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New cheap nanoparticle-based cathode for K+ batteries can survive 40,000 cycles with only modest degradation

The weakest point of the electric vehicle (EV) is arguably the very thing that powers it -- the battery.  Tesla Motor Comp. (TSLA) rates its 245-mile (official range) Roadster at 400 guaranteed charge cycles (a 98k mile life) similar to the General Motors Comp. (GM40-mile (electric) range Chevy Volt, which offers 2,500 charge cycles (a 100K mile all electric range) [source].  While 100,000 miles isn't the worst thing in the world, it also isn't great, considering the high up-front costs of EV ownership.

I. New Electrode is Survival Champion

Virtual all EVs and personal electronics rely on the same old lithium-ion batteries.  But a new type of batteries – potassium-ion batteries -- intrigues with the potential for much greater durability.

Stanford University has cooked up a cathode (high voltage electrode) for this type of battery capable of outlasting virtually any of its lithium ion peers.  Created by graduate student Colin Wessells and his advisor, Yi Cui, an associate professor of materials science and engineering, the new material was able to deliver 80 percent or better charge delivery for 40,000 charges.

Better yet, the cathode material is inexpensive.  Writes the researchers:

Here we show that crystalline nanoparticles of copper hexacyanoferrate, which has an ultra-low strain open framework structure, can be operated as a battery cathode in inexpensive aqueous electrolytes.

Mr. Wessells says the creation of the cathode compound is cheap and easy.  He says he was able to create gram quantities, stating, "We put chemicals in a flask and you get this cathode material. You can do that on any scale.  There are no technical challenges to producing this on a big-enough scale to actually build a real battery."

Prussian Blue cathode
Stanford University has made a platinum free cathode using copper/Prussian Blue nanoparticles.  The cathode is the perfect compliment to potassium ion electrolyte batteries.
[Image Source: Stanford University/Nature Communications]

II. From Paint Dye to Batteries

The chemical compound comprising the cathode -- Copper hexacyanoferrate (Cu4[Fe(CN)6]3]) -- is also cheap from a chemical basis.  It contains commonly occurring organic elements -- carbon and nitrogen (in the cyanide group) -- and metal elements -- copper and iron.  Much research [abstract] has been done into make copper hexacyanoferrate nanoparticles efficiently.  The hexacyanoferrate group is familiar to chemists and doctors as "Prussian Blue" -- a brilliant blue pigment used in paints and as a treatment to heavy metal poisoning.

Prussian Blue
Prussian Blue [Image Source: MadeInChina]

Ion batteries can go through heat-driven degradation in two primary ways.  First, the electrolyte fluid can dry over time, losing moisture due to the heat they're exposed to during charging and use.  Recent research [press release] from Ohio State University indicates that a second major issue is a fouling process involves ions damaging the cathode and becoming lodged inside it, creating compounded degradation.

The new material sounds particularly promising as a solution to the latter problem (the former problem can potentially be solved by some sort of chemical sensor and liquid reservoir).

Prussian Blue based potassium ion batteries have been dabbled with for over a decade.  Some sources erroneously indicate them to have been "invented" in 2004.  In reality, while research [full text; PDF] on one design was published that year, another work using a Prussian Blue cathode and potassium electrolyte was published [PDF] in 1999.  

The key difference between these works and the current one is that they at least in part relied on platinum in their cathodes.  Platinum's downside?  It is very expensive.

Both of these past studies and the current study consider potassium the "perfect" ion for the Prussian Blue cathode, as it's small enough to jump into and out of the loose lattice without damaging it.

Describes Mr. Wessells, "It fits perfectly – really, really nicely.  Potassium will just zoom in and zoom out, so you can have an extremely high-power battery."

III. Uncertainty Remains

Potassium (-2.92 V) is the closest to lithium (-3.04) in standard potential voltage of the common battery chemistries [source].  Information on energy density and cell voltage versus discharge is not widely available, making it difficult, though, to compare the finished product.  

The 1999 design showed a full charge voltage 2.0 V and a half charge voltage of 1.0 V.  That's pretty bad compared to a standard discharge curve for a lithium ion cell that gives around 4.2 V at full charge and 3.8 V at half charge.  But it's still to early to know what optimized commercial designs of potassium might be able to achieve.

That said, an early commercial version has been used to power at least one mobile electronics device.

A bit of a disclaimer on the most bold claim of 40,000 charge cycles, is that the battery was cycled at 17° C (62.6° F), much lower than car batteries or other designs operate at.  The study admits:

Even at a very high cycling rate of 83 C, two thirds of its maximum discharge capacity is observed.

In other words, the "real world" operating efficiency after 40k cycles would likely fall somewhere between 66 and 83 percent.

The researchers also comment that their fancy new cathode must still be paired with a optimal anode (low voltage electrode) and an overall battery design in order to make the perfect potassium ion battery.

While the potential for portable electronics and EV/hybrid applications seems huge for this improved Potassium ion battery design, the authors are focused primarily on grid storage.  Comments Mr. Wessells, "We decided we needed to develop a 'new chemistry' if we were going to make low-cost batteries and battery electrodes for the power grid.  At a rate of several cycles per day, this electrode would have a good 30 years of useful life on the electrical grid." 

Grid storage is a hot field.  With mankind increasingly harvesting wind and solar energy to produce electricity, utility companies are looking to battery banks as a potential solution to the variable nature of these power sources.  Using battery grid storage consumers can be delivered alternative energy without interruption even when the wind isn't blowing and the sun isn't shining.  However, given the already high costs of solar and wind power, finding cheap yet efficient solutions is as critical in the grid storage market as it is in the automotive and personal electronics markets.

Windmills at sunset
[Image Source: Ames Power]

The new research was published [abstract] in the journal Nature Communications.

Sources: Eurekalert, Nature Communications





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