CL-150836
Received: September 2, 2015 | Accepted: October 9, 2015 | Web Released: October 20, 2015
Steric Effects on the Cyclability of Benzoquinone-type Organic Cathode
Active Materials for Rechargeable Batteries
Takato Yokoji,1 Yuki Kameyama,1 Shun Sakaida,2 Norihiko Maruyama,2 Masaharu Satoh,2 and Hiroshi Matsubara*1
1Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531
2Murata Manufacturing Co., Ltd., Nagaokakyo, Kyoto 617-8555
(E-mail: matsu@c.s.osakafu-u.ac.jp)
Benzoquinone derivatives, which undergo reversible two-
electron redox reactions, should afford high capacity as positive
electrode materials for rechargeable batteries. Although some
benzoquinones have been reported as cathode active materials,
their low cycle-life performance is a drawback. We prepared
benzoquinones bearing alkyl groups with various degrees of
bulkiness to investigate the relationship between the steric
effects of the substituents on the benzoquinone skeleton and the
battery performance. The introduction of bulky substituents,
especially a tert-butyl group, on the skeleton significantly
improved the cyclability.
O
O
O
O
O
O
O
O
Me2-BQ
nBu2-BQ
iPr2-BQ
tBu2-BQ
Figure 1. Structures of BQs bearing alkyl groups with various
degrees of bulkiness (Me, nBu, iPr, and tBu) for utilization as
cathode active materials in rechargeable LIBs.
voltages.5e However, methods to improve the cycle-life perform-
ance remain to be investigated. Such a clarification is extremely
important because the cyclability of cells using low-molecular-
weight organic cathode materials is lower than that of inorganic-
based cells. Herein, we report the steric effects of the introduced
substituents on the battery and cycle-life performance; BQs
bearing functional groups with different steric bulks were
prepared and incorporated into cells for battery performance
evaluations (Figure 1). It should be noted that an approach that
utilizes the structural diversity of organic compounds has never
been reported for the improvement of cycle-life performance
in BQ-based cells, apart from the development of new battery
systems.6
Rechargeable lithium-ion batteries (LIBs) that use LiCoO2
as the cathode active material are widely utilized as energy
sources for various electrical devices.1 However, there is an
urgent need to improve the mass energy density of the batteries
to extend their use to high-energy-consuming devices such as
electric vehicles.2 Organic positive electrode materials should
produce higher mass energy densities than inorganic ones,
because the latter generally act as only one-electron acceptors,
whereas organic materials can often accept two or more
electrons and thereby afford higher capacities. Furthermore,
they could surmount the safety and material resource availability
issues of the classical battery using LiCoO2.
Various types of organic cathode active materials have been
reported as effective substitutes for classical inorganic ones.3,4
Benzoquinone derivatives (BQs) fall into the carbonyl class of
cathode active materials, and afford reversible redox reactions
involving two electrons (Scheme 1).5 Although they provide
high capacities because of their low-molecular-weight skeletons,
low discharge voltages, and inadequate cycle-life performance
are drawbacks of BQ-type cathode active materials. Moreover,
the relationship between the structures of the cathode active
materials and their performance is not well understood,
which complicates the research and design of new candidates.
Therefore, we recently attempted to determine the relationship
between the structure of a BQ-type cathode active material
and its discharge voltage, and revealed that the electronic
effects of substituents on the BQ skeleton affect discharge
voltages; indeed, electron-deficient BQs afforded high discharge
2,5-Dibutyl-1,4-benzoquinone (nBu2-BQ)7 and 2,5-diiso-
propyl-1,4-benzoquinone (iPr2-BQ)8 were synthesized according
to the procedure outlined in Scheme S1 (for details of the
synthesis, see the Supporting Information). The other BQs were
purchased and used without further purification.
Positive-electrode composites of coin-type cells containing
almost 10 wt % positive electrode materials were fabricated with
the composition 10:80:10 wt % positive electrode materials-
vapor-grown carbon fiber (VGCF)-poly(tetrafluoroethylene)
(PTFE). A positive disc 12 mm in diameter was made by
pressing the composite followed by drying in vacuo. A porous
polymer film separator was sandwiched between the positive
disc and a Li metal plate, and the resulting material was placed
in a coin-type cell with the electrolyte solution. The electrolyte
solution was tetraethylene glycol dimethyl ether (tetraglyme)
containing 2.75 M LiN(SO2CF3)2 (LiTFSI). The charge-dis-
charge measurements of the cell were performed at 25 °C by the
constant-current method at 0.1 mA in the cutoff voltage range of
¹1
1.5-4.2 V and at a current density of around 50 mA g
.
O
O
O
The charge-discharge curves and cycle-life performance
of R2-BQ (R = Me, iPr, and tBu) over eight cycles are shown
in Figure 2, while their cycle-life performance over fifteen
cycles is exhibited in Figure 3; their battery performance is
summarized in Table 1.9 The cell based on Me2-BQ afforded
a first discharge capacity, 226 A h kg¹1, which was significantly
lower than the theoretical value, 394 A h kg¹1, and its cyclability
+e-
-e-
+e-
-e-
O
O
O
Scheme 1. Reversible redox reactions involving two electrons
in BQs.
© 2015 The Chemical Society of Japan