ORNL team finds cubic garnet material a promising solid electrolyte for high-energy aqueous lithium batteries

21 October 2014

Batteries with an aqueous catholyte and a Li-metal anode (e.g. aqueous Li-air or Li-redox-flow) are of great interest due to their exceptional energy density and high charge/discharge rate. However, long-term operation of such batteries requires that the solid electrolyte separator between the anode and aqueous solutions must be compatible with Li and stable over a wide pH range. No such compound has yet been reported.

Now, in a paper published in the journal Angewandte Chemie, researchers from the US Department of Energy’s (DOE) Oak Ridge National Laboratory report that a cubic garnet material (Li₇La₃Zr₂O₁₂, or LLZO) is highly stable as a Li-stable solid electrolyte in neutral and strongly basic solutions, and is “a promising candidate for the separator in aqueous lithium batteries.”

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The imminent exhaustion of fossil-based fuels demands alternative power sources with comparable energy densities. Recently, various new battery configurations have been proposed to achieve significantly higher energy densities than the state-of-the-art lithium-ion batteries. Despite the different chemistries, these new batteries frequently have several features in common. Pure Li metal is typically used as the anode to maximize the energy density, as it exhibits the highest possible specific capacity for lithium batteries. Additionally, aqueous solutions are often adopted as the “catholyte”, because their unique combination of low cost, high reliability, and super-fast Li⁺ transport circumvents most of the long-standing issues in the conventional Li-ion batteries. Promising batteries showing these characteristics include the aqueous Li–air and Li-redox-flow batteries. In particular, the theoretical energy density of the aqueous Li–air battery reaches 2,450 Wh kg^-1, which is approximately 10 times of that in the conventional Li-ion battery (-1).

To realize long-term operation of aqueous lithium batteries, special attention must be paid to the solid electrolyte separating the Li metal and the aqueous solutions. Amongst the many required characteristics, the stability of the solid electrolyte over a wide pH range is particularly desirable, as the amount of H⁺ in the aqueous catholyte could vary dramatically during charge/discharge. … However, few solid electrolytes show satisfactory performance in this regard; many of them (e.g., LiPON and most sulfides) simply decompose in aqueous environments regardless of the pH value.

In this regard, a recently discovered garnet, the cubic Li₇La₃Zr₂O₁₂ (LLZO) … appears to show great promise as a solid electrolyte. It not only has a high ionic conductivity of 10^-4 S cm^-1, which greatly surpasses that of all the other garnets, but also has excellent stability even in molten Li. Furthermore, unlike many other solid electrolytes, LLZO does not suffer from conductivity degradation upon exposure to humid atmospheres. While these unique properties have attracted considerable research interest, the impact of aqueous environments with different pH values on the structure and chemistry of LLZO has not yet been investigated in detail.

The researchers used atomic resolution imaging to monitor structural changes in LLZO after the samples’ immersion in a range of aqueous solutions. The team’s observations showed that the compound remained structurally stable over time across neutral and extremely alkaline environments.

Although the material underwent a Li⁺/H⁺ exchange in aqueous solutions, its structure remained unchanged even under a high exchange rate of 63.6%. When treated with a 2 M LiOH solution, the Li⁺/H⁺ exchange was reversed without any structural change.

This solid electrolyte separator remains stable even for a pH value higher than 14. It gives battery designers more options for the selection of aqueous solutions and the catholyte.

Researchers have previously tried to avoid the degradation of the separator by diluting the aqueous solutions, which only makes the battery heavier and bulkier. With this new type of solid electrolyte separator, there is no need to dilute the aqueous electrolyte, so it indirectly increases the battery’s energy density.

The researchers intend to continue their research by evaluating the LLZO garnet’s performance in an operating battery.

Resources

• Ma, C., Rangasamy, E., Liang, C., Sakamoto, J., More, K. L. and Chi, M. (2014), “Excellent Stability of a Lithium-Ion-Conducting Solid Electrolyte upon Reversible Li⁺/H⁺ Exchange in Aqueous Solutions,” Angew. Chem. Int. Ed. doi: 10.1002/anie.201408124