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In-depth Analysis of Solid-State Batteries: Technology, Advantages and Future Prospects

2025-5-8

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In-depth Analysis of Solid-State Batteries: Technology, Advantages and Future Prospects
At present, the energy density of traditional liquid lithium batteries has approached the theoretical limit of 350Wh/kg, while they are also facing safety hazards such as battery thermal runaway. With the rapid expansion of the new energy vehicle market, the urgent demand for high energy density and high safety of power batteries has driven the research and development and progress of solid-state batteries.

The development of solid-state batteries can be divided into stages such as semi-solid, quasi-solid, and all-solid-state. Although all-solid-state batteries still need some time to be industrialized due to the immaturity of material technology, preparation technology and high production costs, semi-solid-state batteries have become an ideal transitional technology at present.

In 2023, the domestic shipment volume of semi-solid batteries exceeded the GWh level, indicating that 2024 will usher in a brand-new stage of large-scale production and vehicle installation. Looking ahead, with the continuous advancement of solid-state battery technology and the gradual decline in costs, especially the launch of domestic semi-solid-state battery industrialization, the solid-state battery market is set to experience rapid growth.

Next, we will delve into the concept, working principle of solid-state batteries and their significant advantages over traditional liquid lithium batteries.

2. The significant advantages of solid-state batteries
Solid-state batteries stand out with their high energy density and high safety, and are regarded as the leaders of the next generation of high-performance lithium batteries. From the perspective of performance comparison, solid-state batteries outperform traditional liquid batteries in multiple aspects such as ionic conductivity, energy density, high voltage resistance, high temperature resistance, and cycle life. It successfully combines the high energy density and high safety features that liquid lithium batteries cannot possess simultaneously, making it an ideal choice for fields such as electric vehicles. Its advantages are mainly reflected in the following five aspects:

Outstanding security

Liquid lithium batteries are at risk of thermal runaway. Factors such as overcharging, impact, short circuit or water immersion may all trigger this risk. When the temperature rises to 90°C, the SEI film on the surface of the negative electrode will start to decompose. The lithium-embedded carbon reacts with the electrolyte and generates heat and flammable gas, which may cause the separator to melt and form an internal short circuit. As the temperature rises further above 200°C, the vaporization and decomposition of the electrolyte may cause the battery to burn fiercely or even explode.

In contrast, solid-state batteries are superior in terms of safety due to their five major safety features. Firstly, the high mechanical strength of solid electrolytes can effectively inhibit the growth of lithium dendrites, thereby reducing the risk of short circuits. Secondly, it is not easy to catch fire or explode, ensuring the safety of use. In addition, solid-state batteries also avoid continuous interfacial side reactions, electrolyte leakage, and drying out, and can maintain stable performance or even better performance in high-temperature environments.
(2) High energy density

The energy density of traditional liquid batteries has approached the theoretical upper limit of 350Wh/kg. However, solid-state batteries, with their broad electrochemical window, can withstand high voltages exceeding 5V, thereby expanding the range of material options. The energy density of a battery is proportional to the working voltage and specific capacity, but the overall specific capacity is limited to the lower electrode among the positive and negative electrodes. At present, the specific capacity of graphite anodes in solid-state batteries is 372mA•h/g, the theoretical specific capacity of silicon-based anodes is as high as 4200mA•h/g, and the theoretical specific capacity of lithium metal anodes also reaches 3860mA•h/g. All these significantly exceed the specific capacity of cathodes. Therefore, cathode materials have become the main obstacle to further improving the performance of lithium-ion batteries. However, the emergence of all-solid-state electrolytes has made it possible to be compatible with high specific capacity anodes and conventional cathode materials, and at the same time, they can also be matched with high specific capacity cathode materials, thus potentially achieving an energy density of 500Wh/kg or even higher.
(3) Wide temperature application range

Compared with traditional liquid batteries, solid-state electrolyte batteries show significant advantages in temperature applicability. Because solid electrolytes have no low-temperature solidification problem and perform well at high temperatures, their operating temperature range is wide, reaching -40°C to 150°C, which is far superior to liquid batteries.
(4) Compact size

In traditional liquid batteries, the separator and electrolyte occupy a considerable amount of space and mass. However, in solid-state batteries, these components are replaced by solid electrolytes, significantly reducing the distance between the positive and negative electrodes, which is usually only a few to a dozen micrometers. This innovative design significantly reduces the thickness of the battery, thus making the solid-state battery more compact in volume under the same power demand.

