Solid State Ionics 225 (2012) 594–597

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Solid State Ionics

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Interfacial phenomena in solid-state lithium battery with sulfide solid electrolyte

Kazunori Takada ⁎, Narumi Ohta 1, Lianqi Zhang 2, Xiaoxiong Xu 3, Bui Thi Hang 4, Tsuyoshi Ohnishi,Minoru Osada, Takayoshi SasakiNational Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

⁎ Corresponding author at: National Institute for Matekuba, Ibaraki 305-0044, Japan. Tel.: +81 29 860 4317;

E-mail address: takada.kazunori@nims.go.jp (K. Tak1 Catalyst Research Group, Fuel Cell Cutting-Edge Cen

ciation, 2-3-26 Aomi, Koto-ku, Tokyo 135-0064, Japan.2 National Key Laboratory, Tianjin Institute of Power S

Nankai District, Tianjin 300381, P.R. China.3 Ningbo Institute of Material Technology & Enginee

ences, 519, Zhuangshi Road, Zhenhai District, Ningbo CP.R. China.

4 International Training Institute for Materials Scienceogy, 1 Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam.

0167-2738/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.ssi.2012.01.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 September 2011Received in revised form 5 January 2012Accepted 9 January 2012Available online 1 February 2012

Keywords:Lithium batterySolid electrolyteNanoionicsInterfaceSpace-charge layer

The effects of surface coating on electrode properties of LiMn2O4 in a sulfide solid electrolyte were investigat-ed. The surface coating with LiNbO3 reduced the electrode resistance by two orders of magnitude. Changes inthe electrode properties were very similar to those observed for the corresponding LiCoO2 electrodes, whichstrongly suggest that the space-charge layer formed at the high-voltage cathode/sulfide electrolyte interfaceis rate-determining and must be controlled to improve the rate capability.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Solid-state lithium batteries are expected to be a fundamental so-lution to the safety issues of lithium-ion batteries, which arise fromcombustible organic electrolytes used in the batteries. However, thereplacement of liquid electrolytes with solids lowers the power den-sity, because the ionic conductivities of solid electrolytes are general-ly much lower than those of liquids. Several kinds of sulfides with fastionic conduction have been developed through numerous studiesaiming at fast ionic conduction in solids. The highest ionic conductiv-ity among the sulfides is of the order of 10−3 S·cm−1 [1,2], which hasvery recently risen to 1.2×10−2 S·cm−1 [3]. However, there was stilla high resistance at the interface between the cathode material andthe sulfide solid electrolyte [4]. No matter how high the conductivitybecomes, the power density will not be improved without reducingthe interfacial resistance. We proposed an interfacial design to reducethe interfacial resistance [5]. We speculated that the high interfacialresistance between the Li1− xCoO2 and the sulfide electrolyte

rials Science, 1-1 Namiki, Tsu-fax: +81 29 854 9061.ada).ter Technology Research Asso-

ources, 18 Lingzhuangzi Road,

ring, Chinese Academy of Sci-ity, Zhejiang Province 315201,

, Hanoi University of Technol-

rights reserved.

originates from a highly developed space-charge layer formed at theinterface. The new interfacial structure developed on the basis ofthis speculation effectively reduced the interfacial resistance and in-creased the power density to values comparable with those of liquidsystems.

In the interfacial structure, a thin film of lithium-ion conductiveoxide solid electrolyte was interposed between the Li1− xCoO2 andthe sulfide solid electrolyte; that is, the surface of the LiCoO2 particleswas coated with the oxide solid electrolyte so that the oxide electro-lyte would be between the LiCoO2 and the sulfide electrolyte whenthey were mixed together into a composite electrode. Meanwhile,the coating effects on the electrode properties in liquid electrolytesystems have been reported in many papers as reviewed in Ref. [6],some of which conclude that the coating layer acts to prevent or sup-press the chemical reaction between cathode materials and electro-lytes. A paper on a solid-state battery [7] also reported the presenceand suppression of such chemical reaction layer by surface coating.Either the space-charge layer or the chemical reaction layer can berate-determining in solid-state lithium batteries; however, the im-provement method is different. If the space-charge layer is rate-determining, it is necessary to control interfacial potential using acoating layer. If the latter is the case, the coating layer must blockinterdiffusion.

