In-situ formed shields enabling Li2CO3-free solid electrolytes: A new

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In-situ formed shields enabling Li2CO3-free solid electrolytes: A new route to uncover the intrinsic lithiophilicity of garnet electrolytes for dendrite-free Li-metal batteries Jianfang Wu, Bo-Wei Pu, Da Wang, Siqi Shi, Ning Zhao, Xiangxin Guo, and Xin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18356 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Materials & Interfaces

In-situ Formed Shields Enabling Li2CO3-Free Solid Electrolytes: A New Route to Uncover the Intrinsic Lithiophilicity of Garnet Electrolytes for Dendrite-Free Li-Metal Batteries Jian-Fang Wu1, Bo-Wei Pu2, Da Wang2, Si-Qi Shi2*, Ning Zhao3, Xiangxin Guo3, Xin Guo1* 1Laboratory

of Solid State Ionics, School of Materials Science and Engineering, Huazhong

University of Science and Technology, Wuhan 430074, P.R. China 2 School

of Materials Science and Engineering, Shanghai University, Shanghai 200444, P.R.

China 3College

of Physics, Qingdao University, Qingdao 266071, P.R. China

Abstract: Introduction of inorganic solid electrolytes is believed to be an ultimate strategy to dismiss dendritic Li in high-energy Li-metal batteries (LMBs), and garnettype Li7La3Zr2O12 (LLZO) electrolytes are impressive candidates. However, the current density for stable Li plating/stripping in LLZO is still quite limited. Here we create insitu formed Li-deficient shields by the high-temperature calcination at 900 °C. By this novel process, the formation of Li2CO3 on LLZO is restrained, and then we successfully obtain Li2CO3-free LLZO after removing the Li-deficient compounds. Without any surface modification, Li2CO3-free LLZO shows an intrinsic “lithiophilicity” characteristic. The contact angles of metallic Li on LLZO garnets are assessed by the first-principle calculation to confirm the “lithiophilicity” characteristic of LLZO electrolytes. The wetting of metallic Li on the Li2CO3-free LLZO surface leads to a continuous and tight Li/LLZO interface, resulting in an ultra-low interfacial resistance of 49 Ω cm2 and a homogenous current distribution in the charge/discharge processes of LMBs. Consequently, the current density for the stable Li plating/stripping in LLZO 1

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increases to 900 μA cm-2 at 60 C, one of the highest current density for LMBs based on garnet-type LLZO electrolytes. Our findings not only offer insight into the “lithiophilicity” characteristics of LLZO electrolytes to suppress dendritic Li at high current densities, but also expand the avenue towards high-performance, safe and longlife energy storage systems.

Keywords: solid electrolytes, dendritic Li, interfaces, Li7La3Zr2O12, Li-metal battery

________________________ * Authors to whom correspondence should be addressed. Tel: +86-27-87559804; Fax: +86-27-87559804; E-mail: [email protected]; [email protected]

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1. Introduction Li-metal batteries (LMBs) are considered as the “Holy Grail” of energy storage systems, and vast improvement in energy density could be expected, due to the extremely high theoretical specific capacity (3860 mAh g-1), and the lowest negative electrochemical potential (-3.04 V versus the standard hydrogen electrode) of metallic Li [1]. However, LMBs suffer from a poor cycle life, unsatisfied Coulombic efficiency, and encounter severe safety issues because of uncontrollable formation and growth of Li dendrites [2]. Metallic Li is reactive with most organic electrolytes, causing the formation of a solid electrolyte interphase (SEI) layer on the Li surface; the brittle SEI layer cannot tolerate the volume expansion of metallic Li during plating/stripping, thus it continuously breaks and recovers; the current distribution on metallic Li becomes uneven, and Li dendrites nucleate and grow at the sites with locally high current density [3].

