Microstructure and Charge­Discharge Characteristicsof Ag­AgCl Coated Natural Bamboo Carbon

Chih-Hsien Wang, Fei-Yi Hung+, Truan-Sheng Lui and Li-Hui Chen

Department of Materials Science and Engineering, Center for Micro/Nano Science and Technology,National Cheng Kung University, Tainan, Taiwan 701

Bamboo carbon decomposed by low temperature has a high surface area and contains micro-holes; it belongs to one of the amorphouscarbon materials. Due to the large lithium storage space and high discharge performance rate, lithium batteries enjoy high power consumption.However, there are also some defects of the first higher irreversible capacity and voltage delay. In this study, a natural bamboo carbon powderwas used as an experimental material. After adding AgNO3 and treating the surface with heat at 450 and 650°C, fine Ag/AgCl phases are coatedon the surface of carbon powders. C­Ag and C­AgCl are formed to increase the capacity and reduce the first irreversibility. Also, theconcentration of Ag on the carbon surface increased with the increment of temperature from 450 to 650°C. This not only increased theconductivity but also enhanced the surface bonding of C­Ag powders to promote the performance of charge­discharge cycles.[doi:10.2320/matertrans.M2012254]

(Received July 17, 2012; Accepted March 5, 2013; Published May 25, 2013)

Keywords: bamboo carbon, silver, silver­chloride, charge­discharge

1. Introduction

Carbon material has many advantages such as low cost,large current discharge capability, stable structure, safety etc.However, the capacity of commercial carbon materials whichare produced by petroleum is less than the theoreticalcapacity of graphite (372mAhg¹1).1) Therefore, the develop-ment of high capacity and excellent cycle life anode materialsis an important issue in the application of lithium­ionbatteries.

Bamboo carbon is a green material. It is conducive to thepassage of lithium ions and creates more on lithium storagelocations due to the pore structure, nano-channels and largespecific surface area. Hence, the lithium storage space andlarge-current discharge capability of the bamboo carbon aregreater than that of graphite. However, the bamboo carbonhas a huge irreversible reaction that affects the electrochem-ical performance. Surface modification of carbon materials isan effective method to improve the electrochemical character-istics of carbonaceous electrodes. In recent years, somemethods of surface modification have been attempted viz.,surface oxidation,2) surface fluorination,3) metal or metal-oxide coating4,5) and carbon coating.6)

According to our previous study,5) carbon coating is ahighly efficient interface reaction. Therefore, natural bamboocarbons were used as a negative electrode in this study andthe carbon surface was modified by the ball milling7,8) andsurface treatment methods. The charge­discharge character-istics of natural bamboo carbon before and after the surfacemodification has been explored. Ag has the best conductivity.In this study, the Ag/AgCl phases were coated on the carbonsurface with good bonding interface. The Ag/AgCl-carbonphases, which were formed with a heat treatment (450°C;650°C), had excellent stability and conductivity thusproviding a higher volumetric capacity and longer lifecyclethan conventional graphite anodes. In addition to discussingthe effects of Ag/AgCl coating on heat treatment, the coating

mechanism and lithium ions’ intercalation­deintercalationreaction in the electrode was investigated. Thus the charge­discharge life cycle of C­Ag and C­AgCl was clarified forpossible applications to batteries.

2. Experimental Procedures

A natural bamboo carbon was prepared by sintering at750°C and then soaked in 10mass% of HCl solution,followed by ultrasonic treatment with deionized water. It wasdefined as C (non-coating). The ratio of the mixture of C andAgNO3 was 10 to 1. After applying ball milling for 12 h, thecarbon powders were removed to be dried and calcined inthe furnace at 450 and 650°C respectively in Ar/H2 for 1 huntil it cooled down to room temperature. The size ofD50 particles was controlled below 5µm. Bamboo carbonpowders were modified by two heat treatments, namely CA-450 and CA-650.

