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Three- and four-electrode EIS analysis of water stable lithium electrode with solid electrolyte plate
Electrochimica Acta 81 (2012) 179–185
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Three- and four-electrode EIS analysis of water stable lithium electrode with solid electrolyte plate Tonghuan Yang a , Lin Sang b , Fei Ding b , Jing Zhang b , Xingjiang Liu a,b,∗ a b
School of Chemical Engineering, Tianjin University, Tianjin 300072, China National Key Lab of Power Source, Tianjin Institute of Power Source, Tianjin 300384, China
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 29 July 2012 Accepted 30 July 2012 Available online 4 August 2012 Keywords: Li–air battery Water stable lithium electrode LAGP Interface Electrochemical impedance spectroscope
a b s t r a c t The properties of interfaces of water-stable lithium electrode have been investigated by EIS in Li–air batteries and four-electrode test-cells. The water-stable lithium electrode was prepared with the protection of NASICON-type glass–ceramic plate [0.8Li2 O·0.25Al2 O3 ·1.5GeO2 ·1.5P2 O5 glass–ceramic (LAGP)]. The glass–ceramic plates are water-stable and 200–500 m thick with ion-conductivity of 4 × 10−4 S cm−1 . In Li–air batteries, the AC impedance spectra of LAGP protected lithium electrode contained two semicircles and became three after one week. The composition of impedance of LAGP protected lithium electrode was analyzed, by comparison on the impedance spectra of LAGP protected lithium electrode and bare Li metal electrode in organic electrolyte. Furthermore, in four-electrode test-cell which excluded the LAGP|aqueous-solution interface, the interface impedance of LAGP plate was analyzed. And then by the analysis of impedance, the Li|organic-electrolyte interface caused the main impedance of the waterstable lithium electrode during the discharge. The impedance of water-stable lithium electrode should be decreased by the study of new electrolyte and interface modification. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, lithium–air batteries became a major trend of new chemical power sources system research. This system employs inexhaustible oxygen from air as electrode’s reagent and metallic lithium as electrode with the most negative potential and high capacity of 3860 mAh g−1 . According to air electrode and lithium electrode, the theoretical specific energy of lithium–air battery is around 3500 Wh kg−1 for reaction forming Li2 O2 . However, until now, the lithium–air battery application has to face two problems: one is the insoluble Li2 O2 accumulates and blocks reaction in porous air electrode in organic electrolyte. Another is lithium electrode reacts with the oxygen, carbon dioxide and water, which diffuse from air electrode [1–10]. For avoiding mentioned problems of Li–air battery, Visco et al. and Imanishi et al. [11–14] proposed a water stable lithium electrode (WSLE) that is a metallic lithium electrode protected by a water stable lithium-ion super-ionic conductor glass (NASICON) plate with the interlayer which could be solid, polymer or organicliquid electrolyte. The interlayer, which is between the lithium metal electrode and water stable glass plate, is used to provide
∗ Corresponding author at: National Key Lab of Power Source, Tianjin Institute of Power Source, Tianjin 300384, China. Tel.: +86 022 23959581; fax: +86 022 23383783. E-mail address: [email protected] (X. Liu). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.07.103
better contact and prevent the reaction between the lithium and glass plate. Rather than the non-aqueous Li–air battery, a hybrid electrolyte lithium–air battery (HELB) was developed using WSLE. In HELB, an air electrode in aqueous electrolyte and a metallic lithium electrode in organic or polymer electrolyte was separated by a NASICON glass plate [Li3 M2 (PO4 )], such as Li1+x Ti2−x Alx (PO4 )3 (x = 0.2–0.3) and Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) [12–20]. At the same time, as the application required, HELB can be designed as a high energy density rechargeable battery or a lithium–air fuel cell with aqueous solution cycle system [9]. Contrasting with the simple diffusion process in organic electrolyte, the Li+ migration of HELB includes complex diffusion processes and interface processes, such as the diffusion in solid electrolyte phase, aqueous electrolyte and organic electrolyte, the interface of organic-electrolyte|solid-electrolyte and solid electrolyte|aqueous electrolyte. During Li+ migration, the impedance of these processes caused a more “polarization potential” than that of the non-aqueous Li–air battery. Then, for improving the discharge performance of HELB, it is essential to decrease the Li+ migration impedance of WSLE. Furthermore, it would be required the study of impedance of WSLE to decrease the impedance. The EIS, as a technique for impedance research, have been performed in the study of solid electrolyte plate and HELBs impedance [1–3,17]. The AC impedance was employed to compare and analyze the characteristic of WSLEs. However, the analysis of the impedance of WSLEs was scarcely reported during the discharge. In this
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work, for studying the impedance of WSLE the ac impedance was measured with reference electrode and four-electrode test-cell. During discharging, in order to study the impedance of water-stable lithium electrode, the polarized water-stable lithium electrodes were measured by EIS. As a result, the impedance spectra were analyzed and discussed for proposing the way to decrease the impedance of WSLE. 2. Experimental 2.1. Preparation and physical characterizations of LAGP plate The LAGP glass–ceramic [0.8Li2 O·0.25Al2 O3 ·1.5GeO2 ·1.5P2 O5 (mol%)] exhibited high lithium-ion conductivity and was made into thin electrolyte plates [15–17]. In this work, the LAGP plates were synthesized by reagent grade chemicals, including Li2 CO3 , Al2 O3 , GeO2 , and NH4 H2 PO4 . The four raw materials were weighted and mixed by the stoichiometry of 0.8Li2 O·0.25Al2 O3 ·1.5GeO2 ·1.5P2 O5 . To begin with, the mixed raw materials were heated slowly to 723 K and kept for 1.5 h to release gaseous production. Subsequently, the mixture was heated up to 1723 K and melted for 2 h. Subsequently, the molten mixture was poured in a heated plate mold and pressed by another one. The glass plate, which was casted by the molten mixture, was annealed at 823 K for 2 h and cooled in furnace. After annealing, the glass plate crystallized at 1223 K for 12 h and became a dense opaque white specimen. This specimen was refined shape and polished to a 200–500 m thick plate. In order to exclude the effect of different batches of the LAGP plate, a big LAGP plate was cut into 10–20 pieces of about 1 cm2 small ones for contrast in all physical and electrochemical measurements. The morphologies of the LAGP surfaces were characterized by a Hitachi S-4800 field emission scanning electron microscopy (FESEM). The X-ray diffraction (XRD) patterns of LAGP thin plates were recorded by a Rigaku TTRAX III diffractometer employing a Cu-K␣ source. 