Decorated Binder-Free Cathodes for Rechargeable Lithium-O

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Energy, Environmental, and Catalysis Applications 2

Controllable Electrochemical Fabrication of KO-Decorated BinderFree Cathodes for Rechargeable Lithium-Oxygen Batteries Wei Yu, Huwei Wang, Lei Qin, Junyang Hu, Liang Liu, Baohua Li, Dengyun Zhai, and Feiyu Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02359 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Controllable

Electrochemical

Fabrication

of

KO2-Decorated Binder-Free Cathodes for Rechargeable Lithium-Oxygen Batteries Wei Yu

a,b,§

, Huwei Wang

c,§

, Lei Qin

a,b

, Junyang Hua,b, Liang Liu,d Baohua Li,a

Dengyun Zhaia* and Feiyu Kanga,b,c*

a.

Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

b.

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

c.

Center

for

Environmental

Science

and

New

Energy

Technology,

Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, China d.

Department of Physics & Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China

Coresponding Author *E-mail: [email protected] (D. Zhai); *E-mail: [email protected] (F. Kang)

KEYWORDS:

Superoxide;

Binder-free;

K-O2

Solution-growth mechanism

1

ACS Paragon Plus Environment

battery;

Li-O2

battery;

ACS Applied Materials & Interfaces 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 Understanding the electrochemical property of superoxides in alkali metal oxygen batteries is critical for the design of a stable oxygen battery with high capacity and long cycle performance. In this work, a KO2-decorated binder-free cathode is fabricated by a simple and efficient electrochemical strategy. KO2 nanoparticles are uniformly coated on the carbon nanotube film (CNT-f) through a controllable discharge process in the K-O2 battery, and the KO2-decorated CNT-f is innovatively introduced into the Li-O2 battery as the O2 diffusion electrode. The Li-O2 battery based on the KO2-decorated CNT-f cathode can deliver enhanced discharge capacity, reduced charge overpotential and more stable cycle performance compared with the battery in the absence of KO2. In-situ formed KO2 particles on the surface of CNT-f cathode assist to form Li2O2 nanosheets in the Li-O2 battery, which contributes to the improvement of discharge capacity and cycle life. Interestingly, the analysis of KO2-decorated CNT-f cathodes, after discharge and cycle tests, reveals that the electrochemically synthesized KO2 seems not a conventional electrocatalyst but a partially dissolvable and decomposable promoter in Li-O2 batteries.

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1. INTRODUCTION The rechargeable Li-O2 battery is still one of promising energy transfer devices due to its ultra high theoretical energy density (~ 3500 Wh kg-1) surpassing state-of-the-art Li-ion batteries.1-3 However, the challenging problems such as large overpotential and short cycle life seriously hinder the practical application of present Li-O2 batteries.4-6 Various mechanism researches have been done to explore the fundamental reaction processes in Li-O2 batteries which are the skeleton key to unlock all critical issues.7-11 Briefly, the basic chemistry of a typical aprotic Li-O2 battery is a two-electron transfer electrochemical reaction, which includes the generation and decomposition of lithium peroxides (Li2O2) in the discharge and charge process, respectively.12 Although a recent work reports a stable Li-O2 battery based on lithium superoxide (LiO2), the LiO2 is usually regarded as an intermediate product. Generally, LiO2 is firstly generated on the surface of cathodes through a one-electron oxygen reduction reaction (ORR, Li+ + O2 + e- → LiO2*), and the Li2O2 is subsequently formed by either second electrochemical reduction reaction (Li+ + LiO2* + e- → Li2O2) or chemical disproportionation (2LiO2* → Li2O2 + O2).10,13-16 The stability of superoxides directly influences the nature of the final discharge products Li2O2, such as morphology and conductivity, and then the performances of rechargeable Li-O2 batteries. According to the Pearson’s Hard−Soft Acid−Base (HSAB) theory, the superoxide radical (O2-) is prone to be more stable in existence of cations with higher charge density (e.g. Na+ and K+).17-19 The commercial KO2 is a thermodynamically and kinetically stable superoxide, which is widely employed to explore the stability of the

