A key challenge facing future battery technologies is finding alternatives for currently used raw materials, which are often expensive and are becoming increasingly scarce (e.g. Li, Co, Ni). Thus, to tackle the strongly increasing demand of electrochemical storage, a variety of battery technologies will be needed adjusted to the specific requirements of each individual application.
In recent years, the Li free Al-graphite-dual-ion batteries (AGDIBs) have gained much attention due to their low-cost and sustainability. Commercial Al-foils and natural graphite flakes are employed as electrode materials, AlCl3-based ionic liquids such as AlCl3/[EMIm]Cl as anolyte. Chloroaluminates Al2Cl7– and AlCl4– within the anolyte are actively involved in Al dissolution and deposition on the Al electrode, whereas AlCl4– is (de)intercalated between layers of graphite on the positive electrode. Albeit AGDIBs offer just a moderate energy density similar to lead-acid and nickel-metal hydride batteries, rate capability (20 A/gGraphite), power density (9,000 W/kgGraphite) and cycling stability (N > 500,000 cycles) are enormous. Using an AlCl3/urea deep eutectic solvent anolyte further reduces overall costs while adapting the same working principle and exhibiting acceptable battery performance. AGDIBs fill the gap between supercapacitors and established (high-energy) battery systems and are a promising high-power energy-storage system suitable for stationary and hybrid mobile applications, such as grid stabilization with its highly dynamical requirements.
However, the batteries’ poor shelf life is a challenge for application that has rarely been investigated and is poorly understood. Thus, a set of experiments was designed to quantify the capacity loss due to self-discharge dependent on idling time. For a typical Al || AlCl3/[EMIm]Cl || Graphite cell with Mo current collector, a reversible capacity loss of ca. 10 % is observed after 24 h of idling time. For a longer idling time, these losses increase significantly. However, even after an idling time of one month, no irreversible losses occur, and the battery directly regains its initial capacity after just one recharging step. Post-mortem Raman spectroscopic investigations under inert conditions revealed deintercalation reactions within the graphite cathode as cause for the capacity losses. Influence factors such as electrolyte species, electrolyte impurities, current collector material and applied potential window were investigated, pointing towards electrolyte decomposition products involved in the reaction. It can be anticipated that these first insights will enable a fundamental understanding of self-discharge processes allowing for a stable cycle life of Al-graphite batteries in the future.