In recent years, the rapid development of electric vehicles has placed increasing demands on traction batteries, particularly in terms of energy density, safety, and cycle life. As a core component, the performance of traction batteries directly determines the driving range and reliability of vehicles. However, conventional liquid lithium-ion batteries face intrinsic safety hazards due to the flammability and leakage risks of organic liquid electrolytes, coupled with energy density limitations below 300 Wh/kg when using graphite anodes. These issues have become critical bottlenecks for industry advancement. Solid-state batteries, which replace liquid components with solid electrolytes, offer the potential to expand the intrinsic safety window and enable the use of high-energy-density materials such as lithium metal anodes and lithium-rich cathodes. This makes solid-state batteries an ideal choice for next-generation traction batteries. Despite these advantages, the rigid solid-solid interfaces formed between solid electrolytes and electrodes pose significant challenges, including insufficient contact area, hindered ion transport, interfacial side reactions, and accumulated mechanical stress during cycling. These factors lead to performance gaps between theoretical and practical values, with interface failures causing capacity decay and dendrite-induced short circuits, thereby hindering applications in automotive scenarios that require high-rate charging/discharging, long cycle life, and wide temperature adaptability. Therefore, understanding the microstructure, chemical reactions, and mechanical behavior of solid-solid interfaces is crucial for optimizing the performance of solid-state batteries.
The key distinction between solid-state batteries and traditional liquid lithium-ion batteries lies in the replacement of “solid-liquid interfaces” with “solid-solid interfaces.” Liquid electrolytes, primarily composed of flammable organic solvents, can be substituted with solid counterparts to significantly enhance safety and thermal stability. Additionally, this solidification opens up possibilities for employing high-energy-density materials. In lithium-ion batteries, interfaces serve as critical pathways for ion transport, directly influencing ion transport efficiency, electrochemical stability, and battery lifespan. Thus, it is essential to focus on the intrinsic characteristics of solid-solid interfaces in solid-state batteries, which include point contact properties, ion transport mechanisms, interfacial reaction behaviors, and mechanical changes under long-term cycling.
Solid-solid interfaces exhibit point contact characteristics due to the rigidity of solid materials. In porous electrodes commonly used in lithium-ion batteries, this results in limited actual contact area and significantly increased interfacial impedance, restricting efficient ion transport at the interface. Applying external pressure or using flexible materials, such as polymer-based electrolytes, can improve interface compatibility. For instance, solid-state batteries based on sulfide solid electrolytes typically require pressures around 20 MPa to maintain interface contact, while those with flexible polymer systems need 2–10 MPa. The incorporation of pressure application devices can affect the energy density of solid-state battery systems, making interface modulation to reduce pressure dependence a key development direction for solid-state batteries.
Ion transport in solid-state batteries differs from that in conventional liquid electrolyte systems. In liquid-based batteries, electrodes require good electronic conductivity, and ion transport relies on electrolyte penetration through porous electrodes. In solid-state batteries, the impermeability of solid electrolytes necessitates simultaneous consideration of ion transport within electrodes, requiring the electrode region to possess ionic conductivity. This is often achieved by coating electrode particles with solid electrolytes to build local ion conduction networks. For example, using oxide electrolytes like LLZO (Li7La3Zr2O12) or novel halide electrolytes as cathode coatings can significantly improve ion transport efficiency and reduce interfacial impedance.
Interfacial reaction mechanisms in solid-state batteries also diverge from those in liquid systems. In traditional lithium-ion batteries, solid-liquid interface side reactions primarily involve electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) on the anode surface. While the SEI layer can inhibit further side reactions, its high impedance adversely affects rate performance, and its stability directly impacts cycle life. Solid-state battery systems largely eliminate organic solvents, mitigating issues related to SEI formation. However, new forms of interfacial side reactions emerge, such as electrolyte decomposition due to narrow electrochemical windows, oxidation of sulfide electrolytes at high voltages, and reduction reactions between oxide electrolytes and lithium metal. These can increase interfacial impedance. Nonetheless, solid-state batteries offer advantages in controllability of interfacial reactions; by optimizing interface materials and structures, excessive growth of interfacial films can be suppressed, leading to significantly lower impedance compared to traditional batteries and enhancing performance in high-rate and low-temperature environments.
