Document [original]

Citation: Luc, P.-M.; Buchwald, F.;

Kowal, J. Reproducibility of

Small-Format Laboratory Cells.

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energies

Article

Reproducibility of Small-Format Laboratory Cells

Paul-Martin Luc *, Fabio Buchwald and Julia Kowal

Electrical Energy Storage Technology, Department of Energy and Automation Technology, Faculty IV,

Secr. EMH 2, Technische Universität Berlin, Einsteinufer 11, D-10587 Berlin, Germany

*Correspondence: [email protected]; Tel.: +49-30-314-73859

Abstract:

For the research and development of new battery materials, achieving high reproducibility

of the performance parameters in the laboratory test cells is of great importance. Therefore, in the

present work, three typical small-format lithium-ion cells (coin cell, Swagelok cell and EL-CELL

ECC-PAT-Core) were tested and compared with regard to the reproducibility of their performance

parameters (discharge capacity, internal resistance and coulombic efficiency). A design of experiments

(DOE) with the two factors separator type and anode–cathode ratio (N/P ratio) was carried out for

all cells. For the quality features discharge capacity, internal resistance and coulombic efficiency,

the coefficient of variation is used as a measure of reproducibility. The statistical evaluation shows

that in 83% of all cases, higher reproducibility is achieved when the Freudenberg separator is used

instead of the Celgard separator. In addition, higher reproducibility is achieved in 78% of all cases

if the anode and cathode are the same size. A general statement about which test cell format has

the highest reproducibility cannot be made. Rather, the format selection should be adapted to the

requirements. The examined factors seem to have an influence on the reproducibility but are more

insignificant than other still-unknown factors. Since the production of small-format test cells is a

manual process, the competence of the assembler seems to prevail. In order to mitigate the influence

of as many unknown variables as possible, assembly instructions are proposed for each cell type.

Keywords: coin cell; test cell; reproducibility; design of experiments; cell assembly

1. Introduction

Extensive research and development are required to meet the continuous demand

for high-quality batteries with new requirements. For the investigation of individual

materials or entire cells, it makes sense to work with test cells that can have special sen-

sors/connections, e.g., for temperature monitoring. Furthermore, test cells can be assem-

bled by hand and can be disassembled for analysis if required. Since test cell housings are

often reusable, costs can also be kept low.

Coin cells [

–

], Swagelok [

–

] and EL-CELLs (type: ECC-Std and PAT-Cell) [

–

]

are most commonly used in laboratory studies. A reason why a specific cell format was

chosen for the investigations is usually not given, since all three formats have been used in

research for years.

Studies have already been conducted to maximize the reproducibility of the perfor-

mance of the cell formats mentioned. While [

] focuses on the reproducibility of the

PAT-Cell from EL-CELL, most of the literature refers to coin cell type R2032, which has been

in use for over 35 years [

–

]. These investigations build on each other only slightly or

not at all and sometimes even come to different conclusions. This complicates the selection

of a suitable test cell format as well as its construction and use for future investigations.

Considering the individual findings from the literature presented, this paper intends to

compare the cell formats coin cell, Swagelok cells of two different sizes (big and small) and

EL-CELL ECC-PAT-Core in terms of their reproducibility of cell performance.

Energies 2022,15, 7333. https://doi.org/10.3390/en15197333 https://www.mdpi.com/journal/energies

Energies 2022,15, 7333 2 of 12

2. Experimental

In this section, the utilized tools and materials as well as the three test cell formats

coin cell, Swagelok cell (big and small) and EL-CELL ECC-PAT-Core, with the respective

special features of the structure, are presented in more detail.

2.1. Tools and Materials

The cell assemblies used in this study consist of purchased anodes and cathodes coated

on one side. An anode consists of an 18

m thick copper foil and a 110

m thick graphite

coating. The cathode consists of a 20

m thick aluminum foil coated with 212

m Li-NMC

oxide (lithium (nickel manganese cobalt) oxide). Two separators were used. The first is the

FS3002 from Freudenberg and the second is the H2013 from Celgard. A mixture of ethylene

carbonate (EC) and dimethyl carbonate (DMC) (percent by volume: 50/50) with 1 M of the

salt lithium hexafluorophosphate (LiPF6) is used as the electrolyte.

The cell assembly takes place under a protective atmosphere in a glovebox (M. Braun

Inertgas-Systeme GmbH—UNIlab Pro Glove Box, Malsch, Germany) filled with argon

gas. Round hollow punches of various diameters are used to punch out the electrodes

and the separator. In addition to the hollow punches, a polyoxymethylene (POM) impact

pad and a hammer are used. To ensure that the cell components are not damaged or

cross-contaminated during assembly, three different ceramic tweezers with blunt ends are

used for the anode, cathode and separator. A plunger-operated pipette from Eppendorf is

used to apply the electrolyte, which can be set to values between 10

L and 100

L. The

coin cells are crimped in the HCCCM-100 crimping machine from Xiamen Tmax Battery

Equipments (Xiamen, China) with the housing cover (+) facing downwards.

The Swagelok cells must be screwed tightly with the nuts in the last assembly step.

To ensure that the pressure from the plungers inserted into the housing remains constant

during this time, the Swagelok cells are mounted in a self-constructed bracket. Appropriate

wrenches or multigrip pliers are required for the nuts.

The finished cells are connected to the battery tester BTS-4000 from the company

Neware Technology Limited (Hongkong, China), and the formation and cyclization are

carried out in an oven (universal oven UF55 from Memmert, Büchenbach, Germany) to

ensure a constant temperature.

