Effects of alkaline earth metal elements and their synergistic roles with Ta for Li7La3Zr2O12

Effects of alkaline earth metal elements and their synergistic roles with Ta for the modified Li7La3Zr2O12 (LLZO) are discussed. Li7.1La3Zr1.95M0.05O12 (M = Mg, Ca, Sr, Ba) with the substitution of alkaline earth metal ions for Zr4+ and Li6.5La3Zr1.35Ta0.6M0.05O12 (M = Mg, Ca, Sr, Ba) with the co-substitution of alkaline earth metal ions and Ta5+ for Zr4+ are prepared. The sole substitution of alkaline earth metal elements for Zr in LLZO have little effects on improving ionic conductivity, while the modified LLZO with synergistically co-doping Ta and alkaline earth metal elements can achieve the great enhancement of ionic conductivity. The order of ionic conductivity influenced by Ta5+ and alkaline earth metal ions (Mg2+, Ca2+, Sr2+, Ba2+) co-substitution for Zr4+ demonstrates a strong correlation with ionic radii of Mg2+/Ca2+/Sr2+/Ba2+. Particularly, the enhanced Li6.5La3Zr1.35Ta0.6Mg0.05O12 with the joint substitution of Mg and Ta delivers a highest ionic conductivity of 3.45 × 10−4 S cm−1 at room temperature.


Introduction
Energy plays an important role in many aspects of human lives [1]. To meet the increasing energy demand, renewable energy resources such as wind, solar, biomass, hydropower and hydrogen are always the research hotspot [2]. In recent years, to design suitable electrical energy storage devices has become a prerequisite to realize the potential of these renewable energies. A few famous energy storage devices like fuel cells [3][4][5], solar cells [6][7][8][9], supercapacitors [10,11], rechargeable batteries [12][13][14], etc have been recognized as the most promising candidates for the purpose of alleviating energy crisis and reducing greenhouse gas emissions. Among alternative routes, although lithium ion batteries have world-widely emerged as energy storage devices for electronic products, electric vehicles, energy storage systems and special purpose devices, these conventional lithium ion batteries using organic liquid electrolytes are still suffering from several issues of safety risks and low energy density. Consequently, it is an extremely task to develop new-type lithium ion batteries of good safety and high energy density in the field of rechargeable batteries.
Two stable phases are found in typical LLZO. Tetragonal LLZO (space group I4 1 /acd, no. 142) [26] with low ionic conductivity is stable at room temperature, whereas cubic LLZO (space group Ia3d, no. 230) [27] with high ionic conductivity cannot survive for long time below approximately 150°C [28,29]. So far, many works have been done to obtain cubic LLZO which can be stable at room temperature. Factors such as the synthesis method, the sintering process, the sintering container, as well as the element doping modification are deemed to have close relation to the properties of LLZO [13,[30][31][32][33][34][35][36][37]. It is confirmed that doping elements at Li-site, La-site and/or Zr-site is an effective strategy to assist generating and stabilizing LLZO with improved properties at room temperature [13,31,[38][39][40][41]. As a general knowledge, lithium ion concentration and the mobility have great effects on lithium ionic conductivity, as the conductivity can be expressed as the product of the number of charge carriers, the charge of these carriers and mobility (σ=n·e·μ, where 'n' is the number of carriers, 'e' is the charge and 'μ'is the mobility) [42][43][44]. High-valence cations are often doped in LLZO to introduce vacancies of lithium ions, thereby improving its mobility within the crystal lattice [44]. Ta 5+ is usually used as the substitution for Zr 4+ to achieve the enhancement of LLZO systems. Especially, when the content of Ta is 0.6 per formula unit (pfu) in LLZO, the sample can normally achieve high ionic conductivity [31,[44][45][46]. Additionally, low-valence elements can also be introduced in LLZO to lead to the increased lithium ion concentration and the promoted lithium ion transport [40,[47][48][49][50]. The alkaline earth metal elements as typical low-valence elements, they have also taken attentions. For instance, some works have discussed the roles of alkaline earth ions in LLZO conductors [47,48]. Lincoln J Miara et al [51] apply density functional theory (DFT) [52] to study the defect energies and site preference of all possible dopants in LLZO, indicating that alkaline earth ions differ on their stable cation site. In addition, some investigations have demonstrated that alkaline earth ions can achieve two substitution sites of La 3+ or Zr 4+ sites [47,48,[53][54][55]. Inferentially, it is very likely to constitute good lithium ion transfer pathways through co-doping the high-valence cation and the low-valence cation at the same Li/La/Zr sites [56], and co-doping the high-valence cation and the low-valence cation can balance the lithium ion concentration.
In this work, we try to synthesize Li 7 [1,46,57]. The influences of substituting alkaline earth metal elements on Zr site and their synergistic effect with Ta are comparatively discussed. Alkaline earth metal elements (Mg, Ca, Sr, Ba) and Ta co-doped LLZO can achieve cubic phase and the improved ionic conductivity, and ionic radii of these alkaline earth metal ions make a difference on the ionic conductivity. Too large ionic radius size of alkaline earth metal ions might have a negative impact on ionic conductivity for Li 6 with alkaline earth metal elements and Ta substituted at Zr sites, the preparation process is modified. The schematic diagram of the preparation process for the modified LLZO with alkaline earth metal elements (Mg, Ca, Sr, Ba) and Ta at Zr sites is shown in figure 1. All reagents are purchased from Sinopharm Chemical Reagent Co. Ltd Mg(OH) 2 /CaCO 3 /SrCO 3 /BaCO 3 for alkaline earth metal source and Zr(CH 3 COO) 4 for Zr source and Ta 2 O 5 as Ta source are weighed according to stoichiometric ratio and mixed in acetic acid to prepare precursors (I). Li 2 CO 3 for Li source (30 wt% Li 2 CO 3 excess is used to make up for the loss of Li during subsequent heat treatment.) and La 2 O 3 for La source are mixed to prepare precursors (II). The obtained precursors (I) as well as precursors (II) are transferred to an alumina crucible, and sequentially heated at 400°C for 2 h and then at 750°C for 8 h to obtain calcined powders (i) as well as calcined powders (ii), respectively. The powders (i) and powders (ii) are ground and mixed to obtain mixtures.

