# Lithium Squarate as Sacrificing Electrolyte Additive for Prelithiation: Case Study in Zero‐Excess Lithium Metal Batteries

## TL;DR

Lithium squarate is studied as a prelithiation additive for lithium metal batteries, but it causes issues when used in cathodes or electrolytes due to gas evolution or depletion during SEI formation.

## Contribution

The study reveals the impracticality of lithium squarate as an electrolyte additive due to its reductive depletion during SEI formation.

## Key findings

- Lithium squarate as a cathode additive causes CO and CO2 gas evolution, leading to cathode rupture.
- As an electrolyte additive, lithium squarate is reductively depleted during SEI formation, preventing its oxidation.
- In Li metal cells with preformed SEI, squarate oxidation depends on cathode type and electrolyte formulation.

## Abstract

Among different approaches, the prelithiation via sacrificing additives can be technically relatively easy applied. In this work, lithium squarate (Li2C4O4) as a literature‐known representative is investigated in zero‐excess lithium metal batteries, which are simple in handling and have high active lithium loss (ALL), thus ideal for R&D of sacrificing additives. When incorporated via cathode, Li2C4O4 oxidizes at a relatively low cathode potential (≈4.5 V vs Li|Li⁺) and provides the aimed active Li (= capacity), but ruptures the cathode via gaseous evolution of CO and CO2. Interestingly, when incorporated via electrolyte, the additive oxidation is absent. This can be correlated with its reductive depletion in the course of solid electrolyte interphase (SEI) formation, as hinted via computational analysis by its relatively low energetic level of lowest unoccupied molecular orbital (LUMO), as well as by energy dispersive x‐ray spectroscopy, and decreased gas evolution. Hence, the squarate is concluded to be impractical as an additive in electrolytes when combined with anodes, which in operando form the SEI (e.g., graphite, Si). In Li metal cells, i.e., with an already existing and passivating “preformed” native SEI, the squarate oxidation can be seen again, but the oxidation onset sensitively depends on the cathode type and electrolyte formulation.

Active lithium loss (ALL) and capacity fade can be compensated by prelithiation, apparently simple via sacrificing additives e.g., lithium squarates. However, as a cathode additive it ruptures the cathode via gas evolution, while as an electrolyte additive it gets reductively depleted on anode side in course of solid electrolyte interphase (SEI) formation.