Effect of the Electrode/Electrolyte Contact Resistance on the Energy Storage Capacity and Cycleability of Li/O2 Batteries

There is currently considerable interest in the use of renewable energy sources as clean and efficient energy supplies because of serious concerns about the anticipated doubling of world energy consumption within the next 50 years, and also due to intense interest in the use of low – or even zero – emission energy sources as the world strive to reduce greenhouse gas emissions. Energy in the form of electricity generated from renewable sources, such as solar, wind and tidal, offers enormous potential for powering our future energy demands. Due to the intermittent nature of these energy sources, energy storage technologies with high capacities and superior power capabilities are required to fulfil our forthcoming continuous energy needs.

Because of their inherent simplicity in concept batteries are at the forefront of the electrical energy storage systems.  The amount of energy per mass or volume that a battery can deliver is a function of the cell’s voltage and its capacity. The capacity of a lithium battery system can be enhanced remarkably by using a completely different approach which combines Li as anode directly with oxygen as cathode active material in a Li/oxygen cell. In a Li/O₂ cell, molecular oxygen accessed from environment is reduced catalytically on an air electrode surface providing anions that react with lithium cations supplied by the anode and delivered by the electrolyte to form Li₂O₂on the air electrode surface during discharge. The system is unique since lithium/oxygen couple delivers the highest energy density among different materials yet investigated for advanced battery systems.

 Electrolyte used in the battery is a non-aqueous electrolyte consisted of a lithium salt dissolved in an organic solvent such as propylene carbonate.  It is the only media for the transfer of lithium ions between cathode and anode during charge/discharge. The wettability of the electrode by the electrolyte and also electrolyte diffusion into the porous structure of the electrode are important factors affecting the performance and rate capability of the battery. Incomplete wetting of electrode by electrolyte leads to an increase in the electrolyte/electrolyte interfacial resistance handicapping high current charging and discharging. It is thus essential to improve the contacting behavior of the electrode-electrolyte interface to lessen the cell’s internal resistance for achieving higher energy and power densities.

In this work composite air cathode electrodes based on carbon aerogels with different porous structures are fabricated and electrochemical impedance spectroscopy (EIS) has been applied to study the interfacial changes occurring on the internal surface of electrodes during cell’s charge/discharge processes to understand the effect of the electrode porosity on the electrode/electrolyte contact resistance which has significant effects on the cell performance. The analysis of impedance spectra (shown in Figure 1) indicated that the interfacial resistance between electrode and electrolyte (R_int) decreases from 58.12 to 21.20 Ω/cm²as the size of pores in the porous electrode is increased from 17 to 24 nm. This is explained by decrease in contact surface area between electrode and electrolyte as the electrode’s pore size increases. It has been also shown that the charge-transfer resistance in cell decreases with increasing air electrode's pore size probably due to the increase in ionic conducting paths within the electrode.

The in-situ electrochemical impedance spectroscopy of the cathode electrode in which the impedance spectra are measured concurrently during charge/discharge cycling of the cell showed that the interfacial resistance of the electrode, R_int, increases with cycling due to the build-up of low conductivity discharge products within the carbon structure that cover a higher portion of the electrode’s internal surface hindering electrode/electrolyte contacts with increase in the cycle number.