The challenges for further development of Li rechargeable batteries for electric vehicles are reviewed. Most important is safety, which requires development of a nonflammable electrolyte with either a larger window between its lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) or a constituent (or additive) that can develop rapidly a solid/electrolyte-interface (SEI) layer to prevent plating of Li on a carbon anode during a fast charge of the battery. A high Li⁺-ion conductivity (σ_Li > 10⁻⁴ S/cm) in the electrolyte and across the electrode/electrolyte interface is needed for a power battery. Important also is an increase in the density of the stored energy, which is the product of the voltage and capacity of reversible Li insertion/extraction into/from the electrodes. It will be difficult to design a better anode than carbon, but carbon requires formation of an SEI layer, which involves an irreversible capacity loss. The design of a cathode composed of environmentally benign, low-cost materials that has its electrochemical potential μ_C well-matched to the HOMO of the electrolyte and allows access to two Li atoms per transition-metal cation would increase the energy density, but it is a daunting challenge. Two redox couples can be accessed where the cation redox couples are “pinned” at the top of the O 2p bands, but to take advantage of this possibility, it must be realized in a framework structure that can accept more than one Li atom per transition-metal cation. Moreover, such a situation represents anintrinsic voltage limit of the cathode, and matching this limit to the HOMO of the electrolyte requires the ability to tune the intrinsic voltage limit. Finally, the chemical compatibility in the battery must allow a long service life.

Lithium-ion batteries are important for energy storage in a wide variety of applications including consumer electronics, transportation and large-scale energy production. The performance of lithium-ion batteries depends on the materials used. One critical component is the electrolyte, which is the focus of this paper. In particular, inorganic ceramic and organic polymer solid-electrolyte materials are reviewed. Solid electrolytes provide advantages in terms of simplicity of design and operational safety, but typically have conductivities that are lower than those of organic liquid electrolytes. This paper provides a comparison of the conductivities of solid-electrolyte materials being used or developed for use in lithium-ion batteries.

This short review presents the state-of-the-art knowledge on crystalline, composite and amorphous inorganic solid lithium ion conductors, which are of interest as potential solid electrolytes in lithium batteries and might replace the currently used polymeric lithium ion conductors. The discussion of crystalline Li ion conductors includes perovskite-type Lithium Lanthanum Titanates, NASICON-type, LiSICON- and Thio-LiSICON-type Li ion conductors, as well as garnet-type Li ion conducting oxides. The part on composite Li ion conductors discusses materials containing oxides and mesoporous oxides. In the amorphous Li ion conductor part, mechanical attrition of Li compounds, oxide and sulfide-based glasses as well as LIPON and related systems are presented.

Highly lithium ion conducting glass–ceramics in the system Li₂SP₂S₅ were successfully prepared by a heat treatment of the mechanochmically prepared sulfide glasses. The 80Li₂S·20P₂S₅ (mol.%) glass–ceramic mainly composed of the crystal analogous to the highly conductive thio-LISICON II phase in the system Li_4−xGe_1−xP_xS₄ showed conductivity as high as 10⁻³ S cm⁻¹ at room temperature. The 70Li₂S·30P₂S₅ glass–ceramic, in which the highly ion conductive new metastable phase was formed on heating, showed the highest conductivity of 3.2 × 10⁻³ S cm⁻¹ and the lowest activation energy of 12 kJ mol⁻¹ for conduction. The all-solid-state battery In/80Li₂S·20P₂S₅ glass–ceramic/LiCoO₂ exhibited excellent cycling performance of over 500 times with no decrease in the charge–discharge capacity (100 mAh g⁻¹).

The SnSP₂S₅ glasses as active materials were mechanochemically prepared from SnS and P₂S₅. High performance of these glassy electrode materials was observed in the rechargeable cell of 80SnS·20P₂S₅/80Li₂S·20P₂S₅ glass–ceramic/LiCoO₂, in which a continuous sulfide network between electrode and electrolyte was successfully formed.

