Core–shell microfibers fight fires in Li-ion batteries
A new material for lithium ion batteries solves their flammability issue without lowering battery performance. Though lithium ion batteries are valuable for their high energy densities and durability, the risk of fire remains a significant challenge because the cells often use flammable organic electrolytes. Past attempts to incorporate flame retardant materials into lithium ion batteries resulted in significantly degraded performance. Yi Ciu leads a team of researchers at Stanford University that recently described a solution to this problem in Science Advances: they enclosed a flame-retardant material inside an inert fiber that only releases when the battery temperature becomes dangerously high.
The fire risk of lithium ion batteries stems from their design. The battery cell has four basic parts. As the battery charges and discharges, lithium ions flow back and forth between electrode and cathode materials that connect to the other circuitry of the device. An organic electrolyte liquid, often ethylene carbonate, fills the space between electrode and cathode and allows the ions to flow. If the electrodes were allowed to touch through the electrolyte, the result would be a dangerous short circuit, so a separator material is placed in the middle of the cell. In most batteries, this separator is a simple polymer like polyethylene.
The behavior of the battery is controlled by integrated electronics. These are necessary because the battery can only be safely charged and discharged at or below certain charging rates to remain stable. Fast charging and discharging can produce lithium dendrites that creep from the anode to the cathode, creating a short circuit that causes the battery to begin heating uncontrollably, eventually leading to a fire. If this or any other short circuit occurs, it’s the job of the integrated electronics to shut down the battery. In spite of this design, fires do sometimes occur.
Simple solutions to this problem, like adding flame-retardant compounds directly to the flammable electrolyte, have met with limited success. Though the batteries do become safer, useful amounts of flame retardant severely impact battery performance. Ciu’s team saw an opportunity for a more mediated approach.
The team changed the separator to a new combination of materials that could prevent fire as a result of thermal runaway. The new separator was made from core–shell microfibers: a core of flame-retardant triphenyl phosphate (TPP) encased in an unreactive polymer, poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP). If the temperature of the electrolyte begins to rise, the shell melts and releases the flame retardant.
“This is a clever, beautiful approach,” says Stephen Harris, an expert in lithium ion battery design at Lawrence Berkeley National Laboratory.
To create the fiber, the researchers dissolved the TPP flame retardant and the unreactive shell molecules together in solution and employed the electrospinning technique to create the fiber. In this approach, the solution is pushed from a syringe aimed at a target. By electrically charging the syringe and the target with opposite charges, the material is drawn out into microfibers by the electric field. “The most challenging part is to get fibers with the desired core–shell structure, which is hard to control. We tried a lot of [experimental] conditions, and finally made it,” says Kai Liu, the first author on the article.
The researchers said a combination of factors helped the core–shell structure to form. Because the flame retardant dissolves more poorly in the organic solvent, this material begins to form a solid fiber first during spinning. Due to its polarity, the unreactive polymer is also likely to be more attracted to the electric field during the spinning process, drawing it to the outside of the fiber. Once there, the energy of having the polymer on the surface of the fiber also enforces the core–shell structure. “We do not have direct evidence of which factor should dominate yet,” says Liu.
The core–shell structure was confirmed by scanning electron microscopy with elemental analysis, x-ray photoelectron spectroscopy, and thermogravimetric analysis. The researchers used a beam of electrons, for sputter depth profiling, to etch away the surface of the fiber. They found no phosphorous atoms until they had removed the outer layer of the fiber, showing that the flame retardant was completely enclosed. However, when the fibers were heated with a torch, they did not ignite, while standard commercial separator materials burned readily.
When the fibers were pressed into separator-sized pads, they could be incorporated into common lithium ion battery cells. The charging and discharging properties of the cells showed their performance to remain stable and not degrade, as they would if flame retardant was added directly to the electrolyte. The TPP was not released until the cell reached 160°C, above normal operating temperatures for consumer batteries. Once at this temperature, the fibers breached and the flame retardant diffused into the electrolyte solution.
By incorporating these flame-retardant core–shell microfibers into lithium-ion batteries as separators, the risk of fire could be significantly reduced. In addition to its applications in batteries, the researchers said the core–shell approach could be useful in other contexts for controlling the delivery of important molecules. Liu says, “There are a lot of things we can move on with this microfiber. For example, we are trying to use the core–shell microfibers as materials for environmental applications.”
Read the article in Science Advances.