CHEM-E4185 - Electrochemical Kinetics, 25.02.2019-29.05.2019

Dozens of different primary battery chemistries have been developed, and batteries with various shapes are available for consumers, ranging from coin cells to cylindrical and prismatic ones. In wearable electronics, flexible thin batteries are often used, while in Internet of Things (IOT)- type applications small primary batteries often power the devices. The most common consumer batteries utilize zinc-manganese, zinc-air and lithium-ion chemistries. The major features of primary lithium-ion batteries are similar to those of secondary lithium-ion batteries, but they have a metallic lithium anode. These batteries have been designed for a single use and attempts to recharge then can lead to hazardous side reactions or even an explosion.

9.2.1  Zinc-Manganese chemistry: Lechlanché and alkaline battery

The alkaline battery is equivalent to the acidic Lechlanché battery or zinc-carbon battery, the discovery of which dates back approximately 150 years. Lechlanché batteries are still in use in a number of consumer and professional applications because they are affordable in price and easy to design to meet specific shape and capacity requirements. However, alkaline batteries are now the most popular primary batteries because they are more reliable than Lechlanché batteries and allow the high discharge currents required in portable consumer electronics. The capacity of alkaline batteries also stays relatively stable over a large range of currents and temperatures and they can be stored for relatively long periods. 

The cell voltage of the zinc manganese chemistry is approximately 1.5 V. The structure of the Lechlanché battery with the aqueous solution of NH₄Cl and ZnCl electrolyte is given on the left of Figure 9.3. In these batteries, MnO₂ serves as the cathode material and Zn as the anode. The electrode reactions are:

Anode:       Zn(s) + 2 OH⁻(aq) \(\ce{ \bond{->} }\) ZnO(s) + H₂O(l) + 2e⁻

Cathode:   2 MnO₂(s) + H₂O(l) + 2e⁻ \(\ce{ \bond{->} }\) Mn₂O₃(s) + 2 OH⁻(aq)

Overall:      2 Zn(s) + 2 MnO₂(s) + H₂O(l)  \(\ce{ \bond{->} }\) MnOOH(s) + 2 ZnO(s)

In an alkaline battery, similar electrode materials are used (on the right of Figure 9.3), except that concentrated immobilized KOH is used as the electrolyte. Under these conditions, the following reactions take place:

Anode: Zn(s)  \(\ce{ \bond{->} }\) Zn²⁺ (aq) + 2e⁻

Cathode: 2 MnO₂(s) + 2 H₂O + 2e⁻  \(\ce{ \bond{->} }\) 2MnOOH (s) + 2 OH⁻(aq)

9.2.2 Zinc-air battery

A Zinc-air battery is introduced here as an example of batteries where one of the electrodes participating the reactions is replaced by an electrocatalyst (see Figure 9.4).  Because of the compact structure of the air cathode, high energy densities are achieved with this primary battery. It is therefore suitable for continuous operation in applications requiring high performance at low investment costs. The applications extend from consumer hearing aids to industrial scale devices.

Because of the gelled alkaline KOH electrolyte, the anode reaction is similar to that in an alkaline battery. However, airborne oxygen reacts on the surface of the cathode producing hydroxyl ions. The resulting battery voltage is 1.2 V.

Anode               Zn  \(\ce{ \bond{->} }\) Zn²⁺ + 2e⁻

                          Zn²⁺ + 2OH⁻  \(\ce{ \bond{->} }\) Zn(OH)₂

                          Zn(OH)₂  \(\ce{ \bond{->} }\) ZnO + H₂O

Cathode            ½ O₂ + H₂O + 2e⁻  \(\ce{ \bond{->} }\) 2OH⁻ 

Overall               Zn + ½O₂ \(\ce{ \bond{->} }\) ZnO

In order to obtain an even current distribution, oxygen must be homogeneously distributed over the cathode catalyst. The battery is therefore equipped with diffusion layers. A hydrophobic film covering the cathode catalyst is another special feature related to the air cathode. This functional layer contributes to the battery water management: First, it reduces the effect of the variation of the air humidity on the cathode performance and, second, it prevents evaporation of water from the electrolyte.