Delithiation Behavior of Silver Nitrate as Lithium Storage

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Lithiation/Delithiation Behavior of Silver Nitrate as Lithium Storage Material for Lithium Ion Batteries Peng Li, Hua Lan, Lei Yan, Shangshu Qian, Haoxiang Yu, Xing Cheng, Nengbing Long, Miao Shui, and Jie Shu ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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ACS Sustainable Chemistry & Engineering

Lithiation/Delithiation Behavior of Silver Nitrate as Lithium Storage Material for Lithium Ion Batteries

Peng Li, Hua Lan, Lei Yan, Shangshu Qian, Haoxiang Yu, Xing Cheng, Nengbing Long, Miao Shui, Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, Zhejiang Province, People’s Republic of China

* Corresponding author: Jie Shu E-mail: [email protected]

1

ACS Paragon Plus Environment


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Abstract In this work, AgNO3/CNTs composite is synthesized through a simple solution method. The morphology, electrochemical property and lithium storage mechanism of AgNO3/CNTs are thoroughly investigated and compared with bare AgNO3. For AgNO3, it can deliver an initial charge capacity of 552.3 mAh g-1. After 100 cycles, AgNO3 only retains a capacity of 84.5 mAh g-1 with inferior capacity retention of 15.3 %. In contrast, AgNO3/CNTs composite presents the first charge capacity of 530.3 mAh g-1 with capacity retention of 92.5 % after 100 cycles (482.5 mAh g-1). The enhanced performance can be ascribed to the introduction of carbon nanotube networks interlaced with AgNO3 particles. Furthermore, the reaction mechanism of AgNO3 with Li is also studied by various in-situ and ex-situ methods. It can be seen that the preliminary reaction between AgNO3 and Li leads to the irreversible formation of LiNO3, Li3N, Li2O and Ag. With further reaction at low potentials, the resulting Ag reacts with Li to form Ag3Li10 alloys. Upon a reverse charge process, the lithium storage capacity is associated with the de-alloying reaction of Ag3Li10 to the formation of Ag and Li. In the following cycles, the reversible capacity is maintained by the Ag-Li alloying/de-alloying reaction.

Keywords: Silver nitrate; Anode material; Electrochemical behavior; In-situ X-ray diffraction; Lithium ion batteries.

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Introduction In recent years, energy storage systems are urgently needed to satisfy the growing demands in portable electronic markets and electrical vehicles because of their high energy, high power, cost-effective and environmentally-friendly properties.1-5 Lithium ion batteries (LIBs), an essential and vital energy storage system, are considered as an increasingly important energy storage devices for many applications as they have high energy densities (150-200 Wh kg-1) and large-scale energy storage capacity.6-10 Carbon-based materials, the currently most widely used anode materials in LIBs, have attracted tremendous research interests in recent years owing to their high coulombic efficiency and low cost.11-15 However, the drawbacks of carbonaceous materials such as low specific capacity (372 mAh g-1) and low lithium intercalation potentials (0.1 V vs. Li/Li+) would restrict its use in the future.12-15 Hence, a wide range of novel electrode materials with high lithium storage capacity and stable cycling performance have been proposed and developed in the past ten decades. Among large numbers of anode candidates, metal nitrates and their derivates (such

as

Cu(NO3)2•xH2O,16-18

[Bi6O4](OH)4(NO3)6•4H2O

22

Pb(NO3)2,19-20

(NH4)2Ce(NO3)5•4H2O

21

and

) have attracted considerable attention and investigated

as anode materials due to their large reversible capacity. However, the crystal water in the interior of metal nitrates (Cu(NO3)2•xH2O, and [Bi6O4](OH)4(NO3)6•4H2O) would cause the electrode instability because of the decomposition of active materials during the charge/discharge cycles and leads to an unsatisfactory cycling properties. 3



