On the Energetic Stability and Electrochemistry of Li₂MnSiO₄ Polymorphs

The thermodynamic stability of Li 2 MnSiO 4 polymorphs and their electrochemical properties as electrode for Li batteries are investigated combining experimental and computational methods. Three possible Li 2 MnSiO 4 forms have been considered crystallizing in Pmnb, Pmn2 1 (β-Li 3 PO 4 derivatives) and P 2 1 / n (γ-Li 3 PO 4 derivative) space groups (S.G.). We have first demonstrated that the relative stability of β- and γ-Li 3 PO 4 polymorphs is well-reproduced by density functional theory (DFT) methods (LDA, GGA). For Li 2 MnSiO 4, the Pmnb form is predicted to be 2.4 meV/f.u. and 65 meV/f.u. more stable than the Pmn 2 1 and P2 1 / n forms, respectively (GGA + U results). Computational results indicate that the denser Pmn 2 1 polymorph can be obtained by high pressure/high temperature treatment of the other polymorphs or their mixtures. A sample of Li 2 MnSiO 4 prepared at 900 °C consists of a mixture of polymorphs, as detected by XRD and confirmed by means of SAED and 6 Li MAS-NMR. As expected from DFT results, exposing the as-prepared Li 2 MnSiO 4 sample to high pressure/high temperature (pressure range 2−8 GPa, temperature range 600−900°C) allows to isolate the Pmn 2 1 polymorph. The crystal structure has a minor impact in the average lithium intercalation voltage for the two electron process (GGA+U calculated voltages are 4.18, 4.19, and 4.08 V for Pmnb, Pmn 2 1 and P 2 1 / n, respectively). Major structural rearrangements are expected under lithium deinsertion from the P 2 1 / n polymorph, as previously found for the β-Li 3 PO 4 derivatives, rendering any MnSiO 4 delithiated hosts prompt to transform into a more stable structure or a mixture of them.