2.2. Coin Cells

Figure 1a shows the utilized coin cell structure. According to [

], the spacer (thickness:

1 mm) is placed with the burr facing away from the anode–separator–cathode compound

(ASC) as shown in Figure 1c (green check mark) to avoid damaging the anode. If the burrs

face the separator (red cross), it is more likely that the ASC gets damaged. A total electrolyte

quantity of 100

L is used per coin cell to ensure that both the electrodes and the separator

are completely wetted. During assembly, 50

L is applied to the anode and 50

L to the

separator. Consequently, the amount of electrolyte is approximately 10% of the housing

volume. A Freudenberg FS3002 with a diameter of 16.5 mm was used as the separator. The

cathode is pressed lightly with the tweezers after placement to ensure the cathode does

not float on the electrolyte and remains concentric to the anode during crimping. Table 1

shows the assembly sequence used and suggested.

Energies 2022,15, 7333 3 of 12

Energies 2022, 15, x FOR PEER REVIEW 3 of 13

The spacer is placed on the spring. The same tweezers can be used for this. Care must be

taken to position the spacer so that the burr of the spacer faces the spring. The spacer must be

aligned concentrically with the spring.

The anode is placed on the spacer using tweezers or a vacuum pin. A single-side coated an-

ode has the substrate facing the spacer and the active material facing up.

Half of the required amount of electrolyte is filled with a plunger-operated pipette (~50 μL)

distributed on the anode. Complete wetting of the anode should be aimed for.

With the tweezers used for the spacer and spring, or with different tweezers, the separator or

separators are now placed on the anode. This step can cause the anode to slip or the separa-

tor to float. If the anode and separator are non-concentric, the separator should be removed

and repositioned.

The second half of the required amount of electrolyte (~50 μL) is distributed on the separator.

Complete wetting of the separator should be aimed for. If two separators are used, a small

amount of electrolyte can be applied between them.

The cathode has to be applied with a separate tweezer. For a single-sided cathode, the active

material must point in the direction of the anode. To prevent the cathode from floating, the

cathode can be gently pressed with the tweezers to suppress so much electrolyte that friction

between the layers begins.

Now the housing (+) can be put on. This can be carried out with wide tweezers to ensure the

case is placed parallel. The housing (+) should be pressed slightly to ensure friction between

the cell components before rotating the cell for the crimping machine.

The coin cell is inserted into the crimping machine. A pressure of 950 psi is recommended.

Small amounts of electrolyte may leak during crimping. The cell can be taken out and

whipped clean. The crimping dye should also be wiped.

Figure 1. (a) Coin cell components and assembly order; (b) roll-over and burr of a punched disk;

2.3. Swagelok Cell

The test cell comparison uses two different sizes of Swagelok cells as shown in Figure 2.

With the smaller version, the diameter of the ASC can be as little as 10 mm in diameter.

With the larger housing, it can be up to 12 mm. The middle parts of the housing (8) are

made of metal and must be insulated against short circuits on the inside. A plastic film (7)

made of biaxially oriented polyethylene terephthalate and a thickness of approx. 0.1 mm

is used, as the housing has an inside diameter of 10.3 mm or 12.2 mm. If possible, the

insulating foil should cover the entire interior of the housing up to the clamping rings on

each side. To insert the film into the housing, it must be brought into a cylindrical shape.

In order to avoid a short circuit via the housing, the foil should be slightly larger than the

inner circumference of the housing. The housings of both Swagelok cell sizes are identical

in length. For the rectangular cut foils, the ideal dimensions of 25 mm (length) × 35 mm

(circumference) for the small cell and 25 mm (length) × 41 mm (circumference) for the

large cell have been found. Spacers (CuZn 37.6) with a diameter of 10 mm or 12 mm and

a thickness of 1 mm are used to apply a homogenous pressure on the ASC. The spring (5)

Figure 1.

(

) Coin cell components and assembly order; (

) roll-over and burr of a punched disk;

Table 1. Assembly instruction for coin cells.

Step Instruction

Clean the work surface and set up all tools and cell components within easy reach.

It is also advisable to place all passive cell components close to each other to keep

the assembly time for all sub-steps as equal as possible.

The spring is inserted into the housing cover (−) with tweezers. If it is a disk

spring, the larger radius of the spring points upwards. The spring should be

placed as centrally as possible.

The spacer is placed on the spring. The same tweezers can be used for this. Care

must be taken to position the spacer so that the burr of the spacer faces the spring.

The spacer must be aligned concentrically with the spring.

4The anode is placed on the spacer using tweezers or a vacuum pin. A single-side

coated anode has the substrate facing the spacer and the active material facing up.

Half of the required amount of electrolyte is filled with a plunger-operated pipette

(~50 µL) distributed on the anode. Complete wetting of the anode should be

aimed for.

With the tweezers used for the spacer and spring, or with different tweezers, the

separator or separators are now placed on the anode. This step can cause the

anode to slip or the separator to float. If the anode and separator are

non-concentric, the separator should be removed and repositioned.

The second half of the required amount of electrolyte (~50

L) is distributed on the

separator. Complete wetting of the separator should be aimed for. If two

separators are used, a small amount of electrolyte can be applied between them.

The cathode has to be applied with a separate tweezer. For a single-sided cathode,

the active material must point in the direction of the anode. To prevent the cathode

from floating, the cathode can be gently pressed with the tweezers to suppress so

much electrolyte that friction between the layers begins.

