The safety and solution of lithium battery
With the popularization of mobile phones, digital products, and electric vehicles, lithium-ion batteries are playing an increasingly important role in people's lives. Use problems such as low energy density and limited cycle life are often criticized. However, compared with these problems, the safety of lithium batteries is the focus of attention.
In recent years, accidents caused by battery safety issues abound, and the consequences of many problems are shocking, such as the fire incident on the lithium battery of the Boeing 787 Dreamliner that shocked the industry, and the large-scale battery fire and explosion incident on the Samsung Galaxy Note 7. The safety of lithium-ion batteries once again sounded the alarm.
The composition and working principle of a lithium-ion battery
Lithium-ion batteries are mainly composed of positive electrode, negative electrode, electrolyte, separator, external connection and packaging components. Among them, the positive electrode and the negative electrode contain active electrode materials, conductive agents, binders, etc., which are uniformly coated on the copper foil and aluminum foil current collectors.
The positive electrode potential of lithium-ion batteries is relatively high, often lithium-intercalated transition metal oxides, or polyanionic compounds, such as lithium cobaltate, lithium manganate, ternary, lithium iron phosphate, etc.; lithium-ion battery negative materials are usually carbon materials , Such as graphite and non-graphitized carbon; lithium ion battery electrolyte is mainly non-aqueous solution, composed of organic mixed solvent and lithium salt, the solvent is mostly organic solvent such as carbonic acid, and the lithium salt is mostly monovalent polyanionic lithium salt, such as Lithium hexafluorophosphate, etc.; lithium ion battery separators are mostly polyethylene and polypropylene microporous membranes, which isolate the positive and negative materials, prevent short circuits caused by the passage of electrons, and allow ions in the electrolyte to pass through.
During the charging process, inside the battery, lithium is extracted from the positive electrode in the form of ions, transported by the electrolyte through the diaphragm, and embedded in the negative electrode; outside the battery, electrons migrate from the external circuit to the negative electrode. In the discharge process: lithium ions inside the battery are extracted from the negative electrode, pass through the diaphragm, and are embedded in the positive electrode; outside the battery, electrons migrate from the external circuit to the positive electrode. With charging and discharging, it is "lithium ion" that migrates between the batteries instead of the elemental "lithium", so the battery is called "lithium ion battery".
Second, the safety hazards of lithium-ion batteries
Generally speaking, the safety problems of lithium-ion batteries manifest themselves as burning or even explosion. The root cause of these problems is the thermal runaway inside the battery. In addition, some external factors, such as overcharge, fire, squeeze, puncture, and short circuit Other issues can also lead to security issues. Lithium-ion batteries will generate heat during charging and discharging. If the heat generated exceeds the battery’s heat dissipation capacity, the lithium-ion battery will overheat, and the battery material will decompose the SEI film, electrolyte decomposition, positive electrode decomposition, negative electrode and Destructive side reactions such as the reaction of the electrolyte and the reaction of the negative electrode and the binder.
1 The safety hazards of cathode materials
When the lithium-ion battery is used improperly, the internal temperature of the battery will increase, and the active material of the positive electrode material will be decomposed and the electrolyte will be oxidized. At the same time, these two reactions can generate a lot of heat, causing the battery temperature to rise further. Different delithiation states have very different effects on the lattice transformation of the active material, the decomposition temperature and the thermal stability of the battery.
2 The safety hazards of anode materials
The negative electrode material used in the early days was metallic lithium, and the assembled battery was prone to produce lithium dendrites after repeated charging and discharging, which would then pierce the diaphragm, causing the battery to short-circuit, leak and even explode. Lithium intercalation compounds can effectively avoid the generation of lithium dendrites and greatly improve the safety of lithium-ion batteries. As the temperature increases, the carbon negative electrode in the state of lithium intercalation first reacts exothermically with the electrolyte. Under the same charging and discharging conditions, the heat release rate of the reaction between the electrolyte and the lithium-intercalated artificial graphite is much greater than that of the reaction with lithium-intercalated mesophase carbon microspheres, carbon fibers, coke, etc.
