Synthesis mechanism of lithium nickel oxide using hydrothermal–electrochemical method: Thermodynamic modelling and experimental verification

ARTICLE IN PRESS

Physica B 362 (2005) 76–81 www.elsevier.com/locate/physb

Synthesis mechanism of lithium nickel oxide using hydrothermal–electrochemical method: Thermodynamic modelling and experimental verification Ying Taoa,, Zhenhua Chenb, Baojun Zhub a

Institute of Materials Science and Engineering, Central South University, Changsha 410083, China b Institute of Materials Science and Engineering, HunanUniversity, Changsha 410082, China Received 3 August 2004; received in revised form 30 January 2005; accepted 30 January 2005

Abstract Potential–pOH diagrams of nickel are drawn at various temperatures to predict the reaction of nickel in a 4 M lithium hydroxide solution. Based on these diagrams, the thermodynamic stability of each constituent of nickel in 4 M LiOH solution at various temperatures is evaluated. The oxidation mechanism is studied based on the thermodynamic analysis and the oxidation proceeds in the following order: Ni, Ni(OH)2 or HNiO
2 , NiOOHdH2O, NiOOH, LiNiO2. The thermodynamic model is validated experimentally by the cyclic voltammogram method. r 2005 Elsevier B.V. All rights reserved. PACS: 12.38.Qk; 51.30.+i; 82.45.Aa Keywords: Lithium ion battery; Lithium nickel oxide; Potential–pOH diagram; Thermodynamic stability

1. Introduction LiNiO2 is an attractive cathode candidate because of the relatively abundant natural resources of nickel and its high potentialities in the possible use as insertion electrodes in 4 V rechargeable lithium batteries. Besides, the compound is environmentally benign [1–3]. Nevertheless, it is Corresponding author. Tel.: +07318830420;

fax: +07318876692. E-mail address: [email protected] (Y. Tao).

difficult to synthesize LiNiO2 with satisfactory electrochemical properties [4], which are strongly dependent on the stoichiometry, crystal structure and cation disorder [5–7]. In view of the potential importance of LiNiO2 as a cathode material for lithium-ion batteries, it is valuable to optimize the synthesis conditions. LiNiO2 is usually prepared by solid-state reaction and sol–gel. It is known that a high temperature (above 700 1C) treatment of the sample involved in these methods leads to nonstoichiometry in Li1
xNi1+xO2 (x40) and its cyclic ability is very poor [8–10].

0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.477

ARTICLE IN PRESS Y. Tao et al. / Physica B 362 (2005) 76–81

So an attempt to develop an alternative synthetic route to produce the desired cathode materials in an economical, less energy and material consuming and environmentally friendly way is in progress [11,12]. The hydrothermal–electrochemical method is a suitable choice. By using the electrochemical–hydrothermal approach, crystallized and electrochemically active LiNiO2 films can be effectively prepared in a single synthetic step from nickel metal plates [13,14]. The formation mechanism of lithium nickel oxide prepared by the electrochemical–hydrothermal method including the thermodynamic phase equilibria for the formation of lithium nickel oxide in alkaline has not yet been studied. However, a successful electrochemical–hydrothermal method where both phase and morphology are controlled, requires the optimization of processing parameters that include including temperature, precursor concentrations, current and pH. In all cases, LiOH is used as a pH-adjusting agent and a reagent to induce the formation of LiNiO2. There are some novel features that differ from the present technology in our research. The thermodynamic model has firstly been used to predict processing conditions for the synthesis of the phase-pure LiNiO2 films in lithium hydroxide

77

solution. A series of thermodynamic calculations are made to investigate the optimum LiNiO2 synthesis conditions in the Ni–LiOH–H2O system. The thermodynamic model is validated experimentally by cyclic voltammogram method. Also, possible formation mechanisms of hexagonal LiNiO2 are proposed in the Ni–LiOH–H2O system.

2. Theoretical consideration and evaluation for thermodynamic stability The details of the construction of potential–pOH diagrams are described in the previous paper [15]. The thermodynamic data of chemical species required for drawing potential–pOH diagrams in Ni–H2O systems are summarized in Table 1 [16–18]. In the
table, the standard Gibbs energy

of formation DG 0f ;298:15 and absolute entropy  0  S 298:15 are mainly quoted from the Handbook   of Chemistry and Physics, while heat capacity C p at constant pressure is quoted from the JANAF Thermochemical Table [18]. Gibbs energy of chemical species at given
temperature DG 0f ;T is derived from the following

Table 1 Thermodynamic data of chemical species required for drawing potential–pOH diagrams in Ni–LiOH–H2O system Species

