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.
Ah – Ampere-hour is the unit of cell capacity.
Anode – the negative or reducing electrode that releases electrons to the external circuit and oxidizes during and electrochemical reaction.
Anode Free – a battery cell where the Anode is formed during the cell formation cycles.
Bipolar Battery – The bipolar battery essentially moves the series connections inside the cell. This brings a number of advantages and significant challenges.
Blade Cell – a cell format invented by BYD and optimised for Cell to Pack designs.
BOL – Beginning of Life.
Bottom Vent – a cylindrical or prismatic cell design where the pressure relief vent sits on the can/case bottom rather than the top surface, a design choice that affects pack orientation, busbar routing and the gas venting path.
Calendar Ageing – the capacity loss of the battery with time and without cycling.
Capacity – battery capacity is expressed in ampere-hours (Ah).
Cathode – 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.
Cell Design – a look at all elements of what goes into a battery cell and how to optimise the design of everything from the anode porosity through to the tab to case.
Cells – the lowest definable working battery electrochemical element. Tags: Cell, Cells Category: Cells
CID – Current Interrupt Device, comprises a thin metal diaphragm which is part of the positive terminal. Abnormally high gas pressure inside the cell pushes the diaphragm upwards, breaking the electrical connection, thereby isolating the cell’s jellyroll from the external circuit.
Coin Cell – used a lot in early stage cell chemistry development as it is normally the simplest bilayer.
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.
Critical Self-Heating Temperature – the threshold beyond which thermal runaway becomes almost certain. At T2 the cell is self-heating so rapidly that the reaction cannot be stopped anymore.
For NMC/graphite cells this occurs at around 170 – 190 °C, compared to 230 – 260 °C for LFP/graphite.
Cycle Life – the number of charge and discharge cycles that a battery can complete before losing performance.
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.
DCR Growth – the percentage increase in DC resistance over a cell’s cycle life or storage period, reported by manufacturers alongside — but separately from — capacity or energy retention, as a second, independent ageing metric (e.g. LG’s spec: “Below 25% DCR growth after 2000 cycles”).
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.
Electrolyte – the medium that allows ionic transport between the electrodes during charging and discharging of a cell.
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 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.
Individual Tab Fuse – a fuse built into an individual cell’s interconnect tab, the lowest and most localised tier of a layered fusing strategy, positioned to blow before the module or pack-level fuse in response to a single-cell fault.
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.
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.
Manufacturing Process – the three major sections of this are: Electrode Manufacturing, Cell Assembly and Cell Finishing
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.
Open Circuit Voltage (OCV) – is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.
Overpotential – the extra voltage beyond the thermodynamic equilibrium value needed to drive a given current through a cell, made up of ohmic, activation and concentration contributions.
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.
Power vs Energy Cell – what are the key differences between them.
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.
Self-Heating Onset Temperature – the temperature at which a cell begins self-heating under adiabatic conditions. It describes the lowest temperature at which a cell will begin to heat up, if it cannot pass any heat to its environment. It is therefore a theoretical, worst-case lower value, as cells in real-world battery packs can readily exchange heat.
NMC/graphite cells begin self-heating at approximately 90 – 110 °C, while for LFP/graphite it occurs around 150 – 170 °C.
Separator – material that separates the anode and cathode electrically whilst allowing ions to pass through.
Siemens/Wh – a useful metric when looking for a cell with the lowest internal resistance.
Soft Cell – informal industry shorthand for a cell exhibiting unusually high internal resistance, where voltage sags heavily under load and doesn’t hold well on charge, often caused by extreme cold or electrolyte starvation.
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
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.
Tab Design – the electrodes that carry the anode and cathode need to connect to the positive and negative tabs on the cell case.
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.
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 Runaway – if you heat a battery cell to somewhere above 130°C then exothermic chemical reactions inside the cell will increase the temperature and further reactions will take place. The result is an uncontrolled runaway and increase in temperature.
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 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%.
Wh/kg – gravimetric energy density of a cell.
See also: Abbreviations and Jargon, LCA Definitions & Glossary, System Definitions & Glossary, HV Definitions and Glossary