In recent years, the development of rechargeable batteries has seen significant progress, particularly in the realm of lithium-ion batteries. Conventional lithium-ion batteries predominantly employ liquid electrolytes, which offer high ionic conductivity and excellent wettability. However, these liquid electrolytes are prone to decomposition at the lithium metal interface, leading to reduced battery lifespan. Moreover, they fail to effectively suppress the growth of lithium dendrites, resulting in short-circuit failures. State-of-the-art lithium-ion batteries are approaching their energy density limits, facing challenges in meeting the growing demands of energy storage and power applications. In contrast, solid-state batteries, which utilize solid electrolytes, exhibit a wider electrochemical window, higher energy density, and significantly enhanced safety. All-solid-state lithium-ion batteries are expected to fundamentally address issues such as thermal runaway, sparking increased interest in their development.
Solid-state batteries essentially 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, including sulfide-based, oxide-based, polymer-based, and halide-based solid-state batteries. The performance characteristics of these electrolytes are compared in the following sections.
Classification and Characteristics of Solid-State Batteries
Solid-state batteries are classified according to their electrolyte materials, each with distinct advantages and limitations. The ionic conductivity, electrochemical stability, mechanical properties, and interfacial compatibility vary significantly among these types. Below, we discuss the key features of sulfide-based, oxide-based, halide-based, and polymer-based solid-state batteries.
| Electrolyte Type | Ionic Conductivity (S cm⁻¹) | Electrochemical Window (V) | Mechanical Flexibility | Interface Stability |
|---|---|---|---|---|
| Sulfide-based | 10⁻² to 10⁻³ | ~2.5 | Moderate | Poor |
| Oxide-based | 10⁻⁴ to 10⁻⁵ | ~5.0 | Low | Moderate |
| Halide-based | 10⁻³ to 10⁻⁴ | ~4.0 | High | Good |
| Polymer-based | 10⁻⁸ to 10⁻⁶ | ~4.5 | High | Excellent |
Sulfide-Based Solid-State Batteries
Sulfide-based solid-state batteries utilize electrolytes composed of lithium sulfide combined with elements such as aluminum, phosphorus, or silicon. The large ionic radius of sulfide ions facilitates broader lithium-ion transport channels, resulting in high ionic conductivity and good interfacial compatibility with sulfur-based cathodes. However, these electrolytes have a narrow electrochemical window and exhibit significant interfacial impedance when paired with lithium metal anodes. The unstable electrode-electrolyte interface often leads to lithium dendrite formation, posing safety risks. The ionic conductivity of sulfide electrolytes can be modeled 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. Despite their high conductivity, the challenges associated with interface stability limit the widespread adoption of sulfide-based solid-state batteries.
Oxide-Based Solid-State Batteries
Oxide-based solid-state batteries offer a wider electrochemical window and higher oxidation stability compared to sulfide-based systems. However, their room-temperature ionic conductivity is generally lower, and they suffer from high grain boundary resistance, which impedes ion transport. The inherent brittleness of oxide ceramics leads to poor electrode-electrolyte contact, resulting in increased interfacial resistance during cycling. This often causes rapid capacity fade and reduced battery lifespan. To mitigate these issues, oxide electrolytes are frequently combined with polymer components or ionic liquids to form hybrid or quasi-solid-state batteries. These composites retain the safety advantages of solid electrolytes while improving interfacial contact. The effective conductivity in composite electrolytes can be described by the percolation theory: $$ \sigma_{\text{eff}} = \sigma_p \phi_p + \sigma_o \phi_o $$ where $\sigma_{\text{eff}}$ is the effective conductivity, $\sigma_p$ and $\sigma_o$ are the conductivities of the polymer and oxide phases, and $\phi_p$ and $\phi_o$ are their volume fractions.
Halide-Based Solid-State Batteries
Halide-based solid-state electrolytes have recently gained attention due to their high ionic conductivity, reaching up to 10⁻³ S cm⁻¹, excellent deformability, and wide electrochemical windows. Despite these advantages, their reduction potential is insufficient for direct use with lithium metal anodes, and raw material costs remain high. The development of halide electrolytes, such as Li₃YCl₆ and Li₃YBr₆, has renewed interest in this category. These materials demonstrate room-temperature ionic conductivities of 5.1×10⁻⁴ S cm⁻¹ and 1.7×10⁻³ S cm⁻¹, respectively. The ionic transport in halide electrolytes can be expressed as: $$ \mu_{\text{Li}} = \frac{z e D}{kT} $$ where $\mu_{\text{Li}}$ is the lithium ion mobility, $z$ is the charge number, $e$ is the electron charge, $D$ is the diffusion coefficient, $k$ is Boltzmann’s constant, and $T$ is the temperature. Ongoing research focuses on optimizing halide compositions to enhance compatibility with high-energy electrodes.
