Nanostructured ligament and fiber Al–doped Li7La3Zr2O12 scaffolds to mediate cathode-electrolyte interface chemistry☆
Graphical abstract
Introduction
Among oxide lithium conductors, lithium lanthanum zirconate (LLZO) has attracted significant scientific interest for its potential use in high–energy density batteries. LLZO has a wide voltage window, excellent stability against lithium metal, and high conductivity at room temperature when it is crystalized in the cubic phase [[1], [2], [3]]. The high conductivity was shown to be better stabilized when the LLZO crystal is doped with aluminum (Al-LLZO) [4]. LLZO has been explored extensively as a solid electrolyte for all solid-state batteries [[5], [6], [7], [8], [9]]. Additionally, there is significant interest in porous doped LLZO that can be filled with polymer electrolytes. These hybrid ceramic polymer electrolytes combine the high ionic conductivity values of the garnet ceramic with the lightweight, flexible, and interfacial properties of the polymer electrolyte [[10], [11], [12]].
Several techniques have been employed to fabricate porous garnet electrolyte templates such as electrospinning, supercritical drying, freeze tape casting, and slurry casting on sacrificial templates that can be removed by temperature annealing [11,[13], [14], [15], [16]]. Of these methods, electrospinning is a scalable technique that can be used to precisely control the size and structure of the spun fibers by tailoring the chemical composition of the precursor solutions and the electrospinning parameters. The solvent, bias, flow rates, calcination temperature, calcination duration, and heating rate are critical to controlling the resulting crystal structure and morphology of the produced fibers. Aqueous- and dimethylformamide (DMF)-based precursor solutions are typically used for fiber synthesis [3] because of their favorable dielectric strengths and viscosities enabling suitable draws of precursors. The aqueous-based formulations typically result in fiber coalescence during calcination and the formation of ligament-type structures. On the contrary, the DMF-based formulations tend to maintain the fiber structure after calcination. Hybrid electrolytes based on polyethylene oxide, Li salt, and garnet electrolyte scaffolds have been investigated. The morphology and crystallization phase of the LLZO fibers is strongly dependent on the precursor formulation and the calcination procedure [3]. The ionic conductivity, mechanical reinforcement, and ability of the hybrid electrolyte to block the formation of Li dendrites have shown significant improvements [10,17]. LLZO nano-size domains can crystalize in the cubic phase at low calcination temperature. Increase of the domain size to micrometers was shown to result in a crystal phase change from cubic to tetragonal [13]. The conductivity of the latter phase at room temperature is more than one order of magnitude lower; therefore, the desired phase is the cubic. Aluminum- or gallium-substituted LLZO can better stabilize the cubic phase and result in increased conductivity values for sintered garnet electrolytes with micrometer-size grain morphology [18,19].
Herein, we investigate the use of Al-LLZO scaffolds as cathode additives to stabilize the interface chemistry of LiNi0.6Mn0.2Co0.2O2 (NMC 622). Electrolyte scaffolds with two nanostructured morphologies were made of electrospun fibers. The electrospinning parameters of the Al-LLZO precursors and the calcination procedure were tailored to synthesize dense ligament scaffolds and high–aspect ratio nanofibers that were crystalized in the cubic phase. The garnet electrolytes were dispersed in cathode formulations based on NMC 622 active material. Half-cells were assembled using lithium metal foil and lithium hexafluorophosphate (LiPF6)–based electrolyte. The electrochemical stability of the cells was evaluated at several charge/discharge rates. The results were correlated with the nanostructured morphology of the cathodes and the chemical composition of the cathode electrolyte interface (CEI) because of the dispersed Al-LLZO scaffolds. The dispersion of the Al-LLZO nanofibers in the cathode changed the CEI chemistry significantly and allowed the composite cathodes to better cycle with a standard electrolyte.
Section snippets
Experimental
Aqueous- and DMF-based nitrate precursor solutions were formulated for the synthesis of the Al-LLZO fibers [3]. The Li:La:Zr molar ratio was 7.7:3:2. A 10% excess of Li was used to account for the Li loss during calcination. For the aqueous formulation, deionized (DI) water and acetic acid were used to dissolve and stabilize the reagents [20]. Initially, 2 mmol zirconium oxynitrate, 7.7 mmol lithium nitrate, 3 mmol of lanthanum nitrate hexahydrate, and 0.27 mmol aluminum nitrate nonahydrate
Results and discussion
The electrospun Al-LLZO nanofibers using aqueous formulations are shown in Fig. 1. The as-spun (before calcination) nanofibers were uniform, and no bead structures were observed. Their diameter was 100–200 nm. The calcination process was implemented gradually in several temperature steps to avoid the rapid shrinkage and the pronounced coalescence of the fibers and to remove the PVP that was added to stabilize the grown fibers. After calcination, they coalesced to form dense ligament-type
Conclusions
Garnet electrolyte Al-LLZO scaffolds based on electrospun fibers were synthesized using aqueous- and DMF-based formulations. Calcination of the aqueous-derived Al-LLZO fibers resulted in interconnected coalesced ligament scaffolds. Calcination of the DMF-derived Al-LLZO fibers resulted in scaffolds of high–aspect ratio fibers. Both scaffolds were crystalized in the cubic phase. The scaffolds were used as additives in cathodes. Cathodes filled with fiber scaffolds demonstrated the lowest
CRediT authorship contribution statement
Georgios Polizos: Methodology, Conceptualization, Investigation, Formal analysis, Writing – original draft. Jaswinder Sharma: Investigation, Conceptualization, Writing – review & editing. Charl J. Jafta: Investigation, Formal analysis, Writing – review & editing. Nitin Muralidharan: Investigation, Formal analysis. Gabriel M. Veith: Investigation, Writing – review & editing. Jong K. Keum: Investigation, Formal analysis. Alexander Kukay: Investigation. Ritu Sahore: Investigation. David L. Wood:
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
This research at Oak Ridge National Laboratory, managed by UT Battelle LLC, for the US Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy Advanced Manufacturing Office (Program Manager: Brian Valentine). The SEM and XRD characterization were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. XPS measurements were supported by the DOE Office of Basic Energy
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Notice: This manuscript has been authored by UT-Battelle LLC under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).