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polymeric binders or the organic electrolytes in ion or oxygen batteries.20-22 Recently, KO2 has been formed as the sole discharge product in the K-O2 battery and the one-electron reaction process (K+ + O2 + e- → KO2) contributes to its extremely low overpotential.18,23 In comparison, LiO2 is a thermodynamically unstable superoxide under usual ambient condition, which can be chemically synthesized at liquid ammonia temperature by Peng’s group.14 In previous studies, K+ salts have been introduced into electrolytes of Li-O2 batteries as soluble additives to promote the solution-growth of Li2O2, which conduces to enhanced discharge capacity.24,25 Besides, the potassium impurity in the active carbon cathode has been demonstrated to improve the discharge capacity as well as the cycle life of the Li-O2 battery.26,27 However, to our best knowledge, there is currently no study concerning the effect of KO2 particles (not only the K+-anion) on the electrochemical performances of Li-O2 batteries. Generally, most oxygen electrodes are consisted of carbon-based materials, polymeric binders and catalysts. The formation of the isolated Li2O2 on the surface of porous carbon cathode results in large charge overpotentials and consequent poor cycle performances.28,29 Some solid catalysts, such as noble metals,30 transition metal oxides31-33 and rare-earth oxides,34 are applied to assist the decomposition of Li2O2 and reduce the charge overpotential. However, solid catalytic particles are usually synthesized by some chemical methods and then mechanically mixed and coated on current collectors with conductive carbon materials by polymer binders (Table S1, Supporting Information). The commonly used polymeric binders not only block the

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active sites but also cause some unwanted side reactions, which inevitably leads to the premature death of batteries.22,35,36 On the other hand, the mechanical mixing can not guarantee the homogeneous distribution of the catalysts, which will greatly undermine the catalytic effects. Herein, we develop an electrochemical strategy to fabricate a binder-free carbon nanotube films (CNT-f) cathode decorated with size-controllable KO2 particles, which surprisingly endows the Li-O2 battery with an enhanced discharge capacity and improved cycle life. The morphology evolution of decorated KO2 crystals, from nanoparticles to cubic microcrystals, can be easily achieved by controlling the discharge depth of K-O2 batteries. We observe that the generation and decomposition of Li2O2 dominate the chemistry reaction of the Li-O2 battery even based on the KO2-decorated cathode. According to the characterization toward discharged cathodes, the discharge capacity enhancement is possibly related to the change of growth mechanism of Li2O2. We reveal that the presence of KO2 particles facilitate the formation of Li2O2 nanosheets and toroidal aggregated nanosheet particles, which might contribute to low charge overpotential and long cycle stability of Li-O2 batteries. Our findings not only provide a new approach to electrochemically prepare a bifunctional binder-free cathode but also give further insight into the role of superoxide in the rechargeable Li-O2 battery.

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2. EXPERIMENTAL SECTION 2.1 Materials and cell assembling Free-standing carbon nanotube films (CNT-f) were fabricated by stacking several layers of ultrathin superaligned CNT films, which were creatively prepared by Fan’s group.37-39 The N2-adsorption isotherm of CNT-f exhibits a surface area of 108.6 m2 g-1 and a total pore volume of 0.9787 cm3 g-1 (Figure S1, Supporting Information). The porous and conductive CNT-films with a diameter of 12 mm (~ 0.7 mg) were directly used as the pristine cathode in K-O2 and Li-O2 batteries. The electrolyte used in the K-O2 and Li-O2 cell tests were 0.5 M potassium hexafluorophosphate (KPF6, Sigma-Aldrich) in diethylene glycol diethyl ether (DEGDME, Sigma-Aldrich) and 1 M lithium trifluoromethanesulfonate (LiCF3SO3, Sigma-Aldrich) in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich), respectively. DEGDME and TEGMDE solvents were further distilled and stored with 4 Å molecular sieves until the water content of those solvents were below 20 ppm. In addition, all the salts, separators, pristine cathodes and cell components were dried under vacuum at 110 oC overnight. For R2032 coin cells assembling, K metal (99.5 %, Sigma-Aldrich) and Li metal (99.99 %, China Energy Lithium Co., Ltd) were used as the anodes which were separated from the CNT-f cathode by a piece of glass fiber (GF/A, Whatman). The amount of electrolyte was about 70 µL for each cell. All the coin cells were assembled and disassembled in a highly purified Ar-filled glovebox (H2O ≤ 0.1 ppm, O2 ≤ 0.1 ppm). 2.2 Electrochemical tests

