Enhanced electrochemical performance of garnet-based solid-state lithium metal battery with modified anodic and cathodic interfaces

Deen Yan,Huangwang Mai,Wen Chen,Wei Yang,Hanbo Zou,*,Shengzhou Chen,*

1 School of Chemistry and Chemical Engineering,Guangzhou University,Guangzhou 510006,China

2 Guangzhou Tinci Materials Technology Co.,Ltd.,Guangzhou 510760,China

Keywords:Lithium metal battery Solid-state electrolyte Li-ZnO anode

ABSTRACT Due to high ionic conductivity and wide electrochemical window,the garnet solid electrolyte is considered as the most promising candidate electrolyte for solid-state lithium metal batteries.However,the high contact impedance between metallic lithium and the garnet solid electrolyte surface seriously hampers its further application.In this work,a Li-(ZnO)x anode is prepared by the reaction of zinc oxide with metallic lithium and in situ coated on the surface of Li6.8La3Zr1.8Ta0.2O12(LLZTO).The anode can be perfectly bound to the surface of LLZTO solid electrolyte,and the anode/electrolyte interfacial resistance was reduced from 2319 to 33.75 Ω?cm2.The Li-(ZnO)0.15|LLZTO|Li-(ZnO)0.15 symmetric battery exhibits a stable Li striping/plating process during charge-discharging at a constant current density of 0.1 mA?cm-2 for 100 h at room temperature.Moreover,a Li-(ZnO)0.15|LLZTO-SPE|LFP full battery,comprised of a polyethylene oxide-based solid polymer electrolyte (SPE) film as an interlayer between LiFePO4 (LFP) cathode and LLZTO solid electrolyte,presents an excellent performance at 60 °C.The discharge capacity of the full battery reaches 140 mA?h?g-1 at 0.1 C and the capacity attenuation is less than 3% after 50 cycles.

1.Introduction

All-solid-state metal lithium batteries become more and more attractive because of their obvious virtues over traditional Li-ion ones which use an organic liquid electrolyte in terms of their high safety and specific energy [1–3].The utilities of inorganic solidstate electrolyte (SSE) make it possible to use metal lithium as an anode by effectively blocking lithium dendrites to pierce the separator.Garnet-based lithium metal solid-state batteries,composed of Li7La3Zr2O12(LLZO) as solid electrolytes,are recognized as the most promising candidate powers since LLZO electrolytes have high lithium-ion conductivity,excellent stability to lithium metal,and wide electrochemical stability window(～6 V)[4–6].However,the low spreading ability of molten lithium on the surface of LLZO solid electrolytes and high interfacial impedance between garnetbased SSE and metal lithium significantly decreases the performance of garnet-based batteries.

The modification of the surface of garnet SSEs,called the transition layer,can improve the wettability of lithium on the SSEs and successfully strengthen lithium contact with garnet SSEs and reduce lithium-ion transmission impedance[7–10].Wang[7]reported that the wettability of lithium sheet to garnet SSEs was enlarged,and the resistance of lithium-ion transmission on the interface was reduced from 1900 to 450 Ω?cm2by coating a layer of zinc oxide on the solid electrolytic surface using an atomic deposition method.Aluminum oxide [8],stannic oxide [9],and gold [10] can also be coated on the garnet solid electrolyte by atomic deposition.It is suggested that metal oxide transition layers can be transformed into a lithiummetal alloy by conversion reaction of metal oxide with metallic lithium,thus make lithium sheet closely attach to the surface of the garnet SSEs.However,the atomic deposition equipment is expensive and is not conducive to large scale production.

Alloying is another effective way to reduce the surface energy between garnet SSEs and metallic lithium.Moreover,the Li-metal alloy frameworks can provide continuous pathways for Li-ions and electrons to transfer in the bulk of alloy anode during long stripping/-plating cycles [11].Lu [12] reported that a Li-Al alloy can wholly spread on the solid electrolyte sheet by adding a certain amount of metal Al to the molten lithium and the impedance of interfacial lithium-ion transmission was reduced to less than 1 Ω?cm2.

