Since the Industrial Revolution, energy has been playing an increasingly important role in human survival and development. However, global energy and climate change crisis has stared us in the face as a result of excessive consumption of carbon-based fossil fuels. In order to fulfil the “carbon neutrality” (net zero carbon) ambition embedded in the Paris Agreement to keep the global warming to less than 2 oC (or even 1.5 oC), any actions that lead to emissions must be accompanied by other actions that confidently reduce (or offset) emissions. To reduce the carbon emissions, replacing traditional fossil fuels with clean energy harvested from renewable resources (for example, wind, solar, tidal, geothermal and so on) is considered one of the most promising paths. Unfortunately, most renewable energies possess intermittent and regional nature, which further makes energy-storage technologies indispensable alongside.
Electrochemical energy storage technologies, realizing the mutual conversion between electrical energy and chemical energy, are one of the most promising candidates. Of these, lithium-ion batteries (LIBs), benefiting from high energy density, long cycle life, no memory effect and low self-discharge, have had a profound impact on modern society and have become a ubiquitous commodity in our daily life since the first commercialization by Sony in 1990. Nevertheless, the use of flammable and toxic organic electrolytes posing safety risks, and the fragile supply of key raw materials (for example, lithium and cobalt), in terms of both quantity and price, brings about new sustainability challenge. Thus, the development of alternatives complementary to LIBs becomes increasingly urgent. Aqueous batteries offer tremendous competitiveness in terms of safety, cost and environmental benignity. In recent years, rechargeable aqueous zinc (Zn) metal batteries, featuring intrinsic safety and low cost due to the high abundance of key materials and excellent ease and environmental friendliness in manufacturing, storage, use and recycling, have provoked extensive attentions from researchers. They also have a high energy density benefiting from a high specific capacity (820 mAh g−1 and 5,855 mAh cm−3) and a favourable redox potential (−0.76 V versus the standard hydrogen electrode) of the Zn metal anode. These fascinating merits make Zn metal batteries be considered as a sustainable alternative to LIBs, especially in grid energy storage. However, the implementation of this technology still has to overcome some critical technical barriers involving Zn dendrite growth and side reactions in water (for example, hydrogen evolution and corrosion). The former jeopardizes the cycle life (the number of complete charge discharge cycles a battery can undergo) whereas the latter shortens their calendar life (the time for which a battery can be stored, as inactive or with minimal use) by continuous consumption of both electrolyte and Zn anode, which seriously limits the practicality of Zn batteries as a viable energy storage medium.
Since these problems originate directly or indirectly from the interaction between the metallic Zn anode and the aqueous electrolyte, why not just directly use a completely Zn-compatible organic solvent? But the intrinsic flammability of most organic solvents and the high cost of the known non-flammable fluorine- and phosphorus-containing organic solvents shall compromise the high safety and low cost of Zn batteries, respectively. So, can coupling Zn salts with low-cost flammable organic solvents enable the electrolyte non-flammability? This is highly desirable but has never been achieved as far as we know. To this end, we explored using hydrate tetrafluoroborate (Zn(BF4)2·4.5H2O), an industrial-scale salt that is widely used in electroplating and textile manufacturing as a flame retardant but rarely used in Zn batteries.
In our recent work published in Nature Sustainability, we realized a low-cost hydrous organic electrolyte for sustainable Zn metal batteries by coupling a hydrated Zn(BF4)2 salt with an ethylene glycol (EG) solvent. The electrolyte not only promotes the formation of a favourable ZnF2 passivation layer to enable Zn long-term cycling stability but also embraces excellent non-flammability. Equally intriguingly, we found this hydrous organic electrolyte also shows outstanding operating capability across a wide temperature range from −30 oC to 40 oC without serious performance compromise because of strong hydrogen bonding among the various ingredients. Impressively, the present electrolyte has a competitive price compared with the normally used ZnSO4 electrolyte, which comes from the industrially used and low-cost materials (namely the salt and the solvent). This work not only develops a sustainable electrolyte to help address issues surrounding Zn batteries, but also suggests a promising direction for developing electrolyte solutions for practical Zn batteries and beyond which combine safety, performance and sustainability.
Figure 1 | Properties of the proposed electrolyte.
For more details of this work, please see our recent publication in Nature Sustainability: A non-flammable hydrous organic electrolyte for sustainable zinc batteries, https://www.nature.com/articles/s41893-021-00800-9.