With the rapid advancement of the new energy vehicle industry, the market has imposed higher demands on lithium-ion batteries, particularly regarding energy density, safety, and operating temperature range. Traditional lithium-ion batteries, which rely on organic liquid electrolytes, have reached a bottleneck in energy density due to limitations in their composition. These liquid electrolytes, while offering excellent ionic conductivity and interfacial contact, suffer from side reactions during charge-discharge cycles that shorten battery lifespan and pose safety risks due to their flammability. In contrast, all-solid-state batteries utilizing solid-state electrolytes have emerged as a promising alternative, offering high energy density, superior safety, and excellent material compatibility. A key advantage of solid-state electrolytes is their ability to suppress lithium dendrite growth, significantly enhancing battery stability and reliability. As conventional lithium-ion batteries approach their theoretical energy density limits, solid state batteries hold the potential to achieve energy densities exceeding 500 W·h/kg by enabling the use of high-capacity lithium metal electrodes. Consequently, solid-state electrolyte technology has become a focal point in energy research, poised to drive the next generation of battery innovations.
Solid-state batteries represent a significant leap in battery technology, replacing traditional liquid electrolytes and separators with solid-state electrolytes. This substitution reduces both the mass and volume of battery packs while providing exceptional fire resistance and preventing lithium dendrite penetration, thereby ensuring high safety. The working principle of solid-state batteries is similar to that of liquid batteries, with the solid electrolyte fully replacing the liquid electrolyte and separator. The cathode and anode are mixed with the solid electrolyte to form cathode and anode regions, respectively. Currently, solid-state batteries are categorized based on electrolyte type into polymer, oxide, halide, and sulfide-based systems. Early research primarily focused on polymer electrolytes, but attention has gradually shifted to oxides and sulfides, with halides emerging as a promising option due to recent breakthroughs. Each type of solid-state electrolyte offers distinct advantages: polymer-based systems provide excellent flexibility and processability with simple manufacturing processes suitable for mass production; oxide electrolytes exhibit high safety and stability but may have poor mechanical properties, making large-capacity batteries challenging; halide electrolytes boast high ionic conductivity and wide electrochemical windows, though their technology remains immature; sulfide electrolytes demonstrate the highest ionic conductivity and low electrochemical polarization, enabling high energy and power density in solid state batteries.
In this review, we delve into the recent progress in solid-state electrolytes for all-solid-state batteries, covering polymer, oxide, halide, and sulfide-based systems. We also analyze global patent trends to forecast technological developments and provide insights into future directions for solid-state battery commercialization. The integration of solid-state electrolytes into practical applications requires addressing key challenges such as interfacial stability, ionic conductivity, and cost-effectiveness. Through comprehensive analysis, we aim to outline the current state of research and highlight promising strategies for advancing solid state battery technologies.
Polymer Solid-State Electrolytes
Polymer solid-state electrolytes utilize various high-performance materials as matrices, including poly(ethylene oxide) (PEO), poly(vinylidene fluoride) (PVDF), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)). Among these, PEO has garnered significant attention due to its excellent compatibility with lithium metal, good electrochemical stability, and low cost. However, the high crystallinity of PEO restricts lithium-ion transport, resulting in relatively low ionic conductivity at room temperature. PAN exhibits superior lithium-ion migration compared to PEO but suffers from reduced mechanical strength, leading to brittleness after film formation. Similarly, PMMA and PVDF face challenges in mechanical performance and exhibit low ionic conductivity. To overcome these limitations, researchers have adopted strategies such as blending, cross-linking, copolymerization, and the incorporation of inorganic fillers to enhance ionic conductivity and structural stability in polymer electrolytes.
PVDF contains polar -C-F groups that facilitate the dissolution of lithium salts, enabling rapid Li+ transport in PVDF/lithium salt complexes. PVDF demonstrates excellent electrochemical stability with an electrochemical window exceeding 4.6 V versus Li/Li+, and it shows good interfacial compatibility with lithium anodes, resulting in low polarization voltage. Additionally, PVDF can form a LiF interface layer that inhibits lithium dendrite growth. For instance, researchers have reported PVDF-based solid-state electrolytes that resist lithium dendrite formation. Nevertheless, PVDF’s semi-crystalline nature makes it difficult to achieve high ionic conductivity, with room temperature values typically ranging from 10-5 to 10-7 S·cm-1. To address this, various approaches have been explored: (1) designing polymer chains through block or graft copolymerization, such as introducing hexafluoropropylene into PVDF to reduce crystallinity and improve the overall performance of P(VDF-HFP) electrolytes; (2) incorporating additional components like inorganic fillers, ionic liquids, or plastic crystal molecules. The addition of inorganic fillers has become a mainstream method, enhancing ion transport without compromising the flexibility and processability of polymers, thereby driving the development of organic-inorganic composite solid-state electrolytes.
