Battery Basics

This is my first blog post, and I thought of no better way to start than to cover the basic concept of secondary Li-ion batteries. This post aims to describe the elements which constitute a battery - or rather - an electrochemical cell, and the basic principle of discharge and charge.

Batteries are used for energy storage and are highly relevant in our everyday lives as they are crucial for all portable electronics. This includes both the more obvious electronic devices such as mobile phones and laptops and less obvious devices such as remote controls for TVs and electronic scales. Diving into the science of batteries, it becomes clear that batteries are not just batteries.

A battery is composed of two or more electrochemical cells [1]. Batteries are categorized as either primary or secondary. Primary batteries are non-rechargeable. They can only be discharged once, where the fixed amount of reacting compounds are consumed. An example of this is the alkaline manganese primary battery system. Secondary batteries, on the other hand, are rechargeable. In this case, the electrochemical reaction is reversible, which allows for continuous usage of the battery [2]. LiCoO₂ is the most commonly used positive electrode material for secondary Li-ion batteries [3]. The focus of this post is secondary Li-ion batteries.

Electrochemical Cells

An electrochemical cell generates electric energy from chemical energy [2]. It consists of a positive electrode and a negative electrode separated by an electrolyte solution and an ion-permeable separator. The electrolyte contains dissociated salts, typically LiPF6, in one or more organic solvents [3][4]. The electrolyte enables ion transfer between the two electrodes [3]. It is important that the electrolyte does not transfer electrons as this would result in self-discharge [1]. Figure 1 shows an electrochemical cell. The anode can consist of either lithium metal in which case the battery is named a "Li-metal battery", or it may consist of another compound, such as graphite, in which case the battery is named a "Li-ion battery". Lithium is the lightest metal with a molar mass of 6.94 g/mol and a density of ρ = 0.53 g/cm³ leading to a high energy density, it also has a standard reduction potential of E° -3.04 V against standard hydrogen electrode, SHE. Li-metal batteries struggle with unwanted dendrite formation upon cycling the battery, which leads to explosion hazards [3]. The negative electrode material can be substituted by another compound, such as graphite (thus forming a Li-ion battery), to circumvent this dendrite formation. 98 % of the Li-ion batteries on the market have a negative electrode consisting of graphite [5], which reasons the layered structure in Figure 1. The positive electrode typically consists of an inorganic material, typically an oxide or phosphate with a transition metal [4].

Figure 1: Illustration of an electrochemical cell.

Electrochemistry

The chemical reaction which generates electricity takes place at the electrodes. Each electrode undergoes a half-cell reaction, which in total constitute a redox reaction. The specific redox reaction depends on which active materials the electrodes are made of. Each of these electrodes are connected to a current collector, a metallic component, which allow for electron transfer in an external circuit. During discharge, the electrons lost during the half-cell reaction at the negative electrode are collected by the current collecter. When transported through the external circuit, the electrons can be used to do work before being collected by the current collector at the positive electrode completing the redox reaction [1]. An example of such half-cell reactions for a positive electrode consisting of V₂O₅ and a negative electrode consisting of graphite with Li are found in Reaction 1 and 2. In this example 1 mol of Li is intercalated in 1 mol of V₂O₅.

Reaction 1 (negative electrode): LiC₆ → Li⁺ + e^- + C₆

Reaction 2 (positive electrode): V₂O₅ + Li⁺ + e^- → LiV₂O₅

During discharge of a Li-ion battery, Li-ions are also transferred from the negative electrode to the positive electrode. As previously mentioned, this transfer takes place in the electrolyte. In order to decrease internal resistance, the two electrodes must be as close to each other as possible (this is not shown in Figure 1). The ion-permeable separator prohibits the two electrodes from touching each other, thus preventing the battery from short-circuiting [1].

Terminology

In electrochemical cells, the flow of ions is from the anode to the cathode for both charge and discharge. This means that in a secondary battery the anode and the cathode technically change depending on whether the battery is discharging or charging, which can lead to confusion [6]. In practice, the negative electrode is typically called the "anode" and the positive electrode is typically called the "cathode" when discussing secondary batteries, even though this usage of terminology is strictly not correct.