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Li-stuffed garnet electrolytes: structure, ionic conductivity, chemical stability, interface, and applications

Publication: Canadian Journal of Chemistry
6 April 2022

Abstract

Current lithium-ion batteries have been widely used in portable electronic devices, electric vehicles, and peak power demand. However, the organic liquid electrolytes used in the lithium-ion battery are flammable and not stable in contact with elemental lithium and at a higher voltage. To eliminate the safety and instability issues, solid-state (ceramic) electrolytes have attracted enormous interest worldwide, owing to their thermal and high voltage stability. Among all the solid-state electrolytes known today, the Li-stuffed garnet is one of the most promising electrolytes due to its physical and chemical properties such as high total Li-ion conductivity at room temperature, chemical stability with elemental lithium and high voltage lithium cathodes, and high electrochemical stability window (6 V vs. Li+/Li). In this short review, we provide an overview of Li-stuffed garnet electrolytes with a focus on their structure, ionic conductivity, transport mechanism, chemical stability, and battery applications.

Résumé

Les batteries au lithium-ion actuelles sont largement utilisées dans les appareils électroniques portables et les véhicules électriques ainsi que dans des applications à demande de puissance maximale. Cependant, les électrolytes liquides organiques utilisés dans les batteries au lithium-ion sont inflammables et ne sont pas stables au contact du lithium métal et à des tensions plus élevées. En vue d’éliminer les problèmes de sécurité et de in stabilité, les électrolytes solides (céramiques) ont suscité un intérêt énorme dans le monde entier, en raison de leur stabilité thermique et à haute tension. Parmi tous les électrolytes solides connus à ce jour, le grenat rempli de lithium est l’un des électrolytes les plus prometteurs en raison de ses propriétés physiques et chimiques, notamment la conductivité ionique du lithium total élevée à température ambiante, la stabilité chimique en présence de lithium métal et de cathodes au lithium à haute tension, ainsi que la fenêtre de stabilité électrochimique élevée (6 V vs Li+/Li). Dans ce bref article de synthèse, nous présentons un aperçu des électrolytes à base de grenat rempli de lithium, en nous intéressant à leur structure, à leur conductivité ionique, à leur mécanisme de transport, à leur stabilité chimique et à leurs applications aux batteries. [Traduit par la Rédaction]

