Electrochemical in-Situ Raman Spectroscopic Studies of Highly-Cycled Li(Ni/Co/Mn)O2 Cathode Cells

Abstract

1. IntroductionRaman spectroscopy is a novel non-destructive, non-contact method to study lithium ion battery (LIB) electrode surfaces [1]. We have developed and constructed an original optical LIB cell, which allows for reproducible Raman spectroscopic measurement under electrochemical control. The capability of this EC in-situ Raman cell for spectro-electrochemical and distribution spatial mapping measurements were reported previously [2], where information of Li ion intercalation behavior could be effectively studied for a chosen graphite anode particle. In this study, we have applied the electrochemical in-situ Raman measurement to study the high-capacitance 18650 type LIB cell that was highly-cycled and compared to those of the initially cycled one.2. MethodA LIB coin-cell was constructed with commercially available 18650 type Li(Co0.3Ni0.3Mn0.3)O2 ternary oxide cathode and graphite anode. LIB cells were highly-cycled (500 cycles) at rate of 1C and capacity found to deteriorate to 74% of the initial value. For EC in-situ Raman measurements, the cathode was retrieved from the coin-cell and a LIB half-cell was assembled in a dry and inert Ar atmosphere (<1 ppm O2) glove-box. Lithium metal was used as the counter electrode and electrolyte solution consisted of 1 M LiPF6EC/DEC (3:7). All EC in-situ Raman spectral measurements were measured under confocal alignment at an excitation wavelength of 532 nm. Progressive electrochemical Raman spectra in the charge and discharge processes were obtained during a potential sweep rate of 10 mV/min. High-resolution Raman spectral mapping measurements (Witec Instruments) were also carried out to investigate the distribution of active particles within the electrode sheet using the same excitation wavelength of 532 nm. All mapping measurements were carried out at open circuit potential (OCP).3. Results and DiscussionFigure 1 shows the Raman spectral mapping result of the initial and 500-cycled cathode surfaces. The Raman shift map was constructed using the peak centered at about 595 cm-1 that includes LiCoO2, Co-O (A1g) lattice vibrational modes [3]. The highly-cycled cathode particles showed areas with lower wavenumber centered at 550 cm-1 (blue areas) compared to the initial cycle, where the peak center was observed near 600 cm-1 (red areas). The peak positional shift points to a partial change in the layered rock salt LiCoO2structure and supporting evidence obtained from ICP elemental and metal valance (TEM-EELS) analysis suggest that Ni metal dissolution was most likely the cause. To further investigate the lattice structural change of this endured 500-cycled cathode sample, EC in-situ Raman method was employed to study the electrochemical lithiation/delithiation behavior directly.Figure 2 shows the EC-in-situ Raman spectroscopic measurements at the 500-cycled cathode surface during the progressive charging/discharging processes. The dominant peak centered at 588 cm-1, assigned to the Co-O stretching mode, showed a shift to 592 cm-1 with lithiation of the cathode which was found to be reversible with delithiation. It is also interesting to note that the LiPF6 (£) and carbonate electrolyte ( ) peaks showed an increase in peak intensity near the cathode particle surface with delithiation, indicating to changes in the solid-electrolyte interface (SEI) region.Further details on the result of EC in-situ Raman measurements and interpretation of the degradation mechanisms will be discussed with supportive information obtained for the SEI layer obtained from conventional LIB analysis methods.