With the rapid development of new energy vehicles and 5G communication technology, higher demands have been placed on the comprehensive performance of lithium-ion batteries as power sources. Among various lithium-ion battery technologies, solid-state lithium-ion batteries have garnered widespread attention due to their exceptional energy density and safety. As a key component of lithium-ion batteries, the performance of solid-state electrolytes directly affects the overall battery performance. Designing and manufacturing solid-state electrolytes with excellent properties is crucial for promoting the practical application of lithium-ion batteries. In this article, we discuss the Li+ transport mechanisms in inorganic solid-state electrolytes, polymer solid-state electrolytes, and composite solid-state electrolytes. Based on recent literature, we comprehensively review research progress in improving solid-state electrolyte performance through methods such as ion doping and the introduction of new preparation techniques. We summarize the application of different types of solid-state electrolytes in domestic and international enterprises and prospect the challenges and future development trends of solid-state electrolytes. This review aims to provide valuable insights for developing novel solid-state electrolyte materials with outstanding comprehensive performance, thereby accelerating the rapid industrialization of solid-state electrolytes.
Solid-state batteries represent a transformative advancement in energy storage technology, offering enhanced safety and higher energy density compared to conventional liquid electrolyte-based systems. The core of a solid-state battery lies in its electrolyte, which must exhibit high ionic conductivity, wide electrochemical stability, and excellent mechanical properties. The evolution of solid-state electrolytes has been driven by the need to overcome limitations such as leakage, flammability, and dendrite formation associated with liquid electrolytes. Research into solid-state batteries has expanded significantly, focusing on three main categories: inorganic solid-state electrolytes, polymer solid-state electrolytes, and composite solid-state electrolytes. Each category possesses distinct advantages and challenges, which we explore in detail below.
Inorganic Solid-State Electrolytes
Inorganic solid-state electrolytes (ISEs) are characterized by their high ionic conductivity and excellent thermal stability. They can be classified into oxide-based, sulfide-based, and halide-based electrolytes based on their anion chemistry. The Li+ transport mechanisms in ISEs primarily involve vacancy-mediated diffusion, interstitial diffusion, and interstitial-vacancy pair mechanisms. The concentration and distribution of defects critically influence the ionic conductivity, which can be enhanced through aliovalent doping to create vacancies or interstitial sites.
Oxide Solid-State Electrolytes
Oxide solid-state electrolytes (OSEs) are among the most studied due to their high mechanical strength and chemical stability. Common structures include perovskite-type (e.g., Li3xLa2/3-xTiO3, LLTO), garnet-type (e.g., Li7La3Zr2O12, LLZO), LISICON-type, and NASICON-type electrolytes. Perovskite LLTO exhibits high grain conductivity but suffers from high grain boundary resistance. Doping with elements such as Sn2+ or Ta5+ at the B-site can expand Li+ migration pathways and increase relative density, enhancing total conductivity. For instance, Li0.24La0.587Ti0.98Sn0.02O3 achieves a conductivity of 2.96 × 10−4 S/cm. Garnet-type LLZO stabilizes in a cubic phase at high temperatures, and doping with Ga3+, Ta5+, or Y3+ helps maintain the cubic phase at room temperature, with Li6.4Ga0.2La3Zr1.7Y0.3O12 reaching 1.04 × 10−3 S/cm. NASICON-type electrolytes like Li1.3Al0.3Ti1.7(PO4)3 (LATP) benefit from F-doping, where Li1.3Al0.3Ti1.7P3O11.76F0.48 exhibits a conductivity of 1.14 × 10−3 S/cm due to improved densification and reduced diffusion barriers.
The ionic conductivity in oxides follows the Arrhenius equation: $$ \sigma = A \exp\left(-\frac{E_a}{kT}\right) $$ where ( \sigma ) is the conductivity, ( A ) is the pre-exponential factor, ( E_a ) is the activation energy, ( k ) is Boltzmann’s constant, and ( T ) is the temperature. Doping reduces ( E_a ) by creating favorable diffusion pathways.
