Safety of power lithium-ion battery -Lithium - Ion Battery Equipment

Safety of lithium-ion battery for lithium-ion power -Lithium - Ion Battery Equipment

The factors affecting the safety performance of power lithium battery run through the life cycle of power lithium battery from the selection of battery core to the end of use, so the reasons are complex and varied. The cell material itself, the cell manufacturing process, and the design and operating conditions of BMS (battery management system) and safety in battery integration are all factors affecting the safety performance of lithium-ion batteries. In these links, it is difficult to prevent manufacturing errors and abuse of working conditions in any case. Therefore, under this practical condition, it is particularly important to design the plan for thermal runaway of the battery.

Because of its high energy density, high power density and long service life, lithium-ion battery stands out among chemical energy storage devices. Now it has been widely used in the field of portable electronic products with mature technology. Now, with the support of national policies, the demand in the field of electric vehicles and large-scale energy storage has also increased explosively.

Lithium-ion batteries are generally safe, but reports of safety accidents are presented to the public from time to time. In recent years, Boeing 737 and B787 aircraft batteries caught fire, BYD electric vehicles caught fire, and Tesla MODELS caught fire… The earliest time these lithium-ion battery safety accidents came into the public view can be traced back to 4 or 5 years ago. Up to now, safety is still the key factor restricting the application of lithium-ion batteries in the field of high energy/high power. Thermal runaway is not only the essential reason for safety problems, but also one of the short boards that restrict the performance of lithium-ion batteries.(Lithium - Ion Battery Equipment)

The potential safety of lithium-ion batteries has greatly affected consumer confidence. Although it has been expected that BMS can accurately monitor the safety condition (SOS) and predict and prevent the occurrence of some faults, it is difficult for a technical system to guarantee all the safety conditions it faces in its life cycle due to the complexity and diversity of thermal runaway conditions, so it is still necessary to analyze and study the causes for a safe and reliable lithium-ion battery.

2. Selection of core material

The internal composition of lithium-ion battery is mainly composed of positive electrode | electrolyte | diaphragm | electrolyte | negative electrode. On this basis, the electrode lug is welded and the outer package is wrapped to finally form a complete cell. After the initial charge and discharge, the cell can be converted into partial volume exhaust and other steps before leaving the factory. The first step in this process is the selection of materials. The important factors affecting the safety of materials are their intrinsic orbital energy, crystal structure and material properties.

cathode material

The important use of positive active material in battery is to contribute specific capacity and specific energy, and its intrinsic electrode potential has a certain impact on safety. For example, in recent years, China has widely used the low voltage material LiFePO4 (lithium iron phosphate) as the cathode material of the power lithium battery in vehicles (such as hybrid electric vehicle HEV, electric vehicle EV) and energy storage equipment (such as uninterruptible power supply UPS), but the safety advantage of LiFePO4 in many materials is actually at the expense of energy density, That is to say, it will restrict the endurance of its users (such as EV and UPS). While ternary materials such as NMC (LiNixMnyCo1-x-yO2) have excellent performance in energy density, as an ideal cathode material for power lithium batteries, the safety problem has not been completely solved. In order to study the thermal behavior of cathode materials, researchers have done a lot of work, and found that the intrinsic electrode potential and crystal structure are important factors affecting its safety, such as electrode potentialμ Whether the highest occupied orbital HOMO of the electrochemical window of C and electrolyte is perfectly matched, and whether the lattice can smoothly pass through multiple lithium ions at the same time&hellip& hellip; The safety performance of positive active materials can be enhanced by selecting materials and doping elements.

