Ultrahigh-Performance Cu2ZnSnS4 Thin Film and Its Application in

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Ultrahigh Performance CuZnSnS Thin Film and Its Application in Microscale Thin Film Lithium-Ion Battery: Comparison with SnO 2

Jie Lin, Jianlai Guo, Chang Liu, and Hang Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10730 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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ACS Applied Materials & Interfaces

Ultrahigh Performance Cu2ZnSnS4 Thin Film and Its Application in Microscale Thin Film Lithium-Ion Battery: Comparison with SnO2 Jie Lin, Jianlai Guo, Chang Liu, and Hang Guo* Pen-Tung Sah Micro-Nano Science and Technology Institute, Xiamen University, Xiamen, Fujian 361005, China

ABSTRACT: In order to develop high performance anode for thin film lithium-ion batteries (TFBs, with a total thickness on the scale of micrometer or less), a Cu2ZnSnS4 (CZTS) thin film is fabricated by magnetron sputtering, and exhibits an ultrahigh performance of 950 mAh g-1 even after 500 cycles, which is the highest among the reported CZTS for lithium storage so far. The characterization and electrochemical tests reveal that the thin film structure and additional reactions both contribute to the excellent properties. Furthermore, the microscale TFBs with effective footprints of 0.52 mm2 utilizing the CZTS thin film as anode are manufactured by micro-fabrication techniques, showing superior capability than the analogous TFBs with the SnO2 thin film as anode. This work demonstrates the advantages of exploiting thin film electrodes and novel materials into micro-power sources by electronic manufacture methods. KEYWORDS: Cu2ZnSnS4, thin film, ultrahigh performance, all-solid-state, lithium-ion battery

1. INTRODUCTION The quick development of microelectro mechanical system (MEMS) and integrated circuit (IC) has triggered an urgent demand for micro power devices.1 Microscale thin film lithium-ion batteries (TFBs),2-4 by converting the widely used lithium-ion batteries with bulk configurations into all-solidstate and microscale power sources, has many significant advantages such as intrinsic stability,5 flexible sizes/shapes,6 evitable solid electrolyte interphase (SEI)7 and lithium dendrite.8 However, the 1 ACS Paragon Plus Environment


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specific volumetric capacity and cycle performance of the commonly used anodes (84 and 206 μAh cm-2 μm-1 for graphite and lithium metal)9-10 still cannot meet the increasing requirements for energy storage. Especially in the application of low power electronics,11 stable voltage output12 and robust durability13 are the most important targets. As a result, novel anodes with apparent voltage plateau, high specific volumetric capacity and excellent cycle performance should be developed. Cu2ZnSnS4 (CZTS) materials are well-known to be the absorption layer in low cost solar cells,14 whose constituent elements Sn and Zn are also electrochemically active toward lithium with a high capacity.15 Interestingly, the multiple reaction mechanism will change depending on the thickness and conductivity, which may further influence the voltage profiles of TFBs with CZTS as anode. Lithium phosphorus oxynitride (LiPON),2, 16 which is generally fabricated by sputtering Li3PO4 target in N2 atmosphere, can transport Li+ and has a wide electrochemical stability window (>5.5 V). To date, it remains the most popular solid state electrolyte in TFBs.8,

17-18

Micro-fabrication techniques are

widely used in microelectronic industry,3-4 including lithography, deposition, etching, etc. With the aim of exploiting energy components in realistic conditions, we combine the CZTS anode, LiPON solid electrolyte and LiCoO2 cathode films into microscale integrated power devices through microfabrication process. In this work, a CZTS thin film with homogeneous morphology and composition is fabricated by radio frequency (RF) magnetron sputtering. The electrochemical properties of CZTS thin (300 nm) and thick (1 μm) films are compared with the SnO2 thin film (300 nm), manifesting the remarkable advantages of using CZTS thin film for lithium storage. Moreover, we report the manufacture and measurements of the microscale TFBs with the CZTS film as anode. Surprisingly, the LiCoO2/CZTS TFBs exhibit higher capacity, better cycle performance and more apparent voltage plateaus than the LiCoO2/SnO2 TFBs. The strategy of utilizing thin film material that is thin and conductive enough to induce different mechanism, provides insight into the development of thin film electrodes and micro power devices for MEMS/IC applications.

