Researchers from the University of Texas at Austin have synthesized and characterized a polymer-based, solid composite electrolyte with improved conductivity compared to most of today’s composite polymer electrolytes. The flexible electrolyte consists of a three-dimensional (3D) network of a ceramic lithium-ion conductor surrounded by a polymer matrix. The smooth interaction between the two materials creates continuous conducting paths for the lithium ions, which can move faster in the interphase zone between the polymer and the ceramic. 

While Li-ion batteries are ubiquitous today, users of portable rechargeable devices, as well as the aviation and automobile industry, are looking eagerly toward materials researchers for improved lithium-ion batteries (LIBs) that have a longer overall life, have higher energy/power densities, and are safer than those currently available. Incidents of malfunctions, overheating, melting, and fire are not rare and certainly not something one wishes for their cell phone battery, let alone for avionics backup power supplies and engine start batteries in fighter jets and drones. 

The electrolyte, the component of the battery that facilitates the transport of ions between cathode and anode, influences battery safety significantly. Commercially available LIBs contain toxic flammable organic solvents, which can combust when temperature is elevated, as a result of ionic transport for example or chemical reactions during charge-discharge cycles. Solid polymer electrolytes (SPEs) on the other hand are solvent free, thus leakage is prevented, and also have low flammability. Therefore, they can be a safer option, while at the same time they offer good mechanical, chemical, and thermal stability.

Certain conditions, like cooling an SPE at room temperature or certain discrete compositions, can cause the polymer chains to arrange themselves into interconnected parallel bundles,  not perfectly crystalline but not amorphous either. Since ionic conduction in SPEs reportedly occurs mainly in the amorphous region rather than the crystalline region of the polymer matrix, as a result the ionic conductivities of SPEs are stuck several orders of magnitude below the conductivities of conventional liquid electrolytes. 

At the University of Texas at Austin, researchers in the group of Guihua Yu, head of the Energy Nanomaterials Laboratory, decided to work around this issue by combining a high molecular weight polymer electrolyte (polyethylene oxide or PEO) with a ceramic nanostructured filler (Li_0.35La_0.55TiO₃ or LLTO), thus creating a composite polymer electrolyte (CPE). The presence of lithium cations in the LLTO makes this an “active” filler, meaning that it acts as an ion conductor and improves the conductivity of the electrolyte.

“A big difference from other nanostructured fillers is that ours forms a continuous 3D interconnected structure, while other fillers are still a kind of separated particles, with high aspect ratio,” Yu says. To create this 3D structure, first an LLTO hydrogel was synthesized. The hydrogel was then decomposed and the remaining LLTO 3D framework was heat-treated at 800°C. A solution of the polymer (polyethylene oxide-PEO) was then prepared by dissolving lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in acetonitrile and then adding PEO with viscosity-average molecular weight (M_v)~ 600,000. Stirring at 80°C for 4 h in an argon-filled glove box helped  the polymer to dissolve slowly. The final solution, with a concentration of 1g of PEO in 80 ml acetonitrile, and with a composition corresponding to 10 ether oxygens per lithium ion, was then used to fill the LLTO framework and then the whole system was left to dry at 60°C. The researchers repeated this wetting and drying process until the entire framework was fully embedded in the PEO matrix.

“The composite of interconnected ceramic nanoparticles and polymer matrix demonstrated unexpectedly high conductivity, especially when considering the low conductivity of pure PEO electrolyte (~10^-6 S/cm),” says Jiwoong Bae, PhD candidate in Yu’s research group and first author of the article on this work published in a recent issue of Angewandte Chemie International Edition. The conductivity was measured at different temperatures by means of electrochemical impedance spectroscopy, and was found to be 8.8 × 10^-5 S/cm at room temperature. Bae says that this improvement in conductivity compared to pure PEO was unexpected. According to Bae, this stems from the design of the 3D LLTO framework where a continuous interphase is formed between the ceramic structure and the polymer matrix.

Although the newly developed composite electrolyte is better than the polymer alone, other CPEs with conventional nanoparticle fillers also have conductivities between 10^-5 S/cm ~ 10^-6 S/cm, while for typical solid-state electrolytes such as pure LLTO or Li₇La₃Zr₂O₁₂ (LLZO), the conductivity range is between 10^-3 S/cm and 10^-5 S/cm. The problem with these other solid-state electrolytes, Yu says, is that “they are too brittle and rigid to handle or to be used in practical devices, not to mention for flexible devices. They are also very expensive to make due to a high temperature (>1000^oC) process [required].”

“This is a novel and promising approach for developing high-performance composite polymer electrolytes for the next generation of all-solid-state LIBs. The article presents quite authentic ideas, enriching the field of solid composite lithium-ion electrolytes,” says S. Jayalekshmi, head of the Division for Research in Advanced Materials at Cochin University of Science and Technology, Kerala, India.

Jayalekshmi was not involved in the study, but her team has recently developed a novel type of solid polymer electrolyte (SPE) based on a polymer blend of PEO and PVP. “It would be interesting to look at the performance of the present LLTO-based CPE, serving as the separator and the solid electrolyte, in solid state Li - ion cells,” she says.

“This work provides an artful fabrication strategy for [the] 3D nanostructured hydrogel-derived LLTO framework in composite polymer electrolytes,” says Xiaoxiong Xu, professor at the Ningbo Institute of Material Technology and Engineering in the Chinese Academy of Sciences, who specializes in solid electrolytes with high Li-ion conductivities and was not involved in the study.

Xu says that the pre-percolated LLTO network not only provides a continuous interphase, which serves as a pathway for Li-ion conduction, but also offers continuous Li-ion hopping sites along the surface of the 3D interconnected structure, and thus increases ionic conductivity. “This is a promising approach for high-performance composite polymer electrolytes,” Xu says.

Read the abstract in Angewandte Chemie International Edition.