Cell is the smallest building block of a functional battery.
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 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.
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.
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.
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.
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.
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.
Cell Definitions & Glossary
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 Free – a battery cell where the Anode is formed during the cell formation cycles.
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.
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.
Open Circuit Voltage (OCV) – is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.
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.
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 – within a cell is mostly driven by 3D shape, +ve and -ve tabs and thermal connection to the environment (ie cooling).
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
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.
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.