Research project finds continuous high charge levels damage LFP cells

The study was published in the Journal of The Electrochemical Society. In it, the authors not only state that operating at high charge levels is harmful to LFP cells in the long term but also explain in detail exactly how capacity loss occurs in the cell.

As in NMC cells with nickel, manganese and cobalt in the cathode, the voltage in LFP cells increases with a higher charge level – but with a different curve. If an increased temperature is added to the high voltage (during fast charging with a high charge level), reactions in the cell, especially in the electrolyte, can form harmful compounds. These are then deposited on the anode during the charging process, which means that parts of the anode surface are no longer available for the electrochemical reaction that is actually desired in rechargeable batteries. In other words, the usable capacity of the cell decreases because the lithium ions can no longer ‘dock’ to the surface covered with the deposits – but only to the areas that are not affected by the deposits.

“At higher SoC, there’s higher voltage, negative reactions recurring within the electrolyte get accelerated, consuming the lithium inventory,” the authors said. “Cycling near the top of charge (75%–100% SOC) is detrimental to LFP/graphite cells. Our results show a correlation between the average SOC of battery operation and capacity fade rate, meaning that the lower the average SOC, the longer the lifetime […] Therefore, the time spent cycling at high states of charge is critical to minimise.”

One thing is clear: this research contradicts car manufacturers’ recommendations to charge LFP batteries to 100 per cent regularly. In the manual for the LFP versions of the Model 3 and Model Y, Tesla recommends charging the vehicle to 100 per cent at least once a week, while Ford recommends once a month.

This ‘conflict’ is easily explained: for the study, the research team only analysed the service life or capacity retention within the LFP cell, while the vehicle manufacturers focus on the entire car and the customer benefit, in this case, the battery management system (BMS), for example. That is where the aforementioned different voltage curve of the LFP cells comes into play. LFP battery systems must be calibrated regularly due to the flatter voltage curve so that the BMS has a fixed point. It would theoretically also be possible in an empty state, i.e. with zero per cent SoC. However, it is difficult to convince customers to drive their electric car completely empty once a week or month – charging to 100 per cent is easier to communicate. The BMS then knows the exact charge level of all cells and can subsequently specify the charge level and remaining range more precisely.

The study authors’ conclusion should also be seen against this background. On the one hand, the results show that operation at charge levels of zero to 25 per cent extends the service life of the LFP battery cell. On the other hand, they write: “There is clearly a tradeoff between useful capacity and capacity retention. […] It is not realistic to recommend cycling LFP cells between 0%–25% SoC only, because that is a waste of capacity.”

The authors thus recommend not changing charging habits. It is possible that findings such as these – after all, Tesla battery researcher Jeff Dahn is also involved in the study – will contribute to car manufacturers adapting their charging recommendations one day.

iop.org (study as PDF), insideevs.de