Introduction

All-solid-state batteries (ASSBs) containing a Li metal anode are projected as the ultimate battery technology due to their higher energy density and better safety than the present Li-ion batteries with organic liquid electrolytes. Inorganic solid-state (ceramic) electrolytes could fundamentally eliminate the safety hazards caused by organic electrolytes. The observed inorganic solid-state electrolytes are perovskite, lithium superionic conductors (LiSICON), garnet, sulfur-based solid electrolyte, and halide-based solid electrolyte. Among the various known solid-state inorganic electrolytes for ASSBs, Li-ion conducting garnet-type electrolytes attracted much attention due to their stability toward Li metal, wide electrochemical stability window (>6 V vs. Li/Li+), and ionic conductivity of 10–3 S cm–1 at room temperature (RT).1–3
Garnets have a general formula of A3IIB2III(SiO4)3 (A = Ca, Mg; B = Al, Cr, Fe) where A, B, and Si cations occupy the eight, six, and four coordination sites, respectively; it crystallizes in a cubic structure with space group.2,4 Li containing garnets LixM3M′2O12 (M = trivalent metal ions; M′ = tetra or pentavalent metal ions) are derived from the structural prototype Ca3Al2Si3O12 by replacing the Si with Li. Each unit formula of [Ca3][Al2](Si3)O12 consists of two octahedral AlO6, three antiprismatic CaO8, and three tetrahedral SiO4 polyhedrals. O’Callaghan et al. studied the relationship between Li site occupation and Li-ion conductivity in Li3Ln3Te2O12 (Ln = Y, Pr, Nd, Sm–Lu), where Li+ occupied the tetrahedrally coordinated site, Ln3+ was in the 8-fold coordination site, and Te6+ was in the 6-fold coordination site.5 Li3Ln3Te2O12 exhibits poor Li-ion conductivity because all Li ions in Li3Ln3Te2O12 are occupied in the less mobile tetrahedral sites (24d). When the Li content is increased from Li3 to Li5, Li+ ions are partially distributed over both tetrahedral (24d) and distorted octahedral (96h/48g) sites.
In 2003, Thangadurai et al. first discovered an Li-ion conductivity of 10–6 S cm−1 at RT for Li5La3M2O12 (M = Nb, Ta).6 Murugan et al. further investigated the Li+ transport properties of garnet electrolytes and obtained the Li7La3Zr2O12 (LLZO) by substituting the pentavalent Nb/Ta with tetravalent Zr in Li5La3M2O12.3 LLZO crystallizes in two phases, namely the tetragonal phase with space group I41/acd and the cubic phase with space group . The ionic conductivity of cubic phase LLZO is about two orders of magnitude higher than that of the tetragonal form at RT. The Li distribution of cubic phase Li5La3Ta2O12 and Li7La3Zr2O12 is shown in Fig. 1,7,8 which shows that Li occupies in tetrahedral sites (24d) and two different interstitial octahedral positions (48g/96h). When Ta and Nb were replaced by Zr, extra Li+ can be incorporated into the structure, resulting in a large increase in ionic conductivity to above 10–4 S cm−1. To further improve the ionic conductivity of LLZO, aliovalent ions such as Ta5+,9 Nb5+,10 Te6+,11 W6+,12 Ti4+,13 Al3+,11 and Ga3+14 are substituted for Zr in the LLZO.15 In addition, Wagner et al. observed another cubic modification of Ga-doped LLZO, which is the acentric coarse-grained I-43d.16 By doping those cations, Li+ vacancies increase, and correspondingly, the Li site occupancies decrease in the 24d and or 48g/96h sites. Generally, the ionic conductivity of Li-stuffed garnets increases exponentially with Li content.17 The progress in crystal structure and ionic conductivity of garnet-type electrolytes is shown in Fig. 2.3,6,12,18–40
Fig. 1.
Fig. 1. Distribution of Li in cubic garnet-type (a) Li40La24Ta16O96 (a = 12.8065 Å) and (b) Li56La24Zr16O96 (a = 12.9438(2) Å) structures with space group .6,7 [Colour online.]
Fig. 2.
Fig. 2. Development of a garnet-type solid electrolyte with different compositions (2003,8,18–20 2007,3,21–24 2011,25–28 2013,12,29–32 2015,33 2017,34,35 2019,36–38 and 202139,40). [Colour online.]

