Hydrothermal synthesis of Co-free NMA cathodes for high performance Li-ion batteries☆
Introduction
Currently the electrification of the transportation section relies on Li-ion battery technology that utilizes NMC (LiNixMnyCozO2) and NCA (LiNi0.8Co0.15Al0.05O2) cathode materials with graphite electrodes as anode. These materials provide high energy densities suitable for EV applications but suffer from capacity decay as well as thermal instability [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]]. Increasing demand for electric vehicles introduces high demand stresses on Ni, Mn as well as Co minerals as necessary raw materials for NMC and NCA cathodes. While catering to the increased demand for all the raw minerals poses great challenges, Co is a key concern due to the additional socio-political challenges associated with its mining. Resource availability of critical materials can significantly impact the long-term market stability and techno-economics of electric vehicles. Currently, a single EV battery pack (∼75 kWh) requires close to 2 kT of Co material for NMC-type cathodes which is expected to lead to a significant resource scarcity in the coming decades (Fig. 1b). Transitioning to Co-free cathode materials have numerous technological advantages over and above the economic and socio-political advantages concerning stabilized resource prices, reduced sociological impact and enabling a sustainable battery market. Alternatives to Co in the cathode composition include Al, Mn, Fe, among others which are abundant in the crust of the earth.
High Ni concentration in cathode materials is required for achieving high driving ranges required for EV applications leading to sustained research effort in novel cathode development [[12], [13], [14]]. However, high Ni containing materials still suffer from several technological challenges. A key operational bottleneck is the formation of surface passivation layers (viz. decomposition of Li2O in air to form Li2CO3 which is responsible for gassing and swelling issues via O2 and CO2 evolution; and formation of LiOH which causes slurry gelation during processing) that limit the electrochemical performance of the batteries [[15], [16], [17]]. Specifically, for the NCA co-precipitation process there is a wide disparity in the solubility product constants of Ni2+, Co2+ and Al3+ leads to non-spherical morphologies and strong compositional heterogeneities, limiting large-scale application of NCA. Currently, the coprecipitation method in continuous stirred tank reactors (CSTR) is the typical production route widely employed to synthesize cathode precursor materials in their hydroxide forms. Ammonia (NH3⋅H2O) is used as the chelating agent in this method to control the particle size and morphology. The use of large quantities of ammonia leads to increased production costs, as well as increased concerns with safety, controls, and waste management for the consumed ammonia. Large quantities of water are also used to generate ammonia solutions and for washing of the synthesized precursors which is a serious environmental concern. Hydro/solvothermal synthesis is an alternative process that is pursued for synthesis of high quality, monodispersed spherical cathode material particles with homogenous elemental and chemical composition. Solvothermal processes are typically used to synthesize binary metal oxide anode materials in the presence of urea as chelating and precipitating agent. Limited work is carried out so far on the synthesis of carbonate- or hydroxide-precursors via the urea-mediated hydrothermal processing route due to the weak acidic nature of the resultant solution that forms carbonic acid (from urea decomposition) leading to metal dissolution and non-stoichiometric cathode compositions [18,19].
Fu et al. [20]. synthesized LiNi0.7Co0.15Mn0.15O2 microspheres via an ethanol-mediated solvothermal method with particle sizes ranging from 100 to 500 nm. The effect of excess lithium content (5, 10 and 15%) was evaluated on the electrochemical performance of the resulting material. Samples with 5% excess lithium showed the best discharge capacities and the cycling behavior among the investigated materials. The capacity retention for 5% excess Li NMC is 88.8% compared to 76.9% for the sample synthesized with 10% excess and 84% for the sample synthesized with 15% excess Li. Cao et al. [21]., similarly synthesized NCA (LiNi0.88Co0.09Al0.03O2) spherical particles that showed promising electrochemical performance with an initial discharge capacity of 210.7 mAh g−1 at 0.1C with an 83.2% coulombic efficiency. Recently, our group reported the synthesis of a new class of Co-free materials with general formula LiNixFeyAlzO2 (x + y + z = 1)13, 14. These materials have a high specific capacity of ∼200 mAh g−1 and show good cycling and rate capability (80% capacity retention after 100 cycles). Overall, hydrothermal process has numerous advantages over other cathode preparation methods that renders it uniquely advantageous for commercial applications. Specifically, hydrothermal process generally is carried out at lower reaction temperature that limits formation of unfavorable side products as well as reducing the cost of the process. This process can also avoid both unusual oxidation processes during synthesis and dissolution of oxide impurities at higher temperature facilitating good crystallinity and narrow particle size distributions that are crucial for high performance battery materials.
