There are two main reasons for thermal runaway in energy storage lithium-ion batteries: one is external factors, as energy storage power stations are enclosed spaces that store a large amount of energy. During charging and discharging, electrochemical reactions release heat, which inherently poses a potential risk of thermal runaway; the other is internal factors, as side reactions triggered by the electrochemical reactions in the lithium-ion battery electrolyte can easily lead to thermal runaway.
To address the risk of thermal runaway in energy storage power stations, the industry has proposed solutions from three aspects: modification of lithium-ion battery materials, active safety protection of lithium-ion power stations, and passive safety protection.
Modification of lithium-ion battery materials mainly focuses on three aspects: overcharge protection agents, lithium-ion battery cathode materials, and lithium-ion battery anode materials.
Battery overcharge protector
Overcharging is one of the unavoidable misuses of energy storage lithium-ion batteries. This phenomenon can be effectively avoided by adding an overcharge protectant to the battery electrolyte.
There are two main types of overcharge protectants: redox shuttle additives and overcharge shut-off additives.
Redox shuttle additives can be reversibly oxidized/reduced between electrodes at a specific voltage slightly higher than the end-of-charge voltage, providing overcharge protection; at lower or normal voltages, their molecules are inactive and do not interfere with the internal chemical reactions of the battery.
Currently, typical redox shuttle additives include phenothiazine, triphenylamine, organometallocene, dimethoxybenzene, and their derivatives.
Overcharge protection additives are irreversible additives; once triggered at higher voltages, they permanently stop the battery from operating. Their main drawback is that they cause irreversible oxidation of lithium-ion batteries, thus shortening battery life.
Currently, typical overcharge shut-off additives include xylene, cyclohexylbenzene, biphenyl, 3-thiophene acetonitrile, and 2,2-diphenylpropane.
Modification of lithium battery cathode materials
There are two main technologies for improving the thermal performance of lithium-ion battery cathode materials: element substitution and protective coating.
Element substitution techniques can stabilize crystal structures and effectively improve the thermal properties of layered oxide materials, such as replacing transition metals Co, Ni, and Mn with Al. Doping lithium cobalt oxide with alloying elements such as nickel and manganese can significantly increase the initial decomposition temperature of the cathode, preventing harmful reactions at high temperatures.
The protective coating mainly refers to applying a thin layer of lithium-ion conductive compound as a protective layer to the emergency material of lithium-ion batteries, so that the cathode surface does not come into direct contact with the electrolyte, thereby avoiding side reactions, phase transitions, etc., thus improving structural stability and reducing the disorder of cations in crystal sites.
In addition, the cathode coating material is generally a thermally inert material, which not only adds a protective layer to the cathode but also helps to reduce its heat generation.
Modification of lithium battery anode materials
A key area of research for improving lithium-ion battery anode materials is the development of artificial SEI films to mitigate the electrochemical reaction between the SEI film and the electrolyte, thereby improving its thermal performance. Three main technologies are involved: mild oxidation, metal deposition, and polymer coating.
Researchers have found that, compared to uncoated graphite anodes, graphite anodes with aluminum fluoride coatings have higher initial discharge capacity, longer cycle life, and better capacity retention and rate performance.
