Electrochemical properties of LiCoO2 + x% S mixture as anode material for alkaline secondary battery

Electrochimica Acta 85 (2012) 352–357

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Electrochemical properties of LiCoO2 + x% S mixture as anode material for alkaline secondary battery Yanan Xu a , Dawei Song b , Jia Li c , Li Li a , Cuihua An a , Yijing Wang a,∗ , Lifang Jiao a , Huatang Yuan a a b c

Institute of New Energy Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Nankai University, Tianjin 300071, PR China School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, PR China School of Electronic and Information Engineering, Tianjin Vocational Institute,Tianjin 300192, PR China

a r t i c l e

i n f o

Article history: Received 7 June 2012 Received in revised form 7 August 2012 Accepted 17 August 2012 Available online 30 August 2012 Keywords: Lithium cobalt oxide Hydrothermal reaction Alkaline secondary battery Anode material

a b s t r a c t LiCoO2 is synthesized via a hydrothermal reaction of cobalt salt, LiOH·H2 O and suitable amount of H2 O2 . A series of LiCoO2 + x% S mixtures are prepared by simply mixing LiCoO2 and S powder with different LiCoO2 /S weight ratios, and investigated as anode material for alkaline secondary battery. At the discharge current density of 500 mA g−1 , LiCoO2 + 5% S mixture electrode displays the maximum discharge capacity of 320 mAh g−1 . Meanwhile, the LiCoO2 + 10% S mixture electrode shows the most outstanding cycle performance with a capacity retention rate of over 94% after 150th charge–discharge cycles. Moreover, the charge–discharge reaction mechanism of LiCoO2 + x% S mixture electrodes is also investigated. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Alkaline secondary batteries [1,2] play an important role in the field of electric energy storing devices for several decades. A multitude of researches have contributed to make alkaline rechargeable electrochemical energy storing systems viable for a larger market, including nickel hydride battery, nickel cadmium battery, nickel zinc battery and nickel iron battery [3–6]. Among them, the nickel/metal hydride (Ni/MH) batteries are widely used owing to their outstanding features of high power and energy density, environmental issue and safety [7], but hardly satisfy the need of the society. To meet the ever-increasing demands for application in electric tools, such as electric vehicles (EV) and hybrid electric vehicles (HEV), there is a very urgent need to develop the new rechargeable batteries with high power and energy density. Gao et al. systematically proposed a new type Ni/Co batteries system for the first time in 2009 [9], which employed Ni(OH)2 as cathode material and Co-based materials as anode material in KOH aqueous solution. Some cobalt-containing materials, such as Co(OH)2 , Co–S, Co, Co3 O4 , have been reported with excellent performance as anode materials for alkaline secondary battery [8–11]. As an important class of cobalt-containing materials, lithium cobalt oxide is a versatile material with industrial applications in lithiumion batteries. LiCoO2 has the structure of layered rock-salt shape and the advantages of high electronic conductivity, good rate

∗ Corresponding author. Tel.: +86 22 23503639; fax: +86 22 23503639. E-mail address: [email protected] (Y. Wang). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.092

capability, easy preparation and excellent cycling performance [12,13]. Therefore, we speculate that LiCoO2 have the possibility to be a promising anode materials for alkaline secondary battery, and the reports on this area are few. There are two typical synthesis methods of LiCoO2 : a solid state method and a solution method [14–16]. The solid state method has been used extensively in industrial production, which may bring about some drawbacks such as broader particle size distribution, higher calcination temperature and longer reaction time. The common solution methods, including sol–gel method [17] and hydrothermal method [18–20], allow lower reaction temperature and shorter reaction time. Additionally, the resulting powders also show good stoichiometric control and narrow particle size distribution. In this paper, LiCoO2 is synthesized via the hydrothermal reaction and LiCoO2 + x% S mixtures are prepared by simply mixing of as-prepared LiCoO2 and S powder. The electrochemical properties of LiCoO2 + x% S mixtures are systematically investigated as anode material for alkaline secondary battery. Moreover, the possible electrochemical reaction mechanism of the LiCoO2 + x% S mixture electrode is also investigated. 2. Experiment 2.1. Preparation All chemicals are of analytical grade and used without further purification. In a typical process, 5 mmol Co(NO3 )2 ·6H2 O (Sigma–Aldrich, 98%) and 5 mmol LiOH·H2 O are dissolved in a

700 C

60

70

o

Intensity (a.u.)

