Unraveling the formation mechanism of solid-liquid electrolyte

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Surfaces, Interfaces, and Applications

Unraveling the formation mechanism of solidliquid electrolyte interphases on LiPON thin films Manuel Weiss, Beatrix-Kamelia Seidlhofer, Matthias Geiß, Clemens Geis, Martin R. Busche, Maximilian Becker, Nella M. Vargas-Barbosa, Luca Silvi, Wolfgang G. Zeier, Daniel Schröder, and Jürgen Janek ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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ACS Applied Materials & Interfaces

Unraveling the Formation Mechanism of Solid-Liquid Electrolyte Interphases on LiPON Thin Films Manuel Weiss,†,‡ Beatrix-Kamelia Seidlhofer,¶ Matthias Geiß,†,‡ Clemens Geis,§,‡ Martin R. Busche,†,‡ Maximilian Becker,†,‡ Nella M. Vargas-Barbosa,k Luca Silvi,¶ Wolfgang G. Zeier,†,‡ Daniel Schröder,∗,†,‡ and Jürgen Janek∗,†,‡ †Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany ‡Center for Materials Research (LaMa), Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany ¶Helmholtz-Zentrum Berlin for Materials and Energy, Hahn-Meitner-Platz 1, 14109 Berlin, Germany §Institute of Applied Physics, Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany kDepartment of Chemistry, Philipps University of Marburg, Hans-Meerwein-Strasse 4, 35032 Marburg, Germany E-mail: [email protected]; [email protected] Abstract Most commercial lithium-ion batteries and other types of batteries rely on liquid electrolytes, which are preferred due to their high ionic conductivity and facilitate fast charge-transfer kinetics at the electrodes. On the other hand, hybrid battery concepts

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that combine solid and liquid electrolytes might be needed to suppress unwanted shuttle effects in liquid electrolyte–only systems, in particular if mobile redox systems are involved in the cell chemistry. However, at the then newly introduced interface between liquid and solid electrolyte, a solid-liquid electrolyte interphase forms. In this study, we analyze the formation of such an interphase between the solid electrolyte lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”) and various liquid electrolytes using in situ neutron reflectometry, quartz crystal microbalance, and atomic force microscopy measurements. Our results show that the interphase consists of two layers: a non-conducting layer directly in contact with “LiPON”, and a lithium-rich outer layer. Initially, a fast growth of the solid-liquid electrolyte interphase is observed, which slows down significantly afterward, resulting in a thickness of about 20 nm eventually. Here, a formation mechanism is proposed, which describes the solid-liquid electrolyte interphase growth as fast deposition of a mostly-covering film with only a little degree of remaining porosity. The residual void space is then slowly filled, thus blocking the remaining channels for ionic conduction, which leads to increasing resistance of the interphase. The results obtained imply that hybrid battery concepts with liquid electrolyte and solid electrolyte can be hampered by highly resistive interphases, whose formation cannot be simply slowed down or suppressed. Further research is required regarding possible countermeasures.

Keywords battery, solid liquid electrolyte interphase, LiPON, protective layer, lithium metal anode

1

Introduction

Electrochemical energy storage systems are used in daily life as portable electronic devices and as resilient storage for solar energy, wind energy, and other renewable energy sources. 1,2 Increasingly important is the application in electric vehicles, which requires higher energy and 2


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power density, increased safety and a prolonged cycle life, however. These requirements might not be fulfilled by current lithium-ion battery (LIB) systems based on liquid electrolytes (LEs), which offer high ionic conductivity, fast interface kinetics, and facile, low-cost synthesis routes. However, they suffer from degradation processes and reduced cyclability due to the LE’s ability to transport various anions besides Li+. 3,4 Ultimately, all-solid-state batteries (SSBs) without any liquid components are considered to possibly replace our current LIBs. 5,6 Until SSBs are market-ready, hybrid concepts combining LEs with solid electrolyte (SE) separators might be used to improve state-of-the-art LIBs by allowing the use of high-capacity or high-potential electrodes. Thus, detrimental effects, such as the polysulfide shuttle in Li S batteries 7–9 or shuttling of redox mediators in Li O2 cells, 10,11 can be suppressed, and the use of the lithium metal anode may be enabled. SE separators have already been successfully employed in Li S batteries to avoid polysulfide shuttling. 12–17 In addition, there are reports about hybrid Li O2 systems with ceramic components. 18–20 These SEs do not only improve capacity and cycling stability, however. They also introduce new interfaces, which appear as additional contribution to the overall cell resistance. Further, it was reported that the charge transfer across the solid/liquid interface exhibits the highest kinetic barrier in the entire system. 4,21 Earlier work on the SE/LE interface by Ogumi and co-workers focused on the characterization by electrochemical impedance spectroscopy (EIS). 21–24 More recent studies were also only conducted electrochemically— either by EIS as well 25 or by using DC polarization techniques. 26 In complementary work, Busche et al. employed surface analysis methods and depth profiling in combination with time-dependent EIS measurements to analyze the interface between the solid lithium ion conductor Li1+x Alx Ge2–x (PO4)3 (LAGP) and the binary mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), an LE commonly used in Li–S batteries. The authors found out that the elevated interface resistance results from actual chemical degradation of, both, LE and SE, which forms a solid-liquid electrolyte interphase (SLEI) comprised of decomposition products of SE, sol3



