Researchers at Pacific Northwest National Laboratory (PNNL) have found that adding a small, optimal amount (0.05M) of LiPF6 (lithium hexafluorophosphate) as an additive in LiTFSI–LiBOB dual-salt/carbonate-solvent-based electrolytes significantly enhances the charging capability and cycling stability of Li metal batteries. A paper on their work is published in the journal Nature Energy.
In the paper, they report that using the additive in a Li metal battery with a 4-V Li-ion cathode at a moderately high loading of 1.75 mAh cm−2 resulted in 97.1% capacity retention after 500 cycles along with very limited increase in electrode overpotential at a charge/discharge current density up to 1.75 mA cm−2. The researchers attributed the fast charging and stable cycling performances to the generation of a robust and conductive solid electrolyte interphase at the Li metal surface and stabilization of the Al cathode current collector.
|Li||NMC battery cycling performance with various electrolytes in EC–EMC solvent mixture at the same charge and discharge current density of 1.75 mA cm−2 after three formation cycles at 0.175 mA cm−2 under 30 ˚C. Zheng et al. Click to enlarge.|
Lithium (Li) metal is regarded as the ultimate anode for energy storage systems because of its ultrahigh specific capacity of 3,860 mAh g-1, a very low redox potential (−3.040 V versus standard hydrogen electrode) and a small gravimetric density of 0.534 g cm−3. Secondary Li metal batteries (LMBs) have been extensively studied in the past four decades, and received increasing attention recently because of the growing needs for high-energy-density batteries. However, technical challenges such as unsatisfied coulombic efficiency and dendritic Li growth impede the successful deployment of secondary LMBs. Recently, extensive work has been dedicated to Li metal protection, including the application of polymer or solid-state electrolytes, ionic liquids, concentrated electrolytes or additives, protective layers, interlayers between Li and separator, nanoscale design, selective deposition, Li/reduced graphene oxide composites, and others.
… Here we report that significantly improved charging capability and cycling stability of LMBs using a LiNi0.4Mn0.4Co0.2O2 (NMC, 1.75 mAh cm−2) cathode can be achieved via manipulating the lithium salt chemistry in the electrolyte to generate a highly conductive SEI on Li metal. An optimal additive level (0.05 M) of LiPF6 can greatly alter the interfacial reactions between Li metal and dual-salt electrolyte containing LiTFSI and LiBOB in carbonate solvents. The capacity retention of moderately high areal-capacity Li||NMC batteries could be significantly improved to >97% after 500 cycles at 1.75 mA cm−2.—Zheng et al.
Corresponding author Wu Xu and colleagues were part of earlier PNNL research seeking a better-performing electrolyte. The electrolytes they tried produced either a battery that didn’t have problematic dendrites and was super-efficient but charged very slowly and couldn’t work in higher-voltage batteries, or a faster-charging battery that was unstable and had low voltages.
Next, they tried adding small amounts of a salt that’s already used in lithium-ion batteries—LiPF6lithium hexafluorophosphate—to their fast-charging electrolyte. They paired the new electrolyte with a lithium anode and a lithium nickel manganese cobalt oxide cathode. The result was a fast, efficient, high-voltage battery.
Because the additive is already an established component of lithium-ion batteries, it's readily available and relatively inexpensive. The small amounts needed—just 0.6 wt%—should also further lower the electrolyte’s cost.
Xu and his team continue to evaluate several ways to make rechargeable lithium-metal batteries viable, including improving electrodes, separators and electrolytes. Specific next steps include making and testing larger quantities of their electrolyte, further improving the efficiency and capacity retention of a lithium-metal battery using their electrolyte, increasing material loading on the cathode and trying a thinner anode.
This research was supported by the Department of Energy’s Office of Energy Efficiency and Renewable Energy. Researchers performed microscopy and spectroscopy characterizations of battery materials at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science national User Facility at PNNL. The battery electrodes were made at DOE’s Cell Analysis, Modeling, and Prototyping Facility at Argonne National Laboratory.
Jianming Zheng, Mark H. Engelhard, Donghai Mei, Shuhong Jiao, Bryant J. Polzin, Ji-Guang Zhang and Wu Xu (2017) “Electrolyte Additive Enabled Fast Charging and Stable Cycling Lithium Metal Batteries,” Nature Energy doi: 10.1038/nenergy.2017.12