Extremely Accessible Potassium Nitrate (KNO3) as the Highly Efficient

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Extremely Accessible Potassium Nitrate (KNO3) as the Highly Efficient Electrolyte Additive in Lithium Battery Weishang Jia, Cong Fan,* Liping Wang, Qingji Wang, Mingjuan Zhao, Aijun Zhou, and Jingze Li* State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China S Supporting Information *

ABSTRACT: The systematic investigation of RNO3 salts (R = Li, Na, K, and Cs) as electrolyte additives was carried out for lithium-battery systems. For the first time, the abundant and extremely available KNO3 was proved to be an excellent alternative of LiNO3 for suppression of the lithium dendrites. The reason was ascribed to the possible synergetic effect of K+ and NO3− ions: The positively charged K+ ion could surround the lithium dendrites by electrostatic attraction and then delay their further growth, while simultaneously the oxidative NO3− ion could be reduced and subsequently profitable to the reinforcement of the solidelectrolyte interphase (SEI). By adding KNO3 into the practical Li−S battery, the discharging capacity was enhanced to average 687 mAh g−1 from the case without KNO3 (528 mAh g−1) during 100 cycles, which was comparable to the one with the well-known LiNO3 additive (637 mAh g−1) under the same conditions. KEYWORDS: potassium nitrate, synergetic effect, solid-electrolyte interphase, electrolyte additive, Li−S battery

1. INTRODUCTION Nowadays, to meet the urgently increasing requests for sustainable development and large-scale energy storage, it is extremely desirable to develop rechargeable batteries with high energy density.1−5 Currently, lithium-ion battery (LIB) is one of the best electrochemical power sources and has been commercially applied in electric vehicles and portable electronic devices (computers and mobile phones etc.).6−8 However, due to the relatively low specific capacity of the anode (usually LiC6 with 372 mAh g−1) and the cathode, such as LiFePO4 (170 mAh g−1), the LIB has been operating approaching its theoretical limit.9−11 To solve the capacity problem, the battery community is refocusing on the metallic lithium-battery (Libattery) system, because Li metal as the anode exhibits an extremely high theoretical specific capacity (3860 mAh g−1) and the lowest electrochemical redox potential (−3.040 V vs standard hydrogen electrode (SHE)).12 Unfortunately, the use of Li anode seriously limits the cyclic lifetime and Coulombic efficiency (CE), due to the uncontrolled growth of Li dendrites in the repeated charging−discharging processes.13−22 Once charging, the growth of Li dendrites may reach to the cathode and cause short circuiting of the battery; while during discharging the oxidation of Li atom to Li+ ion starts from the bottom of Li dendrites, probably resulting in their electrical disconnection from Li matrix.23 Although the passivation of the solidelectrolyte interphase (SEI) inherently formed by the initial contact of active Li metal and electrolyte could sort of prevent the worsening of this situation, the volume change induced by Li stripping/deposition during charging−discharging processes © 2016 American Chemical Society

will still cause the SEI to crack and then repeatedly expose fresh Li metal to electrolyte, ultimately leading to continuous consumption of both active Li metal and electrolyte.13 Now that the growth of Li dendrites is inevitable, most attention is in turn focusing on improving SEI stability and uniformity by either adopting Li surface modification strategy or introducing additive into electrolyte to reinforce the SEI film. For instance, ion-conductive Li3N and LiPON have been coated on Li anode surface, respectively.24,25 In the aspect of additive, Aurbach et al. initially presented that lithium nitrate (LiNO3) is the critical additive in Li-sulfur (Li−S) battery to enhance the SEI formation.26,27 The exact composition of the SEI film cannot be fully comprehended due to the chemical and dynamic complexity, but it is clear that the oxidative NO3− ion can be reduced and the possible resulting products like Li3N can quicken and strengthen the SEI formation. The effective utility of LiNO3 on other Li-battery systems has been further confirmed by many groups afterward.28−30 Recently, Xu’s group reported a novel approach to obtain the dendrite-free Li deposition by using alkaline ions (Cs+ and Rb+) as the electrolyte additive.31 The alkaline ions (Cs+ and Rb+) originally possess higher standard redox potentials (−3.026 and −2.980 V vs SHE, respectively) than Li+ ion (−3.040 V), but in fact Cs+ and Rb+ ions were endowed with lower reduction potentials by controlling their active concentration. As a result, rather than be initially reduced, the positively charged Cs+ and Received: April 1, 2016 Accepted: May 30, 2016 Published: May 30, 2016 15399

