Electrolyte Reactivity at the Charged Ni-Rich Cathode Interface and Degradation in Li-Ion Batteries.

Temprano, Israel 
Allen, Jennifer P 
Björklund, Erik 
O'Keefe, Christopher A 

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The chemical and electrochemical reactions at the positive electrode-electrolyte interface in Li-ion batteries are hugely influential on cycle life and safety. Ni-rich layered transition metal oxides exhibit higher interfacial reactivity than their lower Ni-content analogues, reacting via mechanisms that are poorly understood. Here, we study the pivotal role of the electrolyte solvent, specifically cyclic ethylene carbonate (EC) and linear ethyl methyl carbonate (EMC), in determining the interfacial reactivity at charged LiNi0.33Mn0.33Co0.33O2 (NMC111) and LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes by using both single-solvent model electrolytes and the mixed solvents used in commercial cells. While NMC111 exhibits similar parasitic currents with EC-containing and EC-free electrolytes during high voltage holds in NMC/Li4Ti5O12 (LTO) cells, this is not the case for NMC811. Online gas analysis reveals that the solvent-dependent reactivity for Ni-rich cathodes is related to the extent of lattice oxygen release and accompanying electrolyte decomposition, which is higher for EC-containing than EC-free electrolytes. Combined findings from electrochemical impedance spectroscopy (EIS), TEM, solution NMR, ICP, and XPS reveal that the electrolyte solvent has a profound impact on the degradation of the Ni-rich cathode and the electrolyte. Higher lattice oxygen release with EC-containing electrolytes is coupled with higher cathode interfacial impedance, a thicker oxygen-deficient rock-salt surface reconstruction layer, more electrolyte solvent and salt breakdown, and higher amounts of transition metal dissolution. These processes are suppressed in the EC-free electrolyte, highlighting the incompatibility between Ni-rich cathodes and conventional electrolyte solvents. Finally, new mechanistic insights into the chemical oxidation pathways of electrolyte solvents and, critically, the knock-on chemical and electrochemical reactions that further degrade the electrolyte and electrodes curtailing battery lifetime are provided.

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NMC, Ni-rich cathode, degradation, electrolyte reactivity, ethyl methyl carbonate, ethylene carbonate, lattice oxygen, lithium-ion batteries
Journal Title
ACS Appl Mater Interfaces
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American Chemical Society (ACS)
Faraday Institution (FIRG024)
Faraday Institution (via University Of St Andrews) (NEXGenna)
Engineering and Physical Sciences Research Council (EP/P024947/1)
Engineering and Physical Sciences Research Council (EP/R00661X/1)
The present research has been supported by the Faraday Institution degradation project (FIRG001) and EPSRC (EP/S003053/1). W. M. D., M. F. L. D., and C. P. G. acknowledge funding from Cambridge Royce facilities grant EP/P024947/1 and Sir Henry Royce Institute grant EP/R00661X/1. J. P. A. acknowledges financial support from NSERC. C. A. O. acknowledges support from the Faraday Institution next generation Na-ion batteries project (FIRG018). The authors are grateful to A. Jansen, S.E. Trask, B.J. Polzin, and A.R. Dunlop at the U.S. Department of Energy’s CAMP (Cell Analysis, Modeling, and Prototyping) Facility, Argonne National Laboratory, for producing and supplying the electrodes in this work. We acknowledge the EPSRC National Facility for XPS (“HarwellXPS”), operated by Cardiff University and UCL, under Contract No. PR16195. We thank Nigel Howard for assistance with the ICP-OES measurements.