Concentration Gradient Core-Shell Li[Ni0.6Co0.15Mn0.25]O2 for Rechargeable Lithium Batteries: Comparative Study of the Shell Morphologies of Nanoparticles and Nanorods

Abstract

The structure, electrochemistry, and thermal stability of concentration gradient core-shell (CGCS) particles with different shell morphologies were evaluated and compared. We modified the nanoparticles to nanorods in the shell since nanorods can result in a reduced surface area of the shell such that the outer shell would have less contact with the corrosive electrolyte, resulting in improved electrochemical properties. Electron microscopy studies coupled with electron probe X-ray micro-analysis revealed the presence of a concentration gradient shell consisting of nanoparticles and nanorods before and after thermal lithiation at high temperature. Rietveld refinement of the X-ray diffraction data and the chemical analysis results showed no variations of the lattice parameters and chemical compositions of both produced CGCS particles except for the degree of cation mixing (or exchange) in Li and transition metal layers. As anticipated, the dense nanorods present in the shell gave rise to a high tap density (2.5 g cm-3) with a reduced pore volume and surface area. Intimate contact among the nanorods is likely to improve the resulting electric conductivity. As a result, the CGCS Li[Ni0.6Co0.15Mn0.25]O2 with the nanorod shell retained approximately 85.5% of its initial capacity over 150 cycles in the range of 2.7–4.5 V at 60oC. The charged electrode consisting of Li0.16[Ni0.6Co0.15Mn0.25]O2 CGCS particles with the nanorod shell also displayed a main exothermic reaction at 279.4oC releasing 751.7 J g-1of heat. Due to the presence of the nanorod shell in the CGCS particles, the electrochemical and thermal properties are substantially superior to those of the CGCS particles with the nanoparticle shell. A few alternatives have been introduced by our group including Ni-rich Li[Ni0.74Co0.08Mn0.18]O2 core-shell (CS) particles engineered through rearrangement of oxidation states of transition metal elements, in which the inner core (12 μm in diameter) is composed of Li[NiIII 0.8CoIII 0.1MnIII 0.1]O2 to deliver a high capacity while the outer shell (1 μm in thickness) consists of Li[NiII 0.5MnIV 0.5]O2 to provide structural and thermal stabilities. 1- 4 As expected, the CS particles possessed superior cyclability and thermal stability with the help of the Li[Ni0.5Mn0.5]O2 shell. A subsequent trial was performed to further improve the capacity and thermal stability by varying the concentration of transition metal elements in the shell which was approximately 2 μm thick with a chemical composition of Li[Ni0.64Co0.18Mn0.18]O2. Although the diameter of the Li[Ni0.8Co0.1Mn0.1]O2 core is smaller than that of the CS particles, the gradual compositional change from Li[Ni0.8Co0.1Mn0.1]O2 to Li[Ni0.46Co0.23Mn0.31]O2 in the shell is responsible for the compensation of the capacity derived from the core. In addition, the presence of more stable NiIIand the lower concentration of NiIII at the surface, in addition to the rich presence of MnIV in the shell, led to an improved thermal stability relative to the core-shell. Further efforts have been made to confirm the effectiveness of the concentration gradient, where the nickel concentration decreases linearly and the manganese concentration increases gradually from the center (Li[Ni0.86Co0.10Mn0.04]O2) to the outer surface (Li[Ni0.70Co0.10Mn0.20]O2) with an average composition of Li[Ni0.75Co0.10Mn0.15]O2. This structure also exhibited a high capacity due to the nickel-rich core as well as high structural and thermal stabilities due to the manganese-rich outer layers and therefore, a long life.