Author’s Accepted Manuscript Poly (dimethylsiloxane) Modified Lithium Anode for Enhanced Performance of Lithium-Sulfur Batteries Qian Li, Fang-Lei Zeng, Yue-Peng Guan, ZhaoQing Jin, Ya-Qin Huang, Ming Yao, Wei-Kun Wang, An-Bang Wang www.elsevier.com/locate/ensm
PII: DOI: Reference:
S2405-8297(17)30448-8 https://doi.org/10.1016/j.ensm.2018.01.002 ENSM286
To appear in: Energy Storage Materials Received date: 31 October 2017 Revised date: 28 December 2017 Accepted date: 2 January 2018 Cite this article as: Qian Li, Fang-Lei Zeng, Yue-Peng Guan, Zhao-Qing Jin, YaQin Huang, Ming Yao, Wei-Kun Wang and An-Bang Wang, Poly (dimethylsiloxane) Modified Lithium Anode for Enhanced Performance of Lithium-Sulfur Batteries, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2018.01.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Poly (dimethylsiloxane) Modified Lithium Anode for Enhanced Performance of Lithium-Sulfur Batteries Qian Lia,b,1, Fang-Lei Zengc,1, Yue-Peng Guana, Zhao-Qing Jinb, Ya-Qin Huanga, Ming Yaoa, Wei-Kun Wangb,*, An-Bang Wangb,* a Department of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. bMilitary Power Sources Research and Development Center, Research Institute of Chemical Defense, Beijing 100191, China
cSchool of Material Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China.
[email protected]
[email protected].
ABSTRACT Although lithium sulfur (Li-S) battery is a promising candidate for next generation energy storage devices due to the high theoretical specific capacity and energy density, the severe corrosion and the formation of lithium dendrites hinder its practical application. Here, we demonstrated a
simple method which only needed to immerse Li foil into the PDMS-contained DOL/DME solution and then evaporate the DOL/DME solvent. Finally a new protective layer on the surface of Li foil was formed. This protective layer could not only effectively stabilize the interphase of lithium anode, but also suppress the formation of dendritic lithium. With the PDMS-modified Li anode, Li-Cu and Li-S cells all delivered a stable cycling performance with a high Coulombic efficiency and a long-lifespan performance, which were all superior to the cells with the pristine Li anode. Hence, introducing PDMS to protect Li anode provided a new strategy for the improvement of Li-S batteries.
Graphical abstract
Keywords:
Poly
(dimethylsiloxane),
lithium
anode,
surface
modification, long-lifespan, Li-S battery
1 Introduction The ever-increasing demand for portable electronic devices, large-scale energy storage, and electric vehicles has sparked the research in advanced battery systems with low cost and high energy density [1]. In this scenario, lithium-sulfur (Li-S) battery, comprising a lithium metal anode and a sulfur cathode, has attracted tremendous attention from the energy
storage community, mainly due to its high theoretical energy density (≈2567 Wh kg-1), which is much higher than that of conventional Li-ion batteries. And the cathode material-elemental sulfur has a high theoretical specific capacity of 1675 mAh g-1, based on the electrochemical reaction of 16Li + S8 → 8Li2S. The high theoretical energy density and high theoretical specific capacity makes the Li-S system more attractive in the energy storage systems. Also, the low price and abundant availability of sulfur as the cathode material are the attractive features in comparison to state-of-the-art
lithium-ion
technologies
[2-5].
