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J Mater Chem A Mater ; 9(41): 23582-23596, 2021 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-34765222


The transition towards electric vehicles and more sustainable transportation is dependent on lithium-ion battery (LIB) performance. Ni-rich layered transition metal oxides, such as NMC811 (LiNi0.8Mn0.1Co0.1O2), are promising cathode candidates for LIBs due to their higher specific capacity and lower cost compared with lower Ni content materials. However, complex degradation mechanisms inhibit their use. In this work, tailored aging protocols are employed to decouple the effect of electrochemical stimuli on the degradation mechanisms in graphite/NMC811 full cells. Using these protocols, impedance measurements, and differential voltage analysis, the primary drivers for capacity fade and impedance rise are shown to be large state of charge changes combined with high upper cut-off voltage. Focused ion beam-scanning electron microscopy highlights that extensive microscale NMC particle cracking, caused by electrode manufacturing and calendering, is present prior to aging and not immediately detrimental to the gravimetric capacity and impedance. Scanning transmission electron microscopy electron energy loss spectroscopy reveals a correlation between impedance rise and the level of transition metal reduction at the surfaces of aged NMC811. The present study provides insight into the leading causes for LIB performance fading, and highlights the defining role played by the evolving properties of the cathode particle surface layer.

Nat Mater ; 20(1): 84-92, 2021 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-32839589


Ni-rich layered cathode materials are among the most promising candidates for high-energy-density Li-ion batteries, yet their degradation mechanisms are still poorly understood. We report a structure-driven degradation mechanism for NMC811 (LiNi0.8Mn0.1Co0.1O2), in which a proportion of the material exhibits a lowered accessible state of charge at the end of charging after repetitive cycling and becomes fatigued. Operando synchrotron long-duration X-ray diffraction enabled by a laser-thinned coin cell shows the emergence and growth in the concentration of this fatigued phase with cycle number. This degradation is structure driven and is not solely due to kinetic limitations or intergranular cracking: no bulk phase transformations, no increase in Li/Ni antisite mixing and no notable changes in the local structure or Li-ion mobility of the bulk are seen in aged NMCs. Instead, we propose that this degradation stems from the high interfacial lattice strain between the reconstructed surface and the bulk layered structure that develops when the latter is at states of charge above a distinct threshold of approximately 75%. This mechanism is expected to be universal in Ni-rich layered cathodes. Our findings provide fundamental insights into strategies to help mitigate this degradation process.

Nanoscale ; 7(38): 15727-33, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26349897


Nylon-6 based polymer-nanocomposite (PNC) dielectrics containing nano-regions of Ti-only and Ag + Ti have been manufactured by layer-by-layer deposition. By varying the thickness and deposition rate of individual layers, the PNC structure was manipulated at the nano-scale and then studied using various types of transmission electron microscopy (TEM). Enhanced PNC dielectric properties, with a dielectric constant k as high as ∼73, were shown to relate critically to in situ reactions and the detailed nano-arrangement of the resulting Ti (core)-TiOx (shell) and Ag nanoparticles.

Nanotechnology ; 25(47): 475706, 2014 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-25379841


We present a detailed study of the evolution and nature of metallic core-oxide shell particles and the role of nanostructure in the physics of enhanced polarization in polymer-nanocomposite (PNC) based dielectrics. Nylon-6 based PNCs consisting of aluminium (core)-aluminium oxide (shell) nanoparticles were fabricated by a vacuum deposition technique. Their resulting high polarizability was closely related to the formation and chemistry of the core-shell structure that was revealed by transmission electron microscopy to comprise a highly-defective, strained and non-stoichiometric semi-crystalline/amorphous Al-oxide shell.