The work of Lee et al. studied other active material components, and the authors studied the effect of the slurry preparation process sequence on the performance of LiCoO2/conductive agent/PVDF electrode (NMP was used as the slurry solvent). Two different preparation process sequences were compared: the first method loosened dry premixed active material/conductive agent powder in PVDF/NMP solution, while the second method loosened the same active material/conductive agent mixture in into a thicker PVDF/NMP solution (2/5 of the total volume of PVDF+NMP), and then dilute the resulting slurry with the remaining NMP (in three steps, each step takes 1/5 of the total volume of NMP). The second method achieves better functionality. The authors believe that by first loosening the active material/conductive agent mixture in a thicker PVDF/NMP solution, a more uniform conductive agent distribution is achieved, which increases the conductivity of the electrode. In the case of preparing other oxide active materials, Li2.2V3O8/CB/PMMA [ethyl acetate + ethylene carbonate] slurry, it has a similar effect (assuming that the slurry is prepared starting from a thicker solvent, that is, assuming If a binder is added from the beginning, it will have better electrode performance).

　　Therefore, from the above it can be concluded that changes in the order of the preparation process will have different effects depending on the properties of the components:

　　?In the case of graphite/conductive agent/PVDF slurry, if a thinner solvent is used first for loosening, a better adhesive distribution structure will be formed.

　　?In the case of [Li1.2V3O8 or LiCoO2]/conductive agent/[PVDF or PMMA] slurry, assuming the starting solvent is thicker, a better adhesive distribution will result.

　　There are two explanations for the improvement in electrode performance due to different mapping changes: first, it is assumed that an increase in electrode conductivity (due to better active material/conducting agent mixing) is the reason for the improvement in performance when the current density is higher , its effect is the most obvious, as shown in Figure 9a. Second, the mapping changes are related to improvements in the mechanical properties of the electrodes, without significant changes in electrode conductivity. The two types of electrode performance changes caused by changes in the order of the preparation process depend on the active material-conducting agent-binder-solvent interaction in the slurry, which is not only related to the properties of the active material, conductive agent and binder itself, but also Can depend on the specific active material/conductive agent/binder combination.

　　Now, pre-dry mix the slurry components (active material/conductive agent, active material/binder, and active material/conductive agent/binder), and then loosen these mixed powders into the solvent (or adhesive) mixture solution), this has become a trend. Many studies report that the dry mixing process, followed by the addition of solvent (or binder solution) and the pre-dispersion of the mixed powder often have a positive impact on the final performance of the electrode.

　　There are two ways to improve electrode performance by pre-dry mixing:

　　(1) Dry mixing of AM (active material)/CB (conductive agent)/binder mixed powder. The outstanding feature of this method is that the affinity of PVDF powder to CB is significantly higher than the affinity of PVDF to AM, so in fact, a CB/PVDF mixed phase is first formed, and then the AM particles are aggregated loosely and separated by the conductive adhesive mixed phase. (See Figure 12a). During further powder loosening, the slurry maintained the structure shown in Figure 12a, that is, the loose AM particles were separated by the CB/binder mixture, and this shape remained after drying. This allows the electrodes to have higher conductivity and better other functions. The dry powder of the AM/CB/binder mixture is mixed loosely, and the prepared electrode performs better than the high-energy loose electrode (see Figure 12b). Because the energy is too high, the fine distribution structure of the dry mixture will be damaged.

　　(2) The second method is dry mixing of AM/CB (polymer-free binder) and then loosening the resulting mixture into the binder solution. The function of electrodes prepared in this way is closely related to the mixing of AM and CB, which add electronic conduction pathways to the AM particles. However, AM/CB interactions may have complex characteristics. It is reported that during the high-intensity mixing process of AM/CB, AM particles are covered by a thin carbon layer, similar to a carbon coating, consisting of electroless carbon plating. An example is shown in Figure 13. Generally, this plating basically improves the cathode performance (most cathode AM has low conductivity). However, in the case where the amount of CB is insufficient, the conductivity of the electrode may also be low due to the AM/[CB layer]/[adhesive layer] layered structure shown in Figure 13 .

