| 1. |
- Jin, Yanting, et al.
(författare)
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Understanding Fluoroethylene Carbonate and Vinylene Carbonate Based Electrolytes for Si Anodes in Lithium Ion Batteries with NMR Spectroscopy
- 2018
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 140:31, s. 9854-9867
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Tidskriftsartikel (refereegranskat)abstract
- Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF6by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by13C-13C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using29Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.
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| 2. |
- Paul, Souvik, et al.
(författare)
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Anti-site disorder and improved functionality of Mn2NiX (X = Al, Ga, In, Sn) inverse Heusler alloys
- 2014
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 116:13, s. 133903-
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Tidskriftsartikel (refereegranskat)abstract
- Recent first-principles calculations have predicted Mn2NiX (X = Al, Ga, In, Sn) alloys to be magnetic shape memory alloys. Moreover, experiments on Mn2NiGa and Mn2NiSn suggest that the alloys deviate from the perfect inverse Heusler arrangement and that there is chemical disorder at the sublattices with tetrahedral symmetry. In this work, we investigate the effects of such chemical disorder on phase stabilities and magnetic properties using first-principles electronic structure methods. We find that except Mn2NiAl, all other alloys show signatures of martensitic transformations in presence of anti-site disorder at the sublattices with tetrahedral symmetry. This improves the possibilities of realizing martensitic transformations at relatively low fields and the possibilities of obtaining significantly large inverse magneto-caloric effects, in comparison to perfect inverse Heusler arrangement of atoms. We analyze the origin of such improvements in functional properties by investigating electronic structures and magnetic exchange interactions.
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| 3. |
- Paul, Souvik, et al.
(författare)
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First-principles study of the lattice instabilities in Mn2NiX (X = Al, Ga, In, Sn) magnetic shape memory alloys
- 2015
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:3, s. 035401-
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Tidskriftsartikel (refereegranskat)abstract
- Using first-principles based density functional theory, we have investigated the structural instabilities in the austenite phases of Mn2NiX (X = Al, Ga, In, Sn) magnetic shape memory alloys. A complete softening is observed in the acoustic TA(2) branches for all the materials along [xi xi 0] directions leading to instability in the austenite structure which effectively stabilizes into martensitic structure. The reasons behind this softening are traced back to the repulsion from the optical T-2g branches and to the nesting features in the Fermi surfaces. The vibrational density of states, the force constants and the elastic moduli are also computed and analyzed, which reconfirm the underlying mechanism behind the instabilities. The results indicate that the phonon anomalies are related to the occurrence of possible pre-martensitic phases which can be quite complex.
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| 4. |
- Paul, Souvik, et al.
(författare)
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Magnetic properties of Mn2NiSn shape memory alloy
- 2013
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:23, s. 236005-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present the magnetic properties of the inverse Heusler Mn2NiSn alloy computed by ab initio density functional theory (DFT) calculations in order to understand the large magnetic moments observed in experiments in contrast to smaller values obtained in previous ab initio calculations. Our results show that the magnetization in this system is quite sensitive to volume and atomic ordering in the sublattices. The observed variations in the magnetizations are explained from the features in the electronic structures of the Mn d-bands. We conclude that the symmetry of lattice sites and the alloying with Ni atoms are responsible for a high magnetic moment for a range of volumes. This opens a possible route to realize a large magnetization, a requirement for shape memory properties in magnetic alloys, in similar systems with an inverse Heusler structure.
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