Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mössbauer spectroscopy study

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Abstract

The extraction and insertion of lithium in solid-state synthesized LiFePO4 has been followed by in situ X-ray diffraction and Mössbauer spectroscopy in ‘coffee-bag’ cells of type 〈Li-metal | liq. el. | LiFePO4〉 during the first cycle. Two-phase Rietveld refinement of the X-ray diffractograms gives the triphylite (LiFePO4) to heterosite (FePO4) phase-ratios as charging and discharging of the cell proceeds. The Fe3+/Fe2+ ratios at each step, as measured by Mössbauer spectroscopy and X-ray diffraction, were in good general agreement with the amount of lithium calculated from the charge passed through the cell; there was, however, a slight tendency for the Mössbauer technique to record a higher concentration of the oxidized phase. The possible existence of a thin interface region at the phase boundary is discussed.

Introduction

The need for compact, high-energy density, environmentally friendly, rechargeable batteries has led to the development of the lithium-polymer and lithium-ion battery concepts. Transition-metal oxides have tended to be the most attractive candidates as active cathode materials through their high potential vs. Li+/Li; in many of them, a large proportion of the lithium can be inserted/extracted reversibly. While vanadium, cobalt, nickel and manganese oxides have so far attracted most attention, an iron-based system would represent an almost ideal alternative, since iron is one of the most abundant metals in the earth’s crust. Such a material would therefore tend to be cheap and relatively harmless to living organisms.

Lithium insertion has been investigated for several iron-containing compounds; e.g. α-Fe2O3 [1], [2], γ-Fe2O3 [3], Fe3O4 [1], [4], [5], LiFe5O8 [6], spinel ferrites [7], [8], FeS2 [9], γ-FeOOH-derivatives [10], [11], [12], β-FeOOH [13], FeOCl [14], FePS3 [15], [16] and Li3FeN2 [17]. All these compounds (except FeS2) rely on the Fe3+/Fe2+ redox couple and thus have a rather low open-circuit voltage (OCV) vs. Li/Li+. Typically, insertion takes place over the range from 3 V (or lower) down to 1.0–1.5 V and many of them have been suggested as anode materials. On the other hand, LiFeO2 exploits the Fe4+/Fe3+ redox couple, which increases the electrochemical potential vs. Li/Li+ and thus facilitates a higher capacity. However, unlike Mn4+, Fe4+ is relatively unstable, which limits the amount of lithium that can be extracted/inserted reversibly into the structure; delithiation has so far only been reported for a few structural modifications of LiFeO2 [18], [19], [20], [21], [22].

The potential use of different iron phosphates has recently been investigated [23], [24], [25], [26], [27]. The substitution of oxygen atoms by a polyanion (XO43−) like sulfate, phosphate or arsenate lowers the Fermi level of the Fe3+/Fe2+ redox couple and thereby increases the cell potential [23], [24], [25], [26], [27], [28], [29], [30]. This effect is larger for compounds with less covalent Fe–O bonds; X in XO43− determines the strength of the Fe–O covalency via an inductive effect [28], [29]. The Fermi level of the redox couple in lithium iron phosphates is strongly affected by the structure [24]. One of the most promising candidates for a low-power, rechargeable lithium battery is LiFePO4, with a theoretical capacity of 170 mAh/g [23]. It shows a flat voltage curve, with a plateau around 3.5 V vs. Li/Li+. It is inexpensive, non-toxic, non-hygroscopic and environmentally friendly. LiFePO4 occurs in nature as the mineral triphylite, which has the olivine-type structure (Fig. 1) (space-group: Pnma) and often contains varying amounts of manganese, i.e. LiFexMn1−xPO4, and has the oxygen atoms arranged in a slightly distorted, hexagonal close-packed arrangement [31], [32], [33], [34]. The phosphorous atoms occupy tetrahedral sites, while the iron and lithium atoms occupy octahedral sites [denoted M(2) and M(1), respectively]. The FeO6 octahedra are linked through common corners in the bc-plane, and the LiO6 octahedra form edge-sharing chains along the b-axis. One FeO6 octahedron has common edges with two LiO6 octahedra and a PO4 tetrahedron. PO4 groups share one edge with an FeO6 octahedron and two edges with LiO6 octahedra.

