Analysis of lithium-ion battery production key factors

The production of lithium-ion batteries is coodinated by many processing sequences. Overall, the production of lithium-ion battery includes pole piece manufacturing process, battery assembly process and the final process of liquid injection, prefilling, forming and aging. In these three stages of processes, each working procedure can be divided into several key processes. Each step has made great influence on the performance of the battery.

Phase, the plate manufacturing process can be subdivided into slurry preparation, slurry coating, plate rolling, plate cutting, extremely dry process. The battery assembly process, and according to different cell types, roughly divided into winding, into the shell, the welding process, etc. At the end of the liquid phase and including injection liquid, exhaust, sealing, prefilled, into each process such as, aging. Pole piece manufacturing process is the whole lithium electricity. The core content of pool manufacturing, relationship between the electrochemical performance of battery, and the pulp quality is particularly important.

Lithium-ion battery production flow chart

First, the basic theory of slurry

Lithium ion battery electrode paste is a kind of fluid, fluid usually can be divided into Newtonian fluids and non-Newtonian fluid. Can be divided into the expansion of non-Newtonian fluid plastic fluid, time-dependent non-Newtonian fluid, pseudoplastic fluid and Bingham plastic fluid to wait for a few kinds.  After Newton fluid refers to the mechanical deformation extremely easily, and the shear stress and deformation rate is proportional to the low viscous fluid. Any point of the shear stress with shear deformation rate is linear function relation of fluid. Many of the fluid is Newtonian fluid in the nature. The most pure liquid such as water, alcohol, light oil, low molecular compound solution and low speed flow of gas are the Newtonian fluid.

Non-Newtonian fluid, it is to point to not meet the fluid of Newton's law of viscosity experiment, namely the linear relationship between shear stress and shear strain rate is not fluid. Non-Newtonian fluid is widely existing in life, the production and the nature. Polymer, such as concentrated solution and suspension for non-Newtonian fluid.  The vast majority of biological fluid are now defined non-Newtonian fluid. People blood, lymphatic fluid, fluid and other body fluids, and like the cytoplasm "semi-fluid" belong to non-Newtonian fluid.

Electrode paste is a kind of is composed of a variety of different proportion and granularity of raw materials, is mixing dispersion and solid - liquid phase formation of slurry belongs to non-Newtonian fluid.  Lithium battery paste and paste can be divided into the positive and negative paste two kinds, because slurry system (oily, water-based) is different, the nature will vary.  But, judge the character of paste the following parameters:

1. The viscosity of the slurry

Viscosity is a measure of the fluid viscosity, is a fluid flow power of its internal friction phenomenon of a said. When liquid flow, the nature of the internal friction in the intermolecular, known as the viscosity of the liquid, the size of the viscosity with viscosity, said is used to characterize the liquid properties related to the resistance factor.  Viscosity is divided into condition of dynamic viscosity and viscosity.

Viscosity is defined as A pair of parallel plate, covers an area of A, Dr, apart between filled with A liquid. Today to upper exert a force F, make it produce a speed change du. Due to the viscosity of the liquid transfer this force layer upon layer, each layer of liquid also corresponding movement, the formation of a velocity gradient du/Dr, shear rate, expressed as a r ‘. F/A, known as the shear stress to tau said. With the following relationship between shear rate and shear stress:

(F/A) = eta (du/Dr)

Newtonian fluid in accordance with Newton's formula, only related with the temperature, viscosity and shear rate, tau and D as proportional relationship.

Non-Newtonian fluid is not in conformity with the Newton formula tau/D = f (D), expressed in eta a certain viscosity under (tau/D), said the apparent viscosity. Viscosity of non-Newtonian fluid in addition to related to temperature, also associated with shear rate, time, and shear thinned or the change of shear thickening.

2. The pulp properties

Slurry is a kind of non-Newtonian fluid, which is solid-liquid mixture, in order to meet the requirements of the subsequent coating process, sizing agent needs to have the following three characteristics:

1) good liquidity. Liquidity can stir pulp, make its natural flow, observe its continuity. Good continuity, and off shows good liquidity. Liquidity is associated with the solid content and viscosity of slurry,

(2) the flow property. Affect the flowing property of paste is the flatness and uniformity of the coating.

(3) rheology. Rheology is refers to the pulp in the flow deformation characteristics, its nature is good or bad affects the merits of the pole piece quality.