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For preparing the binder-free CNT-f cathode with a certain amount of KO2 decorated (KO2 @ CNT-f), K-O2 batteries with pristine CNT-f cathodes operated under galvanostatic discharging to limited capacities (0.35 mAh, 0.70 mAh, 1.05 mAh and 1.40 mAh) or to the cut-off voltage of 2.0 V (v.s. K/K+). After discharging, the K-O2 batteries were disassembled to get KO2 @ CNT-f cathode, which were assembled into the Li-O2 battery. Note that KO2-decorated CNT-f cathodes were washed with 1,2-Dimethoxyethane (DME) to get rid of K salts as much as possible. The cut-off voltage for the full discharge process of Li-O2 batteries with KO2 @ CNT-f cathodes were 2.3 V (v.s. Li/Li+), and the voltage range for limited capacity cycle tests (500 and 1000 mA h gCNT-f-1, on the basis of the weight of CNT-f cathode) was from 2.3 V to 4.5 V (v.s. Li/Li+). All assembled coin cells were sealed and tested in our home-made devices filling with 1 atm of high-purity O2 (99.995 %). The galvanostatic discharge and charge tests were performed by using a Land 2001A battery testing system at the room temperature (298 K) with the uniform current density 0.1 mA cm-2. The cyclic voltammogram (CV) tests of the coin cells were carried out on a VMP3 electrochemical workstation (Bio Logic Science Instruments, France) with a voltage range between 2.0 V and 4.5 V (v.s. Li/Li+) at a scan rate of 50 mV s-1. 2.3 Characterizations The water contents of the electrolytes were detected by Karl Fischer titration (Metrohm 831). X-ray diffraction (XRD) of the CNT-f cathodes after discharging in K-O2 batteries and the KO2 @ CNT-f cathodes in Li-O2 batteries were performed on a D8 ADVANCE (Bruker, Germany), in which Cu Kα (λ = 0.154 nm) was used as the

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radiation. Note that all CNT-f cathodes for XRD tests were sealed in a glass holder by a Kapton film to isolate the air. Similarly, a home-made sealing test device with quartz window for Raman characterization was applied to prevent the CO2 and moisture in the air. The Raman spectrum was detected by a LabRAM HR 800 which was set up in a 180o reflective mode. X-ray photoelectron spectroscopy (XPS) measurements for CNT-f cathodes after discharge were implemented on PHI VersaProbe II spectrometer with monochromated Al Kα excitation source. The morphologies of cathodes after discharge and cycles were characterized by scanning electron microscopy (SEM, Hitachi SU-8010). The samples for XPS and SEM characterization were prepared in an Ar-filled glovebox, and the transfer devices were also employed. Proton nuclear magnetic resonance (1H NMR) spectra of electrolytes were collected by a Bruker AVANCE III 400 instrument. The residual electrolyte from the separator was extracted by CDCl3.

3. RESULTS AND DISCUSSION 3.1 Characterization of the discharge products in K-O2 batteries

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Figure 1. (a) Galvanostatic discharge curves and (b) XRD patterns of CNT-film cathodes after discharge with different limited capacities, 0.35 mAh (black), 0.70 mAh (red), 1.05 mAh (blue), 1.4 mAh (cyan) and after full discharge (magenta) with a cut-off voltage of 2.0 V (v.s. K/K+) in K-O2 batteries. The K-O2 battery provides a facile and controllable method to synthesize the KO2 particle that is industrially prepared by high-temperature combustion of molten potassium metal at purified air.40 To obtain different KO2-decorated cathodes, a series of K-O2 coin cells based on the CNT-f cathode were assembled and discharged to specific capacities. As shown in Figure 1a, the discharge curves exhibit a stable discharge plateaus around 2.42 V (v.s. K/K+), which is in accordance with the

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formation of KO2 reported in previous studies.18,23 The XRD results of cathodes after discharge cathodes (Figure 1b) demonstrate that the KO2 dominates the final discharge products. Although all obvious diffraction peaks can be indexed to the characteristic peaks of the KO2 crystal (PDF#43-1020), the difference of relative intensity and full width at half maximum (FWHM) implies different sizes of the KO2 crystals. According to the calculation by the Scherrer formula,41 the crystal size of the KO2 particles on the 0.35 mAh discharge cathode is about 60 nm.

