ARTICLES
The existence of a temperature-driven solid
solution in LixFePO4 for 0 ≤ x ≤ 1
CHARLES DELACOURT, PHILIPPE POIZOT, JEAN-MARIE TARASCON AND CHRISTIAN MASQUELIER*
Laboratoire de Réactivité et de Chimie des Solides, CNRS UMR 6007, Université de Picardie Jules Verne, 33 Rue St. Leu, 80039 Amiens Cedex 9, France
*e-mail: [email protected]
Published online: 20 February 2005; doi:10.1038/nmat1335
Lithium-ion batteries have revolutionized the powering
of portable electronics. Electrode reactions in these
electrochemical systems are based on reversible insertion/
deinsertion of Li+ ions into the host electrode material
with a concomitant addition/removal of electrons into the
host. If such batteries are to find a wider market such as
the automotive industry, less expensive positive electrode
materials will be required, among which LiFePO4 is a
leading contender. An intriguing fundamental problem is
to understand the fast electrochemical response from the
poorly electronic conducting two-phase LiFePO4/FePO4
system. In contrast to the well-documented two-phase
nature of this system at room temperature, we give the first
experimental evidence of a solid solution LixFePO4 (0 ≤ x ≤ 1)
at 450 °C, and two new metastable phases at room
temperature with Li0.75FePO4 and Li0.5FePO4 composition.
These experimental findings challenge theorists to
improve predictive models commonly used in the field.
Our results may also lead to improved performances of
these electrodes at elevated temperatures.
A
mong the solid-state electrochemistry community, it is still an
open question as to whether a single-phase or a two-phase
insertion/extraction process is intrinsically advantageous in
terms of kinetics as well as of structural stability. The most obvious
distinction between the two types is that the equilibrium potential
of a single-phase electrode is composition-dependent, whereas
that of a two-phase system is constant over the entire composition
range. These issues have been rekindled with the arrival of LiFePO4.
Eight years after the original work of Padhi1 was published,
phospho-olivines LiMPO4 (M = Fe, Mn, Co) now appear to be
potential candidates as positive electrode materials for rechargeable
lithium batteries. Owing to smart material processing (carbon
coating in particular), Li+ may be easily extracted out of LiFePO4
leading to room-temperature capacities of ~160 mAh g–1 (close to
the theoretical value of 170 mAh g–1)2–13. The room-temperature
insertion/extraction of lithium proceeds, at 3.45 V versus Li+/Li, in
a two-phase reaction between LiFePO4 and FePO4 that crystallize
in the same space group1,14. The crystal structure of LiFePO4 is
well-known, and described in the space group Pmnb with the
following unit-cell parameters: a = 6.011(1) Å, b = 10.338(1) Å,
c = 4.695(1) Å (refs 15–17). Iron is located in the middle of a slightly
distorted FeO6 octahedron, with a Fe–O average bond-length
higher than expected for iron in the +2 valence state in octahedral
coordination. Lithium is located in a second set of octahedral sites
but distributed differently: LiO6 octahedra share edges in order
to form LiO6 chains running parallel to [100]Pmnb (Fig. 1), which
generates preferential rapid one-dimensional Li+-ion conductivity
along that direction18.
Then come the very intriguing questions of how does Li+
extraction exactly proceed, that is, how small are the values
of δ in Li1-δFePO4 and ε in LiεFePO4? On similar concerns,
another intriguing issue emerged out of Zhou’s 2004 paper19,
in which the authors explicitly report on “a significant failure
of the local density approximation (LDA) and the generalized
gradient approximation (GGA) to reproduce the phase stability
and thermodynamics of mixed-valence LixFePO4 compounds”.
We found it quite striking that their calculations converged towards
the existence of intermediate LixFePO4 solid solutions instead of
a two-phase mixture, when lithium is extracted from LiFePO4.
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a
a
x LiFePO4 + (1–x ) FePO4
a
50 °C
x = 0.04
b
LiO6
x = 0.19
x = 0.41
x = 0.49
x = 0.68
x = 0.85
PO4
x=1
FeO6
LiFePO4
FePO4
18
a
20
22
24
26
28
30
32
34
36
38
40
Diffraction angle 2θ (°), CoKα radiation
b
c
b
Li
x = 0.04
(031)
(121)
(200)
350 °C
(120)
(101)
(111)
(021)
(011)
(020)
Lix FePO4
x = 0.19*
x = 0.41
Figure 1 Projections of the triohylite LiFePO4 structure. a, representation of a
TeOc2 layer and b, projection along [010]Pmnb. TeOc2 layers consist of LiO6 and FeO6
octahedra (Oc) and PO4 tetrahedra (Te) that occupy 1/2 and 1/8 of the octahedral
and tetrahedral interstitials, respectively. LiO6 octahedra form chains isolated
from each other by corner-sharing FeO6 octahedral layers perpendicular to [010].
