【Author's Preface】
The traditional EC-based commercial electrolyte (LiPF6 is lithium salt) is difficult to meet the needs of high energy density, wide temperature range and long life of energy storage system. As an intermediate bridge, the solvation structure is affected by the components and their concentrations in the electrolyte on the one hand, and on the other hand, the solvation structure has an important impact on the electrode-electrolyte interface and kinetics, which in turn affects the electrochemical performance of the secondary battery. Therefore, solvation structure design is the basis and premise for realizing micro-meso-macro multi-scale and integrated regulation. Based on the above ideas, previous research and our own work, we are systematically summarizing the solvation chemistry in liquid electrolytes for lithium-based rechargeable batteries. Chem. Soc. Rev., 2023, 52, 5255 - 5316.）。 In order to verify the above idea, we prepared a perfluorinated weakly solvated electrolyte by molecular design (fluorine substitution) to achieve stable cycling of a 4.8 V lithium metal battery (NCM811-Li) in a wide temperature range (A nonflammable electrolyte for ultrahigh−Voltage (4.8 V−Class) Li||NCM811 Cells with a Wide Temperature Range of 100 °C，Energy Environ. Sci. 2022, 15, 2435-2444）。
In this work, the construction of ultra-high temperature liquid lithium-ion batteries is realized through electrolyte design according to actual engineering requirements. At high temperatures, on the one hand, the electrolyte itself is unstable, and thermal interpretation will occur; At the same time, the side reaction between the electrolyte and the electrode material intensifies under the electric field. Designing a high-temperature stable electrolyte and constructing a high-temperature electrode electrolyte interface is an important way to improve the high-temperature performance of liquid lithium rechargeable batteries. Continuing the above solvation chemistry ideas, we designed a high-temperature resistant weak solvation electrolyte, and introduced high-bond energy unsaturated silicon-based additives to construct an organic-inorganic hybrid interface with a unique structure, and realized the construction of high-temperature resistant lithium secondary batteries.
At present, we continue to design the eco-solvent electrolyte and apply it to the design of ultra-low temperature lithium secondary batteries to meet the urgent needs of national defense equipment. I hope that the work of the low temperature chapter of weak solvated electrolyte can meet with you as soon as possible. Although we have done some work on electrolytes, we are still "primary school students" in the field, and we hope to receive criticism and guidance from our predecessors and peers in the field. Today, I just listened to Professor Xu Kang's report at the Institute of Advanced Studies of Peking University, and I was deeply shocked and inspired, and I hope to make our modest contribution in this field.
【Background】
As lithium-ion battery technology matures, there is a growing demand for its application in extreme high-temperature scenarios, including high-temperature environments such as aerospace and desert exploration. However, the acceleration of the electrochemical reaction kinetics of the electrode and electrolyte in the high-temperature environment leads to serious gas production, electrolyte decomposition and interface instability in the battery, and its electrochemical performance and safety performance are worrying, which is not enough to meet its practical application in extremely special fields. Therefore, it is of great significance to design high-performance and high-temperature lithium-ion batteries by adjusting the solvent chemistry of the electrolyte to optimize the interface of lithium-ion batteries in order to solve the causes of high-temperature failure of lithium-ion batteries.
【Brief Introduction】
Recently, the energy materials and devices team of the National University of Defense Technology has improved the interface of LIBs at high temperature by making reasonable use of the synergistic effect of weak solvation and functional additives. In conventional electrolytes, the use of LiODFB instead of LiPF6 greatly reduces the solvent separation ion pairs (SSIPs) in the electrolyte, and promotes the generation of anion-dominated derivatization of inorganic-rich electrode/electrolyte interfaces (EEIs). In addition, by introducing functional unsaturated siloxane additives (PTSE), its high electrochemical activity and strong polymerization ability are used to form heat-resistant polycarbosilanes at the positive and negative electrode interfaces. Under the synergistic effect of anion-derived inorganic composition and silicon-rich organic components, a stable CEI and SEI film with excellent comprehensive performance was formed at the interface between the positive and negative electrodes. Under the premise of not affecting its low-temperature performance, it effectively inhibits the continuous occurrence of interfacial side reactions at high temperatures, and greatly improves the electrochemical performance of lithium-ion batteries at extreme high temperatures (≥100 °C). This work provides a feasible new strategy for the application of lithium ions in high-temperature scenarios. The paper was published in the international high-level journal Angewandte Chemie, entitled "Interface Engineering via Manipulating Solvation Chemistry for Liquid Lithium-ion Batteries Operated ≥ 100 °C" (doi.org/10.1002/anie.202410982).
Figure 1. Mechanism of influence of high-temperature electrolyte on high-temperature performance of lithium-ion batteries.
【Details】
Figure 2. Design principles of weak solvation electrolyte for high-temperature resistant electrolytes.
