The phase diagram of non-hydrated NaxCoO2 has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x = 1 2 ) to a 'Curie-Weiss metal' (around 0.70), and finally to a weak-moment magnetically ordered state (x > 0.75). The unusual properties of the state at 1 2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.Research on oxide conductors has uncovered many interesting electronic states characterized by strong interaction, which include unconventional superconductivity, and charge-or spin-ordered states [1,2]. Recently, attention has focussed on the layered cobaltate Na x CoO 2 . At the doping x ∼ 2 3 , Na x CoO 2 exhibits an unusually large thermopower [3]. Although the resistivity is metallic, the magnetic susceptibility displays a surprising Curie-Weiss profile [4], with a magnitude consistent with antiferromagnetically coupled spin-1 2 local moments equal in number to the hole carriers [5]. The thermopower at 2.5 K is observed to be suppressed by an in-plane magnetic field [5]. This implies that the enhanced thermopower is largely due to spin entropy carried by strongly correlated holes (Co 4+ sites) hopping on the triangular lattice. When intercalated with water, Na x CoO 2 ·yH 2 O becomes superconducting at or below 4 K [6] for 1 4 < x < 1 3 [7,8,9]. These experiments raise many questions. Is the Curie-Weiss state at 2 3 continuous with the 1 3 state surrounding superconductivity? Are commensurability and charge-ordering effects important? To address these questions, we have completed a study of the phase diagram of non-hydrated Na x CoO 2 . As x increases from 0.3 to 0.75, we observe a series of electronic states, the most interesting of which is an insulating state at x = 1 2 that involves charge ordering of the holes together with the Na ions. We identify details specific to the triangular lattice, especially in the metallic state from which the superconducting composition evolves, and comment on recent theories.Starting with powder or single-crystal samples with x ∼ 0.75, we vary x by specific chemical deintercalation of Na (Fig. 1, caption). Powders of Na 0.77 CoO 2 were made by solid-state reaction of stoichiometric amounts of Na 2 CO 3 and Co 3 O 4 in oxygen at 800 C. Sodium deintercalation was then carried out by treatment of samples in solutions obtained by dissolving I 2 (0.2 M, 0.04 M) or Br 2 (1.0 M) in acetonitrile. After magnetic stirring for five days at ambient temperature, they were washed with copious amounts of acetonitrile and multiple samples were tested by the ICP-AES method to determine Na content. Unit-cell parameters were determined by powder X-ray diffraction (XRD) with internal Si standards. For the transport studies, we first grew a boule (with x = 0.75) in an optical furnace by the floating-zone technique. Crystals cleaved from the boul...

Ni-rich materials are appealing to replace LiCoO as cathodes in Li-ion batteries due to their low cost and high capacity. However, there are also some disadvantages for Ni-rich cathode materials such as poor cycling and rate performance, especially under high voltage. Here, we demonstrate the effect of dual-conductive layers composed of LiPO and PPy for layered Ni-rich cathode material. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show that the coating layers are composed of LiPO and PPy. (NH)HPO transformed to LiPO after reacting with surface lithium residuals and formed an inhomogeneous coating layer which would remarkably improve the ionic conductivity of the cathode materials and reduce the generation of HF. The PPy layer could form a uniform film which can make up for the LiPO coating defects and enhance the electronic conductivity. The stretchy PPy capsule shell can reduce the generation of internal cracks by resisting the internal pressure as well. Thus, ionic and electronic conductivity, as well as surface structure stability have been enhanced after the modification. The electrochemistry tests show that the modified cathodes exhibited much improved cycling stability and rate capability. The capacity retention of the modified cathode material is 95.1% at 0.1 C after 50 cycles, whereas the bare sample is only 86%, and performs 159.7 mAh/g at 10 C compared with 125.7 mAh/g for the bare. This effective design strategy can be utilized to enhance the cycle stability and rate performance of other layered cathode materials.

A very large drop in dielectric constant upon application of small magnetic fields is observed at room temperature for LuFe2O4 (see figure). Such behavior is unprecedented and indicates a strong coupling of spins and electric dipoles at room temperature. This behavior of LuFe2O4 is apparently related to its ferroelectricity, which occurs through the highly unusual mechanism of Fe2+ and Fe3+ ordering.