► Aging results of commercial graphite/LFP cells. ► Capacity loss is directly related to the storage temperature. ► Storage SOC influences the capacity loss. ► Loss of cyclable lithium is the main ...source of aging. ► Results suggest the absence of active material losses at the negative electrode.
Graphite/LFP commercial cells are stored under 3 different conditions of temperature (30°C, 45°C, and 60°C) and SOC (30%, 65%, and 100%) during up to 8 months. Several non-destructive electrochemical tests are performed at different storage times in order to understand calendar aging phenomena. After storage, all the cells except those stored at 30°C exhibited capacity fade. The extent of capacity fade strongly increases with storage temperature and to a lesser extent with the state of charge. From in-depth data analysis, cyclable lithium loss was identified as the main source of capacity fade. This loss arises from side reactions taking place at the anode, e.g. solvent decomposition leading to the growth of the solid electrolyte interphase. However, the existence of reversible capacity loss also suggests the presence of side reactions occurring at the cathode, which are less prominent than those at the anode. The analyses do not show any evidence about active-material loss in the electrodes. The cells do not suffer substantial change in internal resistance. According to EIS analysis, the overall impedance increase is 70% or less.
A series of graphite/LFP commercial cells, stored under 3 different conditions of temperature (30, 45, and 60 °C) and SOC (30, 65, and 100%) during up to 8 months, are disassembled and analyzed in ...order to identify aging phenomena. The recovered positive and negative electrodes are studied using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and electrochemical testing. The maximum lithium stoichiometry in the recovered cathodes, derived both from XRD data and from electrochemical titration, decreases with an increase of storage temperature and storage SOC. This result confirms that the capacity fade of the commercial cells is caused by the loss of cyclable lithium. From capacity measurements on individual electrodes, any loss of active material is ruled out. Cyclable lithium loss arises from the growth of the solid electrolyte interphase at the anode, as outlined by the presence of a thick and fluffy film at the graphite particle surface for severe aging conditions (e.g., T = 60 °C and SOC = 100%) and an increase of the impedance. Evidence for side reactions at the LFP electrode is provided as well, as demonstrated by the presence of F-rich particles and an impedance increase for the electrodes that aged the most.
► Postmortem results of calendar-aged commercial graphite/LFP cells. ► Capacity loss is directly related to the severity of storage conditions (temperature and SOC). ► Electrodes develop rich surface chemistry upon storage. ► Loss of cyclable lithium is the main source of aging. ► Results suggest the absence of active material losses at both positive and negative electrodes.
This paper aims to characterize pouch cells based on the parameters external pressure, temperature & C-rate and study its effect on capacity & open circuit voltage (OCV) of the cell. Lithium ...batteries based energy storage devices are gaining prominence in for its role in tapping the potential of renewable energy sources and electro-mobility. For large scale energy storage applications lithium polymer pouch cells are naturally the best choice, due to its packaging efficiency & higher energy density. It is imperative to perform extensive characterization of pouch cells, to study its behavior in various operating conditions. Pouch cells are observed to expand or swell slightly due to aging. Under such conditions, when the pouch cells placed in a battery pack, there are possibilities of external pressure applied on them due to aging. Elaborate studies on the effect of external pressure characterization with temperature & varying C-rates aids in better understanding of pouch cells as an energy storage medium.
Battery management systems (BMS) are essential for a battery pack's safe operation and longevity. This paper presents an active balancing method-based BMS for different cell chemistry structures to ...be used in behind-the-meter storage (BTMS) applications. The proposed system utilizes modular isolated dual active bridge (DAB) DC/DC converters to actively balance the battery pack through a low voltage (LV) bus. A supervisory controller monitors all the cell voltage, current, and state of charge (SOC) values. Based on the estimation of the SOCs, reference currents for the DAB converters are generated by the supervisory controller. Detailed modeling and the control approach of the modular DAB converters are presented in the paper. Moreover, the control strategy of the supervisory control is also analyzed. The proposed method and structure can be extended to any combination of the number of cells to design the battery pack. Simulation results are provided for a system consisting of three cells in parallel to form a cell block and three cell blocks in series to form the battery module. Experimental results are provided for three modular DAB converters operating with a LiFeMnPO4 prismatic cell with 3.2V, 20Ah rated values.
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