Cheap and abundant, sodium is a promising candidate for new battery technology. However, the limited performance of sodium-ion batteries has hindered large-scale application. Now, a research team from the Department of Energy’s Pacific Northwest National Laboratory (PNNL) has developed a new electrolyte enabling a high-voltage sodium-ion battery with greatly extended longevity in laboratory tests. A paper on the work appears in Nature Energy.

Sodium-ion batteries (NIBs) have attracted worldwide attention for next-generation energy storage systems. However, the severe instability of the solid–electrolyte interphase (SEI) formed during repeated cycling hinders the development of NIBs. In particular, the SEI dissolution in NIBs with a high-voltage cathode is more severe than in the case of Li-ion batteries (LIBs) and leads to continuous side reactions, electrolyte depletion and irreversible capacity loss, making NIBs less stable than LIBs.

Here we report a rational electrolyte design to suppress the SEI dissolution and enhance NIB performance. Our electrolyte lowers the solvation ability for SEI components and facilitates the formation of insoluble SEI components, which minimizes the SEI dissolution. In addition to the stable SEI on a hard carbon (HC) anode, we also show a stable interphase formation on a NaNi_0.68Mn_0.22Co_0.1O₂ (NaNMC) cathode.

Our HC||NaNMC full cell with this electrolyte demonstrates >90% capacity retention after 300 cycles when charged to 4.2V. This study enables high-voltage NIBs with long cycling performance and provides a guiding principle in electrolyte design for sodium-ion batteries.

—Jin et al.

A battery electrolyte forms by dissolving salts in solvents, resulting in charged ions that flow between the positive and negative electrodes. Over time, the electrochemical reactions that keep the energy flowing get sluggish, and the battery can no longer recharge. In current sodium-ion battery technologies, this process happens much faster than in similar lithium-ion batteries.

The PNNL team, led by scientists Yan Jin and Phung Le, attacked that problem by switching out the liquid solution and the type of salt flowing through it to create a new electrolyte recipe. The new electrolyte consists of 1.5M sodium bis(fluorosulfonyl)imide (NaFSI) salt in a solvent mixture of dimethyl carbonate (DMC) and tris (2,2,2-trifluoroethyl)phosphate (TFP) (1.5:2 in mole or 1.6:8.4 in wt.).

In laboratory tests, the new design proved durable, holding 90% of its cell capacity after 300 cycles at 4.2 V—higher than most sodium-ion batteries previously reported.

The current electrolyte recipe for sodium-ion batteries results in the protective film on the negative end (the anode) dissolving over time. This film is critical because it allows sodium ions to pass through while preserving battery life. The PNNL-designed technology works by stabilizing this protective film. The new electrolyte also generates an ultra-thin protective layer on the positive pole (the cathode) that contributes to additional stability of the entire unit.

The new PNNL-developed sodium-ion technology uses a naturally fire-extinguishing solution that is also impervious to temperature changes and can operate at high voltages. One key to this feature is the ultra-thin protective layer that forms on the anode. This ultra-thin layer remains stable once formed, providing the long cycle life reported in the research paper.

We also measured the production of gas vapor at the cathode. We found very minimal gas production. This provides new insights to develop stable electrolyte for sodium-ion batteries that may operate at elevated temperatures.

—Phung Le

Although sodium-ion technology still lags behind lithium in energy density, it offers advantages, such as imperviousness to temperature changes, stability and long cycle life, which are valuable for applications of certain light-duty electric vehicles and even grid energy storage in the future.

The research team continues to refine their design. Le noted that the team is experimenting with other designs in an effort to reduce—and eventually eliminate—the need to include cobalt, which is toxic and expensive if not recovered or recycled.

The study was supported by the Department of Energy’s Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Imaging studies were performed at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility at PNNL sponsored by the Office of Biological and Environmental Research.

Resources

• Jin, Y., Le, P.M.L., Gao, P. et al. (2022) “Low-solvation electrolytes for high-voltage sodium-ion batteries.” Nat Energy doi: 10.1038/s41560-022-01055-0