- Email: [email protected]
Multi-substituted garnet-type electrolytes for solid-state lithium batteries
Journal Pre-proof Multi-substituted garnet-type electrolytes for solid-state lithium batteries Shufeng Song, Yongmin Wu, Zhencai Dong, Fan Deng, Weiping Tang, Jianyao Yao, Zhaoyin Wen, Li Lu, Ning Hu, Janina Molenda PII:
S0272-8842(19)33168-2
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.290
Reference:
CERI 23358
To appear in:
Ceramics International
Received Date: 22 August 2019 Revised Date:
27 October 2019
Accepted Date: 31 October 2019
Please cite this article as: S. Song, Y. Wu, Z. Dong, F. Deng, W. Tang, J. Yao, Z. Wen, L. Lu, N. Hu, J. Molenda, Multi-substituted garnet-type electrolytes for solid-state lithium batteries, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.290. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Multi-substituted garnet-type electrolytes for solid-state lithium batteries Shufeng Songa,*, Yongmin Wub, Zhencai Donga, Fan Denga, Weiping Tangb, Jianyao Yaoa,*, Zhaoyin Wenc, Li Lud,e, Ning Huf,g,*, Janina Molendah a
College of Aerospace Engineering, Chongqing University, Chongqing 400044, China
b
State Key Laboratory of Space Power-sources Technology, Shanghai Institute of
Space Power-Sources, Shanghai 200245, China c
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of
Ceramics, Chinese Academy of Sciences, Shanghai 200050, China d
Department of Mechanical Engineering, National University of Singapore, 117575,
Singapore e
National University of Singapore Suzhou Research Institute, Suzhou 215000, China
f
School of Mechanical Engineering, Hebei University of Technology, Tianjin, 300401,
China g
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing
University, Chongqing 400044, China h
AGH Univ Sci & Technol, Fac Energy & Fuels, Al Mickiewicza 30, PL-30059
Krakow, Poland *Corresponding author. E-mail address: [email protected] (S. Song); [email protected] (J. Yao); [email protected] (N. Hu)
ABSTRACT Lithium garnets are considered as advanced solid electrolytes for safe and high-energy-density solid-state lithium batteries. A multi-substituted strategy illustrating in this work is substitution of Li by Ga and Zr by Ta/Ba, therefore stabilizing and densifying the cubic lithium garnets thus improving the ionic conductivities. The influence of Ga substitution on garnet phase, microstructure and conductivity of Li6.7-3xGaxLa3Zr1.55Ta0.4Ba0.05O12 garnets is systematically studied. The
developed
multi-substituted
ambient-temperature
conductivity
garnet-type of
electrolytes
1.02×10-3
S
cm-1.
demonstrate Moreover,
high the
multi-substituted garnet reveals impressive capability of alleviating lithium metal dendrites over 800 h under current of 0.01 mA cm-2 to 0.5 mA cm-2, promising solid-state battery applications. Keywords: Garnet; Solid electrolyte; Solid batteries; Lithium dendrites; Conductivity
1. Introduction Currently, state-of-the-art lithium-ion batteries could not respond the growing demands on energy density, cycle life, and safety etc [1]. Use of solid electrolytes instead of liquid ones in solid batteries could overcome the intrinsic issues of liquid electrolytes, besides, enable emerging battery technologies [2,3]. Within this material class, the lithium garnet-type oxide with nominal formula Li7La3Zr2O12 exhibits modestly high ionic conductivity, wide voltage window, and stability toward metallic lithium [4]. Ion conduction is one of the key demand for solid electrolytes, however, as one of the most promising solid electrolytes, the ion conduction of lithium garnets performs on the order of magnitudes worse than that of liquid ones [5]. The lithium garnets have two reported crystallographic polymorphs of metastable cubic and low-temperature tetragonal form [6]. The cubic Li7La3Zr2O12 is known to possess ionic conductivity of ~10-4 S cm-1 at room temperature, where the Zr and La cations are occupied in 6-coordination and 8-coordination sites, respectively, forming a 3D framework with 9 interstitial sites per formula unit. 7/9 Li cations and 2/9 vacancies are occupied in 24d and 48g/96h sites [7]. On the contrary, the tetragonal Li7La3Zr2O12 exhibits low ionic conductivity of ~10-6 S cm-1 at room temperature, because of fully occupied Li sites [8]. The most effective solution to stabilize cubic phase is element substitution strategy [9]. Al is a popular and effective element to stabilize the cubic phase that stems from contamination of alumina crucibles during calcining and sintering process, and/or
intentional substitution on Li sites [10]. Al substitutes Li and thereby stabilizes the cubic garnet through the reduction in Li content and/or enhancement in Li vacancies. Al-doping Li7La3Zr2O12 garnet shows a room-temperature conductivity of ~3×10-4 S cm-1 [11]. Unfortunately, Al-doped Li7La3Zr2O12 recently has been suggested unstable toward LiCoO2, one of the most prospective cathodes for solid-state lithium batteries. An annealing temperature above 500 oC induced diffusion of Al from garnet into LiCoO2 and resulted in cubic to tetragonal phase transition, the interfacial reaction also produced impurities of Li2CO3, La2Zr2O7, and LaCoO3 etc [12]. Thus it is desirable to find other element substitutions which like Al stabilize the cubic phase by increasing Li vacancies but simultaneously produce favorable interface with LiCoO2 during co-sintering process. Ta substitutes Zr follows this requirement. First, substitution of Ta on Zr sites could decrease Li content to stabilize cubic phase, but could not hinder Li ions transport, which may result in higher conductivity. Normally, Al-free Ta-doping Li7La3Zr2O12 garnet shows higher conductivity at room temperature compared to Al-doping garnet [13]. However, Ta substitution cannot densify the garnets, Al is generally co-doped occasionally or intentionally to enhance the sinterability of the material to achieve higher ionic conductivities [14]. More important, Ta-substituted Li7La3Zr2O12 garnet exhibits good interfacial stability toward LiCoO2, which makes this material system potential for solid-state lithium batteries [15]. Ga has been also substituted on the Li sites to stabilize cubic phase following the same approach with Al substitution [16]. Ga-doping Li7La3Zr2O12 garnet exhibits
