Surface Lithium Carbonate Influences Electrolyte Degradation via Reactive Oxygen Attack in Lithium-Excess Cathode Materials

Abstract

Lithium-excess layered oxide cathode materials (Li(1+x)TM(1–x)O2) for lithium-ion batteries achieve high specific capacities (≥250 mA h/g) via redox participation of both transition metals and oxygen anions. While oxygen is initially present as O2– in the cathode, oxidized oxygen species such as peroxo-like oxygen (O2 2–) and oxygen gas (O2) are known to form on charge. In this work, differential electrochemical mass spectrometry (DEMS) is used to study the mechanisms by which lithium carbonate, a common impurity, influences how these oxygen species form and react within the battery. We first show, in agreement with prior studies, that Li2CO3 oxidizes electrochemically on charge to evolve CO2, but not O2, implying that reactive oxygen species form instead that then react with cell components to form nonvolatile products. To study the effect of Li2CO3 on degradation processes at the cathode surface, a Li-excess cathode material Li1.17(Ni0.2Mn0.6Co0.2)0.83O2 (NMC) is synthesized using a method that prevents formation of carbonate impurities in the synthesized material. Isotopically tagged lithium carbonate is then deposited on the NMC surface through controlled exposure to 13CO2 or C18O2 gas. DEMS results show that when lithium carbonate is present on the cathode surface, organic fragments containing diatomic oxygen are formed on the cathode surface during charge above 4.2 V versus Li/Li+. Isotopic analysis indicates that the diatomic oxygen within these fragments primarily originates from the NMC lattice, with only a minor fraction originating from the Li2CO3 itself. Our results therefore suggest that reactive oxygen released from the NMC lattice is triggered by the oxidation of surface lithium carbonate.

Oxidative decomposition mechanisms of lithium carbonate on carbon substrates in lithium battery chemistries

“…Here our results show that Li 2 CO 3 decomposition in the ether-based electrolyte is mainly an electrochemical process, which is in good agreement with Kaufman et al 33 , but Freiberg et al claimed that Li 2 CO 3 decomposition was a chemical process in LiPF 6 carbonate-based electrolyte and it was induced by water 41 . They argued that the electrolyte was oxidized and decomposed on high potential and it formed some proton-derivatives which chemically react with Li 2 CO 3 to evolve CO 2 .…”

“…Recently, Freiberg et al claimed that Li 2 CO 3 decomposition in lithium hexafluorophosphate (LiPF 6 )-ethylene carbonate (EC)-ethylmethyl carbonate (EMC) electrolyte follows a chemical route reacting with H + , which is induced by electrolyte oxidation at >4.6 V 41 . In contrast, Mahne et al claimed that Li 2 CO 3 decomposition is an electrochemical process 33 , 46 . Here, our results show that the electrolyte salt affects the route and the chemical reaction is likely caused by LiPF 6 , which will be discussed later.…”

“…In Li-ion batteries, Li 2 CO 3 is one of the main components of the solid electrolyte interphase of the anode and exists as a surface contaminant present on lithium transition metal oxides used in the cathode thus it influences the cell performance 32 . For instance, the lithium transition metal oxides cathode materials are usually covered with a layer of Li 2 CO 3 due to the residual lithium precursors reacting with CO 2 from the ambient atmosphere 33 – 35 . Very recently, McCloskey and co-workers have studied the Li 2 CO 3 decomposition mechanism on Li-ion cathodes.…”

“…Very recently, McCloskey and co-workers have studied the Li 2 CO 3 decomposition mechanism on Li-ion cathodes. Using isotopic labeling, they found that when Li 2 CO 3 is present at the cathode surface, organic fragments containing diatomic oxygen are formed on the cathode surface during the charging process above 4.2 V versus Li + /Li and the diatomic oxygen within these fragments mainly originates from the lithium transition metal oxides lattice and only a minor fraction originates from the Li 2 CO 3 itself 33 – 35 . In summary, the decomposition of Li 2 CO 3 is so important that it determines the electrochemical performance in many systems.…”

Thermal Enhancement of Product Conductivity Permits Deep Discharge in Solid State Li-O2 Batteries

“…Alternatively, the CO2 that evolves from the decomposition of any residual carbonate could react with LiOH, releasing H2O in the process. 37,38 Having stated this, the lack of an identifiable LiOH peak on the discharge product Raman spectra (Figure 3B) suggests that perhaps a less defined hydrated product, rather than pure crystalline LiOH, is involved. This species may relate to the unidentified peak at 675 cm -1 .…”