Abstract: The latest progress and development prospects of high-safety lithium-ion battery research are reviewed. Mainly from the high temperature stability of electrolytes and electrodes, the causes and mechanisms of thermal instability of lithium ion batteries are introduced. The shortcomings of existing commercial lithium ion battery systems at high temperatures are clarified. 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 for the development of safe lithium batteries are prospected.

0 Preface 

Lithium-ion batteries have become a typical representative of new energy sources 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 and power tools. 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 vehicle market dominated by electric vehicles has been increasing. 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 resolved.

The temperature change of the battery system is determined by two factors: heat generation and emission. The heat generation of lithium ion batteries is mainly caused by the reaction between thermal decomposition and battery materials. The battery system is safe by reducing the heat of the battery system and improving the high temperature resistance of the system. Unlike small portable devices such as mobile phones and laptops, which have a battery capacity of less than 2 Ah, the power lithium-ion battery used in electric vehicles generally has a capacity greater than 10 Ah. In normal operation, the local temperature is often higher than 55 °C and the internal temperature is 300 °C. Above [2], under the conditions of high temperature or large rate charge and discharge, the exothermic heat of the high-energy electrode and the rise of the temperature of the flammable organic solvent will cause a series of side reactions, which will eventually lead to thermal runaway and burning or explosion of the battery [3]. . In addition to its own chemical reaction factors leading to thermal runaway, some human factors such as overheating, overcharging, and short-circuit caused by mechanical shocks 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 cause of thermal runaway

The thermal runaway of a lithium-ion battery is mainly caused by an increase in the internal temperature of the battery. The most widely used electrolyte system in commercial lithium-ion batteries is a mixed carbonate solution of LiPF6, which has high volatility, low flash point and is very easy to burn. When an internal short circuit caused by collision or deformation, large charge, discharge, and overcharge occurs, a large amount of heat is generated, causing the battery temperature to rise. When a certain temperature is reached, a series of decomposition reactions are caused, and the thermal balance of the battery is destroyed. When the heat released by these chemical reactions cannot be evacuated in time, the reaction will be aggravated and a series of self-heating side reactions will be triggered. The battery temperature rises sharply, which is “thermal runaway”, which eventually leads to the burning of the battery and even an explosion in severe cases.
In general, the cause of thermal runaway of lithium-ion batteries is mainly concentrated on the thermal instability of the electrolyte and the thermal instability of the electrolyte and the coexistence of the positive and negative electrodes.
At present, from a large perspective, safe lithium-ion batteries mainly take measures from external management and internal design to control internal temperature, voltage and air pressure for safety purposes.

2 Solving the problem of thermal runaway

2.1 External management

1) PTC (Positive Temperature Coefficient) component: A PTC component is mounted in a lithium ion battery, which takes into account the pressure and temperature inside the battery. When the battery heats up due to overcharging, the internal resistance of the battery increases rapidly to limit the current. The voltage drop between the negative electrodes is a safe voltage, which enables automatic protection of the battery.
2) Explosion-proof valve: When the internal pressure is too large due to abnormality of the battery, the explosion-proof valve is deformed, and the lead wire for connection inside the battery is cut off to stop charging.
3) Electronic circuit: The battery pack of 2~4 knots can be embedded in the electronic circuit to design the lithium ion protector to avoid overcharging and overdischarging, thus avoiding safety accidents and prolonging 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 can not completely solve the battery safety problem. Therefore, it is necessary to establish an inherent security protection mechanism.

2.2 Improve the electrolyte system

The electrolyte acts as the blood of the lithium ion battery. The nature of the electrolyte directly determines the performance of the battery and plays an important role in the capacity, operating temperature range, cycle performance and safety performance of the battery. At present, the most widely used composition of the commercial lithium ion battery electrolyte system is LiPF6, ethylene carbonate and linear carbonate. The first two are indispensable components, and their use also produces some limitations in battery performance. At the same time, a large number of low-boiling, low-flash carbonate solvents are used in the electrolyte, which will be lower at lower temperatures. Flashing, there is a big safety hazard. Therefore, many researchers have tried to improve the electrolyte system to improve the safety of the electrolyte. In the case where the main material of the battery (including the electrode material, the separator material, and the electrolyte material) does not undergo a subversive change in a short time, improving the stability of the electrolyte is an important way to enhance the safety of the lithium ion battery.