significant high ionic conductivity of ~10-3 S cm-1 at room temperature [17]. The high mobility of Li+ in Ga-doping Li7La3Zr2O12 could be attributed to the preferential tetrahedral 24d location of Ga substituent, leading to a disorder of Li [18], and/or the low eutectic point of Li-Ga-O compounds improves the densification of the garnets through liquid phase at grain-boundaries [19]. The enhancement of density of lithium garnet will be definitely in favour of Li-ions conduction. We found alkaline-earth element substitution on Zr sites in lithium garnets may intrinsically enhance the sinterability [20,21]. It is different from the sinter aids that are always located at grain-boundaries and thus block Li-ions transport. In this paper, a multiple substitution approach is demonstrated where partial replacement of Li with Ga combined to partial replacement of Zr with Ta and Ba. First, Ta substitution on Zr sites gives access to lithium garnet stable with LiCoO2 cathode. Second, Ba substitution on Zr sites increases Li content that boosts other substitution on Li site and intrinsically enhance the sinterability of garnet. Third, we aim to optimize the ionic conductivity with Ga substitution on Li sites. Following this scheme, we synthesize a series of Ga/Ta/Ba-substituted garnets with the nominal formula Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) and study in detail the substitution influence. We also shed light on the capability of the multi-substituted garnet to alleviate lithium metal dendrites. 2. Experimental section 2.1. Synthesis LiOH•H2O (Aladdin, 99%), Ga(NO3)3•9H2O (Diyanghg, 99.99%), La2O3 (Aladdin,
99.9%, dried at 900oC for 12 h), ZrO2 (Aladdin, 99.99%), BaCO3 (Aladdin, ACS), and Ta2O5 (Aladdin, 99.5%) were used as raw materials and weighed in stoichiometric amounts
(15wt.%
LiOH•H2O
3xGaxLa3Zr1.55Ba0.05Ta0.4O12
excess
was
added)
to
reach
Li6.7
−
(x=0, 0.1, 0.15, 0.2). The chemicals were mixed and
ground in a zirconia milling vial for about 12 h under addition of ethanol and subsequently pressed into pellets. The precursor pellets were placed on a MgO pellet to avoid Al contamination from the crucibles and calcined at 900 oC for 6 h in alumina crucibles. The calcined pellets were again ground, pressed, and placed on a MgO pellet in alumina crucibles, and covered by additional garnet powder. The sintering procedure was first heated at 1250 oC for 10 min and subsequently cooled at 1150 oC for 12 h in dry O2. The resulting pellets were stored in a glovebox under Ar atmosphere for further characterization. 2.2. Structure characterization X-ray diffraction (XRD, Bruker, D2 PHASER, Cu-Kα) was conducted to detect the crystallographic structure of the samples. Scanning electron microscopy (SEM, Hitachi, Su8020) was examined on fracture surfaces of sintered pellets to study the microstructure. The general distribution of elements was checked using energy dispersive spectroscopy (EDS) analysis. 2.3. Electrochemical characterization AC impedance spectroscopy measurement was conducted on an Autolab PGSTAT302N Analyzer to determine the ionic conductivity of the materials. The polished pellet was pasted by Ag electrode on both surfaces and loaded into a
Swagelok cell in Ar-glovebox to exclude the adverse influence of air/moisture on conductivity measurement. The AC impedance spectroscopy measurements were performed by applying 10 mV in the frequency range of 1 MHz-0.1 Hz in the temperature range from -78 oC to 80 oC. Li/garnet/Li symmetric cell was fabricated to investigate the interfacial performance of sintering garnet against metallic lithium. The fresh Li foil was integrated with a polished pellet and sealed in CR2016 coin cell in Ar-filled glovebox. The symmetric cell was heated at 60 oC for 24 h before cycling. The galvanostatic cycling was first performed at 60 oC under constant current density of 0.01 mA cm-2 over 40 cycles using an Arbin LBT20083. Li plating/stripping was subsequently took place at ambient temperature and 0.01-0.5 mA cm-2 current densities (2 hours for each plating/stripping cycle) were sequentially applied.
3. Results and discussion Fig.
1
shows
XRD
patterns
3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0,
of
calcined
and
sintered
Li6.7
−
0.1, 0.15, 0.2) at 900 oC and 1150 oC, respectively.
As seen in Fig. 1a, all diffraction peaks of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) could be indexed as cubic garnet structure (PDF 45-0109, Space group Ia-3d) after calcined at 900 oC for 6 h, which will be in favour of the densification of lithium garnets in the following sintering. No detectable impurity phases could be observed for the samples after sintered at 1150
o
C for 12 h, except
Li6.1Ga0.2La3Zr1.55Ba0.05Ta0.4O12 shows slight amount of ZrO2 impurity as shown in Fig.
1b. The weak XRD diffraction of impurities in LLZO was also identifed previously [22,23]. Therefore, the Ga/Ta/Ba-multiple substitution results in single cubic garnet phase with x<0.2.
Fig. 1. Powder XRD patterns of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) Calcined at 900 oC. (b) Sintered at 1150 oC. JCPDS file # 45-109 was used for cubic lithium garnet.
To obtain more meaningful information, SEM was conducted to survey the cross-sectional morphological evolution of sintered Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). As seen in Fig. 2, several key points can be made. First, partial proportion of the fracture mode for Ga-free sample is intergranular, on the other hand, most proportion of the fracture mode for Ga-doping samples is transgranular, implying high grain-boundary strength that would result in low grain-boundary resistances. Second, the Ga-free sample exhibits clear grain boundary, whereas good intergranular connectivity is observed for the Ga-doping samples that would lead to low electrical resistances. Third, the majority of voids are intergranular, there are very
few intragranular voids. Fourth, the grains are fairly isotropic shape with an average intercept grain size of ~5 µm. Therefore, the Ga/Ta/Ba-multiple substitution results in more favorable microstructure, due mainly to the intrinsic enhancement of sinterability by the addition of Ga.
Fig. 2. Morphological evolution of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) x=0. (b) x=0.1. (c) x=0.15. (d) x=0.2. The EDS elemental mapping of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) in Fig. 3 reveals that Zr, Ta, La, and Ba are uniformly distributed. However, the distribution of Ga is distinct. As seen in Fig. 3b, the distribution of Ga in Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12 is homogeneous. As seen in Fig. 3c&d, few Ga-rich phase accumulates at the grain boundary in Li6.25Ga0.15La3Zr1.55Ba0.05Ta0.4O12 and Li6.15Ga0.2La3Zr1.55Ba0.05Ta0.4O12. It implied that the grain-boundary Ga-rich phase are amorphous because it was not detected by the X-ray diffraction as seen in Fig. 1.
Matsuda et al. also suggested that small amounts of amorphous Li-Ga-O oxides presented at the grain boundaries of Ga-doping lithium garnets [19].
Fig. 3. EDS elemental mapping images of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) x=0. (b) x=0.1. (c) x=0.15. (d) x=0.2.
