Crystal Structure and Preparation of Li₇La₃Zr₂O₁₂ (LLZO) Solid-State Electrolyte and Doping Impacts on the Conductivity: An Overview

Abstract

As an essential part of solid-state lithium-ion batteries, solid electrolytes are receiving increasing interest. Among all solid electrolytes, garnet-type Li₇La₃Zr₂O₁₂ (LLZO) has proven to be one of the most promising electrolytes because of its high ionic conductivity at room temperature, low activation energy, good chemical and electrochemical stability, and wide potential window. Since the first report of LLZO, extensive research has been done in both experimental investigations and theoretical simulations aiming to improve its performance and make LLZO a feasible solid electrolyte. These include developing different methods for the synthesis of LLZO, using different crucibles and different sintering temperatures to stabilize the crystal structure, and adopting different methods of cation doping to achieve more stable LLZO with a higher ionic conductivity and lower activation energy. It also includes intensive efforts made to reveal the mechanism of Li ion movement and understand its determination of the ionic conductivity of the material through molecular dynamic simulations. Nonetheless, more insightful study is expected in order to obtain LLZO with a higher ionic conductivity at room temperature and further improve chemical and electrochemical stability, while optimal multiple doping is thought to be a feasible and promising route. This review summarizes recent progress in the investigations of crystal structure and preparation of LLZO, and the impacts of doping on the lithium ionic conductivity of LLZO.

1. Introduction

Many countries around the world are setting up renewable energy plants, reducing the use of carbon emitting fuels, making electric vehicles (EV’s), and taking many other initiatives to tackle the energy crisis that is imminent because of increasing burn rates of fossil fuels; and most of these initiatives need reliable energy storage devices which will ensure the efficient use of the said energy. Therefore, for now, the main challenge regarding effective use of energy is to make energy conversion and storage more efficient and more reliable.

Among many energy storage devices, lithium-ion battery has been the most commonly used technology because of its advantages including high energy density, long duty cycle, low self-discharge, and light weight [1,2,3]. However, there are some concerns about lithium-ion batteries like: safety issues because of the formation of dendrites, reduced power capability because of the formation of solid electrolyte interface (SEI) from continuous charging and discharging, low ionic conductivity, and high cost compared to other existing rechargeable batteries. Extensive research is being carried out by many groups to resolve these issues aiming to improve the performance of lithium-ion batteries.

Electrolyte is one of the most important parts that determine the performance of lithium-ion batteries. According to the state of form, electrolyte materials can be categorized as liquid electrolytes, polymer electrolytes, ionic electrolytes, and solid electrolytes.

Liquid electrolytes are made by dissolving various Li-rich salts like Li₃N, LiPF₆, LiClO₄, etc. into solvents. The main advantage of liquid electrolytes is the high ionic conductivity on the order of 10⁻² S/cm, while the major problem of liquid electrolytes is that they react with electrodes creating a solid electrolyte interface (SEI) layer which lowers battery performance and increases safety concerns [4].

Polymer electrolytes (PE) are defined as a solvent-free salt solution in a polymer host material such as polyethylene oxide (PEO) that conducts lithium ions through the polymer chains. Polymer electrolytes typically have an ionic conductivity in the range of 10⁻⁴ to 10⁻⁸ S/cm [5,6]. The main advantage of polymer electrolytes is that they can form a very thin layer of electrolyte which exhibits a low internal resistance, thus delivering batteries with a high specific power density [7]. However, polymer electrolytes also react with Li electrode leading to the formation of a SEI layer at the interface between electrodes and electrolyte.

Ionic electrolytes are composed of organic salts such as alkylammonium, N-alkyl pyridinium, phosphonium, etc. [8,9]. They show very good electrochemical and thermal stability. Moreover, they are inflammable and recyclable. However, due to the high viscosity, ionic electrolytes usually present a poor ionic conductivity [10].

Solid electrolytes have many advantages such as good chemical stability with Li metal electrode, good thermal stability, high electrochemical voltage window (>5 V), economy, and environmental friendliness. They exhibit an ionic conductivity typically in the range of 10⁻³ to 10⁻⁷ S/cm, which is relatively lower than that of liquid electrolytes. Because solid electrolytes can be used to form all solid-state batteries (ASSBs), they have been intensively studied. Solid electrolytes can be categorized mainly into three types: inorganic solid electrolytes, solid polymer electrolytes and solid composite polymer electrolytes.

Research has been performed on various types of inorganic solid electrolyte: Li₃N type, [11,12,13] LISICON type, [14,15,16] NASICON type, [17,18,19,20,21,22,23,24] beta alumina type, [25,26,27,28,29] LIPON type, [30,31,32] and perovskite type; [33,34] but the problems with these types of inorganic electrolytes are: (1) poor chemical stability due to the reaction with metal electrodes, (2) difficulty to prepare, (3) a low electrochemical voltage window, and (4) low ionic conductivity.

In 2003, Thangadurai et al. first reported the use of inorganic garnet-type lithium oxide with a nominal composition of Li₅La_3×2O₁₂ (X = Nb, Ta) for ASSB and showed the promising capability in overcoming the problems mentioned above [35]. In 2007, the same group reported the most promising garnet-type Li oxide with nominal composition of Li₇La₃Zr₂O₁₂, also known as LLZO, which revealed excellent properties with exceptional results that exceeded all the previous findings of solid electrolytes [36]. Since its first report, LLZO has drawn great attention of the battery research community; in order to use in practical purpose application a considerable amount of research has been undertaken to increase the performance of LLZO.

In 2015, Ramakumar et al. reported that among all garnet-type electrolytes, LLZO with a composition of Li₇La₃Zr₂O₁₂ exhibits the highest ionic conductivity corresponding to the lowest activation energy [37]. At 33 °C, Li₇La₃Zr₂O₁₂ composition exhibited a total (bulk + grain boundary) ionic conductivity of 5 × 10⁻⁴ S/cm with an activation energy of 0.32 eV. However, with further increasing Li content, the total ionic conductivity starts to decrease; in the case of 7.5 molar unit of Li content, the conductivity drops to 1 × 10⁻⁴ S/cm with the activation energy increasing to 0.38 eV [37].

In this paper, we review different crystal structures of LLZO, various synthesis methods for the preparation of LLZO, the molecular dynamics in relation to Li ion diffusion in LLZO, and the effects of different cation dopants on the conductivity of LLZO. The paper starts with an introduction, followed by five sections on the topics of (i) structural analysis, (ii) synthesis techniques for LLZO, (iii) sintering techniques, (iv) Li ion diffusion mechanism, and (v) doping and Li ionic conductivity, and ends with a conclusion section that summarizes the major conclusions and addresses the challenges of on-going LLZO research. The goal of this article is to outline the optimal crystal structure with a doping strategy that may guide the experimental synthesis towards improving the lithium ionic conductivity of LLZO solid electrolyte.

