12V27A battery.Lithium-ion battery technology

　　Lithium-ion battery is a secondary battery (rechargeable battery) that mainly relies on Li+ to intercalate and deintercalate back and forth between two electrodes to work. With the continuous development of downstream industries such as energy vehicles, the production scale of lithium-ion batteries is continuously expanding.

　　This topic is divided into two parts, the upper and lower parts. The first part focuses on the principle, formula and process flow of lithium-ion batteries. The next article explains the production and performance of lithium batteries. This article is the first article of this topic.

　　1. Working principle 1. Positive electrode structure

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%

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　　LiCoO2+conductive agent+adhesive (PVDF)+current collector (aluminum foil)

　　2. Negative electrode structure

　　Graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)

　　3. Working principle

　　3.1 Charging process

　　A power source charges the battery. At this moment, the electrons e on the positive electrode run to the negative electrode through the external circuit. The positive lithium ions Li+ "jump" from the positive electrode into the electrolyte, "crawl through" the small winding hole on the separator, and "swim" "Arrived at the negative electrode, it is combined with the electrons that have already run over. now:

　　The reaction that occurs on the positive electrode is:

　　The reaction occurring on the negative pole is:

　　3.2 Battery discharge process

　　Discharge includes constant current discharge and constant resistance discharge. Constant current discharge actually involves adding a variable resistor in the external circuit that changes with the voltage. The essence of constant resistance discharge is to add a resistor to the positive and negative terminals of the battery to allow electrons to pass through. It can be seen that as long as the electrons on the negative electrode cannot run from the negative electrode to the positive electrode, the battery will not discharge. Both electrons and Li+ move together, in the same direction but in different paths. During discharge, electrons run from the negative electrode through the electronic conductor to the positive electrode. Lithium ions Li+ "jump" into the electrolyte from the negative electrode and "climb" across the barrier. small hole, "swim" to reach the positive electrode, and combine with the electrons that have run over long ago.

　　3.3 Charge and discharge characteristics

　　The positive electrode of the battery cell uses LiCoO2, LiNiO2, and LiMn2O2. LiCoO2 is a crystalline form with a very stable layer structure. However, when x Li ions are removed from LiCoO2, its structure may change, but whether it changes depends on x big and small.

　　Through research, it is found that when x>0.5, the structure of Li1-xCoO2 is extremely unstable, and crystalline collapse will occur. The external manifestation is the collapse of the battery core. Therefore, during use of the battery core, the x value in Li1-xCoO2 should be controlled by limiting the charging voltage. Generally, if the charging voltage is not greater than 4.2V, then x is less than 0.5. At this time, the crystal form of Li1-xCoO2 is still stable.

　　The negative electrode C6 itself has its own characteristics. After the first formation, the Li in the positive electrode LiCoO2 is charged into the negative electrode C6. When discharged, the Li returns to the positive electrode LiCoO2, but after the formation, a part of Li must remain in the center of the negative electrode C6. , to ensure the normal embedding of Li in the next charge and discharge, otherwise the overload of the cell will be very short. In order to ensure that a part of Li remains in the negative electrode C6, it is generally achieved by limiting the lower limit voltage of discharge: the upper limit voltage of safe charging is ≤4.2V, the lower limit of discharge Voltage ≥2.5V.

　　The principle of the memory effect is crystallization, and this reaction hardly occurs in lithium batteries. However, the capacity of lithium-ion batteries will still decrease after repeated charging and discharging. The reasons are complex and diverse. The main reason is the change of the positive and negative electrode materials themselves. From a molecular level, the hole structure holding lithium ions on the positive and negative electrodes will gradually collapse and block; from a chemical point of view, it is the active passivation of the positive and negative electrode materials, causing side reactions to generate stable Other compounds. Physically, there will also be situations where the positive electrode material gradually peels off, which ultimately reduces the number of lithium ions in the battery that can freely move during the charge and discharge process.

　　Overcharging and over-discharging will cause permanent damage to the positive and negative electrodes of lithium-ion batteries. From a molecular level, it can be intuitively understood that over-discharging will cause the negative electrode carbon to excessively release lithium ions, causing its lamellar structure to collapse. Overcharging will force too many lithium ions into the negative carbon structure, making it impossible for some of the lithium ions to be released.

　　Unsuitable temperatures will trigger other chemical reactions inside the lithium-ion battery to produce compounds that we do not want to see. Therefore, many lithium-ion batteries are equipped with protective temperature control barriers or electrolyte additives between the positive and negative electrodes. When the battery heats up to a certain level, the pores of the composite membrane close or the electrolyte denatures, and the internal resistance of the battery increases until the circuit is broken, and the battery no longer heats up, ensuring that the battery charging temperature is normal.

