Introduction: Solid-state battery refer to lithium batteries that use solid electrolytes instead of traditional electrolytes. According to the amount of solid electrolytes used, they can be divided into “semi-solid batteries (SSBs)” and “all-solid-state batteries (ASSB)”:
Semi-solid batteries: The liquid content in the battery is 10% as the boundary. If the liquid content is lower than this ratio, it can be classified as this type, which is between liquid batteries and all-solid-state batteries.
All-solid-state batteries: Completely use solid electrolytes, and the liquid content in the battery is reduced to 0%, which is the ultimate form of solid-state battery technology.
Solid-state lithium batteries are mainly composed of cathode electrodes, anode electrodes and solid electrolytes. The core difference between them and traditional liquid batteries is that solid electrolytes replace liquid electrolytes and diaphragms, achieving the goal of using less or no electrolytes and diaphragms.
Transformation of battery energy storage technology
The iteration of energy storage technology plays a key role in promoting contemporary technological innovation and has made important contributions to the construction of a sustainable energy system. Historically, energy storage has gone through multiple stages of innovation, and each stage has achieved improvements in efficiency, safety, and environmental impact.
At present, the energy storage field is experiencing an important transformation marked by the development of solid-state batteries (SSBs): the evolution towards solid-state energy storage.
On the one hand, traditional liquid electrolyte lithium-ion batteries have significant safety bottlenecks and performance limitations, such as the flammability and volatility of liquid organic electrolytes, high risk of thermal runaway of batteries, easy generation of Lithium Plating during charging, and significant temperature influence on ionic conductivity, which will have a negative impact on battery performance and service life;
On the other hand, breakthroughs in materials science provide support for solid-state energy storage technology. Batteries using solid-state electrolytes can not only achieve higher energy density (which is crucial for multiple scenarios such as consumer electronics and electric vehicles), but also significantly reduce the safety risks brought by liquid electrolytes.
Due to the above limitations, the industry is increasingly eager to explore alternatives such as solid-state batteries, aiming to build a battery system with better performance.
The difference between all-solid-state batteries and Lithium Batteries (traditional)
The main difference between all-solid-state batteries and traditional lithium-ion batteries (LIBs) lies in the electrolyte form: traditional lithium-ion batteries rely on liquid electrolytes to complete ion transfer between electrodes, while all-solid-state batteries use solid electrolytes and exhibit multiple advantages due to this feature.
The liquid electrolyte of traditional lithium batteries performs the function of ion conduction and relies on a diaphragm layer to separate the positive and negative electrodes. This diaphragm only allows ions to pass through to avoid direct contact between the electrodes and cause short circuits. In contrast, in all-solid-state batteries, solid electrolytes have both ion conduction and electrode separation functions, so no additional diaphragm layer is required.
Under ideal conditions, the charging speed of all-solid-state batteries may even exceed that of traditional lithium batteries.
Composition of solid-state batteries
According to the functional classification of materials in solid-state batteries, the material system can be divided into cathode materials, anode materials, electrolyte materials and interface layer materials.
The following focuses on the characteristics and challenges of anodematerials:
Cathode Materials | Solid-state battery
In solid-state lithium batteries, metallic lithium (Li) is an ideal negative electrode material because of its significant advantages of high specific capacity (3860 mAh/g) and low electrochemical potential (-3.04V vs standard hydrogen electrode). However, the practical application of lithium anode electrodes faces multiple bottlenecks:
- Lithium Plating growth: Lithium Plating are easily formed during the charge and discharge cycle, which may pierce the electrolyte and cause internal short circuits, resulting in capacity decay and safety hazards;
- Volume expansion: Lithium metal expands significantly in volume during the cycle, further exacerbating structural damage, shortening cycle life and increasing overpotential.
To overcome these limitations, the industry has explored a variety of alternative anode materials:
Intercalation-type anode materials (such as graphite) have been widely used in lithium-ion batteries. Although their specific capacity is lower than that of metallic lithium, they have mature cycle performance and stability and are still a viable choice for solid-state battery anode electrodes.
Alloy-type anode materials (such as lithium silicon (Li-Si), lithium tin (Li-Sn), and lithium titanate (Li₄Ti₅O₁₂)) have higher specific capacity because they can form alloys with lithium. Among them, the specific capacity of Li-Si is as high as 4.2Ah/g, but its volume expansion and contraction rate during the charge and discharge cycle is as high as 300%, which is prone to capacity decay and cracking; Li-Sn has better cycle stability than Li-Si, but has a lower specific capacity; lithium titanate is concerned because of its small volume change, but its energy density is low.
