Interrogating bulk and interfacial decay pathways in electroactive battery materials

Madsen, Kenneth Earl

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https://hdl.handle.net/2142/115391 Copy

Description

Title

Interrogating bulk and interfacial decay pathways in electroactive battery materials

Author(s)

Madsen, Kenneth Earl

Issue Date

2022-04-20

Director of Research (if dissertation) or Advisor (if thesis)

Gewirth, Andrew A

Doctoral Committee Chair(s)

Gewirth, Andrew A

Committee Member(s)

• Nuzzo, Ralph G
• Sottos, Nancy R
• Rodríguez-López, Joaquín

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Batteries
• Lithium-ion
• Cathode
• Anode
• Solid electrolyte
• Operando methods
• Organic
• Organometallic

Abstract

The objectives of this thesis are twofold. Firstly, this thesis aims to interrogate the chemical, electrochemical, and mechanical pathways affecting the performance of battery materials on repeated charging and discharging. Secondly, it aims to develop approaches to suppress or circumvent these deleterious processes in the interest of developing redox active materials with adequate safety and longevity for practical application. The work contained herein addresses a range of materials including inorganic, organic, and organometallic solids and discusses their efficacies as electrodes and electrolytes for lithium-based energy storage. Both the bulk electrochemical behaviors and the interfacial dynamics of these materials are studied, with the goal of fully understanding the myriad factors that dictate battery stability. For clarity this thesis is divided into chapters based on the chemistry under investigation. A brief description of each chapter’s contents is provided below. Origin of Enhanced Cyclability in Covalently Modified LiMn1.5Ni0.5O4 Cathodes. High-voltage lithium-ion cathode materials exhibit exceptional energy densities; however, rapid capacity fade during cell cycling prohibits their widespread utilization. Surface modification of cathode-active materials by organic self-assembled monolayers (SAMs) has emerged as an approach to improve the longevity of high-voltage electrodes; however, the surface chemistry at the electrode/electrolyte interphase and its dependence on monolayer structure remains unclear. Herein, we investigate the interplay between monolayer structure, electrochemical performance, and surface chemistry of high-voltage LiMn1.5Ni0.5O4 (LMNO) electrodes by the application of silane-based SAMs of variable length and chemical composition. We demonstrate that the application of both hydrophobic and hydrophilic monolayers results in improved galvanostatic capacity retention relative to unmodified LMNO. The extent of this improvement is tied to the structure of the monolayer with fluorinated alkyl-silanes exhibiting the greatest overall capacity retention, above 96% after 100 charge/discharge cycles. Postmortem surface analysis reveals that the presence of the monolayer enhances the deposition of LiF at the electrode surface during cell cycling and that the total surface concentration correlates with the overall improvements in capacity retention. We propose that the enhanced deposition of highly insulating LiF increases the anodic stability of the interphase, contributing to the improved galvanostatic performance of modified electrodes. Moreover, this work demonstrates that the modification of the electrode surface by the selection of an appropriate monolayer is an effective approach to tune the properties and behavior of the electrode/electrolyte interphase formed during battery operation. Direct Observation of Interfacial Mechanical Failure in Thiophosphate Solid Electrolytes with Operando X-Ray Tomography. Herein, the mechanical behaviors of Li10GeP2S12 (LGPS) solid electrolytes during electrochemical cycling using operando X-ray tomography are investigated. It is demonstrated that the bulk mechanical decomposition of LGPS when cycled against lithium is a direct result of electrochemical reduction of the solid electrolyte at the LGPS/Li0 interface. The reductive decomposition of LGPS during lithium plating results in the formation of low-density domains at the electrode/electrolyte interface, which impose sufficient mechanical stress on the underlying LGPS to crack the SE pellet. The critical stress developed prior to pellet fracture is significantly lower than the bulk shear modulus of LGPS, suggesting that the electrochemical instability of LGPS dramatically worsens the mechanical stability of the material near the LGPS/Li0 interface. It is also shown that the application of a highly concentrated liquid electrolyte to the LGPS surface suppresses the reductive decomposition of LGPS, improving both the electrochemical performance and mechanical stability of the bulk LGPS solid electrolyte. Pressure Dependent Electrochemical Behavior of Di-Lithium Rhodizonate Cathodes. Herein, we investigate the electrochemical properties of the high-capacity organic cathode material di-lithium rhodizonate (Li2C6O6) under different applied mechanical loads. We demonstrate, through a combination of pressure-dependent voltammetry and electrochemical impedance spectroscopy, that the charge-transfer kinetics at the cathode/electrolyte interface is strongly impacted by the magnitude of the load applied to the cathode. At low pressures, lithium rhodizonate displays untenably high charge-transfer impedances toward lithiation and delithiation. As the load applied to the cathode material is increased, the charge-transfer impedance decreases, reflecting a reduction in the overpotential associated with lithiation and delithiation. Furthermore, pressure-dependent galvanostatic cycling reveals that cells cycled at high pressures exhibit improvements in their overall capacity retention when compared with their low-pressure counterparts. Using a combination of postmortem X-ray diffraction and first-principles calculations, we show that, in the absence of sufficient external load, lithium rhodizonate converts from its redox-active structure to a redox-inactive structure, resulting in the observed rapid capacity fade. As the pressure applied to the electrode is increased, this phase transition is suppressed, resulting in improvements in both long-term stability and electrochemical kinetics. Demonstration of a Stable Redox-Active Metal Rhodizonate Cathode Material Through Crystal Structure Modification. In this report we describe the electrochemical behavior of several divalent metal rhodizonate complexes (M(II)C6O6, where M is either a first-row transition metal [Mn, Co, or Ni] or a heavy metal [Pb]) and discuss their efficacy as electrode materials for rechargeable lithium-ion batteries. We also compare the electrochemical responses of these materials to those of the better studied Li2C6O6 to gain insight into the origins of and methods to circumvent the known electrochemical instability. We demonstrate that the different rhodizonate complexes display similar structures, with hexagonally close packed layers of rhodizonate ions forming the framework into which ions can intercalate and deintercalated. We likewise demonstrate that structures which coordinate M(II) within these planes of rhodizonate ions universally demonstrate poor capacity retention, likely due to Li+ occupation of low energy tetrahedral sites between the rhodizonate layers during discharge. In contrast, structures which occlude these sites with the metal chelate, as is the case with PbC6O6, suppress this deleterious Li+ coordination to some extent, thereby facilitating improved capacity retention on repeated lithiation and delithiation. This work suggests that careful control over the available Li+-binding sites within oxocarbon structures may lead to reversible high-capacity electrochemical energy storage.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Kenneth Madsen

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