Chinese and foreign researchers jointly develop wide voltage window button cell battery cr1620 electrolytes to promote the commercialization of high-voltage lithium batteries

Since the 1990s, the good antioxidant capacity (~4.3V) of EC-based electrolyte systems and their good matching performance with graphite negative electrodes have laid the foundation for the large-scale application of commercial lithium-ion batteries. However, with people's further desire for electric vehicle mileage in recent years, this system (graphite negative electrode-EC-based electrolyte-NMC/LCO/LFP positive electrode) has gradually failed to meet people's expectations.

Therefore, the development of new high-voltage button cell battery cr1620 systems or further improving the charge cut-off potential and energy density based on the current system is becoming the research focus of high-energy density lithium batteries. Whether it is the development of a new button cell battery cr1620 system with a higher voltage system or the construction of a higher voltage and higher energy density button cell battery cr1620 based on the current positive and negative electrode materials, the narrow voltage window of the electrolyte has become the key to restricting the further development of the next generation of high-energy density lithium-ion batteries. Based on this, Researcher Fan Xiulin of Zhejiang University and Professor Wang Chunsheng of the University of Maryland, USA, sorted out the new electrolyte systems with wide voltage windows in recent years and made prospects for them.

The review article was published in the international journal Chemical Society Review under the title "High-voltage liquid electrolytes for Libateries: progress and perspective".

First of all, it should be pointed out that the breakthrough of organic liquid electrolytes has widened the voltage window of secondary batteries to above 3.0V for the first time, thus getting rid of the limitation of the 1.23V thermodynamic window of aqueous batteries. This is also the basis for lithium-ion batteries to stand out from many secondary batteries (as shown in Figure 1a). According to the formula 1/Q=1/Qa+1/Qc, under the existing NMC positive electrode system, when the capacity of the negative electrode exceeds 1000mAh/g, further increasing the capacity of the negative electrode has almost no effect on the energy density of the entire system (as shown in Figure 1b). In order to further improve the energy density of the button cell battery cr1620, it will be more effective to increase the working cutoff voltage of the existing positive electrode materials or develop new positive electrode materials with higher voltage and high capacity (as shown in Figure 1b). However, the current increase in cutoff voltage is increasingly restricted by the antioxidant capacity of traditional commercial EC ester-based electrolytes.

Recent studies have found that EC solvent is a double-edged sword in commercial electrolyte systems. Although it can form an excellent SEI film on the graphite negative electrode side, it is also the electrolyte matrix component that is least resistant to oxidation in commercial electrolytes. Therefore, the presence of high EC content in commercial electrolytes greatly limits the application of electrolytes in the next generation of high-voltage, high-energy density button cell battery cr1620 systems (as shown in Figure 1c). Fortunately, we can form an excellent SEI/CEI interface by regulating the solvation structure of the electrolyte or film-forming additives, thereby inhibiting side reactions and kinetically widening the button cell battery cr1620's operating voltage window (as shown in Figure 1d)

Figure 1 (a) The development history of representative secondary batteries since 1800; (b) The effect of changes in positive and negative electrode capacity and increasing the positive electrode charge cutoff voltage on the energy density of the button cell battery cr1620 system; (c) Comparison of the operating voltage range of embedded positive electrode materials, which is currently a hot research topic, with the stable window of commercial electrolytes; (d) Schematic diagram of SEI/CEI expanding the electrolyte voltage window

Based on the above analysis, the author sorted out several electrolyte systems that have been hotly studied recently and can effectively improve the electrolyte working window, including EC-free electrolyte system (including EC-free ester electrolyte system, fluorinated electrolyte, sulfone electrolyte, nitrile electrolyte and other high-voltage electrolytes), high-concentration electrolyte and pseudo-high-concentration electrolyte system (including ester-based high-concentration, ether-based high-concentration, other organic high-concentration, water-in-salt and other electrolytes), ionic liquids, electrolyte additives (unsaturated bonds, fluorine-containing, phosphide, nitrile, organic boride, organosilicon, organic sulfide, etc.).