Three types of powder (C, CA-450, CA-650) morphologywere examined by SEM (Philip XL-40FEG), and then theelectrodes were scanned by backscattered electron imaging(BEI) to distinguish Ag/AgCl. The content of carbon andmodified carbon was also calculated semi-quantitatively byusing an energy dispersive spectrometer (EDS). Meanwhilean X-ray diffractometer (XRD, RigakuD-max/IIB, CuK0.15418 nm) identified the powder structure (scan angle of2­90°, scan rate of 2°min¹1), and the particle size of thepowders was measured by a particle size analyzer (HORIBA-300 type).

The electrochemical test was performed by a two-electrodesemi-cell. The working electrode was carbon and the counterelectrode was lithium metal. In the initial stage of the charge­discharge cycle, lithium ions first intercalated into carbonduring the discharge process and then deintercalated fromcarbon into lithium metal in the later charge process. Anelectrode was prepared by mixing 90mass% carbon,6mass% PVDF (polyvinylidene fluoride) and 4mass%conductive carbon black, and subsequently dissolved inNMP (N-methyl-2-pyrrolidone). The slurry was coated onto+Corresponding author, E-mail: fyhung@mail.ncku.edu.tw

Materials Transactions, Vol. 54, No. 6 (2013) pp. 1018 to 1024©2013 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

copper foil (15 µm). The coated films were dried for 2 h at100°C and were cut to a diameter of 13mm. The electrolytewas 1M LiPF6 (lithium hexafluorophosphate) in EC (ethyl-ene carbonate) + DEC (diethyl carbonate) 1 : 1 in terms ofvolume. The assembled cells exhibited 0.2C discharge(intercalation) and 0.2C charge (deintercalation) at first, andthen a cycle life test was implemented at 1C discharge(intercalation) and 1C charge (deintercalation) (ArbinBT2000). The voltage range was 0.01­2V, and the elec-trochemical impedance spectroscopy (EIS) (Potentiostats2273) was performed at a frequency of 1MHz­0.01Hz.

3. Results and Discussion

3.1 Microstructure of powderFigure 1(a) shows the morphology of the bamboo carbons

after ball milling. It shows that the form of the particles isirregular and some are in void and fold structures. The size ofthe particles (D50) was measured as 2.5 µm. Table 1 showsthe element analysis of powders C: the weight percentageof carbon is 84.87mass%, H is 2.17mass% and O is about10mass%. Because the bamboo carbon can be decomposedat low temperature (750°C), the proportion of H and O washigh and they existed on the carbon surface in the form ofhydroxyl and carboxyl groups. The atomic size and chargesof other elements are different from carbon’s, and this couldhave affected the charge­discharge behavior to yield a largerirreversible capacity.9) On the other hand, Fig. 1(b)(c) showthe morphologies of CA-450 and CA-650 after the heattreatment. Tiny phases were generated on the carbon surface;the morphology and size of the C powders did not changeobviously after modification. To corroborate the distributionof surface elements, the electrode’s mapping was carried out.We found that the elements of Ag and Cl were uniformlydistributed on the surface of the electrodes (Fig. 2(a)(b)).Notably, the amount of Cl in the CA-650 electrode wassignificantly less than the CA-450 electrode. EDS analysisfor CA-450 and CA-650 are listed in Table 2. The amount ofCl is significantly more than Ag in the CA-450 electrode.This indicates that Cl not only reacts with Ag to form AgCl,but also bonds with C easily to form COCl or CCl2. This isdue to the existence of the functional groups and themicrovoids.10,11) However, the amount of Ag was more thanCl in the CA-650 electrode (treated at 650°C). It is knownthat AgCl was decomposed into silver and chlorine in thereducing atmosphere. Through BEI scanning of the electrodesurface, it was found that the light spots were aggregated inthe CA-450 and CA-650 electrodes. Table 3 shows the EDSanalysis of the light spot zone. The main components arecarbon and silver in the CA-450 electrode, and a smallquantity of chlorine. Notably, the light spot zones of the CA-650 electrode were carbon and silver, and no chlorine wasdetected. According to the results above, we can know thatwhen the temperature rises, C­AgCl or C­Ag compoundswill form on the surface of bamboo carbons. To understandthe structure of C­AgCl or C­Ag phases, XRD identificationwas carried out.

