On the Energetic Stability and Electrochemistry of Li₂MnSiO₄ Polymorphs

Abstract

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.

In-situ XAS study on Li2MnSiO4 and Li2FeSiO4 cathode materials

“…The reason for observed differences in QS values is not fully understood, however we presume this difference can be ascribed to the different polymorph used in our experiment as it was used by Nyten. It is worth to mention that the existence of different polymorphs in Li 2 FeSiO 4 was not proven yet, however the above opinion is based on the work on Li 2 MnSiO 4 and Li 2 CoSiO 4 [2,3,12,20]. Based on our in-situ Mössbauer experiment we agree with a conclusion given by Nyten [7], that local environment of iron observed with Mössbauer spectroscopy is very similar before and after "phase transition" reflected as a shift of potential from 3.1 to 2.8 V. At the end of Mössbauer part we need to emphasize that Mössbauer parameters of the magnetic phase remain very similar during the oxidation/reduction process in all fitted spectra.…”

“…As-prepared samples are usually mixtures of polymorphs due to small difference in formation energy. Some polymorphs, however, can be isolated with selective synthesis conditions [12]. Although Li 2 FeSiO 4 and Li 2 MnSiO 4 are iso-structural, their electrochemical activities are different [13].…”

Improved Electrochemical Performance of Li[sub 2]MnSiO[sub 4]/C Composite Synthesized by Combustion Technique

“…Thus, LM2 with 20 wt % of acetylene black shows a superior electrochemical stability and a higher rate capability as compared to those reported so far. [15][16][17][18][19] Because the electrical conductivity of Li 2 MnSiO 4 is very low, almost in the insulator range, it is expected that a large impedance would develop at the cathode-electrolyte interface that would hinder the electrochemical intercalation of Li during charge-discharge. The impedance spectra of the cells LM0 and LM2 have been recorded at various stages, before any charge-discharge and during cycling after 10 and 20 cycles.…”

Inorganic & organic materials for rechargeable Li batteries with multi-electron reaction

“…The relationship of crystal structure and electrochemical performance has been investigated by theoretical calculation. Arroyo-deDompablo's group [41] reported that the calculated average 2Li + intercalation potentials are 4.18, 4.19, and 4.08 V for Pmnb, Pmn2 1 , and P2 1 /n, respectively. Fisher et al [39] demonstrated that the orthorhombic Pmn2 1 phase possesses the lowest Li + migration energy barrier among the four phases.…”