In a study published in the journal Joule, a team from the US Naval Research Laboratory find that of the two main approaches to designing structural batteries—monofunctional materials with de-coupled functions vs. multifunctional materials with coupled functions—decoupled structural batteries (relying on monofunctional materials) generally achieve higher elastic moduli and specific-energy values than coupled structural batteries.

Decoupled structural batteries outperform coupled versions. Cell-level specific-energy values versus corresponding elastic moduli of reported structural batteries, numbered by their references. Hopkins et al.

The researchers’ analysis also suggests that further found that ext-generation structural batteries should look to energy-dense aluminum-air and zinc-air batteries.

Structural batteries, i.e., batteries designed to bear mechanical loads, are projected to substantially increase system-level specific energy, resulting in electric vehicles with 70% more range and unmanned aerial vehicles (UAVs) with 41% longer hovering times. By storing energy and bearing mechanical loads, structural batteries reduce the amount of conventional structural materials required by devices.

—Hopkins et al.

The team performed a meta-analysis on reported structural batteries to develop their findings. They also used the equation for flexural rigidity to demonstrate that decoupled structural batteries also have a fundamental advantage because they position load-bearing components on their outermost sur- faces; i.e., the casing.

…our analysis shows that decoupled structural batteries generally outperform coupled versions based on existing full-cell prototypes. Challenges remain, however, for both types of structural batteries. For example, most reported structural batteries use Li-ion chemistries (238 Wh kg_cell^–1 ) that can experience thermal runaway if damaged by mechanical loads. Unfortunately, more energy-dense, li-based chemistries such as lithium-sulfur and lithium-air can also experience thermal runaway.

If battery-casing materials are reaching fundamental performance limits by relying on lightweight metals or car-bon-fiber composites, then damage-tolerant battery materials with higher volumetric and gravimetric energy densities must be used to advance structural-battery performance. We therefore recommend using energy-dense aqueous metal-air batteries that cannot experience thermal runaway, such as primary (single-use) aluminum-air (900 Wh kg_sys^–1 at the system level) and secondary (electrically rechargeable) zinc-air (400 Wh kg_sys^–1).

—Hopkins et al.

Resources

• Hopkins et al. (2020) “High-Performance Structural Batteries,” Joule doi: 10.1016/j.joule.202 0.07.027