Enhanced Lithium Transport by Control of Crystal Orientation in Spinel LiMn₂O₄ Thin Film Cathodes

Abstract

A promising cathode material for rechargeable batteries is LiMn2O4, which exhibits higher operating voltage, reduced toxicity and lower costs as compared to commonly used LiCoO2 cathodes. However, LiMn2O4 suffers from limited cycle life, as excessive capacity fading occurs during battery cycling due to dissolution of Mn into the acidic electrolyte. Here, we show that by structural engineering of stable, epitaxial LiMn2O4 thin films the electrochemical properties can be enhanced as compared to polycrystalline samples. Control of the specific crystal orientation of the LiMn2O4 thin films resulted in dramatic differences in surface morphology with pyramidal, rooftop or flat features for respectively (100), (110), and (111) orientations. All three types of LiMn2O4 films expose predominantly ⟨111⟩ crystal facets, which is the lowest energy state surface for this spinel structure. The (100)-oriented LiMn2O4 films exhibited the highest capacities and (dis)charging rates up to 33C, and good cyclability over a thousand cycles, demonstrating enhanced cycle life without excessive capacity fading as compared to previous polycrystalline studies.

Pulsed Laser Deposited Films for Microbatteries

“…A spinel structure (Fd3m space group) was successfully prepared using different PLD conditions and applied as a positive electrode in thin-film lithium microbatteries (see, for example, [121]). Most prior works focused on the fundamental properties of PLD-grown LMO cathode films, aiming to deposit the highly structured and porous morphology required for a good operating electrode [7,[122][123][124][125][126][127][128][129][130][131][132][133][134][135][136] [123]. The electrochemical features of the Li cell showed a specific capacity as high as 120 mC·cm −2 ·µm −1 in the voltage range of 3.0 to 4.2 V vs. Li + /Li, which was attributed to the high degree of film crystallinity [7,123].…”

“…PLD epitaxial LMO thin films were deposited on oriented Nb:SrTIO 3 substrates maintained at 950 • C from a sintered target with 100 wt.% excess Li 2 O with different surface morphologies and orientations, such as (100)-oriented pyramidal, (110)-oriented rooftop, or (111)-oriented flat structure. The pyramidal-type LMO was cycled at a 3.3C rate, demonstrating a specific capacity of 90 mAh·g −1 after 1000 cycles [129]. Using oriented substrates, i.e., (111) reported a detailed mechanism of the epitaxial LMO film/substrate (current collector) interface formation.…”

“…% excess Li2O with different surface morphologies and orientations, such as (100)-oriented pyramidal, (110)-oriented rooftop, or (111)-oriented flat structure. The pyramidal-type LMO was cycled at a 3.3C rate, demonstrating a specific capacity of 90 mAh·g −1 after 1000 cycles [129]. Using oriented substrates, i.e., (111)Nb:SrTiO3 (STO) and (001)Al2O3 single crystal, LMO films were grown with the (111) orientation under the following synthesis conditions: Li-enriched target (Li/Mn = 0.6), Ts = 650 °C, and PO₂ = 30 Pa [130].…”

Morphology Evolution during Lithium-Based Vertically Aligned Nanocomposite Growth

“…The LMO and LLTO peaks show the presence of highly crystalline oriented spinel and perovskite structures, in good agreement with previous studies of individual LMO or LLTO thin films grown on STO(100) substrates. 23,24 The small extra peaks suggest the presence of minor contributions of tetragonal LLTO (Li 0.56 La 0.33 TiO 3 ), which could be randomly distributed throughout the LLTO matrix or located at specific interfaces. This minority tetragonal LLTO phase will be studied in detail in follow-up research.…”

Lithium-based vertically aligned nanocomposites for three-dimensional solid-state batteries

“…In a good agreement with previous studies of individual LMO or LLTO thin films grown on crystalline STO(100), the LMO and LLTO peaks show the presence of highly crystalline-oriented spinel and perovskite structures. [54][55][56] The LLTO phase is epitaxially strained to the underlying STO substrate as determined by reciprocal space mapping XRD. The LLTO exhibits an outof-plane lattice parameter of ~ 3.85 Å, which corresponds to a crystal structure with a volume equal to a relaxed cubic phase of about Li 0.3 La 0.57 TiO 3 with a lattice parameter of ~ 3.88 Å.…”