Review—Gassing Mechanisms in Lithium-ion Battery

Abstract

This paper provides a holistic view of the different studies related to gassing in NMC/graphite lithium-ion batteries over the past couple of decades of scientific development. It underlines the difficulty of predicting the concentration and the proportion of gas released upon cycling and storage and to get a clear mechanistic insight into the reduction and oxidation pathways of electrolyte solvents, the thermal electrolyte degradation, as well as the reactions that involve secondary sources such as water, NMC surface species and cross-talk reactions. Though many relevant experiments such as operando gas analysis using isotope-labeled solvents or two-compartment cells have been conducted, they failed, for instance, to determine the exact mechanism leading to the generation of CO and CO2 gas. Last but not least, this paper discusses different strategies that are currently proposed to reduce or eliminate gassing such as the use of electrolyte additives that enable singlet oxygen quenching or scavenging, NMC coatings that limit the contact with electrolyte and different lithium salts to prevent thermal electrolyte degradation.

Beyond Composition: Surface Reactivity and Structural Arrangement of the Cathode–Electrolyte Interphase

“…Likewise, in situ NMR spectroscopy clearly shows the formation of OPF 2 (OCH 3 ) from the reaction between DMC and POF 3 upon cycling spinel cathodes . A wide variety of other electrolyte fragments and salt decomposition products are observed upon cycling above 4.3 V due to reaction with oxygen and other reactive byproducts that have not been described in depth here but may play a role in CEI stability at high potential. ,,,,, Once the system exceeds 4.5 V, the solvent may also undergo electrochemical oxidation (in addition to chemical oxidation from reactive oxygen species), generating more protic species (ROH) that exacerbate LiPF 6 decomposition, HF formation, and transition metal dissolution. ,,,, These protons are reduced at the anode and released as H 2 gas, which leads to swelling in pouch cells and compromised safety when cycled >4.5 V. , …”

“…68 A wide variety of other electrolyte fragments and salt decomposition products are observed upon cycling above 4.3 V due to reaction with oxygen and other reactive byproducts that have not been described in depth here but may play a role in CEI stability at high potential. 25,27,54,93,100,166 Once the system exceeds 4.5 V, the solvent may also undergo electrochemical oxidation (in addition to chemical oxidation from reactive oxygen species), generating more protic species (ROH) that exacerbate LiPF 6 decomposition, HF formation, and transition metal dissolution. 6,58,67,144,148 These protons are reduced at the anode and released as H 2 gas, which leads to swelling in pouch cells and compromised safety when cycled >4.5 V. 159,167 From this insight there are obvious routes forward to mitigate electrolyte decomposition in high-energy cathodes for LIBs, and we can project how these characterization methods may adapt to understand the resulting CEI.…”

Seesaw Effect of Substitutional Point Defects on the Electrochemical Performance of Single-Crystal LiNiO2 Cathodes

“…Finally, differential electrochemical mass spectrometry (DEMS) measurements were performed to examine the differences in the gassing behavior, which has also been linked to the degradation of LNO at high potentials. , At the cathode side, CO 2 is the most prominently evolved gas, for which there are three main sources. − First, surface carbonate impurities formed by exposure of the cathode material to humidity and atmospheric CO 2 decompose under CO 2 evolution, especially in the initial cycle. − Second, oxygen, which subsequently leads to chemical oxidation of the electrolyte, results mostly in CO 2 and, to a lesser extent, O 2 evolution. ,, Lastly, electrochemical oxidation of the electrolyte also leads to the decomposition of the solvent molecules under the CO 2 evolution. However, for ethylene carbonate (EC) as the most common electrolyte solvent, the onset of this decomposition has been determined to be around 4.6 V vs Li + /Li. − Figure a–c shows the CO 2 evolution profiles for the three materials previously discussed, as obtained via DEMS during cycling of LIB half-cells at C /10 rate between 3.0 and 4.5 V. Table S2 (Supporting Information) reports the specific capacities and CO 2 amounts obtained in each cycle and for each material.…”

Elucidating Gas Evolution of Prussian White Cathodes for Sodium‐ion Battery Application: The Effect of Electrolyte and Moisture

“…Two other species commonly discussed in gas evolution studies are CO (m/z = 28) and C 2 H 4 (m/z = 26 and 28), with the former formed together with CO 2 during electrolyte decomposition and the latter generated during SEI formation from EC. [67][68][69][70] Indeed, as shown in Figure S4a (Supporting Information), a strong m/z = 28 signal is observed during overcharge. However, CO is also formed as a fragment of CO 2 or DEC in the mass spectrometer, which has to be corrected for.…”

“…[57,58,65,66] During battery operation, reactions of and between electrode materials and electrolyte, such as the SEI formation or the release of lattice oxygen from charged layered oxides, with subsequent oxidation of electrolyte, result in gas evolution, which can be studied in-situ via differential electrochemical mass spectrometry (DEMS). [67][68][69][70] While the gas evolution of LIBs has been studied in great detail, investigations of SIB gassing are still relatively sparse. [67,69,71,72] In previous works on highentropy PBA and PW CAMs for SIBs, some of us have performed DEMS measurements, in which not only the commonly evolved gases H 2 and CO 2 , but also cyanogen [(CN) 2 ] (assumed to be the product of oxidative dimerization of CAM anions, similar to the release of O 2 from layered oxides) were observed.…”