The transition from fossil fuels to renewable energy sources necessitates the development of robust energy storage systems. Lithium-ion batteries (LIBs) are at the forefront of this shift, owing to their high energy density. This thesis focusses on the cathode electrode as it is not only the source of lithium ions (Li+) during operation but also determines key metrics such as practical specific capacity, operating voltage window, rate capability, and lifespan of an LIB. In general, a cathode electrode consists of an electrochemically active material (e.g., lithium nickel manganese cobalt oxide (NMC) or lithium iron phosphate (LFP)), an electronic conductive carbon additive (e.g., Super P), and a polymer binder (e.g., polyvinylidene difluoride (PVDF)). For the electrochemical reactions, Li+ ions are transported through the electrolyte that has penetrated the composite structure, reaching the cathode active material (CAM). To fully exploit electrochemical capabilities of the CAM, efficient ion transport is crucial. While extensive research has focused on improving liquid transport through various types of electrolyte chemistry, electrode tortuosity/porosity, and solid-state diffusion in the CAM, the role of interfacial transport in determining cathode performance remains less understood. Thus, this thesis aims to identify the factors that influence interfacial transport within the cathode electrode. Based on findings, the interfacial transport was designed to enhance ionic transport kinetics to ultimately boost the electrochemical performance of the cathode. In this thesis, Chapter 1 presents a brief introduction to the goals and outline, Chapter 2 provides a comprehensive literature review covering LIBs, cathodes, and ion transport and Chapter 3 details all experimental electrochemical characterisation techniques used in this thesis. In the experimental results chapters (Chapters 4, 5 and 6) pathways to enhance LIB cathode performance by optimising transport kinetics at the cathode – electrolyte interface were explored. In Chapter 4, carbon black (CB), traditionally considered unrelated to ionic transport, was investigated for its role in ionic transport. It was hypothesised that surface oxygen moieties of CB can participate in ion transport, being either detrimental or beneficial. To this end, specific oxygen functional group dominant CB, namely hydroxyl (CB-OH), carbonyl (CB=O), and carboxyl groups (CB-OOH), were synthesised through mild chemical treatments using hydrogen peroxide (30% v/v at 70 celsius for 24 – 72 h) and nitric acid (20% v/v reflux at 140 celsius for 1.5 h). The cathodes were fabricated using LiNi0.6Mn0.2Co0.2O2 (NMC622). The CB-OOH showed a 34% reduced capacity in 0.75C rate compared to CB, suggesting that strong electrostatic interactions from deprotonated carboxylate groups trap Li+ ions resulting in an increased energy barrier for ion transport. In contrast, CB=O promoted the formation of a more effective triple point (CAM - CB - electrolyte interface; Note that the binder was excluded from the triple point, as it is considered inactive with respect to ion transport). By deconvoluting the surface contributed charge storage using cyclic voltammetry (CV), it was shown that the CB=O enhanced surface reactions indicating more effective interfacial transport. The overpotential of CB=O measured via galvanostatic charge discharge (GCD) exhibited 109 mV (when x = 0.9 in LixNMC622), representing a 37% reduction compared to CB. The mechanism by which CB=O enhances the rate performance was similarly observed in other CAMs such as LiNi0.8Mn0.1Co0.1O2 (NMC811), LiMn2O4 (LMO), and LiNi0.8Co0.15Al0.05O2 (NCA). In Chapter 5, building on the Coulombic attraction between Li+ ions and functional groups, this chapter introduced an interfacial layer directly on the CAM surface to utilise Coulombic interaction with Li+ ions. A negatively charged metal-phenolic network (MPN) was trialled in which various thicknesses were formed on the CAM surface via rapid self-assembly of tannic acid and iron ions (Fe3+). It was hypothesised that by utilising a catechol-based MPN layer that Li+ ions would be attracted closer to the cathode – electrolyte interface. Before electrochemical testing, it was confirmed by Synchrotron x-ray absorption spectroscopy (XAS) that the coating process had no influence on the oxidation states and interatomic bonding distances for the NMC622. Operando electrochemical impedance spectroscopy (EIS) showed that MPN reduced electrochemical reaction impedances, and subsequent distribution of relaxation times (DRT) analysis confirmed that this decrease was primarily attributed to Li+ ... MPN interactions improving interfacial transport. In addition, the galvanostatic intermittent titration technique (GITT) revealed that the MPN layer facilitates efficient structural phase transition of the CAM, indicating a more homogeneous intercalation reaction at the CAM surface. In the half-cell format, the MPN coated CAM exhibited a 54% decrease in interfacial resistance and a 20% reduction in overpotential compared to the pristine cathode. In full-cell format, MPN coated CAM demonstrated a 10% increase in specific capacity and a 54% increase in lifespan. This study is the first to reveal that introducing a negatively charged layer at the LIB CAM interface can enhance interfacial transport by increasing Li+ ion flux through charge-attraction principle. Producing cathodes with a higher mass loading of the CAM leads to higher energy density. However, higher mass loading cathode cannot undergo fast cycling due to the transport limitations, which have traditionally been attributed solely to the increased liquid transport length. Given that the ionic concentration gradient driving liquid transport is governed by interfacial transport, it was hypothesised that interfacial transport could be optimised by changing the metal ions in the MPN; and it was examined how interfacial transport influence the liquid transport. In Chapter 6, further types of MPNs were introduced using various metal ions namely, Ni2+, Zn2+, Fe3+, Mn3+, Ti4+, and Zr4+. It is generally accepted that the metal ligand bonding tends to strengthen as the oxidation state of the metal centre increases. As a CAM, LFP was used because of its relatively higher chemical stability and better rate retention compared to NMC. Synchrotron XAS was employed to examine the impact of the MPN formation on LFP crystals. The absorption of tannic acid reduces unstable Fe3+ to Fe2+ which can increase the number of Li+ storage sites. In the half-cell format, various metal ion driven MPN coated LFPs were investigated in the range of mass loading from 2 to 16 mg cm-2 to control liquid transport length. The cycling time constant of ion transport showed that it is controlled by interfacial transport in low mass loading and by liquid transport in high mass loading. In the low mass loading (< 10 mg cm-2), the time constant appeared in the order of Ti(IV) < Zr(IV) < Fe(III) < Mn(III) < Ni(II) < Zn(II). However, in higher mass loading, the slopes of increase in the time constant with respect to mass loading decreased in the order of Ti(IV) < Zr(IV) < Mn(III) < Zn(II) < Ni(II) < Fe(III). Since the ion flux in liquid transport is governed by the electrolyte’s diffusion coefficient and the concentration gradient, fast interfacial transport can increase the concentration gradient between the bulk electrolyte and the CAM interface promoting liquid transport. This suggests that the rate capability of high energy density cathodes potentially depends on the interfacial transport rather than liquid transport.

Donghyuck Park

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School of Chemical and Biomedical Engineering

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