In batteries with high current densities proportioning to its energy densities, the

formation of Li dendrites can be sped up. Extensive studies have been conducted to prevent Li dendrites in LMBs [1, 3-4], including employing polymer electrolytes[5-6], ionic liquids[7], concentrated electrolytes[8] or additives[9-10], artificial SEI layers[11], mechanical protective layers[12-13], nanostructure design[14], homogenous Li-flux through selective deposition[15], and others[16]. In principle, inorganic solid electrolytes with high shear moduli (>1.8 shear moduli of Li) are expected to suppress the formation of dendritic Li

[17],

but many of

them are hindered from LMBs for the low lithium-ion conductivity or the deteriorated stabilities. Fortunately, cubic garnet Li7La3Zr2O12 (LLZO) and its variants show the 3



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highest lithium-ion conductivity of ~10-3 S cm-1 in oxide-based solid electrolytes and extremely good stability against metallic Li [18-20], thus having potential applications in LMBs. Normally, point-to-point contact dominates the LLZO and metallic Li interface, and Li2CO3 on the surface hinders the wetting of metallic Li on the LLZO surface, resulting in insufficient contact between LLZO and metallic Li, and an inhomogeneous current distribution during Li plating/stripping happens, therefore, Li dendrites nucleate at the interface. Solid electrolytes are polycrystalline materials, including grain boundaries, pores, and cracks, along which dendritic Li can grow[21-25]; as a result, the threshold current density for stable Li plating/stripping is limited. Surface modification, enhancing interfacial contact between LLZO and metallic Li, was employed to suppress dendritic Li formation and growth [20, 22, 26-33]. With a ~5 nm thick Al2O3 deposited on the surface of LLZO by atom layer deposition, the current density increased to 200 μA cm-2 at room temperature

[20].

Notably, all the chosen

materials for modifications can easily alloy with metallic Li, which makes metallic Li wet LLZO, i.e. the contact angle θ < 90 º , thus leading to a homogenous current distribution and a high threshold current density. These works come from the fact that metallic Li cannot wet the Li2CO3-coated LLZO surface. If metallic Li can wet the Li2CO3-free LLZO surface, a conformal interface between them, a homogenous current distribution and high threshold current density can be expected. However, it is very difficult to obtain Li2CO3-free LLZO solid electrolytes. Sakamoto et al.

[34]

tried to

remove Li2CO3 on the LLZO surface by polishing and calcination at 400~500 °C, but they observed contact angles > 95 °, therefore, metallic Li did not wet the LLZO 4


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electrolyte, which is due to the residual Li2CO3 and LiOH on the LLZO surface. In a very recent work of Goodenough

[33],

the introduction of carbon, which reacts with

Li2CO3 on the LLZO surface in Ar at 700 °C, eliminated Li2CO3; as a result, metallic Li wetted LLZO and a current density of 400 μA cm-2 were achieved at 65 °C. However, it is noted that carbon can alloy with metallic Li easily. Therefore, the intrinsic surface property, i.e. wettability of Li2CO3-free LLZO to metallic Li, is still a pending question. In this work, we tuned the surface chemistry of solid electrolytes and demonstrated their intrinsic “lithiophilicity”. We created Li-deficient compounds on LLZO by the high-temperature calcination at 900 °C, which inhibited the formation of Li2CO3 on the LLZO surface, and after removing the Li-deficient compounds, Li2CO3-free LLZO was obtained, providing us a perfect platform to answer the fundamental question: whether LLZO garnets are intrinsically “lithiophilicity”? Our results show that the contact angles between Li2CO3-free garnets and metallic Li are below 90 °C and vary with different dopants, which are further verified by the first-principle calculations. The method to realize the wetting of metallic Li on LLZO in this work is more feasible and cost-effective than the surface modification. The wetting of metallic Li on the surface of the LLZO electrolyte produces continuous and tight contact between them and a low interfacial resistance of 49 Ω cm2, which facilitates the suppression of dendritic Li in the LLZO electrolyte, and the current density for stable Li plating/stripping increases to 900 μA cm-2 at 60 °C. In addition to the LLZO materials, this work is also beneficial to other oxide-based ion conductors, because it is easy for Li+ ions to leak out from the highly conductive electrolytes to form “lithiophobic” Li2CO3. 5