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Consequently, the poor structural stability and rapid capacity fading cannot satisfy the demand of the practical application as anode candidates for LIBs. For comparison, Pb(NO3)2 exhibits a superior cycling stability because the structure is free of crystal water.19 Furthermore, metal Pb could form Li-Pb alloys with lithium ions during charge/discharge cycles and this alloying reaction could offer considerable reversible capacity.22-24 Similar with Pb(NO3)2, silver nitrate (AgNO3) is another compound without crystal water in the structure. In addition, from previous reports,25 we know that Ag could also alloy with Li and exhibits a high specific capacity. Hence, we propose an investigation on AgNO3 to check its electrochemical activity in LIBs. Metallic Ag has been recognized as a kind of material that possesses superior electronic conductivity compared with other metals or metal oxides,25, 26 and it has been used as a decorating material for enhancing the electrical conductivity of various electrode materials.27,

28

Using Ag nanorods or nanoparticles as anode materials,

carbon materials like carbon nanotubes (CNTs) or graphene sheets were always introduced to design Ag-contained composites to alleviate large volumetric expansion during Li-Ag alloying/de-alloying process.29-31 Hence, we fabricate AgNO3/CNTs composite by introducing carbon nanotubes to enhance the electronic conductivity and stabilize the structure. In this paper, we adopt a simple solution method to synthesize

AgNO3/CNTs

composite.

The

electrochemical

property

and

lithiation/delithiation mechanism of AgNO3/CNTs are totally investigated. It is found that AgNO3/CNTs exhibit higher reversible capacity and better cycling performance than bare AgNO3. 4


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Experimental Synthesis of AgNO3/CNTs and recrystallized AgNO3 is illustrated in Figure1. In this typical synthesis, 0.5 g AgNO3 was added into 60 ml ethanol solution with consistent magnetic stirring at room temperature for 8 hours to get a colorless and transparent solution A. After stirring for 8 hours, the solution A was transferred into vacuum oven and evaporated at 80 oC for 12 hours to obtain the active powders recrystallized AgNO3. To prepare AgNO3/CNTs composite, AgNO3 and CNTs with a weight ratio of 9:1 were dissolved/dispersed into 60 mL ethanol with continuous magnetic stirring at room temperature. We marked this black solution as solution B. After the ultrasonic vibration several times, the solution was transferred into a 100 mL polytetrafluoroethylene vessel for hydrothermal treatment at 100 oC for 12 hours and then evaporated at 80 oC for 12 hours to obtain the final AgNO3/CNTs composite. In this experiment, original AgNO3 was of analytical grade and purchased from Guanghua Chemical Reagent Shantou Co. Ltd in China. The working electrode was fabricated by mixing the recrystallized AgNO3 or AgNO3/CNTs (80 wt.%) with carbon black (10 wt.%) as conductive additive and polyvinylidene

fluoride

(10

wt.%)

as

binder

(Figure

1)

dissolved

in

N-methyl-2-pyrrolidinone (NMP). The above slurry of the mixture were cast onto stainless foils with a diameter of 15 mm and used as the working electrodes for battery assembling. The average active loading mass of working electrode is about 2.0 mg. The half cells were assembled with the as-prepared electrode as working cathode, 5



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metallic lithium as counter electrode and Whatman glass fiber as separator. The electrolyte was a 1 M LiPF6 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1, v/v). The assembly of the simulated half cells was carried out in an argon-filled glove box. Galvanostatic discharge and charge measurements were evaluated at a current density of 50 mA g-1 (0.16 C) in the 0.0 V to 3.4 V voltage windows by using a multi-channel Land battery test system (Wuhan Jinnuo, China). Cyclic voltammetry (CV) tests were measured on CHI 1000B electrochemical workstation (Shanghai Chenhua, China) within a potential window of 0.0-3.4 V at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) tests were performed using a CHI 660D electrochemical workstation in the frequency range of 0.1 MHz to 0.01 Hz. The thermal stability of as-prepared AgNO3 was analyzed by thermogravimetry (TG) using a thermogravimetric-differential thermal analyzer (Seiko TG/DTA 6300 instrument) at a heating rate of 10 oC min-1 in the temperature range between 20 and 600 oC under air atmosphere. For structural characterization, the X-ray diffractometer (Bruker AXS D8 Focus) with CuKa radiation was used to identify the crystalline phase of the AgNO3 and AgNO3/CNTs powders in the 2θ range of 10-80º. In-situ X-ray diffraction (XRD) patterns were also performed on the same Bruker D8 Focus system with Cu Κα radiation (λ=0.15405 nm) with 40 mA and 40 kV. The scan angle ranges from 10 to 45°. The electrochemical tests during in-situ XRD were performed using a battery tester with a voltage window of 0.01-3.40 V. The in-situ cell was tested for a whole discharge/charge process at a rate of 50 mA g-1. Scanning electron 6


