The factors affecting the safety performance of the power battery run through the life cycle of a power battery from the selection of the battery to the end of use, so the reasons are complex and diverse. The cell material itself, the manufacturing process of the cell, the design and use conditions of the BMS (battery management system) and safety in battery integration are all factors influencing the safety performance of the lithium ion battery. In these links, manufacturing errors and abuse conditions are inevitable in any case, so under this realistic condition, the design of the thermal runaway of the battery is particularly important. This paper summarizes the factors affecting the safety performance of lithium-ion battery, in order to provide a reliable basis for its application and research in high energy / high power.
1 Introduction
Lithium-ion batteries stand out in chemical energy storage devices because of their high energy density, high power density and long service life. Now they have been widely used in the field of portable electronic products. Now, with the support of national policies, The demand for electric vehicles and large-scale energy storage is also exploding.
Lithium-ion batteries are generally safe, but reports of safety incidents are present in the public. The well-known Boeing 737 and B787 aircraft batteries are on fire in recent years, BYD electric vehicles are on fire, Tesla MODEL S is on fire... The earliest time for these lithium-ion battery safety accidents to enter public view can be traced back to 4 or 5 years ago. . Up to now, safety is still a key factor restricting the application of lithium-ion batteries in high energy/high power applications. Thermal runaway is not only the essential cause of safety problems, but also one of the shortcomings that limit the performance of lithium-ion batteries.
The potential safety issues of lithium-ion batteries largely affect consumer confidence. Although people have been expecting BMS to accurately monitor the security status (SOS) and predict and prevent some failures, it is difficult to guarantee all the lifecycles faced by a technical system due to the complexity of the thermal runaway situation. The safety situation, therefore, the analysis and research of the cause is still necessary for a safe and reliable lithium-ion battery.
2. Selection of battery materials
The internal composition of the lithium-ion battery is mainly positive electrode|electrolyte|separator|electrolyte|negative electrode. On the basis of this, the electrode is welded, and the outer package is finally formed into a complete battery core. After the battery is subjected to the initial charge and discharge, and the components are discharged, the battery can be used at the factory. The first step in this process is the choice of materials. The safety factors affecting materials are mainly their intrinsic orbital energy, crystal structure and material properties.
Cathode material
The main role of the positive active material in the battery is to contribute specific capacity and specific energy, and its intrinsic electrode potential has a certain influence on safety. For example, in recent years, China has widely used low-voltage material LiFePO4 (lithium iron phosphate) as a positive electrode material for power batteries in vehicles (such as hybrid electric vehicle HEV, electric vehicle EV) and energy storage equipment (such as uninterruptible power supply UPS). However, the safety advantage that LiFePO4 exhibits in many materials is actually at the expense of energy density, which means that it limits the endurance of its users (such as EV, UPS). While ternary materials such as NMC (LiNixMnyCo1-x-yO2) perform excellent in energy density, as an ideal positive electrode material for power batteries, the safety problem has not been solved satisfactorily. In order to study the thermal behavior of the positive electrode material, the researchers have done a lot of work, and found that the intrinsic electrode potential and crystal structure are the main factors affecting its safety, such as the electrode potential μC and the electrochemical window of the electrolyte occupying the highest orbit HOMO. Perfectly matched, whether the crystal lattice can smoothly pass through multiple lithium ions at the same time... The safety performance of the positive active material can be enhanced by the selection of the material type and the doping of the element.
Anode material
The effect of the negative active material on safety performance is mainly due to its intrinsic orbital energy and the arrangement relationship of electrolyte LUMO and HOMO. In the process of fast charging, the speed of lithium ions passing through the SEI (solid electrolyte interface) film may be slower than that of lithium in the negative electrode. The branch crystal of lithium will grow with the charge and discharge cycle, which may cause internal short circuit and ignite. The flammable electrolyte is thermally out of control, which limits the safety of the anode during fast charging. Only in the case where the difference between the electromotive force of the negative electrode of the lithium alloy containing the carbonaceous material as the buffer layer and the electromotive force of lithium is less than -0.7 Ev, that is, μA < μLi 0.7 eV, it is ensured that the deposition of lithium does not cause a short circuit. For safety reasons, the power battery should use a negative electrode material with an electromotive force of less than 1.0 eV (relative to Li+/Li0) to achieve safe fast charging or to control the charging voltage to be much lower than the deposition potential of lithium. Li4Ti5O12 has a safety advantage in the fast charge and fast release fields because its electromotive force is 1.5 eV (relative to Li+/Li0) and lower than the electrolyte LUMO. There is also a negative electrode material Ti0.9Nb0.1Nb2O7, which can be rapidly charged and discharged for more than 30 weeks at a voltage of 1.3 ≤ V ≤ 1.6V (relative to Li+/Li0), and has a specific capacity of 300 mAhg1, which is higher than LTO. The fast release process is safe during the discharge process because there is no competition for the rate at which lithium ions pass through the SEI film and deposit on the negative electrode.
