Surface films of lithium: an overview of electrochemical studies

doi:10.1016/S0378-7753(97)02771-7

Journal of Power Sources

Volume 72, Issue 2, 21 April 1998, Pages 203-210

https://doi.org/10.1016/S0378-7753(97)02771-7 Get rights and content

Abstract

There is considerable ambiguity in the literature about the importance of lithium surface film on the electrochemical reaction at the lithium/electrolyte interface. Some studies attribute the measured interfacial properties to the surface film on lithium alone and other studies consider the measured properties due entirely to electron-transfer reaction; while a few studies provide due recognition to both the surface film as well as the charge-transfer reaction. Literature on DC polarization and AC impedance spectroscopic studies of Li/non-aqueous liquid and solid polymer electrolyte interface is reviewed. Our electrochemical impedance spectroscopic studies of lithium/solid polymer electrolyte interface are analyzed using simple equivalent circuit models. The models include both the surface film properties as well as the electron-transfer reaction. The experimental results and reviewed literature support that both the surface film on lithium and the electron-transfer reaction are equally important interfacial phenomena, and should be considered simultaneously during an investigation.

Introduction

Lithium metal is extremely attractive as a negative electrode material in electrochemical power sources (i.e., batteries). The importance of Li originates from its low atomic weight (viz., 6.94), high negative electrode potential (viz., −3.03 V) and high specific capacity (viz., 3.86 Ah g⁻¹). These favourable factors, which are possessed by no other metal, are responsible for the development of so called `lithium batteries'. These batteries are able to meet the increasing demand for high energy, light weight and compact electrical power sources for a wide range of applications where portability is often the prime requisite. Unlike the other battery systems which employ an aqueous electrolyte, a lithium battery requires a non-aqueous electrolyte medium due to the high reactivity of Li with water. Even in non-aqueous electrolytes, the solvents of interest for lithium batteries are the dipolar–aprotic solvents, which ensure the absence of labile or active hydrogen atoms [1].

It is known that passivity of metals and alloys is an essential and significant phenomenon responsible for their stability and utility in almost all applications. Most structural metals are viable in an engineering sense because of the existence of a surface oxide film, the thickness of which may be no more than a few nanometers [2]. The film isolates and protects the metal from rapid reactivity with it's environment. For example, aluminium has a free energy of reaction with oxygen as high as that of a fossil fuel but is stable for a variety of structural applications. The continued integrity of the structure is due entirely to the aluminium oxide that exists on the surface. Lithium metal with its passive surface layer is in no way different from the rest of the metals. However, in aqueous and oxygen environments which are highly reactive, the thin passive film on the surface of Li does not protect the metal. A rapid reaction takes place resulting in a total disintegration of the metal. However, in organic aprotic media lithium metal is protected by its surface film.

The importance of the passive layer on lithium and its role on the performance of the lithium as a negative electrode in electrochemical power sources has been under discussion for a long time. While Peled [3], Peled and Gabano [4]and Dey [5]emphasized the need of a passive film on Li for proper functioning of primary lithium cells, Brummer [6]and Newman [7]believed that a passivated anode could offer only a limited cycle life of secondary cells. According to the latter authors, the lithium anode must be free from a passive layer and it should be kinetically stable in the electrolyte in order to obtain a deep-discharge and high cycle life lithium secondary battery. There is innumerable published literature on various aspects of the passive layer on lithium. Literature on electrochemical behaviour of lithium with associated passive film are also large in quantity. A survey of the recent literature on electrochemical measurements of Li/electrolyte interface reveals the existence of a controversy in understanding this system. While some authors attribute the experimental results entirely to the passive film, others ignore the presence of the passive film and explain the results based on electron-transfer reaction. There are only a few papers that consider both the electron-transfer reaction and the passive film present on lithium. The purpose of this article is to overview the existing literature on this controversy and to present the results of the authors' investigations of lithium in solid polymer electrolyte medium.

Section snippets

Overview of literature

Peled [4]had reviewed the electrochemical considerations at the lithium/electrolyte interface. Until the early 1970s, it was generally believed that lithium was kinetically stable in many organic solvents. Although some passivating film was assumed to cover at least part of the surface, it was generally accepted that the rate determining step (r.d.s) of the deposition–dissolution process of lithium was the electron-transfer between the metallic electrode and lithium ion in the solution. $Li^{+} +e^{−} = Li$

Results and discussion

In the present investigations, we have assembled several symmetrical Li/SPE/Li cells having varied the SPE compositions. Electrochemical impedance measurements have been made, and the cell parameters were evaluated by using Boukamp NLLS fit procedure [25]with a good degree of accuracy. Simple Voigt type of equivalent circuits were considered for fitting the data. Since the experimental impedance spectra contained depressed and overlapped semicircles in Nyquist plots, constant phase elements (Q)

Conclusions

There is no ambiguity on the existence of surface film on lithium metal when present in a non-aqueous or a solid polymer electrolyte. However, the ambiguity, concerns the role of the surface film in the electrochemical reaction (Li⁺+e⁻=Li) that is expected at Li/electrolyte interface. The reviewed literature indicates three categories of reports. The first category of studies attributes the measured interfacial property entirely to the charge-transfer reaction, whereas the second category

Acknowledgements

Financial support by US Air Force through European Office of Aerospace Research and Development (UK) under Contract No. C0007 is gratefully acknowledged. The authors thank Mr. Allen Turner for his help in the manuscript preparation.

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Citation Excerpt :
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Surface films of lithium: an overview of electrochemical studies

Abstract

Introduction

Section snippets

Overview of literature

Results and discussion

Conclusions

Acknowledgements

Thin Solid Films

Solid State Ionics

J. Electroanal. Chem.

Solid State Ionics

Electrochim. Acta

J. Power Sources

Solid State Ionics

J. Electroanal. Chem.

J. Electrochem. Soc.

J. Electrochem. Soc.