3. The development path of solid-state batteries
As the proportion of liquid electrolyte gradually decreases, the development of solid-state batteries can be divided into different stages such as semi-solid (5-10wt%), quasi-solid (0-5wt%), and all-solid (0wt%). It is worth noting that the electrolytes used in both semi-solid and quasi-solid batteries are of the solid-liquid hybrid type.

At present, all-solid-state batteries are still in the research and development and trial production stage worldwide. The main challenges in its industrialization lie in the maturity of materials and preparation technologies, as well as the relatively high production costs. Experts predict that it will take at least another five years for all-solid-state batteries to achieve large-scale commercialization.

During this period, semi-solid batteries might become an effective transitional technology. It combines the advantages of solid-liquid mixed electrolytes, with the electrolyte content controlled between 5% and 10%, while increasing the application of coated solid electrolytes. This makes semi-solid batteries similar to traditional liquid lithium batteries in terms of electrochemical principles, thus enabling full utilization of existing mature manufacturing processes. In addition, semi-solid batteries have also achieved significant performance improvements, including better safety, energy density, flexibility, as well as longer cycle life and operating temperature range. Therefore, semi-solid batteries are expected to become an important bridge in the transition from liquid batteries to all-solid batteries. In fact, in 2023, many enterprises have already started to build the production capacity of semi-solid batteries, indicating that they are about to enter the commercial production stage.

4. Three major technical routes of solid-state batteries
There are three mainstream technical routes in the field of solid-state batteries: polymer solid-state batteries, oxide solid-state batteries, and sulfide solid-state batteries. The core difference of these technical routes lies in the different solid electrolytes adopted. Solid electrolytes can mainly be classified into polymer electrolytes, oxide electrolytes and sulfide electrolytes. Among them, polymer electrolytes belong to the organic category, while oxide and sulfide electrolytes belong to inorganic electrolytes.

Ideally, solid electrolytes should possess high ionic conductivity, chemical and electrochemical stability towards lithium metal, the ability to effectively suppress the formation of lithium dendrites, low manufacturing cost, and no need for the use of rare metals. However, at present, these three major technical routes still face challenges to varying degrees in meeting these requirements. Overall, sulfide electrolytes have shown the greatest development potential in the application of all-solid-state batteries.

Polymer electrolytes are renowned for their easy processing characteristics, are highly compatible with existing electrolyte production equipment and processes, and also have excellent mechanical properties. However, its disadvantages include a relatively low ionic conductivity and the need to operate normally at a higher temperature. It has insufficient chemical stability, cannot adapt to high-voltage cathode materials, and there is a risk of combustion in high-temperature environments. In addition, its electrochemical window is relatively narrow. When the potential difference is too large (exceeding 4V), the electrolyte is prone to electrolysis, thereby limiting the upper limit of the polymer's performance.

Oxide electrolytes are renowned for their excellent electrical conductivity and stability. Their ionic conductivity is higher than that of polymers, and their thermal stability can reach up to 1000℃. Moreover, they exhibit good mechanical and electrochemical stability. However, compared with sulfides, its ionic conductivity is slightly insufficient, which to some extent limits the performance improvement of oxide solid-state batteries. In addition, the hardness of oxide electrolytes is relatively high, which leads to problems in interface contact of solid-state batteries. Simple room-temperature cold pressing processes are difficult to meet the working requirements of batteries.
Sulfide electrolytes have shown the greatest development potential in all-solid-state batteries. They have the highest ionic conductivity and possess excellent mechanical properties as well as a wide electrochemical stability window (exceeding 5V). However, it also has some shortcomings, including the issue of interface stability, which is prone to side reactions with the positive and negative electrode materials, resulting in high interface impedance and increased internal resistance. In addition, in terms of the manufacturing process, the production of sulfide solid-state batteries is relatively complex, and the reaction of sulfides with water and oxygen in the air may produce highly toxic hydrogen sulfide gas.

Polymer electrolytes have developed rapidly in terms of technological maturity and commercial application, and have achieved small-scale mass production. Despite this, polymer electrolytes still have disadvantages such as low electrical conductivity, which limits their upper performance limit and thus have not yet been widely promoted.

Oxide electrolytes exhibit balanced performance and are currently making rapid progress. Although its ionic conductivity is lower than that of sulfide electrolytes, it shows good thermal stability and mechanical properties. However, how to maintain the high stability of oxide electrolytes remains an unsolved problem.

In conclusion, making breakthroughs in the key technologies of solid electrolytes is expected to promote the industrialization process of the entire industry.

