Researchers at Pacific Northwest National Laboratory (PNNL) have developed a novel localized high‐concentration electrolyte (HCE) that enables dendrite‐free cycling of lithium‐metal anodes with high Coulombic efficiency (99.5%) and excellent capacity retention (>80% after 700 cycles) of Li||LiNi_1/3Mn_1/3Co_1/3O₂ batteries.

Unlike other HCEs developed earlier, the new PNNL electrolyte exhibits low concentration, low cost, low viscosity, improved conductivity, and good wettability that could bring lithium metal batteries (LMBs) closer to practical applications. The fundamental concept of “localized HCEs”, described in a paper published in the journal Advanced Materials, can also be applied to other battery systems, sensors, supercapacitors, and other electrochemical systems.

Lithium-metal batteries that replace a graphite electrode with a lithium electrode offer the potential for much higher energy density batteries; however, they typically exhibit very poor performance and safety properties due to early failure of Li-metal anodes (LMAs) induced by the instability of the lithium-electrolyte interface and dendrite growth during repeated Li plating/stripping.

Conventional electrolytes used in lithium-ion batteries are not suitable for lithium-metal batteries because they will lead to dendritic growth and very low Coulombic efficiency (CE) of LMAs.

Recently, high-concentration electrolytes (HCEs, e.g., >3 M) have received much attention because their unusual functionalities effectively improve the interfacial stability between electrode and electrolyte in various battery systems. However, the high concentration of salt comes with distinct downsides, including the high cost of lithium salt. The high concentration also increases viscosity and lowers conductivity of the ions through the electrolyte.

The PNNL team addressed this by localizing HCEs—diluting an HCE with an “inert” diluent. The diluent itself exhibits a similar or even wider electrochemical stability window compared to the HCE, which also does not dissolve the salt but is miscible with the solvent and the Li⁺-solvent solvates in the HCE.

We were trying to preserve the advantage of the high concentration of salt, but offset the disadvantages. By combining a fluorine-based solvent to dilute the high concentration electrolyte, our team was able to significantly lower the total lithium salt concentration yet keep its benefits.

—Ji-Guang Zhang, a senior battery researcher at PNNL and corresponding author of the paper

In this process, they were able to localize the high concentrations of lithium-based salt into “clusters” which are able to still form protective barriers on the electrode and prevent the growth of dendrites.

The researchers tested the patent-pending electrolyte in PNNL’s Advanced Battery Facility on an experimental battery cell similar in size to a watch battery. It was able to retain 80% of its initial charge after 700 cycles of discharging and recharging. A similar battery using a standard electrolyte can only maintain its charge for about 100 cycles.

Researchers will test this localized high concentration electrolyte on pouch batteries the size and power of a cell phone battery, to see how it performs at that scale. They say the concept of using this novel fluorine-based diluent to manipulate salt concentration also works well for sodium-metal batteries and other metal batteries.

This research is part of the Battery500 Consortium led by PNNL which aims to develop smaller, lighter, and less expensive batteries that nearly triple the specific energy found in batteries that power today's electric cars. Specific energy measures the amount of energy packed into a battery based on its weight.

This research was supported by the Department of Energy’s Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office. Researchers performed microscopy and spectroscopy characterizations of battery materials at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility at PNNL.

Resources

• Shuru Chen, Jianming Zheng, Donghai Mei, Kee Sung Han, Mark H. Engelhard, Wengao Zhao, Wu Xu, Jun Liu and Ji-Guang Zhang (2018) “High-voltage lithium-metal batteries enabled by localized high concentration electrolytes,” Advanced Materials doi: 10.1002/adma.201706102