However,
the
commercialization of Li-S battery is impeded by the low sulfur utilization, low Coulombic efficiency, fast capacity fade, poor cycle life and serious safety problems. These problems mainly arise from the poor electrical conductivity of sulfur and Li2S, the heavy dissolution and “shuttle effect” of polysulfides, the volume expansion, and the severe corrosion and mossy dendrite of lithium metal anode during cycling [6].So far, significant progress has been achieved recently in hindering the dissolution of polysulfides into electrolyte and improving the conductivity of sulfur by developing various strategies, such as using various
types
of
host
cathode
materials
[7-9],
inserting
polysulfides-blocking interlayers [10-12], modifying the separator [13, 14], and so on. Although, these strategies can greatly improve the performance of Li-S battery, there was long distance far from the
commercial applications for Li-S battery. The main reason is that the safety problem of Li-S battery caused by lithium anode remains unresolved. Lithium metal is an ideal anode material for the development of Li-S batteries with high energy densities because of its high theoretical capacity (3860 mAh g-1), its light weight and electro-negative potential (-3.04 V versus standard hydrogen electrode) [3, 15-18]. However, the use of Li metal as anode for Li-S battery faces several hurdles [6]. The most challenging one is the formation of Li dendrites, originating from the non-uniform deposition of Li, which could puncture separator and contact with cathode, resulting the internal cell short circuit and serious safety concerns [19]. The other challenge is the serious lithium corrosion. Lithium is so reactive which can react with the solvent easily to form a solid electrolyte interphase (SEI) layer on the surface of Li anode. But the SEI layer is unstable to cause a great irreversible capacity loss and low deposition efficiency of Li upon charging [20-22]. The last one for Li-S battery is that the penetrating soluble lithium polysulfides through the separator due to the shuttle effect (by efficient cathode and membrane design, the shuttle effect has been effectively trapped in the cathode side and the coin cells deliver a superior cycling performance of 2000 cycles [23].) would react with the Li anode to form the Li 2S/Li2S2 layer on the surface of the Li anode, which would make the composition of the SEI
film more complex and the formation of a stable SEI film more difficult. Many attempts have been applied to protect the Li anode during the charge-discharge process [24], such as reactive organic [25-27]/inorganic [28-31] additives, polymer coatings, solid-state electrolyte [32, 33], and so on. Additionally, alternative nanostructured Li anodes, such as 3D Cu current collector-based Li anode [34, 35], graphene framework-based Li anode [36, 37] as well as 3D skeleton-based Li anode [38], have also been used to prohibit the formation of Li dendrites and extend the lifespan
of
Li-metal
anodes.
They
can
greatly
enhance
the
electrochemical performance and the safety of Li-S cells. Among these attempts, proposing artificial protective layers [39-44] are the mostly used method of protecting Li anode and blocking the formation of Li dendrites during cycling. It is widely regarded that the ideal interfacial layer should be chemically stable in a strong reducing environment, mechanically strong, and also beneficial to lithium ion transport and Li deposition. Although some successes have been achieved by these attempts, there is still a distance far from the practical application. Zhu et al. [45] have demonstrated a PDMS protective film with nonporous fabricated by hydrofluoric (HF) acid and spin-coating on Cu foil for the uniformly deposition of lithium. However, the method of modifying PDMS and the processes are complicated, which may restrain
the scalability. Here, we demonstrated a simple method which only need immerse Li foil into the PDMS-contained DOL/DME solution and then evaporated the DOL/DME solvent, finally a new protective layer was formed on the surface of Li foil. With the PDMS-modified Li anode, Li-Cu and Li-S cells all deliver a stable cycling performance. In a word, the fabrication method of the protective layer was facile and convenient, which would be benefit for the practical of Li-S batteries.
2 results and discussion 2.1 The characterization of modified lithium anode Li foil was immersed into the solution of 1wt% PDMS-contained DOL/DME (V/V=1/1) and dried under vacuum at room temperature to evaporate the DOL/DME solvent, obtaining a new protective layer on the surface of Li foil(Figure 1).
Figure 1. The schematic process for the PDMS- modified Li foil
Figure 2a and 2b-d display the morphological properties of surface of the
pristine Li foil and the PDMS-modified Li foil, respectively. From Figure 2a, it was obvious there were lots of white debris on the surface of the pristine Li foil, which was mainly composed by Li2O, LiOH and Li2CO3, because of the unavoidable reactions of lithium anode with air components during the preparation process [46]. When modified with PDMS, only little white debris were observed on the surface of Li foil, and it displayed a relatively homogeneous and dense coating on the surface of Li foil though there were some nanoscale bumps (Figure 2c, d). The thickness of the protective layer was about 1.5 μm (Figure 2b). It was obvious that the regular surface of PDMS-modified Li foil was compact as shown in Figure 2b-d. Additionally, the energy-dispersive X-ray spectroscopy (EDX) results of the PDMS-modified Li foil were displayed in Supplementary Materials (Figure S1). The overlays of the C EDX signal (marked as white), the O EDX signal (marked as red) and the Si EDX signal (marked as green) were uniformly distributed on the surface of Li foil, which also confirmed that the a uniform and dense protective layer was formed on the surface of Li anode.