On the positive material side,introducing transition layers is a useful route to effectively connect the SSE with the cathode.Goodenough and co-workers [13] designed a polymer/ceramic/polymer sandwich electrolyte (PCPSE) in which the polymer was made of cross-linked poly (ethylene glycol) methyl ether acrylate and the ceramic was made of NASICON Li1.3Al0.3Ti1.7(PO4)3.The Li|PCPSE|LFP battery delivers a superb long-term electrochemical stability and a signally high Coulombic efficiency of 99.8%–100%at 65 °C.Fan and his co-workers [14] designed a similar sandwich structure electrolyte with a solid polymer electrolyte (SPE)/garnet electrolyte(Ta-doped LLZO,LLZTO)/SPE in which the SPE was made of polyethylene oxide (PEO).The 3D solid-state battery,Li|SPELLZTO-SPE|LFP,exhibits the first discharging capacity of 135 mA?h?g-1at 90°C,and the coulombic efficiency is about 99.6%after 200 cycles.Guo’s group [15] designed an asymmetric solid electrolyte (ASE),covering a very thin (7.5 nm) modified layer of poly(ethylene glycol) methyl ether acrylate (PEGMEA) on the surface of LLZO near the lithium metal anode and a 5.4 μm thick polymer layer of PEGMEA on the side of LFP cathode.The Li|ASE|LFP solidstate battery has a high capacity retention rate of 94.5% and the coulombic efficiency exceeds 99.8% at 55 °C during at least 120 cycles due to the unique asymmetric structure that reduces the specific resistance both inside the battery and on the electrode/SSE interfaces.However,the all-solid-state battery constructed with polymer electrolyte and ceramic electrolyte could not work well at room temperature.

In this work,we introduce a simple method to synthesize a composite anode.Zinc oxide powders are added to molten metallic lithium to form a lithium-zinc alloy,which causes the composite anode to spread out and tightly attach to the surface of the Li6.8La3-Zr1.8Ta0.2O12solid electrolyte and significantly reduces interfacial resistance between SSEs and the negative electrode.Furthermore,Li2O particles,a by-product of the reaction of ZnO and Li,can accommodate the volume change of the composite anode as a buffer layer [16–18].The Li-ZnO anode with large rough surfaces can lower the effective current density of Li nucleation and thus suppress the Li dendrite formation.Besides,the Li-(ZnO)0.15|LLZTOSPE|LFP battery assembled with the asymmetric solid electrolyte has a stable capacity of 140 mA?h?g-1at 60°C.This simple strategy for constructing a composite negative electrode may accelerate the development of garnet-based solid-state batteries.

2.Experimental

2.1.Preparation of solid electrolyte

The LLZTO was synthesized by a solid reaction method[19].Stoichiometric amounts of LiOH?H2O(Macklin,20%(mass)excess,98%),La2O3(Macklin,99%),ZrO2(Macklin,99%),and Ta2O5(Macklin,99.5%) were ball milled adequately with isopropyl alcohol for 6 h.And the mixture was dried at 65°C for 8 h and then sintered at 900°C for 6 h in an alumina crucible and then cooled to room temperature.The obtained powders were ground at 400 rotation per minute for 6 h by ball mill and then pressed into pellets at 300 MPa.The diameter of the pellets is 11.3 mm,and the thickness is 1 mm.Finally,the pellets were calcined at 1230°C for 40 minto obtaina cubicphase LLZTO pellet and then stored in an argon-filled glove box.

2.2.Synthesis of Li-ZnO anode

In an Argon-filled glove box,zinc oxide powders and molten lithium slice were stirred well at 200°C,and then the molten mixture(Li-(ZnO)x)was coated on the surfaces of the LLZTO solid electrolyte pellet and cooled to room temperature.A series of Li-ZnO composite anodes (Li-(ZnO)x) prepared using various mole ratios of metal Li to ZnO powders were denoted as Li-(ZnO)0.05,Li-(ZnO)0.1,Li-(ZnO)0.15,Li-(ZnO)0.2.As a comparison,the Li-Zn anode,consisting of lithium slice (0.18 g) and metallic zinc (0.046 g),was made by the same method.

