New materials for future batteries

Rechargeable lithium (Li)-water and Li-air/O2 batteries are important targets for research as they have at least 10-30 times higher theoretical energy density (~5-12kWh/kg) than the conventional rechargeable electricity storage systems (~0.4 kWh/kg) and thus come close to gasoline (13 kWh/kg). In addition, Li-metal provides a greater specific energy density (3800 Ah/kg) than other metal-O2 batteries (820 Ah/kg for Zn or 2900 Ah/kg for Al). The challenge is to make them work reversibly and safely. Furthermore, extreme pH conditions lead to technical problems such as membrane clogging.

The goal of this collaborative project was to develop a new generation of safe, reversible Li-air (and Li-water) batteries that can be used as stand-alone storage systems providing high energy density. The researchers aimed to develop stable and safe non-aqueous and aqueous electrolytes, new cathode materials that resist high pH and clogging, as well as a full battery protocol.

The new crown-ether-based ionic liquids show superior properties compared with commercial electrolytes. They provide a higher cation transference number (>0.5) than commercial electrolytes alone (0.3–0.4). Their high ionic conductivity, together with high thermal and chemical stability, non-flammability and good electrochemical stability, promise enhanced safety. The newly optimised pH-controlled aqueous electrolyte showed no clogging at the cathode and a reversible redox reaction in oxygen.

After systematically studying the MoS2 lamellar membrane, the researchers now understand the excellent stability of MoS2 in harsh aqueous environments, which lends this material potential utility in the electrolyte solution over a wide range of pH values. The introduction of edges further enhances the hydrogen evolution reaction (HER) activity, which would improve the Li-water battery performance. Benefiting from this promising MoS2 catalytic activity, a stable voltage supply of >2.2V for more than 2.5 days was obtained. It is envisioned that this novel MoS2-based material will promote the construction of a series of new energy conversion and storage systems.

Implications for research

For the first time, systematic investigation of an MoS2 lamellar membrane has revealed properties such as stability, molecular sieving and selectivity diffusion of the lamellar membrane material. The study has also enriched the understanding of molecular transport in 2D channels in an aqueous environment. Moreover, the catalytic activity of MoS2 in a HER shows, for the first time, promising performance for a Li-water battery, lending potential adaptability to electrical energy conversion and storage. The researchers also established a new class of compounds – crown-ether-based ionic liquids ¬– as new and very promising electrolytes that are stable up to 380°C, non-flammable and possess excellent ion-transport properties.

Implications for practice

An easy way of preparing the MoS2 membrane without any need for additives or sticking agents would facilitate mass production. The stability of MoS2 lamellas in a harsh aqueous environment, as demonstrated, warrants potential application in a variety of electrolytes used in energy conversion and storage devices. The successful demonstration of a Li-water battery could lead to an energy and hydrogen (H2) supply system, for example a pure-H2 generator for supplying fuel to a hydrogen fuel cell.

The newly developed ionic liquids can be widely applied in various types of alkali-metal batteries with little modification of design thanks to its tuneable structure. At most, they are directly usable not only in Li-O2 batteries but also in advanced Li-ion batteries because of their high Li+-ionic conductivity and wide-ranging electrochemical stability.

New rechargeable metal-water and metal-air batteries: fundamental science & feasibility