Thermodynamic aspects of the reaction of lithium with SnP2O 7 based positive electrodes

Article abstract of DOI:10.1149/1.2424410

The reaction of lithium with tin pyrophosphate, 4 Li+ +Sn P2 O7 → Li4 P2 O7 +Sn, which yields a nanocomposite formed by tin and lithium pyrophosphate, has been probed electrochemically by step potential electrochemical spectroscopy. The thermodynamic characteristics (i.e., ΔG, ΔH and ΔS) of the formation of nanosized tin particles have been determined from data obtained under equilibrium conditions at different temperatures. A first order change in free energy clearly reflects the transformation of α tin to Β tin at 286±1 K. This shows that for the electrochemically produced nanocrystalline tin phase, the transition is at the same temperature as that expected for bulk materials. Interestingly this transition is not significantly limited by kinetics in the way that bulk tin metal is infamous for in tin plague and so we have been able to derive values for these important thermodynamic parameters. Taking into account the absence of long range order in the nanocomposites, the observations indicate that performance of batteries based upon tin oxides as the anode precursor may be affected by small changes of temperature around the transition point. The thermodynamics for this displacive reaction are found to be of a similar order to those obtained for a typical intercalation reaction. However, it seems that the entropy contribution to the free energy dominates for the displacive reaction, which is likely due to the formation of nanosized tin particles.

Full text of DOI:10.1149/1.2424410

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Journal of The Electrochemical Society, 154 ͑3͒ A217-A220 ͑2007͒
A217
0013-4651/2007/154͑3͒/A217/4/$20.00 © The Electrochemical Society
Thermodynamic Aspects of the Reaction of Lithium with
SnP2O7 Based Positive Electrodes
a
Pierrot S. Attidekou,a F. García-Alvarado,b, Paul A. Connor, and
*
,z
John T. S. Irvinea,
*
aSchool of Chemistry, University of St. Andrews, Fife, Scotland KY16 9 ST
bDepartamento de Quimica, Universidad San Pablo-CEU, Madrid 28668, Spain
The reaction of lithium with tin pyrophosphate, 4Li+ + SnP2O7 → Li4P2O7 + Sn, which yields a nanocomposite formed by tin and
lithium pyrophosphate, has been probed electrochemically by step potential electrochemical spectroscopy. The thermodynamic
characteristics ͑i.e., ⌬G, ⌬H and ⌬S͒ of the formation of nanosized tin particles have been determined from data obtained under
equilibrium conditions at different temperatures. A first order change in free energy clearly reflects the transformation of ␣ tin to
␤ tin at 286 1 K. This shows that for the electrochemically produced nanocrystalline tin phase, the transition is at the same
temperature as that expected for bulk materials. Interestingly this transition is not significantly limited by kinetics in the way that
bulk tin metal is infamous for in “tin plague” and so we have been able to derive values for these important thermodynamic
parameters. Taking into account the absence of long range order in the nanocomposites, the observations indicate that performance
of batteries based upon tin oxides as the anode precursor may be affected by small changes of temperature around the transition
point. The thermodynamics for this displacive reaction are found to be of a similar order to those obtained for a typical interca-
lation reaction. However, it seems that the entropy contribution to the free energy dominates for the displacive reaction, which is
likely due to the formation of nanosized tin particles.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2424410͔ All rights reserved.
Manuscript submitted September 11, 2006; revised manuscript received October 23, 2006.
Available electronically January 18, 2007.
Crystalline SnP2O7 was proposed by Behm and Irvine1 as a po-
tential anode material for lithium batteries. This crystalline cubic
material exhibits a good reversible specific charge capacity of
Ͼ360 mAhg−1 and a capacity retention of 300 mAhg−1 over
100 cycles when cycled between 0.02 and 1.2 V versus lithium
metal. It was shown that the initial first discharge at room tempera-
ture involves 10.4 Li/Sn with a reversible capacity corresponding to
4.4 Li/Sn. The reversible capacity is based on Li4.4Sn alloying and
de-alloying. Furthermore, Attidekou et al.2 have proposed a mecha-
nism elucidating the chemical processes occurring in the material
involving three processes: an irreversible displacive reaction to form
nanocrystalline Sn dispersed in a matrix of Li4P2O7, a Sn/Li4P2O7
composite ͑1͒, irreversible reduction of the Li4P2O7 matrix ͑2͒ and
reversible lithium alloying of Sn to form Li22Sn5, or more recently
former case tends to be quite distinguishable, the second case is not
so easy to detect since perfect plateaux are not usually obtained due
to kinetics limitations.
