稀有金属(英文版) 2020,39(09),1092-1098

Sr²⁺-doped rhombohedral LiHf₂(PO₄)₃ solid electrolyte for allsolid-state Li-metal battery

Qing-Hui Li Chang Xu Bing Huang Xin Yin

作者简介：*Bing Huang e-mail:huangbing@tju.edu.cn;*Xin Yin e-mail:yinxin@hnu.edu.cn;

收稿日期：20 January 2020

基金：financially supported by the National Natural Science Foundation of China (No.51504127);

Sr²⁺-doped rhombohedral LiHf₂(PO₄)₃ solid electrolyte for allsolid-state Li-metal battery

Qing-Hui Li Chang Xu Bing Huang Xin Yin

College of Electrical and Information Engineering,Hunan University

State Key Laboratory of New Ceramics and Fine Processing,School of Materials Science and Engineering,Tsinghua University

School of Materials Science and Engineering,Tianjin University

Abstract：

Solid-state electrolytes in rechargeable all-solidstate Li-metal batteries,which have better safety and higher specific capacity than conventional rechargeable Liion batteries with liquid electrolytes,are limited by the low Li-ion conductivity of the solid electrolyte and the large electrolyte/electrode interfacial resistance.Here,we report a new rhombohedral NAS ICON structure Li_1.4Sr_0.2Hf_1.8(PO₄)₃ with a high Li-ion conductivity of 1.62×10^-5 S·cm^-1 at 25℃,and its conductivity can be improved to 3.4×10^-5 S·cm^-1 after the densification of the pellet by hot pressing.Li_1.4Sr_0.2Hf_1.8(PO₄)₃ coated by a thin layer of polymer electrolyte showed a stable lowimpedance dendrite-free plating/stripping process in a symmetric Li/Li cell for 100 h;moreover,the Li_1.4Sr_0.2Hf_1.8(PO₄)₃ electrolyte had a small interfacial resistance in all-solid-state Li/LiFePO₄ cell,which allows a high Coulombic efficiency and good cycling of the cell.

Keyword：

Solid-state electrolyte; NASICON; Rhombohedral phase; Interfacial resistance; All-solid-state battery;

Received： 20 January 2020

1 Introduction

Developing rechargeable all-solid-state batteries with an alkali-metal anode has attracted great attention because of the low voltage and high capacity of the metallic anode,which can increase the energy density of the batteries.Solid electrolytes as the key component of all-solid-state batteries should have a high Li-ion conductivity,a small interfacial resistance with various electrodes,a good stability in air,and a large electrochemical window.However,it is still quite challenging to develop new solid electrolytes that meet all these requirements [ 1, 2, 3, 4, 5] .

The most studied solid electrolytes include polymer electrolytes,inorganic electrolytes based on sulfides,and oxides.The flexible polymer electrolytes are easy to be prepared and have a small interfacial resistance with Limetal anode.However,most solid polymer electrolytes showed a low Li-ion conductivity of 1×10^-6 S.cm^-1 at room temperature because of the slow segment motion of the polymer chains.The small Li-ion transfer number of the polymer electrolytes (<0.5) results in a concentration polarization in all-solid-state batteries.Moreover,these solid polymer electrolytes are unstable at high voltages (above 4V),which also limited their application in all-solid-state batteries [ 6, 7, 8] .The sulfide electrolytes have high Li-ion conductivity of 1×10^-3 S·cm^-1;however,they are unstable in air and have small electrochemical windows.Their sensitivity to moisture significantly increases the cost of the battery assembly.Moreover,these sulfide electrolytes are unstable with Li-metal anode,and the inhomogeneous current density at the Li/sulfide electrolyte interface leads to the short circuit of the cell at a small current density [ 9, 10] .