The results of XRD are shown in Fig. 3. The structure ofcarbon (C) is amorphous and its degree of crystallization islow. Diffraction peaks at (002) and (100) were broad, and

indicate that some graphite layers were stacked. The d(002)became larger and the solid-state diffusion became faster.This contributed to the acceleration of the charge­dischargerate. The intensity of the peak was higher in the low-angle

(a)

(b)

(c)

2μm

2μm

2μmFig. 1 SEM photograph: (a) C (b) CA-450 (c) CA-650 powders.

Table 1 EA analysis of C powders.

Element N C H O

mass (%) 0.48 84.87 2.17 Bal.

Microstructure and Charge­Discharge Characteristics of Ag­AgCl Coated Natural Bamboo Carbon 1019

region, which reveals that the carbon materials have micro-porous characteristics.12) Furthermore, the XRD pattern ofCA-450 not only shows the existence of carbon phase butalso silver cubic phase and silver chloride cubic phase.According to Scherrer’s equation, the grain size of AgCl isabout 43.2 nm. XRD patterns of CA-650 show an increasingtrend in the heat-treatment temperature, which increased thevolume ratio and the degree of crystallization of silver phase,but silver chloride phase was not significantly affected. Whenthe temperature increased from 450 to 650°C in the reducing

atmosphere, AgCl could decompose more easily into silverand chlorine, and resulting in an increase of silver phase.Based on this, C­Ag phase was obtained in the coatingmechanism of the modified bamboo carbon powders withAg/AgCl and improved the stability of crystallization.

3.2 Charge­discharge behaviorThe coating of the carbon electrode with Ag/AgCl

improved the initial charge­discharge performance. Table 4shows the first charge­discharge capacity and irreversibility.The capacity of CA-450 and CA-650 were greater than thetheoretical capacity of graphite (372mAhg¹1). Moreover,CA-450 showed the highest charge capacity (deintercalation,up to 576mAhg¹1) and the charge capacity of CA-650 was439mAhg¹1. They were larger than the charge capacity ofthe original carbon material (195mAhg¹1).5) The first cycleirreversibility of CA-450 and CA-650 was 46 and 48%respectively, both of which were less than the 60% of theoriginal carbon material. When the lithium ions intercalatedinto carbon, the electrolyte reacted on the carbon electrodeand decomposed to form the passive layers on the electrodesurface. This would have caused some lithium ions to be lost

(a) (b)

Fig. 2 SEM-BEI and mapping: (a) CA-450 and (b) CA-650 electrodes.

Table 2 EDS analysis of CA-450 and CA-650 electrodes.

ElementCA-450 CA-650

Mass% At% Mass% At%

C 82.27 89.58 82.29 91.05

Ag 3.24 0.39 7.03 0.87

Cl 3.34 1.23 1.01 0.38

O 8.70 7.11 7.20 5.98

F 2.45 1.69 2.47 1.73

Table 3 EDS analysis of bright spot on CA-450 and CA-650 electrodes.

ElementCA-450 CA-650

Mass% At% Mass% At%

C 7.96 38.96 14.57 54.91

Ag 87.64 47.75 81.59 34.23

Cl 1.41 2.34 0 0

O 2.98 10.95 3.84 10.86

Diffraction angle, 2θθ / degree

Inte

nsit

y (a

.u.)

Fig. 3 XRD patterns for the original and modified carbon powders.

Table 4 Charge/discharge measurements at 0.2C and 0.01­2Vand specialsurface area (SSA) analysis.

