Lithium-ion batteries have been traditionally used for portable devices—for phones, laptops, and even cars. However, the supply of lithium as an alkali metal is limited, most of it being mined from the mountains of South America. At the moment, lithium-ion batteries are the safest option to go for, but another potential element is making its way into the picture, and it could solve the ongoing problems of traditional batteries: sodium.
The Issues With Lithium-Ion Batteries
While lithium-ion batteries are the most effective batteries at the moment, there are a lot of issues that put their efficiency and their sustainability in question in the long run. Moreover, there have been instances where lithium-ion batteries exploded due to the volatile nature of the element.
To start with, the cost of producing the lithium batteries is high, and it is getting higher as time goes by. The reason for the price increase is simple: the resources of lithium are limited, and the less there is, the more it is going to cost. Since 2015, the price of lithium has increased dramatically, going up to almost €17,500 per tonne (i.e., €17.5 per kilo). Additionally, it takes a large amount of water to separate the lithium when mined, proving that the production process is not as green as most would like it to be.
The next issue is that the demand surpasses the resources. With electric cars making an entrance on the market on a large scale, battery manufacturers are already looking for cheaper and more sustainable alternatives to lithium-ion batteries.
Why Is Sodium a Good Alternative to Lithium?
Sodium and lithium are neighbours on the periodic table. This means that the difference in their atomic number is 1, so they have similarities in their atomic structures and electrochemistry. Researchers believe that if they replace lithium with sodium in batteries, it will roughly have the same energy capacity. And of course, sodium costs only a fraction of the price of the lithium (€0.13 per kilo), and it is available in large quantities, as it is the fourth most abundant element on Earth.
How Batteries Work
Let’s look at the way that the traditional lithium-ion battery works and the chemical elements involved in the process. The anode, or the positive electrode of the battery, is made of lithium-cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), and the cathode, or the negative electrode, is made up from carbon, specifically graphite. The electrolyte in the battery functions as an insulator. The lithium atoms move from the anode to the cathode when the battery is charging, while the lithium atoms return to the cathode when it is discharging.
The first problem when it comes to sodium batteries is that you can’t just replace the lithium with the sodium without adjustment. The sodium does not work well with the graphite at the other end. If the graphite stays in the sodium battery without modifications, too many sodium atoms would be lost. The sodium ions stick to the carbon-based graphite, forming a solid electrolyte interface. Many researchers are on the lookout for different types of materials that could be used at the anode, and they’ve found a few different solutions.
Current Developments on Sodium-Ion Batteries
Researchers from Purdue University decided to stick to the traditional carbon-based anode and take a different approach on the state of matter of the sodium that was used in the batteries. They went for a powder version of the substance. First, the sodium’s exposure to moisture needed to be limited. This would ensure the sodium powder doesn’t combust later. Then, they used ultrasound to make the powder—they melted the sodium by exposing it to the ultrasound, turning it into a purple milky liquid which cools down into powder. This ensures even distribution of the powder’s particles, as it was suspended in hexane solution. In order for this to be successful, during the production of the battery, the researchers needed to add few drops of the suspension on the cathode that gives the battery required stability so that it doesn’t lose the sodium ions in the process.
Another team of researchers, consisting of experts from the University of Birmingham and University of Cambridge, paired the sodium ions with another element instead of graphite: phosphorus. By using a phosphorus compound for the battery’s electrode, the charge capacity of the battery would be increased by seven times compared to the same mass of graphite. Researchers at Stanford University are taking the same approach by using phosphorus for the anode of sodium ion batteries, and a mixture of sodium, oxygen, and carbon for the cathode. They are also experimenting with a substance called myo-inositol. Myo-inositol is an organic substance found in the human body as a part of the cell membrane, and it can be extracted from plants such as rice and nuts.
King Abdullah University of Science and Technology (KAUST) in Saudi Arabia decided to replace the graphite with hard carbon. To create the hard carbon, researchers used copper foil and put it under a high-intensity laser. The product was graphene, but that wasn’t their intention. To create the hard carbon, the needed to add nitrogen while the laser was blasting on the foil. The nitrogen incorporated itself in the structure of the graphene, replacing around 13% of the carbon atoms.
Making Sodium Batteries Commercially Available
Looking at the current scope of the battery industry, the situation looks promising. The key is to find a universal solution for the anode, since it is the most vital part of the battery. One of the potential problems for super-sized batteries is the weight, since sodium weights more than lithium. And when it comes to lithium, research already concluded that adding sulfur to the lithium-ion batteries can increase the battery capacity between three and five times—and also cut down on costs.
There is one final question: Are sodium batteries really worth the effort? They definitely are, yet the current challenges need to be overcome before turning the prototypes into commercially available products. Sodium is cheap, but the expenses to produce a battery at the moment are still high—hence why there are different groups of researchers focusing on different approaches to solve this problem. If a solution which is both environment-friendly and cost-efficient is found, we will be seeing much more of sodium batteries in our day-to-day lives.