Local State of Charge Mapping and Analysis of Lithium-Manganese Rich NMC High Voltage Electrodes

Current R&D efforts in lithium-ion batteries are directed towards increasing energy density with reduced cost, improved cycle-life and safety performance. Among other important factors, developing high energy density lithium-ion cathodes is a high priority area. Recently, there has been significant progress made on a number of high voltage cathode chemistries such as LiMn_1.5Ni_0.5O₄¹ and lithium-manganese rich NMC (LMR-NMC) ^{2, 3} cathodes. However, significant technical challenges still need to be addressed at both materials and electrode level for their practical use. Issues include structural and phase stability under continuous high voltage cycling, transition Metal (TM) dissolution, oxygen evolution etc.  We have carried out confocal-Raman imaging and spectral analysis of LMR-NMC cathode having nominal composition of Li_1.2Mn_0.525Ni_0.175Co_0.1O₂,  maintained at different state of charge (SOC)  and cycled 25 and 200 times  between a voltage window of 4.9-2.5V.

Figure 1. Raman mapping (top) and spectral analysis (bottom) of pristine LMR NMC electrode. The false color maps show carbon rich regions (red), metal oxide (cyan) and mixed colors for both carbon-LMR-NMC regions.

The Raman maps and their spectral behavior change as the electrodes are charged (discharged). Both spectral position and full width at half maximum (FWHM) change with the voltage (SOC) and also under continuous electrochemical cycling. To quantify the inhomegneities of the LMR-NMC electrodes we undertake Raman analysis for cathode particle selected from different regions of electrodes charged at 3.9, 4.2, 4.5 and 4.9V, and compare to pristine electrode. Analysis was carried out at least on 5-6 cathode particles and results are summarized in terms of the ratio of their E_g to A_1g band areal intensities and their respective Raman band positions. There are some interesting trends from this analysis: (i) for a given bulk electrode SOC (or voltage) the cathode particles at different electrode locations vary widely in terms of the areal ratios and band positions; (ii) The A_1gband shifts consistently to red with increasing SOC or voltage. In addition, we notice a relatively larger inhomogeneity for cathode particles for the 4.2 and 4.9 V electrodes measured in terms of local spectroscopic SOC.

Acknowledgement

This research at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy, is sponsored by the Vehicle Technologies Program for the Office of Energy Efficiency and Renewable Energy.

References:

1. R. Santhanam, and B. Rambabu, J. Power Sources, 195, 5442–5451 (2010).
2. M. M. Thackeray, C. Johnson, J. T. Vaughey, N. Li, and S. A. Hackney, J. Mater. Chem. 15, 2257–2267 (2005).
3. S. K. Martha, J. Nanda, G. M. Veith, and N. J. Dudney, J. Power Sources, 199, 220–226 (2012).
4. J. Nanda, J. Remillard, A. O’Neill, D. Bernardi, T. Ro, K. E. Nietering, J. Y. Go, and T. J. Miller, Adv. Funct. Mater. 21, 3282–3290 (2011).
5. S. K. Martha, J. Nanda, G. M. Veith, and N. J. Dudney, J. Power Sources, 216, 179–186 (2012).