650 C JCPDS 50-653 o

600 C o

550 C &

JCPDS 44-145 o

400 C 10

&

20

&

30 40 50 2-Theta (degree)

b

2.2. Compositional and structural characterization

-1 Weight,%

-2

60

1

-3

2

-4 40

3. Results and discussion 3.1. Material characterization X-ray diffraction (XRD) pattern of as-prepared LiCoO2 at different temperature is illustrated in Fig. 1(a). All the diffraction peaks from 550 ◦ C to 700 ◦ C can be indexed to hexagonal LiCoO2 (Space ˚ b = 2.82 A, ˚ c = 13.89 A). ˚ No peaks of other Group: R-3m; a = 2.82 A, impurities are observed, suggesting high purity and good crystallization of the product. As can be seen, the pure LiCoO2 can be obtained above 550 ◦ C, which is consistent with the result of TG experimental test in Fig. 1(b). The intensity of all the peaks strengthens with the extension of temperature, suggesting that the high temperature does favor the crystallization of the LiCoO2 phase. Curiously, the peak at 2 = 66◦ changes into two peaks when temperature rises above 650 ◦ C, meanwhile the standard card change from JCPDS 44-145 (rhombohedral, Space Group: R-3m) to JCPDS 50-653 (rhombohedral, Space Group: R-3m) as reported in other literatures [21]. The TG curve in Fig. 1(b) shows a three-step decomposition pattern in the calcined process. The first weight loss at approximately 185 ◦ C is due to the H2 O loss, then further weight loss (endothermic reaction about 280 ◦ C) may be due to dissociation of nitrate.

DTG (%/min)

80

2.3. Electrochemical measurements Negative electrodes are fabricated with the smearing method. They are constructed by mixing as-prepared material with carbonyl nickel powders and PTFE in a weight ratio of 32:64:4 to form a paste and coated on a piece of Ni-foam. Electrochemical measurements are conducted in a three compartment cell using a Land battery test instrument (CT2001A). The NiOOH/Ni(OH)2 electrode and Hg/HgO electrode in a 6 M KOH aqueous solution are served as the counter electrode and the reference electrode, respectively. The electrodes are charged at 500 mA g−1 for 80 min and discharged to −0.5 V (vs. Hg/HgO) at the different discharge current density (100, 200, 500, 1000 mA g−1 ) after resting for 5 min. Zahner IM6e electrochemical workstation is used for cyclic voltammetry (scan rate: 0.2 mV s−1 ; potential interval: −1.3 V to −0.4 V vs. Hg/HgO). All the tests are performed at room temperature.

80

0

100

The crystal structure and surface configuration of the materials are characterized by X-ray diffraction (XRD, Rigaku MiniFlex II with Cu K␣ radiation), scanning electron microscopy (SEM, SUPRA 55VP Field Emission), transmission electron microscopy (TEM, JEM-2010FEF), X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe), thermogravimetric analysis (TG, NETZSCH TG209).

Co3O4

(113)

o

& CoOOH

(018) (110) (107)

LiCoO2

(104)

a

(101)

beaker with 65 mL deionized water. Then about 6 ml 30% H2 O2 is slowly added to above solution under magnetic stirring to form a homogeneous solution. The mixed solution is transferred to a Teflon-lined stainless steel autoclave (100 mL capacity) with 80% capacity of the total volume and heated at 180 ◦ C for 48 h. After the bomb is cooled down to room temperature, the product is dried at 80 ◦ C in order to ensure the evaporation of all the free water and avoid the loss of lithium ion. And the precursors are heating to the different temperature (500 ◦ C, 550 ◦ C, 600 ◦ C, 650 ◦ C and 700 ◦ C) with a heating rate of 5 ◦ C min−1 for 3 h in tube furnace to form pure LiCoO2 and remove impurities, such as nitrate and crystal water. A series of LiCoO2 + x% S mixtures (x = 0, 2, 5, 10) are prepared by simply mixing of as-prepared LiCoO2 and S powder. That is, the test materials are obtained by physical mixing as-prepared LiCoO2 with S powder under different mass ratios to form uniform powder mixture.

