Addressing Li-ion Battery Degradation and Charging Limitations

one week ago by David Rutland

From telecoms to transport, the last two decades have seen a drive towards efficient, portable, and environmentally-friendly batteries.

Low voltage fixed telephone lines have largely been replaced by their power-hungry mobile alternatives and electric cars are becoming a common sight on European roads—outperforming their hydrocarbon-fuelled rivals in both acceleration and efficiency.

Tobacco products, which kept lung cancer wards filled for decades, have now been overtaken in popularity by electronic cigarettes, which must more palatably fill the air with sweet-scented vapour.

Every one of these devices requires high-density, rechargeable batteries to function; and for better or for worse, lithium-ion batteries have become the standard; after all, lead-acid batteries are bulky, dangerous and inefficient, while the cadmium in NiCad batteries was outlawed in the EU in 2005.

 

The Limits of Li-ion Battery Technology

The advantages of lithium batteries are well documented. They have a very high energy density, meaning that they can hold a considerable charge: they don’t discharge themselves over time, and they require little to no maintenance—particularly as there is no memory and the battery does not require scheduled cycling to prolong its life.

The future looks bright for lithium-powered appliances to the point that electric vehicle manufacturer, Tesla, has ploughed an estimated 4.5 billion euros to establish the first solar-powered gigafactory, which makes lithium-ion-based powerwalls, power packs, and battery cells for the company’s Model S.

The world’s largest lithium-ion battery was built by Tesla in 2017, which when fully charged, is capable of powering up to 30,000 homes for at least an hour.

 

Ageing and Degradation

However, it is an undeniable fact that lithium-ion batteries die over time. Cathodes degrade, and after approximately 1,000 charge cycles, a high-end battery will have lost around 20% of its capacity. Lithium-ion batteries also degrade even if they aren’t used, and after a year of storage, a fully charged battery will also have lost 20% of its capacity; if they fall into a ‘deep discharge’ state, it’s likely that it won’t be charged again.

This degradation is mainly due to the appearance of whiskers, tree-like structures, and surface growth—collectively known as dendrites—on the plating surface.

 

Adverse Environmental Impacts

While there are many facilities across Europe which accommodate the collection and disposal of lithium-ion batteries, few of these end up being recycled. And although Tesla plans to facilitate the recycling of battery cells at its gigafactory, it currently only does so using third-party partners in what is admittedly an inefficient process. Often, dead lithium-ion cells simply end up in landfills.

 

Industrial high-current lithium-ion batteries. Image courtesy of Bigstock.

 

Unfortunately, it is significantly cheaper to mine lithium for batteries than to extract it from cells during the recycling process—lithium battery recycling enterprises rarely, if ever, break even, and so require subsidies from the state.

Although it may make sound financial sense to pull lithium from the ground, it makes poor environmental sense. The history of lithium mining forms a catalogue of environmental damage. Periodic toxic chemical leaks from mines and manufacturing plants play havoc with local ecosystems—poisoning rivers, killing livestock and fish, and leaving previously fertile farmland severely contaminated.

While the metal can be extracted from seawater or brine deposits, the process requires vast quantities of water, which is already in heavy demand. In Chile, legislators are considering limiting lithium mining rights due to the limited availability of water in what is the world’s driest desert.

The lack of recycling capacity and the increasing number of battery-powered products means that the demand will only grow—and with it, the environmental burden.

 

Improvements to Stability and Performance

One way to prevent, or at least ameliorate, the problems associated with lithium-ion batteries is to improve their reliability, ensuring they do not degrade over time and can be kept in use indefinitely.

Increasing Battery Lifespan and Overall Reliability

Researchers from Ulsan National Institute of Science and Technology in Korea have designed a new technology, which promises to restrict dendrite growth on the lithium foils inside cells. This potentially increases the lifespan indefinitely, while simultaneously improving the stability and performance of the batteries themselves.

The technology, which is still in its early stages, involves coating the lithium anode with lithium—the lithiated form of silicon—which creates a far more uniform surface, and limits opportunities for dendrites to form. The process has not yet been trialled in a real battery, and research is ongoing.

Another possibility would involve the industry moving away from lithium-ion and other rare earth batteries altogether—enabling the creation of batteries with materials that can be easily extracted from the air, without polluting the environment.

 

Ohio State University researchers Paul Gilmore and Vishnu-Baba Sundaresan (from left to right) involved with potassium-oxygen battery development. Image courtesy of Ohio State University.

 

The Alternative of Potassium-oxygen Batteries

Potassium-oxygen batteries were first developed in 2013, offering twice the storage capacity and a greater level of efficiency than their lithium-based counterparts. But the battery degraded with each charge, never lasting for more than ten cycles, rendering the technology effectively useless for applications which require reliability and longevity.

The degradation was due to oxygen seeping into the anode, causing it to break down. Recent developments have seen Ohio State University (OSU) researchers incorporating polymers into the cathode, preventing oxygen from reaching and damaging the anode.  Over the last six years, the lifespan of a potassium-oxygen battery has leapt from 10 charge cycles to at least 125. This is still not as good as a lithium-ion battery, but progress is being made.

If the OSU researchers can improve the potassium oxygen cell performance to a similar level, the advantages over lithium-ion cells are obvious. In addition to increased capacity and efficiency, vast gigafactories won’t be required to manufacture them. Potassium-oxygen cells are less than half the cost of lithium cells, they don’t use any exotic materials—and they can be made anywhere.

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