Although it has only been heard not seem, scientists continue to note and anticipate the development of lithium-sulfur batteries because their theoretical storage capacity is more than 5 times that of lithium-ion batteries. Now the University of Michigan has developed a new lithium-sulfur battery separation film and believes this design to be "almost perfect" with the number of charges and discharges exceeding a thousand while fast charging.

Battery life is related to the number of battery charges and discharges. As usage increases, battery capacity will decay normally. Usually after 500 charge and discharge cycles, battery capacity will still maintain more than 80% of the standard but the lifespan of lithium-sulfur batteries under development often do not last that long.

Team leader Nicholas Kotov said that in the past, many studies have reported that lithium-sulfur batteries have been charged and discharged hundreds of times but this achievement had only been obtained from a tradeoff in capacity, charging rate, toughness, and safety. Now, the challenge is to manufacture a battery that meets criteria such as cost while boasting charge-discharge cycles that increase from 10 to several hundred.

The University of Michigan team primarily started with the battery separator. The structure of the lithium-sulfur battery is similar to that of the lithium-ion battery. It is mainly composed of two electrodes, an electrolyte between the electrodes and a separator. The ions first start from the cathode (positive electrode) and pass through the electrolyte, the separator, and finally reach the anode (negative electrode). On the other hand, the problem of lithium dendrites will also be encountered and the by-product polysulfide generated by the chemical reaction returns readily to the lithium electrode, hindering the movement of lithium ions, and further reducing the charging capacity of the battery and lifespan.

（Source: University of Michigan)

For this reason, the team switched to aromatic polyamide (aramid) nanofibers and molded these Kevlar-like nanoscale fibers into a battery structure network and then injected an electrolyte gel, hoping to prevent the electrode from growing dendrites.  The team also created many tiny, slightly charged biomimetic channels that allow positively charged lithium ions to flow freely.

One of authors of the study, Ahmet Emre, described it in one sentence: build a highway for lithium ions and set up checkpoints so that polysulfides cannot pass.

Kotov believes that the design of this ion-selective battery leads to a near-perfect lithium-sulfur battery, pointing out that the efficiency of the device is close to the theoretical limit and capacity is five times that of a standard lithium-ion battery. It also has a service life of up to ten years.

In terms of the battery manufacturing process, the content of sulfur in the earth's crust is also more abundant than the cobalt used in lithium-ion batteries, so there is less of a problem with resource depletion. Aramid can also be obtained from retired bulletproof vests, making the process more environmentally friendly. Kotov believes that the biomimetic engineering of batteries integrates both molecular and nanoscale and, for the first time, integrates ion-selective separators with structural toughness to meet the challenges of lithium-sulfur batteries.

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