LiDFBOP Assists LiPF6 Based Local High Concentration Electrolyte to Improve Low Temperature Performance of Lithium Ion Battery

research background]
The inability of lithium-ion batteries to function properly at low temperatures is one of the main obstacles limiting their use at high altitudes or latitudes, as well as for certain defense and space applications. The influence of electrolyte on the battery at low temperature is fatal. In addition to the intrinsic electrolyte (such as electrolyte ionic conductivity, viscosity), the electrode-electrolyte interface behavior (lithium ion desolvation process, solid electrolyte interface on the surface of positive and negative materials) also has a great influence on the battery. The ideal electrolyte should have high ionic conductivity, low lithium ion desolvation energy and good film forming ability, and the formed SEI film should have low resistance. Based on this, this paper designs a local high concentration electrolyte based on LiPF6 to improve the low temperature performance of lithium ion batteries, and uses LiDFBOP as an electrolyte additive to form an excellent SEI film to ensure the normal use of LiPF6-based local high concentration electrolyte and provide an effective strategy for the commercialization of local high concentration electrolyte.

job introduction]
Recently, researcher Chen Chengmeng and associate researcher Su Fangyuan of Shanxi Institute of Coal Chemistry of the Chinese Academy of Sciences and researcher Xu Guoning of the Institute of Space Information Innovation of the Chinese Academy of Sciences have jointly designed a local high-concentration electrolyte based on LiPF6 to improve the low-temperature performance of lithium-ion batteries. At the same time, additive LiDFBOP are introduced into the local high-concentration electrolyte to regulate the formation of SEI film. The preparation of local high concentration electrolyte can effectively improve the conductivity of Li, and the introduction of LiDFBOP can generate SEI film with low interface impedance to ensure the normal operation of local high concentration electrolyte based on LiPF6. In addition, the effect of lithium ion desolvation kinetics on low temperature performance in local high concentration electrolyte was also discussed. The article was published in the international journal ACS ENERGY LETTERS. Song Ge, Yi Zonlin is the first author of this article.

content expression]
The development of suitable electrolyte is undoubtedly one of the most effective ways to improve the low temperature performance of lithium-ion batteries. It has been reported that local high-concentration electrolyte can improve the low-temperature performance of lithium-ion batteries by improving the electrolyte phase and the interface phase. However, in most of the reported studies on local high-concentration electrolytes, the lithium salts are almost LiFSI and LiTFSI. The production process of this kind of lithium salt is complex, expensive, and the degree of commercialization is low, which greatly hinders the commercialization process of local high-concentration electrolyte. Lithium hexafluorophosphate (LiPF6) is a commonly used lithium salt in commerce, which is favored by commerce because of its low price, simple preparation and high degree of commercialization. At the same time, LiPF6 can effectively alleviate the corrosion of aluminum foil in lithium-ion batteries. Therefore, there is an urgent need to explore and develop the commercial application of LiPF6-based local high concentration electrolyte. In the local high concentration electrolyte, the anion squeezed in the lithium ion solvation shell dominates the formation of the SEI film. Since LiPF6 itself cannot form an excellent SEI film required for low-temperature applications and cannot meet the requirements for low-temperature applications, additional film-forming additives need to be added. LiDFBOP additives can be reduced in preference to the electrolyte solvent and lithium salt to form a thin and uniform SEI film with high lithium ion conductivity. Therefore, in the experiment, LiPF6 was selected as the lithium salt in the local high concentration electrolyte, LiDFBOP as the film forming additive.

Firstly, the formation of LiPF6 based local high concentration electrolyte was proved by Raman spectroscopy and liquid nuclear magnetic resonance (Figure 1 a, B). Further theoretical calculations were used to simulate the special solvation structure of lithium ion-anion-solvent in local high concentration electrolyte, which assisted to prove the formation of local high concentration electrolyte. In (locally) high concentration LiPF6-based electrolytes, PF6 − in the solvation shell is likely to compete with LiDFBOP for decomposition to form SEI films. For this reason, it needs to be explored whether the LiDFBOP will still preferentially decompose the formation of the dominant SEI film. In general, the solvent with the lowest unoccupied molecular orbital (Lowest unoccupied molecular orbital,LUMO) energy will be preferentially reduced. Therefore, by calculating the LUMO value and HOMO value of various solvated structures in the electrolyte, it is speculated that the LUMO value of the LiDFBOP is the lowest in the local high concentration electrolyte containing LiDFBOP, and it is speculated that it will dominate the generation of SEI film (Figure 1 c, d).

