3.1.1. Lamination Structure Observation
The SEM images of the cross-section of the composite film (PCPF-1 and PCPF-2) are shown in Figure 1
. The five-layered structure, including three main layers and two adhesive layers, can be clearly observed. The affiliations of these layers are also added in Figure 1
clearly reveals the five-layer structure in the composite film, where the cross-section morphology of PCPF-1 and PCPF-2 is quite similar. The total thickness of the composite film is about 125 μm. The middle layer is aluminum foil (Al layer) with a thickness of about 40 μm, while on its two sides, the PP layer and PA layer with a thickness of about 40 μm and 30 μm, respectively, can be observed. Between the PP layer and Al foil, there is polyolefin adhesive (thickness 15 μm), which provides adhesion between PP/Al, while on the other hand, there is polyurethane adhesive (thickness 3–5 μm) between the PA layer and Al layer.
These different layers contribute to different properties, which together enable the composite film to meet the critical performance requirement for pouch LIBs: The PP layer provides heat-seal properties, certain resistance of electrolyte and mechanical properties, while the PA layer contributes its superior mechanical properties. The Al layer in the middle gives the laminated film superior gas and water barrier properties. In this way, the easily sealable laminated film with high barrier property and superior mechanical properties for pouched LIB application is obtained.
3.1.2. DSC Analysis
The DSC cooling curves after elimination of thermal history, as well as the subsequent melting curves of each layer are plotted as shown in Figure 2
. The obtained crystallization and melting parameters are listed in Table 1
As can be seen from Figure 2
and Table 1
, the crystallization peak temperatures Tc
of the PP layer and PA layer after elimination of the thermal history are 110.2 °C and 180.5 °C, respectively. For the polyolefin adhesive, its crystallization peak can hardly be observed, indicating that its isotacticity is very low [24
]. No crystallization peak can be observed for the PU adhesive, since it is already crosslinked and cannot crystallize.
From the aspect of melting, the melting points Tm
and degrees of crystallinity Xc
of PP and PA are 159.3 °C and 216.2 °C, 34.2% and 36.1%, respectively. The Tm
of polyolefin adhesive are only 85.4 °C and 7.8%, respectively, reflecting the very poor crystallizability of the polyolefin elastomer [31
]. Moreover, no melting peak of the PU adhesive can be found, indicating that it cannot crystallize.
3.1.3. Tensile Properties of the Laminated Films
Although the gas and water barrier properties of aluminum foil are extremely high, suffering from its poor ductility, permanent deformation and even fracture will easily take place when the aluminum foil is under even slight scraping or stretching [3
]. Therefore, it must be laminated with polymer films to enhance its ductility and to give it the heat seal property.
To deeply investigate the relationship between the multi-layer structure and final properties of the composite film and to clarify the contribution of each layer to the mechanical properties of the composite film, in this section, we carefully prepared the composite films with different constitutions, including PP/Al, PA/Al and PA/Al/PP. Their tensile behaviors are investigated.
a represents the stress-strain curves of PCPF-1 at both MD (machine direction) and TD (transverse direction), and Figure 3
b shows the stress-strain curves of samples in the MD direction, including single-layer PP, Al, PA films, and the composite films of PA-Al, PP-Al, PP-Al-PA. From Figure 3
a, the tensile strength and elongation at the breakage points of the specimens are shown in Table 2
and Table 1
reveal that for the single-layer films, PA exhibits the highest tensile strength of more than 144 MPa, and its elongation at breakage lies between 50–70%; PP has the lowest tensile strength of about 23 MPa; however, it exhibits the highest elongation at a breakage of more than 600%; Al foil has high tensile strength of about 80 MPa, but its elongation at breakage is as low as 9–11%.
After lamination, elongation at breakage of PA-Al is evidently enhanced compared with single Al foil, indicating that using PU adhesive, PA and Al foil are closely bonded, and the poor ductility of Al foil is evidently reinforced. Meanwhile, its tensile strength is also increased, indicating that the tensile stress is released from Al to PA. With respect to the PP-Al composite film, its tensile strength and elongation at breakage are very low compared with PA-Al, which might be attributed to the very low tensile strength of PP. When the tensile stress is applied, the “soft” PP layer cannot efficiently share the tensile stress in Al layer; therefore, the deformation and rupture of each layer mainly occur in their own way, and the final tensile property of PP-Al can hardly be significantly enhanced.
For the PA-Al-PP laminated film, its tensile strength and elongation at break are higher compared with PA-Al, indicating that the addition of the PP layer can further reinforce the tensile properties of the laminated film.
On the other hand, considering the tensile results in MD and TD, only slight differences in their tensile properties can be seen.
