Solid-Solution Vs. Two-Phase Li-Ion Storage in Nanograined Orthosilicate Cathode Materials

Abstract

Lithium metal orthosilicate cathode materials (Li2(Fe,Mn)SiO4 denoted as LMS) have been the subject of significant research interest for the past 10 years because of the high theoretical capacity nearly two times higher than that of the analogous polyanion cathode LiFePO4 (165 mAh/g), if the two lithium ions could be fully extracted and re-inserted in multiple charge/discharge cycles. Despite significant progress, a series of critical issues-questions relating to this strategically important family of cathode materials remain open hampering their full development. One such critical issue is the mechanism of Li ion storage namely, solid-solution behavior (single-phase) vs. two-phase reaction. In analogy with the well understood two-phase separation Li storage mechanism of LiFePO4 cathode the dominant view among those investigating the orthosilicate system is that we have also a two-phase mechanism between monoclinic (P21/n) Li2FeIISiO4 and orthorhombic (Pmn21) one-Li delithiated LiFeIIISiO4. However, in many of these studies the generated discharge curves are not flat but are slanted. Such behavior has been attributed to polarization effects without further characterization leaving an important gap in our understanding. With the view of clarifying this issue we have conducted first principle calculations (1) and in situ (unpublished) and ex situ (submitted) synchrotron XRD and XANES (Fe K-edge and L-edge; Si K-edge and Oxygen K-edge) characterizations of nanostructured LFS cathode in terms of structure evolution and charge compensation at different states of charge (Li(2-x)FeSiO4, x=0, 0.25, 0.50, 0.75, 1.0). This is done by charging/discharging LFS at low current (C/50) to allow for structure relaxation hence unequivocal (quasi-equilibrium condition) probing of the Li storage mechanism: solid solution vs. two-phase reaction. Furthermore, cycling is not limited to the first cycle but is extended to several cycles. For the study we employ a mixed phase (monoclinic/orthorhombic) nanograined Li2FeSiO4 material synthesized via organic-assisted hydrothermal precipitation and annealing at 400 °C (2,3). The data clearly point out towards solid solution mechanism- a finding that should bring new perspective in our pursuit of unlocking the full capacity advantage of the orthosilicate Li-ion host structure.AcknowledgmentsThis work is supported through a Hydro-Québec/Natural Sciences & Engineering Research Council of Canada (NSERC) Collaborative R&D research grant. DTJ acknowledges NSERC support for the work done at the University of Guelph. Synchrotron radiation measurements were performed at the Canadian Light Source, which is supported by CFI, NSERC, University of Saskatchewan, Province of Saskatchewan, Western Economic Diversification Canada, NRC Canada, and CIHR.