Figure 2. SEM images of (a) pristine CNT-f cathode (0 mAh), (b, c, d, e) cathodes after limited 0.35 mAh, 0.70 mAh, 1.05 mAh and 1.40 mAh capacity discharge, respectively, and (f) cathode after discharge to cut-off voltage of 2.0 V (v.s. K/K+). Scale bars, 1 µm. Insert in Figure 2b shows the magnifying images of nanoparticles on the cathodes. Scale bars, 100 nm. (g) Scheme for the nucleation and growth mechanisms of KO2 particles at different depth of discharge. 10


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SEM images in Figure 2a-f show the microscopic morphologies of the cathodes before and after capacity limited discharge. The pristine superaligned CNT-f cathode (Figure 2a) provides an easily observable surface to investigate the nucleation and growth processes of the KO2 crystal in the K-O2 battery. After limited 0.35 mAh capacity discharge (Figure 2b), the CNT-f cathode is uniformly covered with spherical nanoparticles, and the insert image at higher magnification shows the size of the nanoparticles is around 50 - 70 nm, in line with the predicted size by the XRD result above (Figure 1b). In addition to the nanoparticles, several cubic microcrystals are also appeared in Figure 2b, which is consistent with the typical morphology of the KO2 microcrystal in previous studies.42 Unsurprisingly, the aggregated microcrystals gradually dominate the final products with the increasing depth of discharge in Figure 2c-f. The accumulation of the KO2 crystals on the CNT-f cathode blocks the O2 diffusion channel and finally leads to the death of the K-O2 batteries. According to the morphology change of the KO2 crystals above, a mechanism has been proposed to demonstrate growth process of the KO2 particles on CNT-f cathode (Figure 2g). At the beginning of the discharge process (low depth of discharge), O2 is reduced to O2- species at the active sites of the interface between the K+-electrolyte and CNT-f cathode, where the KO2 nuclei is initially formed. Due to the relatively better electron conductivity of the KO2 comparing with the Li2O2 in the Li-O2 battery,43,44 the growth of primary nano-sized KO2 particle can continuously occur at KO2 surface until the electron transfer is limited in KO2 crystals with larger size. This process is similar to the surface-growth mechanism in the Li-O2 battery.11,34 However,

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the better solubility of the superoxide radicals in the electrolyte containing K+-ion apparently assists the superoxide dissolution and diffusion process in the K-O2 battery,17,18,45 which contributes to the formation of the KO2 microcrystals through a solution-growth process, just as the formation of NaO2 in the Na-O2 battery.19,46,47 It should be mentioned that no KO2 nanoparticles (smaller than 100 nm) have been observed in previous reports on the basis of the carbon fiber cathode.42 Theoretically, the CNT-f cathode in our experiment offers obviously more active sites for the surface-growth of KO2 comparing with the carbon fiber,37,39 which might be one of the reasons that KO2 nanoparticles appear on CNT-f cathode. Therefore, we deduce that the initial surface-growth mechanism and following solution-growth mechanism dominate in different depths of discharge of the K-O2 battery based on the CNT-f cathode. In general, the different properties of the cathodes also can significantly influence the growth mechanism of the KO2 in the K-O2 battery, sharing similarity with previous reports in Li-O2 batteries.20,48

3.2 Full-discharge performance of the Li-O2 batteries based on the KO2-coated CNT-film cathode

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Figure 3. (a) Galvanostatic discharge curves, (b) XRD patterns and (c) Raman spectra of full-discharged CNT-f cathodes decorated with 0 mAh (black), 0.35 mAh (red, 500 mAh/gCNT-f) and 0.70 mAh (blue, 1000 mAh/gCNT-f) KO2. (Inset) Specific capacity dependence on the amount of KO2-decorated. The error bar shows the standard deviation of three measurements. (d) 1H NMR spectra of three electrolytes after full discharge. The full discharge tests were performed with a cut-off voltage of 2.3 V (v.s. Li/Li+) in Li-O2 batteries. To insure the sufficient O2 diffusion and ion transfer channel, we chose two as-prepared KO2-decorated CNT-f cathodes with relative low depth of discharge, 0.35 mAh and 0.70 mAh KO2, to investigate the effects of electrochemically synthesized KO2 on the discharge and cycle performance of Li-O2 batteries. EIS measurements are performed in the Figure S2 (Supporting Information). It can be seen that the presence of KO2 particles causes a slight increase in resistance due to the low electronic 13



