To increase the reversible capacity of layered lithium nickel-cobalt-manganese oxide, a Li
Full concentration gradient (FCG) cathode material having nickel-rich core Li [Ni 0.89 Rechargeable lithium-ion batteries have received great attention as power sources for portable electronic device, plug-in hybrid vehicles (PHEVs), and electric vehicles (EVs) because of their high energy density, long cycle life, and excellent rate capability. However, commercialization of rechargeable lithium-ion battery system for the automobile industry requires further improvements in energy density and safety. In order to meet these requirements, numerous researches have been in progress for finding new electrode materials, especially cathode materials.1-4 So far, most of the studies have focused on the layered cathode materials, Li[Ni 1−y−z Co y Mn z ]O 2 , which combine the rate performance of LiCoO 2 , the high capacity of LiNiO 2 , and the structural stability imparted by the presence of Mn 4+ . 5 Particularly, the Ni-rich layered composite oxides Li[Ni 1−y−z Co y Mn z ]O 2 (1-y-z ≥ 0.8), are considered one of the most promising cathode materials because they offer higher capacity than currently commercialized cathode materials. 6,7 However, the Ni-rich materials have poor thermal stability due to the oxygen release at the charged state, which can lead to severe thermal runaway and possible explosion. [8][9][10] Moreover, the unstable Ni 4+ ions can be easily reduced to inactive NiO, resulting in increase of the interfacial impedance, and thus poor cycle life. 11,12To overcome these problems of the Ni-rich Li[Ni 1−y−z Co y Mn z ]O 2 layered cathode materials, we have developed a functional lithium nickel-cobalt-manganese oxide cathode material which have coreshell structure or core-shell with concentration gradient. These cathodes are composed of a Mn-rich outer surface providing the outstanding safety and a Ni-rich core delivering a high capacity. These cathodes show high capacity and good cycle life at high voltage cycling, and improved safety characteristics.3,13,14 Recently, we reported a highenergy cathode material with a full concentration gradient (FCG) of Ni, Co, and Mn ions consisting of rod-shaped primary particles grown in radial directions with a crystallographic texture, which improved the rate capability and safety characteristics. 15 Based on the proposed material design concept, we can design various cathode materials with different gradient compositions.In this study, we report newly designed FCG Li [Ni 0.65 ExperimentalThe FCG precursors were synthesized through the co-precipitation method.15 The Ni-poor aqueous solution (Ni:Co:Mn = 0.58:0.10:0.32 in molar ratio) from tank 2was slowly pumped into a Ni-rich aqueous solution (Ni:Mn=0.96:0.04 in molar ratio) in tank 1. The homogeneous mixture was then fed into a continuously stirred tank reactor (CSTR). At the same time, a 4.0 mol dm −3 solution of NaOH (aq.) and the desired amount of NH 4 OH solution (aq.) (chelating agent) under N 2 atmosphere were also separately pumped into the reactor. The molar ratio of ammonium hydroxide to transition metal...
In this work, nickel-rich, layered-structure LiNi 0.65 Co 0.08 Mn 0.27 O 2 cathode materials were synthesized and compared with materials of the same overall composition, but with a concentration gradient throughout the particles: the Ni concentration is higher at the center of the particles and lower at surface, while the opposite is true for the Mn concentration. The co-precipitation synthesis parameters were optimized, with two different annealing protocols for the final products and the effect of chelating agent concentration during synthesis examined. The gradient materials provided superior capacity and rate capability than their respective non-gradient materials, at normal operating potentials and temperatures, e.g. 30 • C up to 4.3 V vs. Li. The reasons for the improved discharge capacity of the gradient materials were explored through impedance spectroscopy and post-mortem characterization. The gradient structure evolution was examined via TEM and electron diffraction measurements of particle cross-sections. Prolonged cycling, even at elevated temperatures, did not change the initial concentration profiles determined by the synthesis. Additionally, long-term cycling experiments of the second-generation material electrodes vs. graphite electrodes in full cells were performed in order to explore the practical advantage of these novel materials.
Li was incorporated into transition metal layers of Ni-rich Li[Ni 0.95 Co 0.05 ]O 2 by formation of a solid solution with Li 2 MnO 3 (layer notation: Li[Li 0.33 Mn 0.67 ]O 2 ), which can be denoted as (1-x)Li[Ni 0.95 Co 0.05 ]O 2 -xLi[Li 0.33 Mn 0.67 ]O 2 , to understand the effect of Li on the structure, electrochemistry and thermal characteristic of the cathode. Structural analysis data obtained by Rietveld refinement of X-ray diffraction data indicate that the additional Li can be found in the transition metal layers of (1x)Li[Ni 0.95 Co 0.05 ]O 2 -xLi[Li 0.33 Mn 0.67 ]O 2 . An interesting feature is that the average oxidation states of Ni and Mn are 3+ and 4+, respectively, for (1-x)Li[Ni 0.95 Co 0.05 ]O 2 -xLi[Li 0.33 Mn 0.67 ]O 2 as demonstrated by X-ray absorption study. The first discharge capacity was approximately 217 mAh g −1 , and the resulting retention was above 90% for 0.9Li[Ni 0.95 Co 0.05 ]O 2 -0.1Li[Li 0.33 Mn 0.67 ]O 2 (Li[Li 0.033 Ni 0.855 Co 0.045 Mn 0.067 ]O 2 ) in the range of 2.7 to 4.5 V for 100 cycles, which is a surprising result for Ni-rich compounds. Moreover, the thermal stability of (1-x)Li[Ni 0.95 Co 0.05 ]O 2 -xLi[Li 0.33 Mn 0.67 ]O 2 was significantly improved over the cathodes with identical Ni fraction. These results highlight the role of tetravalent Mn ions, even in small amounts, in stabilizing the electrochemical performances and thermal properties in the Ni-rich layer cathodes.
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. References [1] Y.-K. Sun, S.-T. Myung, M.-H. Kim, J. Prakash, K. Amine, J. Am. Chem. Soc. 2005, 127, 13411– 13418. [2] Y.-K. Sun, S.-T. Myung, M.-H. Kim, J.-H. Kim, J. Electrochem. Soc. 2006, 9, A171– A174. [3] Y.-K. Sun, S.-T. Myung, B.-C. Park, K. Amine, Chem. Mater. 2006, 18, 5159 –5163. [4] Y.-K. Sun, S.-T. Myung, H.-S. Shin, Y. C. Bae, C. S. Yoon, J. Phys. Chem. B 2006, 110, 6810 –6815.
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