Elsevier

Nano Energy

Volume 100, September 2022, 107482
Nano Energy

Promoting the performances of P2-type sodium layered cathode by inducing Na site rearrangement

https://doi.org/10.1016/j.nanoen.2022.107482Get rights and content

Highlights

  • Targeted regulation of the arrangement of Na+ at two distinct Wyckoff positions is achieved in P2-type layered cathodes.

  • The modified material exhibits fast and smooth kinetic process with outstanding rate performance.

  • The rearranged Na layer ensures stable and robust structure at the whole voltage range of 2.0–4.3 V.

  • DFT calculation reveals the mechanism of Na+ rearrangement and the role of different Na sites.

Abstract

The electrochemical performance and structural stability of sodium-ion battery is substantially dependent on the occupancy and distribution of Na+ in cathode materials. However, it is challenging to simultaneously regulate the occupancy and optimize the distribution of Na+ in cathodes for higher capacity and superior cyclability. Here we attempt to adjust the arrangement of Na+ in layered cathode materials by applying a combination approach, including enhancing the Na content, disrupting transition metal ordering and strengthening Na+-TMn+ electrostatic force. Through detailed structural characterizations on cathodes, it is revealed that the rearrangement of Na+ at two distinct Wyckoff positions can be realized in Li/Ti-codoped Na2/3Ni1/3Mn2/3O2 cathodes, contributing to outstanding rate performance and smooth kinetic process. In addition, the inhibited P2-O2 phase transition and intact lattice structure is closely related to the rearranged Na layer and strengthened transition metal slab, jointly resulting in excellent long cycling performance with 90.2% capacity retention after 200 cycles at 1 C (150 mA/g). This work sheds new light on the role of different Na sites and provides a universal and practical approach to adjusting the Na+ distribution in P2-type cathode materials.

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The rearrangement of Na+ at two distinct Wyckoff positions can be realized in P2-type Na2/3Ni1/3Mn2/3O2 cathode via Li/Ti codoping. The new Na+ coordination and distribution show high superiority in the Na+ migration and contributes to the high voltage stability, which makes this regulation an effective approach for the design of high-performance sodium layered cathodes.

Introduction

To help address global environmental problems caused by carbon emissions, the demand for developing clean energy is growing rapidly [1]. As one of the most promising technologies, lithium-ion batteries (LIBs) have been widely used in different scenarios [2], [3]. However, the existing lithium resources on earth cannot meet the continuous needs of portable energy storage devices, not to mention large-scale energy storage systems [4]. In this case, rechargeable sodium-ion batteries (SIBs) are good alternative considering the abundant sodium resources and relatively mature technique that can be inherited from LIBs [5], [6]. For multiple components of SIBs, cathode is the key factor that determines the energy density and operative voltage, among which layered transition-metal oxide (NaxTMO2, 0 <x ≤ 1, TM=Ni, Co, Mn, Fe, V, etc.) is one of the most applicable candidates due to its high energy density and simple production methods [7], [8]. According to the different coordination of sodium ions and the sequences of oxygen atoms, NaxTMO2 can be classified as either P2 or O3 type [9]. Generally, O3-type cathode materials are noted for their high capacity, with fully occupied octahedral (O) sites by Na+. Nevertheless, complex phase transitions during the charging process and natural kinetic defects limit its practical application [10], [11]. In contrast, the P2-type cathode materials have more potential in terms of application prospects, as P2-type cathode materials possess unobstructed Na+ migration pathways due to unique local environments of prismatic (P) site [12]. Whereas irreversible structural destruction caused by a large volume change at high voltage suppresses its capacity release and long cycle performance [13]. Moreover, initial Na deficiency in the Na layers restricts the discharge capacity of P2-type cathode materials, especially in full cell usage [14].

