Solid state electrolytes for all-solid-state 3D lithium-ion batteries

The focus of this Ph.D. thesis is to understand the lithium ion motion and to enhance the Li-ionic conductivities in commonly known solid state lithium ion conductors by changing the structural properties and preparation methods. In addition, the feasibility for practical utilization of several studied solid electrolyte materials in 3D all-solid-state lithium ion batteries was investigated. Several inorganic compounds with high ionic conductivity for all solid state lithium ion batteries have been proposed in recent literatures. These are discussed in Chapter 2 in terms of the relationship with the structural features and the lithium ion mobility. The key criteria of high lithium ion mobility in any solid lithium ion conductor are the concentration of the charge carriers and vacancies, the "bottleneck size" which is the cross sectional area that lithium ion has to pass through, the connectivity of the sites where lithium ions are mobile and the polarizability of the anions. Based on the structural features, the very well known Li0.50L0.50TiO3 (LLT) compound with perovskite-type structure was modified to increase the bottleneck size by anionic substitution of oxygen by the relatively larger anion, nitrogen, in Chapter 3. The resulting oxynitride compound contains 0.58 atoms of nitrogen in the formula unit and has a higher lattice volume up to 4.4 %. Although it is expected that lithium ions can move more easily in this structure, impedance measurements show that the ionic conductivity is decreasing with increase in nitrogen content. This has been explained by the distortion in TiO6 polyhedra which is slowing down the lithium ion motion. In addition we observed anionic vacancies which are also changing the chemical environment of the lithium. The anionic vacancies which are formed during the substitution can be prevented by cationic substitution of Ti4+ with Ta5+ to combine with anionic substitution in oxygen positions. This type of approach will not only prevent anionic vacancies but will also increase the electro chemical stability of this compound towards possible reduction when in contact with lithium. Li7La3Zr2O12 (LLZO) with garnet type structure has recently become of high interest due to its potential as a solid-state lithium ion conductor. It has a high ionic conductivity (10- 4 S/cm for the cubic phase at room temperature) as well as a good stability against lithium and moisture. However, LLZO crystallizes in three different phases; low and high temperature cubic and tetragonal. The high temperature cubic phase is preferred because it has a 2 orders of magnitude higher ionic conductivity than the tetragonal phase whereas the synthesis of the cubic phase needs a high calcination temperature which makes it difficult to control the stoichiometry. Recently it has also been found that due to the high calcination temperature, the aluminum contamination from the reaction crucible (Al2O3) enables and stabilizes the cubic phase formation. To have a better control on the chemical composition and prevent contamination, low temperature synthesis by a sol-gel method was investigated in Chapter 4. The tetragonal phase was successfully synthesized at 1073 K. This is 200 K lower than any previously reported results and a new low temperature (973 K) cubic phase was reported for the first time. The ionic conductivities of the tetragonal phase were determined and to be in the same order of magnitude with those of the materials synthesized by conventional solid-state synthesis. Unfortunately, the ionic conductivity of the new cubic phase could not be determined due to the temperature limitation during the densification process which leads porous specimens. Li5La3Ta2O12 (LLTO) and Li6BaLa2Ta2O12 (LLBTO) with garnet type structure are yet other promising candidates as solid lithium ion conductors. They are chemically stabile against lithium and moisture due to the presence of Ta5+ in the garnet compound series. LLTO has a lithium ionic conductivity of 10-6 S/cm, whereas LLBTO exhibits a conductivity of 10-5 S/cm at room temperature due to its larger unit cell and higher lithium ion concentration. The sol-gel synthesis of garnet compounds was investigated in Chapter 5. It is found that nano-sized compounds have better sintering ability and the ionic conductivities are found in the same order magnitude with a slightly increase compared to the compounds synthesized by conventional solid state methods. The sol-gel synthesis of garnet compounds opened up a new approach in the preparation of solid-state lithium ion conductors with 3D structure. This is discussed in detail in Chapter 6 for the preparation of 3 dimensional ordered macraporous (3DOM) materials. LLTO was investigated for 3DOM material preparation experiments due to the relatively lower synthesis temperature resulting in smaller grain sized compounds compared to other members of garnet compounds. 3DOM membranes of Li5La3Ta2O12 (LLTO) for all-solid-state lithium ion batteries were prepared by using colloidal crystal templating of mono dispersed polystyrene (PS) spheres combined with sol-gel synthesis of LLTO precursor. Two different types of solvent (EtOH and HAc/EG) and 3 different sizes of PS spheres (1, 3 and 5 µm) were used for the preparation of 3DOM membranes. The effect of the solvent type and the PS sphere size on the morphology of the 3DOM membranes was investigated. Our investigations show that using a HAc/EG based solution with the template prepared by using 5 µm PS spheres results in the most interconnected and long range ordered membranes. The 3DOM membranes can be used to fabricate all-solid-state lithium ion batteries using a "sandwich structure" which is composed of a dense LLTO layer having the 3DOM layer on both sides. Then by immersing the electrode material in the pores of the 3DOM layer, the all-solid-state battery can easily be fabricated. As an alternative 3D structuring method, nano printing by soft lithography is described in Chapter 7. The sol-gel synthesis of Li0.29La0.54TiO3 (LLT) with perovskite structure and its patterning by soft lithography was studied. A 3D patterned LLT structure was obtained in the micro molding experiments and for the first time ceramic electrolyte materials were deposited with 3 dimensional structures (line and pit patterns) starting with simple sol-gel synthesis were combined with micro molding experiments by soft lithography. The patterning experiments conducted on Si substrates and\or Pt coated Si substrate. The Si substrate was found to be more suitable and yielded better patterns compared to Pt coated Si substrate because the deposition performed on Pt coated Si substrate yielded unwanted residual layers in both line and pit pattern deposition experiments. This may be explained due to the wetting properties of the Pt surface and the durability of Pt coated Si substrates at 973 K. AFM and SEM measurements were done to investigate the morphology of the patterns deposited by soft lithography and it shows that by using the different type of mold it is possible to replicate the desired structure. In conclusion, sol-gel synthesis is a successful method to prepare various lithium ion battery electrolyte materials at low temperature and it makes use of inexpensive precursors of metal salts (nitrates, acetates or oxides) combined with metal alkoxides. Using sol-gel precursor solutions, we demonstrated the successful preparation of 3D structured electrolyte materials by soft lithography and crystal templating for 3D all solid state lithium ion batteries. We found that the soft lithography is a very accurate technique (sub-micrometer precision) and it can easily be applicable to larger scales. In contrast, the accuracy and the applicability of crystal templating is dependent on the template sphere size and the precursor material. As a follow up research, the studied preparation techniques (such as crystal templating and micro patterning) can be combined with impregnation of the electrode material by previously investigated deposition techniques such as; spin-coating, atomic layer deposition (ALD) or chemical vapor deposition (CVD). The micro patterning experiments can easily be used for any type of material which can be prepared by sol-gel synthesis. Since the sol-gel synthesis of active electrode materials was established in literature, 3D all-solid-state lithium ion battery can be easily prepared by deposition of different array of compounds separately on the same substrate. It is also possible with non-oxidic compounds which could be generated by changing from an air atmosphere to different (N2, NH3, H2, etc.) atmospheres during the high-temperature step of the synthesis. Overall, the combination of rather low tech and low-cost processes (sol-gel combined with soft-lithography or crystal templating) using simple starting materials and equipment (vacuum pump and heater plate) makes it possible to create sub-micrometer structures of electrolyte materials in the normal lab environment. It is likely that this manufacturing route can be applied to various other battery materials as well as large scales