Researchers at Georgia Tech have developed a promising new conversion-type cathode and electrolyte system that replaces expensive metals and traditional liquid electrolyte with lower cost transition metal fluorides and a solid polymer electrolyte. A paper on their work is published in the journal Nature Materials.

Rechargeable lithium-ion battery (LIB) is considered to be the most promising candidate to meet the future demands for energy storage devices including portable electronics and electric transportation. Unfortunately, the reduction in commodity cell price and improvements in energy density have plateaued due to the practical limits of intercalation-type cathode materials. Pushing it further may jeopardize cell safety by increasing the risk of runaway reactions that might lead to spontaneous ignition. Additionally, commercial nickel- (Ni) and cobalt- (Co) based minerals are scarce, expensive and toxic. The recent surge of Co prices (up to 90 USD kg^-1) due to the rapid expansion of the LIB market, attracted attention to the violation of environmental norms and child labor laws and serious health hazards artisanal Co miners in Africa are often exposed to. If nothing changes, the future demand of Co for EVs and other applications may likely cause an increase in Co prices in the next decade, which, in turn, may slow down EV adoption.

Conversion-type cathodes, especially transition metal fluorides (MFs), are thus becoming of the highest interest due to their very high specific and volumetric capacities, which allow storing 1.5-2 times and 2-3 times more energy per given unit stack volume and mass, respectively, than those using conventional electrodes. … Among the MFs (including binary metal fluorides such as CuF₂, and CoF₂, or ternary metal fluorides, such as Ni_yFe_1-yF₂), the fluorides of iron, namely FeF₃ or FeF₂, which exhibit high specific capacity and are made from extremely low-cost iron ore (~0.1 USD kg^-1), provide ultimate advantages for the massive adoption of the LIBs by the EVs.

—Huang et al.

Unfortunately, the researchers noted, such MF cathodes suffer from extremely poor performance at elevated temperatures, and suffer from poor ionic conductivity, significant volume (up to 30%) and morphological changes during cycling, active material dissolution, separation of LiF and M clusters, significant resistance buildup, and irreversible growth of the cathode electrolyte interphase.

The Georgia Tech team sought to overcome those obstacles by using the solid polymer electrolyte.

Advantages of robust and elastic cathode solid electrolyte interface (CEI) on MF particles: a, conventional CEI on a MF particle formed in liquid electrolyte exhibits poor elasticity and fails under repeated lithiation/de-lithiation cycles. This leads to electrolyte decomposition, CEI growth and Mn⁺ and F_- leaching and eventual cell failure; b, robust and flexible CEI produced on the MF particle surface by using elastic SPE confined in a strong composite. Elastic CEI effectively prevents continuous electrolyte decomposition and leaching of ions problem and significantly boost the cell-level stability. Huang et al.

For the study, which was sponsored by the Army Research Office, the research team fabricated used iron fluoride for the cathode active material and a solid polymer electrolyte nanocomposite. Iron fluorides have more than double the lithium capacity of traditional cobalt- or nickel-based cathodes. In addition, iron is 300 times cheaper than cobalt and 150 times cheaper than nickel.

To produce such a cathode, the researchers developed a process to infiltrate a solid polymer electrolyte into the prefabricated iron fluoride electrode. They then hot pressed the entire structure to increase density and reduce any voids.

Two central features of the polymer-based electrolyte are its ability to flex and to accommodate the swelling of the iron fluoride while cycling and its ability to form a very stable and flexible interphase with iron fluoride. Traditionally, that swelling and massive side reactions have been key problems with using iron fluoride in previous battery designs.

Cathodes made from iron fluoride have enormous potential because of their high capacity, low material costs and very broad availability of iron. But the volume changes during cycling as well as parasitic side reactions with liquid electrolytes and other degradation issues have limited their use previously. Using a solid electrolyte with elastic properties solves many of these problems.

—corresponding author Gleb Yushin, a professor in Georgia Tech’s School of Materials Science and Engineering

The researchers then tested several variations of the new solid-state batteries to analyze their performance over more than 300 cycles of charging and discharging at elevated temperature of 122 degrees Fahrenheit, noting that they outperformed previous designs using metal fluoride even when these were kept cool at room temperatures.

The researchers found that the key to the enhanced battery performance was the solid polymer electrolyte. In previous attempts to use metal fluorides, it was believed that metallic ions migrated to the surface of the cathode and eventually dissolved into the liquid electrolyte, causing a capacity loss, particularly at elevated temperatures.

In addition, metal fluorides catalyzed massive decomposition of liquid electrolytes when cells were operating above 100 degrees Fahrenheit. However, at the connection between the solid electrolyte and the cathode, such dissolving doesn’t take place and the solid electrolyte remains remarkably stable, preventing such degradations, the researchers wrote.

The polymer electrolyte we used was very common, but many other solid electrolytes and other battery or electrode architectures—such as core-shell particle morphologies—should be able to similarly dramatically mitigate or even fully prevent parasitic side reactions and attain stable performance characteristics.

—co-author Kostiantyn Turcheniuk

In the future, the researchers aim to develop new and improved solid electrolytes to enable fast charging and also to combine solid and liquid electrolytes in new designs that are fully compatible with conventional cell manufacturing technologies employed in large battery factories.

This material is based upon work supported by the Army Research Office under Grant No. W911NF-17-1-0053. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsors.

Resources

• Qiao Huang, Kostiantyn Turcheniuk, Xiaolei Ren, Alexandre Magasinski, Ah-Young Song, Yiran Xiao, Doyoub Kim, and Gleb Yushin (2019) “Cycle Stability of Conversion-Type Iron Fluoride Lithium Battery Cathode at Elevated Temperatures in Polymer Electrolyte Composites,” Nature Materials doi: 10.1038/s41563-019-0472-7