CHEMICAL SCIENCE

Electrolyte design for lithium-ion batteries under extreme conditions


On February 9, 2023, Beijing time, Professor Wang Chunsheng’s research team from the University of Maryland published a new study entitled “Electrolyte design for Li-ion batteries under extreme operating conditions” in the journal Nature.

This study reports and validates a soft solvent-based electrolyte design strategy that balances Li+-solvent interactions, salt dissolution, and electrolyte interface layers to meet the LiNi0.8Mn0.1Co0.1O2 (NMC811) || Graphite lithium-ion batteries support higher voltage (≥ 4.5 volts), fast charging (≤ 15 minutes), charging and discharging within a wide temperature range (± 60°C), and not easy to burn. At -50°C (-60°C), the 4.5-volt NMC811 || Graphite batteries can still retain 75% (54%) of their room temperature capacity.

The corresponding authors of the paper are Oleg Borodin and Professor Wang Chunsheng, and the first author is Xu Jijian.

Current carbonate-based electrolytes cannot meet most of the extreme conditions required by lithium-ion batteries (LIBs) because their voltage window is limited to 4.3V, they have a narrow operating temperature range from -20°C to +50°C, and they are flammable. Previous studies have generally achieved low temperature operation by introducing a series of co-solvents with low freezing points, such as linear carboxylates or ethers, to reduce the freezing point of the electrolyte. However, the electrochemical stability of these carboxylic esters and ethers is narrow. Recent breakthroughs in low-temperature batteries enabled by liquefied gas electrolytes can maintain more than 60% of room temperature capacity even at minus 60°C, but these volatile solvents require redesigning sealed batteries at the pressure required for gas liquefaction. In addition to ionic conductivity, interfacial resistance dominates at low temperatures, which requires the electrolyte to have a low Li+ desolvation energy. In addition, high overpotential at low temperatures reduces the available capacity and leads to the deposition of lithium metal on the graphite surface. Lithium deposition accelerates the capacity decay of the anode and reduces the coulomb efficiency (CE) to less than 99.5%. The accompanying lithium dendrites may short-circuit the battery, causing safety hazards. In order to avoid lithium deposition on the graphite surface at low temperatures, it is common practice to use a relatively high anode/positive electrode capacity ratio. This allows for better safety at the expense of overall energy density. However, lithium deposition may still occur at fast charging or at very low temperatures (below -20 °C) because the dynamics between the graphite anode and the NMC811 cathode are different. An ideal low-temperature electrolyte should form a kinetically matched interface layer on the positive and negative electrodes, resulting in low and identical overpotentials at different temperatures and currents.

Professor Wang Chunsheng’s research team at the University of Maryland proposed the electrolyte design principle that enables high-specific energy lithium-ion batteries to work under extreme conditions. At the heart of this principle is the determination of solvents with relatively low DN values (less than 10) and high dielectric constants (greater than 5), which minimizes the binding energy of Li+ to the solvent while still dissociating lithium salts. At the same time, a component with a high reduction potential is introduced into the electrolyte, which can form a similar LiF-rich interface layer on the negative and positive electrodes. The thermodynamic (capacity) and kinetic (impedance) matching of the negative and positive electrodes makes the NMC811 || Graphite batteries are able to charge quickly and charge and discharge over a wide temperature range without lithium deposits. As a proof of concept, the designed 1 M LiTFSI MDFA/MDFSA-TTE electrolyte enables capacity and impedance matching of negative and positive electrodes under extreme conditions. 4.5 V NMC811 || with an area capacity over 2.5 mAh cm-2 The graphite full battery can operate stably over a wide temperature range (-60 °C to +60 °C). The pouch battery maintains more than 83% capacity after 300 cycles at -30°C, and the average coulombic efficiency exceeds 99.9%.

Figure 1: Electrolyte design strategy.

Figure 2: Physical properties of electrolyte and MD calculations.

Figure 3: NMC811 || Electrochemical performance of graphite full battery.

Figure 4: Characterization of the SEI on the surface of graphite anodes.

This design principle opens up a new direction for batteries with high voltage, fast charging and wide temperature zones. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41586-022-05627-8



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