Abstract: Summary of the latest advances and development prospects for high security lithium-ion battery research. Important from the high temperature stability of electrolytes and electrodes, the causes of thermal instability of lithium ion batteries and their mechanisms have clarified that the existing commercial lithium-ion battery system is inadequate at high temperatures, proposes to develop high temperature electrolytes, positive and negative modifications and External battery management, etc. to design high security lithium-ion batteries.
Outlook on the development of the technical prospect of the development of safety lithium-ion batteries. 0 Introduction Lithium ion batteries become a typical representative of a new type of energy due to its low cost, high performance, high-power, and green environment, widely used in 3C digital products, mobile power and electric tools..
In recent years, due to environmental pollution intensification and national policy guidance, electric vehicle-based electric vehicle market has increased the demand for lithium-ion batteries, in the process of developing high-power lithium-ion battery systems, battery safety issues have attracted extensive attention , Existing problems urgently need to further solve. The temperature change of the battery system is determined by the emergence of heat and distributed two factors..
The occurrence of heat of lithium ion battery is important is caused by the reaction between thermal decomposition and battery material.. Reduce the heat of the battery system and improve the system of anti-high temperature performance, the battery system is safe.
And small portable equipment such as mobile phones, the laptop battery capacity is generally less than 2AH, and the power-type lithium-ion battery capacity used in electric vehicles is generally greater than 10ah, and the local temperature is often higher than 55 ° C during normal operation, and the internal temperature will reach 300 ° C, Under high temperature or large rate charge and discharge conditions, the rise in heat and flammability organic solvent temperature will cause a series of side reactions, eventually leading to thermal out of control and battery combustion or explosion . In addition to its own chemical response factors, some people have a short circuit caused by overheating, overtaking, and mechanical impact, some artificial factors can also lead to the occurrence of a lithium-ion battery to cause safety accidents..
Therefore, it is important to study and improve the high temperature performance of lithium-ion batteries.. 1 thermal out-of-control cause analysis of the thermal out of control of the lithium-ion battery is important because the internal temperature of the battery rises.
At present, the most widely used electrolyte system in commercial lithium-ion batteries is a mixed carbonate solution of LiPF6. Such solvent has high volatility, low flash point, very easy to combust. When the internal short circuit caused by collision or deformed, a large rate charge and discharge and overtake, there will be a lot of heat, resulting in raising battery temperature.
. When reaching a certain temperature, a series of decomposition reactions will cause the thermal balance of the battery to be destroyed..
When the heat released by these chemical reactions cannot be evacuated in time, it will exacerbate the progression of the reaction, and trigger a series of self-heating side reactions.. The battery temperature rises sharply, that is, “thermal out of control”, eventually leading to the burning of the battery, and even an explosion occurs seriously.
. In general, the cause of thermal out of control of lithium-ion battery is important in the thermal instability of the electrolyte, as well as the thermal instability of electrolyte and positive and negative electrode coexistence..
At present, from a large aspect, the safety of lithium-ion batteries is important from external management and internal design to control internal temperature, voltage, and air pressure to achieve safety purposes.. 2 Resolve thermal out-of control strategy 2.
External management 1) PTC (positive temperature coefficient) component: Install the PTC component in a lithium ion battery, which considers the pressure and temperature inside the battery, and when the battery is warmed by overcharge, the battery is 10 The resistance increases to limit the current, and the voltage between the positive and negative poles is reduced to a safe voltage to realize the automatic protection function of the battery.. 2) Explosion-proof valve: When the battery is too large due to abnormal, the explosion-proof valve is deformed, which will be placed inside the battery to be connected, stop charging.
3) Electronics: 2 ~ 4 battery packs can embelfine the electronic circuit design lithium ion protector, prevent overcharge and over-discharge, prevent safety accidents, extend battery life. Of course, these external control methods have a certain effect, but these additional devices have added the complexity and production cost of the battery, and they cannot completely solve the problem of battery safety..
Therefore, it is necessary to establish an intrinsic safety protection mechanism.. 2.
2 Improveing the electrolyte electrolyte electrolyte as a lithium ion battery, the nature of the electrolyte directly determines the performance of the battery, the capacity of the battery, the operating temperature range, the cycle performance and safety performance are important.. At present, commercial lithium-ion battery electrolytic solution systems, the most widely used composition is LIPF6, vinyl carbonate and linear carbonate.