| Electrolyte | Conductivity (S/cm) | Activation Energy (eV) | Modification |
|---|---|---|---|
| Li0.24La0.587TiO3 | 1.52 × 10−4 | 0.39 | None |
| Li0.24La0.587Ti0.98Sn0.02O3 | 2.96 × 10−4 | 0.31 | Sn doping |
| Li6.4Ga0.2La3Zr1.7Y0.3O12 | 1.04 × 10−3 | 0.281 | Ga, Y co-doping |
| Li1.3Al0.3Ti1.7P3O11.76F0.48 | 1.14 × 10−3 | — | F-doping |
Sulfide Solid-State Electrolytes
Sulfide solid-state electrolytes (SSEs) exhibit high ionic conductivity, often exceeding 10−3 S/cm, and good mechanical deformability. However, they are sensitive to moisture, reacting with water to produce H2S, and have narrow electrochemical windows. Crystal structures include glassy (e.g., Li3PS4), glass-ceramic (e.g., Li7P3S11), and crystalline (e.g., Li10GeP2S12, LGPS). LGPS shows a conductivity of 1.2 × 10−2 S/cm but poor stability against Li metal. Doping with Se or using protective coatings like Li7.5La3Zr1.5Co0.5O12 (LLZCO) on cathodes improves stability. For example, Li6.95Zr0.05P2.9S10.8O0.1I0.4 (LZPSOI) achieves 3.01 × 10−3 S/cm with enhanced air stability.
The conductivity in sulfides can be modeled using the Nernst-Einstein relation: $$ \sigma = \frac{n q^2 D}{kT} $$ where ( n ) is the carrier concentration, ( q ) is the charge, and ( D ) is the diffusion coefficient. Elemental substitution (e.g., O for S) reduces H2S evolution.
| Electrolyte | Conductivity (S/cm) | Stability | Modification |
|---|---|---|---|
| Li10GeP2S12 | 1.2 × 10−2 | Poor | None |
| Li6.95Zr0.05P2.9S10.8O0.1I0.4 | 3.01 × 10−3 | Improved | Zr, I co-doping |
| Li7P2S8I | 5.16 × 10−3 | Moderate | None |
| Li7.1P2S8.1I0.9 | 6.27 × 10−3 | High | Li-rich composition |
Halide Solid-State Electrolytes
Halide solid-state electrolytes (HSEs), such as Li3InCl6 (LIC), offer high ionic conductivity (>10−3 S/cm) and wide electrochemical windows (>4 V). They exhibit good compatibility with oxide cathodes but poor stability with Li metal anodes. Doping with Mn2+ or F− enhances performance. For instance, Mn-doped Li2ZrCl6 shows a conductivity of 8 × 10−4 S/cm, and F-doped Li3InCl5.5F0.5 achieves 1.00 × 10−3 S/cm with improved voltage stability. The Li+ migration in HSEs is facilitated by vacancy mechanisms, and doping creates lithium vacancies that enhance diffusion.
The ionic conductivity in halides can be expressed as: $$ \sigma = \sum n_i q_i \mu_i $$ where ( n_i ), ( q_i ), and ( \mu_i ) are the concentration, charge, and mobility of species i. F-doping reduces electronic conductivity and suppresses side reactions.
| Electrolyte | Conductivity (S/cm) | Electrochemical Window (V) | Modification |
|---|---|---|---|
| Li3InCl6 | ~10−3 | ~4.5 | None |
| Li3InCl5.5F0.5 | 1.00 × 10−3 | 4.8 | F-doping |
| Li2ZrCl6 (5% Mn) | 8 × 10−4 | 4.5 | Mn doping |
| Li2.31Y0.98Nb0.02Cl5.31 | ~1.0 × 10−3 | 4.5 | Nb doping |
Polymer Solid-State Electrolytes
Polymer solid-state electrolytes (PSEs) consist of a polymer matrix (e.g., PEO, PVDF, PAN, PMMA) and lithium salts. They offer flexibility, good processability, and excellent electrode compatibility but suffer from low room-temperature ionic conductivity (10−6–10−7 S/cm) due to high crystallinity. The Li+ transport in PSEs occurs primarily in the amorphous phase via segmental motion of polymer chains, described by the Vogel-Tammann-Fulcher equation: $$ \sigma = \frac{A}{\sqrt{T}} \exp\left(-\frac{B}{T – T_0}\right) $$ where ( A ), ( B ), and ( T_0 ) are constants. Strategies to improve conductivity include adding plasticizers, cross-linking, and incorporating nanofillers.
