The safety problem of lithium-ion batteries is a dark cloud that cannot be erased, but flow batteries have attracted more and more attention because of their high capacity and excellent safety characteristics. At present, both 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 batteries in power batteries has made many people think about the possibility and prospects of their application in energy storage batteries, but the safety of lithium ion batteries has always been a key point of concern for the industry, especially in recent years in the world The news of lithium battery fires and explosions reported one after another in the scope has increased the public's safety considerations for the wide application of lithium-ion batteries in the field of energy storage. The following table shows some relevant news of lithium-ion battery explosions in 2019. At present, the thermal runaway problem of lithium-ion batteries has also become a major factor affecting the promotion of lithium-ion batteries and new energy vehicles.
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 intercalation and deintercalation. During charging, lithium ions are extracted from the lattice of the positive electrode material, reach the negative electrode material through the electrolyte, and are embedded in the lattice; while during discharge, after the lithium ions in the negative electrode material are deintercalated, they are re-embedded into the positive electrode material through the electrolyte. , and this reversible intercalation and deintercalation process constitutes the charge-discharge process of Li-ion batteries.
At present, the mainstream lithium-ion batteries in the market are ternary lithium batteries (such as nickel cobalt lithium manganate batteries) and lithium iron phosphate batteries, and a considerable number of batteries use lithium cobalt oxide and lithium manganate as positive electrode materials. Among them, ternary lithium batteries are the most widely used in the field of high-power electric vehicles due to their excellent cost advantages and capacity advantages, as well as better cycle performance. But at the same time, the thermal runaway defect of ternary lithium battery is also more obvious.
The reasons for the thermal runaway of lithium-ion batteries are more complicated. The safety problem is not only related to the heat dissipation design of the battery itself, but also to the properties of the materials used in the battery. The heat dissipation rate inside the battery under certain conditions is less than the heat generation rate. This leads to the accumulation of heat. If the safety problem cannot be effectively solved, the application in the field of energy storage will be quite limited. At present, relevant research believes that the thermal runaway of lithium batteries may be caused by the continuous release of heat from the chemical reaction between the battery electrolyte and the negative electrode material when the lithium ion battery is overcharged, externally fast charged, or subjected to external abuse such as squeezing and collision, or An internal short circuit in the battery creates a massive exothermic condition that can lead to thermal runaway of the battery, which in turn creates a risk of fire and explosion.
In the lithium battery system, the heat source can be 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 phenomenon of battery polarization. The heat generated by the side reaction is the heat release of the thermal decomposition of the positive electrode active material, the negative electrode active material, the electrolyte and the solid electrolyte interface film at different temperatures, as well as the reaction heat release of the metal lithium and the substances in the battery.
This risk of detonation is inseparable from the properties of many materials and structures in lithium-ion batteries. First of all, the electrolyte in lithium-ion batteries is mostly flammable low-melting organic lipids. This organic electrolyte is flammable at high temperatures. In the case of excessive heat release or runaway thermal management of the lithium-ion battery system, it is equivalent to battery explosion. "A big important fuel" for the accident. Secondly, graphite electrodes are mostly used as negative electrodes of lithium ion batteries. The flammable characteristics of graphite also make the safety problems of lithium ion batteries more and more strictly controlled. Of course, silicon anode materials have also been developed at present, but since the silicon anode will expand by more than 300% during the charging and discharging process, this huge expansion is unacceptable for a closed battery system, and will undoubtedly lead to the collapse of the entire battery structure and the battery. of failure.
In addition, current research focuses on replacing the graphite anode with metallic lithium to improve its energy density. However, the problem of the continuous growth of lithium dendrites during the cycling of lithium-ion batteries is still being solved. At present, in the battery system, the growth of lithium dendrites is unavoidable. We can only try to slow down its growth and prolong its service life. When the lithium dendrites grow enough to pierce the separator, the thermal management caused by the short circuit of the positive and negative electrodes is out of control. And the exothermic reaction of the lithium metal negative electrode is accompanied by a greater risk of explosion.
From a probability point of view, if the capacity of a single lithium battery cell is 1KWh, and the risk of explosion is one in a million, then a 1GWh energy storage station will have 1 million cells, then in the whole life cycle The risk of explosion will reach 63%, and the cumulative risk of 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, without the risk of explosion or fire, and the uniformity of flow batteries is good. Taking the all-vanadium redox flow battery that has been successfully applied as an example, the system uses the reversible change between vanadium ions of different valence states to realize the charging and discharging of the battery, thereby achieving the purpose of mutual conversion between chemical energy and electrical energy. The vanadium-containing ions in the all-vanadium redox flow battery are stored in an aqueous solution, and the electrolyte is an aqueous solution of dilute sulfuric acid and vanadium, which is completely different from the low-melting flammable organic solvents used in lithium-ion batteries. Compared with lithium-ion batteries, flow batteries can greatly reduce the risk of batteries overheating and causing explosions. Relevant people also said that as long as they are properly managed, there is almost no danger of explosion of all-vanadium redox flow batteries.
Professor Wang Baoguo of Tsinghua University pointed out that in the development of energy storage technology, safety is the first priority, and it is necessary to give priority to technologies with safer characteristics to develop energy storage, and then consider resource issues, environmental issues, and social and economic benefits. question. In the traditional battery system, although the chemical composition of the solid material constituting the electrode does not change after each charge and discharge cycle, the internal structure of the battery electrode may have changed, and the continuous change of this structure is inevitable. It can affect the performance of the battery, and may even cause a safety accident. In a flow battery, the chemical reaction in the battery is carried out in a solution, and the solid electrode is only responsible for the conduction of current, and is less affected by various side reactions. As a result, flow batteries can often withstand more charge-discharge cycles than conventional batteries without substantially compromising performance.
The high reversibility and low polarizability of vanadium ions in electrochemical reactions, its excellent charge-discharge characteristics, fast response speed of charge-discharge switching, and small performance degradation determine its application space and potential in the field of energy storage. very broad. And in recent years, with the continuous development and application of flow battery technology, its cost has also dropped significantly in recent years. It is predicted that the installed cost is expected to be reduced to 2,000 yuan/KWh by 2025. All in all, considering the safety of energy storage, flow battery energy storage technology may be a better choice than lithium battery energy storage.