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2. EXPERIMENTAL SECTION 2.1. Fabrication. All films were deposited by RF magnetron sputtering systems (JC500-3/D and JS3X-100B). The photolithography was carried out using Karlsuss MA6/BA6 mask aligner. The micro-fabrication process and the corresponding structure of microscale TFBs are illustrated in Figure 1. Firstly, the oxidized Si substrate with an isolation layer SiO2 was patterned with photoresist (BP212) to define the area of current collector. The Cr/Pt films were consecutively sputtered on the SiO2 layer by direct circuit (DC) magnetron sputtering, and then the sample was immersed into acetone to lift off the excess Cr/Pt films. The area of anode was defined as a square opening of 650 μm × 800 μm by lithography. A CZTS film was sputtered on the Pt film with a mass loading of ~0.57 mg cm-2 μm-1, and then selectively lifted off. After photoetching the photoresist to define the area of other films, a solid electrolyte film LiPON was deposited on the CZTS layer. For preventing the expose of LiPON film to air, the LiCoO2/Al films were directly deposited and then selectively removed. Finally, the whole sample was patterned with a polydimethylsiloxane (PDMS) film to protect the whole device. Meanwhile, the LiCoO2/SnO2 TFBs were fabricated in the same process with the substitution of CZTS into SnO2 film. During the deposition of anode and cathode films, the working electrodes for half-cells were also fabricated on Cu and Al substrates, respectively, and then cut into small rounds of 1.13 cm2. All the electrodes were pure active materials without any additives. The fabrication parameters of all films are listed in Table 1.

Figure 1. Fabrication process and corresponding structure of microscale TFBs.



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Table 1. Deposition conditions of all films fabricated by magnetron sputtering. Materials

Target

Mode Power Pressure Atmosphere Thickness -2 (W cm ) (Pa) (nm)

Pt/Cr

Pt/Cr

DC

1.27

1.3

Ar=100%

200

Cu2ZnSnS4 Cu2ZnSnS4 RF

1.53

1.3

Ar=100%

300

SnO2

Sn

RF

1.53

1.3

O2:Ar=1:2

300

LiPON

Li3PO4

RF

1.27

1.3

N2=100%

150

LiCoO2

LiCoO2

RF

1.53

1.5

O2:Ar=1:3

150

Al

Al

DC

1.27

1.3

Ar=100%

200

2.2. Characterization. The surface and cross section morphologies were investigated by using ZEISS Sigma scanning electron microscope (SEM). The crystal structure and elementary composition were studied by using transmission electron microscopy (TEM, FEI Tecnai-F30) equipped with an energy dispersive X-ray spectrometer (EDX) analyzer. X-ray diffraction (XRD) equipment (Rigaku Ultima IV) with Cu Kα radiation was used to identify the microstructure. The Raman spectra (IDSpec ARCTIC) were obtained with a 532-nm laser excitation wavelength. The chemical composition was also tested by X-ray photoelectron spectroscopy (XPS) on PHI Quantum 2000 scanning microprobe. The TEM samples were prepared by depositing a drop of diluted suspension in ethanol on a copper grid coated with a carbon film.

2.3. Electrochemical Tests. The electrochemical properties of TFBs were tested on a two-probe station connected with Arbin-BT2000 battery test instrument and cycled with a charge/discharge current density of 50/10 nA per TFB between 0.1-1.5 V. The electrochemical properties of LiCoO2, SnO2, thin and thick CZTS films were also measured in coin cells (CR2016) assembled in an Arfilled glove box. The above prepared samples were chosen as the working electrodes, lithium metal foils were the counter electrodes, polypropylene films (Celgard 2400) were the separator, and 1.0 M LiPF6 mixed with ethylene carbonate, dimethyl carbonate and diethyl carbonate (EC/DMC/DEC, 1:1:1 in volume) was the electrolyte. After the half-cells were shelved for ~12 h, the galvanostatic 4 ACS Paragon Plus Environment

