The continuous lithium consumption during cycling severely reduces the energy density of the lithium battery, and thus, lithium compensation is essential. Herein, Li_xC₆O₆ (x = 2, 4) was proposed as an air-stable high-efficiency sacrificial additive in the cathode to compensate for the lost lithium ions in solid-state lithium batteries. Below a delithiation (oxidation) potential as low as 3.8 V, Li₂C₆O₆ can release most of its Li⁺ ions (294.8 mAh g⁻¹ in theory). Similarly, Li₄C₆O₆ is also characteristic of low oxidation potential and high delithiation capacity (547.8 mAh g⁻¹ in theory). The feasibility of using Li_xC₆O₆ as the self-sacrificial additive in the cathode was verified with the marked increase of the initial charge capacity of the Li||LiFePO₄ (half) cells and the initial discharge capacity of Cu||LiFePO₄ (full) cells, and the improved electrolyte/cathode interface stability and interface contact, in the solid-state poly(ethylene oxide)-lithium bis(trifluoromethane)sulfonimide (PEO-LiTFSI) electrolyte. In addition, the structure and delithiation of Li_xC₆O₆ and the impacts of its decomposition product on the PEO-LiTFSI solid electrolyte were also evaluated on the basis of the comprehensive physical characterizations and the density functional theory (DFT) calculations. These findings open a new avenue for elevating the energy density and/or elongate the lifespan of the solid-state secondary batteries.

Lithium carbonate (Li₂CO₃) is very common in various types of lithium (Li) batteries. As an insulating by-product of the oxygen reduction reaction on the cathode of a Li–air battery, it cannot be decomposed below 4.75 V (vs. Li⁺/Li) during recharge and leads to a large polarization, low coulombic efficiency, and low energy conversion efficiency of the battery. On the other hand, more than 10% of the Li ions from the cathode material are consumed during chemical formation of a Li-ion battery, resulting in low coulombic efficiency and/or energy density. Consequently, lithium compensation becomes essential to realize Li-ion batteries with a higher energy density and longer cycle life. Therefore, reducing the oxidation potential of Li₂CO₃ is significantly important. To address these issues, we show that the addition of nanoscaled LiCoO₂ can effectively lower this potential to 4.25 V. On the basis of physical characterization and electrochemical evaluation, we propose the oxidization mechanism of Li₂CO₃. These findings will help to decrease the polarization of Li–air batteries and provide an effective strategy for efficient Li compensation for Li-ion batteries, which can significantly improve their energy density and increase their energy conversion efficiency and cycle life.