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Batteries

Solid-State Batteries Could Use Electrolyte Derived From Cellulose

Cellulose from wood could be the key to solid-state batteries.

Solid-state batteries are the holy grail of lithium-ion batteries. Today, most batteries use a semi-liquid paste between the anode and the cathode. Sometimes dendrites — hard spikes of lithium metal — occur in the paste, which can create an internal short circuit. When that happens, the battery cell overheats to the point where the volatile chemicals in the paste catch fire. When one cell ignites, it causes the cells near it to overheat, leading to a full-fledged thermal runaway event.

Solid-state batteries have no volatile chemicals, no dendrites, and no risk of fires. So why aren’t EV manufacturers using them? Because the technology is still confined to laboratories. There are no commercially available solid-state batteries today, although companies like Quantumscape, StoreDot, Solid Power, and others are pouring millions of dollars into making them ready for mass production. The date most often heard when discussing solid-state batteries is maybe in 2025.

Most researchers working on solid-state batteries are using ceramics for the electrolyte, but they are brittle, which limits battery life. Scientists from Brown University and the University of Maryland have come up with a novel idea, however. They are using cellulose nanofibrils found in cellulose derived from wood as their starting point for a solid-state electrolyte. The material they have discovered is paper-thin, which allows it to bend and flex to absorb stress as the battery cycles. The research has been published recently in the journal Nature.

Polymer tubes derived from cellulose are combined with copper to form a solid ion conductor boasting a conductivity similar to ceramics, and up to 100 times better than other polymer ion conductors. According to the research team, adding copper creates space between the cellulose polymer chains which allows “ion superhighways” that let lithium ions travel with record efficiency.

“By incorporating copper with one-dimensional cellulose nanofibrils, we demonstrated that the normally ion-insulating cellulose offers a speedier lithium-ion transport within the polymer chains,” says co-author Liangbing Hu. “In fact, we found this ion conductor achieved a record high ionic conductivity among all solid polymer electrolytes.”

Because the material is paper-thin and flexible, the scientists believe it will better tolerate the stresses of battery cycling. They say it has the electrochemical stability to accommodate a lithium-metal anode and high voltage cathodes. It could also act as a binder material that encases ultra-thick cathodes in high density batteries.

“The lithium ions move in this organic solid electrolyte via mechanisms that we typically found in inorganic ceramics, enabling the record high ion conductivity,” says co-author Yue Qi. “Using materials nature provides will reduce the overall impact of battery manufacture to our environment.”

Executive Summary

For readers who want more technical details, here is the executive summary to the report.

“Although solid-state lithium-metal batteries promise both high energy density and safety, existing solid ion conductors fail to satisfy the rigorous requirements of battery operations. Inorganic ion conductors allow fast ion transport, but their rigid and brittle nature prevents good interfacial contact with electrodes.

“Conversely, polymer ion conductors that are Li-metal-stable usually provide better interfacial compatibility and mechanical tolerance, but typically suffer from inferior ionic conductivity owing to the coupling of the ion transport with the motion of the polymer chains. 

“Here we report a general strategy for achieving high-performance solid polymer ion conductors by engineering of molecular channels. Through the coordination of copper ions (Cu2+) with one-dimensional cellulose nanofibrils, we show that the opening of molecular channels within the normally ion-insulating cellulose enables rapid transport of Li+ ions along the polymer chains.

“In addition to high Li+ conductivity (1.5 × 10−3 siemens per centimetre at room temperature along the molecular chain direction), the Cu2+-coordinated cellulose ion conductor also exhibits a high transference number (0.78, compared with 0.2–0.5 in other polymers) and a wide window of electrochemical stability (0–4.5 volts) that can accommodate both the Li-metal anode and high-voltage cathodes.

“This one dimensional ion conductor also allows ion percolation in thick LiFePO4 solid-state cathodes for application in batteries with a high energy density. Furthermore, we have verified the universality of this molecular channel engineering approach with other polymers and cations, achieving similarly high conductivities, with implications that could go beyond safe, high-performance solid-state batteries.”

Hmm…I am no scientist, but I’m pretty sure LiFePO4 is scientific shorthand for the LFP batteries that have suddenly become popular with automakers like Tesla. Typically, these batteries have a lower energy density than traditional lithium-ion batteries, but if these researchers are correct, that concern could be eliminated one day.

It’s unfortunate that it often takes years for new technology to move out of the lab and into the commercial production. For those who are impatient, we can only say, “Patience, grasshopper.” It took a century to perfect the internal combustion engine. With the state of technological change possible today, it won’t take nearly that long to perfect the batteries we need to move the EV and energy storage revolutions forward.

The prospect of low cost, high power batteries with little to no risk of thermal runaway is exciting stuff. One day our grandkids will see lithium-ion battery cells from 2007 in a museum and marvel that cars once used such crude devices. That’s inevitable. We can’t wait for it to happen!

 
 
 
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Steve writes about the interface between technology and sustainability from his home in Florida or anywhere else The Force may lead him. 3000 years ago, Socrates said, "The secret to change is to focus all of your energy not on fighting the old but on building the new." Perhaps it's time we listened?

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