High Performance Li-Ion Battery Electrode with Hierarchical Ordered Porous Microstructure

Abstract

Rechargeable batteries are important components of many systems and devices and a high capacity battery that can be rapidly charged remains a top research and development priority. Important performance factors like capacity, energy density, and power density strongly depend on chemical nature and the microstructure of the porous electrodes. Highly tortuous porous electrodes and long lithium ion pathway due to pore blockage by inactive material are detrimental to the energy and power density. Our goal is to investigate the potential of fabricating low tortuosity, controlled porosity Li-ion battery electrodes by taking advantage of the hierarchical ordered porous microstructure, which can be obtained using freeze tape casting technique. For this research, Lithium titanate (Li4Ti5O12) 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. 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 Ti4+ reduction to Ti3+ with charge compensation by the Mo6+ dopant, producing a stable mixed-valent Ti4+/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 effect of Mo doping, freezing rate, electrode thickness on electrochemical behavior of electrodes were investigated. The resulting freeze tape cast 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. We show that these engineered porous electrodes of MoLTO have improved energy and power density compared with randomly oriented uniform porosity electrodes – feature of the current generation of battery electrodes.