Air-stable Li2C6O6 and Li4C6O6 as high-efficiency lithium compensation additives in cathode

Solid-state batteries have attracted significant attention due to their high safety and high energy density. However, the continuous lithium (Li) consumption will lead to decrease of the energy density and lifespan of the battery. Efforts have been devoted to exploring new strategies to increase the content of the active Li-ions, among which application of sacrificial additives is one of the most effective ways to compensate for the lost lithium. To spur more research into a stable and high-efficiency additive, researchers in China assessed the current advances of research and proposed Li_xC₆O₆ (x=2 and 4) as high-efficiency Li-compensation additives in cathode.

They published their work on Dec. 3, 2024 in Energy Material Advances.

“Currently, the energy density of the battery is a key factor restricting its development,” said paper author Zhaoxiang Wang, professor of Institute of Physics, Chinese Academy of Sciences (CAS). “Thus, it is vital to develop new strategies to compensate for the lost lithium in the battery.”

Wang explained that, among many methods, addition of sacrificial lithium additives may be the best solution due to the high chemical stability of the additives and their compatibility with the current fabrication process.

“Various compounds have been reported for sacrificial cathode additives including Li₃N, Li₃P, Li₂CO₃, Li₄SiO₄, Li₂C₂O₄, Li₂C₄O₄, Li₅FeO₄, Li₆CoO₄ and composites of LiF, Li₂O, and Li₂S with metals such as Co and Fe. However, the commercial applications of these compounds are hindered with severe problems including high electrochemical delithiation potential and mass residues.” Wang said.

“For instance, Li₂CO₃ is insulating and difficult to be decomposed below 4.0 V (vs. Li⁺/Li) even with cobalt (Co) modification. These severely limit the viable additives. A similar dilemma is also reflected in Li₂C₂O₄ and Li₂C₄O₄. In addition, the solid-state residues (as the decomposition products of Li₃P, Li₅FeO₄ and Li₆CoO₄) or gas products (decomposition of Li₃N) have negative impacts on the energy density and/or safety of the battery.”

Considering these issues, more efforts should be paid to hunt for safer and higher-performance additives. According to Wang, cyclohexanehexone (C₆O₆) is regarded as a potential additive precursor. It was taken as a high-capacity electrode material for Li-ion batteries because it contains six C=O groups and each can take up one Li-ion, corresponding to a theoretical capacity of 957.4 mAh g^-1. Density functional theory (DFT) calculations even predicted the presence of Li₈C₆O₆, which can store more lithium. However, dissolution of C₆O₆ or delithiated Li_xC₆O₆ in the popular carbonate electrolytes severely hinder their applications in the current Li-ion batteries. Although composites such as Li₂C₆O₆/FeF₃/rGO can fettering Li_xC₆O₆ to some extent, dissolution of C₆O₆ remains the major obstacle that prevents them from becoming promising electrode materials.

“However, the rapid dissolution of the delithiated Li_xC₆O₆ becomes an advantage when used as a sacrificial Li-compensation additive, as long as the dissolution of C₆O₆ does not bring negative impacts on the solid-state electrolytes and/or the batteries,” Wang said. “Considering the chemical compatibility of the high reductivity of Li_xC₆O₆ to the cathode materials, their low delithiation potential as well as the Li-ion capacity, Li₂C₆O₆ and Li₄C₆O₆ were selected as sacrificial additives in the cathode for Li compensation in PEO-based solid-state batteries.”

“Therefore, air-stable Li_xC₆O₆ with low delithiation potential were proposed as high-efficiency Li-compensation additives in the cathode. Experimentally, the Li_xC₆O₆ exhibits a high initial charge capacity of 292.6 mAh g^-1 (for x=2) and 446.8 mAh g^-1 (for x=4), respectively, which is sufficient to compensate for the lost lithium,” Wang said. “With the addition of 5wt% Li_xC₆O₆ in the LiFePO₄ cathode, the initial charge capacity and discharge capacity of the PEO-electrolyte-based Cu||LiFePO₄ cell increase significantly.”

This work opens the prospect of developing new and promising cathode sacrificial additives for practical applications in the battery industry.

Wang is also affiliated with the College of Materials Science and Opto-Electronic Technology and School of Physical Sciences in University of Chinese Academy of Sciences (UCAS). Other contributors include Mengyan Cao, Huajun Li, Hang Chu, Xuefeng Wang and Liquan Chen, Institute of Physics, CAS; Bingyun Ma and Tao Cheng, Institute of Functional Nano and Soft Materials, Soochow University; Simeng Zhang, Eastern Institute for Advanced Study Eastern Institute of Technology Ningbo; Yurui Gao, National Center for Nanoscience and Technology, CAS; Xueliang Sun, Department of Mechanical and Materials Engineering, University of Western Ontario.

The following authors have additional affiliations: Mengyan Cao, Huajun Li, Hang Chu and Xuefeng Wang, College of Materials Science and Opto-Electronic Technology, UCAS; Xuefeng Wang, School of Physical Sciences, UCAS and Tianmu Lake Institute of Advanced Energy Storage Technologies Co. Ltd..

The National Natural Science Foundation of China (NSFC No. 22075316, U23A20577, 22005334 and 52172257) and Natural Science Foundation of Beijing (Grant No. Z200013) supported this work.