The ionic conductivity of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 was measured by AC impedance. Fig. 4a shows the Nyquist curves from the AC impedance measurement at ambient temperature. Only an approximate straight-line without any clear semi-circles was revealed. As Ag-blocking electrode was used in the measurement, the appearance of low-frequency tail declares the material which is a single lithium-ion conductor. The disappearance of high-frequency semi-circles implies a feature of material with slight grain-boundary impedance. Several studies done by Kotobuki et al. [24] and Allen et al. [9] support the fact that this disappearance of high-frequency semi-circles
indicates the grain-boundary impedance is ignorable in comparison to the grain impedance, therefore, the intercept of the straight line on the real axis represents the sum response of grain and grain boundary, as the total conductivity of the material. The total Li-ion conductivity rather than grain conductivity is significant important for a practical device. As a result of multi substitution, the total conductivity enhances from 8.3×10-4 S cm-1 for the Ga-free sample (Li6.7La3Zr1.55Ba0.05Ta0.4O12) to an optimal
value
of
1.02×10-3
S
cm-1
for
the
Ga-containing
sample
(Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12), illustrating a positive effect of Ga/Ta/Ba-multiple substitution. It is noted that this conductivity value of ~10-3 S cm-1 is highly repeatable as the careful control of preparation technology, i.e., MgO pellet was used to isolate the alumina crucibles and samples during both the calcining and sintering procedures. In addition, the AC impedance was examined in Ar-filled atmosphere that eliminates the negative influence of CO2 and H2O on the lithium garnets. In view of the high conductivity value of 10-3 S cm-1, the dopant Ga shows its advantage to improve the ionic conductivities of lithium garnets, although the four samples show very close ionic conductivities as seen in the Fig. 4a and the inset. Fig. 4b shows the Arrhenius curves as a function of total Li-ion conductivity of Li6.7-3xGaxLa3Zr1.55Ba0.05Ta0.4O12 and temperature. As seen, the total ionic conductivity of Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12 is the highest at all temperatures, and the activation energy is calculated from Arrhenius equation and the slope of the line in the temperature range of 195-353 K. The activation energy is around 0.38-0.40 eV that is comparable
to
the
other
reported
Ta-Al-substituted
garnet
Li6.4625Al0.1375La3Zr1.875Ta0.125O12 (0.36 eV, 1.03×10-3 S cm-1) [14].
Fig.
4.
Electrochemical
impedance
characterization
of
Li6.7-3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) Nyquist curves. (b) Arrhenius curves. The inset is a local view of Nyquist curves.
A challenge of Li metal being used in practical device is the formation of lithium dendrites that may cause sudden short-circuit giving rise to dangerous battery fire/explosion. Garnet-type inorganic solid electrolytes are expected to block Li dendrites by means of their dense microstructure and high shear moduli, although short circuit still happens. The high solid-solid interfacial impedance between garnet-type electrolyte and Li metal anode is another great challenge. Fresh Li foil was
attached
on
both
surfaces
of
sintered/polished
pellet
(Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12) and sealed in CR2016 coin-cell. As seen in Fig. 5b, the as-prepared symmetric cell exhibits very large impedance due to the interfacial impedance between Li metal and solid electrolyte, the interfacial resistance is around 6.5×106 Ω cm2, whereas, the interfacial resistance significantly decreases to be around
4500 Ω cm2 after activation (i.e., cycling for 80 h at elevated temperature of 60 oC and at current density of 0.01 mA cm-2). The mild interfacial resistance between lithium metal and solid electrolyte after activation would be in favour of the use of lithium metal as an anode. Fig. 5a shows the extended cycling result of the symmetrical cell under different current density of 0.01 mA cm-2, 0.05 mA cm-2, 0.1 mA cm-2, and 0.5 mA cm-2, respectively. The symmetrical cell demonstrates very flat and stable plating/stripping profiles with a small overpotential of 30 mV during the initial 0.01 mA cm-2 cycling (Fig. 5a and the zoom-in profiles). The random voltage fluctuation is possibly due to the current fluctuation, resulting from the low current density and current accuracy. The random voltage fluctuation disappears as the current density is stable at 0.05 mA cm-2. Smooth voltage profiles are observed when the current density is boosted to 0.05 mA cm-2 and 0.1 mA cm-2, and the overpotential is enlarged from 30 mV to 50 mV and 70 mV, respectively. When the current density is further boosted from 0.1 mA cm-2 to 0.5 mA cm-2, a noisy voltage fluctuation is observed during initial dozens of cycles, which is probably owing to the dynamic interfacial contact area or interim break of the sequential electron migration pathway resulted from the fast lithium deposition/dissolution [25]. This phenomenon is generally observed in symmetrical cell with bare lithium and untreated inorganic solid electrolytes cycling at high current densities [26]. The voltage response tends towards stabilization after initial noisy fluctuation, indicating stable lithium plating and stripping. New but weak voltage fluctuation could appear again when the cycling is suddenly interrupted following renewing the measurement (300th cycle). Again, stable voltage response is
gradually produced after voltage oscillation and the voltage profiles are more flat in the successive cycling, indicating ameliorative interfacial contact between lithium metal and solid electrolyte through the long-term lithium plating and stripping. The subsequent electrochemical impedance spectrum of the 400th cycling symmetrical cell exhibits a small partial high-frequency semicircle and a low-frequency depressed semicircle, which can be assigned to the total resistance of garnet electrolyte and charge transfer resistance of the interfaces, respectively. Therefore, a very low interfacial resistance of ~25 Ω cm2 can be obtained after the long-term cycling, which can be comparable to those of symmetrical cells with complex surface/interface modification [27]. Therefore, our study proves that the multi-substituted garnet-type electrolytes can achieve stable and long-term lithium plating and stripping.
Fig. 5. Electrochemical characterizations of Li/garnet Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12
/Li symmetric cell at ambient temperature. (a) Cyclic performance at various current densities. (b) Impedance spectra at different cell status.
4. Conclusion In summary, a multi-substituted strategy is proposed to investigate the garnet-type solid electrolytes, where the substitution of Zr by Ta/Ba targeted at stabilizing the cubic garnet phase and the substitution of Li by Ga at adjusting the Li contents and enhancing the intrinsic sinterability. Cubic Li6.7-3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) was synthesized and an optimal Ga proportion of x=0.1 per formula unit leads to the highest ionic conductivity of 1.02×10-3 S cm-1 at ambient temperature. Importantly, the multi-substituted Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12 exhibits good capability
of
alleviating
lithium
metal
dendrites.
A
symmetrical
Li/Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12/Li cell exhibits good cycling stability for 800 h as the current density increased in steps from 0.01 mA cm-2 to 0.5 mA cm-2, and ultralow interfacial charge transfer resistance of ~25 Ω cm2 upon long-term cycling.
Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 51702030, No. 11632004, and No. U1864208), Shanghai
Aerospace
Science
and
Technology
Innovation
Fundation
(No.
SAST2017-137), Chongqing University, the Fundamental Research Funds for the Central Universities (No. 2018CDXYHK0016), and the Key Program for
International Science and Technology Cooperation Projects of the Ministry of Science and Technology of China (No. 2016YFE0125900).