The effects of conductive additives in composite electrodes on charge–discharge behavior of all-solid-state cells with Li₂SP₂S₅ glass–ceramics as a solid electrolyte were investigated. Under a current density over 1 mA cm⁻², the cell with vapor grown carbon fiber kept larger discharge capacities during 50 cycles than the cell with acetylene black. The design of continuous electron conducting path from a point of view of morphology for conductive additives is important to improve cell performances of all-solid-state lithium secondary batteries.

Lithium metal oxides with the nominal composition Li₅La₃M₂O₁₂ (M = Nb, Ta), possessing a garnetlike structure, have been investigated with regard to their electrical properties. These compounds form a new class of solid-state lithium ion conductors with a different crystal structure compared with all those known so far. The materials are prepared by solid-state reaction and characterized by powder XRD and ac impedance to determine their lithium ionic conductivity. Both the niobium and tantalum members exhibit the same order of magnitude of bulk conductivity (∼10⁻⁶ S/cm at 25°C). The activation energies for ionic conductivity (<300°C) are 0.43 and 0.56 eV for Li₅La₃Nb₂O₁₂ and Li₅La₃Ta₂O₁₂, respectively, which are comparable to those of other solid lithium conductors, such as Lisicon, Li₁₄ZnGe₄O₁₆. Among the investigated materials, the tantalum compound Li₅La₃Ta₂O₁₂ is stable against reaction with molten lithium. Further tailoring of the compositions by appropriate chemical substitutions and improved synthesizing methods, especially with regard to minimizing grain-boundary resistance, are important issues in view of the potential use of the new class of compounds as electrolytes in practical lithium ion batteries.

Lithium garnet-type oxides Li_7−XLa₃(Zr_2−X, Nb_X)O₁₂ (X = 0–2) were synthesized by a solid-state reaction, and their lithium ion conductivity was measured using an AC impedance method at temperatures ranging from 25 to 150 °C in air. The lithium ion conductivity increased with increasing Nb content, and reached a maximum of ∼0.8 mS cm⁻¹ at 25 °C. By contrast, the activation energy reached a minimum of ∼30 kJ mol⁻¹ at the same point withX = 0.25. The potential window was examined by cyclic voltammetry (CV), which showed lithium deposition and dissolution peaks around 0 V vs. Li⁺/Li, but showed no evidence of other reactions up to 9 V vs. Li⁺/Li.

Abstract

High lithium-ion (Li⁺) conductive garnet-structured lanthanum lithium zirconate (LLZ) solid electrolyte is prepared by incorporation of appropriate amounts of silicon (Si) and aluminum (Al). The resultant pelletized LLZ obtains total Li⁺ conductivity of 6.8 × 10^− 4 S cm^− 1 at 298 K. This improved conductivity is nearly identical with the bulk Li⁺ conductivity of the LLZ reported earlier, suggesting that the grain boundary resistance is effectively reduced by the incorporation of Si and Al. Microanalyses by transmission electron microscopy coupled with energy-dispersive X-ray microanalysis and electron energy-loss spectroscopy revealed the presence of amorphous Li–Al–Si–O with nano crystalline LiAlSiO₄ at grain boundaries. Fast lithium-ion transport around the amorphous Li–Al–Si–O/LiAlSiO₄ interface will facilitate Li⁺ transport between the LLZ particles, resulting in the improvement of the total Li⁺ conductivity of LLZ.