2.2. Three-electrode and four-electrode test cell design and assembly The ionic-conductivity of LAGP plate was measured by EIS. A 0.5 m thick Ag coating was deposited on both sides of the LAGP plate by evaporation. The plate coated with Ag was fixed by two stainless blocking electrodes in a blocking test cell. The three-electrode test-cells were assembled with organic electrolyte (EC:DEC:EMC = 1:1:1, 1 M LiPF6 ) and three lithium electrodes, which were employed as electrode (WE, 0.5 cm2 ), counter electrode (CE, 0.5 cm2 ) and reference electrode (Ref., 0.25 cm2 ) respectively. The HELBs with a reference electrode were designed and assembled for impedance spectroscope measurements, as shown in Fig. 1(b). The HELBs were separated into a Li electrode region and an air electrode region by LAGP plate (0.25 cm2 working area) with a thickness of 500 m. The Li electrode region was filled with organic electrolyte (EC:DEC:EMC = 1:1:1, 1 M LiPF6 ) and contained a Limetal electrode (0.5 cm2 ). The air electrode region was filled with aqueous solution (0.5 M LiOH and 0.5 M KCl) and contained an airelectrode (Super-P carbon loaded Ni foam, 1 cm2 ) and a reference electrode (SCE in KCl saturated solution, connected by salt bridge). A four-electrode test-cell was designed for further electrochemical impedance spectroscope measurements, as shown in Fig. 1(c). The test-cells were separated into two regions by 500 m or 200 m thick LAGP plates with 0.25 cm2 working area. There are two metallic lithium electrodes and organic electrolyte (EC:DEC:EMC = 1:1:1, 1 M LiPF6 ) which sealed in each region. In one
Fig. 1. Schematic representations of test-cells: (a) the three-electrode test-cell with organic electrolyte (WE and CE: 0.5 cm2 Li metal, RE: 0.25 cm2 Li metal), (b) the three-electrode test-cell with hybrid electrolyte (WE: 0.5 cm2 Li metal in organic electrolyte, RE: salt bridge + SCE in KCl saturated solution, CE: 1 cm2 air-electrode in aqueous solution) and (c) the four-electrode test cell with hybrid electrolyte (WE and CE: 0.5 cm2 Li metal; RE1 + RE2: 0.25 cm2 Li metal, all in organic electrolyte separated by LAGP) (organic electrolyte: EC:DEC:EMC = 1:1:1, 1 M LiPF6 ; aqueous solution: 0.5 M LiOH and 0.5 M KCl aqueous solution).
region, the two lithium electrodes were used as a work electrode (WE, 0.5 cm2 ) and reference electrode 1 (Ref. 1, 0.25 cm2 ). In the other, the two lithium electrodes were used as a counter electrode (CE, 0.5 cm2 ) and reference electrode 2 (Ref. 2, 0.25 cm2 ). A frequency response analyzer (a Solartron instrument model, 1470E + 1455A), controlled by MultiStatData software, was employed for obtaining impedance spectra at the room temperature (298 K). Unless specified, the AC impedance measurements were performed at OCP with the AC amplitude of 10 mV and frequency ranged from 0.1 to 1 MHz. The collected EIS were fitted using ZView software. 3. Result and discussion 3.1. The physical and electrochemical characterization of LAGP plate For discussing the characteristics, the XRD, SEM and EIS were measured. Fig. 2 provides XRD patterns of the LAGP plate which consists of Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) and AlPO4 crystals [15]. In XRD patterns, the peaks marched the characteristic of Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) and AlPO4 . The Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) phase is dominant in LAGP glass–ceramics, and other peaks indicates the presence of an AlPO4 phase. Fig. 3 shows SEM images of the LAGP surface. The LAGP surface is smooth, close-grained and has visible traces of polishing. Fig. 4
Fig. 2. XRD patterns of LAGP plate.
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Fig. 3. SEM images of LAGP plate surface; (a) 10,000× and (b) 50,000×.
shows the impedance spectra of blocking electrode. In Fig. 4, the impedance spectra of LAGP have a semicircle or distorted one at high frequency and a line at low frequency. The conductivity of LAGP plate can be calculated. It can reach 4 × 10−4 S cm−1 [the formula: ε = d/(S·R); d: thickness of LAGP; S: area of LAGP; R: resistance of LAGP] [21–23]. 3.2. AC impedance spectra of three-electrode test-cells The AC impedance of WSLE comes from metallic lithium electrode impedance, organic electrolyte impedance, solid-state electrolyte impedance and the interface impedance between them. For studying the interfacial phenomenon of one interface, impedance spectra of either HELBs or the symmetric Li–Li test-cells without reference electrode were usually used [1–3,8,10,13,14]. However, a HELB combines the interfacial properties of the WSLE and the air electrode. Thus, the interfacial impedance of WSLE was not exactly reflected by it. Through, in Li–Li test-cells without reference electrode, the impedance of WSLE has been estimated by EIS, the interfacial impedance in WSLE was not analyzed in detail. Then, a series of three- or four-electrode impedance measurements has been conducted to investigate the interfacial impedance on WSLE. At first, three-electrode test-cells with three lithium electrodes were used to investigate the properties of the Li|organic-electrolyte interface. The configuration of three-electrode test cell is shown in Fig. 1(a). As Fig. 5, two overlapped semicircles are shown in the initial impedance spectrum of lithium electrode in three-electrode
Fig. 4. An AC impedance spectrum of Ag/LAGP plate/Ag test cells at 25 ◦ C.
test-cell, in which the high and low frequency semicircle is caused by solid electrolyte interface (SEI) impedance and charge-transfer impedance respectively. It is consistent with the reports of Li electrode impedance [24]. The typical equivalent circuit of Li electrode was used for fitting the impedance spectra. Re is ohmic resistance (21 ), which was estimated by the intercept of the high frequency semicircle with real axis. Rsei is SEI resistance (350 ); CPE1 is a constant phase element, which represents the geometric capacitance of SEI. Rct is charge-transfer resistance (105 ); CPE2 represents a capacitance related to charge-transfer. By contrasting the impedance spectra of the initial and an hour storage, the increase of SEI impedance, corresponding to the semicircle at high frequency, can be observed. The result of fitting analysis is: Re is 27 , Rsei is 549 , Rct is 116 . The Rsei markedly increased with the storage time. The formation of SEI layer is essential and durative process on the surface of metallic lithium [25,26]. In view of the growth of SEI layer within hours or months, the three-electrode test-cells were stood at room temperature and measured impedance at different time. Fig. 6 shows the five impedance spectra of Li electrode at 1 h, 24 h, 72 h, a week and a month. In the impedance spectra, the semicircle at low frequency corresponds to the charge-transfer impedance. The semicircle at high frequency, which corresponds to the SEI impedance, increased and covered the one at low frequency gradually. The two semicircles were combined to form one big semicircle, in which the SEI impedance is dominated. Then, if the Rct is reduced from the resistance of big semicircle, it was
Fig. 5. An AC impedance spectrum and fitting-line for the cell with three Li electrodes at 25 ◦ C.