　　The improvement in electrode performance after dry mixing is related to the increase in AM electronic pathways, which is caused by a more uniform mixing of AM/CA, with smaller CA particles evenly distributed on the surface of larger AM particles. This improvement in electronic pathways is related to the mixed-sequence approach, but the connection is extremely messy. Moreover, low-energy mixing is less effective (for example, powerful ball mill mixers are reported to be more effective than manual grinding and mixing with a bowl and pestle), and super-powerful mixing often has negative effects due to the presence of CB aggregates in AM/CB raw powders. During the slurry preparation process, "over-blending" of AM/CB powder resulted in AM agglomeration (see Figure 14).

　　It is worth noting that the increase in conductivity of AM/CB powder does not guarantee an improvement in the final electrode performance. The AM/CA powder mixture prepared by a high-energy powder mixer (Nobilta) has better conductivity than the low-energy rotating drum mixer, but the conductivity of the electrode prepared by the AM/CB powder mixed with Nobita is significantly lower than that of the rotating drum. Electrodes prepared from AM/CB powder processed in a blender (Fig. 15). In addition, dry powder pretreatment can help achieve better CA distribution and electrode performance only when the selected mixing procedure is suitable for the specific AM/CB properties. Otherwise, pre-dry mixing may damage the final electrode performance.

　　7. Characteristics of slurries containing nanocarbon, graphite and CNT

　　In recent years, the use of graphene-based and carbon nanotube (CNT) materials has been increasing. Such materials are commonly used as conductive additives, negative active materials, and as positive electrode substrates for lithium-air batteries. This requires dealing with the problem of slurries containing nanocarbon materials (CCM) and developing suitable loosening technologies.

　　7.1. CNT

　　CNT is mainly used as a conductive additive. The greater the aspect ratio of the conductive agent particles, the smaller the volume fraction of the conductive additive required to maintain the conductivity of the composite material composed of an insulating matrix and conductive particles. Therefore, CNT and carbon nanofiber (CNF) conductive agents are very suitable electrode components, because the smaller the volume fraction of the conductive agent, the greater the volume fraction of the active material, and the higher the energy density of the electrode. Inspired by this, many researchers are committed to using these high aspect ratio conductive agents in electrode formulations. CNT and/or CNF-based materials are successfully used as conductive additives with various positive and negative electrode materials (LiFePO4, LiCoO2, LiNi0.7Co0.3O2, CFx, LiMn0.8Fe0.2PO, TiO2, Li2O4, TiO2, SnO2, Ti4Ti5O12, Si), and CNT/CNF-based conductive agents have advantages over common low aspect ratio conductive agents. The loose mass of CNT-based materials strongly affects the conductivity of electrodes, and preparing slurries containing high aspect ratio nanoconductive agents is challenging because these conductive agents are prone to bunching. The most common NMP/PVDF slurry solvent is beneficial to the loosening and bundling of CNTs, but water-based slurries require special methods.

　　First, due to strong van der Waals interactions, the surrounding surfaces of CNTs easily adhere to each other. Secondly, during the fluid flow shear mixing process, in addition to the attraction between particles, internal friction in a single fiber can also cause CNT aggregation. Therefore, the stirring and mixing method has a huge impact on the final quality of CNT-containing slurry. Ultrasonic loosening is considered a better method and is often used for CNT loosening. However, during extension sonication, CNTs may crack, so the optimal mixing time and power requirements are optimized based on the results. Alternatively, it may be advantageous to use a particularly compact approach, for example, a combination of high-energy and low-energy ultrasound.

　　CNT bundling will reduce the slurry performance, while the parallel orientation of CNTs is beneficial to conductivity. Therefore, the slurry mixing process needs to combine the CNT unbundling process with the overall parallel orientation process of CNTs after bundling. For example, a mixing process consisting of high-energy shear mixing followed by low-energy shear mixing can produce CNT-epoxy resin composites with better conductivity than composites prepared by extending the high-energy mixing process alone. . The preparation of slurries containing CNTs can also be assisted by surfactants, especially water-based slurries. Although surfactants have similar effects on CNT loosening. Compared with the loosening of common carbon conductive materials, the most obvious difference in the loosening process of CNT is that it requires unbundling. For this reason, surfactants with long hydrophilic portions are more favorable for CNTs to repel each other (repulsive forces act over longer intervals and are also more useful). In contrast, surfactants with too long hydrophobic portions are not good because they interact with two CNT particles at the same time, causing the CNTs to attract each other. Many common surfactants are beneficial to CNT unbundling. In short, the selection of CNT loosening surfactants requires special attention. Generally, the most suitable surfactants contain relatively short, flat and rigid chains with distinct hydrophilic and hydrophobic end groups.

