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Ni0.85Se hexagonal nanosheets as an advanced conversion cathode for Mg secondary batteries
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Ni0.85 Se hexagonal nanosheets as an advanced conversion cathode for Mg secondary batteries Dong Chen , Jingwei Shen , Xue Li , Shun-an Cao , Ting Li , Wei Luo , Fei Xu PII: DOI: Reference:
S2095-4956(20)30033-4 https://doi.org/10.1016/j.jechem.2020.01.018 JECHEM 1071
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Journal of Energy Chemistry
Received date: Revised date: Accepted date:
23 December 2019 19 January 2020 20 January 2020
Please cite this article as: Dong Chen , Jingwei Shen , Xue Li , Shun-an Cao , Ting Li , Wei Luo , Fei Xu , Ni0.85 Se hexagonal nanosheets as an advanced conversion cathode for Mg secondary batteries, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.018
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Ni0.85Se hexagonal nanosheets as an advanced conversion cathode for Mg secondary batteries
Dong Chena, Jingwei Shena, Xue Lia, Shun-an Caoa, Ting Lib,*, Wei Luoc, Fei Xua,*
a
Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and
Mechanical Engineering, Wuhan University, Wuhan 430072, Hubei, China b
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission,
Ministry of Education, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, Hubei, China c
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei, China
* Corresponding authors. E-mail addresses: [email protected] (F. Xu), [email protected] (T. Li).
Highlight Ni0.85Se with different nanostructures is used as cathode for Mg batteries.
A high capacity of 168 mAh g‒1 is obtained as well as good rate capability. A superior long-term cycleability over 500 cycles is achieved. The thin nanosheet structure enhances solid-state Mg2+ diffusion kinetics.
Abstract Mg secondary batteries are promising scalable secondary batteries for next-generation energy storage. However, Mg-storage cathode materials are greatly demanded to construct high-performance Mg batteries. Electrochemical conversion reaction provides plenty of cathode options, and strategy for cathode selection and performance optimization is of special significance. In this work, Ni0.85Se with nanostructures of dispersive hexagonal nanosheets (D-Ni0.85Se) and flower-like assembled nanosheets (F-Ni0.85Se) is synthesized and investigated as Mg-storage cathodes. Compared with F-Ni0.85Se, D-Ni0.85Se delivers a higher specific capacity of 168 mAh g‒1 at 50 mA g‒1 as well as better rate performance, owing to its faster Mg2+ diffusion and lower resistance. D-Ni0.85Se also exhibits a superior cycling stability over 500 cycles. An investigation on mechanism indicates an evolution of Ni0.85Se towards NiSe with cycling, and the Mg-storage reaction occurs between NiSe and metallic Ni0. The present work demonstrates that advanced conversion-type Mg battery cathode materials could be constructed by soft selenide anions, and the electrochemical properties could be manipulated by rational material morphology optimization.
Keywords: Mg secondary batteries; Electrochemical conversion reaction; Ni0.85Se; Hexagonal
nanosheets; Diffusion kinetics
1. Introduction Scalable energy storage batteries are currently attracting considerable attention [1]. Among the potential candidates, rechargeable Mg battery is considered as an attractive energy storage technique owing to the merits of metallic Mg anode such like low redox potential, low cost and high capacity [2‒5]. Most significantly, Mg anode usually undergoes dendrite-free electrochemical deposition, essentially ensuring high safety for the batteries [6,7]. However, promising Mg-storage cathode materials are relatively limited. This is owing to the high electric density of the bivalent Mg2+ cation, which makes the Mg-intercalation process difficult or sluggish [8‒11]. Electrochemical conversion reaction provides an alternative mechanism for Mg-storage beyond intercalation [12,13]. Actually, there are two key factors governing the development of advanced conversion Mg battery cathodes, namely, the material selection criteria and performance optimization strategy. Interestingly, hard and soft acid and base (HSAB) theory can be employed to interpret the Mg-storage performance [14]. As bivalent Mg2+ is an acid with high hardness, soft base such as Se2‒ would have a relatively weak interaction with Mg2+ and is a preeminent anion for the host framework. Furthermore, selection of the cation is also of great significance. Soft acid cations (Cu+ and Ag+) [15‒19] would interact strongly with the soft anions and assist the dissociation of discharge product. In this respect, it is thought that quasi-soft acids, Ni2+ as an example, could also be selected for Mg-storage cathodes.
