The safety issue of lithium-ion batteries is a dark cloud that cannot be erased, while flow batteries are receiving increasing attention due to their high capacity and excellent safety. At present, lithium-ion batteries and all vanadium flow battery energy storage stations in the energy storage industry have entered the stage of commercial operation. The excellent performance of lithium-ion batteries in power batteries has led many people to think about the possibility and prospects of their application in energy storage batteries. However, the safety of lithium-ion batteries has always been a key concern in the industry, especially in recent years, the continuous reports of lithium-ion battery fires and explosions worldwide have increased public safety considerations for the widespread application of lithium-ion batteries in the energy storage field. The table below shows some news related to lithium-ion battery explosions in 2019. At present, the thermal runaway problem of lithium-ion batteries has also become the main factor affecting the promotion of lithium-ion batteries and new energy vehicles.
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News related to fires and explosions of lithium-ion batteries in 2019 [1]
As shown in the figure below (taking LiMO2 [2] as an example), the working principle of lithium-ion batteries can be summarized as the charging and discharging process of lithium ions shuttle between the positive and negative electrodes and are embedded and removed. During charging, lithium ions detach from the lattice of the positive electrode material, pass through the electrolyte, reach the negative electrode material, and embed into its lattice; During discharge, the lithium ions in the negative electrode material are deintercaled and then re embedded into the positive electrode material through the electrolyte. This reversible insertion and deintercalation process constitutes the charging and discharging process of lithium-ion batteries.
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At present, the mainstream lithium-ion batteries in the market are ternary lithium batteries (such as nickel cobalt manganese oxide lithium batteries) and lithium iron phosphate batteries, and a considerable number of batteries use cobalt oxide lithium and manganese oxide lithium as positive electrode materials. Among them, ternary lithium batteries are most widely used in the field of high-power electric vehicles due to their excellent cost and capacity advantages, as well as better cycling performance. However, at the same time, the thermal runaway defect of ternary lithium batteries is also more obvious.
The reasons for thermal runaway of lithium-ion batteries are complex, and their safety issues are not only related to the heat dissipation design of the battery itself, but also closely related to the properties of the materials used in the battery. The heat dissipation rate inside the battery under specific conditions is lower than the heat generation rate, resulting in heat accumulation. If the safety issues cannot be effectively solved, its application in the field of energy storage will be quite limited. At present, relevant research suggests that the possible reason for thermal runaway of lithium-ion batteries is that the chemical reaction between the battery electrolyte and negative electrode material continuously releases heat due to external abuse such as overcharging, rapid external charging, or squeezing and collision, or the internal short circuit of the battery causes a large amount of heat release, leading to thermal runaway of the battery and the risk of combustion and explosion.
In the lithium battery system, the heat source can be specifically divided into reversible reaction heat generation, irreversible reaction heat generation, and side reaction heat generation. The heat generation of reversible and irreversible reactions mainly depends on the battery reaction and the polarization phenomenon of the battery. The side reaction heat generation is the thermal decomposition exothermic of positive active material, negative active material, electrolyte and solid electrolyte interface facial mask at different temperatures, as well as the reaction exothermic of metal lithium with the materials in the battery.
This risk of explosion is closely related to the characteristics of many materials and structures in lithium-ion batteries. Firstly, the electrolyte in lithium-ion batteries is mostly combustible low melting point organic lipids, which are flammable at high temperatures. In the case of excessive heat release or uncontrolled thermal management in lithium-ion battery systems, they are equivalent to "a major important fuel" for battery explosion accidents. Secondly, graphite electrodes are commonly used as negative electrodes in lithium-ion batteries, and the combustible properties of graphite make the safety issues of lithium-ion batteries increasingly require strict control. Of course, currently there are also developments in silicon negative electrode materials, but due to the volume expansion of silicon negative electrodes exceeding 300% during the charging and discharging process, this huge expansion is unacceptable for sealed battery systems, which undoubtedly leads to the collapse of the entire battery structure and the failure of the battery.
In addition, current research focuses on replacing graphite negative electrodes with metallic lithium to improve their energy density. However, the problem of continuous growth of lithium dendrites during the cycling process of lithium-ion batteries is still being further resolved. At present, the growth of lithium dendrites is unavoidable in the battery system, and can only be slowed down as much as possible to extend its service life. When the lithium dendrites grow enough to penetrate the separator, causing a short circuit between the positive and negative electrodes, thermal management loss of control, and the exothermic reaction of the metal lithium negative electrode, accompanied by a greater risk of combustion and explosion.
In terms of probability, if the capacity of a single lithium battery cell is 1KWh and the risk of explosion is one in a million, then there will be one million cells in a 1GWh energy storage station, and the risk of explosion throughout the entire life cycle will reach 63%. The cumulative risk of explosion for a 5GWh energy storage station will reach 99.3%.
Unlike lithium batteries, flow batteries have excellent safety. The energy storage medium of flow batteries is aqueous solution, which is safer and more reliable. There is no risk of explosion or fire, and the uniformity of flow batteries is good. Taking the successful application of all vanadium flow batteries as an example, this system utilizes reversible changes between vanadium ions of different valence states to achieve the charging and discharging of the battery, thereby achieving the goal of mutual conversion between chemical energy and electrical energy. The vanadium ions in all vanadium flow batteries are stored in aqueous solutions, and their electrolyte is a dilute sulfuric acid and vanadium aqueous solution, which is completely different from the low melting point flammable organic solvents used in lithium-ion batteries. This characteristic allows all vanadium flow batteries to significantly reduce the risk of overheating and explosion compared to lithium-ion batteries. Relevant personnel also stated that as long as managed properly, there is almost no risk of explosion in all vanadium flow batteries.
Professor Wang Baoguo from Tsinghua University pointed out that safety is the top priority in the development of energy storage technology. Priority should be given to selecting technologies with safer characteristics for energy storage development, followed by considerations of resource, environmental, and socio-economic benefits. In traditional battery systems, although the chemical composition of the solid materials that make up the electrodes does not change after each charge and discharge cycle, the internal structure of the battery electrodes may have changed. The continuous changes in this structure will inevitably affect the performance of the battery and may even cause safety accidents. In liquid flow batteries, chemical reactions occur in the solution, while solid electrodes are only responsible for conducting current and are less affected by various side reactions. Therefore, flow batteries can often withstand more charging and discharging cycles than traditional batteries while maintaining performance that is basically unaffected.
The high reversibility and low polarization of vanadium ions in electrochemical reactions result in excellent charge and discharge characteristics, as well as fast response speed and low performance degradation during charge and discharge switching, which determines their broad application space and potential in the field of energy storage. And in recent years, with the continuous development and application of flow battery technology, its cost has also significantly decreased. It is predicted that the installation cost is expected to decrease to 2000 yuan/KWh by 2025. In summary, from the perspective of energy storage safety, flow battery energy storage technology may be a better choice compared to lithium battery energy storage.
[1] Bao Yizheng Research on combustion and explosion suppression technology for automotive lithium batteries [D]. Xi'an University of Architecture and Technology, 2021
[2] Wandt, Johannes. (2017) Operando Characterization of Fundamental Reaction Mechanisms and Degradation Processes in Lithium-Ion and Lithium-Oxygen Batteries.
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