For example, Zhang et al. introduced 10 wt% Li6.75La3Zr1.75Ta0.25O12 nanoparticles into a PVDF-based electrolyte system, fabricating a flexible solid-state film with excellent mechanical strength and thermal stability. The La3+ ions efficiently complex with solvents, possessing high electron density states that promote PVDF dehydrofluorination and optimize electrode interface compatibility. This composite electrolyte achieved an ionic conductivity of 5 × 10-4 S·cm-1 at room temperature. Despite these advances, polymer solid-state electrolytes face challenges such as further improving ionic conductivity, optimizing compatibility with other battery components, and reducing production costs. Strategies like copolymerization, blending, and inorganic filler incorporation continue to enhance the performance of polymer solid-state electrolytes, offering new directions for the development of solid state batteries.
The ionic conductivity in polymer electrolytes can be described by the Arrhenius equation for temperature dependence:
$$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{kT}\right) $$
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. For composite polymers, the effective conductivity often follows a percolation theory model, where the conductivity increases significantly above a critical filler concentration.
Oxide Solid-State Electrolytes
Oxide solid-state electrolytes are renowned for their high safety, stability, and include perovskite-type (LLTO), garnet-type (LLZO), and sodium superionic conductor (NASICON)-type oxides. LLTO-type oxides offer high oxidation potential and ionic conductivity but are prone to reduction at low potentials, leading to decreased conductivity. The highest ionic conductivity in LLTO systems is observed in lanthanum lithium titanate (La2/3-xLi3xTiO3), with research focusing on ion substitution, composite formation, and sintering atmosphere control to enhance performance.
NASICON-type oxide solid electrolytes have the general formula Li[A2B3O12], with current research hotspots centered on LiTi2(PO4)3 and LiGe2(PO4)3 systems. To optimize ion transport, ion substitution strategies are widely employed, incorporating elements such as Al, Y, Ga, or Sc to replace Ti or Ge ions, or Si to replace P, thereby modifying ion transport channels and improving conductivity. For instance, Al3+ substitution has led to Li1.3Al0.3Ti1.7(PO4)3 and Li1.5Al0.5Ge1.5(PO4)3 achieving ionic conductivities on the order of 10-3 S·cm-1.
LLZO-type oxide solid electrolytes, with the formula Li7La3Zr2O12, exhibit outstanding ionic conductivity and are considered more promising than previous oxide types. Enhancing the density of LLZO materials is a key research direction, achieved through ion doping, sintering aids, and improved preparation techniques to stabilize the cubic phase structure and boost ionic conductivity. Oxide solid-state electrolytes offer high stability in air and at elevated temperatures, excellent cycling performance, and electrochemical stability. Their relatively low research costs and difficulty make them attractive to battery manufacturers, particularly for semi-solid and quasi-solid-state battery applications, enabling rapid scaling. Many new entrants and Chinese companies, such as Qingtao (Kunshan) Energy Development Group Co., Ltd. and Beijing Weilan New Energy Technology Co., Ltd., have adopted this technology路线.
The conductivity in oxide electrolytes can be modeled using the following equation for ion migration:
$$ \sigma = n \mu q $$
where $n$ is the charge carrier concentration, $\mu$ is the mobility, and $q$ is the charge. For NASICON-type materials, the ionic conductivity is influenced by the crystal structure and defect chemistry, which can be tailored through doping.
