Letter www.acsami.org
Quantitative Analysis of Electrochemical and Electrode Stability with Low Self-Discharge Lithium-Sulfur Batteries Sheng-Heng Chung, Pauline Han, and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: The viability of employing high-capacity sulfur cathodes in building high-energy-density lithium−sulfur batteries is limited by rapid self-discharge, short shelf life, and severe structural degradation during cell resting (static instability). Unfortunately, the static instability has largely been ignored in the literature. We present in this letter a longterm self-discharge study by quantitatively analyzing the control lithium−sulfur batteries with a conventional cathode configuration, which provides meaningful insights into the cathode failure mechanisms during resting. Utilizing the understanding obtained with the control cells, we design and present low self-discharge (LSD) lithium−sulfur batteries for investigating the long-term self-discharge effect and electrode stability. KEYWORDS: lithium−sulfur batteries, self-discharge, shelf life, electrode stability, electrochemistry
L
(CNF)-coated separators, and lithium anodes with a sulfur content of 70 wt % and a sulfur loading of 3.4 mg cm−2.The hierarchical PEG/CNF coatings on the separator were arranged toward the sulfur cathodes for reducing polysulfide diffusion during cell resting.7,11,14 The reduced polysulfide diffusion and the incurring static electrochemical stability were further studied with the LSD cells fabricated with three different hierarchical separators with one, two, and three layers of PEG/ CNF, which are denoted, respectively, as CNF#1, CNF#2, and CNF#3. The analytical data with both the control and LSD batteries offer a better understanding of the self-discharge mechanism, cell shelf life, and electrode stability. Finally, a citation analysis of the quantitative data in consideration of both the electrochemical characteristics and cell-fabrication parameters with the literature values concludes that our LSD lithium−sulfur batteries exhibit the most stable static cell performances. For the purpose of conducting long-term investigations, it is vital to have the control and the LSD lithium−sulfur batteries both attain acceptable electrochemical characteristics. Thus, the dynamic and static battery performances are studied as an initial assessment.14 Dynamic battery performances are investigated at C/10 rate for 500 cycles (Table S1). The control lithium− sulfur cells have a peak charge-storage capacity of 933 mAh g−1, corresponding to a low areal capacity of 2.8 mAh cm−2, gravimetric capacity of 606 mAh g−1, and volumetric capacity of only 373 mAh cm−3 by considering the dimension of the entire
ithium−sulfur batteries are becoming appealing as sulfur offers a high charge-storage capacity of 1,675 mAh g−1 with a two-electron reaction per sulfur as 16 Li + S8 = 8 Li2S at a standard potential of 2.2 V (vs Li/Li+). As a result, lithium− sulfur batteries offer a high theoretical specific energy density of 2600 Wh kg−1 and an anticipated practical energy density value of up to 600 Wh kg−1 in a full cell, a value exceeding significantly that of the current Li-ion batteries.1−3 However, the application of sulfur cathodes is hampered by their dynamic and static electrochemical instability, as the soluble higher-order polysulfide intermediates (Li2Sx, x = 4−8) irreversibly resettle during cell cycling and resting.3−5 Although the static electrochemical instability is one of the two major challenges in developing lithium−sulfur battery technology, the major progress accomplished in recent years focuses almost solely on the improvement in the dynamic battery performances during cell cycling.6−11 Unfortunately, understanding and ameliorating the static electrochemical instability during cell resting still remain a serious problem and need more attention.12−14 We present here a detailed quantitative investigation into the long-term self-discharge effect by first examining control lithium−sulfur batteries fabricated with sulfur cathodes (sulfur content of 65 wt % and sulfur loading of 3.0 mg cm−2), polypropylene membrane, and lithium anode. After understanding the failure mechanisms of the control cells during resting, we then quantitatively analyze the experimental data collected from our low self-discharge (LSD) cells, which were developed in our previous work,14 for exploring (i) the longterm self-discharge effect and (ii) the cathode stability. The LSD batteries are assembled with sulfur cathodes, hierarchical polyethylene glycol (PEG)-supported carbon nanofiber © XXXX American Chemical Society