2. Structural Analysis of LLZO

In 2003, Thangadurai et al. reported Li₅La₃M₂O₁₂ (M = Nb, Ta) oxides as a fast garnet-like Li-ion conductor [35]. Later in 2007, from the same group Murugan et al. introduced a new garnet-type material Li₇La₃Zr₂O₁₂ known as LLZO which has higher ionic conductivity at room temperature [36]. The generic formula for LLZO garnet Li₇La₃Zr₂O₁₂ was derived from the garnet type family Li₇A₃B₂O₁₂, in which Li ions occupy octahedral and tetrahedral sites, A-cations belong to eight-coordination sites and B-cations belong to six-fold coordination sites. LLZO exhibits two phases, namely tetragonal phase (lattice parameter: a = b ≠ c, angle: α = β = γ = 90°, space group I4₁/acd) and cubic phase (lattice parameter: a = b = c, angle: α = β = γ = 90°, space group Ia$\overline{3}$d). Both phases possess the same structural framework but there is a difference in the distribution of Li atoms, which dominantly determines the ionic conductivity of LLZO. In the tetragonal phase, Li ions can occupy tetrahedral 8a, and octahedral 16f and 32g sites as shown in Figure 1, [38] where Li(1) is in 8a tetrahedral site connected to four O atoms; Li(2) and Li(3) belong to 16f and 32g octahedral sites, respectively, and both are connected to six O atoms. Unlike the tetragonal phase, Li in cubic structure occupies only 2 different sites namely 24d tetrahedral sites and 96h octahedral sites as shown in Figure 2 [39]. Therefore, it is clear from the site occupancy perspective that in the cubic phase Li ions have more available sites for migration than in the tetragonal phase.

Therefore, for an 8 per formula unit (pfu) LLZO cell which has 56 Li atoms, the total number of positions available for Li atoms in tetragonal phase is 56 (tetrahedral 8a, octahedral 16f and 32g), but in cubic phase the total number of positions available for Li atoms is 120 (tetrahedral 24d, octahedral 96h). In other words, in the cubic phase Li has 64 empty positions available for migration, but in the tetragonal phase all the positions (the sites 8a, 16f and 32g) are filled—and thus there is no vacancy for movement of Li as shown in Figure 3a [39]. According to Awaka et al., in cubic-phase LLZO the tetrahedral sites are more likely to be occupied than octahedral sites as shown in Figure 3b, where more vacant octahedral sites are available to be occupied [38]. Similar hypothesis has been given by Bernstein et al. [40] as shown in Figure 4, according to Bernstein the 8a sites which are filled in the tetragonal phase are identified as 24d in cubic phase are almost filled; while 16f and 32g sites which are full in the tetragonal phase are identified as 96h in the cubic phase which has more empty positions available (because of Coulomb repulsion) to be filled [40].

Awaka and Bernstein suggested higher occupancy of tetrahedral sites than octahedral sites [39,40]; however, a simulated study done by Meier et al., using the first principles method with molecular metadynamics on CP2K, found the opposite scenario [41,42]. They took 120 randomly generated structures where 56 Li atom can fill 72 available sites (considering 48 octahedral sites because two neighboring octahedral sites cannot be occupied) obeying Coulomb repulsion; they found very interesting and convincing results which indicate that the cubic phase LLZO octahedral site has higher occupancy than the tetrahedral site. They have shown that the most stable structure is the structure that possesses the lowest potential energy, in which Li atoms occupy 11 out of 24 tetrahedral sites (about 46%) and 45 out of 48 octahedral sites (about 94%). Figure 5b indicates that the average occupancy of blue dots is about 60% whereas for red dots it is about 85%, which shows that tetrahedral sites have lower occupancy than octahedral sites [41].

Transition to Cubic Phase from Tetragonal Phase and Vice Versa

Among the two phases of LLZO, the tetragonal phase shows good stability at room temperature but exhibits a low Li ionic conductivity, typically on the order of 10⁻⁶ S/cm; whereas at higher temperatures the tetragonal phase transforms into cubic phase which shows a higher Li ionic conductivity on the order of 10⁻⁴ S/cm, about 100 times that of the tetragonal phase. The transition temperature typically lies between 450–1000 K [36,38,39,40,43,44,45,46,47,48].

The crystal structure shown in Figure 6 reported by Cao et al. reveals that both tetrahedral and cubic phase LLZO have the same structural framework except in Li distribution [49]. Bernstein et al. reported that tetragonal phase LLZO presents good stability due to its ordered geometry. During the phase transformation, as shown in Figure 4a, the tetrahedral 8a site transforms into the tetrahedral 16e site and ultimately converts into tetrahedral 24d site, and octahedral 16f and 32g sites transform into octahedral 96h site in cubic LLZO [40]. Geiger et al. suggested that the ionic conductivity of cubic LLZO increases because of the decrease in distance between neighboring Li pairs [43]; but Meier et al. found no difference in neighboring Li–Li pair distance in their simulated study shown in Figure 7 [41].

In 2015, Klenk and Lai reported the transformation of tetragonal phase into cubic phase using MD simulation. For tetragonal phase they used cage occupancy of four sites (8a, 16e, 16f and 32g) as a function of temperature, and mean squared displacement (MSD) as a function of time [46]. The results published by Klenk are in contrast to the observation revealed by Bernstein et al. [40], according to their observation with the increase in temperature the occupancy of site 8a decreases, and the occupancy of site 16e increases, but the change in octahedral sites 16f and 32g are very negligible, which indicates tetragonal sites 8a and 16e are exchanging Li atoms; but Bernstein et al. stated that there is no exchange of Li atom between 8a and 16e. However, both groups reported the simultaneous increase in MSD of Li atoms of tetrahedral 8a sites and octahedral 32g sites, and pointed out that the change of positions of Li atoms at site 8a and 32g are correlated [40,46].

Using X-ray diffraction (XRD) patterns, Adams and Rao revealed the gradual transformation of tetragonal LLZO into cubic LLZO [48]. According to their report, at low temperature the LLZO shows tetragonal phase characteristic with two Bragg peaks at 211 and 112; but as the temperature increases the 211 peak becomes more pronounced whereas the 112 peak vanishes gradually, after 450 K the 112 peak disappears completely, which clearly indicates the transformation of tetragonal into cubic phase LLZO. Geiger et al. also mentioned, within the range of 400 K to 600 K, the tetragonal phase transforms into octahedral phase [43]. Adams and Rao investigated the transformation of doped LLZO, and interestingly found that the transformation can happen even at room temperature [48].

Mori et al. experimentally found that by adding Ga impurity into LLZO, it is possible to obtain cubic phase Ga-LLZO and it can be reversed into tetragonal phase by doping another elemental cation like Sr [50]. When the doping level of Sr reaches to a maximum limit the cubic phase of GaSr-LLZO transforms into tetragonal phase as shown in Figure 8a [50]. The XRD patterns of the samples at different proportions of Sr content reveal that up to 0.2 pfu the structure remains cubic, but further increase in Sr content converts the material into tetragonal phase, which is shown in the upper three XRD patterns corresponding to the samples which contain 0.3, 0.4 and 0.5 pfu of Sr, respectively. Figure 8b suggests that, with increasing Sr content, the volume of sample also increases linearly up to 0.125 pfu, and then turns to a platform in the range from 0.125–0.2 pfu. With further increasing Sr content above a molar value of 0.2, the lattice parameter increases discontinuously due to a change in the crystal phase from cubic to tetragonal. A similar tendency was reported elsewhere [51,52,53].

