Research team develops semi-solid electrodes to prevent short circuits in next-generation lithium batteries

To push the boundaries of battery design and pack more power and energy into a given space or weight, researchers are exploring a more promising technology that uses solid electrolyte materials, rather than liquid electrolytes, between the two electrodes of a lithium-ion battery.

(Image source: MIT)

However, there has always been a problem with this type of battery, that is, metal dendrites will form on one of the electrodes, eventually connecting the electrolyte and short-circuiting the battery. According to foreign media reports, researchers at the Massachusetts Institute of Technology (MIT) and other institutions have now found a way to prevent dendrite formation, which is expected to enhance the potential of this new type of high-power battery.

MIT researchers included graduate student Richard Park and professors Yet-Ming Chiang and Craig Carter, with additional researchers from Texas A&M University, Brown University and Carnegie Mellon University.

Solid-state batteries have attracted much attention because of their combination of safety and energy density. But "the only way to achieve energy density is to use metal electrodes," said researcher Yet-Ming Chiang. Coupling metal electrodes with liquid electrolytes can achieve good energy density, but it does not achieve the same safety advantages compared to solid electrolytes. Solid-state batteries only make sense with metal electrodes, but the development of such batteries is hampered by the growth of dendrites, which eventually fill the gap between the two electrode plates and cause the battery to short-circuit. It is well known that in the case of fast charging, the greater the current, the faster the dendrites form. Currently, the current density that can be achieved by experimental solid-state batteries is far lower than the requirements of commercial rechargeable batteries. But researchers believe that its development prospects are good because this experimental version of the battery can store almost twice as much energy as traditional lithium-ion batteries.

The team took a compromise between solid and liquid to solve the dendrite problem. The researchers made a semi-solid electrode in contact with a solid electrolyte material. The semi-solid electrode can provide a self-healing surface at the interface, rather than a solid brittle surface, which can cause tiny cracks and pave the way for dendrite formation.

The inspiration came from experimental high-temperature batteries, in which one or both electrodes are made of molten metal. According to reports, such molten metal batteries can reach temperatures of hundreds of degrees Celsius and cannot be used in portable devices. But this work does show that liquid interfaces can achieve high current densities without forming dendrites. Researcher Richard Park said: "The starting point is to develop electrodes based on carefully selected alloys in order to introduce a liquid phase that can serve as a self-healing component of the metal electrode."

The material is more solid than liquid, but similar to the amalgam solid metals dentists use to fill cavities, it's still able to flow and take shape. In this case, it's made from a mixture of sodium and potassium that exists in a state where it's both solid and liquid at normal battery operating temperatures. The team demonstrated that the system could run at currents 20 times greater than using solid lithium without any dendrites forming. The next step will be to replicate this performance with actual lithium-containing electrodes.

In a second version of the solid-state battery, the team introduced a very thin layer of liquid sodium-potassium alloy between the solid lithium electrode and the solid electrolyte. The results showed that this approach also overcomes the dendrite problem, providing another avenue for further research.

The researchers say the new method is applicable to many different versions of solid-state lithium batteries. The team will next demonstrate the system's applicability to a variety of battery architectures.