Mechanical changes at interfaces under long-term cycling present another challenge. Similar to liquid lithium-ion batteries, solid-state batteries experience “breathing effects” with repeated expansion and contraction during cycling. Since all components are rigid, volume changes in electrodes cause cumulative mechanical stress at interfaces. This stress can lead to interface cracking, contact deterioration, or even delamination, severely affecting battery performance. For instance, volume changes in lithium metal anodes during charging/discharging can promote lithium dendrite growth, increasing short-circuit risks. Silicon-based anodes and ternary cathodes also exhibit significant volume expansion, resulting in irreversible stress accumulation that damages electrolyte layers and shortens battery life. Introducing elastic interlayers or using buffer-capable electrolyte materials can alleviate mechanical issues at solid-solid interfaces by reducing stress concentration.
The degradation processes at solid-solid interfaces in all-solid-state batteries involve lithium ions crossing interfaces through solid electrolytes during charge-discharge cycles, accompanied by volume changes, phase transitions, and inter-component reactions. These processes lead to characteristic degradation mechanisms, primarily categorized into interface contact degradation and interfacial side reaction degradation.
Interface contact degradation stems from mechanical stress concentration between electrodes and solid electrolytes, causing interface fracture, separation, and crack formation. During cycling, volume changes in electrode materials induce stress concentration, promoting crack initiation and propagation. In situ X-ray computed tomography (XCT) observations of silicon anodes and sulfide electrolyte interfaces reveal stress-driven vertical “mud-crack” patterns during delithiation, with some cracks closing upon lithiation. Studies show that the microstructure of electrode materials significantly influences interface cracking; comparing Li6PS5Cl electrolytes with different particle sizes indicates that smaller particles enhance microstructural homogeneity in electrode composites, effectively mitigating stress accumulation from silicon particle expansion/contraction and reducing interface debonding and crack formation. Factors affecting interface contact degradation also include material elastic modulus, interface bonding strength, and compatibility between electrodes and electrolytes. Optimizing these parameters can improve mechanical stability and long-term cycling performance.
Interfacial side reaction degradation involves chemical reactions and ion migration at electrode-solid electrolyte interfaces, including lithium dendrite formation, element diffusion, and space charge layer effects. Lithium dendrite growth is a critical challenge in solid-state batteries. Research indicates that dendrite formation is influenced by interface structure and ion transport behavior. Non-uniform ion migration at interfaces increases local current density, promoting dendrite growth. The electronic conductivity of solid electrolytes also plays a role; for example, high electronic conductivity in LLZO and Li3PS4 can lead to lithium ion reduction within the electrolyte, forming dendrites. Element interdiffusion at solid-solid interfaces, such as Co, P, and S diffusion between LiCoO2 cathodes and Li2S-P2S5 solid electrolytes, forms new compounds or amorphous layers, altering interface structure and degrading performance. Space charge layers arise from chemical potential differences between electrodes and electrolytes, causing ion redistribution and charge accumulation. At sulfide solid electrolyte-oxide cathode interfaces, lithium ion migration from the electrolyte to the active material forms a space charge layer, triggering chemical reactions and structural changes. Although typically nanoscale in thickness, the associated local electric field impedes lithium ion migration, significantly increasing interfacial impedance. Thus, high-resolution characterization is needed to observe ion distribution and interface electric fields, while optimization requires matching electrode and electrolyte chemical potentials to reduce differences.
Advanced characterization techniques are essential for understanding solid-solid interface properties in solid-state batteries. These include electrochemical analysis, non-destructive testing, disassembly and cross-section analysis, and online sensing technologies, each offering unique insights into interface evolution and degradation mechanisms.