3. Synthesis Techniques of LLZO (Li₇La₃Zr₂O₁₂)

Pure and doped LLZO can be prepared in many ways, such as the solid-state reaction, sol-gel method, Pechini method, RF magnetron sputtering method, and pulsed laser deposition method.

3.1. Solid-State Reaction Method

Solid-state reactions are a commonly used technique which involve ball milling, grinding, and sintering of the precursor material to get the final solid electrolyte material. In this process the preliminary grinding is generally performed using a ball mill or a planetary milling machine, the output of ground powder may be wet, which is then dried and sintered or annealed at high temperature in a furnace. This process of grinding and sintering needs to be repeated as the first sequence of grinding and sintering is employed to mix the precursor contents and the subsequent grinding along with sintering is repeated to produce a finer and denser particle. Finally, the precursor powder is pelletized using a hydraulic press and then sintered again at elevated temperature to obtain a more compact and non-porous solid electrolyte. This process is preferred because of its low cost, simplicity, and high throughput.

The first LLZO was prepared by Murugan et al. in 2007 using solid state reaction technique where they used sintering temperature of 1230 °C for 36 h [36]. After 2007 this became the most widely used method; however, some researchers reported some concerns regarding this process: (1) the method is difficult in achieving desired stoichiometric structure, (2) high sintering temperature introduces impure phases into the system, (3) the long period of high sintering processing causes too much Li loss from the system, and (4) at high temperature, other foreign elements are introduced from the crucible containing the sample [54,55].

3.2. Sol-Gel Method

To avoid the problems arising from high sintering temperatures during solid state reaction, which causes Li loss and creates impure phase formation in the system, the sol-gel method has been developed which involves hydrolysis and polymerization. The sol-gel method uses relatively low temperatures and yields uniformly distributed finer sized particles without needing intermittent grinding [56].

Li₅La₃Ta₂O₁₂ garnet was first prepared using the sol-gel method by Gao et al. [57] Kokal et al. and Shimonishi et al. reported the preparation of cubic LLZO using the sol-gel technique followed by sintering at the temperatures of 700 °C and 1180 °C, respectively [58,59]. However, Li garnets obtained through sol-gel shows a relatively low ionic conductivity [59,60]. In 2014, Janani et al. introduced a new sol-gel technique which uses additives during the sintering process, yielding Al-doped LLZO with an improved ionic conductivity of 6.1$\times$10⁻⁴ S/cm at 33 °C and relative density of 96% [61].

3.3. Pechini Method

The Pechini method is a modified technique of the sol-gel method which involves the ability to chelate metal ions with the help of carboxylic acids such as tartaric acid, citric acid, etc. and forms a polymeric resin due to polysensitization when heated with polyethylene glycol or another hydroxylic alcohol agent. This process requires lower sintering temperature, shorter reaction time and yields uniformly distributed nanosized particles. Using the Pechini method, Gao et al. synthesized Li₅La₃Bi₂O₁₂ nanocrystalline powder [62], and Jin et al. synthesized stable cubic Al-doped LLZO [63].

3.4. Radio Frequency (RF) Magnetron Sputtering

Radio frequency (RF) magnetron sputtering is a thin film processing technique in which a thin layer of LLZO layer is deposited through RF-powered sputtering. The main advantage of this established process is ease of control. In 2016, RF magnetron sputtering was used by Lobe et al. to prepare thin film cubic LLZO with a substrate temperature of 700 °C [64]. (Earlier in 2012, Kalita et al. reported the successful deposition of amorphous thin film LLZO using the RF sputtering technique [65].) However, it came with limited yield capability and meanwhile the resulted ionic conductivity is poor.

3.5. Pulsed Laser Deposition

Pulsed laser deposition (PLD) has also been used for the fabrication of LLZO. In PLD, a laser beam usually ablates the targets at a suitable angle which helps to preserve the stoichiometry and composition of the target. PLD is very flexible process as it allows the adjustment of conditions that are appropriate for deposition such as the energy of laser density, temperature, and pressure. Also, it reduces the Li loss that occurs during the synthesis process as it allows films to be grown at low temperature in a shorter period. However, a very few studies have been reported so far that used PLD for LLZO preparation. The effect of different substrates on the growth of LLZO at different deposition temperature were investigated by Park et al. [66]. Tan et al. used this method to prepare LLZO on sapphire and SrTiO₃ substrates [67].

Table 1. The sintering temperature, space group, Li ionic conductivity (σ) and total activation energy (E_a) of pure and doped LLZO prepared by various research group using different synthesis processes.