Graphical abstract

Li₇La₃Zr₂O₁₂ electrolytes doped with different amounts of Al (0, 0.2, 0.7, 1.2, and 2.5 wt.%) were prepared by a polymerized complex (Pechini) method. The influence of aluminum on the structure and conductivity of Li₇La₃Zr₂O₁₂ were investigated by X-ray diffraction (XRD), impedance spectroscopy, scanning electron microscopy (SEM), and thermal dilatometry. It was found that even a small amount of Al (e.g. 0.2 wt.%) added to Li₇La₃Zr₂O₁₂ can greatly accelerate densification during the sintering process. SEM micrographs showed the existence of a liquid phase introduced by Al additions which led to the enhanced sintering rate. The addition of Al also stabilized the higher conductivity cubic form of Li₇La₃Zr₂O₁₂ rather than the less conductive tetragonal form. The combination of these two beneficial effects of Al enabled greatly reduced sintering times for preparation of highly conductive Li₇La₃Zr₂O₁₂ electrolyte. With optimal additions of Al (e.g. 1.2 wt.%), Li₇La₃Zr₂O₁₂ electrolyte sintered at 1200 °C for only 6 h showed an ionic conductivity of 2.0 × 10⁻⁴ S cm⁻¹ at room temperature.

Cubic garnets of composition Li_6.75La₃Zr_1.75Ta_0.25O₁₂, Li_6.15La₃Zr_1.75Ta_0.25Al_0.2O₁₂, and Li_6.15La₃Zr_1.75Ta_0.25Ga_0.2O₁₂ were prepared from a co-precipitated precursor and consolidated by hot-pressing to a relative density of ∼96–98%. The total Li-ion conductivities at 298 K and activation energies (in parentheses) of Li_6.75La₃Zr_1.75Ta_0.25O₁₂, Li_6.15La₃Zr_1.75Ta_0.25Al_0.2O₁₂ and Li_6.15La₃Zr_1.75Ta_0.25Ga_0.2O₁₂ were 0.87 mS cm⁻¹ (0.22 eV), 0.37 mS cm⁻¹ (0.30 eV) and 0.41 mS cm⁻¹ (0.27 eV), respectively. The above results suggest that cubic stabilizing substitutions outside of the Li-ion sub-lattice are preferable to obtain faster Li-ion conductivity.

The doping effect of Si, In and Ge in Li₇La₃Zr₂O₁₂ (LLZ) as the solid electrolyte was studied via conventional solid-state reaction. The crystal structure, cross-sectional morphology, element distribution and conductivity were characterized for the undoped and doped LLZ ceramics. Our results showed that Si and In doping deteriorate electrical properties of LLZ as electrolyte, while Ge doping obviously improves properties. Ge might thus be a promising element for further increasing the ionic conductivity of LLZ ceramics.

Abstract

An attempt has been made in this work to increase the bulk and total conductivity of Li₇La₃Zr₂O₁₂ (LLZ) with partial substitution of trivalent Y for a tetravalent Zr using yttria-stabilized ZrO₂ (3% YSZ) as reactant. The small doping of Y for Zr helps to increase the bulk and total conductivity to 9.56 × 10^− 4 and 8.10 × 10^− 4 Scm^− 1, respectively, at 25 °C. The presence of small amount of Y helped to get well sintered pellet relatively at lower temperature with lower sintering time compared to LLZ, which helps to improve the overall conductivity.

Highlights

? Garnet-like Li₇La₃Zr₂O₁₂ (LLZ) has been considered for application in energy storage.? LLZ exhibits high lithium ion conductivity and stable nature against Li metal.? An attempt has been made in this work to increase the ionic conductivity of LLZ.? Higher ionic conductivity was observed with partial substitution of Y for Zr.

An investigation of the interrelationships between strength, crack-resistance (R-curve) characteristics, and grain size for alumina ceramics has been carried out. Results of identation-strength measurements on high-density aluminas with uniform grain structures in the size range 2 to 80 μm are presented. A theoretical fit to the data, obtained by adjusting parameters of a constitutive frictional-pullout relation in a grain-bridging model, allows determination of the critical microstructural parameters controlling theR-curve behavior of these aluminas. The primary role of grain size in the toughness characteristic is to determine the scale of grain pullout at the bridged interface. It is shown that the strength properties are a complex function of the bridged microstructure, governed at all but the finest grain sizes by the stabilizing effect of theR-curve. The analysis confirms the usual negative dependence of strength on grain size for natural flaws that are small relative to the grain size, but the dependence does not conform exactly to the −1/2 power predicted on the basis of classical “Griffith-Orowan” flaws. The analysis provides a self-consistent account of the well-documented transition from “Orowan” to “Petch” behavior.

