Journal of the Korean Physical Society, Vol. 65, No. 3, August 2014, pp. 317∼324

Effect of Copper Content in the New Conductive Material Cu-SPB Used inLow-temperature Li-ion Batteries

Adnan Yaqub, Syed Atif Pervez, Umer Farooq, Mohsin Saleem and Chil-Hoon Doh∗

Korea Electro-technology Research Institute (KERI), Changwon 642-120, Koreaand Electrical Functionality Material Engineering (KERI Campus),

University of Science and Technology, Daejeon 305-333, Korea

You-Jin Lee, Minji Hwang, Jeong-Hee Choi and Doohun Kim

Korea Electro-technology Research Institute (KERI), Changwon 642-120, Korea

(Received 14 January 2014, in final form 28 February 2014)

A new conductive material, copper/Super-P carbon black composite (Cu-SPB), is prepared viaan efficient ion reducing method for use in low-temperature lithium-ion batteries (LIBs). Thepresent study investigated the effects of copper content on the low-temperature performance ofLIBs. Electrodes prepared with a high-copper-content conductive material (Cu = 18.54%) showedremarkably improved performance in terms of capacity retention (around 40%), cycling stability, andcolumbic efficiency. The electrochemical impedance spectroscopy (EIS) analysis revealed that thepresence of higher Cu contents could reduce the cell’s impedance. The results were also confirmedby using a coin-type full cell’s improved capacity retention, which indicated the significance of Cuparticles in enhancing the low-temperature performance of LIBs.

PACS numbers: 82.47.Aa, 82.45.Yz, 81.05.UwKeywords: Graphite anode, Copper supported SPB, Conductive material, Low-temperature LIBsDOI: 10.3938/jkps.65.317

I. INTRODUCTION

Lithium ion batteries (LIBs) are considered state-of-the-art energy storage devices [1]. In 1991, when Sonylaunched the first LIB, shortly thereafter they showedever increasing demand as power sources for portable de-vices such as cellular phones, laptop computers, and digi-tal cameras [2,3]. LIBs show a great deal of power and en-ergy at and around ambient temperatures [4–8]. Becauseof this, LIBs are dominating the market as other batter-ies NiCd and NiMH have failed to satisfy consumers’needs [9]. LIBs are the best candidates for energy stor-age used in space and aerospace applications [10]. Powersources are used to supply of energy but also to performover a wide temperature range [11]. However, as the tem-perature varies (high or low), the poor electrochemicalperformance of LIBs at low temperatures is one of themajor technical barriers to their practical applications asenergy sources [12]. When the temperature falls below−20 ◦C, both the power and the energy of LIBs havebeen reported to be significantly reduced [13].

Usually, graphite is used as the anode in LIBs due toits good cyclic performance, safety features and abun-

∗E-mail: chdoh@keri.re.kr; Tel: +82-55-280-1662

dant availability. However, graphite shows certain limi-tations when the temperature varies [14–16]. Recently,many efforts have been made to enhance the performanceof graphite anodes at low temperatures. Several factorsare responsible for the low rate capability and the poorelectrochemical performance of LIBs at low temperaturessuch as (i) the reduced conductivity of the electrolyteand solid electrolyte interphase (SEI) film, (ii) the highcharge-transfer resistance at the electrolyte-electrode in-terface, and (iii) the limited diffusivity of lithium ionswithin the graphite anode [17, 18]. Many efforts havebeen pursued to overcome such limitations. For this pur-pose, the use of metal particles (i.e., Cu, Ag, Ni, Sn)dispersed in the electrode’s slurry is useful for improvingthe low-temperature performance of the graphite anode[19–21]. Among various metals particles, Cu has beenrevealed to be one of the most productive additives forgraphite electrodes used for low-temperature LIBs. Theaddition of small amounts of metal particles to an anodeactive material has already been reported [22,23]. Mari-naro et al. [1] showed that a new conductive material,the Cu-SPB composite (copper/Super-P carbon black)prepared by using a microwave-assisted procedure, couldimprove the intercalation capacity of anodes at low tem-peratures, but whether or not the low-temperature be-havior is improved is still debatable.