Figure 2. SEM images of Li foil surfaces: pristine Li (a); thickness of protective layer (b); PDMS-modified Li foil (c, d).
To further explore the composition of protective layer on the Li foil surface, the X-ray photoelectron spectroscopy (XPS) spectra of PDMS (Figure 3a, b) and PDMS-modified Li foil (Figure 3c, d) were analyzed as shown in Figure S2 and Figure 3. Figure S2 exhibited the wide-scan XPS spectra of PDMS (black line) and PDMS-modified Li foil (red line) with the Si 2p, Si 2s, C 1s and O 1s peaks. In Figure 3a, the Si 2p spectra of PDMS exhibited two prominent peaks at binding energy of 102.3 ev and 103.8 ev, which ascribed to the Si-C and Si-O bonds in PDMS, respectively. When modified with PDMS on the surface of Li foil, the two above peaks had shift to 102.1 ev and 102.4 ev, respectively, as shown in Figure 3c. Also, the O 1s spectra of Si-O shifted from 532.8 ev
to 532.5 ev when Li foil was treated by PDMS as shown in Figure 3b, d. These results suggested that a reaction occurred between lithium and PDMS and a new protective layer different from the PDMS formed which could be marked as PDMS-Li. FTIR spectra of the PDMS and the PDMS modified Li also confirmed the formation of PDMS-Li. As Figure 3e shown, both of them exhibited a strong Si-O-Si stretching peak around 1035 cm-1 and a symmetric Si-O stretching peak around 795 cm-1. Also, the Si-C stretching peak around 1260 cm-1 and the symmetric Si-CH3 peak around 850 cm-1 could be found in the FTIR spectra. It was worth noting that these above peaks became broad and moved towards slightly lower frequencies for the PDMS modified Li. And for PDMS and the PDMS modified Li, the intensities and the frequencies of the stretching vibration (2964 cm-1) and bending vibration (1454 cm-1) of -CH3 peak were also different. Most important, a new broad peak appeared around 1765 cm-1 in the spectra of the PDMS modified Li. These results sufficiently proved that the PDMS reacted with lithium and formed a new protective layer (PDMS-Li). Although the pure PDMS was not a lithium-ion conductor, the new protective layer (PDMS-Li) might benefit the conduction of lithium-ion [45].
Figure 3. XPS spectra of Si 2p and O 1s for PDMS (a, b); PDMS-modified Li foil (c, d); FTIR spectra of PDMS and PDMS-modified Li (e).
