The role of anionic processes in Li1−xNi0.44Mn1.56O4 studied by resonant inelastic X-ray scattering

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The Electrochemistry of LiNi0.5-xMn1.5+xO4-δ in Li-ion Batteries : Structure, Side-reactions and Cross-talk

The use of Li-ion batteries in portable electronic products is today widespread and on-going research is extensively dedicated to improve their performance and energy density for use in electric vehicles. The largest contribution to the overall cell weight comes from the positive electrode material, and improvements regarding this component thereby render a high potential for the development of these types of batteries. A promising candidate is LiNi0.5Mn1.5O4 (LMNO), which offers both high power capability and energy density. However, the instability of conventional electrolytes at the high operating potential (~4.7 V vs. Li+/Li) associated with this electrode material currently prevents its use in commercial applications. This thesis work aims to investigate practical approaches which have the potential of overcoming issues related to fast degradation of LNMO-based batteries. This, in turn, necessitates a comprehensive understanding of degradation mechanisms. First, the effect of a well-known electrolyte additive, fluoroethylene carbonate is investigated in LNMO-Li4Ti5O12 (LTO) cells with a focus on the positive electrode. Relatively poor cycling performance is found with 5 wt% additive while 1 wt% additive does not show a significant difference as compared to additive-free electrolytes. Second, a more fundamental study is performed to understand the effect of capacity fading mechanisms contributing to overall cell failure in high-voltage based full-cells. Electrochemical characterization of LNMO-LTO cells in different configurations show how important the electrode interactions (cross-talk) can be for the overall cell behaviour. Unexpectedly fast capacity fading at elevated temperatures is found to originate from a high sensitivity of LTO to cross-talk. Third, in situ studies of LNMO are conducted with neutron diffraction and electron microscopy. These show that the oxygen release is not directly related to cation disordering. Moreover, microstructural changes upon heating are observed. These findings suggest new sample preparation strategies, which allow the control of cation disorder without oxygen loss. Following this guidance, ordered and disordered samples with the same oxygen content are prepared. The negative effect of ordering on electrochemical performance is investigated and changes in bulk electronic structure following cycling are found in ordered samples, accompanied by thick surface films on surface and rock-salt phase domains near surface

The Electrochemistry of LiNi0.5-xMn1.5+xO4-δ in Li-ion Batteries : Structure, Side-reactions and Cross-talk

The use of Li-ion batteries in portable electronic products is today widespread and on-going research is extensively dedicated to improve their performance and energy density for use in electric vehicles. The largest contribution to the overall cell weight comes from the positive electrode material, and improvements regarding this component thereby render a high potential for the development of these types of batteries. A promising candidate is LiNi0.5Mn1.5O4 (LMNO), which offers both high power capability and energy density. However, the instability of conventional electrolytes at the high operating potential (~4.7 V vs. Li+/Li) associated with this electrode material currently prevents its use in commercial applications. This thesis work aims to investigate practical approaches which have the potential of overcoming issues related to fast degradation of LNMO-based batteries. This, in turn, necessitates a comprehensive understanding of degradation mechanisms. First, the effect of a well-known electrolyte additive, fluoroethylene carbonate is investigated in LNMO-Li4Ti5O12 (LTO) cells with a focus on the positive electrode. Relatively poor cycling performance is found with 5 wt% additive while 1 wt% additive does not show a significant difference as compared to additive-free electrolytes. Second, a more fundamental study is performed to understand the effect of capacity fading mechanisms contributing to overall cell failure in high-voltage based full-cells. Electrochemical characterization of LNMO-LTO cells in different configurations show how important the electrode interactions (cross-talk) can be for the overall cell behaviour. Unexpectedly fast capacity fading at elevated temperatures is found to originate from a high sensitivity of LTO to cross-talk. Third, in situ studies of LNMO are conducted with neutron diffraction and electron microscopy. These show that the oxygen release is not directly related to cation disordering. Moreover, microstructural changes upon heating are observed. These findings suggest new sample preparation strategies, which allow the control of cation disorder without oxygen loss. Following this guidance, ordered and disordered samples with the same oxygen content are prepared. The negative effect of ordering on electrochemical performance is investigated and changes in bulk electronic structure following cycling are found in ordered samples, accompanied by thick surface films on surface and rock-salt phase domains near surface

Mg-Ni nanoparçacıkların endüksiyon plazma ile sentezi.

