In recent years, the development of rechargeable batteries has seen significant advancements, particularly in lithium-ion technology. Conventional lithium-ion batteries typically employ liquid electrolytes, which offer high ionic conductivity and excellent wettability. However, these electrolytes are prone to decomposition at the lithium metal interface, leading to reduced battery lifespan. Additionally, liquid electrolytes fail to effectively suppress lithium dendrite growth, resulting in short-circuit failures. State-of-the-art lithium-ion batteries are approaching their energy density limits, struggling to meet the growing demands of modern energy storage and power applications. In contrast, solid-state electrolytes exhibit a wider electrochemical window and higher energy density. All-solid-state lithium-ion batteries significantly enhance safety, potentially addressing thermal runaway issues at their root. Consequently, interest in all-solid-state lithium batteries has surged.
Solid-state batteries fundamentally replace liquid electrolytes and separators with solid electrolytes, thereby improving safety and energy density. These batteries can be categorized based on the type of solid electrolyte used, primarily including sulfide-based, oxide-based, polymer-based, and halide-based solid-state batteries. The performance characteristics of these electrolytes are summarized in the following table.
| Electrolyte Type | Ionic Conductivity (S/cm) | Electrochemical Window (V) | Mechanical Properties | Key Challenges |
|---|---|---|---|---|
| Sulfide-based | 10⁻² to 10⁻³ | Narrow (~1.7-2.1) | Brittle | Interface instability, dendrite growth |
| Oxide-based | 10⁻⁴ to 10⁻⁶ | Wide (~5) | Rigid | High grain boundary resistance |
| Halide-based | 10⁻³ to 10⁻⁴ | Wide (~4) | Deformable | High cost, low reduction potential |
| Polymer-based | 10⁻⁶ to 10⁻⁸ | Moderate (~4.5) | Flexible | Low ionic conductivity at room temperature |
Sulfide-based solid-state batteries utilize electrolytes composed of lithium sulfide and elements like aluminum, phosphorus, or silicon. The large ionic radius of sulfide ions creates broader lithium-ion transport channels, resulting in the highest ionic conductivity among solid electrolytes and good compatibility with sulfur-based cathodes. However, their narrow electrochemical window and instability at the electrode-electrolyte interface lead to significant solid electrolyte interphase (SEI) impedance and dendrite formation when paired with lithium metal anodes.
Oxide-based solid-state batteries offer a wider electrochemical window and higher oxidative stability compared to sulfides. Nevertheless, their room-temperature ionic conductivity is generally lower, and substantial grain boundary resistance limits overall performance. The inherent rigidity of oxide electrolytes results in poor electrode-electrolyte contact, causing rapid increases in interfacial resistance during cycling, insufficient anode capacity, and accelerated battery degradation. To mitigate these issues, oxide electrolytes are often combined with polymer components or ionic liquids to form quasi-solid-state batteries, retaining safety advantages while improving interfacial contact.
Halide-based solid-state electrolytes demonstrate ionic conductivities up to 10⁻³ S/cm, good deformability, and wide electrochemical windows, alongside scalability potential comparable to polymers. However, their insufficient reduction potential for compatibility with lithium metal anodes and high raw material costs hindered progress until recent developments. For instance, Li₃YCl₆ and Li₃YBr₆ electrolytes achieved room-temperature conductivities of 5.1×10⁻⁴ S/cm and 1.7×10⁻³ S/cm, respectively, renewing interest in this category.
Polymer-based solid-state batteries leverage electrolytes with superior processability, leak-proof nature, high energy density, flexibility, and reduced reactivity with electrode surfaces. While safer than liquid alternatives, their low voltage stability (e.g., in polyethers) leads to oxidative decomposition at high voltages, whereas oxidation-resistant polymers (e.g., polyesters) are easily reduced by lithium metal anodes. Thus, widening the electrochemical stability window is crucial for high-performance solid polymer electrolytes.
Polymer-inorganic solid-state electrolytes combine the flexibility and processability of polymers with the enhanced ionic conductivity and stability of inorganic materials. This synergy enables compatibility with large-scale, roll-to-roll manufacturing processes used in traditional lithium-ion batteries. The ionic transport mechanism in these composite electrolytes involves lithium salt dissociation by polar groups in the polymer, forming “polymer-Li⁺” complexes. Under an electric field, polymer chain segment motion facilitates lithium ion hopping between coordination sites. Ion conduction primarily occurs in the amorphous regions of the polymer matrix, with crystalline contributions being minimal. The ionic conductivity (σ) can be expressed as:
$$ \sigma = n e \mu $$
where ( n ) is the charge carrier concentration, ( e ) is the elementary charge, and ( \mu ) is the mobility. For polymer electrolytes, the lithium ion transference number (( t_+ )) is often low (<0.3) due to strong ion-polymer interactions. Optimization strategies include adjusting lithium salt concentration, using novel lithium salts, and incorporating inorganic nanofillers. High-concentration electrolytes increase solvent coordination, optimize interfacial chemistry, and form uniform SEI layers, suppressing dendrite growth and electrolyte decomposition. Localized high-concentration electrolytes, diluted with non-coordinating solvents like BTFE or TTE, maintain salt-solvent cluster stability while mitigating issues like high viscosity and cost. For example, a bilayer electrolyte of poly(propylene carbonate) and poly(ethylene oxide) with LiPSTFSI salt enhanced lithium transference numbers by bonding anions to polymer chains, reducing side reactions. Similarly, adding one-dimensional LLTO nanowires to PVDF/LiClO₄ composites achieved an ionic conductivity of 5.8×10⁻⁴ S/cm and a 5.2 V electrochemical window, demonstrating stable cycling in LiFePO₄||Li cells with near-100% coulombic efficiency.
