Introduction of lithium ion power battery

In the development process of new energy vehicles, in addition to high prices, short driving range and insufficient power supply and replacement infrastructure, power safety is the focus of consumers and professionals. This problem also affects the power battery's specific energy increase.

"Developing short circuit prevention, over-charge prevention, heat-proof control, anti-burning, and non-combustible electrolytes is the key to the safety of power batteries." Professor Xinping Ai of Wuhan University stressed on the new energy vehicle industry development peak BBS at the 14th China international industry expo in Shanghai.

Mechanism of unsafe behavior of lithium-ion power battery

Xinping Ai pointed out that in addition to the normal charge and discharge reaction, the lithium-ion battery has many potential exothermic side reactions. When the battery temperature or the charging voltage is too high, these exothermic side reactions are easily caused.

The main superheating side reactions include 1. The SEI film decomposes at a temperature higher than 130 ° C, so that the electrolyte is liberated by a large amount of reduction on the surface of the exposed high-activity carbon negative electrode, resulting in an increase in battery temperature. This is the root cause of the thermal runaway of the battery.

2. The heat of the positive state of the charged state positive electrode, and the decomposition of the electrolyte caused by the active oxygen further exacerbate the heat accumulation inside the battery and promote the thermal runaway.

3. The thermal decomposition of the electrolyte causes the electrolyte to dissipate heat and accelerate the temperature rise of the battery.

4. The reaction of the binder with a highly active negative electrode. The initial temperature of the reaction of LixC6 with PVDF is about 240 ° C, the peak temperature is 290 ° C, and the heat of reaction is 1500 J / g.

The main overcharge side reaction is that the organic electrolyte oxidizes and decomposes to generate organic small molecule gas, which causes the internal pressure of the battery to increase and the temperature to rise.

When the heat generation rate of the exothermic side reaction is higher than the heat dissipation rate of the power battery, the internal pressure and temperature of the battery will rise sharply, and enter an uncontrollable self-heating state, that is, the heat is out of control, causing the battery to burn. The thicker the battery, the larger the capacity, the slower the heat dissipation, and the greater the heat generation, the more likely it is to cause safety problems.

Initiating factors of unsafe behavior of lithium-ion power battery

It mainly includes short circuit caused by the following three conditions: 1 process surface conductive dust, positive and negative pole misalignment, pole piece burr and electrolyte distribution unevenness; 2 metal impurities in the material; 3 low temperature charging, high current charging, negative electrode performance Excessive attenuation leads to the application of lithium, vibration or collision on the surface of the negative electrode.

In addition, there is local overcharge caused by high current charging, extreme over-charge due to the uneven coating of the pole piece, uneven distribution of electro-hydraulic, and over-charging factors such as excessive attenuation of the positive electrode performance.

Progress in lithium-ion power battery safety technology

Conventional methods such as battery safety design and manufacture, PTC current limiting devices, pressure safety valves, heat-sealed diaphragms, and improved thermal stability of battery materials have their limitations and can only reduce the probability of unsafe behavior of batteries to a certain extent. Xinping Ai stressed: To solve the problem fundamentally, it is necessary to study new technologies for preventing short circuit, overcharge prevention, heat loss control, anti-burning and non-combustible electrolyte, and establish a self-excited safety protection mechanism for batteries.

1. Prevent internal short circuit of the battery: Protective coatings such as ceramic diaphragms and negative thermal resistance layers.

2. Anti-overcharge technology.

A, redox power to the additive: A redox electric pair O/R is added to the electrolyte. When the battery is overcharged, R is oxidized to O on the positive electrode, and then O diffuses to the negative electrode and is reduced to R. This internal circulation clamps the charging potential at a safe value, inhibiting electrolyte decomposition and other electrode reactions.

Dimethoxybenzene derivatives have stable voltage clamping ability, but due to low solubility, the clamping capacity is less than 0.5C; the battery self-discharge is large. Further research is needed on the molecular structure of Shuttle.

Reversible overcharge protection not only solves the overcharging problem of the battery, but also facilitates the capacity balance of the single battery in the battery pack, reduces the requirement for battery consistency, and prolongs the battery life.

B, voltage sensitive diaphragm: An electroactive polymer is filled in the micropores of the diaphragm portion. In the normal charging and discharging voltage range, the diaphragm is in an insulating state, and only ion conduction is allowed; when the charging voltage reaches a control value, the polymer is oxidized and doped into an electronically conductive state. A polymer conductive bridge is formed between the positive and negative electrodes to bypass the charging current, thereby avoiding overcharging of the battery.

3. Technology to prevent thermal runaway.

A, temperature sensitive electrode (PTC electrode): The PTC material has good contact with the conductive carbon black dispersed in the polymer matrix at normal temperature, and can form a good electron transport channel and has high electron conductivity; when the temperature rises to the Curie transition temperature of the composite, the polymer The matrix expands, the conductive carbon black is out of contact, and the conductivity of the composite drops sharply.

At high temperatures, the resistance of the PTC coating embedded between the PTC electrode current collector and the electrode active coating increases sharply, cutting off the current transmission, terminating the battery reaction, and preventing the safety of the battery due to thermal runaway.

For example, the PTC lithium cobaltate (LiCoO2) electrode, the experimental results show that at a high temperature of 80 ~ 120 ° C, it shows a good self-excitation heat blocking effect, can prevent the safety of the battery caused by overcharge and external short circuit.

However, the PTC electrode is incapable of internal short circuit. In addition, the temperature response characteristics of the polymer PTC material have yet to be further optimized.

B, heat sealed electrode. A layer of nanospherical hot melt material is modified on the surface of the electrode or membrane. At normal temperature, the accumulation of spherical particles forms porous, which does not affect the liquid phase transport of ions; when the temperature rises to the melting temperature of the spherical material, the sphere melts into a dense membrane, which cuts off ion transport and terminates the battery reaction.

C, heat curing battery: A monomer which can undergo thermal polymerization is added to the electrolyte. When the temperature rises, polymerization occurs, the electrolyte is solidified, ion transport is cut off, and the battery reaction is terminated. For example, experiments have shown that BMI electrolyte additives have little effect on battery charge and discharge. At high temperatures, BMI can inhibit battery charge and discharge.

4. Non-combustible electrolyte to prevent battery burning. The organophosphate has the characteristics of high flame retardancy and strong ability to dissolve electrolyte salts. For example, DMMP (dimethoxymethyl phosphate): low viscosity (CP ~ 1.75, 25 ° C), low melting point, high boiling point (-50 ~ 181 ° C), strong flame retardant (P-content: 25%), the lithium salt has high solubility.

However, the flame retardant solvent has the following problems in the application: poor compatibility with the negative electrode, and low efficiency of charge and discharge of the battery. Therefore, there is a need to find matching film forming additives.

Safety issues that should be paid attention to in the commercialization of power batteries

For the safety of lithium-ion power batteries, Xinping Ai believes that, firstly, since the thermal decomposition of the positive electrode material is only a part of the thermal runaway reaction, theoretically, the lithium iron phosphate battery is not absolutely safe, and the large-capacity battery should be cautious when loading. .

Secondly, due to the probability of battery detection, the power battery tested by safety cannot be proved to be absolutely safe. Strictly speaking, the battery should be tested after a certain number of cycles of full charge and discharge; the battery after low-temperature charging; the battery module and battery pack are tested for safety.

And, in the process of the battery, the vehicle manufacturers as the power battery in 20 ~ 45 ℃ environment temperature control range, so can not only improve the battery life and reliability, also can avoid the low-temperature analysis lithium problems caused by a short circuit and high-temperature thermal runaway.

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