The War Against Dendrites, the Plague of Li-ion Batteries, Wages On
Dendrites have long since plagued lithium-ion batteries with problems that not only shorten their lifespans, but in some situations can be outright dangerous. Here’s how recent research efforts have contributed to the fight against these microscopic adversaries.
Dendrites are tiny, spiky, rigid tree-like structures with needle-like projections called "whiskers" that can grow inside of lithium-based batteries. Both this tree-like structure and its whiskers can cause serious problems both inside and out of the battery.
Most notably, they can pierce and protrude through the polyolefin separator films inside of lithium-ion (Li-ion) battery cells—much like a weed always seems to find a way to poke through even the most well-paved of roads—which can lead to the battery failing. In a best-case scenario, this would be a failure such as a short circuit; in a worst-case scenario, the battery may combust, especially in high-density batteries.
Depiction of dendrite growth in lithium-ion batteries. Image used courtesy of Washington University in St. Louis
It should come as no surprise then that research is heavily invested in solving this issue that stands in the way of the widespread use of higher energy density batteries for critical applications such as electric vehicles.
Here are three recent research examples.
A Non-Porous Separator
Earlier this month, Toray Industries announced a method for fighting dendrite formation, which utilizes high heat resistance aramid polymer design technology. Aramid is a super engineering plastic with high heat resistance and strong synthetic fibers used in aerospace and military applications. It sits just behind polyimide in terms of heat resistance and is used in thin-film circuit materials.
Toray used aramid to create a highly ion-conductive polymer with high heat resistance. This allowed the company to suppress dendrite formation in lithium-metal anode batteries while retaining ion conductivity by using the polymer as a non-porous separator comprised of a pore-free layer atop a microporous separator.
Non-perforated lithium-ion battery separators. Image used courtesy of Toray Industries
In a proof of concept, Toray was able to demonstrate that a battery using this separator was able to suppress dendrite-attributable short circuits and maintain over 80 percent of its capacity after 100 charge/discharge cycles.
The Japanese textiles company plans to accelerate research and development to establish technologies that use lithium metal batteries to drive further progress in safety for future high-density lithium batteries.
It’s not all about Li-ion batteries. While lithium metal (Li-metal) batteries provide a much higher theoretical capacity, the extreme reactivity of Li-metal means that we’ve not yet realized stable, long-life batteries based on it.
To prevent the growth of dendrites, one of the most important yet least understood factors is the formation of solid-electrolyte-interphase (SEI) layers. The constant passing of electrons between the Li-metal anode and the electrolyte in a lithium-metal battery causes the electrolyte layer to grade, resulting in the formation of SEI layers on the electrodes. For SEI films to be effective, they need to act as barriers to stop the transfer of electrons while allowing lithium ions to pass and deposit smoothly on the metal surface.
A simulation of a microscopic view of a stable SEI layer surrounding battery anode nanoparticles. Image used courtesy of Diego Galvez-Aranda, Texas A&M University
Now, recent works conducted at Texas A&M University could lead to protective layers for safer, long-life lithium batteries. In particular, the research team was able to explain the growth of SEI layers at the surface of anodes in Li-ion batteries.
The team also contributed ideas for tuning protective layers by examining nucleation and growth mechanisms of SEI layers derived from the battery’s electrolyte material.
Researchers at Carnegie Mellon University have used an entirely new class of materials to suppress dendrite growth—liquid crystals.
Liquid crystals have properties different from conventional liquids and solids with dendrite suppression occurring due to the tendency of liquid crystal molecules to line up in an ordered arrangement.
Although Carnegie Mellon researchers have achieved superior dendrite suppression using solid electrolytes, these have slower lithium-ion conductivities and cannot be easily integrated into current batteries. In contrast, liquids have faster conductivity but cannot suppress dendrites. Liquid crystals sit nicely in the middle because they have some orientational order (but no positional order like solids) and are easily integrated into existing lithium-ion batteries. They’re also safer and provide spontaneous dendrite suppression.
In their findings, the researchers proposed various design criteria for selecting liquid crystals as battery electrolytes that could “pave the way towards the realization” of a new class of electrolytes for “well-functioning” lithium metal batteries.
The next step for the researchers is to make liquid crystals more stable so that they better satisfy battery design criteria.
The battle to address dendrite growth has been an ongoing one for years. What other methods are you aware of that could abate dendrites and pave a clearer path for Li-ion batteries? Share your thoughts in the comments below.
First off, I’m a BIG fan of the Texan’s battery research. On dendrites & moderator failure, I’d suggest a perpendicular wrapping on top of the SEI layer.
Very educative reading.