MLi2Ti6O14 (M = 2Na, Sr, Ba, Pb) Titanate Anodes for Lithium-Ion Capacitors (LICs)

Abstract

Presence of shared edge, particularly of shared face, decreases the stability of ionic structures, for e.g., the least number of [TiO₆] shared edges (two) makes rutile (hcp) the most stable TiO₂ polymorph (Pauling’s IIIrd rule) [1]. Further, on insertion of (one or three) Li into spinel LiTi₂O₄ (ccp) or Li-substituted defect-spinel Li₄Ti₅O₁₂ (LTO), Li in 8a [LiO₄] sites migrate along with incoming Li to vacant 16c [LiO₆] sites, effectively avoiding face sharing of [TiO₆]-[TiO₆], [LiO₆]-[TiO₆] or [LiO₄]-[TiO₆] polyhedra in the intermediate states. This is unlike the face shared polyhedra that form mid-way during lithium insertion in case of rutile (hcp), brookite (hcp/ccp) or anatase (ccp) [2]. A unique zero-strain two-phase separation occurs favourably at a 1.5 V flat Ti(IV)/Ti(III) redox voltage vs Li, which is far above the deposition potential of lithium. This makes LTO a safe, dendrite-free, high-power, and long-life anode that is widely used commercially (Altair Nano, Toshiba SCiB, Yinlong) as an alternative to graphite. LTO is also reported (Hydro Quebec) to cycle over 30000 times vs LiFePO₄ while retaining 90% capacity.Along these lines, orthorhombic Cmca MLi₂Ti₆O₁₄ (MLTO) (M = 2Na, Sr, Ba, Pb) [3-5] prepared by rapid combustion-synthesis exhibit similar voltage (1.3-1.45 V) and capacity (100-160 mAh/g, upto 4 Li uptake). The first reported compound in this family, BaLi₂Ti₆O₁₄, had a high ionic conductivity (10-3 S/cm, 300ᵒC) and was found by serendipity during attempts to stabilize - a high-temperature type-III superionic Ramsdellite phase (MnO₂) of Li₂Ti₃O₇ - by addition of Ba [6]. Ramsdellite crystallizes in a rutile-type arrangement of 2X1 [TiO₆] polyhedras and reversibly inserts 2.24 Li [7]. Lithium diffusivity - migration pathways, their temperature dependent activation energies and diffusion coefficient - in MLTO was probed using a combination of methods involving bond valence site energy (BVSE) analysis, AC bulk-conductivity analysis, and electrochemistry versus Li was demonstrated. Further, motivated by Amatucci’s pioneering work on high power LTO/Activated Carbon (AC) asymmetric lithium-ion capacitors (LIC) showing electrochemical performance between high-energy battery and high-power capacitors, we employed MLTO electrodes in LICs for the first time. MLTOs were first loaded with lithium (pre-discharged) before assembling them in MLTOs/AC LIC hybrids. AC cathode adsorbs PF6 - ions rapidly in a capacitor-like sloped profile (40 mAh/g) at a higher voltage, while MLTOs (de)intercalate Li (100 mAh/g, 1-2 V) at Ti-redox potential.