About Lithium-ion Battery Safety – Part 2

3 The impact of materials

Generally speaking, the thermal stability of battery materials is an important factor in the safety of lithium-ion power batteries. This is mainly related to the thermal activity of battery materials. When the temperature of the battery rises, many exothermic reactions occur inside the battery. If the generated heat exceeds the heat loss, thermal collapse will occur. The main exothermic reactions between lithium-ion battery materials include: decomposition of SEI film; Electrolyte decomposition; Positive electrode decomposition; The reaction between the negative electrode and the electrolyte; The reaction between the negative electrode and the adhesive; In addition, due to the presence of resistance in the battery, a small amount of heat is generated during use.


3.1 Positive electrode material

The positive electrode material of lithium-ion batteries has always been the key limiting factor for the development of lithium-ion batteries. Compared with negative electrode materials, positive electrode materials have lower energy density and power density, and are also the main cause of safety hazards in lithium-ion batteries. The structure of positive and negative electrode materials has a decisive impact on the insertion and removal of lithium ions, thus affecting the cycling life of batteries. The use of easily removable active materials results in minimal and reversible structural changes during charge discharge cycles, which is beneficial for extending the lifespan of batteries.


Under the conditions of abuse in lithium-ion batteries, as the internal temperature of the battery increases, the positive electrode undergoes decomposition of the active substance and oxidation of the electrolyte. These two reactions generate a large amount of heat, leading to further increase in battery temperature. At the same time, different delithiation states have significant effects on the lattice transformation of the active substance, decomposition temperature, and thermal stability of the battery. Finding positive electrode materials with good thermal stability is the key to lithium-ion power batteries.


3.2 Negative electrode materials

The early negative electrode material used was metallic lithium, and batteries assembled with metallic lithium as the negative electrode are prone to lithium dendrites during multiple charging and discharging processes. Lithium dendrites can puncture the separator, causing battery short circuits, leakage, and even explosions. The use of lithium intercalation compounds avoids the generation of lithium dendrites, greatly improving the safety of lithium-ion batteries.

Recently, there are three types of carbon that have significant value and application prospects in lithium-ion secondary batteries: highly graphitized carbon, soft and hard carbon, and carbon nanomaterials. Currently, most of the negative electrode materials used in lithium-ion batteries use graphite, and the theoretical appropriate specific capacity of graphite is only 372 mAh/g, and the volume specific capacity is only 800 mAh/cm3. Although the medical pyrolysis carbon currently developed has a specific capacity of 700 mAh/g, its volume specific capacity is still very limited.


Due to the need for high power, high energy density metals and metal compounds have attracted widespread attention, and research mainly focuses on the development of small particles (at the nanoscale), single-phase to multiphase, and doped inactive materials. Metal and alloy negative electrodes undergo significant volume changes during cycling, resulting in a short cycling life. To extend the lifespan, approximate methods in metallurgy are used to develop and control the composition and microstructure of alloy materials. Recent research on nanoscale and surface treatment techniques has shown that as the temperature increases, carbon negative electrodes embedded in lithium will first undergo exothermic reactions with the electrolyte.


Under the same charging and discharging conditions, the exothermic rate of the reaction between the electrolyte and lithium embedded artificial graphite is much higher than that of lithium embedded MCMB, carbon fiber, coke, etc. The carbon interlayer spacing of hard carbon materials and soft carbon materials graphite materials is about 0.38nm, 0.34-0.35 nm, and 0.335 m, respectively. When lithium is embedded in the carbon layer, the interlayer spacing is about 0.37lnm. Graphite materials have the smallest interlayer spacing and the greatest deformation during the insertion and extraction process of lithium-ion batteries. The diffusion rate of lithium ions in this type of carbon layer is also slow. When charged and discharged at high currents, the polarization is large and the resistance is high, resulting in poor battery safety. Hard carbon materials, on the other hand, have the opposite effect.


