Facile Synthesis of Al-Stabilized Lithium Garnets by Solution-Combustion Technique for All Solid-State Batteries

: Garnet-type solid electrolytes with cubic modification (c-LLZO, Li 7 La 3 Zr 2 O 12 ) are considered one of the most promising candidates for SSLBs with desirable properties such as high ionic conductivity (about 1 mS cm -1 ) at room temperature, wide electrochemical operational window, and good stability against reduction by Li metal. The synthesis and processing of garnets through conventional wet-chemical, solid-state reaction and nitrate-combustion approaches often requires one or more of the following processing conditions (energy intensive milling steps, multiple and long periods of calcination) to attain conductive cubic phase making synthesis time intensive. Herein, we report a facile fuel-assisted solution combustion method using carbohydrazide-nitrate mixtures to synthesize cubic-Li 6.28 Al 0.24 La 3 Zr 2 O 12 (Al-LLZO); compared to other nitrate-combustion approaches, utilizing a nitrogen containing fuel source (CH 6 N 4 O) offers drastic reduction in the synthesis duration at relatively low temperatures. Selection of the right fuel to oxidizer ratio and annealing conditions are found to be critical for attaining phase purity and particle growth size of LLZO powders. Cubic phase Al-LLZO with particle size up to ~ 200 nm was attained at temperatures as low as 800 ºC upon calcining the as-combusted powders for 4 h. The green pellets attained high relative densities of 90-92% and ionic conductivities up to 0.45 mS cm -1 at low sintering conditions of 1100 ºC for 6 h compared to longer sintering duration (~10-24 h) for LLZO prepared with common solid-state reaction or wet chemical methods using conventional pressure-less sintering methods. Sintered pellets exhibited low activation energy of 0.29 eV likely due to the low grain boundary resistance. Synthesizing sub-micron sized Al-LLZO powders through low-cost facile synthesis approaches are of great importance towards fabrication composite electrolytes and catholytes.


INTRODUCTION
Li-ion batteries (LiBs) with organic solvent-based electrolytes suffer from flammability and show poor electrochemical compatibility with high voltage cathode materials [1,2]. They also suffer from poor rate capabilities at lower temperatures and are un-safe to operate at elevated temperatures (beyond 40 ºC), while the next-generation of LiBs using Li-metal anodes and a solid electrolyte are anticipated to overcome drawbacks associated with liquid-based LiBs and can deliver up to two-fold increase in energy density [3]. A robust ceramic solid electrolyte with ionic conductivity above 1 mS cm -1 at room temperature with excellent chemical and mechanical properties is expected to inhibit dendritic growth causing short circuits, playing a key role in the development of solid-state Li batteries (SSLBs) [3,4]. Among numerous solid Li + ion conductors, the Li-stuffed garnet-type oxide (Li 7 La 3 Zr 2 O 12 (LLZO)) [5] has gained much attention as a potential solid electrolyte for SSLBs owing to its high ionic conductivity at room temperature and good electrochemical stability against metallic Li anode [6]. While LLZO exhibits two polymorphs i.e. cubic (c-LLZO, Ia d) and tetragonal (t-LLZO, I4 1 /acd) phases [7], the latter shows two orders 3 of magnitude lower ionic conductivity compared to the cubic phase. The c-LLZO is stabilized at room temperature by aliovalent doping of metal cations either at the Li + sites using Al 3+ [8], Ga 3+ [9] or the Zr 4+ sites with Ta 5+ [10] or Nb 5+ [11] to overcome formation of thermodynamically favorable t-LLZO. Among all the doped derivatives, Al-doped LLZO still remains an attractive candidate due to its good sinterability and cost competitiveness [12][13][14].
The LLZO is conventionally synthesized using solid-state reaction (SSR), which often requires time-intensive milling periods of precursors and high reaction temperatures (> 1200 ºC) to attain conductive cubic phase [15]. Moreover, high temperature synthesis route generally produces micron-sized particles with large aggregates, making it difficult for processing dense pellets and composite electrode-electrolyte assembly. Even though wet chemical routes like sol-gel [16,17] Pechini [18], co-precipitation [19], and spray pyrolysis [20,21] share the advantage of producing fine particles of LLZO at lower temperatures (< 900 ºC ), these synthesis methods require use of either expensive precursors, multiple drying /calcination steps to remove water and organic solvents etc. along with long durations of annealing to attain phase pure conductive c-LLZO making the process elaborated. For example, Sakomoto et al. [22] synthesized cubic Al-LLZO ~ 900 ºC using low temperature sol-gel synthesis route, however required long intermediate drying steps along with supercritical fluid extraction step to retain nano-scale features. In another study [23], Al-doped LLZO was synthesized using a nitrate-combustion method, but the desired cubic phase (without secondary phases) was attained only at elevated reaction temperatures (> 950 ºC).
Several studies report low-temperature synthesis of Al-doped LLZO below 850 °C [24][25][26][27][28][29], but the calcined powders lacked phase purity or as-synthesized powders required additional milling steps, or long duration sintering (>10 h) to attain dense and conductive pellets. Therefore, producing submicron LLZO powders through less process intensive and low processing temperatures are of importance to prepare dense pellets at low sintering conditions and allowing stable interface contact between LLZO/cathodes to build SSLBs [30][31][32][33]. In addition, nano sized LLZO powders are of great importance which acts as active fillers to improve Li + transport and mechanical properties of solid polymer electrolytes such as poly (ethylene oxide) or polyacrylonitrile (PAN) [34,35].
Herein, an alternative facile fuel assisted wet-chemical route (solution combustion synthesis -SCS) is adopted to synthesizes Al-LLZO having nominal composition Li 6.28 Al 0.24 La 3 Zr 2 O 12 [36]; eliminating intermediate drying steps of precursors and long duration annealing steps involved in wet-chemical and solid-state reaction method. SCS is an attractive method for synthesizing high purity nano-sized functional materials using inexpensive precursors and fuel, wherein the oxidizers and fuels undergo self-sustained and fast redox reactions under the influence of an external heat source [37,38]. Here, we achieve sub-micron sized cubic Al-LLZO powders at low reaction temperatures 800 ºC in 4 h with optimized processing conditions (i.e. fuel ratio, calcination duration). The Al-LLZO pellets attained relative density of ~ 92 % with Li + conductivities up to 0.45 mS cm -1 at room temperature through conventional sintering method.

