Department of Chemical and Biomolecular Engineering, University of California Berkeley, Berkeley, California 94720, United States.
Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
ACS Appl Mater Interfaces. 2023 Apr 19;15(15):18747-18762. doi: 10.1021/acsami.2c16974. Epub 2023 Apr 4.
Lithium-excess, cation-disordered rocksalt (DRX) materials have been subject to intense scrutiny and development in recent years as potential cathode materials for Li-ion batteries. Despite their compositional flexibility and high initial capacity, they suffer from poorly understood parasitic degradation reactions at the cathode-electrolyte interface. These interfacial degradation reactions deteriorate both the DRX material and electrolyte, ultimately leading to capacity fade and voltage hysteresis during cycling. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are combined to quantify the extent of bulk redox and surface degradation reactions for a set of Mn-based DRX oxyfluorides during initial cycling with a high-voltage charging cutoff (4.8 V vs Li/Li). Increasing the fluorine content from 7.5 to 33.75% is shown to diminish oxygen redox and suppresses high-voltage O evolution from the DRX surface. Additionally, electrolyte degradation processes resulting in the formation of both gaseous species and electrolyte-soluble protic species are observed. Subsequently, DEMS is paired with a fluoride-scavenging additive to demonstrate that increasing fluorine content leads to increased dissolution of fluorine from the DRX material into the electrolyte. Finally, a suite of spectroscopy techniques (X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, and solid-state nuclear magnetic resonance spectroscopy) are employed to study the change in DRX composition during charging, revealing the dissolution of manganese and fluorine from the DRX material at high voltages. This work provides insight into the degradation processes occurring at the DRX-electrolyte interface and points toward potential routes of interfacial stabilization.
富锂、阳离子无序岩盐 (DRX) 材料作为锂离子电池潜在阴极材料，近年来受到了广泛关注和研究。尽管它们具有组成灵活性和较高的初始容量，但在阴极-电解质界面存在着一些难以理解的寄生降解反应。这些界面降解反应会破坏 DRX 材料和电解质，最终导致在循环过程中容量衰减和电压滞后。在这项工作中，差分电化学质谱 (DEMS) 和滴定质谱相结合，定量研究了一组锰基 DRX 氟氧化物在高电压充电截止（4.8 V 相对于 Li/Li）下初始循环过程中的体相氧化还原和表面降解反应的程度。结果表明，氟含量从 7.5%增加到 33.75%，可以减少氧的氧化还原，并抑制 DRX 表面的高压 O 演化。此外，还观察到导致形成气态物种和电解质可溶性质子物种的电解质降解过程。随后，DEMS 与氟化物清除添加剂配对，证明增加氟含量会导致氟从 DRX 材料中更多地溶解到电解质中。最后，采用一系列光谱技术（X 射线光电子能谱、电感耦合等离子体发射光谱和固态核磁共振波谱）研究了充电过程中 DRX 组成的变化，揭示了在高压下 DRX 材料中锰和氟的溶解。这项工作深入了解了 DRX-电解质界面上发生的降解过程，并为界面稳定的潜在途径指明了方向。
