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Online electrochemical mass spectrometry
Online electrochemical mass spectrometry (OEMS) is composed of an electrochemical cell with both a vacuum system and a quadrupole mass spectrometer which analyzes the volatile species sampled in the head space of the electrochemical cell.118,147 Figure A. 1. 2 in Appendix shows a schematic view of the whole OEMS system used in this work, which is a closed system allowing for the continuous gas sampling from the electrochemical cell to the mass spectrometer (MS) through a thin capillary. More detailed description of the system can be found in Appendix section A. 1. 2. Figure II. 2 present the electrochemical cell used for the OEMS experiments. The cell is derived from the in-situ pressure cell with one single modification: the quick connector for gas inlet/outlet in the pressure cell has been replaced by a Swagelok adaptor which can be connected to either a connector to the inlet capillary of MS or to the gas filling station.
Rotating ring disk electrode voltammetry
As illustrated in Figure II. 3a, RRDE tip is constructed by embedding in the cylindrical insulator (polytetrafluoroethylene (PTFE) is typically used owing to its great chemical stability and inertness) a central disk electrode and a surrounding ring electrode. Depending on the target usage, different materials and sizes can be selected for the electrodes as well as the insulator. Platinum ring is chosen in this work due to its remarkable catalytic activity for oxygen reduction reaction, as witnessed in the fields of Li-O2 battery and fuel cell.148,149 The disk and the ring electrode are electrically separated by an insulating barrier (a PTFE U-cup as marked with blue arrow in Figure II. 3a). The RRDE experiments are carried out with a bipotentiostat, which controls independently the potentials of the disk and the ring electrode with respect to the reference electrode and measures separately the current going through each electrode (Figures II. 3b & 4). The RRDE tip is also connected with a motor controller through a shaft which allows for rotating both the two electrodes at the same rate (Figure II. 4). A schematic of the whole RRDE system together with a four-electrode electrochemical cell is presented in Figure II. 4 and more details about the set-up can be found in Appendix Section A. 2. 1.
Detailed structural characterization of phase A’
Next, the structure of the A’ phase was determined by combined transmission electron microscopy (TEM), synchrotron XRD and NPD.
TEM data were collected by Dr. A. Abakumov at EMAT, Belgium. The electron diffraction (ED) patterns reflect that the majority of the A’ phase crystallizes in the O3 𝑅3̅𝑚 structure (Figure III. 6b). The [010] ED pattern demonstrates reflection splitting along the c* direction, which is the typical signature of the mirror-twinned O3 𝑅3̅𝑚 structure with the (001) twin plane (Figures III. 6b and 7). The nanosized twinned domains are evident in the low magnification high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure III. 6b). Besides sharp reflections of the O3 structure, diffuse spots are present in the [010] ED pattern of the A’ phase at the positions characteristic of the O1 structure (marked with vertical arrowheads in Figure III. 6b). In contrast to the pristine Li-rich NMC sample,43 the [110] ED pattern of the A’ phase shows only very faint diffuse intensity lines from the “honeycomb” Li-TM ordering (marked with horizontal arrowheads in Figure III. 6b), indicating that this ordering is largely suppressed. This justifies the validity of the simple O3 𝑅3̅𝑚 structure model used for further Rietveld refinement of the A’ structure.
The impact of phase transition on the electrochemical properties
Next, we continued to explore how the phase transition impacts the electrochemical behavior for this Li-rich phase. As previously demonstrated in the subchapter III. 2. 1, following the 4.8 V charge, phase A → A’ transition starts to occur after entering the OCV period, and gradually becomes a single phase A’ with CV hold. Therefore, we tentatively cycled three identical Li1.2Ni0.13Mn0.54Co0.13O2/Li cells between 4.8 V and 2.0 V, but with different first charge protocols: one cell was directly switched to discharge (denoted as CC, black curve) with phase A forming at the end of the first charge, whereas either an intermediate OCV step for 120 h (blue line, denoted as CC – OCV) or CV hold for 8 h (red line, denoted as CC – CV) was imposed to the other two cells to trigger the formation of phase A’. Figure III. 13 compare the first charge-discharge voltage curves, and Figure III. 14 compare the retention for the average discharge voltage and the discharge capacity upon cycling. Notably, phase A’ exhibits a larger first-cycle irreversible capacity and an initial voltage drop as compare to phase A (Figure III. 13), whilst upon cycling, the persistence of the voltage decay and capacity fading (Figure III. 14).
From an electrochemical perspective, the initial voltage drop can be explained by the reduction of transition metal ions (Co and Ni) during OCV or CV step, as revealed by ex-situ X-ray absorption spectroscopy (XAS) measurements (Figure III. 15). The observed capacity drop after imposing the OCV or CV step is closely associated with the gradual phase A → A’ transition. More specifically, the contraction of TM layers and the growing Mn occupation in the octahedral Li sites upon A → A’ phase transition certainly would cause a kinetic hindrance in Li+ intercalation. In addition, the formation of phase A’ is accompanied with O2 release, which are known to induce particle cracking that could also contribute to the performance deterioration.
