Date of Award

May 2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

WILKISTAR OTIENO

Committee Members

KHALIL AMINE, GUILIANG XU, HAMID SEIFODDINI, DAH-CHUAN GONG, PILLAI KRISHNA

Keywords

Alkali-ion batteries, Anode alloys, Microsized, Phosphorus, Silicon, Volume change

Abstract

Modern human societies will not be able to thrive without reliable, cost-effective, and safe energy storage methods. Alkali-ion batteries, a frontier in energy storage systems, have been in the limelight due to their promising specific capacities and low electrochemical potentials. However, State-of-the-art lithium-ion batteries cannot satisfy the worldwide increasing energy demand because of the low specific capacity of the graphite anode. Alloying anode materials are considered among the best candidates for replacing graphite due to their much higher specific capacity. However, their potential is hampered by the significant volume changes experienced during cycling, leading to mechanical degradation and short cycle life. Many structural and design strategies have been utilized to mitigate volume changes and enhance battery performance. Yet, an optimized design and an intensive understanding of the underlying alloying mechanism continue to be debatable and ambiguous due to a lack of operando and in-situ characterization. In this thesis, multiscale synchrotron X-ray diffraction and absorption spectroscopies coupled with in-situ and ex-situ characterization have been used to understand the role of carbon, different composite, different structures, and different electrolytes in the alloying process and in mitigating volume expansion of alloy anode for Li-and Na-ion battery systems.

Firstly, Carbon-sandwiched SnS2, obtained through a hydrogel-embedding method, to confine the growth of few-layer SnS2 nanosheets between nitrogen- and sulfur-doped carbon nanotube (NS-CNT) and amorphous carbon, demonstrate excellent sodium storage properties. In operando small-angle X-ray (SAX) scattering combined with the ex-situ X-ray absorption near edge spectra (XANES) reveal that the redox reactions between SnS2/NS-CNT and the sodium ion are highly reversible. On the contrary, the nanostructure evolution is found to be irreversible, in which the SnS2 nanosheets collapse, followed by the regeneration of SnS2 nanoparticles. This work provides operando insights into the chemical environment evolution and structure change of SnS2-based anodes, elucidating its reversible reaction mechanism, and illustrates the significance of engineered carbon support in ensuring the electrode structure stability.

Silicon and phosphorus both show much higher specific capacity than graphite; however, their practical use is significantly hindered by their large volume changes during cycling. Although significant efforts have been made to improve their cycle life, the initial coulombic efficiencies of the reported Si-based and P-based anodes are still unsatisfactory (<84%). Here, by using a scalable high-energy ball milling approach, we report a practical hierarchical micro/nano-structured P-based anode material for high-energy lithium-ion batteries, which possesses a high initial coulombic efficiency of 91% and a high specific capacity of 2500 mAh.g−1 together with long cycle life and fast charging capability. In-situ high-energy X-ray diffraction and in-situ single-particle charging/discharging were used to understand its superior lithium storage performance. Moreover, proof-of-concept full-cell lithium-ion batteries using such an anode and a LiNi0.6Co0.2Mn0.2O2 commercial battery grade cathode were assembled to show their practical use.

Starting from micrometer-sized silicon and black phosphorus, we have also reported a high-energy silicon-phosphorus/ carbon anode (denoted as mSPC) via a high-energy ball milling process, which demonstrates an average discharge working potential of 0.3 V versus lithium, together with a high reversible capacity of > 2000 mAh.g−1, high initial coulombic efficiency of 84%, excellent cycle stability, and superior rate capability up to 15 A.g−1. Furthermore, in-situ focused ion-beam scanning electron microscopy (FIB-SEM) reveals that the volume change of the mSPC anode during repeated (de) lithiation is effectively alleviated. In contrast, starting from nanometer-sized silicon, the resulting anode (denoted as nSPC) not only presents a lower reversible capacity (1200 mAh.g−1) but also exhibits a higher charge/discharge working potential, leading to reduced energy density. Our results indicate the importance of composition/structure control in tailoring the working potential and specific capacity of alloying-type anodes toward high-energy lithium-ion batteries.

Lastly, micro-sized Sn is a promising anode material for sodium-ion batteries in terms of cost, specific capacity, and volumetric energy density, which, however, suffers from huge volume changes and rapid cell degradation upon cycling. Despite recent advances via nano-structured electrode design and interface engineering, the correlation between mechanical stability, solid-electrolyte interphase (SEI), and reaction kinetics/reversibility remains controversial and elusive. Here, by combining in-situ scanning electron microscopy (SEM) and X-ray absorption spectroscopy (XAS) as well as X-ray photoelectron spectroscopy (XPS), we have investigated the underlying electro-chemo-mechanical behavior and their coupling effects during charge/discharge of micro-sized Sn anode. Our results revealed that micro-sized Sn is pulverized into nano-particles with simultaneous formation of numerous voids and pores upon the 1st charge/discharge, while the electrolytes composition plays a critical role in the consequent parasitic reactions and eventually the sodiation/desodiation reversibility. In contrast to carbonate-based electrolytes, ether-based electrolytes enabled the formation of inorganic species dominated SEI with improved mechanical strength, thus leading to higher specific capacity and improved cycling stability. The present findings are crucial for future development of micro-sized anode materials for rechargeable batteries with high volumetric energy density.

In conclusion, this research offers a holistic view of the design, optimization, and challenges of alloy electrodes for alkali-ion batteries. The findings pave the way for next-generation battery systems with improved stability, longevity, and energy output.

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