DOI: 10.1021/acsami.6b03897 ACS Appl. Mater. Interfaces 2016, 8, 15399−15405

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ACS Applied Materials & Interfaces Rb+ ions were attracted and then coated around the growing Li dendrites, which could slow their further growth. Inspired by this work, we make the hypothesis that the synergetic effect probably exists on potassium nitrate (KNO3): On one hand, K+ ion could play the same role as Cs+ and Rb+ ions since its standard redox potential (−2.931 V) is quite close to Li+ ion (−3.040 V);12 On the other hand, NO3− ion can behave the same functionality for the SEI formation. Also, we notice that the abundance of K element in the earth’s crust (∼2.47%) is significantly higher than Li element (∼0.0065%). Meanwhile, KNO3 is widely used as one of the resources for potash fertilizer in agriculture. Therefore, under the consideration of production cost and environmental benignity, the potential employment of KNO3 is much more accessible than LiNO3. Herein, we undertake the systematic research on KNO3 as the electrolyte additive in Li-battery systems for the first time. The detailed comparison with its close analogues of RNO3 (R = Li, Na, and Cs) as well as the related KR′ (R′ = I) is fully investigated, which proves that KNO3 is an excellent alternative of LiNO3.32 As a result, the Coulombic efficiency of Li−Cu test cell is significantly improved after adding KNO3, with the high and stable value of 97% for 100 cycles. Moreover, for its practical application in Li−S battery, the KNO3-added one exhibits comparable discharging capacity (average 687 mAh g−1) to the LiNO3-added one (average 637 mAh g−1) during 100 cycles.

concentration was not carried out. Besides, it was notable that the low CE values in the initial charging−discharging cycles were observed, which was due to the SEI formation in the interface between the electrolyte and the deposited Li metal. 2.2. Comparison of KNO3 with RNO3 (R = Li, Na, and Cs) in Li−Cu Cell. Figure 2a showed the overall CE curves for 0.1 M RNO3 additive (R = Li, Na, K, and Cs) in the separately fabricated Li−Cu cells under the identical conditions (test current density is 1 mA cm−2). It was straightforward that the Li−Cu cell with KNO3 exhibited the same high and stable CE value (average 97%) as the one based on LiNO3 (average 97%) for 100 cycles. Both are more superior than the ones based on NaNO3 and CsNO3 (∼93 and 89%, respectively). Moreover, after 60 charging−discharging cycles, the Li−Cu cells based on NaNO3 and CsNO3 started to display unstable CE values. The selected charging−discharging curves of RNO3 additive (R = Li, Na, K, and Cs) under different cycle number were further depicted in Figure 2b−e, where small and stable polarization was recorded for KNO3 (b) and LiNO3 (c), but gradually increased or severe polarization was detected for NaNO3 (d) and CsNO3 (e), respectively. The polarization phenomena were much more comprehensive and distinguishable in the transformed time-voltage curves of Figure 2 (Figure S1). Furthermore, the same trend for the CE curves was observed in the parallel Li−Cu cells tested at higher current density of 2 mA cm−2 (Figure S3). To explain the different functionality between KNO3, LiNO3, NaNO3, and CsNO3, we initially paid attention to analyze the RNO3-added Li−Cu cells by electrochemical impedance spectroscopy (EIS). As shown in Figure 3, all the spectra started with the bulk resistance (Ri) of the electrolyte and the electrodes, and then followed by the semicircles corresponding to the charge-transfer resistance (Rct).28 Prior to the charging− discharging measurement (Figure 3a), it was clear that the impedance was quickly raised once mixing NaNO3 or CsNO3 into the basic electrolyte, but in contrast the addition of KNO3 or LiNO3 caused inapparent impedance change. After 10 cycles (Figure 3b), the Li−Cu cell without additive displayed two semicircles related to the interface and/or SEI resistance (high frequency semicircle) and the charge-transfer resistance (low frequency semicircle), respectively. Comparably, the Li−Cu cells based on KNO3 or LiNO3 displayed apparently reduced impedance whereas the reduced impedance was less effective in the case of NaNO3. Remarkably, the CsNO3-added Li−Cu cell exhibited the largest impedance and consequently the poorest charge transfer capability, which could be responsible for the low CE data and the large cell polarization mentioned above. The results also coincided with the low CE value of 76.6% reported by Xu et al. despite of the dendrite-free Li morphology was provided after adding Cs+ ion.31,34 Next, the deposited Li morphologies with different additives in the Li−Cu cells were investigated by scanning electron microscopy (SEM). As depicted in Figure 4, serious Li dendrites were clearly formed without additive, as previously reported.35,36 Expectedly, the smooth and uniform Li surface was observed after adding KNO3, which was almost the same as the effect of LiNO3 and could indicate the formation of Li dendrites was efficiently suppressed. However, the efficacy of NaNO3 and CsNO3 was not as good as KNO3 for the distinctive appearance of the dendrite-like Li morphology. Since Goodman and Kohl have systematically unveiled that Na+ ion could be reduced in their Li-battery system due to its