conductivity of KO2.44 Figure 3a shows the galvanostatic discharge curves of Li-O2 batteries with the KO2-decorated CNT-f cathodes. Figure 3a (insert) shows the average specific capacity for each amount of pre-decorated KO2. The same discharge plateaus at 2.67 V (v.s. Li/Li+) in Figure 3a illustrate that the formation of Li2O2 still dominates the ORR process. Figure 3a shows a large average discharge capacity (7513 mA h gCNT-f-1) for the Li-O2 battery based on 0.35 mAh KO2-decorated CNT-f cathode (6072 mA h gCNT-f-1 for 0.70 mAh KO2-decorated CNT-f), which is much higher than that in a battery with pristine (0 mAh KO2-decorated) CNT-f cathode (4050 mA h gCNT-f-1). In addition, the structure of final products on CNT-f cathodes after full discharge to 2.3 V (v.s. Li/Li+) were detected by XRD and Raman. In Figure 3b, all obvious diffraction peaks are indexed to the standard Li2O2 (PDF#09-0355). The Raman peaks at 790 cm-1 and 258 cm-1 in Figure 3c also demonstrate the generation of Li2O2 on KO2-decorated CNT-f cathodes.49,50 Both XRD and Raman results illustrate that the Li2O2 dominates final products, and no obvious parasitic product can be detected by XRD and Raman at least. However, no observable diffraction signals of the KO2 crystal can be detected by XRD or Raman. This phenomenon may results from the very limited existence of KO2 crystal after full discharge in the Li-O2 batteries compared with the large amount of the Li2O2 crystal. Moreover, 1H NMR characterizations toward three residual electrolytes after full discharge tests show almost the same peaks of TEGDME, which means the KO2 coated on cathode may not trigger parasitic reaction.51,52 Hence, we speculate that electrochemically synthesized KO2 particles on the surface of CNT-film cathode did

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not change the basic discharge reaction of the Li-O2 battery.

Figure 4. SEM images of the products on cathodes after full discharge with a cut-off voltage of 2.3 V (v.s. Li/Li+). (a, b) 0 mAh KO2, (c, d) 0.35 mAh and (e, f) 0.70 mAh KO2-decorated CNT-f cathode. Scale bar in Figure 4a, c and e is 1 µm, and scale bar in Figure 4b, d and f is 200 nm. To investigate the morphology of discharge products, cathodes after full discharge in Figure 3a were characterized by SEM (Figure 4). Typical disc or toroid-shaped Li2O2 appears along the CNT sidewalls in the Figure 4a, b, which implies that the nucleation and growth of Li2O2 particles on the pristine CNT-f mainly relies on the active sites of conductive CNT. The similar results were reported in previous study concerning the evolution of Li2O2 particles based on the CNT cathode.9, 48 As to the cathodes after discharge with KO2 decorated, the surface of CNT-f cathodes (Figure 4c-f) are covered by unique nanosheets and toroidal aggregated nanosheets Li2O2. Moreover, a closer observation reveals that the toroids are relatively loose (Figure 4d, 15



f) whereas toroid-shaped Li2O2 in Figure 4b are highly packed. Hence, we deduce that there exist two different mechanisms contributing to the morphologic differences of final products. At the beginning of discharge, O2 is firstly reduced to form O2- on the active site of pristine CNT, followed by the binding with Li+ to form LiO2. The binding between LiO2 and active sites might hinder the dissolution of LiO2 into the electrolyte. Hence, LiO2, which is trapped by sufficient active sites of pristine CNT-f, will undergo the disproportionation or second electron transfer reaction to form Li2O2 toroids on the surface of CNT-f cathode. In contrast, the pre-decorated KO2 on the active sites might weaken the binding force between carbon cathode and LiO2,35 which is expected to favor a disproportionation pathway in the electrolyte and to form aggregated nanosheets.20,30,53 The former mechanism is mainly limited by the amount of active sites, and the discharge process will quickly terminate when all active sites are covered with insulated Li2O2 toroids. However, the electrochemically decorated KO2 nanoparticles enhance the diffusion of LiO2 away from the cathode surface, which may contribute to the improvement of discharge capacity. In addition, the solubility equilibrium of the formed KO2 particles, especially the nanoscale ones, in the Li+-containing electrolyte will offer a certain amount of the K+ cations and O2anions, which may promote the growth of the Li2O2 in the electrolyte.11,24,25 Generally, Li2O2 nanosheets may give rise to the lower potential on the charge process.30,35

