As global technology and economy continue to evolve, humanity faces increasing challenges such as greenhouse effects, environmental degradation, and the depletion of fossil fuels. In response, initiatives like the “2030 Carbon Peak” and “2060 Carbon Neutrality” plans have been proposed to address environmental issues stemming from fossil fuels. Currently, renewable energy technologies, including wind and solar power, still exhibit limitations due to their intermittent and unpredictable nature, which increases grid regulation pressures and leads to significant energy waste through “wind curtailment” and “solar abandonment.” In this context, the rapid development of rechargeable energy storage systems, particularly lithium-based solid state batteries, has become an inevitable trend. Solid state batteries represent a transformative advancement in secondary chemical power sources, focusing on replacing traditional liquid electrolytes and separators with solid electrolytes to eliminate inherent safety hazards. This article delves into the research and industrialization progress of solid state batteries, covering inorganic solid electrolytes, solid polymer electrolytes, and composite solid electrolytes, while systematically summarizing current limitations and efforts to overcome them.
Traditional lithium-ion batteries, while relatively mature, suffer from potential risks like volatilization, flammability, and explosion due to their liquid electrolytes. Solid state batteries, by utilizing solid electrolytes, fundamentally mitigate these safety concerns. Solid electrolytes offer excellent stability, resistance to corrosion and leakage, and superior mechanical strength that inhibits lithium dendrite growth. Moreover, they enable compatibility with higher-capacity electrode materials, leading to enhanced energy density. The promising prospects of solid state batteries have made them a focal point of recent research. Solid state batteries are categorized based on their electrolyte types: inorganic solid electrolytes (ISEs), solid polymer electrolytes (SPEs), and composite solid electrolytes (CSEs). Each category has unique properties, as summarized in Table 1, which compares their ionic conductivity, mechanical strength, and electrochemical stability.
| Type | Ionic Conductivity (S/cm) | Mechanical Strength | Electrochemical Window (V) | Key Advantages | Limitations |
|---|---|---|---|---|---|
| Inorganic Solid Electrolytes (ISEs) | 10-3 to 10-2 | High | >5 | High stability, wide window | Brittleness, interfacial issues |
| Solid Polymer Electrolytes (SPEs) | 10-6 to 10-4 | Moderate | ~4.5 | Flexibility, ease of fabrication | Low room-temperature conductivity |
| Composite Solid Electrolytes (CSEs) | 10-4 to 10-3 | High | >5 | Balanced properties, enhanced conductivity | Complex synthesis |
Ideal solid electrolytes for solid state batteries must meet several criteria: high lithium-ion conductivity (greater than 10-3 S/cm at room temperature), low electronic conductivity (less than 10-5 S/cm), a wide electrochemical window, good compatibility with electrodes, high mechanical strength, safety, non-toxicity, and cost-effectiveness for large-scale commercialization. The ionic conductivity in solid state batteries often follows 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. This equation highlights the temperature dependence of conductivity, which is crucial for optimizing solid state batteries for various applications.
Inorganic Solid Electrolytes in Solid State Batteries
Inorganic solid electrolytes are a cornerstone of solid state battery technology, offering high ionic conductivity and excellent thermal stability. They can be classified into oxides, sulfides, and nitrides based on their structural atoms. Oxide-based electrolytes, such as LISICON and NASICON types, provide good stability but often exhibit lower ionic conductivity. Sulfide-based electrolytes, like Li10GeP2S12 (LGPS), achieve high conductivity but suffer from poor stability in air. Nitride-based electrolytes, such as LiPON, are used in thin-film solid state batteries due to their moderate conductivity and stability. The general formula for ionic conductivity in these materials can be expressed as: $$\sigma = n \mu q$$ where \(n\) is the charge carrier concentration, \(\mu\) is the mobility, and \(q\) is the charge. Enhancements in these parameters are key to improving solid state battery performance.
LISICON-type electrolytes, for instance, were first developed with Li14Zn(GeO4)4, showing high conductivity at elevated temperatures. Substitutions with sulfur atoms have led to materials like Li10GeP2S12, with room-temperature conductivity up to 1.2 × 10-2 S/cm. Further modifications, such as Li9.54Si1.74P1.44S11.7Cl0.3, have pushed conductivity to 2.5 × 10-2 S/cm, enabling solid state batteries to operate across a wide temperature range from -30°C to 100°C. However, LISICON materials face challenges like limited room-temperature conductivity and poor stability with lithium metal anodes. NASICON-type electrolytes, such as Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li1.5Al0.5Ge1.5(PO4)3 (LAGP), offer conductivities above 10-3 S/cm but are prone to reduction when in contact with lithium. Doping strategies, like incorporating Ca2+ or Sr2+, have improved stability and conductivity, as seen in Li1.2Zr1.9Ca0.1(PO4)3 with 1.2 × 10-4 S/cm at 25°C.
