On the energetic stability and electrochemistry of Li2MnSiO4 polymorphs

The thermodynamic stability of Li2MnSiO4 Polymorphs and their electrochemical properties as electrode for Li batteries are investigated combining experimental and computational methods. Three possible Li2MnSiO4 forms have been considered crystallizing in Pmnb, Pmn2(1) (beta-Li3PO4 derivatives) and P2(1)/n (gamma-Li3PO4 derivative) space groups (S.G.). We have first demonstrated that the relative stability of beta-and gamma-Li3PO4 polymorphs is well-reproduced by density functional theory (DFT) methods (LDA, GGA). For Li2MnSiO4, 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 Li2MnSiO4 prepared at 900 degrees C consists of a mixture of polymorphs, as detected by XRD and confirmed by means of SAED and Li-6 MAS-NMR. As expected from DFT results, exposing the as-prepared Li2MnSiO4 sample to high pressure/high temperature (pressure range 2-8 GPa, temperature range 600-900 degrees C) allows to isolate the Pmn21 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 beta-Li3PO4 derivatives, rendering any MnSiO4 delithiated hosts prompt to transform into a more stable structure or a mixture of them.