As the demand for high-energy-density and safe energy storage systems grows, solid-state batteries have emerged as a promising alternative to conventional lithium-ion batteries. The inherent stability of solid-state electrolytes eliminates flammable liquid components, reducing thermal runaway risks. However, the complex interfacial evolution and failure mechanisms in solid-state batteries pose significant challenges for real-time monitoring. Traditional sensing techniques struggle to adapt to the rigid interfaces and solid-solid contacts in these systems, limiting their sensitivity and compatibility. In this context, ultrasound and optical fiber sensing technologies offer innovative solutions for in-situ, non-destructive characterization of solid-state batteries. This article explores the principles, applications, and future prospects of these techniques, focusing on their role in advancing the development of reliable solid-state batteries.
Ultrasound technology leverages high-frequency mechanical waves to probe internal structures in solid-state batteries. Key parameters such as acoustic impedance, attenuation coefficient, and time-of-flight (ToF) provide insights into material properties like density, elastic modulus, and interface integrity. The acoustic impedance (Z) is defined as:
$$Z = \rho \times V$$
where $\rho$ is the material density and $V$ is the sound velocity. The attenuation of ultrasound waves follows an exponential decay model:
$$P_x = P^* e^{-\alpha x}$$
where $P_x$ is the pressure amplitude at distance $x$, and $\alpha$ is the attenuation coefficient. ToF relates to the material thickness $L$ and sound velocity $V$ as:
$$\text{ToF} = \frac{L}{V} = L \sqrt{\frac{\rho}{E}}$$
where $E$ is the elastic modulus. These equations enable the detection of gas formation, crack propagation, and interfacial degradation in solid-state batteries. For instance, ultrasound imaging can visualize pore evolution and electrolyte wetting, critical for assessing the performance of solid-state batteries.
Optical fiber sensing, particularly Fiber Bragg Grating (FBG) technology, offers high sensitivity to strain and temperature changes. The Bragg wavelength $\lambda_B$ is given by:
$$\lambda_B = 2n_{\text{eff}} \Lambda$$
where $n_{\text{eff}}$ is the effective refractive index and $\Lambda$ is the grating period. External stimuli like strain ($\Delta \varepsilon$) and temperature ($\Delta T$) shift the Bragg wavelength:
$$\frac{\Delta \lambda_B}{\lambda_B} = (1 – p_e) \Delta \varepsilon + (\alpha + \xi) \Delta T$$
Here, $p_e$ is the effective strain-optic constant, $\alpha$ is the thermal expansion coefficient, and $\xi$ is the thermo-optic coefficient. By decoupling these effects, FBG sensors can monitor stress distribution and thermal gradients in solid-state batteries, providing real-time data on mechanical and physicochemical changes.
Interfacial Characteristics Assessment
In solid-state batteries, interfacial degradation and gas evolution are major concerns. Ultrasound technology dynamically assesses electrode-electrolyte contact states, crack formation, and gas generation. For example, studies using ultrasonic imaging have shown that polymer-based solid-state electrolytes exhibit reduced gas production when modified with dynamic disulfide bonds or inorganic fillers. The following table summarizes key findings in interfacial monitoring for solid-state batteries:
| Technique | Application | Key Parameters | Impact on Solid-State Batteries |
|---|---|---|---|
| Ultrasound Imaging | Interface Contact Evaluation | ToF, Acoustic Impedance | Detects physical delamination and chemical passivation |
| Ultrasound Attenuation | Gas Evolution Monitoring | Attenuation Coefficient ($\alpha$) | Quantifies gas generation rates in solid-state batteries |
| FBG Sensors | Strain and Stress Mapping | Bragg Wavelength Shift ($\Delta \lambda_B$) | Reveals interfacial stress evolution during cycling |
Optical fiber sensors integrated into solid-state batteries provide complementary data on interfacial strain. For instance, embedded FBG sensors have demonstrated that modified electrolytes, such as those with single-ion conductors, reduce interfacial stress and gas generation. The synergy between ultrasound and fiber optics enables a comprehensive understanding of failure mechanisms in solid-state batteries.
Mechanical Characteristics Assessment
Mechanical failures, including lithium dendrite penetration and electrode cracking, are critical issues in solid-state batteries. Ultrasound techniques monitor these phenomena through changes in sound velocity and elastic modulus. The relationship between ToF and material properties can be expressed as:
$$\Delta \text{ToF} \propto \frac{\Delta L}{V} + L \frac{\partial}{\partial E} \left( \frac{1}{V} \right) \Delta E$$
where $\Delta E$ represents changes in elastic modulus due to microcracks. Studies have shown that a 0.1% decrease in sound velocity correlates with lithium dendrite-induced cracks in LLZO electrolytes, highlighting the sensitivity of ultrasound for early failure detection.
