The effects of process parameters on the properties of manganese-rich carbonate precursors: A study of co-precipitation synthesis using semi-batch reactors
Introduction
The development of next-generation cathode materials for lithium-ion batteries is critical to enable full implementation of energy storage into the grid and transportation sectors, a fact emphasized by the award of the 2019 Nobel Prize in Chemistry for the development of lithium-ion batteries (LIBs). The most common cathodes in today’s LIBs are transition metal oxides with compositions of LiNiaMnbCocO2 (referred to as NMCs). As demand for new and improved technology continues to grow, critical factors, such as cost and safety, begin to play a deciding role. Lithium- and manganese-rich oxides xLi2MnO3•(1-x)LiMO2 (M = Ni, Mn, Co) (LMR-NMC) are highly lauded as a viable candidate for a next-generation cathode material because of their inherent safety, low cost, and high reversible capacities (>250 mAh/g) (Lu et al., 2002, Thackeray et al., 2018, Thackeray et al., 2005).
Precursors for LMR-NMC cathodes are generally synthesized via co-precipitation using a continuously stirred tank reactor (CSTR). This method has been widely adopted for a variety of reasons. First, it is easier to achieve homogenous mixing of transition metals at the particle level with a co-precipitation method than with a traditional solid-state synthesis. This means that each secondary particle achieves the target transition metal ratio and is neither highly deficient nor highly enriched in any of the transition metals, as has been observed previously using traditional solid-state synthesis (see Fig. 2d in (Dong et al., 2018)). Additionally, the use of a CSTR produces precursors (and final oxides) with spherical particles, which enable high packing densities—a property critical for practical applications such as electric vehicles where volume dedicated for the battery is limited. The two most common precursors made by the co-precipitation method are carbonates and hydroxides. For LMR-NMC materials, the more common precursor is carbonate, although hydroxide precursors have been explored (Xu et al., 2015). The carbonate pathway is very appealing because it is easy to obtain a product that does not contain Mn3+ impurities (e.g., MnOOH and Mn3O4), a result not easily achieved when synthesizing Mn-rich hydroxides (Wang et al., 2013, Xu et al., 2015, Zhou et al., 2010). In the hydroxide pathway, Mn3+ impurities form with relative ease, as Mn(OH)2 readily oxidizes when exposed to air at any stage of production (Zhou et al., 2010). The co-precipitation of carbonates is, therefore, distinctly advantageous when synthesizing precursors for Mn-rich materials.
Several studies have used Mn-rich carbonates (of various compositions) synthesized via CSTR to achieve morphologies that are practically relevant for studying some aspect of LMR-NMCs (Chen et al., 2013, Deng et al., 2009, Lee et al., 2006, Park et al., 2008). Other groups have investigated the performance of LMR-NMCs made with carbonate precursors having some unique morphology, such as a porous structure (Qiu et al., 2017) or hollow microspheres (Jiang et al., 2013). Although useful in some regards, these studies generally offer very little in terms of understanding how the various parameters of co-precipitation (i.e., feed rates, concentration of solutions, temperature, pH, etc.) affect the properties (i.e., tap density, composition, phase purity, etc.) of the resultant carbonate. However, several reports do exist that provide a glimpse into the complexities of producing Mn-rich carbonates via CSTRs. For instance, Wang et al. calculated an optimal pH range that ensures phase purity (Wang et al., 2011). Several studies have investigated the composition and morphology during the early stages of growth and found that a minimum reaction time is required to achieve the desired composition and to obtain spherical particles (Pimenta et al., 2017, Wang et al., 2011). A recent study has also demonstrated that multiple parameters can independently affect the morphology of the precursor, such as pH and solution feed rate (Huang et al., 2019).
Although CSTR co-precipitation synthesis is appealing for the control it offers, one of its drawbacks for lab-scale development is the high level of waste produced. Reaching steady-state reaction conditions requires the volume of the reactor to be “flushed” several times with incoming reactants. This process can be especially wasteful during compositional screening/optimization. In contrast, use of a semi-batch mode, which does not require the reaction to reach steady state, greatly reduces the amount of waste during co-precipitation. However, few results have been reported on how semi-batch synthesis parameters influence the properties of carbonate precursors. Hence, the objective of this work is to examine how varying the synthesis conditions will affect the properties of Mn-rich carbonate precursors produced under semi-batch mode and provide useful guidance for more efficient precursor development. Therefore, several parameters were isolated to gain perspective on the degree of influence each parameter has on the desired properties.
Section snippets
Continuously stirred vs. semi-batch operation
We begin by describing the differences between a co-precipitation reaction carried out in a semi-batch mode versus continuously stirred mode (hereafter referred to as semi-batch and CSTR). Both types of reactions can be carried out using the same tank reactor. The key difference between them is the extent of mass exchange that occurs. To understand this difference, we first need to distinguish between a batch and a semi-batch reaction. A batch reaction is by nature a transient closed system
Baseline CSTR
A baseline sample, with a target composition of Mn0.53Ni0.28Co0.19CO3, was prepared with the intent of producing an LMR-NMC cathode of final composition 0.25Li2MnO3•0.75LiMn0.375Ni0.375Co0.25O2. Briefly, 2 M MSO4, 0.5 M NH3 (aq.), and 2 M Na2CO3 solutions were used for the reaction. A setpoint pH value of 8.3, stirring speed of 1000 RPM, and a constant temperature of 35 °C were used. The flow rates were controlled such that the residence time for the baseline reaction was 6 h. (The residence
Conclusions
The effect of various synthesis parameters for a co-precipitation reaction using a semi-batch method were investigated. The following observations were made.
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We found that within the tested pH range (at 35 °C), higher pH values led to more homogenous particle shapes and increased particle size and tap density. Yet, if the pH was taken too high, agglomeration occurred, leading to a dramatic increase in particle size.
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Although the ammonia concentration did not change the particle shape greatly, the
CRediT authorship contribution statement
Noko Ngoepe: Writing - original draft, Conceptualization, Investigation. Arturo Gutierrez: Writing - review & editing, Conceptualization, Investigation. Pallab Barai: Writing - review & editing, Formal analysis. Jiajun Chen: Investigation, Validation. Phuti E. Ngoepe: Conceptualization, Funding acquisition, Project administration. Jason R. Croy: Conceptualization, Funding acquisition, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Support from the U.S. Department of Energy's Vehicle Technologies Program, specifically from Peter Faguy and Dave Howell, is gratefully acknowledged. Argonne National Laboratory’s work was supported by the U.S. Department of Energy (DOE) Office of Science laboratory under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute
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