Synthesis and Pelletization of Al-LLZO
Li 6.28 Al 0.24 La 3 Zr 2 O 12 (Al-LLZO) was synthesized as described in the flowchart given in Figure 1. 497-18-7) was dissolved in ultra-pure DI water and subsequently mixed/ stirred with the precursor solution at 80 ºC for about 10 mins until gelation occurred. The molar concertation of the fuel was determined based on the total oxidizing and reducing valences as described by Jain et al. [39] and the detailed calculations is provided in the supplementary information file. The dehydrated gel was introduced into a preheated muffle furnace at 500 ºC to trigger combustion and the self-sustained ignition led to voluminous fine foamy product. The as-burnt foamy powder was collected, lightly ground and calcined in MgO crucible (Tateho Ozark Technical Ceramics) between 600 and 900 ºC until cubic phase Al-LLZO was obtained. The Al-LLZO green pellets were consolidated via cold-pressing using a 9 mm diameter die set at 3-ton load for 5 min. The green pellets were kept covered with the sacrificial mother powder (to prevent loss of Li) in MgO crucible with lid covering during sintering. The pellets were held first at 950 °C for 1 h for removing any contamination layers such as LiOH or Li 2 CO 3 , followed by sintering between 1000 °C and 1100 °C for 6 h. The theoretical density of the Al-LLZO pellet was considered as 5.1 g cm -3 [29,53] and the relative density of sintered pellets was calculated based on the volume and weight after sintering. thermal analysis was carried out using a Labsys Evo in air up to 900 ºC with 5 ºC min -1 .

Inductively Coupled Plasma Optical Emission Spectroscopy
Compositional analysis of the Al-LLZO samples was performed using inductively coupled plasma optical emission spectroscopy (ICP-OES). An iCAP 6500 RAD (Thermo Fisher Scientific, USA) and Qtegra software provided by the manufacturer of the instrument was used for all measurements. Detailed information about the used instrumental settings is shown in Table S2.
For sample digestion, borax fusion was used: 50 mg sample (crushed and ground using an agate mortar to ensure homogeneity) were mixed with 0.8 g borax and heated to 1000 °C for 5 h. A HCl/HF/H 2 O mixture (10 m% HCl, 0.8 m% HF) at 70 °C was used to dissolve the solidified fusions. For signal quantification, conventional external calibration using aqueous standards prepared from certified single element ICP-standard solutions was used. Internal standardization (0.2 mg kg -1 Europium) was applied to correct instrumental drifts and variations in sample introduction.