Received: April 21, 2017 Accepted: June 7, 2017 Published: June 7, 2017 A
DOI: 10.1021/acsami.7b05602 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces cathode. The control cells end in a short cycle life and are terminated in 100 cycles due to the low reversible capacity of less than 200 mAh g−1. The rapid capacity fade in the initial several cycles is indicative of severe dynamic polysulfide diffusion, thus causing poor electrochemical utilization and cyclability as shown in the a short cycle life.5,15 These phenomena show the typical dynamic electrochemical instability.12−14,16 In contrast, the LSD lithium−sulfur batteries exhibit enhanced dynamic battery performance with the peak discharge capacity attaining 1329 mAh g−1. Coupling this charge-storage capacity with the mass loading of sulfur translates to a high areal specific capacity of 4.5 mAh cm−2, better than the representative value of electrodes targeted for commercial application (4.0 mAh cm−2).17 Moreover, the corresponding gravimetric and volumetric capacities rise to, respectively, 930 mAh g−1 and 602 mAh cm−3. The high sulfur utilization approaching 80% is facilitated by the triple-layer hierarchical coatings that provide additional conductive pathways for increasing the cell conductivity18−20 and improving the reaction kinetics.21,22 After 500 cycles, the LSD batteries remain at a high reversible discharge capacity of 529 mAh g−1. The high reversible capacities and long cyclability are direct results of the hierarchical coatings that provide excellent polysulfidetrapping capability and high redox-reaction accessibility.19−22 The data points collected from control and the LSD lithium− sulfur batteries14 are further used for, respectively, investigating and reducing the self-discharge effect during cell resting. The self-discharge effect is studied by using 121 control lithium−sulfur batteries and 146 LSD lithium−sulfur batteries. Static battery performances are studied by resting the cells over a time period of 15−365 days for the visualization of selfdischarge effects (Table S2). The control lithium−sulfur cells retain only 35% of their original charge-storage capacity after initially resting for 15 days. These values represent the typical self-discharge behavior and the short shelf life of lithium−sulfur cells.16,23,24 After 150 days, the control lithium−sulfur cells retain only 15% of their original charge-storage capacities. After that, all the 40 examined control cells failed due to the selfdischarge. In contrast, the LSD lithium−sulfur batteries retain up to 95 and 53% of their original charge-storage capacities after resting for, respectively, 15 and 150 days. The LSD batteries further extend the cell shelf life to over one year and retain 50% of their original charge-storage capacity cyclable. To the best of our knowledge, the control and the LSD lithium− sulfur batteries provide, respectively, the long-term selfdischarge investigation and the best improvement (e.g., the best stability and longest cell shelf life) reported thus far for lithium−sulfur batteries (Table S3). According to the long-term self-discharge analysis, both the control and the LSD lithium− sulfur batteries are subjected to (i) a quantitative analysis of self-discharge (Figures 1 and 2),12,23,24 (ii) an investigation into the cathode stability (Figures 2−4).25,26 Figure 1a, b presents the capacity-retention and capacity-fade rates, which quantify the self-discharge behavior based on how much charge-storage capacities are retained or lost after resting for a certain time period12,23,24 capacity‐retention rate (%): Q rem/Q ini100%
(1)
capacity‐fade rate (%): (Q ini − Q rem)/Q ini100%
(2)
Figure 1. Quantitative analysis of self-discharge: (a) capacity-retention rate, (b) capacity-fade rate, and (c) time-dependent capacity-fade rate of the cells after resting for 1 year.
