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The smoke screen that hindered the lithium-ion battery development for twenty years

30 January 2020

“This battery has had a dramatic impact on our society,” Olof Ramström, a chemist and a member of the 2019 Nobel Committee for chemistry, said on October 9 during the announcement of the Nobel Prize. “It’s clear that the discoveries of our three laureates made this possible. It’s really been to the very best benefit of humankind.”

Many of you have probably seen the media coverage of the 2019 Nobel Prize in Chemistry, awarded to Akira Yoshino, Stanley Whittingham, and John Goodenough for development of the lithium-ion battery. However, what this coverage has missed is that the molecular-level understanding about the chemistry mechanism is still insufficient. This short digest focuses on one interesting snippet of lithium-ion battery technology, the misleading electrolyte component that slowed down the field for twenty years.

The twins?! Electrolyte: Ethylene Carbonate (EC), Propylene Carbonate (PC) Disparity

Think of the commercial lithium-ion battery as a sandwich: the cathode and the anode are the two pieces of bread, the liquid electrolyte is the protein in the middle, and the separator is the tomato slice that prevents the two bread pieces from touching each other. When the battery operates and charges, you are pressing harder on one piece of bread. Under this pressure, the inner part of the sandwich will have more contact with the bread and begin to mesh together. The solid (bread) and electrolyte (protein) Interphase, abbreviated to SEI, is the mesh that has attracted numerous scientists to study why some electrolyte components would form a protective interphase to support the reversible lithium-ion battery cycling, while others simply destroy the electrode (bread) structure. Among all the electrolyte components being studied, EC and PC remain the most mysterious duo since 1950.

Looking at the molecular structure of these two battery electrolyte components in the figure below, it’s easy to see that EC and PC only differ by a Methyl Group (CH3). However, this difference allows EC to form almost ideal interphases on the graphite anode, thus widely used in all lithium-ion batteries manufactured nowadays. PC, on the other hand, could not form any protective interphase, resulting in exfoliation of graphite during charging and discharging. For example, think of the exfoliation products you use to clean the dead cells on your face. If you use such a strong scrubber to “clean” your sandwich repeatedly, you will be left with no “bread” on the anode.

Figure 1: Chemical Structure of EC (left) and PC (right)

It took more than two decades for the scientific community to realize the supremacy of EC. In the early 1960s, the government and military drove the first wave of enthusiasm in lithium-based batteries. The majority of the work used PC instead of EC, as PC has a lower melting temperature (-48.8 °C) than EC (36.4 °C). Another hidden assumption of this preference is that PC and EC are essentially the same” in their electrochemical behavior. In the next two decades, EC was only occasionally used as an additive solvent with PC, to help decrease viscosity and increase conductivity.

Figure 2 : The Report published in 1956 that states EC and PC are the same!

In 1967, most scientists holding the general PC supremacy belief reported a graphite anode, PC electrode case. This work was the first documentation of the graphite exfoliation caused by gassing of PC. The researchers did not have a solid grasp of the underlying chemistry. From hindsight, one of the well-respected battery researchers, Dr. Kang Xu, wrote in a review article that “Had an EC-based electrolyte been used instead of PC in this work (1967), the electrochemical Li+ intercalation into graphite could have been accomplished at least 20 years earlier!” Almost two decades after the initial theory was formed, on May 26, 1987, Dr. Akira Yoshino, one of the 2019 Nobel Chemistry Prize winner filed for the secondary battery patent, paving the way for the first success. Next, in 1991, Sony commercialized the lithium-ion battery.  Hidden behind this victory, the EC-PC disparity is unfortunately still a mystery on the molecular level.

For the future scientists who just experienced the emotional roller coaster with me, there is still hope to understand LiB on a more fundamental chemical level. As the other 2019 Nobel Chemistry Laureate, Dr. John GoodEnough, said in his Interview: You know, you live long enough, you never know how it’s going to come out. Now just imagine if researchers in the 1967 hadn’t made assumptions when investigating EC and PC, would the lithium-ion battery have dominated the market and co-led the third Industrial Evolution in the early 1990s?

 

Pu Riley Zhang is a first year graduate student at Stanford University in the Department of Materials Science & Engineering

Banner image photo credit: craig1black