What are the factors affecting the safety of Nickel Hydride No. 5?

The factors affecting the safety performance of power lithium batteries run through the life cycle of a power lithium battery from the selection of battery cells to the end of use, so the reasons are complex, diverse and rich. The battery cell material itself, the manufacturing process of the battery cell, the design and use 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, manufacturing errors and abuse conditions are difficult to prevent in any way, so under this realistic condition, the design of a plan for thermal runaway of the battery is particularly important.

Lithium-ion batteries stand out among chemical energy storage devices because of their high energy density, high power density and long service life. Now they have mature technology and are widely used in the field of portable electronic products. Now, with the support of national policies, the demand in the field of electric vehicles and large-scale energy storage has also exploded.

Lithium-ion batteries are usually safe, but reports of safety accidents are presented to the public from time to time. The more famous ones are the battery fires of Boeing's 737 and B787 aircraft, BYD electric vehicles, and Tesla MODELS in recent years. The earliest time these lithium-ion battery safety accidents entered the public eye 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 fundamental cause of safety problems, but also one of the shortcomings that restrict the performance of lithium-ion batteries.

The potential safety issues of lithium-ion batteries have greatly affected consumer confidence. Although people have always expected BMS to accurately monitor the safety status (SOS) and predict and prevent the occurrence of some faults, due to the complexity and diversity of thermal runaway, it is difficult for a technical system to guarantee all safety conditions faced in its life cycle. Therefore, analysis and research on its causes are still necessary for a safe and reliable lithium-ion battery.

2. Selection of battery cell materials

The internal composition of lithium-ion batteries is mainly positive electrode | electrolyte | diaphragm | electrolyte | negative electrode. On this basis, the welding of the pole ears and the wrapping of the outer packaging are carried out to finally form a complete battery cell. After the initial charge and discharge, the battery cell can be shipped out of the factory for use after the steps of volume fractionation and exhaust. The first step of this process is the selection of materials. The factors that affect the safety of materials are their intrinsic orbital energy, crystal structure and material properties.

Positive electrode materials

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

Negative electrode materials

The impact of negative electrode active materials on safety performance mainly comes from their intrinsic orbital energy and the configuration relationship of electrolyte LUMO and HOMO. During fast charging, the speed at which lithium ions pass through the SEI (solid electrolyte interface) film may be slower than the deposition speed of lithium on the negative electrode. Lithium dendrites will continue to grow with the charge and discharge cycle, which may cause internal short circuits and ignite the flammable electrolyte to cause thermal runaway. This feature limits the safety of the negative electrode during fast charging. Only when the difference between the electromotive force of the negative electrode of the lithium alloy with a carbon-containing material as a buffer layer and the electromotive force of lithium is less than -0.7Ev, that is, μA<μLi0.7eV, can the deposition of lithium be guaranteed not to cause a short circuit. For safety reasons, power lithium batteries should use negative electrode materials with an electromotive force of less than 1.0eV (relative to Li+/Li0) to achieve safe fast charging or be able to control the charging voltage to a range far below the deposition potential of lithium. Li4Ti5O12 has a safety advantage in the field of fast charging and fast discharge because its electromotive force is 1.5eV (relative to Li+/Li0), which is lower than the LUMO of the electrolyte. There is also a negative electrode material Ti0.9Nb0.1Nb2O7, which can be quickly charged and discharged at a voltage of 1.3V1.6V (relative to Li+/Li0) for more than 30 weeks, and has a specific capacity of 300mAhg1, which is higher than LTO. During the discharge process, because there is no competition between the speed of lithium ions passing through the SEI membrane and depositing on the negative electrode, the fast discharge process is safe.

Electrolyte and diaphragm

The important influence of electrolyte and diaphragm on safety is their properties.

The flammability and liquid state of the currently widely used commercial electrolytes are not particularly ideal choices for safety. If a solid electrolyte with a lithium ion conductivity of σLi+>104Scm1 is used, it can prevent lithium branch crystals from piercing the diaphragm to reach the positive electrode to solve the safety problem on the one hand, and on the other hand, it can also solve the stability problem when the negative electrode contacts the carbonate electrolyte and the positive electrode contacts the aqueous electrolyte. Of course, by using an electrolyte with a wider electrochemical window (especially a higher LUMO), adding some flame retardant materials to the electrolyte, and modifying the mixed ionic liquid and organic liquid electrolyte into a non-flammable electrolyte (at the same time, the ionic conductivity σLi will not be reduced too much), etc., safety can also be effectively improved.

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

Manufacturing of battery cells

Starting from the electrode ingredients, it has to go through a series of steps such as stirring, slurry pulling, sheet cutting, powder scraping, powder brushing, rollers, riveting of tabs, welding and connecting, adhesive tape, testing, and formation. In this series of processes, even if all the steps have been completed, there is still a hidden danger of safety problems due to the increase of battery internal resistance or short circuit due to inadequate work. For example: cold welding during welding (between positive/negative electrode sheet and tab, between positive electrode sheet and cap, between negative electrode sheet and shell, large internal resistance between rivet and contact, etc.), material dust, diaphragm paper is too small or not well padded, diaphragm has holes, burrs are not cleaned up, etc. Incorrect capacity ratio of positive and negative electrodes may also cause a large amount of metal lithium to deposit on the surface of the negative electrode. Insufficient slurry uniformity will also lead to uneven distribution of active particles, resulting in large volume changes of the negative electrode during charge and discharge and lithium precipitation, thereby affecting its safety performance. In addition, the quality of SEI film generation in the formation step also directly determines the cycle performance and safety performance of the battery, affecting its lithium insertion stability and thermal stability. Factors affecting the SEI film include the types of negative electrode carbon materials, electrolytes and solvents, current density during formation, temperature and pressure, etc. By properly selecting materials and adjusting the parameters of the formation process, the quality of the generated SEI film can be improved, thereby improving the safety performance of the battery cell.

