Study of the intrinsic electrochemical properties of spinel LiNi0.5Mn1.5O4

Electrochimica Acta 112 (2013) 557–561

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Study of the intrinsic electrochemical properties of spinel LiNi0.5 Mn1.5 O4 Guoqiang Liu ∗ , Yue Li, Beiyue Ma, Ying Li ∗ School of Material and Metallurgy, Northeastern University, Shenyang 110819, China

a r t i c l e

i n f o

Article history: Received 13 July 2013 Received in revised form 14 August 2013 Accepted 3 September 2013 Available online 18 September 2013 Keywords: Li-ion battery Cathode High-voltage

a b s t r a c t In the past, the electrochemical properties of spinel LiNi0.5 Mn1.5 O4 , i.e. rate capability at room temperature and cycle performance at elevated temperature, were not satisfactory. In this study, the influence of glycolic acid on the electrochemical properties of spinel LiNi0.5 Mn1.5 O4 was studied. With some amount of glycolic acid as an additional reactant, spinel LiNi0.5 Mn1.5 O4 was synthesized, and its electrochemical properties were improved. In addition, the mechanism of capacity decay at elevated temperature was also studied, which presents constructive view to further improve the electrochemical properties of spinel LiNi0.5 Mn1.5 O4 . © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Spinel LiNi0.5 Mn1.5 O4 has attracted much attention in the field of lithium ion batteries in recent years due to its high charge–discharge voltage approaching 5 V [1,2]. It is considered as one of the most promising cathode materials for Li-ion batteries that can be deployed in hybrid electrical vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) [3,4]. So far there have been many published research papers about spinel LiNi0.5 Mn1.5 O4 . From these papers it can be seen that the electrochemical properties of spinel LiNi0.5 Mn1.5 O4 at room temperature are satisfactory. However, at elevated temperature such as 55 ◦ C, it suffers serious capacity decay [1,2]. To solve this problem, some strategies have been put forward. Of these strategies, partial substitution of Ni and Mn by other transition elements, such as Cr, Co, Fe, Ru and so on [2–6], is the most commonly employed method. Although there has been some progress in improving the cycle performance of spinel LiNi0.5 Mn1.5 O4 at elevated temperature, no explicit mechanism has been presented to explicate the role of cation substitutions. Therefore the strategy of cation substitutions is still a subject of debate. It is believed that the electrochemical properties of spinel LiNi0.5 Mn1.5 O4 mainly depend on the following factors such as oxygen deficiency, Ni/Mn ordering, the amount of Mn3+ , the presence of rock salt phase and so on. These factors are correlated with each

∗ Corresponding authors. Tel.: +86 024 83673860; fax: +86 024 83687731. E-mail addresses: [email protected] (G. Liu), [email protected] (Y. Li). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.09.015

other. Oxygen deficiency of spinel LiNi0.5 Mn1.5 O4 usually takes place when the temperature increases above 700 ◦ C. This process can be expressed by the following reaction [7,8]: LiNi0.5 Mn1.5 O4 → Lix Ni1−x O + LiNi0.5−y Mn1.5−y O4−x + O2

(1)

As the reaction shows, rock salt is also a product. In order to keep the charge neutrality for the new product LiNi0.5−y Mn1.5−y O4−x , there should be some amount of Mn3+ in the product LiNi0.5−y Mn1.5−y O4−x . The Mn3+ can improve the conductivity and increase the lattice parameter which is beneficial for Li+ to be transported in the crystal [9]. When LiNi0.5 Mn1.5 O4 decomposes into the rock salt phase Lix Ni1−x O, the Ni/Mn ordering in LiNi0.5 Mn1.5 O4 will change in to Ni/Mn disordering. The Ni/Mn disordering can undergo charge and discharge at higher current rates. Thus, better electrochemical properties of spinel LiNi0.5 Mn1.5 O4 can be achieved by employing a properly synthesizing route to control the amount of oxygen deficiency [10]. In this study, we synthesized spinel LiNi0.5 Mn1.5 O4 (sample 1) using some amount of glycolic acid as an additional reactant. Sun et al. used glycolic acid as chelating agent to synthesize LiNi0.5 Mn1.5 O4 previously [11,12], however, we used a different synthesizing procedure (different amount of glycolic acid, synthesizing temperature and time) in this study and achieved a good result. We also synthesized spinel LiNi0.5 Mn1.5 O4 (sample 2) using the same process except that no glycolic acid was used, and investigated the influence of agent glycolic acid on the structure and electrochemical properties of spinel LiNi0.5 Mn1.5 O4 .