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In the present paper, successful application of theCu-SPB composite with a graphite anode for low-temperature LIBs is reported. This study especiallyaimed to investigate the effects of the copper metal con-tent on the electrochemical performance and to deter-mine the optimum value for metal particles paired withan electrode bulk at which maximum capacity retentioncould be achieved. Super-P carbon is usually used as aconductive material to enhance the electronic conductiv-ity of electrodes for LIBs.

In this report, Cu-SPB was prepared by reducingCu ions from solution. This “ion reducing” procedurepresents several advantages. Copper particles are welldispersed within graphite matrix, and the procedure ischeap and easy to perform. Graphite electrodes pre-pared with the new conductive material Cu-SPB showedsignificantly improved capacity retention at temperatureranging from −20 to −32 ◦C compared to graphite an-odes prepared with pristine SPB. In order to explain theimproved capacity retention, we present and discuss thelow-temperature intercalation capacity, and the electro-chemical impedance spectroscopy measurements for Cu-SPB/graphite and SPB/graphite electrodes.

II. EXPERIMENTS

Cu particles supported by super-P carbon black (Cu-SPB) were synthesized by using an “ion reducing” pro-cedure. In a typical synthesis, 125 ml of CuSO4 5H2O(Aldrich; > 99%) 0.01-M solution in ethylene glycolwas mixed with 0.910 g of Super-P black (SPB) in abeaker placed in a heating mantle. The suspension wasstirred for 4 hours; then, 125 ml of a solution of 0.02-MNaH2PO2 H2O (Aldrich) in ethylene glycol were slowlyadded to the suspension at 100 ◦C. Ethylene glycol has aboiling point of 197.3 ◦C, so stirring was continued dur-ing the ion reducing procedure, and the reactant mixturereached a temperature of about 200 ◦C. After the sus-pension had been cooled, it was washed with acetone andcentrifuged at 3000 rpm several times. The solid prod-uct, Cu-SPB with a weight ratio of 8:92, was dried firstin air at 50 ◦C for 4 hours and later at 180 ◦C under vac-uum overnight. In a similar way, other compositions ofthe Cu-SPB composite, 12:88 and 30:70, were prepared.

Knowing the actual amount of copper present in theSPB is important. For this purpose, the actual Cu con-tent was measured by using strong acid to extract themetal from the Cu-SPB powders. Firstly, Cu-SPB pow-der was mixed with HNO3 70% diluted; then, the suspen-sion was stirred at 80 ◦C for 30 minutes. The extract wasquantitatively analyzed by using AAS (atomic absorp-tion spectroscopy) [24]. For the morphological character-ization of the samples, field-emission scanning electronmicroscope (Hitachi FE-SEM S4800), an energy disper-sive X-ray (EDX) analyzer, and an X-ray diffractometer(Philips X’pert-MPD) were used.

The electrochemical performance was evaluated by us-ing both coin-type half and coin-type full cells with astandard 2023 (diameter = 2.0 mm and height = 2.3mm) coin cell hardware. Pitch-coated natural graphite(99.95%; BTR Energy Materials Co., Ltd., China) andLiNi0.6Co0.2 Mn0.2O2 (NCM622; L&F Co., Ltd., Ko-rea) were used as active materials for the anode and thecathode, respectively. Graphite and NCM622 sampleswere adopted in this study with mean particle sizes of17.1 μm and 12.0 μm, and surface areas of 2.11 m2g−1

and 0.22 m2g−1 respectively, as determined by usingthe Brunauer-Emmett-Teller (BET) method. Cu-SPBand pristine SPB were used as conductive material forthe modified and the unmodified electrodes, respectively.Polyvinylidene fluoride (PVDF) was used as a binder,and polypropylene membrane separator (Celgard 2325,Celgard, Inc., USA) was used as a separator. The elec-trolyte was a mixture of 1-M LiPF6 ethylene carbonate(EC)/ethyl methyl carbonate (EMC)/dimethyl carbon-ate (DMC) (1/1/1 by volume).

