The latest progress and development prospects of high-safety lithium-ion battery research are reviewed.

　　Abstract: The latest progress and development prospects of high-safety lithium-ion battery research are reviewed. This article mainly introduces the causes and mechanisms of thermal instability of lithium-ion batteries from the high-temperature stability of electrolytes and electrodes, clarifies the shortcomings of existing commercial lithium-ion battery systems at high temperatures, and proposes the development of high-temperature electrolytes, positive and negative electrode modifications, and External battery management, etc. to design high-safety lithium-ion batteries. The technical prospects of developing safe lithium batteries are prospected. 0 Introduction Lithium-ion batteries have become a typical representative of a new type of energy due to their low cost, high performance, high power, green environment and many other advantages. They are widely used in 3C digital products, mobile power supplies, power tools and other fields. In recent years, due to the intensification of environmental pollution and the guidance of national policies, the demand for lithium-ion batteries in the electric transportation market, mainly electric vehicles, has continued to increase. In the process of developing high-power lithium-ion battery systems, battery safety issues have attracted widespread attention. , the existing problems urgently need to be further solved. The temperature change of the battery system is determined by two factors: heat generation and dissipation. Heat generation in lithium-ion batteries is mainly caused by thermal decomposition and reactions between battery materials. Reduce the heat of the battery system and improve the high temperature resistance of the system, and the battery system will be safer. Different from the battery capacity of small portable devices such as mobile phones and notebooks, which are generally less than 2Ah, the power lithium-ion batteries used in electric vehicles generally have a capacity greater than 10Ah. During normal operation, the local temperature is often higher than 55°C, and the internal temperature will reach more than 300°C. Under high-temperature or high-rate charge and discharge conditions, the heat release of high-energy electrodes and the increase in the temperature of flammable organic solvents will cause a series of side reactions to occur, eventually leading to thermal runaway and battery combustion or explosion [3]. In addition to thermal runaway caused by its own chemical reaction factors, some human factors such as short circuit caused by overheating, overcharging, and mechanical shock can also cause thermal instability of lithium-ion batteries and cause safety accidents. Therefore, it is of great practical significance to study and improve the high-temperature performance of lithium-ion batteries. 1 Analysis of the causes of thermal runaway Thermal runaway of lithium-ion batteries is mainly caused by the rise in the internal temperature of the battery. Currently, the most widely used electrolyte system in commercial lithium-ion batteries is a mixed carbonate solution of LiPF6. This type of solvent has high volatility, low flash point, and is very easy to burn. When an internal short circuit is caused by collision or deformation, a large amount of heat will be generated, causing the battery temperature to rise. When a certain temperature is reached, a series of decomposition reactions will occur, destroying the thermal balance of the battery. When the heat released by these chemical reactions cannot be evacuated in time, the reaction will be accelerated and trigger a series of self-heating side reactions. The temperature of the battery rises sharply, which is called "thermal runaway", eventually causing the battery to burn and even explode in severe cases. In general, the causes of thermal runaway in lithium-ion batteries mainly focus on two major aspects: the thermal instability of the electrolyte and the thermal instability of the coexistence system between the electrolyte and the positive and negative electrodes. At present, from a large perspective, safety lithium-ion batteries mainly take measures from two aspects: external management and internal design, and control internal temperature, voltage, and air pressure to achieve safety purposes. 2 Strategies to Solve Thermal Runaway 2.1 External Management 1) PTC (Positive Temperature Coefficient) Element: Installing PTC elements in lithium-ion batteries takes into account the pressure and temperature inside the battery. When the battery heats up due to overcharging, the temperature inside the battery The resistance increases rapidly to limit the current, reducing the voltage between the positive and negative electrodes to a safe voltage, thereby realizing the automatic protection function of the battery. 2) Explosion-proof valve: When the internal pressure of the battery is too high due to abnormality, the explosion-proof valve deforms and cuts off the leads placed inside the battery for connection, stopping charging. 3) Electronic circuit: Battery packs of 2 to 4 cells can be pre-embedded with electronic circuits to design lithium-ion protectors to avoid overcharge and over-discharge, thereby avoiding safety accidents and extending battery life. Of course, these external control methods have certain effects, but these additional devices increase the complexity and production cost of the battery, and cannot completely solve the battery safety problem. Therefore, it is necessary to establish an inherent security protection mechanism. 