Cell Definitions & Glossary

Cells definitions & glossary is an extension to the Cells page and organized A to Z, hopefully allowing you to find more information.

18650 – Cylindrical cell format where the diameter of the cell is 18mm and the height of the cell is 65mm.

21700 – Cylindrical cell format where the diameter of the cell is 21mm and the height of the cell is 70mm.

3D Electrodes – another way to increase energy density is with 3D electrodes. Increasing the surface area and connection to the active materials can improve a number of features of the cell.
3D Electrodes - Addionics
Ah – Ampere-hour is the unit of cell capacity.

Anode Free – a battery cell where the Anode is formed during the cell formation cycles.

bipolar solid state cell

Bipolar Battery – The bipolar battery essentially moves the series connections inside the cell. This brings a number of advantages and significant challenges.

BOL – Beginning of Life

Capacity – battery capacity is expressed in ampere-hours.

C-rate – a measure of the rate at which a battery is charged or discharged relative to its capacity. It is the charge or discharge current in Amps divided by the cell capacity in Ampere-hours.

CID – Current Interrupt Device

Cylindrical Cell – The layers of the cell are wound in a spiral. Normally these cells have the lower case as the negative terminal and the top centre as the positive terminal.

DCIR – Direct Current Internal Resistance is the internal resistance of the cell.  This is the resistance in charge and discharge to a direct current demand applied across the terminals.

Discharge – the cell has a current demand and hence the state of charge (SOC) is decreasing. Establishing the maximum cell discharge capability is difficult without understanding the design in detail. However, you can work towards establishing this limit with a number of measurements and calculations.

Electrode – the electrical part of a cell and consists of a backing metallic sheet with active material printed on the surface.

EOL – End of Life

Gas Pressure – the gas inside the cell is a mix of CO, CO2, H2, C2H2 and other chemicals. This mix is dependent on cell chemistry.

In a 21700 cell the pressure post formation is ~260mbar for an NMC811/Gr+Si cell.

This pressure changes by ~140mbar over charge/discharge and increases by ~1mbar for every ageing cycle.

heat generation in a battery cell

Heat Generation in a cell can be defined quite simple for the case where the cell is operating within it’s normal limits. The first expression gives the heat flow [W].

The first part of this equation is the irreversible Joule heating term, the I2R term.

The second part is the reversible entropy term or Reaction heat terms. The charge and discharge reaction can be exothermic or endothermic under certain conditions.

Instrumenting Cells – if you are going to instrument a cell you need to be able to do this reliably and robustly. The process flow diagram illustrates the experimental stages employed for cell instrumentation and includes: sensor fabrication, cell modification and sensor insertion. The diagram highlights the different verification stages for assessing LIB performance, operation and ageing.
WMG Smart Cell instrumentation

LFP – Lithium Iron Phosphate, a lithium ion cathode material with graphite used as the anode. This cell chemistry is typically lower energy density than NMC or NCA, but is also seen as being safer.

LRM – Lithium Rich Managanese.

NMC – Lithium Nickel Manganese Cobalt Oxides are a family of mixed metal oxides of lithium, nickel, manganese and cobalt. Nickel is known for its high specific energy, but poor stability. Manganese has low specific energy but offers the ability to form spinel structures that allow low internal resistance.

Ohm Ampere-hour – a useful metric when looking for a cell with the lowest internal resistance.

typical OCV vs SoC
Open Circuit Voltage (OCV) – is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.

Pouch Cell – they look like an aluminium jiffy bag with +ve and -ve terminals protruding from the edge. They need to be supported mechanically and need a controlled pressure applied to the surface to deliver the power and energy over their lifetime.

Prismatic Cell – as the name suggests these are a prismatic block, normally with the outer case made from aluminium.

Ragone Plot – a plot of power density vs energy density.

Siemens/Wh – a useful metric when looking for a cell with the lowest internal resistance.

Solid Electrolyte Interphase – is formed on electrode surfaces from decomposition products of electrolytes. The SEI allows Li+ transport and blocks electrons in order to prevent further electrolyte decomposition and ensure continued electrochemical reactions.

Specific Heat Capacity – for the main lithium ion chemistries the following generic heat capacities for a cell are:
  • Lithium Nickel Cobalt Aluminium Oxide (NCA) = 830 J/kg.K
  • Lithium Nickel Manganese Cobalt (NMC) = 1040 J/kg.K
  • Lithium Iron Phosphate (LFP) = 1130 J/kg.K
SoC definition for Samsung SDI 94Ah cell
State of Charge (SoC) – abbreviated as SoC and defined as the amount of charge in the cell as a percentage compared to the nominal capacity of the cell in Ah.
Temperature – a critical parameter that you need to know before charging or discharging a cell. A cell is a 3 dimensional structure that is also inhomogeneous and hence you will observe temperature gradients within the cell. The temperature limits, gradients and heat rejection rate will define the overall power capability of the battery.
Temperature Gradient – the maximum temperature differential in a cell is normally specified as ~2°C to minimise the degradation in capacity of the cell. This requirement will drive the cell selection versus application along with the cooling system design.
  • High temperature and the SEI layer on the anode grows faster. If the SEI layer grows fast it tends to be more porous and unstable.
  • At low temperatures we see slower diffusion and intercalation with the possibility of lithium plating. Lithium plating removes lithium from the active cell, reducing cell capacity. Also, lithium plating can subsequently into lithium dendrites that can cause electrical shorts.
cell temperature limits
Temperature Limits – these temperatures will change with chemistry and by cell manufacturer, therefore, it is really important to use the limits as advised by the manufacturer. In addition you will need to test the cell to gain the detailed understanding of how the cell behaves in your application versus temperature.

The limits will also be blurred by the design of the battery and control system. One example is the maximum operating temperature for the cell.

Thermal Conductivity – If we look at the active layers of a cell the thermal conductivity in the plane of the layers is approximately 10x to 100x that through the planes.

This should not be unexpected as the electrodes are made from sheets of aluminium and copper. Two of the best materials for thermal conductivity.

These values though have a large range [Ref 1]:

  • 15 to 160 W/mK In-Plane
  • 0.2 to 8 W/mK Through-Plane
Thermal conductivity of a cell
Thermal Runaway Energy vs Electrical Energy
Thermal Runaway vs Electrical Energy – the energy released during Thermal Runaway versus the electrical energy stored in a battery.

The energy released during Thermal Runaway (TR) versus the stored electrical energy. A number has been bandied around for a long time that the energy released in a TR event was 2 to 6 times the electrical energy stored in the cell.

Usable SoC Window – If we want a battery cell to last a lot of cycles, extend the life in a power application or to ensure the available power is consistent then we need to set a usable SoC window that is smaller than 100%.
usable SoC window

Wh/kg – gravimetric energy density of a cell.