The existence of such Li+ ions channels makes the [100]Pmnb direction the most
probable for Li+ ion transport.
x = 0.49
x = 0.68
x = 0.85
x=1
16
18
20
22 24
26
28
30
32
34 36
38 40
Diffraction angle 2θ (°), CoKα radiation
Facing such an experimental/theoretical contradiction, we decided
to take a closer look at the system in order to add new insight into
the intrinsic factors that determine the nature (single versus twophase process) of the lithium electrochemical insertion process in
LiFePO4. It is hoped that such experimental work would help improve
theoretical models such as LDA/GGA. Inspired by the previous
literature20 in which a temperature-driven two-phase to one-phase
insertion process was demonstrated on Chevrel phases, we decided
to investigate the temperature dependence of the LiFePO4/FePO4
phase diagram.
To address the above issues, we investigated the thermal
behaviour of a series of x LiFePO4 / (1–x) FePO4 (0 ≤ x ≤ 1) twophase mixtures (prepared by either chemical oxidation of LiFePO4
or intimate mixtures of the two end-members). As already described
in the literature21, the chemical oxidation is straightforward, and we
obtained well-defined series of x LiFePO4 / (1–x) FePO4 (0 ≤ x ≤ 1)
mixtures, whose compositions were basically governed by the initial
amount of NO2BF4 in the acetonitrile solution. As gathered in
Fig. 2, the corresponding X-ray diffraction (XRD) patterns display
diffraction peaks characteristic of both LiFePO4 and FePO4.
Figure 2 XRD patterns of LixFePO4 global compositions at 50 °C and 350 °C.
a, x LiFePO4 + (1–x) FePO4 mixtures at 50 °C before heating. b, LixFePO4 single
phases with x ranging from 1 to ~0. As indicated by the Bragg positions, pristine
samples are composed of the two end-members, LiFePO4 and FePO4. Chemical
analysis of the Li/Fe ratio, by atomic absorption spectroscopy as well as Rietveld
refinements of the XRD patterns, very satisfactorily converged to the indicated
values of x. These samples were prepared either by chemical delithiation (x = 0.68,
x = 0.41, x = 0.19 and x = 0.04) or simply by mixing the two phases (x = 0.85
and x = 0.49). At 350 °C (*390 °C for x = 0.19), full pattern-matching refinements
allow full indexing in the space group Pmnb of the observed Bragg reflections as
originated from a single LixFePO4 olivine-type phase. A continuous shift of (hk0)
reflections towards higher 2θ angles when x decreases is clearly depicted.
Temperature-controlled XRD under flowing N2 (to avoid
possible oxidation of Fe(II) in LixFePO4, 0 ≤ x ≤ 1) was performed
for each of these two-phase mixtures, and is illustrated in Fig. 3 for
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a
350 °C
x = 0.68
V (Å3)
290
50 °C
130 °C
280
270
Cooling
210 °C
4.78
c (Å)
290 °C
4.74
350 °C
290 °C
4.70
210 °C
10.4
10.2
b (Å)
Heating
130 °C
50 °C
LiFePO4
FePO4
16
18
10.0
9.8
20
22
24
26
28
30
32
34
36
38
40
6.10
a (Å)
Diffraction angle 2θ (°), CoKα radiation
b
6.00
5.90
x = 0.19
5.80
50 °C
0.0
0.2
0.4
0.6
0.8
1.0
x in Lix FePO4
130 °C
Cooling
210 °C
Figure 4 Unit-cell parameters as a function of x in LixFePO4 at 350 °C.
These values were refined in the Pmnb space group from the data of Fig. 3.
Noticeably, from LiFePO4 to FePO4, a and b undergo a linear contraction of 3.7 and
5.5%, respectively, whereas c undergoes a slight expansion of 1.4%.