C CA-450 CA-650

DC (mAhg¹1) 488 1062 848

CC (mAhg¹1) 195 576 439

1st IR (%) 60 46 48

SSA (m2 g¹1) 137.0 141.8 145.7

C.-H. Wang, F.-Y. Hung, T.-S. Lui and L.-H. Chen1020

in the reaction. Therefore the charge capacity (deintercala-tion) was less than the discharge capacity (intercalation),hence the irreversibility of electrode.

Since carbon has a large surface area, the solid electrolyteinterface (SEI) layers which are produced through theresponse of the electrode and electrolyte increased. Accord-ing to the literature13) and Table 4 (special surface area),a carbon surface which is coated with halide, exhibits theenhanced kinetics of lithium intercalation and leads to anincrease in the capacity of modified carbon when the SEIforms. In addition, the modified carbon powders formed amore conductive interface due to the high conductivity ofsilver for lithium ions. During the process of electrochemicaldischarge, the Ag reacted with Li to form AgLi at a lowervoltage (<0.2V), which led to the irruption of more Li toform Ag5Li8.14) It’s also clear that Li migration with theincrease of Ag on the carbon surface. Silver increased theelectron transport rate and enhanced the diffusion velocity oflithium ions between the carbon particles. The carbon surfacewas coated with Ag/AgCl to change the intercalation­deintercalation voltage during the charge­discharge lifecycle.

Figure 4 shows the first charge­discharge curve of thecarbon. The discharge (intercalation) curve of C powders issmooth and the SEI layers are produced through the responseof carbon materials and electrolyte when below 1V. Thecharge (deintercalation) curve has two stages of 0.01­0.9Vand 0.9­2V, and the lithium deintercalation mainly occurredin the 0.01­0.9V stage. Since lithium ions do not departfrom carbon powders easily, the potential platform was notobvious, and the intercalation potential was higher than thedeintercalation, resulting in serious voltage hysteresis.15­17)

For the modified powders, the discharging curves of CA-450and CA-650 were similar. The difference compared with Cpowder is that the discharge (intercalation) curve has a longerplatform at 0.8V. This reveals that Ag/AgCl joined theformation of SEI layers and AgCl was decomposed to LiCland Ag.

The silver particles alloyed with doped lithium at 0V,but the amount of lithium intercalation was limited. Also,it was distinguishable from several platforms below 0.8V,showing that the discharge (intercalation) curve declinedgradually from 0.4 to 0V and a structure of Li-intercalatedcarbon compound formed.18) With regards charging (dein-tercalation), the difference between the original carbon andmodified carbon is that the obvious platform producedmore than 0.5V and this proves that the surface of modifiedcarbon has better ionic conductivity. The voltage curve ofCA-450 and CA-650 lies almost entirely between 0 and 1V,but the CA-450 had a more obvious delithiated platformthan CA-650 between 1 and 2V, showing that the surfaceof CA-450 is modified by chlorine that enhanced the kineticsof lithium deintercalation from the carbon electrode. FromFig. 4 and Table 4, we can see that the coating with silverchloride and silver obviously increased the initial reversiblecapacity of the modified carbon electrode due to thereduction in polarization during the intercalation anddeintercalation of lithium. Furthermore, the initial coulombefficiency increased from 40 to 55­60%. It was confirmedthat the chloride modified SEI layers and the better kineticsof lithium intercalation decreased internal resistance andproduced the obvious delithiated platform thus decreasingvoltage hysteresis.