Now the housing (+) can be put on. This can be carried out with wide tweezers to

ensure the case is placed parallel. The housing (+) should be pressed slightly to

ensure friction between the cell components before rotating the cell for the

crimping machine.

The coin cell is inserted into the crimping machine. A pressure of 950 psi is

recommended. Small amounts of electrolyte may leak during crimping. The cell

can be taken out and whipped clean. The crimping dye should also be wiped.

2.3. Swagelok Cell

The test cell comparison uses two different sizes of Swagelok cells as shown in Figure 2.

With the smaller version, the diameter of the ASC can be as little as 10 mm in diameter.

With the larger housing, it can be up to 12 mm. The middle parts of the housing (8) are

made of metal and must be insulated against short circuits on the inside. A plastic film (7)

made of biaxially oriented polyethylene terephthalate and a thickness of approx.

0.1 mm

Energies 2022,15, 7333 4 of 12

is used, as the housing has an inside diameter of 10.3 mm or 12.2 mm. If possible, the

insulating foil should cover the entire interior of the housing up to the clamping rings on

each side. To insert the film into the housing, it must be brought into a cylindrical shape.

In order to avoid a short circuit via the housing, the foil should be slightly larger than the

inner circumference of the housing. The housings of both Swagelok cell sizes are identical

in length. For the rectangular cut foils, the ideal dimensions of 25 mm (length)

35 mm

(circumference) for the small cell and 25 mm (length)

41 mm (circumference) for the

large cell have been found. Spacers (CuZn 37.6) with a diameter of 10 mm or 12 mm and a

thickness of 1 mm are used to apply a homogenous pressure on the ASC. The spring (5)

used for both Swagelok cell sizes is a helical compression spring (outer diameter: 9.9 mm;

length: 20 mm; wire gauge: 0.9 mm; number of coils: 7). To exert pressure on the cell

stack, an electrically conductive stamp (2) is inserted into the housing from both sides. The

stamps have a diameter of 10 mm or 12 mm for the two case sizes. In order to be able to

measure the Swagelok cells, the stamps have a thread on one side to enable contact via

a screw connection. The stamps are connected to the housing with the aid of clamping

rings (3 and 4) and hexagon nuts (1), and the inside of the housing is sealed off from the

outside. The clamping rings come from Swagelok and are divided into front (4) and rear

clamping rings (3). They are made of non-conductive nylon, polytetrafluoroethylene (PTFE)

or perfluoroalkoxy alkane (PFA) to ensure insulation between the stamp and the housing

and are designed for 10 mm and 12 mm tubing, respectively. The hexagon nuts are 19 mm

or 22 mm (wrench size). Table 2shows the assembly sequence used and suggested.

Energies 2022, 15, x FOR PEER REVIEW 4 of 13

used for both Swagelok cell sizes is a helical compression spring (outer diameter: 9.9 mm;

length: 20 mm; wire gauge: 0.9 mm; number of coils: 7). To exert pressure on the cell stack,

an electrically conductive stamp (2) is inserted into the housing from both sides. The

stamps have a diameter of 10 mm or 12 mm for the two case sizes. In order to be able to

measure the Swagelok cells, the stamps have a thread on one side to enable contact via a

screw connection. The stamps are connected to the housing with the aid of clamping rings

(3 and 4) and hexagon nuts (1), and the inside of the housing is sealed off from the outside.

The clamping rings come from Swagelok and are divided into front (4) and rear clamping

rings (3). They are made of non-conductive nylon, polytetrafluoroethylene (PTFE) or per-

fluoroalkoxy alkane (PFA) to ensure insulation between the stamp and the housing and

are designed for 10 mm and 12 mm tubing, respectively. The hexagon nuts are 19 mm or

22 mm (wrench size). Table 2 shows the assembly sequence used and suggested.

Figure 2. (a) Exploded view of a Swagelok cell; (b) Swagelok cell big (left) and small (right).

Table 2. Assembly instructions for Swagelok cells.

Step

Instruction

The insulating foil is shaped cylindrically by hand with the smooth side inwards and placed

in the middle of the housing. The film can be carefully pressed onto the inside of the housing

with one of the stamps so that there is no free space between the housing wall and the foil.

A front and a rear clamping ring are placed on the shorter stamp (cathode side), whereby the

side with the smaller radius of the two clamping rings must point to the flat side of the stamp

and not to the thread side. The front clamping ring is placed on the stamp so that the front

part of the stamp is approximately 5 mm outside. The stamp is inserted into the housing and

screwed tight with the nut.

The cathode is inserted with tweezers from the other side of the still-open case. In the case of

single-sided electrodes, the active material faces upwards (towards the separator).

Half of the required amount of electrolyte (here: 50 μL) is evenly distributed on the cathode

with a plunger-operated pipette.

The separator is placed on the electrolyte-covered cathode with a second pair of tweezers.

The separator should be pressed lightly with the tweezers. The concentrically aligned separa-

tor can then be pressed lightly at the overlapping points of the separator on the housing wall.

The second half of the electrolyte (here: 50 μL) is applied evenly to the separator.

With a third pair of tweezers, the anode is placed on the separator with the active material

facing down. During this step, extra care must be taken not to turn the anode the wrong way

around when loosening it with the tweezers. A vacuum pin is recommended for this step.

The spacer is placed on the anode and lightly pressed.

The compression spring is placed on the spacer with the flat side down.

A front and rear clamping ring are placed on the longer stamp (anode side) with the same

orientation as on the shorter stamp. This time, the front clamping ring is flush with the end of

the stamp. The stamp is placed with the nut on the body and spring and the nut is tightened

until the stamp can still be moved. After contact with the spring, the stamp is inserted 10 mm

into the housing and the nut is tightened. To keep the compression of the compression spring

the same, a torque wrench is recommended, with which the tightening torque can be ad-

justed. The tightening torque required should be determined in a preliminary test.