2.2.1 Functional Additives Functional additives are characterized by low dosage and strong targeting. That is, certain macroscopic properties of the battery can be significantly improved without increasing or substantially increasing the cost of the battery and without changing the production process. Therefore, functional additives have become a research hotspot in the field of lithium-ion batteries, and are one of the most promising ways to solve the flammability problem of lithium-ion battery electrolytes. The basic function of the additive is to prevent the battery from being overheated and to limit the battery voltage to a controlled range. Therefore, the design of the additive is also considered from the viewpoint of the temperature and the charging potential.

Flame Retardant Additive: Flame retardant additive can be divided into organic phosphorus flame retardant additive, nitrogen compound flame retardant additive, halogenated carbonate flame retardant additive, silicon flame retardant additive and composite flame retardant additive according to different flame retardant elements. 5 main categories.

Organic phosphide flame retardants: mainly include some alkyl phosphates, alkyl phosphites, fluorinated phosphates and phosphazenes. The flame retardant mechanism is mainly the chain reaction of flame retardant molecules interfering with hydroxyl radicals, also known as free radical trapping mechanism. The gasification decomposition of the additive releases a phosphorus-containing free radical that has the ability to trap the chain reaction of hydrogen radicals in the capture system.

Phosphate-based flame retardants: mainly trimethyl phosphate, triethyl phosphate (TEP), tributyl phosphate (TBP) and the like. Phosphazene compounds such as hexamethylphosphazene (HMPN), alkyl phosphites such as trimethyl phosphite (TMPI), tris-(2,2,2-trifluoroethyl), phosphite (TT⁃) FP), fluorinated phosphate such as tris-(2,2,2-trifluoroethyl)phosphate (TFP), di-(2,2,2-trifluoroethyl)-methyl phosphate (BMP) (2,2,2-Trifluoroethyl)-diethyl phosphate (TDP), phenyloctyl phosphate (DPOF), etc. are all good flame retardant additives. Phosphates generally have a relatively high viscosity and poor electrochemical stability. The addition of a flame retardant also has a negative effect on the ionic conductivity of the electrolyte and the cyclic reversibility of the battery while improving the flame retardancy of the electrolyte. The solution is generally as follows: 1 increasing the carbon content of the alkyl group; 2 partially replacing the alkyl group with the aromatic (phenyl) group; 3 forming the phosphate of the cyclic structure.

Organic halogenated species (halogenated solvents): Organic halogenated flame retardants mainly refer to fluorinated organic compounds. When H in a non-aqueous solvent is substituted by F, its physical properties change, such as a decrease in melting point, a decrease in viscosity, and an increase in chemical and electrochemical stability. The organic halogenated flame retardant mainly includes a fluorinated cyclic carbonate, a fluorinated chain carbonate, and an alkyl-perfluoroalkyl ether. OHMI and other comparative fluoroether, fluoroester fluorochemicals showed that 0.33 mol% of the fluorinated compound was added 0.67 mol/L LiClO4/EC+DEC+PC (volume ratio 1:1: 1) The electrolyte has a high flash point, and the reduction potential is higher than the organic solvents EC, DEC and PC, which can rapidly form the SEI film on the surface of the natural graphite, improving the coulombic efficiency and discharge capacity of the first charge and discharge.

The fluorinated material itself does not have the free radical trapping function of the flame retardant as described above, and only serves to dilute the highly volatile and flammable cosolvent, so only when it accounts for the majority of the volume ratio in the electrolyte. (>70%), the electrolyte is not flammable.

Composite flame retardant: The composite flame retardants currently used in electrolytes are P-F compounds and N-P compounds. Representative materials include hexamethylphosphoramide (HMPA) and fluorophosphate. The flame retardant exerts a flame retardant effect by the synergistic action of two flame retardant elements.

FEI et al. proposed two N-P flame retardants MEEP and MEE, the molecular formula of which is shown in Figure 1. LiCF3SO3/MEEP:PC=25:75, the electrolyte can reduce the flammability by 90%, and the conductivity can reach 2. 5 × 10-3 S/cm.