As a combination of Ni2+ and Se2‒, nickel selenides have many advantages including high metallic conductivity, high thermal and chemical stability and low cost [20]. Enlightened by the attractive properties, many studies have focused on utilization of nickel selenides as catalysts [21‒24], supercapacitors [25,26] and Li/Na-ion battery materials [27‒30]. Ni and Se can form varies of stoichiometric and non-stoichiometric compounds (NiSe2, Ni3Se2, NiSe and Ni0.85Se) due to their small difference in electronegativity. Among them, non-stoichiometric Ni0.85Se is of special importance because of its unsaturated atoms and unique electronic configuration [31]. Furthermore, Ni0.85Se is regarded as a promising material for energy storage with its high electroconductivity and high theoretical capacity in several recent works [32,33]. However, its electrochemical Mg-storage properties in rechargeable Mg batteries are far from being reported. Generally, the electrochemical performances strongly depend on the morphologies of electrode materials, and low dimensional nanostructures are preferentially favored [34,35]. Herein, a facile one-step solvothermal method is adopted without usage of expensive reagents or complicated instruments, to fabricate Ni0.85Se nanostructures including dispersive hexagonal nanosheets (D-Ni0.85Se) and flower-like assembled nanosheets (F-Ni0.85Se). Mg-storage performance investigation demonstrates Ni0.85Se could undergo highly reversible conversion with Mg2+ ions, indicating the privilege of using a soft Se2− anion to construct Mg battery cathodes. Moreover, D-Ni0.85Se shows better Mg-storage performance than F-Ni0.85Se, providing a high capacity of 168 mAh g‒1 and a superior cycleability for 500 cycles. This study provides scientific understanding for rational selection of advanced Mg
battery cathodes using HSAB theory, and delivers strategies for facile synthesis and nanostructure tailoring of selenides.
2. Experimental 2.1 Materials and synthesis Ni0.85Se was synthesized via solvothermal reactions. Typically, 2 mmol of nickel acetate tetrahydrate (Ni(ac)2·4H2O) and 8 mmol of Se powder were added to hydrazine (aqueous solution, wt.80%, 40 mL) and ultrasonically treated for 10 min. Then the solution was added to a solvothermal reactor (100 mL) and maintained at 140 ℃ for 2.5 h. The solid D-Ni0.85Se product was obtained by filtration and washed with water for several times. For comparison, F-Ni0.85Se was prepared by the same method by using equal mol of NiSO4·6H2O instead of Ni(ac)2·4H2O. The Mg electrolyte used in this study was a tetraglyme (10 mL, Alfa asear, 99%, anhydrous) solution of magnesium bis(hexamethyldisilazide) (Mg(HMDS)2, 2 mmol, Sigma-Aldrich, 97%), AlCl3 (4 mmol, Alfa asear, 99.999%) and MgCl2 (2 mmol, Alfa asear, 99%). The above solutes were added to tetraglyme in sequence under stirring. The whole process was performed in a Ar glovebox (Mikrouna, Super, H2O and O2 < 0.1 ppm). 2.2 Characterization X-ray diffraction (XRD) was performed on a D8 advance XRD (Bruker, Cu K radiation). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted on FEI Quanta-200 and JEOL JEM-2100FEF,
respectively, to observe the morphology. X-ray photoelectron spectroscopy (XPS) was carried out on VG MultiLab 2000 spectrometer. 2.3 Electrochemical measurements The
electrodes
were
prepared
using
Ni0.85Se,
Ketjen
Black
and
polytetrafluoroethylene (PTFE) as the active material, conductive carbon and binder, respectively, with a weight ratio of 7:2:1. Isopropanol was used for the preparation of the electrode slurry, which was coated onto carbon cloth current collector and dried under vacuum at 70 ℃. The diameter of the electrodes was 12 mm and the active material was ~1.6 mg cm–2. Electrochemical measurements were conducted via bespoke cells (Fig. S1) [36‒38], using the as-prepared electrodes as cathode, Mg foil as anode, a glass microfiber film as separator, and the as-prepared Mg electrolyte. Galvanostatic cycling, cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) were performed on a LAND cycler (Kingnuo Co., China), an electrochemical workstation (CHI 660e) and an impedance measuring unit (IM 6e, Zahner), respectively. For EIS measurements, AC voltage amplitude of 10 mV was applied with frequency ranges from 100 kHz to 0.1 Hz.
3. Results and discussion Fig. 1(a) exhibits XRD patterns of D-Ni0.85Se and F-Ni0.85Se. All the peaks could be indexed to hexagonal Ni0.85Se with a space group of P63/mmc (JCPDS No. 18-0888), indicating the high purity and good crystallinity of the samples. Fig. 1(b and f) shows SEM images of D-Ni0.85Se and F-Ni0.85Se, respectively, and Fig. 1(c and
g) exhibits the corresponding magnified images. It is observed that D-Ni0.85Se appears as dispersive hexagonal nanosheets with varies of sizes (Fig. 1b and c). The diameter of nanosheets ranges from 50 to 200 nm, which can be further illustrated from TEM images (Fig. 1d and Fig. S2). In Fig. 1(e) (HRTEM image), lattice fringe for (101) plane of hexagonal Ni0.85Se is observed (spacing of 0.27 nm). As for F-Ni0.85Se, interconnected irregular nanosheets self-assemble into nanoflowers rather than dispersed structures (Fig. 1f and g). TEM image shown in Fig. 1(h) demonstrates that these nanosheets have a thickness of about 10 nm. The (101) plane is also observed in Fig. 1(i). These results demonstrate that different nickel salts used for the solvothermal process would result in large difference in morphology for the products.
