The development of low-cost, high-performance solid-state electrolytes (SSEs) and compatible solid-state batteries (SSBs) is crucial for advancing next-generation lithium batteries with enhanced energy density and safety. Solid-state batteries, which replace flammable organic liquid electrolytes with solid alternatives, offer significant advantages, including the potential use of lithium metal anodes to achieve higher energy densities. Currently, research institutions worldwide are intensifying efforts in both fundamental research and industrialization of SSBs, with a focus on oxide, sulfide, halide, and polymer-based solid electrolytes. However, challenges such as interfacial issues and poor cycle life hinder the full commercialization of SSBs, leading some companies to adopt hybrid solid-liquid battery strategies as an interim solution. This article analyzes the technology readiness level (TRL) of typical SSE materials and SSB technologies, providing insights for industry development. It emphasizes that the performance of solid-state batteries, as complex systems, depends not solely on the properties of solid electrolytes but requires comprehensive evaluation.
Solid-state batteries have garnered substantial attention due to their potential for higher energy density and improved safety compared to conventional lithium-ion batteries. The transition to solid-state batteries is driven by the limitations of traditional batteries, which include non-active components that reduce energy density and the risk of thermal runaway from organic electrolytes. The ion conductivity of solid electrolytes is a critical parameter, often described by the Arrhenius equation: $$\sigma = \sigma_0 \exp\left(-\frac{E_a}{kT}\right)$$ where $\sigma$ is the ionic conductivity, $\sigma_0$ is the pre-exponential factor, $E_a$ is the activation energy, $k$ is Boltzmann’s constant, and $T$ is the temperature. Achieving high ionic conductivity in solid-state batteries is essential for practical applications, and various materials have been explored to this end.
Oxide-based solid electrolytes, such as garnet-type Li7La3Zr2O12 (LLZO), NASICON-type Li1+xAlxTi2-x(PO4)3 (LATP), and perovskite-type Li3xLa2/3-xTiO3 (LLTO), exhibit high ionic conductivities in the range of 10−4 to 10−3 S/cm. These materials are known for their excellent thermal stability and mechanical strength. However, the rigid nature of oxide ceramics leads to poor solid-solid contact with electrodes, resulting in high interfacial resistance. To address this, interface engineering strategies, such as the introduction of polymer layers or liquid electrolytes, have been developed. For instance, the incorporation of polyacrylic acid (PAA) at the LLZO/lithium metal interface can reduce interfacial resistance significantly. Despite these advancements, the decomposition of oxide electrolytes at high voltages or when in contact with lithium metal poses safety risks. Thermal stability studies show that LLZO remains stable up to 1600°C, but when combined with cathode materials, decomposition can occur at lower temperatures, around 600°C. The TRL for oxide solid electrolytes varies: LATP is at TRL 7-8 due to ton-scale production capabilities, while LLZO and LLTO are at TRL 6-7. Oxide-based solid-state batteries, such as those using LLZO, are at TRL 5-6, with companies like ProLogium developing large-format lithium ceramic batteries.
| Electrolyte Type | Example Materials | Ionic Conductivity (S/cm) | TRL |
|---|---|---|---|
| Garnet-type | LLZO | 10−4 to 10−3 | 6-7 |
| NASICON-type | LATP | 10−4 to 10−3 | 7-8 |
| Perovskite-type | LLTO | 10−4 to 10−3 | 6-7 |
| Thin-film | LiPON | 10−6 to 10−5 | 8-9 |
Sulfide-based solid electrolytes, including Li10GeP2S12 (LGPS) and Li6PS5Cl (LPSCl), demonstrate exceptionally high ionic conductivities, up to 10−2 S/cm, rivaling liquid electrolytes. These materials are soft and malleable, facilitating better interfacial contact in all-solid-state batteries. However, sulfides suffer from poor air stability, generating toxic H2S upon exposure to moisture, and thermodynamic instability against lithium metal and high-voltage cathodes. The electrochemical window of many sulfides is narrow, around 1.6-2.3 V, leading to decomposition at interfaces. Strategies like artificial interlayers or multilayer electrolyte structures have been employed to mitigate these issues. For example, a Li/G-LPSCl-LGPS-LPSCl-G/Li structure can suppress lithium dendrite growth and enable long cycle life. Safety concerns are prominent, as sulfides can react exothermally with cathode materials, releasing significant heat and toxic gases. The TRL for sulfide electrolytes like LGPS and LPSCl is estimated at 4-6, with companies such as Ganfeng Lithium producing kilogram-scale materials. Sulfide-based all-solid-state batteries are at TRL 4-5, with demonstrations of ampere-hour scale cells by organizations like Enpower Greentech and FAW.