Compared with sulfide and solid polymer electrolytes,the Li-ion conducting oxides are more stable in air,and some of them have high Li-ion conductivities at room temperature.For example,garnet electrolyte Li₇La₃Zr₂O₁₂shows a large electrochemical window above 5 V,a high Li-ion conductivity of 1×10^-3 S.cm^-1,and a small activation energy for Li-ion transfer.However,the fast Li⁺/H⁺exchange of garnet electrolytes in air results in the formation of Li-ion insulating Li₂CO₃ layer on the pellet surface,which significantly increases the interfacial resistance of garnet with lithium metal [ 11, 12, 13, 14] .Perovskite oxide Li_3xLa_2/3-x1/3-2xTiO₃ with x=0.1 has a high bulk Li-ion conductivity of 1×10^-3 S·cm^-1 at 25℃(□is the Li-ion vacancy in perovskite A site);however,Ti⁴⁺is reduced by Li-metal anode,and the surface Li₂O evaporation during preparation at high temperature reduces the total Li-ion conductivity to 1×10^-5 S·cm^-1 at room temperature.Moreover,a high sintering temperature (about1300℃) is required to prepare the pure cubic perovskite phase [ 15, 16, 17] .NASICON electrolytes Li_1.3Al_0.3Ti_1.7P₃0₁₂and Li_1.5Al_0.5Ge_1.5P₃0₁₂ with a high Li-ion conductivity have been commercialized,and the strong covalence P⁵⁺-O^2-bond makes these kinds of solid electrolytes quite stable in moist air,but the Ti⁴⁺/Ge⁴⁺are unstable with Limetal anode.Replacing the Ti⁴⁺/Ge⁴⁺by other stable metal ions is useful strategy to increase their stability with Li metal.However,a low Li-ion conducting monoclinic phase rather than the high Li-ion conducting rhombohedral phase was obtained when Zr⁴⁺substitutes Ti⁴⁺'which may be caused by the too large Li-ion transfer channel in LiZr₂(PO₄)3 [ 18, 19, 20, 21, 22] .

In this work,we replace Zr⁴⁺with Hf⁴⁺to prepare rhombohedral LiHf₂(PO₄)3 electrolytes;Sr²⁺was also introduced into the structure to further increase the concentration of Li⁺.We find that the pure rhombohedral LiHf₂(PO₄)3 phase could be obtained at a relatively low sintering temperature,and the Sr²⁺doping increases the conductivity of the NASICON material.Moreover,we employed the hot-pressing sintering technique to further improve the density and reduce the grain boundary resistance of the pellet.The Li_1.4Sr_0.2Hf_1.8(PO₄)3 prepared by hot pressing had a high room-temperature Li-ion conductivity of 3.4×10^-5 S·cm^-1.A symmetric Li/Li cell and an all-solid-state Li/LiFePO₄ cell were also prepared,and the results confirmed the good performance of the Li_1.4Sr_0.2Hf_1.8(PO₄)3 pellet in these cells.

2 Experimental

2.1 Preparation of NASICON Li1+2xSrxHf2-x(PO4)3electrolyte

Stoichiometric Li₂CO₃,HfO₂,SrCO₃,and (NH₄)2HPO₄were thoroughly mixed in an agate mortar and preheated at900℃for 6 h to remove the absorbed water and decompose the carbonates,and 20%excess of Li₂CO₃ was added to compensate for the lithium loss during the high-temperature sintering.The obtained powders were then reground and were pressed into pellets with a diameter of12 mm.The pellet was finally sintered in a Pt crucible with cover at 1100℃for 20 h in air.

2.2 NASICON electrolyte prepared by hot pressing

The pellet after the high-temperature sintering was ballmilled in a ball milling machine for 12 h.The obtained powders were heated in a graphite mold at 1100℃in an argon with a pressure of 60 MPa for 1 h.The pellets were cut into 0.5 mm in thickness with a diamond saw for the symmetric Li/Li cell and all-solid-state Li/LiFeP0₄ cell.

2.3 Characterizations

Powder X-ray diffraction (XRD) patterns for the as-prepared samples were characterized from 10°to 60°with a Bruker D8 Advance.Scanning electron microscopy (SEM)images were examined by a field-emission scanning electron microscopy (SEM,FEI Quanta 650).Surface-enhanced Raman spectroscopy was employed in air to monitor the reaction of the pellet surface in air.Electrochemical impedance spectra (EIS) of Li_1+2xSr_xHf_2-x(PO₄)3(LSHP)(0.10<x<0.30) were carried out from 293 to413 K on a precision impedance analyzer (Agilent 4294A)with the alternative current (AC) amplitude of 10 mV;the applied frequency was 110 MHz-40 Hz.Both sides of the pellets were coated with Li-ion blocking Au electrodes for the EIS testing.The pellet density was determined by measuring the pellet weight and size.The composition and valence states of the Li_1+2xSr_xHf_2-x(PO₄)3 (0.10≤x≤0.30) powders were characterized by X-ray photoelectron spectroscopy (XPS,ESCALab 250Xi,Thermo Fisher Scientific).

