Steven Dunn* and Ellis Davies investigate why the world relies on the use of lithium-ion batteries and look at two developments that have the potential to become alternatives as the price of lithium grows.
In order to power our mobile lives, we have come to rely on metal ion rechargeable or secondary batteries. More specifically, we use lithium ion (Li-ion) batteries due to their combination of high energy – meaning they can do lots of work once charged – and reasonable power density – meaning they can be charged quickly. These parameters are difficult to match using other metal ion battery systems and have made lithium technology the go-to for portable electronics and battery focused energy storage systems (for use of lithium in nucelar applications see here).
The ups and downs of lithium
The advantages of using lithium for batteries are intrinsic to its chemistry. Lithium has the lowest reduction potential of any element, leading to the highest cell potential. The small ionic radii of the singly charged ion associated with the low atomic weight leads to a high overall gravimetric energy density. This provides a highly mobile ion that can diffuse through the battery rapidly leading to advantages in power. Other metal ion systems are limited by a larger mass or charge on the ion that reduces the power of the cell and leads to increased charge times.
However, lithium does have some drawbacks. Although it seems unlikely that there will be a shortage of the element in the near future, there have been significant increases in its price over the past few years. Metalary shows that the price of lithium has almost doubled since 2014, with annual price increases exceeding 15%. Such a rapid increase is limiting the applications of the technology. The transition metals used in lithium ion systems have also raised doubts about its sustainability and ability to sustain global power hunger in future decades.
With the price of lithium continually on the up, and there being no competitive ‘new kid on the block’ since it’s invention in 1980, it is perhaps time to find an alternative as we head into an electrical, and renewables driven future.
Storing it away
Alternatives to lithium include sodium, magnesium, aluminium, oxygen and carbon. Interest in these is growing, but the current cost of lithium ion systems makes them unfavourable for grid load leveling.
Storage systems are becoming increasingly important because of the uptake of renewable energy sources, such as wind and solar. Intermittency issues can keep them from connecting widely to the grid, and they require energy storage systems that currently run at a minimum of around US$100 per KWh and function only in certain locations. Researchers at Massachusetts Institute of Technology , USA, have attempted to tackle this issue by developing an ‘air-breathing’ battery that they claim could be used to store electricity for long durations for about one-fifth the cost of current technologies, with minimal location restraints and zero emissions.
The battery uses an aerated liquid salt solution in the cathode that ‘breathes’ oxygen to balance the charge as ions move between electrodes. The oxygen going into the cathode causes the anode – sulphur dissolved in water, which has the lowest cost per stored charge next to water and air – to discharge electrons to an external circuit, whereas oxygen flowing out pushes electrodes to the anode to recharge the battery.
The chemical cost of the battery is around 1/30 of a Li-ion alternative, and a scaled up system could store energy at about US$20–30 per KWh. Yet-Ming Chiang, Kyocera Professor of Materials Science and Engineering at MIT, highlighted that battery development over the past few decades has been focused on synthesising materials with a greater energy density, and not on finding a cheaper alternative. ‘If we want energy storage at the terawatt scale, we have to use abundant materials.'
Although lithium has a chemical advantage over other elements, there is significant interest and merit in investigating alternatives. There is a range of static applications, such as energy storage, where energy density is not the major driver, but cost is. Under these circumstances, lower theoretical energy systems become of interest. Sodium, for example, is available as salt in the oceans and sodium batteries can use Earth abundant elements such as carbon to further increase the renewable credentials of the battery system. The theoretical energy density of a sodium cell might be lower than for a like-for-like lithium system, but a practical cell would not perform significantly worse.
Working with what you’ve got
Using Earth’s abundant elements is a constraint employed by research institutes ETH Zurich and EMPA, Switzerland, in a recent study that produced a low-cost battery made from scrap metal and waste graphite flakes from steel production. Primarily for use as grid storage – but with the potential for home use – the battery features a graphite cathode and a metal anode – the opposite to a standard Li-ion battery.
The battery has a energy density of 70 Wh/kg at a low cost, making it suitable to grid storage applications – traditional hydroelectric storage systems have an energy density in the range of 0.5–1.5 Wh/kg. It has a Wh/kg three times smaller than a Li-ion battery, but the driver is cost, not energy density.
Dr Kostiantyn Kravchyk of ETH Zurich summarised the project for Materials World. ‘We worked exclusively with the most abundant elements on Earth, such as sodium, aluminium, oxygen and carbon. We wanted to make an electrochemical storage device, and when we screened different materials we decided to use aluminium foil as an anode and graphite flakes for the cathode. This is not an entirely new idea, but when we tested graphite, we realised that each type used had a significant impact on the performance of the battery.’
The team discovered that graphite flakes with a very high crystallinity were needed. Kish graphite flakes, which are a by-product of steel production, have the desired crystallinity, as well as a constant space between layers and limited defects. As Kish graphite is waste material, it is a cheap and easily acquirable component.
Ground natural graphene is used in Li-ion batteries, but can only be used in the form of coarse flakes. This is because the battery uses aluminium chloride as electrolyte, which has thicker anions than those found in lithium, meaning that they are too large to pass through ground graphite. By grinding the graphite particles, the layers become creased like crumpled-up paper. Only small lithium ions are able to penetrate this crumpled graphite, not the new battery’s thick aluminium anions.
The new battery has survived thousands of charging and discharging cycles in lab testing, suggesting that it is long lasting.
Kravchyk highlighted the battery’s major selling point, its use of accessible and abundant materials, and what is next for the study. ‘Components like aluminium foil and graphite flakes are very cheap, and recently we also found that the battery could potentially use even cheaper electrolytes. Right now we only have lab scale batteries, and in order to make real grid energy storage prototypes, we need to invest time and money into the study.’
The global interest in metal ion electrochemical storage is significant and growing annually. To make a real impact, new approaches to existing technology need to be adopted and the understanding of different metal systems must be expanded.
*Steven Dunn owns RSDScientific, a consultancy that produces technical articles, white papers and strategic insight for the applied materials industry with a focus on energy and energy policy.