Electrochemical analysis techniques, such as electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV), are widely used to study electrochemical behavior, reaction kinetics, and performance degradation at solid-solid interfaces. In solid-state batteries, multiple solid-solid interfaces contribute to complex impedance characteristics, making it challenging to isolate individual interface contributions from EIS data. The distribution of relaxation times (DRT) method addresses this by inverting frequency responses to decouple multiple interface effects into distinct components, providing separate impedance spectra for each interface. DRT analysis has been applied to decouple charge transfer resistance and double-layer capacitance at solid-solid interfaces, revealing that electrode-electrolyte interactions at high voltages significantly increase impedance. CV methods assess interface reaction reversibility; for instance, sulfide solid electrolytes with narrow electrochemical windows require cathode coatings to prevent oxidation, and CV curve changes can identify irreversible reactions causing capacity decay. However, electrochemical techniques primarily provide information on electrochemical behavior and lack direct insight into microscopic structural changes, necessitating combination with other interface characterization methods.
Non-destructive testing techniques, including XCT and ultrasonic scanning, enable real-time monitoring of solid-solid interface changes without battery destruction. XCT reconstructs 3D internal structures through multi-angle X-ray scans, allowing observation of interface cracks, voids, and defects at micro- or nanoscale. In solid-state batteries, XCT has shown that interface crack propagation during cycling contributes to capacity fade and that dendrite growth varies with charge rates, affecting performance. Ultrasonic techniques use sound wave reflection and transmission to detect internal defects, assess stress distribution, evaluate interface adhesion, and monitor gas evolution. Studies using ultrasonography have revealed stress inhomogeneity and local delamination at interfaces after long-term cycling, as well as gas accumulation from electrolyte decomposition exacerbating microcrack expansion. While non-destructive techniques offer valuable insights, they often lack precise quantitative analysis for parameters like stress distribution and cannot directly reveal elemental composition or chemical reactions, limiting comprehensive understanding of interfacial physiochemical processes.
Disassembly and cross-section analysis techniques provide direct structural and compositional data on solid-solid interface layers. Due to strong physical bonding under external pressure in solid-state batteries, disassembly often involves cross-sectioning for analysis. Scanning electron microscopy (SEM) visualizes interface morphology, revealing cracks and delamination, while energy-dispersive X-ray spectroscopy (EDS) analyzes elemental distribution across cross-sections. SEM-EDS studies have identified element diffusion at electrode-electrolyte interfaces, especially under high voltages, confirming interfacial side reactions. Atomic force microscopy (AFM) probes mechanical properties such as adhesion strength, hardness, and elastic modulus at nanoscale, assessing micro-deformation and stress state changes during cycling. Techniques like X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyze reaction products at interfaces, informing component optimization. Transmission electron microscopy (TEM) and related electron-based methods enable direct analysis of space charge layers; for example, phase-shift electron holography measures potential distribution to determine electric field strength and gradients, while differential phase contrast in scanning transmission electron microscopy (DPC-STEM) maps net charge density. Electron energy loss spectroscopy (EELS) visualizes lithium ion distribution, highlighting ion enrichment or depletion in space charge regions.
Online sensing technologies represent an emerging direction for real-time monitoring of solid-solid interface states in solid-state batteries, particularly for tracking structural evolution, stress changes, and dendrite growth. Optical coherence tomography (OCT), a non-invasive high-resolution imaging technique, has been used for in situ characterization, capturing dendrite formation and evolution at electrode-electrolyte interfaces during charge-discharge cycles, with results consistent with SEM and XPS. Fiber Bragg grating (FBG) sensors, known for high sensitivity, electromagnetic interference resistance, and small size, monitor internal stress and strain by embedding them in battery structures, revealing volume change effects on interface stability. The application of online sensing technologies promises powerful tools for understanding dynamic interface behavior during battery operation.
Future directions for interface modification in solid-state batteries focus on enhancing stability through material design and process optimization. Composite electrolytes, combining inorganic and polymer systems, offer broader electrochemical stability windows, higher ionic conductivity, and improved mechanical flexibility, thereby enhancing solid-solid interface stability. Strategies include using passive fillers, active fillers, and 3D framework structures to optimize ion transport paths and reduce interfacial impedance. Functional additives play a key role in stabilizing interfaces; for instance, trimethylsilyl compounds in sulfide electrolytes form stable cathode electrolyte interphase (CEI) layers during charging, suppressing side reactions and extending cycle life. Additives like LiBODFP and LiDFOB serve as interface stabilizers for cathodes and anodes, respectively, and their combination in double-layer PEO-based composite electrolytes significantly improves cycling performance and capacity retention. Manufacturing process optimization is critical for interface contact quality and stability. Replacing traditional cold pressing with novel processes can alleviate poor contact issues, reduce interfacial impedance, and enhance rate capability and low-temperature performance. Future research should deepen understanding of interface reaction mechanisms, develop new materials and processes, and advance toward high-performance, long-life solid-state batteries.