Conversion-type anode materials (represented by lithium metal oxides and hydrides) provide high specific capacity through a unique electrochemical mechanism (lithium ions react with materials to form new compounds), but the accompanying volume change is large, and it is difficult to maintain structural integrity during long-term cycles.
Metallic sodium (Na) has the advantages of abundant reserves and low cost as anode materials. Its redox potential is 2.7V (vs standard hydrogen electrode) and its specific capacity is 1165.8 mAh/g. However, its metal anode still has problems of dendrite formation and limited cycle stability, although it is slightly lighter than the lithium system. The future development of sodium-based solid-state batteries depends on whether the challenge of Plating growth can be overcome and stable cycling can be achieved.
In addition, patterning the graphite anode by a simple mechanical imprinting method helps shorten the diffusion length of the electrode/electrolyte interface layer, providing a new path for optimizing the anode performance. Currently, lithium-based solid-state batteries need to continue to make breakthroughs in material modification and structural design, while the research and development focus of the sodium-based system is on plating suppression and cycle stability improvement.
Cathode Materials | Solid-state battery
Similar to liquid lithium-ion batteries, the most commonly used cathode materials in solid-state batteries are lithium metal oxides, such as lithium cobalt oxide (LCO), lithium nickel cobalt aluminum oxide (NCA), lithium nickel manganese cobalt oxide (NMC), lithium manganese oxide (LMO) and lithium iron phosphate (LFP).
Solid-state electrolyte: the core technology of solid-state batteries
The core difference between solid-state batteries and traditional lithium-ion batteries lies in the electrolyte form. Although the room temperature ionic conductivity of solid-state electrolytes is lower than that of liquid electrolytes, they have significant advantages in thermal stability, flame retardancy, structural durability and simplicity of battery design. According to material properties, solid-state electrolytes are mainly divided into the following categories:
| Oxide electrolytes | Typical representatives include LIPON, NASICON and garnet. This type of electrolyte has excellent chemical stability and mechanical strength, but the ionic conductivity is relatively low. The conductivity can be improved by doping modification, nano-sizing and other technical means. |
| Sulfide electrolytes | Represented by LPS and silver sulfide type, they have high ionic conductivity and good flexibility, but weak chemical stability and high synthesis cost. At present, the production cost can be reduced by process optimization such as wet chemical synthesis and mechanical ball milling. |
| Polymer electrolytes | The advantages are good flexibility and easy processing, but there are problems of low ionic conductivity and insufficient thermal stability. By introducing inorganic fillers and optimizing molecular structure, its comprehensive performance can be significantly improved. |
| Other types | Include halide electrolytes, composite electrolytes, mixed solid-liquid electrolytes, etc. |
At present, oxides, sulfides, and polymers are the mainstream technical routes, and each has its own advantages and disadvantages:
- Oxide electrolytes: The best chemical stability, suitable for scenarios with high safety requirements;
- Sulfide electrolytes: The highest ionic conductivity, more conducive to improving battery fast charging performance;
- Polymer electrolytes: The best compatibility with the electrode interface, easy to adapt to the process.
Since no single technical route has an absolute advantage, companies that deploy solid-state batteries around the world generally adopt a multi-route parallel R&D strategy, striving to find a balance between performance, cost and industrialization progress.
Oxide electrolytes
Oxide electrolytes are an important technical direction for solid-state batteries, mainly including LIPON, NASICON and garnet types:
LIPON
is composed of lithium, phosphorus, oxygen and nitrogen (chemical formula LixPOyNz). It began to be developed in the 1970s. It is usually prepared by magnetron sputtering as an amorphous glass film, which is suitable for micro batteries (such as paired with LiCoO₂ positive electrode and lithium metal negative electrode). Its advantages are wide electrochemical window (0–5.5V vs. Li⁺/Li), extremely low electronic conductivity (about 8×10⁻¹⁴S/cm) and excellent mechanical properties, but it has the disadvantages of low cathode loading and high film process cost.
NASICON
First reported by Goodenough’s team in 1976, the general formula is LiX₂(PO4)₃ (X is Ge, Zr, Ti, etc.), and can be derived from materials such as LATP and LAGP through Al³⁺ doping, with room temperature ionic conductivity up to 10⁻³S/cm. LATP has atmospheric stability and low sintering temperature, but the interfacial reaction problem with lithium metal anode needs to be solved by introducing an intermediate layer.
Garnet type (LLZO)
Represented by Li₇La₃Zr₂O₁₂, it has high ionic conductivity and chemical stability, but its production depends on lanthanum, requires high temperature process, high cost, and limited compatibility with some positive electrode materials.
