A recent study evaluating garnet-type solid electrolytes for lithium metal batteries finds that their expected energy density advantages may be overstated. The research reveals that an all-solid-state lithium metal battery (ASSLMB) using lithium lanthanum zirconium oxide (LLZO) would achieve a gravimetric energy density of only 272 Wh/kg, a marginal increase over the 250-270 Wh/kg offered by current lithium-ion batteries. Given the high production costs and manufacturing challenges associated with LLZO, the findings suggest that composite or quasi-solid-state electrolytes may be more viable alternatives.

"All-solid-state lithium metal batteries have been viewed as the future of energy storage, but our study shows that LLZO-based designs may not provide the expected leap in energy density," said Eric Jianfeng Cheng, lead author of the study and researcher at WPI-AIMR, Tohoku University. "Even under ideal conditions, the gains are limited, and the cost and manufacturing challenges are significant."

Solid-state lithium metal batteries are considered a promising next-generation technology due to their potential for improved safety and energy performance. LLZO, a leading candidate for solid electrolytes, is valued for its stability and ionic conductivity. However, detailed modeling of a practical LLZO-based pouch cell challenges the assumption that this material significantly boosts energy density. The study finds that even with an ultrathin 25 μm LLZO ceramic separator and a high-capacity cathode, the battery's performance remains only slightly ahead of the best conventional lithium-ion cells.

One key issue highlighted in the study is LLZO's density, which increases the overall cell mass and reduces expected energy benefits. Although the volumetric energy density reaches approximately 823 Wh/L, the added weight and cost of LLZO hinder its practicality. Additionally, the material's brittleness, difficulty in fabricating defect-free thin sheets, and issues with lithium dendrites and voids at the interface further complicate large-scale implementation. "LLZO is an excellent material from a stability standpoint, but its mechanical limitations and weight penalty create serious barriers to commercialization," Cheng explained.

As an alternative, researchers are exploring hybrid approaches that integrate LLZO with other materials. One promising strategy involves LLZO-in-polymer composite electrolytes, which retain high ionic conductivity while improving flexibility and manufacturability. Another approach is quasi-solid-state LLZO electrolytes, which incorporate a small amount of liquid electrolyte to enhance ionic transport and structural integrity. These hybrid designs have demonstrated improved long-term stability.

"Instead of focusing on a fully ceramic solid-state battery, we need to rethink our approach," said Cheng. "By combining LLZO with polymer or gel-based electrolytes, we can improve manufacturability, reduce weight, and still maintain high performance."

The study, published in Energy Storage Materials, was conducted in collaboration with researchers from Tohoku University, Shanghai Jiao Tong University, MIT, UW Madison, Johns Hopkins University, and St Andrews University. By highlighting the limitations of fully ceramic solid-state batteries, the research emphasizes the need for practical engineering solutions that balance energy performance, manufacturability, and cost.