Energy density versus lifespan: which really matters in stationary storage

In the comparison between storage technologies, there is one figure that is cited more than any other: energy density, watt-hours per kilogram. It is the parameter that has driven twenty years of battery development, for a simple reason: batteries have mainly been developed for things that move. A phone, a tool, a car.

For stationary storage, that number matters much less than people think, and this article explains why—and which parameter to look at instead.

The compromise, put plainly

Le polyanionic NFPP cells they have an energy density of 100–120 Wh/kg. Lithium iron phosphate is between 120 and 180, and nickel-manganese-cobalt is between 220 and 280.

Read from left to right, the table looks like a league table in which sodium comes last. Read across the two columns together, it tells another story: the two quantities move in opposite directions. Nickel-manganese-cobalt has the highest density and the lowest number of cycles. Polyanionic sodium has the lowest density and the highest number of cycles. It is not a coincidence, it is physics.

Why can’t you have both?

A cathode stores energy by trapping ions within a crystalline structure. There are two ways of designing that structure, and they are at odds with one another.

You can return it rich yet light: a few atoms that do not take part in the reaction, plenty of sites available for ions, mass concentrated where it is needed. This is the approach taken by layered oxides, and it produces the highest specific energy. The downside is that the structure expands and contracts with every cycle, and, as a result of this ‘breathing’, it loses its order.

Or you can make it rigid: A three-dimensional framework is formed from phosphate and pyrophosphate groups that hold the lattice in place. Ions move in and out without the structure becoming distorted. The trade-off is that these groups have mass, and that mass does not store energy — it is merely a supporting structure.

The energy density that the polyanionic cathode lacks is precisely the weight of the framework that gives it its longevity.

Asking a chemist to do both is like asking a building to have thin walls and withstand an earthquake.

How much does it really weigh, practically speaking

Translated to a home system: for the same installed capacity, a polyanionic sodium battery is bulkier and heavier than a lithium iron phosphate one. On a cabinet that stands against a wall, in a garage or in a plant room, this translates into a few centimetres of depth and a few tens of kilos more.

It is worth asking when this constitutes a real problem. If the battery has to go up onto a roof, fit into a narrow pre-existing compartment, or be wall-mounted on a lightweight structure, volume and weight must be checked during the site survey — just like any other installation constraint. In a floor-mounted technical room, which is by far the most frequent situation, the difference has no practical consequences.

What never changes is this: a stationary battery is lifted only once, on the day of installation. From that point on, watt-hours per kilogram no longer have any bearing on anything. Not on performance, not on the electricity bill, not on lifespan.

The key figure to look at

If energy density is not the right criterion for a steady state, what is?

It is the cost of energy actually cycled over the system’s lifetime — in the literature LCOS, Levelised Cost of Storage. As a rough estimate, this is calculated by dividing the total cost of the system by the amount of energy that the system will be able to transmit before reaching the end of its service life.

The point is that the denominator consists of cycles. Two batteries with the same list price and the same capacity, but with a different number of cycles, have profoundly different costs per accumulated kWh — and this difference is not visible in any comparison made on purchase price.

There are also three factors that the list price does not reflect:

• the’round-trip efficiency, because every point lost represents energy that is dissipated as heat during every single cycle, throughout the system’s lifetime. The NFPP cells measure 97%;
• the usable discharge depth, because a rated capacity of which only a portion can be utilised is an inflated rated capacity. The 9,000 cycles of the NFPP cells are specified at 95% DoD, i.e. by discharging the battery almost completely in each cycle;
• the converted to actual temperatures at the installation site, not at 25 °C in a laboratory — the subject of a separate in-depth analysis.

We have applied this method to the current generation of Heiwit systems in an article dedicated to LCOS calculation. The updated figures for the polyanionic cell generation will be published upon launch, once the system specifications have been finalised.

When the choice would be the opposite

For the sake of intellectual honesty, it must be pointed out where this compromise falls short.

In an electric vehicle every kilogram of battery is mass to accelerate, brake and carry for the entire life of the vehicle: there energy density is not just one parameter among others, it is the parameter. The same goes for portable electronics, cordless tools and any application where weight costs you with every use.

This is why sodium will not replace lithium everywhere, and anyone presenting it as a technology that “beats lithium” is oversimplifying to the point of being wrong. They are chemistries with different vocations. Stationary storage paired with photovoltaics is the application where the properties of polyanionic sodium — lifespan, thermal stability, cold tolerance, independence from lithium and cobalt — are worth more than what it gives up.

Frequently asked questions

Is a sodium battery larger than a lithium one?

Yes, for the same capacity in kWh. The difference is a few centimetres and a few tens of kilograms on a home system, and only needs to be checked during a site inspection if the available space is restricted.

Does lower Wh/kg mean it performs worse?

No. Energy density describes how much energy is contained in one kilogram, not how much is returned. The energy returned per cycle depends on efficiency — 97% for NFPP cells — and the usable depth of discharge.

What is LCOS?

Levelised Cost of Storagethe cost of each kWh passing through the battery over its entire lifetime, obtained by dividing the total system cost by the total energy cycled. It is the parameter that makes batteries with different prices, capacities and lifespans comparable.

Why don't we design a dense and rigid cathode together?

Because stiffness is achieved by adding groups to the lattice that hold it in position, and those groups have a mass that does not participate in energy storage. Density and structural stability compete for the same space and the same mass.