FEATURED TOPIC

Aluminum-Celmet―Aluminum Porous Metal withThree-Dimensional Consecutive Pores
Hideaki SAKAIDA*, Kengo GOTO, Koutaro KIMURA, Kazuki OKUNO,
Junichi NISHIMURA and Akihisa HOSOE---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Celmet is a material that has a high porosity and a large surface area. Sumitomo Electric Toyama Co., Ltd. has offered nickel andnickel-chromium alloy Celmets. There has been a demand for aluminum porous meatal due to advantages such as the lightweight, thermal conductivity, and high voltage resistance. Therefore, we worked on the development of Aluminum-Celmet andsucceeded in its production. This paper presents the physical properties, machine properties, and electric characteristics of theAluminum-Celmet. We also report on the application of Alminum-Celmet to a lithium ion battery.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: plating, molten salt electrolysis, aluminum, porous structure, battery current collector

1. Introduction
Celmet is a porous metal with a three-dimensionalmesh structure. Sumitomo Electric Toyama Co., Ltd.
(Sumitomo Electric Toyama) currently mass-produces
Celmet in nickel and in a nickel-chromium alloy. Utilizing
its characteristic of having a significantly high surface area
compared with its volume, along with a high porosity of up
to 98%, Celmet is used as a current collector*1 in nickelhydrogen batteries, catalyst carriers, electromagnetic
shielding materials, filters, and as a component in fuelbatteries. In order to expand usage and our Celmet lineup,
we have developed an aluminum version of Celmet—
Aluminum-Celmet.
Table 1 compares the characteristics of nickel andaluminum. Compared to nickel, aluminum is characterized
by its lower specific gravity, lower electrical resistivity, andhigher heat conductivity. Also, aluminum does not dissolve
in high action potential levels in non-aqueous electrolyte
solutions such as in lithium-ion batteries (LIB). This is why
aluminum foil is used as the cathode collector. Utilizingthese features, porous aluminum can contribute to theweight reduction of component materials, and improvement of heat transfer and performance quality of LIBs.
Although various developmental attempts have been
made to date, porous aluminum is not widely available as
an industrial product due to difficulties in the manufacturing processes. For example, aluminum casting,(1) in
Table 1. Nickel and Aluminum Comparison
Nickelwhich melted material is poured into a plaster mold, is notsuitable for manufacturing long items and is costly.
Sintering,(2) another aluminum piece production methodthat uses aluminum powder, cannot deliver sufficiently
high porosity and purity to the final product. To address
such issues, we focused on aluminum plating. Plating is
widely used for our mass-production Nickel Celmet, and if
we could plate aluminum, we should be able to produce
long aluminum pieces while meeting the porous metal
requirements, including porosity and ductility. This paper
discusses a new production method for porous aluminum
utilizing plating. The features of the product and its benefits when utilized in LIBs, are also discussed.

2. Celmet Production Method and Challenges
Figure 1 shows the Celmet production method. The
first step is to apply carbon paint onto an open-cell
urethane foam to make the foam surface conductive.
Secondly, nickel is electroplated onto the carbon painted
foam to create a specified thickness of nickel substrate. The
nickel plated foam is then heated to 800°C to remove theurethane. However, because this process oxidizes thenickel, the structure is then exposed to a hydrogen gas
atmosphere at 1,000°C for nickel oxygen reduction. In thisway, Celmet can be created using any metal—as long as it
can be plated. In other words, we cannot create Celmet
with metals that are unsuitable for plating. Plating usuallyrequires an aqueous solution, however, as shown in Fig. 2,

Alminum
3

Speciﬁc gravity

8.9 g/cm

2.7 g/cm3

Electrical resistivity

69.3 nΩ･m

28.2 nΩ･m

Thermal conductivity

90.9 W/(m·K)

237 W/(m·K)

Melting point

1455°C

660°C

Oxidation

Oxidized at hightemperature
Reducible in hydrogenetc.