References [1] P. Parikh, M. Sina, A. Banerjee, X. Wang, M.S. D’Souza, J.-M. Doux, E.A. Wu, O.Y. Trieu, Y. Gong, Q. Zhou, K. Snyder, Y.S. Meng, Role of polyacrylic acid (PAA) binder on the solid electrolyte interphase in silicon anodes, Chem. Mater. 31 (2019) 2535-2544. [2] K. Kimura, K. Wagatsuma, T. Tojo, R. Inada, Y. Sakurai, Effect of composition on lithium-ion conductivity for perovskite-type lithium strontium tantalum zirconium-oxide solid electrolytes, Ceram. Int. 42 (2016) 5546-5552. [3] F. Zheng, M. Kotobuki, S.F. Song, M.O. Lai, L. Lu, Review on solid electrolytes for all-solid-state lithium-ion batteries, J. Power Sources 389 (2018) 198-213. [4] X. Huang, Z. Song, T. Xiu, M. Badding, Z.Y. Wen, Sintering, micro-structure and Li+ conductivity of Li7-xLa3Zr2-xNbxO12/MgO (x=0.2-0.7) Li-garnet composite ceramics, Ceram. Int. 45 (2019) 56-63. [5] V. Thangadurai, S. Narayanan, D. Pinzaru, Garnet-type solid-state fast Li ion conductors for Li batteries: critical review, Chem. Soc. Rev. 43 (2014) 4714-4727. [6] R. Jalem, Y. Yamamoto, H. Shiiba, M. Nakayama, H. Munakata, T. Kasuga, K. Kanamura, Concerted migration mechanism in the Li ion dynamics of garnet-type Li7La3Zr2O12, Chem. Mater. 25 (2013) 425-430.
[7] H. Xie, J.A. Alonso, Y. Li, M.T. Fernandez-Díaz, J.B. Goodenough, Lithium distribution in aluminum-free cubic Li7La3Zr2O12, Chem. Mater. 23 (2011) 3587-3589. [8] E.J. Cussen, Structure and ionic conductivity in lithium garnets, J. Mater. Chem. 20 (2010) 5167-5173. [9] Z. Cao, X. Cao, X. Liu, W. He, Y. Gao, J. Liu, J. Zeng, Effect of Sb-Ba codoping on the ionic conductivity of Li7La3Zr2O12 ceramic, Ceram. Int. 41 (2015) 6232-6236. [10] Z.L. Hu, H. Liu, H. Ruan, R. Hu, Y. Su, L. Zhang, High Li-ion conductivity of Al-doped Li7La3Zr2O12 synthesized by solid-state reaction, Ceram. Int. 42 (2016) 12156-12160. [11] H. El-Shinawi, G.W. Paterson, D.A. MacLaren, E.J. Cussen, S.A. Corr, Low-temperature densification of Al-doped Li7La3Zr2O12: a reliable and controllable synthesis of fast-ion conducting garnets, J. Mater. Chem. A 5 (2017) 319-329. [12] G. Vardar, W.J. Bowman, Q. Lu, J. Wang, R.J. Chater, A. Aguadero, R. Seibert, J. Terry, A. Hunt, I. Waluyo, D.D. Fong, A. Jarry, E.J. Crumlin, S.L. Hellstrom, Y.M. Chiang, B. Yildiz, Structure, chemistry, and charge transfer resistance of the interface between Li7La3Zr2O12 Electrolyte and LiCoO2 Cathode, Chem. Mater. 30 (2018) 6259-6276. [13] X. Tong, V. Thangadurai, E.D. Wachsma, Highly conductive Li garnets by a multielement doping strategy, Inorg. Chem. 54 (2015) 3600-3607.
[14] Y. Matsuda, Y. Itami, K. Hayamizu, T. Ishigaki, M. Matsui, Y. Takeda, O. Yamamoto, N. Imanishi, Phase relation, structure and ionic conductivity of Li7-x-3yAlyLa3Zr2-xTaxO12, RSC Adv. 6 (2016) 78210-78218. [15] F. Han, J. Yue, C. Chen, N. Zhao, X. Fan, Z. Ma, T. Gao, F. Wang, X. Guo, C.S. Wang, Interphase engineering enabled all-ceramic lithium battery, Joule 2 (2018) 1-12. [16] S. Afyon, F. Krumeichb, J.L.M. Rupp, A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries, J. Mater. Chem. A 3 (2015) 18636-18648. [17] J.F. Wu, E. Chen, Y. Yu, L. Liu, Y. Wu, W.K. Pang, V.K. Peterson, X. Guo, Gallium-doped Li7La3Zr2O12 garnet-type electrolytes with high lithium-ion conductivity, ACS Appl. Mater. Interfaces 9 (2017) 1542-1552. [18] C. Bernuy-Lopez, W. Manalastas, J.M.L. Amo, A. Aguadero, F. Aguesse, J.A. Kilner, Atmosphere controlled processing of Ga-substituted garnets for high Li-ion conductivity ceramics, Chem. Mater. 26 (2014) 3610-3617. [19] Y. Matsuda, A. Sakaida, K. Sugimoto, D. Mori, Y. Takedaa, O. Yamamoto, N. Imanishi, Sintering behavior and electrochemical properties of garnet-like lithium conductor Li6.25M0.25La3Zr2O12 (M: Al3+ and Ga3+), Solid State Ionics 311 (2017) 69-74. [20] S.F. Song, D. Sheptyakov, A.M. Korsunsky, H.M. Duong, L. Lu, High Li ion conductivity in a garnet-type solid electrolyte via unusual site occupation of the doping Ca ions, Mater. Design 93 (2016) 232-237. [21] S.F. Song, M. Kotobuki, F. Zheng, C. Xu, Y. Wang, W. Z. Li, N. Hu, L. Lu, Roles
of alkaline earth ions in garnet-type superionic conductors, ChemelectroChem 4 (2017) 266-271. [22] T. Yang, Y. Li, W. Wu, Z. Cao, W. He, Y. Gao, J. Liu, G. Li, The synergistic effect of dual substitution of Al and Sb on structure and ionic conductivity of Li7La3Zr2O12 ceramic, Ceram. Int. 44 (2018) 1538-1544. [23] D. Mori, K. Sugimoto, Y. Matsuda, K. Ohmori, T. Katsumata, S. Taminato, Y. Takeda, O. Yamamoto, N. Imanishi, Synthesis, structure and ionic conductivity of garnet like lithium ion conductor Li6.25+xGa0.25La3-xSrxZr2O12, J. Electrochem. Soc. 166 (3) (2019) A5168-A5173. [24] M. Kotobuki, H. Munakata, K. Kanamura, Y. Sato, T. Yoshida, Fabrication of all-solid-state rechargeable lithium-ion battery using mille-feuille structure of Li0.35La0.55TiO3, J. Power Sources 196 (2011) 6947−6950. [25] G.T. Hitz, D.W. McOwen, L. Zhang, Z. Ma, Z. Fu, Y. Wen, Y. Gong, J. Dai, T.R. Hamann, L.B. Hu, E.D. Wachsman, High-rate lithium cycling in a scalable trilayer Li-garnet-electrolyte architecture, Mater. Today 22 (2019) 50-57. [26] J. Duan, W. Wu, A. Nolan, T. Wang, J. Wen, C. Hu, Y. Mo, W. Luo, Y.H. Huang, Lithium-graphite paste: an interface compatible anode for solid-state batteries, Adv. Mater. 31 (2019) 1807243. [27] X.B. Cheng, C.Z. Zhao, Y. Yao, H. Liu, Q. Zhang, Recent advances in energy chemistry between solid-state electrolyte and safe lithium-metal anodes, Chem 5 (2019) 74-96.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)33168-2
DOI:
https://doi.org/10.1016/j.ceramint.2019.10.290
Reference:
CERI 23358
To appear in:
Ceramics International
Received Date: 22 August 2019 Revised Date:
27 October 2019
Accepted Date: 31 October 2019
Please cite this article as: S. Song, Y. Wu, Z. Dong, F. Deng, W. Tang, J. Yao, Z. Wen, L. Lu, N. Hu, J. Molenda, Multi-substituted garnet-type electrolytes for solid-state lithium batteries, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.290. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Multi-substituted garnet-type electrolytes for solid-state lithium batteries Shufeng Songa,*, Yongmin Wub, Zhencai Donga, Fan Denga, Weiping Tangb, Jianyao Yaoa,*, Zhaoyin Wenc, Li Lud,e, Ning Huf,g,*, Janina Molendah a
College of Aerospace Engineering, Chongqing University, Chongqing 400044, China
b
State Key Laboratory of Space Power-sources Technology, Shanghai Institute of
Space Power-Sources, Shanghai 200245, China c
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of
Ceramics, Chinese Academy of Sciences, Shanghai 200050, China d
Department of Mechanical Engineering, National University of Singapore, 117575,
Singapore e
National University of Singapore Suzhou Research Institute, Suzhou 215000, China
f
School of Mechanical Engineering, Hebei University of Technology, Tianjin, 300401,
China g
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing
University, Chongqing 400044, China h
AGH Univ Sci & Technol, Fac Energy & Fuels, Al Mickiewicza 30, PL-30059
Krakow, Poland *Corresponding author. E-mail address: [email protected] (S. Song); [email protected] (J. Yao); [email protected] (N. Hu)