2.4 Battery measurement

The obtained NASICON pellet was sandwiched by two lithium foils with a surface area of 0.5 cm² on both sides of the Li/Li symmetric cell.The symmetric cell was cycled at0.025 and 0.05 mA.cm^-2 at 50℃.To reduce the Li/LSHP interfacial resistance in the Li/Li symmetric cell and in the all-solid-state Li/LiFePO₄ cell,a Li-ion conducting polymer with a thickness of 0.2 mm was put between Li and LSHP pellet.The preparation of the conducting polymer was the same as the method in Ref. [ 23] .Poly(ethylene oxide)(PEO,molecular weight of M_w=600,000) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with a EO/Li⁺ratio of 10 were added into acetonitrile and stirred for 24 h at 50℃.10 wt%Al₂O₃ nanopowder was added as a ceramic filler to increase the amorphous phase of PEO.The PEO membrane was obtained by casting the suspension into a polytetrafluoroethylene dish with the following dry at 60℃for 48 h.

Fig.1 a XRD patterns of Li_1+2xSr_xHf_2-x(PO₄)₃ (0.10≤x≤0.30) electrolyte fired at 1150℃for 12 h;b XRD patterns of Li_1.4Sr_0.2Hf_1.8(PO₄)3treated by hot pressing (HP) at 1100℃;cross-sectional SEM images of LY_0.2ZP treated by c regular sintering and d hot pressing

Fig.2 a Li-ionic conductivities (Z'and Z"representing real part and imaginary part of impedance value,respectively) and b Arrhenius plots of Li_1.4Sr_0.2Hf_1.8(PO₄)₃ (T andσrepresenting temperature and Li-ionic conductivities,respectively) prepared by regular sintering and hot pressing

For the all-solid-state battery testing,the LiFePO₄cathode was prepared according to Ref. [ 21] .LiFePO₄,carbon,PEO,and LiTFSI salt were dispersed in acetonitrile with a weight ratio of 70:10:13:7;the slurry was stirred overnight and cast on aluminum foil by the doctor blade.The obtained cathode was cut into 0.5 cm² pellets and dried at 80℃for 48 h.The active material loading was 3-5mg·cm^-2.The charge/discharge curve of the cell was recorded from 3.8 to 2.8 V at 0.025 and 0.05 mA·cm^-2.The interfacial resistance of the electrode/NASICON per square centimeter was calculated by multiplying the resistance of the interfacial resistance and the surface area of the electrode.

Fig.3 XPS spectra of fresh (black line) and aged (red line) Li_1.4Sr_O.2Hf_1.8(PO₄)₃ pellet:a C 1s,b O 1s,c Li 1s,d Hf 4d,e P 2p,and f Raman spectra of fresh (black line) and aged (red line)

3 Results and discussion

Li_1+2xSr_xHf_2-x(PO₄)3 with 0.10≤x≤0.30 was designed because that a high Li-ion conductivity can be obtained by introducing more mobile Li⁺in the M(Ⅱ) site of NASI-CON structure via alio valent doping [ 24] .XRD patterns displayed in Fig.1a showed that all obtained samples could be indexed to the rhombohedral NAS ICON structure,and the diffraction peaks matched well to the standard peaks in the space group of R-3c (PDF No.52-0569).A close inspection of XRD patterns shows that Sr²⁺doping shifts the peak to the smaller diffraction degree because of the large ion radius of Sr²⁺(0.112 nm) than that of Hf⁴⁺(0.071nm).For x>0.20,some impurities existed.The sample Li_1.4Sr_0.2Hf_1.8(PO₄)₃ showed the highest relative density(82%),which was improved to 92.9%by hot pressing.The higher density of the LSHP helps to improve the Li-ion movement in the grain boundary of the pellet.SEM results in Fig.1d also confirmed only a very small amount of closed pores remained in Li_1+2xSr_xHf_2-x(PO₄)₃ prepared by hot pressing.Moreover,the hot-pressing method also increased the mechanical strength of the Li_1+2xSr_xHf_2-x(PO₄)₃pellet,and the Li_1+2xSr_xHf_2-x(PO₄)₃ with x=0.15 showed the highest Vickers hardness of 4.6 GPa than the other compounds.

