In this study, we will show how the oxygen release depends on the Li 2 MnO 3 content of the material and how it affects the actual voltage fading of the material. Thus, we compared overlithiated NCMs (x Li 2 MnO 3 • (1-x) LiMeO 2 ; Me = Ni, Co, Mn) with x = 0.33, 0.42 and 0.50, focusing on oxygen release and electrochemical performance. We could show that the oxygen release differs vastly for the materials, while voltage fading is similar, which leads to the conclusion that the oxygen release is a chemical material degradation, occurring at the surface, while voltage fading is a bulk issue of these materials. We could prove this hypothesis by HRTEM, showing a surface layer, which is dependent on the amount of oxygen released in the first cycles and leads to an increase of the charge-transfer resistance of these materials. Furthermore, we could quantitatively deconvolute capacity contributions from bulk and surface regions by dQ/dV analysis and correlate them to the oxygen loss. As a last step, we compared the gassing to the base NCM (LiMeO 2 , Me = Ni, Co, Mn), showing that surface degradation follows a similar reaction pathway and can be easily To face future issues, as global warming, air pollution, as well as the consumption of fossil fuels, an alternative is required to cover the future demand of energy and mobility in an environmentally friendly and sustainable way. In this context, lithium-ion batteries are viable options for large scale energy storage and for electric vehicles, as they have been used to power consumer electronics for many years. 1,2Since graphite is an excellent anode material at potentials of ≈0.1 V vs. Li + /Li with a roughly 2-fold higher specific capacity of about 360 mAh/g compared to currently used cathode active materials (CAMs), many efforts have been undertaken to increase the specific capacity and energy density of CAMs. As first practical cathode active material Lithium-Cobalt-Oxide (LCO) was investigated by Goodenough et al. in the 1980s, exhibiting a specific capacity of about 140 mAh/g and having a layered structure composed of lithium and transition metal layers.3 As these layered structures showed good structural stability during lithium extraction and insertion, and therefore good capacity retention, many attempts have been undertaken to further develop alternative layered structures which would offer higher capacity. One promising attempt that led to the currently used Lithium-NickelCobalt-Manganese-Oxides (NCMs) is to change the occupancy of the transition metal layer by not using exclusively cobalt, but also introducing nickel and manganese into the transition metal layer; hereby it was found that nickel shows a high redox activity, while manganese helps to stabilize the structure during lithium extraction.4-6 By using different transition metals and metal compositions, a playground has been created that allows to tune the properties of the material: while initially a Ni:Co:Mn ratio of 1:1:1 was used (also referred to as NCM-111), trends nowadays favor the so-called Ni...

Washing is a commonly used method to remove surface impurities of cathode materials for lithium-ion batteries. However, a clear mechanistic understanding of the washing process is missing in the literature. In this study, we will investigate the effect of washing and subsequent drying of nickel-rich NCM cathodes (85% nickel) with respect to gassing and impedance of the washed cathodes. By on-line electrochemical mass spectrometry (OEMS), we will show a drastic reduction of the O 2 release above 80% SOC for the NCM washed with deionized water, suggesting the formation of an oxygen-depleted surface layer on the NCM particle surface. The modification of the surface can be confirmed by a strong impedance buildup of cathodes composed of washed NCM (using a microreference electrode in a full-cell), revealing that the impedance increases strongly with increasing drying temperature after washing. Last, we will propose a comprehensive mechanism on the processes occurring during the washing/drying process of nickel-rich NCM materials and identify the drying temperature after washing as the dominant factor influencing the surface properties.

Lithium-and manganese-rich layered oxide-based cathode active materials (often referred to as HE-NCM) exhibit high reversible specific capacity (≈250 mAh/g) and could improve future lithium-ion batteries in terms of energy density and safety, while offering lower cost. Unfortunately, drawbacks such as voltage-fading, hysteresis, and increasing cathode impedance over charge/discharge cycling have so far hindered its commercialization. In this study, we examine the reasons and the implications of the high resistance build-up of this material in graphite//HE-NCM full-cells. Impedances/resistance were obtained either by electrochemical impedance spectroscopy (EIS) with a micro-reference electrode or by current pulse measurements (so-called direct-current internal-resistance (DCIR) measurements). These data show that the so-called activation of the material above 4.5 V vs. Li + /Li leads to an asymmetric high charge-transfer impedance at low state-of-charge (SOC) between charge and discharge, manifested as an anomalous cell resistance hysteresis which increases over cycling and with increasing upper cutoff potentials. These findings are rationalized by reversible transition-metal migration phenomena.

Li- and Mn-rich layered oxides are a promising class of cathode active materials (CAMs) for future lithium-ion batteries. However, they suffer from fast capacity fading in standard EC-containing electrolytes, and therefore fluorinated alternatives, such as FEC, are required to improve their full-cell performance, which unfortunately increases the cost of the electrolyte. In this study, we will analyze the reasons for the poor cycling performance of EC-containing electrolytes with CAMs that release lattice oxygen at high degrees of delithiation, i.e., either of Li- and Mn-rich NCMs (LMRNCMs) during activation or of NCMs at high cutoff voltages. By on-line electrochemical mass spectrometry (OEMS), we will show that the stability of EC towards electrochemical oxidation is sufficient up to potentials of ≈4.7 V vs Li+/Li, but that its chemical reaction with lattice oxygen released from CAMs negatively affects cycle-life. Furthermore, we will show that the use of EC-based electrolytes above the onset potential for oxygen release leads to a resistance build-up causing a rapid “rollover” fading, while FEC does not show such a dramatic impedance increase. Last, we will demonstrate that the lattice oxygen release from NCM-622 above ≈4.5 V vs Li+/Li requires the use of EC-free electrolytes for stable cycling.