UMD team uses high concentration of LiFSI salt to suppress dendrite formation on Li-metal anode; paired with Ni-rich cathode
Researchers in the Department of Chemical and Biomolecular Engineering (ChBE) at the University of Maryland (UMD), led by ChBE Professor Chunsheng Wang, have recently created a battery chemistry that successfully suppressed dendrite formation in Li-metal batteries by increasing the LiFSI (Lithium bis[fluorosulfonyl]imide) salt concentration in the electrolyte to ~10 M.
In a paper in the journal Chem, they reported achieving a high Coulombic efficiency (CE) of ∼99.3% of Li deposition and stripping, along with an anodic stability of >5.5 V. Pairing a Li-metal anode in this electrolyte with and LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode at high loading (2.5 mAh/cm2) created a NMC622||Li cell, which showed a high capacity retention of 86% after 100 cycles at a high cutoff voltage of 4.6 V.
To further enhance the energy density of batteries, more aggressive chemistries are required, one of which is a Li-metal anode. When coupled with a high Ni-content cathode such as LiNi0.6Mn0.2Co0.2O2 (NMC622), a 500 Wh/kg battery becomes possible.
… Extensive work has been devoted to stabilizing Li-metal anodes through approaches including protective layers, electrode designs at nanoscale, electrolyte additives, and solid-state electrolytes. Among these, ether-based electrolytes present the highest CE and the lowest overpotential, effectively suppressing dendrite growth owing to their low reactivity with Li metal. Especially, the highest cycling CE of 99.1% was recently realized in 1,2-dimethoxyethane (DME). However, ether-based electrolytes are intrinsically unstable against oxidation on cathode surfaces, as characterized by their typical anodic limits of <4 V, which is much lower than those of carbonate-based electrolytes. Thus, ether-based electrolytes can only be applied in low-voltage systems such as Li-S, Li-O2, and Li-LiFePO4. For high energy density LMBs that require Li metal to be paired with a high-voltage, high-capacity cathode such as Ni-rich cathodes, a non-ether electrolyte that can simultaneously stabilize both Li-metal and cathode surfaces must be developed.
… Here, we report that by simply increasing the Li bis(fluorosulfonyl)imide (LiFSI) concentration in carbonate electrolytes (propylene carbonate [PC], dimethyl carbonate [DMC], ethylene carbonate [EC]/DMC), a significantly high CE of $99.3% can be achieved with an extremely high cycling stability.—Fan et al.
|Schematic illustration of the effect of the reactive fluorine content in the concentrated carbonate electrolyte on a Li-metal anode and Ni-rich cathode. Fan et al. Click to enlarge.|
The FSI anion in the concentrated electrolyte will react with the Lithium metal anode to generate a LiF-rich SEI layer, which can suppress dendrite formation and greatly improve the coulombic efficiency, explained Xiulin Fan, ChBE research scientist and first author on the corresponding research paper.
Xiulin Fan, Long Chen, Xiao Ji, Tao Deng, Singyuk Hou, Ji Chen, Jing Zheng, Fei Wang, Jianjun Jiang, Kang Xu and Chunsheng Wang (2018) “Highly Fluorinated Interphases Enable High-Voltage Li-Metal Batteries (link is external).” CHEM 4, 1-12 doi: 10.1016/j.chempr.2017.10.017