considered both formations of triplet or singlet state molecules.
For the triplet, the free energy was 44.85 kcal molꢁ1 whereas for
the singlet state, it was 53.10 kcal molꢁ1 leading to Eꢂ values of
1 V and 0.73 V, respectively. These different Eꢂ values (0.36 V
then 1 V (triplet) or 0.73 V (singlet)) for both insertion steps do
not match the experimental data as the 2 Li are inserted at the
same potential. Therefore, a second set of calculations was
undertaken considering here the same two steps radical path but
with a quasi-simultaneous 2 electron transfer (Fig. 6c), neglecting
kinetic limitation. Depending on the spin state of the fully dis-
charged molecule, we obtained Eꢂ values of 0.54 and 0.72 V for
singlet and triplet states, respectively. These values are closer to
what we experimentally found, even though any conclusion
could be given about the spin state of the molecule that may be,
for instance, pure triplet or a mixture of singlet/triplet. In both
calculations, the experimental emf is the average of the two 1
lithium steps.
capacity still remain to be found; this is the scope of our present
research. Here, we have shown that two polymorphs exist, and
only one allows fast and reversible intercalation. It underlines
a common phenomenon in organic chemistry, when the solvent
used for crystallization directs the structure. What is remarkable
is the apparently robust interlocking of ‘‘p’’ planes, so that
EtOH-2 does not revert to MeOH-2 during intercalation (in
contrast with, for instance, the isomerization of cis–cis (CH)x
into trans–trans after one Li insertion cycle6), but also the very
small polarization, similar to the best inorganic electrode
materials (LiFePO4).
Acknowledgements
The authors want to thank M. Courty for running some TGA
measurements on the samples, C. Masquelier for helping us in
collecting XRD data with capillary, and P. Poizot for discus-
sions.
Conclusion
This study reported a new carboxylate-based molecule capable of
reacting reversibly with almost 2 Li+ at a potential of ꢀ0.65 V
which is, like for dilithium terephthalate, attractive as a negative
electrode in Li-ion batteries as it enables the use of an Al current
collector. Besides, we show that this compound displays high rate
capabilities although large amounts of carbon are necessary
(>40%) to trigger the electrochemical activity of the molecule
even if the Li-driven process enlists radical processes which are
kinetically fast. Such an amount of carbon is well above what is
needed in inorganic systems with less than 20% whatever the type
of particles (1D, 2D or 3D) to reach electronic percolation. The
reason for such differences, common to most of the organic
molecules so far investigated, most likely results from the fact
that we are mainly dealing with weak interactions (e.g. van der
Waals bonded molecules) so that the electronic percolation
requires larger amounts of carbon as electrons must be brought
at the molecular level. The irreversible capacity caused by this
carbon fraction is a large handicap in practice. Even though
carboxylates turn out to be an attractive family of compounds in
the hunt for negative electrodes, especially when the percolation
threshold can be reduced possibly by the use of graphene or
conjugated polymers instead of carbon, means to increase their
References
1 H. Chen, M. Armand, G. Demailly, F. Dolhem, P. Poizot and J.-
M. Tarascon, ChemSusChem, 2008, 1(4), 348.
2 M. Armand, C. Michot, N. Ravet, WO Pat. 99288984, 1999.
3 H. Chen, M. Armand, M. Courty, M. Jiang, C. Grey, F. Dolhem, J.-
M. Tarascon and P. Poizot, J. Am. Chem. Soc., 2009, 131, 8984.
4 M. Armand, S. Grugeon, H. Vezin, S. Laruelle, P. Ribiere, P. Poizot
and J.-M. Tarascon, Nat. Mater., 2009, 8, 120.
5 W. Walker, S. Grugeon, O. Mentre, S. Laruelle, J.-M. Tarascon and
F. Wudl, J. Am. Chem. Soc., 2010, 132(18), 6517.
6 W. Walker, S. Grugeon, H. Vezin, S. Laruelle, M. Armand, J.-
M. Tarascon and F. Wudl, Electrochem. Commun., 2010, 12(10),
1348.
7 M. J. Mio, L. C. Kopel, J. B. Braun, T. L. Gadzikwa, K. L. Hull,
R. G. Brisbois, C. J. Markworth and P. A. Grieco, Org. Lett., 2002,
4(19), 3199.
8 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975,
16(50), 4467.
9 J. Peet, M. L. Senatore, A. J. Heeger and G. C. Bazan, Adv. Mater.,
2009, 21, 1521.
10 M. Morcrette, Y. Chabre, G. Vaughan, G. Amatucci, J.-B. Leriche,
S. Patoux, C. Masquelier and J.-M. Tarascon, Electrochim. Acta,
2002, 47, 3137.
11 M. Xu, L. Zhou, L. Xing, W. Li and B. L. Lucht, Electrochim. Acta,
2010, 55, 6743.
12 T. A. Baker and M. Head-Gordon, J. Phys. Chem., 2010, 114, 10326.
1620 | J. Mater. Chem., 2011, 21, 1615–1620
This journal is ª The Royal Society of Chemistry 2011