To further explore the contributions of the PP layer and PA layer to the PA-Al-PP laminated film, the morphology evolution of PP-Al, PA-Al and PA-Al-PP during the tensile process are further studied. The results are shown in Figure 4
As can be seen from Figure 4
, the deformation and rupture processes of these three types of laminated films are quite different. For PA-Al, when the tensile stress is applied, the PA layer and Al layer deform simultaneously at the beginning, and the width of the specimen decreases gradually, indicating that plastic deformation takes place. As the strain increases, the tensile stress concentrates, which cannot be efficiently dispersed, and the ruptures of the PA layer and Al layer take place simultaneously in the boundaries. The elongation at breakage and the tensile strength of the PA-Al film are 41.2% and 87.5 MPa, respectively, both higher compared with the single Al layer, which can be explained as follows: PA has higher tensile strength compared with Al, which can share a large amount of stress from Al during stretching, resulting in the coordination deformation of PA and Al with the characteristic of plastic deformation.
On the other hand, for PP-Al, when tensile stress is applied, the width of the specimen remains almost unchanged before the rupture of the Al layer. When the strain increases to more than 10%, the Al foil is broken in a brittle rupture manner, while the elongation and plastic deformation of PP layer continue.
For the PA-Al-PP composite film, it can be observed that during the tensile process, the characteristic of plastic deformation is observed, and all the layers deform simultaneously in the same manner; the decrease of width and necking can be seen as the strain increases. At higher strain, many tiny cracks can be seen from the boundary of the Al layer, as indicated by arrows in Figure 4
d. Surprisingly, with the help of the PA and PP layers, the further enlargement of these cracks are restrained, resulting in higher tensile tolerance of the laminated films. Finally, as the strain further increases, the sample is fractured, exhibiting high elongation at a breakage of 53.2% and a high tensile strength of 61.5 MPa.
Based on the experimental results above, we proposed the following mechanism describing the roles of the PA and PP layers in the final tensile behavior of the PA-Al-PP composite film, as shown in Figure 5
As is illustrated in Figure 5
, for PP-Al, since the tensile strength of the PP layer is too low compared with Al, PP cannot efficiently share tensile stress from Al and cannot reinforce the ductility of Al. During stretching, PP and Al are broken individually in their own manner.
For PA-Al, owing to the high tensile strength and plastic film nature of the PA layer, it can share a large amount of stress from the Al layer, helping the Al layer to deform in the plastic deformation manner. However, when cracks appear on the Al layer, PA can hardly restrain the further enlargement of these cracks. The Al layer and PA layer break together.
For the PA-Al-PP laminated film, it can be seen that not only the tensile strength can be efficiently shared from Al layer, but also the small cracks can be restrained with the help of the PP layer (compared with Figure 5
b). In this way, the laminated PA-Al-PP film with balanced tensile strength and elongation at break is obtained.
In summary, in the PA-Al-PP laminated film, PA with high tensile strength can release a large amount of tensile stress from the Al layer during stretching, while the PP layer can restrain the further development of the small cracks on the boundary of the Al layer, preventing the final rupture of the laminate. It should also be noted that the adhesives between the PP-Al and PA-Al layers also play a determining role in the final properties of the laminated film. Tight and even bonding between each layer is a prerequisite and the foundation for obtaining the laminated PA-Al-PP with desired tensile properties [4
3.4. Heat Seal Properties
Since the heat seal properties of the laminated film are directly related to its PP layer, the orientation status of the PP layer might play an important role in the process. Therefore, in this section, the heat seal properties of PCPF-1 and PCPF-2 are investigated, and the roles of heat seal temperature (TH) and dwell time in the heat seal strength (HSS) and failure behavior are studied.
The experiment is performed at the heat seal temperature (TH
) of 170–230 °C, dwell time of 3 s and heat seal pressure of 1 MPa. The obtained heat seal strength (HSS
) is plotted as a function of TH
as shown in Figure 8
As can be seen from Figure 8
, for both PCPF-1 and PCPF-2, the HSS
increases gradually with the increase of TH
. Meanwhile, a critical transition heat seal temperature Tcrit
can be observed: when the heat seal temperature is higher than Tcrit
, the HSS
reaches a plateau region and does not increase evidently with the further increase of TH
Comparing the results of PCPF-1 and PCPF-2, it can be seen that the HSS values in the MD and TD directions are quite similar. At the same TH, the HSS of PCPF-1 in MD direction is slightly higher than that in TD, and the corresponding Tcrit in MD and TD directions is 195 °C and 200 °C, respectively. However, for PCPF-2, the HSS in MD direction is obviously higher than its counterpart in the TD direction in all the heat seal temperature ranges studied, and its Tcrit in the MD and TD directions is 185 °C and 215 °C respectively. Moreover, the HSS values of the samples above the plateau region are also quite different. Generally, from the highest to lowest, the ranking is PCPF-2 MD > PCPF-1 MD > PCPF-1 TD > PCPF-2 TD.