Ion-transport mechanisms of garnet-type solid electrolytes

Ion conduction involves the displacement of ions in the crystal lattice. The ion conductivity of a material is expressed41 as
(1)
where Ci is the Li-ion concentration per cm3, q is the elementary charge (1.6 × 10–19 C), and μi is the mobility of Li ions. The relation between ionic conductivity and diffusivity can be derived by substituting the ionic mobility according to the Nernst–Einstein equation:20
(2)
(3)
where Di is the Li-ion diffusion coefficient, T is the temperature (K), and k is the Boltzmann’s constant. The first reported Li-ion conducting Li-stuffed garnets by Thangadurai et al. in 2003 showed the idealized crystal structure of Li5La3M2O12.6 This composite has shown ionic conductivity (σi) in the magnitude of ∼10–6 S cm−1 at 25 °C and the activation energy (Ea) of Li5La3Nb2O12 is more than that of Li5La3Ta2O12. Subsequently, a wide compositional range of garnets have been prepared with Ca, Sr, and Ba in the 8-coordinate site and Bi, Sb, W, Zr, and Sn in the 6-coordinate site.22,42,43 All of the lithium-stuffed compositions have demonstrated a fast lithium conductivity.2 Among them, garnet-type Li7La3Zr2O12 (LLZO) is a fast Li-ion conductor, receiving a lot of attention as an electrolyte candidate for all solid-state Li-ion batteries (ASSLBs). In the Li-stuffed garnet framework of LLZO, the three-dimensional pathway formed through the incompletely occupied tetrahedral sites bridged by a single octahedron responsible for Li+ conduction.44
Several investigations suggested that LLZO in the form of a cubic structure contains a disordered Li+ arrangement throughout tetrahedral and octahedral sites.3,24 Awaka et al. used single-crystal X-ray structural analysis to explore the precise crystal structure of cubic LLZO.45 The Li (1) atoms inhabited the tetrahedral (24d) sites in the cubic phase, and the Li (2) atoms occupied the deformed octahedral (96h) sites. Fast ion conduction is aided by the disordered cubic structure and partial occupation of Li atoms at the Li (2) locations. In comparison with the cubic phase, the ionic conductivity of the tetragonal polymorph is around two orders of magnitude lower, especially at low temperatures. The structure difference of cubic and tetragonal LLZO is shown in Figs. 3a and 3b.17 Thompson et al. compared both tetragonal and cubic phases of LLZO and revealed that to form a cubic phase of LLZO, 0.4–0.5 Li vacancies per formula unit is required.46 Besides the observed cubic-phase LLZO with space group , coarse-grained Ga-stabilized LLZO can also be obtained via solid state reaction, which has an acentric cubic space group I-43d.16 The crystal parameter decreased when the content of Ga in the structure increased and could result in a better Li-ion mobility compared with Al-doped LLZO.
Fig. 3.
Fig. 3. Crystal structures of Li7La3Zr2O12 with (a) cubic and (b) tetragonal phases and coordinate polyhedral at Li (1) and Li (2) sites. Reprinted with permission: Wang, C.; Fu, K.; Palakkathodi Kammampata, S.; McOwen, D. W.; Junio Samson, A.; Zhang, L., et al. Chem. Rev. 2020, 120 (10), 4257. doi:10.1021/acs.chemrev.9b00427.17 Copyright 2021 American Chemical Society. [Colour online.]
Ohta et al.47 reported the cubic Nb-doped LLZO, Li6.75La3Zr1.75Nb0.25O12, with a high total conductivity of 8 × 10–4 S cm−1 at 25 °C, and Wang and Lai48 recorded Li6.7La3Zr1.7Ta0.3O12 with bulk conductivity of 9.6 × 10–4 S cm−1 at 25 °C. Inada et al. reported that the cubic Li7–xLa3Zr2–xTaxO12 system with x = 0.5 revealed conductivity of 6.1 × 10–4 S cm−1 at RT.49 According to these studies, the substitution of Ta5+/Nb5+ in LLZO results in the highest ionic conductivities. Element doping in LLZO is a method to improve ionic conductivity.46 Having high valent metal cations in the lattice, the structural disorder will increase the entropy of the system and result in the reduction of Gibbs free energy. Thus, higher ionic conductivity can be obtained compared with undoped LLZO garnet.
The influence of sintering temperatures on ionic conductivity was studied by Thangadurai et al.20 and Murugan et al.3 They reported a direct relationship between sintering temperature and ionic conductivity, as well as an inverse relationship between activation energy and lattice expansion (Fig. 4a).50 Besides, higher occupancy of Li-ion sites results in higher Li-ion content, and the Li+ ionic conductivity of the garnet-type electrolyte is growing exponentially when the Li-ion content increases (Fig. 4b)2,24 High occupancy of Li (2) sites, according to O’Callaghan,51 is critical for high Li-ion conductivity in the Li-rich garnets. The Li (2) sites are coupled in a three-dimensional manner, allowing Li+ ions to hop from one edge-shared octahedron site to another. Percival et al.52 later provided experimental evidence, demonstrating that quenching samples from higher temperatures resulted in an increase of low-temperature conductivities. Because the Li (2) site occupancy increases with temperature, as demonstrated by neutron diffraction (ND) tests, quenching the sample from 700 °C resulted in a greater conductivity. This lends credence to the concept that Li (2) site occupancy is more important for ionic conductivity than the Li (1) site in garnet structures.
Fig. 4.
Fig. 4. (a) Composition comparison of the lattice parameter and activation energy of Li5+xBaLa2Ta2O11.5+0.5x (x = 0, 0.50, 0.75, 1, 1.25, 1.50, 1.75, 2). Open square: lattice parameter and solid circle: activation energy. Reproduced with permission from Springer: Murugan, R.; Thangadurai, V.; Weppner, W. Appl. Phys. A, 2008, 91(4), 615. doi:10.1007/s00339-008-4494-2.50 (b) Variation of Li-ion occupancy and RT conductivity in the garnet structure as a function of Li concentration. Total conductivity values of garnet structures as a function of Li content as shown (Li3Tb3Te2O12, Li5La3Ta2O12, Li6BaLa2Ta2O12, and Li7La3Zr2O12). Reproduced with permission from Royal Society of Chemistry: Thangadurai, V.; Narayanan, S.; Pinzaru, D. Chem. Soc. Rev. 2014, 43(13), 4714. doi:10.1039/c4cs00020j.2 [Colour online.]
Several improved solid-state methods have recently emerged, giving significant resources for studying Li+ dynamics in solid conductors. Li solid-state nuclear magnetic resonance (NMR) spectroscopy could measure the Li+ arrangements in the lattice structure.53,54 To obtain a more detailed migration feature, pulsed-gradient spin-echo (PGSE) Li NMR can further help in understanding Li diffusion and related parameters, which detects longer time and wider distance transport in the garnet structure, compared with traditional NMR.53,54

