Redox (II): Storage cells: the Lithium-ion cell.

Edexcel A level Chemistry (2017)

Topic 14: Redox (II): Storage cells: the Lithium-ion cell.

14/15 To be able to understand the application of electrode potentials to storage cells.

The Lithium–ion cell.

Lithium ion (Li-ion) cells are the most popular type of rechargeable cell for most applications.

There is a global market of at least £8bn and predicted to grow to £50bn by 2020.

Li-ion battery offers many advantages over other secondary (or rechargeable) cells:

•  It is lighter than other rechargeable batteries for a given capacity

•  Li-ion chemistry delivers a high voltage

•  Low self-discharge rate (about 1.5% per month)

•  Do not suffer from battery memory effect

•  Environmental benefits: rechargeable and reduced toxic landfill

However Li-ion batteries have also struggled with issues such as:

•  Poor cycle life, particularly in high current applications

•  Rising internal resistance with cycling and age

•  Safety concerns over overheating or overcharging

•  Applications demanding more from Li-ion battery capacity

How a Lithium–ion cell works

In Li-ion batteries, lithium ions move from the anode to the cathode during discharge, and from cathode to anode when charging.

The materials used for the anode and cathode can dramatically affect a number of aspects of the battery’s performance, including capacity.

New higher capacity materials are urgently required in order to address the need for greater energy density, cycle life and charge lifespan, among the other issues faced by Li-ion batteries.

Graphite has traditionally been the anode of choice for commercial use, with typical first generation Li-ion chemistry working as follows:

At the cathode:

LiCoO₂ – Li⁺ – e^– ↔ Li_0.5CoO₂

At the anode:

6C + Li⁺ + e^– ↔ LiC₆

Overall reaction on a Li-ion cell:

                             C + LiCoO₂ ↔ LiC₆ + Li_0.5CoO₂

Materials other than graphite have been investigated, with silicon offering the highest gravimetric capacity.

The volumetric capacity of silicon, i.e. the capacity of silicon taking into account volume increases resulting from lithium insertion, is still significantly higher than that associated with carbon anode materials.

The potential contained within silicon holds great promise for the future of Li-ion batteries, if it can be used without compromising the battery cycle life.

When charging a lithium ion battery, lithium is inserted into the silicon, causing a dramatic increase in volume (up to 400%).

On discharge, lithium is extracted from the silicon which returns to a smaller size.

Repeated expansion and contraction places great strain on the silicon, causing silicon material to fracture or pulverise.

This, in turn, leads to the electrical isolation of silicon fragments from nearest neighbours and a loss of conductivity in the anode of the battery.

For this reason, charge-discharge cycle life for conventional silicon-based anodes is typically short.

This post comes with thanks to nexeon.co.uk