Michael C. Tucker

Joining of dissimilar materials

Abstract Emerging trends in manufacturing such as light weighting, increased performance and functionality increases the use of multi-material, hybrid structures and thus the need for joining of dissimilar materials. The properties of the different materials are jointly utilised to achieve product performance. The joining processes can, on the other hand be challenging due to the same different properties. This paper reviews and summarizes state of the art research in joining dissimilar materials. Current and emerging joining technologies are reviewed according to the mechanisms of joint formation, i.e.; mechanical, chemical, thermal, or hybrid processes. Methods for process selection are described and future challenges for research on joining dissimilar materials are summarized.

7Li and 31 P Magic Angle Spinning Nuclear Magnetic Resonance of LiFePO4-Type Materials

LiFePO 4 and LiMnPO 4 have been characterized using 7 Li and 3 1 P magic angle spinning (MAS) nuclear magnetic resonance spectroscopy. LiFePO 4 was synthesized by a hydrothermal route and LiMnPO 4 was synthesized at high temperature in an inert atmosphere. Both compositions give rise to single isotropic 7 Li resonances. The MAS isotropic peak linewidth for LiFePO 4 is considerably larger than that for LiMnPO 4 , suggesting the presence of local disorder in the Li coordination sphere for LiFePO 4 . In both samples, the isotropic peak is accompanied by a large, asymmetric spinning sideband manifold, arising from bulk magnetic susceptibility broadening and the paramagnetic interaction between the lithium nucleus and transition metal unpaired electrons.

A 7Li NMR Study of Capacity Fade in Metal-Substituted Lithium Manganese Oxide Spinels

Nuclear magnetic resonance (NMR) spectroscopy and electrochemical techniques have been used to determine dominant failure modes of LiMn 2 O 4 -based positive electrode materials for lithium rechargeable batteries, and to elucidate the role of metal-for-manganese substitution in the stabilization of the material toward electrochemical cycling on the 4 V plateau. Various compositions were cycled galvanostatically (C/15 rate) from 3.3 to 4.4 V vs. Li metal at room temperature using a 1 M LiPF 6 in ethylene carbonate/dimethyl carbonate (1:2) electrolyte. A rapid capacity fade was observed for LiMn 2 O 4 , which was mitigated to varying extents by substitution of some of the Mn by other metals After cycling, the 7 Li magic angle spinning (MAS) NMR peaks broadened, and the peaks assigned to lithium near defects increased in relative intensity. These changes were most pronounced for the poor-performing LiMn 2 O 4 , and were almost undetectable in the most robust compositions. The results for the cycled electrodes were compared to results for electrodes which had been exposed to model degradation processes chosen so as to mimic failure by one of the possible mechanisms proposed in the literature. This comparison provided substantial evidence that manganese dissolution and concomitant Li-for-Mn ion exchange at the end-of-discharge is the dominant mode of failure on the 4 V plateau. Cr substitution effectively mitigated this failure mode.

Optimization of electrode characteristics for the Br 2 /H 2 redox flow cell

The Br2/H2 redox flow cell shows promise as a high-power, low-cost energy storage device. The effect of various aspects of material selection, processing, and assembly of electrodes on the operation, performance, and efficiency of the system is determined. In particular, (+) electrode thickness, cell compression, hydrogen pressure, and (−) electrode architecture are investigated. Increasing hydrogen pressure and depositing the (−) catalyst layer on the membrane instead of on the carbon paper backing layers have a large positive impact on performance, enabling a limiting current density above 2xa0A cm−2 and a peak power density of 1.4xa0Wxa0cm−2. Maximum energy efficiency of 79xa0% is achieved. In addition, the root cause of limiting-current behavior in this system is elucidated, where it is found that Br− reversibly adsorbs at the Pt (−) electrode for potentials exceeding a critical value, and the extent of Br− coverage is potential-dependent. This phenomenon limits maximum cell current density and must be addressed in system modeling and design. These findings are expected to lower system cost and enable higher efficiency.

