Technical analysis of unified cell by Powerco (VOLKSWAGEN)

Standardardisation attempts for Lithium-ion cell dimensions

There have been various standardisation attempts in the Lithium-ion cell industry in terms of cell dimensions. Still, it is difficult to find two chemistries, such as LFP and NMC cells, with similar dimensions (capacities may be different). For example, LFP is popular in 26700, 32700 and 33140cylindrical cell formats, whereas NMC is popular in 18650, 21700, and 26650 formats.

4680 cylindrical was the first attempt to make LFP and NCA cells with the exact same form factor. LFP from BYD (FinDreams battery) cell exhibits 15Ah, and NCA from Panasonic (for Tesla) exhibits close to 26Ah capacity. Another such attempt was made by Volkswagen in the prismatic cell form factor, which we will discuss soon in this article.

Many companies manufacture cells up to 100Ah capacity in prismatic form factor with 148mm width in LFP and NMC, but their thickness and height tend to vary. Cells from 150Ah to 280Ah in prismatic form factor have 173mm width and 204mm height (without terminal) and varying thickness. Both 148 mm and 173mm widths come under the VDA cell standard. VDA stands for Verband derAutomobilindustrie (German Association of the Automotive Industry). It is a German interest group of the German automobile industry that proposed standards for the size of the battery cell technology used for the automotive industry.

Volkswagen PowerCo’s UC (unified cell) is a concept one step ahead, where the three dimensions of the cell will be the same and different chemistries will be used for different scenarios. Similarities in the cell dimensions allow for the following advantages:

• Reuse the cell manufacturing equipment for various chemistries; this is recommended to maximise the capacity utilisation factor (CUF) of the plant setup.

• Improved CUF allows costs to be reduced and efficiency improved.

• Similar cell dimensions allow for a lower cost of battery pack assembly for building various packs in terms of sourcing the balance of systems for battery packs (except BMS).

• Similar cell dimensions allow for the creation of low-range and high-range versions of the same electric vehicle. LFP is preferred in short-range vehicles, and NMC is preferred in vehicles where the expected range is higher.

• It saves costs during cell manufacturing and battery pack assembly.

Volkswagen says it will use the Unified cell for 80% of its requirements across many vehicles, leading to a 50% reduction in cost.

80% of the cells would include LFP for lower-range EVs, High Manganese no Cobalt chemistry for mid-range EVs and NMC for high-range EVs. The remaining 20% of the cells would be for specific solutions, which could mean different form factors and chemistry.

Below is the discussion about these chemistries (that would comprise 80% of the cells).

Lithium Iron Phosphate (LFP)

The LFP chemistry cells are expected to be charged at a higher voltage, up to 3.8V, to achieve higher capacity and a volumetric energy density above 430Wh/L in prismatic format. This will allow it to come closer to NMC prismatic cells (which are 500-600Wh/L). Traditionally, LFP prismatic cells have much lower than 400Wh/L volumetric energy density.

Contrary to popular belief that gravimetric energy density is important in EVs, it is actually the volumetric energy density that allows to pack more energy in a given space and enables a higher range. The weight difference is minimal when the gravimetric energy density is improved. For example, if a cylindrical LFP cell is improved to 190Wh/Kg from the existing 175Wh/Kg in a30kWh electric car, the overall weight of the battery pack would be only 20Kg less, and this weight reduction is not significant compared to the weight of the car which is close to 1.5 ton. On the other hand, volumetric energy density allows for more cells to fit and, therefore, allows for a higher range.

Charging a particular type of cell at a higher voltage is not a new concept, but there is a trade-off in cell cycle life. For example, charging an LFP cell up to 3.8V could deliver between 1000 and 1500 cycles, which is almost half of the cycle life when an LFP prismatic cell is charged up to 3.6Vvoltage. But lower cycle life is typically enough for decently long-range EVs, allowing for either a lower depth of discharge (DoD) per cycle or a smaller number of cycles at a higher DoD. Either way, the battery will deliver a high number of kilometres on the odometer over a 10-to-15-year period of EV ownership.

High Manganese Chemistry

Volkswagen did not mention the exact chemistry for this category, though high Manganese refers to LMO, LNMO or LMFP chemistries.

LMO (Lithium Manganese Oxide) faded from the market due to poor cycle life at high temperatures and manganese dissolution problems. Later, it was relaunched as a composite with NMC. Many companies in the LMO+NMC market are now migrating to other chemistries; it is unlikely that Volkswagen will produce this, given the presence of Cobalt.

LNMO (Lithium Nickel Manganese Oxide) is a modified version of LMO with Nickel. LNMO has been gaining popularity in the European region due to its lack of cobalt and high voltage nature. This cell has the highest possible voltage, but its overall energy density is lower than nickel-rich NMC and NCA cells. This has a price advantage, but it has certain commercialisation challenges, such as finding a suitable electrolyte to work with. Existing electrolyte technologies max out at close to 4.6V potential operation, and LNMO operates over 4.6V. Due to Volkswagen’s European nature and no Cobalt in LNMO, this is a highly likely contender for a high Manganese cell. Also, recently, Toshiba launched a cell with LNMO cathode material, although it uses a different anode (NTO).

LMFP (Lithium Manganese Iron Phosphate) is an upcoming cell type in Asia and is gaining popularity to compete with NMC, providing advantages such as similar voltage, better cycle life and and lower costs. LMFP is a modified LMP (Lithium Manganese Phosphate) with the incorporation of Iron. Contrary to popular belief that LMFP is a competitor to LFP, it is actually a competitor to NMC cells. Certain applications have no competition for LFP, such as heavy-duty vehicles and energy storage applications. LFP is priced lower than LMFP and has a higher cycle life and value for money. LMFP could also be a contender for this category, considering that Volkswagen has a partnership with Gotion (China) and that Gotion has already launched LMFP cells at a commercial scale. By the way, most LMFP cells use LMFP+NMC material to provide stable performance, but this mix is an unlikely choice for Volkswagen because of its Cobalt content. Only LMFP might be a likely contender.

This is Volkswagen’s long-term strategy, we will find out more about high Manganese and no Cobalt type cell in times to come.

Lithium Nickel Manganese Cobalt Oxide (NMC)

NMC has been in the news frequently due to its pros and cons.

Pros – High gravimetric energy density, high volumetric energy density (>50% higher than LFP in Nickel rich NMC) for higher EV range, high-power rating of charge for fast charging and high-power rating of discharge for achieving higher driving speeds and its ability to provide a predictable voltage profile.

Cons – Comparatively unsafe, expensive and uses Cobalt (concerns of sustainability and availability).

Because of their high volumetric energy density, NMC/NCA (high Nickel) cells are the absolute choice for long-range cars. Volkswagen would use these cells for their long-range cars.

The remaining 20% of the cells would be for specific solutions, which could mean different form factors and chemistry. Volkswagen is also eyeing solid-state batteries in collaboration with Quantum Scape, where it aims for less than half charging time and 30% more range.

The image are from Volkswagen’s ‘Power Day’, where the unified cell was first unveiled in 2021.

This article was first published in EVreporter March 2024 magazine.

About the author:

Rahul Bollini is an R&D expert in Lithium-ion cells with 9 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.

Also read: Understanding battery energy storage system (BESS) | Part 4