Several techniques have been introduced in order to determine
the reaction mechanism and composition ranges. The use of incre-
mental capacity ͑−ץx/ץE͒ has been proposed by Thompson4 in order
to trace with high resolution various processes occurring during in-
sertion reactions. This technique allows first order ͑biphasic͒ and
continuous ͑solid solution͒ transformation5,6 to be distinguished. Re-
cently, another technique has been proposed that is based on the
analysis of potentiostatic data, step potential electrochemical
spectroscopy7,8 ͑SPECS͒, which can also provide this information
with good accuracy and leads to buildup of the phase diagram in an
insertion system. SPECS is based on the principle of varying the
potential stepwise and allowing the system to approach equilibrium
in every step. The current relaxation versus time ͑t͒ being recorded
at each step allows the determination of whether the system has
reached equilibrium. The current variation with time can give ki-
netic information, whereas the voltage is directly related to the ther-
modynamic equilibrium state. Side reactions, as, for instance, elec-
trolyte oxidation or reduction, can be detected and discounted,
unlike the galvanostatic method.
3
ascribed to Li17Sn4 ͑3͒
4Li+ + 4e− + SnP2O7 → Li4P2O7 + Sn
2Li+ + 2e− + Li4P2O7 → ЉLi6P2O7Љ
͓1͔
͓2͔
͓3͔
xLi+ + xe− + Sn → LixSn ͑0 ഛ x ഛ 4.4͒
The Gibbs free energy can be calculated from
Since the Reaction 1 to form the active material, nanosized tin,
has an effect on the possible application of this as anode material
͑Reaction 3͒ we have focused this work on the thermodynamics
aspect of this initial tin formation reaction and hence its form in the
matrix.
It is well known that phase transformations upon reaction with
lithium can be investigated by using lithium electrochemical cells.
The variation of voltage versus number of electrons or ͑lithium in-
serted in this case͒ tends to have two different forms: an S-shape
continuous variation of the potential which corresponds to insertion
reactions producing a single phase solid-solution region, or a con-
stant potential plateau corresponding to a two-phase region. In the
latter case the two phases formed may be either related if they form
through a topotactic reaction ͑for example, intercalation, displacive
reactions, etc.͒ or very different if they form by reconstructive
mechanisms ͑for example, a decomposition reaction͒. While the
⌬G = − n · F · ⌬E
͓4͔
where ⌬G is the free energy, n the number of electrons, F the Far-
aday constant and ⌬E the equilibrium cell voltage or the electromo-
tive force ͑emf͒. From temperature controlled experiments, ⌬G can
be generated at each temperature and allows the estimation of ⌬S
and ⌬H through the equation ⌬G = ⌬H − T⌬S.
Experimental
SnP2O7 was prepared after the method of Gover et al.9 by heat-
ing SnCl4·5H2O ͑98% + Aldrich͒ and H3PO4 ͑85% w/w; Aldrich͒
at 1000°C. The structure of the materials was characterized by
X-ray powder diffraction and collected on a STOE StadiP diffracto-
meter ͑Cu K␣1 radiation͒. Selected area electron diffraction ͑SAED͒
patterns and high-resolution transmission electron microscopy
͑HRTEM͒ picture were collected using a JEOL 2011 electron micro-
scope at 200 kV and equipped with a side entry sample holder ͑ 20°
tilt͒. Electrode materials were made using the Bellcore method as
previously described.10 MacPile ͑Bio-logic͒ battery testing systems
were used for electrochemical measurements.
*
Electrochemical Society Active Member.
z E-mail: jtsi@st-andrews.ac.uk
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Journal of The Electrochemical Society, 154 ͑3͒ A217-A220 ͑2007͒
Figure 1. ͑Color online͒ Relaxed current during the different voltage steps
corresponding to the compositional range LixMnO2 at T = 30°C between
OCV and 2.5 V.
Figure 2. ͑Color online͒ Voltage profile upon first discharge of emf obtained
using SPECS experiment in a voltage range between the OCV and 3 V
covering the first process.