Several types of high-voltage electrolyte systems involve a wide range of specific components and types, but the essential mechanism or design principles are similar: inhibiting the oxidation or reduction of solvent molecules, while introducing components with better film-forming properties. However, the specific positive and negative electrode materials are often different. For example, for graphite negative electrodes, since the volume expansion rate is only about 13%, the SEI generated by the decomposition of traditional EC and tightly bonded to graphite is sufficient to withstand such a large volume expansion, so commercial lithium-ion batteries can be effectively cycled for thousands of cycles. For electrode materials with much larger volume expansion rates such as Si (volume expansion rate>300%) and Li (volume expansion rate is infinite), the design criteria of SEI should be completely different.

For such electrode systems, the most ideal situation is that there is no SEI generated. If there is no SEI generated, even if the electrode material undergoes volume deformation or particle pulverization, as long as the pulverized small particles can come into contact with the current collector, there will be electrochemical activity. However, there is no such electrolyte/electrolyte at present, so we can only settle for the next best thing, hoping that the generated SEI/CEI can be as stable as possible during the cycle process. At this time, the characteristics of SEI/CEI should be: (i) preferably generated in the expanded state of the electrode material; (ii) the interface energy with the electrode material is as high as possible and does not change with the system deformation of the electrode material (as shown in Figure 2).

Figure 2 Effect of different SEI characteristics on the volume deformation of alloy negative electrode: (a) low interface energy SEI, (b) high interface energy; (c) comparison of bandgap width of common inorganic components in SEI; Li/LiF interface structure (d), and its corresponding state density (e); HAADF-STEM (f) and ABF-STEM (g) of NMC811 after cycling in 1MLiFSIDME/TFEO electrolyte.

Other points to note are:

1) The widening of the button cell battery cr1620 voltage window has both thermodynamic contributions and kinetic inhibition of side reactions such as SEI/CEI, but in most cases, the contribution of SEI/CEI is more obvious. Taking a typical EC-based electrolyte as an example, the thermodynamic reduction potential of the EC electrolyte system is about 1.2V, but the excellent SEI generated on the graphite surface can extend its working potential to below 0.1V; and in recent years, the series of Water-in-salt new wide voltage window aqueous electrolytes developed by Professor Wang Chunsheng's team also benefited from the good SEI generated by salt decomposition, which further extended the window of aqueous electrolytes to >3V. If a salt cannot effectively generate SEI, even at a high concentration, the ability to widen the voltage window is very limited.

2) The electrolyte is not universal. Once any positive and negative electrode system is established, if the electrochemical performance is to be maximized, it is necessary to design an electrolyte system specifically for it. Even if the charge and discharge potential range of the button cell battery cr1620 system is the same, it is necessary to design an electrolyte system specifically for specific positive and negative electrode materials. For example, some electrolyte systems have excellent performance for graphite||LCO system, but the performance will be greatly reduced when transplanted to graphite||NMC system; even if it is the same graphite||NMC532 system, the same electrolyte system will perform differently for single crystal positive electrode and polycrystalline positive electrode.

3) Since there is no electrolyte thermodynamic window that can meet both high voltage positive electrode and low voltage negative electrode at the same time, the effective operation of SEI/CEI is the basis for achieving stable button cell battery cr1620 operation. In addition to meeting the basic physical and chemical requirements of SEI/CEI (such as low electronic conductivity & high ionic conductivity, and a certain binding force with the positive and negative electrode surfaces), the electrolyte system and the SEI/CEI on the positive and negative electrode surfaces also need to be able to self-repair. This is required for any electrolyte, regardless of electrolyte, organic polymer electrolyte, or inorganic all-solid electrolyte.

4) Solid-state batteries have developed rapidly in recent years, but apart from the LiPON thin-film solid-state button cell battery cr1620 developed by Oak Ridge National Laboratory in the 1980s, there are no inorganic all-solid-state batteries that can be commercially produced on a large scale. Therefore, researchers have developed a series of organic-inorganic composite solid electrolytes. In addition to effectively increasing the contact area between the electrolyte and the electrode, organic electrolytes/polymer electrolytes can also effectively generate SEI/CEI similar to liquid electrolytes. In fact, even LiPON generates SEI components similar to liquid electrolyte systems, mainly Li2O. Therefore, for the development of solid-state batteries, we should also learn from the liquid electrolyte lithium-ion batteries that have been commercially used for more than 30 years in terms of the electrode/electrolyte interface. Energy scholar

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