3. The pulp dispersion

Lithium-ion battery electrodes, the anode paste by adhesive, conductive agent, the anode material, etc. The cathode paste is composed of adhesive, graphite, carbon powder, etc. Positive and negative electrode paste preparation includes liquid and liquid, liquid and solid materials mixing, dissolution, between dispersion and a series of technological process, and in this process is accompanied by temperature, viscosity, environment, etc. Lithium ion battery slurry mixing dispersion process can be divided into macro and micro disperse mixing process, the two processes will be accompanied with the whole process of the preparation of lithium ion battery slurry. The paste preparation tend to go through the following stages:

1) powder mix. Between particles in the form of dots, point, point, line,

(2) the dry mud knead phase. After this stage in dry powder mixing, adding binder liquid or solvent, raw material is wet, slimy. After strong stirring mixer, the material is cut by mechanical force and friction, also have internal friction between particles at the same time, under the various forces, the raw material between particles tend to be highly fragmented. At this stage to finished product particle size and viscosity of slurry has crucial influence.

(3) down the stage. After the completion of kneading slowly add solvent slurry viscosity and solid content adjustment. The dispersion and reunite of this stage coexist, and finally achieve stability. At this stage of material spread mainly by mechanical force friction resistance between liquid, powder, scattered high speed shearing force, the influence of the size and container wall impact force.

Second, the impact parameter analysis of pulp properties

And pulp of the pulp need has good stability, is this battery production process to ensure the consistency of an important indicator. As the end of the pulp, stirring stops, the slurry will be condensed and wait for a phenomenon, flocculant sedimentation, produce large particles, on a subsequent coating process caused great influence, etc. Characterization of slurry on the stability of the main parameters have liquidity, viscosity, solid content, density, etc.

1. The viscosity of the slurry

Electrode size needs to be stable and appropriate viscosity, it has a crucial influence on plate coating process.  Viscosity too high or too low are detrimental to pole piece of the coating, high viscosity of the slurry is not easy to precipitate and dispersion will be better, but too high viscosity against flow leveling effect, does not favor the coating;  Low viscosity is also bad, while low viscosity slurry liquidity is good, but the dry hard, reduces the drying efficiency of coating, coating will happen crack, the density of slurry particle reunion, surface consistency is bad.

In our production problems that often appear in the process of a change in viscosity, and here's the "change" and can be divided into: instantaneous change and static.  Instantaneous refers to changes in the viscosity of dramatic changes in the during the test, static change refers to a static placed after a period of time paste viscosity changes.  The change of viscosity or high or low, or high low.  Generally speaking, the factors in the slurry viscosity are stirring speed, time control of the thick liquid material, ingredients order, environmental temperature and humidity, etc.  Many factors, when we meet the viscosity changes should be how to solving?  The viscosity of the slurry in essence, is by the binder decisive influence.  Imaginary, no binder PVDF/CMC/SBR (as shown in figure 2, 3), or binder will be a very good no live material combination, solid material will live form is evenly coated with conductive agent of non-Newtonian fluid? Don't!  Therefore, solving the slurry viscosity change reason, from the nature of the binder and pulp dispersion degree.

(1) the viscosity increases

Different system has different paste viscosity change law of the current mainstream of slurry system is the anode paste PVDF/NMP oil system, the form of graphite anode paste/CMC/SBR water system.

(1) the positive paste viscosity in place after a period of time.  The reason for this is a (short) pulp mixing speed too fast, binder not fully dissolved, placed the PVDF powder dissolves after period of time, viscosity increased.  Generally, PVDF needs at least 3 hours to fully dissolved, no matter how fast the stirring speed can change the influence factors of the so-called "more haste, less speed".  For another (long) is in the process of the slurry let stand, colloidal sol into gel state, as if to slow homogenate, its viscosity can be restored.  The third reason is between colloidal particles and living material, conductive agent formed a kind of special structure, this state is irreversible, unable to recover after the paste viscosity increases.

(2) anode paste viscosity increased. Anode paste viscosity increase is mainly caused by the adhesive molecular structure was damaged, the molecular chain rupture after being oxidized slurry viscosity increased.  If the material being overstretched, larger reducing particle size, also can increase the viscosity of slurry.

(2) reduce the viscosity

(1) the positive paste viscosity reduced.  One of the reasons, adhesive colloid properties changed. Cause a variety of changes, such as process of slurry from strong shear force, the binder absorbs water quality change, lead to changes in the structure in the process of mixing, itself is degraded, and so on. For another, uneven distributed mixing slurry solid material in the large area of subsidence. Third, mixing process of binder by equipment and living creature strong shear force and friction force, under the condition of high temperature change character, resulting in decreased viscosity.

