A. Manuel Stephan

Organic free low temperature direct synthesis of hierarchical protonated layered titanates/anatase TiO2 hollow spheres and their task-specific applications

Layered protonated titanates and corresponding anatase TiO2 shapes, specifically the hollow spheres, are the most important functional materials and have attracted much attention because of their superior performance. Here, a facile organic substrate (both as solvent or surfactant) free, low temperature aqueous solution based chemical protocol for the direct synthesis of 3D arranged hierarchical hollow spheres of protonated layered dititanate (H2Ti2O5·H2O), is proposed. The spheres had a high surface area (as high as 334 m2 g−1), and were prepared through hydrothermal treatment of freshly prepared peroxo titanium carbonate complex in the presence of ammonium hydroxide. Ammonium hydroxide is crucial in the formation of spherical arrangement of titanate sheets and the size of the spheres is tunable by changing the amount of ammonium hydroxide. The titanate spheres can easily be converted to pure anatase TiO2 with identical morphology on subsequent calcination. The synthesized titanate spheres showed very high removal capacity for toxic heavy metals like Pb2+ and methylene blue from aqueous solution. Corresponding anatase TiO2 spheres manifested as a brilliant anode material for lithium ion batteries with excellent cyclability. TiO2 spheres also showed good photocatalytic activity.

Sisal-derived activated carbons for cost-effective lithium–sulfur batteries

Elemental sulfur was successfully impregnated in an activated carbon (AC) matrix obtained from sisal fibers. The impregnation of sulfur in the activated carbon (S-AC) matrix was confirmed by XRD, SEM and Raman analyses. The sulfurized activated carbon (S-AC) composite electrode was employed as a cathode material for lithium–sulfur (Li–S) cell. The Li–S cell delivered a discharge capacity of 950 mA h g−1 at 0.1C-rate. The electrochemical impedance spectroscopy measurements were carried out for the Li–S cell before and after cycling and also at different depth of discharge and depth of charge. A stable cycling was achieved at 1C-rate.

Metal organic framework-laden composite polymer electrolytes for efficient and durable all-solid-state-lithium batteries

A copper benzene dicarboxylate metal organic framework (Cu-BDC MOF) was synthesized and successfully incorporated in a poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonylimide) (LiTFSI) complex. The incorporation of Cu-BDC MOF was found to significantly enhance the ionic conductivity, compatibility and thermal stability of the composite polymer electrolyte (CPE). An all-solid-state-lithium cell composed of Li/CPE/LiFePO4 was assembled, and its cycling profile has been analysed for different C-rates at 70 °C. The appreciable ionic conductivity, thermal stability and cycling ability qualify these membranes as electrolytes for all-solid-state-lithium batteries used in elevated temperature applications.

A flexible zirconium oxide based-ceramic membrane as a separator for lithium-ion batteries

A flexible and thermally stable zirconium oxide (ZrO2)-based porous ceramic membrane (PCM) was prepared with PVdF–HFP as binder. This membrane was found to have 60% porosity with good electrolyte uptake and appreciable Li+-ion transport number. TG analysis revealed that this membrane is thermally stable up to 400 °C. The ZrO2-based membrane exhibited better interfacial and electrochemical properties than the commercially available Celgard 2320 membrane. This ceramic membrane was employed as a separator in a 2032-type coin cell comprising Li/PCM/LiFePO4 and its charge–discharge profile was analyzed at different C-rates.

Charge–discharge studies of all-solid-state Li/LiFePO4 cells with PEO-based composite electrolytes encompassing metal organic frameworks

Lithium batteries with high energy density can be achieved only with a metallic-lithium anode in conjunction with solid polymer electrolyte. Unfortunately, the undesirable interfacial properties and the subsequent formation of a solid electrolyte interface (SEI) layer and poor ionic conductivity at ambient temperature hamper this system from being commercialized. In order to conquer these issues, numerous attempts have been made. Herein we report the preparation and electrochemical properties of a nickel-1,3,5-benzene tricarboxylate metal organic framework (Ni3–(BTC)2–MOF) laden-composite polymer electrolyte with a lithium salt (LiTFSI). The added Ni3–(BTC)2–MOF plays a vital role in enhancing the ionic conductivity, and the mechanical and thermal properties. The scavenging properties of the highly porous MOF significantly improved the Li/electrolyte interfacial properties. A 2032-type coin cell composed of Li/CPE/LiFePO4 was assembled and its cycling profile is discussed.