Characterization
The crystal structure of materials is characterized by x-ray diffraction measurement (XRD, Bruker D8 Advance). All the pellets are ground to powders before XRD testing. Rietveld refinements of the synthesized samples are accomplished by GSAS software. Electrochemical Impedance Spectroscopy (EIS) measurement is applied with a Solartron Impedance Analyzer (1260 & 1287, Ametek) from 5 MHz to 100 Hz with a 10 mV amplitude to measure the ionic conductivity. Before EIS testing, both parallel surfaces of pellets are sputtered with gold as the lithium ion blocking electrode.

Results and discussion
In order to ensure the substitution of the alkaline earth metal elements at Zr sites, the mixtures of precursors are separated into (I) and (II), respectively. The relevant calcined powders are (i) and (ii), respectively (figure 1). XRD patterns of calcined powders (i) for Li 7 (PDF#65-1025). That is to say, monoclinic ZrO 2 will grow, as sole Zr(CH 3 COO) 4 is heated at 400°C for 2 h. However, when the mixture of Zr(CH 3 COO) 4 with incorporation of M (M=Mg, Ca, Sr, Ba) element is calcined at 750°C for 8 h, the patterns of main components exhibit the similar structure to ZrO 2 with tetragonal phase (PDF#50-1089). It is demonstrated that two main factors result in ZrO 2 structure changes from monoclinic phase to tetragonal phase. For one thing, Zr 1.95 M 0.05 O 3.95 (M=Mg, Ca, Sr, Ba) calcined powders (i) (at 750°C) are obtained at higher temperature than that of pure ZrO 2 calcined powders (at 400°C). With the increase of temperature, the monoclinic phase will change into tetragonal phase naturally [58]. For another, alkaline earth oxides are the common stabilizers for tetragonal ZrO 2 , and a small amount of stabilizers can improve the stability performance of tetragonal ZrO 2 at low sintering temperature [59,60]. As a result, in our work, the introduction of Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ can stabilize tetragonal ZrO 2 and help to achieve the transformation from monoclinic to tetragonal at low sintering temperature.
Rietveld refinement results confirm that calcined powders with the incorporation of the alkaline earth metal element M (M=Mg, Ca, Sr, Ba) are mixture of solid solution based on tetragonal ZrO 2 and monoclinic ZrO 2 . The refinement plots for crystal structure analysis and refined structure parameters are given in figure 3 and table S1-S5 is available online at stacks.iop.org/MRX/7/125201/mmedia (supporting information), respectively. The results show that alkaline earth metal elements induce the formation of tetragonal ZrO 2 and they occupy the Zr site (8f) to form Zr 1 When Ta synergistically substitutes Zr with alkaline earth metal element M (M=Mg, Ca, Sr, Ba) for calcined powders (i), XRD patterns seem more complicated. The patterns exhibit intensive peaks referring to tetragonal ZrO 2 structure (PDF#50-1089) and Ta 2 O 5 , while faint peaks referring to monoclinic ZrO 2 (PDF#65-1025) are also detected. The peaks referring to Ta 2 O 5 are in well accord with XRD pattern of Ta 2 O 5 raw material ( figure 2(b)). It indicates that calcined powders (i) for Li 6 (M=Mg, Ca, Sr, Ba). The rest semi-circle is not observed because it is out of the frequency range of the analyzer. In an ideal (modified) Li 7 La 3 Zr 2 O 12 without any impurities, the