From pv magazine 08/2020
Among all the technologically mature battery systems, lithium-ion batteries (LIBs) come closest to internal combustion engines in terms of energy and power densities – especially when compared with other conventional rechargeable batteries. Since their commercialization in the early 1990s, they have grown to become the dominant power storage solution for portable devices and are now seen as the power source of choice for battery-powered electric vehicles (BEVs) and hybrid electric vehicles (HEVs).
Development of BEVs has rapidly passed from demonstration prototypes to very successful commercial products such as Tesla Models X, S, & 3, Nissan LEAF, and others. However, their rapid commercialization necessitates a shift toward higher-density LIBs.
The energy density of a lithium-ion battery is dependent on both the anode and cathode materials. However, the cathode is currently the bottleneck for the development of LIBs, due to low energy density, high costs, sluggish charge and discharge rates, and low voltages.
For example, an expensive metal used in the commercial cathode is cobalt and about 60% of the world’s cobalt supply comes from the Democratic Republic of Congo (DRC). Regrettably, the mining of cobalt has been linked to human rights abuses, corruption, environmental destruction, and child labor. Many companies would therefore like to reduce or eliminate cobalt in lithium batteries, but it is difficult to match its efficiency. Interestingly, top BEV manufacturer Tesla recently announced that its future cars will only use cobalt-free batteries, based on lithium iron phosphate (LFP).
Lithium iron silicate
Over the past few years, my colleagues and I in the HydroMet research group at McGill University have devoted our attention to the development of a cathode made of lithium, iron, and silicate (LFS). Our recent study focusing on the specific phase (crystal structure) of this cathode shows that these affordable materials could prove key for improving the batteries used in electric vehicles. We utilized a polymer coating procedure that might make it possible for iron and silicon – two of the world’s most abundant elements – to step in for cobalt. The breakthrough was analyzed and confirmed with the Canadian Light Source (CLS) at the University of Saskatchewan. The research was funded by NSERC and supported by Hydro-Quebec.
Theoretically, LFS has double the capacity of LFP (which will be used in Tesla’s future cars) and can hold it over several charge-discharge cycles. In practice, however, this hasn’t been achieved, due to poor intrinsic conductivity – meaning slow movement for electrons and ions during the charging and discharging process of the battery.
As it happens, commercially available LFP suffered from the same issues during its research phase. That was resolved using carbon coating for the individual nanocrystals, which usually requires a high-temperature baking step. But in the case of the low-temperature phase of LFS, this would completely change its structure. Therefore, our research team was keen to utilize an alternative technique to coat the individual nanoparticles to improve conductivity, while keeping their original structure.
We used an electronically conductive polymer known as PEDOT, which can essentially coat the nanoparticles similarly to widely used carbon coating methods. However, this was easier said than done, as figuring out a way to apply the coating to the surface of the nanocrystals took almost two years, due to the inherent low voltage of LFS nanocrystals. Once the coating was done, LFS nanocrystals showed a surprisingly big jump in performance over carbon coating.
To validate our work, cathodes were sent halfway across Canada to CLS in Saskatchewan, where they were tested on the synchrotron beamlines. These high-resolution testing techniques allowed researchers to dig deeper and begin to explain why the PEDOT coating treatment and the sub-surface iron-rich layer improved performance so much. While there is still work to be done to understand why and build on this finding, this high-density cathode material opens another pathway for cobalt-free batteries for BEVs.
For the past decade, the McGill team has been working on this “silicate project” with Hydro-Quebec, a known key player in battery development globally. Being able to use iron and silicon in the cathode could radically reduce the price of LIBs. Currently, it is estimated that the cost of the cathode can make up 40% of a battery cell’s price.
So, maybe in the near future the PEDOT coating method or other engineering strategies applied to LFS will be able to unleash its full theoretical capacity, making it a commercially practical cathode for BEVs. Then we may be able to look forward to a battery, essentially made of iron (even rust) and sand (silica), and start approaching the idea of cheap batteries and mass electrification of various transport systems across the globe for a green planet. Of course, it is usually a long way from laboratory results to a commercially viable product. Nevertheless, this recent development has some advantages and opens a pathway to develop sustainable energy density batteries.
About the author
Majid Rasool is a post-doctoral fellow at McGill University in Montreal, Canada. He is a recent Ph.D. graduate from the materials engineering department at McGill. His doctoral research was focused on the quest for alternative cathode materials that are affordable, safe, and possess high-energy-density. Most of his research work was conducted at HydroMet Lab, under the supervision of Prof. George P. Demopoulos, Hydro Quebec’s Center of Excellence in Transportation Electrification and Energy Storage (ETSE), and Canadian Light Source. He is a battery enthusiast and keeps a keen eye on new developments in the battery world.
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