Lithium battery Safety Design: Active materials, electrolytes and diaphragms!

1 Diaphragm protection technology
1.1 Surface modification On the basis of the original polyolefin diaphragm, surface coating can improve the high temperature resistance and electrochemical performance of the diaphragm. The coating modified materials mainly include inorganic nanoparticles and organic polymers.
Inorganic modified coating materials include Al2O3, SiO2, TiO2 and ZrO2 inorganic particles, compared with Al2O3, Boehmite (AlOOH) ceramic coating has higher heat resistance temperature, lower density, low internal resistance and other advantages, the future application potential of AlOOH modified diaphragm greater. Two kinds of composite diaphragms, B1 and B2, were prepared by using 0.741μm and 1.172μm Boehmite powder as coating material, PVDF as binder, and 9μm thick PP diaphragm as substrate, and their properties were tested. The comprehensive performance of Boehmite /PP composite diaphragm is better than that of PP diaphragm. For example, the B0 diaphragm (unmodified PP diaphragm) shrinks by more than 57% at 140 ° C, while the B1 diaphragm is less than 3% and remains intact at 180 ° C; The tensile strength of B1 diaphragm was 18.8% higher than that of B0 diaphragm, and the puncture strength of B2 diaphragm was 54.4% higher than that of B0 diaphragm. Within 30s, the electrolyte could completely infiltrate the B2 diaphragm, while the B0 diaphragm could infiltrate less than 1/2 of the area.
Al2O3, Boehmite and other nano inorganic coatings, although can increase the heat resistance of the diaphragm, but also easy to block the diaphragm pore, hinder the transmission of Li+, for this reason, researchers use polymers as coating materials to modify polyolefin diaphragm. Such polymers include PVDF, PVDC, ANF, PAN, PMMA and PDA. Coating polyolefin membrane with PVDF and copolymer is a mature membrane modification method at present.

1.2 Different diaphragm systems Polyimide (PI) based diaphragms are regarded as the next generation of lithium-ion battery diaphragm materials due to their good heat resistance, chemical stability and ideal mechanical properties. The PI diaphragm prepared by electrospinning method has the advantages of low cost, high controllability and high porosity, but the prepared diaphragm has poor mechanical strength, large pore size and wide pore size distribution, which may aggravate the self-discharge and crosstalk reaction of the battery. In addition, electrospinning method also has problems of low productivity, poor reproducibility and environmental pollution, and it still faces many bottlenecks in industrial scale manufacturing. In this connection,Y. R. Deng et al. prepared a PI aerogel (PIA) diaphragm with uniform porosity, high temperature resistance and good electrochemical performance by using sol-gel method and supercritical drying, and applied it in lithium-ion batteries. The porosity (78.35%) and the absorption rate of electrolyte (321.66%) of PIA diaphragm are high, which is helpful to improve the electrochemical performance of lithium-ion batteries. The LiFePO4-Li half battery with PIA diaphragm can be stably cycled more than 1000 times with a ratio of 1C at 2.8~4.2V, and the capacity retention rate is above 80%. Thanks to the high thermal stability of PIA, the LiFePO4-Li half battery with PIA diaphragm can be stably cycled at 120 ° C. In order to determine the effect of improving the safety performance of lithium-ion batteries, the LiFePO4 positive electrode, PIA separator and graphite negative electrode were assembled into a flexible packaging battery, compared with the Celgard 2400 separator, and the thermal runaway behavior of the whole battery was studied by accelerating calorimeter (ARC). It is found that the thermal runaway temperature of the battery using PIA diaphragm can be increased from 131℃ to 170℃ using Celgard 2400 diaphragm battery, and the increase rate is about 30%.
Among the many system diaphragms, there are polyethylene terephthalate (PET), cellulose, fluoropolymer diaphragms, etc. The main performance parameters of several diaphragms and polyolefin (PP or PE) diaphragms are compared in Table 1.

As can be seen from Table 1, both the thermal stability and liquid absorption rate of these diaphragms have been greatly improved, providing more options for the development of high-safety lithium-ion batteries.

