In recent years, numerous accidents have been caused by battery safety issues, with many resulting in alarming consequences. Examples include the shocking fires involving lithium batteries in the Boeing 787 "Dreamliner" and the widespread battery fires and explosions in Samsung mobile phones, all of which have served as stark reminders of the safety risks associated with lithium batteries. The safety of energy storage batteries is also a crucial indicator in the energy storage field. This article will discuss issues related to lithium battery safety and testing.
I. Composition and Working Principle of Lithium Batteries
A lithium battery is mainly composed of a positive electrode, a negative electrode, an electrolyte, a separator, and external connecting and packaging components. The positive and negative electrodes contain active electrode materials, conductive agents, binders, etc., which are uniformly coated on copper foil and aluminum foil current collectors.
Lithium-ion batteries have a high positive electrode potential and are often lithium-intercalated transition metal oxides or polyanionic compounds, such as lithium cobalt oxide, lithium manganese oxide, ternary lithium, and lithium iron phosphate. The negative electrode material of lithium-ion batteries is usually a carbon material, such as graphite and non-graphitized carbon. The electrolyte of lithium-ion batteries is mainly a non-aqueous solution, composed of a mixture of organic solvents and lithium salts. The solvent is mostly an organic solvent such as carbonic acid, and the lithium salt is mostly a monovalent polyanionic lithium salt, such as lithium hexafluorophosphate. The separator of lithium-ion batteries is mostly a polyethylene or polypropylene microporous membrane, which serves to isolate the positive and negative electrode materials, prevent electrons from passing through and causing short circuits, and at the same time allow ions in the electrolyte to pass through.
During charging, inside the battery, lithium ions are released from the positive electrode, transported by the electrolyte through the separator, and inserted into the negative electrode; outside the battery, electrons migrate from the external circuit to the negative electrode. During discharging, lithium ions are released from the negative electrode, pass through the separator, and are inserted into the positive electrode; outside the battery, electrons migrate from the external circuit to the positive electrode. Because it is lithium ions, not elemental lithium, that migrate between the cells during charging and discharging, the battery is called a "lithium battery."
II. Safety Hazards of Lithium Batteries
Generally, safety issues with lithium batteries manifest as combustion or even explosion. The root cause of these problems lies in thermal runaway within the battery. In addition, external factors such as overcharging, fire sources, compression, puncture, and short circuits can also lead to safety problems. Lithium batteries generate heat during charging and discharging. If the heat exceeds the battery's heat dissipation capacity, the battery will overheat, leading to destructive side reactions such as the decomposition of the SEI film, electrolyte decomposition, positive electrode decomposition, reactions between the negative electrode and the electrolyte, and reactions between the negative electrode and the binder.
1. Safety hazards of cathode materials
When lithium batteries are used improperly, the internal temperature rises, causing the active material in the positive electrode to decompose and the electrolyte to oxidize. These two reactions generate significant heat, further increasing the battery temperature. Different delithiation states have vastly different effects on the lattice transition of the active material, decomposition temperature, and the battery's thermal stability.
2. Safety hazards of negative electrode materials
Early lithium-ion batteries used lithium metal as the anode material. After repeated charge-discharge cycles, lithium dendrites easily formed in these batteries, which could puncture the separator, leading to short circuits, leakage, and even explosions. Lithium-intercalated compounds effectively prevent the formation of lithium dendrites, significantly improving the safety of lithium batteries. As temperature increases, the lithium-intercalated carbon anode first undergoes an exothermic reaction with the electrolyte. Under the same charge-discharge conditions, the exothermic reaction rate between the electrolyte and lithium-intercalated artificial graphite is much greater than the exothermic reaction rate with lithium-intercalated mesophase carbon microspheres, carbon fibers, coke, etc.
3. Safety hazards of the diaphragm and electrolyte
The electrolyte in lithium-ion batteries is a mixture of lithium salt and organic solvent. Commercially available lithium salt is lithium hexafluorophosphate (LiPF6), which is prone to thermal decomposition at high temperatures and undergoes thermochemical reactions with trace amounts of water and organic solvents, reducing the electrolyte's thermal stability. The organic solvent in the electrolyte is a carbonate, which has low boiling and flash points and readily reacts with the lithium salt to release PF5 at high temperatures, making it easily oxidized.
4. Safety hazards in the manufacturing process
During the manufacturing process of lithium batteries, processes such as electrode fabrication and battery assembly can all affect battery safety. Quality control in steps such as mixing positive and negative electrodes, coating, rolling, cutting or punching, assembly, electrolyte addition, sealing, and formation all impact battery performance and safety. The uniformity of the slurry determines the uniformity of the distribution of active materials on the electrodes, thus affecting battery safety. If the slurry fineness is too large, significant expansion and contraction of the negative electrode material will occur during charging and discharging, potentially leading to lithium metal precipitation; if the slurry fineness is too small, it will result in excessive internal resistance. Insufficient coating temperature or drying time can leave solvent residue and partial dissolution of the binder, causing some active materials to easily peel off; excessively high temperatures may cause binder carbonization, leading to active material detachment and internal short circuits within the battery.
5. Safety hazards during battery use
Lithium batteries should be used in a way that minimizes overcharging or over-discharging, especially for batteries with high single-cell capacity, as thermal disturbances may trigger a series of exothermic side reactions, leading to safety issues.
