A safer, cheaper and more flexible battery invented for wearable technology

Fitness tracker. Smart watches. Virtual reality headsets. Even smart clothes and implants. Smart wearable devices are everywhere these days. But to improve the convenience, reliability and longevity of these devices, they need to become more flexible and reduce the size of their energy storage units, which are often frustratingly bulky, heavy and fragile. Furthermore, improvements should not come at the expense of safety.

In recent years, much battery research has focused on the development of flexible microenergy storage (MFESD). A number of different structures and electrochemical principles have been explored, of which aqueous microbatteries offer many advantages.

Water batteries, i.e. those that use a water solution as the electrolyte (the medium that allows ions to be transferred in the battery and thus create an electrical circuit), are not new. They have been around since the late 19th century. a century. However, its energy density, i.e. the amount of energy contained in the battery per unit volume, is too low to be used in electric cars because it requires too much space. Lithium-ion batteries are more suitable for these purposes.

At the same time, water-based batteries are less flammable and therefore safer than lithium-ion batteries. It is also much cheaper. Due to this strong safety and low cost, aqueous options are increasingly being explored as one of the best options for MFESDs. These are called micro-aqueous batteries or simply AMBs.

“Unfortunately, so far, AMBs have not fulfilled their potential,” says Qi Niu, a materials scientist at the Guangxi Key Laboratory of Optical and Electronic Materials and Devices at Guilin University of Technology, and one of the lead researchers on the team. “In order to be used in a wearable device, it must be able to withstand a certain amount of real-world bending and twisting. However, most materials studied so far fail under such loads.”

To overcome this problem, any breaks or errors in the AMB will need to self-heal after this load. Unfortunately, the self-healing AMBs developed to date generally rely on metal compounds as charge carriers in the battery circuit. This has the unwanted side effect of causing a strong interaction between the metal ions and the materials that make up the electrodes (the positive and negative electrical conductors of the battery). This in turn reduces the battery's reaction rate (the speed at which the electrochemical reactions occur at the core of each battery), which significantly limits performance.

“So we started exploring the possibility Non-metallic “They are charge carriers, because they will not have the same difficulties when interacting with electrodes,” added Junjie Shi, another senior member of the team and a researcher at the School of Physics and the Center for Nanoscale Device Characterization (CNCD). Huazhong University of Science and Technology, Wuhan.

The research team discovered ammonium ions, which are obtained from abundant ammonium salts, as ideal charge carriers. It is much less corrosive than other options and has a wide electrochemical stability window.

“But ammonium ions are not the only ingredient in the recipe needed to make our batteries self-healing,” said Long Zhang, the third lead member of the research team, also at CNCD.

To do this, the team incorporated ammonium salts into a hydrogel – a polymer material that can absorb and store a large amount of water without changing its structure. This gives hydrogels amazing flexibility – exactly the kind of self-healing properties needed. Gelatin is probably the best known hydrogel, although in this case the researchers chose polyvinyl alcohol (PVA) hydrogel because of its high strength and low cost.

To improve compatibility with the ammonium electrolyte, titanium carbide – a “2D” nanomaterial with only one atomic layer – was chosen as the anode (negative electrode) material due to its excellent conductivity. For the cathode (positive electrode), manganese dioxide, already widely used in dry cell batteries, was woven into the carbon nanotube matrix (again to improve conductivity).

Tests of the self-healing battery prototype showed that it has excellent energy density, power density, cycle life, flexibility, and self-healing even after ten self-healing cycles.

The team now wants to further develop and improve the prototype to prepare it for commercial production.