02
Challenges and Coping Strategies in the Industrialization Process
Technical challenges and countermeasures

The development of solid electrolytes has encountered three core scientific challenges: the ion transport mechanism of solid electrolytes, the lithium dendrite growth mechanism of lithium metal anodes, and the out-of-control failure mechanism in multi-field coupling systems. The resolution of these issues is an indispensable link for the creation of new solid-state electrolyte materials, the optimization of the physical and chemical properties of solid-state batteries, and the overall development of solid-state batteries.
Solid-state battery electrolytes are facing the challenge of performance balance. From the perspective of material properties, whether it is polymers, oxides or sulfides, as solid electrolytes, each has its own shortcomings. For instance, polymer electrolytes are favored for their ease of processing and low production difficulty, but their ionic conductivity is relatively low, which affects their charging and discharging performance. In contrast, although oxide and sulfide electrolytes have excellent electrical conductivity, safety and mechanical strength, their manufacturing difficulty and cost are relatively high.
Solution idea: A composite electrolyte that integrates the advantages of multiple materials. To overcome the performance bottleneck of solid-state battery electrolytes, an effective solution is to adopt composite electrolytes, that is, to combine different types of materials for use. This strategy aims to fully leverage the advantages of each material to achieve an overall improvement in performance. For instance, polymer/polymer composite electrolytes not only have stronger preparability, but also have improved mechanical strength and ionic conductivity. On the other hand, the composite electrolyte of polymers/inorganic substances (such as oxides or sulfides) combines the flexibility of polymers with the excellent properties of inorganic substances, thereby achieving a comprehensive integration of multiple advantages such as high strength, high flexibility, high electrical conductivity, and easy preparation. Therefore, composite solid-state electrolytes are regarded as an important development direction in the field of solid-state battery electrolytes.

However, all-solid-state batteries still face problems such as slow charging and discharging speeds and rapid capacity attenuation. Ionic conductivity is a key factor affecting the charging and discharging speed of all-solid-state batteries, and the ionic transport performance in solid-state electrolytes is jointly influenced by bulk phase and surface-interface transport processes. Compared with liquid electrolytes, the interionic interaction forces in solid electrolytes are stronger, resulting in a significant increase in the ion migration energy barrier and subsequently a decrease in ionic conductivity.

Furthermore, although solid electrolytes with high mechanical strength can inhibit the growth of lithium dendrites to a certain extent, the problem of uniform deposition of lithium metal has not been completely solved. Studies have shown that inorganic solid electrolytes with high shear modulus are also difficult to completely prevent the penetration of lithium dendrites in solid electrolytes. Therefore, the lithium dendrite problem remains a key challenge hindering the practical application of all-solid-state batteries.

In addition, the reduction in stability caused by solid-solid interface contact is also one of the main reasons for battery failure. In solid-state batteries, due to the solid-solid contact interface, the electrical conductivity is often severely affected by the high contact resistance at the interface between the electrode and the electrolyte. This high impedance not only increases the overpotential, but also leads to capacity attenuation and a decrease in energy density. The main sources of interfacial impedance include the interface issues between the solid electrolyte and the negative electrode, the interface issues with the composite positive electrode, and the microscopic interface issues between the positive electrode active material and the solid electrolyte inside the composite positive electrode.
Solution approach: Conduct interface engineering and modification from the two dimensions of materials and processes. In terms of the material dimension, Li alloy with small volume change was selected as the anode to alleviate the problem of anode expansion. Meanwhile, solid electrolytes with higher stability are adopted to reduce interfacial side reactions. For composite cathodes, surface coating (coating) is adopted to reduce interfacial stress and enhance the efficiency of ion and electron transport. In terms of the process dimension, the interface contact between the solid electrolyte and the electrode can be improved by increasing the preparation pressure, eliminating pores, enhancing interface contact, or by using in-situ solidification technology to inject liquid and heat it to solidify.
In addition, solid-state batteries still face challenges in terms of economy. Due to the imperfect supply chain of raw materials for solid-state batteries and the incomplete battery manufacturing equipment, the manufacturing cost is relatively high. Meanwhile, the electrode materials required for solid-state batteries are all high-tech new materials, which are difficult to produce and expensive. Therefore, technological progress and the market's digestion of the high price are needed to promote its wide application. At present, although there are a few commercial sales examples of solid-state batteries, on the whole, the cost of solid-state batteries is still much higher than that of traditional liquid lithium batteries.
Solution approach: Starting with semi-solid batteries, gradually achieve large-scale production to reduce costs. Given that semi-solid battery technology is relatively mature and closer to traditional liquid lithium-ion batteries, its industrialization is expected to lead to an increase in the production capacity of solid electrolytes, a reduction in raw material costs, and an optimization of processes. These factors working together will continuously reduce the raw material and production costs of semi-solid batteries.

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