Perovskite-type electrolytes, such as Li3xLa2/3-xTiO3 (LLTO), exhibit high bulk conductivity but suffer from low grain boundary conductivity and instability at low voltages. Garnet-type electrolytes, like Li7La3Zr2O12 (LLZO), have gained attention for their wide electrochemical window (~6 V) and compatibility with lithium metal. Doping with elements like Ga3+ or Ge3+ stabilizes the cubic phase, enhancing conductivity to 1.46 × 10-3 S/cm. Sulfide electrolytes, including Li7P3S11 and Li3PS4, achieve high conductivity but react with moisture, producing toxic H2S gas. Nitride electrolytes, such as LiPON, are used in thin-film solid state batteries, offering conductivities around 3.3 × 10-6 S/cm and excellent cycle life. Table 2 summarizes key inorganic solid electrolytes and their properties, highlighting the diversity in solid state battery materials.
| Electrolyte Type | Example Material | Ionic Conductivity (S/cm) | Activation Energy (eV) | Stability Issues |
|---|---|---|---|---|
| LISICON | Li10GeP2S12 | 1.2 × 10-2 | 0.25 | Poor stability with Li |
| NASICON | Li1.3Al0.3Ti1.7(PO4)3 | 10-3 | 0.35 | Reduction at low voltage |
| Perovskite | Li0.34La0.51TiO2.94 | 10-3 (bulk) | 0.40 | Low grain boundary conductivity |
| Garnet | Li7La3Zr2O12 | 10-4 | 0.30 | Interfacial reactions |
| Sulfide | Li7P3S11 | 10-3 | 0.20 | Moisture sensitivity |
| Nitride | LiPON | 3.3 × 10-6 | 0.54 | Low conductivity |
The development of inorganic solid electrolytes for solid state batteries involves optimizing their crystal structures to facilitate lithium-ion migration. For example, in garnet-type materials, the cubic phase promotes higher conductivity due to disordered lithium distribution. The ionic conductivity can be modeled using the Nernst-Einstein relation: $$\sigma = \frac{D n q^2}{kT}$$ where \(D\) is the diffusion coefficient. Efforts to enhance these electrolytes focus on doping and composite formation, which are critical for advancing solid state battery technology.
Solid Polymer Electrolytes for Solid State Batteries
Solid polymer electrolytes (SPEs) offer advantages such as flexibility, ease of processing, and good interfacial contact, making them suitable for large-scale production of solid state batteries. However, their semi-crystalline nature often results in low room-temperature ionic conductivity, typically below 10-6 S/cm. SPEs are primarily based on polymers like poly(ethylene oxide) (PEO), polycarbonates (PC), and others such as poly(vinylidene fluoride) (PVDF). In these systems, lithium salts like LiTFSI or LiFSI dissociate within the polymer matrix, enabling ion transport through segmental motion of polymer chains. The conductivity in SPEs is governed by the Vogel-Tammann-Fulcher (VTF) equation: $$\sigma = A T^{-1/2} \exp\left(-\frac{B}{T – T_0}\right)$$ where \(A\) and \(B\) are constants, and \(T_0\) is the reference temperature related to the glass transition. This equation underscores the temperature dependence, which is a key factor in improving solid state battery performance at ambient conditions.
PEO-based electrolytes have been widely studied since the 1980s, leveraging the coordination between ether oxygen atoms and lithium ions. However, high crystallinity at room temperature limits conductivity, which increases significantly above the melting point (50-80°C). Strategies to reduce crystallinity include cross-linking and blending with other polymers. For instance, interpenetrating networks of PEO and polyacrylonitrile (PAN) have achieved conductivities of 1.06-8.21 mS/cm at elevated temperatures, enabling solid state batteries with stable cycling over 100 cycles. Polycarbonate-based electrolytes, such as those derived from poly(ethylene carbonate) (PEC) or poly(propylene carbonate) (PPC), benefit from the high polarity of carbonate groups, which weakens ion-polymer interactions and enhances conductivity. A PEC-LiTFSI-Pyr14TFSI-SiF composite electrolyte demonstrated a conductivity of 10-5 S/cm at 80°C and delivered a discharge capacity of 100 mAh/g in LiFePO4-based solid state batteries. Similarly, PPC electrolytes modified with ionic liquids reached 8.2 × 10-5 S/cm at room temperature, supporting reversible lithium plating/stripping and improved rate capability.