Figure 2. (a) Exploded view of a Swagelok cell; (b) Swagelok cell big (left) and small (right).

Table 2. Assembly instructions for Swagelok cells.

Step Instruction

The insulating foil is shaped cylindrically by hand with the smooth side inwards

and placed in the middle of the housing. The film can be carefully pressed onto the

inside of the housing with one of the stamps so that there is no free space between

the housing wall and the foil.

A front and a rear clamping ring are placed on the shorter stamp (cathode side),

whereby the side with the smaller radius of the two clamping rings must point to

the flat side of the stamp and not to the thread side. The front clamping ring is

placed on the stamp so that the front part of the stamp is approximately 5 mm

outside. The stamp is inserted into the housing and screwed tight with the nut.

The cathode is inserted with tweezers from the other side of the still-open case. In

the case of single-sided electrodes, the active material faces upwards (towards

the separator).

4Half of the required amount of electrolyte (here: 50 µL) is evenly distributed on

the cathode with a plunger-operated pipette.

The separator is placed on the electrolyte-covered cathode with a second pair of

tweezers. The separator should be pressed lightly with the tweezers. The

concentrically aligned separator can then be pressed lightly at the overlapping

points of the separator on the housing wall.

Energies 2022,15, 7333 5 of 12

Table 2. Cont.

Step Instruction

6 The second half of the electrolyte (here: 50 µL) is applied evenly to the separator.

With a third pair of tweezers, the anode is placed on the separator with the active

material facing down. During this step, extra care must be taken not to turn the

anode the wrong way around when loosening it with the tweezers. A vacuum pin

is recommended for this step.

8 The spacer is placed on the anode and lightly pressed.

9 The compression spring is placed on the spacer with the flat side down.

A front and rear clamping ring are placed on the longer stamp (anode side) with

the same orientation as on the shorter stamp. This time, the front clamping ring is

flush with the end of the stamp. The stamp is placed with the nut on the body and

spring and the nut is tightened until the stamp can still be moved. After contact

with the spring, the stamp is inserted 10 mm into the housing and the nut is

tightened. To keep the compression of the compression spring the same, a torque

wrench is recommended, with which the tightening torque can be adjusted. The

tightening torque required should be determined in a preliminary test.

2.4. EL-CELL ECC-PAT-Core

In this study, the EL-CELL ECC-PAT-Core (EL-CELL) is used in the standard ECC-Std

version, which allows testing of a two-electrode configuration without a reference electrode.

The components of the cell are shown in Figure 3. The EL-CELL consists of a cell base (used

as a positive pole, 3), a lid (used as a negative pole, 10), a non-conductive insulation sleeve

consisting of two parts (5 and 7) into which the ASC is inserted, a lower (aluminum, 6)

and an upper plunger (copper, 8), which act as spacers and transfer the compressive force

evenly to the ASC, a polyethylene sealing ring (4) between the cell base and the lid, which

also serves as insulation to prevent a short circuit, a stamp with a gold-plated compression

spring (9), which is placed in the lid, and the bracket (1), which presses the cell base and lid

together via a screw connection and at the same time exerts pressure on the ASC via the

stamp on the spring.

Energies 2022, 15, x FOR PEER REVIEW 5 of 13

2.4. EL-CELL ECC-PAT-Core

In this study, the EL-CELL ECC-PAT-Core (EL-CELL) is used in the standard ECC-

Std version, which allows testing of a two-electrode configuration without a reference

electrode. The components of the cell are shown in Figure 3. The EL-CELL consists of a

cell base (used as a positive pole, 3), a lid (used as a negative pole, 10), a non-conductive

insulation sleeve consisting of two parts (5 and 7) into which the ASC is inserted, a lower

(aluminum, 6) and an upper plunger (copper, 8), which act as spacers and transfer the

compressive force evenly to the ASC, a polyethylene sealing ring (4) between the cell base

and the lid, which also serves as insulation to prevent a short circuit, a stamp with a gold-

plated compression spring (9), which is placed in the lid, and the bracket (1), which presses

the cell base and lid together via a screw connection and at the same time exerts pressure

on the ASC via the stamp on the spring.

The contact for the reference electrode (2) is tightly screwed into the cell base but is

not connected to any reference electrode. The separator is clamped between the two com-

ponents of the insulation sleeve and is therefore significantly larger in diameter than the

electrodes inserted into the assembled sleeve. Testing different separator sizes has shown

that a diameter of 21 mm is ideal so that the separator is not too big, making it uneven and

not too small, making it not appropriately clamped and slipping out easily. The optimal

maximum diameter of the electrodes was found to be 17.5 mm and thus does not corre-

spond to the manufacturer’s specification of 18 mm. At 18 mm, the electrodes were a bit

uneven. Table 3 shows the assembly sequence used and suggested.

Figure 3. EL-CELL ECC-PAT-Core components.

Table 3. Assembly instructions for EL-CELL ECC-PAT-Core.

Step

Instruction

The insulation sleeve is disassembled into its two parts, the separator is inserted into the

larger half with tweezers and clamped by placing the smaller half on top.

In total, 50% of the electrolyte (here: 100 μL) is applied to the separator from the side of the

larger half of the insulation sleeve using a plunger-operated pipette.

With a second pair of tweezers, the cathode is inserted with its active material facing the sep-

arator.

