In the near future, the targets for lithium-ion batteries concerning specific energy and cost can advantageously be met by introducing layered LiNi x Co y Mn z O2 (NCM) cathode materials with a high Ni content (x ≥ 0.6). Increasing the Ni content allows for the utilization of more lithium at a given cell voltage, thereby improving the specific capacity but at the expense of cycle life. Here, the capacity-fading mechanisms of both typical low-Ni NCM (x = 0.33, NCM111) and high-Ni NCM (x = 0.8, NCM811) cathodes are investigated and compared from crystallographic and microstructural viewpoints. In situ X-ray diffraction reveals that the unit cells undergo different volumetric changes of around 1.2 and 5.1% for NCM111 and NCM811, respectively, when cycled between 3.0 and 4.3 V vs Li/Li+. Volume changes for NCM811 are largest for x(Li) < 0.5 because of the severe decrease in interlayer lattice parameter c from 14.467(1) to 14.030(1) Å. In agreement, in situ light microscopy reveals that delithiation leads to different volume contractions of the secondary particles of (3.3 ± 2.4) and (7.8 ± 1.5)% for NCM111 and NCM811, respectively. And postmortem cross-sectional scanning electron microscopy analysis indicates more significant microcracking in the case of NCM811. Overall, the results establish that the accelerated aging of NCM811 is related to the disintegration of secondary particles caused by intergranular fracture, which is driven by mechanical stress at the interfaces between the primary crystallites.

The strength of metal crystals is reduced below the theoretical value by the presence of dislocations or by flaws that allow easy nucleation of dislocations. A straightforward method to minimize the number of defects and flaws and to presumably increase its strength is to increase the crystal quality or to reduce the crystal size. Here, we describe the successful fabrication of high aspect ratio nanowhiskers from a variety of face-centered cubic metals using a high temperature molecular beam epitaxy method. The presence of atomically smooth, faceted surfaces and absence of dislocations is confirmed using transmission electron microscopy investigations. Tensile tests performed in situ in a focusedion beam scanning electron microscope on Cu nanowhiskers reveal strengths close to the theoretical upper limit and confirm that the properties of nanomaterials can be engineered by controlling defect and flaw densities.Single crystalline metal wires, or metal whiskers, with diameters larger than one micrometer have been routinely fabricated 1 and used for experimentally examining mechanical, 2 ferromagnetic, 3 superconductive, 4 and electronic 5 properties. Interest in metal whiskers became intense once it was observed that they could exhibit strengths close to theoretical (ideal) values. 2,6 Whiskers have been grown by the vapor liquid solid method 7 (VLS) or metal halide reduction, 6 the latter of which have demonstrated high strength. 6 This has been attributed to the near-equilibrium nature of the growth process and the resultant absence of defects and flaws in the samples. Extending the fabrication of high quality metal whiskers to submicrometer diameters, as routinely produced for semiconductors, 7-9 is clearly desired but has been largely elusive. However, single crystalline metal nanowires have only occasionally been successfully fabricated. 10 Here we describe the first time to our knowledge the fabrication of metal single crystalline nanowhiskers (NWs) with diameters as small as 20 nm and no defects, as evidenced by their strengths near the theoretical upper limit. Such nanostructures have the potential to serve as model systems for elucidating intrinsic properties in tiny structures, such as quantum effects, and to be used as building blocks in nanotechnological applications where unique functionalities are required.In this study, free-standing, high aspect ratio, single crystalline nanowhiskers of a variety of different materials (copper, gold, silver, aluminum, and silicon) have been successfully grown from partially C-coated, oxidized and nonoxidized Si (100), (110), and (111) substrates under molecular beam epitaxy (MBE) conditions. Both elevated substrate temperatures (on the order of 0.65 T M of the deposited species) and the partial C layer are necessary to achieve nanowhisker growth. In the remainder of this publication we will focus on results from Cu nanowhiskers. Figure 1 shows scanning electron micrographs of two Cu samples with nominal Cu thicknesses of 45 and 200 nm, respectively. In addi...

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