RSC Advances
COMMUNICATION
A new structured aluminium–air secondary battery
with a ceramic aluminium ion conductor
Cite this: RSC Advances, 2013, 3, 11547
Received 4th May 2013,
Accepted 28th May 2013

Ryohei Mori*

DOI: 10.1039/c3ra42211a
www.rsc.org/advances

A metal–air secondary cell with the abundant metal aluminium
has been assembled by placing the aluminium ion conductor
Al2(WO4)3 both on top of the aluminium anode and underneaththe air cathode. This provides a snapshot of future Al–airbatteries.

New solutions and materials for high energy and power density
storage are required to meet the increasing demand forcompetitive (hybrid) electric vehicles, (H)EVs; existing nickel metal
hydride and lithium-ion cell technologies fall short in terms ofprice, range, charge time and lifetime.1,2 Metal–air cells havegarnered attention as potential high-performance power sources
for electronic devices.3,4 Of these, lithium–air cells are the mostpromising for high persistence applications.5,6 However, lithium is
too sensitive to ambient conditions such as humidity and oxygenand is a scarce natural resource in some regions.
By contrast, aluminium is an abundant, attractive anodematerial for energy storage and conversion because of its highspecific capacity, highly negative standard electrode potential, andenvironmentally benign characteristics. Compared with zinc,lithium and other metals, aluminium has its own uniqueadvantages and disadvantages. Its relatively low atomic weight of
26.98 and its trivalence give a gram-equivalent weight of 8.99 and acorresponding electrochemical equivalent of 2.98 Ah g21, compared with 3.86 Ah g21 for lithium, 2.20 Ah g21 for magnesium
and 0.82 Ah g21 for zinc. Moreover, on a volume basis aluminium
should yield 8.04 Ah cm23, compared with 2.06 Ah cm23 forlithium, 5.85 Ah cm23 for zinc and 3.83 Ah cm23 for magnesium.7
The barrier to commercialisation is aluminium’s high selfcorrosion rate in alkaline solutions both under open-circuit
conditions and during the discharge process. To reduce the selfcorrosion of the aluminium anode, two methods are usuallyemployed: alloying aluminium with other elements8–10 or modifying the composition of the electrolyte.11–15 However, little research
into industrial Al–air cells has been successful. Thus, we have put
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 ß The Royal Society of Chemistry 2013

forward the idea of covering the aluminium anode with a ceramicaluminium ion conductor to prevent anode corrosion due to directcontact with the alkaline electrolyte, while retaining aluminium
ion conduction. On the other hand, Zhou et al. have placed a solid
electrolyte at the air cathode side of a lithium–air cell to prevent
oxygen from penetrating the cell, thus inhibiting the formation of
Li2O2 or Li2O as the by-product.16,17 In this study, we havecombined both approaches by placing Al2(WO4)3 as an aluminium
ion conductor, both on the anode and air cathode sides, andsucceeded in preparing an Al–air cell with stable cell properties.
Hereafter, we shall refer to this Al2(WO4)3 material at anodeside as the ‘‘film’’ since its thickness is on the order of 10–20 mm,
and the cathode side material as the ‘‘lid’’ due to its thickness ofapproximately 2 mm and its function as a cover for the liquidelectrolyte. For simplicity and comparison, we classify and refer tothe Al–air cells presented in this study as follows: (a) aluminium
anode + air cathode (AA), (b) aluminium anode + Al2(WO4)3 film +Fig. 1 A schematic figure of AFLA cell.

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Communication

RSC AdvancesFig. 2 Discharge curve of Al–air cells: (a) AA and AFA cells, (b) AFLA cell.
Fig. 3 Charge–discharge curve of the AFLA cell. Discharge curve of the AFLA cell
after up to 30 charge–discharge reaction cycles.

