The performance degradation of graphite/LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) lithium ion cells, charged and discharged up to 300 cycles at different operating conditions of temperature and upper cutoff potential (4.2V/25 • C, 4.2V/60 • C, 4.6V/25 • C) was investigated. A combination of electrochemical methods with X-ray diffraction (XRD) both in situ and ex situ as well as neutron induced PromptGamma-Activation-Analysis (PGAA) allowed us to elucidate the main failure mechanisms of the investigated lithium ion cells. In situ XRD investigations of the NMC material revealed that the first cycle irreversible capacity is the cause of slow lithium diffusion kinetics. In full-cells, however, this "lost" lithium ions can be used to build up the SEI of the graphite electrode during the initial formation cycle. A new systematic approach to correlate the lithium content in NMC with its lattice parameters (c, a) allows a convenient quantification of the loss of active lithium in aged cells by determining the c/a ratio of harvested NMC cathodes in the discharged state using ex situ XRD. Besides loss of active lithium, transition metal dissolution/deposition on graphite and growth of cell impedance strongly effect cell aging, especially at elevated temperatures and high upper cutoff potentials. Besides their current use in portable power electronics, lithium ion batteries have recently been used for battery electric vehicles (BEV) and are envisioned for large-scale energy storage. For the latter applications, life times of >10 years are required so that it is essential to understand and quantify the mechanisms that contribute to battery failure. Among the commercially available lithium-ion battery chemistries, 1,2 the graphite/LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) system is one of the materials currently envisioned for automotive applications. 3 This cathode material demonstrates high capacity, good structural stability due to its small volume changes (<2%) during Li insertion and extraction, and high thermal stability in the charged state. [4][5][6] In addition, this material could theoretically be operated with high charge cutoff potentials up to 5.0 V, as its bulk structure is claimed to be stabilized by the presence of Mn 4+ , 7 even though other authors suggest that irreversible structural changes occur at these very high potentials and at high temperature.8 Due to its sloped potential profile, the capacity and also the average cell voltage increase with increasing charging potential. 7,9 Despite the improved safety and cycling performance of NMC material, operating NMC based cells (full-cells or half-cells) at elevated temperatures or at high charge potential leads to poor cycle life. [10][11][12][13] During cycling of graphite/NMC full-cells, transition metal dissolution from the NMC material is found to be a crucial factor controlling capacity fade. 11,12 In one of these studies, Zheng et al. demonstrated that upper cutoff potentials of >4.3 V lead to transition metal dissolution from NMC and thus compromise cycling performa...

No abstract

Silicon has several advantages when compared to other thermoelectric materials, but until recently it was not used for thermoelectric applications due to its high thermal conductivity, 156 W K(-1) m(-1) at room temperature. Nanostructuration as means to decrease thermal transport through enhanced phonon scattering has been a subject of many studies. In this work we have evaluated the effects of nanostructuration on the lattice dynamics of bulk nanocrystalline doped silicon. The samples were prepared by gas phase synthesis, followed by current and pressure assisted sintering. The heat capacity, density of phonons states, and elastic constants were measured, which all reveal a significant, ≈25%, reduction in the speed of sound. The samples present a significantly decreased lattice thermal conductivity, ≈25 W K(-1) m(-1), which, combined with a very high carrier mobility, results in a dimensionless figure of merit with a competitive value that peaks at ZT≈ 0.57 at 973 °C. Due to its easily scalable and extremely low-cost production process, nanocrystalline Si prepared by gas phase synthesis followed by sintering could become the material of choice for high temperature thermoelectric generators.