RSC Advances

c3ra44659j

COMMUNICATION

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A novel aluminium–air secondary battery with longterm stability
Ryohei Mori*

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An aluminium–air secondary battery was fabricated by using
an aluminium ion conductor, Al2(WO4)3, and plain salt water
as an electrolyte.

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COM  C3RA44659J_GRABSRSC Advances
COMMUNICATION
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A novel aluminium–air secondary battery withlong-term stability

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Cite this: DOI: 10.1039/c3ra44659j

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Ryohei Mori*

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Received 25th August 2013
Accepted 15th November 2013

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DOI: 10.1039/c3ra44659j

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www.rsc.org/advances

An aluminium–air secondary battery was fabricated by using an

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aluminium ion conductor, Al2(WO4)3, and plain salt water as an electrolyte. The battery functioned for approximately one month. It wasalso possible to enhance the cell capacity and suppress the yield of byproducts, by modifying the cell structure.

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There is an urgent need for clean fuel alternatives for energy
supply and storage created by fossil fuel depletion and air
pollution arising from combustion.1,2 To this end, metal–air
batteries have garnered attention as potential high-performance
power sources for electronic devices,3,4 of which lithium–air cells
are the most promising for high-performance applications.5,6However, lithium is sensitive to ambient conditions such as
humidity and oxygen, and is a scarce natural resource in someregions.
Aluminium, in contrast, is an abundant, attractive anodematerial for energy storage and conversion because of its highspecic capacity, highly negative standard electrode potential[E ¼ 1.7 V vs. standard hydrogen electrode (SHE)], and environmentally benign characteristics. In addition, aluminium isthe most recycled metal in the world and is relatively inexpensive. When compared with zinc, lithium, and other metals,
aluminium has unique advantages and disadvantages. Its
relatively low atomic weight of 26.98 and trivalence confer it agram-equivalent weight of 8.99 and an electrochemical equivalence of 2.98 A h1 g1, compared with 3.86 A h1 g1 forlithium, 2.20 A h1 g1 for magnesium, and 0.82 A h1 g1 forzinc. Moreover, on a per-volume basis, aluminium yields 8.04 Ah1 cm3, compared with 2.06 A h1 cm3 for lithium, 3.83 Ah1 cm3 for magnesium, and 5.85 A h1 cm3 for zinc.7
A major barrier to commercialization is aluminium's highself-corrosion rate in alkaline solutions, both under open-circuit
conditions and during discharge. To reduce the self-corrosion of
the aluminium anode, two methods are usually employed:

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Fuji Pigment Co.Ltd, 2-23-2 Obana, Kawanishi-city, Hyogo Prefecture 666-0015, Japan.
E-mail: moriryohei@fuji-pigment.co.jp; Fax: +81-72-7599008; Tel: +81-72-7598501

This journal is © The Royal Society of Chemistry 2014

alloying aluminium with other elements8–10 or modifying thecomposition of the electrolyte.11–15 However, these research has
been largely unsuccessful for the commercial production of Al–
air cells because by-products such as Al2O3 and Al(OH)3 accumulate at both the anode and cathode. Therefore, we propose tocover the aluminium anode and the air cathode with ceramicaluminium ion conductors, preventing anode corrosion resulting from direct contact with the electrolyte, while retaining
aluminium ion conduction. In addition, this method may
prevent by-product accumulation and suppress electrolyte
evaporation (the major drawback of all metal–air batteries).
In a previous study of ours, Al2(WO4)3 was used as analuminium ion conductor on both the anode and air cathode,and we succeeded in preparing an Al–air cell with secondary
battery properties that was stable for one week.17 However, byproducts such as Al2O3 and Al(OH)3 still formed on thealuminium ion conductor at both the anode and the cathode.
In this study, by using a plain salt water as an electrolyte, wehave fabricated an Al–air cell which functioned for approximately 1 month with the absence of Al2O3 and Al(OH)3.
Furthermore, the cell capacity was largely enhanced by
combining conductive carbon and ceramic aluminium ion
conductor as the air cathode material.
In our previous study, the Al–air cell was termed AFLA as anacronym of its major components: an aluminium anode, an
Al2(WO4)3 lm, an Al2(WO4)3 lid, and an air cathode17 (Fig. 1).
(We refer to the material which covers the air cathode as the“lid”.) An aluminium board was used as the anode. The air
cathode was composed of conductive carbon and polyvinylidenediuoride (PVDF) dissolved in N-methyl-pyrrolidone
on a nickel mesh current collector. The Al2(WO4)3 lm was
prepared by coating the aluminium anode with a printing slurrycomposed of Al2(WO4)3 powder and terpineol, followed byannealing at 500  C. A 10% NaCl aqueous solution was used as
the electrolyte. The pH of the electrolyte was adjusted to 6.7–6.8.
In this study we prepared Al–air cells with two diﬀerent lid
compositions. The rst lid was made of Al2(WO4)3 (AFLA-A cell);the second lid was a mixture of Al2(WO4)3 and conductive carbon

