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NMR Studies of the Electronic Structure, Lithium-Ion Dynamics, and Prelithiation of Lithium-Ion Battery Anode Materials


Type

Thesis

Change log

Authors

Maxwell, Dylan 

Abstract

The development of high energy density Li-ion batteries is essential if we are to succeed in the electrification of the transport system. Central to this is the need for electrodes with increased specific capacity (mAh/g and mAh/cm3) which can run for 100s to 1000s cycles. The manufacturing of these electrodes must also be cheap and scalable in order to meet the demands of future electric vehicles. A Li-ion battery has two types of electrodes, anodes, and cathodes. This work focuses on anodes, specifically graphite and Si anodes. Si anodes have a high theoretical capacity (3579 mAh/g), nearly 10 times more than graphite) and a low (0.1 - 0.4 V vs Li/Li+) operating voltage. However, Si undergoes up to 300% volume expansion upon reaching the fully lithiated Li3.75Si phase. This causes both particle and electrode fracture, leading to continuous electrolyte decomposition at the electrode surface, forming an SEI. This results in irreversible consumption of Li, impedance increase, and capacity fading. Carbon coatings and additives can enhance cyclability, as well as the electrical conductivity of Si electrodes. Carbon coatings can also help manage the stress and strain of the electrode when Si is lithiated, reducing pulverisation. These approaches are costly and not compatible with commercial manufacturing techniques. In this work silicon-graphene composite anodes are produced through a highly scalable, one-step mixing process. Utilising microfluidic technology, graphite is exfoliated into graphene and mixed thoroughly to produce a homogeneous composite anode. The project aims to explore prelithiation of these anodes as a technique to further improve cyclability and then use solid-state NMR (ssNMR) to understand the differences in the lithiation mechanism during two different prelithiation techniques, electrochemical and chemical. Although much progress has been made on Si and Li metal anodes, graphite remains the major active component in Li-ion battery anodes. This is owing to its good specific capacity, 372 mAh/g, low cost, non-toxicity, low voltage hysteresis which is important for high energy efficiency and good cyclability. Graphite also has a low average lithiation voltage, ∼100 mV, which along with its flat voltage profile, results in a high operating voltage in full cells. Graphite is known to lithiate via a staged mechanism following either the Daumas-Hérold or the Rüdorff-Hofmann model. The stages are defined by the number, n, of graphene layers between adjacent Li layers. Some discrepancies exist in the literature as to the exact voltages and capacities the different stages form, especially the earlier stages. This could be due to differences in cycling rates as faster charging could lead to non-equilibrium states and the existence of multiple stages coexisting at once. In addition, what is still not fully understood is how the intercalation occurs when transitioning from the dilute stages to the dense stages. Also what is not known is whether lithium diffuses only in a 2D plane or whether any 3D diffusion occurs. In-situ and ex-situ 7Li ssNMR was used to probe the local chemical environment of Li and its dynamics in electrochemically lithiated graphite. Electrochemical GITT measurements, variable temperature XRD and 7Li ssNMR, including relaxometry techniques (T1, T2 and T1ρ), were used to study the intercalation of lithium into graphite and Li dynamics at different states of charge. Notably, through T1ρ measurements, it was shown that the lithium dynamics in the dilute stages is much faster than in the dense stages. These techniques will enable a deeper understanding of Li-ion dynamics in graphite could help further the understanding of ion dynamics in graphite and other layered intercalation compounds. The 13C NMR peak positions for the lithiated graphite stages 3L - 1 were assigned using ex-situ 13C NMR and XRD measurements. The dense stage LiC12 and LiC6 compounds were shown via 13C and 7Li to be metallic in nature, with both T1 times following the Korringa relation. The 7Li NMR shifts in graphite can be described by a direct Fermi-contact Knight shift while the 13C NMR shifts can be described in part by the combination of an anisotropic spin-dipolar Knight shift and an Isotropic Knight Shift.

Description

Date

2021-12-23

Advisors

Grey, clare

Keywords

NMR, Solid-State NMR, Lithium-Ion Batteries, Graphite, Silicon, Graphene, Knight Shift, Relaxometry, VT NMR, Lithium-Ion Dynamics

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
EPSRC (1804233)
EPSRC