Unlocking the Reactivity of Silicon Negative Electrodes for Sodium-Ion Batteries via Electrochemical Activation

다국어 초록 (Multilingual Abstract)

Sodium-ion batteries （SIBs） have attracted increasing attention as a cost- effective complement to lithium-ion batteries, owing to the natural abundance and even global distribution of sodium resources, which make them particularly appealing for large-scale energy storage where cost is a decisive factor. Their practical development, however, remains constrained by the anode, since the larger ionic radius and distinct chemistry of sodium prevent it from being readily accommodated by the host materials that have been optimized for lithium, and graphite in particular stores very little sodium. Silicon, which offers an exceptionally high theoretical capacity together with natural abundance and low cost, is an attractive anode candidate, yet crystalline silicon is essentially inactive toward sodium because its rigid lattice does not permit appreciable sodium insertion, and the reaction is confined to a shallow surface region. Amorphous silicon, by contrast, is known to store sodium through an alloying reaction, but the existing routes to amorphous silicon do not allow the extent of amorphization to be adjusted in a continuous fashion, and a method to control the degree of disorder, and thereby the reversible sodium capacity, in a systematic and predictable manner has been lacking. This study presents a lithium-mediated electrochemical activation strategy in which crystalline silicon is converted into amorphous silicon through a single lithium pre-cycle, exploiting the well-established fact that silicon does not recover its crystalline order once it has been lithiated and delithiated. It is demonstrated that the degree of amorphization can be systematically tuned simply through the lower cut-off voltage applied during the lithiation step, so that a single, experimentally accessible parameter governs the structural state of the resulting material. Three activation cut- off voltages （0.1, 0.05, and 0.01 V versus Li/Li⁺） were used to produce a series of samples, designated Act-0.1, Act-0.05, and Act-0.01, in which the lithium uptake during activation （881.33, 1833.64, and 3886.19 mAh g⁻¹, corresponding to compositions from Li₀.₉₂Si to the fully alloyed Li₃.₇₅Si） served as a direct measure of the conversion. X-ray diffraction and transmission electron microscopy confirmed a progressive and controllable loss of crystallinity, with Act-0.01 rendered essentially amorphous, while a direct comparison of silicon cycled in sodium and in lithium cells showed that sodium cycling alone leaves the crystalline interior intact, whereas the lithium cycle disorders the entire particle and thereby creates the host structure into which sodium can subsequently be stored. The reversible sodium capacity increased monotonically with the degree of amorphization, rising from 164.5 mAh g⁻¹ for pristine silicon to 360.5 mAh g⁻¹ for Act-0.01, and the activated electrodes retained a stable capacity advantage over the pristine material throughout cycling, indicating that the additional storage created by amorphization is sustained rather than rapidly lost. The smoothly sloping voltage profiles and broad differential-capacity features identified the underlying process as an alloying reaction between sodium and amorphous silicon, whose magnitude scaled with the amount of amorphous material present. Galvanostatic intermittent titration further showed that activation roughly doubles the reaction depth accessible to sodium at a comparable overpotential, and the distinct quasi-equilibrium potential profiles of the pristine and activated electrodes indicated that activation extends a reaction largely confined near the particle surface into a bulk process that proceeds throughout the particle. Ab initio molecular dynamics simulations provided independent support at the atomic scale, confirming that sodium penetrates the amorphous matrix about four times more deeply and diffuses approximately 1.6 times faster than in the crystalline structure. Taken together, these results establish that the controlled disordering of silicon, governed by a single experimentally accessible parameter, converts an essentially sodium-inactive material into a tunable high-capacity anode, and provide a coherent structural, electrochemical, and computational account of the mechanism by which activation enhances sodium storage, offering a practical design principle for silicon anodes in sodium-ion batteries.

목차 (Table of Contents)

• CHAPTER 1. Introduction 7
• 1.1 Research Background 7
• 1.2 Sodium-Ion Batteries and Anode Material Challenges 9
• 1.3 Silicon as an Anode and the Limitation of Crystalline Silicon 9
• 1.4 Amorphous Silicon as a Sodium-Active Phase 10
• 1.5 Electrochemical Amorphization via Lithiation as an Activation Strategy 10
• 1.6 Research Motivation and Objectives 11
• CHAPTER 2. Experimental 13
• 2.1 Materials 13
• 2.2 Electrode Fabrication 13
• 2.3 Lithium-Mediated Electrochemical Activation 14
• 2.4 Sodium Half-Cell Assembly 14
• 2.5 Material Characterization 15
• 2.6 Electrochemical Characterization 16
• 2.7 Ab Initio Molecular Dynamics Simulation details 16
• CHAPTER 3. Results and Discussion 17
• 3.1 Tunable Amorphization through Lithium-Mediated Activation 17
• 3.2 Structural Confirmation of Amorphization by X-ray Diffraction 20
• 3.3 Microstructural Analysis of Crystalline and Activated Silicon 22
• 3.4 First-Cycle Sodiation and De-sodiation Behavior 24
• 3.5 Reversible Sodium Storage Behavior 26
• 3.6 Cycling Performance 28
• 3.7 Expanded Na-Accessible Reaction Domain Revealed by GITT 29
• 3.8 Simulation analysis 33
• CHAPTER 4. Conclusion and Future Directions 36
• References 39
• 국문초록 41