Wesley Chang

This review provides a comprehensive overview of ultrasonic testing (UT) applied to battery research and development, bridging the gap between fundamental acoustic principles and practical applications. We begin by detailing the acoustic physics underlying UT and describing the hardware, software, and signal processing algorithms necessary to extract useful information from battery systems. We then summarize key academic findings and trends in UT analysis of lithium-based batteries, highlighting both foundational studies that have shaped the field and recent advancements pushing the boundaries of UT application. Following this, we provide an overview of lab-scale operando tools that complement UT analysis, illustrating how they can enhance and validate its findings. The discussion is extended beyond academic work to encompass UT applications in battery manufacturing, uniquely incorporating industry perspectives on the challenges and opportunities in this space. Finally, we conclude with a discussion of future directions in battery UT research. This review aims to provide a summary of the current state of UT applied to batteries, equip readers with the tools to contextualize new UT studies and applications, and serve as a practical guide for researchers and engineers seeking to implement UT in their work.
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The increased adoption of lithium-based rechargeable batteries requires advanced metrology tools across the entire battery lifecycle. Ultrasound testing (UT) has emerged as a promising, low-cost, and scalable technique for providing valuable insights at various stages of battery development, production, and operation. This review highlights the fundamental principles of UT and its diverse applications, spanning from lab-scale research to manufacturing and field-deployed monitoring. In addition to a synthesis of the existing battery UT literature, including acoustic detection of battery failure modes and decoupling different physical phenomena, we emphasize emerging battery UT methods and applications. For instance, recent advances in ultrasonic transducer technology may be adapted to enhance battery UT resolution. UT techniques could be used not only for battery cell characterization but also for in-line monitoring of slurry quality and electrodes. Lastly, we draw connections from lab-scale research and development to in-line quality assurance and metrology on an industrial scale. Underscoring this is a discussion of current challenges, including signal processing complexity, the need for large datasets, and, especially pertinent to manufacturing and field-deployed applications, cost and throughput. Addressing these issues through methods such as new signal processing techniques, machine learning, and advanced sensors will help drive UT’s adoption of batteries at an industrial scale.
This review provides an introduction to ultrasonic testing for battery researchers and an overview of current research directions within the field, covering applications ranging from lab-scale experiments to in-line manufacturing quality control. Perspectives are included from both industry and academia, with an emphasis on the ways ultrasound can complement existing measurement techniques.

Nondestructive ultrasonic testing is finding increasing use in battery science. We provide instructions and software for the development of a low cost, modular, and easy to use scanning acoustic microscope. Basic principles of ultrasonic testing are discussed with particular attention to its application for operando characterization of batteries. An example measurement showing real time monitoring of electrolyte wetting in a pouch cell is shown. By providing detailed hardware setup instructions and freely accessible analysis software, this paper aims to make ultrasonic testing accessible to the wider battery research community.

Abstract The chemo-mechanics of lithium-sulfur (Li-S) batteries are unique in lithium-based batteries as sulfur undergoes a solid-liquid-solid transition during each half-cycle. The dissolution of sulfurous species in liquid electrolytes is a primary degradation mode in Li-S systems. While this challenge is well known, tracking and measuring sulfur liquefaction requires ex-situ experiments or hard-to-parallelize X-ray techniques. Here, we show that operando acoustic analyses can track both physicochemical phase changes and the mechanical dynamics of sulfur. We show time-of-flight can monitor sulfur phase changes during density and effective elastic moduli dynamics. Acoustic wave damping is highly sensitive to the state-of-matter transitions of the sulfur electrode. By accounting for cell dilation from Li plating and stripping, we show sulfur’s chemo-mechanical phase changes dominate time-of-flight’s nonlinear, non-monotonic signatures. By utilizing inter-cycle and intra-cycle time-of-flight trends, we develop a semi-quantitative method that can be calibrated to measure the dissolution of sulfur into the electrolyte and verify this with ex-situ TGA and XRD. Lastly, we pair acoustics with voltammetry to observe slow chemo-mechanical dynamics alongside the sluggish kinetics of sulfur utilization. Operando acoustic analyses can elucidate the chemo-mechanical dynamics of the sulfur electrode noninvasively and aid development efforts to slow and mitigate S migration.

Successful integration of metallic lithium anodes into secondary batteries could enhance energy density and enable new forms of electrified transportation. However, the outlook for widespread lithium metal adoption in energy storage devices remains mixed. This comes in part from existing gaps in our understanding of the relationships connecting the initial state of lithium, its evolution with cycling, and end-of-life state. It remains important to develop standardized protocols for material and cell characterization, cycling performance, safety, and recycling procedures for lithium metal-based batteries. In February 2023 a cohort of scientists and engineers from academia, national laboratories, and industry gathered to converge on a list of critical challenges and action items to provide better understanding of lithium metal evolution and to enhance academic, governmental, and industrial partnerships to address these challenges. Here, we highlight the major discussion topics revolving around the manufacturing of lithium metal, its related metrology and integration into battery form factors, and best practices testing its electrochemical performance relevant to automotive applications. We introduce a power-controlled discharge testing protocol for research and development cells, in alignment between major automotive stakeholders, that may reveal lithium metal battery dynamics closer to practical driving behavior. [Display omitted] Rechargeable lithium metal batteries have been researched for decades and are currently in an era where large-scale commercialization of safe, high energy density cells is being attempted. This commentary is a result of discussions across academia, industry, and government to align on useful testing protocols, metrologies, and other characterization efforts of lithium metal batteries and to specifically introduce a power-controlled cycling protocol that may reveal lithium metal cycling dynamics of interest to the automotive industry.

