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Fundamentals of Electrochemical Energy Storage Technologies

Minghua Chen1 and Yu Li2

1Harbin University of Science and Technology, School of Electrical and Electronic Engineering, Department of New Energy Materials and Devices, Key Laboratory of Engineering Dielectric and Applications (Ministry of Education), 52 Xuefu Road, Nangang District, Harbin, 150080, China

2Harbin University of Science and Technology, School of Electrical and Electronic Engineering, Department of Electrical Theory and New Technology, 52 Xuefu Road, Nangang District, Harbin, 150080, China

2.1 Typical Battery Patterns and Corresponding Functions

Of late, numerous electrochemical energy storage systems have been developed to meet the demands of our daily lives, of which five are commonly used and commercialized: lead–acid batteries, lithium-ion batteries (LIBs), nickel metal hydride (Ni-MH) batteries, nickel–zinc (Ni-Zn) batteries, and supercapacitors. Lead–acid batteries are most commonly used in gasoline cars for engine ignition and running the auxiliary electronics when the engine is off. However, they are harmful to the environment, and replacing them with less expensive, safe, and energy-dense electrochemical energy storage technologies is one of the major challenges faced by researchers. Ni-MH batteries and Ni-Zn batteries are typical alkaline batteries using KOH-based electrolytes. Their energy density is slightly higher than that of lead–acid batteries; however, their output voltage is highly restricted by the limited theoretical thermodynamic stable potential (1.23 V). In addition, alkaline electrolytes will continuously etch the current collector and zinc anode, and thus, cycling lifespans of these batteries can rarely be more than 500 cycles without obvious decay [1]. LIBs have been widely used in portable devices, electric vehicles, and grid-scale energy storage systems due to their high output voltage, high energy density, and long cycling lifespans. The lifespan of advanced LIBs can be more than 10,000 cycles. Although the safety of using LIBs is ensured in various portable devices (e.g. phones and laptops), thermal runaway and explosion can occur in grid-scale energy storage systems and electric vehicles due to the growth of lithium dendrite, which induces internal short circuits and releases substantial heat to ignite flammable electrolytes. Dendrite growth is related to overcharging and over-discharging of batteries. These can be prevented by rational battery management, which can be efficiently controlled in a single cell and hardly controlled in the battery pack [2]. The more the number of LIBs, the higher the thermal runaway risk. Supercapacitor is a power-density-superior electrochemical energy storage device that harvests energy via a rapid physical adsorption/desorption process. However, the energy density of supercapacitors is more than tenfold lower than that of batteries, and thus, they cannot be used as a main power source for the functioning of electric equipment [3].

Nevertheless, all the abovementioned electrochemical energy storage technologies have changed our daily lives and have similar component patterns. In general, all of them have two electrodes. If the electrode materials used are different, they can be classified into cathode and anode, which will be introduced in the following sections. Materials used as electrodes should at least be good electronic conductors. Meanwhile, the cathode and anode need to be separated by electrolytes and separators to avoid direct contact between them. The energy storage mechanism of electrochemical energy storage technologies is mainly based on the electrochemical reactions (reversible redox reaction or intercalation) at cathode and anode. It is noteworthy that the redox reactions happened in electrochemical energy storage devices is quite different with chemical reactions. The primary distinction between an electrochemical reaction and a chemical redox reaction is that, in the former, reduction occurs at one electrode and oxidation occurs at the other, while in the latter, both reduction and oxidation occur at the same electrode. This distinction has several implications. In an electrochemical reaction, oxidation is spatially separated from reduction. Thus, the entire redox reaction is divided into two half-cells. The rate of these reactions can be controlled by externally applying a potential difference between the electrodes, e.g. using an external power supply, a feature that is not present in chemical reactors. Furthermore, electrochemical reactions are always heterogeneous; i.e. they always occur at the interface between the electrolyte and an electrode (and possibly a third phase, such as a gaseous or insulating reactant). Even though half-cell reactions occur at different electrodes, reaction rates are coupled by the principles of conservation of charge and electroneutrality.