290 °C
370 °C
390 °C
370 °C
290 °C
Heating
210 °C
130 °C
50 °C
LiFePO4
FePO4
18
20
22
24 26 28 30
32 34
Diffraction angle 2θ (°), CoKα radiation
36
38
40
Figure 3 Temperature-controlled XRD patterns of two selected LixFePO4 global
compositions under N2. a, ‘Li0.68FePO4’ b, ‘Li0.19FePO4’. The transformation of each
of the two two-phase samples x LiFePO4 + (1–x) FePO4 into LixFePO4 single phases
is initiated at ~200 °C, and completed at ~350 °C for Li0.68FePO4 and ~390 °C for
Li0.19FePO4. On cooling, these single phases split into complex mixtures of olivinetype phases. Importantly, this behaviour was observed for the whole series of x
LiFePO4 + (1–x) FePO4 samples gathered in Fig. 2.
two selected compositions, x = 0.68 and x = 0.19. As the temperature
is raised to around 200 °C, the intensities of the diffraction peaks
for both LiFePO4 and FePO4 start to vanish, together with the
appearance of broad diffraction peaks intermediate between the
reflections of LiFePO4 and FePO4. As the temperature increases,
these diffuse intensities increase, giving rise to well-defined
new sets of reflections characteristic of new LixFePO4 phases.
This very broad and continuous temperature range within which
the transformation occurs (∆T ~150–200 °C) leads to no thermal
signature in differential scanning calorimetry on first heating.
Indexing of the XRD reflections of the single phase measured at
350 °C was straightforward using the same space group Pmnb as for
the two end-members. The unit-cell parameters for all the members
of the LixFePO4 series were obtained from full pattern-matching
refinements of XRD data recorded at 350 °C (390 °C for Li0.19FePO4).
Precise structural descriptions of these new LixFePO4 compositions
with fractional occupancy of the Li site and a mixed-valence
Fe3+/Fe2+ site, obtained from temperature-controlled neutron
diffraction data, are given in another paper (C. Delacourt et al., to be
published). Rietveld refinements carried out on neutron diffraction
data for Li0.5FePO4 solid solutions confirmed the existence of
mixed-valence Fe3+/Fe2+ single phases. Moreover, valence-bond
calculations on the iron site were shown to agree very well with
nominal compositions. As shown in Fig. 4, the unit cell constants a
and b linearly decrease by –3.7% and –5.5% from x = 1 to x = 0 in
LixFePO4, respectively, whereas c almost linearly increases by +1.4%
from x = 1 to x ≈ 0.4, and remains fairly unchanged from x ≈ 0.4 to
x = 0. Interestingly, the emptying of Li sites that are gathered as LiO6
chains running along [100]Pmnb (Fig. 1a) significantly diminishes the
average Li–Li site distance within these chains (= a/2) and also leads to
an even larger contraction of the b parameter, which is characteristic
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25 °C
LiFePO4
10.30
10.20
b (Å)
of the interlayer distance between corner-sharing FeO6 planes
(a, c) (Fig. 1b).
On heating at temperatures greater than ~500 °C, LixFePO4
phases start to decompose into mixtures of non-olivine compounds.
The nature and extent of each of the formed phosphates and/or
oxides depend on the value of x. For Li-rich compositions, LixFePO4
transforms into LiFePO4, monoclinic Li3Fe2(PO4)3 (ref. 22), and
iron oxides. As the value of x decreases, the proportion of LiFePO4
decreases and a mixed-valence iron phosphate FeII3FeIII4(PO4)6
appears23. The latter phase was reported24 as a decomposition product
of x LiFePO4 + (1–x) FePO4. This work, however, did not mention
the possible existence of intermediate LixFePO4 phases. As x decreases
even more towards Li0FePO4, the proportions of Li3Fe2(PO4)3 and
FeII3FeIII4(PO4)6 significantly decrease while α-FePO4 (ref. 25) is
formed. One should note that Li-poor LixFePO4 compositions
(x < 0.10) transform into non-olivine phases at relatively high
temperatures, as previously mentioned17 for Li0FePO4.
Having discovered a whole range of LixFePO4 compositions
for ~400 < T < ~500 °C (that is, a new series of mixed-valence
Li-deficient FeII–FeIII phosphates), the next step was to determine
whether such compositions could be stabilized at room temperature.
Overall, as detailed below, the behaviour of LixFePO4 on cooling is
more complicated than on heating, resulting in mixtures of phases
depending on x, as well as on the thermal history of the samples.
