For achieving higher energy density lithium-ion batteries, the improvement of cathode active materials is crucial. The most promising cathode materials are nickel-rich layered oxides LiNi_xCo_yMn_zO₂ (NCM) and over lithiated NCM (often called HE-NCM). Unfortunately, the full capacity of NCM cannot be utilized due to its limited cycle-life at high state-of-charge (SOC), while HE-NCM requires high voltages. By operando emission spectroscopy, we show for the first time that highly reactive singlet oxygen is released when charging NCM and HE-NCM to an SOC beyond ≈80%. In addition, on-line mass-spectrometry reveals the evolution of CO and CO₂ once singlet oxygen is detected, providing significant evidence for the reaction between singlet oxygen and electrolyte to be a chemical reaction. It is controlled by the SOC rather than by potential, as would be the case for a purely electrochemical electrolyte oxidation. Singlet oxygen formation therefore imposes a severe challenge to the development of high-energy batteries based on layered oxide cathodes, shifting the focus of research from electrochemically stable 5 V-electrolytes to chemical stability toward singlet oxygen.

为了获得更高能量密度的锂离子电池，阴极活性材料的改进至关重要。最有希望的阴极材料是富镍层状氧化物LiNi _x Co _y Mn _z O ₂（NCM）和锂化NCM（通常称为HE-NCM）。不幸的是，由于NCM在高荷电状态（SOC）下的循环寿命有限，因此无法利用其全部容量，而HE-NCM需要高电压。通过操作发射光谱法，我们首次显示出当将NCM和HE-NCM充电至SOC约80％以上时，会释放出高反应性单线态氧。另外，在线质谱揭示了CO和CO ₂的演变一旦检测到单线态氧，就为单线态氧与电解质之间的反应是化学反应提供了重要的证据。它是由SOC而不是由电势控制的，就像纯电化学电解质的氧化一样。因此，单线态氧的形成对基于分层氧化物阴极的高能电池的开发提出了严峻的挑战，将研究重点从电化学稳定的5 V电解质转移到化学稳定性，转向单线态氧。