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G. Liu et al. / Electrochimica Acta 112 (2013) 557–561

2. Experimental Stoichiometric amounts of LiNO3 , Ni(NO3 )2 ·6H2 O and Mn(CH3 COO)2 ·4H2 O were dissolved in distilled water and stirred to obtain a clear aqueous solution. A small amount of extra Li salt was added to this solution to compensate for the lithium loss at high temperature. Glycolic acid was used as agent, and the mole ratio of glycolic acid to total metal ions was 0.85:1. The glycolic acid was dissolved in distilled water to form a transparent solution. Two solutions were mixed and stirred; the water was then thermally evaporated at 120 ◦ C to make a gel. The dried gel was calcined at 910 ◦ C for 12 h in air. Finally, it was cooled to room temperature at a cooling rate of 2.5 ◦ C min−1 to form product which was defined as sample 1. In order to make a comparison, sample 2 was synthesized by the procedure but no glycolic acid was employed. X-ray diffraction patterns were obtained with a Philips X-ray diffractometer equipped with Cu K␣ radiation. The powder morphologies of the products were examined with a Hitachi scanning electron microscope (SEM). Raman spectra were obtained with a Renishaw in Via Raman Microscope with a 442-nm blue laser beam. The thermogravimetic (TG) analysis were collected on a PerkinElmer TGA 7 in flowing air. The powder morphology of the sample was examined with a scanning electron microscope. XP spectra were collected with Thermo ESCALAB 25 0 spectrometer. Electrochemical properties were measured with coin half-cells composed of Li [Ni0.5 Mn1.5 ]O4 as cathodes, a metallic-lithium anode, 1 mol L−1 LiPF6 in 1:1 volume mixture of diethyl carbonate/ethylene carbonate as electrolyte, and a Celgard polypropylene separator. The cathodes were prepared by mixing 85 wt% active material with 10 wt% acetylene black and 5 wt% PVDF binder in Nmethyl-2-pyrrolidone (NMP); the slurry was cast on aluminum foil, dried in an oven, and punched into circular discs 0.95 cm in diameter. All coin cells were assembled in an argon-filled glovebox. The cells were galvanostatically cycled at room temperature and 55 ◦ C.

3. Results and discussion Fig. 1(a) shows the XRD pattern of sample 1. There are no additional peaks at 24.34◦ , 26.70◦ , 32.80◦ and 34.70◦ which belong to simple-cubic P43 32 structure, so it can be indexed to the cubic Fd3m space group [10]. There are impurity peaks at 37.5◦ , 43.7◦ and 63.6◦ , indicating the presence of rock salt Lix Ni1−x O phase [13]. Fig. 1(b) shows the XRD pattern of sample 2. The peak intensity of rock salt is less than that of sample 1. There is (2 2 0) peak in this pattern, which indicates the difference of atom sites of transition metals (Ni or Mn) in the structure. The thermogravimetric experiment was carried out from room temperature to 910 ◦ C at 5 ◦ C min−1 for the as prepared samples, and the result is displayed in Fig. 1(c). For sample 1, when the temperature was increased to above 700 ◦ C, weight loss took place, indicating that oxygen was being lost. At the same time, some amount of rock salt separated from the reaction materials. When temperature decreased from 910 ◦ C, the rock salt phase combined oxygen again to change into spinel phase. Because the thermogravimetric experiment was carried out under oxygen atmosphere, the oxygen deficiency existed in the product was eliminated. The rock salt phase originally existed in the product also assimilated oxygen to change into spinel phase. A final weight about 3.8% greater than the starting weight was attained. Therefore, it can be estimated that there was about 3.8% of oxygen deficiency in sample 1. For sample 2, weight about 0.27% less than the starting weight was attained in the thermogravimetric experiment. It can be derived that the rock salt phase and oxygen deficiency in sample 2 were less than those

Fig. 1. Structure of the as prepared products (a) X-ray diffraction pattern of sample 1, (b) X-ray diffraction pattern of sample 2, (c) TG curves of sample 1 and 2 and (d) Raman spectrum of sample 1.

