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 their shorter cycle life. Researchers in the last ~15 years have demonstrated that additives such as Lithium Iron oxide Li₅FeO₄ (LFO) in LFP-based chemistries can irreversibly release their lithium to compensate for LLI. More recently, CATL and Rimac have commercialised LFO in LFP cells, claiming to have achieved zero degradation for extended periods. However, the specifics of how they achieved this are neither fully disclosed by them nor much 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 later part of cell life) can impact cell degradation and its life. We then use the model to find an optimum weight fraction of LFO that can be used in LFP cells to maximize cell 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 the large anode overhang can help prevent cell unbalancing however it 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 doesn’t affect cell degradation. This work shows that achieving longer cell life requires not merely adding lithium-rich additives like LFO but also the ability to control the lithium and oxygen release through appropriate methods.