Fig. 1. (a) XRD patterns of D-Ni0.85Se and F-Ni0.85Se. (b, c) SEM and (d, e) TEM images of D-Ni0.85Se. (f, g) SEM and (h, i) TEM images of F-Ni0.85Se. (j) The diagram of reaction procedure for D-Ni0.85Se and F-Ni0.85Se. The synthesis process is schematically illustrated in Fig. 1(j). The nucleation and growth rates of the products could be manipulated by tuning the precursors which release the reacting ions with a moderate speed. Hence, products with different morphologies could be obtained [39]. The different morphologies between these two samples are reasonably attributed to the different reaction rates. Nickel acetate (Ni(ac)2) forms violet nickel-hydrazine complex upon addition into hydrazine hydrate,
which will react with Se2‒ (formed from Se powder in basic solution) at high temperature with a high rate. In contrast, NiSO4 is difficult to coordinate with hydrazine hydrate, and could only release Ni2+ for the reaction with a low rate, which provides enough time for the growth of nanocrystals into nanoflowers. Because of the excessive Se2‒, Ni0.85Se was generated instead of stoichiometric NiSe. Moreover, a short reaction time of 2.5 h is adopted to prevent reaction of Se2‒ with Se into Se22‒. Predictably, D-Ni0.85Se with nanosheet morphology would provide a higher specific capacity with its short-range Mg2+ diffusion path, while F-Ni0.85Se with its stable hierarchical structure, rather than typical 2D structure, could have a better cycleability. Such Ni0.85Se with 2D or hierarchical structures could be also used in other fields such like catalyst, since these structures provide plenty of active sites and enhanced stability [40]. XPS was conducted to reveal the valance state of the samples. The Ni 2p and Se 3d peaks are well observed in the overall XPS spectra (Fig. 2a). As shown in Fig. 2(b), both of Ni 2p3/2 and Ni 2p1/2 signals are separated into three peaks [41,42]. The peaks at 853.5 and 870.9 eV are attributed to Ni2+ ions, and the two peaks at 856.0 and 874.1 eV originate from Ni3+ ions formed by surface oxide. The Ni2+ oxidation state is also suggested by the satellite peaks at 860.8 and 879.9 eV. In the Se 3d spectra (Fig. 2c), the presence of Se2‒ is demonstrated by Se 3d5/2 (54.2 eV) and Se 3d3/2 (55.0 eV) peaks. Slight surface oxidation is indicated by the broad peak at 59.2 eV, which is common for synthesized selenides but would not obviously affect the electrochemical properties [41,42].
Fig. 2. (a) Full XPS spectra and detailed spectra of (b) Ni 2p and (c) Se 3d for D-Ni0.85Se and F-Ni0.85Se. Prior to the battery tests, the Mg electrolyte was first evaluated by CV and galvanostatic cycling, which demonstrate its high compatibility with metallic Mg anode (Fig. S3). The Mg-storage properties of the Ni0.85Se cathodes were then investigated. Fig. 3(a and b) describes the charge/discharge curves at 50 mA g‒1 for D-Ni0.85Se and F-Ni0.85Se, respectively. Usually, conversion-type electrode would experience an activation process in initial cycles, during which the reversible capacity increases along with voltage polarization decrease. Such an activation process could be observed for both of D-Ni0.85Se and F-Ni0.85Se, but the activation degree of D-Ni0.85Se is obviously higher than that of F-Ni0.85Se, possibly owing to its structure features favorable for the formation of nanograins facilliating reversible conversion reactions. After 60 cycles, one discharge plateau was located at 0.95 V and two charge plateaus were located at 1.65 V and 1.95 V (Fig. 3a and b). Obviously, the degree of activation for F-Ni0.85Se is lower than that for D-Ni0.85Se, which can be also suggested by the cycling performance (Fig. 3c and d). As seen in Fig. 3(c), D-Ni0.85Se shows a remarkable Mg-storage capacity of 168 mAh g–1 after the activation, while F-Ni0.85Se provides a relatively lower reversible capacity (Fig. 3d, 96 mAh g–1). It is noteworthy
that both of the Ni0.85Se cathodes can undergo highly reversible conversion with Mg2+ ions, demonstrating the merits of using soft Se2‒ anion for Mg-storage cathodes. It is observed that the capacity of D-Ni0.85Se displays a slight decreasing tendency after 60 cycles, while F-Ni0.85Se shows a better cycleability, suggesting the privilege of hierarchical nanoflower structure in cycling stability. CV curves of the Ni0.85Se cathodes after 60 cycles were also measured. As shown in Fig. 3(e and f), both of D-Ni0.85Se and F-Ni0.85Se show a reduction peak and two oxidation peaks with different current densities, which agrees well with the charge/discharge profiles above. The difference in capacity and cycleability for the Ni0.85Se cathodes demonstrates the importance of morphology regulation for conversion-type Mg-storage materials. Firstly, construction of nanostructures, especially low-dimensional nanostructures are indispensable to enhance the solid-state Mg2+ ion diffusion kinetics. Secondly, for a better cycleability, employment of hierarchical structure is necessary for the electrode stability during cycling. Usually, simple nanomaterials are facile to collapse during long-term conversion reactions, possibly due to the nanograins aggregation. However, hierarchical micro-/nano-structure have higher material integrity than simple nanomaterials due to their larger micro-structures and lower tendency toward collapse or particle aggregation.
