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 Li 2 MnSiO 4 adopting the Pn space group, prepared by ion-exchange from Na 2 MnSiO 4 . Density-functional theory methods were used to predict the lattice parameters and atom positions of the new polymorph material and those of Na 2 MnSiO 4 and LiNaMnSiO 4, 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 Pmn 2 1 polymorph of Li 2 MnSiO 4, as well as LiNaMnSiO 4 and Na 2 MnSiO 4 . 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 °C, as confirmed by differential scanning calorimetry scans. Both Li 2 MnSiO 4 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 Li 2 MnSiO 4 polymorphs and both exhibit similar capacity fades. LiNaMnSiO 4, however, yields a stable capacity of 45 mA h g –1, whereas Na 2 MnSiO 4 yields an initial capacity of 20 mAh g –1, increasing to 60 mAh g –1 with cycling.