The front is an indispensable ingredient, and their use also has some limitations in terms of battery performance. At the same time, a large amount of low boiling, low flash point of carbonate solvent is used in the electrolyte, which will be at lower temperatures. Flash, there is a big safety hazard.
Therefore, many researchers try to improve the electrolyte system to improve the safety performance of electrolytes.. In the case where the main body material of the battery (including the electrode material, the diaphragm material, the electrolyte material) does not change in a short period of time, the stability of the electrolyte is an important way to enhance the safety of lithium ion batteries.
1 Functional additive function additives have less dosage, targeted feature. That is, it can significantly improve certain macroscopic performance of the battery without changing the production process without changing or substantially no new battery costs..
Therefore, function additives have become a hot spot in today’s lithium-ion battery, which is one of the most promising pathways that are currently the most promising pathogenic solution of lithium-ion battery electrolyte.. The basic use of the additive is to prevent the battery temperature from being too high and the battery voltage is limited to the control range.
. Therefore, the design of the additive is also considered from the perspective of the temperature and charging potential..
Flame retardant additive: The flame retardant additive can also be divided into organic phosphorus flame retardant additives, a nitrogen-containing compound flame retardant additive, a silicon-based flame retardant additive, and a composite flame retardant additive. 5 important categories. Organic phosphorescell-flame retardant: Important include some alkyl phosphate, alkyl phosphite, fluorinated phosphate, and phosphate nitrile compounds.
The flame retardant mechanism is important to the chain reaction of flame retardant molecules interfering with hydrogen free radicals, also known as free radical capture mechanism.. Additive gasification decomposition releases phosphorus-containing free radicals, the ability of the free radicals to terminate a chain reaction.
Phosphate flame retardant: Important phosphate, triethyl phosphate (TEP), tributyl phosphate (TBP), etc.. Phosphate nitrile compound such as hexamethyl phosphazene (HMPN), alkyl phosphite such as trimethyl phosphite (TMPI), three – (2,2,2-trifluoroethyl), phosphite (TT- FP), fluorinated acid ester, such as three-(2,2,2-trifluoroethyl) phosphate (TFP), di-(2,2,2-trifluoroethyl)-methyl phosphate (BMP) , (2,2,2-trifluoroethyl) – diethyl phosphate (TDP), phenylphosphate (DPOF), etc.
is a good flame retardant additive. The phosphate typically has a relatively large viscosity, poor electrochemical stability, and the addition of the flame retardant also has a negative effect on the ionic conductivity of the electrolyte and the circulation reversibility of the electrolyte while increasing the refractiveness of the electrolyte..
It is generally: 1 carbon content of new alkyl groups; 2 aromatic (phenyl) group moiety substituted alkyl group; 3 form a cyclic structure phosphate. Organic halogenated material (halogenated solvent): organic halogenic flame retardant is important to flu flu flu flu. After H is replaced by F, its physical properties have changed, such as decrease in melting point, decrease in viscosity, improvement of chemical and electrochemical stability, etc.
. The organic halogenic flame retardant is important to include fluorocyclic carbonates, fluoro-chain carbonates and alkyl-perfluorodecane ether, etc..
OHMI and other comparative fluororethyl ether, fluoride-containing fluoride compounds showed that the addition of 33.3% (volume fraction) 0.67 mol / lliclo4 / Ec + DEC + PC (volume ratio 1: 1: 1) electrolyte has a more High flash point, the reduction potential is higher than the organic solvent EC, DEC and PC, which can rapidly form a SEI film on the surface of the natural graphite, improve the first charge and discharge of Cullen efficiency and discharge capacity.
. The fluoride itself does not have the use of the free radical capture function of the flame retardant described above, only to dilute high volatile and flammable co-solvents, so only the volume ratio in the electrolyte is mostly (70%) When the electrolyte is not flammable. Composite flame retardant: The composite flame retardant currently used in the electrolyte has a P-F compound and an N-P-class compound, representative substances have an important hexamethylphosphoride (HMPA), fluorophosphate, etc.
. Flame retardant exerts flame retardant effect by synergistic use of two flame retardant elements. FEI et al.