1.3 Thermal closed diaphragm The thermal closed diaphragm is a diaphragm that will have a closed hole at a certain temperature and block the ion channel. The initial thermal sealing diaphragm was to coat the surface of the PP diaphragm with paraffin microspheres, but due to the large size of the microspheres and the uneven coating, the ratio performance of the battery was affected. In addition, the response of the paraffin microspheres is slow when the temperature rises quickly, which is easy to cause the temperature response lag and can not restrain the thermal runaway behavior of the battery. For this reason, W. X. Ji et al. proposed a heat-sealing diaphragm modified with ethylene-vinyl acetate copolymer microspheres. Thanks to the appropriate thermal response temperature (90 ° C), small particle size (about 1μm), and high chemical and electrochemical stability of the ethylene-vinyl acetate copolymer microspheres, the microsphere-modified diaphragm ensures not only the electrochemical performance is not affected, but also the reliable high-temperature thermal shutdown function. The 20Ah lithium cobaltate-graphite flexible packaging battery was assembled with PP diaphragm and modified diaphragm respectively, and the short circuit test was carried out. The results show that: at the beginning of short circuit, the voltage of the battery using PP diaphragm drops sharply, generating a large short circuit current and releasing a large amount of joule heat, so that the internal temperature of the battery quickly reaches 131.2℃, until the voltage drops to 0V, the temperature begins to decrease. When the membrane is coated with ethylene-vinyl acetate copolymer microspheres, the open circuit voltage suddenly rises after a sudden drop at the beginning of the external short circuit, and the maximum surface temperature of the cell is only 57.2℃. This is because the Joule heat caused by the external short circuit causes the copolymer microspheres coated on the surface of the diaphragm to melt and collapse, and after transforming into a dense polymer insulation layer on the surface of the PP diaphragm, the Li+ transmission between the positive and negative electrodes is broken in the battery, so that the battery is in a state of open. It can be seen that the thermal sealing diaphragm can prevent the severe temperature rise of the battery in the case of external short circuit, improve the safety of large-capacity lithium-ion batteries, and show a good application prospect.

1.4 Endothermic diaphragm Z. F. Liu et al. prepared a phase change temperature regulating diaphragm, which can in-situ absorb the heat generated in the battery. The phase change material (PCM) with heat storage function is integrated into the PAN fiber membrane to give the diaphragm the function of temperature regulation. Under abuse conditions, the internal PCM is heated and melted, and accompanied by a large amount of latent heat storage, which can absorb the heat generated inside the battery in time to prevent thermal runaway. Under normal working conditions, due to the high porosity and good electrolyte affinity of the PAN fiber membrane, the battery assembled based on the diaphragm material has the characteristics of low polarization potential, fast ion transport, etc., showing the ideal electrochemical performance. The 63mAh lithium iron phosphate - graphite lithium ion battery assembled based on this kind of diaphragm material can be restored to room temperature within 35s after acupuncture experiment. This shows that the phase change temperature regulating diaphragm has a good temperature regulating ability for the battery after internal short circuit, and provides internal overheating protection for high energy density lithium-ion batteries, and provides a method for improving the safety of lithium-ion batteries. The acupuncture experiment was carried out based on 63mAh lithium iron phosphate - graphite lithium ion battery, the battery capacity is relatively small, and the temperature regulation ability and practical prospect in large-capacity batteries have yet to be verified.

2 Safe electrolyte
2.1 Ionic liquid Ionic liquid is a molten salt with a melting point below 100 ° C, in the molten state, consisting only of cations and anions. The high number of ions in the ionic liquid gives a high conductivity, but also has good thermal stability, chemical stability, electrochemical REDOX stability, non-volatilization and low reaction heat with the active electrode material, more importantly, it is completely non-combustible, so it is expected to become a high safety electrolyte. The complete absence of solvent molecules in the electrolyte will bring a series of problems, such as most ionic liquids can not be decomposed to form a stable SEI film, and carbon-based materials such as graphite anode compatibility is poor, therefore, can only use the higher cost of Li4Ti5O12 or non-carbon anode. The introduction of film forming additives or lithium fluoride sulfonimide (LiFSI), as well as the use of high concentration salt electrolyte, can improve the interface stability, but can not solve the high viscosity of ionic liquid, poor infiltration and low Li+ diffusion coefficient caused by the poor rate performance of electrode materials.
Carbonate solvent has low viscosity and high dielectric constant, can improve the physical and chemical properties of ionic liquid, and can decompose to form stable SEI film. Mixing ionic liquid with carbonate solvent to prepare non-flammable electrolyte is a method to balance the rate performance and safety of battery. The viscosity, wettability and Li+ diffusion coefficient of the blended electrolyte have limited improvement effect. And the electrolyte contains 20% flammable compounds, which will still bring certain safety risks to lithium-ion batteries. The safety of the battery can be further improved by mixing high-flash, non-combustible sulfone solvents with ionic liquids.

2.2 Fluorinated solvent Fluorinated solvent is a kind of lithium ion battery electrolyte solvent that has been studied more deeply at present, and is widely used in high-safety lithium ion battery electrolyte. Fluorine atom has small atomic radius, strong electronegativity, low polarizability, and fluorine solvent has the advantages of low freezing point, high flash point, and good infiltration between electrode and so on.