Sulfide Electrolytes
In 2011, the sulfide electrolyte developed by Kanno’s team at Tokyo Institute of Technology in Japan caused a sensation in the industry. Its ionic conductivity increased from 25mS/cm in 2011 to 32mS/cm in 2023, becoming one of the current mainstream technology routes. This type of material is based on lithium and sulfur, and can introduce phosphorus, silicon, germanium or halides. It has both high ionic conductivity (close to or exceeding liquid electrolytes) and adaptability to cold pressing processing. Costs can be reduced through wet chemical synthesis or mechanical ball milling. The ductility of sulfide electrolytes enables them to form a dense layer with low grain boundary resistance under high pressure, reduce lithium dendrites and optimize electrode contact, and has been selected as a research and development focus by most leading companies.
Polymer Electrolytes
Polymer electrolytes are a transitional solution between liquid and solid technologies. They are composed of a polymer matrix (such as PEO, PVDF), a lithium salt (such as LiTFSI) and additives, and promote lithium ion conduction through polymer chains. Its advantages include good flexibility, easy processing, and strong flame retardancy, but it has problems such as low room temperature ionic conductivity (needs to be enhanced by nanofillers such as γ-LiAlO₂ or Al₂O₃) and insufficient thermal stability. For example, PEO-based electrolytes reduce crystallinity through LiTFSI, and the conductivity of some new systems can exceed 1mS/cm, but the polarization caused by anion migration and lithium dendrite penetration still need to be solved.
Halide Electrolytes
Halide electrolytes have high ion mobility and oxidation stability (especially fluorine and chlorine systems) due to the weak Coulomb interaction between halogen anions and lithium ions, but their early research was limited by low room temperature conductivity. Typical structures such as Li₃MX₆ (M is a rare earth metal, X is F/Cl/Br/I) conduct ions through the vacancy mechanism formed by doping. Although fluorides have a wide electrochemical window, their conductivity needs to be improved and are still in the laboratory exploration stage.
Composite electrolytes
Composite electrolytes (CEs) combine the advantages of ceramic fast ion conductors and polymers:
Inorganic nanoparticles/polymers (INPCs)
Incorporating SiO₂, Al₂O₃ or active fillers (such as LATP) into the polymer matrix to improve mechanical strength and ion conduction. Small particle fillers are more conducive to ion path optimization.
Inorganic nanofibers/polymers (INFPCs)
Replacing particles with nanofibers (such as LLTO nanowires) reduces nodes and builds a three-dimensional conductive network. For example, the conductivity of PAN-LiClO₄-based composite materials can be significantly improved.
This technology makes up for the defects of a single material through “rigid-flexible combination” and is a key direction for balancing performance and process.
Mixed solid-liquid electrolytes
In order to address the pain points of low room temperature conductivity and poor interface contact of solid electrolytes, mixed solid-liquid electrolytes introduce a small amount of liquid components (such as gel electrolytes) to combine the high ion conductivity of liquids with the structural stability of solids. Liquid components can reduce interface resistance and optimize low-temperature performance, while the solid matrix inhibits dendrite growth and leakage risks, becoming a transitional solution to break through the bottleneck of industrialization.
Technology route comparison and trends
At present, the three major routes of oxides, sulfides, and polymers each have their own focus:
- Oxides: Advantages in chemical stability, suitable for high-safety scenarios;
- Sulfides: Good at conductivity, focusing on fast charging needs;
- Polymers: Simplify the process by relying on interface compatibility.
Enterprises generally adopt a multi-route parallel strategy, while composite electrolytes and mixed solid-liquid technologies accelerate the commercialization process through cross-system integration. In the future, material modification (such as nano-sizing, doping) and process innovation (such as low-temperature sintering, 3D printing) will be the key to breaking through performance and cost bottlenecks.
Although Ideal L6 is not equipped with all-solid-state batteries, it has achieved an excellent balance between safety and performance. While enhancing overall safety, its battery system has a single cell energy density of up to 368Wh/kg, helping the vehicle’s range to exceed 1,000 kilometers, fully meeting users’ long-distance travel needs. Not only that, Ideal L6’s fast charging performance is also impressive. It only takes 12 minutes of charging time to increase the range by 400km, greatly improving the efficiency of energy replenishment and bringing convenience to users. It is a high-quality compromise solution under current technical conditions.