In summary, solid-state batteries hold great promise for future energy storage due to their potential for high energy density and safety. However, the complexity and instability of solid-solid interfaces remain major challenges. This article has reviewed advanced characterization techniques, including electrochemical analysis, disassembly and cross-section analysis, non-destructive testing, and online sensing, to analyze interface evolution and inform battery design. Future testing technologies will evolve toward in situ, high-throughput, intelligent, and real-time monitoring. Through interdisciplinary innovation, solid-state batteries can progress toward industrial application in vehicles.
| Characteristic | Liquid Electrolyte Batteries | Solid-State Batteries |
|---|---|---|
| Interface Type | Solid-Liquid | Solid-Solid |
| Contact Area | High (liquid penetration) | Low (point contact) |
| Ion Transport | Through liquid electrolyte | Through solid electrolyte and interfaces |
| Interfacial Reactions | SEI formation | Space charge layers, element diffusion |
| Mechanical Stability | Moderate (buffered by liquid) | Low (stress accumulation) |
| Typical Pressure Requirement | Low | 2–20 MPa |
The ionic conductivity in solid electrolytes can be described by the Arrhenius equation: $$\sigma = A \exp\left(-\frac{E_a}{kT}\right)$$ where $\sigma$ is the ionic conductivity, $A$ is the pre-exponential factor, $E_a$ is the activation energy, $k$ is Boltzmann’s constant, and $T$ is the temperature. This equation highlights the temperature dependence of ion transport in solid-state batteries, which is crucial for interface design.
For interfacial impedance, the total resistance $R_{\text{total}}$ in a solid-state battery can be expressed as the sum of contributions from various interfaces: $$R_{\text{total}} = R_{\text{anode}} + R_{\text{cathode}} + R_{\text{bulk}}$$ where $R_{\text{anode}}$ and $R_{\text{cathode}}$ represent resistances at anode and cathode interfaces, and $R_{\text{bulk}}$ is the bulk resistance of the electrolyte. Techniques like EIS and DRT help decompose these components to identify degradation sources.
| Technique | Application | Advantages | Limitations |
|---|---|---|---|
| Electrochemical Impedance Spectroscopy (EIS) | Interface impedance analysis | Non-destructive, in situ capability | Complex data interpretation |
| Cyclic Voltammetry (CV) | Reaction reversibility assessment | Simple, direct kinetic information | Limited to electrochemical behavior |
| X-Ray Computed Tomography (XCT) | 3D structure imaging | High resolution, real-time monitoring | Limited chemical information |
| Ultrasonic Testing | Stress and defect detection | Non-destructive, mechanical insights | Quantitative challenges |
| Scanning Electron Microscopy (SEM) | Morphology observation | High spatial resolution | Requires sample preparation |
| Transmission Electron Microscopy (TEM) | Atomic-scale structure analysis | Direct imaging of interfaces | Complex operation, expensive |
| Online Sensing (e.g., OCT, FBG) | Real-time state monitoring | Continuous data, embedded capability | Integration complexity |
The space charge layer effect at interfaces can be modeled using the Poisson-Boltzmann equation: $$\frac{d^2\phi}{dx^2} = -\frac{\rho}{\epsilon}$$ where $\phi$ is the electric potential, $\rho$ is the charge density, and $\epsilon$ is the permittivity. This equation describes the potential distribution across interfaces, influencing ion transport and stability in solid-state batteries.
In conclusion, the development of solid-state batteries relies heavily on understanding and optimizing solid-solid interfaces. Through advanced characterization and innovative modification strategies, such as composite electrolytes and functional additives, the performance and reliability of solid-state batteries can be enhanced for widespread adoption in electric vehicles and other applications.