Various temperature controlled experiments were investigated
via the SPECS method on both MnO2 electrolytic manganese diox-
ide ͑EMD͒ and SnP2O7 electrodes. EMD was used in order to vali-
date our experimental methods. Coin cells were made using SnP2O7
or EMD as the positive electrode, metallic lithium as the negative
electrode and LiClO4 + EC/DMC as the electrolyte. Cells based on
SnP2O7 were discharged at temperatures of −8, 5, 9, 15, 20, 25, 30,
43 and 50°C and those made in EMD at 9, 20 and 30°C. The
potential was stepped at 20 mV/15 h or until the current ͑I͒ con-
dition of ͉I͉ Ͻ ͉I0͉/50,000 was satisfied. When the step duration is
interrupted, the next voltage step is applied. Due to the long duration
of the step, the cell voltage ͑⌬E or Ec͒ can be estimated to be at
equilibrium with
procedure has been set up: the thermodynamics of formation of
LixMnO2 with x = 0.2 is determined from Fig. 2 showing the varia-
tion of voltage with composition for cells discharged at 9, 20 and
30°C. Cell voltages were determined at a fixed point halfway along
͑x = 0.1͒ the plateau and ⌬G calculations were performed with
n = 0.2 electron as it represents the completion of the first process
displayed Fig. 1, i.e., the formation of Li0.2MnO2.
The dependence of ⌬G upon temperature is shown in Fig. 3. The
fitting to ⌬G = ⌬H − T⌬S leads to an entropy value of
110 3 J mol−1 K−1 and an enthalpy of 30 1 kJ mol−1 for the for-
mation of Li0.2MnO2. The value of the obtained entropy variation is
similar to those already reported for other intercalation systems such
as LixCoO212 validating the experimental conditions and confirming
that the method is valid and can be applied to SnP2O7.
⌬E = Ec = lim ⌬E.
͓5͔
I\0
SnP2O7.— Figure 4 shows the general discharge shape of
SnP2O7 between 1.5 and 0.02 V at T = −8 and 30°C. Reaction with
lithium takes place through different processes that are related to the
slope of cell voltage variation versus composition. The completion
of electrochemical reactions upon the first discharge was ascertained
Results and Discussion
MnO2 (EMD).— Our experimental method was validated using
EMD in order to assess the thermodynamic results. The electro-
chemical processes that occur with EMD are essentially intercala-
tion and de-intercalation where ordering is preserved and the lithium
ions move back and forth into the vacant sites ͓Eq. 6͔.
xLi + MnO2 ↔ LixMnO2 ͑0 Ͻ x Ͻ 1͒.
͓6͔
The current profile versus voltage of EMD titration upon the first
discharge in the SPECS experiment between 3.6 and 2.5 V at 30°C
showed at least two distinctive processes as reported previously.11
At 30°C, the first process occurs ϳ3.25 V followed by another
process that was detected ϳ2.85 V.
The analysis was focused on the high potential process for which
Fig. 1 shows that relaxation of current is not homogeneous through-
out the range. Therefore the formation of Li0.2MnO2 occurs through
a biphasic transformation even though a nonperfect plateau has been
obtained in the isothermal experiments. The corresponding voltage
composition plots at three different temperatures are shown in Fig.
2. It can be seen in the same figure that at the end of the “pseudo-
plateau” the differences in voltage are not the same as those that can
be read at half plateau likely due to the influence of the second
process, changes in compositional ranges due to the effect of tem-
perature, etc. Taking also into account that ideally a perfect plateau
should be obtained for which the maximum of current is obtained at
half the plateau, as indeed is the case here ͑see Fig. 1͒, a calculation
Figure 3. ͑Color online͒ Diagram of free energy ͑⌬G/kJ mol−1͒ of formation
of Li0.2MnO2 vs temperature ͑K͒ showing a linear variation of free energy
with temperature.
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Journal of The Electrochemical Society, 154 ͑3͒ A217-A220 ͑2007͒
A219
Figure 6. ͑Color online͒ Diagram of free energy variation vs temperature
showing the first order transition around T = 286 K.
maximum of current was observed, regardless the shape of the pla-
teau. This is the same procedure as applied to EMD ͑see above͒.
The free energy values determined from the value of the voltage
corresponding to the formation of Li4P2O7 + Sn are depicted in
Fig. 6.
Figure 4. ͑Color online͒ Voltage variation of cell bearing SnP2O7 based
electrode obtained from SPECS experiment at two different temperatures
͑−8 and 25°C͒. The inset figure shows the current variation upon discharge
from SPECS experiment.