air cathode (AFA), and (c) aluminium anode + Al2(WO4)3 film +Al2(WO4)3 lid + air cathode (AFLA).
A schematic figure of the AFLA cell is given in Fig. 1. The
structure of the proposed AFLA cell can be summarized as:‘‘Al|Al2(WO4)3|NaOH|Al2(WO4)3| air cathode.’’ An aluminium
board was used as an anode. The air cathode was composed ofacetylene black and polyvinylidenedifluoride (PVDF) dissolved in
N-methyl-pyrrolidone on a nickel mesh current collector.
Al2(WO4)3 film was prepared by coating the aluminium anodewith a printing slurry composed of Al2(WO4)3 powder andterpineol, followed by annealing at 600 uC. The Al2(WO4)3 lid
was prepared by mixing Al2(WO4)3 with PVDF and pressing theresulting mixture into a disk. It should be noted here that with thiscell structure, we suggest that the major drawback of all metal–aircells, electrolyte evaporation, can be circumvented.
Fig. 2 shows the discharge curve of Al–air cells prepared in thisstudy at an applied current of 0.2 mA cm22. The capacity of the AAand AFA cells were 17.6 mAh cm22 and 18.9 mAh cm22,respectively (see Fig. 2 (a)). This relatively small difference allows
us to conclude that the Al2(WO4)3 film on the Al anode does not
significantly influence the cell capacity. Fig. 2 (b) presents thedischarge curve of the AFLA cell at an applied current of 0.2 mA
cm22. The capacity of the AFLA cell was 6.9 mAh cm22, smaller
than the AA and AFA cells. Although Al2(WO4)3 is an Al3+ ion
conductor, the thickness of Al2(WO4)3 (approximately 2 mm),reduced Al3+ ion conduction inside the Al2(WO4)3 material, thereby
suppressing the cell capacity. However, it is obvious that the AFLA
cell functions as a secondary cell.
Fig. 3(a) displays the charge-discharge curve of the AFLA cell at
an applied current of 0.2 mA cm22. The charge-discharge reaction
was conducted for approximately one week. To the best of ourknowledge, this is the first report on the charge–discharge curves

11548 | RSC Adv., 2013, 3, 11547–11551

of an Al–air cell for a time period this long.18,19 However, it should
be noted here that because of evaporation of the electrolyte, NaOH
solution was refilled once every 2 days to ensure sufficientelectrical contact. We suggest that macro and micro pores formedin the lid because of the simple sample preparation technique,and these were responsible for electrolyte evaporation; this will befurther discussed in later sections. Fig. 3(b) shows the discharge
curve at an applied current of 0.2 mA cm22 for up to 30 chargedischarge cycles at a cut-off potential of 0.3 V. The capacity was
initially5.3 mAh cm22, but dropped to 5.0 mAh cm22 and 4.4 mAhcm22 at the 10th and 30th cycles, respectively. That is, the capacity
did not deteriorate significantly even after 30 charge-discharge
reactions.
Nyquist plots of the cells are shown in Fig. 4. The cell resistancedifference between AA and AFA cells was small; therefore, theFig. 4 Nyquist plot of Al–air cells prepared in this study.

This journal is ß The Royal Society of Chemistry 2013

RSC Advances

Communication

Fig. 5 SEM images of (a) the Al2(WO4)3 film and (b) the Al2(WO4)3 lid.

resistance of the Al2(WO3)4 film is suggested to be small and does
not affect the cell resistance. This is probably why the cell capacity
difference was also small (see Fig. 2 (a)). However, the cell
resistance of AFLA was obviously than the other two cells. This
could be attributed to the resistance of the Al2(WO3)4 lid, since thethickness of Al2(WO3)4 lid is about 2 mm, its resistance was muchgreater than the Al2(WO3)4 film. The intrinsic conductivity ofAl2(WO3)4 is nearly 4 6 1026 S cm21 at 600 uC.20 It should be
noted here that the resistance of the Al2(WO3)4 lid should be evenlarger than the measured data in Fig. 4 due to its thickness andintrinsic conductivity. We suggest that this is due to NaOH
penetration into the Al2(WO3)4 lid, decreasing the overall
resistance due to its own liquid electrolyte conductivity. This isalso good evidence for electrolyte evaporation via macro and micro
pores in the lid, as discussed earlier.
Fig. 5 shows SEM images of the Al2(WO4)3 film and lid. As seenin Fig. 5(a), the particle sizes range from 100 nm to 2 mm, and theparticles are uniformly dispersed on the Al2(WO4)3 film surface. Itis inferred that owing to this uniform monolith film-like structure,
the Al2(WO4)3 film resistance was small. On the other hand, thestructure and surface of the Al2(WO4)3 lid are more rugged andexhibit micro-cracks (Fig. 5(b)). One can conclude the ion
conductivity could be improved by preparing a thinner and more
densely packed lid, with smaller dispersed particles. In this study,
the Al2(WO4)3 lid was prepared by simple compression ofcommercially available Al2(WO4)3 powders. By applying appropriate dispersion techniques to break down individual aluminium
ion conductor ceramics into small nanoparticles, one can expect
to prepare a densely packed Al2(WO4)3 lid which wouldThis journal is ß The Royal Society of Chemistry 2013

Fig. 6 X-ray diffraction pattern of materials prepared in this study: (a) Al2(WO4)3
powder purchased for this study, (b) Al2(WO4)3 film after charge–discharge reaction
for 3 days with 1.5 M NaOH electrolyte, (c) Al2(WO4)3 lid after charge–discharge
reaction for 3 days with 1.5 M NaOH electrolyte, (d) Al2(WO4)3 lid after soaking in
1.5 M NaOH for 3 days.

