Research Progress and Industrial Applications of Automotive Solid-State Lithium Batteries

With the rapid development of new energy vehicles (NEVs) and the growing demand for grid energy storage, the need for batteries with higher energy density and greater safety has become increasingly urgent. According to the “Energy-Saving and New Energy Vehicle Technology Roadmap” released by China’s Ministry of Industry and Information Technology (MIIT), single-cell energy density targets are set to reach 300 Wh/kg by 2020, 400 Wh/kg by 2025, and 500 Wh/kg by 2030 (as shown in Figure 1). However, current battery systems struggle to achieve 400 Wh/kg energy density, let alone 500 Wh/kg.

1. Performance Requirements for Automotive Lithium Batteries

Automotive power batteries must meet several critical performance criteria: safety, high energy density, high power density, low self-discharge rates, wide operational temperature ranges, long lifespan, and low cost.
For fully battery-driven electric vehicles (EVs), energy-type batteries with high capacity and energy density are essential. To achieve this, lithium-ion power batteries must improve performance metrics, reduce costs, enhance safety, and extend lifespans. Table 1 summarizes these requirements.

2. Advantages of Solid-State Batteries

2.1 Higher Energy Density

Solid-state batteries enable the use of lithium metal as the anode, with a theoretical capacity of 3800 mAh/g—approximately 10 times that of graphite. In conventional liquid electrolyte systems, issues like uncontrollable lithium dendrite growth and significant volume expansion limit the use of lithium metal. Solid electrolytes, with their superior mechanical properties, suppress dendrite formation and allow for the application of lithium metal, thereby enhancing energy density.

2.2 Improved Thermal Stability

Solid-state batteries exhibit better thermal stability than liquid electrolyte batteries, which are prone to decomposition and gas generation at high temperatures. Key advantages include:

1. Polymer Framework: Solid-state electrolytes have amorphous polymer backbones that improve lithium-ion conductivity at elevated temperatures.
2. Inorganic Ceramic Electrolytes: These materials demonstrate high decomposition temperatures and improved ionic diffusion at higher thermal conditions.

For comparison, liquid electrolyte systems begin to degrade at 80–120°C, leading to internal short circuits and thermal runaway. In contrast, most solid electrolytes remain stable beyond 200°C, significantly reducing the need for cooling systems in battery applications.

2.3 Flexible Battery Assembly

Solid-state batteries support innovative internal series connection designs, enabling higher single-cell voltages comparable to multiple liquid cells in series. This reduces packaging requirements and improves assembly efficiency.

3. Advances in Solid-State Battery Technology

3.1 Solid Electrolytes

Solid electrolytes are pivotal to the performance of solid-state batteries. Among the various types, composite solid electrolytes and sulfide electrolytes are most promising for commercial applications.

3.1.1 Composite Solid Electrolytes

Composite solid electrolytes combine organic polymer matrices with inorganic fillers, leveraging the advantages of both components to achieve higher ionic conductivity and mechanical strength.

• PEO-Based Electrolytes: Polyethylene oxide (PEO) is widely studied for its high dielectric constant and lithium-ion solvation capacity. However, its poor ion conductivity at room temperature can be improved by adding inorganic fillers to enhance polymer segment motion.
• PVDF-HFP Electrolytes: Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) provides excellent lithium salt solubility and improved ionic conductivity through crystallinity reduction. Inorganic fillers can further enhance its mechanical properties.

3.1.2 Sulfide Solid Electrolytes

Sulfide-based electrolytes, derived from oxides, replace oxygen with sulfur, reducing lithium-ion binding and enlarging ionic transport channels. This results in superior ionic conductivity, thermal stability, and mechanical properties.

• Challenges: Sulfides are highly sensitive to moisture, reacting with water to produce H₂S gas. To address this, production must occur in dry environments, or stable oxides can partially replace sulfides to improve moisture resistance.

3.2 Interface Engineering

Interface issues in solid-state batteries differ significantly from those in liquid batteries, as solid-solid interfaces dominate. Challenges include physical contact issues, interfacial reactions, and increased impedance.

3.2.1 Anode/Electrolyte Interface

The high reactivity of lithium metal in liquid systems causes safety concerns. Solid electrolytes mitigate these issues but introduce interfacial impedance. Strategies to improve interface performance include:

• Designing elastic interfacial layers using polymers or gels to enable soft contact and reduce impedance.
• Employing in-situ polymerization techniques to create seamless interfaces between solid electrodes and electrolytes.

3.2.2 Cathode/Electrolyte Interface

High-capacity, high-voltage cathodes often face challenges like significant interfacial impedance and element diffusion. To address this, researchers are:

• Using surface modifications and additives to stabilize the cathode-electrolyte interface.
• Designing integrated electrode/electrolyte structures, such as porous solid electrolytes that infiltrate cathode materials.

4. Industrial Applications of Automotive Solid-State Batteries

4.1 Semi-Solid Batteries

Semi-solid batteries incorporate liquid electrolytes into solid-state systems to reduce interface impedance. مثال کے طور پر, Weihong New Energy developed a hybrid solid-liquid battery achieving 300 Wh/kg with a 42 Ah capacity, enabling over 500 km of range in NEVs.

4.2 Fully Solid-State Batteries

Leading advancements include:

• Bolloré Group: Developed PEO-based solid-state batteries with 200 Wh/kg energy density for small electric vehicles.
• Toyota: Targeting 800 km range with sulfide-based solid-state batteries, aiming for commercialization by 2025.
• ProLogium (Taiwan): Created flexible solid-state batteries with up to 833 Wh/L volumetric energy density, achieving significant efficiency improvements in NEV applications.

5. Conclusion

This article compares composite and sulfide solid electrolytes, highlights interface modification strategies, and reviews industrial progress. The combination of advanced solid electrolytes, improved interface engineering, and lithium metal protection will drive the transition from liquid and semi-solid batteries to fully solid-state systems.

Future advancements in solid-state technology will play a pivotal role in achieving safer, higher-performance batteries for next-generation electric vehicles.