The ionic conductivity in sulfide electrolytes can be modeled using the Nernst-Einstein relation: $$D = \frac{\sigma kT}{Nq^2}$$ where $D$ is the diffusion coefficient, $N$ is the carrier concentration, and $q$ is the charge. This highlights the importance of high carrier mobility for achieving superior performance in solid-state batteries.
| Electrolyte Type | Example Materials | Ionic Conductivity (S/cm) | TRL |
|---|---|---|---|
| LGPS-type | LGPS | ~10−2 | 4-6 |
| Argyrodite | LPSCl | ~10−3 | 4-6 |
| Glass-ceramic | Li2S-P2S5 | 10−4 to 10−2 | 4-6 |
Polymer-based solid electrolytes, such as poly(ethylene oxide) (PEO), offer flexibility and ease of processing but typically have lower ionic conductivities (10−8 to 10−4 S/cm) and narrow electrochemical windows. To enhance performance, composites with inorganic fillers (e.g., LLZO, SiO2) or in-situ polymerization methods are used. For instance, PEO combined with LLZO nanoparticles can achieve conductivities over 10−4 S/cm through percolation effects. In-situ polymerization, utilizing monomers like 1,3-dioxolane (DOL) or poly(ethylene glycol) diacrylate (PEGDA), enables seamless electrode-electrolyte integration and improved interfacial contact. Safety remains a concern, as polymers can decompose at high temperatures, but the addition of flame-retardant additives enhances stability. The TRL for PEO electrolytes is high (8-9) due to commercial availability, while in-situ polymerized systems are at TRL 5-6. Polymer-based solid-state batteries, such as those by Bolloré, have been deployed in electric vehicles, indicating a TRL of 7-8.
The conductivity in polymer electrolytes often follows the Vogel-Tammann-Fulcher equation: $$\sigma = \sigma_0 \exp\left[-\frac{B}{T – T_0}\right]$$ where $B$ is a constant and $T_0$ is the glass transition temperature. This model accounts for the segmental motion of polymer chains, which is critical for ion transport in solid-state batteries.
| Electrolyte Type | Example Materials | Ionic Conductivity (S/cm) | TRL |
|---|---|---|---|
| Homopolymer | PEO | 10−8 to 10−4 | 8-9 |
| Composite | PEO-LLZO | 10−5 to 10−4 | 6-7 |
| In-situ polymerized | PEGDA-based | 10−4 to 10−3 | 5-6 |
Halide-based solid electrolytes, such as Li3YCl6 or Li3InCl6, are emerging materials with high ionic conductivities (up to 10−3 S/cm) and good moisture tolerance. However, they are still in the early stages of research, with limited studies on interface stability and scalability. The TRL for halide electrolytes is estimated at 4, indicating a need for further development before commercialization in solid-state batteries.
In terms of industrialization, hybrid solid-liquid batteries have gained traction as a transitional technology. Companies like Guoxuan High-Tech, Ganfeng Lithium, and WeLion New Energy have developed hybrid batteries with energy densities up to 360 Wh/kg, leveraging solid electrolytes in combination with minimal liquid components to improve safety and cycle life. These systems typically contain 5-15 wt% liquid electrolyte, reducing interfacial resistance while maintaining enhanced safety compared to all-liquid systems. The TRL for hybrid solid-state batteries is 5-6, with several products undergoing vehicle testing. All-solid-state batteries, particularly sulfide-based, are progressing, with prototypes demonstrating long cycle life and high energy density. However, challenges in manufacturing, cost, and interface control remain barriers to widespread adoption.
The overall performance of solid-state batteries can be evaluated using metrics such as energy density ($E$), which is given by: $$E = \frac{1}{2} C V^2$$ where $C$ is the capacitance and $V$ is the voltage. Higher voltages enabled by stable solid electrolytes can significantly boost energy density in solid-state batteries.
| Battery Type | Electrolyte System | Energy Density (Wh/kg) | TRL |
|---|---|---|---|
| All-solid-state | Sulfide-based | 250-300 | 4-5 |
| Hybrid solid-liquid | Oxide-polymer composite | 300-360 | 5-6 |
| Polymer-based | PEO with lithium salt | 200-250 | 7-8 |
| Thin-film | LiPON | ~300 | 8 |
In conclusion, the development of solid-state batteries is a multifaceted endeavor that requires advancements in material science, interface engineering, and manufacturing processes. While oxide, sulfide, and polymer electrolytes each offer distinct advantages, their integration into viable solid-state batteries depends on overcoming interfacial and safety challenges. Hybrid systems currently bridge the gap between liquid and all-solid-state batteries, but the ultimate goal remains the commercialization of all-solid-state batteries with superior performance. Future research should focus on optimizing ionic conductivity, enhancing interfacial stability, and reducing costs to accelerate the adoption of solid-state batteries in electric vehicles and grid storage. The technology readiness level analysis underscores the progress made and the hurdles ahead, emphasizing that the success of solid-state batteries hinges on a holistic approach rather than the properties of any single component.
The evolution of solid-state batteries will likely involve continued innovation in electrolyte materials, such as the exploration of new halide compositions or advanced polymer composites. Moreover, scaling up production processes and ensuring compatibility with existing manufacturing infrastructure are critical for commercialization. As solid-state battery technologies mature, they hold the promise of revolutionizing energy storage by providing safer, higher-energy-density solutions for a sustainable future.