2. RESULTS AND DISCUSSION 2.1. With or Without KNO3 in Li−Cu Cell. At first, it was worthy to confirm the effectiveness of KNO3 on improving the SEI stability in Li-battery systems. Therefore, the preferred Li− Cu cells with bis(trifluoromethane)sulfonimide lithium (LiTFSI) as the basic electrolyte were constructed to test the CE by using KNO3 as the additive, since Cu is one of the most-used current collectors.33 As shown in Figure 1, by adding a slight

Figure 1. Coulombic efficiency of Li−Cu cells for different concentrations of KNO3 added in the basic electrolyte (test current density is 1 mA cm−2 and the discharge capacity is 1 mAh cm−2).

amount of KNO3 (0.01 M), the resulting Li−Cu cell displayed obvious improvement on the CE stability (average 31%) for 100 charging−discharging cycles compared to the one without KNO3 (average 13%). Afterward, by increasing the KNO3 concentration to 0.05 M, the impressively stable and high CE value (average 97%) was obtained. By further enhancing the concentration of KNO3 to 0.1 M, the CE value could still maintain as high as 98% at the 100th cycle. However, based on the solubility of inorganic KNO3 in organic solvent and the satisfactory achievements, further augmentation of KNO3 15400


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Figure 2. (a) Coulombic efficiency of the Li−Cu cells for 0.1 M RNO3 additive (R = Li, Na, K, and Cs) into the basic electrolyte; (b)−(e) The selected 1st, 5th, 50th, and 100th charging−discharging curves for RNO3 (R = Li, Na, K, and Cs), respectively.

Figure 3. EIS plots of the RNO3-added Li−Cu cells (R = Li, Na, K, and Cs). (a) Before charging−discharging measurement; (b) after 10 charging− discharging cycles.

relatively high standard redox potential (−2.710 V),12 the element distributions of the deposited Li metal from the RNO3added Li−Cu cells (R = Na, K, and Cs) were reasonably checked by energy dispersive X-ray spectroscope (EDX).

Unexpectedly, neither traces of Na, K, nor Cs elements were detected from these samples except for the residual O and N elements (Figure S4 and Table S2). The inconsistent results further forced us to check whether Na+, K+, and Cs+ ions were 15401


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Figure 4. SEM images of deposited Li metal on different Li−Cu cells. (a) Without additive, (b) KNO3, (c) LiNO3, (d) NaNO3, and (e) CsNO3, respectively (scale bar of 50 μm in I-series and 5 μm in II-series).

reduced or not. So the subsequent cyclic voltammogram (CV) experiments were performed in the corresponding Li−Cu cells (Figure 5). As shown from its amplified part (inset of Figure 5),

Figure 6. Coulombic efficiency of the Li−Cu cells for KNO3 and KI added into the basic electrolyte.