　　2. Formula and process flow of lithium battery

　　1. Positive and negative electrode formulas

　　1.1 Positive electrode formula: LiCoO2 + conductive agent + adhesive + current collector (aluminum foil)

　　LiCoO2(10μm):96.0%

　　Conductive agent (CarbonECP) 2.0%

　　Adhesive (PVDF761) 2.0%

　　The weight ratio of NMP (increased adhesion): solid material is approximately 810:1496

　　a) Positive electrode viscosity control 6000cps (temperature 25 rotor 3);

　　b) The NMP component must be adjusted appropriately to meet the viscosity requirements;

　　c) Pay special attention to the effects of temperature and humidity on viscosity

　　Positive active material:

　　Lithium cobalt oxide: positive active material, lithium ion source, and advanced lithium source for batteries. Non-polar substance, irregular shape, particle size D50 is generally 6-8μm, water content ≤0.2%, generally alkaline, pH value is about 10-11.

　　Lithium manganate: non-polar substance, irregular shape, particle size D50 is generally 5-7μm, water content ≤0.2%, generally weakly alkaline, pH value is about 8.

　　Conductive agent: chain-like substance, water content <1%, particle size generally 1-5μm. Superconducting carbon blacks with excellent electrical conductivity are generally used, such as Ketjen Carbon Black CarbonECP and ECP600JD. Its functions: improve the conductivity of the positive electrode material, compensate for the electronic conductivity of the positive electrode active material; increase the electrolyte absorption capacity of the positive electrode sheet, Increase the response interface and reduce polarization.

　　PVDF adhesive: non-polar substance, chain substance, molecular weight ranging from 300,000 to 3,000,000; after absorbing water, the molecular weight decreases and the viscosity becomes worse. Used to bond lithium cobalt oxide, conductive agent and aluminum foil or aluminum mesh together. Commonly used brands such as Kynar761.

　　NMP: Weak polar liquid, used to dissolve/swell PVDF and dilute the slurry together.

　　Current collector (positive lead): Made of aluminum foil or strip.

　　1.2 Negative electrode formula: graphite + conductive agent + thickener (CMC) + binder (SBR) + current collector (copper foil)

　　Negative electrode material (graphite): 94.5%

　　Conductive agent (CarbonECP): 1.0% (Ketjen superconducting carbon black)

　　Binder (SBR): 2.25% (SBR = styrene-butadiene rubber latex)

　　Thickener (CMC): 2.25% (CMC = sodium carboxymethylcellulose)

　　The weight ratio of water:solid matter is 1600:1417.5

　　a) Negative electrode viscosity control 5000-6000cps (temperature 25 rotor 3)

　　b) The water content needs to be adjusted appropriately to meet the viscosity requirements;

　　c) Pay special attention to the influence of temperature and humidity on viscosity

　　2. Positive and negative mixture

　　Graphite: negative active material, the main material forming the negative reaction; mainly divided into two categories: natural graphite and artificial graphite. Non-polar substances are easily contaminated by non-polar substances and are easily dispersed in non-polar substances; they are not easy to absorb water and are not easy to be dispersed in water. Contaminated graphite easily reassembles after being dispersed in water. The general particle size D50 is about 20 μm. The shapes of particles are diverse and irregular, mainly including spherical, flaky, fibrous, etc.

　　Conductive agent: Its function is:

　　a) Improve the conductivity of the negative electrode sheet and compensate for the electronic conductivity of the negative electrode active material.

　　b) Improve response depth and utilization rate.

　　c) Prevent the generation of dendrites.

　　d) Utilize the liquid absorption ability of conductive materials to improve the reaction interface and reduce polarization. (You can choose to add or not add it according to the graphite particle size distribution).

　　Additive: Reduce irreversible reactions, improve adhesion, increase slurry viscosity, and prevent slurry precipitation.

　　Thickener/anti-sedimentation agent (CMC): high molecular compound, easily soluble in water and polar solvents.

　　Isopropyl alcohol: a weakly polar substance, which can reduce the polarity of the adhesive solution and improve the compatibility between graphite and the adhesive solution; has a strong defoaming effect; can easily catalyze the network cross-linking of the adhesive , improve bonding strength.

　　Ethanol: a weakly polar substance, which can reduce the polarity of the adhesive solution and improve the compatibility between graphite and the adhesive solution; has a strong defoaming effect; can easily catalyze the linear cross-linking of the adhesive and improve the adhesion Knot strength (isopropyl alcohol and ethanol are essentially the same, and you can choose which one to add based on cost considerations when producing large quantities).

　　Water-based adhesive (SBR): Bonds graphite, conductive agent, additive and copper foil or copper mesh together. Small molecule linear chain emulsion, easily soluble in water and polar solvents.

　　Deionized water (or distilled water): diluent, increase as appropriate to change the fluidity of the slurry.

　　Negative lead: Made of copper foil or nickel tape.

　　2.1 Positive electrode mixture:

　　2.1.1 Pretreatment of raw materials

　　1) Lithium cobalt oxide: dehydration. Generally, bake at 120°C and normal pressure for about 2 hours.

　　2) Conductive agent: dehydration. Generally, bake at 200°C and normal pressure for about 2 hours.

　　3) Binder: Dehydrated. Generally, it is baked at 120-140°C under normal pressure for about 2 hours. The baking temperature depends on the size of the molecular weight.

