Exploring Stability of Nonaqueous Electrolytes for Potassium-Ion

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Letter

Exploring stability of nonaqueous electrolytes for potassium-ion batteries Yu Lei, Lei Qin, Ruliang Liu, Kah Chun Lau, Yiying Wu, Dengyun Zhai, Baohua Li, and Feiyu Kang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00214 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Energy Materials

Exploring Stability of Nonaqueous Electrolytes for Potassium-Ion Batteries Yu Lei,†,⊥ Lei Qin,†,⊥ Ruliang Liu, ‡,⊥ Kah Chun Lau,§ Yiying Wu,‖ Dengyun Zhai, *,† Baohua Li,† and Feiyu Kang*,† †

Shenzhen Key Laboratory for Graphene-based Materials and Engineering Laboratory for

Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China. ‡

Materials Science Institute, School of Chemistry, Sun Yat-sen University, Guangzhou

510275, PR China. §

Department of Physics and Astronomy, California State University, Northridge, 18111

Nordhoff Street, Northridge, California 91330-8268, USA. ‖

Department of Chemistry and Biochemistry，The Ohio State University，151 W Woodruff

Ave, Columbus, OH 43210, USA. Corresponding Authors *E-mail: [email protected] (D. Zhai). *E-mail: [email protected] (F. Kang). Author Contributions ⊥

Y.L., L.Q. and R.L. contributed equally to this work.

1 Environment ACS Paragon Plus

ACS Applied Energy Materials 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

ABSTRACT Recently nonaqueous potassium-ion batteries (KIBs) have attracted tremendous attention but a systematic study about the electrolytes remains lacking. Here, the stability of a commonly used electrolyte (KPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC)) at the anodes (e.g. graphite, solid K and liquid Na-K alloy) was studied. Interesting results show that the linear DEC is unstable. Possibly attributed to stronger reducibility against the anodes for KIBs, the decomposition of DEC is initiated by the C(H2)-O bond breaking of the solvent molecule. This study shows that a systematic study to look for more stable electrolyte is critically important for KIBs.

Keywords: potassium-ion battery, electrolyte, diethyl carbonate, anode, interface, nuclear magnetic resonance,


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In recent decades, electrochemical energy storage (EES) devices (e.g. lithium-ion batteries (LIBs)) are not only widely applied in portable energy storage, but also have a huge influence in stationary energy storage, i.e. the grid application.1 However, the shortage of lithium resources (0.0017 wt %) in the Earth’s crust2, 3 have posed a huge challenge to meet the fast growing requirement of EES devices worldwide. Owing to the natural abundance of alkali metal Na and K (2.3 wt% and 1.5 wt%, respectively), sodium-ion and potassium-ion batteries (SIBs and KIBs) with comparable energy density, despite lower than that of LIBs, can potentially be promising candidates in large-scale stationary energy storage system.2, 4 Thus in the past decade, SIBs4, 5 have been investigated intensively, and KIBs2, 3, 6-8 have also attracted significant interests in research and development since 2015. Considering that generally the Na or K solid metal cannot be directly used as the anode for commercial batteries because of the safety concerns that related to the alkali metal dendrites, the main anode materials for SIBs and KIBs are therefore based on carbon materials or alloying compounds.2-5 Analogues to LIBs, the graphite as the standard anode material for LIBs can also be employed as the anode for KIBs by electrochemically forming KC8, i.e. the stage-one K−graphite

intercalation

compound

(K-GIC),

which

is

very

similar

to

the

intercalation/deintercalation process in LIBs.3, 6, 9 However the graphite anode only delivered a very limited capacity for SIBs using traditional ester-based electrolytes, and thus the hard carbon as the alternative anode material has been considered recently.5, 9 Compared to the binary intercalation of Li or K in graphite in ester-based electrolyte, the solvated alkali metal ions (Li+, Na+ and K+) in ether-based electrolyte can achieve ternary GICs in graphite through co-intercalation mechanism.10-12 In addition to graphitic anodes, a novel liquid Na-K alloy as the anode for KIBs has been reported by Goodenough et al. recently.13 The Na-K alloy was absorbed in carbon paper as the membrane above 400 oC, acted as the anode for KIBs in ester-based electrolyte (i.e. the mixture of ethylene carbonate (EC) and diethyl carbonate



(DEC)), and the problem of dendrite growth can be eliminated.13 Similarly, a dendrite-free KO2 battery based on a Na-K alloy anode with diethylene glycol diethyl ether (DEGDME) electrolyte that improved the cycling performance remarkably has been reported by us recently.14 To date, the electrolytes of KIBs are very similar to the conventional electrolytes of LIBs, and the most commonly used electrolyte is the mixture of EC/DEC solvents and KPF6.2, 3, 6, 7, 9