Electrodes were prepared according to the stan-dard procedure. Modified electrodes were preparedby following the “doctor blade” technique with aslurry composition of graphite:Cu-SPB:PVDF (anode)and NCM622:SPB:PVDF (cathode) with mass ratio of90:05:05. In the same way, unmodified electrodes weresynthesized with same mass ratio except for the conduc-tive material: Cu-SPB was replaced by pristine SPB.The homogeneous slurries after magnetic stirring werecoated onto copper foil (anode) and aluminum foil (cath-ode) and dried in an oven at 100 ◦C.

In the case of coin-type half cells, lithium metal (pu-rity: 99.9%) was used as a reference and a counterelectrode with a graphite anode. The cells werecharged/discharged in the CC-CV/CC mode in the po-tential range of 0.005 − 1.5 V (coin-type half cell)/2.5 −4.2 V (coin-type full cell) at current densities that werevaried from 0.1C to 3C at room temperature (25 ◦C)and at various low temperatures from −20 to −40 ◦C byusing a multi-channel battery tester (TOYO TOSCAT-3100U).

III. RESULTS AND DISCUSSION

Three different compositions of the Cu-SPB compos-ite, A (8:92), B (12:88), and C (30:70), were synthesizedby following the “ion reducing” method. In the chemicalreaction, ethylene glycol reduced the metal ions directly.In addition sodium hypophosphite increased the reduc-tion rate. The entire chemical reaction can be explainedas follows [1]:

CH2OHCH2OH → CH3CHO + H2O, (1)2nCH3CHO + 2Mn+

→ 2M + 2nH+ + nCH3COCOCH3, (2)Cu2+ + H2PO−2 + H2O → Cu + H2PO−3 + 2H+. (3)

Effect of Copper Content in the New Conductive Material· · · – Adnan Yaqub et al. -319-

Fig. 1. (Color online) SEM images of Cu-SPB samples: (a)8:92, (b) 12:88, (c) 30:70. White spots indicate the presenceof copper metal particles in a carbon matrix. (d) EDX spectrato analyze quantitatively the copper weight percentage withinSPB.

The FE-SEM images are shown in Fig. 1 for three com-positions of Cu-SPB: (a) 8:92, (b) 12:88, and (c) 30:70.In the images, the small white spots indicate the pres-ence of copper particles in the carbon matrix. Copperparticles are well distributed within the carbon matrixpresent in SPB. The Cu particles exhibit relatively smallsizes with small presence within the SPB matrix. Thesizes of the particles in the Cu-SPB composite were de-termined by using a particle-size analyzer (PSA), whichshowed that the particles in the Cu-SPB samples hadaverage particle sizes of (A) 998, (B) 9.72, and (C) 8.38μm while pristine SPB had a mean particles size of 10.9μm. Furthermore, energy dispersive X-ray (EDX) spec-troscopy was used to confirm the presence of copper par-ticles. Figure 1(d) shows the EDX spectrum with theemission lines of copper, carbon, and a minimal amountof oxygen for Cu-SPB sample C (30:70). The EDX im-ages of other two samples are not reported for the sakeof brevity.

The structural analysis of sample presented informa-tion on the geometry of the material. The X-ray diffrac-tion pattern of the Cu-assisted SPB is presented in Fig. 2.It clearly shows a broad peak centered at about 25◦,assigned to amorphous SPB, and well-defined peaks at43.4, 50.4 and 75◦, indexed as Cu (111), Cu (200) andCu (220), respectively. From Fig. 2, clearly, the Cu-SPBcompositions with higher copper content showed higherCu peaks and lower SPB peaks.