Proposes two N-P flame retardants MEEP and MEE, and its molecular formula is shown in Figure 1.. Licf3SO3 / MeEP :PC = 25:75, the electrolyte can reduce the flammability of 90%, and the conductivity can reach 2.
5 × 10-3S / cm.. 2) Overcharged additive: A series of reactions occur when the lithium-ion battery is overcharged.
The electrolyte component (important is the solvent) inveraffling the surface of oxidative decomposition reactions in the surface of the positive electrode, the gas is generated and the amount of heat is released, resulting in the increase in the internal pressure of the battery and the temperature rise, and the safety of the battery is seriously affected.. From the purpose mechanism, the overchaul protection additive is important to the oxidative stripping power-type and two types of electrical polymerization type.
. From the type of additive, it can be divided into lithium halide, metallocene compound. At present, an overchaled additional additional additional adaprase (BP) and cyclohexylbenzene (CHB) on redox anti-overchard additives are the principle when the charging voltage exceeds the normal cutoff voltage, the additive begins at the positive electrode.
The oxidation reaction, the oxidation product diffuses to the negative electrode, and the reduction reaction occurs.. Oxidation is closed between the positive and negative poles, absorb excess charge.
Its representative substances have a ferrocene and its derivative, ferrid 2,2-pyridine and a complex of 1,10-adjacent glenoline, thiol derivative. Polymerization block anti-filled additive. Representative substances include cyclohexylbenzene, biphenyl and other substances.
When the biphenyl is used as a pre-charged additive, when the voltage reaches 4.5 to 4.7V, the added biphenyl is electrochemically polymerized, forming a layer of conductive film on the surface of the positive electrode, increasing the internal resistance of the battery, thereby limiting the charging current protection battery.
2 Ion liquid ion liquid electrolyte is completely composed of yin and cation. Since the interi ions or cationic volumes are weak, the intermediate is weak, the electron distribution is uneven, and the oan-censoon can be free to move at room temperature, which is liquid..
It can be divided into imidazole, pyrazole, pyridine, quaternary ammonium salt, etc.. Compared to the ordinary organic solvent of lithium ion batteries, ionic liquids have 5 advantages: 1 high thermal stability, 200 ° C can not decompose; 2 vapor pressure is almost 0, do not have to worry about the battery; 3 ionic liquid is not easy to combusture No corrosiveness; 4 has a high electrical conductivity; 5 chemical or electrochemical stability is good.
AN or the like forms PP13TFSI and 1Mollipf6ec / Dec (1: 1) into an electrolyte, which can achieve completely non-fuel effects, and add 2 wt% liboB additive in this system to significantly improve interface compatibility.. The only problem that needs to be solved is the conductivity of the ion in the electrolyte system.
3 Selecting the thermal stability of lithium salt hexafluorophosphate (LiPF6) is a widely used electrolyte lithium salt in a commodity lithium-ion battery.. Although its single nature is not optimal, its overall performance is the most advantageous.
However, LiPF6 also has its disadvantage, for example, LiPF6 is chemical and thermodynamically unstable, and the reaction occurs: LIPF (6S) → LIF (S) + PF (5G), the reaction generated PF5 is easy to attack the organic solvent in oxygen atom Lonely to electrons, resulting in the open loop polymerization and ether bonds of the solvent, this reaction is particularly serious at high temperatures.. Current research on high temperature electrolyte salts is concentrated in organic lithium salt fields.
Representative substances are important with boron-based salts, imine-based lithium salts. LIB (C2O4) 2 (liboB) is a newly synthesized electrolyte salt in recent years. It has many excellent properties, decomposing temperatures 302 ° C, can form a stable SEI film in an negative electrode.
. Improve the performance of graphite in the PC based electrolytic solution, but its viscosity is large, the impedance of the SEI film formed . The decomposition temperature of LIN (SO2CF3) 2 (Litfsi) is 360 ° C, and the ion conductivity at normal temperature is slightly lower than LiPF6.
The electrochemical stability is good, and the oxidation potential is about 5.0V, which is the most organic lithium salt, but it Serious corrosion of Al base set fluid. 2.
2.4 Polymer Electrolyte Many commodity lithium ion batteries use flammable and volatile carbonate solvents, if a leakage is likely to cause fire. This is especially the powerful lithium-ion battery of high-capacity, high energy density.
