Summary:
The dry processing of lithium-ion battery electrodes is easy to achieve powder-to-film and is therefore considered a very promising strategy for lithium-ion battery manufacturing. However, a basic understanding of the effects of the dry mixing involved is still poorly reported. In this paper, the dry mixing degree was monitored by the dry mixing time, and a set of dry process electrodes with different dry mixing degrees were prepared and comprehensively studied. This work novelly reveals that the degree of dry mixing exhibits a significant effect on the morphology, uniformity and PTFE fibrosis of the electrode components, resulting in differences in the mechanical strength and electrochemical properties of the dry electrode. Therefore, it is recommended that for high-performance dry treatment of lithium-ion battery electrodes, moderate dry mixing is preferred.
Preparation of dry electrodes (DPE).
Original LiNi0.6Mn0.2Co0.2O2 (NMC, Targray) powder, conductive carbon black (Denka) and PTFE (601X) powders with a mass ratio of 92:3:5 mixed by ball mill (Retsch Ball Mill MM400) for different durations. The resulting powder mixture is then calendered into a freestanding DPE film, which is then laminated onto a current collector.
Prepared samples are labeled DPE-10, DPE-30, and DPE-60 according to the dry mix (DM) duration of 10, 30, and 60 min. The area loading and thickness of the three prepared DPEs were controlled at about 7.2 mAh/cm and about 153 μm.
Figure 1 Schematic diagram of dry electrode synthesis
Results and discussion
In terms of powder morphology, the material in the DPE-10 powder mixture was unevenly distributed, the PTFE was not completely fibrosized, and residual lumpy PTFE was observed. NMC secondary particles are mostly intact at the micron scale. When the DM time is increased to 30 min (Figure 1c), the DPE30 powder takes on a flaky form and is well mixed with NMC pellets, in addition PTFE binders are fibrosized, and few NMC secondary particles are broken into primary pellets. PTFE-carbonblack networks appear to be denser than those in DPE-10. When the DM time is further increased to 60 min, PTFE appears to become a large network between NMC particles or on the coating. The NMC particle surface became smoother, and no significant PTFE fibers were observed. In addition, the delayed mixing time will not only cause more NMC secondary particles to break into primary particles, but also reduce the PTFE fiber formed. These phenomena of DPE-60 powder mixtures can be attributed to the grinding of the DM process and the heat generated. It is confirmed that DM time directly affects the fibrosis of PTFE binders, the morphology of mixed powders and the distribution of electrode components.
Figure 2 SEM image of powder mixture at low and high magnification
(a and b) SEM image of DPE-10 powder mixture at low and high magnification
(c and d) SEM image of DPE-30 powder mixture at low and high magnification
(e and f) SEM image of DPE-60 powder mixture at low and high magnification
After calendering the powder into electrodes, the three DPEs (Figures 2a, 2b and 2c) exhibited similar morphology, with some carbon-rich regions and some carbon-poor regions between NMC secondary particles, especially between primary NMC particles, caused by the fracture of NMC secondary particles. Carbon and NMC species separation signals SEM EDX element mapping image) further confirms this observation. At higher magnifications (Figures 2d, 2e, and 2f), clear PTFE fibers are not visible on the DPE surface. The morphology of the t-cross-section (Figures 2G, 2H, and 2I) differs significantly from the surface. PTFE fibers were observed in cross-sectional images, especially the DPE-30 electrode (Figure 2h), while no obvious PTFE fibers were found on the surface. This may be due to the higher surface shear forces that break down the NMC secondary particles into primary particles and embed PTFE fibers between the NMC particles. In stark contrast, the cross-sectional SEM images in Figures 2g, h, and 2i are morphologically similar to the mixed powders in Figures 1b, 1d, and 1f, showing PTFE fibers and more complete NMC secondary particles. A similar phenomenon can be seen in Figure S1, where fine NMC primary particles form a dense surface layer. Of the three DM cycles, 30 minutes of DM appears to be more appropriate and produces more PTFE fibers than the other two conditions.
Figure 3 DPE-10/30/60 low/high magnification top-down SEM image and cross-sectional SEM diagram
(a\u2012 c) are DPE-10/30/60 low-magnification top-down SEM images, respectively
(d \u2012 e) are DPE-10/30/60 high-magnification top-down SEM images, respectively
(g\u2012 i) Cross-sectional SEM images of DPE-10/30/60, respectively
In terms of adhesion strength, DM time has an effect on adhesion strength, DPE-10, DPE-30 and DPE-60 strength are 5, 8, 4 N/m, respectively, and DPE-30 with DM time of 30 minutes has the highest adhesion strength, because more PTFE fibers are generated, which increases the contact area between the electrode and the current collector. In short, a short DM time (i.e. 10 minutes) is not enough to completely fibrosify the PTFE, while a prolonged DM time (i.e. 60 minutes) destroys the PTFE fiber. Both conditions result in a decrease in mechanical strength.