2. Experimental Section Preparation of solid electrolytes. Li6.25Ga0.25La3Zr2O12 (Ga-LLZO) powder was prepared via a solid-state reaction by mixing Li2CO3, Ga2O3, La2O3 and ZrO2 in stoichiometric ratio but with 10 mol% Li2CO3 in excess. Ga-LLZO electrolyte, with a relative density of 98.8 %, was obtained by hot-pressing in Ar at 1100 °C for an hour in a graphite die under 10 MPa pressure. The samples were afterwards calcined at 900 °C in air for 24 h. Finally, the samples were polished in a glove-box filled with pure Ar using SiC abrasive papers. Other garnet-type electrolytes, such as Li6.4Al0.2La3Zr2O12 (Al-LLZO) and Li6.4La3Zr1.4Ta0.6O12 (Ta-LLZO), were prepared similarly. Wetting of molten Li on the LLZO electrolyte. Metallic Li was molten in a stainlesssteel container on a hot plate at 350 °C for 10 mins in a glove-box filled with pure Ar. A drop of molten Li was poured on one surface of the heated LLZO pellet, and then waited for a few minutes until the molten Li spread out on the surface. Battery assembly. Via the method in the wetting experiment, symmetric Li/GaLLZO/Li cells were assembled. All-solid-state Li/LiFePO4 batteries were assembled as follows: Firstly, poly (vinylidene fluoride) (PVDF) and Li(CF3SO2)2N (LiTFSI) were dissolved in N-methyl2 pyrrolidon (NMP) by magnetic stirring; carbon coated-LiFePO4 and Ketjen black were mixed by grinding in an agate mortar for one hour, and then it was transferred to the above NMP solution and stirred for another 24 h. The mass ratio of LiFePO4, Ketjen black and PVDF is 75:15:10, and the mass of LiTFSI is 80 % of that of the LiFePO4. 6


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The obtained slurry was coated on the side of the electrolyte pellet without metallic Li. After being pressed with a stainless-steel plate by a pressure of 10 N, the solid electrolyte with electrode coating was dried in vacuum at 80 °C for 12 h to remove NMP and trace moisture. The loading masses of the electrodes were about 2 mg cm-2. Solid Li/LiFePO4 batteries were assembled as follows: Cathode pastes with 70 % LiFePO4, 20 % Ketjen black and 10 % PVDF were prepared on stainless-steel plates. Then a paste was pressed on the side of the electrolyte pellet without metallic Li to get a battery, and 50 μL Li+-containing ionic liquid (1 mmol LiTFSI in 1 ml [EMIM][TFSI]) was added to enhance the Li-ion conduction between the cathode and the solid electrolyte. The LLZO pellets for battery tests had a diameter of ~15 mm and a thickness of ~0.8 mm. All procedures were conducted in an Ar filled glovebox with O2 < 0.1 ppm and H2O < 0.1 ppm. Material Characterizations. Scanning electron microscopy (SEM, Sirion 200) images of the cross section of the LLZO ceramics and the Li/LLZO interface were recorded. Crystalline phases of the ceramics were investigated through X-ray diffraction (XRD, XRD-7000S). Raman spectroscopy (LabRAM HR800) was conducted on the surface of the LLZO samples, and the samples were transferred in a sealed glass-bottle in the glove-box. X-ray photoelectron spectroscopy (XPS, AXIS-ULTRA DLD-600W) was conducted on the LLZO samples to check their surface chemical compositions. The 7Li NMR spectra were gathered on a Bruker Avance III 400 NMR spectrometer, and the chemical shifts of 7Li were calibrated by using a 1M LiCl solution. Alternating current (AC) impedance measurements were conducted in the 7



temperature range of 25 to 60 °C using a Solartron 1260 impedance and gain-phase analyzer in the frequency range of 1 to 5 × 106 Hz at an amplitude of 50 mV. Li-ion blocking Ag electrodes were used. The samples were heated at the desired temperatures for an hour before each measurement. The dimensions of the sample for the AC impedance test were 10×10×1 mm3. Galvanostatic charge and discharge performances of Li/Ga-LLZO/Li cells and LiFePO4/Ga-LLZO/Li batteries were investigated using a LANHE CT2001A charge/discharge system (Wuhan LAND Electronics Co.). The cutoff voltages for the LiFePO4/Ga-LLZO/Li battery were set to be 4.0 and 2.8 V. Before the charge/discharge of the all-solid-state battery, it was heated at 60 °C for 24 h to reach thermal equilibrium. Theoretical calculations. First-principle calculations of the surface energy and the interfacial energy were carried out based on the projector-augmented wave (PAW) method of the density functional theory (DFT), and conducted with the Vienna Ab initio Simulation Package (VASP)

[35-37].