Stability of garnet-type solid electrolyte

The garnet-type ceramic electrolyte (e.g., LLZO) has been reported to have relatively good stability towards the lithium metal anode, but it is unstable in the ambient atmosphere (air). When garnet is exposed to air, water vapor will be absorbed by the surface, which allows for the ion exchange between Li+ and H+ and results in the formation of LiOH·H2O. The formed LiOH·H2O will further react with CO2 and produce the common contaminants that occur in most garnet-type electrolytes, namely Li2CO3.55 From DFT calculations, the garnet-type electrolyte with a formula of Li7La3M2O12 (M = Zr, Sn, Hf) is unstable in the ambient environment and is more reactive towards CO2 compared with moisture.56 The formation of Li2CO3 on the surface of the garnet blocks the migration pathway of Li+, which induces the interfacial resistance between electrolyte and electrode. The reactions of Li7La3M2O12 (M = Zr, Sn, Hf) with H2O and CO2 can be written as follows:56
(4)
(5)
The reported methods to remove the lithium carbonate on the surface of the garnet are physical polishing, rapid acid treatment, high-temperature sintering, and atomic layer deposition (ALD).57–61 Recently, Nb and Ga doping in the Li7La3Zr2O12 (LLZO) garnet structure is found to help reduce the amount of LiOH·H2O and Li2CO3 formed on the surface.62,63 By comparing the Raman spectra, (Ga, Nb)-doped LLZO do not show significant peaks for LiOH·H2O and Li2CO3, even after 7 days of exposure to CO2 and moisture (Figs. 5a and 5b),62 whereas the comparison group of (Ca, Nb)-doped LLZO had these impurities. Additionally, Nb-doped Li7La3Zr2O12 can reverse the Li+/H+ exchange process and retain the cubic phase structure.64 Meanwhile, Hofstetter et al. found that Al2O3 coating on Li7La2.75Ca0.25Zr1.75Nb0.25O12 via ALD enhanced its stability in CO2.65 When exposed to CO2, the sample without the Al2O3 layer has ∼8% of total resistance increase, whereas there is no total resistance change in the sample with the Al2O3 layer (Figs. 5c and 5d).65
Fig. 5.
Fig. 5. (a) Raman spectra of as-prepared, 1-day air-exposed, and 7-day air-exposed (Ca, Nb)-LLZO and (Ga, Nb)-LLZO denoted as (1), (2), and (3) and (4), (5), and (6), respectively. (b) (1) and (2) in Raman spectra represent (Ca, Nb)-LLZO with 50 μL of deionized water and after 7 days of exposure followed by drying under argon at 80 °C for 12 h for the standard Li2CO3. (3), (4) and (5) represent (Ga, Nb)-LLZO with 50 μL of deionized water and after 7 days of exposure followed by drying under argon at 80 °C, respectively. Reprinted with permission: Abrha, L. H.; Hagos, T. T.; Nikodimos, Y.; Bezabh, H. K.; Berhe, G. B.; Hagos, T. M., et al. ACS Appl. Mater. Interfaces, 2020, 12(23), 25709. doi:10.1021/acsami.0c01289.62 Copyright 2021 American Chemical Society. Area impedance spectra (1 MHz – 5 kHz) of LLCZN under dry N2, 400 ppm CO2/N2 for 100 h, and 3000 ppm CO2/N2 100 h for (c) uncoated and (d) ALD-coated. Reproduced with permission: Hofstetter, K.; Samson, A. J.; Dai, J.; Gritton, J. E.; Hu, L.; Wachsman, E. D.; Thangadurai, V. J. Electrochem. Soc. 2019, 166(10), A1844. doi:10.1149/2.0201910jes.65 (e) Schematic diagram of how the modification layer protects the surface of garnet. The “golden bell mantle” acts as the surface modification method that prevents the reaction of the garnet surface with water and CO2. [Colour online.]
Surface modification is another approach to diminish the impurity caused by moisture and CO2.66 As shown in Fig. 5e, the surface modification layer can be considered as a “golden bell mantle”, which is a traditional Shaolin Kung Fu technique used to protect a human from getting hurt by sharp weapons. Application of the “golden bell mantle” to the garnet-type electrolyte can prevent the surface from being contaminated by H2O and CO2. Duan et al. introduced a surface treatment to eliminate Li2CO3 on the garnet. Instead of heating at a high temperature, they introduce NH4F to LLZTO and heat the garnet at a moderate temperature (∼180 °C).67 The hydroxide and carbonate are then transformed into LiF, which helped maintain a stable electrolyte–electrode interface. Alternatively, Meng et al. designed a Li2CO3-affiliative mechanism to modify the surface of the garnet without removing the carbonate layer. After liquid metal painting, the garnet has a better wettability of lithium, which results in a small interfacial area-specific resistance (ASR) (∼5 Ω cm2). This method allows a quick filling of gaps during lithium stripping and plating.68
Rajendran et al. illustrated that the introduction of hexagonal boron nitride (h-BN) nanosheet to the garnet-type electrolyte resists the Li2CO3 formation on the surface for more than 120 hours in moisture conditions.69 Despite being unstable in moisture and CO2, the garnet-type electrolyte is relatively stable in an aqueous saturated LiCl solution.70 Shimonishi et al. demonstrated that the conductivity and X-ray diffraction (XRD) of LLZO after being immersed in saturated LiCl solution did not change.70 Meanwhile, Narayanan et al. also revealed that the pH of the LiCl solution increases by days after the treatment of immersing Li5+2xLa3Ta2–xYxO12 (x = 0.50 and 0.75) in the solution, while the XRD results still present a pure cubic phase of garnet.71 The ionic conductivity after the treatment only dropped slightly. Likewise, LLZO shows significant stability in neutral and strongly basic solutions.72 The structure of the LLZO garnet does not change after being immersed in 2M LiOH solution and the reversed ion exchange of H+/Li+ is observed.
Garnet-type electrolytes can also have good chemical stability towards a lithium anode. Based on first-principle calculation, the reductive potential of LLZO is 0.05 V against Li, resulting in the reduction product of Li2O, Zr3O, and La2O3.73 However, it is critical to notice that such a weak reduction driving force could be hindered by kinetic factors.74 The chemical and electrochemical stability of garnet-type electrolytes is mainly affected by the composition of metal elements; for instance, a garnet structure with Ta5+ is more stable with Li metal whereas Nb5+ is not.75 However, poor interaction between the electrolyte and electrode interface leads to extremely large interfacial resistance and inhomogeneous current distribution. Lithium dendrite formation will then become an inevitable problem. A recent study showed that Li ions can be prematurely reduced at grain boundaries in LLZO where there is a reduced bandgap.76 It provides an explanation for why Li can easily infiltrate the garnet electrolyte when cycling at a low current density.77

