Advantages and Challenges of Solid-State Li-ion Batteries

There are lost of discussions and research on solid-state Li-ion batteries. What exactly are they? What advantages do they offer over traditional Li-ion batteries? And what challenges remain? Below is a brief analysis.

Concept

Lithium-ion batteries consist of four main components: the cathode, anode, electrolyte, and separator. Solid-state batteries replace the electrolyte and separator with a solid-state electrolyte. Therefore, the solid-state electrolyte must fulfill the functions of both these core components. To replace the electrolyte, it must serve as a pathway for lithium ions to migrate between the cathode and anode during charging and discharging. To replace the separator, it must prevent direct contact between the cathode and anode, thereby avoiding short circuits.

During lithium-ion battery charging and discharging, lithium ions must shuttle between the anode and cathode. This process primarily relies on lithium salts dissolved in liquid solvents. Continuing the warehouse-to-cargo analogy, this resembles transporting goods (lithium ions) between two warehouses via “water transport.” In contrast, lithium ions in solid-state electrolytes travel through solid crystals or amorphous structures, akin to goods transported via “land routes”. 

Advantages

What benefits does this design offer, by replacing liquid electrolyte and separator with solid electrolyte?

In terms of safety, solid-state lithium-ion batteries reduce the risk of combustion associated with liquid electrolytes and suppress lithium dendrite penetration. The liquid electrolytes in traditional lithium-ion batteries are often organic compounds, which are flammable (typically with flash points below 60°C). At high temperatures, they decompose and release flammable gases, contributing to fire spread during thermal runaway. Solid-state batteries utilize oxide or sulfide solid electrolytes with high decomposition temperatures reaching hundreds of degrees Celsius, eliminating the combustion hazard. Additionally, the mechanical strength of solid electrolytes significantly exceeds that of separators, effectively slowing lithium dendrite growth and reducing short-circuit risks. In liquid batteries, internal short circuits triggered by lithium dendrites piercing the separator are a primary cause of thermal runaway.

Even if thermal runaway occurs, solid-state batteries release heat at a much slower rate than liquid batteries due to the high decomposition temperature of solid electrolytes. This effectively creates a safety barrier against thermal runaway, buying critical time for system safety measures. Furthermore, the solid-state system avoids continuous side reactions between liquid electrolytes and electrode materials, reducing gas generation and internal pressure buildup, thereby lowering the probability of explosion. Second, solid-state batteries undergo significantly higher manufacturing pressures than liquid batteries to ensure tight contact between solid electrolyte particles and electrode materials. This inherently grants them superior resistance to external compression. Under various mechanical abuse conditions, solid-state batteries deliver more stable current output, whose failure modes typically involves localized melting rather than violent combustion.

In terms of energy density, the active materials are the cathode and anode in lithium batteries, which can convert chemical energy into electrical energy. While, the electrolyte and separator serve as auxiliary materials. From this perspective, previous articles have noted that solid-state batteries do not inherently affect energy density. However, since cathodes and anodes cannot be used independently, these auxiliary materials also influence energy density by affecting assembly and operational processes. Moreover, one key necessity for developing solid-state electrolytes lies in their potential to overcome the theoretical energy density limits of existing liquid lithium-ion batteries, manifested in the following aspects. As analyzed in previous articles, lithium metal represents a critical frontier for anode development. However, the painful lessons from lithium metal battery applications in the 1980s, which underscore addressing the safety concerns is essential for their practical implementation. Solid-state electrolytes can suppress lithium dendrite growth, making the direct use of metallic lithium anodes feasible.

From the cathode material perspective, while transition metal materials are already the optimal choice for cathodes, current cathode materials still fail to achieve their maximum energy density. Because, the limited oxidation resistance of liquid electrolytes restricts the charging cut-off voltage of cathode materials. Solid-state electrolytes possess strong oxidation resistance, enabling compatibility with high-voltage cathode materials such as high-nickel and lithium-rich manganese-based cathodes (>4.5V). This allows individual cell energy densities to exceed 400Wh/kg, representing a 30%-60% improvement over current mainstream liquid batteries.

Additionally, liquid batteries require excess electrolyte and thick separators for safety. Solid-state electrolytes, however, combine ion conduction with electron isolation, allowing significant reduction in inactive materials within the cell—further boosting energy density. For instance, by employing thin-film solid-state electrolytes combined with three-dimensional electrode structures, solid-state Li-ion batteries could achieve volumetric energy densities exceeding 1200 Wh/L, meeting the space-constrained demands of aerospace and similar applications.

Furthermore, solid-state electrolytes can be fabricated to conduct ions in precisely designed shapes. Since liquid electrolytes conduct ions throughout the entire cell, isolating electrolyte compartment must require necessitating separate chambers. By limiting ion conduction to a few specific layers, series connections between different electrode sheets within a single cell become feasible. This enables battery packs to achieve energy densities approaching those of individual cells.

Challenges

Despite significant hype, solid-state batteries have yet to gain widespread market adoption due to numerous unresolved challenges.

Regarding charge/discharge power, ion transport in solid electrolytes relies on crystal-internal pathways. This restricts ion movement compared to the free migration in liquid electrolytes, increasing internal resistance and reducing power output. Taking ionic conductivity (the rate of lithium-ion transport, analogous to the speed of “cargo” movement) as an example: conventional electrolytes typically range from 0.1–10 S/m, sulfide solid-state electrolytes fall within this range, while oxide solid-state electrolytes generally measure < 0.1 S/m. This explains why sulfide solid-state electrolytes remain the primary focus of research for major manufacturers and academic institutions.

Regarding cycle life, the contact interface between the electrode and solid-state electrolyte is susceptible to collapse under external pressure (such as vibration), blocking ion transport pathways. Repeated expansion and contraction during cycling further degrades interface stability. Additionally, whether electron or ion-depleted lack of transport pathways in cathode and anode materials, isolated particles may lose activity. In other words, poor interface stability ultimately leads to accelerated cycle degradation (2-3 times faster) under real-world vehicle conditions (temperature shocks / vibration), despite laboratory tests showing tens of thousands of cycles. This necessitates an extended engineering validation period.

Regarding cost, as mentioned earlier, improving the solid electrolyte interface requires continuously applying high pressure to the battery, which inevitably increases production equipment costs and reduces manufacturing efficiency. Additionally, the material and production costs for solid electrolytes (e.g., sulfide-based systems) are tens of times higher than those for liquid electrolytes. This high cost inherently limits market penetration during commercialization, resulting in disproportionately low market share relative to R&D investment. This mismatch between market demand and R&D intensity also dampens corporate willingness to invest in development.

Regarding safety, solid-state batteries are not without concerns. On one hand, numerous experiments indicate lithium dendrites can grow along solid electrolyte grain boundaries, meaning the risk of internal short circuits persists. On the other hand, achieving high energy density necessitates highly reactive materials like metallic lithium anodes and high-nickel cathodes. These materials release significantly more energy during thermal runaway than conventional batteries, partially negating the thermal stability advantages of solid electrolytes.