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Figure 5. (a) O 1s, (b) Li 1s and (c) K 2p XPS spectra of the cathode after limited 0.70 mAh capacity discharge in a K-O2 battery (KO2 @ CNT-f), the cathode after limited 1.0 mAh capacity discharge in a Li-O2 battery (Li2O2 @ CNT-f) and the cathode after a two-step discharge (Li2O2 @ KO2 @ CNT-f). X-ray photoelectron spectroscopy (XPS) was applied to further identify the composition of the products loaded on the CNT-f cathode after a two-step discharge process, starting with a first step limited 0.70 mAh capacity discharge in a K-O2 battery and followed by a limited 1.0 mAh capacity discharge in a Li-O2 battery (Li2O2 @ KO2 @ CNT-f). The O 1s spectrum of the cathode after limited 0.70 mAh capacity discharge in a K-O2 battery (KO2 @ CNT-f) in Figure 5a3 shows a typical K-O (~ 534 eV) functionality and the corresponding functionality of the K 2p3/2 (~ 293.8 eV) appears in the spin-orbit splitting spectrum of K 2p (Figure 5c2), which indicates that KO2 is the dominated discharge product in K-O2 battery.54-56 Similarly, the Li-O spectrum line in O 1s spectrum (~ 532.3 eV, Figure 5a2) and Li 1s spectrum (~ 55.5 eV, Figure 5b2) are assigned to the discharge products Li2O2 on the pristine 17



CNT-f cathode.57,58 For the O 1s spectrum of the Li2O2 @ KO2 @ CNT-f cathode (Figure 5a1), the Li-O (~ 532.3 eV) and C-O (~ 531.4 eV) functionalities can be obviously observed, which is similar with the O 1s spectrum of the Li2O2 @ CNT-f cathode.58 Those results illustrate that the electrochemically synthesized KO2 on the CNT-f will not change the dominated formation reaction of Li2O2 in the Li-O2 battery, which is consisted with the above analysis based on the XRD and Raman results in Figure 3b and c. Interestingly, although no KO2 diffraction peak can be detected by XRD (Figure 3b), a small K-O (~ 534 eV) functionality still contributes to the O 1s spectrum in Figure 5a1, and appearance of the typical K 2p spectrum in Figure 5c1 implies the existence of the KO2. It seems that a small quantity of KO2 crystals can remain on the after discharge cathode.

3.3 Full-charge performance of the Li-O2 batteries based on the KO2-coated CNT-film cathode

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Figure 6. (a) Galvanostatic charge curves and (b) XRD patterns of full-charged CNT-f cathodes decorated with 0 mAh (black), 0.35 mAh (red) and 0.70 mAh (blue) KO2. (Inset) Specific capacity dependence on the amount of KO2-decorated. The error bar shows the standard deviation of three measurements. (c, d and e) SEM images of full-charged CNT-f cathodes decorated with 0 mAh, 0.35 mAh and 0.70 mAh KO2, respectively. The full charge tests were performed with a cut-off voltage of 4.5 V (v.s. Li/Li+) in Li-O2 batteries. Scale bars, 1 µm. Figure 6a shows the galvanostatic full-charge performances of three CNT-f cathodes decorated with 0, 0.35 mAh and 0.7 mAh KO2, respectively. With a cut-off voltage of 4.5 V, the batteries based on 0.35 mAh and 0.7 mAh KO2 exhibit slightly higher full-charge capacities than full-discharge ones. This phenomenon is possible related to the decomposition of pre-decorated KO2 particles. According to the charge performances in Figure S3 (Supporting Information), Li-O2 batteries with KO2-decorated CNT-f cathodes can be directly charged without any discharge process. SEM images of after-charged cathodes in Figure S4a-c (Supporting Information) show that pre-decorated KO2 particles can be completely decomposed in charge process, and this process contributes to a certain charge capacity. In addition, no obvious diffraction peaks can be detected in Figure 6b and hardly any Li2O2 or KO2 particles can be seen on the surface of KO2-decorated cathodes in Figure 6c-e, which means discharge products might be decomposed after full charge process.