For example, electrospun LA-PAM-PEO membranes achieve a conductivity of 6.1 × 10−4 S/cm and a Li+ transference number of 0.32. PVDF-based electrolytes prepared by horizontal centrifugal casting show enhanced uniformity and stability, enabling stable cycling in LiFePO4|Li cells at 1C for 200 cycles. PAN-based films cross-linked with NPGDA exhibit a conductivity of 1.391 × 10−3 S/cm and maintain 80.7% capacity after 150 cycles in Li|NMC811 cells.
| Polymer Matrix | Conductivity (S/cm) | Li+ Transference Number | Advantages |
|---|---|---|---|
| PEO | 10−6–10−7 | 0.2–0.3 | Flexibility, compatibility |
| PVDF | 10−5–10−6 | 0.3–0.4 | Mechanical strength |
| PAN | 10−4–10−5 | 0.4–0.5 | Thermal stability |
| PMMA | 10−4–10−5 | 0.3–0.4 | Low interfacial resistance |
Composite Solid-State Electrolytes
Composite solid-state electrolytes (CSEs) combine inorganic fillers (active or inert) with polymer matrices to synergistically enhance ionic conductivity, mechanical strength, and interfacial stability. The Li+ transport in CSEs occurs through polymer chains, inorganic fillers, and interfacial regions. Active fillers like LLZO or LGPS provide additional Li+ pathways, while inert fillers reduce crystallinity and promote salt dissociation. For instance, LLZO/PEO films with 60% LLZO achieve a conductivity of 3.57 × 10−4 S/cm and enable stable cycling in Li|LiFePO4 cells for over 60 cycles at 0.1C. Asymmetric双层 CSEs like LPSCl-PEO/LATP-PEO exhibit a conductivity of 3.37 × 10−4 S/cm and suppress dendrite growth in Li symmetric cells for 2,000 hours.
The effective conductivity in CSEs can be estimated using the Maxwell-Garnett equation: $$ \sigma_{\text{eff}} = \sigma_p \frac{1 + 2\phi f}{1 – \phi f} $$ where ( \sigma_p ) is the polymer conductivity, ( \phi ) is the filler volume fraction, and ( f ) is a factor related to filler properties. Optimizing filler content and dispersion is critical for performance.
| Electrolyte | Conductivity (S/cm) | Application | Cycling Performance |
|---|---|---|---|
| LLZO/PEO (60%) | 3.57 × 10−4 | Li|LiFePO4 | 97.26% retention (180 cycles) |
| LPSCl-PEO/LATP-PEO | 3.37 × 10−4 | Li|LiFePO4 | 85% retention (400 cycles) |
| PVDF/LIC (15%) | — | Li|NMC811 | 82.8% retention (200 cycles) |
| PCL/Li6PS5Cl (3%) | 1.36 × 10−4 | Li|LiFePO4 | >80% retention (400 cycles) |
Industrial Applications and Future Outlook
The commercialization of solid-state batteries is advancing globally, with enterprises adopting different electrolyte strategies. Japanese and Korean companies (e.g., Toyota, Samsung) focus on sulfide electrolytes, while Chinese and Western firms (e.g., QuantumScape, CATL) prefer oxide or composite systems. Solid Power and SES are developing sulfide and polymer-based solid-state batteries, respectively. In China, companies like WeLion and QingTao are accelerating semi-solid battery production. Government initiatives, such as China’s 6 billion RMB investment in solid-state battery research, underscore the strategic importance of this technology.
Future research should address key challenges: (1) For OSEs, improving interfacial wettability and reducing sintering temperatures; (2) For SSEs, enhancing moisture stability and cathode compatibility; (3) For HSEs, mitigating reactivity with Li anodes; (4) For PSEs, increasing room-temperature conductivity; (5) For CSEs, optimizing filler-polymer interfaces. Machine learning and computational methods can accelerate material discovery and processing optimization. The integration of solid-state batteries in electric vehicles and grid storage will rely on overcoming these hurdles, ultimately enabling safer, higher-energy-density energy storage solutions.
In conclusion, solid-state electrolytes are pivotal to the next generation of lithium-ion batteries. Continued innovation in material design, doping strategies, and composite engineering will drive the industrialization of solid-state batteries, contributing to a sustainable energy future.