to the one with KNO3, since the relatively inert I− anion herein could make little contribution to the SEI reinforcement. 2.4. Comparison of KNO3 with LiNO3 in Li−S Battery. Finally, to evaluate the efficacy of KNO3 as the electrolyte additive in practical Li-battery systems, we selectively chose the system of Li−S battery to test its practicability. Therein, the cathode sulfur (S) can undergo reversible and multistep reduction processes to Li2S, along with the theoretical capacity of about 1675 mAh g−1, which is among the highest values in the rechargeable Li-battery systems.27 The detailed fabrication procedures were presented in the Experimental Section and the added electrolyte volume should be optimized.39,40 As investigated by Liu and Zong et al., if the electrolyte volume added was less, the insufficient wetting of electrode would bring in low cell capacity. On the contrary, if the electrolyte volume added was more, the dissolution of active lithium polysulfide would increase and cause more serious shuttle effect.41 For comparison, the batteries without additive (basic) and with LiNO3 were also fabricated at the same conditions. Figure 7 depicted the discharging capacities from one group of the fabricated Li−S batteries for the above three cases, where they exhibited close initial discharging capacities of 1037 mAh g−1 (KNO3), 1073 mAh g−1 (LiNO3), and 956 mAh g−1 (basic), respectively. But during 100 cycles, the Li−S battery based on KNO3 could exhibit satisfactory discharging capacity (average 687 mAh g−1), which was higher than the one without additive

Figure 5. CV curves of Li−Cu cells under different RNO3 additives (R = Li, Na, K, and Cs) (inset depicts the amplified part of the reduction onsets).

it surprised us to see that the reduction onsets of the RNO3added Li−Cu cells (R = Na, K, and Cs) were shifted to more negative potentials with respect to the basic LiTFSI, indicating no reductive process would take place before the reduction of Li+ ion started. Meanwhile, X-ray photoelectron spectroscopy (XPS) was carried out for the KNO3-added Li−Cu cell (Figure S5). The obtained spectra were quite like the case without KNO3 and no apparent characteristic peak from K element was observed. Combined with the EDX and CV results, we concluded that all Na+, K+, and Cs+ ions were not reduced in our Li−Cu battery system, which could confirm the synergetic effect of K+ ion. In the other aspect, the inferior effect of NaNO3 cannot be fully explained at the current stage and more investigation is ongoing. 2.3. Comparison of KNO3 with KI in Li−Cu Cell. To further verify the functionality of NO3− ion for assisting the formation of stable SEI, KI additive was selected as the counterpart under the same conditions, since I− ion is an important participant in both Li-battery and dye-sensitized solar cells.37,38 However, as shown in Figure 6, the Li−Cu cell based on 0.1 M KI additive displayed poor CE when compared 15402


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was about 1 mg cm−2. The cathode was finally punched into a disk (Φ = 10 mm) for assembling Li−S cells. Cell Assembly. Three individual Li−Cu cells were parallel fabricated and tested for every RNO3-added case (R = Li, Na, K, and Cs), where the copper foils, glass fibers, and Li foils were used as the working electrode, the separator, and the counter electrode, respectively. Before the assembly, the copper foil was washed with distilled water and ethanol and finally dried for 6 h in vacuum at 110 °C. The cutoff potential of the Li−Cu cells was controlled at 0.5 V (vs Li/Li+), the current density is 1 mA cm−2 and the discharge capacity is 1 mAh cm−2. The Li−S batteries were assembled by the same way except with sulfur electrode as the working electrode. And the volume of the electrolyte added in our Li−S cells was optimized to be 40 μL. Two groups of Li−S batteries were parallel fabricated and tested. Each group contained three Li−S batteries related to the basic electrolyte, KNO3, and LiNO3 respectively. In the Li−S batteries, 1 C means the current density of 1675 mA g−1. The specific capacity was calculated according to the sulfur mass. The Li−S batteries were tested in a voltage range of 1.8−2.6 V (vs Li/Li+) at a rate of 0.2 C. Electrochemical Measurements and Characterization. The cells were tested using a CT2001A cell test instrument (LAND Electronic Co. Ltd.) at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were tested on electrochemical workstation (Parstat2263). CV measurements were performed at a scan rate of 10 mV s−1 in the voltage range of −0.5 V to open-circuit potential (vs Li+/Li). EIS tests were carried out at open-circuit potential in the frequency range of 1 MHz to 10 mHz with a perturbation amplitude of 10 mV. The Li−Cu cells were disassembled after 2 h discharging at the current density of 1 mA cm−2. The resulting Cu foils with the deposited Li films were rinsed with anhydrous DME and then dried in the Ar-filled glovebox at room temperature. The morphology of the deposited Li films was observed with field-emission scanning electron microscopy (FE-SEM, Hitachi, S3400N). The composition of the Li-deposited Cu foil was determined by energy dispersive X-ray spectroscopy (EDX, Oxford INCA PentaFET-x3). X-ray photoelectron spectroscopy (XPS, Kratos XSAM800) was used to analyze the surface composition of the deposited Li metal on Cu substrate using Al−Kα (1486.6 eV) radiation as the primary excitation source operated at 180 W. The binding energies were calibrated using the C 1s level (284.6 eV) as an internal reference. A special transfer system was employed to deliver the air-sensitive Li samples to the SEM and XPS system.30

