How to design a zero-degradation battery

Compensating for loss of lithium inventory in LFP cells with LFO additives

Abstract

Controlling lithium-ion battery degradation is a major global challenge and essential to electrify transport, deploy storage on the grid, and extend the lifetime of portable electronics. Loss of lithium inventory (LLI) caused by side reactions in lithium-ion cells is one of the primary reasons behind reduced cycle life. Researchers in the last ~15 years have demonstrated that additives such as Lithium Iron oxide Li₅FeO₄ (LFO) in LFP-based chemistries can release lithium to compensate for LLI. More recently, CATL has commercialised LFO in LFP cells, while Rimac has also reported to have used LFP-LFO composite cells.  Both claim to have achieved zero degradation for extended periods compared to first-cycle capacity. However, the specifics of how they achieved this are neither fully disclosed by them nor sufficiently explored in the literature.

This work describes how LFO can be employed in commercial LFP cells using a full-cell physics-based model in PyBaMM to achieve so-called ‘zero degradation’. We first attempt to find the optimal methods to control lithium release from LFO by simulating 2000 charge/discharge aging cycles for five cases to investigate how controlled lithium release speed (slow or fast release) and timing (early or late in the cell cycle life) can impact cell degradation and its life. We then use the model to find an optimum volume fraction of LFO that can be added to the LFP cathode-based cell to maximize its life. Model results reveal that slow lithium release maintains the cell balancing and reduces the cell degradation rates. In contrast, rapid lithium release and excessive LFO content can accelerate cell degradation rates, resulting in lower cycle life. The results also reveal that having extra anode capacity can help prevent this accelerated degradation, but presents a trade-off between achieving higher cycle life and maintaining energy density. The model assumes that oxygen released during LFO delithiation can be managed by cell degassing and advanced cathode coating agents, and hence does not affect cell degradation. This work shows that achieving longer cell life requires not merely adding lithium-rich additives such as LFO, but also informed cell design changes, and can benefit from advanced lithium release control methods.