2.3 Organophosphate solvent Organophosphate compounds are characterized by high boiling point, low viscosity and high dielectric constant. Compared with ionic liquids. These compounds have the characteristics of low cost and easy synthesis. In the meantime. It has a similar molecular structure to carbonate. It is a solvent that is expected to achieve flame retardant/non-combustible electrolyte. At present, almost all phosphate ester solvents reported in the literature are incompatible with graphite anode, that is, graphite cannot stably and efficiently undergo reversible lithium impaction in the existing electrolyte with phosphate ester as the solvent. The primary task of developing phosphate ester electrolyte is to solve the compatibility problem between organic phosphate ester solvent and graphite.
The development of existing organophosphate solvent mainly includes phosphate ester, phosphite ester and phosphonate ester solvent. As mentioned earlier, organophosphate solvent is not compatible with graphite negative electrode, charge and discharge, can not form a stable SEI film on the surface of the negative electrode, at the same time, it will lead to co-embedding, destroying the layer structure of graphite, so in the early research on organophosphate ester, it is only used as a flame retardant additive or co-solvent added to the electrolyte to reduce the flammability of the electrolyte. The results show that when the concentration of organophosphate added to the electrolyte is too low (<10%), there is no obvious flame retardant effect; However, when the concentration is higher (>20%), it will inhibit the lithium insertion ability of the graphite negative electrode.

2.4 Phosphoronitrile flame retardants Phosphoronitrile compounds are a type of compound flame retardant additives. It mainly includes polymer linear phosphorus nitrogen compounds and small molecule cyclic phosphorus nitrogen compounds. The main characteristics of phosphonitrile flame retardants are. A small amount of addition (mass fraction of 5% to 15%) can achieve the effect of flame retardant or non-combustible electrolyte. And good compatibility with electrode materials. The effect on the electrochemical performance of lithium-ion battery is small.
Bridgestone's cyclophosphonitrile (PFPN) is an early flame retardant with a high electrochemical oxidation window and has many application cases in high-voltage lithium-ion batteries, such as lithium-ion batteries using high-voltage lithium cobalt oxide cathode materials or 5V high-voltage lithium nickelmanganate materials.

3 Positive electrode coating technology
Surface coating can improve the thermal stability of positive electrode materials and is the main positive electrode protection technology at present. Coating other materials with high stability on the surface of the positive electrode material can prevent the direct contact between the positive electrode material and the electrolyte, so as to inhibit the phase transition of the positive electrode material, improve the thermal stability and reduce the cation disorder on the lattice site. This kind of coating layer should have good thermal stability and chemical inertia, and the coating materials mainly include phosphate, fluoride and solid oxide.
Phosphate with strong PO4 covalent bond is coated on the surface of the positive electrode material, which can improve the thermal stability of the positive electrode material. If the positive electrode coated with AlPO4 is used, it has better thermal stability and shows better performance in overcharge test. M. Yoon et al. reported a room temperature coating synthesis strategy of "coating + pouring". Cobalt boride (CoB) metallic glass was applied to nickel-rich layered cathode material NCM811, which achieved full surface covering and grain boundary wetting of secondary particles of the cathode material, and improved the magnification performance and cycle stability, with 1C cycling at 2.8~4.3V 500 times. The capacity retention rate of the material was increased from 79.2% before coating to 95.0%. The results show that the ideal performance is due to the inhibition of both microstructure degradation and side reactions with the interface. M. Jo et al. used sol-gel method to achieve uniform coating of Mn3(PO4)2 nanocrystals on the positive electrode surface of NCM622 at low temperature. The Mn3(PO4)2 coating reduces the direct contact between the electrolyte and the unstable oxidation anode, thereby reducing the degree of exothermic side reactions.