2.2 Symmetric Li-Li cell tests To investigate the electrochemical behavior of Li plating/stripping and the stability of the PDMS-modified Li electrodes, the symmetric Li-Li
cells were constructed and the corresponding galvanostatic cycling performances of the symmetric cells with different lithium anodes were examined as shown in Figure 4a. The current density of 0.5 mA cm-2 with the capacity density of 0.5 mAh cm-2 was adopted for evaluating the plating/stripping behavior. Obviously, there were differences on the voltage polarization between the symmetric cell with PDMS-modified Li and the cell with pristine Li. The cell with PDMS-modified Li exhibited a stable cycle with low and stable polarization potential difference (~10 mv), while the cell with pristine Li showed a larger polarization potential (~20 mv) after cycling for 200 h. Higher current densities of 1.0 mA cm-2 with 1.0 mAh cm-2 and 2.0 mA cm-2 with 2.0 mAh cm-2 had the similar results that the PDMS-modified Li-Li cells showed lower polarization potential (~ 20 mv and 25 mv) than the pristine Li-Li cells (~ 60 mv and 250 mv) correspondingly at the Figure S3, which corresponded the obvious voltage hysteresis phenomenon. The voltage hysteresis was defined as the sum of the over potential for Li depositing and Li stripping to characterize the reaction dynamics. The larger voltage hysteresis of the cell with pristine Li was possibly caused by the unstable Li/electrolyte interface
and
electrical
disconnection
because
of
repeated
growth/corrosion of dendritic Li. However, when Li anode was modified by PDMS, the growth of dendritic Li has been significantly retarded. Also, introducing the protective layer could help to improve the ionic
conductivity of the lithium/electrolyte interface. In summary, the protective layer could contribute to eliminate the hysteresis of the symmetric cell and enhance the lifespan of the symmetric cell. The EIS results of the symmetric Li-Li cells with pristine Li (Figure 4c) and PDMS-modified Li anode (Figure 4c) at different standing time (0, 12, 24, 48 and 96 h) were also verified the above point of view. As Figure 4b and c shown, the symmetric cell with PDMS-modified Li anode exhibited slightly larger initial impedance than the cell with pristine Li anode, which mainly attributed to the lower Li-ion conduction ability of the PDMS-Li than the electrolyte. Therefore, before cycling, the cell with PDMS-modified Li had higher impedance. However, after prolonged standing, the impedance of the cell with PDMS-modified Li anode increased slowly, while the corresponding impedance of the cell with pristine Li anode increased rapidly. The main reason could be explained as follow. For the cell with pristine Li anode, the harmful parasitical reaction among fresh lithium, electrolyte and lithium salt would occur, leading to an unstable interphase of higher impedance. However, for the cell with PDMS-modified Li anode, the harmful parasitical reaction would be suppressed due to the protection of the protective layer for Li anode. Hence, according the EIS spectra, the protective layer could be mitigating the undesirable parasitical reaction, exhibiting comparatively more stable interphase with lower impedance against electrolyte
components than the pristine ones [47,48]. These result also corresponded the galvanostatic cycling diagram of the symmetric Li-Li cells. (a)
0.3
Pristine Li PDMS-modified Li
0.02
Voltage (v)
0.2
0.00 -0.02
0.1 201
204
207
210
213
0.0 -2 -2 0.5mAcm 0.5mAh cm
-0.1
50
100
150
200
250
300
Cycling time (h)
(c)
700
-Z''/ (ohm)
600 Pristine Li
500 400
0h 12 h 24 h 48 h 96 h
300 200 100
700 600
-Z''/ (ohm)
(b)
PDMS-modified Li
500 400
0h 12 h 24 h 48 h 96 h
300 200 100
0
0 0
100 200 300 400 500 600 700
Z/ (ohm)
0
100 200 300 400 500 600 700
Z/ (ohm)
Figure 4. (a) The galvanostatic cycling diagrams of the symmetric Li-Li cells with the pristine and the PDMS-modified Li anodes at the density of 0.5 mA cm-2 for 0.5 mAh cm-2; (b, c) The EIS plots of the symmetric Li-Li cells with the pristine Li anode and the PDMS-modified Li anode at different standing time.
The surface morphologies of pristine Li and PDMS-modified Li anodes in symmetric Li-Li cells after 200 cycles were shown in Figure 5. As depicted in Figure 5a, the pristine Li was block and porous, which was related to the uneven deposition/stripping of Li. However, the surface of PDMS-modified Li was smoother and no block and porous Li were appeared in Figure 5b, which might contribute to a long-lifespan cycling. The EDX experiment was performed for PDMS-modified Li after 600
cycles in Figure S4.The Si, O and C element were uniformly distributed on the surface of lithium anode.
Figure 5. The SEM images for the pristine Li anode surface (a) and the PDMS-modified Li anode surface (b) after 600 cycles. The current density was 0.5 mA cm-2 with the capacity of 0.5 mAh cm-2.
