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Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method



MyoungYoup Song, ChanGi Park, SunDo Youn, YoungHo Na and HyeRyoung Park
 
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ABSTRACT

LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) were synthesized by milling and solid-state method. Their electrochemical properties were then compared with those of LiNi1-yMyO2 (M = Ga3+ and In3+). All the samples had the structure. LiNi0.95Tl0.05O2 has the largest first discharge capacity 179.8 mAh g-1 and the discharge capacity 113.8 mAh g-1 at the 20th cycle. LiNi0.995Tl0.005O2 has the smallest first discharge capacity 125.4 mAh g-1. The samples exhibit similar cycling performances. LiNi0.975Ga0.025O2 and LiNi0.9In0.1O2 had the best electrochemical properties among the samples substituted by the same element, respectively. Among LiNi0.975Ga0.025O2, LiNi0.9In0.1O2 and LiNi0.95Tl0.05O2, LiNi0.95Tl0.05O2 has the largest first discharge capacity, but has the worst cycling performance. LiNi0.975Ga0.025O2 has the smallest first discharge capacity, but has the smallest capacity degradation rate 0.70 mAh g-1 cycle-1.

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MyoungYoup Song, ChanGi Park, SunDo Youn, YoungHo Na and HyeRyoung Park, 2006. Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method. Trends in Applied Sciences Research, 1: 597-604.

DOI: 10.3923/tasr.2006.597.604

URL: https://scialert.net/abstract/?doi=tasr.2006.597.604

Introduction

The transition metal oxides such as LiMn2O4 (Tarascon et al., 1991; Armstrong and Bruce, 1996; Song and Ahn, 1998), LiCoO2 (Ozawa, 1994; Alcatara et al., 1997; Peng et al., 1998) and LiNiO2 (Dahn et al., 1990, 1991; Marini et al., 1991; Ebner et al., 1994) have been investigated in order to apply them to the cathode materials of lithium secondary battery. LiMn2O4 is very cheap and does not bring about environmental pollution, but its cycling performance is not good. LiCoO2 has a large diffusivity and a high operating voltage and it can be easily prepared. However, it has a disadvantage that it contains an expensive element Co. LiNiO2 is a very promising cathode material since it has a large discharge capacity (Nishida et al., 1997) and is relatively excellent from the view points of economics and environment. On the other hand, its preparation is very difficult as compared with LiCoO2 and LiMn2O4.

It is known that Li1-xNi1+xO2 forms rather than stoichiometric LiNiO2 during preparation. This phenomenon is called cation mixing. Excess nickel occupies the Li sites, destroying the ideally layered structure and preventing lithium ions from easy movement for intercalation and deintercalation during cycling. This results in a small discharge capacity and a poor cycling performance. To solve the problem of cation mixing, Co3+, Al3+, Mn3+ and Ti4+ ions were substituted for lithium ion in LiNiO2 (Gao et al., 1998; Kim and Amine, 2001; Broussely, 1999; Caurant et al., 1996; Zhang et al., 2004; Amriou et al., 2004; Shinova et al., 2005). According to Kim and Amine (2001, 2002) and Gao et al. (1998), the substitution of Ti for Ni resulted in a large discharge capacity and a good cycling performance.

In this study LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) were synthesized by milling and solid-state method and the electrochemical properties of the synthesized samples were investigated. Their electrochemical properties were then compared with those of LiNi1-yMyO2 (M = Ga3+ and In3+) synthesized by the same method in our earlier study (Kim et al., 2005a,b). The substituted Ga and In have the same oxidation number as Tl.