When cooled rather slowly in situ (~1 °C min–1), LixFePO4 solid
solution compositions demix into other olivine-type LixFePO4
phases at temperatures that strongly depend on the global Li-content,
according to XRD patterns: 370 ± 20 °C for Li0.19FePO4, 150 ± 20 °C
for Li0.68FePO4 (Fig. 2). This phase separation occurs in two steps.
The first is a phase separation of LixFePO4 into two other olivinetype phases, whose composition depends on the initial value of x and
on the temperature. Then, below a temperature of 140 ± 20 °C, this
two-phase system turns into a more complex mixture in which the
two end-members LiFePO4 and FePO4 coexist with two other olivinetype phases of specific compositions Lix1FePO4 and Lix2FePO4.
The cell parameters of these two intermediate phases were consistently
determined from the average of a series of refinements of various
LixFePO4 starting compositions: a1 = 5.95(2) Å, b1 = 10.20(1) Å,
c1 = 4.73(1) Å, and a2 = 5.91(2) Å, b2 = 10.08(1) Å and c2 = 4.75(2) Å
(Fig. 5). The values of x1 and x2 were interpolated by assuming
that a and b follow the Vegard’s law, that is, they undergo a linear
variation between Li0FePO4 and LiFePO4 at room temperature.
From interpolation of b, we found with remarkable robustness
the values of x1 = 0.75 ± 0.02 and x2 = 0.52 ± 0.02, confirmed by
those obtained from interpolation of a, that is, x1 = 0.73 ± 0.09
and x2 = 0.55 ± 0.09.
This result, which has never been previously reported, is
of importance as it clearly brings experimental evidence that
intermediate olivine-type phases with well-defined compositions
can be obtained at room temperature. Another point concerns the
values of x1 = 3/4 and x2 = 1/2 suggesting a possible ordering of 3
and 2 Li+ ions out of 4 within the LiFePO4 unit-cell, respectively.
Interestingly, Zhou et al. explored the possible existence of five
intermediate compounds (Li0.25FePO4, Li0.5FePO4-a, Li0.5FePO4-b,
Li0.5FePO4-c and Li0.75FePO4) from LDA and GGA calculations having
lower symmetries (monoclinic or triclinic) than the end members
LiFePO4 and FePO4 are predicted19. We may wonder though if a
Li-ordering within the structure exists (and possibly an inducedcharge ordering on the iron sites), as no structural changes such as
distortion of the orthorhombic unit-cell or superstructure formation
was yet observed. So far, we have only been able to detect, from DSC
measurements upon cooling, a broad and small exothermic peak
centred at 180 °C for intermediate values of x, whose intensity was
maximum for the composition Li0.4FePO4: ∆Hx=0.4 ≈ –30 mJ mol–1.
On second heating, a small endothermic peak is observed at
Li0.5FePO4
10.00
9.90
9.80
(020)
Li0.75FePO4
10.10
FePO4
0.0
0.2
0.4 0.6 0.8
x in Lix FePO4
1.0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
LiFePO4
Li0.5FePO4
16
18
Li0.75FePO4
FePO4
20
22
24
26
28
30
32
34
36
38
40
Diffraction angle 2θ (°), CoKα radiation
Figure 5 XRD patterns of quenched LixFePO4 (x = 0.8, 0.6, 0.4, 0.2) samples,
from 400 °C to 25 °C . Inset: cell parameter b (in Pmnb) as a function of x,
determined from full pattern-matching for each composition. Quenching of LixFePO4
single phases leads to a complex mixture of three or four olivine-type phases,
consisting of the two end-members and in two new metastable phases, namely
Li0.75FePO4 and Li0.5FePO4. The proportion of each phase within the mixture depends
on the initial composition. XRD patterns were recorded with CoKα radiation, by steps
of 4 s per 0.032°.
T ~220–230 °C, which probably corresponds to the reverse
phenomenon.
On aging at room temperature, the dark-green-olive mixture of
LixFePO4 phases (LiFePO4, Li0.75FePO4, Li0.5FePO4 and FePO4) slowly
transforms into LiFePO4 and FePO4 alone, which is a clear signature
of the metastability of both Li0.75FePO4 and Li0.5FePO4 at room
temperature. Whatever the quenching protocol (liquid nitrogen or
room temperature between stainless steel pieces), mixtures of the
four olivine phases were systematically obtained, in proportions
depending on the initial LixFePO4 composition. This observation
is consistent with the relatively high kinetics of demixing of the
intermediate phases at room temperature. From a compilation of
all our XRD data collected on cooling and heating, we successfully
established a temperature phase-distribution diagram of LixFePO4
(Fig. 6). Note that caution should be exercised in not literally
interpreting such a diagram constructed from a still limited number
of starting compositions.