RSC Adv., 2014, xx, 1–6 | 1

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Fig. 1A schematic ﬁgure of AFLA cell: (a) AFLA-A cell and (b) AFLA-AC

cell.

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(AFLA-AC cell). The powders were mixed in respective ratios of
9 : 0 : 1 and 8 : 1 : 1 (Al2(WO4)3 : carbon : PVDF), and pressed
into disks. The structure of the prepared cells can be summarized as Al | Al2(WO4)3|NaCl | Al2(WO4)3| air cathode (AFLA-A,
Fig. 1(a)) and Al | Al2(WO4)3|NaCl | Al2(WO4)3 + carbon | air
cathode (AFLA-AC, Fig. 1(b)).
Fig. 2 show een discharge curves of the prepared Al–air
cells at an applied current of 0.2 mA (cm2) for the AFLA-A

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cell and 0.1 mA (cm2) for the AFLA-AC cell. The cut-oﬀ
potential was 0.2 V. for AFLC-A cell, the capacity was initially8.96 mA h cm2, but dropped to 8.85 mA h cm2 and 8.29 mA
h cm2 at the 5th and 15th cycles, respectively. Capacity issmaller than previous study when NaOH was used as anelectrolyte (Fig. 3).17
Generally, cell capacity is larger when NaOH was used as anelectrolyte.7 However. at the AFLA-AC cell, even with NaClelectrolyte, the initial capacity was 3.12 mA h cm2, but dropped
to 3.04 mA h cm2 and 2.95 mA h cm2 at the 5th and 15th
cycles, respectively. This large capacity diﬀerence is responsible
for the applied current diﬀering between the AFLA-A and AFLAAC cells. It is obvious that the cell capacity is much larger in the
AFLA-AC cell.
However, we had observed that capacity and voltage fadedwith cycle number. We suggest this fading is due to deterioration of the AFLA cell itself. The AFLA cell is under development
as a secondary battery and not yet robust enough. We especiallysuggest that the “lid” needs to be strengthened in order tomaintain high capacity and voltage even aer long-term use.
At the air cathode, the water in the electrolyte reacts withoxygen from the air:O2 + 2H2O + 4e / 4OHThis reaction is accelerated by the close proximity of theelectrolyte to the conductive carbon, which is an active air
cathode material. In other words, existence of conductive
carbon as electron conductor in the lid, electron migration was
accelerated in the AFLA-AC cell. We propose that this is themain reason for the enhanced capacity. However, as thereviewer points out that this is only a suggestion. In order toinvestigate this issue, we will need to conduct more-detailed
future research. For example, we could see the eﬀect of varying
carbon content in the lid. Furthermore, we could attempt to use
other types of air cathode materials such as active carbon,
Mn2O3 and LaFeO3 to investigate variation in conductivity andperformance even further.18–20 In addition, both the AFLA-A and
ALFA-AC cells exhibited stable discharge capacity for een

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Fig. 2

Discharge curve of Al–air cells: (a) AFLA-A cell and (b) AFLA-AC

cell.

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Fig. 3

Discharge curve of Al–air cell from previous article ref. 17.