Electrochemical Impedance Spectroscopy
The sintered LLZO pellets were polished using SiC paper (800, 1000 and 1200 grit sizes) to obtain mirror finish surface followed by gold sputtering (thickness 100 nm). Impedance measurements were carried out from 0 to 60 ºC with 20 ºC steps using a Biologic SP-200 Potentiostat/Galvanostat/FRA from 7 MHz to 10 mHz for estimating conductivity and activation energy. Impedance spectra analysis carried out using ZView software. Prior to each impedance measurements, the pellets were stabilized at the set temperature for ~30 min.

Effect of fuel to oxidizer ratio on morphology, phase purity and formation mechanism
Exothermic nature of the combustion reaction is governed by the type of fuel and fuel to oxidizer ratio (F/O), which in-turn defines the microstructure, crystallinity, and phase purity of combusted product [40]. In the present study, a water-soluble fuel i.e., carbohydrazide (CH 6 N 4 O) having low melting point: ~ 154 °C was utilized for synthesizing Al-substituted LLZO. Previously, carbohydrazide based fuels have been utilized to synthesize various high purity nanostructured ceramics and spinel aluminates [41,42]. In addition, hydrazine-based fuels act as a chelator forming bidentate ligands with metal cations to prevent selective precipitation on dehydration of water promoting homogenous combustion [43][44][45]. To understand formation mechanism of cubic Al-LLZO, F/O ratio was systematically varied from fuel-lean zone (F/O = 0.5) to fuel-rich zone (F/O = 4.0), followed by a calcination step of as-burnt powders at 900 ºC for a duration of 4 h (see Table S3 for nature of the combustion). The calcined powder samples were characterized by XRD and SEM for optimizing the molar concentration of the fuel to control phase purity and morphology of the Al-LLZO powders.
The XRD patterns (see   Table S4), which agrees with that by the solid-state reaction methods [57]. The  Table S4) which is very near to the targeted synthesis composition.
The TGA-DTA analysis of the as-burnt powder with F/O ratio 4.0 was carried out to evaluate the thermal decomposition process up to 900 ºC, as shown in Figure 4d. During the heating process from RT up to 50 ºC, a slight increase in mass was observed (region 1) under the TG curve, possibly due to uptake of moisture from the air. Followed by presence of a weak exothermic peak around 130 ºC with weight loss of ca. 6 % associated due loss of water molecule (as seen in the region 1 of Figure 4d). Furthermore, a sharp endothermic peak observed at 250 o (region 2) can C be assigned due to decomposition of residual nitrates and decomposition of the fuel liberating CO 2 [51]. The presence of a weak exothermic peak observed at 450 ºC is unknown, as La 2 Zr 2 O 7 crystallizes beyond 700 ºC [52]. A significant weight loss is observed in the region 3 between (500 and 650 ºC) with onset of endothermic peak at around 600 ºC suggesting growth of LLZO phase.
Above 650 ºC, there is no significant weight loss or endo/exothermic peaks observed indicating the formation of well-stabilized cubic Al-LLZO phase.

