Researchers have engineered a new composite solid-electrolyte by introducing a new material that they claim provides thermal stability to solid-state lithium-ion batteries while achieving higher energy density.

Vilas Pol, a Purdue University professor leading a team of engineers, studied the two categories of solid electrolytes—ceramic electrolytes composed of sulfide and oxide and polymer electrolytes.

Polymer electrolytes have attracted the attention of many material experts, given attributes like flexibility and ease of processing. Unlike liquid electrolytes used in commercial batteries, solid electrolytes are less prone to overheating, fire, and loss of charge.

In the study, the experts discovered that the newly developed solid-state electrolyte has thermal stability up to 330 degrees Celsius. It gives them higher thermal stability than the conventionally used liquid-state electrolyte or solid-polymer electrolytes (SPE), which could lead to fire in lithium-ion batteries.

“SPE have poor thermal stability, which can cause battery thermal runaway and lead to catastrophic firing. Their ionic conductivity is limited to low temperatures, which can reduce energy efficiency and lifetime. And low-energy-density batteries can cause quick energy depletion and short operating times of devices, considering the limited space for a battery in electronics or electric vehicles,” explained Pol.

Role of the polymer matrix in solid-polymer electrolytes

Polymer matrix materials are made of long chains of organic molecules, generally classified as thermoset or thermoplastic. It is a composite material made of diverse continuous fibers bound together by a matrix of organic polymers.

In the two-part study published in the Chemical Engineering Journal, the team of engineers studied the polymer matrix polyethylene oxide (PEO) because it is highly compatible with diverse lithium salts.

However, the team observed that PEO comes with poor mechanical properties and low room-temperature ionic conductivity, which hinders the use of this polymer matrix in solid-state batteries.

After studying various matrices, the researchers found polyvinylidene fluoride (PDVF) to be a suitable polymer matrix with high ionic conductivity at room temperature.

Combining two solid-state electrolytes

Pol and his team went on to improve polyethylene oxide’s mechanical property and thermal stability in the PDVF-based solid electrolyte by introducing ceramic filler.

The filler is a garnet-type solid-state electrolyte named LIZTO (Li6.4La3Zr1.4Ta0.6O12) which was combined with the PDVF polymer electrolyte.

They found that LIZTO and lithium bis trifluoromethanesulfonylmide (LiTFSI) salt interacted with the PDVF matrix, facilitating lithium salt’s dissociation. The compound interaction led to the formation of a rapid Li+ conductive path at the ceramic-polymer interface, which enhanced the ionic conductivity.

The interaction between stiff ceramic (LIZTO) and PVDF host led to higher crystallinity of the PVDF electrolyte, which improved the mechanical properties and thermal stability of the obtained composite solid-polymer electrolyte (CPSE).

The CPSE developed by the team has a wide voltage window of around 4.8 volts, with an optimized ionic conductivity of around 2.4*10^4 microsiemens.

The team claimed that their solid polymer electrolyte showed enhanced resistance to cell damage, which can help produce safer solid-state lithium-ion batteries.

Professor Pol added, “We have further developed advanced electrolytes with fire-retardant molecules as a quasi-solid-state battery, enhancing the lithium-ion battery safety.”

In another experiment, researchers claimed that solid-state batteries with a disordered arrangement of metals in electrolytes enhance the battery’s performance by increasing the flow of ions.

In January this year, researchers at the ARC Training Centre for Future Energy Storage Technologies claimed that the suppression of the lithium metal creep under controlled conditions can extend the cell cycle life by 40%, resulting in reduced deformation levels of lithium.