III. Lithium Battery Safety Testing Indicators
After lithium batteries are manufactured, they undergo a series of tests before reaching consumers to ensure battery safety and reduce potential safety hazards.
1. Compression test:
Place a fully charged battery on a flat surface, apply a pressure of 13±1KN by a hydraulic cylinder, and then press the battery with a flat steel rod with a diameter of 32mm. Once the pressure reaches its maximum, stop pressing. The battery will not catch fire or explode.
2. Impact test:
After the battery is fully charged, place it on a flat surface. Place a 15.8mm diameter steel column vertically in the center of the battery, and then drop a 9.1kg weight from a height of 610mm onto the steel column above the battery. The battery should not catch fire or explode.
3. Overcharge test:
Fully charge the battery with 1C, then conduct an overcharge test according to 3C overcharge at 10V. When the battery is overcharged, the voltage rises to a certain level and stabilizes for a period of time. When it approaches a certain time, the battery voltage rises rapidly. When it rises to a certain limit, the battery cap breaks off and the voltage drops to 0V. As long as the battery does not catch fire or explode, it is considered a good battery.
4. Short circuit test:
After fully charging the battery, short-circuit the positive and negative terminals of the battery with a wire with a resistance of no more than 50mΩ and test the change in surface temperature of the battery. The highest surface temperature of the battery is 140℃. When the battery cap is opened, the battery does not catch fire or explode.
5. Needle prick test:
Place a fully charged battery on a flat surface and pierce it radially with a 3mm diameter steel needle. Test that the battery does not catch fire or explode.
6. Temperature cycling test:
Lithium-ion battery temperature cycling tests are used to simulate the safety of lithium-ion batteries when repeatedly exposed to low and high temperature environments during transportation or storage. The test utilizes rapid and extreme temperature changes. After the test, the sample should not catch fire, explode, or leak.
IV. Solutions to Lithium Battery Safety Issues
Given the numerous safety hazards associated with lithium batteries in their materials, manufacturing, and usage processes, improving the parts prone to safety issues is a problem that lithium battery manufacturers need to solve.
1. Improve the safety of the electrolyte.
The electrolyte exhibits high reactivity with both the positive and negative electrodes, especially at high temperatures. Therefore, improving the safety of the electrolyte is one of the most effective methods to enhance battery safety. Adding functional additives, using novel lithium salts, and employing novel solvents can effectively address the safety hazards associated with the electrolyte.
Based on their different functions, additives can be mainly divided into the following categories: safety protection additives, film-forming additives, positive electrode protection additives, lithium salt stabilizing additives, lithium precipitation promoting additives, current collector corrosion prevention additives, and wettability enhancement additives.
To improve the performance of commercial lithium salts, researchers have performed atomic substitution, yielding many derivatives. Among these, compounds obtained by substituting atoms with perfluoroalkyl groups possess numerous advantages, including high flash point, similar conductivity, and enhanced water resistance, making them a promising class of lithium salt compounds. Furthermore, anionic lithium salts chelated with boron atoms as the central atom and oxygen ligands exhibit high thermal stability.
Regarding solvents, many researchers have proposed a series of novel organic solvents, such as carboxylic acid esters and organic ethers. Additionally, ionic liquids also constitute a class of highly safe electrolytes; however, compared to commonly used carbonate electrolytes, ionic liquids have viscosity orders of magnitude higher, and lower conductivity and ion self-diffusion coefficients, meaning much work remains before practical application.
2. Improve the safety of electrode materials
Lithium iron phosphate and ternary composite materials are considered low-cost and "safe" cathode materials, and have the potential for widespread application in the electric vehicle industry. Regarding cathode materials, a common method to improve their safety is coating modification, such as coating the cathode material with metal oxides. This can prevent direct contact between the cathode material and the electrolyte, suppress phase transitions in the cathode material, improve its structural stability, and reduce the disorder of cations in the crystal lattice, thereby reducing heat generation from side reactions.
Regarding anode materials, since their surface is often the part of a lithium battery most prone to thermochemical decomposition and heat release, improving the thermal stability of the SEI film is a key method to enhance the safety of anode materials. The thermal stability of anode materials can be improved through weak oxidation, metal and metal oxide deposition, polymer or carbon coating.
3. Improve battery safety protection design
In addition to improving the safety of battery materials, many safety protection measures adopted in commercial lithium batteries, such as setting up battery safety valves, thermal fuses, connecting components with positive temperature coefficients in series, using heat-sealed diaphragms, loading dedicated protection circuits, and dedicated battery management systems, are also means to enhance safety.
V. Lithium Battery Testing Service Providers
In recent years, the lithium battery performance and safety testing industry has become one of the fastest-growing industries globally, with an annual growth rate of around 20%. my country's testing industry has reached a scale of nearly 100 billion RMB, with an average annual growth rate of around 25%. Currently, nearly 700 laboratories have obtained CNAS and CMA accreditations through the Jiayu Testing Network.
VI. Suppliers of Lithium Battery Safety Solutions
As the safety of lithium batteries has become an increasingly important concern, many companies have been conducting research and development specifically targeting the safety hazards in lithium batteries, and have proposed effective battery safety solutions.
