Habtom Desta Asfaw

Emulsion-templated bicontinuous carbon network electrodes for use in 3D microstructured batteries†

High surface area carbon foams were prepared and characterized for use in 3D structured batteries. Two potential applications exist for these foams: firstly as an anode and secondly as a current collector support for electrode materials. The preparation of the carbon foams by pyrolysis of a high internal phase emulsion polymer (polyHIPE) resulted in structures with cage sizes of ∼25 μm and a surface area enhancement per geometric area of approximately 90 times, close to the optimal configuration for a 3D microstructured battery support. The structure was probed using XPS, SEM, BET, XRD and Raman techniques; revealing that the foams were composed of a disordered carbon with a pore size in the <100 nm range resulting in a BET measured surface area of 433 m2 g−1. A reversible capacity exceeding 3.5 mA h cm−2 at a current density of 0.37 mA cm−2 was achieved. SEM images of the foams after 50 cycles showed that the structure suffered no degradation. Furthermore, the foams were tested as a current collector by depositing a layer of polyaniline cathode over their surface. High footprint area capacities of 500 μA h cm−2 were seen in the voltage range 3.8 to 2.5 V vs. Li and a reasonable rate performance was observed.

A one-step water based strategy for synthesizing hydrated vanadium pentoxide nanosheets from VO2(B) as free-standing electrodes for lithium battery applications

The synthesis of two dimensional (2D) materials from transition metal oxides, chalcogenides, and carbides mostly involve multiple exfoliation steps in which hazardous solvents and reagents are used. In this study, hydrated vanadium pentoxide (V2O5·nH2O) nanosheets with a thickness of a few nanometers were prepared via a facile environmentally friendly water based exfoliation technique. The exfoliation process involved refluxing the precursor, vanadium dioxide (VO2(B)), in water for a few days at 60 °C. The proposed exfoliation mechanism is based on the intercalation/insertion of water molecules into the VO2(B) crystals and the subsequent cleavage of the covalent bonds holding the layers of VO2(B) together. The thermal and chemical analyses showed that the approximate chemical composition of the nanosheets is H0.4V2O5·0.55H2O, and the percentage of VV content to that of VIV in the nanosheets is about 80(3)% to 20(3)%. The exfoliated aqueous suspension of the V2O5·0.55H2O nanosheets was successfully deposited onto multi-walled carbon nanotube (MW-CNT) paper to form free-standing electrodes with a thickness of the V2O5·0.55H2O layer ranging between 45 and 4 μm. A series of electrochemical tests were conducted on the electrodes to determine the cyclability and rate capability of lithium insertion into V2O5·0.55H2O nanosheets. The electrodes with the thinnest active material coating (∼4 μm) delivered gravimetric capacities of up to 480 and 280 mA h g−1 when cycled at current densities of 10 and 200 mA g−1, respectively.

Toward Solid-State 3D-Microbatteries Using Functionalized Polycarbonate-Based Polymer Electrolytes

3D microbatteries (3D-MBs) impose new demands for the selection, fabrication, and compatibility of the different battery components. Herein, solid polymer electrolytes (SPEs) based on poly(trimethylene carbonate) (PTMC) have been implemented in 3D-MB systems. 3D electrodes of two different architectures, LiFePO4-coated carbon foams and Cu2O-coated Cu nanopillars, have been coated with SPEs and used in Li cells. Functionalized PTMC with hydroxyl end groups was found to enable uniform and well-covering coatings on LiFePO4-coated carbon foams, which was difficult to achieve for nonfunctionalized polymers, but the cell cycling performance was limited. By employing a SPE prepared from a copolymer of TMC and caprolactone (CL), with higher ionic conductivity, Li cells composed of Cu2O-coated Cu nanopillars were constructed and tested both at ambient temperature and 60 °C. The footprint areal capacity of the cells was ca. 0.02 mAh cm-2 for an area gain factor (AF) of 2.5, and 0.2 mAh cm-2 for a relatively dense nanopillar-array (AF = 25) at a current density of 0.008 mA cm-2 under ambient temperature (22 ± 1 °C). These results provide new routes toward the realization of all-solid-state 3D-MBs.

On the P2-NaxCo1-y(Mn2/3Ni1/3)yO2 cathode materials for sodium-ion batteries: Synthesis, electrochemical performance and redox processes occurring during the electrochemical cycling

P2-type NaMO2 sodiated layered oxides with mixed transition metals are receiving considerable attention for use as cathodes in sodium-ion batteries. A study on solid solution (1 - y)P2-NaxCoO2-(y)P2-NaxMn2/3Ni1/3O2 (y = 0, 1/3, 1/2, 2/3, 1) reveals that changing the composition of the transition metals affects the resulting structure and the stability of pure P2 phases at various temperatures of calcination. For 0 ≤ y ≤ 1.0, the P2-NaxCo(1-y)Mn2y/3Niy/3O2 solid-solution compounds deliver good electrochemical performance when cycled between 2.0 and 4.2 V versus Na+/Na with improved capacity stability in long-term cycling, especially for electrode materials with lower Co content (y = 1/2 and 2/3), despite lower discharge capacities being observed. The (1/2)P2-NaxCoO2-(1/2)P2-NaxMn2/3Ni1/3O2 composition delivers a discharge capacity of 101.04 mAh g-1 with a capacity loss of only 3% after 100 cycles and a Coulombic efficiency exceeding 99.2%. Cycling this material to a higher cutoff voltage of 4.5 V versus Na+/Na increases the specific discharge capacity to ≈140 mAh g-1 due to the appearance of a well-defined high-voltage plateau, but after only 20 cycles, capacity retention declines to 88% and Coulombic efficiency drops to around 97%. In situ X-ray absorption near-edge structure measurements conducted on composition NaxCo1/2Mn1/3Ni1/6O2 (y = 1/2) in the two potential windows studied help elucidate the operating potential of each transition metal redox couple. It also reveals that at the high-voltage plateau, all of the transition metals are stable, raising the suspicion of possible contribution of oxygen ions in the high-voltage plateau.