Chemical Composition	Year Reported	Synthesis Process	Space Group	Ionic Conductivity, σ (mS/cm)	Activation Energy, Ea (eV)	σtotal Measuring Temperature (°C)	Ref.
Li7La3ZrNb0.5Y0.5O12	2018	SSR	Ia$\overline{3}$d	0.830 (T)	0.31	30	[74]
Li6.4Fe0.2La3Zr2O12	2018	SSR	Ia$\overline{3}$d	1.100 (T)	-	RT	[75]
Li6.25Ga0.25La3Zr2O12	2017	SSR	Ia$\overline{3}$d	1.460 (T)	0.25	RT	[76]
Li7La3Zr2O12	2015	SSR	Ia$\overline{3}$d	0.174 (B)	0.26 (Bulk)	25	[77]
Li6.4La3Zr1.4Ta0.3Nb0.3O12	2015	SSR	Ia$\overline{3}$d	0.606 (B)	0.26 (Bulk)	25	[77]
Li6.4La3Zr1.4Ta0.4Nb0.2O12	2015	SSR	Ia$\overline{3}$d	0.455 (B)	0.28 (Bulk)	25	[77]
Li6.4La3Zr1.4Ta0.5Nb0.1O12	2015	SSR	Ia$\overline{3}$d	0.444 (B)	0.27 (Bulk)	25	[77]
Li6.4La3Zr1.4Ta0.6O12	2015	SSR	Ia$\overline{3}$d	0.724 (B)	0.24 (Bulk)	25	[77]
Li6.65La2.75Ba0.25Zr1.4Ta0.5Nb0.1O12	2015	SSR	Ia$\overline{3}$d	0.527 (B)	0.26 (Bulk)	25	[77]
0.5F-LLZO	2015	SSR	Ia$\overline{3}$d	0.450 (T)	-	RT	[78]
1.0F-LLZO	2015	SSR	Ia$\overline{3}$d	0.500 (T)	0.26	RT	[78]
1.5CF-LLZO	2015	SSR	Ia$\overline{3}$d	0.050 (T)	-	RT	[78]
2.0Al-LLZO	2015	SSR	Ia$\overline{3}$d	0.200 (T)	-	RT	[78]
Li7La3Zr2O12	2015	SSR	I41/acd	0.017 (T)	0.47	25	[79]
Li5.9La3Zr1.45W0.55O12	2015	SSR	Ia$\overline{3}$d	0.440 (T)	0.43	25	[79]
Li6.5La3Zr1.75W0.25O12	2015	SSR	Ia$\overline{3}$d	0.490 (T)	0.42	25	[79]
Li6.30La3Zr1.65W0.35O12	2015	SSR	Ia$\overline{3}$d	0.660 (T)	0.42	25	[79]
Li6.1La3Zr1.55W0.45O12	2015	SSR	Ia$\overline{3}$d	0.640 (T)	0.43		[79]
Li6La3Ta1.5Y0.50O12	2015	SSR	Ia$\overline{3}$d	0.126 (T)	0.37	23	[80]
Li6.5La3Ta1.25Y0.75O12	2015	SSR	Ia$\overline{3}$d	0.183 (T)	0.33	23	[80]
Li6.25La3Nb1.375Sc0.625O12	2015	SSR	Ia$\overline{3}$d	0.102 (T)	0.48	25	[81]
Li6.25La3Nb0.75Zr1.25O12	2015	SSR	Ia$\overline{3}$d	0.203 (T)	0.42	25	[81]
Li6.5La1.5Ba1.5Ta2O12	2015	SSR	Ia$\overline{3}$d	0.062 (T)	0.40	33	[37]
Li6.28Al0.24La3Zr2O12	2015	SSR	Ia$\overline{3}$d	0.443 (T)	0.37	30	[82]
Li6.2La3Zr1.2Ta0.8O12	2015	SSR	Ia$\overline{3}$d	2.3 $\times$ 10−3 (T)	0.53	33	[83]
Li5.5La3Zr1.25W0.75O12	2015	SSR	Ia$\overline{3}$d	0.053 (T)	0.43	25	[84]
Li7La3Zr2O12	2014	SSR	I41/acd	9.90 $\times$ 10−3 (T)	0.43	27	[85]
Li6.75La3Zr1.75Ta0.25O12	2014	SSR	Ia$\overline{3}$d	0.410 (T)	0.42	27	[85]
Li6.5La3Zr1.5Ta0.5O12	2014	SSR	Ia$\overline{3}$d	0.610 (T)	0.40	27	[85]
Li6La3ZrTaO12	2014	SSR	Ia$\overline{3}$d	0.210 (T)	0.42	27	[85]
Li6.925La3Zr1.925Sb0.075O12	2014	SSR	Ia$\overline{3}$d	0.340 (T)	0.37	30	[86]
Li7La3Zr2O12	2014	SSR	Ia$\overline{3}$d	0.120 (T)	0.41	30	[86]
Li7La3Nb2ScO12	2014	SSR	Ia$\overline{3}$d	0.068 (T)	0.40	25	[87]
Li6La3Nb1.5Sc0.5O12	2014	SSR	Ia$\overline{3}$d	0.102 (T)	0.40	25	[87]
Li6.25La3Nb1.375Sc0.625O12	2014	SSR	Ia$\overline{3}$d	0.138 (T)	0.36	25	[87]
Li6.5La3Nb1.25Sc0.75O12	2014	SSR	Ia$\overline{3}$d	0.126 (T)	0.39	25	[87]
Li5La3Nb2O12	2014	SSR	Ia$\overline{3}$d	0.022 (T)	0.45	25	[87]
Li5.5La3Nb1.75Sc0.25O12	2014	SSR	Ia$\overline{3}$d	0.052 (T)	0.40	25	[87]
Li6.5La3Ta1.25Y0.75O12	2014	SSR	Ia$\overline{3}$d	0.183 (T)	0.36	23	[88]
Li6.75La3Zr1.75Ta0.25O12 with flow of O2	2014	SSR	Ia$\overline{3}$d	0.740 (T)	0.33	25	[89]
Li6.75La3Zr1.75Ta0.25O12 with flow of Air	2014	SSR	Ia$\overline{3}$d	0.240 (T)	0.39	25	[89]
Li6.5La3Ta0.5Zr1.5O12 30 mol.% excess Li	2014	SSR	Ia$\overline{3}$d	0.433 (T)	-	RT	[54]
Li6La3Zr1.5Te0.5O12	2014	SSR	Ia$\overline{3}$d	2.19 $\times$ 10−3 (T)	0.37	RT	[90]
Li6Ba0.5Sr0.5La2Ta2O12	2014	SSR	Ia$\overline{3}$d	7.19$\times$ 10−3 (T)	0.45	18	[91]
Li7La3Zr2O12	2013	SSR	I41/acd	5.77 $\times$ 10−3 (T)	0.40	RT	[92]
Li6.6La2.6Ce0.4Zr2O12	2013	SSR	Ia$\overline{3}$d	0.0144 (T)	0.48	RT	[92]
Li6.4La2.4Ce0.6Zr2O12	2013	SSR	Ia$\overline{3}$d	0.0126 (T)	0.50	RT	[92]
Li7.06La3Zr1.94Y0.06O12	2013	SSR	Ia$\overline{3}$d	1.0$\times$ 10−3 (B)	0.47	23	[93]
Li7.16La3Zr1.84Y0.16O12	2013	SSR	Ia$\overline{3}$d	1.0 $\times$ 10−3 (B)	0.47	23	[93]
Li7Nd3Zr2O12	2013	SSR	I41/acd	-	0.66	-	[94]
Li7La3Zr2O12	2013	SSR	Ia$\overline{3}$d	0.500 (T)	0.31	24	[51]
Li6BaLa2Ta2O12	2013	SSR	Ia$\overline{3}$d	0.10 (T)	0.39	25	[95]
Li7La3Ta2O13	2013	SSR	Ia$\overline{3}$d	3.21 $\times$ 10−3 (B)	0.55	40	[96]
Li6.75La3Zr1.75Ta0.25O12	2013	SSR	Ia$\overline{3}$d	0.640 (T)	0.30	28	[97]
Li6.75La3Zr1.875Te0.125O12	2013	SSR	Ia$\overline{3}$d	0.330 (T)	0.41	30	[98]
Li6.6La3Zr1.6Sb0.4O12	2013	SSR	Ia$\overline{3}$d	0.770 (T)	0.34	30	[99]
Li6.4La3Zr1.6W0.3O12	2013	SSR	Ia$\overline{3}$d	0.789 (T)	0.45	30	[100]
0.28Al-Li7La3Zr2O12	2012	SSR	Ia$\overline{3}$d	0.350 (T)	0.36	25	[101]
Li6La3ZrTaO12	2012	SSR	Ia$\overline{3}$d	0.260 (T)	0.46	25	[101]
Li6.5La3Zr1.5Ta0.5O12	2012	SSR	Ia$\overline{3}$d	2.0$\times$ 10−3 (T)	0.49	25	[101]
Li6.625La3Zr1.625Ta0.375O12	2012	SSR	Ia$\overline{3}$d	0.520 (T)	0.41	25	[101]
Li7La3Zr2O12	2012	SSR	I41/acd	1.3 $\times$ 10−3 (B)	0.46	20	[102]
0.204Al-Li7La3Zr2O12	2012	SSR	Ia$\overline{3}$d	0.400 (T)	0.26	RT	[55]
Li7La3Zr2O12	2012	SSR	Ia$\overline{3}$d	0.160 (T)	-	RT	[103]
Ge-Li7La3Zr2O12	2012	SSR	Ia$\overline{3}$d	0.763 (T)	-	25	[104]
Li6.7La3Zr1.7Ta0.3O12	2012	SSR	Ia$\overline{3}$d	0.690 (T)	0.36	RT	[105]
Li6.4La3Zr1.4Ta0.6O12	2012	SSR	Ia$\overline{3}$d	1.0 (T)	0.35	25	[106]
Li6.5La2.5Ba0.5ZrTaO12	2012	SSR	Ia$\overline{3}$d	0.090 (T)	0.57	24	[107]
Li7La3Zr2O12	2011	SSR	I41/acd	2.0$\times$ 10−3 (T)	0.49	25	[47]
Li7La3Zr2O12	2011	SSR	Ia$\overline{3}$d	0.400 (T)	0.34	25	[47]
Li7.06La3Y0.06Zr1.94O12	2011	SSR	Ia$\overline{3}$d	0.810 (T)	0.26	25	[108]
Li6.75La3Zr1.75Nb0.25O12	2011	SSR	Ia$\overline{3}$d	0.80 (T)	0.31	25	[109]
Li6La3ZrTaO12	2011	SSR	Ia$\overline{3}$d	0.180 (T)	0.42	25	[110]
Li7La3Zr2O12	2010	SSR	Ia$\overline{3}$d	0.180 (B)	-	25	[111]
Li7La3Hf2O12	2010	SSR	Ia$\overline{3}$d	0.240 (B)	0.29	25	[112]
Li7La3Hf2O12	2010	SSR	I41/acd	9.85 $\times$ 10−4 (B)	0.59	27	[113]
Li7La3Ta2O12	2009	SSR	Ia$\overline{3}$d	2.2 $\times$ 10−3 (T)	0.38	27	[114]
Li7La3Zr2O12	2009	SSR	I41/acd	1.63 $\times$ 10−3 (T)	0.54	27	[38]
Li7BaLa2Ta2O12.5	2008	SSR	Ia$\overline{3}$d	0.097 (T)	0.45	50	[115]
Li5BaLa2Ta2O11.5	2008	SSR	Ia$\overline{3}$d	4.9$\times$ 10−3 (T)	0.51	50	[115]
Li5.5BaLa2Ta2O11.75	2008	SSR	Ia$\overline{3}$d	0.031 (T)	0.47	50	[115]
Li5.75BaLa2Ta2O11.875	2008	SSR	Ia$\overline{3}$d	0.089 (T)	0.44	50	[115]
Li7La3Zr2O12	2007	SSR	Ia$\overline{3}$d	0.774 (T)	0.30	25	[36]
Li7La3Zr2O12	2016	Sol-gel	Ia$\overline{3}$d	0.140 (T)	-	RT	[116]
Li6.4Ga0.2La3Zr2O12	2015	Sol-gel	Ia$\overline{3}$d	0.024 (T)	0.32	RT	[69]
Li6.16Al0.28La3Zr2O12	2014	Sol-gel	I41/acd	3.0 $\times$ 10−4 (T)	-	33	[61]
Li6.16Al0.28La3Zr2O12	2014	Sol-gel	Ia$\overline{3}$d	0.110 (T)	0.38	33	[61]
Li7La3Zr2O12	2013	Sol-gel	Ia$\overline{3}$d	0.400 (T)	0.41	RT	[117]
Li7La3Zr1.89Al0.15O12	2013	Sol-gel	Ia$\overline{3}$d	0.340 (T)	0.33	RT	[118]
Li6La3Zr2O11.5	2011	Sol-gel	Ia$\overline{3}$d	0.139 (T)	-	25	[59]
Li7La3Zr2O12	2011	Sol-gel	Ia$\overline{3}$d	-	-	-	[119]
Li5La3Ta2O12	2010	Sol-gel	Ia$\overline{3}$d	1.54$\times$ 10−3 (T)	0.57	25	[57]
Al-Li7La3Zr2O12	2011	Pechini	Ia$\overline{3}$d	0.200 (T)	-	25	[63]
Li7La3Zr2O12	2011	Sol-gel and Pechini	Ia$\overline{3}$d	-	-	-	[58]
Li7La3Zr2O12	2011	Sol-gel and Pechini	I41/acd	3.12 $\times$ 10−4 (T)	0.67	25	[58]
Li5La3Bi2O12	2010	Pechini	Ia$\overline{3}$d	0.024 (T)	0.40	26	[62]
Al-LLZO	2016	RFMS	Ia$\overline{3}$d	0.120 (T)	0.47	25	[64]
Li7La3Zr2O12	2012	RFMS	I41/acd	4.0 $\times$ 10−4 (T)	0.70	25	[65]
Li7La3Zr2O12	2013	PLD	Ia$\overline{3}$d	2.50 $\times$ 10−3 (T)	0.52	25	[68]
Li7La3Zr2O12	2013	PLD	Ia$\overline{3}$d	0.010 (T)	0.55	25	[68]
Laser annealed Li7La3Zr2O12	2012	PLD	I41/acd	7.36 $\times$ 10−4 (T)	0.32	RT	[67]
Deposited on As Li7La3Zr2O12	2012	PLD	I41/acd	3.35 $\times$ 10−4 (T)	0.36	RT	[67]
Annealed at 800 °C Li7La3Zr2O12	2012	PLD	I41/acd	1.78 $\times$ 10−4 (T)	0.41	RT	[67]