Overall, we bring new insights to the thermal behaviour of the
binary phase diagram between LiFePO4 and FePO4 as a function
of temperature. Previously, only a two-phase electrochemical or
chemical process had been reported between these two end-members
corresponding to a complete miscibility gap at room temperature.
Our findings raise several fundamental questions as to the origin
of the driving force enabling the solid solution system to separate
into different Li-containing phases by lowering temperature, and
furthermore challenge theoretical prediction models commonly
used in the field of Li-insertion chemistry. Among the numerous
approaches, early stage lattice gas-models based on mean-field
theory were used to model the configurational entropy as a function
of temperature20,26. For instance, simulations with the introduction
of a negative term U (indicative of attractive interactions between
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Heating
a
600
α–FePO4
Fe7(PO4)6
Li3Fe2(PO4)3 LiFePO4
550
500
450
Lix FePO4
Temperature (°C)
400
350
300
FePO4 + LiFePO4 + Li~x FePO4
250
200
150
100
FePO4 + LiFePO4
50
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
x in Lix FePO4
Cooling
b
400
350
Lix FePO4
Temperature (°C)
300
250
Liy 1FePO4
200
Liy3FePO4
+ Liy4FePO4
+ Liy 2FePO4
150
100
FePO4 + Li0.5FePO4
50
+ Li0.75FePO4 + LiFePO4
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
x in Lix FePO4
Figure 6 Phase distribution diagrams of LixFePO4 (0 ≤ x ≤ 1) established from
temperature-controlled XRD data. a, On heating and b, on cooling. The filled
symbols refer to the actual compositions x that were investigated. For a given x
composition (x = 0.85, x = 0.75, x = 0.68, x = 0.6, x = 0.50, x = 0.41, x = 0.19 and
x = 0.04) we plotted symbols each time significant changes in the XRD patterns
were detected. Open symbols are the result of extensive refinements of the XRD
data. On heating, x LiFePO4 + (1–x) FePO4 mixtures transform into a single LixFePO4
phase through a succession of continuous stages from ~200 to ~350 °C, and
then decompose into a mixture of non-olivine phases above ~500 °C. On cooling,
LixFePO4 solid solutions separate into a mixture of two olivine-type phases, whose
compositions depend on initial x and T. For instance, the global composition
Li0.19FePO4 was refined, on cooling at 330 °C, as a mixture of Li0.05FePO4 and
Li0.27FePO4 (from Vegard’s law interpolation). Below ~140 °C, a complex mixture of
LiFePO4, Li0.75FePO4, Li0.25FePO4 and FePO4 is obtained, with the proportions of each
phase depending on the initial value of x. Vertical error bars correspond to each
temperature-controlled XRD data collection. Horizontal error bars are estimated
from the standard deviation of the refined cell parameters of each intermediate
phase (Liy1FePO4, Liy2FePO4, Liy3FePO4 and Liy4FePO4).
Li+-ions) was shown to describe superbly well the temperaturedriven phase separation in the LixMo6S8 (0 ≤ x ≤ 1) system for which
room-temperature solid solution undergoes a phase separation (for
T < –6 °C) into a Li-rich and a Li-poor LixMo6Se8 phase20. Such a
term U cannot be determined for LiFePO4 at room temperature,
owing to the two-phase nature of the Li-insertion process. In the
hope of overcoming such limitation, we are currently performing
challenging electrochemical measurements within the temperature
range over which the LixFePO4 system is a solid solution.
Furthermore, although it is tempting to compare the LixFePO4 and
LixMo6Se8 systems, one might use the elastic modulus as a means
to account for the great discrepancy between the temperature
domains in which the transition occurs (around room temperature
for LixMo6Se8 to be compared with ~200–350 °C for LixFePO4):
higher covalency of sulphides and selenides as compared with oxides
leads to higher elasticity of the lattices and thus is more favourable
to single-phase insertion/extraction processes. Another difference
lies in the irreversibility of the transition for the LixFePO4 system:
cooling a LixFePO4 solid solution reveals Li0.5FePO4 and Li0.75FePO4
intermediate phases, which were never observed on heating.