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microscope (Hitachi S3400) and transmission electron microscopy (JEOL JEM-2010) were performed to study the morphology and microstructure of the materials. XPS spectra were obtained on a focused and monochromatized Al Ka radiation with a Kratos Axis Ultra spectrometer.

Results and discussion Figure 2a shows the XRD patterns of the as-prepared AgNO3 and AgNO3/CNTs. As depicted in Figure 2a, the peaks can be assigned to well-crystallized AgNO3 powder in the JCPDS card No.74-2076. Moreover, all the diffraction peaks can be attributed to the AgNO3 single phase and no impurity peaks are detected, revealing the high purity of the received product. The AgNO3 is a kind of orthorhombic crystal structure and the cell parameters are a=6.9583(5) Å, b=7.2953(6) Å and c=10.0794(7) Å. After coating with CNTs, the AgNO3/CNTs also display the same orthorhombic crystal structure and its cell parameters are a=6.9750(9) Å, b=7.3067(2) Å and c=10.0904(9) Å. The two samples reveal the same crystal structure and their parameters without obvious change, indicating that CNTs coating does not change the structure of AgNO3. The typical TG-DTA curves of recrystallized AgNO3 are characterized in the air atmosphere as shown in Figure 2b. There are three steps, 20-252 oC, 252-298 oC and 298-600 oC, appeared in the TG curve during weight loss process. It indicates that AgNO3 starts to decompose at about 252 oC, corresponding to a weight loss of 28.4 % between 252 and 298 oC in the TG curve. It can be attributed to the decomposition of AgNO3 powders into intermediate products. 7



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Furthermore, a following weight loss of 19.4 % between 298 and 600 oC in TG curve may ascribed to the totally decomposition of intermediate products into metallic Ag and other gaseous products. Therefore, it is believed that the AgNO3 and AgNO3/CNTs can maintain their structure during sample and electrode preparation. The particle morphology and cyclic voltammogram (CV) of AgNO3 and AgNO3/CNTs are depicted in Figure 2c-2h. Viewed from Figure 2c and 2d, the AgNO3 is composed of a great number of irregular rod-like particles with the average size of about 5-20 µm. Furthermore, all these particles have a smooth and glossy surface without any agglomeration. For AgNO3/CNTs in Figure 2f, it can be clearly seen that the CNTs are well dispersed in the surface of AgNO3 particles. Moreover, the AgNO3 particles distribute in crosslinked CNTs uniformly and their sizes are about 2-6 µm. As shown in Figure 2g, the single AgNO3 particle with CNTs dispersed in its surface homogeneously forms an electron-conducting three-dimensional network, which could provides larger area for Li+ ions diffusion and suppresses the volume change of active particles more effectively than the bare AgNO3. Therefore, it is expected that AgNO3/CNTs can display an excellent electrochemical performance. The CV curves for the AgNO3 and AgNO3/CNTs electrodes at a scan rate of 0.1 mV s-1 are shown in Figure 2e and 2h. For bare AgNO3, there are two intense peaks at about 3.49 and 1.48 V in its first cathodic sweep and one weak peak at about 1.00 V in its first anodic sweep as depicted in Figure 2e. In the initial curve, the two reduction peaks observed at 3.49 and 1.48 V corresponding to a probable continuous electrochemical decomposition of AgNO3 to LiNO3, Li3N, Li2O and Ag. Furthermore, 8