Interfacial resistance, critical current density (CCD), and device integration

In recent years, various approaches have been used to improve the contact between garnet and Li.78 The most straightforward approach arguably is by removing impurities such as Li2CO3 that form on the surface of garnets upon air exposure. The most noticeable work using a polishing method was done by Sharafi et al.79 The process also involves heating up to 500 °C in argon, resulting in an impressive interfacial resistance reduction to 2 Ω cm2 at RT. Another approach is by introducing a transition layer between garnet and Li. A thin interlayer of polymer or gel electrolytes could effectively reduce the interfacial resistance, such as polyethylene oxide (PEO),80 poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) soaked with liquid electrolyte.81 Depositing a layer of metal or metal oxides, such as Au,82 Si,83 Ge,84 ZnO,66,85 and Al2O3,86 on the garnet surface provides a driving force for Li metal to wet the surface of garnet. The deposition methods include ALD, solution casting, sputtering, chemical vapor deposition, and electron beam thermal evaporation. Among such deposition methods, the most effective to date is Al2O3 deposition by ALD, where the interfacial resistance of 1710 Ω cm2 was reduced to 1 Ω cm2.86 Gutiérrez-Pardo et al. proposed a deposition strategy on a LLZO surface with an organic ionic plastic crystal (OIPC) via spin coating, and by applying this approach, the ASR is significantly reduced (∼120 Ω cm2).87 The full cell, assembled with the following components, Li/OIPC/LLZO/OIPC/LiFPO4 (LFP), can have further enhanced galvanostatic cycling performances. Additionally, the former introduced approach of h-BN nanosheet coating on the garnet surface has notable performance in lithium stripping and plating (over 1400 cycles at 60 °C) and restricts the dendrite formation and has a relatively small ASR of 220 Ω cm2 (Figs. 7a–7d).69 The ASR values after different modification methods are presented in Fig. 6.79,81,85,86,88–105
Fig. 6.
Fig. 6. Area-specific resistance of garnet-type solid electrolyte with different modification methods, deposition,86,88–90 alloy,91–93,103–105 polishing,79,94 sputtering,95,96 wet process,81,85 heating,98,99,106 and others.69,100–102 [Colour online.]
Fig. 7.
Fig. 7. (a) Comparison of electrochemical impedance spectroscopy (EIS) profiles of the uncoated and thin-h-BN-coated LLZT symmetrical cells, measured at 22 °C, the zoomed EIS of the thin-h-BN coated LLZT cell is shown in the inset along with its corresponding curve fitting. (bd) Galvanostatic cycling comparison of (b) thin and thick h-BN-coated LLZT symmetrical cells, (c) uncoated LLZT symmetrical cell, and (d) thin h-BN-coated LLZT at a higher current density. All galvanostatic cycling experiments were done at 60 °C. Reprinted with permission: Rajendran, S.; Thangavel, N. K.; Mahankali, K.; Arava, L. M. R. ACS Appl. Energy Mater. 2020, 3(7), 6775. doi:10.1021/acsaem.0c00905.69 Copyright 2021 American Chemical Society. (e) Long-term cycling behavior of the SnS2-coated battery at 0.2 mA cm–2. (f) Rate performance of the ASSB. Reprinted with permission: Zhou, D.; Ren, G. X.; Zhang, N.; Yu, P. F.; Zhang, H.; Zheng, S., et al. ACS Appl. Energy Mater. 2021, 4(3), 2873. doi:10.1021/acsaem.1c00193.97 Copyright 2021 American Chemical Society. [Colour online.]
