Lithium garnets: Synthesis,structure, Li+ conductivity,Li+ dynamics and applications

Inorganic solid fast Li⁺ conductors based batteries are expected to overcome the limitations over safety concerns of flammable organic polymer electrolytes based Li⁺ batteries. Hence, an all-solid-state Li⁺ battery using non-flammable solid electrolyte have attracted much attention as next-generation battery. Therefore, in the development of all-solid-state lithium rechargeable batteries, it is important to search for a solid electrolyte material that has high Li⁺ conductivity, low electronic conductivity, fast charge transfer at the electrode interface and wide electrochemical window stability against potential electrodes and lithium metal. Hence, significant research effort must be directed towards developing novel fast Li⁺ conductors as electrolytes in all-solid-state lithium batteries. Among the reported inorganic solid Li⁺ conductive oxides, garnet-like structural compounds received considerable attention in recent times for potential application as electrolytes in all-solid-state lithium batteries. The focus of this review is to provide comprehensive overview towards the importance of solid fast lithium ion conductors, advantages of lithium garnets over other ceramic lithium ion conductors and understanding different strategies on synthesis of lithium garnets. Attempts have also been made to understand relationship between the structure, Li⁺ conduction and Li⁺ dynamics of lithium garnets. The status of lithium garnets as solid electrolyte in electrochemical devices like all-solid state lithium battery, lithium-air battery and sensor are also discussed.     

Micro-nano structured VNb9O25 anode with superior electronic conductivity for high-rate and long-life lithium storage

The oxygen vacancies and micro-nano structure can optimize the electron/Li+migration kinetics in anode materials for lithium batteries(LIBs).Here,porous micro-nano structured VNb9O25 composites with rich oxygen vacancies were reasonably prepared via a facile solvothermal method combined with annealing treatment at 800℃for 30 h(VNb9 O25-30 h).This micro-nano structure can enhance the contact of active material/electrolyte,and shorten the Li+diffusion distance.The introduction of oxygen vacancies can further boosts the intrinsic conductivity of VNb9O25-30 h for achieving excellent LIB performance.The as-prepared VNb9O25-30 h anode showed advanced rate capability with reversible capacity of 122.2 mA h g-1 at 4 A g-1,and delivered excellent capacity retention of～100％after 2000 cycles.Meanwhile,VNb9O25-30 h provides unexpected long-cycle life(i.e.,reversible capacity of 165.7 mA h g-1 at 1 A g-1 with a high capacity retention of 85.6％even after 8000 cycles).Additionally,coupled with the LiFePO4 cathode,the LiFePO4//VNb9O25-30 h full cell delivers superior LIB properties with high reversible capacities of 91.6 mA h g-1 at 5C for 1000 cycles.Thus,such reasonable construction method can assist in other high-performance niobium-based oxides in LIBs.     

Crystal structure and ionic conductivity of Li14Zn(GeO4)4 and other new Li+ superionic conductors

This paper reports the synthesis and characterization of a number of new Li⁺ superionic conductors with the type formula Li_16?2xD_x(TO₄)₄, where D is a divalent cation (Mg²⁺ or Zn²⁺), T is a tetravalent cation (Si⁴⁺ or Ge⁴⁺), and 0 < x < 4. One of these materials, Li₁₄Zn(GeO₄)₄, has a resistivity of 8 ω-cm at 300°C, lower than that of any Li⁺-ion conductor so far reported. The structure of this compound, which we have named LISICON (for $\text{Li s}$uper$\text{io}$nic $\text{con}$ductor), has been determined by single-crystal x-ray analysis. The space group is Pnma, with cell parameters $\text{a}\text{=10.828}\text{A}\text{?}$, $\text{b}\text{=6.251}\text{A}\text{?}$, $\text{c}\text{=5.140}\text{A}\text{?}$, and z=1. The structure has a rigid three-dimensional network of Li₁₁Zn(GeO₄)₄. The three remaining Li⁺ ions have occupancies of 55 and 16%, respectively, at the 4c and 4a interstitial positions. Each 4c position is connected to two 4a positions and vice versa. The bottlenecks betweenthese positions have an average diameter that is larger than twice the sum of the Li⁺ and O^2? ionic radii, thus satisfying thegeometrical condition for fast Li⁺ -ion transport. Moreover, all four sp³ orbitals of the O^2? ion are shared by strong tetrahedral covalent bonds with the network cations. Therefore, the anion charge is polarized away from the interstitial Li⁺ ions, weakening the Li1bO bond and increasing the Li⁺-ion mobility.     