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the broad peak at around 0.0 V suggests the Ag-Li alloying process. Besides, the reverse anodic process, only one peak at 1.00 V can be observed except for the broad peak at about 0.1 V, indicating the electrochemical irreversibility of electrode material during the first cycle. In the subsequent two cycles, the main reduction and oxidation peaks cannot re-appear, justifying the irreversibility of AgNO3 and also indicating that the following cycles mechanism have a significant difference with the first cycle. Furthermore, the second and third CV curves show no significant change, indicating a good reproducibility and high reversibility of the redox reaction after the first cycle. Figure 2h presents the CV curves of AgNO3/CNTs and it is clearly can be seen that the CV curves of AgNO3/CNTs have a highly similarity with AgNO3. Except for the redox couple at around 0.0 V, there are two reduction peaks at about 1.41 and 3.48 V and also one weak oxidation peak at 0.95 V in its first sweep. The position of redox peaks of AgNO3/CNTs has a high similarity with the AgNO3, indicating the two samples have identical electrochemical reaction mechanism. By contrast, the peak current of AgNO3/CNTs has a higher relative intensity than the AgNO3, which manifests that the CNTs networks can enhance the Li+ ions diffusion rate and electron conductivity effectively. The charge/discharge curves illustrated in Figure 3a were observed to investigate the electrochemical performance of the AgNO3 and AgNO3/CNTs. The 1st and 30th cycles of AgNO3 and AgNO3/CNTs at a current density of 50 mA g-1 are shown in Figure 3a. For AgNO3 and AgNO3/CNTs, there is a wide discharge plateau at around 3.5 V in their initial cycle, followed by a short plateau at 1.55 V, a slope from 1.55 to 9



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0.02 V and a long platform at 0.01 V. The lithiation plateau at around 3.5 V probably corresponds to the preliminary formation of LiNO3 and metal Ag. For another short platform at 1.55 V, it may contribute to the further decomposition of LiNO3 to form Li3N and Li2O. While in the potential range of 0-0.25 V, metallic Ag exhibits a specific capacity with the formation of different Ag-Li alloys.25 In the charge process, there are two narrow charge platforms appear at around 0.1 and 0.3 V, followed by two slopes from 0.4 to 3.4 V. The discharge curves of AgNO3 and AgNO3/CNTs, are quite different from their initial cycles, consist of two slopes from 3.4 to 0.0 V without lithiation plateaus. However, their charge curves are similar with the initial charge curves. These electrochemical behaviors are in good accordance with its CV curves. In the first cycle, the discharge/charge capacities of AgNO3 and AgNO3/CNTs are 1450.5/552.3 mAh g-1 (a coulombic efficiency of 38.1%) and 1075.8/521.2 mAh g-1 (a coulombic efficiency of 48.4%), respectively. The presence of initial irreversible capacity mainly originates from the irreversible conversion of AgNO3 and irreversible reduction of organic solvents.30 At the 30th cycle, the discharge/charge capacities of AgNO3 and AgNO3/CNTs remain at 345.9/319.7 mAh g-1 (a coulombic efficiency of 92.4%) and 523/506.2 mAh g-1 (a coulombic efficiency of 96.7%), respectively. The higher lithium storage capacity and coulombic efficiency of AgNO3/CNTs indicate the CNTs coating can obviously improve the electrochemical performance of AgNO3. Considering practical application in LIBs, stable cyclic performance is an important factor for electrode materials. The cycling stabilities of the AgNO3 and AgNO3/CNTs were investigated at a rate of 50 mA g-1 between 0.0 and 3.4 V as 10