2.2 Improved electrolyte system The electrolyte is the blood of lithium-ion batteries. The properties of the electrolyte directly determine the performance of the battery, and play an important role in the battery's capacity, operating temperature range, cycle performance and safety performance. Currently, the most widely used components of commercial lithium-ion battery electrolyte systems are LiPF6, ethylene carbonate and linear carbonate. The first two are indispensable components, and their use also creates certain limitations in battery performance. At the same time, a large number of carbonate solvents with low boiling point and low flash point are used in the electrolyte, which will break down at lower temperatures. Flashover is a huge safety hazard. Therefore, many researchers have tried to improve the electrolyte system to improve the safety performance of the electrolyte. As long as the main battery materials (including electrode materials, separator materials and electrolyte materials) do not undergo disruptive changes in a short period of time, improving the stability of the electrolyte is an important way to enhance the safety of lithium-ion batteries. 2.2.1 Functional additives Functional additives have the characteristics of small dosage and strong pertinence. That is to say, certain macroscopic properties of the battery can be significantly improved without increasing or basically increasing the cost of the battery and changing the production process. Therefore, functional additives have become a research hotspot in the field of lithium-ion batteries today and are one of the most promising ways to solve the current flammability problem of lithium-ion battery electrolytes. The basic function of the additive is to prevent the battery temperature from being too high and limit the battery voltage within a controllable range. Therefore, the design of additives is also considered from the perspective of temperature and charging potential. Flame retardant additives: Flame retardant additives can be divided into organophosphorus flame retardant additives, nitrogen-containing compound flame retardant additives, halogenated carbonate flame retardant additives, silicon flame retardant additives and composite flame retardant additives according to different flame retardant elements. 5 main categories. Organophosphide flame retardants: mainly include some alkyl phosphates, alkyl phosphites, fluorinated phosphates and phosphazene compounds. The flame retardant mechanism is mainly the chain reaction of flame retardant molecules interfering with hydroxyl radicals, also known as the free radical capture mechanism. The vaporization and decomposition of the additive releases phosphorus-containing free radicals, which have the ability to capture hydrogen radicals in the system to terminate the chain reaction. Phosphate ester flame retardants: mainly include trimethyl phosphate, triethyl phosphate (TEP), tributyl phosphate (TBP), etc. Phosphazenes such as hexamethylphosphazene (HMPN), alkyl phosphites such as trimethyl phosphite (TMPI), tris-(2,2,2-trifluoroethyl), phosphite (TT- FP), fluorinated phosphates such as tris-(2,2,2-trifluoroethyl)phosphate (TFP), bis-(2,2,2-trifluoroethyl)-methylphosphate (BMP) , (2,2,2-trifluoroethyl)-diethylphosphate (TDP), phenyloctylphosphate (DPOF), etc. are all good flame retardant additives. Phosphate esters usually have relatively high viscosity and poor electrochemical stability. The addition of flame retardants not only improves the flame retardancy of the electrolyte, but also has a negative impact on the ionic conductivity of the electrolyte and the cycle reversibility of the battery. The solutions are generally: ① increase the carbon content of the alkyl group; ② partially replace the alkyl group with an aromatic (phenyl) group; ③ form a phosphate ester with a cyclic structure. Organohalogenated substances (halogenated solvents): Organohalogenated flame retardants mainly refer to fluorinated organic substances. When H in non-aqueous solvents is replaced by F, its physical properties will change, such as lower melting point, lower viscosity, and improved chemical and electrochemical stability. Organic halogenated flame retardants mainly include fluorinated cyclic carbonates, fluorinated chain carbonates and alkyl-perfluoroalkyl ethers. OHMI and other studies comparing fluorinated ethers and fluorinated esters with fluorinated compounds have shown that the 0.67mol/LLiClO4/EC+DEC+PC (volume ratio 1:1:1) electrolyte with 33.3% (volume fraction) fluorinated compounds added has a relatively good It has a high flash point and a reduction potential higher than organic solvents EC, DEC and PC. It can quickly generate an SEI film on the surface of natural graphite, improving the Coulombic efficiency and discharge capacity of the first charge and discharge. Fluorine itself does not have the free radical trapping function like the flame retardants mentioned above, but only plays the role of diluting highly volatile and flammable co-solvents. Therefore, it can only be used when its volume ratio in the electrolyte accounts for the majority. (>70%), the electrolyte is non-flammable. Composite flame retardants: Composite flame retardants currently used in electrolytes include P-F compounds and N-P compounds. Representative substances mainly include hexamethylphosphoramide (HMPA), fluorinated phosphates, etc. Flame retardants exert their flame retardant effect through the synergistic effect of two flame retardant elements. FEI et al. proposed two N-P flame retardants, MEEP and MEE, whose molecular formulas are shown in Figure 1. LiCF3SO3/MEEP:PC=25:75, the electrolyte can reduce the flammability by 90%, and the conductivity can reach 2.5×10-3S/cm.