4. Integration of battery stacks

BMS battery management system

The battery management system (BMS) is expected to solve key problems in the use of power lithium batteries. The management system should manage the battery and its consistency so that it can obtain maximum energy storage, round-trip efficiency and safety under different conditions (temperature, altitude, maximum rate, charge state, cycle life...). BMS includes some common modules: data logger, communication unit and battery status (SOC, SOC, SOP...) evaluation model. With the development of power lithium batteries, the management capabilities of BMS are more and more stringent. New modules such as thermal management module and high-voltage monitoring module have been added... Through the addition of these safety modules, the safety and reliability of power lithium batteries in use are expected to be improved.

Integrated design of battery stack

After the battery has thermal runaway, it will cause destructive behaviors such as smoke, fire, and explosion, endangering the personal safety of users. Even if the safest configuration method is selected in theory, it is not enough to make people rest assured. For example, LiFePO4 and Li4Ti5O12 are used to make positive and negative electrode materials that are safe and suitable for fast charging and discharging batteries. Their electromotive force is located within the electrochemical window of the electrolyte, and SEI membrane is no longer required. However, even so, it will not be enough to cope with the working conditions of the electrode under some working conditions because the redox couple will appear on the top of the P orbital of the anion or overlap with the 4S orbital of the cation. No matter how reasonable the design and manufacture of the battery cell is, it cannot prevent accidents in the use conditions. Only a reasonable integrated design of the battery pack can stop the loss of the battery stack in time when there is a problem with the battery cell.

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 problem of balancing safety and endurance, Tesla Motors Co. Ltd took the lead in setting an example and gave us a good inspiration. Tesla's Model S uses Panasonic Co. Ltd's high-energy-density NCR18650A battery, with more than 7,000 cells in a battery stack. This is a combination with a high probability of thermal runaway, but through the design of battery stack integration and its BMS, many innovative patents are used to greatly reduce the probability of safety accidents in Model S during actual use. Taking Tesla's public patents as an example, the enhancement of the safety performance of cells, modules and battery packs can more or less represent advanced solutions to integration.

Tesla adds fireproof materials and sleeves to the electrodes and shells of the cells, maintains the minimum safety distance between cells, uses gaskets to keep the spacing between cells unchanged after a fire, uses efficient safety valves to predict the location of cell rupture, and cuts off the connection between the cell and the electrical appliance after the cell safety valve is opened, thereby preventing heat diffusion between the cells and the chain reaction caused by thermal runaway. At the same time, by arranging an insulation layer between the battery electrode and the inner surface of the battery shell, arranging an insulation layer between the modules, and protecting the Pack partitions, the heat conduction and runaway diffusion between the modules after thermal runaway occurs are blocked. These measures are taken from the battery cell to the module level, layer by layer, in order to maximize the timely stop of losses after internal thermal runaway occurs.

Thermal runaway contingency plan design

There are many types of contingency plan design methods for thermal runaway. In addition to the safety designs considered during the above-mentioned integration, there are also controlled cooling pipes for battery cooling and thermal runaway active mitigation systems to start spraying cooling liquid to reduce the impact of thermal runaway; the sub-stack safety valve is opened in time to allow the high-temperature gas generated by thermal runaway to be discharged from the system in time and then discharged by the main valve; other built-in systems are used to absorb the energy generated by the high temperature of thermal runaway to reduce the harm... Finally, once a situation that cannot be controlled by the previous means occurs, a bulletproof plate is installed at the bottom of the pack location, and a heat-resistant layer is added between the passenger compartment and the pack layer to minimize the personal injury caused by thermal runaway. These designs can not only timely reduce the energy during internal thermal runaway, but also foresee that after the battery level is completely out of control, the catastrophic consequences are still within the control range, thereby fundamentally ensuring the personal safety of users.

5. Abuse of batteries

Even if lithium-ion batteries are flawless 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 (overcharge and over discharge), ambient temperature (hot box), other abuses (needle puncture, extrusion, internal short circuit), etc., plus the new environmental humidity (seawater immersion) added by the new national standard are all the reasons for safety problems caused by abuse. Overcharging will cause the positive electrode active material crystal to collapse, and the lithium ion deintercalation channel will be blocked, which will cause a sharp increase in internal resistance and a large amount of Joule heat. At the same time, it will also reduce the lithium intercalation ability of the negative electrode active material and cause lithium branched crystals to cause short circuits. 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 electrode active materials with the electrolyte, the decomposition of the positive electrode/SEI film/solvent, and the reaction of the lithium intercalated negative electrode with the binder. Needle puncture/extrusion will cause internal short circuits locally. Like internal short circuits, a large amount of heat will accumulate in the short circuit area, resulting in thermal runaway.

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