Fig. 2. (Color online) XRD spectra of Cu-SPB samples: A(8:92), B (12:88), and C (30:70). Samples with higher coppercontents show higher Cu peaks, but lower carbon peaks.

EDX study shows that the amount of Cu measuredquantitatively and the amount theoretically added aredifferent. Cu-SPB composites A, B and C showed 6.26,10.02, and 14.65% Cu, respectively by weight% throughthe EDX analysis while the samples theoretically hadCu:SPB ratios of 8:92, 12:88, and 30:70, respectively.Therefore, the actual Cu content was measured by usingAAS to analyze the Cu extract. The study results weredifferent from the theoretically added amount and theamount determined through EDX analysis. Copper waspresent 5.91, 8.54, and 18.54% by weight% in the SPBmatrix for compositions A, B, and C, respectively. Com-parison of the data obtained from EDX, and AAS analy-ses with the theoretically added amounts can be seen inTable 1. The reason the Cu contents measured throughAAS were different from the Cu contents obtained viaEDX and via the theoretically-added amounts is as fol-lows: In the EDX analysis, a specific area of the sampleis a focus rather than the entire sample, obtaining anaccurate Cu content through EDX is difficult. If mostof the Cu content is assumed to have been extracted byacid, the extraction method is considered to be betterand more accurate than EDX.

The electrochemical performances of the graphite elec-trodes prepared with the new conductive material Cu-SPB for coin-type half cells were investigated by usingthe CC-CV/CC mode at room temperature (25 ◦C) andat −32 ◦C. Graphite electrodes with pristine SPB werealso prepared for the sake of comparison. Before theexperiment was run at low temperatures, all the coin-type half cells were pre-cycled (formation) two times ata very low C-rate of 0.1C in order to form a stable solidelectrolyte interphase (SEI) over the electrode’s surface.Later cells were tested at 0.2C under room temperaturefor 5 cycles; then, the cells were kept for 4 hours in a low-temperature environment (−32 ◦C). Low-temperatureelectrochemical testing was performed at 0.2C for 5 cy-cles.

Figure 3 shows the evolutions of the charging and dis-

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Table 1. Cu contents present in Cu-SPB composites for samples A, B, and C measured using the EDX and the AAStechniques and compared with theoretical measurements, and analysis of capacity retention achieved at −32 ◦ for a 0.2 C-ratefor a coin half and a coin full cell.

ConductiveCu-content Cu-content Cu-contents C.R. (%) C.R. (%)

Material(%) (%) (%) Coin Half Cell Coin Full Cell

Theoretical EDX AAS −32 ◦C −32 ◦C

A 8 6.26 5.91 15.64 62.02

B 12 10.02 8.54 22.38 64.29

C 30 14.65 18.54 38.79 71.30

SPB 0 - - 13.64 61.44

Fig. 3. (Color online) 1st (RT), 1st (−32 ◦C), and 5th (−32 ◦C) CC-CC/CC mode charge/discharge cycles electrochemicallytested based on copper content: (a) Cu = 5.91%, (b) Cu = 8.54%, (c) Cu = 18.54%, and (d) Cu = 0% (Pristine SPB).

charging capacities. At room temperature, the discharg-ing capacity of both types of cells (modified and unmod-ified) remained around 370 mAhg−1. Figures 3(a)−(c)show that cells prepared with the three types of Cu-SPBconductive material, A (Cu = 5.91%), B (Cu = 8.54%),and C (Cu = 18.54), retained 1st discharging capaci-ties of 57.72, 82.62, and 143.16 mAhg−1, respectively.Meanwhile, cells prepared with pristine SPB shows a1st discharging capacity of 50.35 mAhg−1 (Fig. 3(d)).

It is worth noting that at −32 ◦C, electrodes preparedwith the Cu-SPB composite had a capacity retention ofaround 15 ∼ 40% compared with a capacity retention of13.64% when using pristine SPB.