Herein we report a hydrothermal synthesis method of a new class of Co-free cathode material, LiNi0.9Mn0.05Al0.05O2, termed NMA9055 (Fig. 1a). NMA (LiNi1−x−yMnxAlyO2) is also a new class of cathode materials recently reported, that possesses excellent rate capabilities, thermal stability as well as synthesis tunability for applications in commercial Li-ion batteries. Further work on the optimization of these cathode materials in terms of the morphology, particle sizes and composition is required to extract the highest performance. The electrochemical performance of this cathode material is evaluated with lithium half-cells by galvanometric cycling and cyclic voltammetry which shows 96% retention over 100 cycles. Structural and thermal stability of NMA9055 is investigated by X-ray diffraction and differential scanning calorimetry measurements. The developed NMA material has properties like the conventional Co-based cathodes (NMCs/NCAs) and thus can be seamlessly manufactured and integrated into the current industrial scale manufacturing techniques. Additionally, this material eliminates the need of critical material Co entirely making this cathode suitable for long term energy security and stability (Fig. 1). In summary, the developed NMA9055 material has the potential to address key Co issues facing the battery industry in addition to providing improvements in processing cost, performance, structural and thermal stability.
Section snippets
Results and discussion
Many researchers have utilized hydro/solvothermal route to generate high quality, spherical cathode precursors to enable high performance battery anode and cathode materials [[22], [23], [24]]. Herein, we used a hydrothermal synthesis approach to synthesize precursor α-3Ni(OH)2·2H2O particles and subsequent heat treatment after mixing with LiOH, Mn(NO3)3 and Al(NO3)3 to form the NMA9055 material (Fig. 1c). The XRD pattern of the precursor α-3Ni(OH)2·2H2O is shown in Fig. 1d.
The pattern shows a
Conclusion
LiNi0.9Mn0.05Al0.05O2 (NMA955) cathode material with nickel, manganese and aluminum in its composition was successfully synthesized by a solvothermal process. Ethanol oxidation in the presence of NO3- precursors leads to the precipitation of monodispersed, spherical morphology of the cathode precursors and elimination of ammonia from the cathode synthesis process. NMA shows excellent half-cell performance with an initial discharge capacity exceeding 200 mAh/g and a capacity retention over 80%
Material synthesis
An ethanol assisted hydrothermal method was used to prepare α-3Ni(OH)2.2H2O microsphere precursors. 10 g of nickel (II) nitrate hexahydrate [Ni(NO3)2·6H2O] was dissolved in 100 ml absolute ethanol at room temperature to obtain a clear solution. The resulting solution was transferred to an autoclave (250 ml capacity), which was then heated at 180 °C for 24 h followed by cooling naturally to room temperature. Finally, after the hydrothermal treatment, a green powder of α-3Ni(OH)2.2H2O was
CRediT authorship contribution statement
Rachid Essehli: Conceptualization, Experiments, Writing – original draft, Investigation, Methodology. Anand Parejiya: Data curation, Formal analysis, Investigation, Methodology, Writing – review & editing. Nitin Muralidharan: Data collection, Formal analysis, Writing – review & editing. Charl J. Jafta: X-ray results, Formal analysis, Writing – original draft. Ruhul Amin: Investigation, Formal analysis, Writing – original draft. Marm B. Dixit: Data curation, Formal analysis, Investigation,
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.
Acknowledgements
This research at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE), was sponsored by the Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO), (Program manager: Peter Faguy, and Office Director: David Howell). Characterization was conducted at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. M.B.D. was also supported by Alvin M. Weinberg Fellowship at
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