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A study on multilayered MXenes for rechargeable battery electrodes through surface termination control
Get PDFIn the transition from fossil fuels to renewable energy sources, the demand for rechargeable batteries is rapidly increasing. To meet this demand, a diversification of battery chemistries will be essential, due to limited abundance of currently used battery materials. In addition to Li-ion batteries (LiBs), rechargeable Mg batteries (RMBs) stand out as a theoretically promising battery chemistry, with the potential to yield high energy batteries based on abundant materials. However, to enable practical RMBs, competitive cathode materials need to be established. Potential candidates may come from the family of two-dimensional transition metal carbides and nitrides known as MXenes, which due to their high electrical conductivity, chemical tunability and ion intercalating properties, have been predicted to yield high intercalation capacities for a range of cations, including Mg-ions. Hence, this study aimed towards evaluating the feasibility of MXenes for rechargeable battery electrodes, and in particular for RMBs.
In the first paper of this work, the feasibility of MXenes as an RMB cathode was investigated by using two different MXene compositions: V2C and Ti3C2. To ensure the possibility for Mg-ion desolvation and migration, these electrodes were galvanostatically cycled in cells with four different electrolytes, and at both low (20 oC) and high (60 oC) temperatures. Still, only minimal capacities were obtained, indicating insignificant reversible Mg-intercalation in the MXene particles. However, by including Li-salts to the electrolyte, significant reversible capacities were measured, demonstrating the functionality of the MXene electrodes. To explain this difference, density functional theory (DFT) calculations were implemented and showed that the migration barriers for Mg-ions were significantly higher than for Li-ions, and that the termination groups attached to the surface of the MXene sheets strongly influenced the intercalation voltages for Li and Mg-ions. Denoted as "T" (i.e. V2CTx), these termination groups usually consist of a mixture of OH, O and F, and the DFT calculations showed that multilayered particles of O-terminated MXenes would be ideal for RMB cathodes. Hence, the focus of this work was directed towards controlling the termination groups of MXenes, and especially towards substituting F- with O-terminations.
In the second and third paper of this work, gas hydrolysation was explored as a post etching treatment to substitute F-terminations in multilayered V2CTx and Ti3C2Tx MXenes, respectively. It was found that successful hydrolysation required a MXene structure with an expanded interlayer spacing, to allow the water molecules to penetrate in between the MXene sheets. For the V2CTx phase, this was achieved by intercalated water molecules from the HF etching step, whereas the Ti3C2Tx phase required preintercalation of cations prior to hydrolysation. For both phases, a F-reduction around 70 % was achieved, demonstrating the greatest F-reduction in multilayered particles reported so far. However, due to challenges of quantifying the remaining terminations (O vs. OH vs. unterminated), the exact composition of the final MXenes remained unclear. Although the F-reduction was clear for both MXene phases, no increase in capacity was observed upon cycling in RMB coin cells. To investigate the termination group’s influence on the electrochemical performance, MXene electrodes were thus cycled in LiBs. This resulted in dissimilar changes for the two MXene compositions. Where the hydrolysed V2CTx phase displayed an enhanced rate capability compared to the pristine MXene, the opposite was true for the Ti3C2Tx MXene. The Ti3C2Tx phase also showed increased average voltages and capacities after hydrolysation, which was not observed for the V2CTx phase. Based on these ambiguous changes in electrochemical performance, and the uncertain composition of the resultant termination mixtures, no clear conclusion on the effect of termination change could be drawn from this work. To ensure controlled formation of O-terminated MXenes, further refining of the hydrolysis method will be required, combined with enhanced termination characterisation.
In addition to the published results, the thesis includes new results on a F-free etching method based on hydrothermal treatment in concentrated NaOH solutions. These etching
conditions allowed for F-free Ti3C2Tx MXene, while the synthesis of other MXene compositions was not successful. Whereas all of the Ti2AlC, Ti2AlN, Ti3AlCN, TiMo2AlC2, Nb4AlC3, Nb2AlC, V2AlC and V4AlC3 MAX phase precursors demonstrated significant oxidation, the former three showed possible signs of some unordered MXene. Further investigation, with optimised etching conditions, is therefore required to conclude about this etching method’s compatibility to form other MXene phases than Ti3C2Tx. Nevertheless, due to the mixture of O- and OH-terminations, this etching method is not ideal for optimal termination control. In comparison, a recently reported etching method, based on Lewis acid etching in molten salts, has enabled synthesis of homogeneously terminated MXenes from different precursors, and with various terminations
Pursuing New Cathode Materials for Rechargeable Magnesium Batteries
No full textHigh-performance and safe batteries based on cheap and sustainable materials are highly desirable to accelerate the transition to a renewable energy-based society. The ongoing electrification of the transport sector has largely been enabled by a drastic cost decline in battery energy, and further cost reductions may be enabled by moving beyond the conventional Li-ion battery redox chemistry. Rechargeable magnesium batteries (RMBs) represent one such technology, with truly attractive theoretical properties. Yet, the realization of RMBs have been hampered by the challenging tasks of developing suitable electrolytes and cathode materials. In this work, several cathode materials have been synthesized and investigated as potential candidates for RMBs.