Materials and Methods

The LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) are synthesized by milling and solid-state method under the optimum conditions for the preparation of LiNiO2, previously studied (Kim et al., 2005). LiOH H2O (Kojundo Chemical Lab. Co., Ltd, purity 99%), Ni(OH)2 (Kojundo Chemical Lab. Co., Ltd, purity 99.9%), TlNO3 (Aldrich Chemical, 99.9%) are used as starting materials. The starting materials are mechanically mixed by SPEX milling for 1 h. The mixed materials are preheated at 450°C for 5 h in air, then pressed into pellet and calcined at 750°C for 30 h under oxygen stream. The phase identification of the synthesized samples is carried out by X-ray powder diffraction analysis (Rigaku III/A diffractormeter) using Cu Kα radiation. The scanning rate is 6°min-1 and the scanning range of diffraction angle (2θ) is 10°≤2θ≤80°. The electrochemical cells consist of LiNi1-yTlyO2 as a positive electrode, Li foil as a negative electrode and electrolyte [Purelyte (Samsung General Chemicals Co., Ltd.)] prepared by solving 1M LiPF6 in a 1:1 (volume ratio) mixture of Ethylene Carbonate (EC) and diethyl carbonate (DEC).

Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method
Fig. 1: Experimental procedure for LiNi1-yTlyO2 electrode prepared by solid-state reaction method after milling

The positive electrode consists of 85 wt.% synthesized materials, 10 wt.% acetylene black and 5 wt.% polyvinylidene fluoride (PVDF) binder solved in 1-Methyl-2-pyrrolidinone (NMP). A Whatman glass-filer is used as a separator. The cells are assembled in argon-filled dry box and the cell type is coin-type (2016). All the electrochemical tests are performed at room temperature with a potentiostatic/galvanostatic system. The cells are cycled between 2.7 and 4.2 V at 0.1 C-rate.

Figure 1 shows experimental procedure for the LiNi1-yTlyO2 electrodes prepared by milling and solid-state reaction method.

Results and Discussion

Figure 2 shows X-ray powder diffraction (XRD) patterns of LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.01) calcined at 750°C for 30 h. All the samples have only the phase with Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method structure and do not exhibit the peaks of impurity. The Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method structure is distored in c-axis direction. This is reflected by the split of 006 and 102 peaks and of 108 and 110 peaks in the XRD patterns. The 108 and 110 peaks were split for all the samples.

It is generally known that the cation mixing is small if the intensity ratio of 003 peak to 104 peak (I003/I104) is large (Ohzuku et al., 1993). The value of (I006+I102)/I101, called R-factor, is known to be smaller when the hexagonal ordering is high (Dahn et al., 1990). In addition, the split between 108 and 110 peaks is reported to suggest the smaller cation mixing and the better hexagonal ordering (Ohzuku et al., 1993).

Table 1 gives the lattice parameters a, c, c/a, I003/I104, R-factor and unit cell volume calculated from XRD patterns of LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.01) calcined at 750°C for 30 h. The sample with y = 0.005 has the largest I003/I104 and all the samples have the smallest value of R-factor.

Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method
Fig. 2: XRD patterns of LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) calcined at 750°C for 30 h

Table 1: Data calculated from XRD patterns of LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) calcined at 750°C for 30 h
Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method

Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method
Fig. 3: SEM photographs of LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) calcined at 750°C for 30 h

Figure 3 shows the SEM photographs of LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.01) calcined at 750°C for 30 h. The samples contain small and large particles. The particles become larger as the value of y increases.

Figure 4 shows the variations of the discharge capacity of LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.01) calcined at 750°C for 30 h. The sample with y = 0.05 has the largest first discharge capacity 179.8 mAh g-1 and the discharge capacity 113.8 mAh g-1 at the 20th cycle. The sample with y = 0.005 has the smallest first discharge capacity 125.4 mAh g-1. The samples exhibit similar cycling performance.

Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method
Fig. 4: Variations of discharge capacity at 0.1C-rate with the number of cycles for LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) calcined at 750°C for 30 h

Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method
Fig. 5: Variations of discharge capacity at 0.1 C-rate with the number of cycles for LiNi1-yMyO2 (M = Ga, y = 0.025; M = In, y = 0.1; M = Tl, y = 0.05) calcined at 750°C for 30 h

Table 2: Data calculated from X-ray powder diffraction patterns of LiNi1-yMyO2 (M = Ga, y = 0.025; M = In, y = 0.1; M = Tl, y = 0.05) calcined at 750°C for 30 h
Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method

In our earlier study (Kim et al., 2005a, b), we studied the electrochemical properties of LiNi1-yGayO2 and LiNi1-yInyO2 synthesized by the same method, as mentioned above. Among LiNi1-yGayO2 LiNI0.975Ga0.025O2 had the best electrochemical properties. LiNi0.9In0.1O2 exhibited the best electrochemical properties among LiNi1-yInyO2.