Fig.1.(a)Battery structure.(b)The digital photo of a polished brown LLZTO pellet.(c)XRD patterns of the as-synthesized LLZTO and LLLZTO after treated with molten lithium metal for one hour.(d) Nyquist plot of the LLZTO at room temperature.

2.3.Preparation of LFP cathodes with polymer electrolyte interlayers

LiFePO4,polyvinylidene fluoride(PVDF),and carbon black powders with a mass ratio of 8:1:1 were dispersed in an appropriate amount ofN-methyl pyrrolidone solution to form a slurry mixture by constant stirring.The slurry was coated on aluminum foil evenly,and then was dried at 80°C under vacuum for 12 h.The specific capacity of the battery is calculated on the basis of the mass of LiFePO4in the battery.

The PEO-based solid polymer electrolyte (PEO-based SPE) precursor was consisted of PEO(Mw 600,000,Macklin),lithium bis-tri fluoromethanesulfonimide(LiTFSI),PVDF,and LLZTO powder with the mass ratio of 4:4:1:3.85 and stirred in acetonitrile solvents to form an even suspension.On the surface of LLZTO,20 μl of precursor suspension was spread evenly,and then an LFP positive electrode sheet was covered to the LLZTO layer.After evaporating the solvent at 50 °C for 12 h in a vacuum oven,the LFP positive electrode with an interlayer of PEO-based SPE was obtained.

2.4.Battery assembly

The battery assembly procedure is schematically shown in Fig.1(a).The all-solid-state battery with a sandwich structure consists of a LLZTO solid electrolyte pellet,an LFP positive electrode and a Li-ZnO composite negative electrode.Fig.1(b) shows a brownyellow LLZTO pellet with a thickness of approximately 1 mm after polishing.All the processes were performed in an argon-filled glove box.

2.5.Characterization and electrochemical tests

The X-ray powder diffractometer (XRD) (Cu Kα,PW3040/60)was used to examine the crystal structure of LLZTO and Li-ZnO anode with2θ ranging from 5°to 80°at a scan angle of 4(°)?min-1.A scanning electron microscopy (SEM) was carried out to observe the microstructures by JSM-7001F.The Li-(ZnO)x|LLZTO|Li-(ZnO)xsymmetric battery and Li-(ZnO)0.15|LLZTO-SPE|LFP full battery were assembled in 2032-type coin batteries,and their electrochemical performance was conducted by NETWARE electrochemical testing systems.Electrochemical impedance spectroscopy(EIS)was obtained by Solartron (1260+1287,1 mol?L-1–1 Hz,10 mV).

3.Results and Discussion

3.1.Characterization

As shown in Fig.1(c),the X-ray diffraction pattern of LLZTO powders well matches that of cubic phase garnet Li5La3Nb2O12(-PDF#45–0109),and there are no other impurity peaks,which proves that the cubic LLZTO phase is achieved.By using the Archimedes drainage method,we measure the relative density(relative to theoretical density) of LLZTO to be 90%.Because LLZTO reacts with air slowly to form impurities (such as Li2CO3[20]),all LLZTO pellets are polished and stored in a glove box filled with argon.The ionic conductivity (σRT) of the LLZTO sample at 25 °C is calculated according to the following equation [21].

