a Lithium-ion batteries are being used in increasingly demanding applications where safety and reliability are of utmost importance. Thermal runaway presents the greatest safety hazard, and needs to be fully understood in order to progress towards safer cell and battery designs. Here, we demonstrate the application of an internal short circuiting device for controlled, on-demand, initiation of thermal runaway. Through its use, the location and timing of thermal runaway initiation is pre-determined, allowing analysis of the nucleation and propagation of failure within 18 650 cells through the use of high-speed X-ray imaging at 2000 frames per second. The cause of unfavourable occurrences such as sidewall rupture, cell bursting, and cell-to-cell propagation within modules is elucidated, and steps towards improved safety of 18 650 cells and batteries are discussed. Broader contextFrom portable electronics to grid-scale storage, high energy density Li-ion batteries are ubiquitous in today's society. Such cells can and do fail, sometimes catastrophically, releasing large amounts of energy. To facilitate safer and more reliable cell designs, the importance of understanding failure mechanisms of Li-ion cells is widely recognised. Here, we demonstrate the application of a novel device that is capable of generating an internal short circuit within commercial cell designs, on-demand, and at a predetermined location. This enables us to test more effectively the ability of safety devices of cells and modules to withstand 'worst-case' failure scenarios. By combining the use of this device with high-speed X-ray imaging at 2000 frames per second, we characterise for the first time the initiation and propagation of thermal runaway from a known location within a Li-ion cell. The insights achieved in this study are expected to guide the design and development of safer and more reliable Li-ion cells.
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This study develops two novel charging strategies for lithium-ion batteries, designed to prevent the onset of lithium plating when the cells are charged at low ambient temperatures. Commercially available 3.1 Ah 18650-type cells with NCA and graphite electrodes have been selected for this study. Experimental results highlight that for these cells, lithium plating can be detected when the cells are charged with a traditional constant-current constant-voltage (CC-CV) profile at an ambient temperature of 5 o C and a charge rate of 1C. The occurrence of lithium plating is known to lead to a considerable capacity reduction. To avoid the onset of lithium plating with minimum impact on the charge time, two optimal charging strategies are proposed. The first is based on detecting the onset of lithium plating through the online analysis of the voltage relaxation profile (VRP). The second is to manage the cell charging process to achieve a pre-defined rate of battery degradation per charge cycle. Experimental results highlight that the capacity fade of cells using the proposed charging strategies can be significantly reduced compared to when charged using the conventional CC-CV approach by 45% and 70% respectively while minimizing their impact on the charging speed.
While silicon-based negative electrode materials have been extensively studied, to develop high capacity lithium-ion batteries, implementing a large-scale production method that can be easily transferred to industy, has been a crucial challenge. Here, a scalable wet-jet milling method was developed to prepare a silicon-graphene hybrid material to be used as negative electrode in lithium-ion batteries. This synthesized composite, when used as an anode in lithium cells, demonstrated high Li ion storage capacity, long cycling stability and high-rate capability. In particular, the electrode exhibited a reversible discharge capacity exceeding 1763 mAh g-1 after 450 cycles with a capacity retention of 98% and a coulombic efficiency of 99.85% (with a current density of 358 mA g-1). This significantly supersedes the performance of a Si-dominant electrode structures. The capacity fade rate after 450 cycles was only 0.005% per cycle in the 0.05-1 V range. This superior electrochemical performance is ascribed to the highly layered, silicon-graphene porous structure, as investigated via focused ion beam in conjunction with scanning electron microscopy (FIB-SEM) tomography. The hybrid electrode could retain 89% of its porosity (under a current density of 358 mA g-1) after 200 cycles compared with only 35% in a Si-dominant electrode. Moreover, this morphology can not only accommodate the large volume strains from active silicon particles, but also maintains robust electrical connectivity. This confers faster transportation of electrons and ions with significant permeation of electrolyte within the electrode. Physicochemical characterisations were performed to further correlate the electrochemical performance with the microstructural dynamics. The excellent performance of the hybrid material along with the scalability of the synthesizing process is a step forward to realize high capacity/energy density lithium-ion batteries for multiple device applications.
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