Freeze Tape Cast Thick Mo Doped Li₄Ti₅O₁₂Electrodes for Lithium-Ion Batteries

Abstract

Lithium titanate (Li 4 Ti 5 O 12 ) powders with and without molybdenum doping (LTO and MoLTO respectively) were synthesized by a solid-state method and used to fabricate electrodes on Cu foil using a normal tape-cast method and a novel freeze-tape-cast method. Modest molybdenum doping produces a significant electronic conductivity increase (e.g. 1 mS cm −1 for MoLTO vs 10 −7 mS cm −1 for LTO) that is thought to reflect a partial Ti 4+ reduction to Ti 3+ with charge compensation by the Mo 6+ dopant, producing a stable mixed-valent Ti 4+/3+ state. Freeze-tape-cast electrodes were fabricated by a variant of the normal tape-cast method that includes a rapid freezing step in which the solvent in the Cu-foil-supported slurry is rapidly frozen on a cold finger then subsequently sublimed to create unidirectional columnar macropores in the electrode. The resulting electrodes exhibit high porosity and low tortuosity which enhances electrolyte accessibility throughout the full electrode thickness. Freeze-tape-cast electrodes subjected to galvanostatic charge-discharge testing as cathodes in cells vs. a lithium metal anode exhibit higher specific capacity and lower capacity loss at high discharge rates compared with normal-tape-cast electrodes of the same mass loading, despite the fact that the freeze-tape-cast electrodes are nearly twice as thick as the normal tape cast electrodes. High specific energy is nearly always desirable in battery systems but it is especially important in batteries for electric vehicles. Current electric vehicle batteries achieve specific energy between 50 and 150 Wh kg −1 at a system or pack level, 1,2 but further advances are needed for larger market penetration, particularly in the battery electric vehicle (BEV) area. Andre and co-workers in their recent review on cathode materials for automotive batteries suggest a 2025 full-cell specific energy target of 250 Wh kg −1 to meet a threshold driving range target of 300 miles. A key toward improving specific energy for a particular combination of anode and cathode active material is to maximize the mass fraction of active material, which means minimizing the fraction of inactive material such as the separator, the binder, electrode additives for improving electronic conductivity, and the current collector. A seemingly simple strategy for maximizing the mass fraction of active materials is to make the electrodes as thick as possible.6-8 Thick electrodes are mostly active material, and they can be manufactured at a lower cost per unit of capacity.9 Unfortunately, the thickness of electrodes supported on current collector foils cannot be increased without bound due to limits on transport rates of electrons and lithium ions in the electrodes.8,10 Because of these limitations, current vehicle battery cathodes tend to be limited to areal mass loadings of 15-25 mg cm −2 with average porosities in the range between 25-35%, which results in electrode thicknesses that are usually less than 75 micrometers. 1,2,11Attempts to increase mass loading and electrode...

Drying of Lithium‐Ion Battery Anodes for Use in High‐Energy Cells: Influence of Electrode Thickness on Drying Time, Adhesion, and Crack Formation

“…This seems to be caused by mass transport limitations of lithium ions in the electrolyte phase and restricted electron transport in the solid phase . Research therefore focuses on the structuring of electrodes, e.g., by laser post‐processing steps or by new processing methods to provide pathways from the electrode surface to the substrate for faster Li‐ion transport . Though a broad knowledge base about transport limitations and tailoring of electrode porosity exists, the significant influence of the electrode production process, namely speaking of the coating and drying conditions on electrode microstructure and cell performance, is often not considered …”

“…[4][5][6][7] Research therefore focuses on the structuring of electrodes, e.g., by laser post-processing steps or by new processing methods to provide pathways from the electrode surface to the substrate for faster Li-ion transport. [8][9][10][11] Though a broad knowledge base about transport limitations and tailoring of electrode porosity exists, the significant influence of the electrode production process, namely speaking of the coating and drying conditions on electrode microstructure and cell performance, is often not considered. [10,[12][13][14][15] The mechanism of electrode drying according to latest knowledge is summarized in Figure 1.…”

“…[8][9][10][11] Though a broad knowledge base about transport limitations and tailoring of electrode porosity exists, the significant influence of the electrode production process, namely speaking of the coating and drying conditions on electrode microstructure and cell performance, is often not considered. [10,[12][13][14][15] The mechanism of electrode drying according to latest knowledge is summarized in Figure 1. [16] Directly after coating and before the drying step starts, the wet film has a homogeneous particle and additive distribution within the solvent (Figure 1a).…”

Analysis of Rate-Limiting Factors in Thick Electrodes for Electric Vehicle Applications

“…10 Freeze tape casting method is also potentially useful in creating unidirectional columnar macropores in the thick cathode. 11 Groups have also focused on three-dimensional current collectors to reduce electronic travel distance. Wang et al demonstrated that ultrathick electrodes (1.2 mm thick) could be cycled more than 20 times using aluminum and copper three-dimensional foam current collectors for a LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathode and graphite anode, 12 respectively.…”

External Field Assisted Freeze Casting

“…Freeze casting is a relatively simple alternative that overcomes many of these limitations for fabricating porous anisotropic ceramics [28][29][30], polymers [31,32], metals [33,34] or composites [35,36], often exhibiting high compressive properties [37]. This method has potential applications in cryobiology [38], remediation of contaminated media [39,40], chemical analyses [41], liquid chromatography [42], energy storage [43,44], photocatalysis [45,46], sensors [47,48], pharmaceuticals [49] and the food industry [50,51]. In this review, we highlight important aspects of external field assisted freeze casting, focusing on the theory and experimental results.…”