Apart from the interfacial resistance, critical current density (CCD) is another crucial parameter for the practical application of garnet solid-state electrolytes. CCD is the current density at and above which Li dendrites will penetrate through garnet. It has been indicated that the maximum CCDs garnet could support are typically less than 0.6 mA cm−2,82,107–110 which are significantly lower than the CCDs of liquid electrolytes, which can reach 4–10 mA cm−2 at RT.111 Park et al. introduced a wetting interfacial film of Na–K liquid between Li metal and garnet, increasing the CCD from 0.50 mA cm–2 to 1.21 mA cm–2.112 Sakamoto and colleagues demonstrated that applying high pressure and raising the cycling temperature can significantly increase the CCDs, which are 3.8 ± 0.9 mA cm−2 and 6.0 ± 0.7 mA cm−2 at 40 °C and 60 °C, respectively.113,114 Zhou et al. coated SnS2 onto Li6.5La3Zr1.5Ta0.5O12 (LLZTO) pellets, which improved the wettability, reduced the garnet–Li interfacial impedance to ∼17 Ω cm–2, and increased the CCD to 1.2 mA cm–2.97 A SnS2 layer on the garnet prevents dendrite formation due to the inhomogeneous current distribution. It can have a long lithium stripping and plating cycle at 0.2 mA cm–2 (Figs. 7e and 7f).97 However, cycling Li metal with LLZO at a high current at ambient temperature is still a challenge. A recent study showed that Li ions can be prematurely reduced at grain boundaries in LLZO where there is a reduced bandgap,76 which could explain why Li can infiltrate the garnet electrolyte even when cycling at a low current density.74,77
The interfacial contact between garnet solid-state electrolytes and cathode materials is more problematic than the interfacial contact between the garnet solid-state electrolytes and the anode side.85 The rigid garnet cannot infiltrate a thick, porous cathode to provide the required ion-conducting path. Adding liquid electrolyte into the cathode and forming a hybrid solid–liquid hybrid system may be a viable option to speed up the development of garnet-based solid-state batteries. But whether the safety, energy density, and cycling performance of such type of cell can outperform the state of the art Li-ion cells remains to be tested. Han et al. demonstrated an all-solid-state battery by thermally soldering a LiCoO2 cathode and garnet together through the reaction between the Li2.3C0.7B0.3O3 glass and the Li2CO3 coating on the garnet (Fig. 8).115 It forms a stable and ionically conducting Li2.3–xC0.7+xB0.3–xO3 (LCBO) interphase, and the resulting cell outperforms all-solid-state cells with a Li3BO3 cathode sintering aid. Still, it can only be cycled at a low current rate (0.05C), with a small cathode active material loading (1 mg cm−2) at RT. Apart from LiCoO2, garnet electrolytes have been coupled with varieties of intercalation cathodes such as LiNixMnyCozO2,116 LiNixAlyCozO2,117 and LiFePO481 and conversion cathodes such as sulfur118 and oxygen.119 However, early trials of garnet-based solid-state batteries usually revealed lower cathode capacities than that of the analog cells with liquid electrolytes.120 Although it is challenging to utilize the garnet electrolyte at RT, after raising the temperature to above 200 °C, the ionic conductivity of garnet increases to a more satisfying level, and the solid–solid contact between garnet and Li metal changes to a solid–liquid contact as Li metal melts. High-temperature Li-alloy, Li-metal chlorides, and Li-S/Se batteries operating at above 200 °C have demonstrated excellent performance.121–123
Fig. 8.
Fig. 8. Schematic diagram of the interphase-engineered all-ceramic cathode and electrolyte.115 [Colour online.]

