14 June 2014

How does a lithium-ion battery work, and why are they so popular?

lion batts head
New research from MIT has been making the rounds this week, and while its core insight might seem weak, that very fact highlights just how quickly technology really does move these days. While lithium-ion batteries (LIBs) are all over the world, the truth is we still don’t really know how they work. In particular, as scientists try out more and better new materials for electrodes, each one brings slight variations in function and performance. One of the most promising electrode materials is lithium-iron phosphate, and now researchers have a much better understanding of exactly how it charges and discharges which should hopefully guide the way to improving those processes.

How does a lithium-ion battery work?

First, we need to look at how a lithium-ion battery works in general. Like any other battery, its basic design sees an electrolyte (the “transport medium”) ferrying lithium ions back and forth between the negative electrode and the positive electrode. In a totally discharged batteries, our mobile lithium ions will be entirely connected to the positive electrode their chemical properties keep them bound to the positive electrode material while they lack electrons. If we give them electrons by pumping electricity into the system (recharging), they will naturally dissociate from the positive electrode and migrate back to the negative electrode. Once they’re all lined up on the other side, loaded with nice high-energy electrons, we call the battery “charged.”

This stable state breaks down when we provide an avenue for the electrons now trapped at the negative electrode to travel down their charge gradient to the positive side of the battery this takes away electrons from lithium in the negative electrode and makes them again Li+, causing them to naturally migrate all the way back. We can use that negative-to-positive electron flow to power everything from pacemakers to electric cars, and it all ultimately comes down to the back-and-forth movements of ions. Incidentally, it’s only recently that scientists have discovered exactly why too many back-and-forth reactions cause a battery to slowly die.

Why lithium-ion batteries are popular

The main reason you’ve heard the term “lithium-ion battery” before is energy density; a LIB setup can pack a lot of power into a very small space. More than that, “Li-on” batteries offer decent charge times and a high number of discharge cycles before they die. If you use a pure lithium metal at the electrodes, you’ll get much higher energy storage, but no ability to recharge depending on your choices for electrodes, you can powerfully affect your battery’s performance. Among other things, energy density is related to the number of lithium ions (and thus electrons) the electrodes can hold per unit of surface area.

Solid Solution Zone lines up next to charged and discharged areas of the electrode
This diagram shows how the Solid Solution Zone lines up next to charged and discharged areas of the electrode.

This MIT study [doi: 10.1021/nl501415b - "In Situ Observation of Random Solid Solution Zone in LiFePO4 Electrode] specifically looked at a cathode material lithium-iron phosphate. These lithium-iron phosphate batteries show promise for everything from electric cars (likely) to storage of grid power (less likely), but when it was originally introduced, LiFePO4 showed little promise for battery tech. In its pure form, lithium-iron phosphate shows poor electrical abilities but crush it up into nanoparticles and coat it with carbon and it seems the story changes quite a bit. The incredible jump in ability when turned into nanoparticles is described as a major surprise for battery researchers, and a major win for nanoscience.

The main reason for excitement over the new nano-cathode, beyond its impressive-but-not-amazing storage and discharge abilities, is that it discharges at a totally uniform voltage. This means batteries needn’t incorporate devices to regulate that voltage, which could make them cheaper and smaller, and it also allows them to discharge at full voltage until totally empty. It does this, we now know, by creating a zone called a Solid Solution Zone (SSZ), a buffer area of low lithium density that seems to soften the harsh boundary between charged (LiFePO4) and discharged (FePO4) portions of the electrode during use. This seems to be behind the material’s amazing abilities, and pumping up this SSZ through design could extend make lithium-ion tech last even longer.

Technology does seem to be coming for this aging battery standard, however, and it will need some major upgrades to stay with the times. It’s getting them, with huge design upgrades that hold a lot of promise. Still, everything from improved capacitors to super-batteries based on cotton could supplant lithium as the king of energy storage we may find that improvements in our understanding of conventional batteries are simply too little too late.

Courtesy Extremetech


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