　　4) NMP: dehydration. Use dry molecular sieves to dehydrate or use special reclaiming facilities for direct use.

　　2.1.2 Material ball milling:

　　1) At the end of 4 hours, screen and separate the ball mill;

　　2) Pour LiCoO2 and CarbonECP into the barrel, add grinding balls together (dry material: grinding balls = 1:1), perform ball milling on the roller bottle, and control the rotation speed above 60rmp.

　　2.1.3 Blending of raw materials:

　　1) Dissolution of adhesive (according to standard concentration) and heat treatment.

　　2) Ball milling of lithium cobalt oxide and conductive agent: The powder is initially mixed, and the lithium cobalt oxide and conductive agent are bonded together to improve the aggregation effect and conductivity. After being formulated into a slurry, it will not be dispersed alone in the binder. The ball milling time is generally about 2 hours; in order to prevent impurities from being mixed in, agate balls are generally used as ball milling mesons.

　　2.1.4 Dispersion and wetting of dry powder:

　　Principle: Solid powder is placed in the air. As time goes by, part of the air will be adsorbed on the surface of the solid. After the liquid binder is added, the liquid and gas begin to compete for the solid surface; if the adsorption force ratio between the solid and the gas is the same as that of the liquid. If the adsorption force is strong, the liquid cannot wet the solid; if the adsorption force of the solid and the liquid is stronger than the adsorption force of the gas, the liquid can wet the solid and squeeze out the gas.

　　When the wet angle is ≤90°, the solid is wetted. When the wet angle is >90°, the solid is not wetted.

　　All components in the cathode material can be wetted by the binder solution, so it is relatively easy for the cathode powder to spread.

　　The impact of decentralization methods on decentralization:

　　1) Resting method (long time, poor effect, but does not damage the original structure of the data);

　　2) Mixing method: rotation or rotation plus revolution (short time, good effect, but may damage the structure of individual materials).

　　The influence of mixing paddles on dispersion speed: Mixing paddles generally include snake-shaped, butterfly-shaped, spherical, paddle-shaped, gear-shaped, etc. Generally, serpentine-shaped, butterfly-shaped, and paddle-shaped mixing paddles are used to resist the initial stage of dispersion of materials or ingredients that are difficult to disperse; spherical and gear-shaped mixing paddles are used for conditions where dispersion is less difficult and have good results.

　　Effect of mixing speed on dispersion speed. Generally speaking, the higher the mixing speed, the faster the dispersion, but the greater the damage to the structure of the material itself and the equipment.

　　Effect of concentration on dispersion rate. Under normal circumstances, the smaller the slurry concentration, the faster the dispersion speed, but too thin will lead to the waste of materials and aggravation of slurry precipitation.

　　Effect of concentration on bond strength. The greater the concentration, the greater the softening strength and the greater the bonding strength; the lower the concentration, the smaller the bonding strength.

　　Effect of vacuum degree on dispersion velocity. High vacuum degree is conducive to the discharge of gas from the gaps and surfaces of the material, reducing the difficulty of liquid adsorption; the difficulty of the material being dispersed evenly under the condition of complete weightlessness or reduced gravity will be greatly reduced.

　　Effect of temperature on dispersion rate. At a suitable temperature, the slurry has good fluidity and is easy to disperse. The slurry that is too hot will easily form a skin, and the fluidity of the slurry that is too cold will be greatly reduced.

　　Dilution: Adjust the slurry to a suitable concentration for easy coating.

　　2.1.5 Operation process

　　a) Pour NMP into the power mixer (100L) to 80°C, weigh PVDF and add it into the mixer, and start the machine; parameter settings: speed 25±2 rpm, mix for 115-125 minutes;

　　b) Turn on the cooling system, add the ground positive electrode dry material in four equal parts, each time 28-32 minutes apart, add NMP according to the data requirements for the third addition, add NMP after the fourth addition; power mixer Parameter setting: rotation speed is 20±2 rpm

　　c) After the fourth addition of materials for 30±2 minutes, high-speed mixing is carried out, the time is 480±10 minutes; the power mixer parameter settings: revolution is 30±2 rpm, rotation is 25±2 rpm;

　　d) Vacuum mixing: Connect the power mixer to the vacuum, keep the vacuum degree at -0.09Mpa, and mix for 30±2 minutes; power mixer parameter settings: revolution is 10±2 minutes, rotation is 8±2 rpm

　　e) Take 250-300 ml of slurry and measure the viscosity with a viscometer; test conditions: rotor number 5, rotation speed 12 or 30 rpm, temperature range 25°C;

　　f) Take out the positive electrode material from the power mixer, conduct colloid grinding and sieving, and put a label on the stainless steel basin. After handing over to the operator of the pulping equipment, it can flow into the pulping operation process.

　　2.1.6 Precautions

　　a) Complete and clean up the machinery, equipment and working environment;

　　b) When operating the machine, pay attention to safety to prevent injuries to the head.

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