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Fig. 6. Time dependence of the impedance spectra for the cell with three Li electrodes at 1 h, 24 h, 72 h, a week and month.
estimated that the SEI resistance increased from 500 to around 3100 . The increase of SEI impedance was caused by SEI layer growing at the Li|organic-electrolyte interface [24–26]. This kind of impedance spectra of Li electrode with a single semicircle was also reported [24]. 3.3. Three-electrode impedance spectra of Li–air batteries The WSLE and air electrode was assembled into a HELB, adding a SCE in aqueous solution as reference electrode. After assembly, the impedance spectra were measured and shown in Fig. 7. In Fig. 7, the high frequency semicircle corresponds to the grain boundary impedance, which can be estimated from the diameter of small semicircle and the reports of WSLE [3]. The medium and low frequency semicircle should correspond to the impedance of SEI and charge-transfer, and the interfacial impedance at LAGP plate/liquid electrolyte. However, the medium and low frequency impedance spectra could be a compressed semicircle but could also be two or three overlapped semicircles, by which the number of processes and the impedance of each process cannot be estimated and analyzed exactly. Fig. 8 shows the impedance spectra of WSLE in HELBs at different standing time. The impedance of WSLE increased with time and the medium and low frequency semicircle became two semicircles, which the medium frequency one increased and
Fig. 7. An AC impedance spectrum of WSLE in Li/air battery.
Fig. 8. Time dependence of the impedance spectra and fitting-lines of WSLE in Li/air battery.
Table 1 The resistances of the equivalent circuit of WSLE in Li/air battery at different time.
An hour A week A month
Re
Rg
Rint(N + A)
Rsei
27 30 37
217 218 233
604 770 797
751 1450 4948
reached stable within a week and the low frequency one was still increasing. In Fig. 8, the impedance spectra suggests that the medium and low frequency semicircles correspond to at least two interface processes, and one must correspond to SEI layer impedance, another corresponds to the impedance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface. In respect that the increase of low frequency semicircle is almost equal to the increase of SEI semicircle in Fig. 6, the low frequency semicircle should correspond to the impedance of SEI layer and charge-transfer. Then, the medium frequency semicircle should correspond to the impedance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface. Then, in HELBs, the impedance of WSLE included four parts: 1, the bulk impedance of solution, LAGP plate and current collector; 2, the impedance of LAGP grain boundary; 3, the impedance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface; 4, the impedance of SEI layer and charge-transfer. The semicircle diameter and shape of SEI layer and charge-transfer are dominated by the impedance of SEI layer. As a result, the low frequency semicircle has the similar shape and change with time. Then, an equivalent circuit of three-electrode test-cell can be proposed, as Fig. 8. In equivalent circuit, Re is the bulk resistance of solution, LAGP plate, wire and electrode; Rg is the resistance of LAGP grain boundary; Rint(N + A) is the resistance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface; Rsei is the resistance of SEI layer and Li+ charge-transfer in equivalent circuit. Because the semicircle is distorted, the constant phase element has been chosen to replace the capacitance. The CPE1, CPE2 and CPE3 are the respective constant phase elements of Rg, Rint(N + A) and Rsei. Table 1 shows a resistances list of the equivalent circuit with time, in which Re and Rg was almost constant and Rint(N + A) increased and reached stable within a week. Although the estimated resistances of Rint(N + A) and Rsei were not accurate due to the overlapped two semicircles in the impedance spectrum of an hour standing, the resistances still can be a contrast. The Rsei, corresponding to impedance of SEI layer and charge-transfer, was
T. Yang et al. / Electrochimica Acta 81 (2012) 179–185
Fig. 9. Time dependence of the impedance spectra of WSLE (3-electrode test).
increasing with SEI layer growth and became the main impedance of WSLE in a month. 3.4. Four-electrode impedance spectra of test-cell In the impedance spectra of HELBs with reference electrode, the impedance of each interface cannot be analyzed exactly, then, the impedance spectra of four-electrode test-cell were performed. In four-electrode test-cell, there are only two interfaces which are Li|organic-electrolyte and organic-electrolyte|LAGP interface. By using two reference electrodes, the impedance of organicelectrolyte|LAGP interface was estimated and analyzed. The four-electrode test-cell included a current control and measurement circuit that consists of WE (as work electrode) and CE (as counter electrode), and a potential control and measurement circuit that consists of Ref. 1 (as reference electrode No. 1, connect with sensor 1) and Ref. 2 (as reference electrode No. 2, connected with sensor 2). At first, for contrast with the four-electrode impedance spectra, three-electrode impedance spectra were measured in four-electrode test-cell, in which the RE2 was used as reference electrode; the impedance of Li electrode was also measured in the same test-cell with one reference electrode (Ref. 1). The impedance spectra are showed in Fig. 9. The impedance spectra of an hour standing has two semicircle, the high frequency one is small and corresponds to the impedance of grain boundary, the low frequency one is flat circle and should correspond to the impedance of SEI layer, charge-transfer, and organic-electrolyte|LAGP interface. After a week and a month, similarly with Fig. 8, the one semicircle at low frequency region grew and became two semicircles. The low frequency semicircle, which corresponds to the impedance of SEI layer and charge-transfer, changed with SEI layer formation and growth, which is the same to the phenomenon of WSLE in HELBs. Consequently, the medium frequency semicircle should correspond to the impedance of organic-electrolyte|LAGP interface. Then, in four-electrode test-cell, the impedance composition of WSLE is similar with HELBs, except the LAGP|aqueous-solution interface. The equivalent circuit can be proposed for it, including: Re, Rg, Rsei and Rint(N + N). As symmetric, the test-cell consists of two organic-electrolyte|LAGP interfaces so that the interfacial resistance [Rint(N + N)] was twice. Fig. 10 shows the difference of impedance spectra for threeelectrode test and four-electrode test. The low frequency semicircle with four electrodes that excluded the Li|organic-electrolyte interface is smaller to the low frequency one of three-electrode, and
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Fig. 10. An AC impedance spectra of WSLE (3-electrode and 4-electrode test, a week).