　　2) Overcharge additive: When a lithium-ion battery is overcharged, a series of reactions will occur. The electrolyte components (mainly solvents) undergo irreversible oxidation and decomposition reactions on the surface of the positive electrode, producing gas and releasing a large amount of heat, which leads to an increase in the internal pressure and temperature of the battery, seriously affecting the safety of the battery. In terms of mechanism of action, overcharge protection additives are mainly divided into two types: redox shuttle type and electropolymerization type. In terms of additive types, they can be divided into lithium halides and metallocene compounds. The overcharge additives currently used in large-scale applications mainly include biphenyl (BP) and cyclohexylbenzene (CHB). For redox anti-overcharge additives, the principle is that when the charging voltage exceeds the normal cut-off voltage of the battery, the additives begin to generate energy in the positive electrode. Oxidation reaction, the oxidation product diffuses to the negative electrode, and a reduction reaction occurs. The redox couple shuttles between the positive and negative electrodes, absorbing excess charge. Its representative substances include ferrocene and its derivatives, complexes of 2,2-pyridine and 1,10-phenanthroline of ferrous ions, and thianthrene derivatives. Polymerization blocking additive to prevent overcharge. Representative substances include cyclohexylbenzene, biphenyl and other substances. When biphenyl is used as an anti-overcharge additive, when the voltage reaches 4.5~4.7V, the added biphenyl polymerizes electrochemically, forming a conductive film on the surface of the positive electrode, which increases the internal resistance of the battery, thereby limiting the charging current and protecting the battery. . 2.2.2 Ionic liquid Ionic liquid electrolyte is composed entirely of anions and cations. Because the anions or cations are large in size, the interaction between anions and cations is weak and the electron distribution is uneven. Anions and cations can move freely at room temperature and are in a liquid state. Generally speaking, they can be divided into imidazoles, pyrazole and pyridine, quaternary ammonium salts, etc. Compared with ordinary organic solvents for lithium-ion batteries, ionic liquids have five main advantages: ① High thermal stability and no decomposition at 200°C; ② The vapor pressure is almost 0, so there is no need to worry about bloating in the battery; ③ Ionic liquids are not flammable , non-corrosive; ④ high electrical conductivity; ⑤ good chemical or electrochemical stability. AN et al. formulated PP13TFSI and 1molLiPF6EC/DEC (1:1) into an electrolyte, which can achieve completely non-flammable effect. Adding 2wt% LiBOB additive to this system can also significantly improve the interface compatibility. The only problem that remains to be solved is the conductivity of ions in the electrolyte system. 2.2.3 Choose a lithium salt with good thermal stability. Lithium hexafluorophosphate (LiPF6) is the electrolyte lithium salt widely used in commercial lithium-ion batteries. Although its single property is not optimal, its comprehensive performance is the most advantageous. However, LiPF6 also has its shortcomings. For example, LiPF6 is chemically and thermodynamically unstable, and the following reaction will occur: LiPF (6s) → LiF (s) + PF (5g). The PF5 generated by this reaction can easily attack oxygen atoms in organic solvents. The lone pair of electrons on the solvent leads to ring-opening polymerization of the solvent and cleavage of the ether bond. This reaction is particularly serious at high temperatures. Current research on high-temperature electrolyte salts is mostly focused on the field of organic lithium salts. Representative substances mainly include boron lithium salt and imide lithium salt. LiB(C2O4)2 (LiBOB) is a newly synthesized electrolyte salt in recent years. It has many excellent properties, has a decomposition temperature of 302°C, and can form a stable SEI film on the negative electrode. Improve the performance of graphite in PC-based electrolyte, but its viscosity is high and the resistance of the SEI film formed is large [14]. The decomposition temperature of LiN (SO2CF3)2 (LiTFSI) is above 360°C, its ionic conductivity at room temperature is slightly lower than that of LiPF6, it has good electrochemical stability, and its oxidation potential is about 5.0V. It is the most studied organic lithium salt, but it The corrosion of Al-based current collector is serious. 