High performance multi-functional trilayer membranes as permselective separators for lithium–sulfur batteries

Multi-walled carbon nanotubes (MWCNTs) and magnesium aluminate (MgAl2O4) were coated on either side of a commercially available Celgard 2320 membrane by a simple doctor blade method. This trilayer membrane (TLM) was subjected to ionic conductivity, thermal stability and contact angle measurements. Each layer of the TLM functions for a specific purpose: the MWCNTs provide electronic conductivity while the pores of the Celgard 2320 membrane facilitate lithium-ion transport and MgAl2O4 suppresses the shuttling of polysulfides due to the electrostatic attractive force. The TLM exhibited superior thermal stability and ionic conductivity than the uncoated Celgard 2320 membrane. The Li–S cell with a TLM offered a higher discharge capacity than the one with an uncoated membrane. The results are compared with earlier reports.

Natural, biodegradable and flexible egg shell membranes as separators for lithium-ion batteries

Flexible egg shell membranes (ESM) were obtained from chicken eggs after treatment with hydrochloric acid. The ESMs were subjected to scanning electron microscopy (SEM) and thermogravimetric (TG) and wettability analyses. The morphological studies revealed that the membranes possessed uniform porosity and they were of micron size. The ESM was also found to be thermally stable above 230 °C. The electrochemical properties, such as ionic conductivity, lithium transport number (Lit+) and compatibility of the membrane upon activation in 1 M LiPF6 in ECu2006:u2006DMC (1u2006:u20061 v/v), were analyzed. Finally, a 2032-type coin cell composed of Li/ESM/LiFePO4 was assembled and its cycling profile was also analyzed at different C-rates.

A high-performance BaTiO3-grafted-GO-laden poly(ethylene oxide)-based membrane as an electrolyte for all-solid lithium-batteries

Nanocomposite polymer electrolytes (NCPEs) comprising poly(ethylene oxide) (PEO), barium titanate-grafted-graphene oxide (BaTiO3-g-GO) and lithium bis(trifluoromethanesulfonyl imide) (LiTFSI) were prepared by a simple hot-press technique. An increase of two orders of magnitude in the ionic conductivity was achieved upon incorporation of BaTiO3-g-GO in the polymeric matrix even at 0 °C. The addition of BaTiO3-g-GO as a filler has significantly enhanced the thermal stability and mechanical integrity of the membrane. The BaTiO3-g-GO-laden membrane was found to have better interfacial properties with a lithium metal anode than the filler-free membrane. Charge–discharge studies revealed a stable cycling profile even at 5C-rate and it was found to be superior to those reported earlier.

On the development of a proton conducting solid polymer electrolyte using poly(ethylene oxide)

By mimicking the polymer backbone assisted ‘hop and lock’ lithium ion transport in lithium solid polymer (SP) electrolytes, a new type of proton (H+) transport membrane cum separator is designed which is found to work even in pure water electrolysis. An inexpensive H+ transporting SP membrane (HPEOP) is formulated using perchloric acid (HClO4) as the proton source with a poly(ethylene oxide) (PEO) and polydimethylsiloxane blend as the host structure. The H+ coordinated PEO backbone via the solvation of HClO4 allows easy transport of H+ through PEO segmental motion and inter-segmental hopping. Humidity dependent ionic conductivity measurements on the optimized HPEOP membrane show higher values in comparison to those of Nafion 117, and a considerable ionic conductivity was shown by HPEOP even in an anhydrous environment (3.165 ± 0.007 mS cm−1) unlike Nafion 117 (∼10−7 mS cm−1). Lowering the melting temperature of PEO through HClO4 ‘salting in’ is found to have a considerable effect in enhancing the conductivity of this SP membrane, while addition of HClO4 also modifies the microstructure and mechanical strength of the membrane. Water electrolysis ‘H’ cells are constructed with both pure and protonated water using both HPEOP and Nafion separators (membranes), and studies show the possibilities of highly efficient low cost water electrolysis and fuel cells devoid of expensive Nafion membranes.

Metal–organic framework@SiO2 as permselective separator for lithium–sulfur batteries

The shuttling of polysulfides between the electrodes in a lithium–sulfur battery (Li–S) system remains a challenge to be addressed in order to realize the full potential of this promising technology. In order to overcome this issue several strategies have been adopted. In the present work, UiO-66-NH2@SiO2 was successfully synthesized and coated on a commercial Celgard 2320 membrane. The coating of UiO-66-NH2@SiO2 on a Celgard 2320 membrane has not only enhanced the thermal stability and wettability but also other electrochemical properties such as ionic conductivity, compatibility and charge–discharge behavior. The Li–S cell with the UiO-66-NH2@SiO2-coated membrane delivered higher discharge capacity than the Li–S cell with SiO2-coated and uncoated membranes. The enhanced discharge capacity was attributed to the electrostatic and/or H-bonding interactions between the polysulfide and UiO-66-NH2@SiO2 as evidenced by its positive zeta potential (+56.42 mV). More importantly, the permselective properties of the membrane significantly play against the self-discharge of Li–S cells in which 98.5% of its capacity was retained even after 40 h which is superior to earlier reports.