Thin and dense oxidation
ﬁlm
Irreducible in hydrogen

Durability in non-aqueous Melts at 4.2 V
batteries(Li/Li+)No melting at 4.5 V(Li/Li+)Basematerial

Providingconductivity

Plating

Removing urethane

Redution

NickelNickel oxide

NickelCarbonUrethane framework（Base material）
Urethane foam

Urethane
foam

Paint carbon
to form
conductive
substrate

Form nickel
substrate byelectroplating

Thermally
Nickel oxidation
decompose
reduction incarbon substrate hydrogen atmosphereand urethane
foam in normal
atmosphere

Fig. 1. Celmet Production Procedure

SEI TECHNICAL REVIEW · NUMBER 84 · APRIL 2017 · 87

Oxidation reduction potential (V)

Au
1.0

Ag
Cu

0
Ni
Fe

Platingusing
solution
H2
possible

Cr

-1.0
Ti

Not possible
Al

Fig. 2. Possibility of Plating Using Metal Solutions

because aluminum has a low oxidation-reduction potential,
electrolysis of water starts first. For this reason, aluminumis usually classified as unsuitable for plating.
Therefore, we needed to consider a method to form analuminum substrate with something other than an aqueous
solution. Also, the oxidized aluminum is very stable andirreducible. Therefore, unlike nickel, we needed to find away to remove the urethane foam and avoid aluminum
oxidation.

3. Development of Aluminum Plating
Technology
As mentioned above, plating using aqueous aluminum
solution is not possible. Rather than an aqueous solution,an aluminum substrate can also be formed using either agas-phase method, such as sputtering or vapor deposition,or the hot-dip coating method in which a base material isimmersed in melted aluminum. Challenges in using thegas-phase method include the difficulty of creating a thicksubstrate and the impracticality of creating the substrate in
the foam interior. The challenge of the hot-dip coating
method is that urethane foam is dissolved in the melted
aluminum, which is at a temperature of 660°C or higher. At
this point, we decided to search for an aluminum plating
method utilizing a non-aqueous aluminum solution
working around the above challenges. Table 2 lists potential solutions. These include a molten salt solution,(3) which
is a mixture of salt of alkali metals and aluminum chloride(AlCl3); and an organic solution, such as aluminum chloride-dimethylsulfone (DMSO2-AlCl3).(4) However, as both
Table 2. Features of Different Aluminum Solutions
Types

Molten salt

Organic solvent

Ionic liquid

Composition

NaCl-KCl-AlCl3

DMSO2-AlCl3

EMIC-AlCl3

Plating
temperature

200°C or higher

90~180°C

Room temperature
to 200°C

Surface roughness

RoughControllable byadditives

Controllable byadditives

Impurities

Fe

Fe, S, Cl

Fe

Viscosity

High

High

Low
(20 cP @ 25°C)

EMIC : 1-Ethyl-3-methylimidazolium chloride
DMSO2 : Dimethyl sulfone

solutions need to be at a temperature of more than 90°C for
plating, the urethane foam would be deformed when
immersed in the solution.
We realized the possibility that aluminum chloride
solution in ionic liquid,*2 which is a type of molten saltthat is in a liquid state at room temperature (around 25°C),
could be used for plating on the urethane foam. As a resultof our research, we chose an ionic liquid(5) in which
1-ethyl-3-methylimidazolium chloride (EMIC) and AlCl3
are mixed at a molar ratio of 33:67 as the aluminum plating
solution. EMIC was chosen because it has the highest electrical conductivity and a low melting point among thedifferent types of aluminum ionic liquid.
In this solution, aluminum is separated at the cathodeby the reaction shown in Eq. (1) below, and it is also
dissolved at the anode by the reaction shown in Eq. (2).
Thus, the solution composition remains constant.
4Al2Cl7- + 3e- → Al + 7AlCl4- ............................ (1)Al + 7AlCl4- → 4Al2Cl7- +3e- ............................. (2)However, EMIC-AlCl3 ionic liquid displays highreactivity with water as shown in Eq. (3) and reacts evenwith moisture in the air to produce HCl gas, along with
Al2O3 through the hydrolysis of AlCl3.
2AlCl3 + 3H2O → Al2O3 + 6HCl ....................... (3)Also, oxygen in the liquid causes the immediate
oxidation of the formed aluminum substrate, deteriorating
the plating quality. Therefore, plating must be performed in
an atmosphere of a dry inert gas. Figure 3 shows the settingof our experimental aluminum plating.

Purged with nitrogen gas(–40℃ dew point)

Aluminum mesh
(Anode)DC power

Urethane foam(Cathode)

StirrerFig. 3. Aluminum Plating Experiment Diagram

This experiment was performed in a globe box purged
with nitrogen gas with its temperature controlled to remain
at a dew point of –40°C or lower. Aluminum solution waspoured into a glass beaker, and a piece of aluminum mesh
was set to the anode and a piece of urethane foam coatedwith carbon was set to the cathode. The electroplatingprocess was conducted while stirring the solution.
As a result, aluminum substrate was formed on the

