Lithium-ion batteries are one of the most widely used types of rechargeable batteries today. Sony was the first company to use commercial lithium-ion batteries in mobile phones in 1991. These batteries are lightweight and have high energy capacities compared with other types of battery. This means that they have great future potential, particularly in portable devices and electric vehicles, as we move towards utilising more sustainable sources of energy.
Batteries store chemical energy and convert this into electrical energy; they initiate this process by using an oxidation-reduction (redox) reaction. The battery is connected to a circuit and when discharging occurs, there is reduction at the cathode (positive electrode) and electrons are gained, meanwhile there is oxidation at the anode (negative electrode) and electrons are lost. This does work on charge and produces a voltage between the positive and negative electrodes, which drives electrons around the circuit.
For lithium-ion batteries, the charging and discharging equations are given by:
During discharging, the lithium ions flow from the anode to the cathode through the electrolyte within the battery. The electrons cannot move through the electrolyte, so flow from the anode to the cathode through the outer circuit, causing a current to flow. When the ions and electrons combine at the cathode, lithium is deposited there. During charging, the opposite process occurs, with ions and electrons flowing from the cathode to the anode. When the ions can no longer flow, the battery is fully charged.
The discharging process of a lithium-ion battery can be illustrated using the following diagram from Nexeon:
Lithium-ion batteries typically consist of a graphite anode, a metal oxide cathode, a lithium salt electrolyte and in some cases a polymer separator. The graphite anode is often multi-layered or made into a spinel structure in order to be able to more easily accommodate lithium ions during charging and discharging. The main advantage of this is that the anode remains more stable when charging, increasing cycle life and overcharging tolerance of the battery. There are many sub-types of lithium-ion batteries, which contain additional metals in order to modify the properties of the battery. The types with the fastest growing demand are lithium ion phosphate (LFP) and lithium manganese oxide (LMO).
The trade-off between specific energy and specific power is a key consideration in all batteries when selecting elements and their quantities. The limitations of this compromise can be described using the following bottle analogy from Battery University:
The bottle can be considered as the battery; the amount of charge that the bottle can hold or the capacity is therefore analogous to the amount of energy that the battery can hold (specific energy) and the size of the bottle spout can be thought of as defining the amount of power which can be applied over a certain period of time (specific power). This shows how the specific energy affects the overall energy that the battery is able to supply or run time, whereas the specific power affects power that the battery can supply at a given moment or load.
In general, lithium batteries are useful as they have many desirable properties. The element lithium has only three protons, meaning that it has a low mass, and is particularly useful in lightweight batteries in portable devices. Lithium is part of the Alkali Metals group, which each contain one electron in their outer shell, resulting in another important property of lithium – it reacts readily with water and air. Lithium is also the metal with the highest electrochemical potential. These properties lead to lithium batteries having a higher specific energy than other types of battery, meaning that they provide the highest energy per unit mass. Lithium batteries are also relatively low maintenance and have fewer environmental issues associated with their disposal.
However, these batteries currently have several drawbacks. Due to their high specific energy, lithium-ion batteries have a lower specific power, or power per unit mass. This means that their power output over a short period of time is limited, which could lead to problems where high load is required, such as when starting the engine in a car. The fact that lithium is highly reactive leads to a relatively shorter cycle life, as some unwanted chemical reactions occur inside the battery over time. Relative to other batteries, lithium-ion batteries are relatively expensive and often require a protection circuit in order to ensure safety.
Considering the advantages of lithium-ion batteries, their future potential looks promising. The demand for lithium batteries is increasing, particularly in portable devices where there is a lack of strong competitors. With companies such as Tesla creating the Powerwall home battery and Powerpack and Mercedes-Benz following closely behind, there is clearly a significant amount of interest in expanding the uses of lithium-ion batteries. However, their relatively low specific power, high price and short cycle life currently limit the scope of use of these batteries. There is a great deal of on-going research into improving these areas, which could lead to important advances in the future.
By Charlotte Carty (Studying for BSc Physics at Bristol University)