Other polymer systems, like PVDF-HFP and PAN, provide mechanical strength and thermal stability but require blending with ion-conducting materials. For example, PVCA-based electrolytes exhibited a conductivity of 5.59 × 10-4 S/cm and a wide electrochemical window above 4.8 V, leading to high capacity retention in full cells. Despite these advances, challenges remain in understanding ion transport mechanisms and scaling up production for solid state batteries. Table 3 compares different SPEs, highlighting their conductivity and application potential in solid state batteries.
| Polymer Base | Lithium Salt | Ionic Conductivity (S/cm) | Temperature (°C) | Key Features |
|---|---|---|---|---|
| PEO | LiTFSI | 10-6 to 10-4 | 25-80 | Flexible, but crystalline at RT |
| PEC | LiTFSI | 10-5 | 80 | High polarity, good mechanical strength |
| PPC | LiTFSI + IL | 8.2 × 10-5 | 25 | Enhanced interface compatibility |
| PVCA | LiTFSI | 5.59 × 10-4 | 25 | Rigid backbone, wide window |
| PVDF-HFP | LiClO4 | 10-5 | 25 | Thermal stability, easy processing |
The ion transport in SPEs involves the hopping of lithium ions between coordination sites, which can be described by the following relation for mobility: $$\mu = \frac{q D}{kT} \exp\left(-\frac{\Delta G}{kT}\right)$$ where \(\Delta G\) is the free energy barrier for ion hopping. Research continues to focus on reducing this barrier through polymer design and additives, which is essential for developing high-performance solid state batteries.
Composite Solid Electrolytes in Solid State Batteries
Composite solid electrolytes (CSEs) combine the benefits of inorganic and polymer electrolytes, addressing limitations such as low conductivity in SPEs and interfacial issues in ISEs. CSEs typically consist of a polymer matrix with dispersed inorganic fillers, which can be active (participating in ion transport) or inert (providing mechanical support). Inert fillers like Al2O3, SiO2, or TiO2 enhance ionic conductivity by reducing polymer crystallinity and creating space-charge regions at interfaces, as described by the Maier theory. The effective conductivity in CSEs can be approximated by the Maxwell-Wagner model: $$\sigma_{\text{eff}} = \sigma_p \frac{1 + 2\phi (\sigma_f – \sigma_p)/(\sigma_f + 2\sigma_p)}{1 – \phi (\sigma_f – \sigma_p)/(\sigma_f + 2\sigma_p)}$$ where \(\sigma_p\) and \(\sigma_f\) are the conductivities of the polymer and filler, respectively, and \(\phi\) is the volume fraction of filler. This model helps optimize filler content for solid state battery applications.
For example, incorporating Mg2B2O5 nanowires into PEO-LiTFSI electrolytes improved conductivity to 10-4 S/cm and provided flame-retardant properties, enabling solid state batteries with excellent rate performance and cycle life. Active fillers, such as LATP or LLZO, form continuous ion-conducting pathways within the polymer matrix. A vertically aligned LAGP-PEO composite achieved a conductivity of 1.67 × 10-4 S/cm at room temperature and 1.11 × 10-3 S/cm at 60°C, leading to a capacity retention of 93.3% over 300 cycles in LiFePO4-based solid state batteries. Carbon-based materials like carbon nanotubes or graphene have also been used as fillers to enhance conductivity and mechanical properties. However, challenges include understanding the synergy between components and scaling up production for commercial solid state batteries. Table 4 summarizes key CSE systems and their performance metrics, illustrating the progress in this area.
| Polymer Matrix | Filler Type | Filler Material | Ionic Conductivity (S/cm) | Enhancement Mechanism |
|---|---|---|---|---|
| PEO | Inert | Mg2B2O5 | 10-4 | Reduced crystallinity, Lewis acid-base interaction |
| PEO | Active | LAGP | 1.67 × 10-4 (25°C) | Continuous ion pathways |
| PEO | Inert | SiO2 | 10-5 to 10-4 | Space-charge effect |
| PAN | Active | LATP | 10-4 | Improved interfacial contact |
| PVDF | Inert | TiO2 | 10-5 | Mechanical reinforcement |
The development of CSEs is pivotal for overcoming the trade-offs between conductivity and mechanical strength in solid state batteries. Future research should focus on optimizing filler morphology, distribution, and interface design to achieve holistic improvements. The equation for composite conductivity often involves percolation theory: $$\sigma \propto (\phi – \phi_c)^t$$ where \(\phi_c\) is the percolation threshold and \(t\) is a critical exponent. This approach can guide the design of CSEs for next-generation solid state batteries.
