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Abstract: The formation and the evolution of lithium-containing species on the surface of grains of a layered 4V material such as LiNi1/2Mn1/2O2 along the electrochemical cycling have been followed using 7Li MAS NMR, electrochemical impedance spectroscopy (EIS) and XPS. Materials displaying different specific surface areas and stored in different atmospheres have been investigated in order to study the influence of the surface/volume ratio and the influence of the initial surface state, respectively. It is shown that the presence of an initial interphase of Li2CO3 influences the electrochemical behavior of the electrode, emphasizing the importance of the history of the electrode prior cycling. 7Li MAS NMR experiments performed upon cycling indicate the formation of interphase species in reduction and their partial removal in oxidation, indicating the dynamic character of the interphase upon cycling. Combined NMR, EIS and XPS experiments show the strong influence of the electrode/electrolyte interphase evolution on the electrochemical performance. Such results lead us to draw conclusions on the optimal storage conditions of layered 4V materials for Li-ion batteries such as LiNi1/2Mn1/2O2. [Copyright &y& Elsevier]

Abstract: The changes appearing for LiFePO4–C nano-composites exposed to atmosphere at 120°C have been structurally and chemically examined by the use of TGA, XRD, XPS, Mössbauer, 7Li MAS NMR and electrochemical methods. The results conclude that a highly disordered phase resulting from the aging of LiFePO4 appears on the surface of the grains of the material, is assigned to a phosphate phase and can insert lithium around 2.6V with poor reversibility. The essential role of water has been investigated and clearly demonstrated. Thus, the aging mechanism occurring in hot humid air is completely different from a simple oxidation as well as from the aging process observed above 150°C and involves the incorporation of hydroxyl groups. In addition, Fe2O3 formation has not been observed for such an aging in mild conditions. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of lithium-containing species on the surface of grains of 500nm LiFePO4 and 100nm carbon-coated LiFePO4 materials during the aging process in LiPF6 electrolyte has been followed using coupled 7Li MAS NMR, EIS (Electrochemical Impedance Spectroscopy) and XPS for materials synthesized with and without carbon coating. LiFePO4 undergoes surface reactivity upon immersion in classical LiPF6 electrolyte, although its open circuit voltage (∼3.2V) lies in the thermodynamical stability voltage range. The evolution of the NMR signal shows that the reaction of formation of the interphase is very slow as no evidence of passivation could be found even after 1 month of contact with the electrolyte. 7Li MAS NMR combined with XPS indicates that carbon coating has a strong protective role towards formation of surface species on the material and hinders iron dissolution at elevated temperature. Coupled NMR, EIS and XPS experiments showed that the surface of the material grains is not covered by an homogenous layer. Increasing the storage temperature from 25°C to 55°C promotes the formation of organic species on the surface, most probably covering inorganic species such as LiF, Li x PF y and LiPO y F z . No evidence of the formation of a resistive film is deduced from the evolution of EIS measurements. The interphase growth can accelerate the degradation of the electrochemical performance, leading to a loss of electrical contact within the electrode. [Copyright &y& Elsevier]

Abstract: 7Li MAS NMR, usually a bulk characterization technique, is used here to analyze the positive electrode/electrolyte interphase. The sharpening of the NMR spectra line shape as the amount of surface species increases shows that the observed signal is clearly the sum of signals due to the distribution of lithium ions in the interphase in terms of distance from the bulk of electrode active material. This technique is then used to compare characteristics of the interphase coming from the contact with LiPF6-based electrolyte in the case of storage or electrochemical cycling. A clear influence of the change of potential on the interphase configuration and in particular on its intimacy with the bulk of active material is deduced from the change in NMR spectra lineshape. This information is hardly obtained by other characterization technique, making NMR a powerful tool for the study of interphases and passivation layers in lithium batteries materials. [Copyright &y& Elsevier]

The growth and evolution of the interphase, due to contact with the ambient atmosphere or electrolyte, are followed using 7Li magic-angle spinning nuclear magnetic resonance (MAS NMR) in the case of two materials amongst the most promising candidates for positive electrodes for lithium batteries: LiFePO4 and LiMn0.5Ni0.5O2. The use of appropriate experimental conditions to acquire the NMR signal allows observing only the «diamagnetic» lithium species at the surface of the grains of active material. The reaction of LiMn0.5Ni0.5O2 with the ambient atmosphere or LiPF6 (1 M in Ethylene Carbonated/DiMéthyl Carbonate (EC/DMC)) electrolyte is extremely fast and leads to an important amount of lithium-containing diamagnetic species compared to what can be observed in the case of LiFePO4. The two studied materials display a completely different surface chemistry in terms of reactivity and/or kinetics of the surface towards electrolyte. Moreover, these results show that MAS NMR is a very promising tool to monitor phenomena taking place at the interface between electrode and electrolyte. [ABSTRACT FROM AUTHOR]

Abstract: Decreasing particle size ensures a good accessibility of LiFePO4 to lithium ions and electrons and allows reaching the theoretical capacity and achieving high cycling rates. It nevertheless leads to an increase of the surface area and thus a subsequent amplification of the parasitic reactions at the interface between active material surface and electrolyte. The formation and evolution of the interphase on the surface of LiFePO4 may affect strongly its electrochemical performance. This work aims at monitoring the interphase forming on LiFePO4 upon its operation in a lithium battery, and correlating its evolution with the electrochemical behaviour of the active material. Combined XPS and multinuclear quantitative 7Li, 1H and 19F NMR indicate a trend for the interphase structure that can be described as a mostly stable inner interphase composed of fluorinated inorganic products while outer lithiated organic species undergo a reversible formation at high potential and dissolution at low potential. Although an irreversible accumulation of interphase occurs, spin–spin relaxation time analysis indicates that interphasial species tend to stack on the top of each other rather than cover the whole active material surface. Moreover, the dissolution/precipitation process seems to prevent a blocking of the electrode surface by resistive species such as LiF. [Copyright &y& Elsevier]

Abstract: Investigation of electrode/electrolyte interface of 5V spinel material LiNi0.4Mn1.6O4 was carried out in the presence of glutaric anhydride additive, using combined magic angle spinning NMR spectroscopy and electron energy-loss spectroscopy. After exposure to LiPF6 in EC/DMC liquid electrolyte, oxidation state of +III or lower has been evidenced by EELS for Mn ions, indicating that the presence of glutaric anhydride additive in the electrolyte, not only modifies the interphase, but does not prevent chemical reactions with the active material. Further investigation of the influence of the additive upon storage and cycling was performed using combined 7Li and 19F MAS NMR. The native interphase formed by simple contact of the active material with the electrolyte is partially destroyed at high potential but the new appearing interphase is overall increasing upon cycling, independently from the presence of glutaric anhydride. The use of glutaric anhydride is nevertheless beneficial as it clearly restrains the formation of lithiated interphasial species and alters the interphase composition since the formation of fluorophosphates is promoted, lowering the relative amount of resistive LiF. Although resistive LiF can be formed in significant amount, it is removed by the DMC rinsing while fluorophosphates display a stronger adherence to the active material. [Copyright &y& Elsevier]