Crystal Structure, Energetics, And Electrochemistry of Li₂FeSiO₄ Polymorphs from First Principles Calculations

Abstract

The influence of crystal structure and relative stability on the electrochemical properties of Li2FeSiO4 polymorphs as cathode materials for Li-ion batteries is investigated. Six Li2FeSiO4 forms related to the crystal structure of Li3PO4 have been considered: three known polymorphs crystallizing in layered structures (with space groups P21, Pmnb, and Pmn21) and three forms reported for other LiMSiO4 materials crystallizing in three-dimensional (3D) frameworks (with space groups Pmn21, Pbn21, and P21/n). While Li2FeSiO4 polymorphs are very close in energy, their energies begin to differ substantially upon removal of Li, rendering those with lower electrostatic energy more stable. The change in relative stability of polymorphs upon delithiation results in a driving force for a phase transformation of the exiting two-dimensional (2D)-Li2FeSiO4 polymorphs to a more stable structure having a 3D network of [SiO4] and [FeO4] tetrahedra. The resulting phase exhibits a voltage plateau that is predicted to be 0.3 V below that of the original phase for the first electron process (Li2FeSiO4/LiFeSiO4 couple). The calculated voltage–capacity curves for the first and second cycles of Li2FeSiO4 at room temperature, assuming transformation to the new polymorph occurs after the first cycle, are in excellent agreement with experiments. Independently of the polymorphs, removal of the second lithium ion occurs at too high of a voltage (above 4.7 V) and is accompanied by major structural rearrangements, precluding the utilization of any of the unmodified Li2FeSiO4 polymorph derived from Li3PO4 as high specific capacity material.

Insights into Changes in Voltage and Structure of Li2FeSiO4 Polymorphs for Lithium-Ion Batteries

“…The calculated and experimental structural parameters for the as-prepared polymorphs (β II , γ s , and γ II ) are compared in Table 1 and show general good agreement, as found in other DFT studies. [15][16][17][18]20,21 As noted, the cycled structure has been derived recently from neutron diffraction by Armstrong et al 10 We have taken this experimentally derived structure as the starting point for our structure optimizations. It was first necessary to consider how the Li/Fe ions that share a site in the cycled structure (inverse-β II ) might order.…”

“…Recent DFT work of Saracibar et al 21 35 the energy of the redox couple depends not only on the formal valence state of the transition metal ion but also on the covalent component of the cation−anion bonding, which is influenced by the placement and character of any counterion or polyanion and by the Madelung energy of the ionic component of the bonding, which is, in turn, influenced by the bulk structure. Table 4 presents the calculated cation−oxygen bond lengths and ion−ion separations in the structures of Li 2 FeSiO 4 and the delithiated LiFeSiO 4 .…”

Tuning anisotropic ion transport in mesocrystalline lithium orthosilicate nanostructures with preferentially exposed facets

“…5b) 50,51 . Theoretically, half of the capacity (~165 mAhg −1 ) should be observed at the first charge plateau of~3.1V, corresponding to the oxidation of Fe 2+ to Fe 3+ , and another half capacity should be observed for the second charge plateau of~4.6V (Fe 3+ to Fe 4+ ) 13,47,[49][50][51][52][53] . However, the initial charge capacity at the 3.1 V plateau is less than 165 mA hg -1 (~70 mA hg −1 in this work), which has also been observed in previous work 17,18,20,21,24,27,47,50,51,[53][54][55][56][57][58][59][60][61] .…”

“…), have attracted considerable attention for their high theoretical capacity with insertion/extraction of two Li + ions per formula unit (~330 mA hg −1 ) 11 . However, orthosilicates suffer from intrinsically poor kinetics and capacity fading related to the insertion/extraction of more than one Li + ion from the structure 12,13 . The sluggish kinetics is primarily attributed to poor electronic conductivity and ionic conductivity.…”

“…The sluggish kinetics is primarily attributed to poor electronic conductivity and ionic conductivity. For instance, the most studied orthosilicate material, Li 2 FeSiO 4 , has a poor Li ion diffusivity of~1 × 10 −14 cm 2 s −1 and an extremely low electronic conductivity of~6 × 10 −14 S cm −1 at 298 K, which are four orders of magnitude lower than the conductivity of its phosphate counterpart, LiFePO 4 (~10 -9 S cm −1 ) [12][13][14] . To address the kinetic issues, several efforts such as conductive coatings [15][16][17] , nanostructuring [18][19][20][21] , and heteroatom substitution [22][23][24][25][26] have been pursued to obtain high performance Li 2 FeSiO 4 -based cathodes.…”