Fig. 3. Charge/discharge curves of (a) D-Ni0.85Se and (b) F-Ni0.85Se as Mg battery cathodes. Cycling performance (at 50 mA g–1) of (c) D-Ni0.85Se and (d) F-Ni0.85Se cathodes. CV profiles of (e) D-Ni0.85Se and (f) F-Ni0.85Se cathodes after 60 cycles at a scan rate of 0.1 mV s–1. The rate performance of the Ni0.85Se cathodes is shown in Fig. 4(a‒c). D-Ni0.85Se exhibits a high capacity of 92 mA h g‒1 at 500 mA g‒1, demonstrating the merits of selenides as Mg-storage cathodes. However, F-Ni0.85Se delivers a lower capacity of 61 mA h g‒1 at this current density, illustrating the importance of morphology on the performance. Thus, 2D nanostructure such as hexagonal nanosheets could provide not only a higher Mg-storage capacity but also a better rate capability. Fig. 4(d) compares the rate capability of D-Ni0.85Se with typical Mg batteries cathodes [8,9,10,14,43,44], in which D-Ni0.85Se exhibits an obviously better performance. For the long-term cycleability (Fig. 4e), D-Ni0.85Se experiences a gradual activation and fading process, resulting in a remarkable cycleability for 500 cycles (see Fig. S4 for F-Ni0.85Se). It should be noted that all these electrochemical results are obtained without carbon coating on materials, which is especially important for scalable application of rechargeable Mg batteries.
Fig. 4. Typical charge/discharge curves of (a) D-Ni0.85Se and (b) F-Ni0.85Se. (c) Rate performance of D-Ni0.85Se and F-Ni0.85Se. (d) Rate capability comparison of Mg battery cathode materials in this work and previous literatures [8,9,10,14,43,44]. (e) Long-term cycling of D-Ni0.85Se at 200 mA g‒1. The Mg-storage reaction mechanism is investigated via XRD, XPS and TEM. Fig. 5(a) shows the ex-situ XRD of Ni0.85Se cathodes at different states. Cycled separator is used for the characterization since the active material tends to remain on the separator. As seen in Fig. 5(a), the two peaks at 36.8° and 41.0° are ascribed to the separator (See Fig.S5 for its overall XRD pattern). For the two discharged states (30th and 60th cycles), the diffraction peaks at 32.8°, 44.4° and 49.9° are respectively indexed to be (101), (102) and (110) planes of NiSe (JCPDS No. 02-0892) rather than Ni0.85Se, indicating a phase change along with cycling. The phenomenon of phase change is common for chalcogenides to be used as conversation-type cathodes for Mg batteries [14,45]. When other selenides such like NiSe2 were reacted with Mg2+, similar phenomena were also observed that the electrochemical reaction was mainly based on NiSe [44]. When charged to 2.4 V, the peaks shifted to high-angle region. Such a shift is significant at 30th cycle, but it is not obvious at 60th cycle. This
indicates an irreversible phase transformation with cycling, and the Mg-storage reaction is a conversion reaction rather than intercalation. In the discharge states, MgSe and metallic Ni0 peaks are not observed due to the poor crystallinity, which is usually observed for conversion materials. MgSe and metallic Ni0 form nanograins, which would lead to broad XRD peaks difficult to identify. The Mg-storage mechanism could also be illustrated by XPS results. As seen in Fig. 5(b), the shape of Ni 2p peaks changes to some extent upon mix with carbon, but the peak position does not shift. The peaks emerged at 852.5 and 869.8 eV of the fully discharged state (0.01V) correspond to Ni0 2p3/2 and Ni0 2p1/2 [32], respectively, demonstrating the existence of metallic Ni0. In the charge state (2.4 V), the weak Ni0 peaks indicate a small part of metallic Ni0 is remained, serving as a conductive framework favors the reaction reversibility. In Fig. 5(c), the Mg 2p band at 50.1 eV originates from discharge product of MgSe [46,47]. Furthermore, TEM observation was also carried out for the conformation. As shown in Fig. 5(d,e), D-Ni0.85Se could maintain the hexagonal nanosheets morphology after cycling. In a HRTEM image at the discharged state (Fig. 5f), the (111) plane of cubic metallic Ni0 (JCPDS No. 88-2326) could be clearly observed with a spacing of 0.20 nm.
Fig. 5. (a) Ex-situ XRD patterns at different states after 30 and 60 cycles. XPS spectra of (b) Ni 2p and (c) Se 3d at different states after 60 cycles. (d, e) TEM and (f) HRTEM images of D-Ni0.85Se at discharge state after 60 cycles. According to the XRD, XPS and TEM results above and reported observations [32,33,44], the Mg-storage mechanism of Ni0.85Se is as follows. During the activation process, an irreversible phase transformation from Ni0.85Se to NiSe occurred, providing a low theoretical capacity of about 29 mAh g‒1. Meanwhile, NiSe underwent reversible conversion with Mg2+ ions, which became prominent as the cycling proceeded. Mg ions react with Ni2+ to form Ni0 during the magnesiation process (discharge), and reverse reaction occurs in the demagnesiation process (charge). As a small portion of metallic Ni0 does not participate in the reaction, the reversible capacity is slightly lower than the theoretical capacity (390 mAh g‒1). Further kinetic analyses were also conducted to understand the different performance of the Ni0.85Se cathodes. Fig. 6(a and b) describe CV profiles at different scan rates for D-Ni0.85Se and F-Ni0.85Se cathodes, respectively. As shown in Fig. 6(c
and d), a relationship of i = avb could be simulated between peak current (i) and scan rate (v) [48,49]. In Fig. 6(c and d), D-Ni0.85Se exhibits relatively higher b values than F-Ni0.85Se, indicating the enhanced pseudo-capacitive behavior of hexagonal nanosheet morphology. Fig. 6(e and f) compares the pseudo-capacitive contribution of D-Ni0.85Se and F-Ni0.85Se [50,51]. D-Ni0.85Se exhibits higher pseudo-capacitive contributions than F-Ni0.85Se at all scan rates, which is in good accordance with the b value analysis. The hexagonal nanosheet structure favors faradic reaction at the surface or near-surface, thus leading to a better rate performance for D-Ni0.85Se.