The polymorphic phase transition (PPT) from tetragonal to orthorhombic symmetry was tailored in lead-free piezoelectric ceramics Li_0.08(Na_0.52+xK_0.48)_0.92NbO₃ (LN_0.52+xKN) by controlling excess Na content, which is opposite to the usually reported orthorhombic–tetragonal phase transition. The grain growth was accelerated by adding excess Na, resulting in an increased grain size from 2–3 μm to 7–8 μm and a bimodal grain size distribution for samplesx=0.015–0.04. The optimized piezoelectric properties with, i.e.d₃₃=226 pC/N andk_p=36.8% were obtained in samplex=0.04 due to the PPT effect. The correspondingT_C,P_r, andE_r were 468 °C, 25.5 μC/cm² and 1.20 kV/mm, respectively.

Recent research has shown that certain Li-oxide garnets with high mechanical, thermal, chemical, and electrochemical stability are excellent fast Li-ion conductors. However, the detailed crystal chemistry of Li-oxide garnets is not well understood, nor is the relationship between crystal chemistry and conduction behavior. An investigation was undertaken to understand the crystal chemical and structural properties, as well as the stability relations, of Li₇La₃Zr₂O₁₂ garnet, which is the best conducting Li-oxide garnet discovered to date. Two different sintering methods produced Li-oxide garnet but with slightly different compositions and different grain sizes. The first sintering method, involving ceramic crucibles in initial synthesis steps and later sealed Pt capsules, produced single crystals up to roughly 100 μm in size. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry (ICP-MS) measurements show small amounts of Al in the garnet, probably originating from the crucibles. The crystal structure of this phase was determined using X-ray single-crystal diffraction every 100 K from 100 K up to 500 K. The crystals are cubic with space groupIa3̅d at all temperatures. The atomic displacement parameters and Li-site occupancies were measured. Li atoms could be located on at least two structural sites that are partially occupied, while other Li atoms in the structure appear to be delocalized.²⁷Al NMR spectra show two main resonances that are interpreted as indicating that minor Al occurs on the two different Li sites. Li NMR spectra show a single narrow resonance at 1.2−1.3 ppm indicating fast Li-ion diffusion at room temperature. The chemical shift value indicates that the Li atoms spend most of their time at the tetrahedrally coordinated C (24d) site. The second synthesis method, using solely Pt crucibles during sintering, produced fine-grained Li₇La₃Zr₂O₁₂ crystals. This material was studied by X-ray powder diffraction at different temperatures between 25 and 200 °C. This phase is tetragonal at room temperature and undergoes a phase transition to a cubic phase between 100 and 150 °C. Cubic “Li₇La₃Zr₂O₁₂” may be stabilized at ambient conditions relative to its slightly less conducting tetragonal modification via small amounts of Al³⁺. Several crystal chemical properties appear to promote the high Li-ion conductivity in cubic Al-containing Li₇La₃Zr₂O₁₂. They are (i) isotropic three-dimensional Li-diffusion pathways, (ii) closely spaced Li sites and Li delocalization that allow for easy and fast Li diffusion, and (iii) low occupancies at the Li sites, which may also be enhanced by the heterovalent substitution Al³⁺ 3Li.

