| Résumé | We report on the syntesis 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 hte new polymorph 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 Pmn2, 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 stable polymorph above 370 degrees Celsius, as confirmed by differential scanning calorimetry scans. Both LiMnSiO4 polymorphs display similar charge-discharge curves, except for a marginally lower lithum 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(to the power of)-1 is intially observed for both Li2MnSiO4 polymorphs and both exhibit similar capacity fades. LiNaMnSiO4, however, yields a stable capacity of 45 mAh g(to the power of)-1, whereas Na2MnSiO4 yields an initial capaciy of 20 mAh g (to the power of)-1, increasing to 60 mAh g(to the power of)-1 with cycling. |
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