Solid-state battery
A solid-state battery (SSB) is an electrochemical energy storage device that utilizes a solid electrolyte to facilitate ion transport between the anode and cathode, in contrast to the liquid or gel electrolytes found in traditional lithium-ion batteries.[1] This design eliminates the need for a separate porous separator, as the solid material itself serves both as the ion conductor and a physical barrier, enabling potentially more compact and robust cell architectures.[2] SSBs are primarily based on lithium-ion chemistry but can incorporate advanced anodes like lithium metal to achieve higher theoretical capacities.[3] The primary advantages of SSBs stem from their solid electrolyte, which is typically non-flammable and less prone to leakage, significantly enhancing safety compared to liquid-electrolyte batteries that risk thermal runaway or fires.[1] They promise higher energy densities—potentially up to 500 Wh/kg or more with lithium metal anodes—allowing for longer ranges in electric vehicles and smaller form factors in consumer electronics.[2] Additionally, SSBs exhibit improved cycle life, with some prototypes retaining over 90% capacity after 1,000 cycles, and faster charging capabilities due to better thermal stability and reduced degradation pathways.[2] These attributes position SSBs as a transformative technology for sustainable energy storage in applications ranging from portable devices to grid-scale systems.[1] Despite these benefits, SSBs face significant challenges that have delayed widespread commercialization. Interfacial instability between the solid electrolyte and electrodes can lead to high resistance, dendrite formation in lithium metal anodes, and capacity fade over time.[3] Manufacturing difficulties, including the need for thin, uniform solid electrolyte layers and high-temperature processing, contribute to elevated costs—currently several times higher than lithium-ion batteries—and limit scalability.[2] Various solid electrolyte materials, such as sulfides (e.g., Li₁₀GeP₂S₁₂), oxides (e.g., LLZO), and polymers, offer trade-offs in ionic conductivity, mechanical flexibility, and stability, but none yet fully resolve these issues at ambient conditions.[3] Recent advancements as of 2025 include hybrid electrolyte designs combining ceramics and polymers for better flexibility and interface engineering techniques like LiPON coatings to suppress dendrites and improve cycling performance.[2] Prototypes from research institutions and companies have demonstrated pouch cells with energy densities exceeding 400 Wh/kg and operation across wide temperature ranges, signaling progress toward practical deployment in electric vehicles by the late 2020s.[3] Ongoing efforts focus on cost reduction through scalable fabrication methods and material innovations to overcome remaining barriers.[1]Overview
Definition and Principles
A solid-state battery is an electrochemical cell that utilizes a solid electrolyte to enable the conduction of ions, such as lithium ions, between the anode and cathode, thereby replacing the liquid or gel-based electrolytes common in traditional lithium-ion batteries. This design eliminates flammable liquid components, enhancing inherent safety while maintaining the core function of reversible ion shuttling for energy storage and release.[4] The fundamental structure of a solid-state battery includes an anode—often lithium metal for its high theoretical capacity—a cathode composed of materials like lithium cobalt oxide (LiCoO₂) or layered oxide compounds such as LiNi₀.₈Mn₀.₁Co₀.₁O₂, and a solid electrolyte that serves as both the ion conductor and separator. Exemplary solid electrolytes encompass inorganic ceramics, including garnet-type lithium lanthanum zirconate (LLZO), and sulfide-based materials like lithium germanium phosphorus sulfide (LGPS), which exhibit high ionic conductivities on the order of 10⁻² S/cm at room temperature. Unlike conventional batteries, this configuration contains no liquid phases, allowing for more compact and flexible architectures, such as thin-film or bulk formats.[4] At its core, the battery operates through solid-state ion diffusion, where lithium ions migrate through the crystalline or amorphous lattice of the solid electrolyte during charge and discharge, facilitating redox reactions at the electrodes without relying on solvated ion transport. This mechanism supports wider electrochemical stability windows—potentially extending up to 5 V versus Li/Li⁺—due to the reduced reactivity and higher decomposition voltages of solid electrolytes compared to liquids, enabling compatibility with high-voltage cathodes. Solid-state batteries achieve energy densities of 250–500 Wh/kg in bulk configurations and 300–900 Wh/kg in thin-film variants, reflecting their potential for greater gravimetric capacity through advanced electrode utilization. Additionally, they function across a typical operating temperature range of -50°C to 125°C, benefiting from the thermal robustness of solid materials that resist leakage or evaporation issues at extremes.[4][5][6][7]Comparison to Conventional Batteries
