P2 Hybrid Structures for Room

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Design and Comparative Study of O3/P2 Hybrid Structures for Room Temperature Sodium-Ion Batteries Xingguo Qi, Lilu Liu, Ningning Song, Fei Gao, Kai Yang, Yaxiang Lu, Haitao Yang, Yong-Sheng Hu, Zhao-hua Cheng, and Liquan Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11282 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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ACS Applied Materials & Interfaces

Design and Comparative Study of O3/P2 Hybrid Structures for Room Temperature Sodium-Ion Batteries Xingguo Qi,a,b,# Lilu Liu,a,b,# Ningning Song,c Fei Gao,d Kai Yang,d Yaxiang Lu,a* Haitao Yang,c* Yong-Sheng Hu,a,b* Zhao-Hua Cheng,c and Liquan Chena,b a

Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China b School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China c State Key Laboratory of Magnetism, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China d State Key Laboratory of Operation and Control of Renewable Energy & Storage Systems, China Electric Power Research Institute, Beijing 100192, China

ABSTRACT: Rechargeable sodium-ion batteries have drawn increasing attention in serving as candidates for the post lithium-ion batteries in large scale energy storage systems. Layered oxides are the most promising cathode materials and their pure phases (e.g., P2, O3) have been widely investigated. Here we report a series of cathode materials with O3/P2 hybrid phase for sodium-ion batteries, which possesses advantages

of

both

P2

and

O3

structures.

The

designed

material,

Na0.78Ni0.2Fe0.38Mn0.42O2, can deliver a capacity of 86 mAh g-1 with great rate capability and cycling performance. 66% capacity is still maintained when the current rate reaches as high as 10C, and the capacity retention is 90% after 1500 cycles. Moreover, in situ XRD was performed to examine the structure change during electrochemical test in different voltage ranges, and the results demonstrate 4 V as the optimized upper voltage limit, with which smaller polarization, better structural 1

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stability and better cycling performance are achieved. The results obtained here provide new insights in designing cathode materials with optimal structure and improved performance for sodium-ion batteries.

Keywords: Layered Oxide; O3-type; P2-type; Bi-phase; Sodium-ion Batteries

1. Introduction Lithium-ion batteries (LIBs) have dominated the electronic and portable device markets since the commercialization in 1990s, which gradually brought the cost of raw materials up to ~ 20000 $/ton for Li2CO3.1-2 Moreover, the application trend of pure electric vehicles will further increase the demand and lithium will be a strategic resource like fossil oil because of its limited reservation and unbalanced distribution. Due to the abundance of sodium resources and similarity of electrochemical processes between LIBs and sodium-ion batteries (SIBs), SIBs have attracted much attention for large scale energy storage system.3-5 Much effort has been made all around the world to search for applicable cathode materials, among which layered oxide compounds are regarded as the most promising candidates. As a typical layered oxide, NaxMO2 compounds (M= Co, Ni, Mn, Fe, Cr, Cu, etc.)6-19 have been widely studied. It can be divided into several types, in which O3 and P2 are the most common structures named by Delmas et al.20 Specifically, O or P represents the octahedral or prismatic environment for Na sites and the number 2 or 3 indicates the minimum number of transition metal layers in the cell unit. 2


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Materials with O3 structure have larger capacity due to the higher sodium content than that of P2 type. However, there will be one or more phase transitions during the electrochemical process for O3 structure (O3P3), resulting in the unsatisfactory long cycling performance.6 In contrast, P2 type materials, in spite of delivering less capacity, can maintain the structure and therefore possess a better cycling property, which is of more importance in large scale energy storage system. The rate performance of P2 compounds is also better because Na+ ion diffusion is favored in prismatic site. The detailed explanation will be discussed in the later section. To combine the advantages of both structures, researchers have paid much attention to hybrid phases with O3 and P2 structures recently.21-24 Johnson et al. first reported the O3 type NaNi0.5Mn0.5O2 material with little content of P2 phase and a smooth profile with higher power density was obtained.21 However, the O3P3 phase transition still influenced the cycling performance because O3 is in the majority of

the

hybrid

phases.

Zhou

et

al.

thus

designed

the

P2/O3

Na0.66Li0.18Mn0.71Ni0.21Co0.08O2 compound with P2 type structure as the main phase, which delivers a large capacity of 200 mAh g-1 with excellent rate and cycling performance.22 Most recently, Liu et al. also found that little O3 phase in biphasic Na0.67Mn0.55Ni0.25Ti0.2-xLixO2 could improve the reversible capacity, rate capability, cycling performance and Coulombic efficiency.23 All of these work have turned the multiple-phased compounds into attractive candidates for SIBs. On the basis of the above discussion, it seems necessary to comparatively investigate the hybrid phase with different ratios of O3/P2 so as to achieve a balance 3



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among capacity, cycling stability and rate capability. Owing to the low cost, Fe and Mn based layered oxides have been widely studied,26 however, large polarization was observed to deteriorate the cycling performance. It may be resulted from the migration of Fe3+ to the tetrahedral site in the Na layer, which would block the diffusion path of Na+, as reported by Komaba and Nazar et al.26-27 Here we choose Fe and Mn based layered material with a small amount of Ni substitution to study the hybrid phase. The optimal Na content is found during the investigation. Moreover, compared with other Fe and Mn based materials, it is interesting to find that the polarization is suppressed by the hybrid O3/P2 phase in the voltage range of 2.5 – 4 V, as proved by in situ XRD.28 The low cost material with abundant Fe and Mn elements meets the core merit of SIBs and could be a potential candidate for practical application of sodium-ion batteries.

2. Results and discussion 2.1 Structural analysis

4


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Figure 1. (a) X-ray diffraction patterns of NaxNi0.2Fex-0.4Mn1.2-xO2 with different Na contents (0.7-1.0) and the phase evolution derived from XRD patterns is shown in the right. (b) The schematic illustration of O3 (left) and P2 (right) structures. (C) Observed and calculated XRD profiles for NNFM-0.78 with (d) TEM image of this material.

Inspired by LiCoO2, the typical layered oxide applied in LIBs with highest energy density, materials with layered structure have also been widely studied as cathode for SIBs .29 O3 and P2 structures are mostly investigated so far, of which sodium content as well as temperature, have an impact on the structure finally formed.30 In the experiment,

conventional

solid

state

reaction

was

adopted

to

prepare

Nax[Ni0.2Fex-0.4Mn1.2-x]O2 (x=0.7~1.0) series of materials (abbreviated as NNFM-x). Figure 1a compares the XRD patterns of materials with different sodium contents and the related phase evolution is depicted on the right-side. O3 layered structure tends to 5



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be formed when sodium content is higher than 0.8 and O3/P2 hybrid phase is obtained when x

P2 Hybrid Structures for Room

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