Electrochemically Induced Deposition (ECiD) of Lithium Phosphate and the Effect of Reaction Parameters

Abstract

Electrochemically induced deposition (ECiD) is a versatile method for the synthesis of thin films of oxides, phosphates and metal organic frameworks on conductive substrates. The process is based on the electrochemical reaction of a probase, often water electroreduction which generates H2 and OH− species, with the latter triggering a deposition reaction in the near-surface diffusion layer. In this work, the ECiD of lithium phosphate was investigated experimentally and by means of numerical modelling. In this system, the electrogenerated OH− deprotonates phosphate species near the surface and precipitates Li3PO4 hemispheres. A growth mechanism is proposed for the ECiD of Li3PO4 where surface-adhered H2 bubbles act as nucleation and growth centers for precipitation. During further growth the subsequently formed H2 gas escapes via grain boundaries along the radially oriented needle-shaped grains. Hence, the growth process creates its own “chimney” structure. The effect of the reaction parameters on the reaction, specifically the effect of current density, initial phosphate concentration, and bulk pH were investigated and used to validate the model. The addition of ethanol as co-solvent and surfactant, and cetyltrimethylammonium bromide (CTAB) as surfactant and micellular species, was shown to affect morphology greatly. Strategies for optimizing the parameters towards desired morphologies are discussed.