Energy density of full cells containing layered-oxide positive electrodes can be increased by raising the upper cutoff voltage above the present 4.2 V limit. In this article we examine aging behavior of cells, containing LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523)-based positive and graphite-based negative electrodes, which underwent up to ∼400 cycles in the 3-4.4 V range. Electrochemistry results from electrodes harvested from the cycled cells were obtained to identify causes of cell performance loss; these results were complemented with data from X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) measurements. Our experiments indicate that the full cell capacity fade increases linearly with cycle number and results from irreversible lithium loss in the negative electrode solid electrolyte interphase (SEI) layer. The accompanying electrode potential shift reduces utilization of active material in both electrodes and causes the positive electrode to cycle at higher states-of-charge. Full cell impedance rise on aging arises primarily at the positive electrode and results mainly from changes at the electrode-electrolyte interface; the small growth in negative electrode impedance reflects changes in the SEI layer. Our results indicate that cell performance loss could be mitigated by modifying the electrode-electrolyte interfaces through use of appropriate electrode coatings and/or electrolyte additives. The pursuit of high energy density lithium-ion batteries for transportation applications continues in order to increase the driving range of vehicles on a single charge. Because graphite (Gr) remains the active material of choice in the negative electrode, the search mainly revolves around layered oxides for the positive electrode. In recent years there has been immense interest in the lithium and manganese-rich layered oxides which are capable of delivering high energy density cells. [1][2][3][4][5][6][7][8][9][10] This interest has ebbed somewhat because the voltage fade exhibited by these oxides lowers the usable energy, and complicates state-ofcharge determination, of the battery cells. In this article we focus our attention on the NCM523 oxide which has been the subject of several recent articles. For example, Bak et al. conducted in situ X-ray diffraction studies on various (delithiated) Li x Ni a Co b Mn c O 2 materials and concluded that NCM523 is an optimized composition offering the thermal stability of lower-Ni oxides (such as NCM333) while displaying capacities closer to the higher-Ni oxides (such as NCM811).27 From AC impedance and DC polarization studies Amin and Chiang reported that the electronic conductivity of NCM523 increases with decreasing Li-content, from 10 −7 S/cm for a fully-lithiated oxide (Li 1.0 ) to 10 −2 S/cm for a significantly delithiated (Li 0.25 ) oxide.28 Dixit et al. confirmed from first * Electrochemical Society Member. z E-mail: abraham@anl.gov principle-based simulation studies that during oxide delithiation Ni oxidizes first, followed by Co, while Mn rem...
We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproducible metal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinner films with sharp MITs are discussed, including the Volmer-Weber type growth mode due to the high energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2 substrate, which we quantify using low angle annular dark field scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy. We find that controlling island coalescence on the (001) surface and minimization of cation interdiffusion by using a low growth temperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2 films with abrupt MIT behavior.
A combination of in situ and post-deposition experiments were designed to probe surface roughening pathways leading to epitaxial breakdown during low-temperature (T s ϭ95-190°C) growth of Ge͑001͒ by molecular beam epitaxy ͑MBE͒. We demonstrate that epitaxial breakdown in these experiments is not controlled by background hydrogen adsorption or gradual defect accumulation as previously suggested, but is a growth-mode transition driven by kinetic surface roughening. Ge͑001͒ layers grown at T s տ170°C remain fully epitaxial to thicknesses hϾ1.6 m, while deposition at T s Ͻ170°C leads to a locally abrupt transition from epitaxial to amorphous growth at critical film thicknesses h 2 (T s ). Surface morphology during lowtemperature Ge͑001͒ MBE evolves via the formation of a periodic array of self-organized round growth mounds which, for deposition at T s Ͼ115°C, transform to a pyramidal shape with square bases having edges aligned along ͗100͘ directions. Surface widths w and in-plane coherence lengths d increase monotonically with film thickness h at a temperature-dependent rate. As h→h 1 (T s ), defined as the onset of epitaxial breakdown, deep cusps bounded by ͕111͖ facets form at the base of interisland trenches and we show that epitaxial breakdown is initiated on these facets as the surface roughness reaches a critical T s -independent aspect ratio w/dӍ0.02. h 1 (T s ) and h 2 (T s ) follow relationships h 1(2) ϰexp(ϪE 1(2) /kT s ), where E 1 is 0.61 eV and E 2 ϭ0.48 eV. E 1 is approximately equal to the Ge adatom diffusion barrier on Ge͑001͒ while (E 1 ϪE 2 )ϭ0.13 eV is the free energy difference between crystalline and amorphous Ge. We summarize our results in a microstructural phase map vs T s and h, and propose an atomistic growth model to explain the epitaxial to amorphous phase transition.
In cells containing Li 1.05 ͑Ni 1/3 Co 1/3 Mn 1/3 ͒ 0.95 O 2-based positive and graphite-based negative electrodes, a significant portion of cell impedance rise on aging is known to be from the negative electrode. One possible reason for this impedance rise is the dissolution of transition-metal elements from the oxide electrode that accumulate and create a high-impedance layer at the negative electrodeelectrolyte interface. This article details dynamic secondary ion mass spectrometry ͑SIMS͒ measurements, which provide a relative comparison of Mn, Co, and Ni contents on fresh, formed, and aged graphite electrodes. The data clearly indicate that these transition-metal elements accumulate at the electrode surface and diffuse into the electrode during cell aging.
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