Although the capacity-fade rate is referred to as the selfdischarge rate in some research articles,23,24 a “time-dependent” capacity-fade rate that is calculated in consideration of the effect of resting time (Tres) should be simultaneously evaluated for a fair evaluation (Figure 1c).12,24 This is because the selfdischarge effect is a time-dependent characteristic,12,16,25 and a short cell-storage period would depict a low self-discharge rate in the calculation and would not reflect the true self-discharge behavior.27−31 The time-dependent capacity-fade rate is therefore calculated as time‐dependent capacity‐fade rate (%per day) : (Q ini − Q rem)/Q ini /Tres100%
(3)
The control cells that portray a typical self-discharge behavior show a low capacity-retention rate of 35% in 15 days and a high capacity-fade rate of above 80% after 30 days. Long-term selfdischarge analysis shows that severe self-discharge behavior starts after resting for approximately 15 days. This agrees with the previous literature and suggests that a long-term cell-storage period rather than a short rest period presents a more meaningful opportunity for assessing and developing statically stable lithium−sulfur batteries.23−25,32 Moreover, such electrochemical instability becomes worse as the sulfur loading and content increase.12,14,16,17 In contrast, the LSD lithium−sulfur
where Qrem is the remaining charge-storage capacity after resting and Qini is the initial discharge capacity without resting.12,25 B
DOI: 10.1021/acsami.7b05602 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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In terms of the defect-free rate of cells applied in the selfdischarge analysis, we found no literature reports thus far that note this analysis. The defect-free rate of the cells decreases with extended cell-storage periods, especially in the case of control lithium−sulfur batteries. Therefore, the high defect-free rate of the LSD lithium−sulfur batteries demonstrates not only the improved static electrochemical stability but also the enhanced storage capability and the reliability of the designed cells. As a reference, we defined whether a cell is “cyclable” or “non-cyclable” by two steps. In the first step, if the rested cells cannot be cycled, they were directly classified as noncyclable cells. This situation mainly happens with the control cells, which confirms the severe self-discharge and short shelf life. The second step is to have at least two cells showing similar electrochemical performances, including similar OCV and capacity values as well as overlapping cycling performances after resting (Figure S1). Because the literature reports on evaluating the remaining cyclability and stability of the rested cells are rare, we discuss the analytical data and results in the following sections. The rested cells show a fractional restoration of their chargestorage capacities after the initial cell operation (Figure S1), which is related to the electrode stability and the recovery capability of the active material.25,26 During this initial cell operation, a portion of the relocated polysulfides and the redeposited sulfides are reactivated, recovered, and displayed in their electrochemical activity.8,18 In consideration of the previously defined variables Qrem and Qini with the reversible recharge capacity (Qrch), the capacity-fade rate is broken down into the reversible capacity-fade rate and the irreversible capacity-fade rate26 reversible capacity‐fade rate (%): (Q rch − Q rem)/ Q ini100% (5)
irreversible capacity‐fade rate[%]: (Q ini − Q rch)/Q ini100% (6) Figure 2. Investigation into the electrode stability: (a) defect-free rate, (b) reversible capacity-fade rate, and (c) irreversible capacity-fade rate of the cells after resting for 1 year.