More recent models use density functional theory that gives
information about the energies of various Li-vacancy configurations
in oxide systems. In that respect, classical LDA or GGA have agreed
remarkably well with experiments in predicting miscibility and phase
stability27. Such approaches were also very successful in sorting/
predicting phase metastability within the LixCoO2 system27, and are
taking over from mean-field theory ones. The relative weakness of
such methods is that they tend to overly delocalize electrons so that
they are unable to properly describe the thermodynamics and the
electronic properties of insulating compounds28. Classical LDA/GGA19
led to the prediction that LixFePO4 should be a solid solution at room
temperature over the whole composition range. To counterbalance
the discrepancy between theory and experiment, Zhou et al. brought
an important correction to LDA and GGA models by explicitly
considering coulombic correlation effects through a term U΄ that
corresponds to an ‘effective’ on-site interaction: they calculated that
the threshold value to account for the appearance of the two-phase
process was U΄ ~ 2.5 eV.
Lacking theoretical models to fully account for such behaviour,
we simply rely on thermodynamics and kinetics from a qualitative
point of view. On heating, thermal agitation (entropy) tends to
loosen the ‘attraction’ interactions leading, above 200 °C, to Li+
motion within the structure. Li+–Li+ and Fe3+–Fe3+ (or Fe2+–Fe2+)
‘attractive’ interactions are a consequence of the gain in elastic
potential energy arising from phase separation. The peculiar shapes
of the XRD peaks observed on progressive heating (intermediate
broad signals between those of the two end-members) suggest that Li
motion is of a diffusional type, thus leading to a Li gradient between
the Li-rich part and the Li-poor part of the grain(s). The lack of
any distinguishable thermal effect on first heating suggests a secondorder-type phase transition that progressively occurs, without the
formation of well-defined intermediate phases. The system evolves
through a successive sequence of intermediate states of lower
energies (that is, reverse of spinodal decomposition) that results in
a continuous lowering of ∆G. On the other hand, by decreasing the
temperature (lowering entropy), the single phase LixFePO4 becomes
metastable, as the ‘attraction’ interactions become prominent.
Therefore, phase separation occurs as a succession of intermediate
steps as the temperature is progressively decreased. The first one
consists of a progressive splitting of LixFePO4 into two olivinetype phases whose composition depends on x and T. Then below
140 °C, these intermediate phases transform into a mixture of three
or four olivine-type phases, according to the initial composition x
(LiFePO4, Li0.75FePO4, Li0.50FePO4 or FePO4) (Fig. 6). On aging at
room temperature, Li0.75FePO4 and Li0.50FePO4 slowly separate into
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the end-members LiFePO4 and FePO4. Note, Li0.25FePO4 was never
observed at room temperature, which would suggest that it demixes
very quickly. It is legitimate to wonder whether these intermediate
phases have a thermodynamic existence in a defined range of
temperature, that is, if they belong to the binary diagram.
The onset temperature of a miscibility gap might be influenced
by chemical composition such as that demontrated earlier29 for
the LixMo6SezS8–z system: Se substitution for S results in a great
lowering of the temperature transition due a smaller value of |U|
(ref. 29). Other contributions from Yamada30–32 and Chiang’s group33
deserve some comments with respect to the Fe3+/Fe2+ mixed-valence
LixFePO4 compositions (0 < x < 1) reported here. Yamada’s work on
the Lix(MnyFe1–y)PO4 phase diagram at room temperature showed
the existence of a solid solution domain when lithium is extracted
from Li1–x(Mn2+yFe2+1–y–xFe3+x)PO4 for selected values of y (ref. 31).
On the other hand, Chiang and colleagues33 reported on the possible
existence of Li1–a–xMxFePO4 or Li1–xMxFe1–bPO4 compositions
by ‘doping’ with elements supervalent to Li+ such as Zr or Nb.
Our results suggest that Li-deficient compositions may indeed
exist but we still question34 the possible incorporation of Nb or Zr
into the triphylite structure and its real implication on the overall
electrical conductivity: we have strong evidence, in full accordance
with Subramanya-Herle12, that Fe2P and/or an amorphous (Nb, Fe,
C, O, P) wrap around LiFePO4 particles are produced instead, which
are very conductive (up to 1.6.10–1 S cm–1) with an activation energy
∆E ~ 0.08 eV. Instead of using composition as a variable that would
lead to single-phase electrochemical domains within the olivine
family, our finding also highlights the importance of temperature in
creating conditions favourable to single-phase domains containing
mixed transition-metal valence states and/or partial occupation of
the Li+ sites.
