Rechargeable lithium-ion batteries have great potential as a new large-scale power source for electric vehicles (EVs) due to their high energy density, high voltage, and long cycle life. However, commercialization of these batteries for the automobile industry requires further improvements in energy density and safety. Meeting this challenge has centered on finding new high-capacity electrode materials, especially cathode materials. A layered lithium transition metal oxide, Li[Ni_xCo_yMn_z]O₂ (x+y+z = 1) have been extensively utilized and studied as cathode materials for lithium-ion batteries due to their high capacity [1]. It is well known that an increase of the Ni content in the Li[Ni_xCo_yMn_z]O₂ results in an increase of capacity and total residual lithium content, but a decrease in capacity retention and safety [2]. To resolve the aforementioned drawbacks and to overcome the inherent chemical instability of Ni-rich layered cathodes and to improve capacity retention, we have hybridized Ni-rich and Ni-deficient Li[Ni_xCo_yMn_z]O₂ cathodes at the particle level by establishing concentration profiles within a single particle such that the Ni-rich center maximizes the capacity while the Ni-deficient particle surface ensures high thermal and structural stability in the form of a core-shell (with gradient) [3], full concentration gradient (FCG) [4], and two-slope full concentration gradient (TSFCG) lithium nickel-cobalt-manganese oxide [5]. In addition to increasing the structural stability, Al was incorporated into the FCG and TSFCG cathodes [6]. All the hybridized Li[Ni_xCo_yMn_z]O₂ cathodes demonstrated better electrochemical and thermal properties than their corresponding convention Li[Ni_xCo_yMn_z]O₂ cathodes without concentration (Figure 1).

References

1. J. Katana Ngala, et al, J. Mater. Chem. 14, 214-220, 2004.

2. H.-J. Noh, et al, J. Power Sources, 233, 121-130, 2013.

3. Y.-K. Sun, et al, Nat. Mater. 8, 320-324, 2009.

4. Y.-K. Sun, et al, Nat. Mater. 11, 942-947, 2012.

5. B.-B. Lim, et al, Adv. Fun. Mater. 25, 4673-4680, 2015.

6. U.-H. Kim, et al, Adv. Energy Mater. 6, 1601417, 2016.

Figure 1. SEM and TEM images of hybridized Li[Ni_xCo_yMn_z]O₂ cathodes and their cell performances.