We are a team of three battery researchers and engineers who are working with The Electrochemical Society to develop an “electrochemical techniques and diagnostics for batteries” curriculum, comprised of an online course and an in-person workshop. With a combined 40+ years of experience working in battery research and engineering, ranging from academia to electric vehicle manufacturing, we have noticed that there exists a gap in applied electrochemistry knowledge needed to train the rapidly expanding workforce of battery engineers and scientists. In this backstory, we explain the origin story of our team, our motivations for developing the course and the things we have learned in working together. We share our insights into the emerging electric vehicles business and why we believe electrochemistry education will shape the future of this industry.

Cell formation is an energy and time-intensive empirically-guided process crucial to manufacturing secondary lithium-ion batteries. As the rechargeable battery industry moves towards manufacturing lithium metal batteries-where a metallic lithium negative electrode is used instead of a porous graphite composite-the cell formation process may need reconsidering. The effects of formation rate and cycling protocol on lithium metal battery performance are poorly understood. In this work, we used operando acoustic transmission to measure physical changes during the formation cycles and the effect of formation cycling protocols on the long-term cycling of anode-free lithium metal pouch cells-where all the lithium inventory comes from the positive electrode and is deposited as metallic lithium on copper foil during initial charge. We show that a faster C/3 formation protocol results in comparable cycling performance and cell stiffness change to a slower C/10 formation step. Variations in acoustic metrics across different electrolytes tested are attributed to differences in gas formation, cell swelling, and lithium deposition morphology. NMC811 cathodes paired with a high-concentration ether electrolyte are shown to be particularly prone to gas formation, which is mitigated by using a localized high-concentration ether electrolyte and single-crystal NMC532. The results highlight differences in formation behavior between anode-free lithium metal cells and lithium-ion cells. These are important to consider when bringing new manufacturing plants online for lithium metal batteries.

Recently, non-invasive ultrasonic-based detection has emerged as a powerful tool to estimate the state-of-charge (SOC) and state-of-health (SOH) of lithium-ion batteries with a promising accuracy and efficiency. However, the currently available non-invasive methodology is highly sensitive to experimental setups and conditions, leading to unpredictable and unstable results. To this end, from a more fundamental stress wave propagation perspective, we discover that the quantified change of ultrasonic damping can be an intrinsic physical quantity to correlate with the state-of-charge (SOC) of batteries. We employ time-harmonic waves with different frequencies to obtain the steady-state dynamic response of lithium-ion batteries at various SOCs and a quasi-periodic energy gap can be observed. A mesoscale physics-based model of lithium-ion batteries is established to explain the observed energy gap carrying the multiple reflections of ultrasonic waves within the multi-layered structure of the cell. Finally, the change of ultrasonic damping with SOC is quantified for fast and accurate SOC prediction based on the frequency-domain damping analysis. Results underpin a robust and accurate frequency-domain ultrasonic characterization methodology for batteries and highlight the promise of the fundamental understanding of wave propagation for advanced characterization of batteries.
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•Continuous waves are input as incident signals to conduct in-situ ultrasonic tests.•The wave dissipation mechanism through the pouch cell is revealed.•A meso-scale analytical model of the pouch cell is established.•An acoustic-based methodology for battery SOC estimation is proposed.

Electrochemical cells using rechargeable lithium metal anodes are sensitive to operating temperature and stack pressure. Current understanding generally assumes that temperature drives changes in lithium metal surface chemistry while stack pressure impacts the anode morphology. In this study, we provide quantifiable evidence for these assumptions and propose mechanisms to guide understanding of temperature and pressure effects on lithium metal cell dynamics. Beyond the direct coupling of pressure with mechanics and temperature with kinetics, we also explore possible effects of temperature on cell mechanics and stack pressure on cell chemistry. We investigate an electrolyte composition based on LiDFOB salt, using a range of operando and ex situ techniques. Mechanistic mapping of temperature- and pressure-dependent cell behavior will aid development of improved lithium metal batteries.

The dynamic behavior of the interface between the lithium metal electrode and a solid-state electrolyte plays a critical role in all-solid-state battery performance. The evolution of this interface throughout cycling involves multiscale mechanical and chemical heterogeneity at the micro- and nano-scale. These features are dependent on operating conditions such as current density and stack pressure. Here we report the coupling of operando acoustic transmission measurements with nuclear magnetic resonance spectroscopy and magnetic resonance imaging to correlate changes in interfacial mechanics (such as contact loss and crack formation) with the growth of lithium microstructures during cell cycling. Together, the techniques reveal the chemo-mechanical behavior that governs lithium metal and Li7La3Zr2O12 interfacial dynamics at various stack pressure regimes and with voltage polarization.
All-solid-state batteries are promising alternatives to Li-ion batteries. Here, the authors investigate the chemo-mechanical changes at the lithium metal/solid electrolyte interface via operando acoustic transmission and magnetic resonance imaging.