Coating effects on the cathode properties in sulfide electrolyteshave been studied with LiCoO2 as the cathode material [4,5,7–9]. Onthe other hand, a recently published paper reported that the surfacecoating decreased the electrode resistance of a LiMn2O4/sulfide elec-trolyte composite electrode [10], which suggests that the space-charge layer is rate-determining, because reactivity to the sulfideelectrolyte cannot be the same between LiCoO2 and LiMn2O4. In this

595K. Takada et al. / Solid State Ionics 225 (2012) 594–597

study, the effects of LiNbO3 coating on the electrode properties ofLiMn2O4 were investigated using the identical procedure to that inour previous study on the LiNbO3-coated LiCoO2 system [9] in orderto allow quantitative comparison and confirm which layer is predom-inantly rate-determining and thus must be suppressed in order to im-prove the power density.

2. Experimental

Commercial LiMn2O4 powder (M811, Toda Kogyo; average parti-cle size of 5 μm, Brunauer–Emmett–Teller (BET) surface area of0.40 m2·g−1, and nominal composition of Li1.1Mn1.9O4) was used inthis study. A LiNbO3 layer was formed on the surface of the powderusing the same spray coating procedure as reported in Ref. [9]. Lithi-um metal (Honjo Chemical) was dissolved in anhydrous ethanol(Kanto Chemical) and mixed with niobium pentaethoxide (KojundoChemical Laboratory) to obtain a precursor sol of LiNbO3. The precur-sor sol was sprayed onto the surface of the LiMn2O4 powder by a fine-powder coating machine (SFP-01, Powrex). The samples were heatedat 400 °C for 0.5 h under an oxygen flow in order to decompose thealkoxides and transform them into LiNbO3.

Thickness of the LiNbO3 layer (t) was controlled from 0 nm (pris-tine sample) to 20 nm by changing the dose of the precursor solution,which was calculated from the specific gravity of LiNbO3, BET surfacearea of the LiMn2O4 powder, and the concentration of the solution onthe assumption that the surface was uniformly covered with LiNbO3.Li, Nb, and Mn content in the samples was measured by inductivelycoupled plasma atomic emission spectroscopy (ICP-AES) in order toconfirm the formation of the LiNbO3 layer with the attempted thick-ness. Fig. 1 compares the calculated and analyzed gravimetric frac-tions. They agreed with each other, which verified that the LiNbO3

layer with the attempted thickness was formed on the surface ofthe LiMn2O4 particles.

Electrochemical cells for investigating the electrode properties ofthe LiNbO3-coated LiMn2O4 were constructed with Li3.25Ge0.25P0.75S4as the sulfide solid electrolyte. The Li3.25Ge0.25P0.75S4 was synthesizedusing the procedure reported in Ref. [1] and pressed into a pellet witha diameter of 10 mm. The working electrode was a mixture of theLiMn2O4 and Li3.25Ge0.25P0.75S4 at a weight ratio of 1:1, while the

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counter electrode was In–Li alloy, which was obtained by attachinga small piece of Li foil (b1 mg) to In foil (60 mg). The working elec-trode mixture (15 mg), the ground electrolyte powder, and the In–Li alloy were pressed together at 500 MPa into a three-layer pelletacting as a two-electrode cell with a diameter of 10 mm. The cellswere operated at a constant current density of 20 μA·cm−2. Afterthe cells were charged to 3.68 V, the electrode resistance was evaluat-ed by electrochemical impedance spectroscopy from 106 to 10−2 Hzwith an AC signal of 10 mV. Because the electrode potential of In–Lialloy is 0.62 V vs. Li+/Li, the cell voltages in this paper are presentedby adding 0.62 V as if lithium metal were used as the counter elec-trode, for much easier reading.