vent and conducting salt. As illustrated in Figure 1, the resistance of this interphase RSLEI increases over time. It is believed that this increase stems from increasing thickness of the SLEI, which exhibits high resistance against lithium-ion transport. 4

a

energy

distance

solid electrolyte SLEI

Li+ transport across the SLEI

SLEI growth

b

liquid electrolyte RSLEI RSE

resistance

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RLE time

Figure 1: Current model of the solid-liquid electrolyte interphase (SLEI) growth (a) and resulting temporal evolution of the interphase resistance (b), based on Busche et al. 4 Due to degradation of the solvent, the conducting salt, and the solid electrolyte, an interphase consisting of decomposition products, which exhibits low lithium ion conductivity is formed. While this interphase continues to grow, its resistance RSLEI increases steadily, and diffusioncontrolled growth has been proven. However, this model is based on the temporal evolution of the interfacial resistance and not much is known about the spatial dimensions of the SLEI and its formation mechanism. Therefore, in this paper, we focus on determining the SLEI thickness and propose a possible mechanism for the formation of this interphase. We employ in situ neutron reflectometry (NR) to determine the SLEI thickness, which offers a high resolution for the expected thicknesses in the nm-range, and is able to detect lithium containing compounds due to the high 4


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contrast between the scattering length densities (SLDs) of Li and Si used as the substrate. Neutron reflectometry has already been used successfully to study lithium batteries. 27–29 However, NR requires smooth samples with very low roughness, thus limiting its direct usage on ceramic LAGP pellets. Therefore, the NR studies are carried out on sputtered lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”) thin films, since results similar to those for LAGP were found for the change of the interphase resistance for “LiPON”. 4 Using NR, we find a two-layer SLEI with a non-conducting film directly in contact with “LiPON” and a lithium-rich layer on top. However, the experiments show that the interfacial layer does not grow in thickness, and that the growth appears to be self-limited. The NR experiments are complemented by quartz crystal microbalance (QCM) measurements for an accurate determination of the mass change caused by the SLEI formation. We like to note that we are not aware of other studies combining these two methods. However, QCM has already been used successfully for analyzing the solid electrolyte interphase (SEI) formation on battery electrodes. 30,31 Using the QCM, increasing mass of the SLEI is found in our experiments, which, in conjunction with the constant thickness determined from the NR experiments, leads us to propose a new formation mechanism: The SLEI forms quickly when “LiPON” is immersed in the LE, also forming some empty space such as pinholes. Through these pinholes the LE is still in contact to the “LiPON”. In the subsequent steps, the pinholes are filled with non-ion-conducting deposition products, which blocks the remaining ionic conduction channels through the interphase and, thus, increases its ionic resistance. Our work shows that the formation of highly resistive interphases and their morphological evolution must be considered for the development and optimization of hybrid batteries with SE membranes, separators or protecting films.

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2 2.1

Experimental section Sample preparation

For the neutron reflectometry studies, “LiPON” thin films of about 150 nm thickness were deposited by radio frequency magnetron sputtering on single-side polished n-type Si (1 1 1) wafers (CrysTec GmbH) with a thickness of 1 mm and 50.8 mm in diameter. For some measurements, wafers with 10 mm thickness were used as substrate instead. The Li3PO4 target (76.2 mm diameter) from Kurt J. Lesker Company was mounted at a working distance of 53 mm. The base pressure in the chamber was (0.3–6.9) × 10−7 mbar. During sputtering with N2, a pressure of (1.0–1.4) × 10−3 mbar and a sputtering power of 100 W were used. For QCM measurements, the “LiPON” films were prepared on QS-017 AT quartz crystals (Quarztechnik Daun) with a nominal frequency of 5 MHz. These crystals were 25.4 mm in diameter and coated with Cr-Au electrodes on both sides. They were cleaned in an ultrasonic bath using 2-propanol and dried under vacuum at 60 ◦C. The “LiPON” film was deposited in the center of the electrode measuring 12.8 mm in diameter using a 12 mm mask. After sputtering, the samples were transferred to an argon-filled glovebox without contact to air. As LE a solution of 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (99 %, IoLiTec) in various glymes, 1,3-dioxolane (DOL) (99.8 %, Sigma-Aldrich) or an 1:1 (V /V ) mixture of DOL and 1,2-dimethoxyethane (DME) (≥99.5 %, Sigma-Aldrich) was used. Diethylenglycoldimethylether (diglyme) (≥99.5 %) was also purchased from Sigma-Aldrich. LiTFSI was dried at 170 ◦C under vacuum using a B-585 Glass Oven (BÜCHI Labortechnik) overnight. The solvents were dried and stored over molecular sieves. DOL was distilled prior to use to remove the 75 ppm butylated hydroxytoluene (BHT) redox inhibitor. All work was done inside an argon-filled glovebox. The water content of the LEs was determined to be below 10 ppm by Karl Fischer titration (831 KF Coulometer, Metrohm).

