Surface studies of Li-ion and Mg battery electrodes

Esbenshade, Jennifer L

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Description

Title

Surface studies of Li-ion and Mg battery electrodes

Author(s)

Esbenshade, Jennifer L

Issue Date

2016-01-29

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.
• Suslick, Kenneth S.
• Moore, Jeffrey S.

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
• Li-ion
• Surface studies
• LiMn2O4

Abstract

This dissertation focuses on studies of the surfaces of both Li-ion and Mg-ion battery electrodes. A fundamental understanding of processes occurring at the electrode surface is vital to the development of advanced battery systems. Additionally, modifications to the electrode surfaces are made and further characterized for improved performance. LiMn2O4 Cathodes for Li-ion Batteries: Effect of Mn in electrolyte on anode and Au coating to minimize dissolution: LiMn2O4 (LMO) is known to dissolve Mn ions with cycling. This section focuses on both the effect of the dissolution of Mn2+ into the electrolyte as well as Au coating on the LMO to improve electrochemical performance. Electrochemical quartz crystal microbalance (EQCM) was used to monitor changes in mass on the anode, SEM and AES were used to observe changes in surface morphology and chemical composition, and potentiostatic voltammetry was used to monitor charge and discharge capacity. The effect of Cu2+ addition in place of Mn2+ was also studied, as Cu is known to form an underpotential deposition (UPD) monolayer on Au electrodes. Following this, LMO particles were coated with a Au shell by a simple and scalable electroless deposition for use as Li-ion battery cathodes. The Au shell was intended to limit the capacity fade commonly seen with LMO cathodes by reducing the dissolution of Mn. Characterization by SEM, TEM, EELS, and AFM showed that the Au shell was approximately 3 nm thick. The Au shell prevented much of the Mn from dissolving in the electrolyte with 82% and 88% less dissolved Mn in the electrolyte at room temperature and 65 ºC, respectively, as compared to the uncoated LMO. Electrochemical performance studies with half cells showed that the Au shell maintained a higher discharge capacity over 400 cycles by nearly 30% with 110 mA hr g-1 for the 400th cycle as compared to a commercial LMO at 85 mA hr g-1. Similarly, the capacity fade was reduced in full cells: the coated LMO had 47% greater capacity after 400 cycles over the control. Dimensionally Controlled Lithiation of Thin Film and Multilayer Conversion Li-ion Battery Anodes: Oxide conversion reactions are an alternative approach for high capacity Li-ion batteries, but are known to suffer from structural irreversibility associated with the phase separation and reconstitution of reduced metal species and Li2O. The morphology of the reduced metal species is thought to play a critical role in the electrochemical properties of a conversion material. In this section, a model electrode is used with alternating layers of Cr and CrOx to better understand and control these phase changes in real-time and at molecular length scales. Despite lacking crystallinity at the atomic scale, this superstructure is observed (with XR) to lithiate and delithiate in a purely one-dimensional manner, preserving the layered structure. The XR data show that the metal layers act as nucleation sites for the reduction of chromium in the conversion reaction. Irreversibility during delithiation is due to the formation of a ternary phase, LiCrO2, which can be further delithiated at higher potentials. The results reveal that the combination of confining lithiation to nanoscale sheets of Li2O and the availability of reaction sites in the metal layers in the layered structure is a strategy for improving the reversibility and mass transport properties that can be used in a wide range of conversion materials. Following the Cr/CrOx study, the next step was to study intermetallics which can electrochemically alloy to Li4.4M (M = Si, Ge, Sn, etc.), providing order-of-magnitude increases in energy density. The energy density of Si may be combined with the structural reversibility of an intercalation material using a Si/metal silicide multilayer (ML). In operando XR confirms the ML’s structural reversibility during Li insertion and extraction, despite an overall 3.3-fold vertical expansion. The ML electrodes also show enhanced long-term cyclability and rate capabilities relative to a comparable Si thin film electrode. This intercalation behavior found by dimensionally constraining Si lithiation promises applicability to a range of conversion reactions. Improving Electrodeposition of Mg through an Open Circuit Potential Hold: In this section, in situ XRD, XPS, SEM and electrochemical methods were used to interrogate the mechanism of Mg electrodeposition from PhMgCl/AlCl3 (APC) and EtMgCl electrolytes. An open circuit potential (OCP) pause following Mg deposition led to retained enhancement of Mg deposition and stripping kinetics along with lowered overpotentials for both. In situ XRD demonstrated that the OCP pause led to a more polycrystalline deposits relative to that without the pause, while SEM presented micrographs that showed smaller deposits with an OCP hold. The improvement is attributed to an ‘enhancement layer’ that formed on the electrode during the OCP hold. Analysis of XPS data suggests that this ‘enhancement layer’ consists of Mg and Cl retained on the electrode surface, possibly following electrode depassivation.

Graduation Semester

2016-05

Type of Resource

text

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Copyright and License Information

Copyright 2016 Jennifer Esbenshade

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