SSR—Solid-state reaction, (T)—Total ionic conductivity (Bulk + Grain boundary), (B)-Bulk conductivity, RT—Room temperature, RFMS—Radio frequency magnetron sputtering, PLD—Pulsed laser deposition.

outlines the Li ionic conductivity and the activation energy reported by different research groups where they used different synthesis techniques. Besides the methods discussed above and listed in Table 1, there are also some other methods which have been adopted to prepare solid-state LLZO, such as epitaxial growth [68], combustion method [69,70], sol-gel spin coating [71,72] and nebulized spray pyrolysis [73].

4. Sintering Techniques of LLZO

Sintering is one of the most important steps during the fabrication of solid-state electrolytes as it plays a vital role in densifying the pellet of electrolyte. Dense pellets have many advantages such as: (1) decreasing grain boundary resistance and thus decreasing the activation energy to increase the Li ionic conductivity, (2) suppressing the formation of dendrites and accordingly increasing the safe operation of the battery, and (3) increasing the mechanical strength of the battery. Being not dense enough or possessing high porosity in the solid electrolyte increases the chance of fragility, and hence may cause mechanical failure of the device. There are some techniques like furnace sintering [36], hot pressing [55], field assisted sintering technology (FAST) [120] and spark plasma sintering (SPS) [121] reported in the literature that can be used to improve the density of pellets.

4.1. Furnace Sintering

Furnace sintering is mostly used in conventional solid-state reaction methods, where a crucible loaded with the powder of mixed raw materials is heated at elevated temperatures in vacuum or in the presence of flowing oxygen, air, other gas, or any other agent for a certain period. Different combinations of approach in which optimal parameters are used to make fine and distributed particles that yield a denser pellet with a high Li ionic conductivity have been reported [36,58,59,117].