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charge/discharge tests were conducted on Neware BTS-5 V/1 mA battery test systems (Shenzhen, China) at a current density of 50 μA cm-2. The voltage window was from 0.01-3 and 3-4.2 V for anode and cathode, respectively. The cyclic voltammetry (CV) tests were performed on Arbin-BT2000 battery test instruments between 0-3 V at a scan rate of 0.5 mV s−1. The electrochemical impedance spectroscopy (EIS) plots were measured on CHI660E electrochemical workstations, with an amplitude potential of 5 mV and a frequency range of 0.1-106 Hz. It is worth noting that all fabrication and tests were conducted at room temperature without annealing.

3. RESULTS AND DISCUSSION 3.1. Morphologies and Structures. The surface SEM images of CZTS thin and thick films are compared in Figure 2a-b. Less and smaller aggregations are displayed in the thin film than the thick film. In the magnified image (Figure 2c), the single aggregation in the thin film is not so dense as the thick film, providing more void for volumetric change during repeated lithiation. The TEM image (Figure 2d) confirms that the aggregations are assembled from CZTS nanosheets stacked up closely. The EDX elementary mappings (Figure 2e-h) depict that the Cu, Zn, Sn and S elements are uniformly dispersed in the CZTS thin film. As marked in the high resolution TEM (HR-TEM) image (Figure 2i) and the selected area electron diffraction (SAED) pattern (Figure 2j), the accordant lattice fringes with an interplanar spacing of 0.31 nm and the diffraction ring are all indexed as the (112) crystal plane of tetragonal CZTS (JCPDS No. 26-0575).19 Additionally, single (112) peak is detected in the XRD spectra of CZTS thin film (Figure 2k), verifying the consistent crystal results. In order to distinguish CZTS from other impurities (Cu2SnS3, ZnS, etc.), the Raman spectrum is also detected (Figure 2k). The main peak located at 337 cm-1 is concerned with the symmetric vibration of CZTS, and this spectrum is consistent with the reported CZTS materials.20-21 The morphologies of the thick CZTS, SnO2 and LiCoO2 films are also examined in Figure S1. The elementary composition of CZTS thin film is further investigated by XPS spectra (Figure 2l).22 The Cu 2p peaks are located at 932.4 and 952.2 eV with a peak splitting of 19.8 eV indicating Cu(I). The Zn 2p peak measured at 1022.2 5 ACS Paragon Plus Environment


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eV is associated with Zn(II). The Sn 3d peaks are located at 486.5 and 495.0 eV with a peak separation of 8.5 eV corresponding to Sn(IV). The S 2p peak located at ~164 eV is related to sulfides. All the results indicate that the CZTS thin film was successfully fabricated with homogeneous morphologies and chemical composition.

Figure 2. Surface SEM images of (a) CZTS thin film (300 nm), (b) CZTS thick film (1 μm) and (c) CZTS aggregations in both films. (d) Dark-field TEM image, (e-h) corresponding EDX elementary mappings of Cu, Zn, Sn and S elements, (i) corresponding HR-TEM image and (j) SAED pattern of CZTS thin film. (k) XRD, Raman and (l) XPS spectra of CZTS thin film.


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The optical images of the TFB arrays and single TFB are displayed in Figure 3a-b. The aligned TFBs are free of cracks or flakes on the surface, and the boundary of all films can be seen clearly. The cross section SEM images of the two different TFBs are shown in Figure 3c-d, and the interfaces are flawless without any pinholes or slits. The thickness of all labelled films is measured as listed in Table 1. The coincident thickness of LiCoO2 and LiPON films in the two TFBs ensures the equal capacity delivered by cathode, and indicates the uniformity of sputtered films. Because the ionic conductivity of solid electrolyte is intrinsically inferior than liquid electrolyte,23-24 and the large resistance of thick electrolyte film may lead to the decrease in capacity and cyclability.25-26 The thin thickness of LiPON film will make great contribution to the energy density of whole battery, but it should neither be too thick to add the whole resistance, nor too thin to cause the short-circuit of TFBs. According to the reported properties of LiPON,26-27 the thickness of LiPON is estimated to be ~150 nm in the both TFBs.