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2.2

Neutron reflectometry

Neutron reflectometry relies on the specular reflection of a monochromatic neutron beam on a sample surface or the interface between different layers of a sample. 32,33 During this process, the exit angle θ is equal to the incident angle as shown in Figure 2a. The scattering vector ~q, defined as the difference between the wave vector of the incoming beam ~k0 and that of the reflected beam k~r , is given as 4π |~q| = |~k0 − ~kr | = sin θ λ

(1)

with λ being the wavelength of the neutrons. Below a critical scattering vector qc , only total reflection is observed; above qc the wave is also partly refracted into the next medium under an angle θs . The measured intensity of the reflected wave is then given as the reflectivity R according to the Fresnel equation sin θ − n sin θs 2 R= sin θ + n sin θs

(2)

with the refractive index for neutrons of the material n. 32 The resulting reflectivity curve (R vs. q) decreases rapidly with q −4 . 33 qc is dependent on the material’s SLD for neutrons, which is the product of the atomic density N and its nuclear scattering length bnuc , as p qc = 4 πN bnuc .

(3)

In the case of a multi-layer sample with distinguished SLD for the different layers, the reflectivity curve differs from the q −4 behaviour and shows distinct oscillations instead. These so-called Kiessig fringes 34 occur due to interference of the waves reflected on the surface of the sample and the interfaces between the different layers. The period of these oscillations

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∆q is related to the layer thickness d according to 32

∆q ≈

2π . d

(4)

Surface and interface roughness also influence the reflectivity curve as they are small deviations from the mean layer thickness, which leads to reduced amplitude of the Kiessig fringes for higher scattering vectors. 32 To perform the NR measurements, silicon wafers with the sputtered “LiPON” film were placed upside down in an aluminum tub inside an argon-filled glovebox. The silicon substrate with an SLD of 2.07 × 10−6 Å

−2

ensured high contrast to “LiPON” (expected SLD of

−2

3 × 10−6 Å ). Then, LE was added to fill a cavity below the silicon wafer such that the “LiPON” became immersed. The assembly was placed inside an air-tight aluminum measurement cell as shown in Figure 2b. Aluminum was used because of its low absorption coefficient for neutrons. NR measurements were then carried out at the V6 reflectometer at Helmholtz-Zentrum Berlin (λ = 4.66 Å) in the θ/2θ mode at incident beam angles between 0.07° and 1.60°. The neutron beam was collimated by 40 mm horizontal and 0.5 mm vertical slits. It was guided through the side of the silicon wafer, which thus is the incident medium for the neutrons, i.e. the superphase. The reflected neutrons were detected using 3He detector tubes. 35 Measurements were performed subsequently for up to 40 h per sample. The raw data were corrected for background and footprint prior to further analysis using the Abeles matrix method implemented in the Motofit software package, which returns identical results as Parratt’s recursion algorithm. 36 During this analysis, a simulated reflectivity curve was calculated for a system consisting of multiple layers, each having a unique SLD, thickness d, and roughness σ and was refined against the experimental data.

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a distance from interface

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k0

0

silicon

kr θ

θ θs

LiPON SLEI liquid electrolyte

b

Figure 2: Schematic representation of the measurement setup for neutron reflectometry studies (a) and air-tight measurement cell (b). A silicon wafer with the sputtered lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”) film was placed upside down in the liquid electrolyte consisting of 1,3-dioxolane, 1,2-dimethoxyethane or mixtures thereof. LiTFSI was used as conducting salt. Immersing the “LiPON” thin film in the liquid electrolyte led to the formation of a solid-liquid electrolyte interphase (SLEI). The path of incoming, refracted, and reflected neutrons is indicated by arrows.

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2.3

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Quartz crystal microbalance measurements

A quartz crystal microbalance (QCM) allows for the in situ detection of small mass changes. According to Sauerbrey, 37 the mass of a material ∆m deposited onto an oscillating quartz crystal can be calculated from the frequency change ∆f using 2f0 2 ∆m ∆f = − √ A ρQ µQ

(5)

with the resonating frequency f0 , density ρQ , and shear modulus µQ of the quartz and the active area A. 38 The linear change in mass according to equation (5) only holds for the deposition of uniform, rigid thin films and undamped oscillations. If the quartz crystal is operated in contact to a liquid, however, increased damping is observed due to the shear wave in the quartz crystal being coupled to a damped shear wave in the liquid. Thus, the resonating frequency is reduced depending on the density ρL and the viscosity ηL of the liquid by 39,40 ∆f = −f0

3 2

r

ηL ρ L . πµQ ρQ

(6)

Apart from the initial shift due to the contact with liquid, the relative changes resulting from deposited mass are identical to operation in air. Hence, the mass change can still be calculated using equation (5) as long as the deposited film is flat and rigid. 41 Further details on the principle of QCM and the meaning of the different frequencies (Figure S1) can be found in the Supporting Information. “LiPON”-coated quartz crystals were placed onto two spring-loaded contacts in a measurement cell made from polyether ether ketone (PEEK). Seal rings made from ethylene propylene diene monomer (EPDM) were used to avoid leaking of LE and contact to air. After the cell was filled with LE, it was sealed, removed from the glovebox, and placed into a climatic chamber (Binder) at 25 ◦C. The frequency change was measured using an eQCM 10M quartz crystal microbalance (Gamry Instruments) and recorded with the Gamry Resonator software over multiple days. 10