The lower aluminum plunger is inserted into the sleeve on the cathode.

The sleeve is inserted into the cell base with the lower plunge down. It should be noted that

the sleeve is keyed and there is only one way the sleeve will fit into the cell base. If the sleeve

is only slightly twisted, it jams and cannot be pushed all the way into the cell base.

The second half of the electrolyte is applied to the other side of the separator.

With a third pair of tweezers, the anode is placed on the separator with the copper side up.

The upper plunger is inserted into the sleeve.

Figure 3. EL-CELL ECC-PAT-Core components.

The contact for the reference electrode (2) is tightly screwed into the cell base but

is not connected to any reference electrode. The separator is clamped between the two

components of the insulation sleeve and is therefore significantly larger in diameter than

the electrodes inserted into the assembled sleeve. Testing different separator sizes has

shown that a diameter of 21 mm is ideal so that the separator is not too big, making it

uneven and not too small, making it not appropriately clamped and slipping out easily.

The optimal maximum diameter of the electrodes was found to be 17.5 mm and thus does

not correspond to the manufacturer’s specification of 18 mm. At 18 mm, the electrodes

were a bit uneven. Table 3shows the assembly sequence used and suggested.

Energies 2022,15, 7333 6 of 12

Table 3. Assembly instructions for EL-CELL ECC-PAT-Core.

Step Instruction

1The insulation sleeve is disassembled into its two parts, the separator is inserted

into the larger half with tweezers and clamped by placing the smaller half on top.

2In total, 50% of the electrolyte (here: 100 µL) is applied to the separator from the

side of the larger half of the insulation sleeve using a plunger-operated pipette.

3With a second pair of tweezers, the cathode is inserted with its active material

facing the separator.

4 The lower aluminum plunger is inserted into the sleeve on the cathode.

The sleeve is inserted into the cell base with the lower plunge down. It should be

noted that the sleeve is keyed and there is only one way the sleeve will fit into the

cell base. If the sleeve is only slightly twisted, it jams and cannot be pushed all the

way into the cell base.

6 The second half of the electrolyte is applied to the other side of the separator.

With a third pair of tweezers, the anode is placed on the separator with the copper

side up.

8 The upper plunger is inserted into the sleeve.

9If not already carried out, the stamp is inserted into the compression spring, and

then the stamp and the compression spring are placed on the upper plunger.

10 The sealing ring is placed on the cell base.

The lid is placed on the stamp with the spring and pressed down slightly, allowing

the entire cell to be inserted into the bracket.

Finally, the housing is closed with the wing nut of the bracket. The screw is turned

until a clear resistance can be felt, and the screw ends exactly in the middle

turning position.

2.5. Wetting, Formation and Cyclic Aging

The assembled test cells were placed in a temperature chamber at 30

◦

C. Connected

to the battery tester, each cell format started with a resting phase of 12 h to ensure the

electrolyte reached all pores. Since this paper focuses on comparing reproducibility instead

of achieving the best possible performance, the formation was performed with only one

CCCV (CC: constant current; CV: constant voltage) charging cycle. For the CC phase, a

current of C/10 to 4.2 V was chosen, and for the CV phase, a stop current of C/50 was

chosen. After a 10 s resting phase, the cyclic aging began. With a current of C/2, the cells

were charged and discharged for 30 cycles between 2.5 and 4.2 V, as shown in Figure 4.

Energies 2022, 15, x FOR PEER REVIEW 6 of 13

If not already carried out, the stamp is inserted into the compression spring, and then the

stamp and the compression spring are placed on the upper plunger.

The sealing ring is placed on the cell base.

The lid is placed on the stamp with the spring and pressed down slightly, allowing the entire

cell to be inserted into the bracket.

Finally, the housing is closed with the wing nut of the bracket. The screw is turned until a

clear resistance can be felt, and the screw ends exactly in the middle turning position.

2.5. Wetting, Formation and Cyclic Aging

The assembled test cells were placed in a temperature chamber at 30 °C. Connected

to the battery tester, each cell format started with a resting phase of 12 h to ensure the

electrolyte reached all pores. Since this paper focuses on comparing reproducibility in-

stead of achieving the best possible performance, the formation was performed with only

one CCCV (CC: constant current; CV: constant voltage) charging cycle. For the CC phase,

a current of C/10 to 4.2 V was chosen, and for the CV phase, a stop current of C/50 was

chosen. After a 10 s resting phase, the cyclic aging began. With a current of C/2, the cells

were charged and discharged for 30 cycles between 2.5 and 4.2 V, as shown in Figure 4.

Figure 4. Voltage profile for wetting, formation and cyclic aging.

3. Results

For each cell format, a 22 full factorial experiment was performed. The factors sepa-

rator type (Freudenberg FS3002 (+); Celgard H2013 (−)) and cathode–anode ratio (N/P ra-

tio; N/P ratio = 1 (+); N/P ratio > 1 (−)) were examined.

Since hollow punch diameters are usually only available in 0.5 or 1 mm increments,

it was impossible to achieve a fixed N/P ratio for all cell formats. According to [16], an N/P

ratio of 1.15 is recommended. It was also stated that larger ratios lead to lithium losses

over a large number of cycles. However, this can be neglected here since only the first five

cycles were considered.

Because the production of all test cells is a manual process that requires fine motoric

skills and an understanding of the processes, all tests were performed by one assembler

who produced at least 50 cells in preliminary tests.

