Potassium ion Batteries for Solar PV Systems
Many types of charge carriers are used in solar rechargeable batteries. Lithium has been the most common charge carrier for a long time, but its scarcity as a raw material has prompted the creation of alternatives.
Of all the numerous lithium analogs, potassium is arguably the most popular.
Potassium ion batteries boast high energy density, exceptional ion transport kinetics, and abundant raw material availability.
Technology-wise, potassium batteries are relatively new but have already shown great potential for solar energy storage.
As lithium-ion batteries, potassium ion batteries are known to last for around 200-500 cycles. Our goal in this article is to assess potassium ion batteries and determine how suitable they are for solar PV systems.
What is a Potassium ion Battery?
A potassium ion battery (PIB), also known as a K+ battery is an electrochemical battery leveraging potassium as the charge carrier.
Energy storage in potassium ion batteries is accomplished through electrochemical reactions involving charge transfer (oxidation and reduction).
Like most conventional batteries, potassium ion batteries have 3 main components: the electrodes, electrolyte, and a separator that usually a porous membrane.
Graphite is typically used as the electrode material, but there’re other options as well. A potassium salt solution is commonly used as an electrolyte.
Think of PIBs as your conventional lithium-ion batteries, but with potassium as the charge carrier. Potassium is an alkali that’s closely related to lithium chemically.
However, because it's cheaper and easier to obtain, it makes a better charge carrier.
The PIB technology was invented by Ali Eftekhari, President of the American Nano Society in 2004.
Over the years, the battery has been developed into the low-cost, high-energy-density, and environmentally friendly energy storage system we know today.
Potassium Ion Batteries: Material Analysis
Electrodes
PIBs can work with a vast range of electrodes that fall under two broad categories: organic and inorganic electrodes.
Organic electrodes are synthetically available and have high operational safety. However, they fail for having poor potassium diffusion kinetics and low reversible capacity.
On the other hand, inorganic electrodes have high conductivity and better environmental stability than organic electrodes but due to their low synthetic availability, they’re less preferred than organic electrodes.
With this in mind, we can now look at practical examples of materials used for the anode and cathode of potassium ion batteries.
1. Anodes
| Anode Example | Notes |
| Graphite | Low retention capacity during cyclingWide availability. Cheap to acquireReversible capacity of around 273mAh/g Can be intercalated with potassium |
| Sb | Alloying anode Low capacityHigh reversibility |
| K-Sb | High capacity Low potassiation Large volume changes during alloying and dealloying Fast capacity degradation |
| NP-Sb | Can accommodate volume expansion Have fast ion transport kineticsBetter electrochemical performance |
2. Cathodes
| Cathode Example | Notes |
| Potassium graphite | Very stable Can be intercalated with potassium ions Commonly used for PIBs |
| K3V2 (PO4)3 | 3D Polyanionic electrode High working voltage Complex and costly synthesis process |
| Perylenetetracarboxylic dianhydride (PTCDA) | Organic electrode High bonding rate with K+High capacity (131 mAh g−1) High capacity retention (66.1% after 200 cycles) The greatest potential for use with PIBs |
Electrolytes
Potassium ion batteries can work with the most common electrolyte salts, but KB4 is the most preferred electrolyte.
Even so, a good electrolyte for PIBs should:
- Be stable for use with the highly active K+ and the electrodes
- Have fast ion diffusion kinetics
- Be safe to use
Here are the main types of electrolytes used for potassium ion batteries:
| Type of electrolyte | Notes |
| Traditional liquid electrolytes | Cheaper and readily available Not compatible with K+, which is a highly active chemical element Unsafe to use for PIBs |
| Ionic liquid electrolytes | Stable with conventional graphite anodes Have a more negative redox voltage. Suitable for PIBs |
| Solid polymer electrolytes | Relatively new electrolytes Have enhanced stability and safety A good example is polypropylene carbonate |
How do Potassium Ion Batteries work?
A potassium atom has a zero net charge, but a potassium ion has a +1 net charge. In a potassium ion battery, the positive potassium ions are arranged in a lattice structure between the anode and cathode.
When connected to a solar power source, the positive terminal attracts electrons that flow through an external circuit to the anode.
This creates an imbalance in the electrolyte that makes the potassium to be attracted to the negative terminal and hence also migrate to the anode side.
The potassium ions can move through the electrolyte since the separator used in these batteries is permeable to them.
On the total migration of the K+ ions, the battery is said to be fully charged.
Please note: A fully charged PIB battery is in an unstable state, and the charges are ready to return to their original, stable structure.
When connected to a load, the electrons and potassium flow are reversed. The electrons travel through an external circuit– and through the load– to the anode side (electric current) while the K+ ions flow through the electrolyte back to the metal oxide lattice structure.
Characteristics of Potassium Ion Batteries
Energy density
Energy density is the amount of energy that can be stored in a given mass of a battery.
According to this potassium ion batteries research work, a KFeC2O4 cathode and a carbon anode yielded a potassium ion fuel cell with an energy density of 235 Wh kg−1.
Assuming this as the average energy density range for PIBs, it's on the higher end of lithium-ion batteries’ which is (100-265 Wh kg−1).
Having a high energy density is one of the key features of potassium ion energy storage systems.
Cell potential
The standard reduction potential of K+ is -2.93.
K+ + e- K(s) E◦=-2.93
The reduction potential of Lithium is -3.04 E◦, so potassium is as good as lithium in terms of cell potential.
Cycle life
The average cycle life of potassium ion batteries is 200-500 cycles. This is about 2-3 years of use life which means these energy storage systems are unreliable for solar PV systems.
Thankfully, some hope is on the horizon as the material design of these batteries is being improved to improve their cycle life.
