Lithium battery improvement

1 Use "linear" conductive agent

The so-called "linear" and "granular" conductive agents are the image of the author, which may not be described in academic terms. "Linear" conductive agent is used, mainly including VGCF (carbon fiber), CNTs (carbon nanotubes) and metal nanowires. Their diameters range from a few nanometers to tens of nanometers, and their lengths range from tens of micrometers to even centimeters. At present, the commonly used "granular" conductive agents (such as Super P, KS-6) are generally tens of nanometers in size, and the battery materials are several micrometers in size. The electrode plate composed of "granular" conductive agent and active substance contacts similar points and points, and each point can only contact the surrounding points; In the electrode plate composed of "linear" conductive agent and active substance, there are points and lines, lines and lines in contact. Each point can contact multiple lines at the same time, and each line can also contact multiple lines at the same time. With more nodes in contact, the conductive channel will be more smooth, and the conductivity will be better. Using a variety of conductive agent combinations in different forms can play a better conductive effect. How to select conductive agent is a problem worth exploring for battery production. The possible effects of using "linear" conductive agents such as CNTS or VGCF include: (1) the linear conductive agent can improve the bonding effect to a certain extent, and improve the flexibility and strength of the electrode piece; (2) Reduce the amount of conductive agent (remember it was reported that the conductive efficiency of CNTS is 3 times that of conventional particle conductive agent of the same mass (weight)). In general, (1) the amount of glue may also be reduced, and the content of active substances may be increased; (3) Improve polarization, reduce contact impedance and improve cycle performance; (4) The conductive network has many contact nodes, more perfect network, and better magnification performance than conventional conductive agents; The heat dissipation performance is improved, which is significant for high rate batteries; (5) The absorption property is improved; (6) The material price is high, and the cost rises. 1Kg conductive agent, commonly used SUPER P is only tens of yuan, VGCF is about two to three thousand yuan, and CNTS is slightly higher than VGCF (when the addition amount is 1%, 1Kg CNTs is calculated at 4000 yuan, and the cost increases by about 0.3 yuan per Ah); (7) CNTS, VGCF and other specific surfaces are relatively high. How to disperse them is a problem that must be solved in use, otherwise, poor dispersion will not give full play to their performance. Ultrasonic dispersion and other means can be used. CNTs manufacturers provide well dispersed conductive liquid.

2 Improve dispersion effect

For the slurry with good dispersion effect, the probability of particle contact agglomeration will be greatly reduced, and the stability of the slurry will be greatly improved. The dispersion effect can be improved to a certain extent by improving the formula and batching steps. The ultrasonic dispersion mentioned above is also an effective method.

3. Improve the slurry transfer process

When the slurry is stored, it can be considered to increase the mixing speed to avoid slurry viscosity; For those who use turnover barrels to transfer the slurry, the time from discharging to coating shall be shortened as much as possible, and if possible, pipeline transportation shall be used to improve the slurry viscosity.

4 Extrusion coating (spraying)

Extrusion coating can improve the uneven surface texture and thickness of scraper coating, but the equipment price is high and the stability of slurry is required.

Difficulty in drying

Because of the large specific surface area of lithium iron phosphate and the large amount of binder, the amount of solvent required for preparing the slurry is also large, and it is difficult to dry after coating. How to control the volatilization rate of solvent is a problem worthy of attention. With high temperature, large air volume and fast drying speed, the gap will be large, which may also drive the migration of colloid, resulting in uneven distribution of materials in the coating. If colloid aggregates on the surface, it will hinder the conduction of charged particles and increase the impedance. Low temperature, low air volume, slow solvent escape, long drying time and low production capacity.

Poor bonding performance

The particles of lithium iron phosphate material are small, and the specific surface ratio is much larger than that of lithium cobalate and lithium manganate, so more adhesives are needed. However, if more adhesives are used, the energy density will decrease when the content of active substances is reduced. Therefore, if possible, the amount of adhesives will be reduced as much as possible during battery production. In order to improve the bonding effect, the current common method of lithium iron phosphate processing is to increase the molecular weight of the binder (high molecular weight, improved bonding ability, but the more difficult the dispersion, the higher the impedance), and to increase the amount of the binder. At present, it seems that the results are not satisfactory.

The disadvantage of this method is that the PTC material itself has large impedance and poor conductivity, which is unfavorable to the battery capacity and magnification. In order to solve this problem, other conductive agents can be added to the PTC material.

(3) In addition to the above two methods of coating the electrode material surface with PTC material, Lan Xia of Wuhan University and others coated a layer of PTC material on the surface of the active material lithium cobalate. The PTC material used is poly (3 decyl thiophene). The electrode made of the coated active material can not only sense the battery temperature rise caused by external abuse such as overcharge and internal short circuit, but also inhibit the thermal reaction caused by highly active electrode materials. The protection mechanism is that lithium ions and electrons can be transmitted normally at room temperature. At high temperature, the coating layer becomes an insulating layer. The transmission of ions and electrons is blocked, which plays a role of blocking at high temperature. The coated electrode can effectively protect the electrode material from corrosion by electrolyte while playing a safety role, and improve the chemical stability and thermal stability of the electrode material.

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