ABSTRACT
LiNi0.88Co0.1Al0.02O2 (NCA) is attractive for high-energy batteries, but phase transition and side reactions leave large volume change and thermal runaway. In order to address the drawbacks, orthorhombic Al2(WO4)3, a cheap anisotropic negative thermal expansion material, was synthesized and adopted to modify NCA, and its effects on the electrochemical performance and safety of NCA were investigated using multifarious techniques. Al2(WO4)3 can greatly improve the rate performance, cyclability at different temperatures, thermal stability, and interface behavior and intensify charge transfer as well as decline the deformation and side reactions of NCA. The discharge capacity of the NCA modified with 5 wt % Al2(WO4)3 reaches 170.0 mA h/g at 5.0 C and 25 °C. After 100 cycles, the values of this electrode at 1.0 C and 25 °C and at 3.0 C and 60 °C are 164.2 and 148.7 mA h/g, respectively, much higher than those of the pure NCA under the same conditions. Moreover, Al2(WO4)3 declines the byproducts and cation mixing and decreases the released heat, strain, and charge-transfer resistance after cycles of NCA about 37.1, 33.0, and 32.8%, respectively. The improvement mechanism is discussed. It opens an effective avenue for the applications of energy materials by simultaneously adjusting heat, structure, interface, and deformation.
ABSTRACT
The crystal structure and properties of lithium (cryptand[2.1.1]) ceside, Li+ (C211)Cs-, are reported. Li+ (C211)Cs- is the second ceside and third alkalide with a one-dimensional (1D) zigzag chain of alkali metal anions. The distance between adjacent Cs- anions, 6 A, is shorter than the sum of the van der Waals radii, 7 A. Optical, magic angle spinning NMR, two-probe alternating and direct current conductivity, and electron paramagnetic resonance measurements reveal unique physical properties that result from the overlap of adjacent Cs- wave functions in the chain structure. The properties of cesium (cryptand[2.2.2]) ceside, Cs+ (C222)Cs-, were also studied to compare the effects of the subtle geometric changes between the two 1D zigzag chain structures. Li+ (C211)Cs- and Cs+ (C222)Cs- are both low-band-gap semiconductors with anisotropic reflectivities and large paramagnetic 133Cs NMR chemical shifts relative to Cs- (g). An electronic structure model consistent with the experimental data has sp2-hybridized Cs- within the chain and sp-hybridized chain ends. Ab initio multiconfiguration self-consistent field calculations on the ceside trimer, Cs3(3-), support this model and indicate a net bonding interaction between nearest neighbors. The buildup of electron density between adjacent Cs- anions is visualized through an electron density difference map constructed by subtracting the density of three cesium atoms from the short Cs3(3-) fragment.
ABSTRACT
This is the sixth electride whose crystal structure has been determined and the fourth to show polymorphism. Crystals of the title electride prepared from mixed solvents have a structure similar to that of Li+(cryptand[2.1.1])e-. Electrons occupy cavities that are connected by "ladder-like" channels. The static and spin magnetic susceptibilities of polycrystalline samples that contain this polymorph (called phase α) show Heisenberg 1D antiferromagnetic behavior with -J/kB = 30 K. Similar to other electrides with "localized" electrons, this electride is a poor conductor (σ < 10- 4 ohm-1cm-1). Thin films prepared by high vacuum co-deposition of Rb metal and cryptand[2.2.2] have optical spectra and near-metallic electrical conductivity nearly identical with those of K+(cryptand[2.2.2])e-. These properties would not be expected if the film structure were the same as that obtained for crystals. Rather, they suggest that the films consist of microcrystals whose structure is similar to that of K+(cryptand[2.2.2])e-. Polycrystalline samples prepared by slow evaporation of methylamine from stoichiometric solutions at -78 °C (called phase ß) have properties similar to those of K+(cryptand[2.2.2])e-. The conductivity of samples that contain phase ß is more than an order of magnitude larger than those with phase α. Magnetic and spin susceptibilities show that phase ß samples have much larger electron-electron interactions. As with K+(cryptand[2.2.2])e-, the magnetic susceptibility of phase ß is compatible with alternating linear chain Heisenberg antiferromagnetism, with -J/kB ≈ 300 K and -J'/kB ≈ 240 K. Thin vapor co-deposited films show abrupt changes in the conductivity and optical spectrum at -12 °C that suggest a transition that may be conversion of phase ß to phase α.
