Slurry preparation is the first process of lithium battery production. The mixing process affects the quality of the product in the whole production process of lithium ion battery by more than 30%, which is the most important part of the whole production process. However, what is the matter? To evaluate the quality of the slurry? The test items for evaluating the quality of the slurry are briefly introduced and shared with you. Many of them have not been touched. Please correct me if you are not correct.
(1) Solid content
The solid content refers to the proportion of solid materials such as active materials, conductive agents, and binders in the whole mass of the slurry, and the solids included in the slurry also include additives such as binder dissolved in the solvent. .
Simple measurement method: Take a little slurry, mass W, apply it as a film in the container, dry the solvent at a certain temperature, and then weigh the mass w, then the solid content is N=w/W.
In addition, it can be measured by a quick moisture meter, such as AND's moisture meter, the weighing system and drying system in the equipment. After the sample is added, the sample is automatically weighed, the solvent is automatically dried and then the solid content or moisture content is automatically calculated.
Rapid measurement moisture meter
In the preparation of lithium battery slurry, the solid content is generally not controlled. Usually, according to the coating demand, the final stage of stirring is adjusted to adjust the viscosity of the slurry by adjusting the amount of solvent added. The measured solid content can be compared with the theoretical solid content of the feed, and the weighing is evaluated. Accuracy; Sampling solid content from different positions in the stirring pot can characterize the uniformity of the slurry; sampling and measuring the solid content over time can characterize the sedimentation stability of the slurry.
(2) Density
Density refers to the weight of a substance per unit volume at a specific pressure and temperature. The density of the battery slurry depends to a large extent on the density of the active substance used, and the density of the additive and solvent, as well as the components of the formulation. Volume concentration is related, generally can be measured by PCT cup.
Density cup
(3) Viscosity / rheological curve
Viscosity is the extent to which the fluid internally impedes its flow. It is defined by the formula: viscosity = shear stress / shear rate.
The shear stress τ is the force that the fluid receives on the tangent to the unit area in the shear flow. As shown in the figure, the definition is:
Where F is the shear force and A is the shear force area.
Schematic diagram of fluid shear stress
The shear rate is the velocity gradient between fluid layers. It is the characterization of the speed of fluid movement. Under the action of shear force, the fluid flows along the x-axis. The velocity distribution between the fluid layers is shown in Fig. 3. The shear rate γ is :
Fluid layer velocity distribution
The most common are Newtonian fluids (such as water, most organic solvents, etc.), characterized by: The relationship between shear stress and shear rate is linearly positively correlated, and fluid viscosity is independent of shear rate at a given temperature. Non-Newtonian fluid The viscosity is affected by the shear rate. Pseudoplastic (plastic) Fluid: Viscosity decreases with increasing shear rate (called shear thinning); Expanded fluid: Viscosity increases with increasing shear rate (called Thickened for shearing).
The lithium battery slurry is a shear-thinned non-Newtonian fluid, and the viscosity decreases as the shear rate increases. Therefore, generally, the viscosity of the slurry should define the shear rate condition. The viscosity actually affecting the coating effect is The viscosity value at the actual shear rate of the coating process. Generally, the shear rate during coating is 1000~10000 s.-1.
The rheological curve refers to the relationship between the material viscosity value and the shear rate or shear stress. The viscosity or rheological curve of the slurry is generally measured by a viscometer. The viscometer is simple, the measurement is simple, and the rotation is simple. When the rheometer is tested, because it can cover a wide range of shear rates and stresses, a more comprehensive flow curve can be obtained and the measurement results are more accurate.
Rheometer
(4) Fineness (grain size)
The lithium ion battery slurry is formed by dispersing solid particles such as electroactive materials and conductive agents in a binder solution. When coated, the active material and the conductive agent and other solid components should be uniformly dispersed in the solvent by tiny particles. There should be no granular objects in the coating film. The fineness is commonly used in the industry to detect the dispersion of the granular material in the slurry. Fineness is an important performance index of the battery slurry, and it forms the surface quality of the coating film. , uniformity and storage stability of the slurry have a great influence. The fine particles, the good dispersion of the slurry, the solid particles can be well wetted, the prepared coating is uniform, the surface is flat, no vertical Straight scratches, and the particles are less likely to precipitate, agglomerate, etc. during storage, and the storage stability is good. If large agglomerate particles are present in the slurry, on the one hand, the distribution of additives such as conductive agent is uneven, and the prepared The uniformity of the coating is not good, and the battery consistency is inevitable. In addition, during the coating process, large particles accumulate in the coating knife roll slit or the extrusion coating die discharge slit, and the prepared coating will have vertical bars. Missing .