G. Liu et al. / Electrochimica Acta 112 (2013) 557–561

5.0

in sample 1, but the exact amount of oxygen deficiency can not be known. It can be seen that there is great difference in TG curves between the two samples. The following is an explanation. For sample 1, the chelating agent glycolic acid will consume some oxygen when it is burned away at high temperature, causing a low oxygen atmosphere. The following solid reaction took place in a low oxygen concentration environment, resulting in greater oxygen deficiency in the product when the cooling rate is high. The thermochemical equation can be written as:

a

Voltage (V)

4.5 1C 2C 5C 10 C

4.0

3.5

559

0.5(HOCH2 COO)2 Ni + 1.5(HOCH2 COO)2 Mn + HOCH2 COOLi

3.0



+ 3.25O2 −→LiNi0.5 Mn1.5 O4 + 10CO2 + 7.5H2 O + 3475 (KJ/mol)

0

50

100

150

(2)

-1

Capacity (mAh g )

150 b

-1

Capacity (mAh g )

2C

5C 10 C

100

50

0

5

10

15 20 Cycle number

25

30

5.0

C

RT

Voltage (V)

4.5

o

55 C

RT o 55 C o RT-55 C

4.0

3.5 0

20

40

60 80 100 -1 Capacity (mAh g )

120

140 d

-1

99.2%

100

96.8%

80

95.8% 92.4%

100 o

55 C

RT

60 40

50

20 0

0

50

100

150

Coulombic efficiency (%)

Discharge capacity (mAh g )

150

0 200

The oxygen deficiency for the spinel LiNi0.5 Mn1.5 O4 is also affected by cooling rates, which has been reported in previous paper [14]. This study shows that agent combustion can also create some extent of oxygen deficient. When synthesizing sample 2, there was no oxygen consumed in the atmosphere. The reaction takes place in a normal oxygen atmosphere. Therefore the oxygen deficiency in sample 2 is less than that in sample 1. The degree of Ni/Mn ordering was investigated with Raman Spectroscopy. The octahedral Mn(Ni)O6 structures have Oh symmetry where five active modes ( = A1g + Eg + 3F2g ) are Raman active for Ni2+ /Mn4+ cation ordering [15]. Because there are only two Raman active bands, A1g at 631 cm−1 and F2g at 490 cm−1 , this sample is Ni/Mn disordered. The A1g mode reflects symmetric M O stretching vibration of MO6 groups. The F2g mode can be assigned to the Ni2+ O stretching mode in the structure [16]. Fig. 2 shows the electrochemical properties of sample 1. From Fig. 2(a) and (b), the discharge capacities were 132 mAh g−1 at 1 C, 129.9 mAh g−1 at 2 C, 125 mAh g−1 at 5 C and 117 mAh g−1 at 10 C. It also exhibited good cycle performance at these current rates. It is worth noting that the main part of the discharge profile at 10 C was above 4.5 V. In some recent literature the main part of the discharge profiles at 10 C were below 4.5 V [17,18]. This result indicates that the sample 1 has lower impedance. We define that the 4-V region is the voltage range of 4.40–3.50 V, and the amount of Mn3+ can be calculated as 0.147 per formula unit. The 4.7-V region is defined as the voltage range of 4.9–4.40 V, and the capacity was 110.4 mAh g−1 , so the electrochemically active Ni amount was 0.376 per formula unit. From Fig. 2(c) and (d), the first 100 cycles were carried out at room temperature. The capacity retention was about 95.8% after 100 cycles. The coulombic efficiency was about 99.2%. When the hundred and first cycle was charged to about 4.8 V, the temperature was raised to 55 ◦ C. The charge–discharge capacity then increased. The charge and discharge profiles show that the cell impedance was lower at elevated temperature than at room temperature. After another 100 cycles, the capacity retention was 92.4% at the elevated temperature. The coulombic efficiency was about 96.8%. In the previous literature, the pristine LiNi0.5 Mn1.5 O4 exhibited poor cycle performance at elevated temperature [1,2]. The improved result should be attributed to the specific structure of the as prepared sample. In order to clarify the role of agent glycolic acid, the charge and discharge test for sample 2 was also carried out, as shown in Fig. 3(a). It can be seen that the discharge capacity was 111.3 mAh g−1 at 1 C. The amount of Mn3+ was 0.143 per formula unit, and the electrochemically active Ni amount was 0.307 per

Cycle number Fig. 2. Electrochemical properties of the sample 1 at the voltage range from 3.5 to 4.9 V (a) charge curve at 1 C rate and discharge curves at different rates of 2 C, 5 C and 10 C at room temperature, (b) cycle performance at different discharge rates,

(c) charge–discharge curves at room temperature and elevated temperature, 1 C rate and (d) cycle performance and coulombic efficiency at room temperature and elevated temperature, 1 C rate.

