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Frontiers of Materials Science

ISSN 2095-025X

ISSN 2095-0268(Online)

CN 11-5985/TB

Postal Subscription Code 80-974

2018 Impact Factor: 1.701

Front. Mater. Sci.    2015, Vol. 9 Issue (2) : 199-205    https://doi.org/10.1007/s11706-015-0296-6
RESEARCH ARTICLE
Electrochemical performance of overlithiated Li1+xNi0.8Co0.2O2: structural and oxidation state studies
Roshidah RUSDI1,2,Norlida KAMARULZAMAN1,2,*(),Kelimah ELONG2,4,Hashlina RUSDI3,Azilah ABD-RAHMAN3
1. School of Physics and Materials, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
2. Centre for Nanomaterials Research, Institute of Science, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
3. Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
4. School of Chemistry and Environment, Faculty of Applied Sciences , Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
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Abstract

Pure, layered compounds of overlithiated Li1+xNi0.8Co0.2O2 (x = 0.05 and 0.1) were successfully prepared by a modified combustion method. XRD studies showed that cell parameters of the material decreased with increasing the lithium content. SEM revealed that the morphology of particles changed from rounded polyhedral-like crystallites to sharp-edged polyhedral crystals with more doped lithium. EDX showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. Electrochemical studies revealed the overlithiated samples have improved capacities as well as cycling behavior. The sample with x = 0.05 shows the best performance with a specific capacity of 113.29 mA?h?g-<?Pub Caret1?>1 and the best capacity retention of 92.2% over 10 cycles. XPS results showed that the binding energy of Li 1s is decreased for the Li doped samples with the smallest value for the x = 0.05 sample, implying that Li+ ions can be extracted more easily from Li1.05Ni0.8Co0.2O2 than the other stoichiometries accounting for the improved performance of the material. Considerations of core level XPS peaks for transition metals reveal the existence in several oxidation states. However, the percentage of the+3 oxidation state of transition metals for the when x = 0.1 is the highest and the availability for charge transition from the+3 to+4 state of the transition metal during deintercalation is more readily available.

