Although promising, the current lithium-ion batteries that use organic liquids as electrolytes have many limitations for achieving high energy density. For example, lithium metal, which has the highest gravimetric energy density of all anode materials, cannot be used with a liquid electrolyte because of the compromised cyclability and concerned safety issues. Replacing organic liquid electrolytes with solid electrolytes would bring a new perspective to research on lithium-ion batteries, enabling high-energy battery chemistry with an intrinsically safe cell design.

Nevertheless, solid electrolytes have not been widely used in lithium-ion batteries because their ionic conductivities are generally too low to meet the required current density. Many of the lithium-ion-conducting materials have good compatibility with lithium metal, but their conductivities are still a few orders of magnitude lower than that of the liquid electrolytes. In addition, high interfacial resistance is a challenge for solid electrolytes.

...While the pursuit of high ionic conductivity through innovative chemical structures remains as the momentum for novel lithium-ion conductors, we demonstrate herein that creating nanostructures of existing lithium thiophosphate materials can be another route to achieve improvement of the ionic conductivity in solid electrolytes by several orders of magnitude with the advantage of not interrupting their chemical stabilities. To the best or our knowledge, this is first report of nanostructured lithium thiophosphate.

—Liu et al.

The material has a wide electrochemical window (5 V) and superior chemical stability against lithium metal. The ability to use pure lithium metal as an anode could ultimately yield batteries five to 10 times more powerful than current versions, which employ carbon based anodes.

To make a safer, lightweight battery, we need the design at the beginning to have safety in mind. We started with a conventional material that is highly stable in a battery system—in particular one that is compatible with a lithium metal anode. Cycling highly reactive lithium metal in flammable organic electrolytes causes serious safety concerns. A solid electrolyte enables the lithium metal to cycle well, with highly enhanced safety.

—Dr. Chengdu Liang

Bulk Li₃PS₄ is a γ phase that has a low ionic conductivity; when heated to 195 °C, γ-Li₃PS₄ is converted to the high-conduction β-Li₃PS₄ phase. However, the high- conduction β phase reverts back to the γ phase at temperatures below 195 °C.

Conventionally, Li₃PS₄ has been synthesized through the solid-state reaction of Li₂S with P₂S₅ at 520 °C which produces nonporous crystals of Li₃PS₄. In their research, the team found that the reaction of Li₂S and P₂S₅ can be mediated by tetrahydrofuran (THF) at room temperature; the removal of THF affords pure Li₃PS₄.

The team found that a subtle nanostructure evolves during the removal of solvent.

They also found that surface conduction boosts the overall conductivity of nanoporous β-Li₃PS₄, and that the excellent conductivity of nanoporous β-Li₃PS₄ does not compromise its chemical and electrochemical stability.

They configured a symmetric Li/β-Li₃PS₄/Li cell to demonstrate the cyclability and long-term compatibility of nanoporous β-Li₃PS₄ with metallic lithium. The researchers are continuing to test lab scale battery cells, and a patent on the team’s invention is pending.

In summary, the nanoporous structure enhances the ionic conductivity of Li₃PS₄ by 3 orders of magnitude. Such a porous structure stabilizes the preferred metastable phase and provides a significant surface conduction mechanism. The results will inspire the pursuit of desirable physical properties through revisiting known materials at the nanoscale. Reducing the dimensions of materials leads to very different results, even in contrast to the prevailing view.

—Liu et al.

Resources

• Zengcai Liu, Wujun Fu, E. Andrew Payzant, Xiang Yu, Zili Wu, Nancy J. Dudney, Jim Kiggans, Kunlun Hong, Adam J. Rondinone, and Chengdu Liang (2013) Anomalous High Ionic Conductivity of Nanoporous β-Li₃PS₄. Journal of the American Chemical Society 135 (3), 975-978 doi: 10.1021/ja3110895