Hard Carbon Composite Electrodes for Sodium-Ion Batteries with Nano-Zeolite and Carbon Black Additives

Abstract

To enable fast charging of sodium-ion batteries and eliminate metallic dendrite growth on the electrodes an improvement in electrode design is required. In this work, we show the benefit of a mixed composite electrode containing ionic and electronic conducting additives for a sodium-ion battery negative electrode. Hard carbon electrodes with 5 % additive containing different proportions of zeolite and carbon black are coated. The performance of the electrodes is elucidated through electrochemical and physical characterization methods; fast sodiation, electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration techniques (GITT) and electron microscopy. The addition of zeolite improves the sodium-ion transport diffusivity within the composite electrode by an order of magnitude at low voltages and high states of charge. EIS shows significantly lower series and solid-electrolyte interphase (SEI) resistances in the zeolite containing electrode after cycling. The capacity retention at higher rates is improved and a significant reduction of sodium dendrite growth was observed after cycling. SEM images confirm that porosity is still present in the zeolite containing electrode samples, enabling a pore network for sodium ion transport. These results emphasize the importance and limitations of ionic transport within hard carbon electrodes, and the required optimisation between electronic and ionic conductivity for sodium ion transport in these electrodes.