Negative material

The influence of negative active material on safety performance is mainly from its intrinsic orbital energy and the configuration relationship of electrolyte LUMO and HOMO. In the process of fast charging, the speed of lithium ion passing through the SEI (solid electrolyte interface) film may be slower than the deposition speed of lithium on the negative electrode. The lithium dendrites will continue to grow with the charge-discharge cycle, which may lead to internal short circuit and ignite the combustible electrolyte to cause thermal runaway, which limits the safety of the negative electrode in the process of fast charging. Only when the difference between negative electromotive force and lithium electromotive force of lithium alloy with carbonaceous material as buffer layer is less than -0.7Ev, i.e.μ A<μ In the case of Li0.7eV, the lithium deposition will not cause short circuit. For the sake of safety, the power lithium battery should use the negative electrode material with the electromotive force less than 1.0eV (relative to Li+/Li0) to achieve safe and fast charging or be able to control the charging voltage in the range far below the lithium deposition potential. Li4Ti5O12 has the advantage of safety in the field of fast charging and fast discharging, because its electromotive force is 1.5eV (relative to Li+/Li0), which is lower than the LUMO of electrolyte. There is also a negative material Ti0.9Nb0.1Nb2O7, which can be rapidly charged and discharged at 1.3V1.6V (relative to Li+/Li0) for more than 30 weeks, and has a specific capacity of 300mAh hg1, higher than LTO. In the process of discharge, there is no competition between the speed of lithium ion passing through the SEI film and depositing on the negative electrode, so the fast discharge process is safe.

Electrolyte and diaphragm

Electrolyte and diaphragm have an important impact on safety because of their properties.

The flammability and liquid state of the widely used commercial electrolyte are not particularly ideal for safety. If lithium ion conductivityσ The solid electrolyte with Li+>104Scm1 can prevent the lithium dendrite from penetrating the diaphragm to reach the positive electrode to solve the safety problem, and on the other hand, it can also solve the stability problem when the negative electrode contacts with the carbonate electrolyte and the positive electrode contacts with the aqueous electrolyte. Of course, by using the electrolyte with wider electrochemical window (especially higher LUMO), adding some flame-retardant materials to the electrolyte, and modifying the mixed ionic liquid and organic liquid electrolyte into non-flammable electrolyte (at the same time, the ionic conductivityσ Li will not decrease too much), and other means can also effectively improve the safety.

The mechanical strength (tensile strength and puncture strength), porosity and whether the diaphragm has closing function are the important basis for determining its safety.

Manufacturing of cell

Starting from electrode batching, a series of steps such as mixing, slurry pulling, cutting, powder scraping, powder brushing, roller alignment, pole lug riveting, welding, lamination, adhesive tape, testing, and formation are required. In this series of processes, even if all the steps have been completed, it is still possible that the internal resistance of the battery will rise or short circuit due to poor work, which may lead to potential safety problems. For example, there is faulty welding (between the positive/negative plate and the lug, between the positive plate and the cap, between the negative plate and the shell, and between the rivet and the contact resistance, etc.), material dust, the diaphragm paper is too small or not well padded, the diaphragm has holes, and the burrs are not cleaned. The wrong ratio of the capacity of the positive and negative electrodes may also lead to the deposition of a large amount of metal lithium on the negative electrode surface, and the insufficient uniformity of the slurry will also lead to the uneven distribution of active particles, resulting in large changes in the volume of the charge-discharge negative electrode and lithium precipitation, thus affecting its safety performance. In addition, the formation quality of SEI film in the formation step also directly determines the cycle performance and safety performance of the battery, and affects its lithium intercalation stability and thermal stability. Factors affecting the SEI film include the type of negative carbon material, electrolyte and solvent, the setting of current density, temperature and pressure during the formation, and the quality of the SEI film can be improved through the proper selection of materials and the adjustment of the parameters of the formation process, thus improving the safety performance of the cell.

4. Stack integration

BMS battery management system

Battery management system (BMS) is expected to solve key problems in the use of power lithium battery. The management system shall manage the battery and its consistency to achieve maximum energy storage, round-trip efficiency and safety under different conditions (temperature, altitude, maximum magnification, state of charge, cycle life……). BMS includes some general modules: data collector, communication unit and battery status (SOC, SOC, SOP……) evaluation model. With the development of power lithium battery, the management ability of BMS is also more demanding. Added such as heat management module, high pressure monitoring module&hellip& hellip; With the addition of these safety modules, it is expected to improve the safety and reliability of the power lithium battery in use.