3. Results and discussion

3.1. Nanoionics at Li1− xCoO2/sulfide electrolyte interface

Our previous studies on Li1− xCoO2/sulfide electrolyte interfacebased on nanoionics [11] concluded that the highly developedspace-charge layer at the interface is rate-determining. Thelithium-depleted space-charge layer is formed by the difference inelectrochemical potential of lithium ions between Li1− xCoO2 andthe sulfide electrolyte, as mentioned in the manuscript. When a sul-fide solid electrolyte comes into contact with the Li1− xCoO2, a por-tion of the lithium ions in the sulfide solid electrolyte move to theLi1− xCoO2, because oxide ions have a much stronger attraction ef-fect on lithium ions compared to sulfide ions. The movement in-creases the lithium ion concentration on the Li1− xCoO2 side atthe interface and decreases it on the sulfide electrolyte side. How-ever, the electronic conduction in the Li1− xCoO2 prohibits the in-crease in lithium ion concentration on the Li1− xCoO2 side,because it enables lithium ions to diffuse from the interface to thebulk accompanied by electrons to release the concentration gradi-ent of lithium ion, which promotes further movement of lithiumions. Finally, when the interface reaches equilibrium, the space-charge layer is highly developed on the sulfide electrolyte side,such that lithium becomes depleted to make the space-chargelayer highly resistive.

When the surface of the Li1− xCoO2 is coated with a lithium-ionconductive oxide solid electrolyte, equilibrium is reached betweenthe oxide electrolyte and the sulfide electrolyte instead of that be-tween Li1− xCoO2 and the sulfide electrolyte. At the interface, the lith-ium ion concentration on the oxide side can rise due to the absence ofelectronic conduction. Therefore, only a small amount of lithium ionmovement can make the interface reach equilibrium, because themovement increases the lithium ion concentration on the oxide sideas well as decreases that on the sulfide side, and thus the sulfide elec-trolyte can be connected with the Li1− xCoO2 without the formationof the highly developed space-charge layer.

In other words, when a sulfide solid electrolyte comes into con-tact with the Li1− xCoO2, the noble potential of Li1− xCoO2 lowersthe lithium ion concentration on the sulfide solid electrolyte sideat the interface to make the electrochemical potential, or activity,of lithium ions there reach equilibrium with the noble potentialof Li1− xCoO2, resulting in the formation of a highly resistivelithium-depleted layer. The oxide solid electrolyte interposed atthe interface shields the sulfide electrolyte from the high potentialof Li1− xCoO2 and allows the Li1− xCoO2 to be connected to the sul-fide electrolyte without the formation of the lithium-depletedlayer. Accordingly, the electrochemical potential of lithium ion isan important factor in the formation and suppression of thespace-charge layer. Since Li1− xMn2O4 shows almost the samenoble potential as Li1− xCoO2, a similar situation should arise atthe Li1− xMn2O4/sulfide electrolyte interface if the space-chargelayer is rate-determining.

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596 K. Takada et al. / Solid State Ionics 225 (2012) 594–597

3.2. Coating effect on Li1− xMn2O4/Li3.25Ge0.25P0.75S4 interface

Changes in the electrode resistance of the LiMn2O4 electrode uponLiNbO3 coating are shown in Fig. 2. The coating reduced the electroderesistance of the Li1− xMn2O4/Li3.25Ge0.25P0.75S4. The electrode resis-tance of the uncoated sample (t=0 nm) was 1×104 Ω, and theLiNbO3 coating with t=20 nm reduced it to 200 Ω. That is, theLiNbO3 coating reduced the electrode resistance by two orders ofmagnitude, which was in agreement with that observed for theLi1− xCoO2/Li3.25Ge0.25P0.75S4 interface [9].

We also pointed out in our previous study [4,5,9] that the presenceof the highly developed space-charge layer can be recognized at thevery beginning of the charge curve. When a sulfide solid electrolyteis in contact with a high-voltage cathode material, the lithium-ionconcentration in the interfacial region should decrease to reach equi-librium. Therefore, the space-charge layer is developed while theLiCoO2 is charged to Li1− xCoO2 to show a noble potential of 4 V. Dur-ing the development, lithium ions are removed from the interfacialregion in the sulfide electrolyte, which is observed as an additionaloxidation step at the beginning of the charge curve prior to a 4-V pla-teau corresponding to the lithium deintercalation reaction. Experi-mentally, the additional oxidation step appeared as a potential slopeat the beginning of the charge curve. Also in the case of LiMn2O4 in

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this study, a similar potential slope appeared from 3.0 V to 4.0 V atthe beginning of the charge curve, as shown in the uppermost panelof Fig. 3, where the charge curve is plotted against the capacity (Q)normalized by the BET surface area of the LiMn2O4, because theamount of lithium ions removed from the interfacial region shouldbe proportional to the interfacial area.

