Listening to Batteries: Utilising Acoustic Characterisation Techniques to Optimise Formation Cycle Protocols in Lithium-Ion Batteries
Authors: Fordham Arthur , Owen Rhodri E , Dose Wesley M. , Robinson James B , Brett Dan J L , Jervis Rhodri 
Published: December 2024

The optimisation of the formation cycle in battery manufacturing plays a crucial role in driving the success of the electric revolution. The formation cycle, which involves the initial charging and discharging of lithium-ion batteries (LIB)s to stabilise their electrochemical processes, directly impacts the overall performance and longevity.1 This formation process of LIBs is the most expensive and time-consuming step in the total battery pack cost and process, meaning it is a critical bottleneck in the manufacturing industry and requires substantial capital equipment to deliver reliably.2,3 The long duration is required to ensure that a stable layer of surface interphase grows between the electrode and electrolyte, known as the Solid Electrolyte Interphase (SEI). A stable SEI layer is essential to minimise active lithium loss, electrolyte depletion and capacity fade over the lifetime of the battery, as well as mitigating safety issues within the cell and pack.4 It is therefore vital to develop novel characterisation techniques to improve this formation process.
Acoustic methods are one of the most promising diagnostic and characterisation techniques for batteries. The technique offers the potential for a real-time operando prediction of the State of Health (SoH), State of Charge (SoC) and State of Safety (SoS).5–7 Broadly, acoustic techniques can be divided into Acoustic Emission (AE) and Ultrasonic Testing (UT) methods; each slightly differs in terms of purpose and experimental setup. Both techniques can detect the physical and electrochemical changes within the cell during usage such as gas formation and cracking.8

In this work, conducted at UCL under the Faraday Institution's SafeBatt project, we use a combination of Acoustic Emission and pulse-echo Ultrasonic Testing in a novel dual-sensor approach. Utilising the sensors, we detect the growth of the solid electrolyte interphase (SEI) during the cell's formation cycle under different protocols and with varying additives. Additionally, we investigate the influence of this SEI layer on the long-term performance and State of Health (SoH) of the cell.
We then developed bespoke formation protocols with voltage holds, using acoustic techniques to determine the point at which SEI formation finished and the corresponding success of the passivation layer. This technique can characterise how successfully a cell has formed based on the acoustic signals released, offering a low-cost, non-invasive, and non-destructive approach. We correlated the acoustic results to X-ray photoelectron spectroscopy (XPS), X-ray Computed Tomography, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and in-situ gas volume measurements to confirm the hypotheses made from the acoustic measurements. The results confirmed that as the C-rate increases, a greater amount of acoustic activity is observed, which is attributed to gas formation. Based on the acoustic emission (AE) activity, we were also able to determine the exact voltage at which chemical reactions occur in the electrode electrolyte interphase, including the point where the SEI was developing during the formation cycle. These findings demonstrate the potential for employing acoustic sensors in industry as an advanced characterisation technique to enhance the formation cycle process.

References

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