This suggests that reducing the amount of chemically unstable Ni 3+ and Ni 4+ at and near the surface is the key to obtain the surface stability. It has been hypothesized that the reduced nickel atomic concentration on the surface can reduce the undesired surface reactivity but recent reports suggest that it is the high valence states (Ni 3+ and Ni 4+), not the concentration of Ni, that causes the instability. It has been reported by Amine, Sun, and their colleagues that introducing Ni/Mn-concentration gradient is particularly effective for improving the cycle life of NMC materials 20, 22, 23, 24 however, the factors responsible for the improved stability have not been fully understood. Surface coating, concentration gradient, and core–shell structure are the most widely adopted strategies to improve the material surface stability 18, 19, 20, 21.
#Energy density of nmc cathode materials how to
Therefore, how to modify the surface in order to reduce the undesirable interaction of high-nickel cathode materials with the electrolyte, especially during high voltage charging is still a grand challenge. It is well known that lithium nickel-manganese-cobalt oxide (NMC) cathode materials undergo surface degradation to form rock-salt structure or disordered spinel phase on the primary particle surfaces, which subsequently causes the impedance buildup and induces further irreversible structure degradation originated from the surfaces 17.
The surface/near-surface chemistry of layered oxide materials is another determining factor for the cathode’s performance. These cracks block electrical contact which in turn reduce the utilization of active materials. It also has been reported that the anisotropic volume expansion of the layered materials during delithiation can cause the formation of micro-cracks 13, 14 (crack size >100 nm) and nanocracks 15, 16 (crack size <100 nm) at the secondary and primary particle levels. This migration can induce irreversible structure changes and capacity fading 12. For example, Ni 3+ is chemically unstable and tends to be reduced to Ni 2+, which has an ionic radius close to that of Li + 11, Therefore Ni 2+ has the tendency to migrate into the lithium layer when lithium is extracted from the lattice. However, there are remaining problems for this type of materials, such as structure deterioration 3, 4, 5, unclear surface chemistry, thermal instability 6, 7, 8, and capacity fading 9, 10. In contrast, it is hard to oxidize Co to higher valence state than Co 3.6+ without oxygen redox activity 1, 2. Higher nickel content is able to provide higher specific capacity because Ni 2+ can be fully oxidized to Ni 4+. By increasing nickel content in NMC-333, high-nickel cathode materials increase capacity and reduce cost comparing with LCO and NMC-333. To meet the demand of high energy density, low cost, and long cycle life lithium-ion batteries for electric vehicles, high-nickel-content layered cathode materials have attracted intensive research interests for their higher capacity compared with widely commercialized materials such as LiCoO 2 (LCO) and LiNi 1/3Mn 1/3Co 1/3O 2 (NMC-333).
The result suggests creating an oxidation state gradient that hides the more capacitive but less stable Ni 3+ away from the secondary particle surfaces is a viable principle towards the optimization of high-nickel content cathode materials. The nickel valence gradient material shows superior cycling and thermal stability than the conventional one. To isolate nickel’s valence gradient effect and understand its fundamental stabilization mechanism, we design and synthesize a LiNi 0.8Mn 0.1Co 0.1O 2 material that is compositionally uniform and has a hierarchical valence gradient. Even though promising, the fundamental mechanism of the nickel concentration gradient’s stabilization effect remains elusive because it is inseparable from nickel’s valence gradient effect. To circumvent this problem, nickel concentration-gradient materials have been developed to enhance high-nickel content cathode materials’ thermal and cycling stability. However, the structural and surface instability may cause poor capacity retention and thermal stability of them. High-nickel content cathode materials offer high energy density.
0 kommentar(er)