capacity‐fade rate = reversible capacity‐fade rate + irreversible capacity‐fade rate
where Qrch is indicative of the amount of charge-storage capacity in the rested cells that is possible for reactivation during the initial cycle.25,26 In Figure 2b, the control lithium−sulfur cells display a reversible capacity-fade rate trend rapidly approaching zero. This trend can be attributed to a loss of polysulfides, which either diffuse out from the cathode or convert to insulating sulfide depositions. These relocated and insulating sulfurcontaining species are unable to be reactivated in the subsequent discharge and charge cycles.16,23 Therefore, the control cells establish higher irreversible capacity-fade rates approaching 80% (Figure 2c). According to these, the capacityfade rate along with the reversible and irreversible capacity-fade rates sketch the self-discharge behavior in the details.23,33,34 The high static irreversible capacity-fade rates give rise to the increasing high capacity-fade rates resulting from the irreversible relocation of diffusing polysulfides. Meanwhile, the high irreversible capacity-fade rates also lead to a low and limited reversible capacity-fade rate due to the inability to reactivate the migrated polysulfides and the converted polysulfides.24−26 The consumption of active material and the
batteries have an improved static battery stability over the control cells, with a high capacity-retention rate of 50% after resting for 365 days. The advancements in the LSD lithium− sulfur batteries which give rise to high capacity-retention rates and long cell-storage period are reflected in the time-dependent capacity-fade rates. The LSD lithium−sulfur batteries employing the monolayer, dual-layer, and triple-layer hierarchical separators attain low time-dependent capacity-fade rates of, respectively, 0.20, 0.19, and 0.14% per day. This feature demonstrates that the LSD batteries improve the static electrochemical stability and prolong the cell shelf life by method of layered polysulfide-trapping interfaces in the cathode.14,23,32 The static electrochemical investigation with a long cellstorage period is highly based on the good stability and repeatability of the cells (Figure 2a). The electrode stability could be assessed by the defect-free rate defect‐free rate (%) = no. of cyclable cells/no. of assembled cells100%
(7)
(4) C
DOI: 10.1021/acsami.7b05602 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. Capacity recovery characteristics: discharge curves of the LSD lithium−sulfur batteries after a long-term storage of (a) 180 and (b) 365 days as well as (c) unstable discharge curves of the control lithium−sulfur batteries after resting for 180 and 365 days.
recovery after the initial cell operation after a long-term storage periods of 180 days (Figure 3a) and even 1 year (Figure 3b). The rested LSD lithium−sulfur batteries after a long-term storage of 180 and 365 days show the initial capacity recovery with an increase in the charge-storage capacities of, respectively, 113−211 and 32−224 mAh g−1. This indicates that the PEG/ CNF coatings used in the LSD lithium−sulfur batteries assist in reactivating the trapped active material and maintaining good cycle stability after a long-term cell storage. The discharge curves of the control lithium−sulfur batteries that were identified as failed cells were also collected for a reference (Figure 3c). After a long-term resting, the control cells have no/low remaining charge-storage capacity and detectable capacity recovery after cycling. The above analysis quantitatively evaluates the self-discharge behavior by taking into account the static electrochemical instability regarding polysulfide diffusion and the electrode degradation during resting.16,25,26 It is helpful to consider the ways in which static polysulfide diffusion influences the cyclability of the rested cells. Figure 4a, b depicts the cyclability of the rested LSD lithium−sulfur cells after resting for, respectively, 180 and 365 days. After cycling the rested cells for 100 cycles, the dynamic cycle stability is evidenced by the high capacity-retention rate and a low capacity-fade rate of, respectively, 95−98 and 0.03−0.06% per cycle with the LSD lithium−sulfur batteries that are rested for 180 days. The LSD lithium−sulfur batteries that are rested for an extended period of 365 days also exhibit high active-material retention and
increase in the inactive area eventually cause the severe selfdischarge.33,34 The LSD lithium−sulfur batteries, as opposed to the control lithium−sulfur cells, depict an increasing trend in their reversible capacity-fade rates. As the capacity-fade rate increases with the rest period, the hierarchical coatings act as a good reactivation host for the trapped active material. This conductive matrix offers smooth reactivation during the initial cell operation.25,26 The LSD lithium−sulfur batteries employing the monolayer, dual-layer, and triple-layer hierarchical separators all exhibit an increase in the tendency of the static reversible capacity-fade rates, showing average values of, respectively, 8.1, 9.8, and 13.5%. On the other hand, the irreversible capacity-fade rates of the monolayer, dual-layer, and triple-layer hierarchical separators are, respectively, 72.6, 64.8, and 35.0% after a 365-day rest period. The low irreversible capacity-fade rate and limited capacity-fade rate suggest that the favorable features provided by the hierarchical coatings create the layered polysulfide-trapping interfaces for (i) hindering excess polysulfide diffusion through the layered configuration and (ii) entrapping the blocked polysulfides within the PEG/ CNF matrix for succeeding reutilization.22−26 Aside from demonstrating greatly improved polysulfideretention capability, the better reversible capacity-fade rates reflect good reutilization of trapped active material that converts into different chemical compounds during cell storage.25,26 The rested LSD lithium−sulfur batteries that have high reversible capacity-fade rates exhibit good capacity D