ABSTRACT Lithium garnets are considered as advanced solid electrolytes for safe and high-energy-density solid-state lithium batteries. A multi-substituted strategy illustrating in this work is substitution of Li by Ga and Zr by Ta/Ba, therefore stabilizing and densifying the cubic lithium garnets thus improving the ionic conductivities. The influence of Ga substitution on garnet phase, microstructure and conductivity of Li6.7-3xGaxLa3Zr1.55Ta0.4Ba0.05O12 garnets is systematically studied. The
developed
multi-substituted
ambient-temperature
conductivity
garnet-type of
electrolytes
1.02×10-3
S
cm-1.
demonstrate Moreover,
high the
multi-substituted garnet reveals impressive capability of alleviating lithium metal dendrites over 800 h under current of 0.01 mA cm-2 to 0.5 mA cm-2, promising solid-state battery applications. Keywords: Garnet; Solid electrolyte; Solid batteries; Lithium dendrites; Conductivity
1. Introduction Currently, state-of-the-art lithium-ion batteries could not respond the growing demands on energy density, cycle life, and safety etc [1]. Use of solid electrolytes instead of liquid ones in solid batteries could overcome the intrinsic issues of liquid electrolytes, besides, enable emerging battery technologies [2,3]. Within this material class, the lithium garnet-type oxide with nominal formula Li7La3Zr2O12 exhibits modestly high ionic conductivity, wide voltage window, and stability toward metallic lithium [4]. Ion conduction is one of the key demand for solid electrolytes, however, as one of the most promising solid electrolytes, the ion conduction of lithium garnets performs on the order of magnitudes worse than that of liquid ones [5]. The lithium garnets have two reported crystallographic polymorphs of metastable cubic and low-temperature tetragonal form [6]. The cubic Li7La3Zr2O12 is known to possess ionic conductivity of ~10-4 S cm-1 at room temperature, where the Zr and La cations are occupied in 6-coordination and 8-coordination sites, respectively, forming a 3D framework with 9 interstitial sites per formula unit. 7/9 Li cations and 2/9 vacancies are occupied in 24d and 48g/96h sites [7]. On the contrary, the tetragonal Li7La3Zr2O12 exhibits low ionic conductivity of ~10-6 S cm-1 at room temperature, because of fully occupied Li sites [8]. The most effective solution to stabilize cubic phase is element substitution strategy [9]. Al is a popular and effective element to stabilize the cubic phase that stems from contamination of alumina crucibles during calcining and sintering process, and/or
intentional substitution on Li sites [10]. Al substitutes Li and thereby stabilizes the cubic garnet through the reduction in Li content and/or enhancement in Li vacancies. Al-doping Li7La3Zr2O12 garnet shows a room-temperature conductivity of ~3×10-4 S cm-1 [11]. Unfortunately, Al-doped Li7La3Zr2O12 recently has been suggested unstable toward LiCoO2, one of the most prospective cathodes for solid-state lithium batteries. An annealing temperature above 500 oC induced diffusion of Al from garnet into LiCoO2 and resulted in cubic to tetragonal phase transition, the interfacial reaction also produced impurities of Li2CO3, La2Zr2O7, and LaCoO3 etc [12]. Thus it is desirable to find other element substitutions which like Al stabilize the cubic phase by increasing Li vacancies but simultaneously produce favorable interface with LiCoO2 during co-sintering process. Ta substitutes Zr follows this requirement. First, substitution of Ta on Zr sites could decrease Li content to stabilize cubic phase, but could not hinder Li ions transport, which may result in higher conductivity. Normally, Al-free Ta-doping Li7La3Zr2O12 garnet shows higher conductivity at room temperature compared to Al-doping garnet [13]. However, Ta substitution cannot densify the garnets, Al is generally co-doped occasionally or intentionally to enhance the sinterability of the material to achieve higher ionic conductivities [14]. More important, Ta-substituted Li7La3Zr2O12 garnet exhibits good interfacial stability toward LiCoO2, which makes this material system potential for solid-state lithium batteries [15]. Ga has been also substituted on the Li sites to stabilize cubic phase following the same approach with Al substitution [16]. Ga-doping Li7La3Zr2O12 garnet exhibits
significant high ionic conductivity of ~10-3 S cm-1 at room temperature [17]. The high mobility of Li+ in Ga-doping Li7La3Zr2O12 could be attributed to the preferential tetrahedral 24d location of Ga substituent, leading to a disorder of Li [18], and/or the low eutectic point of Li-Ga-O compounds improves the densification of the garnets through liquid phase at grain-boundaries [19]. The enhancement of density of lithium garnet will be definitely in favour of Li-ions conduction. We found alkaline-earth element substitution on Zr sites in lithium garnets may intrinsically enhance the sinterability [20,21]. It is different from the sinter aids that are always located at grain-boundaries and thus block Li-ions transport. In this paper, a multiple substitution approach is demonstrated where partial replacement of Li with Ga combined to partial replacement of Zr with Ta and Ba. First, Ta substitution on Zr sites gives access to lithium garnet stable with LiCoO2 cathode. Second, Ba substitution on Zr sites increases Li content that boosts other substitution on Li site and intrinsically enhance the sinterability of garnet. Third, we aim to optimize the ionic conductivity with Ga substitution on Li sites. Following this scheme, we synthesize a series of Ga/Ta/Ba-substituted garnets with the nominal formula Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) and study in detail the substitution influence. We also shed light on the capability of the multi-substituted garnet to alleviate lithium metal dendrites. 2. Experimental section 2.1. Synthesis LiOH•H2O (Aladdin, 99%), Ga(NO3)3•9H2O (Diyanghg, 99.99%), La2O3 (Aladdin,
99.9%, dried at 900oC for 12 h), ZrO2 (Aladdin, 99.99%), BaCO3 (Aladdin, ACS), and Ta2O5 (Aladdin, 99.5%) were used as raw materials and weighed in stoichiometric amounts
(15wt.%
LiOH•H2O
3xGaxLa3Zr1.55Ba0.05Ta0.4O12
excess
was
added)
to
reach
Li6.7
−
(x=0, 0.1, 0.15, 0.2). The chemicals were mixed and
ground in a zirconia milling vial for about 12 h under addition of ethanol and subsequently pressed into pellets. The precursor pellets were placed on a MgO pellet to avoid Al contamination from the crucibles and calcined at 900 oC for 6 h in alumina crucibles. The calcined pellets were again ground, pressed, and placed on a MgO pellet in alumina crucibles, and covered by additional garnet powder. The sintering procedure was first heated at 1250 oC for 10 min and subsequently cooled at 1150 oC for 12 h in dry O2. The resulting pellets were stored in a glovebox under Ar atmosphere for further characterization. 2.2. Structure characterization X-ray diffraction (XRD, Bruker, D2 PHASER, Cu-Kα) was conducted to detect the crystallographic structure of the samples. Scanning electron microscopy (SEM, Hitachi, Su8020) was examined on fracture surfaces of sintered pellets to study the microstructure. The general distribution of elements was checked using energy dispersive spectroscopy (EDS) analysis. 2.3. Electrochemical characterization AC impedance spectroscopy measurement was conducted on an Autolab PGSTAT302N Analyzer to determine the ionic conductivity of the materials. The polished pellet was pasted by Ag electrode on both surfaces and loaded into a