The charge (deintercalation) capacity of CA-450 and CA-650 as a function of cycle numbers is shown in Fig. 5(a).5)

After the 30th charge­discharge cycle, the current was at 1Crate, the charge (deintercalation) capacity’s retention of CA-650 and CA-450 were 79.9 and 59.2%. Because the SEIlayers of the first charge­discharge cycle did not fully form,fast cyclic degradation occurred. The beginning of thecharge­discharge cycles experienced larger capacity degra-dation due to the current (1C). This current raised the reactiverate of lithium ions and was able to reduce the degradationrate of charge­discharge cycles. CA-450 has a better lithiumintercalation capability, but its higher chloride content had aninsulating effect which hindered charge transfer reaction anddecreased the amount of lithium intercalation. By increasingthe charge­discharge current and cycles, the internalresistance increased, and this resulted in a decline in capacityand the quality of cycle life. On the contrary, a large amountof Ag also increased the lithium diffusion rate on the carbonsurface, and stimulated the diffusion of lithium ions withinthe carbon particles, effectively preventing the generationof dendrites.14) Comparing the columbic efficiencies withcharge capacity during 30 cycles, the charge­dischargecurrent was 1C, as shown in Fig. 5(b). It shows that CA-650 has better columbic efficiency and stability during theinitial cycle stage, which indicates that the surface structureof CA-650 is stable and has a better cyclability than theother carbons. This reveals a close relationship between themodified structure and the cyclability.

To understand the charge­discharge cyclic characteristicsof the modified carbons, electrochemical impedance spec-troscopy (EIS) was carried out after the 30th cycle(Fig. 6(a)). There were two obvious semicircles from highto middle frequency in CA-650. The high frequencysemicircle was attributed to the passivation film formed onthe carbon. The semicircle in the middle frequency region

0 200 400 600 800 1000 12000

0.5

1

1.5

2

2.5

CCA-450CA-650

Capacity, C / mAh g-1

Vol

tage

, V /

V

Fig. 4 Discharge­charge curves of the first cycle for the carbon powders at0.2C and 0.01­2V.

Microstructure and Charge­Discharge Characteristics of Ag­AgCl Coated Natural Bamboo Carbon 1021

was derived from the charge-transfer process at the electro-lyte/electrode interface. The sloping line at low frequencieswas attributed to the diffusion of lithium ions within thecarbon electrode. Moreover, the charge-transfer impedance ofCA-450 after the 30th cycle was higher than that of CA-650.The main reason is that CA-650 forms a higher conductiveinterface of Ag which improves the intercalation anddeintercalation of lithium ions. In order to understand theability of lithium to diffuse within the carbon materials, theplot of Zre and ½¹1/2 was drawn, and · (Warburg factor)values can be obtained by the fitting method (Fig. 6(b)). The· value is equivalent to the slope of Zre with the reciprocalof the square root of frequency (½¹1/2) in the low frequencydomain, while the diffusion coefficient was inverselyproportional to the · value.14,19) The · value can becalculated, which in the case CA-450 was 10.87 and CA-650 was 9.85, respectively. The diffusion of lithium ions inthe CA-650 electrode was higher than that of CA-450. Itwas confirmed that the C­Ag system had excellent charge­discharge characteristics.

3.3 C­Ag coating mechanismTo understand the interface phenomenon of lithium­ion

transfer, Fig. 7 shows the electrode morphology of CA-450and CA-650 after 30 charge­discharge cycles. The surfacesof the CA-450 and CA-650 electrodes were covered with acompound layer due to the initial irreversible reaction andrepeated charge­discharge reaction. Large white depositsformed on the electrodes and it consisted of decompositionproducts such as ROCO2Li mixtures.20) Ag and Cl stilldistributed evenly on the electrode according to mappingobservation. EDS (Table 5) analysis on the electrodesrevealed that there was significant more Ag than Cl. Thecharge­discharge cycles show that AgCl gradually decom-posed to distribute in the electrolyte or compounds.