Figure 3. (a, b) Optical images of TFB array and single TFB. Cross section SEM images of TFBs with (c) CZTS and (d) SnO2 thin films as anode materials. 7 ACS Paragon Plus Environment


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3.2. Electrochemical Tests in Half-cells. The charge/discharge curves of SnO2 thin film are shown in Figure 4a. The steep discharge curves are mainly attributed to the formation of Li-Sn alloys.28 The charge voltage profiles sharply increase over 1.7 V, implying that the conductivity of SnO2 thin film is not high enough to induce the reversible “conversion reaction”29 of Li2O and Sn into Li and SnO2.30 The detailed electrochemical mechanism of CZTS thick film have been reported previously,15 but the CZTS thin film shifts much from the thick film. This is confirmed by the charge/discharge curves (Figure 4b-c) of CZTS thick and thin films. An obvious charge plateau appears at ~2.2 V as circularly marked in Figure 4c but is absent in Figure 4b, corresponding to the partial reversible conversion reaction for CuxS materials31-32 as shown in eq. 1.

Cu xS  2Li  2e  xCu  Li2S.

(1)

The cycle performance of all films is compared in Figure 4d. The capacity of SnO2 thin film decays greatly during cycling, compared to the SnO2 thick film (Figure S2), suggesting the aggregation and volumetric issues are not effectively released even in the 300 nm thin film. Also, the capacity of CZTS thick film decays drastically in the first several cycles from ~530 to 30 μAh cm-2 μm-1. On the contrast, the capacity of CZTS thin film maintains above 350 μAh cm-2 μm-1 (614 mAh g-1), and even increases after 150 cycles because of the gradual activation process in metal sulfide electrodes as reported.31, 33 In details, the charge plateau at ~2.4 V as marked in rectangle in Figure 4c becomes evident in the 170th cycle, and contributes to the gradually increasing capacity, due to the added reversible reactions including eq. 1. This is also confirmed by the differential capacity plots in different cycling steps (Figure S3). Finally, the value reaches 540 μAh cm-2 μm-1 (950 mAh g-1) after 500 cycles. Even though the specific capacity value of CZTS is ~847 mAh g-1 based on the mechanisms reported before, higher experimental capacity values are still presented,33-34 indicating other reactions can increase the capacity. In addition, the Coulombic Efficiency (CE) of CZTS thin film promptly raises up to ~100% and keeps stable even after 500 cycles, which is obviously superior than other samples. All the electrochemical results demonstrate the excellent capability and durability of CZTS thin film.


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The electrochemical properties of this CZTS thin film are specifically compared with the reported CZTS anodes as listed in Table 2, and the rate performance is also compared as shown in Figure S4. By comparing the differences between this work and the listed results, the high performance of CZTS thin film can be understandably ascribed to the following reasons: (i) the reported CZTS without further process are almost bulk materials, so the active materials in this thin film can be completely utilized. The Li+ diffusion path is shortened, the conductivity is enhanced, and the close contact in this quaternary compound is advantageous for the additional Li+ storage between Cu, Zn, Sn and Li2S matrix. (ii) Different from the reported synthesis methods, the one-step physical deposition applied here can greatly avoid the loss of active materials and the pollution of other impurities. Moreover, the as-prepared CZTS thin film is pristine without any additives such as binders or conductive agents, so the specific capacity is higher than the common anodes made from slurry mixture. (iii) The additional charge plateau at ~2.4 V can only be observed in the CZTS materials with high capacity,33-34 and is more evident in this CZTS thin film because of the high purity and nanoscale thin thickness.

