We know that as a large number of electric vehicles are put into use, the quality problem of poor battery consistency is gradually being exposed. Data from the State Administration of Market Supervision and Administration show that in 2018, there were at least 40 fire accidents involving new energy vehicles in our country, which is equivalent to an average of about 3 fire accidents per month. At the same time, new energy vehicles still have outstanding safety issues and high recall rates for three-electric vehicles. The safety situation of new energy vehicles deserves vigilance.
From the perspective of the chemical structure of the lithium battery itself, the fire and explosion of the lithium battery is caused by the thermal runaway of the lithium battery. The root cause is mainly the violent chemical exothermic reaction between the electrode and the electrolyte, caused by the unlimited diffusion of heat.
Studies have shown that in the thermal analysis of the battery system, the thermal stability of lithium iron phosphate is the best in terms of material. The safety of the battery first depends on the safety of its own materials. To increase the safety of batteries, high-energy batteries may need to include specific protection devices in the system design, such as cooling systems, explosion-proof systems, etc.
At the monomer level, in addition to conventional thermal safety design, it is more important to establish monomer self-excitation thermal protection. Let the monomer adjust its current output or power output according to the temperature it feels. If the battery can turn off the reaction, its heat production will be terminated.
Among them, the PTC thermistor material that plays the role of temperature sensor monitoring has an important feature that when the temperature rises to a certain extent, the material will change from a good conductive state to an insulating state, which will be one of the important paths in monomer thermal protection technology.
The Causes of Lithium Battery Failure
The main failure methods of batteries during transportation and use include mechanical indiscriminate use (impact, collision, acupuncture, etc. ), electrical indiscriminate use (overcharge, over discharge, etc. ), thermal indiscriminate use (overheating), etc. These reasons will cause the destruction of the positive and negative electrode structure of the internal material of the battery, the thermal decomposition of the SEI film, the decomposition and exothermic of the electrolyte, and the heat shrinkage of the diaphragm, resulting in a short circuit inside the battery. When a violent chemical or electrochemical reaction occurs inside the battery, a large amount of heat accumulation will occur, causing serious thermal runaway. At present, the safety protection for lithium batteries can be mainly divided into: external protection, built-in self-excitation protection, and improved material stability. External protection mainly includes setting up a safety valve on the battery housing, external series PTC components, hot melt fuses and BTMS thermal management.
What is PTC in Lithium Battery?
PTC thermistor, that is, positive temperature coefficient thermistor, that is, PTC (positive temperature coefficient). It is a type of temperature sensor material whose resistance value increases with temperature. The application of PTC in lithium-ion batteries has been studied for many years, and it is currently widely used in some consumer batteries and power batteries. After installing a PTC thermistor element in a lithium-ion battery, when the battery heats up due to overcharge, the internal resistance of the battery increases rapidly, thereby limiting the current and realizing automatic protection of the battery.
Externally protected PTC components are often connected in series on the battery housing. The working principle is that when the external current of the battery is overloaded, the temperature of the PTC element exceeds its Curie temperature, causing the resistance of the PTC element to increase rapidly by several orders of magnitude, thereby achieving the current limiting effect. Due to the slow heat transfer speed of the internal materials of the battery, the temperature difference between the inside and outside of the battery is often large. The PTC element connected in series on the housing cannot respond to the internal temperature in a timely manner, so it cannot effectively suppress the thermal runaway of the battery.
Specifically, the PTC thermistor element is installed in the lithium-ion battery, taking into account the pressure and temperature inside the battery. When the battery is heated up due to overcharge, the internal resistance of the battery increases rapidly, thereby limiting the current and reducing the voltage between the positive and negative electrodes to a safe voltage, realizing the automatic protection function of the battery.
The Basic Requirements for PTC Materials:
- -High conductivity at room temperature, does not affect the electrochemical properties of the battery within the normal operating temperature range；
- -Stable chemical and electrochemical properties, does not react with other components of the battery, and has good compatibility；
- -PTC thermal response speed is fast；
- -The resistance change temperature is appropriate (100~130℃), and the lift-to-resistance ratio is large.
Two Types of PTC used in Lithium Battery
PTC materials used in lithium batteries mainly include two types: one is based on a polymer polymer resin, which forms a composite with a conductive material. The corresponding substrates of this kind of polymer-based PTC are epoxy resin, polymethyl methacrylate, polyethylene-vinyl acetate, polyethylene, etc. ; Conductive agents are carbon black, nano-metal particles and various nano-fiber conductors. The working principle of the above-mentioned PTC material is: good contact between the conductive particles at room temperature, with high electrical conductivity; the expansion of the polymer matrix at high temperature increases the spacing of the conductive particles, the conductive network is destroyed, and the electronic conductivity of the complex drops sharply, showing PTC properties.
The other category is conductive polymers, including alkyl-substituted thiophene, alkylpyrrole, etc. The principle is that the doped conductive polymer has high electrical conductivity at room temperature, and the thermal movement of the polymer chain at high temperature causes the doped ions to be thermally impurity-free, resulting in a significant decrease in the conductivity of the polymer, showing PTC characteristics.
The Differences Between NTC and PTC
1.DIFFERENT RAW MATERIALS
PTC thermistors are resistors made of a mixture of barium titanate, titanium oxide, tantalum, silica and manganese. Within the temperature range, the resistance value increases as the temperature rises and decreases as the temperature decreases.
NTC thermistors are resistors sintered from two or more metal oxides such as manganese, copper, silicon, cobalt, and iron. Within the temperature range, the resistance value decreases as the temperature rises and increases as the temperature decreases.
2. DIFFERENT FUNCTIONS
PTC thermistors can also be used as heating elements. At the same time, they play the role of “switches”. They have three functions: sensitive components, heaters and switches, in addition to being used in industry for temperature measurement and control. They are called “thermal switches.”
NTC thermistors can be used for temperature measurement, temperature control, temperature compensation, etc., and can also be used for liquid level sensing.
It is undeniable that in the future and for some time to come, the safety of batteries will become more serious with the increase of the specific energy of new energy vehicle batteries. Summarizing the battery safety protection methods, it can be seen that positive temperature coefficient electrodes have great application prospects in the prevention of thermal runaway of lithium batteries. In the future battery preparation process streamline, the surface modification of the collector fluid (surface coating, surface roughening, corona treatment, etc. ) and the synthesis of surface PTC coating and other technologies are of great significance to obtain high magnification performance, high temperature resistance, and high cycle life lithium batteries. The research and development of suitable temperature-sensitive electrodes will continue to be a continuous hot spot in the development of lithium battery technology.
I believe that in the future, with the development of temperature sensor detection technology such as PTC thermistors, it will definitely help solve the old difficult problem of overheating protection of automotive lithium batteries. We will wait and see.