.
Polymer fibrillation requires materials with unique characteristics. Currently, the most optimal selection for a binder is limited to PTFE due to its exceptional mechanical qualities, high crystallinity (achieving 97% or more post-sintering)
[77[59][60][61],
78,79], and its ability to produce fibers. PTFE exhibits flexibility and reduced resilience compared to the properties of other polymers, with moderate tensile strength and high elongation at the break
[80,81][62][63]. The material undergoes deformation under specific pressure conditions, while retaining the correct dimensions. The above behavior perfectly matches the processing requirements for the SF method of electrode stretch molding.
3.2.1. Molecular Structure
PTFE possesses the highest chemical resistance, a high dielectric constant, and a wide range of operating temperatures
[82][64]. The properties of PTFE result from its high crystallinity, high molecular weight, and unbranched structure. The radius of the F atom in PTFE is more significant than that of the H atom, and the C-F bonding energy is relatively vital (485 kJ mol
−1)
[80,81,82,83][62][63][64][65]. Therefore, the adjacent -CF
2- units in the molecular chain structure cannot present a transverse cross-orientation conformation, as does polyethylene, and instead appear to be helically arranged in the overall conformation
[84][66]. This spiral arrangement of the F atoms surrounds the carbon main chain. It covers the entire surface of the molecular chain, forming a protective layer of low surface energy around the C-C main chain. This non-polar and inert dense layer produces high intermolecular van der Waals repulsion.
3.2.2. The Principle of Polymer Fibrillation
PTFE SF binder particles show an average particle size of 500 μm, with many oblate spheroidal particles. These particles are comprised of several folded lamellar crystals (0.54 μm long, 0.25 μm wide)
[82,86][64][67]. When applying a shear load to PTFE, the oblate spheroidal particles exhibit flexibility, allowing them to elongate and form a fiber. This process is called “fibrillation” in
Figure 85a,b
[87][68].
Why can fibrillations occur using PTFE? There are two primary factors contributing to this phenomenon. One is the dislocation slip in the PTFE polymer crystals
[28][69]. The polymer’s crystal structure determines the fibrillation behavior caused by dislocations. As shown in
Figure 85c, the crystal structure of PTFE can be divided into four phases
[29,88][70][71]: pseudo-hexagonal crystal (Phase I), trilobal crystal (Phase II), planar zig-zag crystal (Phase III), and hexagonal crystal (Phase IV). Taking the hexagonal crystals of Phase IV as an example, the pre-fibrillation operation in the range of 19–30 °C transforms the crystalline phase of PTFE from Phase II to Phase IV. The PTFE’s repeating distance along the molecular axis increases from 1.65 nm in the triple-diagonal crystals to 1.95 nm in the hexagonal crystals. Additionally, the repeating helical structure in the molecular chain expands -CF
2- from 13 to 15, accompanied by a slight expansion in the structure of the helical repeating units
[88][71]. The structure of the helical repeating unit is slightly expanded in
Figure 85d
[89][72]. In this phase of PTFE, the cohesion between the neighboring chains is weak and easily dislocated. In order to avoid polymer chain breakage, dislocations can generally only slide along planes parallel to the polymer chains, most commonly observed as chain slip and lateral slip. The sliding deformation of PTFE along the chain (c-axis of the hexagonal system) has been reported to be easier to deform into nanofiber structures under a high aspect ratio, as shown in
Figure 85e
[89][72].
Figure 85. (
a) PTFE spheres are stretched into banded fibers
[87][68]. (
b) SEM image (2 μm) of PTFE fibrillation
[87][68]. (
c) Phase diagram of PTFE
[88][71]. (
d) The individual PTFE polymer chain with helical structure and its simplified cylinder model
[89][72]. (
e) The PTFE crystals with slip dislocations occurring under shear
[89][72].
3.2.3. Factors Affecting PTFE Fibrillation
Currently, there are fewer studies dedicated to discussing the influence of fibrillation properties. In the field of SF batteries research, Maxwell’s experimental data show that the impedance of the original fibrillated electrode film is related to the feed rate and the shear force
[44][73]. Related reports in other fields also assist in investigating the mechanical behavior of PTFE. Aimin Zhang et al.
[90][74] explored the fibrillation mechanism, crystallization behavior, and mechanical properties of in situ fiber PTFE-reinforced PP composites. The experimental results showed that the shear rate is the key parameter affecting the morphological evolution of PTFE, and the processing time also affects the morphology of PTFE, to a certain extent.
The influencing factor of PTFE deformation may consist of a single variable or a synergistic effect of multiple variables. As a binder in polymer fibrillation, PTFE should possess a smaller particle size and a higher molecular weight.
