4-1 Introduction
Lithium batteries demand the use of non-aqueous electrolytes. These have been under study since lithium battery research started in the 1950's [1]. Several good reviews are available that give the history of organic electrolyte development [2-3]. Rechargeable lithium batteries place more stringent requirements on organic electrolytes and this has been further heightened by introduction of highly oxidising (i.e. 4 V vs. Li/Li+) intercalation compounds as cathode materials, such as LiMn2 0 4, LiCo02 and LiNi0 2, in the 1980's [1,4]. Use of these electrodes is essential in lithium ion cells to avoid the otherwise lower cell voltage due to the use of carbon as an anode. For 4 V cathodes and hence lithium ion batteries the issue of electrolyte stability is critical. A stable electrolyte must resist the very oxidising conditions at the cathode and reducing conditions at the anode. Electrolyte oxidation can jeopardise the capacity and cycle life of the cell, as well as compromise the safety of the system by generating, for instance, gaseous reaction products that increase the internal pressure of the cell. However, electrolyte stability at the anode has been extensively studied at lithium and more recently carbon. Since we focus on cathodes in this thesis and since stability to high voltage cathodes is a relatively new requirement, we concentrate mainly on this aspect in the present chapter.
It is known that the voltage at which lithium can be extracted from lithiated Mn, Co, Ni oxide materials exceeds + 4.0 V (Fig 1-3-2) and so a competitive electrolyte should possess a stable anodic window comparable to this voltage. Of the electrolytes
traditionally favoured in 3 V lithium cells, i.e. tetrahydrofuran (THF), 2- methyltetrahydrofuran (2-MeTHF) and propylene carbonate (PC) solvents [6-15], only PC is suitable for 4 V lithium batteries because of its high resistance against oxidation at high voltage. Because of the interest in 4 V cathodes there is now much research on alternatives to PC including alkyl carbonates and ether based electrolytes, e.g. ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), diethoxyethane (DEE), as well as their mixtures, such as PC+EC, EC-kDMC and EC+DEE [4, 5, 16-22]. Mixed solvents also offer improvements in other physical and chemical properties such as conductivity and operating temperature [4].
The electrochemical stability of an electrolyte includes contributions from the solvent and salt. In order to study the electrochemical stability of organic solvents it is normal to add salts such as LiClO^, KPFg or tetrabutylammonium perchlorate (TBAP) to overcome the high resistivity of the solvents. Unfortunately, these salts can often limit the extent of the electrochemical window due to metal deposition, reduction of the cation or oxidation of the anion. These reactions make it difficult to determine the electrochemical behaviour of the solvents alone. Microelectrode techniques can solve this problem. The use of microelectrodes minimise the effect of solution resistance (i.e. iR drop) in an electrochemical cell [23, 24] and hence permit electrochemical investigation of highly resistive media. This technique has recently been applied to the study of solvents in the absence of added salts [6,7].
Traditionally [10-16], platinum has been used as the working electrode material in the study of electrolyte stability often in the belief that this is an inert source or sink of electrons. Of course all electrodes add some chemical specificity so that the problem of determining electrolyte stability will always depend to some extent on the electrode material. Metals other than platinum, e.g. stainless steel (SS) and aluminium, are employed as substrate materials (i.e. current collectors in lithium batteries, on which active electrode materials are mounted) because of cost, mechanical property and relative stability. The influence of these metal electrodes on electrolyte decomposition
must not be neglected [12, 25-28]. As an essential component of the cells, however, the role of metal current collectors on electrolyte stability window has not been given sufficient emphasis.
In this chapter, several Li+ electrolyte solutions are investigated, studies include (1) determination of the electrochemical stability windows of solvents by means of Pt microelectrodes; (2) the anodic behaviour of electrolytes on A1 and SS electrodes; and (3) the influence of lithium manganese oxide spinels on electrolyte decomposition. The ultimate purpose of these studies is to determine the potentials corresponding to anodic breakdown of these electrolyte solutions at composite electrodes containing lithium manganese oxide spinel mounted on SS or A1 substrates, and hence to establish cut-off potentials for the cycling of cells.
4-2 Electrochemical stability windows of some solvents
The molecular formulae for PC, EC and DMC presented in Fig 4-2-1, the first two possess a five member ring. In this section the electrochemical stability of these solvents without addition of salt is explored using microelectrodes. The voltammetric experiments were run at a sweep rate of 200 mV s"^ and at a 25 pm diameter Pt microelectrode in a two electrode cell. The two electrode cell consists of a Pt microelectrode and a Li electrode as described in CHAPTER THREE (Fig 3-4-3).
Potentials were first scanned anodically and then reversed ending at -2 V versus Li+ (1M)/Li. Fig 4-2-2 shows voltammograms for pure PC, EC, DMC and tetraglyme (CH3 0(CH2-CH2 0)4-CH3) solvents. The voltammograms in Fig 4-2-3 correspond to mixtures of EC with PC and DMC in a 50 : 50 ratio by volume, respectively.
P ^
P ----CH
3 0 =I o==<
O CH3
propylene carbonate ethylene carbonate dimethyl carbonate Fig 4-2-1 Molecular formulae.