the impedance difference is almost equal to the impedance of Li electrode in Fig. 11. It suggests that the organic-electrolyte|LAGP impedance can be reflected by a single low frequency semicircle in four-electrode impedance spectra, instead of three-electrode impedance spectra in which the both of the SEI layer and organicelectrolyte|LAGP impedance cannot be exactly analyzed with an overlapped semicircle. Since the time constant of each interface processes is close and the diameters corresponding to impedance are lengthy, the all semicircles usually overlap and are looked like one semicircle [1–3,13,14]. However, the impedance spectra of four-electrode test made that the impedance of WSLE can be reflected and analyzed. By this method, the grain boundary and organicelectrolyte|LAGP were measured and used for discussing the organic-electrolyte|LAGP impedance. Then, the impedance of each in four-electrode test-cell can be estimated. In Fig. 11, Rsei was around 630 , which was estimated by the diameter of semicircle. The resistances of equivalent circuit can be estimated from Fig. 10 (Re ≈ 310 , Rg ≈ 240 , Rint(N + N) ≈ 1500 ). Consequently, the resistance of a single organic-electrolyte|LAGP interface is around 750 . The impedance of LAGP|aqueous-solution can be estimated and calculated by the impedance of organic solution|LAGP|aqueoussolution and organic solution|LAGP. It is about 20–30 . The
Fig. 11. An AC impedance spectrum of Li electrode (3-electrode test, a week).
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T. Yang et al. / Electrochimica Acta 81 (2012) 179–185 Table 2 The resistances of the equivalent circuit of WSLE with thin LAGP plate in fourelectrode test cell at different time (3 electrodes test).
An hour A day A week
Re
Rg
Rint(N + N)
Rsei
335 337 349
47 53 60
494 608 833
528 642 996
Fig. 12. An AC impedance spectrum and fitting-line of WSLE (3-electrode test).
impedance of the symmetric aqueous solution test-cell is about 100 , corresponding to impedance of both LAGP|aqueous-solution interfaces. That means about 40–50 for each interface, which is similar with estimated result, but more accurate. The impedance spectra of WSLE are affected by the thickness and ion conductivity of LAGP plate, as Fig. 12. In four-electrode test-cell, the impedance spectrum with a reference electrode has two semicircles which correspond to grain boundary impedance at high frequency region and organic-electrolyte|LAGP interface at low frequency region respectively. The semicircle corresponding to LAGP grain boundary cannot be distinguished, since it was small and overlapped by the semicircle of SEI. According to the equivalent circuit model, the line of fitting calculation is shown in Fig. 12, as a result, Re ≈ 335 , Rg ≈ 47 , Rint(N + N) ≈ 494 , Rsei ≈ 528 . Since the semicircle of Rg is indistinct small, the calculation result of it is inaccurate; nevertheless the results of other resistances are worth to analyze. Fig. 13 shows the impedance of WSLE with LAGP plate is increasing with time. The impedance spectra were analyzed by fitting lines and the resistance of each was estimated and listed in Table 2. The Rsei increased with time in a month, which can be attributed to the SEI layer growth. The Rint(N + N) increased within a week and kept stable subsequently, which the phenomena also described above. It suggests that the SEI impedance would become the main impedance of WSLE by the growth of SEI with time at OCP. About
Fig. 13. Time dependence of the impedance spectra and fitting-lines of WSLE (3electrode test).
Fig. 14. AC impedance spectra of Li electrode at different polarized potential (3electrode test, a week).
the impedance increase of the organic-electrolyte|LAGP interface within a week, it may be caused by a kind of interface layer on the LAGP, which conduct Li ion and affect the Rint(N + N) by formation and growth. Although the impedances spectra of WSLE were measured and analyzed in different cell at OCP, the main impedance of polarized WSLE may change while discharging. During discharge polarization, the impedance spectra were measured in four-electrode test-cell for discussing the polarization influence of each part of the impedance. Fig. 14 shows that the impedance of Li electrode rapidly decreased with enhance of polarization potential, which measured by three-electrode EIS in four-electrode test-cell. When the Li electrode polarized at 0.2 V, Rsei decreased to around 260 from around 645 , which was caused by the break of SEI layer at the Li metal surface. In contrast, Fig. 15 shows
Fig. 15. AC impedance spectra of WSLE (3-electrode and 4-electrode test, a week).
T. Yang et al. / Electrochimica Acta 81 (2012) 179–185
the Rg was almost constant and Rint(N + N) decreased slightly. When the polarization potential between two reference electrodes was 0.2 V, Rint(N + N) of two organic-electrolyte|LAGP interfaces decreased from 1565 to 1277 . Consequently, resistance of organic-electrolyte|LAGP decreased from 782 to 633 . During the polarization, the SEI impedance decreased much more than the organic-electrolyte|LAGP impedance. It suggests that although the SEI impedance was more than the organicelectrolyte|LAGP impedance at OCP, the organic-electrolyte|LAGP impedance was almost constant and became the main while discharging. So the Li+ migration at the organic-electrolyte|LAGP interface is an process which cannot be intensively affected and controlled by potential. Therefore, the Li+ migration between two kinds of electrolyte is a process including diffusion and electro-migration, and mainly affected the discharge current density. Then, to improve the discharge performance of Li–air battery, the decrease of organic-electrolyte|LAGP impedance is essential, which should be achieved by the research of new solid and non-aqueous electrolyte and modification of the interface. 4. Conclusion Summarily, the four-electrode EIS was employed to analyze the impedance of the organic-electrolyte|LAGP interface, which can eliminate the combination effect of semicircles in AC impedance spectra. The impedance of WSLE protected by LAGP was investigated during the discharge. In the impedance spectra tested at OCP, the SEI impedance became the main impedance of WSLE as the growth of SEI with time. While discharging, the SEI impedance rapidly decreased with polarization potential, but the organicelectrolyte|LAGP impedance changed a little and became the major impedance. Then, the impedance on organic-electrolyte|LAGP interface was the main problem to improve the discharge performance, which should be decreased by the study of new electrolyte and interface modification.