The first part of the work assessed the feasibility of using MXenes as an RMB cathode material. MXenes, a relatively new large group of 2D materials, have been theoretically predicted to be promising electrode materials for RMBs, with marginal experimental efforts reported. Two of the most well-known MXenes, Ti3C2Tx and V2CTx were successfully synthesized with high phase purity and a controlled particle size. However, negligible Mg2+ intercalation was obtained (capacities generally < 5 mAh/g), regardless of an elevated testing temperature of 60 °C and different electrolytes. The absence of Mg2+ intercalation in MXenes were examined by DFT calculations, revealing challenges with high Mg2+ migration barriers, non-spontaneous intercalation and a propensity for unwanted side reactions. Guidelines for further studies were also provided. In particular, oxygen-terminated multi-layered V2CO2 may be a viable cathode candidate, but will be non-trivial to practically realize.
In the second part, cathodes based on dipenthamethylene thiuram tetrasulfide (PMTT) were investigated and reported with one of the highest capacities for an organosulfur compound for RMBs (295 mAh/g). The high capacity was likely enabled by a solid-liquid conversion mechanism, greatly benefiting from bypassing solid-state Mg2+ diffusion. On the other hand, such a mechanism also caused serious challenges regarding cycle life due to transport and subsequent reduction of soluble cathode species on the Mg anode, analogous to the polysulfide shuttling seen for regular sulfur cathodes. Still, an improved cycling stability was reported compared to a conventional S8 cathode. Possible reasons for the improved performance and the detailed reaction mechanism were discussed. Two different active material preparations and a strong effect of cycling conditions on the electrochemical performance were also reported.
The work from part one and two culminated in the third and last part, where MXene-based composite cathodes with PMTT or S8 as active material were evaluated. Several current collector- and binder-free cathode architectures were fabricated through a facile vacuum-filtration technique. A synergistic effect of MXenes and carbon nanotubes were confirmed, where the carbon nanotubes offered a high surface area for faster reaction kinetics and could prevent MXene restacking, and MXene’s polar surface chemistry was attributed to mitigate polysulfide shuttling and improve capacity retention. A MXene interlayer between the cathode and the separator was found to effectively reduce self-discharge and thereby increase capacity utilization and average voltage, ultimately enabling a capacity of 530 mAh/g at a voltage of ~1.4 V with a capacity retention of 83% after 25 cycles.
All in all, this study aimed to direct further efforts on RMB cathodes in a more viable direction, by 1) narrowing the feasible MXene cathode candidates and emphasize the MXene-specific challenges, 2) report the electrochemical performance of a new organosulfur compound (PMTT) and 3) demonstrate a beneficial role of MXenes as a polysulfide-scavenging conductive additive in the cathode architecture in Mg-S batteries
Fordeler og ulemper med litium-svovel batterier for en bærekraftig framtid
Get PDFLitium-svovel batterier er et type batteri som har vekket stor interesse på grunn av dens høye spesifikke kapasitet. Det er derimot en del utfordringer assosiert med teknologien som en shuttle effekt, dendrittdannelse, volumekspansjon og svovels dårlige ledningsevne. Disse fører til selvutladning, redusert kapasitet og faremomenter som må fikses før batteriene kan bli brukt kommersielt. Disse blir alle diskutert i denne bacheloroppgaven sammen med noen potensielle løsninger.Lithium-sulphur batteries is a type of battery that has gained a lot of traction due to its high specific capacity. There are however quite a few challenges associated with the technology such as a shuttle effect, dendrite growth, volume expansion and sulphur’s low conductive skills. These lead to self-discharge, reduced capacity and risk of use, and must be fixed before the batteries can be used commercially. These are all discussed in this bachelor assignment along with some potential solutions
