Nanocrystals Conversion Chemistry within Slit-like 2D-Nanogap for High-Rate Cyclic Stability of Lithium-Ion Battery Anodes
Authors: Choi Sungho , Hong Yu-Rim , Lee In-Su , Park Soojin 
Published: February 2024

A deep understanding of nanocrystal (NC) conversion chemistry is necessary to access alternative synthetic routes for naturally inaccessible nanostructures and fundamental solutions of any interference impeding their practical applications. For durable catalytic processes and practical energy storage devices, this can enable sustaining full reversibility connecting the active NC components under long, harsh operational conditions. Redox convertible late transition metal oxide (TMO) NCs are excellent anode materials for Li-ion batteries (LIBs) with many superior properties but have deviated from the reversible lithiation/delithiation mechanism (MxOy + 2yLi+ + 2e- xM0/yLi2O). Thus, undesired NC-coalescence and irreversible solid-electrolyte-interphase (SEI) formations result in significant capacity loss with repeated charge/discharge cycles and at high cycling rates (>2.0 C). Several methods, involving immobilization on conductive supports, manipulated crystalline phases, and fabricating hierarchical assemblies, have been investigated to resolve this issue. Sheet-like 2D-TMO nanostructures, which electrochemically convert to reduced metal NC-arrays in 2D-geometry, have proven effective at improving reversible and cycling anode performance at moderate current density ranges (0.2 ~ 1.0 C). However, for practical LIB devices, an increased storage performance, especially at quick charging conditions (> 3.0 C), is required, which may be realized through optimizing 2D-TMOs for improved conversion reversibility. However, as the attempted 2D-TMOs have rough thickness uniformity mostly in the 10 ~ 100 nm range long with the lateral size above 250 nm and often fabricated in the carbon-hybrid form, precise interpretation of thickness-dependent effects of 2D-TMO electrodes, especially within a few-nm-thick range, is still unaccomplished. Hence, we attempted fabrication of thinner 2D-TMOs with well-controlled uniform few-nm thicknesses and assess their effectiveness as LIB anodes based on transformation behavior under redox-switching conditions. The confinement of in-situ created NCs inside the few-nm-thin 2D-nanospace, as pre-defined by the thickness of initial 2D-TMO, would hinder free movement, agglomeration, and sintering of included NCs during their phase transition, improving conversion reversibility. The use of atomic-thin 2D-TMOs was envisioned to place single laterally aligned NC arrays at the ultrathin 2D nanogap, ensuring fully reversible interconversion between reactive NC redox-pairs. However, atomic-thin 2D nanosheets of late TMOs, which are intrinsically non-layered materials, remain a synthetic challenge under conventional methods.
Herein, we adopted the nanospace confinement strategy to thermally transform delaminated layered double hydroxides (NiCo-LDH) within a 2D-SiO2 envelope, forming isomorphic 2D-TMOs (Ni3Co1Ox, NCOs) of uniform 1 nm and 7 nm thicknesses from LDH precursors. A thermal conversion study of the 2D-TMOs under the 2D-SiO2-shell protection, as well as an electrochemical investigation within the in-situ formed 2D SEI-envelope identified a substantial confinement effect on sustaining the fully cyclic transformation behavior of the derived NC arrays from 1 nm-thin 2D-TMOs. Thus, the atomic-thin 2D-TMO anode demonstrated highly reversible capacity of 848.36 mAh g-1 at the first cycle and a high-rate capability (a specific capacity of 61.2% at 5.0 C relative to 0.2 C) and, the long-term stable Li-ion storage capacity delivered 1169 mAh g-1 and 913 mAh g-1 of specific capacity during 1000 cycles even at rapid charge/discharge conditions of 3.0 C and 5.0 C rates, respectively.