Ionometallurgical Step-Electrodeposition of Zinc and Lead and its Application in a Cycling-Stable High-Voltage Zinc-Graphite Battery

Metals find many applications in numerous fields, including energy storage and microelectronics. The extraction of metals from minerals and naturally occurring ores and the associated waste recovery is a growing research area. In particular, there is an urgent need to develop less energy-intensive, cost-effective and ecosystem-friendly methods for extracting metal oxides from their natural resources and producing metals from their respective compounds to reduce the amount of greenhouse gases emitted into the atmosphere. Ionic liquids (ILs) create a favorable reaction environment for sustainable electrodeposition of metals owing to their remarkable properties. This process, also known as ionometallurgical, is a promising alternative to traditional electrometallurgical processes.

ILs have many uses in chemistry. Most importantly, emerging IL analogies have been used as electrolytes for metal electrodeposition in rechargeable batteries. Such batteries mostly used anode electrodes made of zinc (Zn) and lead (Pb). Despite the remarkable properties of Zn, zinc batteries exhibit low cell voltage and poor cycling performance due to lack of cathode materials, high voltage electrolytes during charging and Zn dendrite problems. On the other hand, lead sulfate and lead dioxide, the commonly used compounds in lead-acid batteries, are insoluble in first-generation chloridoalumiante and second-generation air- and water-stable ILs. Thus, a highly efficient ionometallurgical process is required for the efficient electrodeposition of zinc and lead.

Previous research findings revealed that the ionic liquid betainium bis((trifluoromethyl)sulfonyl)imide, [Hbet][NTf₂] is suitable for this process attributed to its redox-stability and specific-functionalization. Although there are several reports on the electrodeposition of metals from their oxides dissolved in [Hbet][NTf₂], there is limited study on their electrochemistry for zinc and lead. Moreover, there is inadequate information regarding the electrochemical processes, the stability of the ILs and the influence of the process parameters on the product morphologies.

On this account, PhD graduate Peng Chen, Ms. Janine Richter, Dr. Gang Wang, Mr. Dongqi Li, Mr. Tobias Pietsch and Professor Dr. Michael Ruck from Technische Universität Dresden conducted an ionometallurgical electromechanical deposition study of metallic lead and zinc and their corresponding application in developing high-voltage and cyclic-stable zinc-graphite batteries (ZGBs). The thermal and electrochemical stability of the PbO and ZnO solutions, their ionic conductivity in [Hbet][NTf₂], as well as the effects of potential and temperature on the morphology of the deposited materials, were examined. Also, the performance of zinc layer deposited from the ionic solution in 3D copper foams assembled in high voltage ZGB was tested. Their work is currently published in the journal, Small.

The research team showed that the IL was relatively stable under the selected conditions enabled by the [Hbet][NTf₂]. The temperature and potential conditions exhibited a remarkable influence on the morphology of the electrodeposited metal. For the IL solutions consisting of dissolved PbO, ZnO and MgO, the deposition of metallic Pb and Zn under potentiostatic control was achieved by either co-electrodeposition or constructive step-electrodeposition fashion. Furthermore, Zn was successfully deposited on 3D copper foam and subsequently assembled into high-voltage ZGB. The resulting ZGB exhibited a working voltage up to 2.7V, an output midpoint discharge voltage of 2.16V, improved rate and cycling performance and 98.6% increase in capacity retention after 150 cycles.

In summary, the study reported an ionometallurgical step-electrodeposition of zinc and lead dissolved in [Hbet][NTf₂]. Generally, step-electrodeposition is suitable for deposition of metals separately with distinct electrochemical potential from complex IL solutions. [Hbet][NTf₂] provided suitable conditions for electrodeposition due to its ability to dissolve various naturally occurring metal oxides and sulfides. Moreover, the electrodeposited metal was used to fabricate Zn/3D-Cu anode that showed excellent performance attributed to inhibition of Zn dendrite formation due to its large specific surface area in high-voltage ZGBs. In a statement to Advances in Engineering, Professor Dr. Michael Ruck explain their study findings could contribute to sustainable ionometallurgical processes for large-scale industrial production of high-performance zinc-graphite batteries.