2.3 Li-Cu cell tests To further research the functionalities of the protective layer, the galvanostatic cycling test of Li-Cu cells were constructed. Among them, Cu foil was served as the working electrode, and Li foil as the counter/reference electrode. Li was deposited and stripped between the two electrodes at different current densities. Coulombic efficiency (CE) is a vital parameter which represents the ratio of the amount of stripped Li versus that of deposited Li in each cycle. A high and stable value of Coulombic efficiency typically represents a stable interface of electrode and electrolyte and a long cycle life. Figure 6 showed the electrochemical performance of the cells with and without the PDMS-modified anode at different current densities. It was clear that the Li-Cu cell with PDMS-
modified anode showed a more stable CE (above 95%) and longer cycle life under various current densities (Figure 6a, 6c, 6e). In comparison, Li-Cu cell with pristine anodes presented a quick decay in CE and unstable cycling. As Figure 6a show, after about 150 cycles, the CE of Li-Cu cell with pristine Li anodes droped to 70% at 0.2 mA cm-2. And when cycling at 0.5 and 1 mA cm-2 current density, the cycle number of the cell with PDMS-modified anodes were 85 and 40 cycles, which was better than the pristine one (30 cycles and 18 cycles) (Figure 6c, 6e). These results indicated that the protective layer was stable during cycling, largely improving the electrochemical stability of the electrodes. Meanwhile, the charge-discharge profiles at different rates were shown in Figure 6b, 6d and 6f. From Figure 6, the cell with pristine Li anode presented a larger voltage hysteresis than the cell with PDMS-modified Li anode. Especially, at the current density of 1 mA cm-2, the voltage hysteresis of the cell with pristine Li anode reached above 90 mV after 30 cycles, indicating that the internal resistance increased after long cycles, which mainly resulted from the nonuniform deposition of Li and the unstable interphase. However, for the cell with PDMS-modified anodes, the voltage hystersis maintained just 25 mV upon 90 cycles (Figure 6f) due to the stable interface with the help of the protective layer.
Figure 6. Cycling performance of pristine Li anode and PDMS-modified Li anode at different current densities: Coulombic efficiencies at (a) 0.2 mA cm-2, (c) 0.5 mA cm-2, and (e) 1.0 mA cm-2; potential profiles at (b) 0.2 mA cm-2, (d) 0.5 mA cm-2, and (f) 1.0 mA cm-2.
In order to further investigate the Li metal deposition behavior, ex-situ SEM observations were carried out on pristine Li and PDMS-modified Li anodes after cycles at 0.2 mA cm-2. As Figure 7a, b shown, there were lots of dendrite-like Li on the surface of the pristine Li anode. In contrast, when Li anode was modified with PDMS, the plating of Li metal showed a distinct morphology-a dense and flat surface without obvious dendrites-like Li after deposition (Figure 7c, d). These results proved that
the protective layer was benefit for the Li deposition.
Figure 7. The SEM images for the pristine Li anode surface (a, b) and the PDMS-modified Li anode surface (c, d) after 400 cycles. The current density was 0.2 mA cm-2 with the capacity of 0.4 mAh cm-2.
2.4 Li-S cell In order to demonstrate the positive effect of the protective layer on the electrochemical performance of Li-S cells, the sulfur cathodes with areal loading of 2.0-2.5 mg cm-2 were fabricated by using the C/S composite materials with a high sulfur content of 80% (Figure 8). As Figure 8a shown, the cell with pristine Li anode demonstrated an initial specific capacity of 990 mAh g-1 at 100 mA g-1 and rapidly decreased to 441 mAh g-1 after 100 cycles, meaning that only 44.5% of the initial capacity was preserved. While the cell with PDMS-modified Li anode showed better
cycling performance. The initial discharge capacity of the cell with PDMS- modified Li anode was 1139 mAh g-1, which was higher than that of the cell with pristine Li anode. After 100 cycles, the capacity of the cell with PDMS-modified Li anode still remained at 761 mAh g-1. Therefore, it can be concluded that the shuttle effects have been greatly hindered by the protective layer of Li anode. Figure 8b showed the discharge/charge curves of the cells with and without PDMS-modified Li anodes after 100 cycles at a current density of 100 mA g-1. The both cells exhibited similar voltage plateaus: a short upper plateau at 2.35 V, a long lower plateau at 2.10 V during the discharge process, and a long plateau during the charge process. These plateaus represent the reduction process of solid sulfur to long-chain polysulfides (Li2Sx, 4