Figure 4. Galvanostatic charge/discharge curves of (a) SnO2 thin film (300 nm), (b) CZTS thick film (1 μm) and (c) CZTS thin film (300 nm) in half-cells. (d) Cycle performance based on charge capacity. 9 ACS Paragon Plus Environment


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Table 2. Comparison of various CZTS materials reported for lithium storage. Materials

Method

Voltage Current Capacity Cycles Thickness Reference (V) (mA g-1) (mAh g-1) (nm)

2D CZTS thin film

Sputtering

0.01-3 260

950

500

300

This work

3D coated CZTS film Sputtering

0.01-3 100

668

100

1000

[15]

CZTS microflower

Solvothermal 0.01-3 100

800

100

\

[33]

CZTS nanocrystal

Solvothermal 0.8-3

100

100

30

\

[35]

CZTS nanoflower

Solvothermal 0-3

200

150

50

\

[36]

CZTS nanocrystal

Microwave

0-2.75 100

250

30

\

[37]

Porous CZTS film

Dip pyrolysis 0-4

600

55

3000

[34]

100

The electrochemical mechanisms of the thin SnO2, thick CZTS and thin CZTS films are analyzed in details by CV curves as marked in Figure 5a-c. The cathodic peaks are relevant with the lithiation of Sn, Cu and Zn sulfides, and the anodic peaks are involved in the de-lithiation of Li-Sn, Li-Cu and Li-Zn alloys. In the first cycle, the highest cathodic peaks are concerned with the formation of Li2O or Li2S and SEI film in all samples.37-39 The CV peaks of SnO2 film (Figure 5a) are consistent with the SnO2 materials.38-39 The CV peaks of CZTS thick film (Figure 5b) has been studied as reported before:15 the de-lithiation is mainly dominated by the anodic peaks at ~0.75 and 1.6 V for Li-Sn and Li-Zn alloys. On the contrast, the highest anodic peak of CZTS thin film located at ~2.2 V (Figure 5c) is ascribed to the de-lithiation of Li-CuxS alloys31-32 as described by eq. 1, contributing to the whole capacity and will further induce an apparent discharge plateau in the LiCoO2/CZTS TFBs. In the following cycles, the anodic peaks do not change obviously, indicating the remarkable reversibility of CZTS thin film. Considering the reversible reactions do not occur in the CZTS thick film, we speculate that the conductivity of CZTS is enhanced and the Li+ diffusion path length is shortened in the thin film,18 leading to the distinct mechanism. The EIS curves of thin SnO2, thick CZTS and thin CZTS films after cycling are plotted in Figure 5d-e, and the equivalent circuit model is shown in Figure 5f. The Bode plots are also provided in Figure S5. The origin intercept is the internal resistance of cell (Rs), 10 ACS Paragon Plus Environment

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the semicircle in high and middle frequency represents the SEI resistance (Rf) and the charge transfer resistance (Rct), and the oblique line in low frequency represents the Warburg impedance of Li+ diffusion (Zw). In addition, the constant phase elements of CPE1, CPE2 and CPE3 are associated with the SEI film, charge transfer interface, and Li+ accumulation and exhaustion in electrodes after cycling, respectively. The fitted parameters are listed in Table 3. The inherent resistance of cell in all samples are almost equal, but the SEI and charge transfer resistances are largely reduced in the CZTS thin film, promoting the reversible lithiation and contributing to the high capacity value.

Figure 5. (a-c) CV curves of thin SnO2, thick CZTS and thin CZTS films in half-cells. (d) EIS curves of thin SnO2, thick and thin CZTS films in half-cells after 100 cycles, (e) corresponding detailed view and (f) equivalent circuit model for fitting.

Table 3. Fitted parameters for EIS plots. Material

Rs(Ω)

Rf(Ω)

Rct(Ω)

SnO2

3.32

344

2658

Thick CZTS

3.24

593

5657

Thin CZTS

3.43

7.11

426



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3.3. Electrochemical Tests in Thin Film Batteries. The LiCoO2 electrode is also fabricated for comparing the voltage profiles of TFBs. As depicted in Figure 6a, the initial broad charge plateau is associated with the de-lithiation of LiCoO2 into LixCoO2 (0.5

Ultrahigh-Performance Cu2ZnSnS4 Thin Film and Its Application in

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