Transition Metal Dissolution in State-of-the-Art and Next Generation Li-Ion Batteries Studied By Spatially Resolved Operando X-Ray Absorption Spectroscopy
Authors: Freiberg Anna Teresa Sophie , Solchenbach Sophie , Strehle Benjamin , Siebel Armin , Tromp Moniek , Gasteiger Hubert A. 
Published: February 2020

High energy Li-Ion batteries rely on the pairing of a transition metal oxide cathode with a graphite anode. Pushing the upper cut-off voltage and state of charge (SOC) to higher values, the cycle-life of the battery is drastically reduced. Besides electrochemical electrolyte oxidation leading to poor coulombic efficiencies and a large growth of cell impedance, leaching of transition metal ions from the cathode active materials is one of the major obstacles to increase the power density of the cell without change of active materials. While the capacity loss which can be ascribed to the overall loss of active material is minor, cell aging is largely aggravated by liberated transition metal ions present in the electrolyte, as they diffuse through the separator and deposit on the graphite anode due to its low potential. A severe impedance growth of the anode and loss of active lithium by chemical delithiation of the graphite has been observed by many studies. Especially for manganese, the amount of active lithium loss has been found to be a multifold higher than the total amount of manganese ions present in the electrolyte and deposited onto the graphite anode, indicating a catalytic role of transition metal ions on active lithium loss (1).
Comparisons of the transition metal leaching behavior for different cathode active materials is missing so far, as studies vary widely in test procedures (potential range, SOC) and environment (electrolyte and temperature). We herein present operando analysis of the intrinsic stability of several cathode active materials upon charge to high SOC and potential with our unique spectroscopic cell enabling spatially resolved operando X-ray absorption spectroscopy, monitoring the concentration and oxidation state of transition metals in the electrolyte and in the graphite anode (2, 3). Aside layered mixed transition metal oxide materials (Li1+w[NixCoyMnz]1-xO2) also the spinel structure is tested to better understand the effect of crystal destabilization at high SOC. The formerly standard layered oxide NCM111 (x=y=z=0.33) was already studied earlier, focusing on manganese dissolution (3). In this talk we present time and potential resolved analysis of all transition metals released from NCM111 (see Fig. 1) in comparison to a nickel-rich NCM811 (x=0.8, y=z=0.1), a lithium- and manganese-rich layered oxide material (HE-NCM; w=0.17, x=0.22, y=0.12, z=0.66 (4)) and the high-voltage spinel LiNi0.5Mn1.5O4, (LNMO). This data allows conclusions on the extent of the two major causes for transition metal leaching from oxide based cathode materials: chemical decomposition induced by protic electrochemical electrolyte oxidation products (5) and crystal lattice destabilization of layered transition metal oxides caused by high degrees of delithiation (high SOC).

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Acknowledgements

Financial support for S. S. and B. S. by the BASF SE through its Research Network on Electrochemistry and Batteries is gratefully acknowledged. XAS data were gathered at the European Synchrotron Radiation Facility (ESRF) and SOLEIL Synchrotron (France).We are grateful to Dr. Debora Motta Meira at the ESRF, Grenoble (France) for providing assistance in using beamline BM 23 as well as Dr. Emiliano Fonda for assistance on the SAMBA beamline at SOLEIL Synchrotron.

Fig. 1: Transition metal dissolution of an NCM111 (5.5 mAh/cm2)//C6 cell upon cycling in the conventional potential range (0-6.5 h: 60% SOC until ≈4.2 VLi) and upon high degrees of delithiation (7-11 h: ≈100% SOC until 5.1 VLi) measured in our operando XAS cell using glass fiber separators and standard LP57 electrolyte (1 M LiPF6 in EC:EMC [3:7]) at 25 °C. The concentration of transition metal ions within the graphite electrode (filled symbols) and in the electrolyte (hollow stars, shown as electrolyte share in the graphite pores [concentration measured in the separator multiplied by the porosity of the graphite anode]) is monitored operando.

Figure 1