Journal of The Electrochemical Society, 160 (10) A1781-A1784 (2013) A1781

0013-4651/2013/160(10)/A1781/4/$31.00 © The Electrochemical Society

Fluorinated Natural Graphite Cathode for Rechargeable Ionic

Liquid Based Aluminum–Ion Battery
J. Vatsala Rani,z V. Kanakaiah, Tulshiram Dadmal, M. Srinivasa Rao, and S. Bhavanarushi
FluoroOrganic Division, Indian Institute of Chemical Technology, Hyderabad 500007, Andhra Pradesh, India

An aluminum–ion battery comprised of fluorinated natural graphite cathode, aluminum anode and AlCl3 containing imidazolium
based ionic liquid as electrolyte is reported for the first time. Electrochemical method of preparation of fluorinated natural graphite
lead to formation of non-covalent C-F bonds. The cycle life studies (40 cycles) of the battery indicated very stable electrochemical
behavior and the discharge capacity of the battery is 225 mAh g−1 .
© 2013 The Electrochemical Society. [DOI: 10.1149/2.072310jes] All rights reserved.

Manuscript submitted May 6, 2013; revised manuscript received August 8, 2013. Published August 20, 2013.

Over last few years intense scientific research is in progress in the of the cell presented interesting and encouraging results. The semi-
area of higher energy density batteries. Magnesium and aluminum ionic graphite fluoride cathode material prepared electrochemically
metal anodes due to their multivalent nature can be used to obtain in triethylamine trihydrogen fluoride showed discharge capacity of
higher energy density (energy per unit volume) improved safety and 548 mAhg−1 in rechargeable magnesium-ion battery.18
lower initial and cycle-life costs than state-of-art lithium batteries. The
introduction of electric vehicles (EVs) and plug-in hybrid vehicles
(PHVs) via the use of battery technologies as stand-alone energy Experimental
sources is slowly transforming the face of automobile industry and Natural graphite foil, 0.5 mm thick (Alfa-Aesar) was cut into 100
adding pressure, on battery research. mm × 200 mm strip and used as working electrode in the fluorination
In this regard multivalent ions such as Mg2+ ,Ca2+ ,Zn2+ , Yl3+ and cell. Bromine based ionic liquid 1,3-di-n-butylimidazolium bromide
Al3+ ion battery system are acquiring more interest as post-lithium ([bim] [Br]) was prepared by mixing 1-n-butyl imidazole and n-butyl
system.1–4 Aluminum is the third most abundant element in the earth’s bromide (Sigma Aldrich) in 1:1.2 molar ratio, at 90◦ C for 12 hrs,
crust, additionally its lower reactivity and easier handling might offer the excess n-butyl bromide was removed under reduced pressure.19,20
noteworthy cost savings and safety improvements than its counter- AlCl3 0.5 M was added to ionic liquid, the molar ration of AlCl3 to
parts, an aluminum–based redox couple, which engages three elec- ionic liquid was 0.5 : 1.0.
tron transport during the electrochemical charge /discharge reactions, The electrochemical fluorination cell was teflon container with
offers viable storage capacity relative to the single electron Li-ion three electrode assembly, Pt plate counter electrode, Pd wire reference
battery.5,6 Aluminum anode in aqueous electrolytes presents decreased electrode (an inert film of oxide was deposited on its surface by dipping
voltage and cell efficiency due to formation of a passive oxide film7 it in strong acid to withstand HF fumes) electrolyte was pyridinium
on the electrode surface and intrinsic hydrogenation which has been poly(hydrogen fluoride) (Sigma Aldrich). Fluorination of graphite was
avoided by designing reserve systems with the electrolyte added just carried out at room temperature in inert atmosphere by potential sweep
before use. Were as, exploration of ionic liquid–based electrolytes technique, the potential was scanned from −1.0 V to 1.0 V at 20 mV/s,
with aluminum electrode, indicated a non-passive electro-deposition for 0.5 h. The electrochemical cell for charge-discharge studies was
on the surface of the electrode.8 With a suitable electrolyte and an high assembled by using Al foils (100 gm) as anode and graphite fluoride
energy density aluminum anode in hand, the only remaining limitation (0.25 cm−2 / 60 mg) as cathode in ionic liquid.
to develop rechargeable Al-ion battery is cathode. The graphite fluoride was characterized by Scanning Electron
A key prerequisite for attaining high energy density of Al-ion Microscope and Energy-dispersive X-ray spectroscopy (SEM and
battery is a cathode which accommodates and releases the Al3+ EDAX, Hitachi with field emission gun), Transmission electron mi-
ions while discharge and charge. Cathode materials such as spinel croscopy (TEM, Tecnai F12,15 kV), X-ray photoemission spectra
λ-Mn2 O4 and V2 O5 nano-wire were exploited with some promis- (KRATOS AXIS 165 ESCALAB, MgKα anode), Confocal Micro
ing results in rechargeable Al-ion battery with Al-ion conducting Raman spectrum (Horiba Jobin –Yvon labRam, He-Ne laser source)
ionic liquid electrolyte.9,10 Graphite fluorides have been widely stud- and surface area (BET, Micromeritics). Electrochemical measure-
ied as cathode materials in lithium11–15 and to certain extent in Mg ments cyclic voltammetry was performed on IM6ex, Zahner-Elektrik,
and Al batteries.16 Covalent graphite fluorides yield discharge capac- Germany make work station, charge –discharge studies of the cell
ity of about 900 Ah kg−1 linked to a discharge potential close to were performed on WonATech multichannel potentiostat/galvanostat
2.1 V versus Li+ /Li. In aluminum batteries, the discharge potential (WMPG1000, Gyeong Gido, Korea).
and capacity are close to 1 V versus Al3+ /Al and 500 Ah kg−1 , re-
spectively, though a decrease of the capacity is observed concurrently.
Graphite fluoride material, Cx F (5 > x > 2) capable of intercalation Results and Discussion
of fluorine reversibly was prepared in the presence of liquid hydro- Cyclic voltammogram of natural graphite in pyridinium
gen fluoride at room temperature.17 The carbon-atom sheets in this poly(hydrogen fluoride) Fig. 1 shows prominent oxidation and reduc-
material are planar and electrochemically reducible, the bifluoride tion peaks at 0.25 V and −0.25 V vs palladium reference electrode,
salt C12 + HF2 − formed as an intermediate, makes it a superior electri- the oxidation and reduction peaks evolve as a result of intercalation
cal conductor (increase in positive holes due to salt formation) than and de-intercalation of fluoride ions in graphite. The CV at 50th scan
graphite itself. V2 O5 Nano-wire cathode material8 presented 240 Wh was still symmetric with hardly any decrease in the peak currents,
kg−1 energy density for rechargeable Al battery. indicating facile movement of fluorine in the graphene layers, it also
We present a novel Al-ion battery system using fluorinated natu- signifies that the C-F bond formed in the graphite foil is non-covalent.
ral graphite (prepared electrochemically) as cathode combined with The CV of electrochemical fluorination of graphite in 47% aqueous
an aluminum anode in an ionic liquid. (1,3-di-n-butylimidazolium HF solution presented a reaction which was not reversible and after
Bromide [bim] [Br]) based electrolyte. The electrochemical behavior several cycles of the potential sweep, the intensity of these peaks was
reduced.21
The X-ray photoelectron spectra of fluorinated graphite Fig. 2 in-
z
E-mail: vatsala@iict.res.in; j21_vat@rediffmail.com dicates different binding energies of C1s, 284.6, 286.1, & 287.6 eV.