Solid-state batteries differ fundamentally in design from conventional lithium-ion batteries, which rely on flammable liquid electrolytes and porous separators. By replacing these with non-flammable solid electrolytes, solid-state batteries eliminate the risk of electrolyte leakage and enable the use of high-capacity lithium metal anodes, which offer a theoretical capacity of 3860 mAh/g compared to the 372 mAh/g of graphite anodes in lithium-ion batteries.[8][2] This design shift also allows for more compact stacking without traditional cooling systems, potentially reducing overall battery volume by up to 40%.[8] In terms of performance, solid-state batteries promise higher theoretical energy density, reaching up to 500 Wh/kg gravimetrically with lithium metal anodes, compared to 250–350 Wh/kg in conventional lithium-ion batteries.[8] They also support faster charging, with prototypes demonstrating full charges in as little as 3 minutes due to improved ion transport at the electrode-electrolyte interface.[9] Additionally, the wider electrochemical stability window of solid electrolytes—often exceeding 5 V—enables operation at higher voltages than the typical 4.2 V limit of liquid electrolytes in lithium-ion systems.[2] Safety is a key advantage, as the non-flammable nature of solid electrolytes prevents ignition and thermal runaway, which can occur in conventional batteries when liquid electrolytes decompose under abuse conditions like overcharging or puncturing. Unlike lithium-ion batteries that require safety vents and fuses, solid-state designs provide a mechanical barrier that can help suppress dendrite penetration, further mitigating short-circuit risks.[8] However, solid electrolytes generally exhibit lower ionic conductivity, typically in the range of 10^{-3} to 10^{-4} S/cm, compared to 10^{-2} S/cm for liquid electrolytes, resulting in higher initial internal resistance and power limitations in early prototypes.[10] Regarding lifecycle, solid-state batteries show potential for superior longevity, with prototypes achieving over 10,000 cycles, surpassing the 500–2000 cycles typical of lithium-ion batteries before significant degradation.[9] This endurance stems from the mechanical stability of solid electrolytes, which reduces side reactions and volume changes during cycling.[8]History
Early Developments (Pre-2010)
The foundational concepts of solid-state batteries trace back to the 19th century, when Michael Faraday discovered ionic conduction in solid materials. In 1834, Faraday identified silver sulfide (Ag₂S) and lead fluoride (PbF₂) as the first known solid electrolytes, observing that these compounds exhibited electrical conductivity due to the movement of ions under an applied voltage, particularly when heated.[11] This breakthrough laid the groundwork for understanding ion transport in solids, though practical applications remained elusive for over a century due to the low conductivity of these early materials at ambient temperatures.[4] Progress accelerated in the mid-20th century with the development of higher-conductivity solid electrolytes. In 1967, researchers at Ford Motor Company, including Y. Y. Yao and J. T. Kummer, invented β-alumina (specifically β''-alumina) as a sodium-ion conductor, enabling the first viable sodium-sulfur batteries that operated at elevated temperatures around 300°C.[12] These batteries used molten sodium as the anode and sulfur as the cathode, separated by the β-alumina ceramic electrolyte, marking an early milestone in solid-state energy storage for potential use in electric vehicles. Concurrently, initial lithium-based solid-state prototypes emerged in the 1970s, exploring materials like lithium iodide for thin-film cells, though they suffered from limited capacity and cycle life.[13] The 1980s and 1990s saw advancements in thin-film solid-state batteries tailored for microelectronics and miniaturized devices. In 1986, Kanehori et al. at NTT Laboratories demonstrated titanium disulfide (TiS₂) thin films as cathodes in lithium-based solid-state cells, fabricated via plasma CVD, achieving initial discharge capacities suitable for small-scale applications.[14] Building on this, in the early 1990s, scientists at Oak Ridge National Laboratory developed lithium phosphorus oxynitride (LiPON) as a stable amorphous solid electrolyte through reactive sputtering, enabling the first commercial thin-film batteries with energy densities around 100-200 Wh/L for uses like smart cards and medical implants.