Synthesis of Nanostructured Li[Ni1/3Co1/3Mn1/3]O2 via a Modified Carbonate Process

Abstract

Introduction There is great deal of interest in using lithium transition metal oxides, Li[Ni1/3Co1/3Mn1/3]O2, as the positive electrode material for high-energy and high-power lithium-ion rechargeable Li-ion secondary batteries. One of the drawbacks of Li[Ni1/3Co1/3Mn1/3]O2 is that the its synthesis is not easy. An appropriate synthesis method needs to be employed, otherwise the transition metal component often saturates, resulting in oxide impurities such as NiO, Co3O4, Mn2O3, or MnO2. Therefore, selection of an appropriate preparation method is critical to attain a final product with high purity. Here, we report electrochemical properties of nanostructured Li[Ni1/3Co1/3Mn1/3]O2 powder synthesized via a newly modified carbonate process, utilizing a chelating agent. With this new method, nanostructured Li[Ni1/3Co1/3Mn1/3]O2 powder with improved uniformity and superior rate capability can be synthesized.

Experimental Spherical (Ni1/3Co1/3Mn1/3)CO3 was prepared as follows. An aqueous solution of NiSO4, CoSO4, and MnSO4 (cationic ratio of Ni/Co/Mn = 1:1:1) with a concentration of 2.0 mol-3 was pumped into a continuous stirred tank reactor (CSTR, capacity 4 L) under CO2 atmosphere. At the same time, Na2CO3 solution (aq.) of 2.0 mol-3 and desired amount of NH4OH solution (aq.) were also separately fed into the reactor. NH3 2+ Ions are activated by a chelating agent. The solution was maintained at 60℃ for 12 h while closely monitoring the concentration of the solution (2M), pH (7.5), temperature (60℃), and stirring speed (1000 rpm) of the mixture. Then, the spherical (Ni1/3Co1/3Mn1/3)CO3 powder was filtered, washed, and vacuum-dried at room temperature. Transmission electron microscopy(TEM, JEOL 2010) and Powder X-ray diffraction (XRD, Rigaku Rint-2000) employing Cu Kα radiation was used to characterize the prepared powders. Charge-discharge tests were performed with a coin type cell (CR2032) with a current density of 20 mA g-1 at 30℃.

Results and discussion As can be seen in Figure 1a and b, the as-prepared carbonate particles have spherical shapes with an average diameter of 10 ㎛. TEM observation showed that the particles were composed of 5-8-nm sized primary particles. The spherical carbonate particles shown in Figure 1b have a unique structure in which nanosized primary particles were tightly agglomerated into a spherical shape. Figure 2 shows that the discharge capacity at high currents was well retained during cycling. This enhanced discharge capacity at accelerated rates clearly demonstrates the advantages of our nanostructured electrode. Since primary particles are held by comparatively weak secondary bonds, the volumetric changes involved with Li migration can be readily accommodated by relaxation of the binding forces among primary particles.