2) Overcharge additive: When the lithium ion battery is overcharged, a series of reactions will occur. The electrolyte component (mainly solvent) undergoes an irreversible oxidative decomposition reaction on the surface of the positive electrode to generate a gas and release a large amount of heat, thereby causing an increase in the internal pressure of the battery and an increase in temperature, which seriously affects the safety of the battery. From the action mechanism, the overcharge protection additive is mainly divided into two types: redox shuttle type and electropolymer type. From the type of additive, it can be further divided into lithium halides and metallocene compounds. At present, the overcharge additives for large-scale applications mainly include biphenyl (BP) and cyclohexylbenzene (CHB) for redox-based anti-overcharge additives. The principle is that when the charging voltage exceeds the normal cut-off voltage of the battery, the additive starts to occur at the positive electrode. In the oxidation reaction, the oxidation product diffuses to the negative electrode, and a reduction reaction occurs. The redox pair shuttles between the positive and negative electrodes, absorbing excess charge. Representative materials are ferrocene and its derivatives, complexes of 2,2-pyridine and 1,10-phenanthroline of ferrous ions, and thiazolidine derivatives.

The polymerization block type anti-overcharge additive. Representative materials are cyclohexylbenzene, biphenyl and the like. When biphenyl is used as an anti-overcharge additive, when the voltage reaches 4.5 to 4. 7 V, the added biphenyl is electrochemically polymerized to form a conductive film on the surface of the positive electrode, which increases the internal resistance of the battery, thereby limiting The charging current protects the battery.

2.2.2 Ionic Liquid The ionic liquid electrolyte consists entirely of anions and cations. Due to the weak interaction between the anion and cation, the interaction between the anion and the cation is weak, the electron distribution is not uniform, and the anion and cation are free to move at room temperature, which is in a liquid state. Generally, it can be classified into imidazoles, pyrazoles and pyridines, quaternary ammonium salts, and the like. Compared with common organic solvents for lithium ion batteries, ionic liquids have five main advantages: 1 high thermal stability, no decomposition at 200 °C; 2 vapor pressure is almost 0, no need to worry about battery swelling; 3 ionic liquid is not flammable , non-corrosive; 4 has high conductivity; 5 chemical or electrochemical stability.

AN et al. formulated PP13TFSI and 1 mol LiPF6 EC / DEC (1:1) into an electrolyte to achieve a completely non-combustible effect. Adding 2wt% LiBOB additive to the system can also significantly improve the interface compatibility.
The only problem to be solved at present is the conductivity of ions in the electrolyte system.

2.2.3 Selecting a lithium salt with good thermal stability Lithium hexafluorophosphate (LiPF6) is an electrolyte lithium salt widely used in commercial lithium ion batteries. Although its single nature is not optimal, its overall performance is the most advantageous. However, LiPF6 also has its disadvantages. For example, LiPF6 is chemically and thermodynamically unstable, and the following reactions occur:

LiPF(6s)→LiF(s)+ PF(5g),
The PF5 formed by this reaction easily attacks the lone pair of electrons on the oxygen atom in the organic solvent, resulting in ring-opening polymerization of the solvent and cleavage of the ether bond, which is particularly decomposed at high temperatures.
At present, research on high temperature electrolyte salts is mostly concentrated in the field of organic lithium salts. Representative materials mainly include a boron-based lithium salt and an imido lithium salt. LiB(C2O4)2 (LiBOB) is an electrolyte salt newly synthesized in recent years. It has many excellent properties and has a decomposition temperature of 302 °C, which can form a stable SEI film on the negative electrode. Improve the performance of graphite in PC-based electrolytes, but its viscosity is large, and the impedance of the formed SEI film is large [14]. The decomposition temperature of LiN(SO2CF3)2(LiTFSI) is above 360 °C, the ionic conductivity at room temperature is slightly lower than that of LiPF6, and the electrochemical stability is good. The oxidation potential is about 5.0 V, which is the most studied organic lithium salt. However, it is highly corrosive to the Al-based current collector.

2.2.4 Polymer electrolytes Many commercial lithium-ion batteries use flammable and volatile carbonate solvents. If liquid leakage occurs, it may cause a fire. This is especially true for high-capacity, high-energy, high-density lithium-ion batteries. The use of non-flammable polymer electrolytes instead of flammable organic liquid electrolytes can significantly improve the safety of lithium-ion batteries.

