Aug 15, 2019 Pageview：117
The analysis and discussion of powder removal in the preparation process of lithium ion battery
At present, the production process of lithium cobalt oxide basically does not fall out of powder. Factors influencing the possibility of falling out of powder in the production process include:
Formula proportion is improper, such as too little adhesive, capacitance less resulting in uneven mixing.
Adhesive baking temperature is too high, so that the adhesive structure is destroyed.
The slurry has not been stirred for enough time.
The temperature of coating is too low and the sheet is not dried.
Uneven coating amount, thickness difference is too large.
The sheet is not baked before being rolled and absorbs a lot of moisture in the air.
Roll pressure is too high, so that the powder and collector fluid stripping.
When the rollers of the plate is not the right way, resulting in uneven force plate.
Bad battery items and causes
1. The capacity is low
A. Less supplementary materials;
B. There is a big difference in the amount of attached materials between the two sides of the pole sheet;
C. Plate fracture;
D. less electrolyte;
E. low conductivity of electrolyte;
F. The anode and cathode plates are not well matched;
G. small porosity of diaphragm;
H. Adhesive aging adhesives fall off;
I. excessively thick core (not dried or penetrated by electrolyte);
J. Not fully charged when capacitance is divided;
K. Anode and cathode materials are smaller than capacitance.
2. High internal resistance
A. Virtual welding of negative plate and pole ear;
B. Virtual welding of positive plate and pole ear;
C. Blind welding of positive lug and cap;
D. False welding of anode lug and shell;
E. Large internal resistance between rivet and press plate;
F. No conductive agent is added to the positive electrode;
G. No lithium salts in electrolyte;
H. There was a short circuit in the battery;
I. diaphragm paper has low porosity.
3. Low voltage
A. Side reactions (electrolyte decomposition; Positive pole has impurity; There is water);
B. Not properly formed (SEI film is not formed safely);
C. Leakage of the customer's circuit board (refer to the cell returned by the customer after processing);
D. The customer fails to spot weld (the cell processed by the customer) as required;
F. micro-short circuit;
G. The negative electrode produces dendrites.
4. The super thick
The reasons for the overthickness are as follows:
A. Weld leakage;
B. Electrolyte decomposition;
C. Undried moisture;
D. poor sealing of the cap;
E. the shell wall is too thick;
F. The shell is too thick;
G. Too thick coil core (too many attachments; Pole sheet not compacted; The diaphragm is too thick.
5. The causes are as follows
A. Incomplete SEI film (incomplete and dense);
B. High baking temperature adhesive aging stripping;
C. Low negative specific capacity;
D. More positive terminal materials and less negative terminal materials;
E. leakage of cap and welding seam;
F. Electrolyte decomposition, reduced conductivity.
6. The explosion
A. The sub-container is faulty (resulting in overcharging);
B. Poor closing effect of diaphragm;
C. internal short circuit
7. A short circuit
B. Shell fashion is broken;
C. Ruler scraping (small diaphragm paper is too small or not padded);
D. Uneven winding;
F. The diaphragm has holes;
8. Open circuit
A. Pole ear and rivet are not well welded, or the effective solder joint area is small;
B. The connecting piece is broken (the connecting piece is too short or too close to the pole piece for spot welding)
Safety features of lithium-ion batteries
Lithium ion battery has been widely used in People's Daily life, so its safety performance should be the first assessment index of lithium ion battery. For the safety performance of lithium ion battery assessment indicators, the international provisions of very strict standards, a qualified lithium ion battery in the safety performance should meet the following conditions.
1) short circuit: no fire or explosion;
2) overcharge: no fire or explosion;
3) hot box test: no fire, no explosion (150 ℃ temperature 10 min)
(4) needle: no explosion with Φ 3 mm nail through batteries);
5) flat plate impact: no fire, no explosion; (10kg weight falls on the battery from a height of 1 meter);
6) incineration: no explosion (gas flame test battery)
In order to ensure the safe and reliable use of lithium ion batteries, experts have carried out very strict and thorough battery safety design to meet the battery safety assessment indicators.
1) diaphragm 135 ℃ automatic shutoff protection: adopt international advanced Celgard2300PE - PP, PE three-layer composite film. In the case of battery temperature reached 120 ℃, the composite film on both sides of the PE film hole closed, battery internal resistance increases, the battery internal form a large area of open circuit, the battery no longer heating up.
2) composite structure of battery cover: the battery cover is of Nick explosion-proof structure. When the battery heats up, the pressure will reach a certain degree of Nick rupture and air release.
3) various environmental abuse tests: conduct various abuse tests, such as external short circuit, overcharge, acupuncture, plate impact, incineration, etc., to investigate the safety performance of the battery. At the same time, temperature shock test, vibration, drop, impact and other mechanical properties test were carried out on the battery to investigate the performance of the battery in the actual use environment.
Lithium ion battery protection line (PCM)
Lithium ion battery needs at least three protections: overcharge protection, overdischarge protection and short circuit protection. Then its protection circuit is generated according to the above three protection requirements.
Overcharge protection: the principle of overcharge protection IC is that when the external charger charges the lithium battery, in order to prevent the rise of internal pressure caused by temperature rise, the charging state shall be terminated. At this time, the protection IC needs to detect the battery voltage. When the battery voltage reaches 4.25v (assuming the overcharge point of the battery is 4.25v), the overcharge protection will be started, and the power MOS will be switched from on to off, and then the charging will be cut off.
