Operando Monitoring of Early Ni-mediated Surface Reconstruction in Layered Lithiated Ni–Co–Mn Oxides

Abstract

Nickel-rich layered lithiated Ni−Co−Mn oxides (NCMs) are emerging as the most promising candidates for next-generation Li-ion battery cathodes. Progress, however, is hindered by an incomplete understanding of processes that lead to performance-limiting impedance growth and reduced cycling stability. These processes typically involve surface reconstruction and O 2 release at the cathode surface, both of which are difficult to monitor in the working cell. We demonstrate that online electrochemical mass spectrometry can be used to measure the gas release from NCMs of varying Ni content at practically relevant potentials and under operando electrochemical conditions. We find that for cathode potentials up to 4.3 V (vs Li + /Li) there is virtually no trade-off between Ni-mediated specific-charge enhancement and parasitic surface reactions. However, at potentials greater than 4.3 V, surface-reconstruction processes giving rise to substantial CO 2 and O 2 release occur, implying that surfacereconstructed layers a few nanometers thick may form already after the first charge. Ni content and the Ni/Co ratio are found to govern the onset, rate, and extent of these surface-reconstruction processes. These results provide novel insights into the role of Ni in governing the surface stability and performance of Li-ion layered oxides.

Oxygen Release and Surface Degradation of Li- and Mn-Rich Layered Oxides in Variation of the Li2MnO3Content

“…As already shown in a previous study, oxygen release caused by a chemical layer-tospinel transformation leading to the formation of a resistive surface layer and concomitant oxygen release from near-surface regions can explain the phenomena of oxygen release for HE-NCMs 25 as well as for classical NCM materials. 9,10 In these reports as well as in our present study, the main part of the oxygen evolution is detected during the first charge of the material, while rather small amounts of O 2 evolution can be detected during the second cycle. The continuous oxygen consumption during the cycling procedure indicated by the decreasing O 2 concentration after the first charging cycle (see middle panel in Figure 3) is ascribed to a gradual reduction of oxygen to Li 2 O 2 on the lithium counter electrode, shown by Yabuuchi et al to occur below 3.0 V vs. Li + /Li.…”

“…x surface layer = n O meas 2 n O theo 2 [9] To translate the molar fraction into a surface layer thickness, the approximate particle radius is estimated from the BET areas (A BET ) given in Table I according to Equations 10, with ρ HE-NCM being the crystallographic density of the pristine material (ρ HE-NCM = 4.2 g/cm 3 ). r = 3 A BET ρ HE−NCM [10] From the radius and the molar fraction of the spinel phase, the surface layer thickness can be easily assumed using Equations 11 and 12.…”

“…6,7 However, it has been shown that all NCM materials show structural instabilities at high state-of-charge (SOC), i.e., at a high degree of delithiation, leading to oxygen release from the near-surface region accompanied by the formation of a resistive surface layer; ultimately, this leads to rapid capacity fading and limits the practical capacity of NCM materials to <200 mAh/g. [8][9][10] * Electrochemical Society Member. * * Electrochemical Society Fellow.…”

Elucidating anionic oxygen activity in lithium-rich layered oxides

“…Electrolyte (EC/DEC) is electrochemically stable until ~4.8 V with a blank electrode composed of conductive carbon and PVDF. 52, 53 We do not attempt to ascribe a capacity to the CO 2 evolution, as CO 2 is likely evolved from surface Li 2 CO 3 that remains after synthesis. Of note, any non-volatile electrolyte decomposition products that cannot be detected using DEMS could contribute to the irreversible capacity on charge.…”

Anode-Dependent Impedance Rise in Layered-Oxide Cathodes of Lithium-Ion Cells

“…This potential corresponds either to the onset of direct oxidation of the electrolyte by the cathode surface or evolution of oxygen from the nickel-rich layered oxide. 16,34 As suggested by Figure 6, this tentative agent X can be gaseous. It is known that some of these gases, including H 2 and CO 2 , can be consumed on the lithiated Gr electrode 17 but not on the LTO electrode, which would explain why pairing of the NCM cathodes with the Gr electrodes can be equivalent to gas venting (Figures 10a and 10b).…”