Rohit Bhagat

Characterization of the FFC Cambridge Process for NiTi Production Using In Situ X-Ray Synchrotron Diffraction

To date, the characterization of the reduction pathway for the Fray Farthing Chen (FFC) Cambridge process has been achieved through ex situ studies, leading to some ambiguities. This study employs a synchrotron X-ray diffraction technique to monitor in situ the FFC reduction of NiTiO3 to NiTi, yielding an unmatched level of detail on the electrochemical and chemical reactions involved. The reduction pathway consists of rapid initial reduction of NiTiO3 to form CaTiO3 and Ni, then the transformation of Ni to Ni3Ti, and finally the consumption of CaTiO3 and Ni3Ti to produce NiTi. The phases observed agree with thermodynamic predictions [J. Electrochem. Soc., 155, E171 (2008)] and allow the mapping of the reduction pathway on an electrochemical predominance diagram. Ni3Ti is a short-lived transient phase in the reduction pathway. Ni3Ti was found in significant quantities in ex situ studies [J. Electrochem. Soc., 155, E171 (2008); Chin. Sci. Bull., 51, 2535 (2006)]. The authors propose that Ni3Ti forms during furnace cooling to ambient temperature if extracted before a complete reduction. Neither Ni2Ti4O nor CaO was observed. The progression of the reduction front is clearly in line with existing observations and models [Metall. Mater. Trans., B, Process Metall. Mater. Proc. Sci., 35, 223 (2004); J. Phys. Chem. B, 109, 14043 (2005)].

Towards High Capacity Li-ion Batteries Based on Silicon-Graphene Composite Anodes and Sub-micron V-doped LiFePO4 Cathodes

Lithium iron phosphate, LiFePO4 (LFP) has demonstrated promising performance as a cathode material in lithium ion batteries (LIBs), by overcoming the rate performance issues from limited electronic conductivity. Nano-sized vanadium-doped LFP (V-LFP) was synthesized using a continuous hydrothermal process using supercritical water as a reagent. The atomic % of dopant determined the particle shape. 5 at. % gave mixed plate and rod-like morphology, showing optimal electrochemical performance and good rate properties vs. Li. Specific capacities of >160 mAh g−1 were achieved. In order to increase the capacity of a full cell, V-LFP was cycled against an inexpensive micron-sized metallurgical grade Si-containing anode. This electrode was capable of reversible capacities of approximately 2000 mAh g−1 for over 150 cycles vs. Li, with improved performance resulting from the incorporation of few layer graphene (FLG) to enhance conductivity, tensile behaviour and thus, the composite stability. The cathode material synthesis and electrode formulation are scalable, inexpensive and are suitable for the fabrication of larger format cells suited to grid and transport applications.

Enhancing cycling durability of Li-ion batteries with hierarchical structured silicon–graphene hybrid anodes

Hybrid anode materials consisting of micro-sized silicon (Si) particles interconnected with few-layer graphene (FLG) nanoplatelets and sodium-neutralized poly(acrylic acid) as a binder were evaluated for Li-ion batteries. The hybrid film has demonstrated a reversible discharge capacity of ∼1800 mA h g-1 with a capacity retention of 97% after 200 cycles. The superior electrochemical properties of the hybrid anodes are attributed to a durable, hierarchical conductive network formed between Si particles and the multi-scale carbon additives, with enhanced cohesion by the functional polymer binder. Furthermore, improved solid electrolyte interphase (SEI) stability is achieved from the electrolyte additives, due to the formation of a kinetically stable film on the surface of the Si.

Heteroatom Doped-Carbon Nanospheres as Anodes in Lithium Ion Batteries

Long cycle performance is a crucial requirement in energy storage devices. New formulations and/or improvement of “conventional” materials have been investigated in order to achieve this target. Here we explore the performance of a novel type of carbon nanospheres (CNSs) with three heteroatom co-doped (nitrogen, phosphorous and sulfur) and high specific surface area as anode materials for lithium ion batteries. The CNSs were obtained from carbonization of highly-crosslinked organo (phosphazene) nanospheres (OPZs) of 300 nm diameter. The OPZs were synthesized via a single and facile step of polycondensation reaction between hexachlorocyclotriphosphazene (HCCP) and 4,4′-sulphonyldiphenol (BPS). The X-ray Photoelectron Spectroscopy (XPS) analysis showed a high heteroatom-doping content in the structure of CNSs while the textural evaluation from the N2 sorption isotherms revealed the presence of micro- and mesopores and a high specific surface area of 875 m2/g. The CNSs anode showed remarkable stability and coulombic efficiency in a long charge–discharge cycling up to 1000 cycles at 1C rate, delivering about 130 mA·h·g−1. This study represents a step toward smart engineering of inexpensive materials with practical applications for energy devices.