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 A1782 Journal of The Electrochemical Society, 160 (10) A1781-A1784 (2013)

Figure 1. Cyclic voltammogram of natural graphite in pyridinium

poly(hydrogen fluoride) at sweep rate of 10 mVs−1 .

The probable compositions of graphite fluoride formed based on the

binding energy can be deduced as C2.5 F, C2.2 F & C2 F.17 The observed
binding energy of F1s in graphite fluoride is 684.5 eV. The binding
energies of the F ligands prepared electrochemically in CxF is more
ionic than F in LiF (F1s: 684.9 eV) compound and covalent F in
Teflon (F1s: 689.1 eV).22,23 The results indicate that the graphite fluo-
ride formed electrochemically had semi-ionic properties, CV studies
additionally indicate the non-covalent nature of CxF.
The scanning electron micrographs of non-fluorinated graphite
Fig. 3a shows sharp flat steps that extend straight along the edges,
where as in fluorinated graphite Fig. 3b the edges are rough with dis-
continuous steps and some of graphitic panes appeared to be curled
up. The morphological changes suggests, fluorine incorporation in
graphite preferably along the edges of the natural graphite. Fig. 4
shows transmission electron micrographs of graphite fluoride (soni-
cated solution of graphite fluoride foil) the micrograph indicates pres- Figure 3. SEM image of (a) non-fluorinated (b) fluorinated graphite material.
ence of rectangular graphene sheets. The graphene sheets were not
observed in non-fluorinated natural graphite solution. The resistiv-
react with the Al anode to form [Al2 Cl6 Br]− complex species, which
ity of fluorinated graphite was almost halved (1.2 × 10−3 cm) on
reacts with cathode to form aluminum intercalated CxF discharge
fluorination when compared with natural graphite. BET surface area
product. To assess the viability of the synthesized ILs based elec-
analysis indicated threefold increase (0.9086 m2 g−1 ) in surface area
trolyte and the electrochemically prepared conducting (semi–ionic)
of the graphite foil after fluorination.
graphite fluoride cathode for Al-ion battery application, electrochem-
To examine the reversibility of Al-ion in fluorinated graphite suit-
ical properties were explored by cyclic voltammetry and galvanostatic
able electrolyte was chosen. The ionic liquid based electrolyte com-
cycling studies.
position with 0.5:1 molar ration of AlCl3 to [bim] [Br] was found to
Cyclic voltammogram Fig. 5 of the CxF cathode and Al metal in
yield effective electrochemical insertion and dissolution of aluminum
0.5:1 molar ratio AlCl3 to [bim] [Br] at room temperature, displays
for the study. During discharge the wide-spread [AlCl3 Br]− anion will