Figure 1(a) Raman spectra of solvent DMC, diluents HFE, HCE, LHCE, HCE-P, LHCE-P and RCE; (B) 7Li-NMR results of five electrolytes at room temperature 25°C. (c) Five Solvated Complexes and (d)HOMO/LUMO Energies Calculated from Statistical Five Solvated Complexes in AIMD Simulation Results
The first cycle CV of the graphite/Li half-cell shows that the additive LiDFBOP can be reduced in the common commercial electrolyte in preference to the solvent and lithium salt, dominating the generation of the SEI film (Figure 2a). It can be seen from TEM that the LiDFBOP in the local high concentration electrolyte can form a thin and uniform SEI film with higher mechanical strength (Figure 2c-h). The combination of XPS and argon ion sputtering technique shows that the LiDFBOP-derived SEI film is in good agreement with the classical double-layer structure. (Fig. 3) Its inner layer is mainly composed of inorganic substances (LiF), while its outer layer is composed of organic polymers (-LixPOyFz, Li2C2O4). Such a dense SEI film can suppress the decomposition of the electrolyte by preventing the electrolyte from directly contacting the graphite, thereby improving the cycle stability. LiF and-LixPOyFz are beneficial to improve the lithium ion conductivity of the SEI film and accelerate the transmission of lithium ions.

Fig. 2 (a) first cycle CV curve and (B) first charge-discharge curve of graphite/lithium battery with five electrolytes;(c) TEM image of graphite; TEM images of SEI film in different electrolytes, respectively (d)HCE, (e)LHCE, (f)HCE-P, (g)LHCE-P and (h)RCE

Fig. 3 XPS peaks of (a)C 1s and (c)F 1s on graphite negative electrode surface under different Ar sputtering times and their corresponding (B)(d) summary results
Through theoretical calculation, it is theoretically proved that the special solvation structure of lithium ion-anion-solvent in local high concentration electrolyte can reduce the desolvation energy. Based on the geometry of the solvated complexes in Figure 1c, the energy of desolvation of lithium ions in these complexes was calculated. According to the calculation results, the desolvation energy of the complex (one lithium ion and four DMC molecules) in the commercial electrolyte is 6.00 eV, which is higher than that of other solvated complexes. This means that the special solvation structure of lithium-anion-solvent may be easier to carry out the desolvation process and improve the kinetic process of lithium ion. Further, by testing the EIS impedance information of the battery at different temperatures, the activation energy (I. e. energy barrier) of each stage in the process of lithium ion intercalation into graphite is obtained by Arrhenius equation, which further verifies that the special solvation structure of Li -PF6-solvent in local high concentration electrolyte can reduce the desolvation energy, improve the kinetic process of lithium ion intercalation into graphite at low temperature, and obtain excellent low temperature performance. Specifically, firstly, the local high concentration in the LHCE-P makes lithium ions need to overcome relatively low ion migration energy; secondly, due to the special solvation structure of lithium ion-anion-solvent, the energy required for lithium ion desolvation is reduced, so that the lithium ion desolvation kinetics is accelerated; thirdly, the SEI film derived from LiDFBOP in the LHCE-P is thin and uniform, with high ionic conductivity, the energy barrier for lithium ions passing through the SEI film at low temperatures is reduced. Thanks to the high ionic conductivity of the electrolyte, excellent SEI film and easier desolvation process, the graphite/lithium half-cell using LHCE-P can maintain its performance at 320 mAh g − 1(0.1 C) at − 10 ° C. Around (Figure 5-17a). Even at − 20°C, the best performance of 250 mAh g − 1 is exhibited. In sharp contrast to the commercial base electrolyte, EC, which has a higher melting point in RCE, increases the impedance of the electrolyte itself. At the same time, with the decrease of temperature, the desolvation barrier of lithium ions is further intensified, so it always shows the lowest performance at low temperature.

Fig. 4 graphite/lithium half-cell with four electrolytes (a) rate performance at room temperature, (B) EIS after 3 cycles at room temperature and (c) corresponding fitted impedance values;(d)LHCE-P improvement of lithium-ion battery kinetics schematic diagram;(e) specific capacity of graphite/lithium battery at different temperatures and different current rates and (f) low temperature cycle performance of four electrolytes;(d) Specific capacity of LHCE-P and RCE NCM622/graphite soft package at different rates (− 20 ℃)
[Conclusion]]
(1) LiPF6, which is the most widely used and has the best comprehensive performance, can be used as the lithium salt of local high concentration electrolyte to improve the low temperature performance of lithium ion batteries.
(2) the introduction of LiDFBOP is conducive to the formation of stable low interface impedance SEI film, to ensure the use of LiPF6 based local high concentration electrolyte.
(3) Through electrochemical tests and theoretical calculations, it is proved that the special solvation structure of lithium ion-anion-solvent in local high concentration electrolyte can reduce the desolvation energy and improve the dynamic process of lithium ion intercalation in graphite at low temperature.