Conclusions

The garnet-type solid electrolyte is a promising candidate for ASSB, owing to its high stability towards lithium metal, wide electrochemical window, and high ionic conductivity. The conduction mechanism of garnet-type electrolytes is related to the partial occupancy of the Li+ octahedral sites, and the ionic conductivity is related to the sintering temperature. Although the garnet electrolyte is stable towards the lithium metal anode, it is not stable in air. Different surface modification methods were introduced in this review to eliminate or prevent Li2CO3 formation on the garnet surface. Interfacial resistance and CCD are two key parameters to dominate the practical applications of garnet-type solid electrolytes; the interfacial contact between the garnet-type electrolyte and cathode is more problematic than that on the anode side. Forming liquid–solid hybrid electrolyte or mixing cathode and electrolyte with thermal solders were demonstrated to improve the cycling performance of the battery. Overall, the fundamental aspects of garnet-type solid electrolytes have been well studied, and further development can focus on thin-film processing and device integration.

Acknowledgements

The Natural Sciences and Engineering Research Council of Canada (NSERC) supported this work through discovery grants to Venkataraman Thangadurai (award number: RGPIN-2021-02493).

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Published In

cover image Canadian Journal of Chemistry
Canadian Journal of Chemistry
Volume 100Number 5May 2022
Pages: 311 - 319

History

Received: 14 November 2021
Accepted: 6 January 2022
Version of record online: 6 April 2022

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Key Words

  1. solid electrolytes
  2. Li-rich garnets
  3. ionic conductivity
  4. chemical stability
  5. interface
  6. solid-state Li batteries

Mots-clés

  1. électrolytes solides au lithium-ion
  2. grenats riches en Li
  3. conductivité ionique
  4. stabilité chimique
  5. interface
  6. batteries à électrolyte solide au lithium

Authors

Affiliations

Bowen Chen
Department of Chemistry, University of Calgary, 2500 University Dr NW, Calgary, AB T2N 1N4, Canada.
Subhajit Sarkar
Department of Chemistry, University of Calgary, 2500 University Dr NW, Calgary, AB T2N 1N4, Canada.
Sanoop Palakkathodi Kammampata
Department of Chemistry, University of Calgary, 2500 University Dr NW, Calgary, AB T2N 1N4, Canada.
Chengtian Zhou
Department of Chemistry, University of Calgary, 2500 University Dr NW, Calgary, AB T2N 1N4, Canada.
Venkataraman Thangadurai [email protected]
Department of Chemistry, University of Calgary, 2500 University Dr NW, Calgary, AB T2N 1N4, Canada.

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