Semi-Solid-State Batteries ：Bridging the Gap Between Liquid and Solid Electrolytes

In recent years, the rapid growth of the electric vehicle and energy storage markets has driven increasing demands for batteries with higher energy density, enhanced safety, and longer cycle life. Against this backdrop, semi-solid-state batteries, a transitional technology between traditional liquid-state lithium batteries and fully solid-state batteries, have garnered significant attention. This article delves into the technical principles and key features of semi-solid-state batteries.

What is a Semi-Solid-State Battery?

Simply put, semi-solid-state batteries are an evolution of traditional liquid lithium batteries. While conventional batteries rely entirely on liquid electrolytes, semi-solid-state batteries partially or entirely replace the liquid electrolytes with solid-state materials, while still retaining a small amount of liquid electrolyte to improve ionic conductivity. As such, they are often referred to as “solid-liquid hybrid electrolyte batteries.”

Based on the composition and state of the solid electrolyte, semi-solid-state batteries can be further classified into the following types:

• Gel Polymer Electrolyte Batteries:These utilize liquid electrolytes impregnated into a polymer matrix to form a gel-like structure. This design provides flexibility and decent ionic conductivity but has relatively low mechanical strength.
• Solid-Liquid Hybrid Electrolyte Batteries: These combine solid electrolyte powders with liquid electrolytes, resulting in a hybrid that offers the safety of solid electrolytes alongside the high ionic conductivity of liquids.
• In-Situ Solidified Electrolyte Batteries: These involve converting a liquid precursor into a solid-state electrolyte after assembly, using methods such as heating or UV exposure. This approach improves interface contact between the electrolyte and electrodes.

Technical Principles of Semi-Solid-State Batteries

The operating principle of semi-solid-state batteries is fundamentally similar to that of traditional lithium-ion batteries, relying on the insertion and extraction of lithium ions between the electrodes during charge and discharge cycles. The key distinction lies in the use of a solid or solid-liquid hybrid electrolyte.

In these batteries, the electrolyte facilitates ionic transport. During charging, lithium ions detach from the cathode, migrate through the electrolyte, and embed into the anode. The process reverses during discharge.

However, since the ionic conductivity of solid electrolytes is typically lower than that of liquid electrolytes, optimizing ion transport efficiency is crucial. Common strategies include:

• Using high-conductivity solid electrolyte materials: Examples include sulfides and oxides.
• Optimizing the microstructure of the electrolyte: For instance, creating nanoscale solid electrolyte particles to increase contact with the electrodes.
• Adding small amounts of liquid electrolyte: This can boost ionic conductivity, though its proportion must be carefully managed to ensure safety.

Key Features of Semi-Solid-State Batteries

Compared to traditional liquid lithium batteries, semi-solid-state batteries offer the following advantages:

• Enhanced Safety: With reduced or partially replaced flammable liquid electrolytes, these batteries exhibit improved thermal stability, significantly lowering the risk of thermal runaway.
• Higher Energy Density: Solid electrolytes can support a wider electrochemical stability window, enabling the use of higher-capacity electrode materials.
• Extended Cycle Life: Solid electrolytes mitigate the volumetric expansion and contraction of electrode materials, enhancing battery longevity.
• Wider Operating Temperature Range: The conductivity of solid electrolytes is less affected by temperature, allowing the battery to function effectively in diverse environments.

That said, there are challenges:

• Interface Contact Issues: Poor contact between solid electrolytes and electrodes can increase ionic resistance, impairing performance.
• Higher Costs: Advanced solid electrolyte materials are often expensive, and manufacturing processes can be complex.
• Technical Maturity: Compared to well-established liquid lithium battery technologies, semi-solid-state batteries remain in the development phase and require further refinement.

Application Prospects

With their advantages in safety and energy density, semi-solid-state batteries hold great promise in electric vehicles, energy storage systems, and consumer electronics. Particularly in the electric vehicle sector, they are poised to become a mainstream technology for next-generation power batteries, driving improvements in range and safety.

Conclusion

As an important direction in battery technology, semi-solid-state batteries blend the benefits of solid and liquid electrolytes, offering notable advancements in safety and energy density. Despite current challenges, ongoing technological progress and cost reductions are likely to accelerate their adoption across various applications in the future.