Effect of Sintering Temperatures on Microstructure and Ionic conductivity
The Al-LLZO powders calcined at 800 °C, 4 h were selected for fabricating green pellets followed by sintering between 900 and 1200 °C for 6 h to evaluate the microstructural and Li+ transport View Article Online densification of pellet led to poor Li + conductivity (as seen in the Nyquist plot, Figure S1). On sintering pellets at 1000 ºC for 6 h led to increase in relative density reaching up to ~ 86 % and improved Li + ionic conductivity to 0.2 mS cm -1 . However, fractured SEM images of the pellets showed presence of uniformly distributed pores as seen in Figure 5b. Interestingly, pellet sintered at 1100 °C did not exhibit Li 2 ZrO 3 minor phases (Figure 5d) in absence of excess Li source and as well no Al-rich secondary phases such as LaAlO 3 were observed leading to resistive grain-boundary transport [55]. A minor peak of La 2 Zr 2 O 7 detected at 2θ ~ 29° on the surface of the pellet but did not show any severe impact on the Li + ion conductivity.
The ICP-OES analysis confirmed a slight loss in the Li content of pellet sintered at 1100 °C (Li 6 [36]. EDS mapping carried out on the cross-section of the pellets confirms no presence of Al-rich phases at the grain boundaries ( Figure S1). Pellets sintered at 1200 ºC / 6 h, showed prominent non-conductive La 2 Zr 2 O 7 pyrochlore phase ( Figure S2) causing drop in Li-ion conductivity below < 0.1 mS cm -1 , but had superior theoretical density up to 95% as La 2 Zr 2 O 7 (6.04 g cm -3 ) phase has high density compared to cubic Al-LLZO phase (5.10 g cm -3 ). The complex plane impedance patterns of the Al-LLZO pellets sintered under optimal condition (1100 °C, 6 h) recorded from 0 to 60 °C are shown in Figure 6a. At lower temperatures (0 and 20 °C), the impedance spectra show a high-frequency arc representing a resistor connected in parallel to a constant phase element (R || CPE), followed by a strong increase of the imaginary part of the complex impedance towards lower frequencies which is attributed to the ideal ionically blocking electrodes represented by R e || CPE e connected in series. In general, the total ionic conductivity in a ceramic conductor is a function of both bulk and grain boundary contributions represented by a distinctive high and mid frequency arcs [56]. Based on the capacitance value, C i = (R 1-n CPE i ) 1/n , where n is a fitting parameter describing deviation from the ideal that is Debye response (n=1), the high frequency arc is assigned to grain boundary processes indicating that the long-range Li + transport is dominated by grain boundaries (see Table 1). As seen in the Figure 6a, the arcs shift towards higher frequencies with increasing temperature according to ω=1/RC, where ω is the resonance frequency, due to decreasing resistance. The temperature dependence total Li + conductivity (σ total = σ gb ) of Al-LLZO pellet sintered at 1100 °C recorded between 0-60 °C is shown using Arrhenius plot in Figure S4. The activation energy E a for the linear behavior was calculated using σ total T = σ 0 exp(−E a /(k B T)), where k B is Boltzmann's constant and σ 0 the pre-factor. The activation energy of the Al-LLZO pellet prepared by solutioncombustion method was found 0.29 eV, which is lower compared to that by solid-state reaction method and comparable to samples prepared by wet-chemical routes (refer Table 2). Low values of the activation energy (calculated in the given temperature range) could be attributed to superior grain boundary contacts and excellent sinterability of Al-LLZO powders prepared by solution combustion route [17,21]. is also faster without any milling or long sintering durations (>10 h) (see Table 2 for comparisons).

CONCLUSIONS
In summary, submicron sized cubic Li 6.28 Al 0.24 La 3 Zr 2 O 12 (Al-LLZO) was synthesized by solutioncombustion method using carbohydrazide-nitrate mixtures. The presented synthesis route eliminates long ageing/ drying steps, multiple calcination steps usually involved with the wetchemical route, produces cubic Al-LLZO powders through a single low-temperature calcination step (800 °C, 4 h). The synthesis parameters such as fuel to oxidizer ratio, and calcination temperatures were studied and optimized to produce single-phase cubic Al-LLZO powders without any intermediated phases. The pellets sintered at 1100 ºC for 6 h with 92 % relative density exhibited Li + conductivity up to 0.45 mS cm -1 at room temperature. The Al-LLZO pellets sintered at 1100 °C showed activation energies of 0.29 eV in the temperature range between (0-60 °C), which were lower compared to those prepared by the solid-state reaction method (0.37 eV) or similar to the values attained by wet-chemical routes. Evidently, the carbohydrazide-nitrate based solution-combustion can be a potential viable method for commercially producing Al-LLZO powders at low temperatures, utilizing inexpensive precursors and less time intensive processing conditions. In addition, access to submicron sized powders using a facile route would provide opportunities for preparing composite high voltage cathodes through co-sintering at lower temperatures and could be used as a low-cost nanoscale LLZO filler for polymer-ceramic composite membranes.

Supporting information
Experimental procedures to synthesize Al-LLZO, theoretical calculations to arrive at fuel to oxidizer ratios, nature of combustion reaction and phase purity of Al-LLZO with varying fuel to oxidizer ratios, phase summary and lattice parameters of Al-LLZO powders at various temperatures, ICP-OES analysis of calcined and sintered pellets, Impedance pattern of pellet sintered at 900 C, EDS mapping of pellet sintered at 1100 C, XRD pattern of pellet sintered at 1200 C and Activation energy plot of pellet sintered at 1100 C / 6 h .

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