Great progress has been made in the study of polymer electrolytes, especially gel-type polymer electrolytes. It has been successfully used in commercial lithium ion batteries. According to the classification of polymer bodies, there are mainly three types of gel polymer electrolytes: PAN-based polymer electrolyte, PMMA polymer electrolyte, PVDF-based polymer electrolyte.

However, gel-type polymer electrolytes are actually the result of compromises between dry polymer electrolytes and liquid electrolytes. Gel-type polymer batteries still have a lot of work to do.

2.3 cathode material

It can be determined that the positive electrode material is unstable when the charging state voltage is higher than 4 V, and it is easy to generate heat at a high temperature to liberate oxygen, and the oxygen and the organic solvent continue to react to generate a large amount of heat and other gases, thereby reducing the safety of the battery [2, 17- 19]. Therefore, the reaction of the positive electrode with the electrolyte is considered to be the main cause of thermal runaway. For the positive electrode material, a common method for improving the safety is coating modification. If the surface of the positive electrode material is coated with MgO, A12O3, SiO2, TiO2, ZnO, SnO2, ZrO2, etc., the reaction between the positive electrode and the electrolyte after Li+ removal can be reduced, and the oxygen release of the positive electrode can be reduced, and the phase change of the positive electrode material can be suppressed. , to improve its structural stability, reduce the disorder of the cations in the crystal lattice, thereby reducing the side reaction heat generation during the cycle.

2.4 Carbon materials

At present, a power battery having higher safety requirements generally uses a spherical carbon material having a lower specific surface area, a higher charge and discharge platform, a less charge state activity, and a relatively better thermal stability, such as a mesophase. Carbon microspheres (MCMB), or spinel-structured Li9Ti5O12, have better structural stability than layered graphite [20]. Current methods for improving the performance of carbon materials mainly include surface treatment (surface oxidation, surface halogenation, carbon coating, cladding metal and metal oxide, polymer coating) or introduction of metal or non-metal doping.

2.5 diaphragm

At present, the most widely used separator in commercial lithium ion batteries is still a polyolefin material, and its main disadvantages are heat shrinkage at high temperatures and poor electrolyte wettability. In order to overcome these shortcomings, the researchers tried many methods, such as looking for a heat-stable material instead, or adding a small amount of Al2O3 or SiO2 nano-powder diaphragm, which not only has the function of a common diaphragm, but also improves the thermal stability of the cathode material. effect.
A polyimide nano-nonwoven membrane prepared by an electrospinning method such as MIAO. Characterizations such as DSC and TGA show that they are not only thermally stable at 500 °C, but also have better electrolyte wettability than Celgard separators.
WANG et al. prepared Al2O3-PVDF nano-scale composite microporous membrane, which showed good electrochemical performance and thermal stability, meeting the requirements of lithium ion battery separator.

3 Summary and outlook

Lithium-ion batteries for electric vehicles and energy storage have much larger capacity than small electronic devices, and the use environment is more complicated. In summary, we can see that its security performance has not been solved yet, and it has become the technical bottleneck of current applications. Subsequent work needs to go deep into the thermal effects that the battery may cause after abnormal operation, and explore an effective way to improve the safety performance of lithium-ion batteries. At present, the use of fluorine-containing solvents and flame retardant additives is the main direction for the development of safe lithium-ion batteries. How to balance electrochemical performance and high-temperature safety will be the focus of future research. For example, a high-performance composite flame retardant integrating P, N, F, and Cl is developed, and an organic solvent having a high boiling point and a high flash point is developed to prepare a high-safety electrolyte. Composite flame retardants, dual-function additives will also become the future development trend. For lithium ion battery electrode materials, the surface chemistry of the materials is different, and the sensitivity of the electrode materials to the charge and discharge potential is also inconsistent. 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 develop different battery systems for specific electrode materials. At the same time, a polymer lithium ion battery system with high safety was developed or an inorganic solid electrolyte with single cation conduction and fast ion transport and high thermal stability was developed. In addition, improving the performance of ionic liquids and developing simple and inexpensive synthetic processes are also important aspects of future research.