The quality features examined are the discharge capacity, the internal resistance and

the coulombic efficiency (CE). Both the discharge capacity and the internal resistance were

determined for the fifth cycle since the discharge capacities of all cells from the experi-

ments only stabilized after the fifth cycle due to the chosen formation. The CE best illus-

trates reactions with water or other foreign particles from the production process in the

first few cycles of a lithium-ion battery [18,19]. For this reason, the CE, the ratio of the

amount of charge removed to the amount of charge introduced, is particularly suitable for

Figure 4. Voltage profile for wetting, formation and cyclic aging.

3. Results

For each cell format, a 2

full factorial experiment was performed. The factors separator

type (Freudenberg FS3002 (+); Celgard H2013 (

−

)) and cathode–anode ratio (N/P ratio;

N/P ratio = 1 (+); N/P ratio > 1 (−)) were examined.

Energies 2022,15, 7333 7 of 12

Since hollow punch diameters are usually only available in 0.5 or 1 mm increments, it

was impossible to achieve a fixed N/P ratio for all cell formats. According to [

], an N/P

ratio of 1.15 is recommended. It was also stated that larger ratios lead to lithium losses over

a large number of cycles. However, this can be neglected here since only the first five cycles

were considered.

Because the production of all test cells is a manual process that requires fine motoric

skills and an understanding of the processes, all tests were performed by one assembler

who produced at least 50 cells in preliminary tests.

The quality features examined are the discharge capacity, the internal resistance and

the coulombic efficiency (CE). Both the discharge capacity and the internal resistance were

determined for the fifth cycle since the discharge capacities of all cells from the experiments

only stabilized after the fifth cycle due to the chosen formation. The CE best illustrates

reactions with water or other foreign particles from the production process in the first few

cycles of a lithium-ion battery [

]. For this reason, the CE, the ratio of the amount of

charge removed to the amount of charge introduced, is particularly suitable for assessing

the quality of the electrode coating, cell structure and electrolyte filling and distribution.

Since the charge loss is most pronounced in the first cycle of the experiment and then

quickly approaches 100%, this performance parameter was only recorded in the first cycle.

The empirical coefficient of variation

VarK

was used as a measure for evaluating the

reproducibility. It is defined by the quotient of the empirical standard deviation (

) and the

arithmetic mean of the measurements X(X) according to Formula (1).

VarK(X) = s

X(1)

Table 4shows an example of the 2

-factor plan for the small Swagelok format with all

three performance parameters.

Table 4. Measurement series for the small Swagelok cell.

Separator N/P Ratio

Cell 1 Cell 2 Cell 3 Cell 4 Cell 5

Energies 2022, 15, x FOR PEER REVIEW 7 of 13

assessing the quality of the electrode coating, cell structure and electrolyte filling and dis-

tribution. Since the charge loss is most pronounced in the first cycle of the experiment and

then quickly approaches 100%, this performance parameter was only recorded in the first

cycle.

The empirical coefficient of variation 𝑉𝑎𝑟𝐾 was used as a measure for evaluating the

reproducibility. It is defined by the quotient of the empirical standard deviation (𝑠) and

the arithmetic mean of the measurements 𝑋 (𝑋

) according to Formula (1).

𝑉𝑎𝑟𝐾󰇛𝑋󰇜=𝑠

𝑋

 (1)

Table 4 shows an example of the 22-factor plan for the small Swagelok format with

all three performance parameters.

Table 4. Measurement series for the small Swagelok cell.

Separato

N/P ratio

Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 𝑿



s Var(X) in

+Freudenberg +11.5 mm/11.5 mm (1)

−Celgard −11.5 mm/11 mm (>1)

Discharge Capacity in mAh

+ + 1.98 1.97 1.74 1.17 1.48 1.67 0.31 18.55

+ − 1.59 1.55 1.52 1.61 1.69 1.59 0.06 3.59

− + 1.30 1.38 1.58 1.48 1.69 1.49 0.14 9.35

− − 1.46 1.53 1.60 0.88 1.20 1.33 0.27 20.00

Internal Resistance in Ω

+ + 32.29 34.51 39.49 36.85 18.78 32.38 7.21 22.28

+ − 41.71 50.73 57.27 50.83 44.74 49.06 5.41 11.02

− + 45.96 92.52 65.11 51.10 42.63 59.46 18.22 30.65

− − 31.22 44.83 28.07 59.09 67.96 46.23 15.46 33.43

Coulombic Efficiency in %

+ + 67.07 63.96 62.49 51.14 55.73 60.08 5.81 9.66

+ − 61.10 60.62 58.85 59.86 63.51 60.79 1.56 2.57

− + 57.44 50.25 59.52 53.90 61.79 56.58 4.10 7.24

− − 57.70 62.92 61.79 14.73 28.15 45.06 19.82 43.99

Figure 5 shows the corresponding boxplots of all three performance parameters on

the left (Figure 5a–c) and the coefficients of variation required for the comparison on the

right (Figure 5d–f). In addition, only two instead of four boxplots are shown for the coin

cell and the EL-CELL. Since the cathode and the anode in the EL-CELL fit perfectly and

cannot slip, it was decided to change the separator as the only factor. Only one cell from

the coin cells made formation for each measurement series with the Celgard separator.