In the coating process, the surface of the LiMn2O4 particles is grad-ually covered with the LiNbO3. Therefore, the interfacial area wherethe highly developed space-charge layer is formed, i.e. the uncoatedsurface area, gradually decreases with increasing dose of LiNbO3. Itshould gradually decrease the Q in the potential slope, which can beclearly seen in the figure.

3.3. Comparison between Li1− xMn2O4/Li3.25Ge0.25P0.75S4 andLi1− xCoO2/Li3.25Ge0.25P0.75S4 interfaces

Since the lithium ion concentration at the interface is governed bythe difference in the potential between the interface and the bulk[11], similar potentials between Li1− xCoO2 and Li1− xMn2O4 predictthat the above consistencies should be quantitative. Fig. 3 alsoshows the charge curves for Li1− xCoO2/LiNbO3/Li3.25Ge0.25P0.75S4

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Fig. 3. Changes in the charge curves upon surface coating with LiNbO3. The solid anddashed lines are charge curves of the In–Li//LiMn2O4 and In–Li//LiCoO2 [9] cells, respec-tively. Q is the capacity normalized by the BET surface area of the LiMn2O4 or LiCoO2.

597K. Takada et al. / Solid State Ionics 225 (2012) 594–597

interface system [9] as comparisons, where Q for the latter wasalso normalized by the BET surface area of the LiCoO2

(0.26 m2·g−1). As clearly seen in the figure, the potential slopeobserved for the Li1− xMn2O4/Li3.25Ge0.25P0.75S4 interface is ingood agreement with that for the Li1− xCoO2/Li3.25Ge0.25P0.75S4 in-terface. Moreover, the changes in the potential slope uponLiNbO3 coating closely resemble each other.

The Q in the slope should correspond to the number of lithiumions that are removed from the interface during the formation ofthe lithium-depleted layer. Since the lithium ion concentration inLi3.25Ge0.25P0.75S4 can be calculated from the crystallographic data,the thickness of the lithium-depleted layer can be roughly estimatedfrom Q. Lattice constants of Li3.25Ge0.25P0.75S4 are a=13.395(3)Å,b=22.979(5)Å, c=18.1981(18)Å, and β=92.591(14)°, and its Zformula units are 9 [1], which give a lithium ion concentration of5.2×1021 cm−3. The Q in the potential slope was 0.23 and0.19 μAh·cm−2 for the Li1− xMn2O4/Li3.25Ge0.25P0.75S4 and Li1− x

CoO2/Li3.25Ge0.25P0.75S4 interfaces, respectively. This Q correspondsto the number of lithium ions in a thickness of 9.7 nm and 8.3 nm, re-spectively, which is reasonable for the thickness of the space-chargelayer in super-ionic conductors [11–15]. Of course, these values maybe somewhat underestimated, because lithium ion concentrations inthe interfacial region will not be completely zero and not all of thesurface acts as the electrochemical interface. However, this wouldnot be a poor estimation, because high interfacial resistances indicatethat the ion concentration is extremely low, and high utilization ofthe active materials suggests that the portions of the surface area act-ing as the electrochemical interface are not small.

4. Conclusions

The effects of surface coating on the electrode properties ofLiMn2O4 in the sulfide electrolyte were very similar to those observedfor LiCoO2. The similarities provide strong evidences that the highlydeveloped space-charge layer is the reason for the high resistances

in composite electrodes comprising high-voltage cathode and sulfideelectrolyte, and the surface coating suppresses the formation of thespace-charge layer to reduce the electrode resistance.

Acknowledgment

This work was partially supported by the New Energy, IndustrialTechnology Development Organization (NEDO), a Grant-in-Aid forScientific Research on Priority Areas, “Nanoionics 439”, and theWorld Premier International Center Initiative (WPI Initiative) on Ma-terials Nanoarchitectonics, Ministry of Education, Culture, Sports, Sci-ence and Technology (MEXT), Japan.

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