The room-temperature impedance plots of Li _1+2x Sr_xHf_2-x(PO₄)3 (0.10≤x≤0.30) pellets are presented in Fig.2a.The semicircle at high frequency corresponds to the Li-ion resistances of the bulk and grain boundary,and the tails at low frequency are caused by the Li-ion blocking Au electrodes.Rhombohedral Li_1.4Sr_0.2Hf_1.8(PO₄)₃ has the highest conductivity of 1.62×10^-5 S·cm^-1,which is much higher than that of LiHf₂(PO₄)₃.The increased Li-ion concentration in the NAS ICON framework increased the Li-ion conductivity.The Li-ion conductivity of Li_1.4Sr_0.2Hf_1.8(PO₄)₃ at room temperature was further improved to 3.4×10^-5 S·cm^-1 when the pellet was further densified by hot pressing.The lower porosity of the sample prepared by hot pressing increased the Li-ion transfer in the grain boundary of the pellet.The conductivity of the pellets measured at different temperatures is shown in Fig.2b;all the samples showed a similar linear temperature dependence behavior from 25 to 80℃.The activation energy for Li-ion transfer of Li_1.4Sr_0.2Hf_1.8(PO₄)₃ prepared in a box furnace and by hot pressing is 0.36 and 0.32 eV,respectively.

Fig.4 a,c Nyquist plots of symmetric cells with (Li/P/LiSr_0.2HP/P/Li) and without polymer layers (Li/LiSr_0.2HP/Li) at 50℃;b voltage profile of Li/LiSr_0.2HP/Li cell cycled at current densities of 0.025 and 0.05 mA·cm^-2;d voltage profiles of Li/P/LiSr_0.2HP/P/Li cell cycled at a current density of 0.05 mA·cm^-2 (size of metallic lithium being 0.5 cm²)

The stability of solid electrolyte influences the Li-ion transport in the interface of all-solid-state Li-metal batteries [ 25, 26] .For the oxide electrolytes,H⁺or H₃O⁺in moist air can exchange with Li⁺in the solid electrolyte to form some Li-ion insulating impurities (e.g.,LiOH and Li₂CO₃),and these impurities will increase the solid electrolyte/electrode interfacial resistance.We have checked the stability of Li_1.4Sr_0.2Hf_1.8(PO₄)3 in air by aging the fresh sample in air for one month.The fresh and aged samples had the same XRD and EIS results,indicating the good stability of Li_1.4Sr_0.2Hf_1.8(PO₄)3 in air.The surface of both samples was also investigated by XPS,and the C,O,Li,Hf,and P spectra results are shown in Fig.3;all elements from the aged sample showed the same peak positions and intensities as the fresh one.In Fig.3a,the peaks at 285 eV corresponded to the adventitious carbon from the environment,and the peaks related to C 1s in carbonate at290 eV did not appear in both samples.These results indicate that the strong Hf⁴⁺-O^2-and p⁵⁺-O^2-bonds increase the NAS ICON stability in moist air [ 27] ,and the good stability in air can reduce the Li-ion interfacial resistance in the all-solid-state batteries.The fresh and aged Li_1.4Sr_0.2Hf_1.8(PO₄)3 samples showed the same Raman spectra,confirming the good stability of NASICON structure.