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3

Results and discussion

3.1

Unraveling the formation of interphases on “LiPON” by neutron reflectometry

To monitor the thickness of the expected interphase in situ, NR experiments were performed. The NR measurements require samples with low roughness. Therefore, the surface of the sputtered “LiPON” films was analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results are shown in Figure S2 in the Supporting Information, indicating a sufficiently low roughness of the “LiPON” for NR experiments. Figure S3a shows an NR curve of “LiPON” on silicon without any LE. Due to the absence of Kiessig fringes, accurate fitting of this measurement is impossible. The recorded NR curve without LE differs clearly from the ones with “LiPON” in contact with the three different LEs (also shown in Figure S3). An NR curve recorded three hours after immersion of a “LiPON” thin film with about 150 nm thickness in an LE consisting of 1 m LiTFSI in a mixture of DOL and DME is depicted in Figure 3a. Figure 3b shows the layered model used for fitting this system. The model consists of four layers: Si|“LiPON”|SLEI|LE. The SLD profile depicted in Figure 3b shows the change of the SLD throughout the sample. Therefore, the SLD is plotted against the distance from the interface, on which the neutron beam was focused during measurement. “LiPON” film: As Si was the superphase (incident medium for the neutrons) and the beam −2

was focused on the Si|“LiPON” interface, Si with an SLD of 2.07 × 10−6 Å , which was fixed for the model, is located at a distance of 0 nm. Since the wafers were handled without contact to air, only a minimal amount of SiO2 is expected to be present on the Si surface. During refinement of the model against the measured data, the SiO2 thickness always approached zero for the 1 mm thick Si wafers used in this work. Hence, a layer of SiO2 was omitted for the respective model. Thus, “LiPON” is found to be in direct contact with Si. To get the best fit to the experimental data, two different layers are used to model the “LiPON” film—a 11



thin one with an SLD of about 2.5 × 10−6 Å

−2

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directly in contact with Si and a much thicker

−2

outer layer with 3 × 10−6 Å . This corresponds to a calculated SLD of 2.970 × 10−6 Å

−2

for Li1.7PO1.95N0.85 with an assumed density of about 2 g cm−3 , which is slightly lower than that of the Li3PO4 target as expected for thin films. For all the measurements the SLD −2

used to model “LiPON” is in the range (2.84 × 10−6 –3.30 × 10−6 ) Å , with an average of −2

3.12 × 10−6 Å . These values can be correlated to densities in the range (1.9–2.2) g cm−3 for the given composition, which agrees with reported densities ((2.1–2.2) g cm−3 ) for similar compositions. 42 The observed two-layer structure in the “LiPON” might be caused by a lower density of “LiPON” at the bottom due to the formation of crystallization nuclei. Another possible explanation might be a higher lithium concentration in the layers deposited initially due to the lower temperature at the start of deposition. During the sputtering process the temperature increases to about 200 ◦C at the end of deposition as a result of the energy transfer from the plasma onto the substrate. Overall, the thickness of the “LiPON” film is determined to be approximately 160 nm, which agrees well with the thickness adjusted during deposition. The SLEI : Surprisingly, two different SLEIs can be identified: It appears that two distinct layers with different SLDs have formed on the “LiPON”. The inner layer (SLD −2

≈ 3.5 × 10−6 Å ), denoted SLEI 1 here, may correspond to Li2CO3, which has an SLD of 3.485 × 10−6 Å

−2

and is a known decomposition product of solid lithium ion conductors. 4,43,44

Since Li2CO3 is non-conductive for lithium ions, it explains the high ionic resistance of the interphase. In contrast, the outer layer of the SLEI, denoted as SLEI 2, exhibits a negative SLD and, thus, might be a lithium-rich and/or hydrogen-rich layer, presumably HCH2OLi, since the SLD of elemental Li (H2) is −0.88 × 10−6 Å LE was fixed to 0.765 × 10−6 Å

−2

−2

−2

(−2 × 10−6 Å ). The SLD of the

in this case, which corresponds to the calculated value for

1 m LiTFSI in the 1:1 (V /V ) mixture of DOL and DME. Based on the layer model discussed above, NR measurements at different points in time and for the other solvents were analyzed. The results for the time-dependent changes of 12


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a

0

lg R

−1

−2 Measurement Fit

−3

0.01

0.02

0.03

0.04

q / Å−1

b 3

SLD / 10−6 Å−2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LiPON 1

SLEI 1

LiPON 2

2 Si 1

LE

0

−1

SLEI 2 0

50

100

150

distance from interface / nm

Figure 3: Neutron reflectivity curve (a) measured three hours after immersing a sputtered lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”) film into the liquid electrolyte (LE) (1 m LiTFSI as conducting salt; a mixture of 1,3-dioxolane and 1,2-dimethoxyethane as solvent). The line shows the best fit to the data according to the scattering length density profile (b). A solid-liquid electrolyte interphase (SLEI) consisting of two layers with different scattering length densities (SLEI 1 and SLEI 2) was found to be present between “LiPON”, which was modeled by two layers (LiPON 1 and LiPON 2) as well, and liquid electrolyte (LE).