Coulombic efficiency
Coulombic efficiency also called current efficiency is the ratio of charge drawn from a battery to the charge fed into the battery in a single cycle.
The higher the coulombic efficiency, the lower the chance of irreversible occurring in the cells. For PV storage systems, this means more reliability for continuous cycling.
Impressively, potassium ion batteries have high coulombic efficiency and they can be as high as 98%.
Performance at high temperatures
PIBs are very resistant to elevated temperatures. With temperatures as high as 60°C, you may only notice a small decrease in the performance of these batteries.
Potassium ion batteries can be used at slightly high temperatures with no risk of fire or explosion.
Operational safety
Modern potassium ion batteries use non-flammable and stable electrodes/electrolytes; thus, they're safe energy storage for PV systems.
Obviously, you'd not want to use a battery system that'd wreck your entire solar PV system, so PIBs definitely gather some points here.
Transport kinetics
The Stokes radius of solvated potassium ion is smaller compared to other commonly used charge carriers in batteries. This increases the rate of chemical diffusion of K+ in the electrolyte, thus improving the transport kinetics of PIBs.
Discharge rate
Potassium ion batteries have an average discharge rate of 250 mAh g−1. This is a decent discharge rate that makes these batteries suitable for emergency and backup situations.
Self-discharge rate
Potassium ion batteries have a low self-discharge rate which means they can store their charge for long when no loads are connected. Nevertheless, you should not leave your potassium ion batteries unused for a long period.
Charging speed
Potassium ion batteries charge faster than most conventional PV batteries. This is, of course, due to the fast ion transport kinetics of potassium ions in the electrolyte.
It's reassuring to know that your PV battery can be charged within a short period, so you can have backup power even when the sun isn't shining a lot.
Battery applications
As PIBs are currently not cyclable, their applications are limited to small-scale projects. On the contrary, potassium’s abundance and even distribution make PIBs, a promising candidate for large-scale PV energy storage applications.
Pros of Potassium Ion Batteries
Materials are abundant and evenly distributed
Potassium is widely available and abundant throughout the world. Being the main component of potassium ion batteries, potassium indicates the practicality and possibilities of this energy storage system.
It’s unlikely that salt deposits containing potassium will be exhausted anytime soon. So we don’t expect that material availability will hinder the development of PIBs as it has for lithium-ion batteries.
High energy density
As already discussed, PIBs excel for having high energy density, which can directly translate into better storage capacity.
Even better, if properly developed, potassium ion batteries have the greatest potential for large-scale applications both as solar PV and grid energy storage systems.
Fast ion transport kinetics in the electrolyte
Potassium ions travel fast in the electrolyte hence causing fast charging and discharging.
This fast ion transport characteristic makes potassium ion batteries suitable for intermittent solar power storage.
Cost-efficient
Potassium ion batteries have a simple design that translates to a low fabrication cost. Additionally, the abundance of potassium reduces the material cost for PIBs.
Since you've already spent a ton on your solar PV system, it makes sense to get batteries like PIBs that won't dig deeper into your pockets.
Cons of Potassium Ion Batteries
In spite of PIBs' exceptional features, the technology has one big shortcoming: unstable cycling.
Unstable cycling
A potassium ion battery with unstable cycling is defined as having a low number total of charge/discharge cycles when the capacity reduces by 50%.
This feature makes PIBs unsuitable for applications needing high energy pulses or rapid charge/discharge cycles.
The Future of PIBs
Moving forward, potassium ion batteries will continue to be fabricated to boost their electrochemical properties and effectiveness in storing solar energy. The most focus will be given to the electrodes as follows:
For the cathode, materials with a high energy density and cycling stability such as transition metal oxides will be developed.
The KFe2C2O4F cathode mentioned earlier is a perfect example here. By coupling it with a graphite anode, the electrode can clock the 2000 cycle life, with 94% capacity retention.
For the anode, the focus should be on improving conversion reactions and modification of the anode material intercalation with potassium.
In summary, potassium ions need to be fabricated to be low-cost, high-performance energy storage systems for PV applications.
FAQs about Potassium Ion Batteries
What ions are present in potassium ion batteries?
Only K+ ions are present in potassium ion batteries. The K+ ions migrate between the cathode and the anode through a microporous membrane during charging and discharging.
Electrons are also present in potassium ion batteries but these flow through an external circuit again during charging and discharging.
In solar PV systems, can other potassium batteries be used?
Today, there’s a wide range of rechargeable potassium batteries that can be used for energy backup or storage.
For PV systems, though, the most feasible ones are potassium oxygen and potassium sulfur batteries. These energy storage systems offer advantages such as high energy densities and low costs.
Are potassium ion batteries better than lithium-ion batteries?
Potassium ion batteries are as good as lithium-ion batteries in terms of electrochemical properties.
They're definitely better in terms of material availability and energy density, but they need to improve on cycle stability and cycle life to be at par with lithium-ion batteries.
Conclusion
Potassium ion batteries have been around for less than 20 years but have already been a favorite for many due to their improved energy storage density, reliable battery kinetics, and reduced storage costs.
Because of the abundant and even distribution of potassium, PIBs are also gradually becoming the go-to PV energy storage solution for small and large solar projects worldwide.
But one aspect makes this battery technology not good enough for solar PV application: poor cycle stability and cycle life.
With a solar power system, you'd expect to cycle your batteries daily, which is obviously not something PIBs can reliably support.
Ultimately, potassium ion batteries will get better, and soon there may be ways to deal with these cycle issues.
Eventually, these batteries could be the most viable, feasible, and reliable energy storage system we have for our amazing solar PV systems.
