Li-ion batteries are widely used in consumer electronics, power batteries, energy storage and other fields, which is convenient for people’s production and life at the same time, there are certain safety problems, related to li-ion batteries fire, explosion events occur from time to time.
Safety problems are inherent in Li-ion batteries, which themselves are an energy storage device and generate a lot of heat when the energy stored inside is released instantaneously. In addition, due to the high operating voltage, Li-ion batteries are difficult to use inorganic materials, and the use of organic materials system, is one of the main causes of safety problems. The factors that cause Li-ion battery safety problems can be divided into external factors and internal factors. Internal factors are related to the design, material selection and production process of the battery, such as the battery itself due to metal impurities, electrode burr and other reasons to Pierce the diaphragm, resulting in the positive and negative internal short circuit. External factors refer to the fire and explosion caused by the thermal runaway of the internal materials of Li-ion batteries after they are subjected to external stresses such as overcharge, short circuit, vibration and extrusion.
For the safety problems of Li-ion batteries, the safety performance of batteries can be improved by improving the safety of electrolyte, improving the safety of electrode materials and improving the safety protection design of batteries.
Solid state battery
In terms of electrolyte, it is an effective strategy to replace the traditional liquid electrolyte with non-combustible solid electrolyte. One of the most prominent advantages of solid-electrolyte batteries, known as solid-state batteries, over traditional Li-ion batteries is their safety. Solid state battery cathode material is not easy to oxygen evolution, the negative electrode can contain Li metal, is not easy to continue with Li side reaction, is not easy to thermal runaway, not easy to flatulence, high temperature stability is good.
The solid electrolyte’s insulation makes it a good barrier between the positive and negative electrodes of the battery, avoiding the short circuit caused by the contact between the positive and negative electrodes and acting as a diaphragm. The core of solid state battery technology is electrolyte innovation, the ultimate goal is to realize the solid-state electrolyte; With the increasing demand for battery energy density, technical problems are also increasing. Mixed solid-liquid battery can be used as an important transition technology for all-solid-state battery. In the process of technological innovation, the application of liquid electrolyte is gradually reduced, and the goal of liquid electrolyte is gradually realized to semi-solid state, quasi-solid state, and finally all-solid state. At present, semi-solid – state batteries have been gradually mass-produced.
LTO battery
In terms of electrode materials, researchers have pointed out that the positive electrode of Li-ion battery charging state will release oxygen and only generate less heat, that is, the battery will not be ignited by internal short-circuit heat; However, in the presence of a negative electrode, the strongly reducing negative electrode (carbon-based material) will consume the oxygen produced by the positive electrode and generate a lot of heat, eventually causing the entire battery to experience thermal runaway. In other words, the carbon material itself is flammable, coupled with the Li embedded in the carbon material will form the compound LiC6, whose properties are close to the elemental lithium, which can directly spontaneously ignite in the air. Therefore, the safety weakness of Li battery is due to the existence of carbon anode material, which is a safety hazard of the material itself. In view of this, LTO battery attention.
LTO material breaks through the limitation of the two-dimensional layered structure of the traditional graphite anode, and has a stable three-dimensional crystal structure. In the process of charging and discharging, the material structure hardly changes, so it is called “zero-strain material”. Compared with graphite anode material, LTO material almost does not form SEI film with poor thermal stability, which avoids the hidden danger of fire and explosion of battery caused by thermal runaway caused by decomposition of SEI film at high temperature.
LTO material has higher potential to Li metal, which avoids the formation of dendrites in the process of overcharge, and has better safety and stability. At the same time, with three-dimensional Li ion transmission channel, high rate of charge and discharge can be achieved in the extremely low temperature range of -50℃ to 60℃ ultra-high temperature range.
Semisolid vs. LTO
For the safety performance of semi-solid battery and LTO battery, some researchers have carried out safety tests. According to relevant standards, the researchers carried out acupuncture test, limit overcharge test, short circuit test and limit heating test on the two kinds of batteries respectively, and evaluated the application of the two kinds of batteries in rail transit based on the test results.
- Performance parameters of the two batteries
- Test Results of Semisolidvs. LTO
- Acupuncture experiment
Through acupuncture test, there was only a little pressure drop and a certain temperature rise in the two groups of test subjects, but no obvious phenomenon
- Limit overcharge test
In the ultimate overcharge test, the lithium titanate battery caught fire, and the semi-solid lithium iron phosphate battery deflated and overcharged to 19V after a period of bulging. According to the test curve, the overcharge duration of the semi-solid li-iron phosphate battery is only 10min, and the SOC is 117% when the overcharge fault occurs. The overcharge duration of the LTO battery is 48 minutes, and the SOC is 218% when the overcharge fault occurs. Before the fire, the temperature rise rate of the LTO battery was lower than that of the semi-solid li-iron phosphate battery, and the temperature suddenly changed at the moment of fire. Both kinds of batteries can meet the requirements of relevant standards, and the overcharge capacity (overcharge tolerance time) of LTO battery is much higher than that of semi-solid li-iron phosphate battery.
- Short circuit experiment
In the short-circuit test, the maximum short-circuit current of both kinds of batteries is greater than 800A, and the maximum temperature rise is about 100K. The semi-solid lithium iron phosphate battery has obvious bulge phenomenon, while the lithium titanate battery has no obvious phenomenon.
- Limit heating experiment
During the limit heating test, the temperature of the two groups of batteries suddenly changed below 200℃, and the voltage quickly decreased to 0V, accompanied by a relatively violent smoking phenomenon. The overtemperature capacity of the two samples is much higher than the standard 130℃, which has high safety.
The above is an introduction to the safety of semi-solid battery and LTO battery. Battery safety is no small matter, and it is also a comprehensive issue, not determined by a single factor. Lithium safety needs to be improved from technology, policy, industry, market and other aspects, not overnight work, the future lithium safety has a long way to go.