Rising global demand for energy supply, storage and portability in a sustainable manner needs significant improvements to be made in the next generation of batteries, both in terms of performance and lifetime. However, despite their importance, battery electrode microstructures remain relatively poorly understood. The performance of the battery is dependent on the nano and micro-structure achieved during manufacture. Furthermore microstructural evolution during operation may degrade electrochemical performance. The growth of dendrites represents a limiting failure mechanism in some battery systems; in particular this can be a challenge in zinc-air batteries. Tomographic techniques allow the direct 3D imaging and characterisation of complex microstructures, including the observation and quantification of dendrite growth. Here we present results from 3D x-ray and FIB-SEM tomography of Zn dendrite formation in a zinc-air battery, down to resolutions of tens of nanometers, enabling analysis of dendrite micro-structure. This approach is shown to be effective in understanding how dendrites grow, and demonstrates that tomography coupled with modeling can provide new insights into degradation mechanisms associated with dendrite growth.
This paper presents a fast cost-effective technique for the measurement of battery impedance online in an application such as an electric or hybrid vehicle. Impedance measurements on lithium-ion batteries between 1 Hz and 2 kHz give information about the electrochemical reactions within a cell, which relates to the state of charge (SOC), internal temperature, and state of health (SOH). We concentrate on the development of a measurement system for impedance that, for the first time, uses an excitation current generated by a motor controller. Using simple electronics to amplify and filter the voltage and current, we demonstrate accurate impedance measurements obtained with both multisine and noise excitation signals, achieving RMS magnitude measurement uncertainties between 1.9% and 5.8%, in comparison to a high-accuracy laboratory impedance analyzer. Achieving this requires calibration of the measurement circuits, including measurement of the inductance of the current sense resistor. A statistical correlation approach is used to extract the impedance information from the measured voltage and current signals in the presence of noise, allowing a wide range of excitation signals to be used. Finally, we also discuss the implementation challenges of an SOC estimation system based on impedance.
Modern applications of batteries in portable electronics, electric vehicles and grid level energy storage, demands long lifetime in various operating conditions. Monitoring its state-of-health in real time and optimising its operation in a cost-effective manner presents a real challenge. Differential thermal voltammetry (DTV) is a novel in-situ diagnosis technique for tracking degradation in lithium-ion batteries based on relatively simple cell surface temperature measurements. DTV only requires temperature and voltage measurements and is carried out by measuring the change in cell surface temperature during a constant current charge or discharge. It is a complimentary method to the incremental capacity or slow rate cyclic voltammetry analysis as it has additional information on the entropy characteristics as a result of temperature measurements. Accelerated aging experiment was carried out on commercial lithium-ion cells where the cells were aged differently in a controlled environment. The cells were characterised regularly using the new technique. Each dT/dV peak in the resulting curve represents a combination of different contributions from positive and negative electrode phases (Figure 1). By decoupling these peaks and by monitoring the evolution of the peak parameters it is possible to quantitatively determine stoichiometric drift in the cell. Cell 1 which was stored at 55degC and held at 4.2V showed significant shift in electrode stoichiometry. On the other hand, cell 2 stored at 55degC and cycled at 1C showed little shift despite the similar capacity fade of approximately 10%. This novel application of DTV presents an opportunity to estimate the state of health of a lithium-ion battery through tracking of decoupled dT/dV peaks. These results can then feed in to adaptively control the cell operation based on the diagnosis to optimise cell lifetime and available energy. Its low software and hardware requirements make it a practical solution to existing and future systems. Figure 1. Decoupled dT/dV peaks for two aged cells at approximately 10% capacity fade. Cell 1 (left) was stored at 55degC and held at 4.2V. Cell 2 (right) was stored at 55degC and cycled at 1C. Schematics of combined phases are shown above each plot. Note the visible stoichiometric drift in cell 1. [1] Y. Merla, B. Wu, V. Yufit, R.F. Martinez-botas, N.P. Brandon, G.J. Offer, Novel application of differential thermal voltammetry as an in-depth state of health diagnosis method for lithium-ion batteries, J. Power Sources. 307 (2016) 308–319. doi:10.1016/j.jpowsour.2015.12.122. [2] B. Wu, V. Yufit, Y. Merla, R.F. Martinez-botas, N.P. Brandon, G.J. Offer, Differential thermal voltammetry for tracking of degradation in lithium-ion batteries, J. Power Sources. 273 (2015) 495–501. doi:10.1016/j.jpowsour.2014.09.127. Figure 1
This paper will demonstrate the concept of a new, low-cost, on-line technique for monitoring battery state of health (SOH) and state of charge (SOC) using electrochemical impedance spectroscopy (EIS). A particular focus will be electric vehicles (EVs), where the SOC accuracy over existing battery management systems (BMS) will improve range prediction accuracy, although the proposed technique is also applicable to other electrochemical energy storage devices. While currently there exist few methods to measure the battery state of charge online, these methods are generally categorized as "indirect" methods which are prone to errors due to environmental changes and require additional hardware/costs for implementation. In this paper the EIS excitation signal will be generated by the system's existing power converter without requiring extra hardware but only requires software upgrade. The main objective of the proposed method is to minimize the impact on the main operation of the power converter in the system.
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