Advantages and technical barriers of solid-state batteries
Revolutionary advantages of solid-state batteries
Solid-state batteries have ushered in a new era of energy storage. Compared with traditional liquid lithium-ion batteries, their advantages are not only the gradual improvement of performance, but also a disruptive technological breakthrough, which is reflected in six dimensions:
- Safety Leap: Completely abandon flammable liquid electrolytes, eliminate the risk of fire and explosion from the root, and completely solve the core safety hazards of liquid batteries.
- Energy density breakthrough: Can be adapted to lithium metal negative electrodes (theoretical capacity 3860mAh/g, 10 times that of graphite).
- Compact structural design (no liquid electrolyte management components are required), energy storage per unit volume is increased by more than 30%, and some semi-solid batteries have achieved a single cell energy density of 368Wh/kg.
- Significantly extended life: Solid electrolytes have stronger chemical stability and better cycle attenuation resistance than liquid systems. Under ideal conditions, the cycle life can reach more than 4,500 times, reducing the frequency of battery replacement and environmental burden.
- Wide temperature adaptability: In the range of -25℃ to 60℃, the fluctuation of ion conduction performance is less than 15%, breaking through the bottleneck of liquid battery’s low-temperature internal resistance surge and high-temperature thermal runaway, and is suitable for extreme scenarios such as polar scientific research vehicles and high-temperature industrial energy storage.
- Design freedom release: There is no restriction on liquid components, and the production of flexible batteries, ultra-thin batteries and other special-shaped structures can be customized, providing possibilities for innovative designs such as curved integration of wearable devices and integrated battery packs for automobile chassis.
These advantages are derived from the physical properties of solid electrolytes – through ion conduction rather than liquid diffusion, the technical synergy of “high safety, high energy, and high reliability” is achieved, making it the core direction of the next generation of energy storage.
Challenges of solid-state battery industrialization
Despite the promising prospects, solid-state batteries still need to overcome five major technical barriers for large-scale commercial use:
(1) Solid-solid interface impedance problem
Core problem: The rigid contact between the solid electrolyte and the electrode leads to interfacial voids. The volume change of the electrode during charging and discharging (such as the expansion rate of lithium metal reaches 400%) further tears the contact layer, and the interface resistance can rise sharply to more than 10³Ω・cm², far exceeding the 10²Ω・cm² of liquid batteries.
Solution: Develop a gradient composite interface layer (such as LiPON/Li₃PO₄ nano-transition layer), or introduce a gel-like intermediate phase to reduce contact stress.
(2) Large-scale manufacturing bottleneck
Process complexity: Sulfide electrolytes require inert atmosphere glove box production, and oxide electrolytes rely on high-temperature sintering above 1200℃, with a yield of only 60%-70%, and the cost is 3-5 times that of liquid batteries.
Breakthrough direction: Explore low-temperature printing technology (such as aerosol jet deposition) and roll-to-roll continuous production process, with the goal of reducing the investment in single GWh production capacity from 120 million yuan for liquid batteries to less than 80 million yuan.
(3) Material system cost dilemma
Dependence on precious metals: Elements such as lanthanum (used in LLZO garnet electrolytes) and germanium (used in LAGP sulfides) are expensive, and global reserves are concentrated (e.g. 80% of lanthanum resources are distributed in China), resulting in significant supply chain risks.
Alternative path: Promote the development of rare earth-free oxides (such as Li₂O₂-ZrO₂-Ta₂O₅ system) and organic-inorganic hybrid electrolytes, with the goal of reducing material costs to below $150/kWh.
(4) Mechanical brittleness and plating risk
Pain point of ceramic electrolytes: The fracture toughness of oxide/sulfide ceramics is only 1/10 of that of liquid electrolytes, and microcracks are easily generated during cycling, resulting in the growth of lithium dendrites along defects (the critical penetration thickness is only 50μm).
Innovative technology: Adopting a “rigid and flexible” composite structure – wrapping ceramic particles (such as PEO/LATP nanocomposites) with a polymer network to increase the fracture energy to more than 200J/m², close to the level of traditional diaphragms.
(5) Thermal management technology gap
The gap in heat dissipation efficiency: The thermal conductivity of solid electrolytes (0.1-0.5W/m・K) is only 1/5 of that of liquid electrolytes (1-2W/m・K). In high-power scenarios (such as 800V supercharging), local hot spots are easily formed (temperature differences can reach more than 20°C).
System-level solution: Develop a solid-state battery module with built-in microchannels, combining phase change materials (PCM) and graphene thermal conductive networks, with the goal of increasing the heat flux density to more than 500W/m².