The variation of the free energy with temperature shows a linear
dependence as expected according to ⌬G = ⌬H − T⌬S, but with
two distinctive regions above and below 13°C ͑286 K͒. The sharp
transition between the two lines suggests a phase transition that is at
least close to first order transition. This transition occurred around
13°C matching with the temperature of the ␣ to ␤ transformation for
Sn. This leads to a conclusion that ␣ tin is formed within the cell
below 286 1 K, whereas it is ␤ tin above. Interestingly this tran-
sition is not significantly limited by kinetics in the way that bulk tin
metal is infamous for in “tin plague” and so has enabled us to derive
values for these important thermodynamic parameters.
The slope of the line in the low temperature region indicates an
entropy value of 1400 10 J mol−1 K−1 and an enthalpy of
93 4 kJ mol−1 for the reaction involving ␣-Sn. In the high tem-
perature region the corresponding values are: an entropy of
1600 10 J mol−1 K−1 and an enthalpy of 62 4 kJ mol−1 for
␤-Sn.
It is rather important to note that results obtained for this trans-
formation indicate that the electrochemically generated nanocom-
posite does not contain amorphous Sn but nanocrystalline Sn. There-
fore if X-ray diffraction data could not be collected it was because
the coherence length needed for diffraction phenomenon was not
reached for the very small crystallites formed.
Considering the reactions ͓Eq. 1 and 6͔, it can be noticed that
4 moles of electrons contribute to Eq. 1, whereas Eq. 6 needs just
0.2 mol of electrons for x = 0.2. This might explain the high value
of ⌬H and ⌬S observed in the case of tin pyrophosphate when
compared to those of formation of Li0.2MnO2. Table I shows the
values of ͑dV/dT͒ corresponding to both cases, which is also related
to ⌬S derived from the Maxwell relation as the value of entropy is
determined from the variation of cell voltage as function of tempera-
ture.
by the analysis of the current relaxation ͑inset Fig. 4͒ in the SPECS
experiment, that leads to the description of three different regions
related to Eqs. 1-3 as already reported.2
The region corresponding to 0 Ͻ x Ͻ 4 lithium uptake per Sn is
depicted by a plateau, which is temperature dependent and consti-
tutes the main focus of this work. This plateau describes a biphasic
region as the relaxation of the current plot versus time exhibits
anomalous behavior. It has been also shown previously by 31P
nuclear magnetic resonance that in that region, SnP2O7 reacts with
lithium to form metallic tin plus a lithiated phosphate matrix that
was characterized as Li4P2O7 ͑see Eq. 1͒. According to x-ray dif-
fraction experiments the composite formed was amorphous but se-
lected area electron diffraction ͑SAED͒ patterns for samples just
after the plateau ͑x = 5͒ exhibit rings that suggest nanocrystallinity
͑Fig. 5a͒ and the particles sizes were estimated between 30 and
40 nm from Fig. 5b.
Figure 4 also shows the influence of temperature on the shape of
the plateau. An almost perfect plateau is obtained at 25°C between
0 ഛ x ഛ 4 from which the thermodynamic equilibrium voltage can
be undoubtedly determined. However, at low temperature the pla-
teau is not perfect, most likely due to kinetic effects. Therefore the
thermodynamic equilibrium voltage was read for all the tempera-
tures at half plateau mark, this is ϳx = 2 Li/formula, where the
Table I. The „dV/dT… for the formation of both Li0.2MnO2 and
Li4P2O7 + Sn starting from EMD and SnP2O7 respectively, and
the extrapolated voltage at 0 K.
EMD
SnP2O7 ͑T Ͻ 286͒ SnP2O7 ͑T Ͼ 286͒
Figure 5. ͑Color online͒ ͑a͒ SAED diffractogram for samples just after the
plateau ͑x = 5͒, ͑T = 25°C͒; h, k, ᐉ values for rings associated with ␤-Sn
are shown; ͑b͒ HRTEM picture for samples reduced till just after the plateau
͑x = 5, T = 25°C͒.
