Ni-rich layered oxides (LiNi x Mn y Co z O2, x ≥ 0.6, x + y + z = 1) are promising positive electrode materials for high energy density lithium-ion batteries thanks to their high specific capacity. However, large-scale application of Ni-rich layered oxides is hindered by its poor structural and interfacial stability, especially during cycling at a high cutoff potential (i.e., ≥ 4.3 V, versus Li+/Li). Herein, we demonstrate that lithium difluoro­(oxalato)­borate (LiDFOB) as a film-forming additive plays a dual role on the electrode|electrolyte interphase formation in a LiNi0.83Mn0.05Co0.12O2||graphite cell, meaning that it can not only be reduced on the graphite negative electrode but also oxidized on the nickel-rich oxide LiNi0.83Mn0.05Co0.12O2 positive electrode cycled at a high cutoff potential (4.4 V, versus Li+/Li) prior to typical carbonate-based electrolyte constituents. As a result, the addition of 1.5 wt % LiDFOB greatly reduces the polarization and improves the cycling stability of the LiNi0.83Mn0.05Co0.12O2||graphite cell, which shows a high discharge capacity of 198 mA h g–1, and more than 83.1% of the initial capacity was retained after 200 cycles at C/3 (the capacity retention obtained at the same cycling condition is only 59.9% for the cell without LiDFOB additive). Furthermore, the employ of LiDFOB additive also significantly suppresses the self-discharge of the LiNi0.83Mn0.05Co0.12O2||Li cell during high-temperature and long-term room-temperature storage at 4.4 V. These electrochemical performance enhancements could be attributed to the participation of LiDFOB in forming a stable and Li+ transfer favorable protective layer that is rich in inorganic boron, fluorine, and carbonate compounds on both the surface of the LiNi0.83Mn0.05Co0.12O2 positive electrode and the graphite negative electrode, thus suppressing the electrolyte decomposition on the positive electrode and negative electrode surfaces and decreasing the dissolution of transition-metal ions from the positive electrode bulk.

Ni-rich layered oxides, such as LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), are considered as promising cathode materials for lithium-ion batteries due to their high energy density. However, Nirich layered oxides are prone to react with water and carbon dioxide in ambient air forming residual lithium compounds, resulting in deterioration of electrochemical performance and bringing a challenge to the cathode electrode preparation. In this work, we have, for the first time, demonstrated that the chemical stability of the NCM811 material in ambient air can be significantly enhanced by passivating the surface with a hydrophobic self-assembled monolayer (SAM) of octadecyl phosphate (OPA). As a result, the degradation reaction between the NCM811 material and ambient air and thus the electrochemical performance deterioration were significantly suppressed during ambient air exposure. Specifically, the 5C-rate capacity retention deterioration of the NCM811 sample during 14-day ambient air exposure has been decreased from 12 to 2% by OPA passivation. Furthermore, the 200-cycle capacity retention deterioration of the NCM811 sample after 7day ambient air exposure has been improved from 23 to 0.7% by OPA passivation. These results are very important for the practical application of Ni-rich oxide since no need for controlling of humidity is required on the cathode manufacture; thus, the cost can be reduced. The concept of molecular self-assembly on the NCM811 material also open vast possibilities to design reagents for surface passivation of Ni-rich layered oxides.

Current studies in the Li-battery field are focusing on building systems with higher energy density than ever before. The path toward this goal, however, should not ignore aspects such as safety, stability, and cycling life. These issues frequently originate from interfacial instability, and therefore, precise surface chemistry that allows for accurate control of material surface and interfaces is much in demand for advanced battery research. Molecular self-assembly as a surface chemistry tool is considered to surpass many conventional coating techniques due to its intrinsic merits such as spontaneous organization, molecular-scale uniformity, and structural diversity. Recent publications have demonstrated the power of self-assembled monolayers (SAMs) in addressing pressing issues in the battery field such as the chemical stability of Li, but many more investigations are needed to fully explore the potential and impact of this technique on energy storage. This perspective is the first of its kind devoted to SAMs in batteries and related materials. Recent research progress on SAMs in batteries is reviewed and mainly falls in two categories, including the improvement of chemical stability and the regulation of nucleation in conversion electrode reactions. Future applications and consideration of SAMs in energy storage are discussed. We believe these summaries and outlooks are highly stimulative and may benefit future advancements in battery chemistry.

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