5.1 Battery materials

Electrical energy is high-quality energy, as it can be easily converted to do many different types of work (e.g. heating, lighting, mechanical and electrochemical). Due to mismatches between the time of energy generation and usage, as well as portability for versatility, storing electrical energy is of high importance. Storage can be accomplished physically (through capacitors, flywheels, hydroelectric, thermal) or through electrochemical processes. This was first done to a useful degree in the early nineteenth century by Alessandro Volta by selecting materials with different redox potentials and building them into the voltaic pile based on copper and zinc [1]. Since then, the study of chemistry and specifically inorganic materials has led to a much deeper understanding in storage of electrical energy and, thereby, advancements in battery technology. There are many ways to store and release electrical energy, which can dictate how a device is constructed. Two performance indicators on how much energy can be stored and how quickly it can be released are, respectively, the energy density (in Watt-hours per unit volume or mass) and the power density (in Watt per unit volume or mass). A way to summarize this is with a Ragone plot, showing these two densities on respective axes (example in Figure 5.1.1). Capacitors can quickly charge and discharge, and thus have a high power density; however, their energy density is very limited. Batteries typically have a high energy density but limited ability to charge or discharge quickly. Supercapacitors, or electrochemical capacitors, started to bridge the gap between the two. However, the ultimate goal lies at the top right of the plot; a device that can store a lot of energy and access it quickly. It is clear from Figure 5.1.1 that different chemistries result in different distributions across this plot. The discussion here will be limited to batteries, which consist of positive (cathode) and negative (anode) electrodes, separated by an ion-transporting but electron-blocking electrolyte. Batteries can be primary (designed for a single discharge followed by disposal) or secondary (able to be reliably recharged). During discharge, electrons are moved through a circuit due to driving forces of different redox potentials between the electrodes, where electrochemical reactions take place, generally involving metals changing their oxidation state. Charging reverses these processes by an applied potential difference. Because battery use involves physically moving material around inside the cell (through electrolyte movement, plating reactions, intercalations, etc.), knowledge of chemical structures, phase transitions, as well as diffusion, and hence the intrinsic inorganic material properties, is essential for understanding real-world performance and commercial application.

Figure 5.1.1: Battery technologies in terms of specific power density (W/kg) and energy density (Wh/kg). Adapted from [2].

5.1.1 The lead acid battery

The lead acid battery, invented in 1859, is still used in the automotive industry as the standard for energy storage for starting, lighting and ignition. Fully charged, the anode consists of Pb metal and the cathode of PbO2, with an electrolyte of H2SO4(aq) (concentrated sulfuric acid). During discharge, lead metal at the anode reacts with bisulfate anions (from the dissociation of one H+ from H2SO4) to produce protons, electrons and lead sulfate (Pb(+II)):

At the cathode, lead oxide (Pb(+IV)O2) reacts with bisulfate and the produced protons, accepting the electrons through the circuit, to produce amorphous lead sulfate (Pb(+II)) and water:

The end result is two plates of lead sulfate, effectively an electrochemical comproportionation reaction, separated by dilute sulfuric acid due to the participation and depletion of the electrolyte in the overall reaction. This means that the state of charge is directly proportional to the concentration of H2SO4, and hence the specific density of the electrolyte that can thus be easily measured. This was one of the first battery chemistries rechargeable by reversing the current; however, the capacity can be impeded by the crystallization of PbSO4 rather than amorphous deposition; this is not reversible, but BaSO4 can be used as an additive to intentionally promote a reversible crystallization of PbSO4 that does not block the active material. Despite the toxicity of lead, this battery class has the benefits of ability to operate in a wide temperature range (–20 to 50 °C) with low self-discharge, as well as ease and widespread ability of recycling. However, energy density is fairly low, especially considering the materials (such as casing, and electronic battery management systems) that need to be added to go from the operating potential of a single cell to that of the battery pack with cells parallel and in series.

5.1.2 The alkaline battery

Ubiquitous alkaline batteries, reliant on zinc and manganese oxide electrodes, are good examples of primary batteries that normally cannot be recharged. At the anode, metallic zinc reacts with hydroxide in the electrolyte to produce zinc oxide (Zn(+II)):

At the cathode, manganese oxide Mn(+IV) is reduced to manganese oxide Mn(+III), with water participating to form hydroxide with the lost oxygen of MnO2 to replenish the electrolyte:

Alkaline batteries are advantageous in that they can be produced cheaply and in high volume for consumer applications (everything from toys, to cameras, to household appliances). However, their disadvantages are foremost that they cannot practically be recharged, in addition to their capacity being a strong function of the rate of discharge as well as being prone to leaking their caustic hydroxide electrolytes.

5.1.3 The nickel-cadmium and nickel-metal-hydride type

The nickel-cadmium (Ni-Cd) chemistry uses a similar electrolyte as the alkaline battery and is widely known for its use in earlier rechargeable cells and as battery packs in electric tools due to its ability to be readily recharged. These cells consist of a cadmium anode, NiOOH (nickel oxide-hydroxide) cathode and hydroxide electrolyte, typically KOH(aq). During discharge, at the anode (metallic cadmium), cadmium hydroxide (Cd(+II)) is formed:

At the cathode, nickel oxide-hydroxide (Ni(+III)) accepts electrons through the circuit and loses oxygen to become nickel hydroxide (Ni(+II)):

These cells can suffer from a “memory effect” where a partial discharge can lead to limited future discharge capacity, though this can be alleviated by deep discharge cycles making sure that all inorganic material is being converted. The cell can deliver up to around 1.3 V and the main benefits of this class of batteries are their cycle-life (over 1,000 cycles typically before their capacity reduces to below 50%, including the ability to handle deep discharge) and much higher energy density than the lead acid battery. However, cadmium, being a toxic heavy metal, is of concern for production and end-of-life.

The nickel-metal-hydride (NiMH) chemistry also relies on nickel oxide-hydroxide, but instead of cadmium, an intermetallic compound (M) is used, typically of composition AX5, where A is a rare-earth metal and X is Ni, Co, Mn or Al; the electrolyte is typically aqueous KOH. During discharge, the intermetallic compound at the anode, having already been converted to a hydride, is reduced, moving an electron into the circuit and producing water:

At the cathode, nickel oxide-hydroxide (Ni(+III)) reacts with water to regenerate OH– and form nickel hydroxide (Ni(+II)):