Chemical Weathering of Layered Ni-Rich Oxide Electrode Materials: Evidence for Cation Exchange

Abstract

Lithiated ternary oxides containing nickel, cobalt, and manganese are intercalation compounds that are used as positive electrodes in high-energy lithium-ion batteries. These oxides undergo changes, when they are stored in humid air or exposed to moisture, that adversely affect their electrochemical performance. There is a new urgency to better understanding of these "weathering" mechanisms as manufacturing moves toward a more environmentally benign aqueous processing of the positive electrode. Delithiation of the oxide and the formation of lithium salts (such as hydroxides and carbonates) coating the surface, are known to occur during moisture exposure. The redox reactions which follow this delithiation are believed to trigger all the other transformations. In this article we suggest another possibility: namely, the proton -lithium exchange. We argue that this hypothesis provides a simple, comprehensive rationale for our observations, which include contraction of the c-axis (unit cell) lattice parameter, rock salt phase formation in the subsurface regions, presence of amorphous surface films, and the partial recovery of oxide capacity during electrochemical relithiation. The detrimental effects of water exposure need to be mitigated before aqueous processing of the positive electrode can find widespread adoption during cell manufacturing. Improving the performance of lithium-ion batteries (LIBs) for electrically powered vehicles gives urgency to the development of high-capacity, high-voltage electrode materials, and layered Ni-rich oxides are presently among the most technological advanced materials that allow operation above +4.0 V vs Li + /Li. 1 Such materials (also known as NCMxyz materials) have the general composition of LiNi x/10 Co y/10 Mn z/10 O 2 , where x+y+z = 10. In these ternary oxides, the manganese stays in the Mn 4+ state during cell cycling, and the capacity mainly originates from the Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ redox couples. The monolayers of octahedra containing transition metal (TM) ions form a structural scaffold with the layers of mobile Li + ions placed in between. The crystalline material retains the structure of the progenitor material of the family, viz. α-LiCoO 2 , 2 with the unit cell belonging to the rhombohedral R(-3)m space group. During electrochemical cycling, the lithium ions intercalate into (and deintercalate from) the layered crystals as the redox state of (mainly the) nickel ions changes to provide the overall charge neutrality.In this study, we examine one such material, NCM523, which represents a particular trade-off between the cycling rate (that improves with Co content), safety (that improves with Mn content), and capacity (that increases with Ni content).1 The safety concerns mainly relate to the thermal and chemical stability, including oxidation of the organic electrolyte by catalytic centers at the oxide surface 3 and phase transitions in the delithiated material that occur when a significant fraction of nickel ions is reduced to Ni 2+ ions and molecular oxygen ...

Effect of Ambient Storage on the Degradation of Ni-Rich Positive Electrode Materials (NMC811) for Li-Ion Batteries

“…A similar initial potential peak was reported in the literature upon storage of NMC532 21 and LiNiO 2 . 22 The (partial) decomposition of the surface impurity film is supported by the absence of the initial potential peak feature in the second and third charge cycle (Figure 1).…”

“…This underlines the significant damage to the material after one year of storage at ambient air. Interestingly, the specific capacity of the 1-year old NMC increases significantly over the first ∼25 cycles, analogous to what was reported for NMC532 21 over the course of a short 4-cycle test. We believe this is due to a more incomplete decomposition of the carbonate species within the first cycles, as the upper cutoff voltage was reduced compared to the data in Figure 1 (note that the initial voltage peak on the 1-year aged NMC811 electrode was at 4.22 V vs. Li/Li + , which is slightly above the upper cutoff voltage used for Figure 6).…”

Editors' Choice—Growth of Ambient Induced Surface Impurity Species on Layered Positive Electrode Materials and Impact on Electrochemical Performance

“…Importantly, it should be noted that the Ni 2p line shapes for materials such as Ni 3+ OOH and Ni 2+ (OH) 2 , which could form upon proton exchange or insertion, are shifted to higher energies compared to NiO, and are similar to Ni 3+ in layered oxides. 46 While the Ni 2p lineshape for samples annealed at temperatures ≥350…”

“…7) to produce new LiHCO 3 , Li 2 CO 3 and H 2 O. 1,46,14,48,49 LiOH + CO 2 → LiHCO 3 [4] 2LiOH [7] Since many of the reactions between surface impurities (Eqs. 5-7) produce water as a by-product, these mechanisms of surface impurity development would likely be autocatalytic and would continue until the source of Li or CO 2 is impeded.…”

Cooling Induced Surface Reconstruction during Synthesis of High‐Ni Layered Oxides

“…Such surface reconstruction, involving a Li-deficient layer as well as Li 2 CO 3 at the surface, is detrimental to the electrochemical performance as reported previously, [28,[44][45][46] and may even become dominant. Such surface reconstruction, involving a Li-deficient layer as well as Li 2 CO 3 at the surface, is detrimental to the electrochemical performance as reported previously, [28,[44][45][46] and may even become dominant.…”