Fluorine-incorporated interface enhances cycling stability of Li metal batteries with Ni-rich NCM cathodes
A joint research team led by Professor Nam-Soon Choi and Professor Sang Kyu Kwak in the School of Energy and Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST) has developed an ion concentrate electrolyte using a solvent containing fluorine atoms. The electrolyte evenly formed a protective film on the negative electrode and the positive electrode of the lithium metal battery, increasing the lifespan and output of the entire battery.
Lee et al.
Li metal anodes and Ni-rich layered oxide cathodes with high reversible capacities are promising candidates for the fabrication of high energy density batteries. However, low Coulombic efficiency, safety hazards from likely vertical Li growth, and morphological instability of Ni-rich cathodes hinder the practical applications of these electrodes.
Here, we report that fluorinated compounds can be employed as interface modifiers to extend the applicable voltage range of ether-based electrolytes, which have been used specifically so far for lithium metal batteries with charging cut-off voltages lower than 4 V (vs. Li/Li+).
A complementary electrolyte design using both 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and fluoroethylene carbonate in concentrated ether-based electrolytes significantly improves the capacity retention (99.1%) in a Li|LiNi0.8Co0.1Mn0.1O2 full cell, with a high Coulombic efficiency of 99.98% after 100 cycles at 25 °C. Thus, the modified electrolyte system is promising for addressing the reductive and oxidative decompositions of labile ether-based electrolytes in high energy density Li metal batteries with Ni-rich cathodes.—Lee et al.
The charging and discharging of a lithium metal battery or a lithium ion battery occurs when a lithium ion moves between a positive electrode and a negative electrode. At this time, the passage through which lithium ions move is the electrolyte, and the electrolyte itself reacts on the surface of the electrode (cathode / anode) to form a protective film. However, when this protective film is formed non-uniformly, a problem arises.
Lithium metal can rise sharply on the negative electrode (dendrification), causing a short circuit, or modifying the positive electrode to reduce battery performance. Therefore, it is important to make an ideal type of protective film, and the electrolyte components must be effectively controlled for this purpose.
Professor Nam’s research team developed a new composition containing fluorine (F) to protect both the negative and positive electrodes at the same time and increase the battery output. Fluorine reacted with lithium to form a protective film on the surface of the lithium electrode, and also repaired when the protective film was partially destroyed.
The fluorine-containing electrolyte formed a protective film on the anode, and the electrolyte was decomposed at a high voltage of 4V or more and the adhesion to the anode was solved. This allows the implementation of high-voltage, long-life lithium metal batteries that was not available in the electrolytes of conventional lithium-ion batteries.—Yongwon Lee (Department of Energy Engineering at UNIST), a senior researcher at LG Chem
Professor Kwak’s team used theoretical calculations to identify reaction trends and reaction mechanisms for fluorine-containing solvents. In particular, the fluorinated ether solvent, which has a reduction reaction more easily than conventional fluorine, has a property of easily emitting fluorine, thereby promoting the formation of a protective film (fluorinated interface) on the cathode.
This calculation principle will contribute to the development of functional electrolyte materials and additives for the high performance of lithium metal batteries.The electrode interfacial stabilization mechanism will be used to design the electrolyte system for high energy density cell development.
It is expected to be of great help in improving the electrochemical performance of next generation high-energy-density batteries, including lithium ion batteries using the same positive electrode as lithium metal batteries.—Professor Kwak
The findings of this research have been published in Nano Energy. This research has been supported through the Technology Innovation Program by the Ministry of Trade, Industry and Energy (MOTIE) and the Technology Development Program to Solve Climate Changes by the Ministry of Science and ICT (MSIT).
Yongwon Lee et al. (2020) “Fluorine-incorporated interface enhances cycling stability of lithium metal batteries with Ni-rich NCM cathodes,” Nano Energy doi: 10.1016/j.nanoen.2019.104309