4. SF Process with PTFE Binder
4.1. Positive Characteristics
Figure 96a shows a comparison of the wet and dry processes, and
Figure 96b shows the SEM images of the electrodes for the wet and dry processes (polymer fibrillation). Polymer fibrillation stands out in the SF approach to batteries because of the following five advantages.
Figure 96. (
a) Scheme of the PTFE-based SF and SC processes
[101][75]; (
b) the top surface of slurry-based LNMO electrodes and SF coated eletrodes
[101][75].
- I.
-
It is environmentally friendly and suitable for large-scale production.
-
NMP solvent is toxic, unfriendly to the environment, and needs to be recycled using the traditional wet process. The SF process does not require solvent in the electrode coating process to reduce baking and solvent recovery, the process is simpler, the equipment covers a smaller area, and the method is more suitable for the large-scale production of electrodes.
- II.
-
It exhibits a flatter electrode shape than that from the wet process.
-
Because the wet method requires solvent, after the solvent evaporation, the active substance and conductive agent will leave more spaces between the gaps, leading to the low compaction density of the material. The SF method does not exist in the drying process, so there is no solvent evaporation left after the gap, and the contact between the particles is closer.
- III.
-
It offers greater compaction density.
-
After compaction under dry conditions, there are fewer cracks, micropores, and other problems. The compacted density of lithium iron phosphate and SF battery energy density may be improved. According to Maxwell’s experimental data
[44][73], the energy density of the SF electrode can be more than 300 Wh/kg, and has the possibility to realize 500 Wh/kg.
- IV.
-
It improves the performance of the battery
-
In the wet process, after the battery has gone through many cycles, the stresses within the active particles continue to accumulate, leading to cracks in the profile, which ultimately reduces the performance of the battery. In the SF process, the fiber network is wrapped around the surface of the active material, and the mesh structure remains intact after many cycles of charging and discharging.
- V.
-
It allows for the possibility of prepare solid-state batteries.
-
Empowered by SF technology, the manufacturing process for creating solid-state battery electrodes can be completely dried, eliminating the problem of solvent molecules remaining after drying in the wet process. In addition, the use of the original fibrillation manufacturing solid electrolyte film can reduce manufacturing costs so that solid-state batteries can also be more productive.
4.2. Development Status of SF Process with PTFE Binder
4.2.1. Effect of PTFE on SF Batteries
The properties of PTFE determine the performance of the solvent-free batteries and influence the SF process at the fundamental level. Relevant studies have been devoted to exploring whether changing the PTFE has an impact on the battery system, and the standard variables include the side reactions of positive and negative electrodes, ultra-low content (0.1–0.5 wt%), crystallinity, modified molecules of PTFE, substitutes for PTFE, and synergistic polymer binder.
- I.
-
Side reactions of PTFE binders
-
The reaction between PTFE and Li
+ on the surface of the negative electrode will preferentially react to generate lithium fluoride, weakening the bonding effect and even destroying the PTFE electrode fiber network, leading to a rapid decline in electrode performance. This phenomenon was reported by Wu et al. in 2019
[102][76]. The chemical reaction between PTFE and Li
+ can cause a low initial reversible capacity (<70%) of the anode of the LiNi
0.6Mn
0.2Co
0.2O
2 (NMC622)/graphite full battery. Chen’s group raised This problem as early as 1996
[103][77].
Does PTFE reveal a similar problem in the cathode? In 2023, Tao et al.
[105][78] compared the changes in the batteries’ cathode electrolyte interface layer (CEI) using electrolytes containing LiPF
6 or LiClO
4. Using LiClO
4 can eliminate other possible F sources, thereby probing the decomposition of PTFE.
- II.
-
Crystallinity
-
In addition to the PTFE content, the crystallinity also affects the SF procedure. To investigate the effect of PTFE crystallinity on all-solid-state batteries (ASSBs), a class of sulfide-based Li
6PS
5Cl -ASSBs was constructed by Dongsoo Lee
[86][67] in 2023. The PTFE formulations used in this experiment were non-crystalline (18.8%), semi-crystalline (41.3%), and highly crystalline (88.1%), respectively. The high crystallinity PTFE exhibited more substantial mechanical properties and closer contact with the active substance particles. Because it promotes uniform charge transfer in the battery, this PTFE can significantly improve the performance of ASSBs.
- III.
-
Modified materials
-
Battery-grade PTFE exhibits the limitation of being difficult to store. Previous studies have attempted to modify or replace PTFE. In 2022, Hong et al.
[108][79] developed a modified PFTE material: poly (tetrafluoroethylene-co-perfluoro (3-oxo-4-pentanesulfonic acid)) lithium. The cross-sectional SEM images of the cathode without adhesive, containing PTFE and the ionic polymer, are shown in
Figure 117.