The exchange-correlation part of the density

function was treated using the generalized gradient Perdew-Burke-Ernzerhof approximation [38]. The plane wave cutoff energy and the Monkhorst-Pack k-point mesh was set to be 600 eV and 3×3×1, respectively, for the Li (001)/garnet (001) interfaces. The self-consistency convergence criterion for the energy was set to be 10−4 eV, and geometry relaxation was converged when all forces were less than 0.05 eV Å-1. The Li/garnet interface models were constructed by Li (001) slab and garnet (001) slab, which are the low-energy surfaces. The surface energy of Li (001) containing six atomic layers was calculated to be 0.447 J m-2 with its structure built based on ICSD8


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642110, and the garnet structures were constructed based on the neutron diffraction results reported in literature [39]. Structural details after optimization are listed in Table S1. The Li (001) surface was modeled as a (3×3) slab, of which the in-plane lattice translation vectors is 13.065 Å, approaching that of garnet slabs (~ 13 Å). The coherent interface approximation was applied, in which the soft Li (001) (3×3) surface slab (108 Li) is strained to match the dimensions of garnet (001) (1×1) surface slabs (8 Li7xM0.125La3Zr2-yO12,

M = Ga, Al, x = 0.375, y = 0; M = Ta, x = 0.125, y = 0.125). A

vacuum region of 15 Å was included in the interface supercell. As proposed by Sharafi et al.[34], the back half atoms farthest away from the interface of each slab were fixed at their bulk-like positions, and thus only one interface was contained in each supercell. Interface configurations are show in Figure 3 and Figure S3. The calculation of the contact angle between Li (001) and LLZO variant (001) is based on the Young-Dupré equation [40]: (1)

𝑊ad = 𝜎Li(1 + cos 𝜃)

where Wad is the interfacial work of adhesion, Li is the surface energy of Li and θ is the contact angle. Wad can be evaluated according to [41]:

Wad 

Einterface  ELi-slab  Egarnet-slab S

(2)

where Einterface, ELi-slab and Egarnet-slab refer to the total energies of the Li (001)/garnet (001) interface supercell, isolated Li (001) and garnet (001) surface slabs, respectively, and S refers to the interfacial area.

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3. Results and discussion Li2CO3-free LLZO solid electrolytes. Highly densified and conductive GaLLZO samples with coarse grains were prepared by hot pressing. The microstructure, crystalline phase and conductivity of the Ga-LLZO electrolyte were characterized, and results are shown in Figure 1 and Figure S1. As shown in Figure 1a, the electrolyte is well sintered without any visible pores, and a high density of 98.8% is demonstrated by the Archimedes method. Since the grain size of the sample is tens of micrometers, the number of the grain boundaries in the direction perpendicular to the surface is low. Li2CO3 easily forms on the LLZO surface in humid air (Figure 1b) [33, 42]; to avoid the formation of Li2CO3 on the LLZO surface, we calcined the LLZO electrolytes at a high temperature of 900 °C. After the high-temperature calcination at 900 °C in air for 24 h, Li-deficient lanthanum-zirconium compounds were formed in-situ on the GaLLZO surface (Figure S1) due to the decomposition of LLZO; the Li-deficient compounds protect the inner cubic garnet from Li2CO3 (Figure 1c and S1). After removing the Li-deficient compounds, cubic garnet without any impurity phase was obtained (Figure 1d). The conductivity of the electrolyte is 1.24 mS cm-1 at 25 °C and it increases to 2.71 mS cm-1 when temperature increases to 60 °C, as calculated from the AC impedance spectra (Figure 1e), and an activation energy of 0.2 eV is determined from the Arrhenius plot (Figure 1f). According to our previous works

[43-44],

the Ga-LLZO

solid electrolyte is a pure ionic conductor with a lithium-ion transference number approximating to 1. 10


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To confirm that there is not any Li2CO3 on the Ga-LLZO surface, XPS and Raman analyses were conducted. Figure 2a shows the XPS results of O 1s and C 1s on the GaLLZO surface. The characteristic peak for Li2CO3 at around 290 eV does not show up in the C 1s spectrum, and the peaks in the spectrum can be indexed to the absorbed carbon on the LLZO surface

[34, 45].