Ferric iron can arise in the LiFePO4 structure in two ways: either by ion replacement, 3Fe2+→2Fe3+, leaving a vacancy in the M(2) site, or by the replacement process LiFe2+→Fe3+, leaving a vacancy in the M(1) site [35]. The phase formed on lithium extraction is the isostructural FePO4 (heterosite) [23], [31], [36]. For triphylite containing both Mn and Fe ions, this oxidation process is believed to proceed via an intermediate phase-ferrisicklerite (Li1−xFe3+xMn2+1−xPO4). The phase-domain boundaries between the mineral ferrisicklerite and heterosite are usually quite sharp [37].

Despite its rather compact structure, the cycling capability of LiFePO4 is surprisingly good at low current densities. Approximately 0.6 Li+ ions can be withdrawn from solid-state synthesized LiFePO4 and reinserted reversibly at a current density of 2 mA/g [23]. It has been suggested by Padhi et al. [23] that the rate-determining step is the lithium diffusion through the diminishing LiFePO4/FePO4 interface as lithium is reinserted into the structure. Since the surface area of FePO4 is decreasing, the amount of lithium that can pass through the interface is insufficient to sustain the current, leading to a decrease in the reversible capacity at higher current densities. It was therefore suggested that a larger amount of lithium could be extracted and reinserted reversibly in samples with smaller grain sizes [25]. LiFePO4 has also been prepared by a ‘new synthetic route’ (particle size ca. 4 μm); 95–100% utilization and good cyclability has been claimed [38].

Many lithiation processes have been followed by Mössbauer spectroscopy. One of the earliest in situ Mössbauer studies was indeed used to follow electrochemical insertion of lithium into KFeS2 [39]. More recently, results from a combined in situ XRD and Mössbauer study of the electrochemical reaction of lithium with mechanically alloyed Sn2Fe have been reported [40], [41]. It was found that a number of the phases formed could be detected more easily by Mössbauer spectroscopy than by X-ray diffraction. We return to this point in our discussion. Mössbauer spectroscopy is known to be a particularly efficient tool for studying Fe oxidation states and coordination in different compounds. The hyperfine interaction of antiferrimagnetic LiFe0.8Mn0.2PO4 at low-temperature has been investigated [42]. Mössbauer spectroscopy studies of triphylite and of the related minerals ferrisicklerite and purpurite (Mn-rich FexMn1−xPO4) have given evidence of a next-nearest neighbour effect [43]. Deganello has also studied the oxidation of triphylite in the temperature range 20–900°C in an inert atmosphere and under oxidizing conditions [44], [45].

In this paper, we report a study of the process of electrochemical delithiation and relithiation of LiFePO4 in a laminated electrochemical cell, as studied by in situ X-ray powder diffraction and Mössbauer spectroscopy.

Section snippets

Synthesis

LiFePO4 was prepared by the solid-state reaction of stoichiometric amounts of Li2CO3 (p.a, Merck), FeC2O4·2H2O (>99%, Sigma–Aldrich) and (NH4)2HPO4 (p.a, Merck) [46]. The materials were handled, as far as possible, in an Ar glovebox (<3 ppm H2O and O2) to avoid oxidation and/or water absorption. The synthesis was performed in three consecutive steps; before each step, the material was ground and mixed thoroughly. The material was first heated to 300°C in a vacuum oven to ‘predecompose’ the

Results

A typical charging curve is shown in Fig. 2. The points at which each X-ray and Mössbauer experiment were performed are indicated.

Discussion

Both XRD and Mössbauer studies imply (as we would expect) that the amount of Fe3+ increases and the amount of Fe2+ decreases (Table 2, Table 4) as charging proceeds; the two methods are in reasonable agreement with values expected from the amount of charge passed at each step. Note, however, that in the most charged state, the Mössbauer study gives a significantly higher value (by as much as 9% units) than that calculated from the electrochemical data (Fig. 5). This is also displayed in Fig. 6a

Acknowledgements

This work has been supported by grants from The Swedish Natural Science Research Council (NFR) and The Swedish Board for Technical Development (NUTEK).

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