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How To Improve the Safety of Electrolyte in Lithium-Ion Batteries?
Introduction
lithium- ion batteries are being widely used in power fields such as portable electronic devices, electric vehicles, and large-scale energy storage power stations [1-3]. With the transformation of the energy structure and the upgrading of large electrical equipment, lithium-ion batteries have undoubtedly brought great changes and convenience to people's lives, but at the same time, their safety issues always exist. Electrolyte, as the most flammable component in lithium- ion batteries, has been considered to be closely related to its safety. The main flammable components in the electrolyte are organic carbonate solvents, and it is not enough to improve the thermal stability of lithium salts alone. The most direct way is to add flame retardants to the solvent or to completely abandon flammable solvents. Considering the complexity of the whole electrolyte system, large changes may lead to the failure of the complete electrochemical performance, and preliminary studies have mainly focused on the use of small amounts of flame-retardant additives. They often increase the flash point of the electrolyte, making it less flammable. This paper discusses the characteristics, action mechanism and research progress of various flame retardants in the electrolyte, aiming to provide relevant researchers with design ideas for new electrolytes.
Phosphorus Flame Retardant Additive
Organophosphorus flame retardant additives have been extensively studied at the beginning because of their rich variety, low toxicity, good physical compatibility, and low cost [4]. Common phosphorus flame retardants used in Li- ion battery electrolytes include trimethyl phosphate (TMP) [5], triethyl phosphate (TEP) [6] , tributyl phosphate (TBP) [7] , methylphosphine di Methyl ester (DMMP) [8 ,?9] , diethyl phosphate (DEEP) [10] , triphenyl phosphate ( TPP) [11] and 4-isopropylphenyl diphenyl phosphate ( IPPP) [12] (Figure 1).
Figure 1:?Structures of several common phosphorus flame retardant additives
Generally speaking, the flame retardant performance of these preliminary researched phosphorus flame retardants is worthy of recognition, and its lower price is also a great advantage in practical applications. However, under the premise of limited dosage, it is still difficult to affect the overall electrolyte. Their high viscosity risks causing degradation of electrochemical performance. Another common problem with phosphorus flame retardant additives is that their electrochemical window is not wide enough, and reductive decomposition reactions often occur on the anode surface, resulting in increased impedance and capacity degradation [13]. It is necessary to further modify these phosphates to further improve the flame retardancy and reduce the dosage.
Fluorine Flame Retardant Additive
Halogen flame retardant additives are also commonly used in Li-ion battery electrolytes due to their high flash point and good thermal stability. The halogen-containing flame retardants used in Li-ion batteries are mainly organic fluorides (Figure 2).
Figure 2:?Structures of several common fluorine flame retardant additives
Different from organophosphorus additives, organofluorine additives have low viscosity, high solubility and good performance at low temperature [14]. Because the bond energy of the C-F bond (105.4 kcal mol?1) is greater than the bond energy of the C-H bond (98.8 kcal mol?1), this means that the energy required to break the CF bond is greater, that is, the organic fluorine additive has better thermal stability (Figure 3). In addition, by replacing hydrogen atoms with fluorine atoms, H free radicals can be reduced, which greatly reduces the flammability of materials [15]. Therefore, organic fluorides are considered to be one of the best choices for constructing non-flammable solvents. Arai et al synthesized chloroethylene carbonate (ClEC) using trifluoro?propylene carbonate (TFPC) as a co-solvent [16]. Compared with PC/TFPC and EC/TFPC electrolytes, the ClEC/TFPC binary electrolyte containing 1 mol L-1 LiPF6 exhibited good cycle performance on both graphite anode and Li1+xMn2O4 anode materials.
Figure 3:?Interpretation of the non- flammable film-forming properties of fluoroflame retardants from the aspects of bond energy and LUMO
For high-voltage Li- ion batteries, Xia et al. reported a nonflammable electrolyte (HFPM) using 1,1,1,3,3,3 - hexafluoroisopropylmethyl ether (HFPM) as a co-solvent at 4.9 Under the high cut-off voltage of V, 82% capacity can still be maintained after 200 cycles[17]. Therefore, fluorinated flame retardant additives have advantages in maintaining electrochemical performance compared to phosphate flame retardants. This is because the lowest unoccupied molecular orbital (LUMO) of these organofluorine additives is lower than that of the electrolyte solvent [18]. Under the influence of the unique electronic effect of fluorine atoms, organic fluorine additives can increase the reduction potential of the electrolyte solvent and form a more stable SEI film on the anode (Fig. 3).
