Optimizing Stability and Capacity for Lithium-Air Batteries

icon Optimizing Stability and Capacity for Lithium-Air Batteries

T. Batcho, D. Kwabi, D. Perego, R. Omampuliyur, Y. Shao-Horn, C. V. Thompson
Sponsorship: Skoltech Center for Electrochemical Energy Storage

Lithium-air batteries hold promise for the next generation of electric vehicles and other applications. By reacting oxygen directly with lithium ions to form Li2O2 on discharge, they can achieve energy densities 3-5 times higher than current lithium-ion batteries. However, a number of challenges exist for implementing lithium-air batteries, including poor rate capability, poor cyclability, high overpotentials upon charging, and electrode and electrolyte instability. We seek to address these issues by developing new electrode materials and architectures and performing studies of Li2O2 formation under various discharge conditions.
Aligned arrays of carbon nanotubes (CNTs) provide ideal conductive scaffolding materials for Li2O2, while having high void space and low mass. CNTs of 5-10 nm in diameter are grown in aligned forests on catalyst deposited silicon wafers. These forests can be delaminated and placed directly into our cell. We observed near ideal gravimetric capacities and high volumetric capacities. However, carbon has been found to decompose in lithium-air cells and promote electrolyte decomposition. This leads to poor cycling performance and high overpotentials on charge. To avoid these effects, we coated materials such as TiN onto CNTs using atomic layer deposition to chemically

passivate the carbon surface. These coated CNTs are capable of supporting Li2O2 growth (Figure 1). We are currently working on optimizing conductivity of the deposited films and testing electrochemical performance during charge and cycling.
Another challenge in designing Li-O2 is obtaining optimal volumetric discharge capacity, which can be achieved by promoting the growth of large toroidal deposits of Li2O2 as opposed to thin films, which electrically passivate and cut off cell discharge prior to full void space filling of the electrode. We seek to study the mechanisms of nucleation and growth in order to control these processes. For this study we used carbon paper electrodes for greater reproducibility and facility in modeling. We test these electrodes with potentiostatic discharges, which use a fixed driving force. We can then adapt existing models for electrodeposition to our system to extract rates of surface nucleation and growth based on current transients (Figure 2). By further studying the dependence of these rates on solvent type, potential, and electrode surface, we can find optimal conditions for greater cell capacity and better understand mechanisms of Li2O2 evolution.

 

 

 

SEM micrograph

transients from potentiostatic

FURTHER READING
  • R. R. Mitchel, B. M. Gallant, Y. Shao-Horn, and C. V. Thompson, “Mechanisms of Morphological Evolution of Li2O2 Particles during Electrochemical Growth,” Journal of Physical Chemistry Letters, vol. 4, no. 7, p. 1060, Mar. 2013.
  • B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thompson, and Y. Shao-Horn, “Chemical and Morphological Changes of Li–O 2 Battery Electrodes upon Cycling,” The Journal of Physical Chemistry C, vol. 116, no. 39, pp. 20800–20805, Oct. 2012.
Last Updated on Thursday, 21 July 2016 20:22