88 · Aluminum-Celmet―Aluminum Porous Metal with Three-Dimensional Consecutive Poressurface of the urethane foam. An SEM*3 image of theplated aluminum substrate is shown in Photo 1.
Although we successfully created the plated
aluminum substrate, it was fragile and crumbled during theprocess of washing and drying. We assumed the cause ofthis crumbliness was because the substrate did not form as
a dense film but as a group of grains, as shown in Photo 1.
We then considered the use of an additive that is also usedin aqueous solution plating to create a denser film.
Referring to the chemicals used as additives to aluminumbased ionic liquid, we researched various substances,including organic solvents,(6) metallic ions,(7) and organic
materials.(8) We finally discovered that adding a small
amount of phenanthroline*4 to the aluminum solution
created a smooth aluminum substrate on the surface of theurethane foam and this substrate retained its shape withoutcrumbling (Photo 2).

Potential thermal
treatment temperature
for Aluminum Celmet
Thermaltreatment
temperature
Urethane

350℃

400℃

500℃

660℃

Urethane decomposition
C+O2→CO2

CarbonSolid

LiquidAluminumFig. 4. Temperatures Required for Thermal Treatment of Aluminum-Celmet

Aluminum oxidation must also be taken into accountwhen determining the optimum temperature. As mentioned
earlier, it is extremely difficult to thermodynamicallyreduce the oxidized surface of aluminum; therefore, conditions that prevent aluminum oxidation are essential.
We eventually obtained the optimum conditions forthe thermal base removal process and this optimized
process successfully removed the urethane foam andcarbon paint without excess oxidation of the aluminum
surface. The Aluminum-Celmet thus produced had good
flexibility and was bendable as shown in Photo 3.

Photo 1. SEM Image of Plated Aluminum Substrate

10mm
Photo 3. Flexibility of Aluminum-Celmet

5. Features of Aluminum-Celmet

Photo 2. SEM Image of Plated Aluminum Substrate with Additive

4. Urethane Foam Removal Method
To obtain Celmet from the unfinished piece, the baseurethane foam and conductive carbon coating must be
removed by thermal decomposition. Figure 4 summarizes
the changes in the status of the urethane foam, carbon, andaluminum at different temperatures. Thermal decomposition must be carried out at temperatures less than 660°C,
the melting point of aluminum. At the same time, thetemperature when carbon vaporizes as CO2 by oxidation is
500°C or higher. Thus, the removal process must be
performed within an oxygenated atmosphere at a temperature of between 500 and 660°C.

5-1 Physical Properties of Aluminum-Celmet
Table 3 lists the physical properties of the developed
Aluminum-Celmet. Tensile strength and electrical resistivity were measured using a test piece of 20 mm x 70 mm
in size. The tensile strength was determined from thestress-strain curves obtained from a tension test with a 30
mm chuck distance and 1.0 mm/min pulling speed.
Electrical resistivity was measured using the four pointprobe method.
Table 3. Physical Properties of Aluminum-Celmet
Electricalresistivity(μΩ･m)

Thickness(mm)

Average pore
diameter
(μm)

0.8

450

112

0.65

2.0

1

550

100

0.50

1.8

Plating density(g/m2)

Tensilestrength
(MPa)

1

550

140

0.80

1.5

2

900

280

0.51

2.4

SEI TECHNICAL REVIEW · NUMBER 84 · APRIL 2017 · 89

The thickness and pore diameter can be controlled by
the plating conditions, while the coating density iscontrolled by the applied current. These conditions can beused to design the product according to its purpose.
5-2 Physical Properties of Aluminum-CelmetThis section reports the SEM observation results concerning
the structural surface and cross-section of 1-mm thickAluminum-Celmet with 550 μm pores and 140 g/m2 density.
Photo 4 presents the surface of the structure. We
confirmed that the aluminum was evenly coated throughout
the structure. Also, the surface was smooth. Figure 5 showsa cross-section view. We confirmed that the aluminum
substrate was sufficiently formed around the interior structure of the urethane foam.

5μm

Photo 5. Aluminum-Celmet Cross-Sectional Crystaline Structure

6. Aluminum-Celmet Applications
6-1 Examples of Aluminum-Celmet Applications
Possible applications of Aluminum-Celmet are listed
in Table 4, taking account of its unique structure as a
Celmet, as well as the aluminum-derived features, such as
high thermal conductivity, high electrical conductivity, andlight weight. Out of these various applications, this section
discusses applicability as the cathode collector in the LIB.