Fig.2.(a)XRD pattern of Li-(ZnO)0.15 anode material.(b)The CV curve of garnet LLZTO.(c)The digital photo of poor contact interface between Li and LLZTO and(d)the photo of good contact interface between Li-(ZnO)0.15 anode and LLZTO.

wheredandSrepresents the thickness and area of LLZTO pellets respectively,andRs(sum ofRs1 andRs2)[22–25]is the ionic resistance of the LLZTO sample obtained by EIS.Rs1 is the bulk resistance,andRs2 is the grain boundary resistance of LLZTO,respectively,as shown in the insert equivalent circuit of Fig.1(d).According to Eq.(1),the Li-ion conductivity of the as-prepared Li6.8-La3Zr1.8Ta0.2O12electrolyte reaches l.52×10-4S?cm-1.

Fig.3.SEM image of the interface between Li-(ZnO)0.15 anode and LLZTO electrolyte.

The structural stability of LLZTO connected with lithium metal anode is very important to the long cycling life of the batteries.To evaluate the high-temperature stability of Li/garnet,LLZTO powders and molten lithium metal were thoroughly mixed at mass ratio of 1:1 for 1 h.The mixture is then washed with ethanol several times,and then the remaining solids are dried and detected by XRD.As shown in Fig.1(c),the characteristic XRD peaks of cubic LLZTO are retained,indicating the good chemical and structural stability of as-prepared LLZTO to lithium metal at 200 °C.The XRD diffraction patterns of the Li-(ZnO)0.15composite anodes are shown in Fig.2(a).It can be found that the XRD diffraction peaks of Li-Zn alloy (PDF#03-0954),metallic lithium (PDF#01-1131)and lithium oxide(PDF#12-0254)are present,and no peaks of zinc oxide are observed.This indicates that zinc oxide powders are fully reduced by excess metallic lithium,and form Li-Zn alloy.As shown in Fig.2(c),due to the massive difference in specific surface energy between molten lithium and garnet,lithium metal cannot be spread on the surface of LLZTO.When zinc oxide powders are added to molten lithium,the Li-(ZnO)0.15composite can be spread entirely on the surface of LLZTO,as shown in Fig.2(d).When the ZnO powder is mixed with molten lithium,Li-Zn alloy and Li2O were produced,which is proved by the XRD results.According to reports [11,26,27],various component Li-Zn alloys,such as Li0.95-Zn0.05,have intimate contact with LLZTO.So,the increasing wettability of LLZTO against Li-(ZnO)xanode is mainly due to the formation of Li-Zn alloy component by adding ZnO to molten Li metal.

Fig.4.The SEM images of negative electrodes:(a) Li;(b) Li-Zn0.15;(c) Li-(ZnO)0.05;(d) Li-(ZnO)0.10;(e) Li-(ZnO)0.15;(f) Li-(ZnO)0.20.

From the electron micrograph of the cross-section of the electrode,Fig.3,it could be seen that the Li-(ZnO)0.15composite is closely attached to the surface of the LLZTO solid electrolyte without voids.The electron micrographs of the pure Li,Li-Zn alloy and Li-ZnO electrodes with different components are shown in Fig.4.It can be seen that the surfaces of the Li-ZnO electrodes are rougher than pure Li and Li-Zn alloy electrodes.The more zinc oxide powders are added,the more surface protrusions are found.The roughness and a lot of small protrusions on the surfaces of Li-ZnO electrodes are favorable to buffer the large volume change of the negative electrode.

3.2.Electrochemical performance

The electrochemical stable window of the LLZTO pellet is studied by cyclic voltammetry.The results are shown in Fig.2(b).In the coin battery,the LLZTO pellet is sandwiched between the stainless steel as the working electrode and the lithium foil as the reference and counter electrode,and the scanning speed is 1 mV?s-1.The representative redox peaks appear at about-0.21 V and 0.20 V,which indicates that the polarization voltage of the lithium plating or stripping process is about 0.2 Vvs.Li+/Li.No other noticeable redox current is observed in the whole test voltage range,which indicates that as-prepared LLZTO has electrochemical stability in a wide voltage range up to 6 V.