Industrialization Progress of Solid State Batteries
The industrialization of solid state batteries has gained momentum globally, with countries emphasizing different aspects based on their strategic goals. In China, policies like the “New Energy Vehicle Industry Development Plan (2021-2035)” and the “Technology Roadmap 2.0” set targets for energy densities of 350 Wh/kg by 2025, 400 Wh/kg by 2030, and 500 Wh/kg by 2035. Chinese companies, such as Beijing Weilan New Energy, Qingtao Energy, and Ganfeng Lithium, are actively developing solid state batteries, focusing on hybrid solid-liquid systems for near-term commercialization. For instance, Weilan New Energy aims to achieve 360 Wh/kg batteries for electric vehicles with a range of 1000 km, with plans to scale production to 20 GWh by 2026. Qingtao Energy has established production lines for solid state batteries with energy densities of 368 Wh/kg, while HuiNeng Technology targets 440-485 Wh/L using silicon oxide/graphite anodes.
Globally, European and American firms prioritize safety, favoring oxide and polymer-based solid state batteries, while Japanese and Korean companies focus on sulfide systems to enhance capacity and stability. The transition to solid state batteries involves addressing challenges in large-scale manufacturing, such as producing thin, robust electrolyte films and ensuring good electrode-electrolyte contact. Hybrid solid-liquid electrolytes offer a pragmatic approach, leveraging existing lithium-ion battery infrastructure while improving safety. The growth in solid state battery production is expected to accelerate, with pilot plants already operational and mass production anticipated by 2024-2025. Table 5 outlines key players and their progress in solid state battery industrialization, highlighting the collaborative efforts needed across the supply chain.
| Company/Region | Electrolyte Type | Energy Density (Wh/kg or Wh/L) | Current Status | Future Goals |
|---|---|---|---|---|
| Weilan New Energy (China) | Hybrid oxide-liquid | 360 Wh/kg | Pilot production | 400 Wh/kg by 2025 |
| Qingtao Energy (China) | Hybrid oxide-liquid | 368 Wh/kg | Mass production line | Scale to 10 GWh |
| HuiNeng Technology (China) | Mixed oxide solid-liquid | 440-485 Wh/L | Prototype testing | Mass production by 2024 |
| European Firms | Oxide/Polymer | 300-350 Wh/kg | R&D focus | Safety enhancement |
| Japanese/Korean Firms | Sulfide | >400 Wh/kg | Material optimization | Solve stability issues |
The commercialization of solid state batteries hinges on overcoming technical barriers, such as interface degradation, gas evolution, and cost-effective manufacturing. The overall performance of a solid state battery can be evaluated using the energy density formula: $$E = \frac{1}{2} C V^2$$ where \(C\) is the capacitance and \(V\) is the voltage. Innovations in material science and process engineering are crucial to realizing the full potential of solid state batteries for applications in electric vehicles and grid storage.
Challenges and Future Perspectives in Solid State Battery Development
Despite significant advancements, the widespread adoption of solid state batteries faces several challenges. Key issues include the high-cost and scalable production of solid electrolytes with consistent performance, the formation of stable interfaces between electrolytes and electrodes, and the prevention of lithium dendrite growth. For instance, inorganic solid electrolytes often exhibit brittleness and poor contact with electrodes, leading to high interfacial resistance. Solid polymer electrolytes suffer from low conductivity at room temperature, while composite electrolytes require precise control over filler distribution. Additionally, solid state batteries may experience side reactions, gas release, and mechanical failure during cycling, necessitating advanced failure analysis techniques.
Future research directions for solid state batteries should focus on developing novel materials with enhanced ionic conductivity and mechanical properties. For example, exploring new crystal structures or amorphous phases could lead to breakthroughs in solid state battery performance. The use of machine learning and computational modeling can accelerate the discovery of optimal compositions and interfaces. Moreover, integrating solid state batteries with renewable energy systems could address intermittency issues, contributing to carbon neutrality goals. The continuous improvement in solid state battery technology will rely on interdisciplinary collaboration among academia, industry, and government agencies. Ultimately, solid state batteries hold the promise of safer, higher-energy-density storage solutions, but realizing this potential requires persistent innovation and investment.
In summary, solid state batteries represent a pivotal advancement in energy storage, with progress in inorganic, polymer, and composite electrolytes driving their development. The industrialization efforts worldwide underscore the growing importance of solid state batteries in the global energy landscape. By addressing existing challenges and leveraging emerging technologies, solid state batteries can play a critical role in the transition to sustainable energy systems.