2.1.1 Cathode

To date, many electrochemical battery systems have been proposed. The term “cathode” is a relative concept. A cathode in one electrochemical system can be an anode in another system. For example, MnO2 is a cathode material in Zn-Mn batteries but an anode material in LIBs. Hence, whether a material is a cathode or anode depends on the relative redox potential of the material, which, in an electrochemical device, can be deduced based on standard reduction potential. Standard reduction potential describes the ability of a material to release electrons. The lower the standard reduction potential, the stronger the ability to release electrons. Therefore, when two materials are assembled into an electrochemical system, one with the higher reduction potential is the cathode, and the other one is the anode. Theoretically, any material can be assembled into electrochemical batteries with appropriate electrolytes. The output voltage of an electrochemical system depends on the potential difference between the cathode and anode. Some electrodes commonly used in diverse electrochemical energy storage technologies are LiCoO2, LiFePO4, LiMn2O4, LiNiCoMnO2, and Ni(OH)2. All these possess unique crystal structures (e.g. layer structure and sodium super ionic conductor (NASICON) structure) and have sufficient space to store ions and ensure rapid ion transportation (Table 2.1).

Table 2.1 The standard reduction potential of typical materials.

2.1.2 Anode

In an electrochemical system, anode is the electrode that reacts at a lower potential among the two electrodes. Metals and graphite are some of the commonly used anode materials in state-of-the-art energy storage technologies. Usually, graphite and other carbon materials are used as host materials to store energy through reversible ion intercalation. The capacity of these materials is usually less than 300 mAh g−1. In metal anodes, energy storage occurs via reversible ion plating/striping. Theoretically, the capacity of metal anodes is much higher than that of carbon materials. However, in metal anodes, irreversible etching and uneven plating/striping lead to the formation of dendrites and “dead metal,” thus presenting them with poor cycling stability and high safety hazard. Designing advanced electrolytes and artificial solid electrolyte interphase (SEI) for ensuring uniform ion plating/striping and protecting irreversible etching is an interesting topic in the field of electrochemical energy storage.

2.1.3 Electrolyte

The electrolyte is an essential component that ensures rapid ionic conductivity inside the battery and prevents direct contact between cathodes and anodes. Electrolytes used in various electrochemical energy storage technologies should generally meet the following basic requirements [4]: (i) high ionic conductivity, at least 1 × 10−3 ∼ 2 × 10−2 S cm−1; (ii) high thermal and chemical stability, with no separation in a wide voltage range; (iii) a wider electrochemical stability window (ESW) to keep the electrochemical performance stable in a wider voltage range; (iv) good compatibility with other parts of the battery, such as electrode materials, electrode current collectors, and separators; and (v) safe, nontoxic, and nonpolluting.

Based on the energy storage mechanism of electrode materials, various electrolytes have been developed, which can primarily be classified into three categories: aqueous electrolytes, organic electrolytes, and solid-state electrolytes. In aqueous electrolytes, water is used as the solvent. The following are the major advantages of aqueous electrolytes: low cost, high ionic conductivity (compared with organic electrolytes and solid-state electrolytes), and being nonflammable and recyclable. Their primary disadvantage is the limited thermodynamic ESW of water. Theoretically, water decomposes above a voltage of 1.23 V. Considering the overpotential of water splitting, the voltage of aqueous electrolytes is less than 2.0 V. Since energy density is highly dependent on output potential, electrochemical energy storage technologies with aqueous electrolytes usually have much lower energy density than those using organic electrolytes and solid-state electrolytes. Recently, numerous strategies have been developed to make the ESW of aqueous electrolytes wider, e.g. water-in-salt (WIS) electrolytes, deep eutectic solvents, and artificial SEI. In particular, using well-designed WIS...