Roles of Mn and Co in the Air Synthesizability of Layered Oxide Cathodes for Lithium-Based Batteries

Abstract

High-nickel layered oxides (LiNi1–x–y Mn x Co y O2) are the prevailing cathode materials for high-energy density lithium-based batteries, but they are plagued with deleterious surface air instabilities stemming from residual lithium formation. These issues severely hinder mass production as cathode calcination is limited to a flowing oxygen atmosphere, which entails high manufacturing costs as opposed to simpler and more economical air calcination. While higher Ni contents are known to worsen air instabilities, the influence of Mn and Co contents in impacting these phenomena is less elucidated. We herein present the synthesis in ambient air and flowing oxygen atmospheres of three cathode variants with the same Ni contents but varying Mn and Co contents: LiNi0.7Mn0.3O2, LiNi0.7Mn0.15Co0.15O2, and LiNi0.7Co0.3O2. It is found that the critical parameter influencing the air stability of the cathodes is the average Ni oxidation state, which is greatly dependent on the Mn and Co contents. Substitution of Mn for Ni drives down the Ni oxidation state as Mn exists as Mn4+ and reduces surface residual lithium formation, which vastly improves the overall air stability and therefore the synthesizability in air but with a penalty of lowered capacity. In contrast, substitution of Co for Ni maintains Ni3+ as Co exists as Co3+, offering increased initial capacity, but worsens the air stability and cyclability as the driving force for residual lithium formation and surface reactivity is increased.

All-Dry Synthesis of NMC from [Ni,Mn,Co]3O4 Spinel Precursors

“…This causes an increase in Ni 3+ concentration, a reduction in the Ni 2+ , and consequently a reduction in Li + /Ni 2+ mixing. 35,36 An accompanying effect is a decrease in unit cell volume, which is observed here as well (compare Tables S2 and S5). The capacity retention of these samples, shown in Fig.…”

“…This is a well-known result of oxygen sintering since it results in a higher concentration of Ni 3+ in the NMC, reducing the concentration of Ni 2+ , and thereby reducing the Li + /Ni 2+ mixing. 35,36 NMC622 samples made from x = 0.070 and 0.091 precursors have slightly higher but comparable inhomogeneity relative to air sintering. The NMC622 samples made from x = 0.048 has smaller inhomogeneity compared to air sintering, and on par with the other two precursor compositions (compare Figs.…”