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G. Liu et al. / Electrochimica Acta 112 (2013) 557–561

Table 1 Discharge behaviors of the as prepared samples at 1/1 C current rate. Discharge capacity at 1 C/1 C (mAh g−1 ) a

Sample 1 Sample 2 a b c d

b

4.7 V region

4 V region

110.4 90.3

21.6 21

Amount of Mn3+ per formula unit of LiNi0.5 Mn1.5 O4 c

Amount of electrochemically active Nid

0.147 0.143

0.376 0.307

The 4.7 V region is the voltage range of 4.90–4.40 V. The 4 V region is the voltage range of 4.40–3.50 V. The amount of Mn3+ was calculated by the theoretical capacity of LiNi0.5 Mn1.5 O4 which is about 147 mAh g−1 . The amount of electrochemically active Ni was calculated by the theoretical capacity of LiNi0.5 Mn1.5 O4 which is about 147 mAh g−1 .

formula unit. Therefore, the agent glycolic acid improved the electrochemically active Ni amount. A comprehensive comparison of the electrochemical properties is summarized in Table 1. Fig. 3(b) shows the Mn (2p) spectra by XPS. Binding energies of 643.6 eV, corresponds to Mn4+ , and 641.8 eV, corresponding to Mn3+ . By analyzing the XPS, it is evident that the surface composition contains a larger content of Mn3+ . It is necessary to explore the reason that the spinel LiNi0.5 Mn1.5 O4 experienced fast capacity decay at elevated temperature. We conducted the charge–discharge test in the voltage range of 4.5–4.9 V at 55 ◦ C. In this voltage range, the Mn3+ /Mn4+ oxidation/reduction couple can be avoided. The charge–discharge profiles and cycle performances are shown in Fig. 4(a) and (b). After 100 cycles, the capacity retention reached 94.9%. This result is better than the one cycled from 3.5 to 4.9 V. Based on this result, we can deduce the mechanism of capacity decay of spinel LiNi0.5 Mn1.5 O4 .

When it is discharged at around 4.0 V, Mn4+ will accept an electron to become Mn3+ . Mn3+ is electrochemically active and contributes a small plateau at around 4 V. A portion of the Mn3+ ions on the surface of the sample may undergo the disproportionation reaction 2Mn3+ = Mn2+ + Mn4+ , and the Mn2+ tends to dissolve in the electrolyte and migrate to the anode to degrade cell, especially at elevated temperature, causing capacity loss during cycling [19]. Therefore, Mn3+ played a dual role in the charge–discharge process. The Mn3+ in the bulk can increase conductivity and lattice parameter, which is favorable to the migration of Li+ in the lattice. However, the existence of Mn3+ in the surface is harmful to cycle performance of spinel LiNi0.5 Mn1.5 O4 . At last, the morphological feature of the sample 1 was characterized by scanning electron microscopy, as shown in Fig. 5. The sample was polyhedral shape and well crystallized. There was

a

5.0

5.0

a

Voltage (V)

Voltage (V)

4.5

4.5

4.0

cycle 1 cycle 2 cycle 100

4.0

3.5

3.0

3.5

1 C 0.5 C

0

50

100

150 -1

0

20

40

60

80

100

120

Specific capacity (mAh g )

140

-1

Capacity (mAh g )

160 b -1

Relative intensity

Mn

b

Capacity (mAh g )

2P3/2 2P1/2

120 94.9%

80

40

635

640

645

650

655

660

665

Binding energy (eV) Fig. 3. (a) Charge and discharge curves of sample 2 at 0.5 and 1 C and (b) XP spectra of sample 1.

20

40 60 Cycle number

80

100

Fig. 4. Electrochemical properties of the as prepared product at the voltage range from 4.5 to 4.9 V at 55 ◦ C (a) charge and discharge curves at 1 C rate and (b) cycle performance.

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References

Fig. 5. Morphology of the as prepared sample 1.

serious particle aggregation. Most particles ranged from 1 to 3 ␮m in size. 4. Conclusions In summary, we used some amount of glycolic acid as the chelating and dispersing agent to synthesize spinel LiNi0.5 Mn1.5 O4 . By this method, the oxygen deficiency was increased in the as prepared sample. The product exhibited excellent rate capability. At 1 C (charge)/10 C (discharge) rate, its discharge capacity could reach 117 mAh g−1 , and its main discharge voltage was above 4.5 V. It also displayed acceptable cycle performance at elevated temperature. After 100 cycles at 1 C/1 C at 55 ◦ C, its capacity retention was about 92.4%. In addition, the mechanism of capacity decay at elevated temperature of LiNi0.5 Mn1.5 O4 was investigated. It was deduced that the Mn3+ on the surface is one of the main reasons for the fast capacity decay. It is possible to further improve the electrochemical properties of spinel LiNi0.5 Mn1.5 O4 at elevated temperature through limiting the amount of Mn3+ on the surface. Acknowledgements We gratefully acknowledge financial support received from National Natural Science Foundation of China (No. 51274057) and National High Technology Research and Development Program of China (863 Program) (No. 2013AA030902).

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