We report here 18650-type sodium-ion battery (NIB) with Prussian Blue Analogue Na 2 Fe 2 (CN) 6 in both monoclinic and rhombohedral phases as the cathode and hard carbon (HC) as the anode using the glyme-based non-flammable 1 mol dm −3 NaBF 4 electrolyte. Rhombohedral-Na 2 Fe 2 (CN) 6 (RPB) vs HC 18650-type cell delivered an energy density of 43 Wh kg −1 , achieving good high rate response up to 4.0 C, stable cycling over 100 cycles with 99.99% average coulombic efficiency and 94.8% average roundtrip-energy-efficiency. A comparison of the calorimetric studies performed on 18650-type cells revealed lower heat generation in RPB vs HC compared to monoclinic-Na 2 Fe 2 (CN) 6 .2H 2 O (MPB) vs HC counterpart. Moreover, the RPB vs HC cell demonstrated lower heat generation than commercial NMC vs graphite 18650-type lithium-ion cells. Internal resistance, which is the major contributor to heat generation, is assessed by analysing the impedance spectra of the cells. Furthermore, variation in subcomponents of internal resistance across different depths of discharge determined by fitting impedance data using an equivalent circuit model and analysis using distribution of relaxation times (DRT) method is presented for 18650-type sodium-ion cells for the first time. The obtained results indicate that these efficient and safe 18650-type NIBs open-up new opportunities for exploring innovative storage systems for stationary applications.
In this manuscript, the impact of operating conditions such as voltage window, and operating temperature on electrochemical performance and cycle life of Zn-substituted Na3.2V1.8Zn0.2(PO4)3 (NVZP) vs hard carbon (HC) coin cells filled with 1 mol dm−3 NaBF4 in tetraglyme is presented. Initially, the cells are cycled for 500 times at C/2 charge and 1 C discharge in three different voltage windows (4.20–1.00 V, 4.05–1.00 V and 4.05–1.50 V) and at two temperatures (28 °C and 40 °C) and are subjected to periodic internal resistance and impedance measurements. The elemental composition of the electrodes harvested after cycling reveals that vanadium dissolution with accompanying deposition on the HC electrode and irreversible loss of sodium causes increased cell impedance. The identified degradation mechanisms, which causes severe capacity fade, are found to be accelerated in the cells cycled over wider voltage windows, particularly at elevated temperature. The best cycling performance and lowest impedance are recorded for the cells cycled within 4.05–1.50 V at 28 °C owing to negligible vanadium dissolution. Under these optimized testing conditions, a prototype 18650 cell, shows impressive capacity retention of 77% after 1000 cycles.
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