Does energy density really matter?
By Apoorv Shaligram
The blog title may seem like a very silly question to even discuss. Surely, the energy density of batteries matters, doesn’t it? Lighter batteries will give more range and better energy efficiency. Right?
To correctly answer the question, we need to look at how much of a benefit higher energy density provides. Let’s do a quick comparison. Consider a mid-range EV sedan (let’s call it 1EV). 1EV has a kerb weight of 1080 kg without the battery. The battery size is 25 kWh. With an NMC powered battery, it provides an energy consumption of 125 Wh/km. The graph below shows the (drive cycle estimated) range of the vehicle as a function of battery weight. I have considered three existing battery types:
- High energy density NMC batteries (NMC-Graphite)
- Medium energy density LFP batteries (LFP-Graphite)
- Low energy density LTO batteries (NMC-LTO and LMO-LTO)
To understand the impact of increasing energy density, I have added two more hypothetical batteries, named Novel 1 (N1) and Novel 2 (N2) with far higher energy density than what is achievable today.
As we can see, going from NMC to LMO-LTO (an increase of 400 kg in battery weight) leads to a 45km drop in range. Alternatively, a hypothetical increase in energy density to 700 Wh/kg increases the range by 16km. I have summed up the results of the simulation in the table below:
Thus, if we consider the high energy NMC batteries as our benchmark, a 62% drop in energy density causes a 16% drop in range, whereas a 180% increase in energy density offers only an 8% increase!
So, I will rephrase the earlier question: Does improving energy density really give us a significant benefit, considering the effort and cost going into developing newer higher energy density? The benefit of increasing energy density clearly has diminishing returns. So, the logic that lighter batteries will give us better range is clearly flawed. The only way to significantly increase driving range seems to be either increasing the capacity of the battery or improving the energy efficiency of the vehicle (normalized by its weight).
Increasing the capacity needs additional space on the vehicle, and also adds to the cost of the vehicle. Let us assume that the space is available (after all, we considered a LMO-LTO battery of the same capacity, so consistency demands that the space is available for even a 3x bigger NMC battery; considering that Tesla model S offered a 85 kWh pack, it seems like a reasonable assumption to make as well). The cost is still a problem. What also needs to be looked at is how many times do people drive more than 200 km straight without a break, to actually need a bigger range.
The answer perhaps lies in looking at faster charging over higher energy density. The ability to accept faster charging comes with an added benefit: the ability to accept more regenerative braking, which will improve the base energy efficiency of the vehicle itself. Hence, more range as well (without increasing energy density, if I may add)!! If the control electronics of the vehicles are designed to allow as much regenerative braking as the batteries can take, the 62% drop in energy density for NMC-LTO batteries would correspond to almost 4–5% drop in range. With a 5–10 minute fast charge option at that!!!
Now imagine, what if it were possible to increase the cell level energy density without changing chemistry? Food for thought…