Through density functional theory (DFT) calculations, it can be seen that the binding energy of Li+-ODFB- (-0.58 eV) is much smaller than that of Li+-PF6- (-0.38 eV), indicating that the solubility of EC or DEC in LiODFB-based electrolyte (HT electrolyte) is weaker than that of LiPF6-based electrolyte (BASE electrolyte, 1PTSE electrolyte), resulting in the formation of weakly solvated electrolyte. PTSE containing unsaturated C≡C and high-bond-energy Si-O is used as an additive to evolve into a heat-resistant cross-linked polymer (polycarbosilane) through surface polymerization. Through molecular dynamics simulation (MD) and NMR, it was further revealed that compared with the LiPF6-based electrolyte, more anions were involved in the solvation structure of LiODFB-based electrolyte, which was a weakly solvated electrolyte.
Figure 3. Electrochemical performance of HT electrolyte at high temperature and compatibility with LiCoO2 cathode.
The three electrolytes of BASE, 1PTSE and HT were assembled into LiCoO2||Li battery, to explore its electrochemical performance at high temperature and its compatibility with LiCoO2 cathode materials. It can be clearly seen that the comprehensive performance of HT electrolyte is better than that of the other two electrolytes at different temperatures, especially at 80 °C, the HT electrolyte maintains high capacity while maintaining a high capacity, the capacity retention rate after 500 cycles is as high as 96.1%, and the capacity is not attenuated after high rate current, which means that it still has good reversibility at high temperatures. In addition, compared with room temperature, the BASE electrolyte has a significant volume decay at 80 °C, while the HT electrolyte can still maintain an ultra-high specific capacity at 120 °C, and its limit temperature is much higher than that of the BASE electrolyte.
In order to clarify the differences in the structure and composition of CEI membranes between HT electrolyte and BASE electrolyte at high temperature, the LiCoO2 cathode interface was analyzed by SEM, TEM, XPS and other tests. From the SEM and TEM images, it can be clearly found that compared with the HT electrolyte, the LiCoO2 cathode particles in the BASE electrolyte are obviously cracked after 100 cycles at a temperature of 80°C, and an uneven CEI film with a thickness of 38nm is formed. It can be seen from the XPS test that PTSE (Fig. 4h) and LiODFB (Fig. 4i) are involved in the formation of CEI films, and unlike the BASE electrolyte and 1PTSE electrolyte, the LiF content in the HT electrolyte gradually decreases with time, which means that the LiPF6 in the BASE and 1PTSE electrolytes continues to decompose at high temperatures, while the HT electrolyte forms a more stable interface at high temperature due to the outstanding film-forming effect of ODFB-. Effectively inhibits the continuous decomposition of electrolyte. From TOF-SIMS, the content of C2HO, the solvent decomposition product of the electrolyte, in the CEI formed by HT electrolyte is much smaller than that in the BASE electrolyte, which indicates that the excessive decomposition of solvent in HT electrolyte is inhibited.
Figure 4. Effect of HT electrolyte on the structure and composition of CEI membranes.
The three electrolytes of BASE, 1PTSE, and HT are assembled into graphite||For Li batteries, it can be found that the HT electrolyte still has the best electrochemical performance at different temperatures. Further confirming the above conclusions, the analysis of the structure and composition of the SEI film shows that the SEI film formed by HT electrolyte is thin and uniform, while the SEI film formed by BASE electrolyte is rough and uneven, which is caused by the continuous reaction between the electrolyte and the graphite anode during the cycling process of the BASE electrolyte. It can be seen from the XPS test results. In the SEI formed by HT electrolyte, the peak intensities of C-O, C=C and C=O are much lower than those of BASE or 1PTSE electrolytes. In addition, due to the low LUMO energy level of anion (ODFB-) and PTSE additives in HT electrolyte, it is easy to be reduced on graphite to form thermally stable organic and inorganic heterophases rich in Si, B and F, thus forming highly conductive and highly stable SEI films, which effectively inhibit the decomposition of organic solvents (EC, DEC).
Figure 5. HT electrolyte at high temperature in LiCoO2||Electrochemical properties of graphite full batteries.
In order to evaluate the practical application potential of HT electrolytes in high-temperature scenarios, the above three electrolytes were assembled into Ah-grade LiCoO2||From the analysis of its electrochemical performance at different temperatures (25 °C-120 °C), cycling performance at 80 °C, gas production, metal dissolution and other aspects, it can be determined that at high temperatures, compared with conventional electrolyte (BASE), HT electrolyte can maintain high electrochemical performance and have good safety performance.
【Conclusion】
In this work, a novel ecosolvated electrolyte with functional additives was carefully designed, guided by DFT calculations and MD simulations. At the same time of adjusting the solvation structure, a stable and heat-resistant inorganic organic heterogeneous phase was successfully constructed on the surface of lithium cobalt oxide cathode and graphite cathode by introducing silicon-containing unsaturated functional additives (PTSE), which effectively inhibited the continuous side reaction at the interface between the electrode and the electrolyte, thereby improving its electrochemical performance at high temperature. LiCoO2||at 80 °CLi, graphite||Li half-cells have excellent long-cycle stability. In addition, when the electrolyte is applied in Ah-grade lithium cobalt oxide||The graphite battery works at temperatures up to 120 °C with a discharge capacity of 862.9 mA h, which is 89.9% at room temperature. This means that lithium-ion batteries have the possibility of practical application in extremely high temperature environments.
Original link:
https://onlinelibrary.wiley.com/doi/10.1002/anie.202410982
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