Difference in First Cycle Irreversible Loss between Li-ion batteries with Graphite as Anode and Na-ion Batteries with Hard Carbon as Anode
Get PDFHensikten med denne artikkelen er å gjennomgå effekten av første syklus irreversibelt tap av kapasitet i litium-ion batterier (LIBs) som inneholder en grafittanode og natrium-ion batterier (SIBs) som inneholder en anode av hard karbon. Batteristruktur, grafitt og hardkarbons struktur og egenskaper, solid elektrolytt interfase (SEI) og førstesyklustap vil bli diskutert, samt alternative anodematerialer kort. Hovedfokuset i denne artikkelen er å se på årsaken til irreversibelt tap i første syklus. Dette er hovedsakelig grunnet dannelsen av et SEI-lag på anodematerialet (i tilfellet for LIBs og SIBs). SEI er nødvendig for at batteriet skal fungere skikkelig over tid, noe som betyr at for å redusere første syklus irreversibelt tap, er en optimalisering av SEI nødvendig. I tillegg til dette vil energitettheten og prisen på LIB og SIB bli vurdert, samt miljøvennlighet. På bruksområder hvor pris og/eller miljøvennlighet er viktigere enn energitetthet til batteriet, vil SIB-er kunne vise seg å være en erstatning for LIB-er, i en fremtid hvor LIB-er og SIB-er sameksisterer.The purpose of this paper is to review the effects of first cycle irreversible loss of capacity in lithium-ion batteries (LIBs) containing a graphite anode and sodium-ion batteries (SIBs) containing a hard carbon anode. Battery structure, graphite’s and hard carbon’s structure and properties, solid electrolyte interphase (SEI) and first cycle loss will be discussed, as well as alternative anode materials briefly. The main discussion in this paper is the cause of first cycle irreversible loss. This is caused by the formation of a SEI layer on the anode material (in the case of LIBs and SIBs). The SEI is necessary for the battery to function properly over time, meaning in order to reduce first cycle irreversible loss, an optimization of the SEI is necessary. In addition to this, the energy density and the price of LIBs and SIBs will be considered, as well as environmental friendliness. In areas where price and/or environmental friendliness is more important than energy density, SIBs could prove to be a substitute for LIBs, in a future where LIBs and SIBs coexist
Prelithiation of High-Energy Li-ion Batteries by Sacrificial Salts
No full textDenne oppgaven undersøkte effekten av prelitiering ved bruk av et offersalt, Li2C2O4 (litiumoksalat, LO), på LiNi0,5Mn1,5O4 (LNMO)||Si celler med høy arealkapasitet (3.5 – 3.9 mAh/cm2). LNMO-katoder med enten 0, 3,9, 7,4, eller 10,7 vekt% LO ("LNMO", "LO1", "LO2", "LO3") ble laget og koblet sammen med enten Li-metall eller Si som anode. Si-halvceller ble også laget. Halv- og fullcellene ble så elektrokjemisk evaluert via sykling.
Si-halvcellene oppnådde en lav litieringskapasitet (1323,8 ± 379.0 mAh/gSi) i første syklus, med høyt irreversibelt tap (14,2 ± 2,8%). Langtidssykling ved C/5 i 100 sykler resulterte i lav kapasitetsbevaring (11,5 ± 5,5%). Den dårlige ytelsen skyldtes trolig høye arealkapasiteter, (gjen-)dannelse av SEI-laget, tap av aktivt materiale samt andre degraderingsmekanismer.
Fullcellene oppnådde økende ladekapasiteter ved økende LO-innhold, fra 142,2 ± 1,2 til 165,0 ± 3,4, 182,4 ± 1,1 og 204,1 ± 1,9 mAh/gLNMO for LNMO, LO1, LO2 og LO3. Utladningskapasitetene økte også med økende LO-innhold. Den gravimetriske energitettheten til LNMO (354 ± 5 Wh/kg) økte med 13 ± 3%, 18 ± 0% og 17 ± 3% for LO1, LO2 og LO3.
Langtidssykling ved C/3 i 100 sykler resulterte i høyest kapasitetsbevaring for LO1 (64,5 ± 16,7%), etterfulgt av LNMO (56,1 ± 8,7%), LO2 (45,1 ± 12,1%) og LO3 (39,3 ± 6,3%). På grunn av store standardavvik var ikke forskjellene signifikante, men de ble tolket som en indikator på at noe porøsitetsøkning som følge av LO-dekomponering er gunstig, mens for høy porøsitet påvirker ytelse negativt i form av høy polarisering og mulig tap av aktivt materiale.