Figure 117. Cross-sectional SEM images of cathodes prepared (
a) without binder, (
b) with PTFE, and (
c) with the ionic polymer. Cross-sectional SEM images of cycled (300 cycles) cathodes prepared (
d,
g) without binder, (
e,
h) with PTFE, and (
f,
i) with the ionic polymer
[108][79].
4.2.2. Influence of Components Other Than Binders
- I.
-
Conductive additives
-
For the variable of conductive additives, in 2023, Yang et al.
[110][80] prepared a full battery, with different carbon active materials (graphite, stiff carbon, and soft carbon) as the negative electrode, and LiNi
0.5Co
0.2Mn
0.3O
2 as the positive electrode. As shown in
Figure 128a, the hard and soft carbon negative electrodes exhibited better cycling stability than that of graphite due to their small volume expansion during charge storage, and this work successfully expanded the application scope of the PTFE-based SF process. Their group also investigated similar work, as shown in
Figure 128b
[111][81].
Figure 128. (
a) Cross-sectional SEM images of a graphite, hard carbon, and soft carbon SF anode
[108][79]. (
b) SF-LFP electrode procedure
[111][81].
- II.
-
Electrode materials
-
The electrode material will not only have an effect on the SF electrode method alone, but it will also affect the overall capacity of the SF batteries. For SF electrode technology, measures such as developing electrode materials with higher specific capacity, increasing the proportion of active substances, and selecting active substances in the appropriate voltage range could enhance the overall performance of the SF batteries. Based on the method using the PTFE solvent-free electrode to adjust for the variation of other components, it will be expected to obtain higher performance batteries. This means that there is an excellent opportunity to prepare high-capacity batteries from the perspective of the PTFE solvent-free process.
- III.
-
Collectors
-
Although there are previous works reporting on collector-free LIBs, most of them focus on the design of an integrated battery structure concept. Currently, the SF batteries usually obtained using the polymer fibrillation method use temperatures of 180 °C and above [] to hot press and hold electrode self-supporting film for a period of time, while employing collectors such as aluminum foil, copper foil, carbon coated aluminum foil, etc.
4.2.3. Innovative Technology and System
The exploration of battery processes and systems is also essential. Nevertheless, several efforts have also focused on novel SF battery systems, such as those involving high-voltage, solid-state, and high-load battery systems.
- I.
-
High-speed airflow technology
-
In 2020, Zhou et al.
[114][82] used a high-speed airflow impact to defibrillate PTFE, along with the use of conductive additives and the inclusion of active materials. Subsequently, they successfully prepared SF-LFP electrodes by combining hot rolling and hot covering processes.
- II.
-
Lithium-sulfur (Li-S) batteries
-
In 2023, Magdalena et al.
[115][83] achieved the preparation of SF all-solid-state lithium-sulfur batteries with high sulfur utilization (ASSB-LiS) by using a high-energy ball milling method. The group pioneered the monitoring of the thickness of ASSB-LiS electrode sheets prepared using the SF process. This approach enabled a deeper understanding of the charging and discharging behavior of ASSB-LiS.
- III.
-
High-voltage batteries
-
High-voltage batteries are crucial for the development of the sustainable LIBs market. Moreover, high-voltage cobalt-free batteries with high energy density and low cost-effectiveness offer new possibilities for the battery industry. In 2023, Yao et al.
[101][75] developed a 5V-grade cobalt-free battery based on a PTFE-based SF process, which enabled the successful preparation of a highly loaded spinel-type oxide LiNi
0.5Mn
1.5O
4 (LNMO) electrode. The wet electrode surface loading performance starts to decrease at 4.0 mAh cm
−2. However, the battery electrochemical performance continues to remain stable up to 9.5 mAh cm
−2 (240 μm thick) of the SF electrode.
- IV.
-
High-load batteries
-
In 2023, Tao et al.
[116][84] constructed a whole battery consisting of a highly loaded graphite (6.6 mAh cm
−2) as the negative electrode and LiNi
0.6Mn
0.2Co
0.2O
2 (6.0 mAh cm
−2) as the positive electrode by using PTFE as the binder using the SF process in order to compare it with the wet approach. The SF procedure exhibits significant advantages over the SC process in regards to multiplication performance and capacity retention for the entire battery.
- V.
-
Solid-state batteries
-
The development of the SF-solid-state battery system should not be underestimated. Dong et al.
[30][85] investigated the factors affecting the performance of SF solid-state batteries by exploring the content of the cathode active material and the solid electrolytes. The data from the batteries show that the most critical design criterion for SF solid-state batteries is the need for a reasonable and balanced conduction path in the SF composite electrode.