“The membrane we made has superior thermal and electrochemical stability to high voltage (greater than 6V),” says team leader Eric Wachsman. “Its high mechanical stability also allows it to block the formation of unwanted Li dendrites, so allowing for a high current density of 0.2 mA/cm2 for a battery that was tested for around 500 hours. What is more, the fact that it is flexible means that it can be made using conventional low-cost roll-to-roll processing techniques.”

Rechargeable lithium-ion batteries are ubiquitous in portable electronics today and consist of two electrodes – anode and cathode – separated by an electrolyte. When the battery is being charged with electrical energy, lithium ions move from the cathode through the electrolyte to the anode, where they are absorbed into the bulk of the anode material.

Metallic lithium anodes

However, as portable devices become ever more energy-hungry, researchers are looking to boost the amount of energy that can be stored in the batteries. To this end, they are developing anodes made from metallic lithium anodes rather than the conventional ion intercalation anode materials. Lithium has the highest specific capacity (of 3860 mAh/g) and the lowest negative electrochemical potential (of around 3.040 V), which could maximize the capacity density and operating voltage window for increased battery energy density.

Using lithium metal in organic liquid electrolytes is not without challenges however – both in terms of safety and battery performance. For example, lithium-sulphur batteries suffer from the fact that intermediate polysulphides produced during operation dissolve in the electrolyte. This can cause severe parasitic reactions on lithium metal surfaces, which degrade the metal and make lithium cycling inefficient.

Lithium-oxygen batteries fare no better because chemically instable electrolytes on the oxygen electrode limit battery cycling too.

Dendrite growth is a problem

One of the main challenges to overcome when using lithium metal is lithium dendrite growth on lithium metal anodes. These dendrites short-circuit the battery because they penetrate the device separator and touch the cathode. Other problems include solid-electrolyte interphase formation (SEI) during uneven lithium deposition. SEI eventually consumes the Li metal and dries up the electrolyte, and so increases the resistance of the cell and decreases its efficiency.

One way to stop the Li dendrite and SEI problems would be to employ a solid-state electrolyte that is mechanically strong enough to suppress the spread of dendrites and which eliminates SEI production. The new membrane, which is based on garnet-type solid-state electrolyte nanofibre networks, developed by the Maryland team happily fits the bill here.

The researchers made their ceramic nanofibres by electrospinning precursors of garnet-type (Li6.4La3Zr2Al0.2O12 or LLZO) ) Li-ion conductors. They then calcinated the material at high temperatures to obtain a network of fibres.

The team, reporting its work in PNAS doi: 10.1073/pnas.1600422113, is now busy optimizing and improving the electrochemical performance of its flexible, solid-state membrane. “We will be be trying to apply the membrane to different types of Li batteries to improve their performance,” Wachsman tells nanotechweb.org.