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presented in Figure 3b. The charge capacity of AgNO3/CNTs remains at 482.5 mAh g-1 after 100 cycles with the capacity retention of 92.5 %, indicating its stable cyclability. In contrast, the AgNO3 only delivers a much lower capacity of 84.5 mAh g-1 after 100 charge/discharge cycles, with its capacity retention of 15.3 %. Moreover, the average coulombic efficiency for AgNO3/CNTs is maintained at 97 % during the repeated cycles. For comparison, the cycling coulombic efficiency for the AgNO3 keeps an average value at about 91 %. Furthermore, AgNO3/CNTs also exhibits good electrochemical performance compared with other anode materials, such as Ag, Pb or other metal nitrates as revealed in Table S1 (Supporting Information). Considering low weight percent of CNTs (10 wt.%) in AgNO3/CNTs, the contribution of reversible capacity from CNTs is negligible (10 wt.%×300 mAh g-1 = 30 mAh g-1, Figure S1 in Supporting Information). It suggests that crosslinked CNTs stabilize the host structure of AgNO3 upon lithiation/delithiation process. The rate performances of AgNO3 and AgNO3/CNTs cycled at various current densities are shown in Figure 3c. The average cycling charge capacities of the AgNO3/CNTs electrode are 566.4, 525.6, 476.1, 441.3, 395.9 and 348.8 mAh g-1 at the current density of 50, 100, 150, 200, 250 and 300 mA g-1, respectively. For comparison, the AgNO3 delivers the average charge capacities of 588.3, 489.7, 412.3, 336.9, 272.8 and 210.0 mAh g-1 at 50, 100, 150, 200, 250 and 300 mA g-1. Clearly, the decline in the charge capacity with an improvement in current density is slower for AgNO3/CNTs than AgNO3, especially at a rate of 300 mA g-1. When the current density returns to 50 mA g-1, the charge capacity of AgNO3/CNTs recovers to 551.3 11



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mAh g-1 and maintains at 533 mAh g-1 after 20 cycles. However, the AgNO3 electrode only displays a capacity of 504.2 mAh g-1 at 50 mA g-1 and it rapidly reduces to 414.7 mAh g-1 after 20 cycles. The enhanced rate performance of the AgNO3/CNTs is mainly related to the following factors: (1) the three-dimensional network which formed by AgNO3 particles dispersed in carbon nanotubes serves as protective layer to maintain the structural integrity of electrode during the redox reaction, thus leading to a good structural stability; (2) the high surface area of three-dimensional network increases the contact between the electrode and electrolyte, provides larger area for Li+ ions diffusion and accommodates large volumetric expansion to slow down the rate of electrode pulverization; (3) the existence of the carbon layer improves the electronic conductivity of the electrode. Therefore, AgNO3/CNTs shows enhanced cycling properties and rate performance compared with bare AgNO3. Furthermore, the Nyquist plots of the two samples before cycling are shown in Figure 3d. As depicted in Figure 3d, the resulting Nyquist plots displaye depressed semicircles in the high-frequency region and straight lines in the low-frequency region. Apparently, the initial horizontal intercept and the diameter of the semicircle for the AgNO3/CNTs in the high-frequency region is smaller than that for the AgNO3, suggesting that the AgNO3/CNTs has lower contact and charge transfer resistances. According to the calculated data from the equivalent circuit inserted in Figure 3d, AgNO3/CNTs reveal an Rct value of 18.26 Ω. For comparison, the AgNO3 shows the Rct of 78.68 Ω, indicating the faster charge transfer kinetics for AgNO3/CNTs. The results correlate well with the high reversible capacity and superior cycle stability of 12