Keywords overlithiation      LiNi0.8Co0.2O2      interstitial doped      Li1.05Ni0.8Co0.2O2      Li1.1Ni0.8Co0.2O2     
Corresponding Author(s): Norlida KAMARULZAMAN   
Issue Date: 23 July 2015
 Cite this article:   
Roshidah RUSDI,Norlida KAMARULZAMAN,Kelimah ELONG, et al. Electrochemical performance of overlithiated Li1+xNi0.8Co0.2O2: structural and oxidation state studies[J]. Front. Mater. Sci., 2015, 9(2): 199-205.
 URL:  
https://academic.hep.com.cn/foms/EN/10.1007/s11706-015-0296-6
https://academic.hep.com.cn/foms/EN/Y2015/V9/I2/199
Fig.1  XRD refinement results of (a) LiNi0.8Co0.2O2, (b) Li1.05Ni0.8Co0.2O2 and (c) Li1.1Ni0.8Co0.2O2 at 700°C for 24 h.
Sample a (= b) /? c /? V /?3 c/a Rw c2 s.o.f.
Li (3a) Li (3b) Ni (3b) Co (3b) Total (3b) O
LiNi0.8Co0.2O2 2.8717 14.1926 101.361 4.94 2.16 1.43 1.00 0.03 0.73 0.20 0.96 1.00
Li1.05Ni0.8Co0.2O2 2.8695 14.1768 101.099 4.94 2.21 1.20 1.00 0.00 0.69 0.21 0.90 1.00
Li1.1Ni0.8Co0.2O2 2.8696 14.1747 101.088 4.94 2.23 1.25 1.00 0.01 0.70 0.20 0.91 0.98
Tab.1  Crystallographic parameters of the samples from the Rietveld refinements of XRD
Fig.2  SEM results of (a) LiNi0.8Co0.2O2, (b) Li1.05Ni0.8Co0.2O2 and (c) Li1.1Ni0.8Co0.2O2 at 700°C for 24 h.
Sample Calculated value EDX value
LiNi0.8Co0.2O2 Li1.05Ni0.8Co0.2O2 Li1.1Ni0.8Co0.2O2
Ni 80 79.8 79.3 79.0
Co 20 20.2 20.7 21.0
Tab.2  EDX values for all samples
Fig.3  Cyclic voltammogram of LiNi0.8Co0.2O2 (a), Li1.05Ni0.8Co0.2O2 (b) and Li1.1Ni0.8Co0.2O2 (c) at 700°C for 24 h.
Fig.4  Specific charge–discharge capacity of (a) LiNi0.8Co0.2O2, (b) Li1.05Ni0.8Co0.2O2 and (c) Li1.1Ni0.8Co0.2O2 at 700°C for 24 h.
Cycle Specific discharge capacity /(mA?h?g-1)
LiNi0.8Co0.2O2 Li1.05Ni0.8Co0.2O2 Li1.1Ni0.8Co0.2O2
Cy1 94.71 113.29 112.32
Cy2 90.50 110.99 109.30
Cy3 90.01 109.54 105.80
Cy4 88.90 109.42 103.74
Cy5 86.12 108.82 102.66
Cy6 85.17 108.09 102.42
Cy7 81.81 107.97 101.33
Cy8 78.03 106.76 100.00
Cy9 74.30 104.47 98.43
Cy10 73.50 101.93 96.50
Capacity fading 21.55% 7.78% 12.37%
Tab.3  Specific discharge capacity of LixNi0.8Co0.2O2 materials anneal at 700°C for 24 h
Fig.5  XPS results of Ni 2p3/2: (a) LiNi0.8Co0.2O2, (b) Li1.05Ni0.8Co0.2O2, and (c) Li1.1Ni0.8Co0.2O2 at 700°C for 24 h.
Fig.6  XPS results of Co 2p3/2: (a) LiNi0.8Co0.2O2, (b) Li1.05Ni0.8Co0.2O2, and (c) Li1.1Ni0.8Co0.2O2 at 700°C for 24 h.
Fig.7  XPS results of O 1s: (a) LiNi0.8Co0.2O2, (b) Li1.05Ni0.8Co0.2O2, and (c) Li1.1Ni0.8Co0.2O2 at 700°C for 24 h.
Fig.8  XPS results of Li 1s: LiNi0.8Co0.2O2 (a), Li1.05Ni0.8Co0.2O2 (b) and Li1.1Ni0.8Co0.2O2 (c) at 700°C for 24 h.
Oxidation state Binding energy (BE) /eV
LiNi0.8Co0.2O2 Li1.05Ni0.8Co0.2O2 Li1.1Ni0.8Co0.2O2
Li+ 54.27 53.53 54.22
Ni2+ 852.81 852.90 852.97
Ni3+ 853.78 853.97 853.90
Ni4+ 855.00 855.25 855.25
Co2+ 778.89 778.64 778.80
Co3+ 779.71 779.63 779.73
Co4+ 782.83 782.45 783.38
O1 528.32 528.03 528.28
O2 530.60 530.08 530.55
Tab.4  Binding energy of LiNi0.8Co0.2O2, Li1.05Ni0.8Co0.2O2 and Li1.1Ni0.8Co0.2O2 cathode materials
Sample Oxidation state Fraction of oxidation state of transition metals /% Total oxidation state of transition metals
+2 +3 +4 +2 +3 +4
LiNi0.8Co0.2O2 0.282 1.746 0.827 9.88 61.16 28.97 2.855
Li1.05Ni0.8Co0.2O2 0.313 1.903 0.438 11.79 71.70 16.50 2.654
Li1.1Ni0.8Co0.2O2 0.191 2.0 0.551 6.97 72.94 20.09 2.742
Tab.5  The ratio of the oxidation states of the transition metals and the total oxidation states for all cathode materials
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