4 Negative electrode modification strategy
The graphite itself is relatively stable, but the lithium-embedded graphite will continue to react with the electrolyte at high temperatures, exacerbating the initial heat accumulation of thermal runaway, and promoting the thermal runaway chain reaction. SEI film can isolate the direct contact between the negative electrode and the electrolyte and improve the stability of the negative electrode. Therefore, the construction of high thermal stability SEI film is a key method to isolate the side reaction between the negative electrode and the electrolyte and restrain the thermal runaway. The structure and properties of SEI film can be improved by introducing film forming additives into the electrolyte. For example, ammonium perfluorooctanoate (APC), vinylidene carbonate (VC) and vinylidene carbonate (VEC) can be preferentially reduced and decomposed in the electrolyte, forming a uniform and dense polymer film on the surface of the graphite negative electrode, and improving the thermal stability of the SEI film. Starting from the material surface modification, the thermal stability of anode materials can be improved by constructing artificial SEI film such as metal and metal oxide deposition layer, polymer or carbon coating layer. As the temperature rises, the SEI film constructed by the above two methods will always decompose, and at higher temperatures, the exothermic reaction between the lithium fossil ink cathode and the electrolyte will be more intense.
In addition, when charging with high current, the lithium evolution reaction of the graphite anode will also cause the risk of thermal runaway of the lithium-ion battery. The charging current ratio determines the Li+ flux per unit area of the anode material. When the solid phase diffusion process of Li+ in the negative electrode is slow (such as when the temperature is too low and the charge state is high), and the charging current density is too high, the negative electrode surface will trigger the lithium evolution reaction, and the precipitated lithium dendrites will puncture the diaphragm, resulting in an internal short circuit, which will cause combustion, explosion and other disastrous consequences. The solid phase diffusion of Li+ between graphite layers can be accelerated by shortening the diffusion path of Li+ between graphite layers and increasing the spacing of graphite layers.

5 Conclusion and Prospect

Lithium-ion battery technology is mature, suitable for large-scale application and mass production, and is the key development direction of electric vehicles and large-scale energy storage technology. At present, the energy density of lithium-ion batteries continues to increase, and the requirements for battery safety are greater, therefore, safety is an important indicator of the development of lithium-ion batteries. Based on the diaphragm, electrolyte and electrode materials, this paper systematically summarizes the existing methods to prevent thermal runaway and improve the safety of lithium-ion batteries. Based on the summary of the current research on improving the safety of lithium-ion batteries, combined with the new mechanism of thermal runaway, several key directions for the development of safety materials for lithium-ion batteries in the future are proposed:

(1) The surface modification of polyolefin membrane with inorganic nanoparticles can improve the thermal stability of the membrane, but the improvement effect is limited. The diaphragm with high thermal stability and high mechanical strength will provide more options for high-safety lithium-ion batteries. In addition, intelligent thermal response diaphragms can also be designed, such as heat-sealing diaphragms that can cut off ion transport at high temperatures, fireproof diaphragms that release flame retardants and phase change heat-absorbing diaphragms. The above safety diaphragm design strategy starts from the thermal runaway caused by diaphragm melting, but the internal short circuit is not the only factor that triggers the thermal runaway of lithium-ion batteries. At high temperature, the intense REDOX reaction between reactive oxygen species released by the positive electrode and electrolyte and lithium fossil ink negative electrode is also the main reason for triggering thermal runaway. How to block the crosstalk reaction of reactive oxygen species released by the positive electrode while ensuring the high temperature resistance of the diaphragm is an important measure to develop a safe diaphragm in the future.

(2) The flash point of commercial lithium-ion battery electrolyte is generally low, and it is easy to burn or even explode at high temperature, and the development of flame retardant/non-combustible electrolyte to reduce the flammability of the electrolyte is one of the measures to improve the safety of lithium-ion batteries. Based on this method, people have carried out extensive research on flame retardant/non-combustible electrolyte, including ionic liquid, fluorinated solvent, organophosphate solvent, phosphazene flame retardant and high concentration salt electrolyte. Based on the timing characteristics of thermal runaway, the combustion of the electrolyte is the main energy source in the late stage of thermal runaway, and the exothermic side reaction between the electrolyte and lithium fossil ink after the SEI film breaks in the early stage contributes to the heat accumulation in the early stage of thermal runaway. Direct repair of broken SEI film in real time from electrolyte. Inhibit the reaction between lithium fossil ink and electrolyte. Would be a strategy to suppress thermal runaway.

(3)Direct contact between cathode material and electrolyte at high temperature will lead to irreversible phase transition on the surface of cathode material. Reduce the thermal stability of the material. The design of the safe cathode material mainly focuses on the isolation of direct contact between the active cathode material and the electrolyte, including the surface coating of the cathode material and the use of monocrystalline ternary cathode material without lattice gap. In addition to the safe cathode material design strategies summarized by the authors of this paper, active oxygen capture coatings can also be developed to quench the active oxygen released by the thermal decomposition of cathode materials such as ternary, lithium cobaltate and lithium manganate, so as to avoid reactive oxygen with the electrolyte or lithium fossil ink negative electrode reaction.

(4) Bare Li-embedded graphite has high reactivity with electrolyte. The traditional improvement strategy is to add film forming additives or construct artificial SEI film in electrolyte. The failure of the SEI film at high temperatures will eventually lead to the reaction of the lithium-embedded graphite with the electrolyte. Therefore, it is necessary to develop a technology that can repair the SEI film in real time in situ to block the reaction between the lithium fossil ink and the electrolyte.