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Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Three- and four-electrode EIS analysis of water stable lithium electrode with solid electrolyte plate Tonghuan Yang a , Lin Sang b , Fei Ding b , Jing Zhang b , Xingjiang Liu a,b,∗ a b
School of Chemical Engineering, Tianjin University, Tianjin 300072, China National Key Lab of Power Source, Tianjin Institute of Power Source, Tianjin 300384, China
a r t i c l e
i n f o
Article history: Received 15 May 2012 Received in revised form 29 July 2012 Accepted 30 July 2012 Available online 4 August 2012 Keywords: Li–air battery Water stable lithium electrode LAGP Interface Electrochemical impedance spectroscope
a b s t r a c t The properties of interfaces of water-stable lithium electrode have been investigated by EIS in Li–air batteries and four-electrode test-cells. The water-stable lithium electrode was prepared with the protection of NASICON-type glass–ceramic plate [0.8Li2 O·0.25Al2 O3 ·1.5GeO2 ·1.5P2 O5 glass–ceramic (LAGP)]. The glass–ceramic plates are water-stable and 200–500 m thick with ion-conductivity of 4 × 10−4 S cm−1 . In Li–air batteries, the AC impedance spectra of LAGP protected lithium electrode contained two semicircles and became three after one week. The composition of impedance of LAGP protected lithium electrode was analyzed, by comparison on the impedance spectra of LAGP protected lithium electrode and bare Li metal electrode in organic electrolyte. Furthermore, in four-electrode test-cell which excluded the LAGP|aqueous-solution interface, the interface impedance of LAGP plate was analyzed. And then by the analysis of impedance, the Li|organic-electrolyte interface caused the main impedance of the waterstable lithium electrode during the discharge. The impedance of water-stable lithium electrode should be decreased by the study of new electrolyte and interface modification. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, lithium–air batteries became a major trend of new chemical power sources system research. This system employs inexhaustible oxygen from air as electrode’s reagent and metallic lithium as electrode with the most negative potential and high capacity of 3860 mAh g−1 . According to air electrode and lithium electrode, the theoretical specific energy of lithium–air battery is around 3500 Wh kg−1 for reaction forming Li2 O2 . However, until now, the lithium–air battery application has to face two problems: one is the insoluble Li2 O2 accumulates and blocks reaction in porous air electrode in organic electrolyte. Another is lithium electrode reacts with the oxygen, carbon dioxide and water, which diffuse from air electrode [1–10]. For avoiding mentioned problems of Li–air battery, Visco et al. and Imanishi et al. [11–14] proposed a water stable lithium electrode (WSLE) that is a metallic lithium electrode protected by a water stable lithium-ion super-ionic conductor glass (NASICON) plate with the interlayer which could be solid, polymer or organicliquid electrolyte. The interlayer, which is between the lithium metal electrode and water stable glass plate, is used to provide
∗ Corresponding author at: National Key Lab of Power Source, Tianjin Institute of Power Source, Tianjin 300384, China. Tel.: +86 022 23959581; fax: +86 022 23383783. E-mail address: [email protected] (X. Liu). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.07.103
better contact and prevent the reaction between the lithium and glass plate. Rather than the non-aqueous Li–air battery, a hybrid electrolyte lithium–air battery (HELB) was developed using WSLE. In HELB, an air electrode in aqueous electrolyte and a metallic lithium electrode in organic or polymer electrolyte was separated by a NASICON glass plate [Li3 M2 (PO4 )], such as Li1+x Ti2−x Alx (PO4 )3 (x = 0.2–0.3) and Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) [12–20]. At the same time, as the application required, HELB can be designed as a high energy density rechargeable battery or a lithium–air fuel cell with aqueous solution cycle system [9]. Contrasting with the simple diffusion process in organic electrolyte, the Li+ migration of HELB includes complex diffusion processes and interface processes, such as the diffusion in solid electrolyte phase, aqueous electrolyte and organic electrolyte, the interface of organic-electrolyte|solid-electrolyte and solid electrolyte|aqueous electrolyte. During Li+ migration, the impedance of these processes caused a more “polarization potential” than that of the non-aqueous Li–air battery. Then, for improving the discharge performance of HELB, it is essential to decrease the Li+ migration impedance of WSLE. Furthermore, it would be required the study of impedance of WSLE to decrease the impedance. The EIS, as a technique for impedance research, have been performed in the study of solid electrolyte plate and HELBs impedance [1–3,17]. The AC impedance was employed to compare and analyze the characteristic of WSLEs. However, the analysis of the impedance of WSLEs was scarcely reported during the discharge. In this
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work, for studying the impedance of WSLE the ac impedance was measured with reference electrode and four-electrode test-cell. During discharging, in order to study the impedance of water-stable lithium electrode, the polarized water-stable lithium electrodes were measured by EIS. As a result, the impedance spectra were analyzed and discussed for proposing the way to decrease the impedance of WSLE. 2. Experimental 2.1. Preparation and physical characterizations of LAGP plate The LAGP glass–ceramic [0.8Li2 O·0.25Al2 O3 ·1.5GeO2 ·1.5P2 O5 (mol%)] exhibited high lithium-ion conductivity and was made into thin electrolyte plates [15–17]. In this work, the LAGP plates were synthesized by reagent grade chemicals, including Li2 CO3 , Al2 O3 , GeO2 , and NH4 H2 PO4 . The four raw materials were weighted and mixed by the stoichiometry of 0.8Li2 O·0.25Al2 O3 ·1.5GeO2 ·1.5P2 O5 . To begin with, the mixed raw materials were heated slowly to 723 K and kept for 1.5 h to release gaseous production. Subsequently, the mixture was heated up to 1723 K and melted for 2 h. Subsequently, the molten mixture was poured in a heated plate mold and pressed by another one. The glass plate, which was casted by the molten mixture, was annealed at 823 K for 2 h and cooled in furnace. After annealing, the glass plate crystallized at 1223 K for 12 h and became a dense opaque white specimen. This specimen was refined shape and polished to a 200–500 m thick plate. In order to exclude the effect of different batches of the LAGP plate, a big LAGP plate was cut into 10–20 pieces of about 1 cm2 small ones for contrast in all physical and electrochemical measurements. The morphologies of the LAGP surfaces were characterized by a Hitachi S-4800 field emission scanning electron microscopy (FESEM). The X-ray diffraction (XRD) patterns of LAGP thin plates were recorded by a Rigaku TTRAX III diffractometer employing a Cu-K␣ source. 