The optimum amount of metal particles in the elec-trode bulk for improving the low-temperature perfor-mance of graphite electrodes has not yet been discov-ered. In this study, improved low-temperature resultsbased on copper content are shown. Figure 4 shows a

Effect of Copper Content in the New Conductive Material· · · – Adnan Yaqub et al. -321-

Fig. 4. (Color online) Relation between copper content(%) and capacity retention (%) to evaluate the electrochemi-cal performance of LIBs at low-temperature when using con-ductive materials of types A, B, C and pristine SPB.

Fig. 5. (Color online) Nyquist plots recorded at −32 ◦Cfor Cu-modified electrodes: Cu = 5.91% (red), Cu = 8.54%(blue), Cu = 18.54% (green), and pristine SPB (black).

direct relation between the capacity retention and thecopper content. This phenomenon can be explained asfollows:

y = 1.4279x + 10.846,

where x is the copper content (%) and y is the capac-ity retention (%). With increasing copper content, thecapacity retention increased.

Copper particles played an important role in improv-ing the intercalation capacity, especially at low tempera-tures. This phenomenon was confirmed through electri-cal impedance spectroscopy (EIS) measurements. Fourtypes of coin-type half cells, three with Cu-SPB basedon different Cu contents and one with pristine SPB, wereprepared. After pre-cycling, the lithium-ion battery cellwas kept at low-temperature for 4 hours, and charged(0.005 V). Then, the impedance was checked at the stateof charge 100% (SOC). All these steps were carried out

Fig. 6. (Color online) Electrochemical testing of coin-typefull cells: 5 cycles at 0.2C @RT, 5 cycles at 0.2C @ −32 ◦C, 5cycles at 0.2C @ RT (in order to check effects on cell after low-temperature testing) for (a) a modified electrode containingCu = 18.54% and (b) an unmodified electrode containingpristine SPB.

at −32 ◦C. The low-temperature EIS response of thegraphite anode containing the Cu-SPB composite as aconductive material is compared with that of the anodecontaining pristine SPB.

An EIS analysis provides information on the bulk re-sistances (Rbulk) of the electrolyte, separator, and elec-trode. The first semi-circle high-frequency region rep-resents the SEI film resistance (Rfilm) of the electrodematerial. The second semi-circle is the medium fre-quency range, which indicates the charge-transfer resis-tance (Rct) of lithium ions at the electrode surface [25].Figure 5 shows Nyquist plots for the electrodes underinvestigation acquired at T = −32 ◦C, E = 0.15 V, andfi = 500 kHz (kHz = kilohertz) ∼ ff = 1 mHz (mHz= milihertz). Similar kinds of behaviors were observedfrom both types of electrodes. High-frequency semicir-cles, which are hard to notice due to overlapping by themain semicircle, reveal the presence of a solid electrolyteinterface. From the EIS analysis, clearly, the overallimpedance of the Cu-SPB-modified electrodes is lowerthan that of the unmodified electrode.

The EIS behavior was also observed based on the cop-per content. Electrodes prepared with conductive ma-

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Fig. 7. (Color online) Comparison of discharge capacities of coin-type full cells prepared with Cu-SPB (Cu = 18.54%) atC-rates of 0.1C. 0.2C, 0.5C, 1C, and 3C at room temperature to those of coin-type full cell prepared at (a) −20, (b) −32, and(c) −40 ◦C. (d) Effects of temperatures, 25, −20, −32, and −40 ◦C on the cell’s performance at C-rates of 0.1C. 0.2C, 0.5C,1C, and 3C.

terials that contained higher Cu contents showed lowSEI film resistance (Rfilm) and charge-transfer resis-tance (Rct) than electrodes prepared with lower Cu con-tents. The other effect that contributed to enhancedlow-temperature electrochemical behavior was the de-creased resistance of the electrolyte, separator, and elec-trode (Rbulk).

