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 Matériaux et composites pour électrodes de batteries au lithium |
 Characterization of Electrode / Electrolyte interphase on Positive Electrodes for Li-ion Batteries.  (version française) |
People involved in the research topic :
Nicolas DUPRE (CR)
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Description of the research topic:
Understanding and monitoring the evolution of the surface layer during storage and electrochemical cycling. Correlation with the electrochemical performances of the electrochemical cell.
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The so-called solid electrolyte interphase (SEI) between the negative electrode and the electrolyte of a Li-ion battery is known to monitor the overall battery behavior in terms of irreversible capacity loss, charge transfer kinetics and storage properties. More than ten years of research in this field have led to excellent control and optimization of the SEI layer. Surface formulation and/or coatings of the positive electrode have been shown more recently to influence the battery performance as well. Interfacial reactions and the growth of a passivation layer at the electrode surface upon cycling have been evidenced for different positive electrode materials (Fig 1).
 
Figure1: SEM pictures of  a): pristine LiNi0.5Mn0.5O2 and b): LiNi0.5Mn0.5O2 after contact with electrolyte
Interfacial problems at positive electrode have been identified of paramount importance because they lead to performance degradation of the battery upon aging and cycling. The chemical, physical and structural properties of the interfacial layer at positive electrode, and its modification upon cycling, are still poorly known. In order to understand interfacial problems on advanced positive electrodes of Li-ion batteries, controlling the long term life duration and intercalation / deintercalation kinetics, we need to characterize the positive / electrolyte interphase evolution upon aging/cycling. For this purpose, we chose two of the most promising positive electrode materials : the layered LiNi0.5Mn0.5O2, operating at 4 V and the 3D LiFePO4 operating at 3.5 V. Our purpose is to Understand the evolution of the surface layer during storage and electrochemical cycling and to correlate it with the electrochemical performances of the electrochemical cell. In order to monitor the formation of the surface layer, different syntheses of the electrode material are performed leading to various grain sizes and surface textures.
MAS-NMR, as a local probe, is a useful tool to obtain informations on the chemical and structural local environment of the nucleus under observation (6Li, 7Li, 1H…etc), complementary to EIS measurements, to long range probe such as X-ray diffraction and surface probes such as XPS. MAS-NMR has been recently successfully applied to the observation of interphase layers on lithium nickel oxide based positive electrodes of lithium-ion batteries [1, 2].
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Figure 2 : 7Li MAS NMR spectra for LiNi0.5Mn0.5O2 acquired with a Hahn-echo pulse sequence (green), single pulse (red) and Li2CO3 (blue). |
The dipolar interaction between the Li ions within the host matrix and the paramagnetic transition metal (Ni2+, Mn4+) is too strong to be averaged out even under relatively high spinning frequencies MAS conditions. Using a single pulse with a dead time longer than the T2 relaxation time of this signal allows to observe only the « diamagnetic » Li at the surface.This surface diamagnetic layer displays also a strong interaction with the paramagnetic bulk but no hyperfine Fermi-contact shift (due to orbitals overlapping) is detected and thus, no chemical bond between the Li in the surface layer and the bulk of the electrode’s active material. In fact, The observed line is extremely broad and featureless and an important set of intense sidebands is observed due to the strong paramagnetism and the proximity of the bulk of the LiNi0.5Mn0.5O2 compound. Not only the line width is broader than the typical quadrupolar shape of Li2CO3 alone: the intensity of the sidebands do not reflect only a strong dipolar interaction with paramagnetic centers distributed around the nucleus under observation but reflects also the overall paramagnetism of the LiNi0.5Mn0.5O2 bulk seen by a secondary phase i.e. the surface layer.
(Fig. 2 ).
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Our first results show a different interaction strength depending on the origin of the surface layer. For instance, we noticed a stronger interaction with the Li2CO3 formed from contact with air than with mixed or milled Li2CO3 (Fig 3). It appears that the line grows broader with the intimacy with the paramagnetic bulk. Spin-lattice (T1) relaxation curves can be fitted using a stretched exponential function describing a T1 distribution and a progressive change in the T1 value with the distance from the paramagnetic centre(s) and allows to follow a change in the layer thickness as well as in the intimacy of the two phases.
Moreover, the sidebands manifolds for soaked and cycled materials are completely different (Fig. 4). This shows clearly the influence of the potential on the surface layer: storage and cycling have different effects on the structure and/or composition of the surface layer. The surface layer for the cycled sample includes additional species with weaker dipolar interaction with the paramagnetic bulk as the intensity of the sidebands decreases faster. This result supports the layered structure of the surface film suggested by XPS for instance.
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Figure 3 : 7Li MAS NMR spectra for LiNi0.5Mn0.5O2 stored in ambient atmosphere (blue), ball-milled with Li2CO3 (red) and mixed manually with Li2CO3 (green).
Figure 4 :7Li MAS NMR spectra for LiNi0.5Mn0.5O2 samples soaked in electrolyte without any electrochemical cycling (blue) and after 1 cycle at 2.5V (red).
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From these exploratory studies, it appears that Li MAS-NMR is a promising tool to study the electrode/electrolyte interphase. The combination of such a technique with EIS and XPS measurements will allow to follow both structural and chemical evolution of the surface layer on positive electrode materials. |
[1]. M. Menetrier, C. Vaysse, L. Croguennec, C. Delmas, C. Jordy, F. Bonhomme and P. Biensan, Electrochem. And Solid State Lett., 7 (6) (2004) A140.
[2]. B. Meyer, N. Leifer, S. Sakamoto, S. Greenbaum, C.P. Grey, Electrochem. And Solid State Lett. 8(3) (2005) A145. |
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Page updated: may, 16, 2007
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