The experiment is carried out in a 1L CSTR with water bath jacket (60 oC) to synthesize Ni1/3Mn1/3Co1/3(OH)2. The reaction proceeds with the addition of 5.0 M NH3(aq) at 15 mL/h and 2.0 M MSO4 (M = Ni, Co, and/or Mn in desired ratios) at 20 mL/h.(1) A pH meter with feedback to the pump is used to monitor the pH value, and 4.0 M NaOH solution is automatically added to the reaction contents by the pump to maintain the desired pH. Samples are prepared with two solution pH levels of 10.6 and 11.4. An overhead rotator is used to stir the solution, and nitrogen gas continuously bubbled into the solution throughout the synthesis. Sample synthesized in the pH 10.6 solution is referred as sample A, while the one synthesized at the high pH level of 11.4 is referred as sample B. The total reaction time is 3 hrs.
Ex-situ samples are drawn from the reactor with pipette during the reaction at certain timer intervals. These samples are immediately diluted in vials to avoid further co-precipitation reaction before characterizing their particle size distribution using particle size analyzer (Cilas 1190LD, Cilas Particle Size).
Results and Discussion
Our analysis focuses on the growth kinetics of the hydroxide primary and secondary particles. Figure 1 (black-square line) shows the change in the median particle size of sample A as a function of reaction time. The secondary particle grows from 1.5 to 16 µm in 3 hours. Three stages of particle growth could be observed: (I) growth and agglomeration of primary particles (before 45 mins); (II) mixed stage (45~100 mins); and (III) surface smoothing of secondary particles (after 100 mins). In stage I, primary particles are formed through instant nucleation, growth, and agglomeration after the addition of transition metal sources. The growth rate is as high as 11.0 µm/h, which is due to the immediate agglomeration of newly formed primary particles, decreasing their surface energy. In contrast, the growth rate of the particles dropped significantly to 0.9 µm/h in stage III. During this stage, the newly added transition metal ions form smooth layers in the void space of the secondary particles surfaces, which consists of disordered nanoplates. This could be proven by the SEM images (not shown here). In stage II, the growth rate of the secondary particles is about 3.2 µm/h, representing a mixed effect of stage I and stage III. This suggests that the growth and agglomeration of the primary particles as well as the surface smoothing of the secondary particles occur at the same time. The change in the growth mechanism is directly related to the concentration of total transition metal ions and hydroxide in the reactor. A quick calculation shows the concentration of transition metal is 0.05 M and 0.1 M at the first turning point (1 hour) and second turning point (2 hour), respectively.
Also shown in Figure 1 (red-circle line) is the change in the secondary particle size as a function of reaction time for sample B synthesized at pH 11.4. In this case, the secondary particles grow from 1.5 to 8 µm in 3 hours. Unlike sample A, only two stages of particle growth can be observed: (I) growth and agglomeration of primary particle at 5.1 µm/h for reaction time smaller than 40 mins; and (II) surface smoothing of the secondary particles at 0.9 µm/h for longer reaction time. The lack of the mixed stage in sample B indicates the primary particle agglomeration is not taking place on the formed secondary particles after 40 mins. This observation is also supported by the SEM images.. The concentration of transition metal in the reactor at the turning point (i.e. 40 mins) is about 0.03 M.
- A. van Bommel and J. R. Dahn, Chem Mater, 21, 1500 (2009).