Challenges and opportunities of solid-state batteries
Although solid-state batteries still have many problems to be solved, compared with the limitations of traditional liquid batteries, the advantages of solid-state batteries are very obvious. The challenges and opportunities of solid-state batteries at this stage can be summarized as follows:
| Challenges | Opportunities |
| Lower ionic conductivity -Reduced power density -Increased internal resistance -Limited high current applications | -Advanced electrode structure design -Pressure and temperature control -Introduction of functional additives -Use of mixed electrolytes -Development of nanostructured materials |
| Lower cycle life -Limited number of charge and discharge cycles -Shorter overall battery life -Cycle and calendar aging | -Development of composite ceramic/polymer electrolytes -Preparation of low ionic conductivity materials -Use of techniques such as electrolyte wetting to achieve uniform lithium deposition -Improvement of mechanical properties of materials, such as increasing shear modulus |
| Degradation of solid electrolytes -Li Plating growth -Side reactions -Crack formation and propagation -Oxygen vacancy formation -Electrolyte decomposition -Grain boundary migration -Interface instability -Volume change -Li deposition from metal anodes -Exposure to moisture and air | -Introduction of additives and dopants in the electrolyte -Development of the electrode-electrolyte interface with the help of suitable interlayers or coatings -Improvement of material microstructure to control grain boundary mobility -Control of charging conditions to reduce lithium deposition at the metal anode -Development of nanostructured electrode materials -Development of robust sealing technology -Development of solid electrolytes with high mechanical strength and chemical stability |
| Fast charging performance -Safety issues related to lithium dendrites -Stability and interfacial resistance -Decomposition of solid electrolytes (SSE) at high overpotentials -Strain and tension on electrodes -Polymerization-induced uneven diffusion of lithium ions (Li+) | -Formation of thin electrodes -Development of electrode coating technology -Use of materials with higher reduction stability -Research on multi-ion conductors that enable multiple ion conduction paths -Development of parallel cell configurations to reduce current density -Implementation of dynamic charging profiles |
| Low stability -Low thermal stability -Low chemical stability -Low reduction stability -Low oxidation stability | -Research on thermal barrier coatings -Implementation of efficient thermal management systems -Development of electrolyte-inert electrodes -Selection of electrode materials compatible with specific electrolytes -Application of passivation layers on electrodes |
| Manufacturing scalability -Production methods are complex and time-consuming -Limited large-scale production capabilities -Difficult to maintain quality consistency when producing on a large scale -High cost of electrolyte materials | -Improve processes and develop efficient processes to reduce production costs -Economies of scale in large-scale production -Research different low-cost coating and doping/mixing techniques -Improve quality during manufacturing |
Solid-state battery industry chain
The solid-state battery industry chain can be divided into upstream, midstream and downstream:
- Upstream: covers basic materials and equipment. In terms of basic materials, there are raw minerals, as well as positive and negative electrode materials and electrolytes used in the manufacture of battery cells; in terms of equipment, it involves various types of battery production equipment. These are the cornerstones of solid-state battery production.
- Midstream: Focus on battery pack processing and preparation. It not only includes battery packaging and integration, but also involves the design of power management systems, energy management systems and other solutions, aiming to create complete and efficient battery products.
- Downstream: It is a wide range of application areas. It covers multiple industries such as new energy vehicles, consumer electronics, energy storage, and power tools, providing power and energy storage support for various fields.
Future development of solid-state batteries
As a key track of future energy technology, solid-state batteries have extremely broad development prospects. Under the influence of the three major driving forces of technology, policy and market, the prospects for large-scale commercial application are promising.
Technological innovation breaks through bottlenecks
Continuous innovation in the fields of materials science and electrochemistry is gradually overcoming the technical difficulties of solid-state batteries. For example, by enhancing ion conduction through material composites and optimizing interfaces to reduce impedance, it is expected to significantly improve ion conductivity and fast charging performance, laying a solid technical foundation for its large-scale application.
Policy support and vigorous development
The Chinese government attaches great importance to the solid-state battery industry and has intensively introduced a series of support policies, from R&D subsidies to industrial planning, to create a clear orientation and a relaxed environment for the development of the industry, helping solid-state battery companies to obtain strong support in terms of funds, technology, market access, etc.
Market expansion demand surges
The rapid expansion of the new energy vehicle market and the blowout growth of energy storage demand have opened up a broad market space for solid-state batteries. With the increase in the penetration rate of new energy vehicles and the large-scale deployment of energy storage systems, the demand for high-safety and high-energy-density batteries will grow exponentially, and solid-state batteries are expected to seize the market high ground with their own advantages.