Jean-Frédéric Martin

Aging of the LiNi1 ∕ 2Mn1 ∕ 2O2 Positive Electrode Interface in Electrolyte

The evolution of lithium-containing species on the surface of grains of layered LiNi 1/2 Mn 1/2 O 2 material during the aging process in LiPF 6 (ethylene carbonate/dimethyl carbonate, 1 M) electrolyte has been followed using 7 Li magic angle spinning NMR spectroscopy. Materials displaying different surface areas have been investigated in order to study the influence of the surface/ volume ratio. The evolution of the NMR signal shows that the reaction of the active material with the electrolyte is extremely fast during the first moments of exposure and tends to slow down for longer exposure times. Coupled NMR, electrochemical impedance spectroscopy, and transmission electron microscopy experiments showed that the surface of the material grains is not covered by a homogeneous layer, indicating that the reaction with electrolyte cannot be considered as a real passivation reaction. The aging process performed on a sample initially stored in ambient atmosphere clearly demonstrates the dissolution of a pristine Li 2 CO 3 surface layer and the growth of an interphase made primarily of fluorinated compounds.

Quantitative MAS NMR characterization of the LiMn1/2Ni1/2O2 electrode/electrolyte interphase

The conditions in which degradation processes at the positive electrode/electrolyte interface occur are still incompletely understood and traditional surface analytical techniques struggle to characterize and depict accurately interfacial films. In the present work, information on the growth and evolution of the interphases upon storage and cycling as well as their electrochemical consequences are gathered in the case of LiNi(1/2)Mn(1/2)O(2) with commonly used LiPF(6) (1M in EC/DMC) electrolyte. The use of (7)Li, (19)F and (31)P MAS NMR, made quantitative through the implementation of empirical calibration, is combined with transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to probe the elements involved in surface species and to unravel the inhomogenous architecture of the interphase. At room temperature, contact with the electrolyte leads to a covering of the oxide surface first by LiF and lithiated organic species are found on the outer part of the interphase. At 55°C, not only the interphase proceeds in further covering of the surface but also thickens resulting in an increase of 240% of lithiated species and the presence of -POF(2) fluorophosphates. The composition gradient within the interphase depth is also strongly affected by the temperature. In agreement with the electrochemical performance, quantitative NMR surface analyses show that the use of LiBOB-modified electrolyte results in a Li-enriched interphase, intrinsically less resistive than the standard LiPF(6)-based interphase, comprised of a mixture of resistive LiF with non lithiated species.

Detection of surface layers using 7Li MAS NMR

Magic angle spinning solid state NMR is generally used to characterize bulk materials. We show here that it is also a promising tool to detect and characterize the diamagnetic surface layer on a paramagnetic material, which is a novel development. We apply this technique to the very hot topic of positive electrode/electrolyte characterization in the field of lithium rechargeable batteries. We report a 7Li MAS NMR study of physisorbed surface layers on LiNi1/2Mn1/2O2, a positive electrode material for lithium ion batteries. 7Li MAS NMR signals arising from surface layers formed by mixing the material with lithium carbonate or from contact of the material with ambient atmosphere, as well as with electrolyte are collected and analyzed. The progressive broadening of the line shape of the MAS NMR spectra reflects the increasing intimacy of the surface layer or secondary phase with the bulk material and therefore gives extremely useful complementary structural information on the surface not available using XPS or IR. We show that relaxation time measurements can be used as a probe of surface layers, allowing for discrimination of interphases from different origins. We propose a detailed analysis of the relaxation curves with a stretched exponential model allowing the description of the distribution of environments inside the surface layer. Our work indicates that MAS NMR can provide useful information for fine surface characterization of materials.

Interphase Evolution at Two Promising Electrode Materials for Li‐Ion Batteries: LiFePO4 and LiNi1/2Mn1/2O2

The present review reports the characterization and control of interfacial processes occurring on olivine LiFePO(4) and layered LiNi(1/2) Mn(1/2)O(2), standing here as model compounds, during storage and electrochemical cycling. The formation and evolution of the interphase created by decomposition of the electrolyte is investigated by using spectroscopic tools such as magic-angle-spinning nuclear magnetic resonance ((7)Li,(19)F and (31)P) and electron energy loss spectroscopy, in parallel to X-ray photoelectron spectroscopy, to quantitatively describe the interphase and unravel its architecture. The influence of the pristine surface chemistry of the active material is carefully examined. The importance of the chemical history of the surface of the electrode material before any electrochemical cycling and the strong correlation between interface phenomena, the formation/evolution of an interphase, and the electrochemical behavior appear clearly from the use of these combined characterization probes. This approach allows identifying interface aging and failure mechanisms. Different types of surface modifications are then investigated, such as intrinsic modifications upon aging in air or methods based on the use of additives in the electrolyte or carbon coatings on the surface of the active materials. In each case, the species detected on the surface of the materials during storage and cycling are correlated with the electrochemical performance of the modified positive electrodes.