Fluorine flame retardants usually exhibit good electrochemical performance and flame retardant effect, and their low viscosity helps to reduce the attractive force between solvent molecules and improve electrical conductivity. In addition, fluorine can improve the composition and morphology of the SEI film, which can alleviate the capacity fading caused by the use of high-concentration electrolyte additives. However, in the case of large doses, the compatibility of LiPF6 with fluorinated flame retardants is generally poor. Therefore, it is necessary to explore suitable lithium salts or modify fluorinated flame retardants to improve their compatibility with LiPF6?based electrolytes. In addition, the current cost of fluorinated flame retardants is still high, requiring special equipment and strict preparation processes.
Ionic Liquid Flame Retardant Additives
As a non-volatile, non -flammable, non-polluting liquid, ionic liquids have a wide electrochemical window, and have been synthesized and reported as electrolyte additives in recent years [19]. The chemical structures of some of the studied lithium-ion battery electrolyte ionic liquids are shown in Fig. 4.
Figure?4:?Structures of several common ionic liquid flame retardant additives
Ionic liquids generally refer to liquid salts composed entirely of anions and cations at room temperature. Therefore, ionic liquid electrolytes are expected to replace traditional organic electrolytes and improve the safety of lithium- ion batteries. The basis of good electrochemical performance is to ensure a suitable conductivity and a suitable electrochemical window. However, the use of ionic liquids may be limited because common ionic liquids would decompose in graphite anodes, thereby affecting the thermal stability of SEI films. Based on trifluoro?methane?sulfonimide (TFSI) with good electrochemical performance and thermal stability, it has been intensively studied as an anion. used an ionic liquid composed of 1-ethyl-3-methylimidazole (EMI) and TFSI mixed with commercial electrolytes including EC and DEC. The resulting hybrid electrolyte has comparable electrochemical performance to common liquid electrolytes [20]. When 40% ionic liquid was added, the hybrid electrolyte was not flammable (Fig. 5).
Figure 5:?Flammability test of different ECDEC-VC mixtures with EMI-TFSI electrolyte
Ishikawa reported a pure ionic liquid containing bi-sfluoro?sulfonimide (FSI) anion and EMI cation as well as Nmethyl-n-propylpyrrole pyridine (P13) and the compatibility of graphite was analyzed in detail, for semi According to the test results of the battery, the reversible capacity of the graphite anode can reach 360 mAh g -1?[21] .
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In summary, ionic liquids, as a new type of safe electrolyte, have high thermal stability and low volatility, and have good development prospects. However, the ubiquitous problem of ionic liquids is that the high viscosity leads to low conductivity at room temperature, and the high purity requirements limit the synthesis process. Another very important aspect is the high cost of ionic liquids compared with conventional electrolytes, which largely limits the current practical applications. Therefore, the vast majority of related research is still the combination of ionic liquids and traditional organic electrolytes.
Composite Flame Retardant Additive
Over the years, research on flame retardant additives for electrolytes has never stopped. There are many kinds of flame retardant additives for lithium-ion batteries, but few additives can significantly improve the flame retardant efficiency while ensuring electrochemical performance with a small amount of addition. A single flame retardant often needs to be added in a large amount, and the solubility of the additive and the compatibility of the electrolyte are very limited. Therefore, it is necessary to configure more suitable additives for Li- ion batteries by integrating the advantages and disadvantages of various types of flame retardants [22]. The introduction of composite additives is beneficial to reduce the amounts of additives and improve the flame retardant efficiency. And several flame retardant elements can play a synergistic effect [23], even improving the cycle performance while improving the thermal stability. Composite flame retardant electrolyte additives currently studied mainly include phosphorus-nitrogen compounds [24] and phosphorus-fluorine compounds [23].
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Fluorinated phosphates are typical FP compound additives. Fluorination of alkyl phosphates is one of the most effective methods to reduce viscosity and degree of fluorination, and the position and type of fluorinated groups have significantly different effects on flame retardant and electrochemical performance. Shiga et al. found that by alkyl fluorination, the thermal stability of TFEP and its mixture with charged electrodes can be improved compared to non-fluorinated phosphates such as TMP [25]. Zhu et al. used diethyl?phosphonate, dimethylformamide, trifluoroethanol and trimethylamine to synthesize TFEP compounds as electrolyte flame retardant additives. Ordinary carbonate electrolyte can be completely inflammable when the amount of TFEP is 20% [26].
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