| Type | General Formula | Advantages | Challenges | Typical Ionic Conductivity (S·cm-1) |
|---|---|---|---|---|
| Perovskite (LLTO) | La2/3-xLi3xTiO3 | High oxidation potential | Reduction at low potentials | ~10-3 |
| NASICON | Li[A2B3O12] | Good stability | Limited by grain boundaries | 10-3 – 10-4 |
| Garnet (LLZO) | Li7La3Zr2O12 | High ionic conductivity | Sintering requirements | 10-3 – 10-4 |
Halide Solid-State Electrolytes
Halide solid-state electrolytes, with the general formula LiaMXb where M is a metal element and X is a halogen, have recently gained attention due to their high ionic conductivity and voltage stability. Mainstream halide electrolytes include LiaMCl6, LiaMCl4, and LiaMCl8, where M is a transition metal. Among these, LiaMCl6 types exhibit the highest room temperature conductivity, often exceeding 10-3 S·cm-1, attributed to the octahedral coordination structure shared by transition metal ions and lithium ions. For example, Asano et al. prepared Li3YCl6 and Li3BrCl6 via ball milling and sintering, achieving room temperature ionic conductivities of 5.1 × 10-4 S·cm-1 and 1.7 × 10-3 S·cm-1, respectively, sparking significant interest in halide electrolytes. Further, Liang et al. found that Li3ScCl6 exhibits excellent room temperature ionic conductivity of 3.02 × 10-3 S·cm-1, with studies indicating that LiaMCl6 structures offer optimal conductivity.
Sun et al. discovered that Li3InCl6 halide electrolyte not only demonstrates excellent cathode stability but also high crystallinity. Through ball milling and sintering, Li3InCl6 achieved a practical room temperature ionic conductivity of 1.49 × 10-3 S·cm-1, underscoring its potential for practical applications. Additionally, Zr4+-based electrolytes like Li2ZrCl6 have emerged as new research hotspots due to their cost advantages. Although Li2ZrCl6 itself has relatively low room temperature conductivity, heterovalent substitution or doping strategies can introduce vacancies and interstitials, significantly enhancing conductivity to 10-3 S·cm-1.
Despite the early stage of corporate involvement in halide solid-state electrolytes, ongoing technological breakthroughs are steadily increasing ionic conductivity beyond 10-3 S·cm-1, highlighting their inherent low-cost advantages and high chemical stability. This progress positions halide electrolytes as one of the most promising solutions for solid-state batteries, accelerating research and development in this area.
The ionic conductivity in halide electrolytes can be expressed in terms of the Nernst-Einstein relation:
$$ \sigma = \frac{D n q^2}{kT} $$
where $D$ is the diffusion coefficient, $n$ is the carrier density, $q$ is the charge, $k$ is Boltzmann’s constant, and $T$ is temperature. For halides, the conductivity is highly dependent on the crystal structure and the presence of defects, which can be engineered through compositional tuning.
Sulfide Solid-State Electrolytes
Since the 1980s, sulfide electrolytes have been a core research focus in the solid-state battery field, with attention on glassy multi-component systems such as Li2S-GeS2, Li2S-SiS2, and Li2S-P2S5-Li composites, which exhibit room temperature ionic conductivities around 10-4 S·cm-1. In the 21st century, breakthroughs occurred with Kanno et al. reporting a new crystalline thio-LISICON-type Li3.25Ge0.25P0.75S4 electrolyte, and Tatsumisago et al. discovering the Li7P3S11 crystalline phase in the Li2S-P2S5 system, which not only improved conductivity but also opened avenues for glass-ceramic sulfide electrolytes. Subsequently, inspired by the argyrodite structure, Deiseroth et al. synthesized Li6PS5X (X=Cl, Br, I) materials, with Li6PS5Cl showing particularly outstanding performance with an ionic conductivity of approximately 2.0 × 10-3 S·cm-1.
Further exploration led to the development of new sulfide electrolytes with continuously rising ionic conductivities. In 2011, Kanno et al. developed Li10GeP2S12 (LGPS), which achieved a room temperature ionic conductivity of 1.2 × 10-2 S·cm-1, comparable to commercial organic liquid electrolytes. In 2016, Kato et al. innovated with an LGPS-derived electrolyte, Li9.54Si1.74P1.4S11.7Cl0.3, which reached a remarkable ionic conductivity of 2.5 × 10-2 S·cm-1 at room temperature, setting a new record in the field. Concurrently, argyrodite-type sulfide electrolytes also saw improvements; for example, Nazar et al. reported a lithium-rich argyrodite electrolyte, Li7.5PS4.5Cl0.5, with a room temperature ionic conductivity of 1.2 × 10-2 S·cm-1, demonstrating that composition optimization and structural design can effectively enhance ion transport.
