Browsing by Author "Sehrawat, D"

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    Alkali metal-modified P2 NaxMnO2: crystal structure and application in sodium-ion batteries
    (American Chemical Society, 2020-08-18) Sehrawat, D; Rawal, A; Cheong, S; Avdeev, M; Ling, CD; Kimpton, JA; Sharma, N
    Sodium-ion batteries (NIBs) are an emerging alternative to lithium-ion batteries because of the abundance of sodium resources and their potentially lower cost. Here we report the Na0.7MnO2 solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type P63/mmc space group symmetry, and the other adopts orthorhombic Pbma space group symmetry. The phase ratio of P2 to the orthorhombic phase is 55.0(5):45.0(4). A single-phase P2 structure is found to form at 1000 °C after modification with alkali metals Rb and Cs, while the K-modified form produces an additional minor impurity. The modification is the addition of the alkali elements during synthesis that do not appear to be doped into the crystal structure. As a cathode for NIBs, parent Na0.7MnO2 shows a second charge/discharge capacity of 143/134 mAh g–1, K-modified Na0.7MnO2 a capacity of 184/178 mAh g–1, Rb-modified Na0.9MnO2 a capacity of 159/150 mAh g–1, and Cs-modified Na0.7MnO2 a capacity of 171/163 mAh g–1 between 1.5 and 4.2 V at a current density of 15 mA g–1. The parent Na0.7MnO2 is compared with alkali metal (K, Rb, and Cs)-modified NaxMnO2 in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, 23Na solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy and in terms of electrochemical performance and structural electrochemical evolution using in situ or operando synchrotron X-ray diffraction. © 2020 American Chemical Society
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    Investigation of K modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 as a cathode material for sodium-ion batteries
    (Royal Society of Chemistry, 2018-11-19) Sehrawat, D; Cheong, S; Rawal, A; Glushenkov, AM; Brand, HEA; Cowie, BCC; Gonzalo, E; Rojo, T; Naeyaert, PJP; Ling, CD; Avdeev, M; Sharma, N
    Sodium-ion batteries (NIBs) are emerging as a potentially cheaper alternative to lithium-ion batteries (LIBs) due to the larger abundance of sodium and in some cases the similar intercalation chemistry to LIBs. Here we report the solid state synthesized K-modified P2 Na0.7Mn0.8Mg0.2O2 which adopts hexagonal P63/mmc symmetry. The second charge/discharge capacity for the as-prepared material is 115/111 mA h g−1 between 1.5–4.2 V at a current density of 15 mA g−1, which reduces to 61/60 mA h g−1 after 100 cycles. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS) analysis shows a heterogeneous distribution of K and solid state 23Na NMR illustrates that the presence of K perturbs the local environment of Na within the P2 Na0.7Mn0.8Mg0.2O2 crystal structure. Larger scale X-ray absorption near-edge structure (XANES) data on the K L-edge also illustrate that K is present on the surface of electrodes in preference to the bulk. In situ synchrotron X-ray diffraction (XRD) data illustrates that the P2 structural motif is preserved, featuring a solid solution reaction for most of charge–discharge except at the charged and discharged states where multiple phases are present. The K-modified sample of P2 Na0.7Mn0.8Mg0.2O2 is compared with the K-free samples in terms of both structural evolution and electrochemical performance. © The Royal Society of Chemistry 2019