Amorphous, binder-free cobalt manganese phosphate cathodes prepared by SILAR method for asymmetric supercapacitors: Harnessing cationic synergy

The rational design of electrode materials with structural flexibility and robust electroactive sites governed by the synergy of cations in bimetal compounds is obligatory to maximize the efficiency of energy storage devices. In light of this, amorphous nanoparticles of cobalt manganese phosphate thin film [Co_xMn_3-x(PO₄)₂·nH₂O] electrodes (S-CMP series) with different compositions of Co and Mn cations are prepared via the successive ionic layer adsorption and reaction (SILAR) method in the present work. The cobalt manganese phosphate (CMP) nanoparticles thin films were directly used as binder-free active electrodes, and synergy between cations (Co:Mn) at optimal composition (∼0.75:0.25) provides a maximum specific capacitance of 743 F g⁻¹ at 2.8 A g⁻¹, with 90.6 % capacitance retention over 4000 cycles. Additionally, an asymmetric aqueous supercapacitor (AAS) and an asymmetric solid-state supercapacitor (ASSS) devices were evaluated in 1 M KOH and PVA-KOH as aqueous and gel electrolytes, respectively, using as-prepared CMP (S-CMP-4) as the cathode material and rGO as the anode material. High specific energies of 45.31 Wh kg⁻¹ and 16.29 Wh kg⁻¹ at specific powers of 0.79 kW kg⁻¹ and 0.82 kW kg⁻¹ are displayed by the AAS and ASSS devices, respectively. Based on the practical demonstration of an ASSS device to power 201 red LEDs, the cobalt manganese phosphate thin film cathodes seem to offer insights into commercialization. Overall, the remarkable electrochemical performances of both AAS and ASSS devices thus demonstrate that amorphous, nanoparticle-like cobalt manganese phosphate thin film prepared by SILAR are efficient binder-free cathodes for prospective applications in energy storage devices.