Electrolyte and SEI Decomposition Reactions of Transition Metal Ions Investigated by On-Line Electrochemical Mass Spectrometry

Sophie Solchenbach,Hubert A Gasteiger,Anna Teresa Sophie Freiberg,Roland Jung,Gloria Hong

doi:10.1149/2.0511814jes

Sophie Solchenbach, Hubert A Gasteiger + Show 3 more

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https://doi.org/10.1149/2.0511814jes

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Abstract

We use on-line electrochemical mass spectrometry (OEMS) to elucidate and quantify the electrolyte reduction on graphite caused by transition metal ions. To have a controlled system, we use ethylene carbonate (EC) with 1.5 M LiPF6 and representative amounts of Ni(TFSI)2 or Mn(TFSI)2 as model electrolytes, combined with a 2-compartment cell in which anolyte and catholyte are separated by an impermeable solid lithium ion conductor. Focusing on C2H4 evolution as a marker for EC reduction, we find that both Ni2+ and Mn2+ lead to enhanced gas evolution on pristine graphite electrodes once the potential is decreased to below the TM2+/TM0 redox potential, demonstrating that the reduced transition metals are active toward electrolyte reduction. If the electrodes are preformed in a TM-free electrolyte and subsequently cycled in an electrolyte containing either Mn2+ or Ni2+, the activity of nickel toward electrolyte decomposition is greatly reduced, whereas the electrolyte with manganese still shows a strong ongoing C2H4 generation. The use of vinylene carbonate during formation partially suppresses the gas evolution from manganese. Using OEMS and post-mortem ATR-FTIR, we finally show that reduced manganese can decompose organic SEI components into Li2CO3, thereby compromising the integrity of the SEI and enabling the additional reduction of electrolyte.

Highlights

Transition metal (TM) dissolution is a long-known degradation phenomenon of lithium manganese spinel-type cathode active materials for Li-ion batteries
This represents the situation in commercial Li-ion cells prior to battery formation, where dissolved transition metals are observed upon electrolyte storage of the pristine materials,[26] likely formed by the reaction of cathode active materials with HF, which is present in commercial LiPF6 based electrolytes at the level of several tens of ppm, or at even higher concentration if cell components are dried improperly.[57,58]
We applied on-line electrochemical mass spectrometry (OEMS) to follow the reactions triggered by manganese and nickel that lead to the loss of active lithium and poor capacity retention in full-cells

Summary

Introduction

Transition metal (TM) dissolution is a long-known degradation phenomenon of lithium manganese spinel-type cathode active materials for Li-ion batteries It is amplified by temperature,[1,2] high cathode potentials,[3] and large BET surface area of the particles.[3] As layered lithium nickel cobalt manganese oxide (NCM) cathode materials are cycled to higher cutoff potentials to maximize the energy density of Li-ion cells, transition metal dissolution becomes significant for NCMs.[4,5,6,7,8,9,10,11] Experimental and ab initio modeling studies suggest that the electrochemical oxidation of LiPF6-based electrolytes at high voltages[8,12,13,14,15,16,17] and/or follow-up reactions of the electrolyte with oxygen released from the NCM host lattice[18,19,20,21,22] can generate HF, which corrodes the layered transition metal oxide cathode materials. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract)

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