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SLD and thickness for all three tested LEs are displayed in Figure 4. Depending on the exact setup used, some adjustments have to be made. In the case of the wafers with 10 mm −2

thickness, an SiO2 layer with fixed SLD (3.475 × 10−6 Å ) and a thickness of a few Å has to be added because these wafers had contact with air before the experiment. This, however, does not affect the results for the other layers in comparison to those on 1 mm wafers without SiO2. Moreover, the SLD of the LE changes based on the solvent, to 1.071 × 10−6 Å DOL and 0.458 × 10−6 Å

−2

1 M LiTFSI in DOL/DME

SLD / 10−6 Å−2

−2

for

for DME. 1 M LiTFSI in DOL

5.1 a

1 M LiTFSI in DME

b

c

LiNO3

SLEI 1

3.5

Li2CO3

2.3

LiF

LiPON 2 0.8 LiPON 1 0.0 SLEI 2

DOL 0

−0.9

thickness / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Li

200 d 150 100 Σ(LiPON)

e

f

15 10 SLEI 1 5 SLEI 2 0

0

10

20

time / h

30

0

10

time / h

20

0

10

time / h

20

Figure 4: Time-dependent scattering length density (SLD) (top) as well as thickness (bottom) of the different layers for lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”) thin films immersed in 1 m LiTFSI in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (a, d), pure DOL (b, e), and pure DME (c, f). On the SLD axis the calculated values for the involved materials (labeled on the right) are shown. The sum of the thicknesses of both “LiPON” layers is shown in d–f. For DOL/DME as well as pure DOL the two-phase solid-liquid electrolyte interphase (SLEI) consists of a thin lithium-rich layer with low scattering length density and a thicker non-conducting layer (presumably Li2CO3). In the case of DME, the SLD of SLEI 2 is higher, indicating LiNO3. Additionally, the lithium-rich layer is the thicker one for DME, whereas the total SLEI thickness does not differ from the DOL and DOL/DME cases. For all solvents, the SLD and the thickness of the layers does not change significantly during the experiment. 14


30

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Comparing the results for the mixture of DOL and DME to those of pure DOL as the solvent does not reveal siginificant differences (Figures 4a, 4d and Figures 4b, 4e). The thickness of “LiPON”—the sum of the thicknesses of both layers used to model “LiPON” is plotted here—is not identical, which can be expected for different samples. The SLDs of the different layers are very similar and also the thicknesses do not vary much from sample to sample. It can be seen that the inner SLEI is about twice as thick as SLEI 2. However, immersing the “LiPON” in the electrolyte using DME as solvent resulted in a completely different SLEI. Indeed, although the two-layer structure of the SLEI remains, the SLD of −2

SLEI 1 increases on average to about 4.4 × 10−6 Å , which suggests that its composition −2

changed from Li2CO3 to LiNO3 (5.128 × 10−6 Å ) with reduced density instead. In addition, the thicknesses of both SLEI layers swapped, resulting in a thinner inner layer and a thicker outer Li-rich layer. The most remarkable observation for this set of experiments is that, regardless of the LE, the thickness and SLD of the SLEI does not change over time during the experiments lasting over one day in all cases. Thus, the increase of the interphase resistance determined using EIS measurements 4 cannot be simply explained by a growing thickness of the interphase. We like to note that we cannot prove these findings at this point with NR.

3.2

Change in mass during the formation of solid-liquid electrolyte interphases

To further investigate the amount of deposited material, we apply a complementary analysis with a quartz crystal microbalance (QCM). The results for QCM measurements with “LiPON” immersed in 1 m LiTFSI in (a) 1:1 DOL/DME (V /V ), (b) DME, and (c) diglyme are shown in Figure 5, where the time-dependent changes in frequencies as well as the mass change are plotted. For this analysis, the deposited mass was calculated according to Equation (5) using the serial resonance frequency fs and the first measured value for fs set as f0 . A decrease in fs indicates an increase in mass, whereas an increase in the difference 15



fp − fs indicates an increase in roughness, the growth of a viscoelastic film, or changes in the viscosity and density of the deposited film 31,45,46 (an explanation of the different measured frequencies and further details can be found in the Supporting Information). Upon injection of LE into the QCM with an oscillating “LiPON”-coated crystal, the observed frequency should decrease according to equation (6). Simultaneously, the frequency divergence is expected to increase due to increased damping. 31 Figure S1b in the Supporting Information shows the response of the crystal oscillating in argon to the injection of an LE consisting of 1 m LiTFSI in an 1:1 (V /V ) mixture of DOL and DME. While fp − fs increases by about 2000 Hz, fs decreases by the same amount. According to equation (6), a frequency shift by 950 Hz can be expected taking into account the viscosities and densities of the employed LEs. 47–54 The deviation between the observed 2000 Hz and expected 950 Hz stems from the fact that equation (6) only applies to ideally flat surfaces. For real systems, a two- or threefold value of the calculated one is usually observed due to the impact of the surface roughness. 55 For DME, the calculated frequency shift was lower than for the binary mixture, while for DOL a higher value was predicted. This is also in line with the measured shifts, resulting in

∆fmeasured ∆fcalculated

≈ 2.1 for all cases.