Figure 5 shows the variations of the discharge capacity of LiNi1-yMyO2 (M = Ga, y = 0.025; M = In, y = 0.1; M = Tl, y = 0.05) calcined at 750° for 30 h. The sample LiNi0.95Tl0.05O2 has the largest first discharge capacity 179.8 mAh g-1 and the discharge capacity 113.8 mAh g-1 at the 20th cycle.

Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method
Fig. 6: X-ray powder diffraction patterns of LiNi1-yMyO2 (M = Ga, y = 0.025; M = In, y = 0.1; M = Tl, y = 0.05) calcined at 750°C for 30 h

Image for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method
Fig. 7: SEM photographs of LiNi1-yMyO2 (M = Ga, y = 0.025; M = In, y = 0.1; M = Tl, y = 0.05) calcined at 750°C for 30 h

The sample LiNi0.975Ga0.025O2 has the smallest first discharge capacity 131.4 mAh g-1, but it has the best cycling performance. LiNi0.975Ga0.025O2 has the discharge capacity 117.5 mAh g-1 at the 20th cycle, showing the discharge capacity degradation rate of 0.70 mAh g-1 cycle-1.

Figure 6 shows XRD patterns of LiNi0.975Ga0.025O2, LiNi0.9In0.1O2 and LiNi0.95Tl0.05O2. All the samples have the phase with RImage for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method structure. In addition, LiNi0.9In0.1O2 shows the peaks for LiInO2.

Table 2 gives the lattice parameters a, c, c/a, I003/I104, R-factor and unit cell volume calculated from XRD patterns of LiNi1-yMyO2 (M = Ga, y = 0.025; M = In, y = 0.1; M = Tl, y = 0.05). LiNi0.9In0.1O2 has the largest I003/I104 and all the samples have the similar values of R-factor.

Figure 7 shows the SEM photographs of LiNi0.975Ga0.025O2, LiNi0.9In0.1O2 and LiNi0.95Tl0.05O2 calcined at 750°C for 30 h. All the samples have small and large particle. The particle size increases roughly from LiNI0.975Ga0.025O2 to LiNi0.9In0.1O2 and then to LiNi0.95Tl0.05O2. The particles of LiNi0.95Tl0.05O2 are agglomerated.

Conclusions

LiNi1-yTlyO2 (y = 0.005, 0.01, 0.025, 0.05 and 0.1) were synthesized by milling for 1 h, preheating at 450°C for 5 h in air, then pelletizing and finally calcining at 750°C for 30 h under oxygen stream. All the samples had the RImage for - Electrochemical Properties of Cathode LiNi1-yTlyO2 Synthesized by Milling and Solid-State Reaction Method m structure. LiNi0.95Tl0.05O2 has the largest first discharge capacity 179.8 mAh g-1 and the discharge capacity 113.8 mAh g-1 at the 20th cycle. LiNi0.995Tl0.005O2 has the smallest first discharge capacity 125.4 mAh g-1. The samples exhibit similar cycling performances. LiNi0.975Ga0.025O2 and LiNi0.9In0.1O2 had the best electrochemical properties among the samples substituted by the same element, respectively. Among LiNi0.975Ga0.025O2, LiNi0.9In0.1O2 and LiNi0.95Tl0.05O2, LiNi0.95Tl0.05O2 has the largest first discharge capacity, but has the worst cycling performance. LiNi0.975Ga0.025O2 has the smallest discharge capacity, but has the smallest capacity degradation rate 0.70 mAh g-1 cycle-1.

Acknowledgment

This study was supported by grant No. R01-2003-000-10325-0 from the Basic Research Program of the Korea Science and Engineering Foundation.

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