(2) anode paste viscosity reduced. One of the reasons for the CMC is mixed with impurities, most of the impurities of CMC are difficult soluble polymer resin, when the CMC and the mixed solvent such as calcium, magnesium, will reduce its viscosity. For another, CMC is hydroxyl methyl cellulose sodium, the main is the combination of C/O, bond strength is very weak and easily destroyed by shear force when the stirring speed too fast or too long that may damage the structure of the CMC.CMC in anode slurry thickening and stable effect, scattered for raw materials play an important role at the same time, in the event of damage, its structure is bound to cause slurry settlement, viscosity reduced. The third reason is the destruction of the SBR binder. In actual production often choose the CMC and the SBR to work together, the two different roles. SBR is mainly have the effect of binder, but its demulsification under long time stirring easily happened, lead to failure, bonding paste viscosity decreased.

(3) special circumstances (a jellylike timely high low)

In the process of preparing the anode paste into "jelly" would happen sometimes. There are mainly two reasons for this: first, moisture. Consider living material moisture absorption, moisture control for mixing process is not good, raw materials absorb water or mixing environment humidity is higher, after lead to PVDF absorb water into jelly. Second, the pH or filler materials. The higher the pH value, control of the water requires stricter, especially the NCA, NCM811 high nickel material mixing.

Paste viscosity and high or low, one reason may be not entirely stable, in the process of slurry test slurry viscosity is affected by temperature. Especially after being rapid dispersion, pulp internal temperature has certain temperature gradient, sampling different viscosity are also different. Second reason is the difference in pulp dispersion, live material, binder, conductive agent without good spread out, paste, there is no good liquidity, natural slurry viscosity and high or low.

2. The size of particle size

After alloy slurry, the need to measure the granularity, granularity measurement method usually used scraper method. Granularity is an important parameter of the pulp quality and particle size for the coating process, rolling process and battery performance has important influence, theoretically slurry particle size as small as possible. When the particle size is too large, the stability of the slurry will be affected, in sedimentation, pulp consistency, etc. Will appear in the process of extrusion type coating after plugging material, extremely dry pitting, etc., pole piece quality problem. In subsequent rolling process, the inferior coating due to the uneven, easily cause pole piece fracture, local micro cracks, the cycle of the battery performance, performance and safety performance ratio has caused great harm.

Is the cathode active material, adhesive, conductive material, such as the main material particle size, density, will appear in the process of stirring mixing, extruding, friction, together wait for a variety of different ways of contact. There are gradually blending raw material, solvent, wetting, large materials rupture and gradually stabilized, there will be a material mixing unevenness, dope dissolve bad serious reunion, and adhesive properties change, and so on and so forth, will lead to larger particles.

When we find out the cause of the particles appear to suit the remedy to the case, to solve these problems. On dry powder mixing, personal feel mixer speed influence on powder mixed degree is not big, but both need plenty of time to ensure dry powder blending. Some manufacturers choose powder dope some choice now liquid dissolve good adhesive, two kinds of different bonding agent determine the process is different, with powder binder take longer to dissolve, otherwise will be seen later in the swelling, the spring back, such as viscosity change. Reunion between fine particles are inevitable, but we have to ensure there is enough friction between the material, can make reunion particles appear extrusion, crushing, mixing. This needs us control the slurry at different stages of solid content, low solid content will affect the friction between granules dispersion.

3. The solid content of slurry

The solid content of slurry and slurry stability is closely related, the same technology and formula, the higher the solid content of slurry, the greater the viscosity, and vice versa. Within a certain range, the higher the viscosity, the higher the stability of slurry. When we design the battery, the general working backward from the battery capacity core winding thickness to the design of the pole piece, then the sheet was designed and surface density, live only physical parameters such as density, thickness. The parameters of the plate are made by coating machine and rolling machine to adjust the results, the solid content of slurry has no direct effect on its. So, the discretion of the slurry solids is irrelevant?

(1) solid content to improve the efficiency of mixing and coating efficiency has a certain impact. The higher solid content, paste shorter mixing time, the less solvent consumption, the higher the coating drying efficiency, save time.

(2) the solid content on the device has certain requirements. The loss of high solid content in slurry on the equipment is higher, because of the high solid content, the more serious the equipment wear and tear.

(3) the higher stability of high solid content in slurry, part of the slurry stability test results show that (pictured), regular mixing of TSI (instability index) are higher than 1.05 0.75 TSI value high viscosity mixing process, so the mixing process of slurry with high viscosity stability is superior to conventional mixing process. But the high solid content in slurry will affect its liquidity, very challenge coating process equipment and technical personnel.

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(4) the high solid content in slurry can reduce between the coating thickness, reduce the battery internal resistance.

4. The slurry density

The density of slurry is one of the important parameters of reaction slurry consistency. Testing slurry density in different location can verify slurry dispersion effect. Above all, I believe you can make good electrode slurry.

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