Sacrificial additives play a critical role in cathode pre-lithiation, compensating for irreversible lithium loss during the initial cycles of lithium-ion battery operation. These additives decompose during the first charge, releasing additional lithium ions into the system, thereby improving the overall capacity and longevity of the battery. Among the most studied sacrificial additives for cathode pre-lithiation are lithium sulfide (Li₂S) and lithium nitride (Li₃N), each with distinct reaction pathways and efficiency benefits.

Lithium sulfide (Li₂S) is a promising sacrificial additive due to its high theoretical capacity (1,166 mAh/g) and ability to release lithium ions upon oxidation. During the first charge cycle, Li₂S undergoes an irreversible electrochemical reaction, yielding sulfur and lithium ions. The reaction proceeds as follows:
Li₂S → 2 Li⁺ + S + 2 e⁻

The released lithium ions migrate to the anode, compensating for the lithium consumed during solid electrolyte interphase (SEI) formation. The sulfur byproduct remains in the cathode, where it may participate in reversible redox reactions in certain battery chemistries, though its primary role here is as a sacrificial agent. The efficiency of Li₂S as a pre-lithiation additive depends on factors such as particle size, distribution within the cathode, and electrochemical stability. Fine Li₂S particles ensure uniform decomposition, while larger particles may lead to incomplete oxidation and reduced effectiveness.

Lithium nitride (Li₃N) is another widely investigated additive, known for its high lithium content and low decomposition potential (~0.44 V vs. Li/Li⁺). Upon charging, Li₃N decomposes into lithium ions and nitrogen gas:
Li₃N → 3 Li⁺ + ½ N₂ + 3 e⁻

The nitrogen gas evolves and exits the system, leaving no residual compounds in the cathode. This characteristic makes Li₃N particularly attractive, as it avoids introducing electrochemically inactive byproducts that could degrade performance. However, the release of gaseous nitrogen requires careful cell design to prevent pressure buildup or electrolyte displacement. The decomposition of Li₃N is highly efficient, with nearly all lithium ions being released, but its hygroscopic nature demands strict handling under inert conditions to prevent moisture-induced degradation prior to cell assembly.

The choice between Li₂S and Li₃N depends on specific application requirements. Li₂S offers higher lithium release per unit mass but introduces sulfur into the system, which may interact with other cell components. Li₃N, while less lithium-dense, provides a cleaner decomposition pathway without residual solids. Both additives must be uniformly dispersed within the cathode to ensure consistent lithium release. Mechanical blending or in-situ synthesis during electrode fabrication are common methods to achieve homogeneous distribution.

Efficiency gains from sacrificial additives are measurable in terms of first-cycle capacity loss reduction and long-term cycling stability. Cells incorporating Li₂S or Li₃N exhibit significantly lower initial irreversible capacity losses compared to untreated electrodes. For example, studies have shown that cathodes with 5-10 wt% Li₂S can reduce first-cycle capacity loss by 20-30%, depending on the cathode material and electrolyte composition. Similarly, Li₃N additives have demonstrated the ability to compensate for up to 15-25% of lithium loss in high-nickel cathodes, extending cycle life by minimizing active lithium depletion.

The electrochemical stability window of the electrolyte must also be considered when using sacrificial additives. Li₂S and Li₃N decompose at relatively low voltages, meaning the electrolyte must remain inert during these reactions to avoid parasitic side reactions. Common carbonate-based electrolytes generally meet this requirement, but additives like fluoroethylene carbonate (FEC) may further enhance stability by forming a robust SEI on the anode.

Despite their advantages, sacrificial additives present challenges. The decomposition of Li₂S can lead to polysulfide formation in certain electrolytes, which may shuttle to the anode and reduce Coulombic efficiency. Li₃N, while free of such issues, requires precise control over moisture exposure during handling. Additionally, the optimal loading of these additives must balance lithium compensation with cathode energy density—excessive additive content dilutes active material, reducing overall capacity.

Advanced formulations are being explored to mitigate these limitations. Composite additives, such as Li₂S-coated conductive carbon or Li₃N embedded in polymer matrices, improve dispersion and reactivity while minimizing adverse effects. Researchers are also investigating ternary systems where multiple additives work synergistically to enhance pre-lithiation efficiency.

In summary, sacrificial additives like Li₂S and Li₃N provide a practical solution for cathode pre-lithiation, directly addressing lithium loss in lithium-ion batteries. Their distinct reaction pathways offer flexibility in design, while their efficiency gains are well-documented in both experimental and commercial contexts. Continued optimization of these materials—focusing on particle engineering, compatibility with emerging cathode chemistries, and scalable synthesis methods—will further solidify their role in advancing battery performance.

The development of next-generation sacrificial additives remains an active area of research, with efforts directed toward higher-capacity compounds, improved stability, and integration with solid-state electrolytes. As battery systems evolve toward higher energy densities and longer lifetimes, the strategic use of pre-lithiation additives will remain a cornerstone of electrode design.