Swagelok cell in Ar-glovebox to exclude the adverse influence of air/moisture on conductivity measurement. The AC impedance spectroscopy measurements were performed by applying 10 mV in the frequency range of 1 MHz-0.1 Hz in the temperature range from -78 oC to 80 oC. Li/garnet/Li symmetric cell was fabricated to investigate the interfacial performance of sintering garnet against metallic lithium. The fresh Li foil was integrated with a polished pellet and sealed in CR2016 coin cell in Ar-filled glovebox. The symmetric cell was heated at 60 oC for 24 h before cycling. The galvanostatic cycling was first performed at 60 oC under constant current density of 0.01 mA cm-2 over 40 cycles using an Arbin LBT20083. Li plating/stripping was subsequently took place at ambient temperature and 0.01-0.5 mA cm-2 current densities (2 hours for each plating/stripping cycle) were sequentially applied.
3. Results and discussion Fig.
1
shows
XRD
patterns
3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0,
of
calcined
and
sintered
Li6.7
−
0.1, 0.15, 0.2) at 900 oC and 1150 oC, respectively.
As seen in Fig. 1a, all diffraction peaks of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) could be indexed as cubic garnet structure (PDF 45-0109, Space group Ia-3d) after calcined at 900 oC for 6 h, which will be in favour of the densification of lithium garnets in the following sintering. No detectable impurity phases could be observed for the samples after sintered at 1150
o
C for 12 h, except
Li6.1Ga0.2La3Zr1.55Ba0.05Ta0.4O12 shows slight amount of ZrO2 impurity as shown in Fig.
1b. The weak XRD diffraction of impurities in LLZO was also identifed previously [22,23]. Therefore, the Ga/Ta/Ba-multiple substitution results in single cubic garnet phase with x<0.2.
Fig. 1. Powder XRD patterns of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) Calcined at 900 oC. (b) Sintered at 1150 oC. JCPDS file # 45-109 was used for cubic lithium garnet.
To obtain more meaningful information, SEM was conducted to survey the cross-sectional morphological evolution of sintered Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). As seen in Fig. 2, several key points can be made. First, partial proportion of the fracture mode for Ga-free sample is intergranular, on the other hand, most proportion of the fracture mode for Ga-doping samples is transgranular, implying high grain-boundary strength that would result in low grain-boundary resistances. Second, the Ga-free sample exhibits clear grain boundary, whereas good intergranular connectivity is observed for the Ga-doping samples that would lead to low electrical resistances. Third, the majority of voids are intergranular, there are very
few intragranular voids. Fourth, the grains are fairly isotropic shape with an average intercept grain size of ~5 µm. Therefore, the Ga/Ta/Ba-multiple substitution results in more favorable microstructure, due mainly to the intrinsic enhancement of sinterability by the addition of Ga.
Fig. 2. Morphological evolution of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) x=0. (b) x=0.1. (c) x=0.15. (d) x=0.2. The EDS elemental mapping of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) in Fig. 3 reveals that Zr, Ta, La, and Ba are uniformly distributed. However, the distribution of Ga is distinct. As seen in Fig. 3b, the distribution of Ga in Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12 is homogeneous. As seen in Fig. 3c&d, few Ga-rich phase accumulates at the grain boundary in Li6.25Ga0.15La3Zr1.55Ba0.05Ta0.4O12 and Li6.15Ga0.2La3Zr1.55Ba0.05Ta0.4O12. It implied that the grain-boundary Ga-rich phase are amorphous because it was not detected by the X-ray diffraction as seen in Fig. 1.
Matsuda et al. also suggested that small amounts of amorphous Li-Ga-O oxides presented at the grain boundaries of Ga-doping lithium garnets [19].
Fig. 3. EDS elemental mapping images of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) x=0. (b) x=0.1. (c) x=0.15. (d) x=0.2.
The ionic conductivity of Li6.7−3xGaxLa3Zr1.55Ba0.05Ta0.4O12 was measured by AC impedance. Fig. 4a shows the Nyquist curves from the AC impedance measurement at ambient temperature. Only an approximate straight-line without any clear semi-circles was revealed. As Ag-blocking electrode was used in the measurement, the appearance of low-frequency tail declares the material which is a single lithium-ion conductor. The disappearance of high-frequency semi-circles implies a feature of material with slight grain-boundary impedance. Several studies done by Kotobuki et al. [24] and Allen et al. [9] support the fact that this disappearance of high-frequency semi-circles
indicates the grain-boundary impedance is ignorable in comparison to the grain impedance, therefore, the intercept of the straight line on the real axis represents the sum response of grain and grain boundary, as the total conductivity of the material. The total Li-ion conductivity rather than grain conductivity is significant important for a practical device. As a result of multi substitution, the total conductivity enhances from 8.3×10-4 S cm-1 for the Ga-free sample (Li6.7La3Zr1.55Ba0.05Ta0.4O12) to an optimal
value
of
1.02×10-3
S
cm-1
for
the
Ga-containing
sample
(Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12), illustrating a positive effect of Ga/Ta/Ba-multiple substitution. It is noted that this conductivity value of ~10-3 S cm-1 is highly repeatable as the careful control of preparation technology, i.e., MgO pellet was used to isolate the alumina crucibles and samples during both the calcining and sintering procedures. In addition, the AC impedance was examined in Ar-filled atmosphere that eliminates the negative influence of CO2 and H2O on the lithium garnets. In view of the high conductivity value of 10-3 S cm-1, the dopant Ga shows its advantage to improve the ionic conductivities of lithium garnets, although the four samples show very close ionic conductivities as seen in the Fig. 4a and the inset. Fig. 4b shows the Arrhenius curves as a function of total Li-ion conductivity of Li6.7-3xGaxLa3Zr1.55Ba0.05Ta0.4O12 and temperature. As seen, the total ionic conductivity of Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12 is the highest at all temperatures, and the activation energy is calculated from Arrhenius equation and the slope of the line in the temperature range of 195-353 K. The activation energy is around 0.38-0.40 eV that is comparable
to
the
other
reported
Ta-Al-substituted
garnet
Li6.4625Al0.1375La3Zr1.875Ta0.125O12 (0.36 eV, 1.03×10-3 S cm-1) [14].
