Thermodynamic considerations of same-metal electrodes in an asymmetric cell

M H Braga,A J Murchison,N S Grundish,J B Goodenough

doi:10.1186/s41313-018-0014-8

M H Braga, A J Murchison + Show 2 more

Open Access

https://doi.org/10.1186/s41313-018-0014-8

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Journal: Materials Theory	Publication Date: Jan 25, 2019
Citations: 4	License type: open-access

Affiliation: The University of Texas at Austin

Abstract

An electrochemical cell contains three open thermodynamic systems that, in dynamic equilibrium, equalize their electrochemical potentials with that of their surrounding by forming an electric-double-layer-capacitor at the interface of the electrolyte with each of the two electrodes. Since the electrode/electrolyte interfaces are heterojunctions, the electrochemical potentials or Fermi levels of the two materials that contact the electrolyte at the two electrodes determine the voltage of a cell. The voltage is the sum of the voltages of the two interfacial electric-double-layer capacitors at the two electrode/electrolyte interfaces. A theoretical analysis of the thermodynamics that gives a quantitative prediction of the observed voltages in an asymmetric cell with an S8 relay at the positive electrode is provided. In addition, new discharge data and an X-ray photoelectron spectroscopy analysis of the lithium plated on the positive electrode of a discharged cell is presented. Ab initio, DFT methods were used to calculate the band structure and surface-state energies of the crystalline S8 solid sulfur relay. The theoretical exposition of the thermodynamics of the operative driving force of the chemical reactions in an electrochemical cell demonstrate that our initial experimental data and conclusions are valid. Other reported observations of lithium plating on the positive electrode, observations that were neither exploited nor their origins specified, are also cited.

Highlights

In order to clarify that our observations (Braga et al 2017) of the transfer on discharge of a lithium anode to the cathode conforms to the laws of thermodynamics, the First Law of Thermodynamics for a closed and an open system is rehearsed (The first law of thermodynamics); the electrode/electrolyte interfaces introduce applied forces that perform work on the mobile charges of a cell
Our thermodynamic analysis provides quantitative predictions of our observed voltages for a cell with a sulfur relay. It is illustrated how the electric-double-layer capacitors (EDLCs) are formed at the electrode/electrolyte interfaces in order to equalize the electrochemical potentials of the two materials at an interface
In a battery containing a negative electrode and a positive electrode each electrode equalizes its electrochemical potential with the electrochemical potential of the electrolyte by the formation of an EDLC at each electrolyte interface through the movement of ions in the electrolyte, see Fig. 2

Summary

Introduction

In order to clarify that our observations (Braga et al 2017) of the transfer on discharge of a lithium anode to the cathode conforms to the laws of thermodynamics, the First Law of Thermodynamics for a closed and an open system is rehearsed (The first law of thermodynamics); the electrode/electrolyte interfaces introduce applied forces that perform work on the mobile charges of a cell. A battery cell can be considered a closed system, it contains three open systems (the negative and positive electrodes and the electrolyte) to which an external electric field from the material contacted is applied.

Results

Conclusion