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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 Na 0.7 MnO 2 solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type P 6 3 / 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 Na 0.7 MnO 2 shows a second charge/discharge capacity of 143/134 mAh g -1 , K-modified Na 0.7 MnO 2 a capacity of 184/178 mAh g -1 , Rb-modified Na 0.9 MnO 2 a capacity of 159/150 mAh g -1 , and Cs-modified Na 0.7 MnO 2 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 Na 0.7 MnO 2 is compared with alkali metal (K, Rb, and Cs)-modified Na x MnO 2 in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, 23 Na 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.

Peptide-based nanofibres are a versatile class of tunable materials with applications in optoelectronics, sensing and tissue engineering. However, the understanding of the nanofibre surface at the molecular level is limited. Here, a series of homologous dilysine-diphenylalnine tetrapeptides were synthesised and shown to self-assemble into water-soluble nanofibres. Despite the peptide nanofibres displaying similar morphologies, as evaluated through atomic force microscopy and neutron scattering, significant differences were observed in their ability to support sensitive primary neurons. Contact angle and labelling experiments revealed that differential presentation of lysine moieties at the fibre surface did not affect neuronal viability; however the mobility of phenylalanine residues at the nanofibre surface, elucidated through solid- and gel-state NMR studies and confirmed through tethered bilayer lipid membrane experiments, was found to be the determining factor in governing the suitability of a given peptide as a scaffold for primary neurons. This work offers new insights into characterising and controlling the nanofibre surface at the molecular level., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

To meet the growing energy demands, sodium-ion batteries can be a potential substitute for lithium-ion batteries. Here, we report the solid-state synthesized alkali Rb- and Cs-modified Na 0.7 Mn 0.8 Mg 0.2 O 2 , which adopts hexagonal P 6 3 / mmc symmetry. The second charge/discharge capacity for the as-synthesized Rb- and Cs-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 are 118/114 and 130/125 mA h g -1 , which reduces to 62/62 and 77/76 mA h g -1 , respectively, after 100 cycles. In situ synchrotron X-ray diffraction data illustrate that a solid solution reaction occurs for most of the charge/discharge process in both cases. Rb-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 shows multiple phases near the charged state, whereas Cs-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 shows the formation of a new phase (P2 new ) at about 2.5 V and multiple phases below 1.7 V. The P2 new phase is found to evolve in conjunction with the original P2 phase until about 1.7 V, where the P2 reflection appears to split into multiple reflections and a single P2 phase is recovered at 2.7 V on the second charge. The Cs-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 shows better energy density in comparison with the K- and Rb-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 and comparable to the parent P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 . Ex situ scanning electron microscopy images show no noticeable change in surface morphology of the Cs-modified Na 0.7 Mn 0.8 Mg 0.2 O 2 , whereas in the case of Rb-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 , rods and irregular-shaped particles are observed after the 100th cycle. The solid-state 23 Na NMR shows a distinct shift in the peak position in comparison with the parent P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 , and two Na environments are observed with no local disordering in Rb- and Cs-modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 samples. Overall, this article illustrates the influence of using larger alkali ions to modify P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 and compares this scheme in terms of phase transitions and electrochemical performance., Competing Interests: The authors declare no competing financial interest.

A large piezoelectric coefficient of 76 pm/V along the diameter direction, approaching that of lead-based piezoelectrics, is observed in hydrothermally synthesized Pb-free Bi 0.5 (Na 0.8 K 0.2 ) 0.5 TiO 3 nanotubes. The 30-50 nm diameter nanotubes are formed through a scrolling and wrapping mechanism without the need of a surfactant or template. A molar ratio of KOH/NaOH = 0.5 for the mineralizers yields the Na/K ratio of ∼0.8:0.2, corresponding to an orthorhombic-tetragonal (O-T) phase boundary composition. X-ray diffraction patterns along with transmission electron microscopy analysis ascertain the coexistence of orthorhombic and tetragonal phases with (110) and (001) orientations along the nanotube length direction, respectively. 23 Na NMR spectroscopy confirms the higher degree of disorder in Bi 0.5 (Na 1- x K x ) 0.5 TiO 3 nanotubes with O-T phase coexistence. These findings present a significant advance toward the application of Pb-free piezoelectric materials.