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 P2 1 /n (γ-Li 3 PO 4 derivative) space groups (S.G.). We have first demonstrated that the relative stability of βand 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 Pmn2 1 and P2 1 /n forms, respectively (GGA + U results). Computational results indicate that the denser Pmn2 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 Pmn2 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, Pmn2 1 and P2 1 /n, respectively). Major structural rearrangements are expected under lithium deinsertion from the P2 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.

Electrodes F 3000On the Energetic Stability and Electrochemistry of Li2MnSiO4 Polymorphs. -The electrochemical properties as electrodes for Li batteries of three possible Li2MnSiO4 polymorphs crystallizing in the space groups Pmnb, Pmn21, and P21/n are characterized by DFT calculations and experimental methods. The Pmnb-Li2MnSiO4 polymorph is 2.4 eV/f.u. and 65 meV/f.u. more stable than the Pmn21 and P21/n forms, respectively. The Pmn21 polymorph is isolated by treating a mixture of different Li 2 MnSiO 4 polymorphs at high pressure and high temperature. Calculated average lithium intercalation voltages for the two electron process are 4.18, 4.19, and 4.08 V, for Pmn21, Pmnb, and P21/n, respectively. From an application point of view, having a mixture of Li2MnSiO4 polymorphs or a unique Li2MnSiO4 polymorph in the electrode material seems to be of minor importance. -(ARROYO-DEDOMPABLO*, M. E.; DOMINKO, R.; GALLARDO-AMORES, J. M.; DUPONT, L.; MALI, G.; EHRENBERG, H.; JAMNIK, J.; MORAN, E.; Chem. Mater.