Figure 2. XPS of electrochemically fluorinated graphite electrode displaying

the different C1s peaks, inset figure displays the recorded spectrum for graphite Figure 4. TEM image of graphene sheets, formed in fluorinated graphite
fluoride. electrode.

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 Journal of The Electrochemical Society, 160 (10) A1781-A1784 (2013) A1783

Figure 6. Voltage vs Sp.capacity plot of Al-ion cell at a constant current

discharge of 60 mA g−1 , inset figure shows charge-discharge cycles of Al-ion
cell.
Figure 5. Cyclic volammogram of Al-ion cell using graphite fluoride cathode
and aluminum anode in ionic liquid at sweep rate of 10 mVs−1 .

highly reversible cathodic and anodic peaks (20cycles) in the potential

window of 0.0 to 1.5 V vs Al electrode. The CV presents three cathodic
peaks shown in figure as 1A, 2A &3A and their corresponding anodic
peaks are shown as 1C, 2C & 3C respectively, which features to
intercalation and deintercalation of Al ions in three oxidation states,
into and from the graphite fluoride cathode. The peak position and
peak current values were nearly identical even after 20 cycles, the
results demonstrate the electrochemical stability of the battery.
Advance assessment of electrochemical properties of the novel
Al-ion battery was made by galvanostatic charge /discharge studies, Figure 7. Cycle life studies of Al-ion cell at a constant current discharge of
which were performed at the cell voltage of 1.5 V to 0.2 V at a con- 60 mA g−1 .
stant current drain of 60 mA g−1 .The cut of voltage while discharge
was 0.2 V, discharge at lower cell potentials (0.2–0.1 V) would in-
volve reactions leading to irreversible processes, mainly related to the In order to assess the cathode material before and after charge dis-
electrolyte decomposition.24 The discharge curve Fig. 6 of Al-ion cell charge cycles, XRD studies were carried out. X-ray diffraction spectra
(the charge-discharge cycles are shown in the figure as inset) the mid of natural graphite, fluorinated graphite and graphite electrode after
potential is ∼0.65 V. The cycle-life studies Fig. 7 of the cell displays completing 40 cycles is presented in Fig. 8, the positions (2-theta
charge and discharge capacity of Al-ion cell, results indicate marginal values) of the four characteristic peaks of natural graphite, showed
increase in cell capacity initially and later it stabilizes and reached small variation in peak intensity. The d-spacing values of the four
225 mAh g−1 at 40th cycle. The columbic efficiency of the cell was hkl indexes (002,100,101,004,) show increase on fluorination, the d-
75%. spacing values of the discharged graphite cathode were higher than

Figure 8. XRD of natural graphite, the inset table

shows XRD data of hkl indexes of natural, fluorinated
and discharged graphite material.

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 A1784 Journal of The Electrochemical Society, 160 (10) A1781-A1784 (2013)

natural graphite but lower than fluorinated graphite. The results indi- Acknowledgments
cate that fluorination of natural graphite electrochemically, enlarges
The authors are thankful to CSIR, New Delhi under the TAPSUN
the space between the graphene sheets in Å, and intercalation of alu- program (NWP-0056) for funding. Authors also thank Dr. S. Gopuku-
minum ions in the graphene sheets occurs without disordering the mar, Senior Principal Scientist, CECRI, Karaikudi.
basic hexagonal symmetry of the graphite crystal.
High degree of reversibility and capacity retention are observed
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