Nanoscale origins of degradation of Ni-rich NMC Li-ion battery cathodes

Session

Stream 1: EMAG - Energy and Energy Storage Materials

Authors

Mr Jedrzej Morzy (2, 3), Dr. Wesley Dose (3, 5), Amoghavarsha Mahadevegowda (2, 4), Clare Grey (1, 4), Prof. Michael de Volder (3), Prof. Caterina Ducati (2)

Affiliations

1. Department of Chemistry, University of Cambridge
2. Dept. of Materials Science and Metallurgy, University of Cambridge
3. Institute for Manufacturing, University of Cambridge
4. The Faraday Institution
5. Yusuf Hamied Dept. of Chemistry, University of Cambridge

Keywords

batteries, energy materials, EELS, STEM, FIB-SEM, focused ion beam, tomography, spectroscopy, cathode

Abstract text

Ni-rich cathode materials for Li-ion batteries such as LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) exhibit high volumetric and gravimetric specific capacities and low cost compared to other, isostructural materials with lower nickel content, which makes NMC811 a strong candidate for new generation of cathodes for Li-ion batteries. However, these layered transition metal oxides suffer from complex degradation mechanisms, where an interplay between lattice parameter changes during cycling, oxygen release at high states of charge, phase transformations at the surface, inter- and intragranular cracking, side reactions with electrolyte and transition metal (TM) dissolution all interact with each other, leading to capacity loss and impedance rise.^1–3

Here, we use strategically designed electrochemical protocols (varying the time at high voltages, upper cut-off voltage, degree of (de)lithiation and number of cycles) aiming to decouple various degradation mechanisms. Based on full cell (NMC811/graphite) cycling data, coupled with area specific impedance from electrochemical impedance spectroscopy and hybrid pulse power characterisation, we show that the time at high voltages (even at 4.3 V) does not cause significant impedance rise, while the most severe cell capacity loss and impedance rise is present when the cells are cycled to >4.2 V during cycling. Moreover, capacity fade and impedance rise are also higher when the high upper cut-off voltage cycling is combined with large state-of-charge changes. 

To further investigate the impedance rise mechanisms, we complement the electrochemical data with electron microscopy of pristine and electrochemically stressed NMC811. We use scanning transmission electron microscopy ((STEM) imaging and electron energy loss spectroscopy (EELS) in a FEI Tecnai Osiris operated at 200 kV to probe the local, nanoscale structure and chemistry of NMC811 particles. Using a comprehensive data analysis approach, we report oxidation state evolution over cycling, where the average degree of TM reduction at the surfaces is correlated with the amount of impedance rise of the cells (in which the NMC811 cathode is the main contributor). Such behaviour points towards the surface reduced layer as the main culprit for impedance rise of NMC811/graphite cells. The EELS and electrochemistry results are supported by FIB-SEM tomography analysis of the samples. Pristine samples exhibit similar levels of cracking at secondary particle level as the cycled ones, which points to electrode manufacturing having a more significant impact on the cracking of NMC811 particles compared to their electrochemical history. 

In summary, by using tailored electrochemical protocols and advanced electron microscopy techniques, we identify the nanoscale changes in the chemistry of the surface layers as the main cause of impedance rise in NMC811/graphite cells. Using EELS, we find evolution of the chemistry of the surface reduced layer during cycling for the first time.  

References

1.        Kondrakov, A. O. et al. Charge-transfer-induced lattice collapse in Ni-rich NCM cathode materials during delithiation. J. Phys. Chem. C 121, (2017).

2.        de Biasi, L. et al. Chemical, Structural, and Electronic Aspects of Formation and Degradation Behavior on Different Length Scales of Ni‐Rich NCM and Li‐Rich HE‐NCM Cathode Materials in Li‐Ion Batteries. Adv. Mater. 31, 1900985 (2019).

3.        Jung, R., Metzger, M., Maglia, F., Stinner, C. & Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNi_xMn_yCo_zO₂ (NMC) Cathode Materials for Li-Ion Batteries. J. Electrochem. Soc. 164, A1361–A1377 (2017).