Polymer-Based Solid-State Batteries
Polymer-based solid-state batteries leverage solid polymer electrolytes, which offer advantages such as excellent processability, leak-proof nature, high energy density, flexibility, and reduced reactivity with electrode surfaces. However, their room-temperature ionic conductivity is low, typically between 10⁻⁸ and 10⁻⁶ S cm⁻¹, and they exhibit limited electrochemical stability. For instance, polyether-based polymers are susceptible to oxidation at high voltages, while polyester-based polymers may be reduced when in contact with lithium metal anodes. To address these issues, researchers are developing advanced polymer electrolytes with widened electrochemical windows. The ion transport mechanism in polymer electrolytes involves segmental motion, where lithium ions hop between coordination sites along the polymer chain. The conductivity is governed by the Vogel-Tammann-Fulcher equation: $$ \sigma = \sigma_0 \exp\left(-\frac{B}{T – T_0}\right) $$ where $\sigma_0$, $B$, and $T_0$ are constants specific to the polymer system.
Polymer-Inorganic Solid Electrolytes
Polymer-inorganic composite electrolytes combine the flexibility and processability of polymers with the enhanced ionic conductivity and stability of inorganic materials. These hybrids are compatible with large-scale manufacturing processes, such as roll-to-roll production, making them promising for commercial applications. The synergistic effects between the polymer and inorganic phases improve overall electrolyte performance, including ion transport and interfacial stability.
Ion Transport Mechanisms
In all-solid-state polymer electrolytes, lithium ions are dissociated by polar groups in the polymer and coordinated to form “polymer-Li⁺” complexes. Under an electric field, lithium ions migrate via segmental motion of the polymer chains, hopping from one coordination site to another. Ion conduction primarily occurs in the amorphous regions of the polymer matrix, with crystalline regions contributing minimally. At room temperature, the predominance of crystalline phases results in low ionic conductivity. Additionally, the strong interaction between lithium ions and polymer functional groups leads to low lithium-ion transference numbers (typically below 0.3). To optimize ion transport, several strategies are employed:
- Lithium Salt Concentration Adjustment: High-concentration electrolytes increase solvent coordination, optimizing interfacial chemistry and forming stable solid electrolyte interphases (SEI). However, they suffer from high viscosity, low conductivity, and poor electrode wetting. Localized high-concentration electrolytes, incorporating diluents like bis(2,2,2-trifluoroethyl) ether (BTFE) or 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy) ethane (TTE), maintain salt-solvent clusters while reducing overall salt concentration.
- Novel Lithium Salt Additives: Salts such as lithium poly(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide (LiPSTFSI) can enhance lithium-ion transference numbers by bonding anions to polymer chains, reducing accumulation at the anode and minimizing side reactions.
- Inorganic Nanofillers: The incorporation of one-dimensional nanofillers, like LLTO nanowires, into polymer matrices (e.g., PVDF/LiClO₄) creates composite membranes with improved ionic conductivity (e.g., 5.8×10⁻⁴ S/cm) and wider electrochemical windows (e.g., 5.2 V). These composites demonstrate excellent stability in full-cell configurations, with negligible lithium dendrite formation and high coulombic efficiency.
The ion transport in composite electrolytes can be modeled using effective medium theory: $$ \sigma_{\text{comp}} = \sigma_p \left(1 + \frac{3\phi_f (\sigma_f – \sigma_p)}{\sigma_f + 2\sigma_p – \phi_f (\sigma_f – \sigma_p)}\right) $$ where $\sigma_{\text{comp}}$ is the composite conductivity, $\sigma_p$ is the polymer conductivity, $\sigma_f$ is the filler conductivity, and $\phi_f$ is the filler volume fraction.
Electrode/Electrolyte Interfaces
The interface between electrodes and electrolytes is critical for the performance of solid-state batteries. During cycling, chemical side reactions can form passivating interphases, such as the cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI), increasing resistance and degrading battery life. Recent advances focus on stabilizing these interfaces through material design and additive engineering.
| Interface Type | Stabilization Method | Key Components | Impact on Performance |
|---|---|---|---|
| Cathode/Electrolyte (CEI) | Fluorine-rich interphases, gradient doping | LiF, M-F bonds, spinel phases | Enhanced kinetics, reduced oxygen release, improved rate capability |
| Anode/Electrolyte (SEI) | Additives (e.g., FEC, LiDFOB), aqueous SEI formation | LiF, Li₂CO₃, carboxylates | Suppressed dendrite growth, stable cycling, high coulombic efficiency |
Electrolyte and Cathode Interface
Stable CEI layers can be formed by incorporating protective coatings or enhancing the oxidative stability of the electrolyte. Fluorine-rich interfaces, for example, improve interfacial kinetics and stability. In one study, a gradient fluorination approach was used to create a stable spinel phase with strong M-F bonds, reducing irreversible oxygen release and inducing uniform LiF deposition. This method achieved a capacity of 133 mAh g⁻¹ at 5C and 81.9% capacity retention after 100 cycles at 1C. The formation energy of such interfaces can be described by: $$ \Delta G_{\text{CEI}} = -RT \ln K_{\text{eq}} $$ where $\Delta G_{\text{CEI}}$ is the Gibbs free energy change, $R$ is the gas constant, $T$ is temperature, and $K_{\text{eq}}$ is the equilibrium constant for interface formation.