353

(003)

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-5

3

100

200

300

400

500

600

700

800

o

Temperature, C Fig. 1. XRD pattern of LiCoO2 calcined at different temperature (a) and TG curve (b) of the precursors via hydrothermal synthesis.

As we all know, the decomposition products of transition metallic nitrate include volatile NO2 and non-volatile metallic oxide. This could explain why the XRD pattern of the product at 400 ◦ C exists the weak peaks of Co3 O4 in Fig. 1(a). However, with the temperature further increase there is a predominant peak at around 435 ◦ C and it may be due to conversion of CoOOH and Co3 O4 to LiCoO2 with a amount of H2 O loss. This could explain why the pure LiCoO2 can obtain above calcining temperature of 550 ◦ C in Fig. 1(a). Therefore, we believe that the products will undergo similar thermo-chemical decomposition and conversion process in the calcined process. From the scanning electron microscopy (SEM) images in Fig. 2, LiCoO2 shows different morphologies and sizes at variable calcined temperatures. It can be seen that all the samples display the nanometer level particle sizes and irregular hexagonal rock structure with slight agglomeration phenomenon. The size and thickness of LiCoO2 nanoparticle increase with the extension of calcined temperature. TEM image (Fig. 2(d)) evidences the presence of hexagonal platelet-like rock salt structure, which is in good agreement with the results in other papers [18–20]. 3.2. Electrochemical performance The cycle performances of LiCoO2 + x% S mixture electrodes at a high current rate of 500 mA g−1 are shown in Fig. 3. It is observed that all the LiCoO2 electrodes allow an electrochemical activation process before their intrinsic capabilities are realized. Notably, the LiCoO2 + x% S electrodes show a higher discharge capacity and a better cycle performance, as compared with the LiCoO2 electrode. Among these electrodes, the LiCoO2 + 5% S mixture electrode exhibits the maximum discharge capacity of 320 mAh g−1 . Meanwhile, the LiCoO2 + 10% S mixture electrode shows the most

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Fig. 2. SEM images of LiCoO2 ((a) 550 ◦ C, (b) 600 ◦ C, (c) 650 ◦ C) and TEM image of LiCoO2 at 650 ◦ C (d).

outstanding cycle performance with a capacity retention rate of over 94% after 150 cycles. One reason for capacity fading might be the dropping and corrosion of active material in alkaline solution during long circulation process. The other one may be caused by the volume change of electrode slices in the electrochemical reaction process. To the best of our knowledge, the discharge capacity of S powder electrode is nearly zero (Fig. 3), but the LiCoO2 + x% S mixture electrodes exhibit higher discharge capacity than the LiCoO2 electrode, this is an interesting phenomenon. As can be seen from the diagram, discharge capacity increases with the increasing amount of S powder. Nevertheless, the well doped LiCoO2 + 5% S electrode displays the maximum discharge capacity, and the capacity of electrode decreases when the adding amount of S increase to 10%. The contributing factor could be too much S powder doping in the electrode affect the electrochemical capacity of LiCoO2 + x% S mixture electrodes. To investigate the kinetic properties of the LiCoO2 electrode, the rate capability and cycling ability at different discharge current densities are measured as shown in Fig. 4. At discharge current density of 500 mA g−1 , the LiCoO2 electrode displays a low capacity

of about 230 mAh g−1 and it is greatly improved to 320 mAh g−1 with the doping of 5% S. The LiCoO2 + 5% S mixture electrode displays the maximum discharge capacity of 342 mAh g−1 at the discharge current density of 200 mA g−1 . It is worth noted that the LiCoO2 + 5% S electrode still displays a better cycle performance even at the high-rate discharge current density of 1000 mA g−1 . Noticeably, the activation processes of LiCoO2 electrode have been efficiently shortened with the existence and dissolution of S powder in charge–discharge process. Charge–discharge curves of the LiCoO2 electrode at the discharge current density of 500 mA g−1 are illustrated in Fig. 5(a), which is similar to other Co-based materials reported before [8–11,22]. The discharge curves present a flat potential plateau around −0.78 V (vs. Hg/HgO) and it is in good agreement with that of Co(OH)2 electrode in Fig. 6. There are two potential plateaus in the charge curves. With the cycle number increasing, the first charge potential plateaus shift positively while the second ones shift negatively. After reaching the steady state, the first charge potential plateau appear at around −0.88 V, which is attributed to the transformation from Co(OH)2 to Co [8]. And the second ones

