A Tale of Two Sites: On Defining the Carrier Concentration in Garnet‐Based Ionic Conductors for Advanced Li Batteries

Abstract

energy storage. While the primary strategy for improving performance has focused on electrode materials, the development of new solid state ionic conducting electrolytes has been overlooked as a potential means to revolutionize electrochemical energy storage. Examples of some of the technologies ( Figure 1 ) could include: (i) dual electrolyte Li-S batteries, (ii) solidstate batteries employing Li metal anodes, and (iii) all oxide, solid-state Li-ion batteries. Indeed, the need for ionic conducting solid-state electrolytes is clear, but relatively few have been identifi ed as promising candidates. The garnet mineral structure represents a family of complex oxides spanning a broad range of natural and synthetic compositions.Initial work identifi ed Li 5 La 3 M 2 O 12 (M = Nb 5+ , Ta 5+ ) as a promising formulation having a total conductivity of 0.04 mS cm −1 at room temperature. [5][6][7] Recently, Weppner and co-workers discovered a formulation exhibiting up to 0.4 mS cm −1 at room temperature: Li 7 La 3 Zr 2 O 12 (also known as LLZO). [ 8 ] For comparison, conventional battery separators soaked in liquid electrolyte have a total conductivity of 0.4 mS cm −1 at room temperature. [ 9 ] In LLZO, edge-sharing ZrO 6 octahedra and LaO 8 dodecahedra form an isotropic skeleton framework through which Li-ions and Li-ion vacancies form a percolative network of tetrahedral and distorted octahedral sites (Figure 1 ). In addition to high conductivity, LLZO has been reported to have the unprecedented combination of stability against metallic Li, and stability in dry air. [ 8,[10][11][12] LLZO is also referred to as a "stuffed" garnet because of its relatively high Li concentration (7 moles) compared to Li 3 La 3 M 2 O 12 . In the garnet family of materials, as the Li content is increased from Li = 3 mols toward Li = 7 mols, the ionic conductivity increases by several orders of magnitude. [ 13 ] At Li contents near Li = 6.5 mols, the cubic structure undergoes a reduction of symmetry to a tetragonal polymorph and the ionic conductivity again decreases. [ 14,15 ] A maximum in the ionic conductivity at room temperature has been observed for compositions near the critical Li concentration to stabilize the cubic phase. [16][17][18][19] As such, the majority of the work in the literature has focused on understanding how the cubic phase is stabilized at high Li concentrations. [14][15][16][17][18][19][20][21] However, the understanding of the fundamental parameters that control the Li conductivity has Solid electrolytes based on the garnet crystal structure have recently been identifi ed as a promising material to enable advance Li battery cell chemistries because of the unprecedented combination of high ionic conductivity and electrochemical stability against metallic Li. To better understand the mechanisms that give rise to high conductivity, the goal of this work is to correlate Li site occupancy with Li-ion transport. Toward this goal, the Li site occupancy is studied in cubic garnet as a function of Li concentration over the com...

Development of Lithium-Stuffed Garnet-Type Oxide Solid Electrolytes with High Ionic Conductivity for Application to All-Solid-State Batteries

“…The Ea of each sample are estimated from the slope of σT data plotted in Figure 8 and summarized in Table 2, together with σ at 27°C. The sample without Ba substitution has the highest Ea = 0.42 eV, which is higher than an Al-doped LLZT sample with a similar composition (Li et al, 2012b) but nearly the same as the Al-free sample prepared by hot pressing (Thompson et al, 2015). The Ea of LLBZT is in the range of 0.34-0.40 eV and tends to decrease with increasing Ba substitution levels as shown in Figure 7C.…”

“…Partial substitution of the Zr 4+ site in LLZ by other higher valence cations, such as Nb 5+ (Ohta et al, 2011;Kihira et al, 2013), Ta 5+ (Buschmann et al, 2012;Li et al, 2012b;Logéat et al, 2012;Wang and Wei, 2012;Inada et al, 2014a;Thompson et al, 2014Thompson et al, , 2015 Ren et al, 2015a,b), W 6+ (Dhivya et al, 2013;Li et al, 2015), and Mo 6+ (Bottke et al, 2015) is also reported to be effective in stabilizing the cubic garnet phase, and their conductivity at room temperature is greatly enhanced up to ~1 × 10 −3 S cm −1 by controlling the contents of dopants and optimizing Li + concentration in the garnet framework. Although a demonstration of a solidstate battery with Nb-doped LLZ as SE has been already reported (Ohta et al, 2012(Ohta et al, , 2014, it has also been reported that the chemical stability against a Li metal electrode of Ta-doped LLZ is much superior to a Nb-doped one (Nemori et al, 2015 (Thangadurai and Weppner, 2005;Murugan et al, 2007b;Awaka et al, 2009b;Kihira et al, 2013;Thangadurai et al, 2014Thangadurai et al, , 2015.…”

Effects of Sublattice Symmetry and Frustration on Ionic Transport in Garnet Solid Electrolytes

“…Other cations occupy the 24c (B) and 16a (C) sites, and oxygen occupies the 96h site. It has been recognized that Li site arrangements vary with Li content x, but there are conflicting reports of the trends coming from experimental measurements [4][5][6]. Li arrangement is difficult to refine, even with neutron scattering, and the nature of ionic conduction is impossible to observe directly.…”

Electrochemical Stability of Li6.5La3Zr1.5M0.5O12 (M = Nb or Ta) against Metallic Lithium

“…However, it is known that stoichiometric Li7La3Zr2O12 (LLZO) results in the tetragonal polymorph with Li-ion conductivities in the 10 −5 S/cm range at 25°C (Awaka et al, 2009;Wolfenstine et al, 2012;Thompson et al, 2014). We and others have demonstrated that ~0.4-0.5 mol of Li vacancies are required to stabilize the higher conductivity (~10 −4 to 10 −3 S/cm at 25°C) cubic garnet type polymorph (Geiger et al, 2011;Rangasamy et al, 2012;Thompson et al, 2014Thompson et al, , 2015. For example, when approximately >0.2 mol of Al 3+ or Ga 3+ substitute for Li, >0.4 mol of Li vacancies are created in the LLZO lattice, thus stabilizing the cubic garnet type polymorph (Geiger et al, 2011;Rangasamy et al, 2012).…”

“…Similarly, when approximately 0.25-0.5 mol of Nb or Ta substitute for Zr, 0.25-0.5 mol of Li vacancies are created, respectively, thus stabilizing the cubic garnet type polymorph (Ohta et al, 2011;Adams and Rao, 2012;Miara et al, 2013). It has been shown that the latter doping scheme (doping on the Zr site) is the approach that results in the highest bulk ionic conductivities approaching 1 mS/cm at 25°C (Ohta et al, 2011;Miara et al, 2013;Thompson et al, 2015). Overall, correlating LLZO formulations with conductivity is reasonably well understood, but understanding the effect of cubic garnet type stabilizing dopants on stability against Li metal is not.…”