Electrolyte and diaphragm
The main impact of electrolytes and membranes on safety is their traits.
The flammability and liquid state of commercially available electrolytes, which are currently widely used, are not particularly desirable for safety. If a solid electrolyte with a lithium ion conductivity σLi+>104 Scm1 is used, the lithium branch crystal can be prevented from piercing the separator to the positive electrode to solve the safety problem, and on the other hand, the contact between the negative electrode and the carbonate electrolyte and the positive electrode and the aqueous solution can be solved. Stability problems caused by electrolyte contact. Of course, by using an electrolyte with a wider electrochemical window (especially higher LUMO), some flame retardant materials are added to the electrolyte to modify the mixed ionic liquid and organic liquid electrolyte into a non-flammable electrolyte (with At the same time, the ionic conductivity σLi does not decrease too much, and the like can also effectively improve safety.
The mechanical strength (tensile and puncture strength) of the diaphragm, porosity and the ability to shut down are important basis for determining its safety.
Battery manufacturing
From the beginning of the electrode ingredients, it is necessary to go through a series of steps such as stirring, drawing, cutting, scraping, brushing, rolling, pole riveting, welding contiguous, adhesive tape, testing, and formation. In this series of processes, even if all the steps have been completed, there is a possibility that the internal resistance of the battery is increased or short-circuited due to the failure of the work, which poses a safety problem. Such as: welding in the process of welding (positive / negative electrode and the ear, between the positive pole and the cap, between the negative pole and the shell, rivet and contact internal resistance, etc.), dust, diaphragm paper is too small or not Padded, the diaphragm has holes, and the burrs are not cleaned. The wrong ratio of the positive and negative electrodes may also cause a large amount of metallic lithium to deposit on the surface of the negative electrode. The insufficient uniformity of the slurry may also result in uneven distribution of active particles, resulting in large volume change of the charge and discharge anode and lithium deposition, thereby affecting its safety performance. . In addition, the formation quality of the SEI film in the formation step directly determines the cycle performance and safety performance of the battery, affecting its lithium intercalation stability and thermal stability. The factors affecting the SEI film include the type of negative carbon material, electrolyte and solvent, the current density at the time of formation, the setting of parameters such as temperature and pressure, and the appropriate selection of materials and parameter adjustment of the formation process can improve the formation of SEI film. Quality, which improves the safety of the battery.
4. Stack integration
BMS Battery Management System
The Battery Management System (BMS) is placed in the hope of solving critical problems in the use of power batteries. The management system needs to manage the battery and its consistency to achieve maximum energy storage, round trip efficiency and safety under different conditions (temperature, altitude, maximum rate, charge state, cycle life...). The BMS includes some common modules: data collector, communication unit and battery status (SOC, SOC, SOP...) evaluation models. With the development of power batteries, the management capabilities of BMS are also more demanding. Adding, for example, thermal management modules, high-voltage monitoring modules... Through the increase of these safety modules, it is expected to improve the safety and reliability of the power battery during use.
Integrated design of the stack
When the battery is out of control, it will cause damaging behavior such as smoke, fire, explosion, etc., which will endanger the user's personal safety. Even if you choose the most secure configuration in theory, it is not enough to give you peace of mind. If LiFePO4 and Li4Ti5O12 are used to make the positive and negative materials suitable for fast charge and discharge batteries, their electromotive force is located in the electrochemical window of the electrolyte, and the SEI film is no longer needed. However, this is also because the redox couple will appear on top of the anion P or overlap with the cation 4S orbit and is not sufficient to cope with the operation of the electrode under some operating conditions. The reasonable design and manufacture of the battery core can not avoid the accidental situation in the working condition. Only a reasonable battery pack integrated design can make the stack stop loss in case of a problem with the battery core.
As mentioned earlier, the safety and endurance of the battery is a contradictory result at the material level. In order to solve the problem of balance between safety and endurance, Tesla Motors Co.Ltd took the lead in making a model that gave us a good inspiration. Tesla's Model S uses Panasonic Co. Ltd's high-energy-density NCR18650A battery, which uses more than 7,000 cells in a stack. This is a combination of high probability of thermal runaway, but through the design of the stack integration and its BMS, many innovative patents have been used, which greatly reduces the probability of a Model S in the event of a safety accident. Taking Tesla's published patent as an example, the enhancement of monomer safety performance, module module safety performance and battery pack assembly safety performance can more or less represent an advanced approach to integration.