Table 2. Temperature and time duration used in high temperature sintering.

Chemical Composition	Sintering Temperature (°C)	Sintering Duration (Hours)	Total Li Ionic Conductivity (mS/cm) at (Temperature)	Reference
Li7La3ZrNb0.5Y0.5O12	1230	15	0.830 (30 °C)	[74]
Li7La3Zr2O12	1230	36	0.30 (RT)	[36]
Li6.75La3Zr1.75Ta0.25O12 (0 wt.% Li2O)	1230	6	0.220 (28 °C)	[97]
Li7La3Zr2O12 (1.7 wt.% Al + 0.1 wt.% Si)	1230	36	0.680 (25 °C)	[122]
Li7La3Zr2O12 (1.3 wt.% Al)	1230	36	0.240 (25 °C)	[122]
Li6.75La3Zr1.75Ta0.25O12 (4 wt.% Li2O)	1200	6	0.440 (28 °C)	[97]
Al-Li7La3Zr2O12	1200	36	0.014 (RT)	[123]
Al-Li7La3Zr2O12	1200	6	0.020 (RT)	[63]
Li6.75La3Zr1.75Nb0.25O12	1200	36	0.80 (RT)	[109]
Li6La3Zr2O11.5	1180	32	0.0140 (RT)	[59]
Li6.75La3Zr1.75Ta0.25O12 (1 wt.% Li3PO4)	1175	6	0.720 (25 °C)	[124]
Li6.75La3Zr1.75Ta0.25O12 (6 wt.% Li2O)	1170	6	0.640 (28 °C)	[97]
Li5.9Al0.2La3Zr1.75W0.25O12	1150	12	0.490 (RT)	[84]
Li7La3Zr1.89Al0.15O12	1150	1	0.0340 (RT)	[118]
Li6.75La3Zr1.75Ta0.25O12 (in Oxygen)	1140	9	0.740 (25 °C)	[89]
Li6.75La3Zr1.75Ta0.25O12 (in Air)	1140	9	0.240 (25 °C)	[89]
Li6.75La3Zr1.75Ta0.25O12 (in Nitrogen)	1140	9	0.210 (25 °C)	[89]
Li6.75La3Zr1.75Ta0.25O12 (in Argon)	1140	9	0.180 (25 °C)	[89]
Li6.4La3Zr1.75Ta0.6O12	1140	16	1.00 (RT)	[106]
Li7La3Zr2O12	1100	5	0.0140 (RT)	[116]
Li6.42Al0.32La3Zr1.91O12.02	1000	7	0.0015 (RT)	[125]
Li7La3Zr2O12	1000	4	0.040 (RT)	[117]
Al-Li7La3Zr2O12	900	5	0.0019 (RT)	[126]
Li7La3Zr2O12	900	5	0.0003 (RT)	[58]

summarizes the sintering temperatures and times used in synthesizing differently doped LLZO and the resultant Li ionic conductivity.

In 2013, Li et al. measured the relative density and the ionic conductivity of Ta-doped LLZO after 9 h of sintering at 1140 °C using different atmospheres including oxygen, air, nitrogen and argon [89]. They found that the relative densities of the pellets prepared under these atmospheres were about 96%, 93%, 91% and 90%, respectively. Figure 9 shows the SEM images of samples sintered at different atmospheres. It can be clearly seen that the use of oxygen as an environment yields the densest pellets with a big grain size and small grain boundary (Figure 9a), whereas the use of argon results in the least dense pellets with a relatively small grain size and large porosity (Figure 9d) [89].

4.2. Hot Pressing

Hot pressing involves heat and pressure, where samples are sintered at high temperatures under a certain pressure. Simultaneous application of high temperature and pressure used in this process helps to stabilize the structure and densifies the pellets better. This method is also known as hot isostatic pressing (HIP).

Table 3. Relative density and ionic conductivity reported by researchers used hot isostatic pressing (HIP).

Chemical Composition	Sintering Temperature (°C)	Sintering Duration (Hours)	Total Li Ionic Conductivity (mS/cm) at (Temperature)	Relative Density in %	Reference
Al2O3-doped Li7La3Zr2O12	1180	127	0.990 (25 °C)	99.1	[127]
Li6.55Ga0.15La3Zr2O12	1160	120	1.130 (25 °C)	97.5	[128]
Li6.6La2.6Ce0.4Zr2O12	1050	40	0.014	96.0	[92]
Li6.24La3Zr2Al0.24O11.98	1000	-	-	97.0	[129]
Li6.25La3Zr2Ga0.25O12	1000	40	0.350 (RT)	91.0	[130]
Li6.24La3Zr2Al0.24O11.98	1000	40	0.400 (RT)	98.0	[55]

shows the sintering temperature used in hot pressing by different groups for different doped LLZO along with the final sintering time duration, total Li ionic conductivity and relative density. It is evident that the pellets prepared in this process yield a much higher ionic conductivity. However, to obtain dense pellets, this method requires a significant amount of time for sintering.

4.3. Field-Assisted Sintering Technology (FAST)

Field-assisted sintering technology (FAST) is a sophisticated modern technology used to sinter samples at a high heating rate. The process of FAST is similar to that of hot isostatic pressing (HIP), however, a dc current passes through the tool along with the uniaxial pressure to assist in sintering process. In other words, the additional heat is generated by an external electrical circuit. FAST has many advantages over other sintering methods in terms of the fast-sintering process that saves time and reduction in Li ion loss that usually occurs due to a long sintering duration. As the synthesis of raw materials and sintering are done together at one time, it avoids the process of tedious intermittent grinding and repeated long-time sintering.

Table 4. Relative density and ionic conductivity reported by researchers using the field-assisted sintering technology (FAST) method.

Chemical Composition	Sintering Temperature (°C)	Sintering Duration (Hours)	Total Li Ionic Conductivity (mS/cm) at (Temperature)	Relative Density in %	Reference
Al doped LLZO	1150	10	0.570 (RT)	99.8	[120]
Al doped LLZO	1000	10	0.332 (RT)	96.5	[131]
Li6.49La3Zr2Al0.17O12	950	50	0.330 (25 °C)	96.0	[132]
Al doped LLZO	900	10	0.034 (RT)	88.9	[131]

shows the sintering temperature used in FAST by different researcher groups for various doped LLZO along with the sintering time duration, total Li ionic conductivity and relative density. In comparison to furnace sintering method, FAST can yield much denser Al-doped LLZO pellets with a higher ionic conductivity at room temperature.

4.4. Spark Plasma Sintering (SPS)

Spark plasma sintering (SPS) is similar to FAST except the use of pulsed direct current (DC). SPS uses a high current to create spark plasma between the sample particles; then the sample is compressed uniaxially for densification. SPS also has the advantages of reducing Li loss, making fine and distributed nanosized particles, and reducing boundary resistance.

Table 5. Relative density and ionic conductivity of LLZO electrolytes synthesized with the spark plasma sintering (SPS) method.