For the long-time QCM measurements, data recording was started after LE had already been filled into the measurement cell. Thus, the initial frequency shifts are not visible in the data. Figure 5 reveals that fs decreases over the entire duration of the experiment for all solvents, thus resulting in increasing mass. Especially in the first (12–24) h, which is a duration comparable to that of the NR experiments, we observe a sharper decrease in fs than in the later stages of the experiment. The frequency divergence fp − fs increases, which suggests that either the roughness of the films increases or viscoelastic material is deposited. For using DME as the solvent, the steeper increase of the deposited mass in the initial 24 h is not as pronounced as for the other solvents. This is possibly related to the swapped SLEI layer structure observed in the NR measurements. For better comparison to the NR data, the thickness of the deposited film was calculated 16


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from the mass change. To do this, the density of the film was assumed to be 2 g cm−3 , which is reasonable for potential SLEI materials such as Li2CO3 or LiNO3. The frequency change in the beginning can be correlated to a film thickness of about (1–2) nm after 24 h. Conversely, the thickness determined via NR is about (20–25) nm (Figure 4). In the following, we provide arguments to explain this discrepancy.

3.3

Consequences for the formation mechanism

The available information about the SLEI formation on “LiPON” are relatively scarce and contradictory at first glance: (a) The interfacial resistance for lithium ions to migrate trough the SLEI increases linearly over time; 4 (b) using neutron reflectometry the interface thickness was determined to remain constant at (20–25) nm; (c) QCM measurements show an increase in mass due to the formation of the SLEI on top of the “LiPON” film. To reconcile these observations, we propose the following formation mechanism of the SLEI. The SLEI is formed quickly after immersing “LiPON” in the LE before the first NR or QCM measurement is started. However, since the SLEI does not grow homogeneously, there are some voids that allow contact between the “LiPON” and the LE (we shall call these voids: pinholes). As the SLEI presumably mainly consists of non-ion-conducting Li2CO3, transport of lithium ions can then only occur through these pinholes. As the exposure time of the “LiPON” film to the LE increases, the pinholes are continuously filled with SLEI material to form a denser film. In this process, the cross-sectional area of the pinholes decreases and, thus, the ionic resistance through the SLEI increases (compare Figure 6). Consequently, the thickness detected via NR remains constant as the pinholes only account for a small fraction of the total sample area. Using the QCM with a mass resolution of about 0.5 × 10−9 g, increasing mass is detected. Additionally, we observe an increase in the frequency divergence fp − fs , which suggests an increase in roughness. Contrarily, the filling of the pinholes, while being visible as a mass increase in QCM measurements due to additionally deposited material, 17



600

Δfp−Δfs 10

400 200

5

0 −200

0

−400 −600

mass

b frequency change / Hz

DOL/DME

Δfs

8

Δfp−Δfs

400

6 200 4 0 2 −200 0 −400

c

DME

mass

600

Δfs

10

Δfp−Δfs

400

8

200

6

0 4

−200

2

−400 −600

diglyme 0

mass 0

25

50

75

100

mass change per area / µg cm−2

Δfs


15

800


frequency change / Hz

a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

125

time / h

Figure 5: Frequency change during quartz crystal microbalance measurements for lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”) immersed in 1 m LiTFSI in (a) a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME), (b) DME as well as (c) diethylenglycoldimethylether (diglyme). The change in mass (referred to the electrochemically active area of the quartz) can be related to the deposited amount of the solid-liquid electrolyte interphase. The duration comparable to that of the NR experiments is marked gray.

18


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should result in reduced roughness. However, we suspect that while the pinholes are filled, the morphology of the remaining surface area of the film changes, possibly due to uneven deposition of small amounts of SLEI material in these areas. Thus, leading to an overall increase in roughness even though the pinholes are closed. Another possible explanation might be the formation of a viscoelastic film. Using NR, a two-layer SLEI was found with a negative SLD of the top layer. A feasible material exhibiting such a negative SLD is HCH2OLi, an organic compound. In comparison to inorganic salts like Li2CO3 or LiNO3 with large shear moduli and low viscosities, 46 HCH2OLi should be softer. The inorganic salts are supposedly located below the soft material (probably HCH2OLi), directly in contact with “LiPON”. Hence, more of the viscoelastic material HCH2OLi is deposited the longer “LiPON” is immersed in the LE and the frequency divergence increases. The SLEI thickness of about (1–2) nm calculated from the deposited mass after 24 h (marked gray in Figure 5) corresponds to less than 10 % porosity in the (20–25) nm thick (determined via NR) film. Thus, a pore volume of about (5–10) % is filled within approximately 24 h. Due to this small difference between the pre- and post-measurement state, a significant change in thickness—which does not change apparently—or SLD—because of the removal of LE with different SLD out of the pinholes—is not observed in the NR measurements. Atomic force microscopy (AFM) measurements were performed to strengthen this reasoning (compare Figure S4 in the Supporting Information). The area fraction on “LiPON”, which remained uncovered after being immersed in 1 m LiTFSI in DOL/DME for 24 h, was about 5 % for that particular film, as ascertained from the AFM image. An additional (5–10) % in porosity were determined to be filled before recording of the AFM images, i.e. during the first 24 h after the start of the QCM measurement. Therefore, the porosity in the beginning of the experiments equals approximately 15 %. Furthermore, the flattening of the mass increase after the first day in the QCM experiments also hints at a low degree of remaining porosity at this point. 19



