Low cost & quasi solid state Na₂Mn_0.5Ni_0.5Fe(CN)₆//Na_xFe₂O₃ hybrid Na-ion batteries for solar energy storage

Herein, we have explored low cost Prussian blue analogues (PBA), namely Na₂MnFe(CN)₆ (Mn-PBA) and Na₂Mn_0.5Ni_0.5Fe(CN)₆ (MnNi-PBA), as positive electrodes and iron oxides, namely Fe₂O₃ and Na-incorporated Fe₂O₃ (Na_xFe₂O₃), as negative electrodes for quasi solid state Na-ion batteries. The detailed electrochemical characterization reveals the fact that MnNi-PBA shows superior positive electrode performances to Mn-PBA, while Na_xFe₂O₃ depicts improved negative electrode performances with respect to Fe₂O₃. The comparative electrode performance data are corroborated by bond-valence energy landscape (BVEL) and density of states (DOS) calculations. Moreover, a 1.6 V full cell with MnNi-PBA positive and Na_xFe₂O₃ negative electrodes is assembled in organic-inorganic hybrid hydrogel electrolyte, which demonstrates a specific capacity of 48 mA h g⁻¹ (@ 50 mA g⁻¹), an energy density of 77 W h kg⁻¹ (@ 118 W kg⁻¹), a power density of 803 W kg⁻¹ (@ 56 W h kg⁻¹) and good cycle life (80% capacity retention after 250 cycles at 50 mA g⁻¹) in the 0-1.6 V regime. Further, to validate the solar energy storage applications, a 3.2 V prototype device is fabricated by the series connection of two 1.6 V cells, which is capable to illuminate a 3 V/270 mW light-emitting diode (LED) setup, and successfully demonstrates a one week-long solar-charge/LED-discharge cycling module.