Fig.
4.
Electrochemical
impedance
characterization
of
Li6.7-3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2). (a) Nyquist curves. (b) Arrhenius curves. The inset is a local view of Nyquist curves.
A challenge of Li metal being used in practical device is the formation of lithium dendrites that may cause sudden short-circuit giving rise to dangerous battery fire/explosion. Garnet-type inorganic solid electrolytes are expected to block Li dendrites by means of their dense microstructure and high shear moduli, although short circuit still happens. The high solid-solid interfacial impedance between garnet-type electrolyte and Li metal anode is another great challenge. Fresh Li foil was
attached
on
both
surfaces
of
sintered/polished
pellet
(Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12) and sealed in CR2016 coin-cell. As seen in Fig. 5b, the as-prepared symmetric cell exhibits very large impedance due to the interfacial impedance between Li metal and solid electrolyte, the interfacial resistance is around 6.5×106 Ω cm2, whereas, the interfacial resistance significantly decreases to be around
4500 Ω cm2 after activation (i.e., cycling for 80 h at elevated temperature of 60 oC and at current density of 0.01 mA cm-2). The mild interfacial resistance between lithium metal and solid electrolyte after activation would be in favour of the use of lithium metal as an anode. Fig. 5a shows the extended cycling result of the symmetrical cell under different current density of 0.01 mA cm-2, 0.05 mA cm-2, 0.1 mA cm-2, and 0.5 mA cm-2, respectively. The symmetrical cell demonstrates very flat and stable plating/stripping profiles with a small overpotential of 30 mV during the initial 0.01 mA cm-2 cycling (Fig. 5a and the zoom-in profiles). The random voltage fluctuation is possibly due to the current fluctuation, resulting from the low current density and current accuracy. The random voltage fluctuation disappears as the current density is stable at 0.05 mA cm-2. Smooth voltage profiles are observed when the current density is boosted to 0.05 mA cm-2 and 0.1 mA cm-2, and the overpotential is enlarged from 30 mV to 50 mV and 70 mV, respectively. When the current density is further boosted from 0.1 mA cm-2 to 0.5 mA cm-2, a noisy voltage fluctuation is observed during initial dozens of cycles, which is probably owing to the dynamic interfacial contact area or interim break of the sequential electron migration pathway resulted from the fast lithium deposition/dissolution [25]. This phenomenon is generally observed in symmetrical cell with bare lithium and untreated inorganic solid electrolytes cycling at high current densities [26]. The voltage response tends towards stabilization after initial noisy fluctuation, indicating stable lithium plating and stripping. New but weak voltage fluctuation could appear again when the cycling is suddenly interrupted following renewing the measurement (300th cycle). Again, stable voltage response is
gradually produced after voltage oscillation and the voltage profiles are more flat in the successive cycling, indicating ameliorative interfacial contact between lithium metal and solid electrolyte through the long-term lithium plating and stripping. The subsequent electrochemical impedance spectrum of the 400th cycling symmetrical cell exhibits a small partial high-frequency semicircle and a low-frequency depressed semicircle, which can be assigned to the total resistance of garnet electrolyte and charge transfer resistance of the interfaces, respectively. Therefore, a very low interfacial resistance of ~25 Ω cm2 can be obtained after the long-term cycling, which can be comparable to those of symmetrical cells with complex surface/interface modification [27]. Therefore, our study proves that the multi-substituted garnet-type electrolytes can achieve stable and long-term lithium plating and stripping.
Fig. 5. Electrochemical characterizations of Li/garnet Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12
/Li symmetric cell at ambient temperature. (a) Cyclic performance at various current densities. (b) Impedance spectra at different cell status.
4. Conclusion In summary, a multi-substituted strategy is proposed to investigate the garnet-type solid electrolytes, where the substitution of Zr by Ta/Ba targeted at stabilizing the cubic garnet phase and the substitution of Li by Ga at adjusting the Li contents and enhancing the intrinsic sinterability. Cubic Li6.7-3xGaxLa3Zr1.55Ba0.05Ta0.4O12 (x=0, 0.1, 0.15, 0.2) was synthesized and an optimal Ga proportion of x=0.1 per formula unit leads to the highest ionic conductivity of 1.02×10-3 S cm-1 at ambient temperature. Importantly, the multi-substituted Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12 exhibits good capability
of
alleviating
lithium
metal
dendrites.
A
symmetrical
Li/Li6.4Ga0.1La3Zr1.55Ba0.05Ta0.4O12/Li cell exhibits good cycling stability for 800 h as the current density increased in steps from 0.01 mA cm-2 to 0.5 mA cm-2, and ultralow interfacial charge transfer resistance of ~25 Ω cm2 upon long-term cycling.
Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 51702030, No. 11632004, and No. U1864208), Shanghai
Aerospace
Science
and
Technology
Innovation
Fundation
(No.
SAST2017-137), Chongqing University, the Fundamental Research Funds for the Central Universities (No. 2018CDXYHK0016), and the Key Program for
International Science and Technology Cooperation Projects of the Ministry of Science and Technology of China (No. 2016YFE0125900).
