Interface Engineering via Li ₂ C ₄ O ₄ Prelithiation: Boosting Activated Carbon Electrode Performance in Lithium‐Ion Capacitors

Pre‐lithiation is an essential step in lithium‐ion capacitors (LICs) due to the lack of Li + in both electrodes. The integration of dilithium squarate (Li 2 C 4 O 4 ) into the positive electrode of LICs is considered one of the most promising pre‐lithiation strategies. Therefore, the ability of Li 2 C 4 O 4 decomposition products to modify the solid electrolyte interphase has been recently disclosed, although their impact on the positive electrode surface has not been studied yet. In this work, the improvement of the electrochemical performance when Li 2 C 4 O 4 was included has been investigated by analyzing the surface of activated carbon‐based electrodes with and without Li 2 C 4 O 4 by scanning electron microscopy and X‐ray photoelectron spectroscopy. The decomposition of Li 2 C 4 O 4 leads to the formation of a surface layer on the positive electrode that remains unaltered regardless of the applied potential, as well as after an aging test. Thus, the improved electrochemical performance is attributed to the presence of a pseudocapacitive charge storage mechanism enabled by the surface layer. Lastly, the cells are modified to reveal the main components participating in the surface layer formation. These findings provide valuable insights into the impact, benefits, and limitations of Li 2 C 4 O 4 , which will accelerate the development of other suitable alternative sacrificial salts.