Conversion Electrodes for Rechargeable Li-Sulfur, Na-Ion and Zn-Air Batteries
Authors: Brehm Wolfgang , Kowal Julia 
Published: August 2024

Rechargeable batteries are reliable and highly efficient energy storage devices providing high energy density at high voltages with the lead of the Li-ion technology since the early 1990s. Despite these promising advantages, the strongly limited abundance of Li and also that of other elements contained in a Li-ion battery (LIB) leads to the search for low-cost and more abundant alternatives. Especially for stationary storage devices alternative battery technologies are required. Alternative chemistries, such as those based on lithium conversion or alloying, can actually lead to higher energy density compared to the lithium (de)intercalation one.Among them, the lithium-sulfur (Li/S) conversion process appeared as one of the most promising candidates to achieve a lithium battery with enhanced performances compared to the state-of-art. In fact, sulfur electrode can deliver in lithium cell a theoretical capacity and energy density as high as 1675 mAh gS
−1 and 2600 Wh kg−1 Unfortunately, soluble polysulfides can migrate and directly react with the lithium anode or shuttle between anode and cathode throughout a continuous process without any charge accumulation. This leads to efficiency decrease, active material loss or even to short circuits and cell failure, whilst insoluble polysulfides can precipitate into the cell and cause resistance increase and capacity fading. The characteristic electrochemical process involving at the cathode side the electro-deposition/dissolution of soluble species focused the attention on the nature of the current collector. Hence, flat and thin metal supports (e.g., bare Al current collector) may lead to poor performances due to high overall impedance of the cell and modest ability in allowing the complex multi-step reaction pathway, whilst thicker porous supports (e.g., gas diffusion layer, GDL) can enhance the cell response, reduce the impedance and actually boost the kinetics of the Li/S process, but suffer from its higher thickness, lowering down the volumetric energy density of the overall cell.In this view, graphene-based materials can actually allow the thinnest configuration of a carbon-coated metal support and hold, at the same time, a suitable Li/S process due to their characteristic morphology, mechanical stability and enhanced electronic conductivity. In regard to this, research activities are going to be presented about an exploited binder-free few layer graphene (FLG) thin carbon-coated Aluminum support for application in a Li/S cell with excellent characteristics in terms of stability, efficiency and delivered capacity.

Conversion electrodes play also an important role for sodium-ion batteries (SIBs) being a promising alternative, since Na cells show similar properties to the Li analogues in many cases, while they are based on more environmentally friendly and/or more abundant materials. The use of SIBs often results slightly lower energy densities and cell voltages than those of LIBs, but they still provide significantly better values than lead-acid batteries, which are currently dominating the market in terms of annually manufactured capacity. Tin (Sn) and its compounds are well investigated alloy electrodes for SIBs as tin provides a very high theoretical specific capacity of 847 mAh g−1. Sn is an example that shows even better capacity retention when used in SIBs compared to its use in LIBs. Moreover, transition metal compounds containing phosphorus (P) and/or sulfur (S) are promising conversion electrodes as well, since these elements enable high specific volumetric and/or gravimetric capacities in LIBs and SIBs.

In addition to LIBs and SIBs, zinc batteries are also attracting research interest due to their environmental friendliness and their applicability with aqueous electrolytes. However, especially zinc-air batteries are well established for use as primary cells, but when considered for use as secondary cells, they suffer from their low rechargeability. Despite great research efforts to solve these problems, no established system has yet been able to achieve reasonable rechargeability in the context of using the commonly considered alkaline electrolytes. However, Sun et al. (2021) found a promising approach using Zn(OTf)2 salt in a small amount of water as an electrolyte. This approach enables the formation of zinc peroxide (ZnO2) instead of formation the irreversible zinc oxide (ZnO) in alkaline electrolytes, which deposits on the gas diffusion layer electrode, blocking the oxygen evolution reaction. ZnO2, on the other hand, has been shown to be highly reversibly malleable. In our current project WaZABi, we are trying another approach using a freeze-cast zinc anode, which enables high porosity, and the application of a hybrid electrolyte under participance of a polymer for rechargeable high performance zinc air batteries.