Abstract

The low ionic conductivity is a bottleneck of the inorganic solid state electrolyte used for lithium ion battery. In ceramic electrolytes, grain boundary usually dominates the total conductivity. In order to improve the grain boundary effect, an amorphous silica layer is introduced into grain boundary of ceramic electrolytes based on lithium–lanthanum–titanate, as evidenced by electron microscopy. The results showed that the total ionic conductivity could be to be enhanced over 1 × 10⁻⁴ S/cm at room temperature. The reasons can be attributed to removing the anisotropy of outer-shell of grains, supplement of lithium ions in various sites in grain boundary and close bindings among grains by the amorphous boundary layer among grains.

Garnet-structure related metal oxides with the nominal chemical composition of Li₅La₃Nb₂O₁₂, In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ and K-substituted Li_5.5La_2.75K_0.25Nb₂O₁₂ were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li₅La₃Nb₂O₁₂. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In³⁺(r_CN6: 0.79 Å) for smaller Nb⁵⁺ (r_CN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K⁺ for La³⁺ and In³⁺ for Nb⁵⁺ in Li₅La₃Nb₂O₁₂ exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25–300 °C. Among the compounds investigated, the In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ exhibits the highest bulk lithium ion conductivity of 1.8×10⁻⁴ S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity (“component diffusion coefficient”) obtained from the AC conductivity and powder XRD data falls in the range 10⁻¹⁰–10⁻⁷ cm²/s over the temperature regime 50–200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li₅La₃Nb₂O₁₂ was found to be unsuccessful under the investigated conditions.

Abstract

The garnet-related oxide with general formula Li₆La₃ZrTaO₁₂ was prepared by conventional solid-state reaction. X-ray diffraction (XRD) and AC impedance were used to determine phase formation and the lithium-ion conductivity. The structures were refined by the Rietveld method from powder X-ray diffraction data. Li₆La₃ZrTaO₁₂ is cubic and crystallizes in the space groupIa-3d with room-temperature lattice parameter a = 12.8873 Å. The bulk and total lithium ion conductivities of Li₆La₃ZrTaO₁₂ at 25 °C were 2.5 × 10⁻⁴ S cm⁻¹ and 1.8 × 10⁻⁴ S cm⁻¹, respectively; the activation energy was about 0.42 eV in the temperature range 298–450 K.

Highlights

? A garnet-related oxide Li₆La₃ZrTaO₁₂ was obtained by solid-state reaction. ? The bulk and total Li⁺-ion conductivities at 25 °C are 2.5 × 10⁻⁴ and 1.8 × 10⁻⁴ S cm⁻¹. ? The activation energy is about 0.42 eV in the temperature range 298–450 K. ? It is stable on contact with a metallic lithium anode.

Garnet-like structured metal oxides with the general formula Li₆ALa₂Nb₂O₁₂ (A=Ca, Sr, Ba) have been prepared by solid-state reaction using appropriate amounts of corresponding metal oxides, nitrates, and hydroxides. The powder X-ray diffraction data reveal that Li₆ALa₂Nb₂O₁₂ compounds are isostructural with the parent compound Li₅La₃Nb₂O₁₂. The cubic lattice parameter was found to increase with increasing size of the alkaline earth ions. The grain size decreases considerably with the substitution of La by the alkaline earth elements under the same preparation conditions. The Ca-substituted compound exhibits both bulk and major grain boundary contributions to the total resistance, while the Sr- and Ba-substituted compounds show mainly bulk resistance with a rather small grain boundary contribution (∼14% of the total resistance at 20°C) with further decrease with increasing temperature. In comparison, the ionic conductivity decreases with decreasing ionic radius of the alkaline earth elements. Among the investigated compounds, the Ba-compound Li₆BaLa₂Nb₂O₁₂ shows the highest ionic conductivity of 6 × 10⁻⁶ S/cm at room temperature (22°C) and lowest activation energy of 0.44 eV compared with 0.55 and 0.50 eV for the corresponding Ca- and Sr- compounds, respectively. The ionic conductivity is comparable with that of parent Li₅La₃Nb₂O₁₂ and other fast lithium ion conductors known so far.