However, the stability of the electrolytes in KIBs has not been systematically studied .2, 7, 15

Considering a wide variety of the cathode materials for KIBs, in this work we will focus on the basic study about chemical/electrochemical stability of the EC/DEC electrolyte at the anodes in special designed cells using experimental galvanostatic discharge/charge measurement, nuclear magnetic resonance (NMR) analysis and X-ray diffraction (XRD) characterization. Besides the commonly used graphite, solid K3, 6, 9, 10, 16 and Na-K liquid alloy13, 14, 17 which were used in KIB and K-O2 battery systems were also employed as the anode materials. To compare against EC/DEC electrolyte stability, the performance of DEGDME-based electrolyte in KIBs is investigated. To fulfill the above purpose, two different half cells, i.e. K/graphite and Na-K/graphite that contain three kinds of anode materials mentioned above, were assembled. In particular, the Na-K alloy is in liquid phase at room temperature, therefore a special configuration for NaK/graphite cell design is adopted. From a previous study,13 the Na-K alloy was immobilized in carbon paper matrix using heat treatment over 400 oC. Based on that cell set-up, the crossover stacking of carbon microfibers may divide the whole Na-K alloy into large amounts of small isolated droplets (Figure S1a in the Supporting Information), and the Na/K ratio of some isolated droplets can change inhomogeneously during the cell cycling, and subsequently lead to the transformation from the initial liquid alloy anode to the unexpected


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solid Na and K according to the phase diagram of Na and K14, 18. Thus the structure of Na-K immobilized in carbon paper would be unfavorable to the investigation into intrinsic reactions between the liquid electrolyte and the liquid Na-K anode. To cope with this problem, we designed a special cell configuration for KIBs (Scheme S1 in the Supporting Information), similar to our previous work.14 Based on this unique cell configuration, the free contact between the liquid Na-K alloy and the electrolyte (Figure S1b in the Supporting Information) can facilitate more uniform plating/stripping processes (including the possible side reactions as well) along the liquid alloy/liquid electrolyte interface. After assembling the Na-K/graphite and K/graphite half cells, the galvanostatic potassiation/depotassiation process was conducted at 0.1 mA/cm2 in EC/DEC and DEGDME electrolyte, respectively, as shown in Figure 1. For Na-K/graphite cell, the initial potassiation and depotassiation capacity of the graphite in EC/DEC electrolyte is only 39 and 15 mAh/g (Figure 1a), respectively, far less than the previous reports,3, 9, 13, 19 and the capacity drops rapidly after a few cycles. Compared Figure 1c with Figure 1a, the initial potassiation and depotassiation capacity of the K/graphite cell within EC/DEC electrolyte increases to 224 and 153 mAh/g, respectively, and the cycling stability is also improved dramatically when the Na-K alloy is replaced by the solid K. For K/graphite cell, the depotassiation capacity is lower than the theoretical value of 279 mAh/g via the formation of KC8, probably attributed to the presence of KC24, i.e. the stage-two KGIC, which is formed along with KC8 as confirmed by XRD spectra (Figure S2 in the Supporting Information).3 For the half cells based on EC/DEC electrolyte (Figure 1a and c), the slope region from 0.8 to 0.2 V in the first potassiation process disappears in the following cycles, similar to LIBs when the solid electrolyte interphase (SEI) layer is formed on the surface of graphite electrode.20, 21 In contrast to EC/DEC based system, the voltage profiles of the half cells based on DEGDME electrolyte (Figure 1b and d) both demonstrate excellent electrochemical



reversibility and nearly reversible potassiation/depotassiation voltage plateau through the ternary intercalation/deintercalation process in graphite, which is found to be consistent with the previous reports.10, 12 In particular for the Na-K/graphite cell, the depotassiation capacity retention of the cell is close to 90% after 45 cycles and is significantly better than the K/graphite cell (88% after 20 cycles), which suggested that the employment of Na-K liquid anode can improve the cycling performance in DEGDME electrolyte.14 Although the DEGDME electrolyte exhibits outstanding stability for KIBs, the potassiation/depotassiation voltage plateau around 1.0 V vs. K+/K gives rise to a narrower electrochemical voltage window for full KIBs and a lower energy density. Based on these observations, one can conclude that the distinct electrochemical performance of these cells primarily depends on the compatibility of anode with the electrolytes.

