For any type of commercially rechargeable battery, lithium batteries possess the highest energy density. For this reason, they find vast applications in electric vehicles. However, these batteries contain lithium salts dissolved in flammable solvents. If the battery is damaged, there is a high risk of explosion. Moreover, battery misuse can lead to rise in temperatures leading to dangerous exothermic reactions.
A solution to all these concerns can be found in the production of solid-state batteries. Polymer-lithium salt electrolytes have been analyzed for lithium battery applications. It has been found that polymer electrolytes are safe in the sense that they are nonvolatile, less reactive, less flammable and do not leak. Poly(ethylene oxide) displays higher conductivity when combined with lithium salts than other polymer electrolytes. This polymer can solvate well a good number of salts. Particularly, poly(ethylene oxide) doped with lithium bis-(trifluoromethanesulfonyl)imide salt exhibits enhanced ionic conductivity. Poly(ethylene oxide) can also be combined with strong polymers like polystyrene to form a robust block copolymer electrolyte.
Unfortunately, little is known about solid electrolyte electrochemical stability. Solid polymer electrolyte degradation may occur in the course of charging and discharging. By-products from side reactions may consume active materials leading to low energy density and reduced battery lifespan. For this reason, Professor Daniel Hallinan Jr. and colleagues developed an electrochemical method to lessen the effects of mass transport, enabling them to determine equilibrium reaction potentials and degradation of the solid electrolyte. Their work is published in Chemical Engineering Science.
The authors prepared two and three-electrode cells in an argon glove box. The cells comprised lithium metal counter electrode, and aluminum, copper, gold or carbon electrodes. They used poly(ethylene oxide) doped with lithium bis-(trifluoromethanesulfonyl)imide electrolyte. For the three-electrode cell, the authors placed a small lithium metal strip between two electrolyte spacers. They assembled the cell with the prepared polymer electrolyte (and a reference electrode) placed between the counter and working electrodes. These cells were then sealed and taken for electrochemical analysis.
They observed that aluminum corrosion in the poly(ethylene oxide) based electrolyte with bis-(trifluoromethanesulfonyl)imide was passivated. However, this was not the case with liquid electrolyte containing bis-(trifluoromethanesulfonyl)imide and ethylene carbonate. The team also noticed no effect of salt concentration in the voltammetry analysis of copper-polystyrene–b–poly(ethylene oxide)-lithium cell. However, current was a function of temperature. The open circuit voltage of the prepared cells corresponded to Cu/Cu2+ stripping, but current passage in both the linear sweep voltammetry and the variable reversed linear voltammetry analyses detected Cu/Cu+ reaction.
The study also found that gold was not an inert electrode for anodic reaction analyses in lithium cells. Actually, gold electrode posted an anodic reaction lower that the theoretical potential. The oxidative degradation of the solid polymer electrolyte was quantified using glassy carbon electrodes and Butler–Volmer kinetics. The study concluded that oxidative degradation of the polymer electrolytes is a slow reaction with high-activation energy, making them promising candidates for use with high voltage cathodes.
“We are excited about our approach to measuring electrochemical reaction kinetics in solid electrolytes, because such measurements have not been previously performed. Our highlighted work indicates that PEO-based electrolytes should be compatible with advanced (high voltage) cathodes for lithium batteries. We are currently applying our approach to reversible reactions on lithium battery electrodes. We anticipate these results to yield insight into limitations of lithium polymer batteries and enable more accurate modeling of battery performance.” Said Professor Daniel Hallinan Jr.
Daniel T. Hallinan Jr.1,2, Alexander Rausch1,2, and Brandon McGill1,2. An electrochemical approach to measuring oxidative stability of solid polymer electrolytes for lithium batteries. Chemical Engineering Science, volume 154 (2016), pages 34–41.Show Affiliations
- Florida A & M University – Florida State University College of Engineering, Chemical and Biomedical Engineering, 2525 Pottsdamer Street, Tallahassee, FL 32310, United States
- Florida State University, Aero-propulsion, Mechatronics and Energy Center, 2003 Levy Avenue, Tallahassee, FL 32310, United States
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