State

  DG0f ;298:15 kJ mol
1

  S0298:15 ðabsÞ J mol
1 K
1

  C p J mol
1 K
1 A

H2O H2 O2 H+ OH
Li+ Ni Ni(OH)2 NiO NiOOH NiOOH H2O Ni2+ HNiO
2 LiNiO2

l g g aq aq aq c c c c c aq aq aq


237.18 0 0 0
157.29
293.8 0
459.1
211.7
321.7
564.4
45.6
349.2
561.4

69.91 130.58 205.03
5 10.2 9.2 29.9 87.9 38 66.98 150.6
170.7 62.8 34.91

B

C
2

70.699 4.228  10 27.28 3.264  10
3 29.957 4.14  10
3 Cation Anion Cation 16.987 2.946  10
2 18.106
3.686  10
2 34.25 5.267  10
2 30.186
3.523  10
2 87.388
8.284  10
2 Cation Acid oxyanion 121.21
0.18786


6.903  105 5.028  104
1.674  105

1.172  105
5.005  105 1.423  105 8.661  105

1.663  106

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way using DG 0f ;298:15 ;S 0298:15 and Cp: Z T DG 0f ;T ¼ DG0f ;298:15 þ C P dT 298:15 Z T ðC P =TÞ dT
DT S0298:15 ,
T

3. Results and discussion

ð1Þ

298:15

where T represents absolute temperature and DT ¼ T
298:15: Cp at given temperature is calculated from the following equation: C P ¼ A þ BT þ CT
2 ,

(2)

where A, B and C represent constants calculated using the Cp values at different temperature. The reaction equations of chemical species with Li+ and OH
in Ni–LiOH–H2O system are shown in Table 2. The changes of Gibbs energy at various temperatures for each reaction equation in Table 2 are calculated using a personal computer. Based on these values, the relationship between potential and pOH is evaluated and the potential–pOH diagrams in Ni–H2O systems are drawn at 100 and 150 1C. Based on this information, the thermodynamic stabilities of nickel constituents are evaluated in 4 M LiOH solutions at various temperatures.

Table 2 Reaction equations of chemical species in Ni–LiOH–H2O system

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

8

7

9

(b)

0.4 Ni(OH)2(NiO)

4

5

0.0 -

HNiO2

2

– 0.4

(a) (– 4)

– 0.8

Ni

1

3 (– 6)

– 1.2 –2

0

2

4

pOH Fig. 1. Potential–pOH diagrams in the Ni–H2O system at 100 1C.

1.2

H2O+e-OH
+H(g) O2+2H2O+4e-4OH
Ni+2OH
-Ni(OH)2+2e Ni+2OH
-NiO+H2O+2e Ni+3OH
-HNiO
2 +H2O+2e NiO+H2O-Ni(OH)2 Ni(OH)2+OH
-HNiO
2 +H2O NiO+OH
-HNiO
2 Ni(OH)2+OH
-NiOOH+H2O+e NiO+OH
-NiOOH+e HNiO
2 -NiOOH+e Ni(OH)2+OH
-NiOOHdH2O+e NiO+OH
+H2O-NiOOHdH2O+e HNiO
2 +H2O-NiOOHdH2O+e NiOOH+H2O-NiOOHdH2O NiOOH+Li++OH
-LiNiO2+H2O Ni+Li++4OH
-LiNiO2+2H2O+3e

NiOOH H2O(NiOOH)

0.8

NiOOH

0.8

NiOOH·H2O (b)

8

9

0.4 E/(V)

(a) (b)

Figs. 1 and 2 show the potential–pOH diagrams in Ni–H2O systems at 100 and 150 1C, respectively. As can be seen from these figures, the numbers on the lines represent the reaction equations given in Table 2. The red lines (a) and (b) show the equilibrium potentials for hydrogen and oxygen evolution reactions in Table 2, respectively. The small numbers between 0 and
6 show the logarithms of the activity of ionic species, and

E/(V)

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7

0.0 5

– 0.4

HNiO

Ni(OH)2

-

(a)

2

– 0.8 – 1.2

1 (– 6) (– 4)

– 1.6 –2

2

Ni

0

2

4

pOH Fig. 2. Potential–pOH diagrams in the Ni–H2O system at 150 1C.

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in LiOH solution [21]. In addition, the equilibrium potential for Ni(OH)2/NiOOHdH2O couple in 4 M LiOH solution is thought to more easily shift toward the negative direction at 150 1C than at 100 1C. In LiOH solution, there is a possibility of a Li+/H+ exchange to obtain LiNiO2 from bNiOOH [22]. According to the results of thermodynamic calculation, the reaction (Eq. (14) in Table 2) occurs in the left range of lines as shown in Fig. 3. To sum up, the oxidation proceeds in the following order: Ni, Ni(OH)2 or HNiO
2, NiOOHdH2O, NiOOH, LiNiO2. Cyclic voltammetry measurements are applied to verify the process of reaction. The cyclic voltammetry measurements are made in a conventional three-electrode cell using nickel as the work electrode, Pt as an auxiliary electrode, and Ag/ AgCl as the reference electrode which are immersed in a 4 M LiOH solution at 100 1C. The apparent surface area of the working electrode is 1 cm2. Fig. 4 shows a cyclic voltammogram of Ni in 4 M LiOH solution. Starting at an open-circuit potential of about 0.05 V versus Ag/AgCl, potentiodynamic scans were made for potentials as positive as 1.15 V. Prior to the scan, Ni was oxidized to Ni(OH)2 or HNiO
2 on the basis of previous work by others [23]. This assignment is consistent with the phase indicated in the Pourbaix 1.2