RSE

RSLEI

−Im(Z)

LiPON SLEI

−Im(Z)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

RSE

Re(Z) liquid electrolyte

GSLEI Re(Z)

Figure 6: Proposed formation mechanism for the solid-liquid electrolyte interphase (SLEI) on lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”). The SLEI is formed quickly after immersing “LiPON” in the liquid electrolyte, with some pinholes left. While the overall film thickness stays constant afterward, the remaining pinholes are filled, thus blocking the remaining conduction pathways for lithium ions. This increases the observed interfacial resistance (compare ref. 4 ). The Nyquist plot for a possible equivalent circuit for impedance measurements with a Gerischer impedance element G for the interphase in comparison to the response of the commonly used RQ element is also shown. Considering the herein proposed formation mechanism carefully, we want to suggest that impedance data of “LiPON” in contact with an LE might be interpreted from an alternative viewpoint. In the past, the interface between solid and liquid electrolyte was always described by an RC or RQ element, i.e. a resistance R and a capacitance C or a constant phase element Q in parallel, in the equivalent circuits used to model impedance data. 4,21–25 For time-dependent measurements this is a reasonable approach to describe an interphase with increasing thickness, which will, thus, result in a higher interphase resistance. Since the underlying mechanism of the SLEI formation is based on the filling of pinholes, we would like to suggest that another impedance element besides the RQ element can be an option to fit the behavior observed—the Gerischer element G. This element was originally used to model a chemical reaction followed by an electrochemical reaction. 56 Other applications of a Gerischer impedance were reported for porous 57,58 and composite electrodes. 59 Generally, a Gerischer element describes the combination of a diffusion process and a chemical reaction. 56,58 Thus, it 20


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might be a suitable model for the filling of pinholes presumably taking place on the “LiPON” surface in an LE. In the present case, the LE components diffuse into the pinholes where they react to form additional SLEI material subsequently, which could be interpreted as a Gerischer-type behavior. A viable explanation comprising an RQ element for the SLEI could be the following: After the initial SLEI layer is formed quickly, conduction occurs mainly through the remaining pinholes. Then, these pinholes are filled during further immersion in a presumably complicated mechanism. While the exact elucidation of this complex mechanism is out of the scope of this study, we want to provide some thoughts on the applicability of a simple RQ element. If newly formed SLEI material is mainly deposited on the walls of the pinholes, the effective area through which ionic conduction can occur will decrease. Hereby, we assume that the area already covered by the SLEI is ionically insulating and thus does not contribute to the cross sectional area of the conductive path. Hence, the decreasing area results in an increasing interphase resistance. On the other hand, if there is preferential deposition of new SLEI material onto “LiPON” located on the bottom of the pinholes, the thickness of this small portion of SLEI inside the pinhole will increase in the following, also resulting in increased resistance. While both theories are able to explain the increasing resistance when using an RQ element, the AFM images of a “LiPON” film after 24 h of immersion in LE (compare Figure S4 in the Supporting Information) show areas, in which pristine “LiPON” is still visible. Thus, our first proposition, the deposition of further SLEI material on the walls of the pinholes, seems to be more probable. In Figure 6 the two equivalent circuits—one with two RQ elements and another with an RQ element and Gerischer impedance—are shown for comparison. For capacitances, which are very close to each other like that of a “LiPON” thin film and a thin interphase, the semi-circles overlap and thus a distinction between the two cases and a decision for one over the other is difficult. Measured impedance spectra of “LiPON” films being immersed in different LEs were analyzed with the two different equivalent circuits, the results are shown 21



in Figure S5. Depicted are Nyquist plots of measurements carried out shortly after the beginning of immersion and after (124–144) h. The spectra recorded at the beginning could be fitted with, both, the equivalent circuit containing a Gerischer impedance as well as the one using an RQ element for the SLEI with comparable goodness of fit. At the end of the impedance measurements after multiple days, however, the RQ element resulted in a better fit to the data, thus, indicating that at this point the pinholes are completely filled and the increasing resistance stems from increasing SLEI thickness then. Due to the similarity between the two proposed equivalent circuits (Figure 6) and the comparable goodness of fit, additional experiments, such as the NR and QCM measurements conducted in our work, are necessary for in-depth interpretation of recorded impedance spectra.

