Lithium-ion batteries are all around us; they are in our phones, our laptops, and even our cars. However, these batteries have not yet reached their potential and properties such as lifetimes and energy densities remain to be optimized. One of the major challenges is reducing the weight of these batteries. Lithium-ion batteries are currently filled with liquid or gel electrolytes, where the weight of these cannot easily be altered. In addition, the liquid is often flammable, which can be dangerous, especially during the fabrication process. A study published in the January 14 issue of Advanced Functional Materials (DOI: 10.1002/adfm.201402538; p. 184) shows promise for a new, all-solid lithium-ion battery that could potentially cut down the weight of the batteries.
“This would be a big deal for cars,” said Petra E. de Jongh, a materials scientist at Utrecht University in The Netherlands and a co-author of the study. With lighter batteries in place, cars can achieve better gas mileage—this is particularly important to environmentally friendly vehicles such as hybrids.
Lithium-ion batteries rely on liquid or gel electrolytes for ion transport, where these typically comprise lithium salts. Positive lithium ions shuttle between the anode and cathode of the battery upon charging and discharging, where this drives the flow of current in the external circuit. Liquids and gels have been the go-to for battery electrolytes because ion transport is fast and reliable, said de Jongh. But beyond weight and safety issues, liquids and gels don’t make for long-lived batteries. Lithium dendrites can form, stretching between the electrodes and resulting in short-circuits.
To create longer-lived and lighter batteries, de Jongh turned to her work with hydrogen-storage materials. Replacing the liquid and gel electrolytes with a solid electrolyte would be ideal, but the electrolyte would have to have high ion-transport properties. For de Jongh, a metal hybrid family known as borohydrides, such as Mg(BH4)2 and LiBH4, became of interest. When used for hydrogen storage, this material had shown high efficiency in transporting ions. In collaboration with Didier Blanchard, a materials scientist at the Technical University of Denmark, de Jongh decided to trial the ability of this material to transport lithium ions in batteries.
This drawing depicts lithium ions (blue balls) moving fast through LiBH4 near the interface with a silica nanoscaffold. Graphic: Christopher Ege.
The researchers encased the LiBH4 in a nanoporous silica, which has a low ionic and electronic transport efficiency, and pores with tunable shapes. By creating pores of different shapes and sizes, the researchers aimed to gain a better understanding of the effects confinement had on the ion-transport efficiency of LiBH4.
They created scaffolds of silica dioxide that had cylindrical pores with a volume of 0.88–0.97 cm3/g. The pores were filled with LiBH4 through melt infiltration, where the scaffold and samples of LiBH4 were autoclaved at 295°C, allowing the LiBH4 to infiltrate the scaffold. It became clear that the lithium ions were still highly mobile even at room temperature.
“[The speed] wasn’t much different than what happens in a liquid,” said de Jongh. The mechanism behind the fast conductance remains a bit of a mystery, she said, though it is clear that it is related to the interface between the scaffold and the LiBH4.
John B. Goodenough, a mechanical engineer at the The University of Texas at Austin, who also studies lithium batteries, has a theory however. He said that it’s been known for some time that the introduction of oxides, such as the dioxide in the silica scaffold, allows for faster conductance of lithium ions. “The morphology of the cylindrical pores in the porous SiO2 creates connected interfacial regions for a better bulk Li+ conduction,” said Goodenough, who was not affiliated with the current research.
But while the increased conductance of lithium ions in a solid electrolyte is interesting, said Goodenough, a major hurdle remains in putting it into practice. So far, all-solid batteries haven’t achieved a stable interface where the solid electrolyte and the electrodes meet, he said. “This problem has so far restricted all-solid Li-ion batteries to those with thin electrodes,” said Goodenough, “and, therefore, a reduced capacity of stored electrical energy.”