[15] These LiPON-based cells offered improved safety over liquid electrolytes but were constrained to low-power, low-capacity formats due to deposition challenges and interface issues.[13] Into the 2000s, research expanded to alternative solid electrolyte structures like perovskites and NASICON-type materials, aiming to boost room-temperature ionic conductivity for broader viability. Perovskite oxides, such as lanthanum lithium titanate (LLTO), were explored for their potential lithium-ion pathways, while NASICON (Na Super Ionic Conductor) analogs like Li₁₊ₓAlₓTi₂₋ₓ(PO₄)₃ showed promise with conductivities up to 10⁻⁴ S/cm, though still orders of magnitude below liquid electrolytes.[16] Despite these efforts, persistent challenges with grain boundary resistances and low overall conductivity limited prototypes to laboratory scales, hindering widespread adoption.[17] Early commercialization attempts faltered due to exorbitant costs; for instance, estimates from pre-2010 technologies pegged a 20 Ah solid-state cell at around $100,000, far exceeding practical thresholds for consumer or vehicular use.[18]Recent Progress (2010-Present)
In the early 2010s, significant breakthroughs in solid electrolyte materials propelled solid-state battery research forward. A pivotal advancement came in 2011 with the discovery of the lithium superionic conductor Li₁₀GeP₂S₁₂ (LGPS), a sulfide-based electrolyte exhibiting room-temperature ionic conductivity of 12 mS/cm, surpassing many liquid electrolytes used in conventional lithium-ion batteries. This material, developed by researchers at Tokyo Institute of Technology, enabled higher lithium-ion mobility and opened pathways for more efficient all-solid-state designs. Concurrently, Toyota intensified its research into automotive applications, unveiling a prototype solid-state battery in 2012 that demonstrated improved energy density and safety for electric vehicles.[19] Meanwhile, QuantumScape was founded in 2010 as a Stanford University spinout focused on solid-state lithium-metal batteries, achieving a public listing via SPAC merger in November 2020 to accelerate commercialization efforts.[20] From 2020 to 2023, industry collaborations and prototypes marked a surge toward practical implementation. Volkswagen deepened its partnership with QuantumScape, investing over $300 million by 2020 to co-develop solid-state cells for automotive use, emphasizing scalability and integration with existing EV platforms.[21] Toyota announced plans for hybrid solid-state batteries in hybrid electric vehicles by 2025, aiming to combine solid electrolytes with conventional components for enhanced range and faster charging in production models.[22] In manufacturing milestones, Murata began mass production of small solid-state batteries in 2021, targeting wearables like earphones with capacities up to 25 mAh and high safety profiles due to their non-flammable oxide electrolytes.[23][24] Maxell introduced 200 mAh cylindrical all-solid-state cells in 2023, 25 times the capacity of prior ceramic-packaged versions, suited for industrial applications with operating temperatures from -50°C to 125°C.[25] Panasonic revealed a drone prototype in 2023 featuring solid-state batteries capable of 80% charge in 3 minutes, leveraging sulfide electrolytes for high-rate performance in compact, high-power devices.[26] Advancements in 2024 and 2025 highlighted durability and material innovations. In January 2024, Volkswagen's PowerCo confirmed that a QuantumScape prototype retained 95% capacity after over 1,000 charge-discharge cycles, equivalent to approximately 500,000 km of driving, exceeding industry benchmarks for longevity.[27] Emerging research on chloride-based electrolytes gained traction for their superior electrochemical stability against lithium metal anodes, reducing dendrite formation and enabling safer, higher-voltage operations compared to traditional sulfides.[28] Toyota reported a surge in solid-state battery patents, filing over 1,000 related applications by mid-2025, and announced pilot production lines in Japan for all-solid-state cells targeting 2027 commercialization, supported by partnerships like Sumitomo Metal Mining for cathode materials.[29] Research trends since 2020 have increasingly emphasized all-solid-state architectures, eliminating liquid components to boost energy density and safety, alongside integration of silicon anodes to leverage their 10-fold higher capacity than graphite while mitigating volume expansion through solid electrolyte buffering.[30] These developments signal a transition from lab-scale prototypes to viable commercial products, driven by automotive and electronics sectors.Materials and Components
Solid Electrolytes