Figure III. 13 The first charge-discharge voltage curves of Li1.2Ni0.13Mn0.54Co0.13O2 cycled with different protocols. One cell was charged to 4.8 V and then directly discharged to 2.0 V, denoted as CC (black line), whereas an intermediate OCV step for 120 h (blue line, denoted as CC – OCV) or CV hold for 8 h (red line, denoted as CC – CV) was respectively imposed to the other two cells. All the cells were cycled at C/3.
Mn migration triggered by oxygen redox and its consequence of extra low-voltage electrochemical activities
As previously discussed, phase A’ forming under harsh oxidizing condition enlists Mn migration, we further investigated the influence of depth of charge on the Mn migration. For that, we prepared several Li1.2Ni0.13Mn0.54Co0.13O2/Li cells that were electrochemically oxidized till Δx = 0.4, 0.6, 0.8, and 1.0 followed by 120 h OCV (Δx refers to the amount of Li+ removal on charge or the amount of Li+ insertion on discharge), prior to sampling the electrodes for ex-situ synchrotron X-ray diffraction measurements. By employing the same Rietveld methodology as before, we could deduce that while the Δx = 0.4 charged sample shows no Mn migration in interlayer octahedral sites (defined as in [Li1.0-ΔxMn][Li0.2Ni0.13Mn0.54-Co0.13]O2), a progressive increase of Mn migration () was observed with increasing Δx from 0.6 up to 1.0 (Figure III. 16a). Interestingly, further maintaining the Δx = 1.0 charged sample at 4.8 V for 8 hours to obtain the A’ phase did not change the amount of Mn migration () within the accuracy of this method, but slightly decreased the c parameter from 14.2058(4) Å to 13.9722(14) Å (Tables III. 3 & A. 3. 1). This is consistent with the decrease in Li+ content from 0.02 (forx = 1.0 charged sample) to 0.005 (for CC – 8 h CV charged sample) according to ICP-OES, which leads to less charged oxygen ions thereby weaker oxygen-oxygen repulsion. In parallel, X-ray photoemission spectroscopy (XPS) measurements were carried out for the various charged samples to determine the fraction of oxygen involved in the anionic redox process (Figure III. 16c). To no surprise, this fraction increases with increasing Δx alike the degree of Mn migration (), hence, implying their correlation. In contrast, note that neither Mn migration (or the percentage of oxidized oxygen correlated with O2 release (Figure III. 16).
Table of contents :
Table of contents
Broader context and thesis outline
Outline of thesis
Chapter I General introduction
I. 1 Basic concepts of Li-based batteries and insertion-type electrodes
I. 1. 1 Intercalation chemistry and the advent of Li battery
I. 1. 2 From Li battery to Li-ion battery
I. 1. 3 Positive electrode materials for Li-ion batteries and their energy limitation .
I. 2 Increase the energy density of positive electrode materials for Li-ion batteries
I. 2. 1 M/M’ chemical substitution
I. 2. 2 Li substitution and the participation of oxygen redox
I. 2. 3 Positive electrode materials with elevated (de)intercalation voltage
I. 3 Searching for new chemistries with potentially higher energy storage
I. 3. 1 Conversion chemistry
I. 3. 2 Li-O2 battery
I. 4 Conclusions
Chapter II Monitor the oxygen release at high potentials in Li-rich layered oxides
II. 1 Background and motivation
II. 2 Description of the in-situ gas analysis techniques
II. 2. 1 In-situ pressure cell
II. 2. 2 Online electrochemical mass spectrometry
II. 2. 3 Rotating ring disk electrode voltammetry
II. 3 Results and discussion
II. 4 Chapter conclusion and outlook
Chapter III Revisiting the structural evolutions and electrochemical properties in the first cycle of Li-rich NMC
III. 1 Background and motivation
III. 2 Results and discussion
III. 2. 1 Phase transition and O2 release on deep oxidation
III. 2. 2 Detailed structural characterization of phase A’
III. 2. 3 The impact of phase transition on the electrochemical properties
III. 2. 4 Mn migration triggered by oxygen redox and its consequence of extra low-voltage electrochemical activities
III. 3 Discussion and chapter conclusion
Chapter IV Li-CO2 battery: a new system for energy storage and CO2 conversion
IV. 1 Background and motivation
IV. 2 Li−O2/CO2 batteries: CO2 conversion via electrogenerated superoxide (O2●¯)
IV. 2. 1 Introduction
IV. 2. 2 Results and discussion
IV. 2. 3 Discussion and conclusion
IV. 3 Li−CO2 batteries: CO2 conversion mediated by quinone derivatives 121
IV. 3. 1 Introduction
IV. 3. 2 Results and discussion
IV. 3. 3 Conclusion
Chapter V General conclusion and outlook
A. 1 Design of experimental equipment
A. 1.1 Gas filling station
A. 1. 2 Description of OEMS systems
A. 2 Supporting information for Chapter II
A. 2. 1 Methods
A. 2. 2 Supporting figures
A. 2. 3 Calculation of the quantity of released O2 gas
A. 3 Supporting information for Chapter III
A. 3. 1 Methods
A. 3. 2 Supporting figures
A. 4 Supporting information for Chapter IV
A. 4. 1 Methods
A. 4. 2 Supporting figures
Bibliography