Integrated design of stack

The thermal runaway of the battery will cause smoke, fire, explosion and other destructive behaviors, which will endanger the personal safety of users. Even if the safest configuration mode in theory is selected, it is not enough to make people feel at ease. For example, LiFePO4 and Li4Ti5O12 are selected as safe and suitable positive and negative materials for fast charge-discharge batteries. Their electromotive force is located in the electrochemical window of the electrolyte, and SEI film is no longer required. However, even in this case, the redox pair will appear on the top of the P orbital of the anion or overlap with the 4S orbital of the cation, which is not enough to cope with the working conditions of the electrode under some working conditions. No matter how reasonable the design and manufacture of the battery core can prevent the occurrence of accidents in the working conditions. Only a reasonable integrated design of the battery package can make the stack stop loss in time in case of a problem in the battery core.

As mentioned earlier, the safety and endurance of the battery are a pair of contradictory results at the material level. In order to solve the balance between safety and endurance, Tesla Motors Co. Ltd took the lead in making a model and gave us good inspiration. Tesla's Model S uses Panasonic Co. Ltd's NCR18650A battery with high energy density, and uses more than 7000 power saving cells in a stack. This is a combination method with high probability of thermal runaway, but through the design of the stack integration and BMS, many innovative patents are used, which greatly reduces the probability of safety accidents in the actual use of the ModelS. Taking Tesla's open patent as an example, the enhancement of monomer safety performance, module safety performance and battery pack assembly safety performance can more or less represent an advanced solution to integration.

Tesla maintains the minimum safety distance between the monomers by adding fireproof materials and sleeves at the electrode and shell of the cell, uses gaskets to keep the spacing of the monomers unchanged after the fire, uses efficient safety valves to predict the fracture position of the monomers, and cuts off the connection between the monomers and the electrical appliance after the opening of the monomer safety valve, thus preventing the heat diffusion between the monomers and the chain reaction caused by the thermal runaway. At the same time, thermal insulation layer is arranged between the battery electrode and the inner surface of the battery shell, and the insulation layer is arranged between the modules to protect the pack by zones, so as to block the heat conduction and uncontrolled diffusion between the modules after the occurrence of thermal runaway. These measures, from the core to the module level, are protected layer by layer in order to stop the loss in time to the maximum extent after the internal thermal runaway occurs.

Thermal runaway plan design

There are many types and levels of plan design methods after the occurrence of thermal runaway. In addition to the safety design considered during the above integration, there are also distributed cooling pipes to spray cooling liquid for battery cooling and thermal runaway active mitigation system to reduce the impact of thermal runaway; The safety valve of the sub-reactor shall be opened in time to allow the high temperature gas out of control to be discharged from the system in time, and then discharged from the main valve; Use other built-in systems to absorb the energy generated by uncontrolled heat and high temperature to reduce the hazard&hellip& hellip; Finally, in case of any situation beyond the control of the preceding means, the bullet-proof plate shall be installed at the bottom of the pack location, and the thermal barrier layer shall be added between the passenger compartment and the pack layer to minimize the personal injury caused by thermal runaway. These designs can not only reduce the energy in time when the internal heat is out of control, but also predict that the catastrophic consequences will still be under control after the battery level is completely out of control, thus fundamentally ensuring the personal safety of users.

5. Battery abuse

Even if the lithium ion battery is perfect in the manufacturing and integration process as described above, it is difficult to prevent abuse in the actual working conditions of users. The charging and discharging system (overcharging and discharging), environmental temperature (hot box), other abuse (acupuncture, extrusion, internal short circuit), and the new environmental humidity (seawater immersion) added in the new national standard are all the reasons for the safety problems caused by abuse. Over-charge will cause the crystal collapse of the positive active material, and the lithium ion de-insertion channel will be blocked, resulting in a sharp increase in internal resistance and a large amount of joule heat. At the same time, it will also reduce the lithium intercalation capacity of the negative active material, resulting in the consequences of short circuit caused by lithium dendrites. Overheating of the ambient temperature will cause a series of chain chemical reactions inside the lithium-ion battery, including the melting of the diaphragm, the reaction of the positive/negative active materials with the electrolyte, the decomposition of the positive/SEI film/solvent, and the reaction of the lithium-embedded negative electrode with the binder. Needling/extrusion is the result of local internal short circuit. Like internal short circuit, a large amount of heat is accumulated in the short circuit area, resulting in thermal runaway.



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