New fluoride electrolyte could revolutionize lithium metal batteries

Sodium fluoride, a compound of fluorine, is commonly found in toothpaste to protect teeth against decay. However, scientists at the U.S. Department of Energy’s Argonne National Laboratory have made an unexpected discovery regarding the practical uses of fluorine compounds. They have developed a fluoride electrolyte that has the potential to safeguard the performance of next-generation batteries.

According to Zhengcheng (John) Zhang, a group leader in Argonne’s Chemical Sciences and Engineering division, an exciting new generation of battery technologies for electric vehicles is on the horizon. These non-lithium-ion batteries offer significantly higher energy storage capacity compared to lithium-ion batteries, allowing for longer driving distances and potentially powering long-haul trucks and aircraft. The widespread use of such batteries could help mitigate climate change. However, these batteries suffer from rapid declines in energy density after repeated charge and discharge cycles.

One promising contender for next-generation batteries features a lithium metal anode instead of the commonly used graphite in lithium-ion batteries. Referred to as a “lithium metal” battery, it utilizes a cathode composed of a metal oxide containing nickel, manganese, and cobalt (NMC). While it delivers more than double the energy density of lithium-ion batteries, its outstanding performance diminishes rapidly within a few dozen charge-discharge cycles.

The team at Argonne addressed this issue by focusing on the electrolyte, the liquid medium through which lithium ions travel between the cathode and anode during charging and discharging. In lithium metal batteries, the electrolyte consists of a liquid solution containing a lithium-containing salt dissolved in a solvent. The problem lies in the electrolyte’s inability to form a sufficient protective layer on the anode’s surface during the initial cycles. This protective layer, known as the solid-electrolyte-interphase (SEI), acts as a barrier that allows lithium ions to freely move in and out of the anode for charging and discharging the battery.

To overcome this challenge, the team discovered a new fluoride solvent that maintains a durable protective layer for hundreds of cycles. This solvent combines a positively charged fluorinated component (cation) with a negatively charged fluorinated component (anion), forming what is known as an ionic liquid—a liquid composed of positive and negative ions.

The key difference in their new electrolyte lies in substituting hydrogen atoms with fluorine atoms in the ring-like structure of the cation component. This modification proved crucial in maintaining high performance over numerous cycles in a test lithium metal cell.

To gain a deeper understanding of the atomic-scale mechanism behind this improvement, the team utilized the high-performance computing resources of the Argonne Leadership Computing Facility (ALCF), a Department of Energy user facility. Through simulations on the ALCF’s Theta supercomputer, they discovered that fluorine cations adhere to and accumulate on the anode and cathode surfaces before any charge-discharge cycling. This, in turn, leads to the formation of a resilient SEI layer during the initial cycling stages, surpassing the performance of previous electrolytes.

Further investigation using high-resolution electron microscopy at Argonne and the Pacific Northwest National Laboratory confirmed the presence of a highly protective SEI layer on both the anode and cathode, resulting in stable cycling.

The team achieved optimal properties by fine-tuning the proportion of fluoride solvent to lithium salt, including an SEI thickness that is neither too thick nor too thin. Thanks to this optimized layer, lithium ions can efficiently flow in and out of the electrodes during charge and discharge for hundreds of cycles.

In addition to its improved performance, the team’s new electrolyte offers several other advantages. It is cost-effective, as it can be produced with high purity and yield in a single step, unlike multiple-step processes. It is also environmentally friendly, requiring less solvent, which is volatile and can release contaminants the team’s new electrolyte offers many other advantages as well. It is low cost because it can be made with extremely high purity and yield in one simple step rather than multiple steps. It is environmentally friendly because it uses much less solvent, which is volatile and can release contaminants into the environment. And it is safer because it is not flammable.

Zhengcheng (John) Zhang expressed optimism about the impact of lithium metal batteries with their fluorinated cation electrolyte on the electric vehicle industry. He believes that this electrolyte could significantly boost the electric vehicle sector. Furthermore, he emphasizes that the electrolyte’s usefulness extends beyond lithium-ion batteries to other types of advanced battery systems.

The findings of the research conducted by the team at Argonne National Laboratory were published in the journal Nature Communications.

Source: Argonne National Laboratory

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