Fig. 6. CV profiles at various scan rates for (a) D-Ni0.85Se and (b) F-Ni0.85Se. Log (peak current, i) vs. log (scan rate, v) for (c) D-Ni0.85Se and (d) F-Ni0.85Se. (e) Pseudo-capacitive contribution of D-Ni0.85Se at 1.0 mV s‒1. (f) Pseudo-capacitive contributions for D-Ni0.85Se and F-Ni0.85Se at various scan rates. Soild-state Mg2+ diffusion kinetics was also determined using the CV results above (see Table S1 for the details) [52,53]. The apparent Mg2+ diffusion coefficients (DMg2+) of peaks C and A are 4.8×10‒11 and 7.9×10‒11 cm2 s‒1 for D-Ni0.85Se, respectively, which are much higher than those of F-Ni0.85Se (1.2×10‒11 and 2.1×10‒11 cm2 s‒1). The hexagonal nanosheets of D-Ni0.85Se establish short-range Mg2+ diffusion
path, thus enhancing the solid-state Mg2+ diffusion kinetics. Moreover, EIS results in Fig. S6 reveal that the resistance of D-Ni0.85Se is much lower than those of F-Ni0.85Se at fully discharge state after 60 cycles, suggesting a faster near-surface charge transfer reaction in the 2D nano-structure [54].
4. Conclusions In summary, a facile one-step solvothermal synthesis is used to prepare two Ni0.85Se nano-materials including dispersive hexagonal nanosheets and flower-like assembled nanosheets, whose morphologies exhibit large difference owing to the different reaction rates. The electrochemical performance as Mg-storage materials are systematically
investigated
for
a
comprehensive
understanding
of
the
structure-property relationship. D-Ni0.85Se deliveries high reversible capacities of 168 and 92 mAh g‒1 at 50 and 500 mA g‒1, respectively, which is benefited from the 2D structure facilitating the pseudo-capacitive behavior and solid-state Mg2+ diffusion. Moreover, D-Ni0.85Se exhibits a remarkable long-term cycling stability for 500 cycles. The Ni0.85Se cathodes experience a phase transformation toward NiSe during initial cycles, and the Mg-storage reaction is mainly between NiSe and metallic Ni0. This work not only explores new Mg-storage materials, but also provides significant understanding for the development of advanced conversion Mg battery cathodes.
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. Declaration of interests
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 This work was financially supported by Intergovernmental International Science and Technology Innovation Cooperation Project (2019YFE010186), the Hubei Provincial Natural Science Foundation (2019CFB452 and 2019CFB620) and the Fundamental Research Funds for the Central Universities.
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TOC
Ni0.85Se with different structures is used as Mg secondary battery cathode, exhibiting high capacity, remarkable long-term cycleability and fast solid-state Mg2+ diffusion kinetics.
Ni0.85 Se hexagonal nanosheets as an advanced conversion cathode for Mg secondary batteries Dong Chen , Jingwei Shen , Xue Li , Shun-an Cao , Ting Li , Wei Luo , Fei Xu PII: DOI: Reference:
S2095-4956(20)30033-4 https://doi.org/10.1016/j.jechem.2020.01.018 JECHEM 1071
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
23 December 2019 19 January 2020 20 January 2020
Please cite this article as: Dong Chen , Jingwei Shen , Xue Li , Shun-an Cao , Ting Li , Wei Luo , Fei Xu , Ni0.85 Se hexagonal nanosheets as an advanced conversion cathode for Mg secondary batteries, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.018
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Ni0.85Se hexagonal nanosheets as an advanced conversion cathode for Mg secondary batteries
Dong Chena, Jingwei Shena, Xue Lia, Shun-an Caoa, Ting Lib,*, Wei Luoc, Fei Xua,*
a
Key Laboratory of Hydraulic Machinery Transients, Ministry of Education, School of Power and
Mechanical Engineering, Wuhan University, Wuhan 430072, Hubei, China b
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission,
Ministry of Education, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, Hubei, China c
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei, China
* Corresponding authors. E-mail addresses: [email protected] (F. Xu), [email protected] (T. Li).
Highlight Ni0.85Se with different nanostructures is used as cathode for Mg batteries.
A high capacity of 168 mAh g‒1 is obtained as well as good rate capability. A superior long-term cycleability over 500 cycles is achieved. The thin nanosheet structure enhances solid-state Mg2+ diffusion kinetics.