400

-1

200 S electrode LiCoO2 LiCoO2+2% S

100

LiCoO2+5% S LiCoO2+10% S

0 0

50

100

150

Cycle number (n) Fig. 3. Cycle performances of LiCoO2 + x% S mixture electrodes (discharge current density of 500 mA g−1 ).

300

200

a Discharge capacity (mAh g )

Discharge capacity(mAh g )

300

-1

Discharge capacity (mAh g )

b

100

0

0

400

-1

300 LiCoO2+5% S electrode 200 100 0 0

1000 mA g 50 100 150 Cycle number (n)

-1

100 mA g -1 500 mA g -1 200 mA g -1 500 mA g

200

50 Cycle number(n)

100

Fig. 4. Cycle life of the LiCoO2 electrode (a) and the LiCoO2 + 5% S mixture electrode (b) at the different discharge current density.

Y. Xu et al. / Electrochimica Acta 85 (2012) 352–357

1.2

1.0

1 st

1 st 2 nd 3 rd 4 th 24 th

24 th

0.8

LiCoO2

S

Co(OH)2

Co

Ni

Discharged (100th cycle)

Intensity (a.u.)

-Potential (V vs.Hg/HgO)

a

355

Charged (100th cycle) Discharged (2nd cycle)

Charged (2nd cycle)

0.6

Original LiCoO2 + S

0

100

200

300 -1

Discharge capacity (mAh g )

10

20

30

40 50 60 2-Theta (degree)

70

80

Fig. 7. XRD patterns of the LiCoO2 + 5% S electrode at different cycles.

1.2

-Potential (V vs.Hg/HgO)

b

1.0 1 st 2 nd 3 rd 4 th 24 th

24 th 1 st

0.8

0.6

0

100

200

300

electrode presents a long and flat discharging plateau in the initial cycles and shows high discharge capacity, compared with the undoped LiCoO2 electrode. And it is consistent with the shortening of activation course after S-doping as mentioned in Fig. 4. The charge curves also display two potential plateaus, the first one negatively shifts to −0.93 V (vs. Hg/HgO) and the second one appears at around −1.1 V (vs. Hg/HgO) equally. It might be that the doping S powder conduces the higher charge plateaus and lower discharge plateaus in the electrochemical process, which is in good agreement with Co(OH)2 mixing with S [23]. 3.3. Electrochemical reaction mechanism

-1

Discharge capacity (mAh g ) Fig. 5. Charge–discharge curves of the LiCoO2 electrode (a) and the LiCoO2 + 5% S mixture electrode (b) at different cycles.

-Potential (V vs.Hg/HgO)

appear at around −1.1 V only correspond to electrolysis reaction of water and keeps practically constant in the later cyclic process. Fig. 5(b) displays the charge–discharge curves of the LiCoO2 + 5% S mixture electrode at the discharge current density of 500 mA g−1 . The potential plateaus of the discharge curves keep around −0.77 V (vs. Hg/HgO) at the steady-state process, which is also attributed to the transformation from Co to Co(OH)2 and accorded with experimental results reported in literatures [8,9,21,22]. It also can be confirmed from discharge curve in Fig. 6. The LiCoO2 + 5% S

0.8

0.7

the undoped LiCoO2 LiCoO2+ 5% S Co(OH)2

0.6

0

100

200

300 -1

Discharge capacity (mAh g ) Fig. 6. Discharge curves of the LiCoO2 and Co(OH)2 electrode at the discharge current density of 100 mA g−1 .