4

Conclusions

In this work, the interphase formation on lithium phosphorous oxide nitride (Lix POy Nz , “LiPON”) in contact with various ether-based liquid electrolytes was examined by neutron reflectometry in combination with quartz crystal microbalance measurements. Using neutron reflectometry, a solid-liquid electrolyte interphase (SLEI) was detected to form on top of a sputtered “LiPON” film immersed in various liquid electrolytes. Surprisingly, the SLEI was found to consist of a non-conducting layer and a second lithium-rich layer. Its thickness, determined via neutron reflectometry, remained fairly constant over the duration of the experiment, whereas complementary quartz crystal microbalance and electrochemical impedance spectroscopy 4 measurements suggest an increase of mass and interfacial resistance, respectively. From these observations, we propose a SLEI formation mechanism that is based on the filling of initially present pinholes in the SLEI. During that, the resistance of the interphase increases as the pinholes are filled with non-ion-conducting material, which will limit the application of hybrid batteries consisting of liquid electrolyte and solid electrolyte. The typical method to reduce interfacial resistances—increasing the interfacial area

22


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by microstructuring the surface of the SE 4 —however, should also work for the mechanism proposed here. Alternatively, the addition of inhibitors has to be investigated, which will block the SLEI formation completely or fill the pinholes with material showing higher ionic conductivity instead. In the end, this might enable better hybrid Li S or Li O2 batteries, which require solid electrolyte separators.

Acknowledgement The authors acknowledge financial support by BASF SE within the International Network for Electrochemistry and Batteries. M.W. thankfully acknowledges the financial support from HZB. W.G.Z furthermore gratefully acknowledges the financial support through start-up funding provided by the Justus Liebig University Giessen. We thank HZB for the allocation of neutron radiation beamtime.

Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at DOI: • Details about the QCM measurements with explanation of the various measured frequencies and equivalent circuit; response of the QCM to the injection of LE; SEM and AFM images of pristine “LiPON” films; neutron reflectivity curves of a pristine “LiPON” film and of “LiPON” films immersed in different LEs; AFM images of a SLEI grown on “LiPON”; impedance spectra of “LiPON” immersed in different LEs (PDF)

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Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical TOC Entry k0 silicon

kr θ

θ

LiPON SLEI

θs

liquid electrolyte

31


a

energy

ACS Applied Materials & Interfaces Page 32 of 38

distance

1 solid electrolyte 2 Li+ transport 3 SLEI across the SLEI 4 5 6 7 8 9 10 liquid electrolyte b11 12 13 14 15 16 17 18 19 ACS Paragon Plus Environment 20 21 22

SLEI growth

RSLEI

resistance

RSE

time

RLE

a

distance from interface

Page 33 of ACS 38 Applied Materials & Interfaces

1 2 0 3 4 5 6 7 8 b9 10 11 12 13 14 15 16 17 18 19 20 21

k0 silicon

kr θ

LiPON SLEI

θ θs

liquid electrolyte


a

0

ACS Applied Materials & Interfaces Page 34 of 38

SLD / 10−6 Å−2

lg R

1 2 −1 3 4 5 6 −2 7 8 Measurement 9 Fit 10−3 0.01 0.02 0.03 0.04 11 −1 12 q/Å 13 b14 15 SLEI 1 LiPON 2 16 3 LiPON 1 17 18 19 2 Si 20 21 1 LE 22 23 0 24 25 SLEI 2 26−1 ACS Paragon Plus Environment 27 0 50 100 150 28

distance from interface / nm

1 M LiTFSI in DOL/DME Page 35 of 38 5.1 a

1 M LiTFSI in DME LiNO3 Li2CO3

SLD / 10−6 Å−2

SLEI 1 1 3.5 2 3 2.3 LiPON 2 4 LiPON 1 5 0.8 6 0.0 SLEI 2 7 −0.9 8 9 200 d 10150 11100 Σ(LiPON) 12 13 15 14 15 10 SLEI 1 16 17 5 SLEI 2 18 0 19 0 10 20

1 M LiTFSI in DOL ACS Applied Materials & Interfaces b c

LiF DOL 0 Li

f

thickness / nm

e

ACS Paragon Plus Environment 20

time / h

30

0

10

time / h

20

0

10

time / h

20

30

15 ACS Applied Materials & Interfaces Page 36 of 38 Δf DOL/DME s

600

Δfp−Δfs


10

5

0

mass Δfs

8

Δfp−Δfs

6 4 2 0

DME

mass Δfs

10

Δfp−Δfs 8 6 4 2

diglyme 0

mass

ACS Paragon Plus Environment 25

50

75

time / h

100

125



1 400 2 200 3 4 0 5 6 −200 7 −400 8 9 −600 10 b11 12 13 400 14 15 200 16 17 0 18 19 20−200 21 22−400 23 c24 600 25 26 400 27 28 200 29 30 0 31 32−200 33 34−400 35 36−600 37 0 38


800



a

−Im(Z)

1 2 3 4 5 6 7 8 9 10 11 12

LiPON SLEI

RSE


RSLEI

Re(Z) liquid electrolyte

−Im(Z)

Page 37 of 38

RSE


GSLEI Re(Z)

ACS Applied Materials & Interfaces Page 38 of 38 k k 0

silicon LiPON 1 SLEI 2

3 4

r

θ

θ

θs


liquid electrolyte

Unraveling the formation mechanism of solid-liquid electrolyte

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