In summary, sulfide electrolytes have made significant strides, with their superior ionic conductivity making them top contenders among solid electrolyte materials. The advantages of sulfide solid-state electrolytes include high conductivity and excellent performance, though they face challenges such as susceptibility to oxidation and interfacial instability. Despite higher costs and research difficulties, sulfide electrolytes are highly favored by Japanese and Korean companies for all-solid-state battery applications.
The ionic conductivity in sulfide electrolytes can be modeled using the following equation for amorphous or crystalline systems:
$$ \sigma(T) = A T^{-1/2} \exp\left(-\frac{B}{(T – T_0)}\right) $$
where $A$ and $B$ are constants, $T$ is temperature, and $T_0$ is the Vogel temperature, often used for glassy systems. For crystalline sulfides, the conductivity is dominated by ion hopping mechanisms.
| Material | Type | Ionic Conductivity (S·cm-1) | Year | Notes |
|---|---|---|---|---|
| Li3.25Ge0.25P0.75S4 | Thio-LISICON | ~10-3 | 2000s | Crystalline phase |
| Li7P3S11 | Glass-ceramic | ~10-4 – 10-3 | 2007 | Discovered in Li2S-P2S5 |
| Li6PS5Cl | Argyrodite | 2.0 × 10-3 | 2008 | High performance |
| Li10GeP2S12 (LGPS) | Crystalline | 1.2 × 10-2 | 2011 | Breakthrough |
| Li9.54Si1.74P1.4S11.7Cl0.3 | LGPS-derived | 2.5 × 10-2 | 2016 | Record high |
| Li7.5PS4.5Cl0.5 | Argyrodite | 1.2 × 10-2 | 2019 | Lithium-rich |
Trends in Solid-State Electrolyte Development
To analyze the technological trends in solid-state electrolytes, we conducted a patent analysis using the incoPat database, focusing on all-solid-state batteries with solid-state electrolytes. The search query included terms related to solid-state batteries and electrolytes within the IPC classification H01M, resulting in 8494 relevant patents as of June 25, 2024. It is important to note that patent application statistics are based on publicly available data, and there is typically a 3 to 18-month delay in publication after filing, meaning the numbers for 2023 and 2024 may not fully reflect the actual application volume.
The annual trend in patent applications for solid-state electrolytes reveals three key development phases: (1) From 2005 to 2010, applications remained below 30 per year with slow growth, indicating relative stagnation; (2) From 2010 to 2015, applications began to increase moderately, signaling a gradual recovery; (3) Post-2015, applications surged dramatically, reflecting intense global research interest and establishing solid-state electrolytes as a hotspot in energy storage technology. This surge underscores the growing recognition of solid state batteries as a transformative technology for next-generation energy solutions.
Table 1 lists the top 10 applicants in the solid-state battery electrolyte field, dominated by Japanese and Korean entities, with five from Japan and three from Korea. The remaining include China’s蜂巢能源科技股份有限公司 (Honeycomb Energy Technology Co., Ltd.) and the United States’ General Motors Global Technology Operations LLC. Additionally, Chinese enterprises and universities are increasing their patent filings, with companies like Beijing Weilan New Energy Technology Co., Ltd., Jiangxi Ganfeng New Energy Technology Co., Ltd., and research institutions such as Harbin Institute of Technology and Central South University appearing in the top 20. This indicates a competitive landscape where Chinese players are catching up through collective efforts, despite individual strengths lagging behind those in Japan, the U.S., and Korea.