To research the interface performance between SSEs and several anode composites,the lithium-ion interfacial transfer impedance of the symmetric batteries was quantified by EIS,as shown in Fig.5.Since the LLZTO electrolytes used in the cases are the same,the significantly higher area-specific resistance(ASR)in the battery may be due to the poor interfacial contact of electrodes with the LLZTO electrolyte.From the equivalent circuit diagrams and fitting results(R2,Supplementary Material ),divided by two,and then normalizing the electrolyte surface area(1 cm2),we can determine the interfacial ASR on either side of symmetric batteries according to the equation,ASR=R2/2S,whereSis the area of LLZTO sheets.Among the interfacial ASRs of LLZTO electrolyte and various anodes (Li,Li-Zn,Li-(ZnO)x),shown in Fig.5,the symmetric Li|LLZTO|Li battery has the highest value of ASR (2319 Ω?cm2) and the ASR of Li-(ZnO)0.15/LLZTO is the lowest(33.79 Ω?cm2).From the above data,adding ZnO or metallic zinc to pure lithium can dramatically lower the interfacial ASR of symmetric batteries.This may be attributed to the following factors:the increase of the wettability of Li-(ZnO)xanode electrodes on LLZTO and the inhibition of the formation of non-ion-conducting impurities,such as Li2CO3on the surface of LLZTO during processing [20].

Fig.5.Nyquist plots of the symmetric electrodes:(a) Li;(b) Li-Zn0.15;(c) Li-(ZnO)0.05;(d) Li-(ZnO)0.10;(e) Li-(ZnO)0.15;(f) Li-(ZnO)0.20.

Through the galvanostatic cycling experiment,we study the transferability and stability of lithium-ions crosses the interfaces of anode and solid electrolyte.As shown in Fig.6(a),the symmetric Li|LLZTO|Li battery displays a fluctuating potential with high voltage polarization,indicating that ions are not uniformly transmitted at the interface.When the current density is 0.1 mA?cm-2,the symmetric Li-Zn0.15|LLZTO|Li-Zn0.15and Li-(ZnO)0.05|LLZTO|Li-(ZnO)0.05batteries have a short-circuiting after 2–2.5 h of stable cycling,as shown in Fig.6(b)and Fig.S1,indicating the two batteries cannot withstand long charge–discharge cycling at a current density of 0.1 mA?cm-2.On the other hand,as shown in Fig.6(c)–(e),the symmetric Li-(ZnO)0.15|LLZTO|Li-(ZnO)0.15battery is very stable,with a small overpotential of 20 mV through charge-discharging at 0.1 mA?cm-2at room temperature.The voltage value of the Li-(ZnO)0.15|LLZTO|Li-(ZnO)0.15battery increases to 60 mV when the charge-discharging current reaches 0.2 mA?cm-2.After that,the voltage gradually declines for continuous 100-h cycles,and the last voltage value is about 30 mV.The reduction of polarization voltage during long time cycling may be caused by the penetration of metallic lithium gradually into the solid-state electrolyte,which eventually short-circuited the electrodes[28].As shown in Fig.6(e),it is apparent that the symmetric Li-(ZnO)0.20|LLZTO|Li-(ZnO)0.20battery is the most stable with an overpotential of 22 mV thorough charge–discharge at 0.1 mA?cm-2.When the current density is 0.2 mA?cm-2,the voltage of the battery increases and stabilizes at 45 mV.In summary,the addition of metallic zinc and zinc oxide to metallic lithium can alleviate the formation of lithium dendrites,and the addition of zinc oxide has the most obvious effect.In the range of 0 to 0.2 the higher the ratio of ZnO/Li is,the better the short-circuit suppression effect occurs on the symmetric electrodes.

Fig.6.Cycling curves of symmetrical cells:(a) Li;(b) Li-Zn0.15;(c) Li-(ZnO)0.10;(d) Li-(ZnO)0.15;(e) Li-(ZnO)0.20.

Fig.7.The electrochemical performance of the all-solid-state battery Li-(ZnO)0.15|LLZTO-SPE|LFP:(a)Cycling stability at 0.1C;(b)Rate capability performed at varied current rates;(c) Galvanostatic discharge/charge curves (the 1st,20th,50th cycles) and (d) Nyquist plots of the battery before and after cycling.

