Analysis of Heat Generation and Impedance Characteristics of Prussian Blue Analogue Cathode-based 18650-type Sodium-ion Cells

We report here 18650-type sodium-ion battery (NIB) with Prussian Blue Analogue Na2Fe2(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 NaBF4 electrolyte. Rhombohedral-Na2Fe2(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 round-trip-energy-efficiency. A comparison of the calorimetric studies performed on 18650-type cells revealed lower heat generation in RPB vs HC compared to monoclinic-Na2Fe2(CN)6.2H2O (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.