Furthermore, XRD identified the structure of the CA-450and CA-650 electrodes after 30 charge­discharge cycles(Fig. 8). With the charge­discharge reaction, the (002) peakof C phase was not obvious, due to various compoundscovering the electrode. In addition to the original Ag phase,the distinctive peak of Li2CO3 formed at the same time inCA-650, but the peak was not obvious in CA-450 (Table 5,

(b)

(a)

0 10 20 3060

70

80

90

100

110

120CA-450CA-650

Cap

acit

y, C

/ m

Ah

g-1C

oulo

mb

effi

cien

cy, C

e / %

Cycle number, N

Cycle number, N

Fig. 5 (a) Cycle performance (b) coulomb efficiency of CA-450 and CA-650 were subjected to discharge and charge at 1C rate and 0.01­2V.

0 20 40 60 80 100 120 140 160 180 200 2200

50

100

150

200

250

300

-Zim

g (o

hms)

CA-450CA-650

(b)

(a)

4 5 6 7 8 9 10

ω −1/2

100

120

140

160

180

200

220

240CA-450CA-650

Zre, Z / ohms

Zre

,Z /

ohm

s

Fig. 6 (a) Impedance spectra (b) the relationship between Zre and ½¹1/2 atthe low-frequency region of the carbon after and 30th discharge­chargecycles.

C.-H. Wang, F.-Y. Hung, T.-S. Lui and L.-H. Chen1022

the oxygen content of CA-650 is greater). The main reasonfor this effect is that the formation of Li2CO3 and thereductive decomposition of EC had generated CO3

2¹.21) TheLi2CO3 which formed on the electrode surface contributedto suppress the decomposition of electrolyte and allowedthe intercalation and deintercalation of lithium ions. Thiscan protect carbon materials from being destroyed by theintercalation of solvent molecules22) and thus promotes thecycle life. Figure 9 is a schematic illustration of the naturalbamboo carbon after surface modification. In the early stage,fine AgCl particles were coated on the carbon surface.Because of the ball milling process, carbon materialscombined well with the modified substance. With theincrease of the heat treatment temperature (AgCl decom-posed), a large number of Ag particles were precipitatedamong AgCl particles resulting in better crystallizationof Ag attached to the carbon surface. Meanwhile, themodified effect contributed to the charging and dischargingperformance.

4. Conclusion

After implementing natural bamboo carbon surface treat-ment, Ag/AgCl formed on the carbon surface, enhancingthe first reversible capacity of carbon and improving theirreversible nature. With increasing heat treatment temper-ature, the Ag phase increased as well as the crystallizationbeing enhanced. After 30 charge­discharge cycles at 1C,the impedance of CA-650 with rich Ag phase had lowerimpedance than CA-450. Under multiple cyclic tests, thederivative of non-active material reduced the charge­discharge cycle life.

Table 5 EDS analysis of CA-450 and CA-650 electrodes after 30thdischarge­charge cycles.

ElementCA-450 CA-650

Mass% At% Mass% At%

C 32.20 52.47 33.36 52.76

Cl 1.05 0.58 0.86 0.46

Ag 10.98 1.99 9.72 1.71

O 13.9 17.0 17.02 20.21

F 3.77 3.88 2.39 2.39

P 38.10 24.08 36.66 22.48

(a) (b)

Fig. 7 BEI and mapping of electrodes after 30th discharge­charge cycles:(a) CA-450 (b) CA-650.

Diffraction angle, 2θθ / degreeIn

tens

ity

(a.u

.)

AgLi2CO3

Cu

Fig. 8 XRD patterns of electrodes after 30th discharge­charge cycles.

Temperaturelow high

Ele

ctro

chem

ical

per

form

ance

Low capacity

High irreversibility

C

CA-AgCl

CA-AgHigh capacity

Low irreversibility

Good cyclability

High conductibility

High capacity

Low irreversibility

Fig. 9 A schematic illustration of the surface coating mechanism.

Microstructure and Charge­Discharge Characteristics of Ag­AgCl Coated Natural Bamboo Carbon 1023

Acknowledgements

The authors are grateful to the Center for Micro/NanoScience and Technology (D101-2700) of National ChengKung University, Taiwan and NSC 101-2221-E-006-114 forthe financial support.

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