2.2. Three-electrode and four-electrode test cell design and assembly The ionic-conductivity of LAGP plate was measured by EIS. A 0.5 m thick Ag coating was deposited on both sides of the LAGP plate by evaporation. The plate coated with Ag was fixed by two stainless blocking electrodes in a blocking test cell. The three-electrode test-cells were assembled with organic electrolyte (EC:DEC:EMC = 1:1:1, 1 M LiPF6 ) and three lithium electrodes, which were employed as electrode (WE, 0.5 cm2 ), counter electrode (CE, 0.5 cm2 ) and reference electrode (Ref., 0.25 cm2 ) respectively. The HELBs with a reference electrode were designed and assembled for impedance spectroscope measurements, as shown in Fig. 1(b). The HELBs were separated into a Li electrode region and an air electrode region by LAGP plate (0.25 cm2 working area) with a thickness of 500 m. The Li electrode region was filled with organic electrolyte (EC:DEC:EMC = 1:1:1, 1 M LiPF6 ) and contained a Limetal electrode (0.5 cm2 ). The air electrode region was filled with aqueous solution (0.5 M LiOH and 0.5 M KCl) and contained an airelectrode (Super-P carbon loaded Ni foam, 1 cm2 ) and a reference electrode (SCE in KCl saturated solution, connected by salt bridge). A four-electrode test-cell was designed for further electrochemical impedance spectroscope measurements, as shown in Fig. 1(c). The test-cells were separated into two regions by 500 m or 200 m thick LAGP plates with 0.25 cm2 working area. There are two metallic lithium electrodes and organic electrolyte (EC:DEC:EMC = 1:1:1, 1 M LiPF6 ) which sealed in each region. In one
Fig. 1. Schematic representations of test-cells: (a) the three-electrode test-cell with organic electrolyte (WE and CE: 0.5 cm2 Li metal, RE: 0.25 cm2 Li metal), (b) the three-electrode test-cell with hybrid electrolyte (WE: 0.5 cm2 Li metal in organic electrolyte, RE: salt bridge + SCE in KCl saturated solution, CE: 1 cm2 air-electrode in aqueous solution) and (c) the four-electrode test cell with hybrid electrolyte (WE and CE: 0.5 cm2 Li metal; RE1 + RE2: 0.25 cm2 Li metal, all in organic electrolyte separated by LAGP) (organic electrolyte: EC:DEC:EMC = 1:1:1, 1 M LiPF6 ; aqueous solution: 0.5 M LiOH and 0.5 M KCl aqueous solution).
region, the two lithium electrodes were used as a work electrode (WE, 0.5 cm2 ) and reference electrode 1 (Ref. 1, 0.25 cm2 ). In the other, the two lithium electrodes were used as a counter electrode (CE, 0.5 cm2 ) and reference electrode 2 (Ref. 2, 0.25 cm2 ). A frequency response analyzer (a Solartron instrument model, 1470E + 1455A), controlled by MultiStatData software, was employed for obtaining impedance spectra at the room temperature (298 K). Unless specified, the AC impedance measurements were performed at OCP with the AC amplitude of 10 mV and frequency ranged from 0.1 to 1 MHz. The collected EIS were fitted using ZView software. 3. Result and discussion 3.1. The physical and electrochemical characterization of LAGP plate For discussing the characteristics, the XRD, SEM and EIS were measured. Fig. 2 provides XRD patterns of the LAGP plate which consists of Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) and AlPO4 crystals [15]. In XRD patterns, the peaks marched the characteristic of Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) and AlPO4 . The Li1+x Alx Ge2−x (PO4 )3 (x = 0.5) phase is dominant in LAGP glass–ceramics, and other peaks indicates the presence of an AlPO4 phase. Fig. 3 shows SEM images of the LAGP surface. The LAGP surface is smooth, close-grained and has visible traces of polishing. Fig. 4
Fig. 2. XRD patterns of LAGP plate.
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Fig. 3. SEM images of LAGP plate surface; (a) 10,000× and (b) 50,000×.
shows the impedance spectra of blocking electrode. In Fig. 4, the impedance spectra of LAGP have a semicircle or distorted one at high frequency and a line at low frequency. The conductivity of LAGP plate can be calculated. It can reach 4 × 10−4 S cm−1 [the formula: ε = d/(S·R); d: thickness of LAGP; S: area of LAGP; R: resistance of LAGP] [21–23]. 3.2. AC impedance spectra of three-electrode test-cells The AC impedance of WSLE comes from metallic lithium electrode impedance, organic electrolyte impedance, solid-state electrolyte impedance and the interface impedance between them. For studying the interfacial phenomenon of one interface, impedance spectra of either HELBs or the symmetric Li–Li test-cells without reference electrode were usually used [1–3,8,10,13,14]. However, a HELB combines the interfacial properties of the WSLE and the air electrode. Thus, the interfacial impedance of WSLE was not exactly reflected by it. Through, in Li–Li test-cells without reference electrode, the impedance of WSLE has been estimated by EIS, the interfacial impedance in WSLE was not analyzed in detail. Then, a series of three- or four-electrode impedance measurements has been conducted to investigate the interfacial impedance on WSLE. At first, three-electrode test-cells with three lithium electrodes were used to investigate the properties of the Li|organic-electrolyte interface. The configuration of three-electrode test cell is shown in Fig. 1(a). As Fig. 5, two overlapped semicircles are shown in the initial impedance spectrum of lithium electrode in three-electrode
Fig. 4. An AC impedance spectrum of Ag/LAGP plate/Ag test cells at 25 ◦ C.
test-cell, in which the high and low frequency semicircle is caused by solid electrolyte interface (SEI) impedance and charge-transfer impedance respectively. It is consistent with the reports of Li electrode impedance [24]. The typical equivalent circuit of Li electrode was used for fitting the impedance spectra. Re is ohmic resistance (21 ), which was estimated by the intercept of the high frequency semicircle with real axis. Rsei is SEI resistance (350 ); CPE1 is a constant phase element, which represents the geometric capacitance of SEI. Rct is charge-transfer resistance (105 ); CPE2 represents a capacitance related to charge-transfer. By contrasting the impedance spectra of the initial and an hour storage, the increase of SEI impedance, corresponding to the semicircle at high frequency, can be observed. The result of fitting analysis is: Re is 27 , Rsei is 549 , Rct is 116 . The Rsei markedly increased with the storage time. The formation of SEI layer is essential and durative process on the surface of metallic lithium [25,26]. In view of the growth of SEI layer within hours or months, the three-electrode test-cells were stood at room temperature and measured impedance at different time. Fig. 6 shows the five impedance spectra of Li electrode at 1 h, 24 h, 72 h, a week and a month. In the impedance spectra, the semicircle at low frequency corresponds to the charge-transfer impedance. The semicircle at high frequency, which corresponds to the SEI impedance, increased and covered the one at low frequency gradually. The two semicircles were combined to form one big semicircle, in which the SEI impedance is dominated. Then, if the Rct is reduced from the resistance of big semicircle, it was
Fig. 5. An AC impedance spectrum and fitting-line for the cell with three Li electrodes at 25 ◦ C.