Abstract Mg secondary batteries are promising scalable secondary batteries for next-generation energy storage. However, Mg-storage cathode materials are greatly demanded to construct high-performance Mg batteries. Electrochemical conversion reaction provides plenty of cathode options, and strategy for cathode selection and performance optimization is of special significance. In this work, Ni0.85Se with nanostructures of dispersive hexagonal nanosheets (D-Ni0.85Se) and flower-like assembled nanosheets (F-Ni0.85Se) is synthesized and investigated as Mg-storage cathodes. Compared with F-Ni0.85Se, D-Ni0.85Se delivers a higher specific capacity of 168 mAh g‒1 at 50 mA g‒1 as well as better rate performance, owing to its faster Mg2+ diffusion and lower resistance. D-Ni0.85Se also exhibits a superior cycling stability over 500 cycles. An investigation on mechanism indicates an evolution of Ni0.85Se towards NiSe with cycling, and the Mg-storage reaction occurs between NiSe and metallic Ni0. The present work demonstrates that advanced conversion-type Mg battery cathode materials could be constructed by soft selenide anions, and the electrochemical properties could be manipulated by rational material morphology optimization.
Keywords: Mg secondary batteries; Electrochemical conversion reaction; Ni0.85Se; Hexagonal
nanosheets; Diffusion kinetics
1. Introduction Scalable energy storage batteries are currently attracting considerable attention [1]. Among the potential candidates, rechargeable Mg battery is considered as an attractive energy storage technique owing to the merits of metallic Mg anode such like low redox potential, low cost and high capacity [2‒5]. Most significantly, Mg anode usually undergoes dendrite-free electrochemical deposition, essentially ensuring high safety for the batteries [6,7]. However, promising Mg-storage cathode materials are relatively limited. This is owing to the high electric density of the bivalent Mg2+ cation, which makes the Mg-intercalation process difficult or sluggish [8‒11]. Electrochemical conversion reaction provides an alternative mechanism for Mg-storage beyond intercalation [12,13]. Actually, there are two key factors governing the development of advanced conversion Mg battery cathodes, namely, the material selection criteria and performance optimization strategy. Interestingly, hard and soft acid and base (HSAB) theory can be employed to interpret the Mg-storage performance [14]. As bivalent Mg2+ is an acid with high hardness, soft base such as Se2‒ would have a relatively weak interaction with Mg2+ and is a preeminent anion for the host framework. Furthermore, selection of the cation is also of great significance. Soft acid cations (Cu+ and Ag+) [15‒19] would interact strongly with the soft anions and assist the dissociation of discharge product. In this respect, it is thought that quasi-soft acids, Ni2+ as an example, could also be selected for Mg-storage cathodes.
As a combination of Ni2+ and Se2‒, nickel selenides have many advantages including high metallic conductivity, high thermal and chemical stability and low cost [20]. Enlightened by the attractive properties, many studies have focused on utilization of nickel selenides as catalysts [21‒24], supercapacitors [25,26] and Li/Na-ion battery materials [27‒30]. Ni and Se can form varies of stoichiometric and non-stoichiometric compounds (NiSe2, Ni3Se2, NiSe and Ni0.85Se) due to their small difference in electronegativity. Among them, non-stoichiometric Ni0.85Se is of special importance because of its unsaturated atoms and unique electronic configuration [31]. Furthermore, Ni0.85Se is regarded as a promising material for energy storage with its high electroconductivity and high theoretical capacity in several recent works [32,33]. However, its electrochemical Mg-storage properties in rechargeable Mg batteries are far from being reported. Generally, the electrochemical performances strongly depend on the morphologies of electrode materials, and low dimensional nanostructures are preferentially favored [34,35]. Herein, a facile one-step solvothermal method is adopted without usage of expensive reagents or complicated instruments, to fabricate Ni0.85Se nanostructures including dispersive hexagonal nanosheets (D-Ni0.85Se) and flower-like assembled nanosheets (F-Ni0.85Se). Mg-storage performance investigation demonstrates Ni0.85Se could undergo highly reversible conversion with Mg2+ ions, indicating the privilege of using a soft Se2− anion to construct Mg battery cathodes. Moreover, D-Ni0.85Se shows better Mg-storage performance than F-Ni0.85Se, providing a high capacity of 168 mAh g‒1 and a superior cycleability for 500 cycles. This study provides scientific understanding for rational selection of advanced Mg
battery cathodes using HSAB theory, and delivers strategies for facile synthesis and nanostructure tailoring of selenides.