Fig. 4 Mechanical adhesion strength of DPE at different DM times
In terms of mechanical strength, DPE-10 showed the worst mechanical strength of the electrodes tested and failed the preloading phase of the tensile test. It can be seen from the adhesion test that poor mechanical properties may be due to insufficient DM, resulting in uneven material distribution and insufficient PTFE fibrisation. With the increase of blending time, and the increase of PTFE fibrilation, the ultimate tensile strength of the independent electrode of DPE-60 increased to 0.43MPa, and the maximum strain experience of the independent electrode was slightly improved. Unlike the trend seen in adhesion testing, increased mixing can improve the cohesion of the self-standing electrode, thereby improving the ultimate tensile strength and ductility of the electrode. However, this increase in mechanical strength does seem to come at the expense of adhesion, which makes DPE-30 an overall better electrode because it has comparable strength but slightly less ductility.
It should be noted that not only DM methods and formulations can lead to large changes in the mechanical properties of individual electrodes, especially the tensile strength. To demonstrate this, Navitas Systems tensile tested 92wt% and 96wt% LiNi0.8Mn0.1Co0.1O2 cathodes using materials mixed in a twin screw extruder (TSE). Results TSE-based electrodes can exhibit higher ultimate tensile strength, even though the 96wt% LiNi0.8Mn0.1Co0.1O2 cathode has a lower PTFE content (5% PTFE for 92% cathode, 2% PTFE for 96% cathode). Therefore, tensile testing concludes that the increase in fibrisis may be caused by DM time or method, thereby improving the mechanical properties and stability of DPE.
In terms of EIS, DPE-10, DPE-30 and DPE-60 have low and comparable ohmic resistances of 2.7, 2.4 and 2.9Ω, respectively. In the mid-high frequency region, the values are 228.6, 183.1, and 240.1Ω, respectively. The larger values of DPE-10 and DPE-60 may be related to the distribution of the PTFE-carbon black network, which is more uniform in DPE-30.
Figure 5 EIS test results for different DPEs
In terms of rate performance, all three electrodes exhibited excellent initial coulomb efficiencies of 91.6%, 92.8% and 91.4% for DPE-10, DPE-30, and DPE-60, respectively. They also have an excellent reversible discharge capacity of about 180mAh/g, which indicates that the obtained DPE is electrochemically stable and the NMC utilization rate is high at low magnification. In short, at low current densities, DM did not show a significant effect on the prepared DPE.
Figure 6 (a\u2012 c) shows the charge/discharge curves of DPE-10, DPE-30, and DPE-60 at small current densities of C/10, respectively
In order to further investigate the rate performance of the prepared DPE, charge/discharge tests at different current densities were performed. While DPEs over three DM cycles exhibited comparable discharge capacity at low discharge magnifications (≤C/5), DPE-30s showed the highest discharge capacity at medium magnifications (C/3-C/2). Due to the limitation of mass transport in high-load electrodes, discharge capacities are similar at higher magnifications (7.2 mAh/cm2). The average discharge capacity of DPE-30 at various magnifications (C/5, C/3, C/2, 1C, 2C and 3C) was 171.7, 150.8, 88.6, 12.8, 4.3 and 2.8 mAh/g. The reversible capacity of DPE-30 is higher at 138.2mAh/g (C/3). The performance performance of DPE-30> DPE-10 > DPE-60 is consistent with the test results of EIS.
Figure 7 Yield test results of different DPEs
The discharge curves at the low current magnification of C/10 are highly overlapping, while the significant voltage drop can be observed at the high current magnification of C/2. For example, the voltage variation of DPE-10, DPE-30, and DPE-60 at a normalized discharge capacity of 0.9 is 0.347, 0.322, and 0.380 V, respectively, indicating that the polarization of DPE-30 is the smallest of them. This is also consistent with the EIS data and magnification performance discussed above, indicating that DPE-30 has the fastest electrochemical kinetics. Higher polarization not only reduces discharge capacity, but also limits efficiency.
Figure 8 Normalized discharge curves for DPE-10, DPE-30, and DPE-60 at current density
In view of the above results and analysis, the effect of DM time on DPE is mainly attributed to PTFE fibrosis, distribution of electrode component materials, and related effects on mechanical and electrochemical properties. Short DM time does not completely fibrosify the PTFE adhesive, while DM time may break the PTFE fibers due to overheating and turn them into PTFE-carbon film. Both reduce the interface area between PTFE, carbon black and NMC particles and weaken the mechanical strength of DPE, including cohesion and adhesion strength. As a result, the charge transfer resistance increases and the rate performance decreases. It is worth noting that the optimal DM time depends on the DM technique and conditions that determine the mixing of energy and heat generation. It may vary when using different DM equipment and conditions.
conclusion
For the first time, the effect of DM degree on the morphology, mechanical and electrochemical properties of DPE was studied. DM time or degree of DM is critical for PTFE fibrosis. Insufficient DM results in low PTFE fibrosis, and too much DM time causes PTFE fibers to break and form PTFE-carbon black films. Both can weaken the mechanical properties of the electrode and impair the rate performance. Conversely, proper DM timing can improve the fibrosis, mechanical properties, and electrochemical properties of PTFE. It is recommended that in order to obtain a good high load DPE, a moderate degree of DM is advantageous.
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文章名：Unraveling the impact of the degree of dry mixing on dry-processed lithium-ion battery electrodes
Publication Date:2023-07-01
DOI:https://doi.org/10.1016/j.jpowsour.2023.233379