Chemical Composition	Sintering Temperature (°C)	Sintering Duration (Hours)	Total Li Ionic Conductivity (mS/cm) at (Temperature)	Relative Density in %	Reference
Li7−xLa3Zr1.5Ta0.5O12	1100	50	1.35 (25 °C)	-	[133]
Li3xLa2/3-3xTiO3	1100	39	1.00 (22 °C)	100	[121]
Li3xLa2/3-3xTiO3	1050	40	5.8 × 10−3 (RT)	98.5	[134]

shows the sintering temperature used in SPS for Ta/Ti doped LLZO along with the sintering time duration and total Li ionic conductivity with relative density. The SPS method can generate denser pellets and provide relatively higher Li ionic conductivity compared to other methods. However, it is a low yield and time consuming method.

5. Li Ion Diffusion Mechanism

The tetragonal structure of LLZO has an ordered geometry in which all the sites, tetrahedral 8a, octahedral 16f and octahedral 32g, are fully occupied but the tetrahedral 16e sites are empty [38,40]. Tetragonal LLZO shows good stability at room temperature. On the other hand, the cubic structure possesses disordered geometry because of partially filled tetrahedral 24d and octahedral 96h sites. These partially occupied sites allow Li ions to migrate from one site to another empty site while maintaining the cubic structure even at high temperature [39].

As the Li arrangement in the tetragonal and cubic LLZO is responsible for Li migration, many studies have been performed to reveal the diffusion mechanism of Li. According to Awaka et al. as shown in Figure 3b for cubic LLZO, the tetrahedral 24d sites are almost full whereas the octahedral 96h sites have empty spaces available. As a consequence Li ions move to their neighboring empty positions and form a loop structure as shown in Figure 10 [39].

Zhang et al. also proposed the similar migration pattern of Li ions in cubic LLZO, the migration of Li ions from a octahedral site (LiO₆) to adjacent tetrahedral site (LiO₄) and then to another adjacent octahedral site (LiO₆) [135]. They also reported a bottleneck size of Li and O for Li ion migration, and suggested that the bottleneck size of Li-O can be adjusted by proper elemental doping which will allow the Li ions to migrate from the octahedral site to tetrahedral site due to reduced activation energy [135]. Wu et al. reported two possible migration pathways for Ga doped LLZO: (1) the Li ions jump from tetrahedral site 24d to tetrahedral site 24d; however, this pathway is very unlikely because the jump is limited by the migration pathway from tetrahedral site 24d to octahedral site 96h then to tetrahedral site 24d (24d → 96h → 24d), and (2) Li ions jump from octahedral site 96h to octahedral site 96h (96h → 96h); jumping through this pathways occurs at a much higher rate [136].

Meier et al. performed a first principles-based simulation that refuted the assumption made by Awaka and Bernstein on Li site occupancy for cubic LLZO as shown in Figure 5 [41]. Through metadynamics simulation, Meier et al. found that in a tetragonal structure the Li ions are in an ordered arrangement and as a result their motion mechanism are synchronized to each other, i.e., the migration of Li ions are collective in nature; a group of Li ions make the movement at the same time to change their positions as shown in Figure 11a, where activation energy needed for migration is 0.44 eV [41]. But in cubic LLZO the migration of Li ions is not synchronous in nature, and can be treated as a single-ion jump. The arrangements of Li ions in cubic LLZO are not in ordered fashion due to site vacancies, thus triggering a single-ion jump occurs at lower energies, around 0.23 eV, as shown in Figure 11b [41]. According to Meier et al., in cubic LLZO the migration of Li from face sharing tetrahedral site to octahedral site or vice versa (24d → 96h/96h → 24d) can occur concomitantly at lower potential in comparison to edge sharing octahedral sites (96h → 96h) because of the smaller distance.

6. Doping and Li Ionic Conductivity

After the first report of LLZO by Murugan et al., researchers have tried to find ways to stabilize the cubic LLZO and increase the ionic conductivity of LLZO operating at room temperature [36]. Researchers state similar positions regarding the effects of elemental doping, that it not only stabilizes the cubic phase of LLZO but also improves the ionic conductivity. The cell size of cubic LLZO can shrink or enlarge due to added dopants. This means dopants are responsible for the increased migration of Li ions from one site to another causing the higher ionic conductivity. Also, the concentration of the Li can be adjusted by elemental doping which produces a more disordered lattice arrangement that helps to stabilize the structure at room temperature [137].

In LLZO, 3 sites of Li⁺, La³⁺ and Zr⁴⁺ can be doped using three types of dopant: subvalent, isovalent and supervalent dopants. Many investigations have been performed to find suitable dopant(s) for the suitable site(s) which can produce stable cubic LLZO with a high ionic conductivity [47,55,123,138,139]. Studies show that Al and Ga can give the best results when they are substituted on Li site [47,140,141,142]. Researchers have made efforts in revealing the effects of changing Li ion concentration on the stability and ionic conductivity by adjusting the number of lithium ions in per formula unit (pfu) [36,59,99,100,104,108,143,144,145]. The highest Li ionic conductivity was found for Li in the range of 6–7 pfu using supervalent dopants [37,88,137,145].Using low energy potential, an impressive first principles-based study was performed by Miara et al.; they summarized the stability of LLZO when a specific site of the LLZO is doped with different elements in the periodic table. The results are shown in Figure 12 [146], and the total ionic conductivity and activation energy for different cation doping at Li, La and Zr sites are summarized in

Table 6. Ionic conductivity and activation energy reported by different groups for single and co-doping with different cation dopants.

Chemical Composition	Total Ionic Conductivity, σtotal (mS/cm)	Activation Energy, Ea (eV)	Reference
Li site substitution
Li6.28Al0.24La3Zr2O12	0.44 (30 °C)	0.37	[82]
Li6.22Al0.26La3Zr2O12	0.01 (25 °C)	0.40	[84]
Li6.25Ga0.25La3Zr2O12	1.46 (RT)	0.25	[76]
Li6.4Fe0.2La3Zr2O12	1.10 (RT)	-	[75]
Li6.4Al0.05Ga0.15La3Zr2O12	0.88	0.26	[162]
La site substitution
Li6.6La2.6Ce0.4Zr2O12	0.01 (RT)	0.48	[92]
Li7Nd3Zr2O12	-	0.66	[94]
Sr doped LLZO	0.50 (25 °C)	0.31	[51]
Zr site substitution
Li6La3ZrTaO12	0.18 (25 °C)	0.42	[110]
Li6.75La3Zr1.75Nb0.25O12	0.80 (25 °C)	0.31	[109]
Li6.5La3Nb1.25Y0.75O12	0.27 (25 °C) (Bulk)	0.36	[163]
Li6.75La3Zr1.875Te0.125O12	0.33 (30 °C)	0.41	[98]
Li6.55La3Hf1.55Ta0.45O12	0.35 (22 °C)	0.44	[145]
Li6.6La3Zr1.6Sb0.4O12	0.77 (30 °C)	0.34	[99]
Li6.7La3Zr1.7Ta0.3O12	0.69 (RT)	0.36	[105]
Li6.4La3Zr1.6W0.3O12	0.79 (30 °C)	0.45	[100]
Li6.5La3Ta1.25Y0.75O12	0.18 (23 °C)	0.36	[88]
Li6.5La3Zr1.75W0.25O12	0.49 (25 °C)	0.42	[79]
Co-doping: substitution to two or more sites
Li6.6La2.5Y0.5Zr1.6Ta0.4O12	0.23 (27 °C)	0.39	[164]
Li6BaLa2Ta2O12	0.09 (50 °C)	0.42	[115]
Li6Ba0.5Sr0.5La2Ta2O12	7.1 $\times$ 10−3 (18 °C)	0.45	[91]
Li6Sr0.5Ca0.5La2Ta2O12	3.2 $\times$ 10−3 (18 °C)	0.50	[91]
Li6.5La2.5Ba0.5ZrTaO12	0.09 (24 °C)	0.57	[107]

.