In the Raman spectrum (Figure 2b), all the peaks

can be indexed to cubic garnet LLZO. Therefore, there is no Li2CO3 on the polished Ga-LLZO, and Li2CO3-free LLZO is a perfect platform to demonstrate the intrinsic “lithiophilicity” characteristic of LLZO electrolytes. Intrinsic “lithiophilicity” characteristic of LLZO electrolytes. The wetting process of molten metallic Li on the Li2CO3-free Ga-LLZO surface is shown in Figure 3a. Initially, the contact angle (θ) of molten metallic Li on the Ga-LLZO surface is about 56º. After being heated at 350 °C for 2 min, the molten metallic Li spreads out and the contact angle gets smaller and approaches ~0º. According to the SEM images of the cross-section of the Li/LLZO interface in Figure 3b, metallic Li is conformally coated on Ga-LLZO, and a continuous and tight interfacial contact is observed, demonstrating the “lithiophilicity” of the Ga-LLZO electrolyte. The same phenomenon is also observed for other LLZO variants, such as Li6.4Al0.2La3Zr2O12 (Al-LLZO) and Li6.4La3Zr1.4Ta0.6O12 (Ta-LLZO), as shown in Figure S2, and their initial contact angles are about 59.5º and 65º, respectively. In previous works

[20, 22, 27-29, 32-33],

most

researchers claimed a “lithiophobicity” characteristic of the LLZO electrolytes and the “lithiophilicity” can only be realized with the help of surface modifications, but they

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attributed the “lithiophobicity” characteristic to Li2CO3 on the LLZO surface. In this work, we demonstrate that the “lithiophilicity” characteristic is intrinsic to LLZO. To further verify the “lithiophilicity” of the LLZO garnets, we conducted firstprinciple calculation to obtain the contact angles of metallic Li on the LLZO garnets, and the results are presented in Figure 3c and d, Figure S3 and Table S2. The contact angles

for

Li/LLZO,

Li/Ga-LLZO

(Li6.625Ga0.125La3Zr2O12),

Li/Al-LLZO

(Li6.625Al0.125La3Zr2O12) and Li/Ta-LLZO (Li6.875La3Zr1.875Ta0.125O12) interfaces are about 73°, 57°, 53° and 68°, respectively, which are smaller than 90°, manifesting that metallic Li wetting the Li2CO3-free LLZO electrolytes is a general phenomenon, in spite of different compositions. We can also regulate the surface chemistry of garnets by doping. Moreover, the calculated contact angles are quite consistent with the experimental observations in Figure 3(a) and Figure S2. Performances

of

symmetric

Li/LLZO/Li

cells.

The

“lithiophilicity”

characteristic of the LLZO electrolyte makes a continuous and tight contact between metallic Li and the electrolyte, which facilitates the lithium ion transport across the interface, leading to a low interfacial resistance. The resistance of the symmetric Li/LLZO/Li cell was investigated by the AC impedance spectrometer at 25 ºC, and the result is shown in Figure 4a. The Li/LLZO interfacial resistance is calculated by subtracting the electrolyte resistance from the total cell resistance, dividing by two, and then normalizing to the sample surface area. The total resistance of the symmetric cell is 233 Ω cm2, the resistance of the electrolyte is calculated from the lithium-ion conductivity and the dimension of the sample, and then the Li/LLZO interfacial 12


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resistance is determined to be 49 Ω cm2. Many researchers measured the interfacial resistances between metallic Li and LLZO electrolytes

[20, 22, 27-30, 32-33, 46-49],

but their

reported values are far larger than 100 Ω cm2 (Figure 4b). The existence of Li2CO3 on the LLZO surface leads to a poor interfacial contact and a high interfacial resistance [46, 50].

Han et al. decreased the Li/LLZO interfacial resistance to a very low level of 1 Ω

cm2 with a 5 nm thick Al2O3 layer deposited on the LLZO electrolyte amorphous Si

[22],

ZnO

[27],

Ge

[29]

and Al

[28],

[20].

With

one could decrease the Li/LLZO

interfacial resistance to ~100 Ω cm2 as well (Figure 4b). With the aid of carbon, the Li/LLZO interfacial resistance was reduced to 28 Ω cm2 and 105 Ω cm2 by Goodenough [33] and Yang [32], respectively. However, it has to be pointed out that we achieve a low interfacial resistance of 49 Ω cm2 without any surface modification. Wetting Ga-LLZO by metallic Li leads to a low interfacial resistance as well as a homogenous current distribution under galvanostatic cycling, thus the capability of the Ga-LLZO electrolyte to suppress the formation of dendritic Li is enhanced. Figure 4c exhibits cycling at room temperature and under different current densities of 100, 200 and 300 μA cm-2 in both directions for 200 h. The corresponding voltages are stable under a constant current, but increase with increasing current density, which indicates no formation of dendritic Li in the solid electrolyte. During the cycling process, the cell resistance (Figure 4d) decreases after 55 cycles and then keeps stable, which is related to the surface activation process

[27].

