This paper investigates the role of lithium squarate (Li₂C₄O₄) as a sacrificial additive for prelithiation in zero-excess lithium metal batteries (ZELMBs). These battery systems are attractive because they can achieve very high energy density by plating lithium directly onto a copper current collector during charging, eliminating the need for excess lithium metal. However, ZELMBs suffer from significant active lithium loss (ALL) due to inefficient lithium plating, dendrite formation, and unstable solid electrolyte interphase (SEI) formation, which rapidly depletes the lithium inventory and causes severe capacity fading. To compensate for lithium losses, the study explores prelithiation using sacrificial additives, which oxidize during the first charge to release additional lithium into the cell. Lithium squarate is examined because it has a relatively low oxidation potential and has previously been suggested as a promising additive. The work compares two incorporation strategies: adding the additive directly into the cathode composite or dispersing it in the electrolyte. Electrochemical tests using NCM622||Cu cells show that lithium squarate incorporated in the cathode oxidizes during the first charge at around 4.5 V vs. Li|Li⁺, generating extra lithium and increasing the initial charge capacity by approximately 70 mAh g⁻¹ compared to additive-free cells. This additional lithium creates a temporary lithium reservoir that improves capacity retention during early cycles. However, the oxidation of lithium squarate also produces gaseous CO and CO₂, leading to visible structural damage in the cathode composite, including increased porosity and void formation. These effects degrade the electrode architecture and limit the long-term practicality of this approach.

In contrast, when lithium squarate is added to the electrolyte, no oxidation plateau is observed during the first charge. Instead, the additive is reductively consumed during SEI formation on the anode, preventing it from acting as a prelithiation source. Computational density functional theory calculations support this observation, indicating a relatively low LUMO energy for lithium squarate, which favors reduction at the anode surface. Post-mortem analyses using SEM, EDX, ICP-OES, and GC-BID confirm that the additive modifies SEI composition and improves lithium plating morphology, although it does not provide prelithiation when added to the electrolyte. Additional overcharge experiments reveal that the oxidation onset of lithium squarate strongly depends on cathode chemistry, electrolyte composition, and anode behavior, particularly whether the anode forms an SEI during operation.
Overall, the results demonstrate that lithium squarate can provide prelithiation when incorporated in the cathode but suffers from gas evolution and electrode degradation. When added to the electrolyte, it is consumed during SEI formation and therefore cannot function as a sacrificial prelithiation additive.