DOI: 10.1021/acsami.7b05602 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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batteries is a practical solution. The resulting LSD lithium− sulfur batteries demonstrate significant improvements in shelf life and static cell performance. Moreover, to the best of our knowledge, with the parameters used in the cathode fabrication and electrochemical analysis,4,5,12,15−17,37,38 the LSD lithium− sulfur batteries in comparison with their control cells represent the best optimization and comprehensive observation in establishing long-term static battery performance reported thus far in the literature (Figures S2−S5 and Table S3).16,23−33 In conclusion, this study presents the long-term analysis of (i) the severe self-discharge with the control lithium−sulfur batteries and (ii) the development of LSD lithium−sulfur batteries with the longest shelf life and electrode stability. The long-term self-discharge data are subsequently summarized with a quantitative analysis for exploring the failure mechanisms causing the static electrochemical instability and understanding the key benefits of the LSD cells. In the first case, the severe self-discharge is known to be governed by the chemical conversion from sulfur to polysulfides and the subsequent irreversible relocation of polysulfides during cell resting. According to this understanding, in the second case, the LSD lithium−sulfur batteries are designed with hierarchical separators to reduce static polysulfide diffusion and therefore they demonstrate superior stability with the longest 365-day shelf life and excellent remaining cyclability. All experimental and quantitative data are further compared with the lithium−sulfur literature reporting self-discharge effect for offering insights and for reconfirming the superior static electrochemical performance of our LSD lithium−sulfur batteries.
Figure 4. Capacity recovery characteristics: capacity recovery and cyclability of the LSD lithium−sulfur batteries after a long-term storage of (a) 180 and (b) 365 days.
remaining excellent charge-storage capability. The 365-day rested cells display a high capacity-retention rate and a low capacity-fade rate of, respectively, 86−92 and 0.08−0.13% per cycle after 100 cycles. The remaining high electrochemical stability confirms that the LSD cell design enhances the static electrochemical characteristics of lithium−sulfur batteries and their performances. The results indicate that the hierarchical coatings effectively reduce the static polysulfide diffusion. However, the control cells lack the hierarchical coatings for suppressing the static polysulfide diffusion. Thus, the rested control cells end in failure. These key findings demonstrate that the static polysulfide-retention capability is integral to good static cell performances.14,25,30 According to the above quantitative analysis and electrochemical inspection of both the control and the LSD lithium− sulfur batteries, the static electrochemical instability is known to be akin to the dynamic instability and is a result of the formation, dissolution, and migration of polysulfides.14,23−26,35 During cell resting, solid-state sulfur initially embedded within the cathode reacts with lithium ions in the liquid electrolyte, forming soluble polysulfide species. As the polysulfides dissolve into the ether-based electrolytes, they diffuse out from the cathode structure, randomly migrate within the cell, and undergo further reduction into insoluble Li2S2 and Li2S mixtures, which agglomerate on the electrode surface.16,23 These insulating deposits form a solid-electrolyte interphase (SEI) layer, passivating the metallic lithium anode35 and blocking the fast ion/electron transport pathways.23,24,36 As a result of this irreversible polysulfide diffusion, the control lithium−sulfur batteries bear low charge-storage capacities and degradation of the electrode materials during cell resting in a short resting period. The static electrochemical instability would intensify with an increase in the resting period, which eventually could cause cell failure. To improve polysulfide retention and active-material reutilization, the hierarchical coating configuration used in the design of LSD lithium−sulfur
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05602. Experimental methods, cell cyclability after resting for half and one year, comparative analysis summarizing all lithium−sulfur research articles reporting self-discharge in the literature (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Arumugam Manthiram: 0000-0003-0237-9563 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), under Award Number DE-EE0007218.
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