The Li/Ga-LLZO interfacial resistance after the

surface activation process decreases to 35 Ω cm2. Impressively, when the working temperature goes up to 60 ºC, the current density for the stable Li plating/stripping in 13



Ga-LLZO increases to 900 μA cm-2 (Figure 4e), which is one of the highest values for ceramic solid electrolytes. To verify no formation of dendritic Li in the LLZO sample after cycling, the sample was carefully polished to dislodge the Li electrodes, and then sealed in a quartz tube for static 7Li NMR test. In Figure 4f, only a narrow signal at around 0 ppm corresponding to lithium ions in Ga-LLZO is recorded in each spectrum. The narrow character without any satellite transition agrees with the high mobility of lithium ions in the Ga-LLZO electrolyte. Notably, the typical resonance peak for metallic Li, in the range of 200 to 300 ppm, does not appear in the samples before and after cycling, which is a direct evidence for the absence of dendritic Li in the Ga-LLZO electrolyte. Therefore, the Ga-LLZO electrolyte can suppress the formation of dendritic Li at a current density up to of 900 μA cm-2. According to Chiang et al.

[25],

there are flaws at the insufficiently contacting

Li/LLZO interface, and once uneven current is formed, metallic Li nucleates, fills in the flaws, and drives crack propagation at these sites; then the cracks expand and penetrate the electrolyte in the following charge/discharge processes. Continuous and tight interface between LLZO and metallic Li, originating from the “lithiophilicity” of LLZO, can dismiss these flaws effectively and produce a homogeneous current distribution, and the metallic nuclei are suppressed at the interface. Consequently, the capability of LLZO electrolytes to suppress dendritic Li is greatly enhanced, and the current density of stable Li plating/strapping increases.

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Lithium ions in LLZO or other highly conductive oxide-based solid electrolytes can easily react with H2O and CO2 in moist atmosphere to form Li2CO3 [33, 42], which seriously deteriorates the wettability of metallic Li on the surface [20]. For example, in the presence of Li2CO3, the contact angle gets to be about 127 °, as shown in Figure S2(c). As a result, the contact at the Li/LLZO interface is insufficient, the interfacial resistance becomes high, and the current density for the stable Li plating/stripping is quite limited. Surface modifications by means of materials that can alloy with metallic Li were used to improve the performances of LLZO electrolytes [20, 22, 26-30]. However, Li-deficient shields can be formed in-situ at high temperatures due to the decomposition of solid electrolytes, which restrains the formation of Li2CO3 on the surface, then metallic Li wets the electrolyte well and the capability of the electrolyte to suppress lithium dendrites is enhanced. Such a strategy is feasible and cost-effective, makes the solid electrolytes very suitable for practical application in LMBs. Performances of solid LiFePO4/LLZO/Li batteries. To demonstrate the application of the Ga-LLZO electrolyte in LMBs, two kinds of batteries with metallic Li anode and LiFePO4 composite cathode were assembled and tested. All-solid-state and solid LMBs underwent long charge/discharge cycling testing between 2.8 and 4.2 V, with results being given in Figure 5. The all-solid-state battery delivers a specific capacity of 148 mA h g-1 (Figure 5a and b) in the first cycle at a charge/discharge rate of C/10 at 60 ºC, which reaches 87% of the theoretical capacity of LiFePO4, and the battery maintains a specific capacity of 116 mA h g-1 after 40 cycles. The capacity decrease can be attributed to the unstable contact between the cathode and the 15



electrolyte after several cycles

[51].

