Electrodeposition of high-rate Prussian blue type cathodes for Li-ion batteries

Efficient energy storage solutions are quintessential for diverse applications - renewable energy harvesting, transportation, portable electronics, automation etc. Regardless of category, high power and high energy are inevitable requirements. In general, the bottle neck is always the energy/power density of the cathode materials. For cathodes, layered metal oxides suffer from issues like low redox potential, irreversible structural changes at high potentials (>3V), poor thermal stability and cationic disorders whereas polyanionic compounds exhibit low gravimetric capacity (owing to their large molecular weight) and low cycling stability.

In this regard, Prussian blue analogues (PBAs) are promising alternative high voltage host materials which have shown promising results for alkali metal ions including Na⁺ and K⁺. They are coordination polymers having promising capacity (> 200 mAh/g), excellent rate capability (up to 50 C in certain cases), and high discharge potential (~3.5 – 4.2 V), along with synthetic ease for development of wide variety of compounds. PBA’s or metal hexacyanoferrates have the general formula, A_xM[Fe(CN)₆]_y which can reversibly store mobile cations like Li⁺, Na⁺, K⁺ etc. Since M can be replaced by many transition metals and even solid solutions of two different metals, it is possible to realize different types of PBAs depending upon the choice of the starting precursors. This provides two redox active centres per formula unit of PBAs i.e M^+2/+3 and Fe^+2/+3, which can reversibly store alkali metal ions. In addition, they are structurally stable with almost no lattice strain upon insertion of alkali ions through their large ionic channels, ideally suited for application as high-rate insertion type cathode materials for alkali metal ion batteries.

PBAs are usually synthesized via solvent-based precipitation method owing to their extremely low solubility product constants (K_sp ~ 10^-15). This causes spontaneous precipitation, usually resulting in large number of vacancies, coordinated solvent molecules in the structure. Such defect rich structures show low capacity, high overpotential and meagre electrochemical stability. An alternative route is to electrodeposit PBA’s, where their precipitation can be triggered electrochemically on the surface of the electrodes by choosing appropriate reactants and electrochemical conditions. Solution based electrodeposition of PBA’s provides more synthetic control and possibility to engineer these open framework structures compared to a solution-based process.

The prospective student will optimize electrodeposition of Fe/Mn based hexacyanoferrate to obtain low-defect PBA thin films. Various parameters like solvent additives, applied current, concentration of the reacts will be evaluated. Li⁺ will then be introduced electrochemically to obtain high voltage cathodes followed by electrochemical characterization.