　　Another way to add looseness to CNTs is CNT surface modification, which involves covalent attachment of different groups and/or molecules to the surrounding surfaces of the CNT and/or termination; recovery treatment, which treats the CNT to be negatively charged (i.e., convert it into a "nano" Tube"). Such nanotubes are surrounded by cations, similar to polymer electrolytes. These methods make CNT/CNF highly loose. However, CNT modification may prevent Li+ and electron transfer in the final electrode.

　　7.2. Graphene

　　Graphene is a two-dimensional carbon material that is used as an active material for the negative electrode of lithium-ion batteries and as a conductive additive for the positive electrode. Negative electrodes are generally made from graphene and a binder alone, or graphene, a binder and a 3D nanoscale carbon additive. The reason for mixing with 3D carbon is that graphene is a planar problem with a relatively large scale, which blocks Li+ ion migration to a certain extent. This spatial effect can be dealt with by introducing 3D nanoscale carbon black and 1DCNT as graphene sheets. The filling phase between them provides a loose pathway for Li+.

　　Another type of graphene-based negative electrode is a mixture of graphene and other negative electrode materials. First, graphene is often used as a substrate for the preparation of other active material/graphene composites. In this case, the tight bond between the active material and the graphene is established before the slurry is prepared. Second, graphene can also be used in the same way as general conductive agents, that is, as a slurry conductive agent component.

　　In the cathode, most workshops focused on arranging graphene shapes during AM/graphene composite composition prior to slurry preparation. When graphene is added as a positive conductive agent in slurry preparation, the graphene sheets may be re-deposited, which may damage the electrode performance. Similar to CNTs, graphene can also be loosened by ultrasonic waves into commonly used NMP/PVDF solvents or mixed by high-intensity shear hydrodynamics. Dispersing graphene and/or graphene-based materials in water-based slurries is also a challenging task, typically using surfactants and/or surface modification of graphene.

　　8. Contact between slurry characteristics and industrial preparation technology

　　In industrial production, the electrode is prepared by coating the current collector with a wet slurry of a predetermined thickness and then drying it. A die-kneading high-speed coating machine is the preferred equipment. As shown in Figure 16, the prepared electrode should have a uniform thickness without coating defects, and the coating process should be highly productive (i.e., the coating speed should be high). For this purpose, the hydrodynamic parameters of the lithium-ion battery electrode slurry (generally a non-Newtonian liquid) should be satisfactory to obtain a uniform and defect-free coating on the substrate foil.

　　First, the slurry coating should be cast evenly, minimizing wet coating thickness fluctuations (such thickness changes are inevitable with die knead coating), and the wet slurry leveling should be fast enough to match the coating speed , low viscosity is conducive to rapid leveling. Second, as shown in Figure 13a, the coating method should be stable, which requires the capillary number to be within a stable area below the Boderline line as shown in Figure 16a, that is, the coating window. (Capillary number, Ca=(μV)/σ, is a function of slurry viscosity μ, slurry surface tension σ and substrate speed V).

　　Coating production requires suitable slurry viscosity. However, slurry viscosity control should not compromise the ultimate electrode performance. For viscosity adjustment, the method of adjusting the solid content of the slurry is often used. The electrode performance will also be affected by the solid content of the electrode slurry. If the solid content is too low, AM/CA will easily separate during the drying process.

　　Another method of adjusting slurry viscosity is to use surfactants. However, this method should also be used with caution. On the one hand, there is an optimal concentration of surfactant, which is difficult to master. On the other hand, surfactant remaining on the electrode may damage the electrode performance.

　　9. Conclusion, summary and outlook

　　This article provides an overview and review of the current technology in AM/CA/adhesive slurry preparation and its possible future developments. Many examples of slurry preparation techniques are cited, with advantages and disadvantages related to the ultimate lithium-ion battery electrode function. This article explores the capabilities and potential of various stirring and mixing techniques and highlights that differences in electrode shape and function also depend on the preliminary slurry properties. In addition to the influence of the stirring and loosening process on the shape of the electrode (i.e. AM/CA/binder distribution and electrode porosity), some special loosening processes can also change the structure of the electrode components (AM, CA and binder), changing Interaction of adhesives and AM/CA surfaces, especially ball milling and ultrasonic slurry preparation methods.

　　The electrode slurry preparation technology should be selected appropriately to ensure the uniformity of the slurry and the

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