Electrolyte and Anode Interface
SEI layers typically exhibit a mosaic structure comprising inorganic salts (e.g., LiF, Li₂CO₃) and organic compounds (e.g., lithium ethylene dicarbonate, LEDC). Multilayer SEI structures, with dense inorganic inner layers and porous organic outer layers, can improve stability but may increase interfacial impedance. Additives like fluoroethylene carbonate (FEC) and lithium difluoro(oxalate)borate (LiDFOB) have been shown to enhance SEI stability by increasing LiF and carboxylate content. In aqueous systems, the introduction of fluorinated/oxidized polymers facilitates stable SEI formation by consuming water in the Li⁺ solvation shell and promoting anion decomposition. The SEI growth kinetics can be modeled as: $$ \frac{dL_{\text{SEI}}}{dt} = k \exp\left(-\frac{E_a}{RT}\right) $$ where $L_{\text{SEI}}$ is the SEI thickness, $k$ is the rate constant, and $E_a$ is the activation energy.
Industrialization of Solid-State Lithium Batteries
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 considered the next-generation successor to liquid lithium-ion batteries, particularly for electric vehicles and consumer electronics. Governments worldwide are implementing policies to support electric vehicle and energy storage technologies, providing a foundation for solid-state battery commercialization. Several companies have made significant strides in electrolyte materials and manufacturing processes, as summarized below.
| Company | Key Developments | Target Metrics | Timeline |
|---|---|---|---|
| Chang’an | Semi-solid state batteries | 350-500 Wh/kg, 750-1000 Wh/L | 2025 onwards |
| SAIC | All-solid-state production line | High-volume manufacturing | 2024 Q3 |
| Toyota | Sulfide-based all-solid-state batteries | 500 km range, enhanced safety | 2025 |
| BYD | Solid-state battery integration | Improved safety, reduced short-circuit risk | Next 10 years |
| Samsung SDI | Ultra-fast charging technology | 80% charge in 9 minutes | 2026 |
| Quantum Scape | Oxide-based solid-state batteries | 16-layer cells, 500 cycles | Pilot production |
Toyota is developing sulfide-based solid electrolytes to enhance energy density and safety, targeting electric vehicle ranges exceeding 500 km. Samsung SDI’s ultra-fast charging technology optimizes lithium-ion pathways and reduces resistance, enabling 80% charge in 9 minutes, with mass production expected by 2026. BYD plans to integrate solid-state batteries into its electric vehicle lineup, emphasizing safety through reduced short-circuit and thermal runaway risks. Quantum Scape, in collaboration with Volkswagen, has produced 16-layer solid-state batteries capable of 500 charge cycles and is advancing toward 20-layer designs. These efforts highlight the ongoing innovation and investment in solid-state battery technology, paving the way for widespread adoption.
Conclusions and Future Perspectives
Solid-state electrolytes, particularly polymer-based systems, have emerged as a focal point in battery research due to their high safety and flexibility. Compared to commercial lithium-ion batteries, all-solid-state lithium batteries offer greater safety and higher energy density potential, contributing to the adoption of electric vehicles and the achievement of carbon neutrality goals. The future development of solid-state lithium batteries is promising, with key areas for advancement including:
- Enhanced Energy Density: The pursuit of higher energy density involves developing advanced anodes (e.g., lithium metal, silicon) and cathodes, alongside electrolyte optimization. The theoretical energy density of solid-state batteries can be calculated as: $$ E_{\text{density}} = \frac{Q \times V}{m} $$ where $Q$ is the capacity, $V$ is the voltage, and $m$ is the mass. Targets exceed current liquid lithium-ion batteries, enabling longer-range electric vehicles and high-performance electronics.
- Improved Safety: Solid electrolytes inherently reduce fire and explosion risks, with superior thermal stability allowing operation in extreme conditions. This is crucial for applications in automotive and portable electronics where safety is paramount.
- Expanded Applications: Beyond electric vehicles, solid-state batteries hold potential in aerospace, grid storage, and consumer electronics. As technology matures and production scales, their versatility and market demand are expected to rise.
In summary, the progress in solid-state battery technology, driven by material innovations and industrial partnerships, positions it as a transformative solution for future energy storage. Continued research into ion transport mechanisms, interface engineering, and scalable manufacturing will be essential to realizing the full potential of solid-state lithium batteries.