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RSC Advancesdemonstrate a higher ion conductivity.21 This lid preparation
method may also prevent electrolyte evaporation.
Fig. 6 displays the X-ray diffraction patterns of the Al2(WO4)3powder, film, and lid after each experiment. Fig. 6(a) shows theX-ray diffraction pattern of the commercially available Al2(WO4)3
powder used in this study, which was composed of Al2(WO4)3 andWO3 phases. After 3 days of charge-discharge experiments, thefilm was composed of Na2 (WO4)(H2O)2 and Al(OH)3, as shown in
Fig. 6(b). Al(OH)3 formation in the Al–air cell during the chargedischarge reaction can be explained by the following consecutivereactions.
The discharge reaction at the aluminium anode can be writtenas:
Al + 4OH2 A Al(OH)24 + 3e
while at the air cathode, the water in the electrolyte reacts withoxygen from the air:O2 + 2H2O + 4e2 A 4OH2 Hence, the overall Al–air cell
reaction would be:
4Al + 3O2 + 6H2O + 4OH2 A 4Al(OH)42Eventually, aluminium hydroxide precipitates:
Al(OH)42 A Al(OH)3 + OH2Fig. 6(c) presents the result for the Al2(WO4)3 lid after 3 charge–discharge cycles. Na2 (WO4)(H2O)2 and Al2O3 were formed insteadof the Al(OH)3 phase. It is surprising that Na2 (WO4)(H2O)2 wasformed after the charge–discharge reaction and Al2(WO4)3 was nolonger observed. In contrast, simply soaking the Al2(WO4)3 lid in
1.5 M NaOH for 3 days, resulted in a pure Al2O3 phase, aspresented in Fig. 6(d). This means that Al was replaced with Na toform Na2 (WO4)(H2O)2.Al2(WO4)3 which is a known inorganic ion
exchanger.22,23 However, the crystalline phase would not change
just by adsorbing metallic ions onto the surface. Generally, the Al3+
ion can be easily replaced with other metallic ions via conventional
solid-state reactions.24 In the present case, however, a different
crystalline phase was formed by electrochemical reaction rather
than by a solid-state reaction. Substitutional solid solution by anelectrochemical reaction is very rare, especially for inorganic oxidematerials, and very few cases have been reported.25
Indeed, Adachi et al. have reported that Al2(WO4)3 can bedoped by (Lu) and (Eu) ions by biasing the voltage between
Al2(WO4)3 and Ln2(WO4)3 (Ln = Lu, Eu) in direct contact. They also
insisted that with this electrochemical doping procedure, metal
doping can be achieved even if the ion radii differ, which isimpossible by conventional solid-state reactions (Al3+: 0.0675 nm,Lu3+: 0.1001 nm, Eu3+: 0.1030 nm).26 The same reaction could beexpected in the present study since Na+ radii is 0.102 nm.27 Adachi
et al. claim that they were the first to discover this phenomenon,
namely, electrochemical doping of inorganic oxide materials; weutilized this to create a new type of Al–air secondary cell. Since weonly observed the Al2O3 phase after soaking the Al2(WO4)3 lid inNaOH, a completely different process must have occurred via theelectrochemical reaction. Apart from these suggestions, one could

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also infer that the Al ion was dissolved and released into aqueous
NaOH, and Na was replaced with Al via an electrochemical
reaction from the electrolyte. Conversely, the Na+ ion in NaOH
reacted with WO3 in the commercial powder to form Na2(WO4)(H2O)2. Further research is necessary to understand thisphenomenon. One should note that Al(OH)3 and Al2O3 were still
produced as by-products, which are the major inhibitors for thecharge-discharge reaction. However, the present Al–air cell worked
for one week, which is longer than any other period reported todate.
In summary, we successfully assembled a functional Al–air cellby placing the aluminium ion conductor Al2(WO4)3 both on top of
the aluminium anode and underneath the air cathode. We are
currently extending the working period of the device to more thana week by applying another electrolyte or aluminium ion
conductor with better Al3+ ion conductivity.28,29 Most importantly,
one can consider using aluminium as a secondary cell material, as
it is more abundant, safer, and easier to handle than other metals
such as lithium, zinc, magnesium, and sodium.

Acknowledgements
The author wishes to express thanks to Dr. H. Yoshioka for
SEM images, and Dr. Sadayoshi Mori and Mr. Kazuo Sakai for
helpful discussions.

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