Particle Surface Cracking Is Correlated with Gas Evolution in High-Ni Li-Ion Cathode Materials.

High-Ni layered oxide cathode materials (LiNixTM(1-x)O2, where x > 0.8) are of great interest because they offer increased capacity compared to current commercial materials within a narrow voltage range. However, recent studies have shown that these materials in their current form suffer from capacity fading when an upper cutoff voltage above 4.3 V vs Li/Li+ is used. While many studies have focused on the H2 → H3 transition as the primary cause of capacity fading, gas evolution studies show that degradation processes cannot be attributed to the H2 → H3 transition alone. In this work, differential electrochemical mass spectrometry (DEMS) is combined with titration mass spectrometry (TiMS) to measure gases evolved in a lithium half-cell during cycling as well as surface species which evolve gas upon addition of strong acid to an extracted cathode. Along with qualitative observations of particle cracking by scanning electron microscopy (SEM), these results reveal correlations between particle cracking, electrolyte reactivity, and carbonate oxidation and deposition on the cathode surface during the first charge of high-Ni cathode materials.

Altering Surface Contaminants and Defects Influences the First-Cycle Outgassing and Irreversible Transformations of LiNi0.6Mn0.2Co0.2O2.

By altering the surface of LiNi0.6Mn0.2Co0.2O2 (NMC622) we show that surface defects and contaminants dominate the outgassing and irreversible surface transformations during the first electrochemical cycle. To alter the surface defects and contaminants without changing the bulk structure of the NMC622, we perform mild methanol and water rinses, a water soak, a water rinse and subsequent heat treatment, as well as purposeful increase of the surface Li2CO3. By combining isotopic labeling; gas analysis; and peroxide, hydroxide, and carbonate titrations we observe that these alterations change the surface Li2CO3, surface hydroxides, and the local defects, which in turn alter the nature and extent of the outgassing to O2 and CO2. Our results highlight that outgassing of Li-ion cathode materials is highly dependent on the synthesis and storage routes and comparison of varying compositions must take into account these differences to make any meaningful conclusions. We also show that simple rinsing procedures may be an effective route to controlling interfacial reactivity of Li-ion active materials.