Energy conversion and storage, producing alternative fuels, and environmental purification are of growing importance to realize sustainable societies. All-solid-state lithium ion rechargeable batteries (All-solid-state LIBs) using oxide-based solid electrolytes have the potential to drastically improve the high energy, high power and high safety of current (liquid-type electrolytes) LIBs. Many researchers are focusing on bulk-type all-solid-state LIBs. However, these LIBs are well-known to have key performance problems. The difficulties lie in decreasing defects in materials and constructing well-connected interfaces between different materials because these are general causes of scattering of conductive carries. I believe that crystal science approaches will definitely help with solving the bottleneck problems of all-solid-state LIBs.
Single crystals of Li6.5La3Zr1.5Nb0.5O12 samples were synthesized by FZ method. The typical single-crystal rod size of Li6.5La3Zr1.5Nb0.5O12 were about 7 mm in diameter and 80 mm length. Single-crystal diffraction data were collected for the structure analysis by single-crystal X-ray and single-crystal neutron diffraction methods. Li ions in the crystal structure occupied two kinds of crystallographic sites; those were distorted tetrahedral 96h site and distorted octahedral 96h site. The crystal structure was refined to the conventional values of RX-ray = 4.25% and Rneutron = 7.09% for single-crystal X-ray and neutron diffraction data. Electrochemical charge and discharge tests of the all-solid-state secondary lithium battery were performed using a two-electrode flat cell. The solid electrolyte was a single-crystal plate with a diameter of 6 mm and a thickness of 0.7 mm, which were obtained by cutting a single-crystal rod. The positive electrode was formed on the solid electrolyte by the sol-gel method.
Bulk single crystals of the solid state electrolyte and cathode materials in the lithium ion batteries are necessary to precious measurements of the electrochemical properties in the lithium ion batteries and development of all single-crystalline lithium ion battery. La2/3-xLi3xTiO3 (LLT) and LiCoO2 are expected as the candidates of solid state electrolyte and cathode in the all solid state batteries. In this paper, growth of the bulk single crystals of LLT and LiCoO2 by the traveling solvent floating zone (TSFZ) method are reported. Inclusion-free LLT single crystals of approximately 5 mm φ and 40 mm long were grown successfully using a LLT stoichiometric feeds and a 10 mol% La2Ti2O7-poor composition solvent relative to the stoichiometric LLT in a four-mirror type image furnace, since LLT melt incongruently to La2Ti2O7 and a liquid. Also high quality LiCoO2 single crystals of about 5 mm φ and 70 mm long were successfully in an ambient pressure of argon using a LiCoO2 stoichiometric feeds and a Li-excess solvent of an atomic ratio Li:Co=85:15 at a tilting angle 𝜃= 10° in a mirror-tilted FZ furnace.
Bulk single crystals of the perovskite LixLa(1-x)/3NbO3, which is one of the materials used as the solid electrolyte in all-solid lithium-ion batteries, have been grown for the first time by the directional solidification method. The ionic conductivity measured in the growth direction of the single crystal wafer of LixLa(1-x)/3NbO3 and the anisotropy of ionic conduction in solid electrolyte were experimentally confirmed for the first time by using LixLa(1-x)/3NbO3 single crystals. Here, the results of four experiments on LixLa(1-x)/3NbO3 bulk single crystals are presented:
(1) growth of solid electrolyte LixLa(1-x)/3NbO3 bulk single crystals, (2) ionic conduction in LixLa(1-x)/3NbO3 single crystal, (3) anisotropy of ionic conduction in LixLa(1-x)/3NbO3 single crystal and (4) microstructure analysis of LixLa(1-x)/3NbO3 single crystal.
We have demonstrated strategic approaches on crystal growth of lithium ion conducting oxide-based materials in molten salt (flux), leading to control morphologies and surface atomic arrangement for the efficient lithium ion transportation. This paper briefly summarized our current research activities regarding on the perfect surface formation and accompanying related emergent properties in crystalline battery materials. We believe that The morphology-controlled growth and mixed anion surface formation, introduced in this article, can be one approach towards solving the challenges for ensuring compatibility among high volumetric energy density and quick charging and lifetime without requiring a particularly large cost increase.
Sulfide glasses with high Li+ concentration are suitable electrolytes for application to all-solid-state lithium batteries because of their high conductivity and ductility (processability). A metastable phase with superior Li+ conductivity such as Li7P3S11 is precipitated by crystallization of its mother sulfide glass. Crystallization processes including nucleation and crystal growth are important to control for achieving highly-crystallized metastable phases. Viscosity of supercooled liquid at glass transition temperature is also a significant factor for understanding the crystallization process from glassy state. By optimizing the crystallization process for Li7P3S11, high Li+ conductivity of over 10-2 S cm-1 at 25℃ is achieved in the glass-ceramic electrolyte.
Metal anodes are promising materials for rechargeable batteries because of the higher capacity. However, low cycle efficiency due to non-uniform deposition prevents them from realization. In this review, we report the crystal orientation dependence of the deposition shape of Li and Zn, which are known as attractive materials for anodes. The result shows that the both of Li and Zn morphology has crystal orientation dependence.