The use of lithium metal as an anode offers a range of benefits for those seeking rechargeable high-energy density systems: ultrahigh theoretical capacity (3860 mAh g⁻¹); low density (0.59 g cm⁻³); and the most negative electrochemical potential (−3.040 V vs standard hydrogen electrode). However, viable application has been hampered by several issues, including safety threats resulting from dendrite formation and poor cycling performance.

Now, researchers in China have used a dual-salt electrolyte—Lithium difluorophosphate (LiPO₂F₂) and lithium bis(fluorosulfony)imide (LiFSI) in the same proportion—to deliver improved long-term cycling performance along with high coulombic efficiency for lithium metal batteries.

In a paper in the Journal of Power Sources, they report that with 1 M LiPO₂F₂ and 1 M LiFSI in 1,2-dimethoxyethane as the electrolyte, a Li/Cu cell can be cycled at 1 mA for more than 300 cycles with a deposited capacity of 1.0 mAh. An homogeneous and dendrite-free solid electrolyte interphase layer is generated on the Cu electrode. The researchers attributed this to the favorable stability of LiPO₂F₂ and high ionic conductivity of LiFSI.

Over the last four decades, numerous researchers have carefully analyzed the failure mechanisms of lithium metal batteries and considered various strategies in view of these issues, such as structured anode, electrolyte additive, solid electrolytes, artificial solid-electrolyte interphase (SEI) and super con- centrated electrolyte etc. Among these approaches, the role of the electrolyte is a critical factor for stabilizing the lithium metal surface.

… Most recently, our groups found that the cyclic stability of lithium-based batteries can be significantly enhanced with Li_xPO_yF_z contained in the SEI layer. We utilized the Lithium difluorophosphate (LiPO₂F₂) as electrolyte additive to optimize compositions in the SEI layer, which improved the cyclability of LIBs with both LiCoO₂ and graphite electrodes. However, LiPO₂F₂ is not appropriate for electrolyte salt alone due to its low ionic conductivity. In view of that another salt lithium bis(fluorosulfony)imide (LiFSI) shows high ionic conductivity and great compatibility with lithium metal, the combination of LiPO₂F₂ and LiFSI as new electrolyte salts have a promising application for improved cyclic performance.

—Dong et al.

The researchers said that their work validates an optimized electrolyte system for a lithium-metal anode, and provides new strategies for next-generation high energy systems.

Resources

• Ning Dong, Guanghua Yang, Hao Luo, Hewei Xu, Yonggao Xia, Zhaoping Liu (2018) “A LiPO₂F₂/LiFSI dual-salt electrolyte enabled stable cycling of lithium metal batteries,” Journal of Power Sources, Volume 400, Pages 449-456 doi: 10.1016/j.jpowsour.2018.08.059