2. Experimental 2.1 Materials and synthesis Ni0.85Se was synthesized via solvothermal reactions. Typically, 2 mmol of nickel acetate tetrahydrate (Ni(ac)2·4H2O) and 8 mmol of Se powder were added to hydrazine (aqueous solution, wt.80%, 40 mL) and ultrasonically treated for 10 min. Then the solution was added to a solvothermal reactor (100 mL) and maintained at 140 ℃ for 2.5 h. The solid D-Ni0.85Se product was obtained by filtration and washed with water for several times. For comparison, F-Ni0.85Se was prepared by the same method by using equal mol of NiSO4·6H2O instead of Ni(ac)2·4H2O. The Mg electrolyte used in this study was a tetraglyme (10 mL, Alfa asear, 99%, anhydrous) solution of magnesium bis(hexamethyldisilazide) (Mg(HMDS)2, 2 mmol, Sigma-Aldrich, 97%), AlCl3 (4 mmol, Alfa asear, 99.999%) and MgCl2 (2 mmol, Alfa asear, 99%). The above solutes were added to tetraglyme in sequence under stirring. The whole process was performed in a Ar glovebox (Mikrouna, Super, H2O and O2 < 0.1 ppm). 2.2 Characterization X-ray diffraction (XRD) was performed on a D8 advance XRD (Bruker, Cu K radiation). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted on FEI Quanta-200 and JEOL JEM-2100FEF,
respectively, to observe the morphology. X-ray photoelectron spectroscopy (XPS) was carried out on VG MultiLab 2000 spectrometer. 2.3 Electrochemical measurements The
electrodes
were
prepared
using
Ni0.85Se,
Ketjen
Black
and
polytetrafluoroethylene (PTFE) as the active material, conductive carbon and binder, respectively, with a weight ratio of 7:2:1. Isopropanol was used for the preparation of the electrode slurry, which was coated onto carbon cloth current collector and dried under vacuum at 70 ℃. The diameter of the electrodes was 12 mm and the active material was ~1.6 mg cm–2. Electrochemical measurements were conducted via bespoke cells (Fig. S1) [36‒38], using the as-prepared electrodes as cathode, Mg foil as anode, a glass microfiber film as separator, and the as-prepared Mg electrolyte. Galvanostatic cycling, cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) were performed on a LAND cycler (Kingnuo Co., China), an electrochemical workstation (CHI 660e) and an impedance measuring unit (IM 6e, Zahner), respectively. For EIS measurements, AC voltage amplitude of 10 mV was applied with frequency ranges from 100 kHz to 0.1 Hz.
3. Results and discussion Fig. 1(a) exhibits XRD patterns of D-Ni0.85Se and F-Ni0.85Se. All the peaks could be indexed to hexagonal Ni0.85Se with a space group of P63/mmc (JCPDS No. 18-0888), indicating the high purity and good crystallinity of the samples. Fig. 1(b and f) shows SEM images of D-Ni0.85Se and F-Ni0.85Se, respectively, and Fig. 1(c and
g) exhibits the corresponding magnified images. It is observed that D-Ni0.85Se appears as dispersive hexagonal nanosheets with varies of sizes (Fig. 1b and c). The diameter of nanosheets ranges from 50 to 200 nm, which can be further illustrated from TEM images (Fig. 1d and Fig. S2). In Fig. 1(e) (HRTEM image), lattice fringe for (101) plane of hexagonal Ni0.85Se is observed (spacing of 0.27 nm). As for F-Ni0.85Se, interconnected irregular nanosheets self-assemble into nanoflowers rather than dispersed structures (Fig. 1f and g). TEM image shown in Fig. 1(h) demonstrates that these nanosheets have a thickness of about 10 nm. The (101) plane is also observed in Fig. 1(i). These results demonstrate that different nickel salts used for the solvothermal process would result in large difference in morphology for the products.
Fig. 1. (a) XRD patterns of D-Ni0.85Se and F-Ni0.85Se. (b, c) SEM and (d, e) TEM images of D-Ni0.85Se. (f, g) SEM and (h, i) TEM images of F-Ni0.85Se. (j) The diagram of reaction procedure for D-Ni0.85Se and F-Ni0.85Se. The synthesis process is schematically illustrated in Fig. 1(j). The nucleation and growth rates of the products could be manipulated by tuning the precursors which release the reacting ions with a moderate speed. Hence, products with different morphologies could be obtained [39]. The different morphologies between these two samples are reasonably attributed to the different reaction rates. Nickel acetate (Ni(ac)2) forms violet nickel-hydrazine complex upon addition into hydrazine hydrate,
which will react with Se2‒ (formed from Se powder in basic solution) at high temperature with a high rate. In contrast, NiSO4 is difficult to coordinate with hydrazine hydrate, and could only release Ni2+ for the reaction with a low rate, which provides enough time for the growth of nanocrystals into nanoflowers. Because of the excessive Se2‒, Ni0.85Se was generated instead of stoichiometric NiSe. Moreover, a short reaction time of 2.5 h is adopted to prevent reaction of Se2‒ with Se into Se22‒. Predictably, D-Ni0.85Se with nanosheet morphology would provide a higher specific capacity with its short-range Mg2+ diffusion path, while F-Ni0.85Se with its stable hierarchical structure, rather than typical 2D structure, could have a better cycleability. Such Ni0.85Se with 2D or hierarchical structures could be also used in other fields such like catalyst, since these structures provide plenty of active sites and enhanced stability [40]. XPS was conducted to reveal the valance state of the samples. The Ni 2p and Se 3d peaks are well observed in the overall XPS spectra (Fig. 2a). As shown in Fig. 2(b), both of Ni 2p3/2 and Ni 2p1/2 signals are separated into three peaks [41,42]. The peaks at 853.5 and 870.9 eV are attributed to Ni2+ ions, and the two peaks at 856.0 and 874.1 eV originate from Ni3+ ions formed by surface oxide. The Ni2+ oxidation state is also suggested by the satellite peaks at 860.8 and 879.9 eV. In the Se 3d spectra (Fig. 2c), the presence of Se2‒ is demonstrated by Se 3d5/2 (54.2 eV) and Se 3d3/2 (55.0 eV) peaks. Slight surface oxidation is indicated by the broad peak at 59.2 eV, which is common for synthesized selenides but would not obviously affect the electrochemical properties [41,42].