Na-K/graphite cell in EC/DEC electrolyte

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0

50

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Figure 1．． Potassiation/depotassiation curves of graphite electrodes in Na-K/graphite cells using (a) EC/DEC electrolyte, (b) DEGDME electrolyte (Inset is the first fifth dischargecharge curves), and in K/graphite cells using (c) EC/DEC electrolyte, (d) DEGDME electrolyte, respectively.


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To further investigate the failure of EC/DEC based half cells, the cells after the cycling test in Figure 1 were disassembled in glove box for further characterization. The possible decomposition byproducts from the electrolyte degradation were analyzed using 1H NMR measurements, as shown in Figure 2. The DEGDME is found to be remarkably very stable, while the EC/DEC is found to decompose severely in both cells. From our NMR analysis, the linear DEC solvent molecule that undergoes the reduction process21,

22

is found likely to

decompose into potassium ethyl (δ=4.35 and 3.65, marked “4” and “5”, respectively) and carbonates, whereas the decomposition of the DEC solvent in Na-K/graphite cell is more severely than in K/graphite cell based on the intensity changes of the NMR peaks “1” and “2” (δ=4.20 and 1.30) that are associated with the DEC solvent (Inset in Figure 2a). In the XPS spectra of graphite anode for K/graphite cell after ten cycles, the existences of K-C (294.0 and 296.7 eV) and CO32- (289.5 eV) species can be identified in K 2p and C 1s signals respectively (Figure 2c).23-25 The O 1s peak can be deconvoluted into three peaks: K-O (533.0 eV), C=O (531.4 ev), and C-O (530.3 eV) (Figure 2d).8,16 The peaks at 293.1 eV, 295.8 eV, and 683.4 eV (Figure S5) were attributed to KF.26,27 Meanwhile, there are no detectable NMR signals attributed to the decomposition of the cyclic EC solvent although it was reported that the decomposition of the small amount of EC presented in SEI film at the interface of the graphite

anode

in

LIBs.21

Based

on

both

NMR

results

(Figure

2)

and

potassiation/depotassiation voltage profiles (Figure 1), it suggests that the high stability of the DEGDME electrolyte guarantees the electrochemical reversibility, and whereas the rapid cell capacity fading observed (Figure 1a and 1c) can be attributed to the decomposition of the DEC solvent in both K/graphite and Na-K alloy/graphite cells. Figure S3 shows the color change of the anode in Na-K/graphite cell before and after the potassiation/depotassiation process. The Na-K alloy in EC/DEC electrolyte became brown in color after the potassiation/depotassiation process, while the Na-K alloy in DEGDME remained unchanged



in color as the pristine, and this observation is consistent with the NMR analysis. It is noteworthy to point out that the DEC in KIBs is found to decompose more drastically than in LIBs, which be possibly attributed to the stronger reducibility of the metal K compared with Li, and interestingly this finding has never reported in previous studies.3, 6, 9, 13 In addition, the more reactivity of the Na-K liquid alloy than the solid K in EC/DEC electrolyte may further accelerate the decomposition process of DEC, and subsequently lead to the rapid capacity fading of the Na-K/graphite cell in EC/DEC electrolyte. Based on the discussion above, an important conclusion is that DEC solvent is generally found to be relatively unstable for the common anodes of KIBs. 4

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1DEGDME

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K2p & C1s

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O1s

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K-C 8.0k

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CPS

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CPS

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282

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Figure 2． The 1H-NMR spectra of (a) EC/DEC- and (b) DEGDME-based electrolytes in Na-K/graphite and K/graphite cells after the galvanostatic potassiation/depossiation test, respectively. Inset shows the intensity changes of peaks corresponding to the DEC and DEGDME solvents, respectively. XPS spectra of graphite anode from K/graphite cell with EC/DEC-based electrolyte after 10 cycles: (c) K 2p, C 1s, and (d) O 1s.


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For the K/graphite cell in EC/DEC electrolyte, it is noted that the decomposition of the DEC solvent can be further traced back to two different interfaces (i.e., the K/electrolyte and the graphite/electrolyte). To investigate and differentiate the possible decomposition processes at two distinct interfaces, we fabricated two kinds of symmetrical cells in each of which there is only one kind of interface (Scheme 1): the K/K cell consisting of two K/electrolyte

interfaces

and

the

KCx/KCx

cell

consisting

of

two

potassiated

graphite/electrolyte interfaces. To enable a working graphite/graphite (G/G) cell, we innovatively fabricated a

