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The biomimetic membrane will finally allow the next generation of lithium-sulfur breakthrough batteries to reach their full potential.
A team at the University of Michigan used recyclable Kevlar (the same material found in bulletproof vests) to create a network of nanofibers similar to cell membranes. They then used it to solve fundamental problems for a next-generation battery type called lithium-sulfur.
Until now, the cycle life of this type of battery — the number of times it can be charged and discharged — has not been sufficient for commercial use in electric vehicles, despite their capacity advantages.
Lithium-sulfur batteries hold five times the power of the industry-standard lithium-ion batteries used in everything from smartphones and laptops to pacemakers.
However, the inherent instability of lithium-sulfur battery cathodes, with a 78% dimensional change with each charge cycle, means their use in consumer electronics is highly impractical.
The breakthrough potential of lithium-sulfur batteries means that research institutions around the world are scrambling to try and make the technology viable, with previous breakthroughs focusing on the use of flexible cathodes.
There are many reports claiming hundreds of cycles for lithium-sulfur batteries, but this comes at the expense of other parameters: capacity, charge rate, resilience, and safety.
A life expectancy of 1,000 cycles means that the average car battery needs to be replaced roughly every 10 years, while using materials that are far more abundant and less damaging to the environment than those used in lithium-ion batteries.
Achieving record levels of multiple parameters for multiple material properties is what is now required for car batteries. It’s a bit like Olympic gymnastics – you have to be perfect in all aspects, including the sustainability of their production.
The invention of the Li-S battery dates back to the 1960s, when Herbert and Ulam filed a patent in 1962, a lithium or lithium alloy as the anode material, sulfur as the cathode material, and an electrolyte composed of aliphatic saturated amines.
Several years later, the technique was improved by introducing organic solvents such as PC, DMSO, and DMF, resulting in 2.35-2.5 V batteries. By the late 1980s, a rechargeable Li-S battery was demonstrated using ethers, especially DOL, as solvents for the electrolyte.
The potential of Li-S batteries has been highlighted due to scientific advances in this field. Over the past two decades, lithium-sulfur batteries have undergone renewed and growing popularity. In particular, strategies to suppress or mitigate the polysulfide “shuttle” effect have been intensively studied and investigated by many researchers.
Manthiram identified the key parameters needed to gain commercial acceptance. Specifically, Li-S batteries need to achieve >5 mg cm-2 sulfur loading, <5% carbon content, <5 μL mg-1 electrolyte/sulfur ratio, <5 μL mg-1 The electrolyte/capacity ratio of -1 is 5 μL (mA h)-1, and in pouch cells, the positive and negative capacity ratio is <5.
In 2021, researchers announced the use of sugar-based anode additives to prevent the release of polysulfide chains from the cathode that contaminate the anode. A prototype battery demonstrated 1,000 charge cycles with a capacity of 700 mAh/g.
The chemical processes in Li-S batteries include the dissolution of lithium from the anode surface during discharge (and incorporation into alkali metal polysulfide salts) and the reverse plating of lithium onto the anode during charging.
At the anodic surface, dissolution of the metallic lithium occurs, with the production of electrons and lithium ions during the discharge and electrodeposition during the charge.
Similar to lithium batteries, the dissolution/electrodeposition reaction can lead to unstable solid-electrolyte interface growth over time, creating active sites for lithium nucleation and dendrite growth. Dendritic growth is the cause of internal short circuits in lithium batteries and causes the battery itself to die.
In lithium-sulfur batteries, energy is stored in the sulfur cathode (S8). During discharge, lithium ions in the electrolyte migrate to the cathode, where sulfur is reduced to lithium sulfide. Sulfur is re-oxidized to S8 during the recharging stage.
In fact, the sulfur reduction reaction of lithium sulfide is much more complex and involves the formation of lithium polysulfides in order of decreasing chain length.
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