2.2.4 Polymer electrolyte Many commercial lithium-ion batteries use flammable and volatile carbonate solvents. If leakage occurs, it is likely to cause a fire. This is especially true for power lithium-ion batteries with large capacity and high energy density. The use of non-flammable polymer electrolytes instead of flammable organic liquid electrolytes can significantly improve the safety of lithium-ion batteries. Research on polymer electrolytes, especially gel polymer electrolytes, has made great progress. It has been successfully used in commercial lithium-ion batteries. According to the main polymer classification, gel polymer electrolytes mainly include the following three categories: PAN-based polymer electrolyte, PMMA polymer electrolyte, and PVDF-based polymer electrolyte. However, gel polymer electrolytes are actually the result of a compromise between dry polymer electrolytes and liquid electrolytes. Gel polymer batteries still have a lot of work to do. 2.3 The positive electrode material can determine that the positive electrode material is unstable when the charged state voltage is higher than 4V, and is prone to thermal decomposition at high temperatures to release oxygen. Oxygen and organic solvents continue to react to generate a large amount of heat and other gases, reducing the safety of the battery [2, 17-19]. Therefore, the reaction between the positive electrode and the electrolyte is considered to be the main cause of thermal runaway. For cathode materials, a common method to improve their safety is coating modification. For example, surface coating of the positive electrode material with MgO, A12O3, SiO2, TiO2, ZnO, SnO2, ZrO2 and other substances can reduce the reaction between the positive electrode and the electrolyte after Li+ removal, while reducing the oxygen release of the positive electrode and inhibiting the phase change of the positive electrode material. , improve its structural stability, reduce the disorder of cations in the crystal lattice, thereby reducing the heat generated by side reactions during the cycle. 2.4 Carbon materials Currently, power batteries with higher safety requirements usually use spherical carbon materials with lower specific surface area, higher charge and discharge platform, smaller charged state activity, relatively good thermal stability and high safety. For example, mesocarbon microspheres (MCMB) or spinel-structured Li9Ti5O12 have better structural stability than layered graphite [20]. Current methods to improve the performance of carbon materials mainly include surface treatment (surface oxidation, surface halogenation, carbon coating, coating of metals and metal oxides, polymer coating) or the introduction of metal or non-metal doping. 2.5 Separator The most widely used separator in commercial lithium-ion batteries is still polyolefin material. Its main disadvantages are heat shrinkage at high temperatures and poor electrolyte wettability. In order to overcome these shortcomings, researchers have tried many methods, such as looking for thermally stable materials instead, or adding a small amount of Al2O3 or SiO2 nanopowder separators, which not only have the function of ordinary separators, but also have the ability to improve the thermal stability of the cathode material. effect. Polyimide nano-nonwoven prepared by MIAO and others using electrospinning methodCloth separator. Characterization methods such as DSC and TGA show that it can not only maintain thermal stability at 500°C, but also have better electrolyte wettability than Celgard separators. WANG et al. prepared an Al2O3-PVDF nanoscale composite microporous membrane. The composite microporous membrane exhibits good electrochemical performance and thermal stability and meets the requirements for use as a lithium-ion battery separator. 3 Summary and Outlook Lithium-ion batteries used in electric vehicles and energy storage have much larger capacities than small electronic devices, and their use environments are more complex. To sum up, we can see that its security performance is far from being solved and has become a technical bottleneck in current applications. Follow-up work needs to delve into the thermal effects that may result from abnormal operation of the battery and explore effective ways to improve the safety performance of lithium-ion batteries. Currently, the use of fluorinated solvents and flame retardant additives is the main direction for developing safe lithium-ion batteries. How to balance electrochemical performance and high-temperature safety will be the focus of future research. For example, develop high-performance composite flame retardants integrating P, N, F, and Cl, develop organic solvents with high boiling points and high flash points, and then prepare electrolytes with high safety performance. Composite flame retardants and dual-functional additives will also become the future development trend. For lithium-ion battery electrode materials, due to the different surface chemical properties of the materials and the different sensitivity of the electrode materials to the charge and discharge potential, it is impossible to design all battery structures with one or a limited number of electrodes/electrolytes/additives. Therefore, in the future, efforts should be made to research and develop different battery systems for specific electrode materials. At the same time, develop and construct polymer lithium-ion battery systems with high safety or develop inorganic solid electrolytes with single cation conductivity, fast ion transport, and high thermal stability. In addition, improving the performance of ionic liquids and developing simple and cheap synthesis processes are also important aspects of future research.

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