Single-material lithium-ion battery

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2015

A single compound based on lithium, germanium, phosphorus and sulfur was used to prepare an all-solid-state lithium battery.

KEY CONCEPTS
• An all-solid-state lithium-ion battery has now been developed because of electrolyte conductivity problems and a highly resistive interface.
• New research has led to the development of a single-material solid-state lithium-ion battery known as an LGPS battery that eliminates the interfacial contact problems.
• Additional work needs to be done to evaluate the durability of the battery.

CURRENTLY USED LITHIUM-ION BATTERIES OPERATE WITH AN ANODE, cathode and a liquid electrolyte also known as an ion-conducting membrane. Problems with longevity and flammability have hindered progress in using this battery type for electric vehicles.

A recent TLT article highlighted one of the problems seen with a lithium battery (1). Researchers found that metal fibers of lithium or other metals form metallic filaments known as dendrites that originate at the anode and create a network of fern-like structures through the electrolyte. If left unchecked, these dendrites will eventually short circuit the battery leading to overheating and potentially a fire. To counteract this effect, the researchers developed a new electrolyte based on aramid nanofibers and poly(ethylene oxide).

Chunsheng Wang, associate professor in the department of chemical and biomolecular engineering at the University of Maryland in College Park, Md., agrees that moving from a liquid electrolyte to a solid electrolyte is desired. He says, “An all-solid-state lithium-ion battery prepared with a solid electrolyte is desired for safety reasons because of concern about the flammability of the liquid solvent currently used in the electrolyte of lithium-ion batteries.”

But there are problems with commercializing all-solid-state lithium-ion batteries due to electrolyte conductivity problems and a highly resistive interface. Wang says, “Thin-film solid-state lithium-ion batteries are stable during charge, discharge cycles but they have limited storage energy. Increasing the thickness from 15 microns to 600 microns to produce a bulky battery has improved energy density but not to the extent of batteries with liquid electrolytes.”

Wang continues, “Difficulty in achieving adequate contact between solid electrodes and solid electrolytes due to wettability and compatibility limits lithium-ion transport hindering battery performance.” Efforts to synthesize new high temperature stable coatings on various materials were tried, but battery performance remained too low.

These current efforts to develop an all-solid-state lithium-ion battery were not successful in part because different materials were used for the electrodes and the electrolyte. If one material could be used for all three battery components, then problems with interfacial contact should be overcome leading to a battery with higher performance. Research has now been conducted to achieve this goal.

LGPS ALL-SOLID-STATE BATTERY
Wang and Fudong Han, his Ph.D. student, have used a single compound based on lithium, germanium, phosphorus and sulfur (LGPS) to prepare an all-solid-state lithium battery. The specific substance used is Li₁₀GeP₂S₁₂. The battery has a thickness of 600 microns meaning that it can store and transfer more energy than the conventional thin-film type solid-state batteries.

The reason LGPS is used is because this material has the highest ionic conductivity of all-solid-state electrolytes. Figure 3 shows a diagram of the LGPS all-solid-state battery. The layer in the middle is the electrolyte with the cathode on top and the anode on the bottom. The different coloration in both the anode and the cathode is due to the use of carbon in both electrodes to facilitate the transfer of charge.


Figure 3. A diagram of an all-solid-state LGPS battery is shown with the cathode on the top, the anode on the bottom and the electrolyte in the middle. The different coloration for the anode and cathode is due to the presence of carbon in both electrodes. (Figure courtesy of the University of Maryland.)

LGPS is prepared by mixing lithium sulfide, phosphorus pentasulfide and germanium disulfide in the appropriate molar ratio in an argon-filled glove box. The raw materials are mixed in a high-energy vibrating mill and the resulting powered pressed into pellets that were heated in a furnace placed in the glove box at 550 C for eight hours.

Initial evaluation of the LGPS battery was done with a liquid electrolyte. Han says, “We chose to initially do a fundamental study with a liquid electrolyte because it has better contact with the electrodes and our objective was to determine if LGPS could function as an anode and a cathode.”

In moving to a solid electrolyte, the researchers obtained a lower level of battery performance. Han says, “We believe that the performance problem can be attributed to a particle size that is too big. In our initial battery, the particles are micron sized. We need to reduce them to nanometer level in order to realize better performance.”

Future work is involved with making nanosized LGPS particles that can be coated in the electrodes uniformly with carbon. Wang says, “Our objective has been to prove the concept that a single-material solid-state lithium-ion battery can work. Now that we have shown the concept will work, we are moving to find out if it is a candidate for commercialization. We intend to carry out charge, discharge cycle studies to assess the durability of the battery.”

A second future objective is to prepare a single-material all-solid-state lithium battery based on oxides instead of sulfides. The reason is oxides are more environmentally friendly.

Wang and Han also indicate that other materials besides LGPS can be used not just in lithium-ion batteries but other battery types. Additional information can be found in a recent article (2) or by contacting Wang at cswang@umd.edu. 

REFERENCES
1. Canter, N. (2015), “Dendrite-suppressing battery technology, TLT, 71 (4), pp. 14-15.
2. Han, F., Gao, T., Zhu, Y., Gaskell, K. and Wang, C. (2015), “A battery made from a single material,” Advanced Materials, 27 (23), pp. 3473-3483.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.