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AgNO3/CNTs and confirm that the CNTs can improve the conductivity of the overall electrode. The in-situ XRD and electrochemistry data related to a full charge/discharge cycle are gathered in Figure 4a-4b. The first XRD pattern collected before cycles displays the featured diffraction peaks of AgNO3. Here, (111), (102), (020), (211), (210), (113), (122), (400), (131) and (024) planes are presented in Figure 4b. The change process of these main diffraction peaks is depicted in the images. For a better illustration of the mechanism in lithiation/delithiation process, selected evolutions are plotted in Figure 4c-4f and Figure S2 (Supporting Information). Upon lithiation, it is clear that the diffraction peaks of the starting phase (AgNO3) at 19.50º, 21.50º, 24.20º, 29.54º, 31.78º, 32.67º, 38.99º, 40.03º and 43.36º progressively vanish in the discharge process and do not recover to their previous positions in the reverse charge process. Moreover, two new characteristic peaks of metallic Ag appear at 38.15º and 44.28º in the initial discharge process, which means the gradual decomposition of AgNO3 into Ag. Upon further lithiation, the two peaks gradually shift to 38.26º and 44.38º, which are assigned to the characteristic peaks of Ag3Li10, indicating the phase transformation from Ag to Ag3Li10 during the deep lithiation. During de-lithiation process, there is a continuous left-shift observed for the two diffraction peaks of Ag3Li10 at 38.26º and 44.38º and the two peaks return to their previous Bragg positions of Ag at 38.15º and 44.26º at the end of charge process, indicating the irreversible reaction between AgNO3 and Li. Therefore, Ag-Li alloying/de-alloying reaction should be responsible for the reversible lithium storage capacity in the following cycles. 13



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To reveal the Li-storage mechanism of the AgNO3/CNTs anode, we performed XPS analysis to track the chemical states of Ag, N and Li elements in the AgNO3/CNTs specimens as a function of working potential. Samples are classified as pristine state, discharged state (lithiation process) and charged state (delithiation process), and the spectra are displayed in Figure 5. As depicted in Figure 5a, the XPS results of Ag 3d at different lithiated/delithiated states are presented. For the pristine state, the peaks located at 368.0 and 373.8 eV are indexed to the Ag 3d5/2 and Ag 3d3/2 of Ag+, which is also verified by previous report about Ag+.32 Consequently, the peaks at 368.0 and 373.8 eV are ascribed to Ag+ in the original AgNO3/CNTs specimens. After a lithiation process in 0.0 V, the peaks shift to 368.5 and 374.5 eV, respectively, which can be assigned to the characteristic peaks of Ag0.6 Hence, this also confirms the formation of Li-Ag alloys during discharge process. Besides, no signal of Ag+ peak can be detected in the discharged state, indicating the complete decomposition of AgNO3. When the AgNO3/CNTs is charged up to 3.4 V, the peaks maintain at 368.5 and 374.5 eV and no Ag+ signals can be observed. Firstly, this means the AgNO3 or AgO cannot generate after a charge process, which in accordance with the results of in-situ XRD. Secondly, the formation of metallic Ag testifies the reaction of Ag-Li alloys is fully reversible. From Figure 5b, the N 1s spectra in AgNO3/CNTs specimens at different states are clearly illustrated. For pristine state, the peaks of N 1s locate at 398.9 and 407.1 eV, indicating the existence of N5+ in sample according to the handbook of XPS.33 This also demonstrates the original structure of AgNO3 in the pristine electrode. After discharge to 0.0 V, the binding energies of N3- and N5+ peaks 14