3 The safety hazards of diaphragm and electrolyte
The electrolyte of lithium ion battery is a mixed solution of lithium salt and organic solvent. The commercial lithium salt is lithium hexafluorophosphate. The thermal stability of the electrolyte. The organic solvent of the electrolyte is carbonate, which has a low boiling point and flash point, and is easy to react with lithium salt to release PF5 at high temperature, and is easy to be oxidized.
4 Hidden safety hazards in the manufacturing process
During the manufacturing process of lithium-ion batteries, processes such as electrode manufacturing and battery assembly will have an impact on the safety of the battery. The quality control of various processes such as positive and negative electrode mixing, coating, rolling, cutting or punching, assembling, filling electrolyte, sealing, and forming all affect the performance and safety of the battery. The uniformity of the slurry determines the uniformity of the active material distribution on the electrode, thereby affecting the safety of the battery. If the fineness of the slurry is too large, the negative electrode material will undergo relatively large changes during charging and discharging, and the precipitation of metallic lithium may occur; if the fineness of the slurry is too small, the internal resistance of the battery will be too large. If the coating heating temperature is too low or the drying time is insufficient, the solvent will remain, and the binder will be partially dissolved, causing some active materials to be easily peeled; too high temperature may cause the binder to be carbonized, and the active materials may fall off and cause internal short circuits in the battery.
5 potential safety hazards during battery use
Lithium-ion batteries should minimize overcharging or over-discharging during use. Especially for batteries with high monomer capacity, thermal disturbance may cause a series of exothermic side reactions, leading to safety issues.
Three lithium-ion battery safety testing indicators
After the lithium-ion battery is produced, before it reaches the consumer, a series of tests are required to ensure the safety of the battery as much as possible and reduce potential safety hazards.
1. Squeeze test: Put the fully charged battery on a flat surface, apply a pressure of 13±1KN by a hydraulic cylinder, and squeeze the battery from the flat surface of a steel rod with a diameter of 32mm. Once the squeeze pressure reaches the maximum stop Squeeze, the battery does not catch fire, just don't explode.
2. Impact test: After the battery is fully charged, place it on a flat surface, place a steel column with a diameter of 15.8mm vertically in the center of the battery, and drop a 9.1kg weight freely from a height of 610mm onto the steel column above the battery. The battery does not catch fire or explode.
3. Overcharge test: Fully charge the battery with 1C, and perform an overcharge test according to 3C overcharge 10V. When the battery is overcharged, the voltage rises to a certain voltage and stabilizes for a period of time. When it is close to a certain period of time, the battery voltage rises rapidly. When a certain limit is reached, the top cap of the battery is pulled off, the voltage drops to 0V, and the battery does not catch fire or explode.
4. Short-circuit test: After the battery is fully charged, the positive and negative electrodes of the battery are short-circuited with a wire with a resistance of not more than 50mΩ, and the surface temperature of the battery is tested. The maximum temperature of the battery surface is 140℃. The battery cap is opened, and the battery does not catch fire or explode. .
5. Acupuncture test: Place the fully charged battery on a flat surface, and pierce the battery in the radial direction with a steel needle with a diameter of 3mm. The test battery does not catch fire or explode.
6. Temperature cycle test: The temperature cycle test of lithium ion battery is used to simulate the safety of lithium ion battery when it is repeatedly exposed to low temperature and high temperature environment during transportation or storage. The test is to use rapid and extreme temperature Changes are made. After the test, the sample should not fire, explode, or leak.
Four lithium-ion battery safety solutions
In view of the many hidden safety hazards of lithium-ion batteries in the material, manufacturing and use process, how to improve the parts that are prone to safety problems is a problem that lithium-ion battery manufacturers need to solve.
1 Improve the safety of electrolyte
There is a high reaction activity between the electrolyte and the positive and negative electrodes, especially at high temperatures. In order to improve the safety of the battery, improving the safety of the electrolyte is one of the more effective methods. The potential safety hazards of electrolyte can be effectively solved by adding functional additives, using new lithium salts and using new solvents.
According to the different functions of additives, they can be divided into the following categories: safety protection additives, film-forming additives, positive electrode protection additives, stabilizing lithium salt additives, lithium precipitation promoting additives, current collector anticorrosive additives, and wettability enhancing additives.
