Li-free Al-graphite dual-ion batteries (AGDIBs) have attracted attention for their raw material availability and lower costs compared to lithium-ion batteries.[1] Given the high power density (up to 9 kW/kgGraphite)[2] and the long cycle life of over 25,000 cycles in lab test cells, AGDIBs represent a promising energy storage solution for dynamic applications such as grid stabilization. AGDIBs, as high-power batteries, fill the gap between supercapacitors and established (high-energy) battery types.
AGDIBs utilize commercial Al-foils as anode material, natural graphite flakes as cathode material, and AlCl3-based ionic liquids, like 1-ethyl-3-methyl-imidazolium chloride [EMIm]Cl/AlCl3, as electrolyte. The system operates on reversible Al deposition and dissolution at the anode and intercalation/deintercalation of AlCl4- between the graphite layers of the cathode, see figure 1a.[1,3]
Operando-XRD can be employed to determine the intercalation mechanism of AGDIBs during the charging/discharging process, clarify the self-discharge mechanism[4], and assist in developing a structural model based on experimental results. However, the operando-XRD method faces challenges in finding an X-ray radiation-permeable window and adapting the cell design to the AGDIBs due to the electrolyte’s corrosive properties. To address this, various window materials were tested, and components in direct contact with the electrolyte were replaced with electrolyte-resistant materials, such as tantalum or PTFE.
The AGDIBs are characterized using in-house operando-XRD in reflection mode during galvanostatic cycling.
Initial measurements indicate different structural changes between the charging and discharging processes, see figure 1b. Notably, the experimental XRD measurements confirmed a stage-3 graphite intercalation compound, supporting previous DFT calculations and ex-situ data.[5]
To better understanding the structure-property relationship of the cathode materials, various graphite materials were investigated with respect to crystallinity and morphology. Insights into the intercalation behavior of different graphite materials will allow for further enhancements in the performance of AGDIBs.