Pristine Surface of Ni-Rich Layered Transition Metal Oxides as a Premise of Surface Reactivity

Abstract

The surface reactivity of Ni-rich layered transition metal oxides is instrumental to the performance of batteries based on these positive electrode materials. Most often, strong surface modifications are detailed with respect to a supposed ideal initial state. Here, we study the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode material in its pristine state, hence before any contact with electrolyte or cycling, thanks to advanced microscopy and spectroscopy techniques to fully characterize its surface down to the nanometer scale. Scanning transmission electron microscopy–electron energy loss spectroscopy (STEM-EELS), solid-state nuclear magnetic resonance (SS-NMR), and X-ray photoelectron spectroscopy (XPS) are combined and correlated in an innovative manner. The results demonstrate that in usual storage conditions after synthesis, the extreme surface is already chemically different from the nominal values. In particular, nickel is found in a reduced state compared to the bulk value, and a Mn enrichment is determined in the first few nanometers of primary particles. Further exposition to humid air allows for quantifying the formed lithiated species per gram of active material, identifying their repartition and proposing a reaction path in relation with the instability of the surface.

A Thermodynamic Model for the Insertion Electrochemistry of Battery Cathodes

“…However, Ni-rich cathodes offer a higher energy density and the natural abundance of nickel is higher, but they suffer from poor structural performance stability and energy efficiency. [1,2] At a macroscopic level, this problem manifests in energy losses during a charging-discharging cycle. This phenomenon could be a consequence of charge-discharge hysteresis which is an inherent feature of not only Ni-based cathodes, [3][4][5] but also many other battery materials [6][7][8] including the prototypical LiFe 2 O 4 .…”

“…Li‐ion batteries based on LiCoO 2 or LiNi x Co y Mn y O 2 (NMC, 1 : 1 : 1) cathodes have been trend‐setting for the last three decades. However, Ni‐rich cathodes offer a higher energy density and the natural abundance of nickel is higher, but they suffer from poor structural performance stability and energy efficiency [1,2] . At a macroscopic level, this problem manifests in energy losses during a charging‐discharging cycle.…”

Residual Li2O degrades PVdF during the preparation of NMC811 slurries for Li-ion batteries

“…The powders were obtained placing the pristine industrial powder under three controlled atmospheres, allowing to attain different surfaces and reactivities, as highlighted in a previous study. 8 Sample 1 and Sample 2 powders were obtained after aging in air under relative humidity RH = 30% for 15 minutes and 2 days, respectively. On the other hand, the pristine powder was left in synthetic air atmosphere with RH E 70% and without any carbon dioxide for 2 days to prepare Sample 3.…”

“…S1, ESI †) in addition to the four contributions (Li 2 CO 3 , LiOH, Li 2 SO 4 , Li 2 O) already confirmed for the pristine powder as detailed in our previous work. 8 The method combining NMR and XPS data is explained in ref. 8.…”

“…8 The method combining NMR and XPS data is explained in ref. 8. It was thence possible to estimate the amount of each species (in mmol g AM À1 ) found in the six (powders and slurries) samples (also collected in Table S1 in ESI † together with the parameters used for the fit).…”