Impact of Sulfate Adsorption on Particle Morphology during Precipitation of Ni-Rich Precursors for Li-Ion Cathode Active Materials

Abstract

Current standard industrial manufacturing of lithium layered transition metal oxides (LiMO2) as cathode active material (CAM) can be divided into two major process steps: The first step involves coprecipitation of mixed transition metal hydroxide (M(OH)2) particles (M consisting mainly of Ni, Co, and Mn) by mixing a transition metal sulfate solution (MSO4(aq.)) and a sodium hydroxide solution (NaOH(aq.)) in a stirred tank reactor. The resulting hierarchally structured secondary particles are composed of many primary particles which are used as precursors for CAMs (referred to as pCAM). Subsequently, the obtained pCAM is mixed with a lithium source such as Li2CO3 or LiOH and calcined at elevated temperatures to yield LiMO2.1 Recently, reports in the literature have shown clear correlations of CAM morphology with their electrochemical performance in a battery cell.2 Furthermore, it has been demonstrated that the electrochemical performance of a CAM is affected by the morphology of the associated pCAM.3, 4 However, a detailed understanding of the impact of the process parameters on the course of precipitation reaction and on the physical properties of the pCAM precipitate is still lacking.In order to gain a mechanistic understanding of the M(OH)2 pCAM particle formation, ten distinctive Ni0.8Co0.1Mn0.1(OH)2 particle lots were prepared by the coprecipitation method in a stirred tank reactor. The precipitation pH-value was systematically varied between pH = 8.6-12.7, while all other process parameters were kept constant. The S-content of the resulting pCAM powders, representing the residual sulfate (SO4 2-) content, was investigated by S-combustion and is depicted as a function of precipitation pH-value in Figure 1a. In the pH-value range between 8.57 and 10.27 (red), a slight increase in residual SO4 2- from 10.29 mol% to 12.60 mol% is observed until it decreases again to 11.10 mol%. Between pH = 10.27 and pH = 10.74, a sharp transition takes place and residual SO4 2- is reduced by a factor of 4 from 11.10 mol% to 2.60 mol%, which even decreases further with increasing pH-value, namely from 2.60 mol% at pH = 10.74 to 0.35 mol% at pH = 12.69 (green). Interestingly, the turning point in S-content between pH = 10.27 and pH = 10.74 (dashed blue line in Figure 1a) coincides with the point-of-zero-charge (pzc) of Ni(OH)2, which is reported to be at pH = 10.50-10.60.5 This result is rationalized by a pH-dependent SO4 2- adsorption equilibrium that is governing the SO4 2- uptake during the precipitation reaction, as depicted schematically in Figure 1b. For a precipitation below the pzc, positive charged surface hydroxyl-groups of Ni0.8Co0.1Mn0.1(OH)2 attract SO4 2-, resulting in high SO4 2- uptake during M(OH)2 formation and vice versa.In light of these results, it is further demonstrated by x-ray diffraction analysis that SO4 2- adsorption not only governs the crystallinity of the Ni0.8Co0.1Mn0.1(OH)2 material, but also suppresses the vertical crystal growth in the 001-direction during particle formation. This in turn seems to affect the vertical primary particle size as well as the secondary particle porosity, both observable by SEM imaging. The morphological trend is quantitatively verified by extracting the primary particle size distribution from SEM images and by measurements of the secondary particle porosity via nitrogen physisorption. As proof-of-concept for the proposed adsorption mechanism, precipitation reactions at pH = 12.0 with different metal feed sources (MX(aq.), with X = SO4 2-, (NO3 -)2, (CH3COO-)2) were conducted. The resulting clearly distinct physical properties of the Ni0.8Co0.1Mn0.1(OH)2 particles obtained from the different anion systems with different anion adsorption affinities can be well understood based on the Fajans-Paneth-Hahn law for crystallization.6 Finally, desorption experiments indicate options to reduce the residual SO4 2- amount after the Ni0.8Co0.1Mn0.1(OH)2 particle formation has been completed.Based on the results of this study, guidelines for pCAM design are formulated and discussed with respect to composition and subsequent manufacturing steps in industrial CAM production.