Lifetime - Battery Design

For primary cells the shelf storage time or calendar ageing discharge rate is the most important factor with respect to lifetime as this will determine how long you can store the cell before using it.

For secondary cells or rechargeable cells we are interested in the calendar ageing and the cycle ageing. The calendar ageing will show how the capacity reduces with time, even when the battery cell is not being used. The cycle ageing will show you how many cycles the cell can deliver at a given charge and discharge rate.

As a rule of thumb we would expect to get 1000 complete cycles for an energy cell and 3000 complete cycles for a power cell before the capacity dropped to 80% of the original Ah value when new.

Ageing of a battery manifests itself in two ways:

Capacity fade – the available energy at a defined discharge rate will decrease.
Increasing internal resistance – as the internal resistance increases the voltage will drop lower under load. Hence the available power before hitting the minimum voltage will decrease. Also, the I²R heating of the cell will increase.

Causes of degradation:

Time
High Temperature
High Voltage/SoC
Current Load
Low Temperature
Stoichiometry
Mechanical Stress
Low Voltage/SoC

Li, Kirkaldy et al [4] look at five degradation mechanisms: These five degradation mechanisms depicted below are closely coupled together. To start with, SEI layer growth consumes useable lithium, reducing lithium inventory (increasing LLI), and solvent, potentially leading to electrolyte drying. The electrolyte drying, in turn, renders part of the active surface area inaccessible to intercalation reactions. Mechanical loss of active material from the electrodes reduces the active surface area. As a result, the remaining active surface area sustains higher interfacial current density under applied current, thus accelerating SEI growth, particle cracking, and mechanically induced LAM. Particle cracking exposes fresh electrode surface area to the electrolyte, which triggers rapid SEI layer growth. The increased interfacial current density also accelerates lithium plating.

The degradation model includes five degradation sub-models, labelled 1 to 5. They are (1) SEI growth, (2) electrolyte dry-out, (3) lithium plating, (4) loss of active material and (5) particle cracking due to mechanical stress.

The plated lithium can itself react with electrolytes form SEI and become completely inactive. It must be noted that not all mechanisms play a significant role in limiting battery performance when the cell is subjected to a specific operating condition. Therefore, not all mechanisms are typically needed to fit one specific ageing dataset.

Ageing Predictions

The standard method used by industry has two fundamental test regimes as inputs:

calendar ageing – where the cells are held at different fixed temperatures and SoC.
cycle ageing – where the cells are cycled through a complete charge discharge at different fixed temperatures.

At regular intervals the capacity of the cells are measured. This data is then used as inputs to the ageing predictions, a simple summation of these two parts gives the overall ageing model [3].

This approach allows the two parts to be modelled and tested independently.

Useful Life Plot showing capacity loss does not equal energy loss

Capacity Loss ≠ Energy Loss in Battery Aging

Specification sheets and battery cell lifetime is always about capacity loss, the percentage of the original capacity in Ah available versus number of cycles and time. However, the range of your vehicle and how long your battery can support your house for is dependent on the amount of energy available. Hence this paper caught our attention.

These results highlight the importance of reporting cell aging based on energy fade to avoid overestimating battery lifetime in full systems [5].

plot of cell cycle life versus volumetric energy density

Cycle Life and Energy Density

Interesting that as energy density increases the cycle life of the cell decreases.

Mileage Equals Wear

When we look at this picture, we see the over-simplicity of the ‘mileage equals wear’ mindset. Figure 1 shows reported SOH as a function of miles-driven for Nissan Leaf EVs. Similar distributions are reported by Myall and co-authors [1] who analysed 1382 reported SOH data points from 283 Nissan Leaf EVs manufactured between 2011 and 2017. At 60,000 miles, the discrepancy in reported SOH is almost 30%. How is it possible that two identical EVs, with the same odometer reading have such different SOH?

Developing a Lifecycle Test Procedure – Where to start with the task of developing a lifecycle test procedure. Battery cell ageing is such a complex subject that it makes it difficult to know where to start. One of the first things to do is to break the problem into Calendar and Cycle Ageing.