Over discharge protection, over discharge protection IC principle: in order to prevent the over discharge of the lithium battery, assuming that the lithium battery connected to the load, when the lithium battery voltage is lower than the discharge voltage testing point (2.5 V) is assumed to start over discharge protection, make the power MOSFET by open to cut off by the discharge, in order to avoid the battery discharge phenomenon, and keep the battery in low static current standby mode, the current is only 0.1 uA. When the lithium battery is connected to the charger, and the lithium battery voltage is higher than the overdischarge voltage, the overdischarge protection function can be removed. In addition, considering the situation of pulse discharge, the delay time of over discharge detection circuit is set.
Analysis of low capacitance of lithium ion battery
1. High compaction density;
2, very little powder attached;
4. Small amount of electrolyte;
5. Incomplete formation;
6. Incomplete charge and discharge of detection capacity;
7. High humidity (water absorption);
8. Long battery storage;
9. Low specific capacity of materials;
10, very piece virtual welding, very ear virtual welding;
11. Environmental control during the manufacturing process, such as temperature, humidity...
Cause and control of cell expansion
Swelling often occurs in the manufacturing and use of lithium ion cells. Through analysis and research, it is found that there are mainly the following two reasons:
1. Thickness variation due to lithium ion imbedding
When a cell is charged, lithium ions detach from the positive electrode and embed into the negative electrode, resulting in the increase of the spacing between the negative electrode layers and the appearance of expansion. Generally speaking, the thicker the cell, the greater its expansion.
2. Expansion caused by poor process control
In the manufacturing process, such as slurry dispersion, C/A ratio discrete, temperature control will directly affect the extent of cell expansion. Water in particular, because the highly reactive lithium-carbon compounds formed by charging are very sensitive to water, resulting in violent chemical reactions. The gas produced by the reaction causes the voltage inside the cell to rise, which increases the expansion behavior of the cell. So in production, in addition to strictly deal with plate dehumidification, more should be used in the process of the injection fluid dehumidification equipment, guarantee the air dry degree for HR2 %, * * (the wet air in the atmosphere due to the temperature drop, make contains water vapour saturated state and temperature) of the condensation of less than 40 ℃. In very dry conditions, and the use of vacuum injection, greatly reduce the plate and electrolyte water absorption rate.
Why do we need to add VGCF carbon tubes in the positive and negative active materials of lithium ion batteries?
1. Both positive and negative active materials will have expansion and contraction problems. Generally, negative carbon materials have 20% expansion and contraction rate, while positive materials like LFP have 6% expansion and contraction rate. In the case of multiple charges and discharges, the positive and negative active wood particles have less contact with each other, larger gap, and even some of them are separated from the collector, resulting in discontinuous phase of the electron and ion transmission path, becoming dead active wood and no longer participating in the electrode reaction. So the cycle life goes down. VGCF carbon tube has a large aspect ratio, even after the expansion and contraction of positive and negative active wood, the gap between the active wood particles can be bridged by VGCF carbon tube, the transmission of electrons and ions will not be interrupted.
2. As the VGCF carbon tube microstructure is hollow multi-tube wall, positive and negative electrodes can absorb more electrolyte, so that lithium ions can be smoothly and quickly embedded or unembedded, which is conducive to high rate charge and discharge.
3.VGCF is a high-strength fibrous material with a large aspect ratio, which can increase the flexibility of the electrode plate. The adhesion between particles of positive or negative active materials or between particles and the electrode plate is stronger, and the powder will not crack due to deflection.
4. The essence of VGCF is high conductivity and high thermal conductivity. The positive active material has poor electrical conductivity.
When dissecting the battery, I encountered some situations listed below. I don't know what your predecessors think of these situations:
1. The positive electrode which is obviously easy to break becomes soft after injection.
2. Folding of positive plate (inner layer)?
3. The edge and inner layer of the newly removed negative plate will be dark purple, which is different from the color of the middle part of the plate (golden yellow in the middle).
4. Why is there a lot of white matter in the head of the negative plate (the first piece) opened each time? Is it lithium?
5. Why there is copper on the positive electrode plate after short circuit, whether the copper of the negative electrode is electrolyzed, and why the copper is absorbed most at the head of the positive electrode;
6. The anode ear is black, is it the phenomenon of short circuit (traces of large current passing through) or the dissolution of anode graphite?
7. Observe whether excessive positive material is dripping on the negative plate, and see whether the fire is ignited.
Answer search :(statement no standard answer, mainly live)
First: the pole has rebounded after charge and discharge, must become soft, popular point, not so dead, loose inside;
The 2nd: that is normal ~ the press close to when a few turn coil are rao roll needle, affirmation has crease... Unless you use very thick needles, hehe, this is impossible
The 3rd: did not charge gray, half fill dark purple, full fill golden yellow, that kind of circumstance thinks oneself, clew: infiltration rate;
Fourth: the negative chip head (the first small piece) will have a lot of white material, other places if not, is your design problem, is to analyze lithium;
The 5th: this problem is not clear, do not know you that what circumstance, it is countercharge, it is whole or part, also possible short circuit;
Sixth: negative ear black, depending on the situation, generally short circuit;
Seven: water drop who teach you? Haven't heard. The positive charge is excessive, the negative charge is obvious, and of course you have to exclude the external cause.
1. If the diaphragm is yellow or has black spots, whether there has been a large current through the diaphragm, breakdown, short circuit, may be dust, may also be your diaphragm originally have holes, of course, there is also the possibility of materials;
2. When the battery is packaged, the current is not stable or the current is too large when the rivets are spot-welded, so that the diaphragm next to the exposed negative electrode ear is burnt out, but whether the high-temperature glue will be burned off.
This has not seen, the general spot welding is instantaneous, the energy can be burned inside the diaphragm has not really seen, high temperature glue is just a temperature high point, if you have a 1000 degrees like the end of the explosion of the battery you can see, high temperature adhesive paper into ash.
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