Citation for published version:
Armstrong, AR, Kuganathan, N, Islam, MS & Bruce, PG 2011, 'Structure and lithium transport pathways in
Li
2
FeSiO
4
cathodes for lithium batteries', Journal of the American Chemical Society, vol. 133, no. 33, pp.
13031-13035. https://doi.org/10.1021/ja2018543
DOI:
10.1021/ja2018543
Publication date:
2011
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Peer reviewed version
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1
Structure and Lithium Transport Pathways in Li
2
FeSiO
4
Cathodes for Lithium Batteries
A. Robert Armstrong
†
, Navaratnarajah Kuganathan
‡
, M. Saiful Islam
‡
and Peter G. Bruce
†
*
†
School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, Scotland
‡
Department of Chemistry, University of Bath, Bath, BA2 7AY, UK
p.g.bruce@st-and.ac.uk
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
ABSTRACT. The importance of exploring new low cost and safe cathodes for larger scale lithium
batteries has led to increasing interest in Li
2
FeSiO
4
. The structure of Li
2
FeSiO
4
undergoes significant
change on cycling, from the as-prepared γ
s
form to an inverse β
II
polymorph; therefore it is important to
establish the structure of the cycled material. In γ
s
half the LiO
4
, FeO
4
and SiO
4
tetrahedra point in
opposite directions in an ordered manner and exhibit extensive edge sharing. Transformation to the
inverse β
II
polymorph on cycling involves inversion of half the SiO
4
, FeO
4
and
LiO
4
tetrahedra, such that
they all now point in the same direction, eliminating edge sharing between cation sites and flattening the
oxygen layers As a result of the structural changes, Li
+
transport paths and corresponding Li-Li
separations in the cycled structure are quite different from the as-prepared material, as revealed here by
computer modelling, and involve distinct zig-zag paths between both Li sites, and through intervening
unoccupied octahedral sites that share faces with the LiO
4
tetrahedra.
2
Introduction
There is intense interest in investigating Li intercalation compounds that might find application as
cathodes in new generations of lithium-ion batteries. In this regard, one advantage of polyoxyanion
intercalation compounds compared with transition metal oxides is that the binding of oxygen in the
polyoxyanions enhances stability and thus safety. Phosphate cathodes, particularly LiFePO
4
, have been
extensively studied and continue to be important. The reason that silicates such as Li
2
FeSiO
4
have
attracted increasing attention recently is that iron and silicon are among the most abundant, and therefore
lowest cost, elements on Earth. Developing cheap and safe cathode materials is a prime target for large
scale lithium batteries in the future. Previous studies of Li
2
FeSiO
4
have reported that more than 160
mAhg
-1
of charge can be extracted from Li
2
FeSiO
4
, with reversible capacities of 120-140 mAhg
-1
.
1,2
Two other related Li intercalation hosts are known, Li
2
MnSiO
4
and Li
2
CoSiO
4
.
3
The structure of as-
prepared Li
2
FeSiO
4
was reported recently.
4
However, it has been shown that the structure, and as a result
the voltage, polarisation (and hence kinetics) of this cathode change during the first few cycles, then
remain constant.
1b
Half of all the Li, Fe and Si ions rearrange on cycling, which results in a significant
change in the Li
+
diffusion pathways. Therefore, establishing the cycled structure of Li
2
FeSiO
4
and its
Li
+
diffusion pathways is important, in order to provide a platform on which future optimization of
Li
2
FeSiO
4
as a cathode for Li-ion batteries can be based. Here we report the crystal structure of cycled
Li
2
FeSiO
4
established using powder neutron diffraction and explore the Li
+
migration pathways using
atomistic simulation techniques.
Experimental
Synthesizing phase pure Li
2
FeSiO
4
has proved a challenge in the past; here we use the following
method by Gong et al., which is known to produce a single phase material.
2d
Stoichiometric amounts of
lithium acetate dihydrate (Acros), iron (II) acetate (Strem), and tetraethyl orthosilicate (Aldrich) were
mixed in ethanol with 2ml acetic acid. After stirring, the suspension was transferred to a Teflon-lined
autoclave and heated to 130
o
C for 12 h. The resulting gel was dried under vacuum at 80
o
C then mixed
3
with sucrose and ball-milled under acetone for 30 minutes. Following acetone evaporation, the sample
was annealed under flowing nitrogen at 600
o
C for 10 h. All subsequent handling was carried out in an
Ar filled glove box (oxygen and water levels < 1 ppm). Powder X-ray diffraction data were collected on
a Stoe STADI/P diffractometer operating in transmission mode with FeKα
1
radiation (λ = 1.936Å) to
eliminate Fe fluorescence.
Composite electrodes (Li
2
FeSiO
4
, super S carbon and Kynar Flex 2801 -a co-polymer based on
PVDF), with weight ratios 75:18:7, were constructed and incorporated into electrochemical cells with a
lithium metal counter electrode and electrolyte (1 molal LiTFSI in ethylene carbonate/ diethyl carbonate
1:1). Electrochemical measurements were carried out at 50
o
C using a Maccor Series 4200 battery
cycler. After cycling, cells were opened in an Ar-filled glove box, the active material removed and
rinsed with dry solvent to remove residual electrolyte and binder and dried. The samples were then
transferred to 2mm quartz capillaries for neutron diffraction measurements. Time-of-flight powder
neutron diffraction data were obtained on the Polaris instrument at ISIS at the Rutherford Appleton
Laboratory. Since lithium is a neutron absorber the data for as-prepared materials were corrected for
absorption. The structures were refined by the Rietveld method using the program TOPAS Academic.
5
The atomistic modelling techniques are well-established and detailed elsewhere;
6
the interatomic
interactions are treated by effective shell-model Buckingham potentials and a three-body term for the
SiO
4
units (Table S2). The transferability of this approach has proved successful in the modelling of
structural, defect and ion transport properties of Li
2
MnSiO
4
,
7
LiFePO
4
,
8
and a range of apatite-silicates
and zeolites.
6,9
An important feature of the methods is the treatment of full lattice relaxation of a large
number of ions (> 700) around the migrating lithium ion, which is modelled by the Mott-Littleton
method (embodied in the GULP code).
6
Results and Discussion
The powder X-ray diffraction pattern of as-prepared Li
2
FeSiO
4
is shown in Figure 1 and is in excellent
agreement with previously reported data.
1,2,4
Electrochemical cells were constructed as described above
and subjected to cycling; the load curve is shown in Fig. 2. Consistent with previous reports it exhibits a
4
change of potential and a reduction in polarisation (separation of charge and discharge curves) during
the first few cycles, indicative of a change in structure, which nevertheless stabilises after 5 cycles.
1,2
Mössbauer measurements indicate a small amount of Fe(III) (~10%), in this sample. We have prepared
materials with up to 30% Fe (III) which exhibit similar phase transition behavior as shown by the
powder X-ray diffraction data in fig. S1.
20 30 40 50 60 70 80 90
0
50
100
150
200
250
300
350
Counts
2 / degrees (FeK
1
)
Figure 1. Powder X-ray diffraction pattern for as-prepared Li
2
FeSiO
4
.