FBG sensors offer high-resolution strain monitoring in solid-state batteries. The stress ($\sigma$) is derived from strain ($\varepsilon$) using Hooke’s law:
$$\sigma = E \varepsilon$$
where $E$ is the Young’s modulus of the fiber. Advanced FBG designs, such as micro-FBGs with reduced diameters (e.g., 30 μm), minimize invasiveness while providing accurate stress measurements. The table below compares mechanical monitoring techniques for solid-state batteries:
| Method | Measured Parameter | Resolution | Applicability to Solid-State Batteries |
|---|---|---|---|
| Pulse-Echo Ultrasound | Sound Velocity ($V$) | 0.1% change | Detects dendrite penetration and microcracks |
| FBG Strain Sensing | Strain ($\varepsilon$) | 1.04 pm/με | Maps stress distribution in electrodes and interfaces |
| Ultrasound Phase Analysis | Phase Shift | High temporal resolution | Monitors state of charge (SoC) via elastic moduli changes |
Birefringence effects in FBG sensors allow for the separation of axial and transverse stresses, providing multidimensional stress analysis in solid-state batteries. This capability is crucial for understanding the anisotropic stress generated during lithium plating and stripping in solid-state batteries.
Physicochemical Characteristics Assessment
Monitoring physicochemical properties, such as temperature and chemical composition, is essential for the safe operation of solid-state batteries. Fiber optic sensors enable distributed temperature sensing with high spatial resolution. The temperature-induced wavelength shift in FBG sensors is given by:
$$\frac{\Delta \lambda_B}{\lambda_B} = (\alpha + \xi) \Delta T$$
where $\alpha$ and $\xi$ are material constants. In solid-state batteries, internal temperature gradients can indicate hot spots prone to thermal runaway. Similarly, ultrasound techniques detect physicochemical changes through variations in acoustic properties. For example, the attenuation coefficient $\alpha$ increases with gas formation or phase transitions in solid-state electrolytes.
The integration of fiber optics with spectroscopic methods, such as Raman or near-infrared sensing, allows for the detection of chemical byproducts and phase changes in solid-state batteries. However, the opaque and rigid nature of solid-state electrolytes poses challenges for optical penetration, necessitating innovative sensor designs. The following equation models the relationship between ultrasound parameters and material density changes:
$$\Delta \rho = \frac{\Delta Z}{V} – \frac{Z \Delta V}{V^2}$$
where $\Delta \rho$ is the density variation, and $\Delta Z$ is the change in acoustic impedance. This approach facilitates the monitoring of densification or porosity evolution in solid-state batteries during cycling.
| Sensor Type | Physicochemical Parameter | Monitoring Principle | Benefits for Solid-State Batteries |
|---|---|---|---|
| FBG Temperature Sensor | Internal Temperature | Thermo-optic effect | Early warning of thermal gradients |
| Ultrasound Spectroscopy | Material Density | Acoustic impedance ($Z$) | Detects phase transitions and byproduct formation |
| Lab-on-Fiber Platforms | Chemical Composition | Refractive index modulation | Identifies interfacial reactions in real-time |
Despite these advancements, quantifying multiple physicochemical parameters simultaneously remains challenging due to signal coupling in solid-state batteries. Future work should focus on multisensor fusion and advanced signal processing to decouple overlapping responses.
Conclusion and Future Perspectives
Ultrasound and optical fiber sensing technologies provide powerful tools for in-situ monitoring of solid-state batteries. Their ability to non-destructively probe interfacial, mechanical, and physicochemical properties enables a deeper understanding of failure mechanisms in solid-state batteries. However, several challenges must be addressed to fully leverage these techniques in commercial solid-state batteries.
First, signal decoupling in complex environments requires advanced algorithms to distinguish between gas evolution, crack propagation, and stress accumulation. Multimodal sensing systems that combine ultrasound, fiber optics, and electrochemical methods could provide correlated data for comprehensive diagnostics. Second, sensor integration must be optimized to minimize invasiveness. Micro-FBG sensors and high-frequency ultrasound probes offer promising pathways for embedded monitoring in solid-state batteries without compromising performance. Third, the development of standardized protocols for data interpretation is essential for translating lab-scale findings to industrial applications.
Future research should prioritize the following areas: (1) designing hybrid sensors for simultaneous multiparameter detection in solid-state batteries, (2) enhancing spatial resolution through advanced imaging techniques like 3D ultrasound tomography, and (3) leveraging machine learning for real-time anomaly detection in solid-state batteries. By addressing these challenges, ultrasound and optical fiber technologies can accelerate the development of intelligent solid-state batteries with self-diagnostic capabilities, ultimately paving the way for safer and more efficient energy storage systems.