3.4 Cycle performances of the Li-O2 batteries with limited capacity

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Figure 7. (a) First galvanostatic discharge-charge curves of the Li-O2 batteries based on the different CNT-film cathodes with 0 (black), 0.35 (red) and 0.70 mAh (blue) KO2 decorated, respectively. (b, c and d) the following cycles of the Li-O2 batteries based on these three different KO2-decorated CNT-film cathodes. (e) Coulombic efficiency and cycle performance. The current density is 0.1 mA cm−2 with a limited capacity of 500 mA h gCNT-film−1 and a voltage range from 2.3 to 4.5 V (v.s. Li/Li+). The cycling performance of Li-O2 batteries with 0 mAh, 0.35 mAh and 0.70 mAh KO2-decorated CNT-f cathodes were investigated at a current density of 0.1 mA cm−2 with a limited capacity of 500 mAh g(CNT-film)−1. As is shown in Figure 7a, the initial 20


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discharge-charge curves of Li-O2 batteries with three different cathodes demonstrate a significant reduction of charge potentials in the existence of KO2. Meanwhile, the discharge plateaus remains at 2.65 V (v.s. Li/Li+), which is consisted with the results in Figure 3a. Moreover, Li-O2 batteries with KO2-decorated cathodes show better cycle stability than the pristine one (Figure 7b-d). As shown in Figure 7e, the Li-O2 battery based on pristine CNT-f cathode demonstrates an obvious decrease in discharge-charge coulombic efficiency after 35 cycles. In comparison, the batteries with KO2 @ CNT-f cathodes, both 0.35 mAh and 0.70 mAh KO2-decorated, keep the 100 % coulombic efficiency over 60 cycles. It seems that the electrochemically synthesized KO2 can effectively enhance the rechargeability of the Li-O2 battery. Similar improved cycle performance of the Li-O2 battery with 0.35 mAh KO2-decorated CNT-f cathode has been shown in Figure S5, where the limited capacity is 1000 mAh/gCNT-f. Furthermore, SEM images of KO2-decorated cathodes after 1000 mAh/gCNT-f limited capacity cycles are shown in Figure S6 (Supporting Information), and the whole O2 diffusion channels are completely blocked by parasitic products. Raman spectra in Figure S7 (Supporting Information) illustrate that the residues may compose of LiOH and LiCO3. Hence, the generation and aggregation of some parasitic products during the long time cycle will finally lead to the degradation of the Li-O2 battery. The lower charge overpotential and better rechargeability hints that the discharge products can be easily charged, which is possibly related to the formation of the Li2O2 nanosheets and nanosheet aggregated toroids on KO2-decorated CNT-f cathodes in

21



Figure 4d, f.30,35 SEM images of 0.35 mAh KO2-decorated cathodes after 1st limited capacity (1000 mAh/gCNT-f) discharge and charge (Figure S8b and d, Supporting Information) demonstrate the generation and decomposition of Li2O2 nanosheets, which is consistence with the morphology of discharge products in Figure 4d. Interestingly, Li2O2 can also be formed on the surface of cubic KO2 microcrystals during limited capacity discharge (Figure S9a, Supporting Information), and KO2 remains almost the same morphology after charge although Li2O2 particles are disappeared (Figure S9b, Supporting Information). In addition, the charge overpotential reduction of the initial cycle was further confirmed by higher depth of limited capacity discharge-charge cycle (1500 and 3000 mA h gCNT-f-1) in Figure S10 (Supporting Information). Note that SEM images in Figure S11 (Supporting Information) show that cubic KO2 microcrystals also remain the same morphology after initial cycle, and XRD results in Figure S12 (Supporting Information) demonstrate that it is the formation and decomposition of Li2O2 that dominates the initial cycle with 3000 mAh/gCNT-f limited capacity. Besides, the cycle performances of Li-O2 batteries based on CNT-f cathodes decorated with higher loading KO2 (1.40 mAh and 1.56 mAh) were also tested. As predicted, those Li-O2 batteries can not even discharge to the limited capacity 500 mAh g(CNT-film)−1 (Figure S13, Supporting Information), which is possibly induced in the clogging of the O2 transfer channels by the fully covered large scale KO2 crystals.