Instead of using unscrupulous polymer electrolytes instead of flammable organic liquid electrolytes, it can significantly improve the safety of lithium-ion batteries.. The research of polymer electrolyte, especially gel-type polymer electrolyte has made great progress.
At present, it has been successfully used in commercial lithium-ion batteries. According to the polymer body classification, the gel polymer electrolyte is important with the following three categories: PAN-based polymer electrolyte, PMMA polymer electrolyte, PVDF-based polymer electrolyte. However, the gel-type polymer electrolyte is actually a result of a compromise of a dry polymer electrolyte and a liquid electrolyte compromise, and gel-type polymer batteries still have many work to do.
. 2.3 The positive material can determine that the positive electrode material is unstable when the charging state voltage is above 4V, and it is easy to generate a heat dissolved in high temperatures to decompose oxygen, oxygen and organic solvents continue to react a large amount of heat and other gases, reduce the safety of the battery [2, 17-19].
Therefore, the reaction of the positive electrode and the electrolyte is considered to be an important cause of heat.. Regarding the normal material, improve the common method of its safety is coating modification.
For the surface coating of the positive electrode material with MgO, A12O3, SiO2, TiO2, ZnO, SnO2, ZrO2, etc., can reduce the reaction of Die +-rear positive and electrolyte while reducing the chromatography of the positive electrode, inhibiting the phase change of the positive electrode substance. Improve its structural stability, reduce the disorder resistance of cation in lattice, thereby reducing the secondary reaction of the circulation process.
2.4 Carbon material currently uses a low specific surface area, a higher charge and discharge platform, a small charge and discharge platform, a relatively high thermal stability, a relatively good thermal state, a relatively high thermostability, a relatively high thermostability, a relatively high thermostability. Such as intermediate phase carbon microspheres (MCMB), or Li9Ti5o12 of spinel structure, which is better than the structural stability of laminated graphite .
The method of currently improving the performance of carbon material is important to surface treatment (surface oxidation, surface halogenation, carbon cladding, coating metal, metal oxide, polymer coating) or introducing metal or non-metallic doping.. 2.
5 The diaphragm currently applied in commercial lithium-ion batteries is still a polyolefin material, and its important disadvantages are hot and electrolytic fluid infiltration is poor.. In order to overcome these defects, the researchers have tried many ways, such as looking for thermal stability materials, or add a small amount of Al2O3 or SiO2 nanopowdia, which not only has a common diaphragm, but also has a thermal stability of the positive electrode material.
use. MIAO et al, polyimide nano nonwoven fabrication prepared by electrostatic spinning method. DR and TGA-like characterization means show that it can not only maintain thermal stability at 500 ° C, but also have better electrolyte infiltration relative to the CELGARD diaphragm.
. WANG et al prepared AL2O3-PVDF nanoscopic microporous membrane, which exhibits good electrochemical properties and thermal stability, satisfying the use of lithium-ion battery separators. 3 Summary and look forward to lithium-ion batteries for electric vehicles and energy storage, which is much larger than small electronic equipment, and the use environment is more complicated.
. In summary, we can see that its security is far from resolving, and has become the current technical bottleneck..
Subsequent work should be in depth to the thermal effect that the battery may result in after abnormal operation, and find an effective way to improve the safety performance of lithium ion battery.. At present, the use of fluorine-containing solvent and flame retardant additives is an important direction for developing a safety-type lithium-ion battery.
How to balance electrochemical performance and high temperature safety will be future research focus.. For example, a high-performance composite flame retardant integral integrated set P, N, F, and CL is developed, and an organic solvent having a high boiling point, a high flash point is developed, and an electrolytic solution of high safety performance is produced.
. Composite flame retardants, dual function additives will also become future development trends..
Regarding the lithium ion battery electrode material, the surface chemical properties of the material are different, the degree of sensitivity of the electrode material on the charge and discharge potential is inconsistent, and it is impossible to use one or limited several electrode / electrolyte / additives to all battery structural design.. Therefore, in the future, we should focus on developing different battery systems for specific electrode materials.
. At the same time, it is also developing a polymer lithium-ion battery system with high security or the development of inorganic solid electrolyte having single cation conductive and fast ion transport and high thermostability..
In addition, improving ionic liquid performance, developing simple and cheap synthetic systems is also an important part of the future research.