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Fig. 6. Time dependence of the impedance spectra for the cell with three Li electrodes at 1 h, 24 h, 72 h, a week and month.
estimated that the SEI resistance increased from 500 to around 3100 . The increase of SEI impedance was caused by SEI layer growing at the Li|organic-electrolyte interface [24–26]. This kind of impedance spectra of Li electrode with a single semicircle was also reported [24]. 3.3. Three-electrode impedance spectra of Li–air batteries The WSLE and air electrode was assembled into a HELB, adding a SCE in aqueous solution as reference electrode. After assembly, the impedance spectra were measured and shown in Fig. 7. In Fig. 7, the high frequency semicircle corresponds to the grain boundary impedance, which can be estimated from the diameter of small semicircle and the reports of WSLE [3]. The medium and low frequency semicircle should correspond to the impedance of SEI and charge-transfer, and the interfacial impedance at LAGP plate/liquid electrolyte. However, the medium and low frequency impedance spectra could be a compressed semicircle but could also be two or three overlapped semicircles, by which the number of processes and the impedance of each process cannot be estimated and analyzed exactly. Fig. 8 shows the impedance spectra of WSLE in HELBs at different standing time. The impedance of WSLE increased with time and the medium and low frequency semicircle became two semicircles, which the medium frequency one increased and
Fig. 7. An AC impedance spectrum of WSLE in Li/air battery.
Fig. 8. Time dependence of the impedance spectra and fitting-lines of WSLE in Li/air battery.
Table 1 The resistances of the equivalent circuit of WSLE in Li/air battery at different time.
An hour A week A month
Re
Rg
Rint(N + A)
Rsei
27 30 37
217 218 233
604 770 797
751 1450 4948
reached stable within a week and the low frequency one was still increasing. In Fig. 8, the impedance spectra suggests that the medium and low frequency semicircles correspond to at least two interface processes, and one must correspond to SEI layer impedance, another corresponds to the impedance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface. In respect that the increase of low frequency semicircle is almost equal to the increase of SEI semicircle in Fig. 6, the low frequency semicircle should correspond to the impedance of SEI layer and charge-transfer. Then, the medium frequency semicircle should correspond to the impedance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface. Then, in HELBs, the impedance of WSLE included four parts: 1, the bulk impedance of solution, LAGP plate and current collector; 2, the impedance of LAGP grain boundary; 3, the impedance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface; 4, the impedance of SEI layer and charge-transfer. The semicircle diameter and shape of SEI layer and charge-transfer are dominated by the impedance of SEI layer. As a result, the low frequency semicircle has the similar shape and change with time. Then, an equivalent circuit of three-electrode test-cell can be proposed, as Fig. 8. In equivalent circuit, Re is the bulk resistance of solution, LAGP plate, wire and electrode; Rg is the resistance of LAGP grain boundary; Rint(N + A) is the resistance of organic-electrolyte|LAGP and LAGP|aqueous-solution interface; Rsei is the resistance of SEI layer and Li+ charge-transfer in equivalent circuit. Because the semicircle is distorted, the constant phase element has been chosen to replace the capacitance. The CPE1, CPE2 and CPE3 are the respective constant phase elements of Rg, Rint(N + A) and Rsei. Table 1 shows a resistances list of the equivalent circuit with time, in which Re and Rg was almost constant and Rint(N + A) increased and reached stable within a week. Although the estimated resistances of Rint(N + A) and Rsei were not accurate due to the overlapped two semicircles in the impedance spectrum of an hour standing, the resistances still can be a contrast. The Rsei, corresponding to impedance of SEI layer and charge-transfer, was
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Fig. 9. Time dependence of the impedance spectra of WSLE (3-electrode test).
increasing with SEI layer growth and became the main impedance of WSLE in a month. 3.4. Four-electrode impedance spectra of test-cell In the impedance spectra of HELBs with reference electrode, the impedance of each interface cannot be analyzed exactly, then, the impedance spectra of four-electrode test-cell were performed. In four-electrode test-cell, there are only two interfaces which are Li|organic-electrolyte and organic-electrolyte|LAGP interface. By using two reference electrodes, the impedance of organicelectrolyte|LAGP interface was estimated and analyzed. The four-electrode test-cell included a current control and measurement circuit that consists of WE (as work electrode) and CE (as counter electrode), and a potential control and measurement circuit that consists of Ref. 1 (as reference electrode No. 1, connect with sensor 1) and Ref. 2 (as reference electrode No. 2, connected with sensor 2). At first, for contrast with the four-electrode impedance spectra, three-electrode impedance spectra were measured in four-electrode test-cell, in which the RE2 was used as reference electrode; the impedance of Li electrode was also measured in the same test-cell with one reference electrode (Ref. 1). The impedance spectra are showed in Fig. 9. The impedance spectra of an hour standing has two semicircle, the high frequency one is small and corresponds to the impedance of grain boundary, the low frequency one is flat circle and should correspond to the impedance of SEI layer, charge-transfer, and organic-electrolyte|LAGP interface. After a week and a month, similarly with Fig. 8, the one semicircle at low frequency region grew and became two semicircles. The low frequency semicircle, which corresponds to the impedance of SEI layer and charge-transfer, changed with SEI layer formation and growth, which is the same to the phenomenon of WSLE in HELBs. Consequently, the medium frequency semicircle should correspond to the impedance of organic-electrolyte|LAGP interface. Then, in four-electrode test-cell, the impedance composition of WSLE is similar with HELBs, except the LAGP|aqueous-solution interface. The equivalent circuit can be proposed for it, including: Re, Rg, Rsei and Rint(N + N). As symmetric, the test-cell consists of two organic-electrolyte|LAGP interfaces so that the interfacial resistance [Rint(N + N)] was twice. Fig. 10 shows the difference of impedance spectra for threeelectrode test and four-electrode test. The low frequency semicircle with four electrodes that excluded the Li|organic-electrolyte interface is smaller to the low frequency one of three-electrode, and
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Fig. 10. An AC impedance spectra of WSLE (3-electrode and 4-electrode test, a week).