To further study the performance of Li-ZnO composite electrode,we designed a full solid-state lithium-ion battery (Li-(ZnO)0.15|LLZTO-SPE|LFP),shown in Fig.1(a).LFP was chosen to be the positive active material as it has as an excellent chemical resistance against the LLZTO solid-state electrolytes.The mass loading of LFP is 2.65 mg?cm-2.A PEO-based SPE interlayer was designed as an ion transport medium to reduce the interface ion transfer resistances of cathode and LLZTO SSEs.After 50 cycles at 0.1 C (1 C=150 mA?g-1) at 60 °C,the discharge capacity of Li-(ZnO)0.15|LLZTO-SPE|LFP is maintained at 140 mA?h?g-1,degenerating less than 3%,and the Coulombic efficiency is 95%.The low coulombic efficiency of the solid battery is probably related to the fact that the lithium ion transference number is quite below 1 in the polymer-based electrolyte under certain charge/discharge rates [29,30].The rate performance is shown in Fig.7(b).The battery exhibits the discharge specific capacities of 141,138,and 128 mA?h?g-1at varied current rates of 0.1,0.2,0.3,and 0.5 C,respectively,indicating the battery can deliver stable charges under these conditions.Fig.7(c)shows the galvanostatic discharge/charge profiles of the 1st,20th,50th cycles.The charge and discharge potential platforms of Li-(ZnO)0.15|LLZTO-SPE|LFP are 3.44 V and the 3.39 V,respectively.The very low potential difference between charge and discharge also shows the battery has good reversibility.It is noted that the discharge capacities of 1st,20th,50th cycle are 136.5,145.7,and 143.1 mA?h?g-1and the corresponding Coulombic efficiencies are 92%,95%and 95%.A few increases of discharge capacity and corresponding Coulombic efficiency imply that the full solid battery undergoes an active process.The Nyquist plots and fitting results (Fig.7(d) and Table S3) show the resistance of the full battery is mainly derived fromR1(bulk resistance),R2(ion transfer interfacial impedance of negative electrode),and theR3(ion transfer interfacial impedance of positive electrode) before and after cycling.It can be seen that the initial total resistance of Li-(ZnO)0.15|LLZTO-SPE|LFP is 899.34 Ω?cm2,but the total resistance rises to 1850.07 Ω?cm2after 50 cycles at 60 °C.In Table S3,the fitting results show that the value ofR2is significantly lower thanR3after cycling (increasing to 159.1 from 66.74 Ω?cm2forR2and from 720.2 to 1591 Ω?cm2forR3).This implies that the increase of impedance of the full battery after cycling mainly originates from the interface of the positive electrode/LLZTO electrolyte,and further improving the performance of the battery should design more effective LLZTO/cathode interlayer.

4.Conclusions

In this work,a rough interface of lithium metal and LLZTO solidstate electrolyte was successfully constructed by in situ conversion reaction of zinc oxide powder with metallic lithium at 200 °C.The structure not only dramatically reduces the interfacial impedance of anode and LLZTO electrolyte,but also increases the cycling stability of all-solid-state battery Li-(ZnO)0.15|LLZTO-SPE|LFP.The rough Li-ZnO anode is beneficial to the even current distribution and alleviates the generation of Li dendrites during the Li plating/stripping process so that the Li-(ZnO)0.15|LLZTO|Li-ZnO battery can cycle steadily over 100 h at 0.1 mA?cm-2.This method is effective and straightforward to reduce the interfacial ion transfer resistance for garnet-based batteries.

Declaration of Competing Interest

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.

Acknowledgements

This work was subsidized by the National Natural Science Foundation of China (21776051),the Nation Science Foundation of Guangdong (2018A030313423),and the College Students’ Innovation and Entrepreneurship Training Program (S201911078039).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.03.030.