With the rapid development of solid-state batteries toward higher energy density and enhanced safety, there is an increasing demand for real-time monitoring of complex internal interface evolution and failure behaviors. Traditional sensing techniques struggle to adapt to the rigid interfaces and solid-solid contact characteristics of solid-state batteries, posing challenges in原位 detection sensitivity and compatibility. In this review, we explore the latest advancements in ultrasound and optical fiber sensing technologies for solid-state batteries. Ultrasound technology, leveraging its non-destructive testing advantages, enables dynamic analysis of electrode-electrolyte interface contact states, crack evolution, and gas generation processes. Optical fiber sensing, through integration with electrodes or electrolytes, allows for real-time monitoring of internal stress distribution, temperature gradients, and changes in chemical byproducts, offering new insights for interface optimization and failure mechanism studies. We systematically summarize the application status of these two sensing technologies in solid-state batteries from three aspects: interface characteristics, mechanical properties, and physicochemical properties, and discuss future development opportunities and key challenges.
Solid-state batteries represent a significant advancement in energy storage technology, offering improved safety and higher energy density compared to conventional lithium-ion batteries. The use of solid-state electrolytes eliminates flammable liquid components, reducing the risk of thermal runaway. However, the inherent challenges of solid-solid interfaces, such as poor contact, dendrite growth, and mechanical stress, necessitate advanced monitoring techniques. Ultrasound and optical fiber sensing have emerged as powerful tools for in-situ characterization, providing real-time data on internal processes without compromising battery integrity. In this article, we delve into the principles, applications, and future prospects of these technologies in the context of solid-state batteries.
Fundamental Principles of Ultrasound and Optical Fiber Sensing
Ultrasound technology relies on high-frequency mechanical waves (typically above 20 kHz) to interrogate material properties. The interaction of these waves with internal structures provides information on density, elasticity, and defects. Key parameters include acoustic impedance (Z), attenuation coefficient (α), and time-of-flight (ToF), which are mathematically expressed as:
$$Z = \rho \times V$$
where ρ is the material density and V is the sound velocity. The attenuation of ultrasound waves follows an exponential decay model:
$$P_x = P_0 \times e^{-\alpha x}$$
where P_x is the pressure amplitude at distance x, and P_0 is the initial pressure. ToF is related to the material thickness L and sound velocity V:
$$\text{ToF} = \frac{L}{V} = L \times \sqrt{\frac{\rho}{E}}$$
where E is the elastic modulus. These relationships enable non-destructive evaluation of interface degradation, gas formation, and mechanical changes in solid-state batteries.
Optical fiber sensing, particularly Fiber Bragg Grating (FBG) technology, utilizes periodic modulation of the refractive index in the fiber core to reflect specific wavelengths. The Bragg wavelength (λ_B) is given by:
$$\lambda_B = 2n_{\text{eff}} \Lambda$$
where n_eff is the effective refractive index and Λ is the grating period. External stress or temperature changes cause shifts in λ_B, which can be decoupled using:
$$\frac{\Delta \lambda_B}{\lambda_B} = (1 – p_e)\Delta \epsilon + (\alpha + \xi)\Delta T$$
where p_e is the effective strain-optic constant, Δε is the strain change, α is the thermal expansion coefficient, and ξ is the thermo-optic coefficient. This allows for simultaneous monitoring of stress and temperature in solid-state batteries.
| Technology | Key Parameters | Applications in Solid-State Batteries | Advantages | Limitations |
|---|---|---|---|---|
| Ultrasound | Acoustic impedance, ToF, attenuation | Interface contact, gas detection, crack monitoring | Non-destructive, high penetration depth | Limited spatial resolution, signal noise |
| Optical Fiber (FBG) | Wavelength shift, strain, temperature | Stress distribution, thermal mapping, chemical sensing | High sensitivity, EMI immunity, miniaturization | Integration complexity, temperature cross-sensitivity |
Application in Interface Characteristics Evaluation
The interface between electrodes and solid-state electrolytes is critical for the performance of solid-state batteries. Ultrasound technology has been employed to dynamically assess interface contact states and gas generation. For instance, in polymer-based solid-state electrolytes, ultrasound imaging can reveal the effects of interface “activation” and self-healing mechanisms. Studies have shown that modifications with dynamic disulfide bonds or inorganic fillers reduce gas evolution and improve contact stability. The following equation relates ultrasound signal changes to interface degradation:
$$\Delta S = k \times \Delta G + m \times \Delta C$$
where ΔS is the signal change, ΔG is the gas volume change, ΔC is the contact area change, and k and m are constants. This allows for quantitative analysis of interface behavior in solid-state batteries.