At present, the fineness of the scraper is basically used to measure the fineness. The scraper fineness gauge is a polished flat plate with a wedge-shaped groove from zero to several micrometers deep, and the groove has a tick mark indicating the depth of the groove. When measuring, drop the sample into the deepest part of the groove, then use a two-blade polished blade to vertically contact the plate, pull the paint through the entire length of the groove at an appropriate speed, and then observe the incident angle of 30°. The depth at which the particles are uniformly exposed in the groove, and the thickness of the sample is expressed by the thickness.
Scraper fineness meter
(5) Membrane impedance
According to the principle of four-probe membrane impedance test, in the field of lithium ion batteries, this method is often used to test the impedance of the slurry membrane, and the distribution state of the conductive agent in the slurry is quantitatively analyzed by electrical resistivity, thereby judging whether the slurry dispersion effect is good or bad. The test procedure is as follows: The slurry is uniformly coated on the insulating film by an applicator, then dried by heating, the thickness of the coating is measured after drying, and the sample is cut to meet the infinity requirement (greater than four times the probe pitch). Finally, the electrode impedance of the electrode is measured by four probes, and the resistivity is calculated according to the thickness.
The four-probe film resistance test method avoids the contact resistance of the probe to the sample, and the test current direction is parallel and the coating also avoids substrate shunting. Therefore, the method can accurately measure the absolute resistance value of the battery pole piece coating. The method can only characterize the resistance of the thin layer on the surface of the coating. For thicker and component gradient polymer coatings, the pole piece resistance cannot be fully characterized. In addition, it cannot test the contact between the coating and the substrate in the real pole piece. resistance.
Membrane impedance tester
(6) Morphology and distribution: SEM/EDS/freezing electron microscope
Scanning electron microscopy can directly observe the morphology of the slurry and analyze the dispersion degree of each component with the energy spectrum. However, during the sample preparation process, the composition may be redistributed when the slurry is dried, and the cryo-electron microscope can maintain the original distribution state of the slurry. Recently, it has also been applied to the analysis of slurry properties. For example, Sanghyuk Lim et al. studied the effect of binder on the rheological properties and microstructure of lithium ion battery anode slurry by cryo-electron microscopy.
Microstructure of negative electrode slurry for lithium ion battery by cryo-electron microscopy
(7) Surface tension / contact angle
The rotary drop interface tension meter is mainly used to measure the interface energy between liquid and liquid. The rotary drop interface tension meter is mainly composed of a camera, a light source, a measuring window and a capillary tube (sample tube) (as shown in the figure), and the sample tube is in the motor. It can be rotated at different speeds. When measuring, the sample tube is filled with a high-density phase, and then a low-density phase (droplet) is injected into the high-density phase. The sample tube is rotated by the motor, in the centrifugal force. Under the action, the droplet is on the central axis of the sample tube and is stretched and deformed. During the change of the droplet, the software control system keeps track of the shape of the droplet and fits its contour. At the same time, the interfacial tension value is automatically calculated.
Interfacial tension test schematic
For example, Boris Bitsch et al. studied the effect of n-octanol as an additive on the properties of lithium battery pastes, and determined the relevant interfacial tension and contact angle (see schematic). They determined that the surface tension of CMC solution was 72.4 mN/m. It is close to water, and the surface tension of n-octanol is 27.3 mN/m, and the interfacial tension between the two is 10.6 mN/m. The contact angle of CMC solution with graphite in air is 74°, while n-octanol is in contact with graphite. The angle is 0°, the three-phase contact angle of graphite, CMC solution and n-octanol is 46°. Therefore, adding different amounts of octanol has a significant effect on the battery slurry, as shown in the figure.
Schematic diagram of wetting of living material particles and binder solution
Effect of octanol addition on slurry properties
(8) ZETA potential
The ZETA potential refers to the potential of the Shear Plane, also called the motor potential or the zeta potential (ζ-potential or ζ-potential), which is an important indicator for characterizing the stability of the colloidal dispersion. A measure of the strength of mutual exclusion or attraction.
The smaller the molecular or dispersed particles, the higher the absolute value (positive or negative) of the Zeta potential, the more stable the system is, that is, the dissolution or dispersion can resist aggregation. Conversely, the lower the Zeta potential (positive or negative), the more likely it is to coagulate or coagulate. , that is, the attraction exceeds the repulsive force, and the dispersion is destroyed and coagulation or coagulation occurs.