The electrode-electrolyte interface is critical in solid-state batteries, where passivating interphases form, increasing resistance. The cathode-electrolyte interface (CEI) and solid-electrolyte interphase (SEI) at the anode significantly impact performance. Fluorine-rich interphases enhance interfacial kinetics and stability. For instance, gradient fluorination in high-voltage spinel phases introduces strong M-F bonds, reducing irreversible oxygen release and inducing uniform LiF deposition for improved lithium transport. This approach achieved 133 mAh g⁻¹ at 5C and 81.9% capacity retention after 100 cycles at 1C. SEI layers typically exhibit a “mosaic” structure of disordered inorganic salts (e.g., Li₂CO₃, LiF) and organic compounds like LEDC. Multilayer SEI structures, with dense inorganic inner layers and porous organic outer layers, can improve performance but may increase impedance. Additives like FEC form protective SEI layers, shielding lithium anodes from adverse reactions with succinonitrile and polymer backbones. LiDFOB incorporation increases carboxylate and LiF content in SEI, enhancing stability for high-voltage tolerance. In aqueous systems, stepped heterocyclic polymers in water-in-salt electrolytes facilitate stable aqueous SEI formation via proton-coupled reactions, consuming water in the Li⁺ solvation shell and triggering organic anion decomposition.
The industrialization of solid-state lithium batteries is gaining momentum due to their superior safety, energy density, cycle life, and operational temperature range. They are regarded as the most promising next-generation technology beyond current liquid lithium-ion batteries, particularly for electric vehicles and consumer electronics. Government policies worldwide supporting electric mobility and energy storage have laid the groundwork for solid-state battery commercialization. Key enterprises have made notable strides in electrolyte materials and manufacturing processes, as summarized below.
| Company | Progress |
|---|---|
| Toyota | Developing sulfide-based solid-state batteries targeting over 500 km range; plans mass production by 2025. |
| Samsung SDI | Developing ultra-fast charging technology enabling 80% charge in 9 minutes; aims for mass production by 2026. |
| BYD | Focusing on solid-state battery safety; plans gradual integration into EVs over the next decade. |
| Quantum Scape | Produced 16-layer solid-state batteries with 500 cycles; developing 20-layer variants with pilot lines. |
| Various Chinese Firms | Investing in semi-solid and all-solid production lines targeting 350-1000 Wh/kg energy densities. |
Toyota’s sulfide-based solid-state electrolyte development aims for higher energy density and safety, with a target of over 500 km driving range. Samsung SDI’s ultra-fast charging technology optimizes lithium-ion pathways and reduces resistance, potentially revolutionizing EV charging times. BYD emphasizes safety in its solid-state designs, aiming to minimize short-circuit and thermal runaway risks. Quantum Scape’s oxide-based solid-state batteries have attracted partnerships, such as with Volkswagen, highlighting their potential. Continuous investment and innovation from these companies are solidifying the foundation for solid-state battery industrialization.
In conclusion, solid-state electrolytes are becoming a focal point in the electric vehicle industry due to their enhanced safety. Polymer electrolytes, with their simplicity in preparation and flexibility, offer excellent electrode contact and have garnered widespread attention. Compared to commercial lithium-ion batteries, all-solid-state lithium batteries provide higher safety and greater energy density potential, supporting the widespread adoption of new energy vehicles and contributing to carbon neutrality goals. The future of solid-state lithium batteries is promising, with key directions including:
- Enhanced energy density through high-energy anodes (e.g., lithium metal, silicon) and cathodes, alongside electrolyte optimization.
- Improved safety via inherent stability and thermal resistance, reducing risks in extreme conditions.
- Expanded applications in aerospace, energy storage, and portable electronics, driven by technological advances and scaling production.
The ionic conductivity in polymer-inorganic composites can be modeled using the Arrhenius equation for temperature dependence:
$$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{kT}\right) $$
where ( \sigma_0 ) is the pre-exponential factor, ( E_a ) is the activation energy, ( k ) is Boltzmann’s constant, and ( T ) is temperature. For interface stability, the Gibbs free energy change (( \Delta G )) for SEI formation can be expressed as:
$$ \Delta G = -nFE $$
where ( n ) is the number of electrons transferred, ( F ) is Faraday’s constant, and ( E ) is the cell potential. These fundamental principles guide the ongoing optimization of solid-state batteries for future energy storage solutions.