Notably, in the first charge/discharge process, the

Coulombic efficiency is 94 %, and it increases to above 99.5 % in the following cycles, which is a very high value for LMBs [1]. With the aid of Li-containing ionic liquid, the recession of the capacity (Figure 5c and d) is much slower at room temperature. The charge/discharge behavior and the high Coulombic efficiency demonstrate no dendritic Li penetrating the Ga-LLZO electrolyte. The high current density for stable Li plating/stripping in LLZO electrolytes is one of the critical factors determining the performances of high-energy LMBs. Except for the Li/LiFePO4 system, LLZO electrolytes show potential applications in burgeoning Li-air and Li-sulfur batteries because of the ability to tolerate high current density [1]. For example, when the weight of active material in the cathode of a Li-sulfur battery reaches 5 mg cm-2, a current density of ~840 μA cm-2 is needed at a charge/discharge rate of 0.1 C, which is still within the tolerance of the Ga-LLZO electrolyte. 4. Conclusions We obtain Li2CO3-free LLZO surfaces using a novel method and demonstrated the intrinsic “lithiophilicity” characteristic of LLZO electrolytes. After the high temperature calcination at 900 ºC, Li-deficient compounds are in-situ formed on the LLZO surface, which shields LLZO from the formation of Li2CO3, and after removing Li-deficient compounds, Li2CO3-free LLZO is obtained. The Li2CO3-free LLZO clearly shows an intrinsic “lithiophilicity” characteristic, without any surface modification. This is also confirmed by the first-principle calculations, in which all the calculated contact angles between metallic Li and Li2CO3-free garnets are below 90 °, 16


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though varying with different dopants. The “lithiophilicity” characteristic of LLZO electrolytes leads to a continuous and tight interfacial contact between metallic Li and the LLZO electrolytes, which facilitates the lithium-ion transport across the interface. As a result, a relatively low interfacial resistance of 49 Ω cm-2, as well as a homogenous current distribution in the charge/discharge processes of LMBs is achieved, increasing the current density for the stable Li plating/stripping in LLZO to a high level of 900 μA cm-2 at 60 ºC. Owing to the ability to suppress dendritic Li, garnet electrolytes potentially enable high-energy LMBs. In addition to the particular LLZO materials, this work is also beneficial to other oxide-based ion conductors for high-performance, safe and long-life energy storage systems.

Supporting information The Supporting Information is available free of charge on the ACS Publications website. XRD patterns of the calculated LLZO; XPS plots of samples: freshly calcined, exposed in air for 7 days, and polished and exposed in air for 7 days; Wetting of molten Li on different garnet electrolytes: Al-LLZO, Ta-LLZO, and polished-Ga-LLZO exposed in air for 7 days; Interfacial structure, calculated works of adhesion (Wad) and contact angles (θ) between metallic Li (001) and Al-LLZO (001) and Ta-LLZO (001); Lattice parameters of cubic lithium, LLZO and its variants obtained from first-principle calculation; Dimension parameters, interfacial works of adhesion, contact angles of Li (001)/garnet (001) interface.

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Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 51672096, 51622207, U1430104 and 51802187).

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Figure 1 (a) SEM image of Ga-LLZO ceramic. Schematics of interactions between (b) unprotected LLZO surface and humid air, and (c) surface protected by the Li-deficient compound and humid air. (d) XRD pattern, (e) AC impedance spectra, and (f) Arrhenius plot of Ga-LLZO ceramic.

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Figure 2 (a) XPS results of O 1s (up) and C 1s (bottom), and (b) Raman spectrum of the calcined and polished Ga-LLZO electrolyte.

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Figure 3 (a) Wetting of molten Li on Ga-LLZO surface, (b) SEM image of the interface between metallic Li and Ga-LLZO. Interfacial structure, calculated works of adhesion (Wad) and contact angles (θ) between metallic Li (001) and (c) LLZO (001) and (d) GaLLZO (001).

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Figure 4 (a) AC impedance spectrum of the Li/Ga-LLZO/Li symmetric cell, (b) comparison of the Li/LLZO interfacial resistances [20, 22, 27, 30, 32-33, 46-49]. (c) The voltage profiles, and (d) AC impedance spectra of the Li/Ga-LLZO/Li symmetric cell during the galvanostatic cycling at room temperature. (e) The voltage profiles of Li/GaLLZO/Li symmetric cell with a current density of 900 μA cm-2 at 60 ºC. (f) Static 7Li NMR spectra of Ga-LLZO solid electrolyte before and after cycling test.

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Figure 5 (a, c) Plots of cell potential versus specific capacity, (b, d) charge capacity, discharge capacity and Coulombic efficiency of the (a, b) all-solid-state, and (c, d) solid batteries during galvanostatic charge/discharge cycling. The insets show LEDs lit by the batteries.

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In-situ formed shields enabling Li2CO3-free solid electrolytes: A new

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