References [1] P. Parikh, M. Sina, A. Banerjee, X. Wang, M.S. D’Souza, J.-M. Doux, E.A. Wu, O.Y. Trieu, Y. Gong, Q. Zhou, K. Snyder, Y.S. Meng, Role of polyacrylic acid (PAA) binder on the solid electrolyte interphase in silicon anodes, Chem. Mater. 31 (2019) 2535-2544. [2] K. Kimura, K. Wagatsuma, T. Tojo, R. Inada, Y. Sakurai, Effect of composition on lithium-ion conductivity for perovskite-type lithium strontium tantalum zirconium-oxide solid electrolytes, Ceram. Int. 42 (2016) 5546-5552. [3] F. Zheng, M. Kotobuki, S.F. Song, M.O. Lai, L. Lu, Review on solid electrolytes for all-solid-state lithium-ion batteries, J. Power Sources 389 (2018) 198-213. [4] X. Huang, Z. Song, T. Xiu, M. Badding, Z.Y. Wen, Sintering, micro-structure and Li+ conductivity of Li7-xLa3Zr2-xNbxO12/MgO (x=0.2-0.7) Li-garnet composite ceramics, Ceram. Int. 45 (2019) 56-63. [5] V. Thangadurai, S. Narayanan, D. Pinzaru, Garnet-type solid-state fast Li ion conductors for Li batteries: critical review, Chem. Soc. Rev. 43 (2014) 4714-4727. [6] R. Jalem, Y. Yamamoto, H. Shiiba, M. Nakayama, H. Munakata, T. Kasuga, K. Kanamura, Concerted migration mechanism in the Li ion dynamics of garnet-type Li7La3Zr2O12, Chem. Mater. 25 (2013) 425-430.
[7] H. Xie, J.A. Alonso, Y. Li, M.T. Fernandez-Díaz, J.B. Goodenough, Lithium distribution in aluminum-free cubic Li7La3Zr2O12, Chem. Mater. 23 (2011) 3587-3589. [8] E.J. Cussen, Structure and ionic conductivity in lithium garnets, J. Mater. Chem. 20 (2010) 5167-5173. [9] Z. Cao, X. Cao, X. Liu, W. He, Y. Gao, J. Liu, J. Zeng, Effect of Sb-Ba codoping on the ionic conductivity of Li7La3Zr2O12 ceramic, Ceram. Int. 41 (2015) 6232-6236. [10] Z.L. Hu, H. Liu, H. Ruan, R. Hu, Y. Su, L. Zhang, High Li-ion conductivity of Al-doped Li7La3Zr2O12 synthesized by solid-state reaction, Ceram. Int. 42 (2016) 12156-12160. [11] H. El-Shinawi, G.W. Paterson, D.A. MacLaren, E.J. Cussen, S.A. Corr, Low-temperature densification of Al-doped Li7La3Zr2O12: a reliable and controllable synthesis of fast-ion conducting garnets, J. Mater. Chem. A 5 (2017) 319-329. [12] G. Vardar, W.J. Bowman, Q. Lu, J. Wang, R.J. Chater, A. Aguadero, R. Seibert, J. Terry, A. Hunt, I. Waluyo, D.D. Fong, A. Jarry, E.J. Crumlin, S.L. Hellstrom, Y.M. Chiang, B. Yildiz, Structure, chemistry, and charge transfer resistance of the interface between Li7La3Zr2O12 Electrolyte and LiCoO2 Cathode, Chem. Mater. 30 (2018) 6259-6276. [13] X. Tong, V. Thangadurai, E.D. Wachsma, Highly conductive Li garnets by a multielement doping strategy, Inorg. Chem. 54 (2015) 3600-3607.
[14] Y. Matsuda, Y. Itami, K. Hayamizu, T. Ishigaki, M. Matsui, Y. Takeda, O. Yamamoto, N. Imanishi, Phase relation, structure and ionic conductivity of Li7-x-3yAlyLa3Zr2-xTaxO12, RSC Adv. 6 (2016) 78210-78218. [15] F. Han, J. Yue, C. Chen, N. Zhao, X. Fan, Z. Ma, T. Gao, F. Wang, X. Guo, C.S. Wang, Interphase engineering enabled all-ceramic lithium battery, Joule 2 (2018) 1-12. [16] S. Afyon, F. Krumeichb, J.L.M. Rupp, A shortcut to garnet-type fast Li-ion conductors for all-solid state batteries, J. Mater. Chem. A 3 (2015) 18636-18648. [17] J.F. Wu, E. Chen, Y. Yu, L. Liu, Y. Wu, W.K. Pang, V.K. Peterson, X. Guo, Gallium-doped Li7La3Zr2O12 garnet-type electrolytes with high lithium-ion conductivity, ACS Appl. Mater. Interfaces 9 (2017) 1542-1552. [18] C. Bernuy-Lopez, W. Manalastas, J.M.L. Amo, A. Aguadero, F. Aguesse, J.A. Kilner, Atmosphere controlled processing of Ga-substituted garnets for high Li-ion conductivity ceramics, Chem. Mater. 26 (2014) 3610-3617. [19] Y. Matsuda, A. Sakaida, K. Sugimoto, D. Mori, Y. Takedaa, O. Yamamoto, N. Imanishi, Sintering behavior and electrochemical properties of garnet-like lithium conductor Li6.25M0.25La3Zr2O12 (M: Al3+ and Ga3+), Solid State Ionics 311 (2017) 69-74. [20] S.F. Song, D. Sheptyakov, A.M. Korsunsky, H.M. Duong, L. Lu, High Li ion conductivity in a garnet-type solid electrolyte via unusual site occupation of the doping Ca ions, Mater. Design 93 (2016) 232-237. [21] S.F. Song, M. Kotobuki, F. Zheng, C. Xu, Y. Wang, W. Z. Li, N. Hu, L. Lu, Roles
of alkaline earth ions in garnet-type superionic conductors, ChemelectroChem 4 (2017) 266-271. [22] T. Yang, Y. Li, W. Wu, Z. Cao, W. He, Y. Gao, J. Liu, G. Li, The synergistic effect of dual substitution of Al and Sb on structure and ionic conductivity of Li7La3Zr2O12 ceramic, Ceram. Int. 44 (2018) 1538-1544. [23] D. Mori, K. Sugimoto, Y. Matsuda, K. Ohmori, T. Katsumata, S. Taminato, Y. Takeda, O. Yamamoto, N. Imanishi, Synthesis, structure and ionic conductivity of garnet like lithium ion conductor Li6.25+xGa0.25La3-xSrxZr2O12, J. Electrochem. Soc. 166 (3) (2019) A5168-A5173. [24] M. Kotobuki, H. Munakata, K. Kanamura, Y. Sato, T. Yoshida, Fabrication of all-solid-state rechargeable lithium-ion battery using mille-feuille structure of Li0.35La0.55TiO3, J. Power Sources 196 (2011) 6947−6950. [25] G.T. Hitz, D.W. McOwen, L. Zhang, Z. Ma, Z. Fu, Y. Wen, Y. Gong, J. Dai, T.R. Hamann, L.B. Hu, E.D. Wachsman, High-rate lithium cycling in a scalable trilayer Li-garnet-electrolyte architecture, Mater. Today 22 (2019) 50-57. [26] J. Duan, W. Wu, A. Nolan, T. Wang, J. Wen, C. Hu, Y. Mo, W. Luo, Y.H. Huang, Lithium-graphite paste: an interface compatible anode for solid-state batteries, Adv. Mater. 31 (2019) 1807243. [27] X.B. Cheng, C.Z. Zhao, Y. Yao, H. Liu, Q. Zhang, Recent advances in energy chemistry between solid-state electrolyte and safe lithium-metal anodes, Chem 5 (2019) 74-96.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: