A1460
Journal of The Electrochemical Society, 149 ͑11͒ A1460-A1465 ͑2002͒
0013-4651/2002/149͑11͒/A1460/6/$7.00 © The Electrochemical Society, Inc.
Synthesis and Study of New Cyclic Boronate Additives for
Lithium Battery Electrolytes
,z
*
*
**
H. S. Lee, X. Sun, X. Q. Yang, and J. McBreen
Brookhaven National Laboratory, Upton, New York 11973, USA
Two novel boronate compounds, 2-͑pentafluorophenyl͒-tetrafluoro-1,3,2-benzodioxaborole ͑1͒ and 2-͑pentafluorophenyl͒-4,4,5,5-
tetrakis͑trifluoromethyl͒-1,3,2-dioxaborolane ͑2͒, have been synthesized as additives for lithium battery electrolytes. These cyclic
boronate compounds have a much more significant effect on conductivity enhancement of LiF salt in dimethoxyethene ͑DME͒ or
ethyl carbonate-dimethyl carbonate ͑EC-DMC͒ than either borane or borate additives we previously synthesized. The conductivity
of a composite electrolyte containing compound 1 and LiF has reached 9.54 ϫ 10Ϫ3 S/cm in DME and 4.79 ϫ 10Ϫ3 S/cm in
EC-DMC ͑1:2͒. This is due to the lower molecular weight and less steric hindrance effects of compound 1. In the case of
compound 2, the enhanced performance also comes from the improved solubility in polar solvents. Composite electrolytes
containing LiF and either compound 1 or compound 2 have excellent electrochemical stability in the EC-DMC solvent, with
respective electrochemical windows of 4.05 and 5.1 V. The composite electrolyte containing LiF and compound 2 shows high
cycling efficiency and cyclability in both Li/LiMn2O4 and Li/LiNi0.8Co0.2O2 cells.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1513559͔ All rights reserved.
Manuscript submitted March 11, 2002; revised manuscript received May 18, 2002. Available electronically October 3, 2002.
The development of portable electronic devices and electric ve-
deficient borate or borane compounds with various fluorinated aryl
or alkyl groups.13-15 The strong anion coordination between these
boron compounds and halide anions ͑Cl-, Br-, and I-͒ has been veri-
fied by near-edge X-ray absorption measurement. Some of these
boron anion receptors can promote the dissolution of normally in-
soluble salts, such as LiF, in several nonaqueous solvents. The ad-
ditives increase the solubility of LiF from Ͻ10Ϫ5 to Ͼ1.0 M and
conductivity to ϳ10Ϫ3 S/cm in carbonate solvents. Most of the
boron-based anion acceptors have high solubility in conventional
nonaqueous solvents. Several of the compounds have exceptional
chemical, electrochemical, and thermal stability. Using these mate-
rials it will be possible to make many new electrolytes and to
consider lithium salts that are normally not soluble in nonaqueous
solvents.
In this paper, we describe the synthesis of two new cyclic fluori-
nated boronate compounds. We have found that the boronate com-
pounds yield higher conductivity and better solubility than
either the borate and borane compounds we reported before. The
electrochemical stability of these new boronate compounds was also
studied. The performance of electrolytes containing these two com-
pounds in Li/LiMn2O4 and Li/LiNi0.2Co0.8O2 cells was also
investigated.
hicles increases the demand for power sources with high energy
density and rapid rechargeability. During the past decade, recharge-
able lithium batteries have surpassed other rechargeable battery
technologies because of their intrinsic superior energy density. An
important goal in lithium battery research is to increase both the
ionic conductivity and the cation transference number of the elec-
trolyte. The low conductivity of nonaqueous electrolyte is mainly a
result of the ion-pairing or low solubility of the lithium salts in
organic solvents. A great effort has been focused on synthesis of
new lithium salts with anions of highly delocalized charge and new
solvents with high electric constant to enhance the ion-pair
dissociation.1-4 Recently, an alternative approach is the development
of new neutral ligands as additives, which have the ability to coor-
dinate with either the cation or anion.5-7 It has been demonstrated by
Gilkerson et al. that the addition of strong ligands could effectively
break down ion pairs due to the dispacement of the anion by ligands
in the vicinity of cations or displacement of the cation by ligands
which selectively coordinated with anions.8,9
Anion coordination ligands are much more favorable than cation
coordination ligand for a lithium battery electrolyte, since it in-
creases both ionic conductivity and lithium transference number
which are necessary to achieve high power density and good re-
chargeablity for lithium batteries.10 Anion receptors are a very active
field of research with most of the work aimed at molecular recogni-
tion to mimic how ion-binding protons control ion transport in bio-
logical membranes. Present anion receptors are based either on posi-
tively charged sites, hydrogen bonding, or Lewis acid metal centers.
None of these are suitable for use in nonaqueous electrolytes. Two
new families of stable anion complexing agents have been synthe-
sized and reported by our group recently. One of these is based on
aza-ether compounds, and they greatly increase the conductivity of
nonaqueous electrolytes, such as lithium salt solutions in tetrahydro-
furan ͑THF͒.11,12 The H atom on the N of these aza-ether com-
pounds is replaced by an electron-withdrawing group such as
Experimental
Synthesis and characterization.—Ethylene carbonate ͑EC͒
͑Fluka, purity Ͼ99%) and dimethyl carbonate ͑DMC͒ ͑Aldrich, pu-
rity Ͼ99%) were dried over 4 Å molecular sieves before use ͑mois-
ture Ͻ20 ppm). The Li1.04Mn2O4 cathode material in the test cells
was purchased from EM Industries, Inc. The LiNi0.15Co0.85O2 cath-
ode materials were obtained from Sumitomo. All other chemical
1
reagents were purchased from Aldrich Chemical Co. HNMR spec-
tra were recorded on a Hitachi R-1200 ͑60 MHz͒ nuclear magnetic
resonance ͑NMR͒ spectrometer using tetramethylsilane as reference.
An Infinity 3000 spectrometer was used to record Fourier transform
infrared ͑FTIR͒ spectra. Melting points were measured using a Kof-
ler hot plate microscope.
Two cyclic boronate compounds were synthesized. The chemical
structures of the synthesized compounds and several borate and bo-
rane compounds ͑discussed in the paper͒ are shown in Fig. 1. The
synthesis steps for the cyclic boronate compound 1 and 2 are
sketched in Fig. 2.
CF3SO2 . The electron-withdrawing group imparts a small positive
Ϫ
charge to the N atoms, hence the complexation of anions. They
function with anions in much the same way as crown ethers act with
cations. The main shortcomings of the aza anion receptors are their
high molecular weight, low solubility in nonaqueous solvents, and
instability at elevated temperatures. More recently we have synthe-
sized another family of anion complexing agents based on electron
2-(Pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole, 1.—
Tetrafluorocatechol ͑3.64 g, 0.02 mol͒ and pentafluoroboronic acid
͑4.2 g, 0.02 mol͒ were mixed in 40 mL of toluene. The mixture was
heated to reflux. Water from condensation reaction was removed by
azeotropic distillation. After 4 h of reaction, the solvent was evapo-
* Electrochemical Society Active Member.
** Electrochemical Society Fellow.
z E-mail: xhsun@bnl.gov
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