Effects of Nanofiber Architecture and Antimony Doping on the

Apr 26, 2018 - To cure these issues, herein, the synthesis of high-performance antimony-doped LLO nanofibers by an electrospinning process is put forw...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 16561−16571

Effects of Nanofiber Architecture and Antimony Doping on the Performance of Lithium-Rich Layered Oxides: Enhancing Lithium Diffusivity and Lattice Oxygen Stability Ruizhi Yu,†,‡,§ Zhijuan Zhang,∥ Sidra Jamil,†,‡,§ Jiancheng Chen,†,‡,§ Xiaohui Zhang,†,‡,§ Xianyou Wang,*,†,‡,§ Zhenhua Yang,∥ Hongbo Shu,†,‡,§ and Xiukang Yang*,†,‡,§ †

National Base for International Science & Technology Cooperation, School of Chemistry, ‡National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, School of Chemistry, §Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, and ∥School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, Hunan, China S Supporting Information *

ABSTRACT: Li-rich layered oxides (LLOs) with high specific capacities are favorable cathode materials with high-energy density. Unfortunately, the drawbacks of LLOs such as oxygen release, low conductivity, and depressed kinetics for lithium ion transport during cycling can affect the safety and rate capability. Moreover, they suffer severe capacity and voltage fading, which are major challenges for the commercializing development. To cure these issues, herein, the synthesis of high-performance antimony-doped LLO nanofibers by an electrospinning process is put forward. On the basis of the combination of theoretical analyses and experimental approaches, it can be found that the one-dimensional porous micro-/nanomorphology is in favor of lithium-ion diffusion, and the antimony doping can expand the layered phase lattice and further improve the lithium ion diffusion coefficient. Moreover, the antimony doping can decrease the band gap and contribute extra electrons to O within the Li2MnO3 phase, thereby enhancing electronic conductivity and stabilizing lattice oxygen. Benefitting from the unique architecture, reformative electronic structure, and enhanced kinetics, the antimony-doped LLO nanofibers possess a high reversible capacity (272.8 mA h g−1) and initial coulombic efficiency (87.8%) at 0.1 C. Moreover, the antimony-doped LLO nanofibers show excellent cycling performance, rate capability, and suppressed voltage fading. The capacity retention can reach 86.9% after 200 cycles at 1 C, and even cycling at a high rate of 10 C, a capacity of 172.3 mA h g−1 can still be obtained. The favorable results can assist in developing the LLO material with outstanding electrochemical properties. KEYWORDS: lithium-rich layered oxide material, antimony doping, nanofiber, stabilized lattice oxygen, expanded layered phase lattice trigger the excessive oxidation of lattice O2− and then results in O2 gas evolution accompanied by the irreversible deintercalation of Li+ from TM layers with a continuous voltage plateau at above 4.4 V, which induces TM ion migration into the Li vacant sites, causing structure rearrangement and the subsequent failure of partial Li+ ion re-intercalation during discharging.12,13 The continuous capacity fading and discharge voltage decay can lead to a decreased energy density and low accuracy in terms of measuring the state of charge, which is currently the bottleneck of commercialization of the LLOs.14,15 These degenerations have been demonstrated to be associated with the phase transformation from the layered structure to the spinel structure.16 Numerous studies have determined that slight octahedral TM ions in TM layers, especially the Mn4+,

1. INTRODUCTION In the past decade, lithium-rich layered oxides (LLOs) with composite structures composed of rhombohedral LiTMO2 (space group: R3̅m, TM = Mn, Ni, Co, etc.) and monoclinic Li2MnO3 (space group: C2/m) nanodomains have received considerable attention as high-energy-density cathode materials because of the delivery of high capacity, often exceeding 250 mA h g−1, through applying a high charging voltage of 4.6 V.1−4 Recent research studies have demonstrated that the high capacity resulted from joint redox processes of TMa+/TM(a+1)+ and O2−/O22−.5−7 Despite the high capacity, these materials suffer from high initial irreversible capacity, severe discharge voltage decay and insufficient cycling performance, in addition to the inferior rate capability.8−10 The high initial irreversible capacity originates from the activation of the Li2MnO3 phase.11 During the initial delithiation, the large overlap of the TM 3d band and the O 2p band at highly charged states within the Li2MnO3 phase can © 2018 American Chemical Society

Received: March 2, 2018 Accepted: April 26, 2018 Published: April 26, 2018 16561

DOI: 10.1021/acsami.8b03608 ACS Appl. Mater. Interfaces 2018, 10, 16561−16571

Research Article

ACS Applied Materials & Interfaces

Figure 1. SEM images of (a,b) PAN-F and (c,d) S/PAN-F precursor fibers, and corresponding (e,f) C-LMNCO and (g,h) S-LMNCO. (i) Schematic diagram of the Li+ diffusion pathway of the LLO nanofibers. (j) XRD patterns of C-LMNCO and S-LMNCO. XPS spectra of (k) Ni 2p, (l) Co 2p, (m) Mn 2p, and (n) Sb 3d + O 1s of C-LMNCO and S-LMNCO.

would migrate to tetrahedral sites in Li layers during the charging process. Upon discharging, some of the TM ions may be trapped in the tetrahedral sites or migrate to octahedral sites in Li layers, which initiates the transformation from the layered structure to the spinel structure.17−19 Furthermore, excessive anionic redox and cationic migration during cycling can lead to large lattice distortions and rearrangements, which trigger particle cracking accompanied by the movement of the grain boundary, resulting in further structural degradation.20,21 Meng’ group presented that the visualization of the Litet−V(TM)Li− Litet dumbbell [Litet is the Li+ ions in tetrahedral sites, and V(TM)Li represents Li vacant sites in the TM layers] and O vacancy would facilitate the migration of TM ions.22 Cho et al. also proposed that the O2 gas loss related to activation of the Li2MnO3 phase can promote the phase transformation.23 As we know, the redox potential ( 0 when x = 0.5; this can further certify that Sb doping can suppress the O2 gas loss and mitigate the degradation of the layered structure during the charging−discharging process. Accordingly, the DFT results commendably account for the improvement of electrochemical performance and structural stability for the S-LMNCO sample.

4. METHODS Details on materials and methods are available in the Supporting Information. 16569

DOI: 10.1021/acsami.8b03608 ACS Appl. Mater. Interfaces 2018, 10, 16561−16571

Research Article

ACS Applied Materials & Interfaces

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03608. Experimental details: material preparation, materials characterizations, theoretical calculations, and electrochemical measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected] (X.Y.). ORCID

Xianyou Wang: 0000-0001-8888-6405 Zhenhua Yang: 0000-0002-3967-6249 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the Hunan Provincial Innovation Foundation for Postgraduate (no. CX2016B229), Natural Science Foundation of Hunan Province (nos. 2015JJ6103 and 2015JJ2137), and Key Project of Strategic New Industry of Hunan Province (nos. 2016GK4005 and 2016GK4030).

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DOI: 10.1021/acsami.8b03608 ACS Appl. Mater. Interfaces 2018, 10, 16561−16571

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DOI: 10.1021/acsami.8b03608 ACS Appl. Mater. Interfaces 2018, 10, 16561−16571