Study sheds light on energy density of solid-state lithium batteries

A study evaluating garnet-type solid state electrolytes for lithium metal batteries has found that their unexpected energy density advantages may be overstated. The research found that an all-solid-state lithium metal battery (ASSLMB) using lithium lanthanum zirconium oxide (LLZO) achieves 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 manufacturing challenges associated with LLZO, the research findings suggest that composite or quasi-solid electrolytes may be more viable alternatives. Eric Jianfeng Cheng, a researcher at Tohoku University, said all-solid-state lithium metal batteries are viewed as the future of energy storage, but his research shows that LLZO-based designs may not provide the expected leap in energy density.

“Even under ideal conditions, the gains are limited, and the cost and manufacturing challenges are significant,” Cheng said.

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

A 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 of LLZO hinders its practicality. 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 commercialisation,” Cheng said.

Researchers are exploring hybrid approaches that integrate LLZO with other materials as an alternative. 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. By combining LLZO with polymer or gel-based electrolytes, we can improve manufacturability, reduce weight and still maintain high performance,” Cheng said.

By highlighting the limitations of fully ceramic solid-state batteries, the research emphasises the need for practical engineering solutions that balance energy performance and manufacturability.

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