The safety of lithium batteries and lead-acid batteries has always been a point of controversy among users. Some people say that lithium batteries are safer than lead-acid batteries, but others think the opposite. From the perspective of battery structure, the current lithium battery packs are basically 18650 batteries for packaging, and lead-acid batteries are basically maintenance-free lead-acid batteries with good sealing performance, and the risk factors of the two are basically the same. Who is safer, just look down and you will know!
Lithium batteries are a type of batteries that use lithium metal or lithium alloy as the negative electrode material and use a non-aqueous electrolyte solution. Lithium batteries can be roughly divided into two categories: lithium metal batteries and lithium-ion batteries. In 1912, lithium metal batteries were first proposed and studied by Gilbert N. Lewis. Due to the very active chemical properties of lithium metal, the processing, storage, and use of lithium metal have very high environmental requirements. Therefore, lithium batteries have not been used for a long time. With the development of science and technology, lithium batteries have now become mainstream.
Lead-acid battery (VRLA) is a storage battery whose electrodes are mainly made of lead and its oxides, and whose electrolyte is a sulfuric acid solution. In the discharge state of a lead-acid battery, the main component of the positive electrode is lead dioxide, and the main component of the negative electrode is lead; in the charged state, the main components of the positive and negative electrodes are lead sulfate.
The nominal voltage of a single-cell lead-acid battery is 2.0V, which can be discharged to 1.5V and can be charged to 2.4V. In applications, 6 single-cell lead-acid batteries are often used in series to form a 12V lead-acid battery. There are also 24V, 36V, 48V and so on.
Which is safer, lithium battery or lead-acid battery?
From the perspective of battery safety protection, safety valves are designed on the 18650 cells, which can not only release excessive internal pressure, but also physically disconnect the battery from the external circuit, which is equivalent to physically isolating the cell to ensure that The safety of other battery cells in the battery pack. In addition, lithium battery packs are usually equipped with BMS protection boards, which can accurately control the status of each cell in the battery pack, and directly solve the problem of overcharge and overdischarge from the root cause.
Lithium battery BMS battery management system can provide full protection to the battery, functions include: charge / discharge high and low temperature protection; single cell overcharge / overdischarge voltage protection; charge / discharge overcurrent protection; cell balance; short circuit protection; Reminders and more.
The electrolyte of the lithium battery pack is a mixed solution of a lithium salt and an organic solvent, of which the commercially available lithium salt is lithium hexafluorophosphate. This material is prone to thermal decomposition at high temperatures and undergoes a thermochemical reaction with trace amounts of water and organic solvents to reduce Thermal stability of the electrolyte.
The power lithium battery mainly uses lithium iron phosphate. The PO bond in the lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperature or overcharge, it will not collapse and generate heat or form strong oxidizing substances like lithium cobaltate. Good security. It is reported that in actual operation, a small number of samples were found to burn during acupuncture or short-circuit experiments, but no explosion event occurred. The safety of lithium battery packs has been greatly improved.
In contrast, lead-acid batteries lack the protection of a BMS system. Lead-acid batteries seem to be lacking in safety protection except for safety valves. BMS protection is almost non-existent. Many inferior chargers can not even power off after being fully charged. Safety protection is far from lithium batteries. Coupled with a low-quality charger, it is good for you to be in good condition.
Spontaneous combustion explosions in electric vehicles often occur, most of which are caused by battery charging and discharging. Some experts have explained that lead-acid batteries take too long to charge, and when they are charged to the end, after the two poles are converted into effective substances, if they continue to charge, a large amount of electricity will be generated. Hydrogen, oxygen gas. When the concentration of this mixed gas accounts for 4% in the air, it is too late to escape. If the exhaust hole is blocked or there is too much gas, it will explode when it encounters an open flame. It will damage the battery in light, and hurt people and damage in serious cases. That is, once the lead-acid battery is overcharged, it will increase the chance of explosion. At present, the lead-acid batteries on the market have not done any "overcharge protection", which makes the lead-acid batteries in the charging, especially at the end of charging, extremely dangerous.
Finally, if the battery structure is damaged due to accidental collision, lead-acid batteries seem safer than lithium batteries. However, in this level of accident, the battery material has already been exposed to the open environment, and the explosion is impossible to talk about.
From the above safety hazards of lead-acid batteries and lithium iron phosphate batteries, it can be seen that the biggest safety hazard of lead-acid batteries lies in their constituent materials. The electrodes of lead-acid batteries are mainly made of lead and its oxides, and the electrolyte is a sulfuric acid solution. The stability of these constituent materials is not very high. If a leakage or explosion accident occurs, the harm caused will be much higher than that of lithium batteries.
Why does a lithium battery explode?
1. The water content is too high
Moisture can react with the electrolyte in the lithium battery to produce gas. When charging, it can react with the generated lithium to generate lithium oxide, which causes the capacity of the battery to be lost. It is easy to overcharge the battery to generate gas, and the water is decomposed. The voltage is low, and it is easy to decompose and generate gas when charging. When this series of generated gas will increase the internal pressure of the battery cell, the lithium battery will explode when the battery cell shell cannot bear it.
2. Internal short circuit
Due to the internal short-circuit phenomenon, the cell discharges at a large current, which generates a lot of heat and burns the diaphragm, resulting in a larger short-circuit phenomenon. This will cause the cell to generate high temperatures, which will cause the electrolyte to decompose into gas, causing excessive internal pressure. When the battery's case cannot withstand this pressure, the battery will explode.
When the cell is overcharged, the excessive discharge of lithium from the positive electrode will change the structure of the positive electrode, and too much discharged lithium will not easily be inserted into the negative electrode, and it will also easily cause lithium precipitation on the negative electrode surface. When the voltage reaches 4.5V or more, The electrolyte decomposes to produce a large amount of gas. All of the above may cause explosions.
4. External short
The external short circuit may be caused by improper operation or misuse. Due to the external short circuit, the battery discharge current is very large, which will cause the battery to heat up, and the high temperature will cause the internal diaphragm of the battery to contract or completely break, causing the internal short circuit of the lithium battery. So exploded.
From the perspective of battery safety and redundancy design, qualified lithium batteries and lead-acid batteries can fully ensure the safety of users, and there is no obvious safety difference. Is lithium battery or lead acid battery safer? At this stage, the safety factor of lithium batteries is still higher.