Optical fiber sensors, when integrated at the interface, can monitor stress variations due to volume changes during cycling. For example, FBG sensors embedded in lithium metal anodes detect strain fluctuations associated with dendrite formation. The stress (σ) can be calculated from the wavelength shift using Hooke’s law:
$$\sigma = E \times \epsilon$$
where E is Young’s modulus and ε is the strain derived from FBG data. This provides real-time insights into interface mechanical stability in solid-state batteries.
| Interface Aspect | Ultrasound Technique | Optical Fiber Technique | Key Findings |
|---|---|---|---|
| Contact Loss | ToF variations | Strain mapping | Ultrasound shows reduced contact resistance after activation; FBG reveals stress concentration at interfaces |
| Gas Evolution | Attenuation analysis | N/A (limited direct gas sensing) | Ultrasound detects gas pockets in polymer electrolytes; modified electrolytes show suppressed gas generation |
| Self-Healing | Signal amplitude recovery | Stress relaxation monitoring | Dynamic bonds in electrolytes improve interface repair; FBG tracks strain recovery during cycling |
Application in Mechanical Characteristics Evaluation
Mechanical failures, such as crack propagation and dendrite penetration, are major concerns in solid-state batteries. Ultrasound techniques, including pulse-echo and continuous wave methods, enable in-situ tracking of elastic modulus changes and micro-crack formation. For example, in LLZO solid electrolytes, a slight decrease in sound velocity (0.1–0.2%) precedes short-circuit events, indicating dendrite-induced cracks. The relationship between sound velocity V and crack density D can be modeled as:
$$V = V_0 \times (1 – \beta D)$$
where V_0 is the initial velocity and β is a material constant. This allows for early warning of mechanical failure in solid-state batteries.
Optical fiber sensors offer high-resolution strain monitoring within electrodes and electrolytes. Micro-FBG sensors with reduced diameters (e.g., 30 μm) minimize invasiveness and provide accurate stress measurements. The birefringence effect in FBGs allows for multi-axial stress analysis, with the wavelength split given by:
$$\Delta \lambda_x – \Delta \lambda_y = \gamma \times \sigma_{\text{lateral}}$$
where γ is a calibration factor and σ_lateral is the lateral stress. This capability is crucial for understanding stress evolution in solid-state batteries during cycling.
| Mechanical Parameter | Ultrasound Method | Optical Fiber Method | Impact on Solid-State Battery Performance |
|---|---|---|---|
| Elastic Modulus | ToF and velocity measurements | N/A (indirect via strain) | Decreases during lithiation; correlates with phase transitions in electrodes |
| Crack Density | Attenuation and scattering | Strain localization | Ultrasound detects micro-cracks before failure; FBG identifies stress hotspots |
| Dendrite Growth | Velocity changes pre-short circuit | Strain spikes during cycling | Early detection possible with ultrasound; FBG shows reversible strain in dendrite-prone areas |
Application in Physicochemical Characteristics Evaluation
Physicochemical properties, such as temperature distribution and chemical reactions, play a vital role in the safety and efficiency of solid-state batteries. Ultrasound techniques can infer state-of-charge (SoC) and state-of-health (SoH) through changes in acoustic parameters. For instance, phase shifts in high-frequency ultrasound correlate with SoC in LiCoO2-based solid-state batteries:
$$\Delta \phi = \kappa \times \Delta \text{SoC}$$
where κ is a proportionality constant. This enables real-time SoC tracking without invasive probes.
Optical fiber sensors excel in thermal monitoring and chemical sensing. Short FBG (sFBG) arrays can decouple temperature and strain, providing internal temperature maps with high resolution. The temperature sensitivity of FBG is typically around 10–12 pm/°C, allowing for detection of minor thermal fluctuations during operation. Additionally, advanced lab-on-fiber platforms integrate spectroscopic techniques (e.g., Raman) for in-situ chemical analysis, though this is less developed for solid-state batteries due to optical opacity of electrolytes.
| Physicochemical Aspect | Ultrasound Approach | Optical Fiber Approach | Challenges and Solutions |
|---|---|---|---|
| Temperature Distribution | Indirect via sound velocity changes | Direct with FBG arrays | Ultrasound lacks precision; FBG offers mm-resolution but requires integration |
| SoC/SoH Estimation | Phase and amplitude analysis | Limited to indirect methods | Ultrasound correlates with electrode modulus; FBG primarily for stress/temperature |
| Chemical Reactions | Gas detection via attenuation | Spectroscopic integration (emerging) | Ultrasound sensitive to gas bubbles; optical fibers face penetration issues in solids |
Conclusion and Future Perspectives
Ultrasound and optical fiber sensing technologies offer transformative potential for the development of reliable solid-state batteries. By providing real-time, multi-parameter insights into interface, mechanical, and physicochemical behaviors, these techniques address critical challenges in solid-state battery research. However, several hurdles remain, including signal decoupling in complex environments, sensor integration without performance degradation, and scalability for large-format batteries.
Future work should focus on hybrid sensing platforms that combine ultrasound, optical fiber, and electrochemical methods for comprehensive diagnostics. Miniaturization of sensors, such as micro-FBGs, will enhance compatibility with solid-state components. Additionally, machine learning algorithms can be employed to analyze multi-modal data and predict failure modes. The integration of these advanced monitoring systems will pave the way for intelligent solid-state batteries with self-diagnostic capabilities, ultimately accelerating their commercialization and adoption in electric vehicles and grid storage.
In summary, the continued advancement of ultrasound and optical fiber sensing is essential for unlocking the full potential of solid-state batteries. Through collaborative efforts in materials science, sensor engineering, and data analytics, we can overcome existing limitations and achieve high-performance, safe, and durable energy storage solutions.