Normally, there are two ways to prolong the lifespan of P2-type cathode materials. One is to narrow the working voltage range, which can avoid structural evolution at high voltage but will inevitably sacrifice a part of capacity [15]. The other is to modify transition metal (TM) layers by introducing active or inactive metal ions (Cu2+, Fe2+, Mg2+,Ti4+) [16], [17], [18], [19]. As a result, the solid-solution reaction is reported to be sustained over a wide voltage range, and the high voltage P2-O2/OP4 phase transition is relieved, which ensure smoother and longer cycling performance. Many researchers ascribed these improvements to the break of Na+/vacancy ordering, which is highly involved with the Na+ arrangement in Na layers [20]. Generally, two different Na sites co-exist in the Na layer of P2-type cathode. Empirically, the Na sites sharing faces with two MO6 octahedra are labeled as Naf sites, while the Na sites sharing only edges with MO6 octahedra are labeled as Nae sites [21]. Na+ resided in Nae site typically appears to extract faster than Na+ in the Naf site [12]. The two distinct Wyckoff sites are occupied as a particular manner in the balance of Columbic forces between Na+-TMn+, Na+-Na+ and electrons [22]. Recently, more and more researches focused on the substantial effect of Na+ distribution on Na+ kinetic transport and high voltage structural stability. For instance, Na+ coordinating with different TMn+ is verified to have quite inequable migration barrier. The incorporation of specific TM atoms can facilitate better kinetic process [23]. In addition, a well-controlled phase transition process is verified to be concerned with the uniform distribution of Na+ at high voltage [24]. Thus, it can be assumed that rational manipulation on the arrangement of Na+ in Na layers is the key to obtain high-performance P2-type cathode materials.

To adjust the Na+ distribution rationally, quite a few factors should be taken into account. First, the arrangement of Na+ in the Na layer is quite sensitive to the Na content. On the one hand, Na+/vacancy ordering patterns vary with Na stoichiometries in the pristine materials and this ordering will go through rearrangement during the charging process due to further migration of Na+ out of the Na layers, which heavily limits the diffusion of Na+ [25], [26]. In addition, cathode materials with a higher Na content tend to maintain more Na+ in the interlayers when the same amount of Na is deintercalated; thus, TMO2 slab sliding is inhibited due to lowered TMn+-TMn+ and Na+-TMn+ electrostatic repulsions [27]. On the other hand, the distribution of Na+ in the lattice is sensitive to the arrangement of TM layer. By introducing exotic elements into TM layers to disrupt TM atom ordering, suppressed Na+/vacancy ordering and lower Na+ diffusion barriers are observed in Na layers, which has been demonstrated both experimentally and theoretically [20], [23], [28], [29]. Moreover, Na+ is ascertained to have preferred occupation around a particular TMn+, by breaking which improved electrochemical performance can be achieved [30]. Despite these attractive findings, it is still unclear how to initiatively regulate the distribution of Na+ in Na layers, to promote electrochemical processes and to understand the underlying working mechanism.

Herein, Li+ and Ti4+ were chosen to be doped into the TM layer of P2-type Na2/3Ni1/3Mn2/3O2 oxide for the targeted regulation of Na+ distribution. According to the previous reports, Li+ and Ti4+ are electrochemically inactive in sodium layered cathode and could successfully substitute Ni2+ and Mn4+ without impurity, which makes them good candidates for the research [19], [31]. Besides, the incorporation of Li+ could help enhance the Na content in the cathode, and Ti4+ doping was proven to have the effect on disrupting the Na+/vacancy ordering, which create favorable conditions for the Na+ rearrangement [20], [27]. As a result, both TMn+ and Na+/vacancy ordering were effectively suppressed and a continuous variation of Nae/Naf ratio is successfully induced. The fabricated P2-type Na7/9Li1/9Ni2/9Mn5/9Ti1/9O2 cathode with rearranged Na+ exhibits distinguished electrochemical performance, especially with regards to rate capacity of 113 mAh/g at 10 C. Smooth diffusion of Na+ is demonstrated by galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV) tests at different scan rates, which is closely related to the lower Na+ migration barrier brought on by the adjusted Na+ distribution. Moreover, the P2 structure was monitored to be entirely preserved within the whole voltage range of 2.0–4.3 V in the Na+ rearranged cathode, and the particles of charged cathode were detected to be almost intact and thermally stable. From all of the above facts, it is believed that proper adjustment of the Na layers is an effective method to design high-performance sodium cathode materials.

Section snippets

Structural characterization

Four samples containing different types of transition metal elements i.e. Na7/9Ni1/3Mn2/3O2, Na7/9Ni1/3Mn5/9Ti1/9O2, Na7/9Li1/9Ni2/9Mn2/3O2, Na7/9Li1/9Ni2/9Mn5/9Ti1/9O2, were successfully synthesized by solid state reaction and labeled as NM, NMT, LNM, and LNMT, respectively. Here the four samples are designed as the same Na content for better comparison purposes. Their accurate compositions were obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements. The

Conclusion

In summary, we realized Na+ rearrangement in the Na layer of the P2 type NiMn-based cathode through a multi-adjusted strategy. The incorporation of Li+ and Ti4+ breaks the original NM ordering and affects the formation energy of the two different Na+ sites. Thus, intrinsic Na+/vacancy ordering is successfully disrupted and an enhanced Nae/Naf ratio is gradually exhibited in the modified cathodes, corresponding with smoother electrochemical reaction and fast Na+ kinetics. The DFT calculation