The results above indicate that unlike PCPF-1, the heat seal properties of PCPF-2 in the MD and TD directions are quite different from each other, which should be related to its orientation in the PP layer.
To understand fully, the failure behavior of the heat sealed samples during tensile stretching is also determined. Generally, the failure behavior can be summarized into four failure types (Types A–D) as shown in Figure 9
Type A: When the tensile stress is applied, the heat seal frontier remains unchanged. Meanwhile, the adhesion between PP layer and Al foil is broken. With the further increase of the strain, only the plastic deformation of freestanding PP layer takes place;
Type B: The failure first takes place in the heat seal frontier, which is split into two pieces. With a further increase in the tensile stress, the separation between the PP layer and Al layer, as well as the failure of PP layer take place at the same time;
Type C: Similar to Type B, the failure first takes place in the heat sealed frontier. Interestingly, with a further increase n the tensile stress, the PP layer is split into many tiny pieces, accompanied with the separation between PP and Al;
Type D: At the beginning of the tensile stress, no evident failure is observed. Instead, evident necking in the laminate film near the heat seal frontier takes place (as indicated by arrows in Figure 9
D), and the width of the laminate film decreases gradually. Meanwhile, the width of the heat seal frontier remains unchanged. With the increase of tensile stress, the strain of the laminate film along the tensile direction increases evidently, and finally, failure takes place in the heat seal frontier or in the laminate film nearby. This type of failure usually happens when HSS
reaches 90 MPa or higher, reflecting that all the layers are tightly laminated, and the two PP layers are perfectly sealed.
As can be clearly seen from Figure 9
and Table 4
, the failure behavior of the specimen is closely related to HSS
. Generally, when HSS
≤ 70 MPa, mainly the failure mode of Type A occurs, supplemented with Type B; when 70 MPa ≤ HSS
≤ 90 MPa, the mixed failure types of Type B and Type C take place; for HSS
higher than 90 MPa, only the failure mode of Type D can be observed. In summary, it is found that the failure type is an indicator of the HSS
of the sample. In fact, the HSS
value can be far higher than the tensile strength of the PP layer, which can be even higher than the tensile strength of the laminate film, only if each layer is tightly laminated and shares the tensile stress efficiently.
values of samples heat sealed at different dwell times are measured and plotted as a function of heat seal temperature as shown in Figure 10
. To benefit comparison, the Y
-axis scales of Figure 10
a–d are the same.
suggests that for all the samples, at given TH
, the HSS
value increases with the increase in dwell time, and the critical transition heat seal temperature Tcrit
However, the increase margin of HSS and decrease rate of Tcrit of the samples are quite different from each other. For PCPF-2 MD, when the dwell time is 6 s, the HSS value will not increase significantly with a further increase in dwell time. For PCPF-1 MD, 9 s and higher dwell time has a similar effect on the HSS value; for PCPF-1 TD and PCPF-2 TD, the HSS value increases continuously with the increase in dwell time. Moreover, when the dwell time is 12 s, the HSS values of the samples can be ordered from high to low as: PCPF-2 MD > PCPF-1 MD > PCPF-1 TD > PCPF-2 TD, indicating that given sufficient dwell time and heat seal temperature, the HSS value that the samples can achieve is still related to its orientation status of the PP layer.
On the other hand, for PCPF-1 MD and PCPF-1 TD, the critical transition heat seal temperature (Tcrit) decreases gradually from around 205 °C to 190 °C with an increase in dwell time from 3 s to 12 s, while for PCPF-2 MD and TD, the Tcrit decreases from 185 °C and 210 °C to 180 °C and 205 °C, respectively, indicating that the variation of Tcrit of the specimens is also closely related to its orientation in the PP layer.
With respect to the failure behavior, an evident relationship between HSS and failure type is clearly observed again in this section, which is in accordance with the observation above.
In summary, the important heat seal properties of the laminated film are closely related to the orientation status of the PP layer. The qualitative variation of the heat seal properties with the increase of orientation degree in the MD direction is listed in Table 5
Since the PP layer is directly related to the heat seal properties of the laminate film, its orientation status is a determining factor of the heat seal properties, i.e., the HSS, the dwell time needed to obtain the desired HSS, the critical transition heat seal temperature (Tcrit), the highest HSS value on the platform and the difference of HSS between MD and TD.
A higher orientation degree of the PP layer in the MD direction leads to a higher difference in HSS between the MD and TD direction. With the increase of the orientation degree in MD, the HSS increases, the time needed to obtain the desired HSS decreases, and both the critical transition heat seal temperature (Tcrit) and the highest HSS value on the platform increase; while in TD, as the orientation degree in MD increases, the HSS decreases, the time needed to obtain the desired HSS increases, and both the critical transition heat seal temperature (Tcrit) and the highest HSS value on the platform decrease.