1.Possible causes of thermal runaway
Energy storage batteries generally experience thermal runaway under abuse conditions, and common abuse conditions are divided into three categories: mechanical abuse, electrical abuse, and thermal abuse.
The most common occurrence of thermal runaway due to electrical abuse. In abusive working conditions, lithium-ion batteries not only release reaction heat, ohmic heat, and polarization heat, but also include heat released by internal short circuits and heat released by reactions. The heat released by internal short circuits and side reactions (including SEI membrane decomposition, positive and negative electrode electrolyte reaction, membrane dissolution, electrolyte decomposition) is much greater than the heat generated under normal operating conditions, which can lead to a rapid increase in battery temperature and easily lead to uncontrolled heating
(1) Mechanical abuse
The main characteristic of mechanical abuse is the relative displacement of battery cells and modules under external forces. The main forms for battery cells (monomers) include collision, compression, and puncture. At the module (battery pack) level, vibration issues also need to be considered.
In mechanical abuse, the most dangerous is puncture, where the conductor is inserted into the battery body, causing a direct short circuit between the positive and negative poles. Compared to collisions, squeezing, and other situations where internal short circuits only occur with probability, the generation of heat during the puncture process is more intense, resulting in a higher probability of runaway heating
(2) Electricity abuse
Electrical abuse generally includes several forms such as overcharging, over discharge, or external short circuit, and overcharging is the most likely to develop into thermal runaway. Due to the high energy content of batteries, overcharging is the most harmful form of electrical abuse, and the generation of heat and gas are two common characteristics in the overcharging process. Heating comes from Ohmic heat and side reactions.
Firstly, due to excessive lithium insertion, lithium dendrites grow on the anode surface, and the stoichiometric ratio of the cathode to the anode determines when lithium dendrites begin to grow. Secondly, excessive detachment of lithium leads to the collapse of the positive electrode structure due to heating and oxygen release, which accelerates the decomposition of the electrolyte and generates a large amount of gas. Due to the increase in internal force, the safety valve opens, and the battery opens. After the active substance in the battery cell comes into contact with air, it reacts violently and releases a large amount
(3) Heat abuse
Heat abuse rarely exists independently and often develops from mechanical and electrical abuse, and is ultimately a part of contact with heat runaway. Local energy is a typical thermal abuse situation that occurs in battery packs. In addition to overheating caused by mechanical and electrical abuse, it has been confirmed that overheating may also be caused by loose connection contacts
2.Process of thermal runaway
The thermal runaway process of lithium-ion batteries can generally be summarized as follows: ① SEI decomposition; ② Lithium embedded negative electrode reacts with electrolyte; ③ Membrane melting; ④ The positive electrode undergoes a decomposition reaction; ⑤ The electrolyte undergoes a decomposition reaction on its own; ⑥ Electrolyte vaporization and combustion.
1) During the first stage of normal charging, the surface temperature of the battery is relatively low (26-30 ° C). Lithium ions normally detach from the positive electrode and insert into the negative electrode, resulting in a slow increase in battery voltage. When the battery voltage is around 3.6V, the negative electrode of the battery tends to saturate
2) During the second stage of slight overcharging, the surface temperature of the battery significantly increased (39-46 ° C). The positive electrode severely loses lithium, and the lithium ion tends to saturate from being embedded in the negative electrode. Lithium ions will precipitate on the surface of the negative electrode and tend to deposit in the edge area of the negative electrode closer to the positive electrode. Previous studies have shown that lithium dendrites precipitated on the surface of the negative electrode will react with the organic binder of the negative electrode.
Previous studies have shown that lithium dendrites precipitated on the surface of the negative electrode will react with the organic binder of the negative electrode to generate hydrogen lithium metal precipitation and severe lithium removal from the positive electrode, resulting in a continued increase in battery voltage.
3) In the third stage, lithium dendrites undergo a side reaction with the electrolyte to generate heat, leading to an increase in the internal temperature of the battery. When the temperature exceeds 90 ℃, it will trigger the decomposition of the SE film and generate gas
4) In the fourth stage, when the internal temperature of the lithium-ion battery reaches around 130 ℃, the separator melts, causing a large area of short circuit in the battery and generating heat. The high temperature caused by heat accumulation forms a positive feedback on the internal reaction, generating gas, and the battery begins to undergo uncontrollable self accelerating reactions, further causing the temperature of the battery to rise.
In the range of 200~300 ℃, the electrolyte itself will undergo decomposition reactions, producing gas and ultimately leading to fire and even explosion accidents. The harm caused by thermal runaway of a single battery is generally limited, but in the application scenario of energy storage power plants, the number of single batteries is large and arranged tightly. When a single battery experiences thermal runaway, the heat generated may be transmitted to the surrounding batteries, causing the thermal runaway to spread and the harm caused will be expanded.
3.Detect characteristic parameters
1) The internal resistance of the battery decreases with the increase of temperature within the normal operating temperature range. However, when the battery experiences thermal runaway and causes abnormal temperature rise, there is a significant increase in its internal resistance. However, the sudden change in battery internal resistance can also be influenced by other factors, such as external disturbances or poor contact caused by some reasons, which can also lead to a sudden increase in battery internal resistance. Therefore, relying solely on changes in resistance to determine whether a battery has experienced thermal runaway is not accurate, and it needs to be combined with other characteristic parameters to determine.
2) Temperature is an important parameter of thermal runaway in lithium-ion batteries, as there is a mutually reinforcing relationship between temperature and side reactions when the battery experiences thermal runaway, forming a positive feedback. Many battery warning devices and battery management systems are equipped with temperature sensing devices to monitor battery temperature. Once the temperature exceeds the preset threshold, an alarm signal will be issued or corresponding actions will be taken.
A three-level warning strategy has been proposed for the 18650 lithium-ion battery and battery pack: when the battery temperature exceeds 50 ℃, the capacity will decay, and the temperature will rise slowly in the range of 50-80 ℃, with 70-80 ℃ being the slowest. Therefore, the three-level warning temperatures are set to 50 ℃, 70 ℃, and 80 ℃ respectively. However, this method of monitoring surface temperature has hysteresis, as the heat generated internally takes a certain amount of time to transmit to the surface, and there is also heat dissipation during the transmission process (heat exchange between the battery and the environment).
3) When the battery is in the early stage of thermal runaway, these characteristic gases will gradually increase in concentration from scratch, indicating a significant change in characteristics. Therefore, using corresponding gas sensors for early warning of battery thermal runaway is also an important way.