The electrochemical window (EW) of a substance is the electrode

electric potential Electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as electric potential energy per unit of electric charge. More precisely, electric potential is the amount of work (physic ...

range between which the substance is neither

oxidized Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

nor reduced. The EW is one of the most important characteristics to be identified for solvents and

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

s used in

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typi ...

applications. The EW is a term that is commonly used to indicate the potential range and the potential difference. It is calculated by subtracting the reduction potential (cathodic limit) from the oxidation potential (anodic limit). When the substance of interest is water, it is often referred to as the ''water window''. This range is important for the efficiency of an electrode. Out of this range, the electrodes will react with the electrolyte, instead of driving the electrochemical reaction. In principle,

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

has an extremely small electrochemical window, but thermodynamically-favored reactions less than 1 V outside the window are very slow. Consequently, the electrochemical window for many practical reactions is much larger, comparable to water.

Ionic liquids An ionic liquid (IL) is a salt in the liquid state at ambient conditions. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as water and gasoline are ...

famously have a very large electrochemical window, about 4–5 V.

The importance of electrochemical window (EW) in organic batteries

The electrochemical window (EW) is an important concept in organic electrosynthesis and design of batteries, especially organic batteries. This is because at higher voltage (greater than 4.0 V) organic electrolytes decompose and interferes with the oxidation and reduction of the organic cathode/anode materials. For this reason, the best organic electrolytes should be characterized by a wider range of electrochemical window, ''i.e.'', greater than the working range of the battery cell voltage. For example, the electrochemical window of Lithium bis- (trifluoromethanesulfonyl)imide, commercially known as LiTFSI is about 3.0 V because it can operate in the range of 1.9 -4.9 V. On the other hand, for electrolytes that are characterized by narrow electrochemical window, they are prone to irreversible decomposition, which in turn triggers the battery capacity decaying during subsequent battery cycling. The electrochemical window of organic electrolyte depends on many factors that include temperature, molecular frontier orbitals such LUMO (Lowest Unoccupied Molecular Orbital) and HOMO (Highest occupied Molecular Orbital) because the mechanisms of reduction (electron gaining) and oxidation (electron loss) are governed by band gap between

HOMO ''Homo'' () is a genus of great ape (family Hominidae) that emerged from the genus ''Australopithecus'' and encompasses only a single extant species, ''Homo sapiens'' (modern humans), along with a number of extinct species (collectively called ...

and

LUMO In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for ''highest occupied molecular orbital'' and ''lowest unoccupied molecular orbital'', respectively. HOMO and LUMO are sometimes collectively called the ''frontie ...

. Solvation energy also plays an important role in defining the electrochemical window of the electrolyte. In order to safeguard the thermodynamic stability working conditions of the electrode materials in a given electrolyte, the electrochemical potentials of the electrode materials (

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

and cathode) must be comprised within the electrochemical stability of the electrolyte. This condition is very succinct because electrolyte might be oxidized when the cathode material possess an electrochemical potential, which is less than the electrolyte oxidation potential. When the electrochemical potential of the anode material is quite higher than the reduction potential of the electrolyte, the electrolyte will be degraded through reduction process.

Limitation of Electrochemical window

One of the shortcoming of electrochemical window (EW) in predicting the stability of the electrolyte towards anode or cathode materials ignores the voltage and the ionic conductivity, which are also important.

References

{{Reflist Electrochemistry