Tesla maintains a minimum safety distance between the cells by adding fireproofing materials and sleeves to the electrodes and the outer casing of the cell. The gasket is used to keep the spacing of the cells after fire, and the high efficiency safety valve is used to predict the monomer. In the rupture position, the single safety valve opens and disconnects the unit from the appliance, thereby preventing heat dissipation between the cells and the chain reaction caused by thermal runaway. At the same time, by arranging a heat insulating layer between the electrode of the battery and the inner surface of the battery case, an insulating layer is arranged between the modules to protect the Pack partition, thereby blocking heat conduction and runaway diffusion between the modules after thermal runaway occurs. . These measures are layered from the core to the module level, in order to maximize the timely stop loss after the internal thermal runaway occurs.
Thermal runaway plan design
For the design of the thermal runaway after the occurrence of various types, multi-layer, in addition to the above-mentioned various safety considerations in the integration, there is also a cooling duct for the battery cooling and thermal runaway active mitigation system to start the cooling liquid to reduce The effect of thermal runaway is realized; the sub-pile safety valve is opened in time, so that the high-temperature gas generated by the thermal runaway is discharged out of the system in time, and then discharged by the total valve; the built-in other system absorbs the energy generated by the thermal runaway high temperature, and reduces the hazard... Finally, In the event that the pre-order means cannot be controlled, a bullet-proof plate is installed at the bottom of the pack, and a heat-resistant layer is added between the passenger compartment and the pack layer to minimize the possibility of the occurrence of thermal runaway. hurt. These designs can not only reduce the energy in the internal thermal runaway, but also predict that after the battery level is completely out of control, the catastrophic consequences are still under control, thus fundamentally protecting the user's personal safety.
5. Battery abuse
Even if the lithium-ion battery is flawless in the manufacturing integration process as described above, it is difficult to avoid abuse in the actual use conditions of the user. Charge and discharge system (overcharge and over discharge), ambient temperature (hot box), other abuse (acupuncture, extrusion, internal short circuit), etc., plus the new national standard increased environmental humidity (seawater immersion) is caused by abuse The cause of the security issue. Overcharge will cause the cathode active material crystal to collapse, and the lithium ion deintercalation channel is blocked, so that the internal resistance is sharply increased, a large amount of Joule heat is generated, and the lithium intercalation ability of the anode active material is lowered to cause the short circuit of the lithium branch crystal. . Overheating of the ambient temperature causes a series of chain chemical reactions inside the lithium ion battery, including melting of the separator, reaction of the positive/negative active material with the electrolyte, decomposition of the positive/SEI membrane/solvent, reaction of the lithium intercalated anode with the binder, and the like. Acupuncture/extrusion is caused by internal short-circuiting locally, and a large amount of heat is accumulated in the short-circuited area as in the internal short-circuit, causing thermal runaway. The above research has been a lot, and this article will not go into details.
6. Summary
The safety performance of the power battery determines the market and future of the lithium-ion battery in the power field. The factors affecting the safety performance of the power battery run through the life cycle of a power battery from the selection of the battery core to the end of use, so the reasons are complex and diverse. The intrinsic orbital energy, crystal structure and properties of the material itself determine the intrinsic safety performance of a cell; the degree of excellence in each process step in the manufacturing process of the cell, the degree of automation and the setting of the formation conditions determine its cycle performance and safety. Performance, affecting its lithium insertion stability and thermal stability; battery design in terms of BMS and safety can effectively ensure the safety of the battery, battery manufacturing and use conditions can not always be in an ideal state, manufacturing errors And the abuse of the working conditions is in any case difficult to avoid. Under this realistic condition, various pre-planning designs for the thermal runaway of the battery are particularly important. Through the study of the patents disclosed by Tesla, we can learn from the method of preventing the heat transfer from the cell to the battery system to prevent thermal runaway chain diffusion; using the cooling sprinkler system, the internal parts of the safety valve consume high heat. The effect of heat loss control is reduced; the reinforcement of the carrier is designed to minimize the degree of personal injury after the occurrence of thermal runaway.
In short, the research on the safety of lithium-ion battery has a long way to go. Only the theory and the actual innovation can usher in the true glory of high-energy/high-power applications.
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