Researchers from the University of Ulm and the University of Freiburg in Germany have recently developed a new positive electrode material that enables high storage capacity for aluminum-ion batteries, solving the biggest drawback that has prevented this promising battery type from wide use.
The scientists used an organic redox polymer based on phenothiazine as a new electrode material that exhibits a surprisingly high storage capacity that surpasses that of graphite electrodes.
Aluminum-ion batteries have long been attractive as an alternative to conventional lithium-ion batteries. This is mainly because aluminum, a common and widely available material, is recyclable and inexpensive compared to lithium, a rare and expensive raw material. However, the main drawback of aluminum-ion batteries is their insufficient storage capacity. The challenge is the lack of appropriate host electrode materials that can enable the reversible insertion of complex aluminum ions.
The storage capacity known as specific capacity (mAh/g) is the amount of electric charge (mAh) delivered by the electrode per gram of material. Since the specific capacity is mainly determined by electrode material, the researchers have been experimenting with various positive electrode materials to increase an aluminum-ion battery’s electric charge. Their work and results were published in the journal Energy & Environmental Science.
Electrode material that inserts complex aluminum anions
To improve the electrode material for Al-ion batteries, the scientists’ approach was to find a more effective mechanism to insert complex aluminum anions in the electrode with high reversibility. Generally, p-type organic compounds can be reversibly oxidized at high potentials, storing and releasing anions at fast C rates. The researchers focused on developing new organic redox-active materials that exhibit high performance and reversible properties. For the first time, they successfully demonstrated a reversible two-electron redox process for a phenothiazine-based electrode material.
As a positive electrode material, the researchers used an organic redox polymer capable of reversibly inserting two anions (AlCl4), providing higher specific capacity compared to graphite. These electrodes are marked as X-PVMPT, which means cross-linked poly(3-vinyl-N-methylphenothiazine). The tested battery uses a liquid EMIm chloroaluminate electrolyte, which is considered the best electrolyte for Al-ion batteries when comparing the cost, electrochemical stability and temperatures that maintain the electrolyte in a liquid state.
The obtained experimental specific capacities were up to 167 mAh/g, which is higher than the graphite electrode (the graphite-specific discharge capacity is limited to 120 mAh/g). The higher specific capacity of an electrode indicates that more aluminum ions can bind to the electrode. Practically, this provides a larger battery storage capacity, which would mean, for example, longer driving ranges for electric vehicles.
Additionally, the new electrode provides superior cyclability at fast C rates, a measure of how fast the battery is charged or discharged. For example, 1C means full battery charging/discharging in one hour, 0.5C in 2 hours, and 10C in 6 minutes. A higher C rate means more stress on the battery, and dangerous dendrites can be formed on the electrode, shortening the lifespan and increasing the possibility of cell failure. The battery’s ability to recover capacity after a higher C rate provides the possibility of fast charging and high discharging current in demanding applications.
The experimental results showed that the battery holds 88% of its capacity even after 5,000 cycles at a 10C rate. The tests were performed at an extremely high 100C rate where the battery remained at 64 mAh/g. At lower C rates, the battery’s original capacity remained unchanged.
X-PVMPT-based electrodes insert negative ions (anions AlCl4 or Al2Cl7) at average charge potentials of 0.81 and 1.65 V versus positive aluminum ions. As the difference between the electrode potentials is larger, the electromotive force (EMF), or cell voltage, is higher. Thus, the cell is capable of producing a higher amount of energy. Cell voltage is determined by the compatibility of all battery parts—the anode, cathode and electrolyte. The experiment also showed that the plateau at a higher potential (average 1.48 V) favorably contributes to a higher amount of the specific capacity than the plateau at a lower potential (average 0.74 V), proving that a larger amount of charge can be stored in the new electrodes at the higher potential.
“With its high discharge voltage and specific capacity, as well as its excellent capacity retention at fast C rates, the electrode material represents a major advance in the development of rechargeable aluminum batteries and thus of advanced and affordable energy storage solutions,” said researcher Birgit Esser in a press release issued by the University of Freiburg.
This research represents the first insight into the aluminum-ion battery performance that could be achieved using the phenothiazine-based electrode with a reversible two-electron redox process. With its high discharge voltage and specific capacity, as well as its excellent ability to retain capacity at fast C-rates, this research could enable the development of advanced and affordable Al-ion batteries. The results could also initiate further research and improvements on positive electrode materials based on organic redox polymers.