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appear at 398.6, 403.0 and 407.2 eV, which can be observed at the discharged state in Figure 5b. In the reverse charged state, the characteristic peaks at 398.6, 403.0 and 407.2 eV remain unchanged and also testify the existence of N3- and N5+ in the charged samples. In Figure 5c, there are no characteristic peaks of Li 1s in the pristine state of AgNO3/CNTs. When the AgNO3/CNTs electrode is deeply discharged to 0.0 V, two new lithium states appear with double peaks at the binding energy of 55.0 and 55.7 eV, revealing the formation of Li0 and Li+ during the electrochemical reaction, indicating the formation of Li-Ag alloys (Ag3Li10) and LiNO3 during the lithiation process. In the reverse charge process, the XPS peak 55.0 eV becomes weaker owing to the Li de-alloying reaction from Li-Ag alloys. In contrast, the remaining peak at 55.7 eV suggests the irreversible formation of LiNO3 in the discharge process. Thus, it further demonstrates that Ag-Li alloying/de-alloying reaction is the main electrochemical process for repeated lithium storage in AgNO3. For further testify the Li storage mechanism, ex-situ FTIR is also used to study the structural evolutions of AgNO3/CNTs in 0.0-3.4 V. As depicted in Figure 6, we arrange 12 samples for ex-situ FTIR analysis. In Figure 6a-6b, six lithiated samples are named as discharge 1 (pristine state, 3.4 V), discharge 2 (discharged to 2.7 V), discharge 3 (discharged to 2.0 V), discharge 4 (discharged to 1.3 V), discharge 5 (discharged to 0.6 V), discharge 6 (discharged to 0.0 V) and six de-lithiated samples are marked as charge 1 (charged to 0.1 V), charge 2 (charged to 0.7 V), charge 3 (charged to 1.4 V), charge 4 (charged to 2.1 V), charge 5 (charged to 2.8 V) and charge 6 (charged to 3.4 V). Obviously, the Li-Ag bond (AgLix) cannot be detected by 15



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ex-FTIR technique. In contrast, the intensity variation of NO3- can be observed during lithiation/de-lithiation process. Figure 6c shows the FTIR spectra of different lithiated and de-lithiated samples in powder by means of a KBr method. The characteristic peak observed at 1384 cm-1 in these curves can be attributed to stretching vibrations of NO3-.34 As can be seen clearly, the intensity of NO3- gradually reduces with the process of lithiation and it does not disappear at the end of discharge process, indicating the probable phase transformation reaction from AgNO3 to LiNO3 and further electrochemical decomposition of partial LiNO3 to form Li3N and Li2O. While in Figure 6d, the infrared characteristic peak of NO3- at 1384 cm-1 shows no evolution in the charge process, manifesting that AgNO3 cannot be regenerated by LiNO3 in the charge process. In addition, the electrochemical behavior of Ag/CNTs is also performed and compared with AgNO3/CNTs as shown in Figure S3 (Supporting Information). It is clear that the lithiation/delithiation behavior of AgNO3/CNTs is similar with that of Ag/CNTs from the second cycle, indicating that the electrochemical reaction between AgNO3 and Li is irreversible and the reversible lithium storage capacity in the following cycles is associated with Ag-Li alloying and de-alloying processes. This phenomenon can also be demonstrated by ex-situ TEM observations (Figure S4, Supporting Information), which reveal the presence of Ag particles after repeated cycles. Here, the probable equations of lithiation and delithiation behaviors in AgNO3 are proposed as follows. Decomposition reaction:

AgNO3 + Li + + e − → Ag + LiNO 3

(1) 16


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LiNO 3 + 8Li + + 8e − → Li 3 N + 3Li 2 O

(2)

Alloying reaction:

3Ag + 10Li + + 10e − ↔ Ag 3 Li10

(3)

Based on the above results, the reaction mechanism for AgNO3 in LIBs can be described as follows: First, the AgNO3 irreversibly decomposes into LiNO3, Li3N, Li2O and metal Ag. Then, the resulting metal Ag further reacts with Li to form Ag3Li10 alloys at low working potentials (0-0.25 V) during the initial discharge process. In the reverse charge process, the Ag3Li10 alloys can reversibly decompose into metal Ag and Li, and the reversible lithium storage capacity of AgNO3 electrode in the subsequent cycles is mainly based on the reversible alloying reaction between Li/Ag and Ag3Li10.