Potensiostatisk elektrokjemisk impedansspektroskopi ble utført for å undersøke effekten av LO og LO-dekomponering på impedansen til LNMO- og LO3-katoder. Kontaktmotstanden ble konkludert til å stamme fra grenseflaten mellom aluminiumsfolien og katoden. Høy ladningsoverføringsmotstand i LO3 minket betydelig etter sykling: dette skyldtes trolig dekomponering av LO med sin dårlige ledningsevne. Tolagskapasitaten til LO3 minket betydelig mer etter sykling enn LNMO sin: dette ble forklart som at økt porøsitet etter LO-dekomponering fører til betydelig elektrisk isolering av aktivt materiale. Dette forklarer muligens også den dårlige kapasitetsbevaringen til LO3-fullcellene.
Prelitiering med LO som offersalt ble konkludert til å være vellykket i å kompensere for irreversibelt kapasitetstap i første syklus, samt øke den gravimetriske energitettheten til LNMO||Si fullceller med høyt innhold av aktive materialer. Levetiden ble også forbedret opp til et visst LO-innhold. Optimalisering av LO-innhold ble konkludert til å være avgjørende i celler med høyt innhold av aktive materialer, for å unngå problemer med store polariseringseffekter, tap av aktivt materiale, og rask kapaitetsfalming. Ytterligere optimalisering av prelitieringsprosessen ble konkludert til å være nødvendig for å sikre fullstendig LO-dekomponering i første syklus.This thesis studied the effect of prelithiation with a sacrificial salt, Li2C2O4 (lithium oxalate, LO), on LiNi0.5Mn1.5O4 (LNMO)||Si cells with high areal capacities (3.5 – 3.9 mAh/cm2). LNMO cathodes were made and enriched with 0, 3.9, 7.4, or 10.7 wt% LO ("LNMO", "LO1", "LO2", "LO3"), and paired with Li metal or Si anodes. Si half cells were also made. The half and full cells were then cycled to evaluate their electrochemical performance.
The Si half cells attained a low first-cycle lithiation capacity (1323.8 ± 379.0 mAh/gSi) with a high irreversible loss (14.2 ± 2.8%). Long-term cycling at C/5 for 100 cycles resulted in low capacity retention (11.5 ± 5.5%). The poor performance was attributed to high loadings, solid-electrolyte interphase (re-)formation, loss of active material, and other degradation mechanisms.
The full cells’ first-cycle charge capacities increased with increasing LO content, from 142.2 ± 1.2 to 165.0 ± 3.4, 182.4 ± 1.1, and 204.1 ± 1.9 mAh/gLNMO for LNMO, LO1, LO2, and LO3. The discharge capacities also increased with increasing LO content. LNMO’s specific energy (354 ± 5 Wh/kg) increased by 13 ± 3%, 18 ± 0%, and 17 ± 3% in LO1, LO2, and LO3.
Long-term cycling of the full cells at C/3 for 100 cycles resulted in LO1 achieving the highest capacity retention (64.5 ± 16.7%), followed by LNMO (56.1 ± 8.7%), LO2 (45.1 ± 12.1%), and LO3 (39.3 ± 6.3%). Large standard deviations meant the differences could not be considered significant; however, the results were interpeted as an indicator that a degree of increased porosity following LO decomposition is beneficial, while excessive porosity has negative effects on performance, such as high polarisation effects and possible loss of active material.
Potentiostatic electrochemical impedance spectroscopy was conducted to investigate the effect of LO and LO decomposition on impedance in LNMO and LO3 cathodes. Contact resistance was concluded to originate in the Al current collector/cathode interface. High charge-transfer resistance in LO3 decreased significantly after cycling: this was attributed to decomposition of poorly conductive LO. Charge-transfer double-layer capacitance decreased more in LO3 than LNMO after cycling: this was attributed to increased porosity in LO3 causing electrical isolation of active material particles and loss of active material. This could also explain the poorer capacity retention of the LO3 full cells.
Prelithiation with LO as a sacrificial salt was deemed successful in compensating for first-cycle irreversible loss and increasing gravimetric energy density in LNMO||Si full cells with high loadings. Cycle life was also improved up to a certain LO content. Optimisation of LO content was concluded to be crucial in cells with high loadings, to avoid issues with high polarisation effects, loss of active material, and rapid capacity fade. Further optimisation of the prelithiation process was deemed necessary to ensure complete LO decomposition in the first cycle
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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