| Rank | Applicant | Country | Patent Count | Percentage of Total |
|---|---|---|---|---|
| 1 | Toyota Motor Corporation | Japan | 1297 | 15.27% |
| 2 | LG Energy Solution, Ltd. | Korea | 703 | 8.28% |
| 3 | Murata Manufacturing Co., Ltd. | Japan | 316 | 3.72% |
| 4 | Hyundai Motor Company | Korea | 308 | 3.63% |
| 5 | Samsung SDI Co., Ltd. | Korea | 175 | 2.06% |
| 6 | Honda Motor Co., Ltd. | Japan | 166 | 1.95% |
| 7 | Panasonic Energy Co., Ltd. | Japan | 145 | 1.71% |
| 8 | Honeycomb Energy Technology Co., Ltd. | China | 131 | 1.54% |
| 9 | General Motors Global Technology Operations LLC | USA | 103 | 1.21% |
| 10 | TDK Corporation | Japan | 93 | 1.09% |
Table 2 shows the distribution of priority countries/regions for solid-state electrolyte patent applications. Since applicants typically file first in their home countries, this distribution indirectly indicates the sources of technology. China leads with 2987 applications, accounting for 35.17% of the total, followed by the United States (1507), Japan (1455), and Korea (790). This dominance reflects China’s aggressive push in new energy technologies, where numerous companies are investing in solid state battery research. Although individual Chinese firms may not yet match the prowess of their Japanese, American, or Korean counterparts, the collective strength of China’s新能源 sector is driving a rapid catch-up,预示着 intense future competition in solid state battery development.
| Rank | Country | Patent Application Count |
|---|---|---|
| 1 | China | 2987 |
| 2 | United States | 1507 |
| 3 | Japan | 1455 |
| 4 | Korea | 790 |
| 5 | Canada | 41 |
Further analysis of the 8494 patents reveals that Chinese new energy companies are focusing their patent布局 on several key technical efficacies, as illustrated in Figure 3. These include: (1) Improving interfacial stability, a critical factor affecting overall performance and cycle life in solid state batteries; (2) Enhancing safety, a inherent advantage of solid-state batteries over liquid counterparts, providing a solid foundation for commercialization; (3) Increasing energy density, a core goal in solid state battery development essential for practical applications; (4) Boosting ionic conductivity, a key technical aspect influencing charge-discharge rates and efficiency; (5) Reducing costs, a major barrier to large-scale adoption, driving research into low-cost electrolyte materials and manufacturing processes. The patent布局 around these efficacies not only advances solid state battery technology but also lays the groundwork for commercial viability.
The focus on these areas highlights the multifaceted approach required to commercialize solid state batteries. For instance, interfacial stability is often addressed through composite materials or surface modifications, while cost reduction involves scalable synthesis methods. The collective efforts in patenting reflect a global race to overcome the remaining hurdles in solid state battery technology, with significant contributions from both established and emerging players.
Conclusion
Solid-state batteries represent a future direction jointly emphasized by the battery technology community and industry, with the potential to fundamentally address the limitations of traditional liquid lithium-ion batteries in safety, energy density, and cycle life. However, transitioning this promising technology from the laboratory to market and achieving large-scale commercialization requires overcoming a series of technical challenges, including high stability, safety, energy density, ionic conductivity, and cost-effectiveness. Polymer solid-state electrolytes have seen notable progress but仍需解决提升室温离子电导率、优化配合及降低成本等挑战, with strategies like copolymerization, blending, and inorganic filler addition providing new pathways for improvement. Oxide solid-state electrolytes, favored for their stability and lower costs, are suitable for semi-solid and quasi-solid-state batteries, attracting attention from Chinese companies. Halide solid-state electrolytes, though in early stages of corporate involvement, are gaining traction due to technological breakthroughs that enhance their low-cost and high chemical stability advantages. Sulfide electrolytes, with their high conductivity and performance, remain a research hotspot despite challenges like cost and interfacial stability, particularly in Japanese and Korean efforts for all-solid-state batteries.
Patent analysis indicates that China has surpassed Japan, the U.S., and Korea in patent布局 for solid-state batteries, marking significant progress in research and development. However, the domestic landscape is fragmented, with many companies involved, leading to weaker individual patent strength compared to leading firms in other countries. Among the top 10 global applicants, Japanese and Korean companies dominate. Therefore, advancing solid-state battery technology requires enhanced collaboration among domestic enterprises to jointly address technical hurdles and better compete in the global solid-state battery industry. The future of solid state batteries hinges on continued innovation in electrolyte materials, interfacial engineering, and manufacturing processes, with the ultimate goal of delivering safe, high-performance, and affordable energy storage solutions for a sustainable future.
In summary, the development of solid state batteries is a complex yet rewarding endeavor. By leveraging the strengths of various electrolyte types and fostering international collaboration, the vision of widespread solid state battery adoption can become a reality, revolutionizing energy storage for applications ranging from electric vehicles to grid storage. The ongoing research and patent activities underscore the dynamic nature of this field, promising exciting advancements in the years to come.