1.1 100°C

E/(V)

the activity of the 10
6 M line is selected for evaluating the equilibrium line between the soluble and insoluble species on the diagrams. The pOH values of 4 M LiOH solution, which is frequently used as an electrolyte in the fabrication of LiNiO2 film by the electrochemical–hydrothermal method, are –0.88 and –0.77 at 100 and 150 1C, respectively. At 100 1C and pOH ¼
0:88; the equilibrium potentials for Ni/Ni(OH)2 and Ni/HNiO
2 (at the activity of 10
6 M)couples (Eqs. (1) and (3)) are
0.834 and
0.951 V versus NHE, respectively. At 150 1C and pOH ¼
0:77; the equilibrium potentials are 0.850 and 1.024 V, respectively. The potentials become more negative at relatively high temperatures. At 100 1C and pOH ¼
0:88; metallic Ni dissolves as HNiO
2 ion, whose concentration is more than 10
6 M. This is because the equilibrium
6 potential for Ni/HNiO
M) 2 (at the activity of 10 couple (Eq. (3) in Table 2) in 4 M LiOH solution is
0.951V versus NHE, and the potential is more negative than the equilibrium potential for the H2/H2O couple (
0.900 V), as seen from Fig. 1. At 150 1C and pOH ¼
0:77; the concentration of
4 HNiO
M. This is because 2 ion is more than 10 the equilibrium potential for Ni/HNiO
2 couple at the activity of 10
4 M is
0.938 V versus NHE, and the potential is more negative than the equilibrium potential for the H2/H2O couple (
0.916 V), as seen from Fig. 2. Comparing both the figures, it can be seen that the stable pOH range of HNiO
2 at 150 1C expands, suggesting that Ni(OH)2 would easily dissolve as HNiO
2 at higher temperature. On the other hand, it can also be seen that HNiO
2 is stable in a wide pOH range at 150 1C, and the stable pOH range expands as the temperature increases. Ni(OH)2 is more stable than NiO in a wide potential and pOH ranges [19]. HNiO
2 and Ni(OH)2 are two bivalent species in 4 M LiOH solution. Moreover, the valence state of Ni increases from bivalent to trivalent as the potential increases. The two bivalent species are further oxidized to NiOOHdH2O which is thermodynamically more stable than NiOOH at the charged positive electrode [20]. Then, NiOOHdH2O transforms into NiOOH which has two kinds of species, bNiOOH and g-NiOOH. Only b-NiOOH can exist

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1.0 150°C 0.9

0.8 – 1.0

– 0.5

pOH Fig. 3. The scope of generating LiNiO2.

0.0

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LiNiO2 could not be reduced in this condition, so the LiNiO2 film could be synthesized on a nickel substrate by the cyclic voltammogram method. After several cyclic voltammetry experiments, a brown film is prepared on the nickel substrate. The XRD pattern of the nickel electrode is shown in Fig. 5. There are well-defined peaks which correspond to LiNiO2 in the XRD pattern of the nickel electrode.

Current / (mA/cm2)

0.16 0.12 0.08 0.04 0.00 0.0

0.2

0.4

0.6

0.8

1.0

4. Conclusion

Potential / (V)

Intensity / (a.u.)

Fig. 4. Cyclic voltammogram of the Ni/Pt electrode in 4 M LiOH solution.

Ni LiNiO2

˚ ˚ 10

20

˚˚ 30

40

˚

50

˚

60

In summary, the metal nickel is active at relatively high temperatures and is easily oxidized to form hydroxides or ions in LiOH solution. Thermodynamic analysis indicates that the oxidation proceeds in the following order: Ni, Ni(OH)2 or HNiO
2 , NiOOHdH2O, NiOOH and LiNiO2. The result is concordant with the cyclic voltammogram analysis.

References 70

80

90

2θ / (deg) Fig. 5. XRD patterns of the film obtained after the cyclic voltammogram analysis.

diagram for our solution pOH of about
0.88 (Fig. 1) and
0.77 (Fig. 2). During the positive potential scan, one oxidation peak appeared at about 0.4 V, corresponding probably to the oxidation of HNiO
2 or Ni(OH)2 to NiOOHdH2O or NiOOH. There was no other anodic peak which means that the Ni ion remains with the value of +3 at the end of the positive scan. According to Melendres [23], the reductive potential of Ni3+ (NiOOHdH2O or NiOOH) to Ni2+(Ni(OH)2 or HNiO
2 ) was little below the anodic peak, but there was no visible indication of a cathodic peak in our voltammogramm, which implies that at the end of the positive scan, it did not consist of NiOOHdH2O or NiOOH. At the end of the positive scan, corresponding to pOH ¼
0.88 and 1.15 V, which is in the left range of lines as shown in Fig. 3, NiOOH could transfer to LiNiO2.

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