Cell design requires inputs from chemistry, electrical, thermal and mechanics.
Cell Stack
The core building block of any battery cell is the stack:
- Cathode
- current collector
- active cathode
- Separator
- Electrolyte
- Anode
- active anode
- current collector
Within this sandwich we must include the electrolyte. Each of these elements can be broken down further, but initially it is worth thinking about the fundamentals of this layered sandwich.
A, B, C, D Sample Maturity and what is expected through the cell development process.
Cathode
The Particle Size Distribution (PSD) of active materials directly impacts their tap density and compactibility—two critical factors that determine how densely an electrode can be packed during calendaring. A higher tap density allows for denser coatings, which is essential when aiming for higher energy density in cells.
Effect of Electrode Thickness on Cell Energy Density
In Li-ion batteries, the cathode thickness will heavily influence the energy density of the cell. A thicker cathode means there will be more cathode active material in the electrode. This increases the capacity of the electrode and hence increase the energy density of the cell.
Cathode Coating Porosity and ASR
Engineers often assume that reducing porosity improves conductivity and therefore lowers ASR. This idea comes from the expectation that a denser structure provides better particle contact and a stronger electronic network. However the real behavior of porous electrodes is more complex.
Electrode Microstructure Architecture Determines Cell Performance
In lithium-ion batteries, electrode performance is governed not only by material chemistry but by the three-dimensional microstructure formed by particles, pores, binder, and conductive additives.
Before production begins, this microstructure already defines how ions and electrons will move through the electrode. Once manufacturing starts, many of these structural characteristics can no longer be fundamentally changed.
Optimizing Thermal Management in Cylindrical from Design
Manufacturers leave the final turn of the anode electrode uncoated—exposing a bare copper foil. This uncoated segment acts as a thermal conductor, channelling the heat from the inner windings directly to the cell casing, where it can dissipate more effectively.
Power versus Energy Cells
Comparing power versus energy cells we see there are some fundamental differences. A high energy cell will have better volumetric and gravimetric energy density at the expense of the ability to deliver a high current. The power cell will have a low internal resistance and will be optimised to deliver current over energy density.
Relationship Between DCIR and Peak Discharge C-Rate in Lithium-Ion Cells
There is an inversely proportional relationship between DCIR and peak discharge C-rate. When the internal resistance is high, the Joule heating (I²R) increases as the current drawn from the cell increases, which makes thermal management difficult and limits the current that can be drawn.
Anode
Natural versus Synthetic Graphite
A comparison of the material properties and then a look at the advantages / disadvantages at cell level.
Adding silicon to the anode brings more challenges as silicon expands more than graphite, increasing the mechanical strain on the stack.
In lithium-ion cell design, one of the most critical relationships shaping performance is the link between anode porosity and energy density. As energy density increases, conventional design logic suggests reducing anode porosity to pack more active material into the electrode. And up to a certain point, this is exactly what happens.
However, beyond a specific energy density threshold, this trend reverses.
The format of the cell and within that how the stack is arranged can have a significant impact on the overall cell cost and performance. Therefore, it is important to understand this within the overall design.
Stacked vs Wound Cells
The stacked cell has higher energy density, better electrical and thermal performance.
The wound cell is faster to make and possibly higher yield, but is it safer?
Addressing Cell Pouch Corrosion through Cell Design
A commonly used material for the pouch cell structure is aluminium laminated film. This pouch material typically consists of a layer of aluminium (Al), coated on the outside with a layer of nylon and/or polyethylene terephthalate (PET) and on the inside with a layer of polypropylene (PP). There are not many configurations of this material out there, most fall under a ~85, ~110, or ~150 µm thickness, depending on thickness of the components and whether PET is present as an outermost layer.
Electrolyte
The electrolyte and additives to the electrolyte play a significant part in the overall cell design.
Electrolyte Mass, Capacity and C-Rate
The electrolyte mass-to-capacity ratio should not be treated as a fixed design rule. Instead, it must be adjusted according to the target C-rate. Cells operating at higher discharge rates generally require more electrolyte per ampere-hour to offset losses, prevent dry-out, and sustain performance.
Enclosure / Case
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
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.
Cell Vent Design
The cell vent needs to remain intact at normal cell operational pressures and operate with a safety margin at a pressure that is lower than that which will cause an uncontrolled structural failure.
Cell Tab Design
The tab serves as the final current-carrying element in this path. Its current-carrying capacity is therefore a critical factor that affects both electrical efficiency and thermal stability. Any limitation in tab conductance can result in increased ohmic losses, localized heating, and potential degradation of cell lifetime and safety.
Golden Ratio for Cell Design
There is no single best length-to-width ratio for a cell. Where the tabs sit and which edge is cooled decide it, so a shape that suits one layout can be the worst choice for another. The post sweeps aspect ratio across five tab and cooling layouts to show where the electrical and thermal optimums fall.
See also: Electrode, Battery Module Design, Mechanical, Case-studies on Cell Design and Construction using the CAMS model