Performance and polarization studies of the magnesium–antimony liquid metal battery with the use of in-situ reference electrode

This work presents the performance and polarization studies of a magnesium–antimony liquid metal battery with the use of an in-situ pseudo reference electrode at high operating temperature (ca. 700 °C). Due to the immiscibility of the contiguous salt and metal phases, the battery appears as three distinct layers: (1) positive electrode, (2) electrolyte and (3) negative electrode layers. The configuration of the in-situ reference electrode within the three floating liquid layers is described and is to avoid direct electrical contact/short circuit with the other electrodes. Electrochemical tests, including linear sweep voltammetry, impedance spectroscopy and galvanostatic cycling, evaluate the performance of a magnesium–antimony battery under a range of operating temperatures and current densities. Through the polarization studies, the area resistance of the negative and positive electrodes and the overall battery are found to be ca. 0.55, 0.65 and 1.20 Ω cm−2, respectively. In a typical 1 h charge/discharge per cycle experiment, average voltage efficiencies of ca. 64% are obtained at 60 mA cm−2 with a slight deterioration after subsequent cycles. In these tests, the half-cell measurements also indicate that the sprayed layer of boron nitride at the reference electrode is chemically stable and shown to be an effective electrical insulator for prolonged operation at high temperature (ca. 700 °C).

Production of Ni-35Ti-15Hf Alloy via the FFC Cambridge Process

The NiTiHf system alloy is considered to be one of the most attractive shape memory alloy (SMA) at elevated temperatures. This paper outlines how the FFC Cambridge Process was applied to produce the Ni-35 atom % Ti-15 atom % Hf (henceforth referred to as NiTiHf) alloy from sintered precursors of NiO, TiO(2) and HfO(2). In order to illuminate the reduction pathway, a number of partial reductions were completed at different reduction times. The samples were characterised by SEM, X-EDS and XRD. It was found that the key stages of reduction involved: (1) the reduction of NiTiO(3) and NiO to Ni, (2) the reduction of CaTiO(3) to Ti and the simultaneous formation of Ni(3)Ti, (3) the reaction of Ni(3)Ti with CaTiO(3) to form NiTi, (4) the reduction of HfO(2) and CaHfO(3) to form NiTiHf alloy and finally (5) the deoxidation and the Ti/Hf homogenisation of NiTiHf alloy. The sintered oxides precursors were reduced to metal alloy after 9 h reduction. After twenty-four hours reduction, a homogeneous alloy was formed with an oxygen content of 1600 ppm. DSC analysis shows that the austenite transformation temperature of the produced NiTiHf alloy was close to that seen in literature. An electrochemical predominance diagram for the Hf-Ca-Cl-O system was constructed to help understand the reactions during reduction.

Electrochemical Evaluation and Phase-related Impedance Studies on Silicon–Few Layer Graphene (FLG) Composite Electrode Systems

Silicon-Few Layer Graphene (Si-FLG) composite electrodes are investigated using a scalable electrode manufacturing method. A comprehensive study on the electrochemical performance and the impedance response is measured using electrochemical impedance spectroscopy. The study demonstrates that the incorporation of few-layer graphene (FLG) results in significant improvement in terms of cyclability, electrode resistance and diffusion properties. Additionally, the diffusion impedance responses that occur during the phase changes in silicon is elucidated through Staircase Potentio Electrochemical Impedance Spectroscopy (SPEIS): a more comprehensive and straightforward approach than previous state-of-charge based diffusion studies.

Binder-free Sn–Si heterostructure films for high capacity Li-ion batteries

This study fabricated and demonstrated a functional, stable electrode structure for a high capacity Li-ion battery (LIB) anode. Effective performance is assessed in terms of reversible lithiation for a significant number of charge–discharge cycles to 80% of initial capacity. The materials selected for this study are silicon and tin and are co-deposited using an advanced manufacturing technique (plasma-enhanced chemical vapour deposition), shown to be a scalable process that can facilitate film growth on 3D substrates. Uniform and hybrid crystalline–amorphous Si nanowire (SiNW) growth is achieved via a vapour–liquid–solid mechanism using a Sn metal catalyst. SiNWs of less than 300 nm diameter are known to be less susceptible to fracture and when grown this way have direct electrical conductivity to the current collector, with sufficient room for expansion. Electrochemical characterisation shows stable cycling at capacities of 1400 mA h g−1 (>4 × the capacity limit of graphite). This hybrid system demonstrates promising electrochemical performance, can be grown at large scale and has also been successfully grown on flexible carbon paper current collectors. These findings will have impact on the development of flexible batteries and wearable energy storage.

Precursor preparation for Ti-Al-V-Y alloy via FFC cambridge process

This paper presents the work done for the preparation of precursor for producing Ti-Al-V-Y alloy via FFC Cambridge process. The aim of the work is also to investigate the uniformity of the phases formed during the pre-processing of the precursor.The importance of the alloy for mechanical and medical applications is well known. Titanium oxide (TiO2), vanadium oxide (V2O5), aluminium oxide (Al2O3) and yttrium oxide (Y2O3) were selected as raw materials for precursor. The expected composition for the new alloy is Ti-6Al-4V-0.5Y. Water was used as a binder for the precursor. The materials were pre-mixed by ball milling for 24 hours and pressed using 13 mm die. The pressed mixtures were then sintered in the furnace at 900°C for 24 hours. The sintered samples were analysed using the optical microscope, electron micrograph with EDX and XRD. The result of the optical micrograph showed that the raw materials were uniformly mixed and well distributed with the presence of porosities. Electron micrograph further verified the morphology of the materials and the elements distribution in the precursor. The overlapping of yttrium and vanadium, Y(VO4) was observed and verified by XRD. The derived formulated precursor was then ready for further work of reduction to Ti-Al-V-Y alloy using FFC Cambridge process.