Performance and cycling of the iron-ion/hydrogen redox flow cell with various catholyte salts

A redox flow cell utilizing the Fe2+/Fe3+ and H2/H+ couples is investigated as an energy storage device. A conventional polymer electrolyte fuel cell anode and membrane design is employed, with a cathode chamber containing a carbon felt flooded with aqueous acidic solution of iron salt. The maximum power densities achieved for iron sulfate, iron chloride, and iron nitrate are 148, 207, and 234xa0mWxa0cm−2, respectively. It is found that the capacity of the iron nitrate solution decreases rapidly during cycling. Stable cycling is observed for more than 100xa0h with iron chloride and iron sulfate solutions. Both iron sulfate and iron chloride solutions display moderate discharge polarization and poor charge polarization; therefore, voltage efficiency decreases dramatically with increasing current density. A small self-discharge current occurs when catholyte is circulating through the cathode chamber. As a result, a current density above 100xa0mAxa0cm−2 is required to achieve high Coulombic efficiency (>0.9).

The Influence of Covalence on Capacity Retention in Metal-Substituted Spinels 7 Li NMR, SQUID, and Electrochemical Studies

Variable-temperature 7 Li nuclear magnetic resonance and superconducting quantum interference device (NMR and SQUID) magnetometry have been used to determine the supertransferred hyperfine coupling constant, an indicator of Li-O-Mn bond covalence, for metal-substituted spinels, LiM x Mn 2-x O 4 (M = Li, Zn, Al, Cr). The Curie and Weiss constants are reported for all compositions studied. The covalence of the Li-O-Mn bond is observed to increase upon substitution for all substituting metals studied. The greatest enhancement in covalence was achieved by substitution of ions that are similar in size to the Mn 3.5 ion. The covalence values are compared to electrochemical capacity retention of the same compositions when cycled on the 4 V plateau. A positive correlation between the effectiveness of a particular substituent at improving capacity retention and the effectiveness of that substituent at enhancing covalency is observed. The bond covalence for LiCr 0.175 Mn 1.825 O 4 remains unchanged upon electrochemical cycling, whereas that of LiMn 2 O 4 increases with cycling.

Origin of chemical shift of manganese in lithium battery electrode materials—a comparison of hard and soft X-ray techniques

Abstract L-edge X-ray absorption near edge spectroscopy and K β emission spectroscopy were applied to monitor ex situ chemical valence changes of manganese in battery electrode materials as they appear during electrode processing and battery operation. We found that significant chemical shifts of the spectra occur already during electrode fabrication, prior to any electrochemical treatment. Employment of these two different techniques allows for the qualitative separation of contributions originating from the surface and from the bulk of the electrode particles.

All‐Iron Redox Flow Battery Tailored for Off‐Grid Portable Applications

An all-iron redox flow battery is proposed and developed for end users without access to an electricity grid. The concept is a low-cost battery which the user assembles, discharges, and then disposes of the active materials. The design goals are: (1)u2005minimize upfront cost, (2)u2005maximize discharge energy, and (3)u2005utilize non-toxic and environmentally benign materials. These are different goals than typically considered for electrochemical battery technology, which provides the opportunity for a novel solution. The selected materials are: low-carbon-steel negative electrode, paper separator, porous-carbon-paper positive electrode, and electrolyte solution containing 0.5u2009m Fe2 (SO4 )3 active material and 1.2u2009m NaCl supporting electrolyte. With these materials, an average power density around 20u2005mWu2009cm(-2) and a maximum energy density of 11.5u2005Whu2009L(-1) are achieved. A simple cost model indicates the consumable materials cost US

A 7Li Nuclear Magnetic Resonance Study of Metal-Substituted Lithium Manganese Oxide Spinels

6.45 per kWh(-1) , or only US

Cathode Contact Materials for Solid Oxide Fuel Cells

0.034 per mobile phone charge.