10μm

Table 4. Aluminum Celmet Applications

100μm

Photo 4. Aluminum-Celmet Structure

FeatureUseHigh thermal conductivity

Heat transfer fin (air conditioners, cars)
Heat sink (electronic devices)High electrical conductivity

Current collector for power storage devices(LIBs, capacitors)

Electromagnetic wave shielding Electromagnetic wave shields

Aluminum
Pore

Thickness

100μm

Fig. 5. Cross-Section View of Aluminum-Celmet

Photo 5 represents the crystalline structure of thealuminum plating. This is an observation made using a low
voltage SEM with 5 kV of accelerating voltage, on thesame sample as used for the cross-section view shown in
Fig. 5 after polishing with a cross-section polisher (CP).*5
Aluminum-Celmet crystal is characterized by its fine pillar
structure despite having undergone thermal processing.
Although the ICP-MS*6 analysis results showed theexistence of Fe, Si, Cr, Mn, and Zn, the product’s aluminum
purity was proven to be 99.9% or higher.

Catalyst carrier

Optocatalytic carriers
Oxidation catalytic carriersLight weightLight weight frames

Filtering

Various filter materials

6-2 Application to Lithium Ion Batteries
If Celmet is used as a current collector in a battery, it
can retain the active material within its 3D mesh structure
in a uniform manner regardless of the thickness of the electrode, which in turn increases the battery’s capacity. In anickel-hydrogen battery, Nickel-Celmet is used as thecathode collector, contributing to capacity increase in thebattery. However, the LIB uses a non-aqueous solution anda high voltage is applied to the cathode. As this would
cause nickel to melt into the solution, nickel cannot be usedas the cathode in the LIB. In contrast, aluminum is passivated, and therefore it is not dissolved in an electrolyte. This
is why aluminum foil is suitable for use as the cathode in
the LIB. If we could use Aluminum Celmet in the LIB, we
can expect it to increase capacity and improve the cycle
characteristics, just as Nickel Celmet achieves in a nickelhydrogen battery. To appraise this applicability ofAluminum-Celmet to LIBs, we confirmed the points shownin Table 5. We also show the voltage resistance of
Aluminum-Celmet in Fig. 6.
Between 3 and 4.5 V (vs. Li-/Li) applied to the LIB
anode, no dissolving current occurred, confirming that we
can use Aluminum-Celmet as the LIB cathode collector.

90 · Aluminum-Celmet―Aluminum Porous Metal with Three-Dimensional Consecutive PoresTable 5. Aluminum-Celmet/LIB Applicability Checklist
Item

Requirement

Separators

Verification method

High voltage
No dissolution occurs between
corrosion
3 to 4.5 V (vs. Li+/Li)
resistance

Checked no dissolution occurredat specified voltage using
electrochemical measurement
methodCathodedischarge
capacity

Capacity equivalent to cathodeusing aluminum foil collector

Discharge capacity confirmedfor LIB with Aluminum Celmet
collector

Capacity
increase

Capacity per active material unit Discharge capacity confirmed
weight is constant when active with LIB with different active
material densities
material density is highCathode collector: Aluminum Celmet
Cathode active material: Ternary (NMC)
Anode collector: Copper Celmet
Anode active material: Graphite
Separator: PE microporous film
Electrolyte: LiFSI/EC solution*7

CathodeAnodeFig. 7. Structure of Test LIB

5.5

Percentage compound composition
Cathode: Active material : Conductive material : Binder = 95 : 3 : 2
Anode: Active material : Binder = 95 : 5

5

3.0

4.5

NickelVoltage（V）

Dissolving current (mA/cm2)

Electrolyte
1M LiPF6Ethylene carbonate : Diethyl carbonate = 1 : 1

2.0

Dissolution
1.0

4
3.5
3
2.5

Charge: CC 0.1C - 4.2 V

2Aluminum

Discharge: CC 0.1C – 2.7 V

1.5

0.0

1
0

50

100

150

Anode usage capacity (mAh/g)

-1.0
1

2
3
4
5
Potential level (V vs Li/Li+)

6

Fig. 8. Charge/Discharge Curves

Fig. 6. Voltage Corrosion Resistance of Aluminum-Celmet and Nickel Celmet

Discharge capacity (mAh)