Conclusion In summary, we propose a promising anode material (AgNO3/CNTs) by a simple solution method for the first time, which shows improved electrochemical performances as anode material for LIBs due to its unique three-dimensional networks. Electrochemical evaluations present that the AgNO3/CNTs exhibits high initial charge capacity of 530.3 mAh g-1 at 100 mA g-1, and maintains a reversible capacity of 482.5 mAh g-1 after 100 cycles. The structure transformation of AgNO3 in AgNO3/CNTs is thoroughly studied by in-situ XRD, ex-situ FTIR, and ex-situ XPS techniques during the charge/discharge cycle. It is known that the electrochemical reaction between AgNO3 and Li is irreversible. In fact, the reverse process for 17



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AgNO3/CNTs electrode is mainly composed of the de-alloying reaction of Ag3Li10 to form metal Ag and Li. As a result, the reversible lithium storage capacity of AgNO3 is mainly based on the reversible conversion reaction between Li/Ag and Ag3Li10.

Acknowledgement Authors acknowledge the financial support by National Natural Science Foundation of China (U1632114), Ningbo Key Innovation Team (2014B81005), Ningbo Natural Science Foundation (2016A610068) and K.C. Wong Magna Fund in Ningbo University.

Supporting Information. The charge/discharge curves of bare CNTs, selected in-situ XRD patterns of AgNO3/CNTs, electrochemical behaviors of Ag/CNTs, TEM images of AgNO3/CNTs after cycles and a comparison of the electrochemical performances between AgNO3/CNTs and Ag, Pb, and other metal nitrates

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Figure 1. Schematic illustration of the fabrication process of the recrystallized AgNO3, AgNO3/CNTs and the working electrode.

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Figure 2. (a) XRD patterns of AgNO3 and AgNO3/CNTs; (b) TG-DTA curves, (c, d) SEM images and (e) CVs of AgNO3; (f, g) SEM images and (h) CVs of AgNO3/CNTs.

25



4

30th

b

1st

Potential/ V

3

AgNO3/CNTs

2

AgNO3 1

0

Specific capacity/ mAh g-1

a

1500 100

1000

80

AgNO3/CNTs AgNO3

60 500 40

Coulumbic efficiency/ %

0 0

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800

0

20

40

-1

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100

Cycle number

d

800

AgNO3/CNTs

400

90

AgNO3

600

50 mA g-1 100 mA g-1 150 mA g-1

200

50 mA g-1

-Z''/ ohm

c

20

1200

Specific capacity/ mAh g

Specific capacity/ mAh g-1

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

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AgNO3/CNTs AgNO3

30

-1

200 mA g 250 mA g-1 300 mA g-1

0 0

20

40

60

0 0

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30

60

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120

Z'/ ohm

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Figure 3. (a) The 1st and 30th discharge/charge curves, (b) cycling performance, (c) rate capability and (d) EIS patterns of AgNO3 and AgNO3/CNTs.

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Figure 4. (a) Discharge/charge curve of AgNO3/CNTs collected during in-situ observation, (b) Overall in-situ XRD patterns of AgNO3/CNTs during the initial charge/discharge process, (c) local in-situ image for intensity evolution in 19-25º, (d) local image for intensity evolution in 29-33º, (e) local image for intensity evolution in 37-42º, (f) local image for intensity evolution in 42-45º.

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Figure 5. XPS investigation of AgNO3/CNTs at different lithiated/delithiated states. (a) Ag 3d. (b) N 1s. (c) Li 1s.

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Figure 6. (a, b) Lithiation/de-lithiation curve obtained for ex-situ FTIR analysis during the initial charge-discharge process, (c) ex-situ FTIR spectra of AgNO3/CNTs in the initial discharge process, (d) ex-situ FTIR spectra of the de-lithiated samples in the reverse charge process.

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Table of Contents Graphic

Lithiation/Delithiation Behavior of Silver Nitrate as Lithium Storage Material for Lithium Ion Batteries

Peng Li, Hua Lan, Lei Yan, Shangshu Qian, Haoxiang Yu, Haojie Zhu, Nengbing Long, Miao Shui, Jie Shu*

As anode material, AgNO3/CNTs composite shows the potential as lithium storage host owing to its electrochemical reversibility.

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Delithiation Behavior of Silver Nitrate as Lithium Storage

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