( ⌬V ⌬T) 0.0057 V K−1
0.0036 V K−1
0.0042 V K−1
ր
V ͑0 K͒
1.53 V
0.242 V
0.162 V
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Journal of The Electrochemical Society, 154 ͑3͒ A217-A220 ͑2007͒
been able to derive values for these interesting thermodynamic pa-
rameters. The thermodynamics for this displacive reaction are found
to be of a similar order to those obtained for a typical intercalation
reaction. However, it seems that the entropy contribution to the free
energy dominates for the displacive reaction, which is likely due to
the formation of nanosized tin particles. A series of ongoing long-
term cycling experiments seek to determine the influence of the Sn
nanophase upon cyclability, taking care to separate these effects
from those due to changing temperature.
The extrapolated values of potential at 0 K are related to
⌬HT=0 K and have no entropy contribution.
It is well known in the case of EMD that crystallinity remains
upon intercalation.11 Taking into account the lattice gas model for
intercalation compounds,13,14 the change in entropy is likely related
with the configurational entropy arising from the different possibili-
ties of lithium distribution among the vacant sites in the Mn-O net-
work. This high entropy value can be seen in ͑⌬V/⌬T͒ data com-
pared to SnP2O7 as it is related to the entropy per lithium ion. The
case of SnP2O7 is different since the increase of entropy may have
another origin. The long range order is reduced to short range order
as a nanocomposite materials is generated. In this displacive reac-
tion the entropy contribution to the free energy is considerably more
important when compared to the intercalation reaction producing
Li0.2MnO2.
Acknowledgments
This work was supported financially by GS Yuasa, and the Uni-
versity of St. Andrews. We thank The Royal Society for Spain-UK
Joint project grant no. 2004-2006.
University of St. Andrews assisted in meeting the publication costs of this
article.
Conclusion
References
1. M. Behm and J. T. S. Irvine, Electrochim. Acta, 47, 1727 ͑2002͒.
2. P. S. Attidekou, P. A. Connor, P. Wormald, D. P. Tunstall, S. M. Francis, and J. T.
S. Irvine, Solid State Ionics, 175, 185 ͑2004͒.
Step potential electrochemical spectroscopy at different tempera-
tures has been utilized to study the thermodynamics of the displa-
cive reaction SnP2O7 + 4Li → Li4P2O7 + Sn. A phase transition
that is at least close to first order has been detected in the variation
of the free energy. The temperature of the transition ͑286 1 K͒ fits
with the well known ␣-␤ tin transformation. This indicates that the
Sn produced by the displacive reaction, which is the active material
of SnP2O7 based electrodes, is nanocrystalline rather than amor-
phous. Note that for the electrochemically produced tin phase, the
transition is at the same temperature as that expected for bulk ma-
terials. Taking into account the absence of long range order in the
nanocomposites, the observations indicate that performance of bat-
teries based upon tin oxides as the anode precursor may be affected
by small changes of temperature around the transition point. Inter-
estingly, this transition is not significantly limited by kinetics in the
way that bulk tin metal is infamous for in tin plague and so we have
3. G. R. Goward, N. J. Taylor, D. C. S. Souza, and L. F. Nazar, J. Alloys Compd.,
329, 82 ͑2001͒.
4. A. H. Thompson, J. Electrochem. Soc., 126, 608 ͑1979͒.
5. W. R. McKinnon and J. R. Dahn, Solid State Commun., 48, 4 ͑1983͒.
6. W. R. McKinnon, J. R. Dahn, J. J. Murray, R. R. Haering, R. S. McMillan, and A.
H. Rivers-Bowerman, J. Phys. C, 19, 5135 ͑1986͒.
7. Y. Chabre, J. Electrochem. Soc., 138, 329 ͑1991͒.
8. Y. Chabre, Prog. Solid State Chem., 23, 1 ͑1995͒.
9. R. K. B. Gover, N. D. Withers, S. Allen, R. L. Withers, and J. S. O. Evans, J. Solid
State Chem., 166, 42 ͑2002͒.
10. A. S. Gozdz, C. N. Schmutz, J.-M. Tarascon, and P. C. Warren, U.S. Pat. 5,552,239
͑1996͒.
11. W. Bowden, C. P. Grey, S. Hackney, F. Wang, Y. Paik, N. Iltchev, and R. Sirotina,
J. Power Sources, 153, 265 ͑2006͒.
12. Y. Ryenier et al., Phys. Rev. B, 70, 174304 ͑2004͒.
13. J. R. Dahn, W. R. McKinnon, and S. T. Coleman, Phys. Rev. B, 31, 484 ͑1985͒.
14. T. Coleman, W. R. McKinnon, and J. R. Dahn, Phys. Rev. B, 29, 4147 ͑1984͒.
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