Understanding slurry formulations to guide solution-processing of solid electrolytes☆
Introduction
To increase the adoption of electrified mobility alternatives, safe, high–energy density batteries must be developed [1,2]. Conventional lithium-ion batteries are not inherently safe because the liquid electrolyte contains flammable solvents [3]. Solid-state batteries (SSBs), which replace the liquid electrolyte with its solid counterpart, are theoretically safer because they do not contain flammable solvents [[4], [5], [6], [7], [8], [9]]. Despite many efforts in material development, cell architecture optimizations, and processing inventions, SSBs have not yet been commercialized [10,11]. The US Department of Energy's performance and cost goals for electric vehicle batteries are energy density greater than 350 Wh kg−1 and costs lower than $60/kWh at the cell level [2,12]. These goals require the development of next-generation batteries with highly ionic conducting electrolytes and the use of energy-dense anodes and high-voltage, high-loading cathode materials. Use of lithium-metal anodes in conventional lithium-ion batteries is limited by various parameters such as safety and side reactions [13]. Furthermore, liquid electrolytes have narrow electrochemical stability windows, which hinder the use of high-voltage cathode materials [14]. However, a solid electrolyte (SE) allows the use of lithium metal because a dense SE can theoretically prevent dendrite formation [15,16], and many SEs have wide electrochemical stability windows, allowing integration of high-voltage cathode materials [3].
To develop a safe, energy-dense, and cost-effective battery, liquid electrolytes must be replaced by their solid counterparts [[17], [18], [19]]. Some challenges must be addressed concerning the ionic conductivity of SEs, chemical and electrochemical stability of anode | SE and cathode | SE, and the processability of SEs [10,20]. Many garnet SEs have ionic conductivity in the range of 10−4 to 10−3 S/cm and have a very wide electrochemical and chemical stability window [[21], [22], [23]]. Despite these favorable electrochemical, chemical, and transport properties, garnet-based SSBs have not been commercialized because garnets have processing challenges coupled with mechanical issues of brittleness, high Young's modulus and low fracture toughness [24]. These properties are not favorable for attaining intimate contact between lithium-metal | SE and SE | cathode, and the solutions to these issues are not cost-effective, generally requiring additional processing steps and materials [[25], [26], [27], [28]]. Furthermore, garnets have a higher density than other SEs, which adversely affects the gravimetric energy density when processed in currently employed form factors (pellets).
To enable garnet-based SE research, thick (1–2 mm) SE pellets are used at lab scale [11]. SE thickness is a critical parameter that enables garnet-like, high-density SEs to achieve gravimetric energy-density metrics relevant to electric vehicle applications [29,30]. Currently, average lithium, SE, and cathode thicknesses used in all-solid-state batteries are 100, 500, and 30 μm, respectively [11]. The energy density metrics for cells with such thicknesses are lower than those of conventional lithium-ion batteries (approximately 10–100 Wh kg−1). Furthermore, SSBs are cycled at very high pressures in the range of approximately 10 MPa to obtain intimate contact between solid–solid interfaces in the SSB (anode | SE and cathode | SE) [31,32]. The brittle nature of garnets limits the maximum allowable pressure on the cell. Transitioning to a thin SE film can mitigate these issues by decreasing the overall material cost, providing facile mechanical properties for ease of handling, and reducing the ion transport path to enable fast charging capabilities of the battery. Thus, developing strategies to manufacture thin, dense-defect–free SEs at scale is necessary for a successful deployment of SSBs in electric vehicles. However, efforts to address this challenge are not well documented [10,30]. A thin and very dense layer of electrolyte may be created via physical vapor deposition or atomic layer deposition, but these methods pose concerns of scalability and costs associated with large-scale production [33]. Tape casting, or roll-to-roll processing, is the most promising technology for manufacturing thin, dense-defect–free SEs SE sheets. Roll-to-roll processing of SEs can also use the existing infrastructure for conventional lithium-ion battery manufacturing for successful deployment of SSBs [34].
Significant challenges must be overcome to facilitate slurry processing for making thin, dense electrolytes. This process is illustrated in Fig. 1. The structure and properties of roll-to-roll–coated SEs are a function of the interactions that occur among the constituents in the dispersion. Slurry processing involves coating, drying, calendering, delaminating, and sintering electrolyte films at high temperatures to eliminate solvent-binder-plasticizer systems. These systems affect the lithium lanthanum zirconate oxide (LLZO) properties, including microstructure, density, hardness, shrinkage, and conductivity. Significant work has been devoted to slurry engineering for electrodes in fuel-cell technologies and conventional lithium-ion batteries [30,[35], [36], [37], [38], [39], [40]]. In this study, the underlying principles of ternary component interactions in the dispersions were leveraged to achieve roll-to-roll coatings of free-standing aluminum-doped lithium lanthanum zirconate oxide (LALZO) SE films. The slurry composition of the solvent-binder-plasticizer system was optimized to form a defect-free green tape that can be delaminated to obtain free-standing LLZO thin films. Various sintering strategies were evaluated to obtain a flat, dense-defect–free SE film. Lithium loss was optimized in the SE densification to achieve a homogeneous composition of LLZO. Experimental strategies and best practices for solution processing of SEs can also be leveraged for other SEs.
Section snippets
Slurry preparation
Commercial LALZO was used for this study. Slurries were prepared using two solvent systems: (1) isopropyl alcohol (IPA) and toluene and (2) ethanol and toluene. Fish oil was used as a dispersant, benzyl butyl phthalate was used as a plasticizer, and polyvinyl butyral (PVB) was used as a binder. Yttrium-stabilized zirconium balls were used as a milling media. Four slurries were prepared using the compositions listed in Table 1. The constituents were mixed in specific ratios and ball milled in a
Coating stability window model
The stability windows for slot-die coatings of non-Newtonian slurries are well documented [[41], [42], [43], [44], [45], [46], [47]]. The model used for predicting coating stability windows is briefly described as follows. A full derivation of the model can be found elsewhere [[41], [42], [43], [44], [45], [46], [47]]. Fig. 2 shows a diagram of fluid flow through a slot die coater with key elements of the flow profile identified. A uniform coating window is specified in terms of the fluid flow
Results and discussion
The development of successful roll-to-roll processing for SEs requires the engineering of stable, functional dispersions. Dispersion stability is a key requirement for achieving high-quality coatings and improving the shelf life of the slurry [36]. Dispersion stability is dictated by the competing interactions between the solvent, dispersed particles, binders, and surfactant. These interactions typically include van Der Waals, electrostatic, steric, and depletion interactions [[48], [49], [50],
Conclusion
Scalable processing of thin SEs is crucial for the development of high–energy density SSBs. Factors that affect the slurry-based processing of LALZO-based thin films were investigated. Specifically, IPA/toluene and ethanol/toluene slurries at different solid loadings were used to investigate the influence of component interaction in the dispersion phase on the resulting film microstructure and processability. Improved component interactions within the IPA/toluene slurries yielded
CRediT authorship contribution statement
Anand Parejiya: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. Marm B. Dixit: Data curation, Formal analysis, Investigation, Methodology, Software, Writing – original draft, Writing – review & editing. Dhrupad Parikh: Formal analysis, Investigation, Methodology, Writing – review & editing. Ruhul Amin: Writing – review & editing, Investigation, Methodology. Rachid Essehli: Writing – review & editing,
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, for the US Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by Laboratory Directed Research and Development (LDRD) Program at Oak Ridge National Laboratory, and the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) (Director: David Howell) Applied Battery Research subprogram (Program Manager: Peter Faguy). Marm Dixit is supported by the Alvin M. Weinberg
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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).