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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.

Fan1
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.

Resources

  • 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

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