These results open the door to a systematic investigation of
the temperature and composition dependence of Li insertion
electrodes operating at room temperature through a two-phase
process, with the end members having a barely distinguishable
structure. Along that line, Li4Ti5O12 appears to be among the most
suitable candidates to generalize such a finding. Finally, a more basic
question to address is whether insertion reactions in insulating
compounds can ever proceed as a single-phase process, and thus
could solid solution reactions be indicative of an enhancement of
intrinsic electronic conductivity. This topic will remain a fruitful
area for further investigations.
METHODS
SYNTHESIS
Pristine LiFePO4 was obtained as fine and dispersed particles (~500 nm) from a straightforward aqueous
solution route involving a FeIII precursor, as we reported earlier35. An aqueous solution containing Li, Fe
and P in stoichiometric proportions was first prepared at room temperature by dissolving Fe(NO3)3.9H2O
(Sigma Aldrich, 98+%) and LiH2PO4 (Sigma Aldrich, 99%). Slow evaporation under continuous stirring
was then followed by a classical two-step thermal treatment at 350 °C and 550 °C under N2/H2 during
10 h. x LiFePO4 / (1–x) FePO4 two-phase mixtures (0 ≤ x ≤ 1)were prepared by chemical oxidation
of pristine LiFePO4 with nitronium tetrafluoroborate in acetonitrile (NO2BF4, Acrös Organics,
97%). 1 g of LiFePO4 was immersed in 50 ml of a solution of NO2BF4 in acetonitrile for two days
in an environmentally controlled dry box at 20 °C under vigorous magnetic stirring21. The depth of
delithiation (1–x) was adjusted by the concentration of NO2BF4 in
the solution.
CHEMICAL ANALYSIS
Each of the x LiFePO4 / (1–x) FePO4 two-phase mixtures was dissolved in an acidic solution of 10 wt%
HCl and 10 wt% HNO3 at ~50 °C under vigorous stirring. Atomic absorption spectroscopy using a
Perkin Elmer AAnalyst 300 monitored by the AAWinlabTM software determined the atomic Li/Fe ratios
in the obtained solution. Standard solutions of 1.035, 2.07, 2.5, 5 and 6.21 mg l–1 were used for the
calibration of Fe. Calibration of Li was carried out using standard solutions of 1, 2 and 3 mg l–1.
TEMPERATURE-CONTROLLED XRD
The values of x in x LiFePO4 / (1–x) FePO4 two-phase mixtures (which very satisfactorily agree with
those determined by chemical analysis) were determined by refining XRD data collected from a Philips
PW 1710 diffractometer (Cu Kα radiation, back monochromator). Instrumental parameters had been
previously determined using a LaB6 calibrated powder (NIST) as a standard. Temperature-controlled
XRD under N2 was performed on a Bruker D8 Diffractometer (Co Kα radiation, Göbel mirror, radial
slits and PSD counter) equipped with an Anton Parr Chamber HTK 1,200 °C. Each pattern (every 20 °C
or 50 °C) was recorded for ~30 minutes at constant temperature with a step of 0.032° and an acquisition
time of 2 s. Between each fixed temperature, the powder was heated at a rate of 6 °C min–1. The same
conditions were used for cooling. All refinements, either full pattern matching or Rietveld refinements
were carried out using the Fullprof suite36.
DIFFERENTIAL SCANNING CALORIMETRY
Differential scanning calorimetry was carried out between room temperature and 400 °C on heating
and cooling with a rate of 5° min–1 in sealed Al crucibles under Argon using a Mettler DSC 25 apparatus
with the TC11 interface. 80–90 mg of x LiFePO4 + (1–x) FePO4 mixtures (with x = 1, 0.8, 0.6, 0.4, 0.2
and 0) were realized by intimate mixing of the powders. To prevent any parasitic thermal effect, FePO4
powders obtained from chemical delithiation of FePO4 were previously heat treated at 400 °C for 24 h.
Received 4 November 2004; accepted 17 January 2005; published 20 February 2005.
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Acknowledgements
The authors express their sincere gratitude to D. Murphy and M. Touboul for helpful comments and
discussions on this manuscript. They also thank UMICORE (Belgium) and C. Wurm for providing the
pristine LiFePO4 powder.
Correspondence and requests for materials should be addressed to C.M.
Competing financial interests
The authors declare that they have no competing financial interests.
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