- Ionic Transport Mechanism
High ionic conductivity is key to improving the charging and discharging speed of all-solid-state batteries: The ionic conductivity of solid electrolytes and the multi-scale interfacial properties of solid-state batteries jointly determine the electrochemical performance of solid-state batteries. In comparison, ion migration at the interface of solid-state batteries is relatively slow, which is also the key to improving electrochemical performance. The main application bottlenecks currently faced by solid-state batteries include slow charging and discharging speeds and rapid capacity decay, which are closely related to the physicochemical properties of solid electrolytes. Unlike liquid electrolytes, strong interionic forces and high migration barriers in solid electrolytes (more than 10 times that of liquids) lead to low ionic conductivity. Therefore, clarifying the conditions for high ionic conductivity is key to developing high-performance solid electrolytes and improving the charging and discharging speed of all-solid-state batteries.
Ion transport performance depends on the transport speed at the interface: The ionic transport performance in solid electrolytes is determined by the transport process of ions in phases and interfaces. In polycrystalline solid electrolytes, interfacial ion transport (grain boundary and intergrain boundary ion transport) is considered the rate-limiting step in the ion transport process. However, current research on the structural composition and transport mechanism of interfaces is insufficient, requiring the industry to continue developing more advanced characterization techniques and computational methods to deeply study lattice dynamics and interfacial ion transport mechanisms.
At present, methods such as doping, developing nano-scale structures, and interfacial engineering are mainly used to improve ionic conductivity: Recent research has also found that the optimization of bulk phase conductivity can be achieved by controlling crystal structure characteristics, such as lattice volume, transport bottleneck size, lattice distortion, defects, etc. Overall, the current industry's understanding of the ion transport mechanism is far from sufficient, and there are significant differences in the ion transport mechanisms of different solid electrolyte systems, which still require a detailed and comprehensive study of the ion transport process to reveal the ion transport mechanisms that can be used in various solid electrolyte systems.
- Lithium Dendrite Growth Mechanism
Lithium dendrites growing inside the battery are prone to cause safety risks: Although solid electrolytes have high mechanical strength, they still find it difficult to completely suppress the growth of lithium dendrites and achieve uniform deposition of lithium metal. Lithium metal may form dendrites on the surface of the anode and even nucleate inside the solid electrolyte, leading to a short circuit in the battery and thus posing safety risks.
According to the famous Monroe and Newman model, in lithium metal battery systems based on polymer electrolytes, lithium dendrite growth can be suppressed when the shear modulus of the solid electrolyte is more than twice that of the lithium metal. Based on this theory, inorganic solid electrolytes with high shear modulus are considered to effectively solve the dendrite problem of lithium metal anodes. However, for inorganic solid electrolytes with high shear modulus, lithium dendrites are also prone to form during cycling under limited current density.
Additives and structural design can inhibit the growth of lithium dendrites: For polymer solid electrolytes, their soft characteristics make it difficult to prevent the formation of dendrites, but the formation of lithium dendrites can also be improved by increasing ionic conductivity, adding inorganic fillers, and adding additional polymers; for inorganic solid electrolytes, the formation of lithium dendrites can be inhibited by changing microstructural defects, increasing relative density, reducing electronic conductivity, and managing current density.
- Solid-Solid Interface Issues
Solid-solid interface issues directly affect the cycle life and other performance of solid-state batteries: In most cases, the contact between the solid-solid interface is point contact with a small contact area. In some battery systems, the interface may initially be in surface contact, but as the battery cycles, the electrode material inevitably undergoes volume expansion, which can deteriorate the originally good contact and increase the interface impedance, causing the battery performance to continue to deteriorate. At the same time, continuous stress accumulation may also lead to the formation of micro-cracks in the cathode and solid electrolyte layer, worsening the contact between the cathode and the electrolyte, and accelerating the battery performance decay.
The solid electrolyte and metallic lithium undergo electrochemical reactions under external potential, and the contact interface between the solid electrolyte and lithium metal is usually fragile, with a relatively large contact resistance. If the interface is unstable, it may cause intense interfacial reactions, leading to rapid degradation of interfacial performance. In contrast, in liquid electrolyte systems, a dynamic SEI layer forms on the surface of lithium metal, which can alleviate side reactions between the electrolyte and lithium metal to a certain extent while maintaining lithium ion conductivity. In addition, liquid electrolytes have good contact and wetting properties, which can self-repair or re-form the SEI layer to a certain extent, adapting to changes in the surface morphology during lithium metal deposition and making the formation and growth of lithium dendrites more easily controlled, because under the action of liquid electrolytes, lithium can be more uniformly deposited.
Solid-state is more prone to thermal runaway due to interface issues: Once cracks form in the solid electrolyte or the contact with lithium metal is poor, it is not like the liquid electrolyte that can form an SEI membrane and has self-healing properties, which is more likely to lead to the breakage of lithium ion transport channels, forming lithium dendrites. The continuous growth of dendrites may penetrate the electrolyte, causing a short circuit in the battery, generating a large amount of heat, and increasing the temperature. High temperatures may cause the cathode to decompose, and high-capacity ternary cathode materials may produce oxygen during thermal decomposition, which can react with lithium metal anodes, causing exothermic reactions and further increasing the battery temperature and thermal runaway.
Interface engineering and modification can effectively solve solid-solid interface issues: In response to the solid-solid interface issues of solid-state batteries, the current mainstream approach is to improve through interface engineering and modification, achieving improvements in two dimensions: materials and processes. 1) Material dimension: Choose Li metal anodes with smaller volume changes and coated composite cathodes. 2) Process dimension: For macroscopic interface issues, increase the pressure during the preparation process to eliminate pores and enhance interface contact.
All-solid-state batteries are expected to start mass production and vehicle installation in 2027: Since 2022, the development and industrialization of solid-state batteries have made significant progress, especially with the mass production and vehicle installation of semi-solid-state batteries represented by Chinese companies such as Weilan New Energy and Ganfeng Lithium, marking the industrialization of semi-solid-state batteries. We expect the large-scale production and vehicle application of solid-state batteries to start around 2027. The market size of solid-state batteries will exceed 250 billion yuan in 2030: It is expected that by 2030, the global shipment of solid-state batteries will reach 614.1GWh, with an estimated penetration rate of about 10% in the overall lithium battery market, and its market size will exceed 250 billion yuan. The main shipped batteries will still be semi-solid-state batteries. As the most important part of solid-state batteries, the demand for solid electrolytes in 2030 will exceed 60,000 tons, with a market size of over 6 billion yuan.