The stability of Li_1.4Sr_0.2Hf_1.8(PO₄)₃ with lithium metal was tested in a symmetric Li/Li cell,and a black layer was formed on the Li_1.4Sr_0.2Hf_1.8(PO₄)₃ surface after contact with the metallic lithium anode.It is reported to be Li-ion conducting Li₃P that is from the reaction between lithium metal and the NASICON material [ 21] .The formation of Li₃P is apparently due to the distinct crystalline structure of Li_1.4Sr_0.2Hf_1.8(PO₄)₃ surface that features O deficiency arrangement in the NASICON framework (rather than edge-to-edge stacking of M0₆ octahedra,P0₄ tetrahedra in the unit').During the cycling,Li ions and electrons are transferred from the lithium metal side to the NASICON side.The inserted lithium ions diffused into the Li interstitial sites,and the electrons reduced non-bridged O-P-O anions near the edges of the NASICON crystalline.Thus,in situ formation of Li₃P acts as a passivating but ionically conducting barrier layer,which bonds the lithium metal surface tightly and gives rise to a continuous Li⁺transfer [ 28, 29] .Figure 4a shows the impedance of the symmetric Li/Li cell with a Li_1.4Sr_0.2Hf_1.8(PO₄)3 pellet;the metallic lithium anode had a small interfacial resistance of 450Ω·cm² with Li_1.4Sr_0.2Hf_1.8(PO₄)₃.Figure 4b shows the voltage profiles of Li/LSHP/Li cell at a current density of0.025 and 0.05 mA·cm^-2 with an overpotential of 0.08 and0.15 V,respectively.The symmetric Li/Li exhibits a stable lithium plating and stripping without short circuit for100 h.Because the solid-solid contact is between the metallic lithium anode and the Li_1.4Sr_0.2Hf_1.8(PO₄)₃ electrolyte,the interfacial resistance is still large.To reduce the interfacial resistance and increase the cycling stability of the symmetric cell,we coated the surface of LSHP electrolyte with a thin PEO electrolyte [ 30] ,which has a conductivity of 7×10^-4 S·cm^-1 at 50℃.The symmetric Li/PEO-coated LSHP/Li cell (Li/P/LiSr_0.2HP/P/Li) had a much smaller total resistance than the symmetric cell without polymer (Li/LiSr_0.2HP/Li),and the overpotential of the cell was also reduced to 0.1 V at 0.05 mA·cm^-2.The symmetric cell showed very stable cycling at 50℃for 100h with no short circuit,indicating the stable interface between the metallic lithium anode and the PEO-coated LSHP electrolyte.

Fig.5 Electrochemical performance of LiFePO₄/LiSr_0.2HP/P/Li all-solid-state cell at 50℃:a Nyquist plot,b charge and discharge profiles at different current densities,and c cycle stability at 0.05 mA·cm^-2 (area of lithium metal and LiFePO₄ being 0.5 cm²)

The LSHP/cathode interfacial resistance was also studied in all-solid-state Li-metal batteries.The EIS data of the all-solid-state Li-metal cell are shown in Fig.5a;the cell had a total resistance of 2200Ωat 50℃,and the Li_1.4Sr_0.2Hf_1.8(PO₄)₃ pellet showed an interfacial resistance of 420Ω·cm².These values are competitive with those of the solid-state batteries with garnet electrolyte.The melting point of the PEO is about 60℃,and the composite cathode should have good contact with LiFePO₄ and Li_1.4Sr_0.2Hf_1.8(PO₄)₃ pellet.The charge/discharge profiles and the cycling performance of the all-solid-state cell are shown in Fig.5b,c.The initial discharge capacity of the all-solid-state cell is 138 and 125 mAh·g^-1 at 0.025 and0.05 mA·cm^-2,respectively.The cell also showed a small overpotential of 0.2 V at 0.05 mA·cm^-2.About 110mAh·g^-1 retained after 100 cycles at 0.05 mA·cm^-2,and the cell showed a high Coulombic efficiency of about99.5%.The good Li/LSHP and LSHP/LiFePO₄ interfaces enable long cycling of the all-solid-state Li-metal cell.

4 Conclusion

A new rhombohedral NASICON Li_1.4Sr_0.2Hf_1.8(PO₄)₃ was prepared as a solid electrolyte in all-solid-state Li-metal batteries.The sample had a Li-ion conductivity of1.62×10^-5 S.cm^-1 at 25℃,which is higher than that of LiHf₂(PO₄)₃;the Li-ion conductivity of the pellet was further improved by the hot-pressing technique.Moreover,the hot pressing increased the mechanical strength of the pellet.There action between Li_1.4Sr_0.2Hf_1.8(PO₄)₃ and lithium metal increased the contact between Li_1.4Sr_0.2Hf_1.8(PO₄)₃ and a metallic lithium anode.The thin-film coating on the surface of Li_1.4Sr_0.2Hf_1.8(PO₄)₃further decreased the resistance of Li/Li_1.4Sr_0.2Hf_1.8(PO₄)₃interface,which increased the cycling stability of the symmetric Li/Li cell.Because of the good stability of Li_1.4Sr_0.2Hf_1.8(PO₄)₃,the all-solid-state Li/LiFePO₄ cell showed a small resistance,and the cell had stable cycling with a high capacity and Coulombic efficiency.

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