Cell

Cell is the smallest building block of a functional battery.

The open circuit voltage is dependent on the chemistry, the capacity is dependent on the amount of active material and the power is dependent on the chemistry, active area and active material thickness.

OK, there are a few more parameters in there and you can read about those below.

Formats

There are many different formats of battery cells, from button or coin cells, cylindrical, prismatic and pouch. There are also a number of other formats and lots of sub-formats.

button or coin cell
Tesla 4680
prismatic cell
LG Chem pouch cell

Cathode, Anode and Electrolyte

Cathode

When discharging a battery, the cathode is the positive electrode, at which electrochemical reduction takes place. As current flows, electrons from the circuit and cations from the electrolytic solution in the device move towards the cathode. Although these processes are reversed during cell charge in secondary batteries, the positive electrode in these systems is still commonly, if somewhat inaccurately, referred to as the cathode, and the negative as the anode.

Anode

The Anode is the negative or reducing electrode that releases electrons to the external circuit and oxidizes during and electrochemical reaction. In a lithium ion cell the anode is commonly graphite or graphite and silicon.

Electrolyte

Electrolyte is an ionic transport medium. It can be liquid or solid. Liquid electrolytes transport ions between the electrodes and thus facilitate flow of electrical current in the cell or batteries.

Power versus Energy Cells

In simple terms the energy cell has thicker layers of active material, thinner current collectors and less of them.

This means the energy cell will have a higher electrical internal resistance meaning it will generate more heat based on I2R heating.

The energy cell will have poorer thermal conductivity in-plane and through-plane. Thus, it will need a higher temperature gradient to reject the heat.

Cell Sample Maturity

Cell Sample Maturity is normally defined by the A, B, C, D sample definitions. These stages of the cell design, production line development and material supply are key to the relationship between the cell manufacturer and cell customer. The customer needs to have confidence in the cell design, robustness and quality and this is only possible if they know the process that the manufacturer is working to.

cell sample maturity

Cell Electrode Pressure

This is required to keep the cell operating at it’s peak performance over it’s lifetime. As the cell is charged lithium ions move into the graphite anode and the cell will increase in thickness. Silicon in the anode will increase this swelling significantly. The layers of the cell are likely to fatigue and fracture over a lifetime of charging and discharging. The external pressure can help to maintain the contact of the layers over time. Also, gas generation can cause the active layers to delaminate, hence reducing the active working area of the cell and reducing capacity and power capability. Applying a pressure normal to the active planes will keep the layers working together.

Cell Expansion

As you charge a cell it expands, when you discharge a cell it contracts and as the cell ages over its lifetime we see a continuing cell expansion. Thus the cell expansion can be divided into:

  • Reversible cell expansion
  • Irreversible cell expansion
cell enclosure functions

Function of the Cell Can or Enclosure

The function of the cell can or enclosure is to contain the chemistry over the lifetime of the battery cell and to allow the electrical, mechanical and thermal connections. It must also work in the extremes and have a controlled failure mode.

Manufacturing Process

cell manufacturing schematic

In order to engineer a battery pack it is important to understand the fundamental building blocks, including the battery cell manufacturing process. This will allow you to understand some of the limitations of the cells and differences between batches of cells. Or at least understand where these may arise.

The energy required to make a cell appears to be between 50 and 180kWh/kWh.

Cell to Cell Variation is an aspect that needs to be controlled in production, this is normally cell capacity and/or internal resistance.

Cell Manufacturers

There is a wide array of cell manufacturers, some of these have a very niche market based on their history or technology. We will not have listed them all here, but if you think we have missed a company then do please drop us a line.

pie chart showing breakdown in cell manufacturing capex costs

Lithium Battery Manufacturing Equipment CAPEX

First indicator in the breakdown of a total ~$36 million/GWh Capex cost. Of which 1/3 of that is for formation and aging.

Cell Roadmap

When looking at a product roadmap there will be a request to understand how the energy storage system will improve throughout the lifetime of the product. Also, you will need to understand if the roadmap contains disruptive elements that might require a redesign of the product.


Cannot find what you are looking for? Try the Cell Definitions & Glossary page for an A to Z approach.