2025 Theses Doctoral

Chemomechanics of Lithium Batteries via operando Acoustics

Thorsteinsson, Gunnar

This dissertation explores the application of acoustics as a characterization technique for three anode materials in lithium-based battery systems. The first focus is on the formation process of anode-free lithium metal batteries, examining how various parameters and properties influence lithium metal plating and stripping dynamics. Across three interconnected chapters, the key finding is that higher current density on formation supports cycling performance. Graphite is the second anode material, with a special consideration of observing its staging behavior. This is done using resonance, a novel method for extracting acoustic features of battery systems where conventional “chirping” falls short. The third and last anode material is silicon. Paired with a solid-state electrolyte, its phasing dynamics, expansion, and pitting are observed in the chemomechanical domain.

Chapter 2 examines how electrolyte composition and formation rate affect the performance of anode-free lithium metal batteries. A faster C/3 formation protocol achieves cycling performance and cell stiffness changes comparable to a slower C/10 formation step. Differences in acoustic metrics across electrolytes are linked to variations in gas formation, cell swelling, and lithium deposition morphology. NMC811 cathodes with a high-concentration ether electrolyte exhibit a tendency for significant gas formation, which is mitigated by using a localized high-concentration ether electrolyte and single-crystal NMC532.

Chapter 3 introduces a novel acoustic apparatus capable of dual-mode, spatially resolved acoustic interrogation. Medium-frequency pitch/catch in parallel with high-frequency pulse/echo enables simultaneous tracking of cell-level and layer-level chemomechanical dynamics. The apparatus is applied on the same gas-prone system studied in Chapter 2 during formation to detect gas localization and plating/stripping dynamics. The findings reinforce the takeaway from Chapter 2 that fast formation may be beneficial for lithium metal batteries.

Chapter 4 further investigates the effects of anode-free lithium-metal battery formation parameters, focusing on the interplay between temperature, stack pressure, and current density in multilayered cells. Accelerated-rate cycling is used to evaluate the impact of formation protocols on performance. Higher temperature, stack pressure, and C-rate are shown to improve lithium morphology after formation. Ultrasound transmission during cycling reveals that these improvements gained during formation lead to better mechanical behavior during cycling, although cathode dynamics and electrolyte side reactions complicate interpretation of electrochemical performance.

Chapter 5 shifts the focus from lithium metal batteries to Li-ion batteries. There we introduce resonance—a method distinct from, yet related to, the chirping technique used in earlier chapters. Acoustic resonance is demonstrated as a viable tool for State-of-Charge (SoC) characterization at both the module level and at the cell level, particularly for challenging cell geometries such as cylindrical cells. By sweeping through frequencies approximately two orders of magnitude lower than previously reported, changes in spectral density and signal energy are linked to known electrochemical processes.

Chapter 6 considers silicon. It is a coveted electrode material due to its high energy density but suffers from poor cycle life due to a threefold expansion during lithiation—and contraction with commensurate pitting on delithiation—which continually breaks and reforms the solid-electrolyte interphase. In this work, it is paired with an argyrodite solid-state electrolyte to approach silicon’s theoretical energy density. Through differential acoustic analysis, the two-phase lithiation and one-phase delithiation dynamics are observed for the first time in the chemomechanical domain. Peak broadening and shifting are related to increased cell impedance and heterogeneous lithiation.