Solid electrolytes are the core component of solid-state batteries, facilitating ion transport without the use of liquid media, thereby enhancing safety and enabling higher energy densities. These materials must exhibit high ionic conductivity, wide electrochemical stability windows, and sufficient mechanical strength to prevent issues like dendrite formation during cycling. Inorganic solid electrolytes, in particular, dominate research due to their superior performance compared to polymers in certain aspects, though hybrids are emerging to combine benefits. Solid electrolytes are broadly classified into several types, each with distinct chemical compositions and properties. Oxide-based ceramics, such as garnet-structured Li₇La₃Zr₂O₁₂ (LLZO) and NASICON-type Li₁.₅Al₀.₅Ge₁.₅(PO₄)₃ (LAGP), offer excellent chemical stability but require careful doping to achieve high conductivity. Sulfide electrolytes, including Li₁₀GeP₂S₁₂ (LGPS) and Li₃PS₄ (LPS), provide the highest room-temperature ionic conductivities among inorganics, often exceeding those of oxides, due to their softer lattice structures. Polymer electrolytes, typically based on polyethylene oxide (PEO) complexed with lithium salts, are flexible and processable but suffer from lower conductivity at ambient temperatures. Halide electrolytes, such as chloride-based Li₃YCl₆, have gained attention for their high conductivity and compatibility with lithium metal anodes, offering improved stability against high-voltage cathodes. As of 2025, superionic variants of Li₃YCl₆ achieve conductivities exceeding 10 mS/cm through mechanisms like collective anion motion, enhancing cycling stability in solid-state cells.[31] Key properties of solid electrolytes directly influence battery performance. Ionic conductivity (σ) is a primary metric, governed by the equation σ = n q μ, where n represents the density of charge carriers (e.g., Li⁺ ions), q is the ion charge (e.g., +e for lithium), and μ is the ion mobility. This relationship, derived from fundamental transport theory, highlights how material design can enhance σ by increasing carrier concentration or improving ion diffusion pathways; for instance, defect engineering in oxides boosts n, while soft lattices in sulfides enhance μ. Representative conductivities include ~10⁻⁴ S/cm for LLZO and LAGP at 25°C, up to 10⁻² S/cm for LGPS sulfides, 10⁻⁵ to 10⁻³ S/cm for PEO-based polymers (higher at elevated temperatures), and 10^{-3} to 10^{-2} S/cm for chloride halides like Li₃YCl₆ (with recent superionic variants >10^{-2} S/cm). Electrochemical stability windows reach up to 5 V vs. Li/Li⁺ for oxides and halides, enabling compatibility with high-voltage cathodes, while sulfides are narrower (~4 V) but can be extended via doping. Mechanically, a high shear modulus is crucial for suppressing lithium dendrite growth, as per the Monroe-Newman criterion requiring it to exceed twice that of lithium metal (~4.2 GPa); LLZO exhibits ~60 GPa shear modulus for robust suppression, sulfides offer ~25 GPa for better electrode contact but moderate dendrite resistance, PEO is soft (~1 GPa) and prone to penetration, and halides provide balanced ductility (~30-50 GPa) for interfacial adaptability.[32] Synthesis methods for solid electrolytes vary by type to optimize density and minimize defects. Bulk ceramics like LLZO are typically produced via solid-state sintering at high temperatures (1000-1200°C) to achieve dense pellets, while thin films such as lithium phosphorous oxynitride (LiPON), developed in the 1990s for early thin-film batteries, use radio-frequency sputtering for precise deposition. For sulfides and halides, solution-based approaches like mechanical milling or melt-quenching are preferred to avoid oxidation, often followed by hot-pressing. A major challenge is grain boundary resistance, which can reduce overall conductivity by 1-2 orders of magnitude in polycrystalline materials; mitigating this involves additives or single-crystal growth, though scalability remains difficult. Recent advancements since 2020 focus on hybrid electrolytes that integrate sulfide and oxide components to leverage complementary properties, such as combining LGPS sulfides with LLZO oxides for enhanced conductivity (>10⁻³ S/cm) and stability. For example, composites of Li₆PS₅Cl with LLZTO achieve ~4×10^{-5} S/cm, while polymer hybrids reach ~2×10^{-4} S/cm at room temperature, improving mechanical integrity and reducing grain boundary impedance for longer cycling stability in prototype cells.[33][34]| Type | Examples | Ionic Conductivity (S/cm at 25°C) | Electrochemical Window (V vs. Li) | Shear Modulus (GPa, approx.) |
|---|---|---|---|---|
| Oxide Ceramics | LLZO, LAGP | 10⁻⁴ | Up to 5 | 50-65 (LLZO) |
| Sulfides | LGPS, LPS | 10⁻³ to 10⁻² | ~4 | 20-30 |
| Polymers | PEO-based | 10⁻⁵ to 10⁻³ | 3-4 | ~1 |
| Halides | Li₃YCl₆ chlorides | 10⁻³ to 10⁻² (recent >10⁻²) | Up to 5 | 30-50 |