Solid-state batteries represent a significant advancement in energy storage technology, offering improved safety and energy density compared to conventional liquid electrolyte systems. However, thermal runaway remains a critical concern, necessitating robust safety mechanisms. One promising approach involves integrating endothermic reactions within battery casings to absorb excess heat during decomposition events. This article explores the role of endothermic materials, particularly borates, in suppressing thermal runaway in solid-state batteries.

Thermal runaway occurs when heat generation within a battery exceeds dissipation, leading to uncontrolled temperature increases. In solid-state systems, the absence of flammable liquid electrolytes reduces fire risks, but high temperatures can still degrade solid electrolytes and electrode materials. Endothermic materials embedded in battery casings provide a passive thermal management solution by absorbing heat through chemical reactions or phase changes.

Borates are a class of compounds widely studied for their endothermic properties. When exposed to high temperatures, borates decompose, absorbing significant amounts of heat in the process. For example, zinc borate decomposes at temperatures above 290°C, releasing water vapor and forming a glassy residue that further insulates the battery. The heat absorption capacity of zinc borate is approximately 500–700 J/g, making it effective for mitigating thermal runaway. Other borates, such as calcium borate and magnesium borate, exhibit similar endothermic behavior but differ in decomposition temperatures and heat absorption rates.

The efficiency of borate-based suppression depends on several factors, including particle size, distribution within the casing, and compatibility with other battery materials. Fine particles increase surface area, enhancing heat absorption kinetics, while uniform distribution ensures consistent thermal management. However, excessive borate loading can compromise mechanical integrity, necessitating optimization to balance safety and structural performance.

In addition to borates, other endothermic materials have been explored for solid-state battery casings. Hydroxides like aluminum hydroxide decompose endothermically at around 180–200°C, absorbing heat and releasing water vapor. Carbonates, such as calcium carbonate, decompose at higher temperatures (600–800°C), providing protection against extreme thermal events. The choice of material depends on the specific thermal profile of the battery system and the desired activation temperature for suppression.

Material compatibility is critical when integrating endothermic compounds into battery casings. Borates and other additives must not react with solid electrolytes or electrode materials under normal operating conditions. For instance, some borates may release acidic byproducts during decomposition, which could degrade certain solid electrolytes. Pre-screening tests, such as thermogravimetric analysis and differential scanning calorimetry, are essential to evaluate chemical stability and endothermic performance.

The placement of endothermic materials within the battery casing also influences suppression efficiency. Layered designs, where endothermic compounds are sandwiched between thermally conductive barriers, can localize heat absorption and prevent propagation. Alternatively, composite casings incorporating dispersed borate particles offer homogeneous protection but may require advanced manufacturing techniques like hot pressing or additive manufacturing.

Experimental studies have demonstrated the effectiveness of borate-enhanced casings in delaying or preventing thermal runaway. In tests involving solid-state lithium-ion batteries, zinc borate additives increased the time to critical failure by up to 30% under abusive conditions. The glassy residue formed during decomposition also acted as a physical barrier, inhibiting oxygen diffusion and further exothermic reactions.

Despite these advantages, challenges remain in scaling endothermic suppression technologies for commercial applications. Cost and weight penalties associated with borate additives must be minimized to avoid compromising energy density. Long-term stability of endothermic materials under cyclic thermal loads also requires further investigation, as repeated exposure to high temperatures may degrade their performance.

Future research directions include the development of multifunctional casings that combine endothermic properties with mechanical reinforcement. Hybrid materials, such as borate-polymer composites, could offer synergistic benefits by improving both thermal and structural performance. Advanced modeling techniques, such as finite element analysis, can optimize material selection and casing design to maximize suppression efficiency while minimizing trade-offs.

In summary, endothermic reactions in solid-state battery casings provide a passive yet effective means of mitigating thermal runaway. Borates, with their high heat absorption capacity and residue-forming properties, are particularly promising for this application. Careful material selection, distribution optimization, and compatibility testing are essential to ensure reliable performance. As solid-state battery technology matures, integrating advanced thermal management solutions like endothermic casings will be crucial for enabling safer, high-performance energy storage systems.