In order to clearly understand the reaction mechanism of the LiCoO2 + x% S mixture in this experiment, the XRD patterns and CV curves of the LiCoO2 + 5% S electrode at different cycles are carried out. And the X-ray photoelectron spectra of Co 2p and S 2p from the LiCoO2 + 5% S sample after 100th charge–discharge process is also conducted. To investigate the structure change in charge–discharge process, XRD patterns of LiCoO2 + 5% S electrode at different cycles are compared in Fig. 7. All peaks of initial electrode can be attributed to LiCoO2 and S powder. At the fully charged state of the 2nd cycle, the diffraction peaks of LiCoO2 become weak and the peaks of Co(OH)2 and Co appear, illustrating that some LiCoO2 transforms into Co(OH)2 and Co during the activation process. When discharged to −0.5 V, the diffraction peaks of Co(OH)2 strengthen and those of Co weaken, illustrating that metallic Co is oxidated to Co(OH)2 during the discharge process. After the 100th charged, only diffraction peaks of Co and Co(OH)2 can be observed, demonstrating that LiCoO2 has completely transformed into Co and Co(OH)2 . After the 100th discharged, the diffraction peaks can be assigned to Co(OH)2 except a small amount of Co. It can be concluded that Co(OH)2 is reduced to metallic Co during the charge process, and metallic Co is oxidated to Co(OH)2 during the discharge process. The result is fairly accorded with experimental results reported in other reports [8,23]. With the reaction going on, the diffraction peaks of S powder disappear basically. So it is inferred that most of S powder is dissolved in KOH aqueous solution during the charge–discharge process. Cyclic voltammetry (CV) curves of the LiCoO2 + 5% S mixture electrode at the 4th and 100th cycles are shown in Fig. 8. A pair of obvious redox peaks are detected, indicating that the reversible capacity is mainly based on the faradaic redox electrochemical mechanism. The potential locations for the observed current peaks are far from the equilibrium potentials of S, so it is unreasonable

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(x 10 )

0.2

a

100th

Co 2p3/2

4th

Intensity (CPS)

Current (A)

24

0.0

-0.2

-0.4

780.5 eV

LiCoO2+5% S electrode

Co 2p3/2

20

780.7 eV

LiCoO2 electrode

16

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

790

Potential (V vs.Hg/HgO)

785

780

775

770

Binding Energy (eV)

Fig. 8. Cyclic voltammetry (CV) of the LiCoO2 + 5% S mixture electrode at a scan rate of 0.2 mV s−1 .

2

(x10 )

Charge :

LiCoO2 + 2H2 O + e → Co(OH)2 + 2OH− + Li+

Co(OH)2 + 2e → Co + 2OH− Discharge :

−

Co + 2OH → Co(OH)2 + 2e

(1) (2) (3)

After activated process, charge–discharge reaction occurring on the electrode transforms into the reversible redox reaction: Co + 2OH−

discharge



charge

Co(OH)2 + 2e

(4)

In the initial several charge–discharge cycles, LiCoO2 completely transforms into Co and Co(OH)2 and the electrochemical reaction occurring on the electrode is only reversible conversion

b

S 2p (166.8 eV)

4.4

Intensity (CPS)

to assign redox peaks to a simple electrochemical reduction and oxidation of S. The curve shape and peak voltage of the 100th cycle are very similar to those of metallic Co powder electrode [22], so the reduction peak is attributed to the reduction from Co(OH)2 to metallic Co and the oxidation peak is assigned to the oxidation from metallic Co to Co(OH)2 . However, two reduction peaks are existed in the CV curves of 4th cycle, which means two reduction reactions occurred in the charge process. One of reduction peaks (about −1.05 V (vs. Hg/HgO)) could correspond with the reduction reaction of Co(OH)2 to Co, while the other one (about −0.98 V (vs. Hg/HgO)) could belong to the transform from LiCoO2 to Co(OH)2 . The discharge capacity of the electrode after activation process is mainly attributed to the electrochemical redox reaction between Co and Co(OH)2 . XPS measurement in Fig. 9(a) is carried out on the peaks of Co 2p from LiCoO2 sample after the 100th charge–discharge cycle. The binding energy of Co 2p3/2 obtained from the LiCoO2 electrode in the present study are 780.7 eV, existing in the form of Co(OH)2 during the cyclic process [24,25]. The slightly peaks shifting of Co 2p after S-doping is not significant, showing that Co(OH)2 also exists during the discharge process in the LiCoO2 + 5% S electrode. This is in line with the results via cyclic voltammetry test in Fig. 8. From Fig. 9(b), the binding energy of S 2p is 166.8 eV, showing that S element still exists in 0 oxidation state inside the electrode. It demonstrates that the S powder does not participate in the electrode redox reaction. Combining with phenomena that diffraction peak of S disappears basically in the XRD curves, we infer that dissolution of S powder increases the contact area between alkaline solution and active material, improving the electrochemical properties of the LiCoO2 electrode as mentioned earlier. Through above discussion, it can be concluded that the charge–discharge reaction occurring on the electrode in the initial cycles is as follows:

LiCoO2+5% S

4.0

3.6

170

165

160

155

Binding Energy (eV) Fig. 9. The XPS spectrum of Co 2p from the LiCoO2 electrode and the LiCoO2 + 5% S electrode (a) and S 2p from the LiCoO2 + 5% S electrode (b) after 100th charge–discharge cycle.

between Co and Co(OH)2 at cyclic steady-state process. According to this redox reaction, utilization of Co material is one of the most important factors influencing the discharge capacity of LiCoO2 electrode, which is largely dependent on the contact area between active material and alkaline aqueous. The electrochemical properties of LiCoO2 material can be greatly improved by doping S powder. This is because S powder dissolving in KOH aqueous solution increases the interspace in the electrode slices and the contact area between the active material and the KOH aqueous solution, further improving the utilization of this Co-based material. 4. Conclusion In summary, LiCoO2 synthesized via hydrothermal reaction is investigated as anode material for alkaline secondary battery. The electrochemical properties of LiCoO2 materials can be greatly enhanced with doping S powder. Most of reversible discharge capacity can be attributed to the redox reaction between Co and Co(OH)2 . Significantly, the doping S powder increases the contact area between alkaline solution and active material but does not participate in the redox reaction. The LiCoO2 + 5% S mixture electrode shows the maximum discharge capacity of 320 mAh g−1 . And the LiCoO2 + 10% S mixture electrode exhibits the most outstanding cycle performance with a capacity retention rate over 94% after 150 cycles. With the increasing consumption in lithium ion battery, spent LiCoO2 from lithium ion battery has become a significant source of the environmental pollution. A new way was put forward

Y. Xu et al. / Electrochimica Acta 85 (2012) 352–357

to recycle the waste lithium ion battery if the feasibility of LiCoO2 used for the new type Ni/Co batteries could be proved. Further detailed researches are still underway. Acknowledgments This work was financially supported by 973 Project (2011CB935900), NSFC (50971071, 51071087), 111 Project (B12015), Nature Science Foundation of Tianjin (11JCYBJC07700, 10SYSYJC27600), KLAEMC-OP201101. References [1] A.K. Shuklaa, S. Venugopalan, B. Hariprakasha, Journal of Power Sources 100 (2001) 125. [2] U. Köhler, C. Antonius, P. Bäuerlein, Journal of Power Sources 127 (2004) 45. [3] S.R. Ovshinsky, M.A. Fetcenko, J. Ross, Science 260 (1993) 176. [4] X.Y. Xiong, H. Vander Poorten, M. Crappe, Electrochimica Acta 41 (1996) 1267. [5] G. Bronoel, A. Millot, N. Tassin, Journal of Power Sources 34 (1991) 243. [6] K. Vijayamohanan, T.S. Balasubramanian, A.K. Shukla, Journal of Power Sources 34 (1991) 269. [7] M.L. Soria, J. Chacón, J.C. Hernández, Journal of Power Sources 102 (2001) 97. [8] L. Li, Y.P. Wang, Y.J. Wang, Y. Han, F.Y. Qiu, G. Liu, C. Yan, D.W. Song, L.F. Jiao, H.T. Yuan, Journal of Power Sources 196 (2011) 10758.

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