Electrochemical stability, electrolytes

Fig. 9.1 Electrolyte electrochemical stability in relation to electrode potentials...

One of the main goals of the predictive calculations of and ox is to establish trends in solvent, salt, additive, and overall electrolyte electrochemical stability that can be used to guide the selection and combination of electrolyte components. Most conveniently, computational results obtained should - in one way or the other - be converted to a relative potential scale, preferentially vs. LP/Li, to ease the comparison of experimental and calculated results. Thus, a grand summary of the work traversed in this chapter would be to present the electrochemical stabilities of the materials covered in this chapter in a master figure relative the LP/Li potential. However, because of the many methods and model approaches implanented, and the different procedures used to project the computational results onto an experimental scale, this would be either a hopeless endeavour or end in an overwhelmingly complex figure. 

In the tradition of previous reviews [1-22], this section addresses various aspects of nonaqueous electrolytes, including intrinsic properties, such as local structures caused by ion-ion and ion-solvent interactions and bulk properties, such as ionic conductivity, viscosity, and electrochemical stability (voltage window), and their relationships to intrinsic properties. 

The electrochemical stability range determines the usefulness of nonaqueous electrolytes for electrochemical studies as well as for applications. It indicates the absence of electrochemical oxidation or reduction of solvent or ions, and of faradaic current... 

However, even if electrolytes have sufficiently large voltage windows, their components may not be stable (at least ki-netically) with lithium metal for example, acetonitrile shows very large voltage windows with various salts, but is polymerized at deposited lithium if this reaction is not suppressed by additives, such as S02 which forms a protective ionically conductive layer on the lithium surface. Nonetheless, electrochemical stability ranges from CV experiments may be used to choose useful electrolytes. 

Tahle 7. Electrochemical stability ranges or anodic stability limits of several nonaqueous electrolytes... 

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

Skotheim et al. [286, 357, 362] have performed in situ electrochemistry and XPS measurements using a solid polymer electrolyte (based on poly (ethylene oxide) (PEO) [363]), which provides a large window of electrochemical stability and overcomes many of the problems associated with UHV electrochemistrty. The use of PEO as an electrolyte has also been investigated by Prosperi et al. [364] who found slow diffusion of the dopant at room temperature as would be expected, and Watanabe et al. have also produced polypyrrole/solid polymer electrolyte composites [365], The electrochemistry of chemically prepared polypyrrole powders has also been investigated using carbon paste electrodes [356, 366] with similar results to those found for electrochemically-prepared material. 

It can be seen that an energy of ca. 150 kJ/kg, comparable to that accumulated in Pb02-Pb or Ni-Cd batteries, can be obtained at voltages of 4V. Somewhat lower energy (100 kJ/kg) is accumulated at a voltage of 3V. Consequently, the searched system carbon/electrolyte should be characterised by (i) specific capacity. > 160 F per gram of activated carbon and (ii) electrochemical stability window at the level of ca. >3V. 

Application of new types of graphite, found to be more oxidation-proof (in particular, TEG and TEG modified by boron), can largely increase the electrochemical stability of materials used in aqueous electrolyte media. Their high resistance to oxidation and enhanced long-term cycling stability create realistic prerequisites for wide range of applications for such graphite... 

In this sense, Lilm favors solvents with a low dielectric constant. Electrochemical stability tests were carried out on a GC electrode, and Im was found to be stable against oxidation in EC/DMC up to 2.5 V vs a Ag+/Ag reference, which translates to 5.0 V vs Li, an oxidation limit lower than those for L1BF4 and LiPFe,but still high enough to be practical. The morphology of cycling lithium in Lilm-based electrolytes is apparently superior to that in other salt-based electrolytes. 

The implication of such a picture of the solution structure on the microscopic level not only concerns ion transport but also further relates to the electrochemical stability of the electrolytes in lithium ion cells, because these solvent molecules in the solvation sheath, such as EC or PC, migrate with the ions to electrode surfaces and are probably more involved in the oxidative or reductive processes than the noncoordinating, low- solvent molecules, such as the linear carbonates. This could have a profound impact on the chemical nature of the electrolyte/electrode interfaces (section 6). 

The cycle life of a rechargeable battery depends on the long-term reversibility of cell chemistries, and the electrochemical stability of the electrolyte plays a crucial role in maintaining this reversibility. In electrochemistry, there have been numerous techniques developed to measure and quantify the electrochemical stability of electrolyte components, and the most frequently used technique is cyclic voltammetry (CV) in its many variations. 

As a compromise between the above two approaches, the third approach adopts nonactive (inert) materials as working electrodes with neat electrolyte solutions and is the most widely used voltammetry technique for the characterization of electrolytes for batteries, capacitors, and fuel cells. Its advantage is the absence of the reversible redox processes and passivations that occur with active electrode materials, and therefore, a well-defined onset or threshold current can usually be determined. However, there is still a certain arbitrariness involved in this approach in the definition of onset of decomposition, and disparities often occur for a given electrolyte system when reported by different authors Therefore, caution should be taken when electrochemical stability data from different sources are compared. 

This section will discuss the electrochemical stabilities of different solvents and salts used in state-of-the-art electrolytes that were determined with nonactive electrodes (i.e., in the first and the third approaches). When active rather than inert electrodes are used as working surfaces, many complicated processes, including the reversible electrochemical redox chemistries as well as surface passivation, occur simultaneously. These related materials will be dealt with in a dedicated section (section 6). 

Table 4 lists selected electrochemical stability data for various lithium salt anions that are commonly used in lithium-based electrolytes, with the measurement approaches indicated. Although it has been known that the reduction of anions does occur, sometimes at high potentials, the corresponding processes are usually sluggish and a definite potential for such reductions is often hard to determine. The reduction of solvents, occurring simultaneously with that of anions on the electrode, further complicates the interpretation efforts. For this reason, only the anodic stability of salt anions is of interest, while the cathodic limit of the salt in most cases is set by the reduction of its cation (i.e., lithium deposition potential). 

Table 5. Electrochemical Stability of Electrolyte Solvents Nonactive Electrodes...

Unfortunately, this approach for electrochemical stability determination has not been widely adopted. The few exceptions include the seminal electrolyte work by Guyomard and Tarascon. In the formulation of new electrolytes for lithium ion technology, the spinel composite electrode was used as the standard working surface in all of the voltammetric measurements. The oxidative decomposition limits of the new electrolytes thus determined are summarized in Table 7 along with a handful of stability data that were determined in a similar approach for other electrolyte systems. 

The anodic limit for the electrochemical stability of these carbonate mixtures has been determined to be around 5.5 V in numerous studies.Thus, new electrolyte formulations are needed for any applications requiring >5.0 V potentials. For most of the state-of-the-art cathode materials based on the oxides of Ni, Mn, and Co, however, these carbonate mixtures can provide a sufficiently wide electrochemical stability window such that the reversible lithium ion chemistry with an upper potential limit of 4.30 V is practical. 

Unfortunately, these aza-ethers showed limited solubility in the polar solvents that are typically preferred in nonaqueous electrolytes, and the electrochemical stability window of the LiCl-based electrolytes is not sufficient at the 4.0 V operation range required by the current state-of-the-art cathode materials. They were also found to be unstable with LiPFe. Hence, the significance of these aza-ether compounds in practical applications is rather limited, although their synthesis successfully proved that the concept of the anion receptor is achievable by means of substituting an appropriate core atom with strong electron-withdrawing moieties. 

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