CRediT authorship contribution statement

Taolve Zhang: Conceptualization, Investigation, Formal analysis, Visualization, Writing – original draft. Haocheng Ji: Conceptualization, Investigation, Formal analysis, Visualization, Writing – original draft. Xiaohui Hou: Investigation, Methodology. Wenhai Ji: Investigation, Resources. Hui Fang: Investigation, Data curation. Zhongyuan Huang: Methodology, Writing – review & editing. Guojie Chen: Investigation. Tingting Yang: Investigation, Resources. Mihai Chu: Formal analysis. Shenyang Xu:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The research was financially supported by the National Key Research and Development Program of China (2020YFA0406203), National Natural Science Foundation of China (Nos. U2032167, and 52072008), Guangdong Basic and Applied Basic Research Foundation (Grant No. 2019B1515120028), Shenzhen Fundamental Research Program (No. GXWD20201231165807007-202008071214001). We are grateful for the allocation of beamtime at KMC2 and RGBL beamlines, BESSY II, HZB, Germany. And we would like to thank Dr. Götz

Taolve Zhang received his B.S. degree from School of Material Science and Engineering at Central South University, P.R. China in 2019. Now he is pursuing his master's degree at School of Advanced Materials, Peking University under the supervision of Prof. Yinguo Xiao. His research interests mainly focus on the design, synthesis and characterization of cathode materials for lithium and sodium batteries.

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    Taolve Zhang received his B.S. degree from School of Material Science and Engineering at Central South University, P.R. China in 2019. Now he is pursuing his master's degree at School of Advanced Materials, Peking University under the supervision of Prof. Yinguo Xiao. His research interests mainly focus on the design, synthesis and characterization of cathode materials for lithium and sodium batteries.

    Haocheng Ji is currently pursuing his master’s degree at the School of Advanced Materials, Peking University. His research interests are on research of cathode materials for Li- and Na-ion batteries, novel sensing materials, and advanced in-situ/operando characterization techniques.

    Xiaohui Hou received his B.S. degree from Lanzhou University in 2019. Now he is a master student under the supervision of Prof. Jiaxin Zheng at Peking University. His research interests include computational materials and energy materials.

    Dr. Wenhai Ji is a postdoctoral researcher at School of Advanced Materials in Peking University since September 2020. He obtained his PhD degree in 2020 from RWTH Aachen University based on work conducted in Jülich Centre for Neutron Science in Research Centre of Jülich, Germany. His current research interests focus on the structural characterization of materials using X-ray/neutron diffraction, small-angle scattering and X-ray absorption spectroscopy.

    Hui Fang received his B.S. degree from School of Metallurgy and Environment, Central South University in 2020. He is currently pursuing his M.E. degree under the supervision of Prof. Yinguo Xiao in the School of Advanced Materials, Peking University. His research mainly focus on the synthesis and structural characterization of cathode materials of sodium-ion batteries based on X-ray and neutron diffraction methods.

    Zhongyuan Huang received his B.S. degree in Materials Science and Engineering from University of Science and Technology Beijing, P.R. China in 2018. He is pursuing his Ph. D. degree in Prof. Yinguo Xiao’s group at School of Advanced Materials, Peking University, P.R. China. Currently, his research interest is structure characterization of LIBs electrode materials based on X-ray and neutron diffraction methods.

    Guojie Chen received his B.S. degree in Nuclear Engineering and Technology from Sichuan University, P.R. China in 2021. He is pursuing his M.S. degree in Prof. Yinguo Xiao’s group at School of Advanced Materials, Peking University, P.R. China. He is focus on layered cathode materials of lithium and sodium ion batteries through X-ray and neutron diffraction methods.

    Dr. Tingting Yang received her Ph.D. degree in Materials Science from Yanshan University in 2021. In 2021, she was granted by a joint postdoctoral exchange program between Helmholtz Research Center and the Office of China Postdoc Council. Now she is a postdoctor at Peking University Shenzhen Graduate School. Her current research interest mainly focuses on the in situ TEM and cryogenic low-dose investigation of battery materials.

    Mihai Chu received his M.S. degree from School of Advanced Materials, Peking University in 2021. He is currently pursuing his Ph.D. degree under the supervision of Prof. Jie Li in the Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano. His research interests mainly focus on the synthesis of cathode materials for Li- and Na-ion batteries and their structural characterization based on X-ray and neutron diffraction methods.

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    These authors contributed equally to this work.

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