Bernstein et al. used supervalent Al³⁺ and Ga³⁺ cations into dope Li sites, and found that there is a critical concentration, x = 0.2 pfu for Li_7−2xM_xLa₃Zr₂O₁₂ (here M = Al and Ga), after this concentration a stable cubic structure is possible [40]. Rangasamy et al. reported an experiment where they used Ce⁴⁺ as a dopant into La sites and found a stabilized structure with the highest ionic conductivity of 1.4 $\times$ 10⁻⁵ S/cm for 0.2 pfu of Ce; but the ionic conductivity starts to decrease when Ce content exceeded 0.2 pfu [92]. Doping of the Zr site still is a source of interest as Zr is a common element of Li garnets [16,147,148], so it is possible to explore new information using Zr substitutes; a lot of research has been carried out by doping Zr sites with different cation dopants such as Ta [43,83,149,150,151,152,153,154,155,156], Y [108], Mo [157], Sb [86,99], Nb [158,159], W [79] and Ge [160]. Besides the single site doping, extensive research is also in progress on co-doping of LLZO, in which two or three sites are doped simultaneously to produce a stable structure, however poor ionic conductivity at room temperature is a major concern for co-doping [91,161].

Elemental doping significantly helps to stabilize the structure and transform the phase from tetragonal to cubic. Mori et al. prepared pellets of Li_6.25+xGa_0.25La_3-xSr_xZr₂O₁₂ that contain different pfu values of Sr content as shown in Figure 13, and found that the pellets at x = 0.1 pfu exhibit good stability and an ionic conductivity as high as 1.3 $\times$ 10⁻³ S/cm with activation energy of 0.25 eV [50]. From the observation of alternating current (AC) impedance spectroscopy as shown in Figure 13a, they found that in the x = 0.0–0.15 pfu range cubic phase GaSr-LLZO with higher ionic conductivity was formed because of the low impedances at the grain-boundary and electrode interface; but with further increasing Sr content, the intercept of the semicircle also increases as shown in Figure 13b. Figure 13c shows the ionic conductivity observed for various Sr composition of x = 0.0–0.5 [50]. Similar results were reported by other groups [52,76,165,166,167]. It was else reported that the thickness of the pellet might affect the value of conductivity, causing the ionic conductivity to vary for the samples which are even the same in composition and crystal structure [36,168].

Recently, Wu et al. performed a comparative study of the change in ionic conductivity and phase transformation of Ga doped Li_7-3xGa_xLa₃Zr₂O₁₂ [76]. Based on the XRD patterns and Raman spectra shown in Figure 14, they reported that, for the Ga content, when x < 0.2 pfu the samples exhibited tetragonal phase structure with lower conductivity and higher activation energy; but for x $\ge$ 0.2 pfu the samples presented cubic nature with higher ionic conductivity and lower barrier potential [76]. They also reported that the La site can be doped by a Rb cation, yielding cubic LLZO with a high ionic conductivity of 1.62 mS/cm at room temperature; it was attributed to large ionic radius of Rb compared to La, thus facilitating more free space for the migration pathways of Li [138].

Generally, in the sintering process, alumina and platinum crucibles are used which affect the final phase of the sample [138]. Geiger et al. prepared LLZO samples at high temperature using an alumina crucible and found stabilized high conductive cubic phase. This was because at high temperature some the Al from the crucible substituted Li and increased the Li vacancy which helps to stabilize the geometry and increases the mobility of Li ions responsible for conductivity. However, when they used a platinum crucible in sintering, they found that the samples were tetragonal phase [43]. Kumazaki et al. reported that Al from a crucible and Si from the pulverized bowl may reduce the grain-boundary resistance and thus increase the Li ionic conductivity to 6.8 $\times$ 10⁻⁴ S/cm [122]. However, Xia et al. stated that Al-doped LLZO pellets sintered using a platinum crucible gave better stability, higher Li ionic conductivity and lower grain-boundary resistance because of an increased relative density in contrast to the pellets sintered using a alumina crucible [169]. Liu et al. mentioned that the use of an alumina crucible acted to introduce impure phases in the sample [54]. In 2009, Awaka et al. successfully prepared transparent tetragonal phase single crystal LLZO by using a gold crucible [38].

7. Conclusions

Cubic-phase LLZO has proved to be a preferred solid electrolyte because of its high ionic conductivity, low electronic conductivity, wide electrochemical voltage window, good chemical stability with metal Li electrode, and high yield productivity. Intensive research on LLZO is going on to address challenging issues and make the material a practically feasible solid electrolyte for applications in renewable energy storage, electric vehicles, laptops, phones, battery operated electronic devices, etc.

The widely accepted understandings about LLZO solid electrolyte include: (1) LLZO has two structural phases: tetragonal structure (I4₁/acd) and cubic structure (Ia$\overline{3}$d); the former is stable at room temperature but exhibits relatively low ionic conductivity, while the latter is stable at high temperatures and shows significantly higher ionic conductivity than the tetragonal phase; (2) in the tetragonal phase, Li ions fully occupy tetrahedral 8a sites and octahedral 16f + 32g sites; but in cubic phase Li ions partially occupy tetrahedral 24d sites and octahedral 96h sites and, at higher temperatures, a transformed cubic phase can be achieved from the tetragonal phase; (3) using a proper sintering technique, it is possible to make denser pellets with a higher conductivity at room temperature due to reduction of grain-boundary resistance; (4) Li ions migrate from the original site to the neighboring site (24d to 96h to 24d) and jump from 96h to 96h; and (5) proper doping can stabilize the cubic structure and increase the ionic conductivity. However, there are some debates regarding the crystal structure and the fabrication of LLZO, involving: (1) the occupancy of Li atoms in tetrahedral 24d and octahedral 96h sites of cubic LLZO; (2) the Li–Li distances in tetragonal and cubic LLZO; (3) the influence of crucibles during high-temperature sintering; and (4) the methods and approaches used to prepare cubic LLZO in the cases of different types of dopant.

Overall, the major challenges in the LLZO research are: (1) to solve chemical instability problem arising from the electrolyte–electrode interface reaction; and (2) find proper composition with elemental cation doping that can result in a higher Li ionic conductivity and can maintain a wide electrochemical voltage window and good chemical stability.