rocking-chair potassium cell that contains two same

graphite/electrolyte interfaces (Scheme 1b and Figure S4 in the Supporting Information) with slightly different electrochemical potentials. By comparing the electrochemical performance of these two symmetry cells (i.e. K/K and G/G cells), it is expected that the electrochemical stability of EC/DEC electrolyte at two different interfaces can be evaluated. For direct comparison, the two cells were discharged/charged for a constant time for each process at 0.1 mA/cm2, as shown in Figure 3. It shows that the overpotentials for plating/stripping process in K/K cell and potassiation/depotassiation process in G/G cell are found to be at about 20 and 40 mV, respectively. To further compare the decomposition process of the DEC solvent at two different interfaces, the two symmetric cells were set to run for the same time duration, ~200 hours, and subsequently both electrode interfaces were characterized using 1H NMR. The NMR data in Figure 4a shows that the DEC solvent at two different interfaces decomposed distinctively from each other. At K/electrolyte interface, the decomposition degree of DEC is relatively more drastic, which further suggested that the stronger reducibility of the metal K can facilitate the decomposition process of DEC solvent during the stripping/plating process at the metal K electrode. In addition, the chemical stability of the EC/DEC electrolyte in which the metal K foil was immersed was also being studied, and the NMR data is shown in Figure 4b. In the first 24 h,



we found that this carbonate based electrolyte is fairly stable. However, when the immersion time increases to 200 h, the decomposition of the DEC solvent becomes substantial. Therefore, one can conclude that the DEC solvent is chemically unstable when exposed to metal K. Based on the comparison in NMR data (inset in Figure 4b), we also found that the electrochemical decomposition of the DEC solvent in K/K cell running for 200 h is substantially more severe relative to the immersion time for 200 h. Thus, this suggests that the decomposition of the DEC solvent in K/K cell could possibly be driven by both electrochemical and chemical reactions. In contrast to the K/K interface, the decomposition of the DEC solvent at G/G interface during the potassiation/depotassiation process is relatively mild (inset in Figure 4a). In this unique G/G interface, the decomposition process that proceeded between the electrolyte and the K-GICs could be possibly closely correlated to the formation and stability of the SEI layer at the graphitic electrode, which would have a significant effect on a working full KIB using the graphite anode, and the further systematic studies will be discussed in our future studies. Scheme 1. The configuration and assembly of (a) K/K and (b) G/G cells.


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Figure 3. Voltage vs. time plots of (a) the symmetrical K/K cell and (b) the G/G cell at a constant current of 0.1 mA. The discharge/charge test is conducted for a constant time of (a) 10h and (b) 20 min, respectively (further details see the experiment section in the Supporting Information). It was noted that the G/G cell stood for 1 min between every switching current and thus a full cycle lasted for 22 min. In Figure 3b the whole test time (~220 h) contains the running time of 200 h and the standing time of 20 h.

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Figure 4． The 1H-NMR spectra of the EC/DEC electrolyte (a) in K/K and G/G cells after the discharge/charge test , and (b) when the metal K was immersed for 24, 200 h. Inset shows the intensity changes of peak “1” corresponding to the DEC solvent.



In summary, we found that the DEC solvent in the standard DEC/EC carbonate based electrolyte which is commonly used in LIBs is both chemically and electrochemically unstable against the common anode materials (i.e. the graphite and the metal K) in KIBs. The possible main reason might due to the strong reducibility against the metal K and K-GIC, which lead to the CH2-O bond breaking in linear DEC solvent molecule. To support this hypothesis, the ingenious symmetric cell design was used to confirm this finding. Based on symmetry cells with the all-metal- or all-carbon-electrode, the experimental observation further indicated that the DEC decomposition process occurred at two different interfaces (i.e., the K/electrolyte and the graphite/electrolyte), and we found the DEC decomposition at K/electrolyte interface is more severe. This new finding will lead us to pay more attention and deeper understanding of the electrolytes for KIBs. Meanwhile, to fully exploit the practical potential of KIBs, a systematic study that focus on the stability of the electrolyte for some prevailing cathode materials of KIBs, such as Prussian blue analogues and layered metal oxides is needed and will be subjected to our future studies.

■ ASSOCIATED CONTENT

Supporting Information This PDF file includes: Experimental section; Figure S1 to S5 and Scheme S1; The characterization of materials (XRD & XPS), and electrochemical performance of materials and batteries (charge/discharge data) (PDF).

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (D. Zhai). *E-mail: [email protected] (F. Kang). 12 Environment ACS Paragon Plus

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Author Contributions ⊥

Y.L., L.Q. and R. L. contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 51232005 and No. 51772167), National Key Basic Research Program of China (No. 2014CB932400) and Shenzhen Basic Research Project (No. JCYJ20170412171311288 and JCYJ20170817162443934).

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Exploring Stability of Nonaqueous Electrolytes for Potassium-Ion

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