Poly(acrylic acid) + zinc diacetate composites: High temperature service and electric conductivity

Poly(acrylic acid) + zinc diacetate hybrid composites have been prepared by precipitation from aqueous solutions and drying. The lowest glass transition temperature T_g determined by differential scanning calorimetry (DSC) is 392°C, providing a service temperature range unusually large for polymer-based materials (PBMs). Thermogravimetric analysis (TGA) shows that thermal decomposition begins some 10–20 K above each T_g. The alternating current impedance was determined in the nitrogen atmosphere from 370°C to 530°C. Dynamic dielectric measurements were performed between 20°C and 350°C, also under nitrogen. In contrast to typical PBMs, there is evidence of ionic conduction in some of the composites. The dynamic dielectric properties depend on the composition. The materials obtained are usable in medical applications and as high durability low friction coatings. Received: 19 February 1999 / Reviewed and accepted: 13 March 1999     

Color tunable and thermally stable luminescence of Tb3+ doped Li4SrCa(SiO4)2 phosphors

Tb³⁺ ions activated Li₄SrCa(SiO₄)₂ phosphors were synthesized using a solid state reaction method. The phase impurity was checked by XRD. The photoluminescence (PL) excitation spectrum, emission spectra at room and high temperature, decay curves of samples with different Tb³⁺ ions concentration were studied in detail. Cross-relaxation and the Inokuti–Hirayama model were used to analyze the experimental results. Li₄SrCa(SiO₄)₂:xTb³⁺ are thermally stable and color tunable phosphors.     

Microstructure and ionic conductivity of (La1/2Li1/3+x )TiO3 perovskite-like solid solutions

The lattice parameters of (La_1/2Li_1/3+x )TiO₃ (x = 0, 1/10, 1/6, 1/5, 1/4) ceramics have been determined as functions of temperature, and their microstructure and transport properties have been studied. The results indicate that the oxides retain orthorhombic symmetry upon the structural changes at ?925 K. According to atomic force microscopy results, the grains in the ceramics consist of nanocrystallites 60 to 120 nm in size. Analysis of the grain bulk and grain boundary contributions to the ionic conductivity of the (La_1/2Li_1/3+x )TiO₃ solid solutions shows that their bulk conductivity σ_bulk decreases with increasing x because reducing the concentration of A-site vacancies impedes ionic transport.     

S‐Doped Carbon Fibers Uniformly Embedded with Ultrasmall TiO2 for Na+/Li+ Storage with High Capacity and Long‐Time Stability

Building a rechargeable battery with high capacity, high energy density, and long lifetime contributes to the development of novel energy storage devices in the future. Although carbon materials are very attractive anode materials for lithium‐ion batteries (LIBs), they present several deficiencies when used in sodium‐ion batteries (SIBs). The choice of an appropriate structural design and heteroatom doping are critical steps to improve the capacity and stability. Here, carbon‐based nanofibers are produced by sulfur doping and via the introduction of ultrasmall TiO₂ nanoparticles into the carbon fibers (CNF‐S@TiO₂). It is discovered that the introduction of TiO₂ into carbon nanofibers can significantly improve the specific surface area and microporous volume for carbon materials. The TiO₂ content is controlled to obtain CNF‐S@TiO₂‐5 to use as the anode material for SIBs/LIBs with enhanced electrochemical performance in Na⁺/Li⁺ storage. During the charge/discharge process, the S‐doping and the incorporation of TiO₂ nanoparticles into carbon fibers promote the insertion/extraction of the ions and enhance the capacity and cycle life. The capacity of CNF‐S@TiO₂‐5 can be maintained at ≈300 mAh g^?1 over 600 cycles at 2 A g^?1 in SIBs. Moreover, the capacity retention of such devices is 94%, showing high capacity and good stability.