Based on this result, we constructed a laminate cell asshown in Fig. 7 to verify that the LIB battery could use
Aluminum-Celmet for the cathode. The electrode size was
5 cm x 5 cm.
The LIB charge and discharge curves obtained in thisverification test are shown in Fig. 8. The discharge capacity
at 0.1C in the Aluminum-Celmet LIB was 123 mAh/g. As
the discharge capacity of the aluminum foil LIB is 120
mAh/g, we confirmed that we could secure comparable
capacity to the LIB with an aluminum foil collector.
We also created LIBs with different active material
densities in the Aluminum-Celmet electrode. Figure 9shows the plot of discharge capacities at 0.1C for different
active material densities. The reference line shows thedischarge capacity per active material density when using
an aluminum foil electrode with a discharge capacity of
120 mAh/g. The discharge capacities per increasing active
material density in the Aluminum-Celmet electrode remainon this reference line and demonstrate that the discharge
capacity per physical weight does not deteriorate even if
the active material density is increased.
In other words, using Aluminum Celmet as an electrode can increase the active material density—the amountof active material per unit space—compared to analuminum foil electrode. As this can decrease the numberof electrode substrates, it is also possible to reduce components that do not directly contribute to power generation,such as separators, while also increasing battery capacity.

900

Electrode of commercial
LIB (Aluminum foil)

800
700
600
500
400

●Aluminum foil
■Aluminum Celmet---Reference line

300
200
100
0
0

40

80

120

160

200

240

280

Active material density (mg/cm2)
Fig. 9. Correlation Between Active Material Density and Discharge Capacity

7. Conclusions
A wide range of applications for Aluminum-Celmet is
expected, for its Celmet-specific nature offering a 3D meshstructure, as well as the aluminum-derived features ofbeing light in weight, and having high thermal conductivity
and corrosion resistance at high voltage. We have developed an aluminum plating technology, which has beendifficult to achieve to date, and we successfully created
Aluminum-Celmet by optimizing the thermal processing
conditions. The product already enjoys a number of inquiries and it is gaining a reputation in different areas of theindustry. We will now focus on the development of roll-toroll*8 manufacturing technology with mass-production inview. We will also develop other technologies to control

SEI TECHNICAL REVIEW · NUMBER 84 · APRIL 2017 · 91

porosity, pore diameter, and thickness in order to enhance
the Aluminum-Celmet lineup.
• ‌Celmet and Aluminum-Celmet are trademarks or registered trademarks of
Sumitomo Electric Industries, Ltd.

Technical Terms

Current collector: Material to collect an electrical＊1	current in batteries. Generally in lithium ion batteries,
aluminum foil is used as the cathode collector andcopper foil is used as the anode collector.
＊2	Ionic liquid: Refers to chemical compounds (salts)
formed by ionic bonds and in a liquid state at room
temperature.
＊3	SEM: Abbreviation for Scanning Electron Microscope.
Observations are made by detecting secondary
electrons emitted from the observing object throughthe projection of an electron beam.
Phenanthroline: A heterocyclic organic compound＊4	that contains nitrogen. Often used as an indicator for
titrimetric analysis of iron.
＊5	Cross section polisher (CP): Produces smooth crosssections of a material using an ion beam to prepare
for SEM observation or other purposes.
ICP-MS: Inductively Coupled Plasma Mass＊6	Spectrometry. Using plasma as the ion source, theions generated from the plasma are detected in themass spectrometry analyzer chamber.
＊7	LIFSI/EC solution: Ethylene carbonate (EC) organic
solvents that dissolve lithium bis (fluorosulfonyl)
imide.
＊8	Roll-to-Roll manufacturing: An efficient manufacturingmethod to produce an item in a rolled sheet format bycontinuously processing it between two moving rolls.

Contributors The lead author is indicated by an asterisk (*).

H. SAKAIDA*• ‌Energy and Electronics Materials Laboratory

K. GOTO• ‌Energy and Electronics Materials Laboratory

K. KIMURA• ‌Sumitomo Electric Toyama Co.,Ltd.

K. OKUNO• ‌Assistant Manager, Energy and Electronics Materials
Laboratory

J. NISHIMURA• ‌Manager, Sumitomo Electric Toyama Co.,Ltd.

A. HOSOE• ‌Group Manager, Energy and Electronics Materials
Laboratory

References(1)	H. Nakae, Chuzo kogaku,. 74, 12 (2002)(2)	Mitsubishi Materials Corporation, Patent WO2010/116679 A1(3)	B. Nayak: J Appl. Electrochem., 7 (1977)(4)	T. Hirato eta: J Electrochem Soc.,148 (4) C280-C283(2001)(5)	S. Takahashi, Denki Kagaku, 59,14 (1991)(6)	S. Takahashi, Hyomen Gijutsu, 49, 361 (1998)(7)	T. Tsuda, eta:Electrochem.Acta, 46, 1891 (2001)(8)	S. Takahashi, Hyomen Gijutsu, 55, 6 (2004)

92 · Aluminum-Celmet―Aluminum Porous Metal with Three-Dimensional Consecutive Pores