However, some people also believe that increasing the degree of graphitization can reduce the activation performance of lithium ion diffusion, which is conducive to the diffusion of lithium ions. However, hard carbon materials, due to the presence of a large number of voids, perform similarly to metallic lithium negative electrodes during high current charging and discharging, and their safety is not good. In the exploration of new materials, lithiated transition metal nitrides and transition metal phosphates are good examples. Further research on these materials may inject new vitality into the development of negative electrode materials for lithium-ion batteries.


3.3 Diaphragm and electrolyte

The diaphragm itself is a non good conductor of electrons, but it also allows electrolyte ions to pass through. In addition, the separator material must also have good chemical, electrochemical stability, and mechanical properties, as well as maintain high wettability of the electrolyte during repeated charging and discharging processes. The interface compatibility between the separator material and the electrode, as well as the electrolyte retention of the separator, have a significant impact on the charging and discharging performance, cycling performance, and other aspects of lithium-ion batteries.


The electrolyte plays a role in transporting Li+between the positive and negative electrodes of lithium-ion batteries, and the compatibility between the electrolyte and the electrode directly affects the performance of the battery. The research and development of the electrolyte is very important for the performance and development of lithium-ion secondary batteries.


From the perspective of battery safety, it is required that organic electrolytes have good thermal stability and remain stable under high temperature conditions generated by battery heating, so that the entire battery will not experience thermal runaway. The impact of organic electrolytes on the safety of lithium-ion power batteries is mainly studied from three aspects: solvents, electrolyte lithium salts, and additives. The fundamental solution to the safety issues of lithium-ion batteries should be ionic liquid electrolytes.


4 Manufacturing Process and Safety of Batteries

The manufacturing process of lithium-ion batteries can be divided into liquid and polymer lithium-ion battery manufacturing processes. Regardless of the structure of the lithium-ion battery, electrode manufacturing, battery assembly and other manufacturing processes will have an impact on the safety of the battery. The quality control of various processes such as mixing positive and negative electrodes, coating, rolling, cutting or punching, assembly, sealing of electrolyte, and formation all affect the performance and safety of the battery.


The uniformity of the slurry determines the uniformity of the distribution of active substances on the electrode, thereby affecting the safety of the battery. If the fineness of the slurry is too large, there may be significant changes in the expansion and contraction of the negative electrode material during battery charging and discharging, which may lead to the precipitation of metallic lithium; A too small fineness of the slurry can lead to excessive internal resistance in the battery.


Low coating heating temperature or insufficient drying time can cause excessive internal resistance in the battery. If the heating time of the coating is too low or the drying time is insufficient, solvent residue and partial dissolution of the binder will occur, causing some active substances to be easily peeled off; Excessive temperature may cause carbonization of the binder, detachment of active substances, and formation of short circuits within the battery


5 Safe use of batteries

The safety of lithium-ion batteries has received much attention and is closely related to their expected applications. For lithium-ion power batteries, regardless of the individual capacity, a combination of batteries is inevitably used. If precise balance control cannot be achieved, it is equivalent to abuse for a single cell.


The number of battery cycles and the charging and discharging system have a significant impact on the safety of batteries. During use, it is advisable to minimize overcharging or discharging of individual cells, especially for batteries with high capacity. Thermal disturbance may cause a series of exothermic side reactions, ultimately leading to safety issues.


Lithium ion batteries also have a very poor “aging” characteristic. After being stored for a period of time, even if not recycled, some of its capacity will be permanently lost. The reason behind this is that the positive and negative electrodes of the battery have already begun their depletion process since leaving the factory. The rate of aging varies under different temperatures and battery levels. The higher the storage temperature and the fuller the charge, the more rapid the battery capacity loss will be. Therefore, it is not recommended to store lithium-ion batteries in a saturated state for a long time. For storing batteries, try to store them at low temperatures.


6 Summary

Lithium ion batteries have made significant progress in recent years, and lithium-ion power batteries have emerged in the market. Currently, it is still in the development stage and is being improved to be suitable for high rate charge discharge cycles, high and low temperature conditions, harsh environments, and low maintenance in industrial environments. With the in-depth research on safety issues such as battery systems and battery materials, it is necessary to work together from the design, production, and use sides to solve the safety of lithium-ion batteries, avoid unsafe factors, and promote the healthy development of lithium-ion power batteries.


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