Li2MnSiO4 polymorphs display similar charge-discharge curves, except for a marginally lower lithium extraction voltage during the first charge of the Pn structure that may be due to the presence of sodium ion impurities. A discharge capacity of 110 mA h g(-1) is initially observed for both Li2MnSiO4 polyrnorphs and both exhibit similar capacity fades. LiNaMnSiO4, however, yields a stable capacity of 45 mA h g(-1), whereas Na2MnSiO4 yields an initial capacity of 20 mAh g(-1), increasing to 60 mAh g(-1) with cycling.