the impedance difference is almost equal to the impedance of Li electrode in Fig. 11. It suggests that the organic-electrolyte|LAGP impedance can be reflected by a single low frequency semicircle in four-electrode impedance spectra, instead of three-electrode impedance spectra in which the both of the SEI layer and organicelectrolyte|LAGP impedance cannot be exactly analyzed with an overlapped semicircle. Since the time constant of each interface processes is close and the diameters corresponding to impedance are lengthy, the all semicircles usually overlap and are looked like one semicircle [1–3,13,14]. However, the impedance spectra of four-electrode test made that the impedance of WSLE can be reflected and analyzed. By this method, the grain boundary and organicelectrolyte|LAGP were measured and used for discussing the organic-electrolyte|LAGP impedance. Then, the impedance of each in four-electrode test-cell can be estimated. In Fig. 11, Rsei was around 630 , which was estimated by the diameter of semicircle. The resistances of equivalent circuit can be estimated from Fig. 10 (Re ≈ 310 , Rg ≈ 240 , Rint(N + N) ≈ 1500 ). Consequently, the resistance of a single organic-electrolyte|LAGP interface is around 750 . The impedance of LAGP|aqueous-solution can be estimated and calculated by the impedance of organic solution|LAGP|aqueoussolution and organic solution|LAGP. It is about 20–30 . The
Fig. 11. An AC impedance spectrum of Li electrode (3-electrode test, a week).
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T. Yang et al. / Electrochimica Acta 81 (2012) 179–185 Table 2 The resistances of the equivalent circuit of WSLE with thin LAGP plate in fourelectrode test cell at different time (3 electrodes test).
An hour A day A week
Re
Rg
Rint(N + N)
Rsei
335 337 349
47 53 60
494 608 833
528 642 996
Fig. 12. An AC impedance spectrum and fitting-line of WSLE (3-electrode test).
impedance of the symmetric aqueous solution test-cell is about 100 , corresponding to impedance of both LAGP|aqueous-solution interfaces. That means about 40–50 for each interface, which is similar with estimated result, but more accurate. The impedance spectra of WSLE are affected by the thickness and ion conductivity of LAGP plate, as Fig. 12. In four-electrode test-cell, the impedance spectrum with a reference electrode has two semicircles which correspond to grain boundary impedance at high frequency region and organic-electrolyte|LAGP interface at low frequency region respectively. The semicircle corresponding to LAGP grain boundary cannot be distinguished, since it was small and overlapped by the semicircle of SEI. According to the equivalent circuit model, the line of fitting calculation is shown in Fig. 12, as a result, Re ≈ 335 , Rg ≈ 47 , Rint(N + N) ≈ 494 , Rsei ≈ 528 . Since the semicircle of Rg is indistinct small, the calculation result of it is inaccurate; nevertheless the results of other resistances are worth to analyze. Fig. 13 shows the impedance of WSLE with LAGP plate is increasing with time. The impedance spectra were analyzed by fitting lines and the resistance of each was estimated and listed in Table 2. The Rsei increased with time in a month, which can be attributed to the SEI layer growth. The Rint(N + N) increased within a week and kept stable subsequently, which the phenomena also described above. It suggests that the SEI impedance would become the main impedance of WSLE by the growth of SEI with time at OCP. About
Fig. 13. Time dependence of the impedance spectra and fitting-lines of WSLE (3electrode test).
Fig. 14. AC impedance spectra of Li electrode at different polarized potential (3electrode test, a week).
the impedance increase of the organic-electrolyte|LAGP interface within a week, it may be caused by a kind of interface layer on the LAGP, which conduct Li ion and affect the Rint(N + N) by formation and growth. Although the impedances spectra of WSLE were measured and analyzed in different cell at OCP, the main impedance of polarized WSLE may change while discharging. During discharge polarization, the impedance spectra were measured in four-electrode test-cell for discussing the polarization influence of each part of the impedance. Fig. 14 shows that the impedance of Li electrode rapidly decreased with enhance of polarization potential, which measured by three-electrode EIS in four-electrode test-cell. When the Li electrode polarized at 0.2 V, Rsei decreased to around 260 from around 645 , which was caused by the break of SEI layer at the Li metal surface. In contrast, Fig. 15 shows
Fig. 15. AC impedance spectra of WSLE (3-electrode and 4-electrode test, a week).
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the Rg was almost constant and Rint(N + N) decreased slightly. When the polarization potential between two reference electrodes was 0.2 V, Rint(N + N) of two organic-electrolyte|LAGP interfaces decreased from 1565 to 1277 . Consequently, resistance of organic-electrolyte|LAGP decreased from 782 to 633 . During the polarization, the SEI impedance decreased much more than the organic-electrolyte|LAGP impedance. It suggests that although the SEI impedance was more than the organicelectrolyte|LAGP impedance at OCP, the organic-electrolyte|LAGP impedance was almost constant and became the main while discharging. So the Li+ migration at the organic-electrolyte|LAGP interface is an process which cannot be intensively affected and controlled by potential. Therefore, the Li+ migration between two kinds of electrolyte is a process including diffusion and electro-migration, and mainly affected the discharge current density. Then, to improve the discharge performance of Li–air battery, the decrease of organic-electrolyte|LAGP impedance is essential, which should be achieved by the research of new solid and non-aqueous electrolyte and modification of the interface. 4. Conclusion Summarily, the four-electrode EIS was employed to analyze the impedance of the organic-electrolyte|LAGP interface, which can eliminate the combination effect of semicircles in AC impedance spectra. The impedance of WSLE protected by LAGP was investigated during the discharge. In the impedance spectra tested at OCP, the SEI impedance became the main impedance of WSLE as the growth of SEI with time. While discharging, the SEI impedance rapidly decreased with polarization potential, but the organicelectrolyte|LAGP impedance changed a little and became the major impedance. Then, the impedance on organic-electrolyte|LAGP interface was the main problem to improve the discharge performance, which should be decreased by the study of new electrolyte and interface modification.
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