This journal is © The Royal Society of Chemistry 2014

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cycles, suggesting that they were quite stable when NaCl wasused as the electrolyte.
Fig. 4(a)–(d) display charge–discharge curves collected overtwenty-eight days at an applied current of 0.2 mA (cm2) for
the AFLA-A cell and 0.1 mA (cm2) for the AFLA-AC cell. Each

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RSC Advancesgure displays the charge–discharge curves of the rst andlast four days of the measurement period. To the best of ourknowledge, this is the longest measurement period reported
for charge–discharge curves of an Al–air cell.21,22 Previous
studies regarding Li–Air battery performance similarly
provided data of charge–discharge voltage against time toevaluate secondary battery functionality.23–25 However, it
should be noted here that due to evaporation of the electrolyte, the NaCl solution was relled once every ve days toensure suﬃcient electrical contact. We suggest that theelectrolyte evaporation was attributable to macro- and micropores formed in the lid because of the simplicity of the lid
preparation technique. This issue needs to be further investigated and improved for future study.26
Fig. 5 shows the voltage proles of the AFLA-AC cell as afunction of the time. The cell was discharged galvanostaticallyunder diﬀerent rates from 0.05 mA cm2 to 0.5 mA cm2. The
cut-oﬀ voltage was 0.1 V in all cases. At discharge rates

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Fig. 5

Discharge curves of AFLA-AC cell at diﬀerent discharge rates.

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Fig. 4 Charge–discharge curve of AFLA cell. (a) 1st four days of AFLA-

A cell. (b) Last four days of AFLA-A cell. (c) 1st four days of AFLA-AC cell.
(d) Last four days of AFLA-AC cell.

This journal is © The Royal Society of Chemistry 2014

Fig. 6

Cyclic votammograms of AFLA-AC cell.

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between 0.05 and 0.2 mA cm2, the voltage proles were kept
almost constant. With increasing discharge rates, theprogressive decline of discharge voltage was observed. Whendischarge current was 0.05 mA cm2, the capacity was 1.572
mA h cm2. As discharge current increases as 0.1,and 0.2 mA hcm2, the capacity were 2.904 and 5.472 mA h cm2, respectively. But for 0.5 mA cm2, the discharge curve of cell becamea slope instead of plateau, and discharge capacity decreased to4.68 mA h cm2.
Fig. 6 is the cyclic voltammograms of the AFLA-AC cell forthe rst, 3rd, 5th, 10th and 20th cycles measured between
0 and 2.0 V to characterize the redox reactions. Anodic current
increases largely at the 3rd cycle compared to 1st cycle,
although anodic current lowers as cycle number increases andbecome similar to the 1st current (at 20 th cycle). The reduction and oxidation peak positions for every cycle's are relatively
the same at 1.2–1.3 V and 1.4–1.5 V respectively. This
demonstrates that the redox pairs contribute to the gain andloss of electrons in AFLA-AC cell during cell charge–discharge
reaction. The reason for high intensities at 3–10 th cycle isunknown, however this fact indicates the higher electrochemical reactivity at 3–10 th cycle.
Fig. 7 shows X-ray diﬀraction patterns of the Al2(WO4)3
powder, lid, and lm, before and aer each experiment.
Fig. 7(a) shows the X-ray diﬀraction pattern of the commercially available Al2(WO4)3 powder used in this study, which wascomposed of Al2(WO4)3 and WO3 phases. Aer one month ofcharge–discharge experiments, the lid composed of Al2(WO4)3
still exhibited both Al2(WO4)3 and WO3 phases (Fig. 7(b)). On
the other hand, the lid composed of Al2(WO4)3 and carbon had
a NaCl phase aer the same period (Fig. 7(c)). Fig. 7(d) shows
the X-ray analysis result of the Al2(WO4)3 lm of the AFLA-A.
The Al2(WO4)3 lm had dissolved and an Al metal phaseoriginating from the substrate was detected. Fig. 7(e) displays
the magnied image of Fig. 7(d) between 10 and 40 degrees in
2q. NaAl23O35(b-alumina), Na11WO3, AlO (aluminium oxide (II))
and Al(OH)3 phases were observed, and Al2(WO4)3 was stilldetectable. When the lid was composed of Al2(WO4)3 andcarbon, the Al anode substrate also was exposed due toAl2(WO4)3 dissolution and AlO(OH) (aluminium hydroxide
oxide) was observed on the substrate lm (Fig. 7(f) and (g)).
However, typical by-products such as AlOH3 and Al2O3 that
inhibit the secondary battery reaction were not observed. One
might infer that this is why we were able to run charge–
discharge measurements for one month with a higher cell
capacity than for the AFLA-A cell.
We attempt to explain the yield of the diﬀerent crystalline
phases on the anode as compared to when a NaOH electrolyte
was used in our previous study, by analyzing the reactions
taking place using a NaCl electrolyte.27
OH is produced at the cathode as in the following reaction:

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Fig. 7 X-ray diﬀraction pattern of materials prepared in this study: (a)Al2(WO4)3 powder purchased for this study, (b) the lid after charge–
discharge reaction for one month with NaCl electrolyte for AFLA-A
Cell. (c) The lid after charge-discharge reaction for one month with
NaCl electrolyte for AFLA-AC Cell. (d) The ﬁlm after charge-discharge
reaction for one month with NaCl electrolyte for AFLA-A Cell. (e)Magniﬁed image of Fig. 4(d) between 10 and 40 degrees in 2q. (f) The

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O2 + 2H2O + 4e / 4OHﬁlm after charge-discharge reaction for one month with NaCl electrolyte for AFLA-AC Cell. (g) Magniﬁed image of Fig. 4(f) between 10
and 40 degrees in 2q.

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Then, OH migrates through the electrolyte to reach the Al
anode, where the following discharge reaction occurs:22
Al + 4OH / Al(OH)4 + 3e

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However, OH must rst transit the Al2(WO4)3 thin lm
before reaching the Al metal anode. We reason that the porosityof the Al ion conductor due to our nano-size particle dispersion
preparation method is responsible for allowing OH to travel
through pores in the Al ion conductor lm and reach the Al
anode in order to continue the reaction.
Hence, the overall Al–air cell reaction is:
4Al + 3O2 + 6H2O + 4OH / 4Al(OH)4
Eventually, aluminium hydroxide precipitates:
Al(OH)4 /Al(OH)3 + OH

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In our previous study, when NaOH was used as the electrolyte, AlOH3 and Al2O3 were observed in the lm and the lid.
When NaOH was used as an electrolyte, Al(OH)3 is the chargecarrying ion species.28 On the other hand, when NaCl is used as
an electrolyte, Al(OH)3 and transitory compounds such as
Al(OH)2Cl, Al(OH)Cl2, and AlCl3 are the charge carriers.29 These
diﬀerent charge-carrying species may be the reason for theyields of diﬀerent crystalline phases at the anode. Thus far, thetypical by-products of the aluminium ion battery that inhibit
the cell reaction have been reported to be Al(OH)3 and Al2O3.30
In this study, various types of other chemicals were detected,
including NaAl23O35, Na11WO3, AlO and AlO(OH). For example,
b-alumina is known to be a good sodium ion conductor and has
been used as a solid electrolyte in NaS batteries.31 However, theinuence of these chemicals needs to be claried since theireﬀects on aluminium air batteries have not yet been studied.7
On the other hand, a diﬀerent crystalline phase precipitated on
the lid aer the measurement. In fact, when the lid was soaked
in aqueous NaCl without applying an electrical bias, the X-ray
diﬀraction showed evidence of phases of Al2(WO3)4 in the AFLAA cell and NaCl in the AFLA-AC cell, respectively (data notshown). One can infer that NaCl tends to precipitate on carbon
more readily than on Al2(WO3)4.

Conclusions
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We successfully assembled a functional Al–air cell with an
AFLA structure using plain salt water as an electrolyte. Moreover, by combining the air cathode active material and thesolid electrolyte, we were able to achieve a higher cell capacity.
Future work will be required to fabricate a cell in which thealuminium ion conductor lm does not dissolve at the anode,
by using either another electrolyte or another aluminium ion
conductor with better Al3+ conductivity.32,33 Importantly, we
have shown that aluminium is a reasonable option for asecondary cell material, as it is more abundant, safer, andThis journal is © The Royal Society of Chemistry 2014

easier to handle than other candidate metals such as lithium,zinc, magnesium, and sodium.

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Acknowledgements

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The author wishes to express thanks to Dr Sadayoshi. Mori and
Mr.Kazuo Sakai for helpful discussions.

Notes and references

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