Figure 7. Discharging capacities from one group of three Li−S batteries for the basic electrolyte, KNO3, and LiNO3, respectively.

(average 528 mAh g−1) and comparable to the LiNO3-added one (average 637 mAh g−1). The discharging capacities from the other group of Li−S batteries also demonstrated the same trend for the practicability of KNO3 (Figure S7). Noticeably, the obtained discharging capacities from our KNO3-added Li−S batteries were comparable or even more superior than the LiNO3-added Li−S batteries previously reported by other groups.27−29,42

3. CONCLUSIONS Due to the possible synergetic effect of K+ and NO3− ions, the effective suppression on the growth of lithium dendrites was clearly observed by employing KNO3 as the electrolyte additive, where the K+ ion possessing higher standard redox potential but not being reducible could surround the growing lithium dendrites by electrostatic attraction and then delay their further growth, while simultaneously the nearby NO3− ion could be reduced and subsequently profitable to the SEI reinforcement. By further evaluating its practical application in Li−S battery, the constructed battery displayed satisfactory discharging capacities of average 687 mAh g−1 for 100 cycles, higher than the one without additive and comparable to the one with LiNO3. 4. EXPERIMENTAL SECTION Preparation of Electrolyte. The basic electrolyte is 1 M bis(trifluoromethane)sulfonimide lithium (LiTFSI) solution, which is composed by 1,2-dimethoxyethane (DME) and 1,3dioxolane (DOL) in the volume ratio of 1:1 (v:v = 1:1) (Zhangjiagang Guotai-Huarong New Chemical Materials Co. Ltd.). And the modified electrolyte was prepared by further adding x M KNO3 (x = 0.01, 0.05, and 0.1), 0.1 M LiNO3, 0.1 M NaNO3, 0.1 M CsNO3, and 0.1 M KI into the basic electrolyte, respectively. All the additives were purchased from Aladdin Co. Ltd. with high purity of 99.99%. Fabrication of Sulfur Cathode. The carbon/sulfur (C/S) composite as the cathode was prepared by heating the mixture of carbon black (Ketjen Black) and sublimed sulfur powder (99.5%, Alddin) in the tube furnace filled with Ar atmosphere. The C/S composite was mixed by the weight ratio of 1:2 and heated at 155 °C for 12 h. Afterward, the C/S composite, acetylene black, and LA132 (binder, Chengdu Indigo Power Sources Co., Ltd.) with the weight ratio of 8:1:1 were manually mixed with deionized water to form a uniform slurry. Subsequently, the slurry was casted on Al foil and dried at 60 °C in vacuum for 12 h. The sulfur mass loading on the cathode

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03897. The time and voltage curves of the Li−Cu cells with different RNO3 additives (Figure S1); Coulombic efficiency of the Li−Cu cells with KNO3 and LiNO3 for 200 cycles (Figure S2); Coulombic efficiency of Li− Cu cells with RNO3 additives tested at 2 mA cm−2 (Figure S3); EDX signals of Li films deposited on Cu substrates (Figure S4); XPS spectra for the deposited Li metal on Cu substrate with KNO3 (Figure S5); the selected charging−discharging curves from Figure 7 (Figure S6); the discharging capacities from the other 15403


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group of three Li−S batteries (Figure S7); the SEM images of Li anodes from three Li−S batteries (basic, LiNO3 and KNO3) after 100 cycles (Figure S8) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected]. *[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Startup Grant of UESTC (NO. ZYGX2015KYQD058); the National Science Foundation of China (21073029, 11234013, 21473022, and 51502032); the Science and Technology Bureau of Sichuan Province of China (NO.2015HH0033); and Fundamental Research Funds for the Central Universities (ZYGX2012Z003).

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