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

Abstract

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 LiNi 0.33 Mn 0.33 Co 0.33 O 2 (NMC111) and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (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/Li 4 Ti 5 O 12 (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.

Advances in studying interfacial reactions in rechargeable batteries by photoelectron spectroscopy

“…Ni-rich, Li-rich materials) and can react in ambient air or in contact with the electrolyte. 19,54,55 The nano-length-scales of the interphases formed at battery electrodes, and their intricate chemical structure are the main reasons why their properties and relations to performance remain so elusive, but also why XPS is commonly the chosen technique to characterise them.…”

Transition metal dissolution mechanisms and impacts on electronic conductivity in composite LiNi0.5Mn1.5O4 cathode films

“…8,77 The understanding that Mn dissolution at high voltages in LNMO is driven by electrolytefacilitated reduction of surface transition metal centers can help us understand the similarities and differences in transition metal dissolution between spinels and layered oxide cathode materials (e.g., NMC) that also have highly reactive surfaces and undergo structural rearrangement during electrochemical cycling. 78,79 During NMC charging, electrolyte oxidation at the particle surface results in a concomitant release of oxygen that results in the production of HF 37,80,81 as well as a reduction of surface transition metals that forms an oxygen-depleted rocksalt layer composed of Ni 2+ , Mn 2+/3+ and Co 2+ . [82][83][84] Thus, in both cathode structures, the surface reconstruction process provides both a source of reduced transition metals and HF needed for metal dissolution, but the extent of dissolution will be highly dependent on the exact composition of the surface layer, which is typically determined by the original stoichiometry of the active material.…”

“…82,87,88 Preferential Mn dissolution over Ni and Co has been observed in NMC cells and often deviates from what might be expected based on the stoichiometry of the material. [80][81][82]88,89 This preferential Mn dissolution has been attributed to the higher solubility of Mn 2+ in non-aqueous electrolytes compared to Ni and Co 10,81,82 as well as the tendency of surface Ni in Ni-rich NMCs to form a stable NiO rocksalt phase. 88,90 One study on the comparative aging between spinels and layered cathodes showed that LMO/LNMO exhibit at least 3´ more transition metal dissolution compared to NMC-type cathodes during aging, 40,44 but how transition metal dissolution proceeds with different cathodes during electrochemical cycling is still not well understood.…”

Surface reduction in lithium- and manganese-rich layered cathodes for lithium ion batteries drives voltage decay

“…Such discrepancies heavily complicate the interpretation of different suggested degradation mechanisms. [29][30][31][32] Thus, it is important to understand overarching degradation patterns that are inherent to Li-and Mn-rich cathodes irrespective of synthesis routes, and under well-controlled characterisation conditions.…”