For this reason, these two series of measurements are assessed as non-functional and are

not taken into account in the further course of the evaluation.

sVar(X) in %

+Freudenberg +11.5 mm/11.5 mm (1)

−Celgard −11.5 mm/11 mm (>1)

Discharge Capacity in mAh

+ + 1.98 1.97 1.74 1.17 1.48 1.67 0.31 18.55

+−1.59 1.55 1.52 1.61 1.69 1.59 0.06 3.59

−+ 1.30 1.38 1.58 1.48 1.69 1.49 0.14 9.35

− − 1.46 1.53 1.60 0.88 1.20 1.33 0.27 20.00

Internal Resistance in Ω

+ + 32.29 34.51 39.49 36.85 18.78 32.38 7.21 22.28

+−41.71 50.73 57.27 50.83 44.74 49.06 5.41 11.02

−+ 45.96 92.52 65.11 51.10 42.63 59.46 18.22 30.65

− − 31.22 44.83 28.07 59.09 67.96 46.23 15.46 33.43

Coulombic Efficiency in %

+ + 67.07 63.96 62.49 51.14 55.73 60.08 5.81 9.66

+−61.10 60.62 58.85 59.86 63.51 60.79 1.56 2.57

−+ 57.44 50.25 59.52 53.90 61.79 56.58 4.10 7.24

− − 57.70 62.92 61.79 14.73 28.15 45.06 19.82 43.99

Figure 5shows the corresponding boxplots of all three performance parameters on

the left (Figure 5a–c) and the coefficients of variation required for the comparison on the

right (Figure 5d–f). In addition, only two instead of four boxplots are shown for the coin

cell and the EL-CELL. Since the cathode and the anode in the EL-CELL fit perfectly and

cannot slip, it was decided to change the separator as the only factor. Only one cell from

Energies 2022,15, 7333 8 of 12

the coin cells made formation for each measurement series with the Celgard separator. For

this reason, these two series of measurements are assessed as non-functional and are not

taken into account in the further course of the evaluation.

Energies 2022, 15, x FOR PEER REVIEW 8 of 13

Figure 5. (a)–(c) Boxplots and (d)–(f) coefficient of variation for performance parameters of coin cell,

Swagelok cell (big and small) and EL-CELL ECC-PAT-Core.

The apparent differences in the boxplot values are due to the different electrode sizes.

However, when considering the coefficients of variation, it becomes clear that none of the

cell formats considered has a low coefficient of variation for all three performance param-

eters. Instead, for each of the performance parameters considered, there is a cell format

that performs better than the others.

Figure 5.

(

–

) Boxplots and (

–

) coefficient of variation for performance parameters of coin cell,

Swagelok cell (big and small) and EL-CELL ECC-PAT-Core.

The apparent differences in the boxplot values are due to the different electrode sizes.

However, when considering the coefficients of variation, it becomes clear that none of

the cell formats considered has a low coefficient of variation for all three performance

parameters. Instead, for each of the performance parameters considered, there is a cell

format that performs better than the others.

3.1. Influence of Factor Combinations on the Discharge Capacity

Figure 5d shows that the coin cell with the Freudenberg separator and an N/P ratio

of one has the highest reproducibility. The next best is the small Swagelok cell with the

Celgard separator and an N/P ratio greater than one.

3.2. Influence of Factor Combinations on the Internal Resistance

Figure 5e shows that the large Swagelok cell with the Celgard separator and an N/P

ratio of one has the highest reproducibility. The next best is the small Swagelok cell with a

Freudenberg separator with an N/P ratio greater than one.

Energies 2022,15, 7333 9 of 12

3.3. Influence of Factor Combinations on the Coulombic Efficiency

Figure 5f shows that the large Swagelok cell with the Freudenberg separator and an

N/P ratio of one has the highest reproducibility. The next best is the large Swagelok cell

with the Celgard separator with an N/P ratio of one.

The average of the coefficients of variation of all evaluable measurement series for

the coin cell is 14.45%. The large Swagelok cell has an average coefficient of variation

of 14.57%, while the small Swagelok cell has a value of 17.69%. The mean coefficient of

variation for EL-CELL is 17.23%. The coin cell has the best average value. However, since

the differences are not very large, the number of cells produced is small at five per factor

combination and two measurement series of the coin cell were assessed as non-functional,

it is not meaningful to claim which design generally has the greatest reproducibility.

A separate consideration of the factors in the form of main effect and interaction

diagrams is more informative at this point.

To determine the effects of changing factor settings on systems, plots of the main

effects, as shown in Figure 6a–c, can be derived from the corresponding coefficients of

variation. When interpreting main effects plots, the steeper a line for a factor, the more

influence that factor has on the system.

Energies 2022, 15, x FOR PEER REVIEW 10 of 13

Figure 6. (a)–(c) Main effect diagrams for performance parameters; (d)–(f) interaction diagrams

between the factor separator type and N/P ratio.

3.4. Influence of Individual Factors on the Internal Resistance

Figure 6b shows that the Freudenberg separator has higher internal resistance repro-

ducibility in all cells except the EL-CELL. The separator type has the least impact on the

large Swagelok cell. Looking at the N/P ratio, it turns out that the coin cell and the large

Swagelok cell are more reproducible when the cathode diameter is equal to the anode

diameter (N/P ratio = 1). In contrast, the small Swagelok cell gives better results with a

smaller cathode diameter. The small Swagelok cell is also the least affected by the N/P

ratio.

Figure 6.

(

–

) Main effect diagrams for performance parameters; (

–

) interaction diagrams between

the factor separator type and N/P ratio.

Energies 2022,15, 7333 10 of 12

3.4. Influence of Individual Factors on the Internal Resistance

Figure 6b shows that the Freudenberg separator has higher internal resistance repro-

ducibility in all cells except the EL-CELL. The separator type has the least impact on the

large Swagelok cell. Looking at the N/P ratio, it turns out that the coin cell and the large

Swagelok cell are more reproducible when the cathode diameter is equal to the anode diam-

eter (N/P ratio = 1). In contrast, the small Swagelok cell gives better results with a smaller

cathode diameter. The small Swagelok cell is also the least affected by the N/P ratio.