In order to improve the performance of commercial lithium salts, researchers have substituted atoms on them and obtained many derivatives. Among them, compounds obtained by substituting atoms with perfluoroalkyl groups have many advantages such as high flash point, similar conductivity, and enhanced water resistance. , Is a kind of lithium salt compound with great application prospects. In addition, the anionic lithium salt obtained by chelating the boron atom with the oxygen ligand has high thermal stability.
Regarding solvents, many researchers have proposed a series of new organic solvents, such as carboxylic acid esters and organic ethers. In addition, ionic liquids also have a class of electrolytes with high safety, but relatively commonly used carbonate-based electrolytes. The viscosity of ionic liquids is orders of magnitude higher, and the conductivity and ion self-diffusion coefficient are low. There is still a lot of work before practicality. To do.
2 Improve the safety of electrode materials
Lithium iron phosphate and ternary composite materials are considered to be low-cost, "excellent safety" cathode materials, and may be popularized in the electric vehicle industry. For the positive electrode material, the common method to improve its safety is coating modification. For example, the surface coating of the positive electrode material with a metal oxide can prevent the direct contact between the positive electrode material and the electrolyte, inhibit the phase change of the positive electrode material, and improve Its structural stability reduces the disorder of cations in the crystal lattice to reduce heat generation by side reactions.
For the negative electrode material, because the surface is often the most prone to thermochemical decomposition and heat generation in the lithium ion battery, improving the thermal stability of the SEI film is a key method to improve the safety of the negative electrode material. Through weak oxidation, metal and metal oxide deposition, polymer or carbon coating, the thermal stability of the negative electrode material can be improved.
3 Improved battery safety protection design
In addition to improving the safety of battery materials, commercial lithium-ion batteries adopt many safety protection measures, such as setting battery safety valves, thermal fuses, connecting components with positive temperature coefficients in series, using thermally sealed diaphragms, loading dedicated protection circuits, and dedicated battery management System, etc., is also a means to enhance security.
Five lithium-ion battery safety solution provider
As the safety of lithium-ion batteries has attracted more and more attention, many companies have conducted research and development specifically for potential safety hazards in lithium-ion batteries, and put forward effective battery safety solutions.
As the earliest researcher of domestic power battery thermal runaway warning and safety technology and the pioneer of battery box special automatic fire extinguishing device, Chuangwei New Energy pioneered the "lithium-ion battery thermal runaway model", which promoted battery box thermal runaway monitoring and automatic fire extinguishing. Large-scale application of technology.
The "Lithium-ion battery thermal runaway model" is divided into three dimensions: vertical, horizontal and vertical. The vertical direction is the data redundancy of multiple sensors, that is, multiple sets of sensor data under the same environment are fitted to simulate the data characterization curve of different materials and different environments; the horizontal direction is the continuous time algorithm for the historical data of the sensor to eliminate noise Interference effectively solves the problems of false alarms, false alarms, and early warning lag in the threshold method; vertical puncture, blunt needle backlog and other methods are used to simulate the thermal runaway process of different types of power batteries.
Through three-dimensional fusion, mathematical methods, based on a large number of experiments and real operating data, the internal relationship between various variables caused by thermal runaway is summarized, and neurological principles are used to form an extremely early, highly reliable, and self-operating "lithium ion" Battery thermal runaway model" realizes early warning and intelligent control of hidden dangers in battery life.
A large number of early warning examples occurred in actual vehicle operation proved the effectiveness and advancement of this model, making it the core technology of current battery box thermal runaway warning and automatic fire extinguishing.
Shenzhen Benwei battery is a high-tech enterprise specializing in R&D, production and sales of lithium-ion batteries. Its product application areas cover: electric vehicle lithium batteries, lithium power batteries, energy storage lithium batteries, etc. The company and battery cell manufacturers maintain long-term stability Cooperative relationship, and apply the latest technological achievements and concepts to the entire series of product development processes. The manufacturing workshop is equipped with advanced production equipment and first-class testing instruments. At the same time, it has a group of professional production and quality management teams, strictly every step of the production link, and through continuous optimization and improvement in the process to ensure battery safety.