ionic conductivity only depends on the bulk and the grain boundary impedance [22,40,64]. Hence, the equivalent circuit of (R b CPE b )(R gb CPE gb )(CPE el ) (figure 5(c)) associated with these two semi-circles and inclined tail line is applie for understanding electrochemistry behaviors [54,[63][64][65][66]. R b and R gb are referring to the bulk resistance and the grain boundary resistance, respectively, while CPE b and CPE gb are corresponding constant phase elements. The inclined tail line observed in low frequency is related to the blocking electrodes, which is represented by CPE el . The total resistance R t is determined by the value of R b +R gb [31,35]. Thus, as with most articles [13,67,68], the intercept of the linear tail on the horizontal axis represents the total resistance arising from the bulk and grain boundary contributions.
Total ionic conductivities of  (table 2). Interestingly, the order of total ionic conductivity (σ total ) influenced by four alkali metal ions substituted for Zr 4+ is Ca 2+ >Mg 2+ >Sr 2+ >Ba 2+ , whereas the order of both σ b and σ gb differ. Obviously, Mg-doped Li 7  substitutes Zr with alkaline earth metal elements (Mg, Ca, Sr, Ba) in LLZO, all the samples, namely Li 6.5 La 3 Zr 1.35 Ta 0.6 M 0.05 O 12 (M=Mg, Ca, Sr, Ba), achieve a great enhancement of the ionic conductivity. Further, the order of total ionic conductivity influenced by four alkaline earth metal ions (Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ ) and Ta 5+ co-substitution for Zr 4+ is Mg 2+ >Ca 2+ >Sr 2+ >Ba 2+ . Clearly, sorting by total ionic conductivity demonstrates strong correlation with ionic radii of these alkaline earth metal ions. More definitely, the sequences of ionic radii of metal ions [44,53,69] is (1) Mg 2+ <Ca 2+ <Sr 2+ <Ba 2+ and (2) Ta 5+ <Zr 4+ . In addition to substitution of Ta 5+ for Zr 4+ , alkaline earth metal ions with the smaller ionic radii are likely to bring the better gain effect on ionic conductivity. This finding agrees with the discussion from Yuki Kihira et al [53] in spite of the substitution of alkaline earth metal ions for La 3+ in their work. The samples of Li 6.5 La 3 Zr 1.35 Ta 0.6 Mg 0.05 O 12 and Li 6.5 La 3 Zr 1.35 Ta 0.6 Ca 0.05 O 12 deliver the higher ionic conductivities than that of Li 6.5 La 3 Zr 1.35 Ta 0.6 O 12 (1.95×10 −4 S cm −1 ) in the previous reported work [46]. Synergetic effects should be deemed to be responsible for the improvement and the two factors are proposed. First, Ta substitution for Zr can induce the formation of cubic phase which exhibits short Li-Li distance in the migration pathway and isotropic three-dimensional Li-diffusion network that allow for easy and fast Li diffusion [27,28]. Second, low valence alkaline earth ions substitution for Zr 4+ can balance the Li content, as high temperature sintering and Al element from crucible can decline Li concentration of LLZO system. However, Li 6  is supposed to be stable on Zr sites, while the substitution changes of Ca 2+ , Sr 2+ and Ba 2+ in LLZO prefer to occur at La sites [51,54]. For another, the ionic radii of both Mg 2+ and Ta 5+ are small, which might stand a good chance to structure a better lithium ion conductive framework, thereby enhancing lithium ion migrations. The

Additional information
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