3.5. Influence of Individual Factors on the Discharge Capacity

The separator from Freudenberg has better values for the reproducibility of the dis-

charge capacity (Figure 6a). The large Swagelok cell is the only test cell where the Celgard

separator performs better. For the EL-CELL, the slope of the diagram is almost zero, which

is why the choice of separator seems irrelevant here. If the N/P ratio of one is chosen, the

reproducibility of the discharge capacity of the coin and large Swagelok cells is higher.

The small Swagelok cell shows the least influence on the cathode size; here, an N/P ratio

greater than one is advantageous.

3.6. Influence of Individual Factors on the Coulombic Efficiency

In all cases, the coulombic efficiency shows higher reproducibility when the Freuden-

berg separator is used (Figure 6c). Changing the separator when using the large Swagelok

cell has the least influence on the reproducibility of the coulombic efficiency. Regarding

the N/P ratio, all cases achieve better values with a cathode of the same size as the anode.

Here, the coin cell is at least affected by the N/P ratio. However, it should be noted that

all cells except the coin cell are cleaned and reused, while the coin cell housing, the spacer

and the spring are always new. Since the degree of pollution is thus lower, the coin cell is

favored when comparing the coulombic efficiency. It should also be noted that EL-CELL

generally recommends changing the stamps for each cell structure.

Summarizing all the findings of the main effect diagrams, it is found that the separator

from Freudenberg caused a higher reproducibility in 83% of the cases than the separator

from Celgard. A similar picture emerges when choosing the cathode diameter. In 78% of

the cases, an N/P ratio of one turns out to be beneficial.

3.7. Interaction between Factors

The selected experimental design also allows the interactions of the factors examined

to be considered. They provide information about how much the reproducibility of the

respective quality feature can change due to the factors’ interaction and are presented in

Figure 6d–f. The greater the slope of the diagrams, the more the interactions of the factors

play a role in the reproducibility of the quality feature. The EL-CELL is not included in the

graphs shown because it was only examined for one factor; thus, there is no interaction

between the factors.

Figure 6d–f clearly shows that the interaction of the two factors examined has the least

influence on the reproducibility of the three quality characteristics when using the large

Swagelok cell.

4. Conclusions

This work dealt with the comparison of small-format lithium-ion batteries in terms

of the reproducibility of their performance parameters. Since preliminary tests showed

that both a tri-layer (PP/PE/PP; Celgard H2013) and a composite separator (PE/ceramic;

Freudenberg FS3002) produced sufficiently functional cells in all designs, the separator

type was chosen as a first factor in the presented main tests. In addition to the separator

material, the anode–cathode ratio (1 and ~1.15–1.3) was given as a second factor. The quality

characteristics that were examined for their reproducibility were the discharge capacity, the

internal resistance and the coulombic efficiency. A total of four 2

full factor plans were

performed with five cells per factor combination. The reproducibility was quantified by

Energies 2022,15, 7333 11 of 12

the coefficient of variation. Due to the small differences, no general statement can be made

as to which design has the highest reproducibility when considering the mean value of

the coefficients of variation of all evaluable measurement series. Depending on the series

of measurements, it turned out that the highest reproducibility of the internal resistance

was achieved with the large Swagelok cell, the factor combination of the Celgard separator

and the same size cathode as the anode. Regarding the discharge capacity, the coin cell

with a Freudenberg separator and a cathode of the same size as the anode showed the best

reproducibility. For the coulombic efficiency, the best result was the large Swagelok cell with

the Freudenberg separator and using the same size for the cathode and anode. However,

based on the main effect diagrams, it can be seen that in ten out of twelve cases (83%),

the separator from Freudenberg showed better reproducibility values than the separator

from Celgard. With regard to the cathode diameter, better results could be achieved in

seven out of nine cases (78%) with the same diameter as the anode. It is noted that the

factors considered are not critical to optimizing reproducibility. Instead, it could be shown

that all three test cell formats considered are suitable for laboratory tests. It is assumed

that parameters that have not yet been examined, such as the assembler’s dexterity or fine

motor skills, represent one of the greatest influencing factors on the reproducibility of test

cells. For this reason, the assembler should not be changed in a series of measurements, or

the assembly instructions should be as detailed as possible.

Author Contributions:

Conceptualization, P.-M.L.; methodology,

P.-M.L.

and F.B.; software,

P.-M.L.

and

F.B.; validation, F.B.; formal analysis, F.B. and P.-M.L.; investigation, F.B. and P.-M.L.; resources, P.-M.L.

and F.B.; data curation, F.B.; writing—original draft preparation, P.-M.L. and F.B.; writing—review and

editing, J.K.; visualization, P.-M.L. and F.B.; supervision, P.-M.L. and J.K.; project administration, J.K.;

funding acquisition, P.-M.L. and J.K. All authors have read and agreed to the published version of

the manuscript.

Funding:

This research was funded by Deutsche Forschungsgemeinschaft (DFG), grant number

KO4626/9-1. We acknowledge support by the German Research Foundation and the Open Access

Publication Fund of TU Berlin.

Data Availability Statement: Not applicable.

Acknowledgments:

The authors would like to thank Julian Long for incorporating all the measuring

devices used here.

Conflicts of Interest:

The authors declare no conflict of interest. The funders had no role in the design

of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or

in the decision to publish the results.

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