Electrodes prepared with Cu-SPB composites withhigher Cu contents showed better results than those pre-pared with Cu-SPB with lower Cu contents. Furtherexperiments with coin-type full cells by using Cu-SPBcomposition C (Cu = 18.54%) with a graphite anode anda NCM622 cathode (modified cell) were performed. Forthe sake of comparison, coin-type full cells with graphiteanodes prepared with pristine SPB and NCM622 cath-odes (unmodified cell) were also assembled. All the ex-perimental steps were the same as there for the cointypehalf cell except the voltage window, which was 2.5 ∼4.2 V. In Figs. 6(a) and (b), both types of cells showed

almost the same cell capacities (1.15 mAh) at room tem-perature. At −32 ◦C, modified cells showed a 1st dis-charging capacity of 0.819 mAh, resulting in a capacityretention of 71.30%, while unmodified cells exhibited acapacity retention of 61.44%. From Fig. 6(a), clearly, themodified cell showed stable cycling behavior and an aver-age columbic efficiency of 98% at low temperatures com-pared to the unmodified cell in Fig. 6(b). With the addi-tion of copper particles, there was an increase of 9.86%in the capacity retention, which once again confirmedthe important role of metal particles in enhancing thelow-temperature performance.

Rate characteristic test was performed under varioustemperatures on a coin-type full cell prepared with Cu-SPB (Cu = 18.54%) as a conductive material. Figure7 shows the average 1st discharge capacity of cells atC-rates of 0.1C, 0.2C, 0.5C, 1C, and 3C under temper-atures of 25, −20, −32, and −40 ◦C. Final results areimproved with the new conductive material, especially

Effect of Copper Content in the New Conductive Material· · · – Adnan Yaqub et al. -323-

Table 2. Comparison of 1st average discharge capacitiesof coin-type full cells electrochemically tested at C-rates of0.1C, 0.2C, 0.5C, 1C, and 3C under temperatures of 25, −20,−32, and −40.

Capacity Capacity Capacity

C-rates Retention (%) Retention (%) Retention (%)

−20 ◦C −32 ◦C −40 ◦C

0.1C 86.76 74.76 69.78

0.2C 81.27 71.30 59.12

0.5C 75.18 59.89 20.78

1C 70.18 25.92 13.05

3C 22.67 1.003 0.8081

at low temperatures.Table 2 presents data on the capacity retentions

achieved at different low temperatures. At −20, −32,and −40 ◦C, more than a 50% of capacity retention wasachieved at 1C or lower C-rates, 0.5C or lower C-rates,0.2C or lower C-rates respectively. Based on these re-sults, we can predict that there is no harm in applying1C or lower C-rates for electrochemical testing of low-temperature LIBs. Higher C-rates than 1C can causedeterioration and capacity loss.

IV. CONCLUSION

Cu-SPB, a new conductive material for graphite an-odes for low-temperature LIBs, has been prepared byusing an ion-reducing method. White spots in the SEMimages confirm the presence of copper particles well dis-persed within carbon matrix. Electrodes prepared withhigher-copper-content Cu-SPB showed significantly im-proved results in terms of capacity retention (around40%), cycling performance and columbic efficiency, es-pecially at low temperatures. The effects of copper con-tent can be justified from the EIS behavior. Higher Cucontent helps in reducing the Rbulk, Rfilm and Rct ofa coin-type half cell. Results were also confirmed for acoin-type full cell. An increase of 9.86% in capacity re-tention is seen in the modified cells compared with theunmodified cells. A Cu/SPB composite with a high Cucontent can enhance the performance of LIBs at temper-atures below −32 ◦C, even at high C-rates.

ACKNOWLEDGMENTS

This work was supported by the Next Generation Mil-itary Battery Research Center Program of The Defense

Acquisition Program Administration and the Agency forDefense Development.

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