3.5 Exploring the decrease of the charge overpotential

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Figure 8. Cyclic voltammograms of (a) the first cycle and (b, c and d) the following five cycles of the Li-O2 batteries based on the different CNT-f cathodes with 0, 0.35 mAh and 0.70 mAh KO2-decorated, respectively. Cyclic voltammograms were carried out to further investigate the electrochemical performance of KO2 particles on the CNT-f cathode. As shown in Figure 8a, the ORR peaks of the initial cycle exhibit almost the same onset potential and current density, which implies that the KO2 particles on the CNT-f cathodes might not change electrochemical oxygen reduction reaction. Hence, those batteries would share the same ORR kinetic overpotentials, which is in agreement with the results of galvanostatic tests above (Figure 3a and Figure 7a). However, the OER onset potentials of the initial cycle were much lower in the existence of KO2 particles, besides that the higher current densities were also observed in Figure 8a. In previous studies concerning solid electrocatalysts, researchers usually judge the electrocatalytic activity of catalysts by comparing discharge and charge onset 23



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potentials and current densities in one certain cycle.33,35,58,59 To avoid arousing unnecessary misunderstandings toward the real effect of KO2 particles in the Li-O2 battery, CV curves of the first five cycles were shown in Figure 8b-d. Actually, we found that higher current densities in the existence of KO2 were not sustainable in our system. Therefore, we speculate the charge potential decrease in initial cycles is probably related to the decomposition of several KO2 crystals, more precisely, KO2 nanoparticles, because the cubic KO2 microcrystals still exist in cathodes after initial cycle in Figure S9 and S11 (Supporting Information). Moreover, the possible generation of K+ on initial charge processes may assist the following discharge process in turn, and it might be an integrated reaction mechanism that contributes to the better cycle performance of the Li-O2 battery based on KO2-decorated CNT-f cathode. Therefore, the electrochemically synthesized KO2 particles in our experiment seem not a typical electrocatalyst that always keep its intrinsic solid state during life-long cycles. By contrast, the KO2 is more like a dissolvable and decomposable promoter that optimizes the growth mechanism of Li2O2 and changes the morphology of Li2O2. Of course, more experimental characterizations or theoretical computations are

urgently

needed

to

comprehensively

understand

the

properties

of

electrochemically synthesized KO2 in the future.

4. CONCLUSION In summary, a KO2-decorated binder-free CNT-film cathode for the Li-O2 battery was constructed by a facile pre-discharging method in the K-O2 battery. By simply controlling the depth of discharge, a series of KO2 crystals with different morphologies were loaded on freestanding CNT cathodes. The Li-O2 batteries with

24


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KO2-decorated CNT-f cathodes exhibited bifunctional improvements in both discharge capacity and cycle stability. Specifically, the existence of the KO2 nanoparticles and cubic microcrystals on the CNT-f cathode enhanced discharge capacity of Li-O2 batteries, from 4050 to 7513 mA h gCNT-f-1, by changing the growth mechanism of the dominated discharge products, Li2O2 crystals. Meanwhile, the cycle life of Li-O2 batteries based on KO2-decorated cathodes can largely increase to 63 cycles compared with 35 cycles for the batteries with pristine CNT-f cathodes. We suggested that the significant improvements in the cycle and discharge performances were attributed to the formation of Li2O2 nanosheets and aggregated nanosheet toroids with the assistance of KO2 particles. More importantly, electrochemically synthesized KO2 particles in the Li-O2 battery could be considered as a partially dissolvable and decomposable promoter that mediate discharge and charge process by a solution-phase related mechanism. In general, tailoring a promoter-loaded binder-free cathode by a simple electrochemical process in a battery provides a feasible route to construct a multifunctional electrode not only for the alkali metal oxygen battery but also for other energy devices, such as sulfur batteries and ion batteries. ASSOCIATED CONTENT Supporting Information. Comparison of the carbon cathodes, Nitrogen adsorption -desorption isothermal, Nyquist plots of impedance, Galvanostatic discharge-charge curves, Raman spectra, SEM images of the cathodes. AUTHOR INFORMATION Coresponding Author

25



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*E-mail: [email protected] (D. Zhai); *E-mail: [email protected] (F. Kang) Author Contributions §

W.Y. and H.W. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENT 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

JCYJ20170412171311288).

26


Research

Project

(No.

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Decorated Binder-Free Cathodes for Rechargeable Lithium-O

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