There is a considerable interest in developing magnesium and magnesium alloys in the form of nanoparticles for hydrogen storage purposes. In the current study, induction thermal plasma was used to synthesize Mg-Ni nanoparticles. In this technique, precursors, which are normally powders, are fed to the thermal plasma where they are evaporated and nanoparticles are derived from condensation of this vapor in the quenching zone. RF induction plasma system used in the current work was operated at 25 kW and the reactor incorporated two injectors axially located in the torch, one from above and the other from below. This allowed injection of precursor powder in different temperature zones. Precursors used were mainly elemental Ni and Mg powders, but experiments were also carried out with pre-alloyed Mg2Ni. The study has shown that Mg, Ni, Mg2Ni with the additional amount of MgO phase could be synthesized in the form of nanopowders less than 100 nm in size. Upon feeding elemental powders Mg and Ni, Mg2Ni could be synthesized; but, its fraction was quite low. The fraction was maximized and reached a weight fraction of 0.55 when Ni was fed from the top and Mg from the bottom injector the tip of which was located just below the quenching gas inlet. Hydrogenation of these powders showed fast kinetics, but there was no significant decrease in the dehydrogenation temperature. So as to further reduce the particle size and also to prevent oxidation, additional experiments were carried out where a fraction of argon used as carrier gas was replaced with methane. This has led to a core-shell structure in derived nanopowders. In the case of Ni, nanoparticles of varying sizes, e.g. 5-200 nm were encapsulated by 3-9 graphitic layers. For Mg, core-shell structure with a comparable clarity was not observed. Instead, nanoparticles which had been embedded in carbonaceous matrix were obtained. The size of these particles could be as small as 5 nm.M.S. - Master of Scienc

FEN BİLİMLERİ ENSTİTÜSÜ/LİSANSÜSTÜ TEZ PROJESİ

MAGNEZYUM TEMELLİ NANOPARÇACIKLARIN ISIL PLAZMA İLE SENTEZ

Size reduction in Mg rich intermetallics via hydrogen decrepitation

A study was carried out into hydrogen decrepitation of Mg rich intermetallics, namely Mg2Ni and Mg2Cu. These intermetallics are quite similar to each other except for the fact that Mg2Ni hydrides directly forming Mg2NiH4, whereas Mg2Cu when hydrided disproportionates into a two-phase structure. A total of ten sorption cycles was applied to the alloys and the resulting size reductions were monitored. The results showed that Mg2Ni decrepitate quite fast with cycling, the greatest size reduction occurring within the first three cycles. The size reduction in Mg2Cu, on the other hand, was quite sluggish. This was attributed to the disproportionation of the alloy which involve more extensive diffusion of the metallic species, counteracting some decrepitating effect of cycling due to ensuing particle sintering and growth

Carbon Coating of magnesium via thermal plasma

Magnesium is a material of considerable interest for electrochemical as well as thermal energy storage. Carbon coated magnesium or magnesium nanopartciles emebedded in a graphitic matrix could provide solutions to some of the problems currently faced in these application areas. The current study was undertaken to develop Mg-C composites at a variety of length scales. The synthesis was achieved by co-feeding magnesium and methane into an RF thermal plasma reactor. This yielded carbonaceous material with magnesium particles 5-10 nm in size embedded in graphitic matrix. A further reduction down to 2-3 nm was possible but required reductions in the precursor feed rate. It was found that 2 wt.% carbon was sufficient to fully protect magnesium particles of approx. 260 nm in size. Light milling, however, disrupts the continuity of graphitic envelop and the particles then react both with oxygen and hydrogen. The potential of carbon coated magnesium as electrodes in rechargeable batteries are discussed

Carbon coating of magnesium particles

Magnesium when hydrided has low thermal and electrical conductivity and carbon coating would be useful to remedy this for a variety of purposes. In this study, carbon coating was achieved by co-feeding magnesium and methane into a thermal plasma reactor. This yielded carbonaceous material with magnesium particles 5-10 nm in size embedded in graphitic matrix. A further reduction down to 2-3 nm was possible but required reductions in the precursor feed rate. 2 wt% carbon was sufficient to fully protect magnesium particles of approx. 260 nm in size. Light milling, however, disrupts the continuity of graphitic envelop and the particles then react both with oxygen and hydrogen

Concentrated LiFSI-\u80\u93Ethylene Carbonate Electrolytes and Their Compatibility with High-Capacity and High-Voltage Electrodes

The unusual physical and chemical properties of electrolytes with excessive salt contents have resulted in rising interest in highly concentrated electrolytes, especially for their application in batteries. Here, we report strikingly good electrochemical performance in terms of conductivity and stability for a binary electrolyte system, consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethylene carbonate (EC) solvent. The electrolyte is explored for different cell configurations spanning both high-capacity and high-voltage electrodes, which are well known for incompatibilities with conventional electrolyte systems: Li metal, Si/graphite composites, LiNi0.33Mn0.33Co0.33O2 (NMC111), and LiNi0.5Mn1.5O4 (LNMO). As compared to a LiTFSI counterpart as well as a common LP40 electrolyte, it is seen that the LiFSI:EC electrolyte system is superior in Li-metalâ\u80\u93Si/graphite cells. Moreover, in the absence of Li metal, it is possible to use highly concentrated electrolytes (e.g., 1:2 salt:solvent molar ratio), and a considerable improvement on the electrochemical performance of NMC111-Si/graphite cells was achieved with the LiFSI:EC 1:2 electrolyte both at the room temperature and elevated temperature (55 °C). Surface characterization with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of thicker surface film formation with the LiFSI-based electrolyte as compared to the reference electrolyte (LP40) for both positive and negative electrodes, indicating better passivation ability of such surface films during extended cycling. Despite displaying good stability with the NMC111 positive electrode, the LiFSI-based electrolyte showed less compatibility with the high-voltage spinel LNMO electrode (4.7 V vs Li+/Li)