Fig. 2. (a) Full XPS spectra and detailed spectra of (b) Ni 2p and (c) Se 3d for D-Ni0.85Se and F-Ni0.85Se. Prior to the battery tests, the Mg electrolyte was first evaluated by CV and galvanostatic cycling, which demonstrate its high compatibility with metallic Mg anode (Fig. S3). The Mg-storage properties of the Ni0.85Se cathodes were then investigated. Fig. 3(a and b) describes the charge/discharge curves at 50 mA g‒1 for D-Ni0.85Se and F-Ni0.85Se, respectively. Usually, conversion-type electrode would experience an activation process in initial cycles, during which the reversible capacity increases along with voltage polarization decrease. Such an activation process could be observed for both of D-Ni0.85Se and F-Ni0.85Se, but the activation degree of D-Ni0.85Se is obviously higher than that of F-Ni0.85Se, possibly owing to its structure features favorable for the formation of nanograins facilliating reversible conversion reactions. After 60 cycles, one discharge plateau was located at 0.95 V and two charge plateaus were located at 1.65 V and 1.95 V (Fig. 3a and b). Obviously, the degree of activation for F-Ni0.85Se is lower than that for D-Ni0.85Se, which can be also suggested by the cycling performance (Fig. 3c and d). As seen in Fig. 3(c), D-Ni0.85Se shows a remarkable Mg-storage capacity of 168 mAh g–1 after the activation, while F-Ni0.85Se provides a relatively lower reversible capacity (Fig. 3d, 96 mAh g–1). It is noteworthy
that both of the Ni0.85Se cathodes can undergo highly reversible conversion with Mg2+ ions, demonstrating the merits of using soft Se2‒ anion for Mg-storage cathodes. It is observed that the capacity of D-Ni0.85Se displays a slight decreasing tendency after 60 cycles, while F-Ni0.85Se shows a better cycleability, suggesting the privilege of hierarchical nanoflower structure in cycling stability. CV curves of the Ni0.85Se cathodes after 60 cycles were also measured. As shown in Fig. 3(e and f), both of D-Ni0.85Se and F-Ni0.85Se show a reduction peak and two oxidation peaks with different current densities, which agrees well with the charge/discharge profiles above. The difference in capacity and cycleability for the Ni0.85Se cathodes demonstrates the importance of morphology regulation for conversion-type Mg-storage materials. Firstly, construction of nanostructures, especially low-dimensional nanostructures are indispensable to enhance the solid-state Mg2+ ion diffusion kinetics. Secondly, for a better cycleability, employment of hierarchical structure is necessary for the electrode stability during cycling. Usually, simple nanomaterials are facile to collapse during long-term conversion reactions, possibly due to the nanograins aggregation. However, hierarchical micro-/nano-structure have higher material integrity than simple nanomaterials due to their larger micro-structures and lower tendency toward collapse or particle aggregation.
Fig. 3. Charge/discharge curves of (a) D-Ni0.85Se and (b) F-Ni0.85Se as Mg battery cathodes. Cycling performance (at 50 mA g–1) of (c) D-Ni0.85Se and (d) F-Ni0.85Se cathodes. CV profiles of (e) D-Ni0.85Se and (f) F-Ni0.85Se cathodes after 60 cycles at a scan rate of 0.1 mV s–1. The rate performance of the Ni0.85Se cathodes is shown in Fig. 4(a‒c). D-Ni0.85Se exhibits a high capacity of 92 mA h g‒1 at 500 mA g‒1, demonstrating the merits of selenides as Mg-storage cathodes. However, F-Ni0.85Se delivers a lower capacity of 61 mA h g‒1 at this current density, illustrating the importance of morphology on the performance. Thus, 2D nanostructure such as hexagonal nanosheets could provide not only a higher Mg-storage capacity but also a better rate capability. Fig. 4(d) compares the rate capability of D-Ni0.85Se with typical Mg batteries cathodes [8,9,10,14,43,44], in which D-Ni0.85Se exhibits an obviously better performance. For the long-term cycleability (Fig. 4e), D-Ni0.85Se experiences a gradual activation and fading process, resulting in a remarkable cycleability for 500 cycles (see Fig. S4 for F-Ni0.85Se). It should be noted that all these electrochemical results are obtained without carbon coating on materials, which is especially important for scalable application of rechargeable Mg batteries.
Fig. 4. Typical charge/discharge curves of (a) D-Ni0.85Se and (b) F-Ni0.85Se. (c) Rate performance of D-Ni0.85Se and F-Ni0.85Se. (d) Rate capability comparison of Mg battery cathode materials in this work and previous literatures [8,9,10,14,43,44]. (e) Long-term cycling of D-Ni0.85Se at 200 mA g‒1. The Mg-storage reaction mechanism is investigated via XRD, XPS and TEM. Fig. 5(a) shows the ex-situ XRD of Ni0.85Se cathodes at different states. Cycled separator is used for the characterization since the active material tends to remain on the separator. As seen in Fig. 5(a), the two peaks at 36.8° and 41.0° are ascribed to the separator (See Fig.S5 for its overall XRD pattern). For the two discharged states (30th and 60th cycles), the diffraction peaks at 32.8°, 44.4° and 49.9° are respectively indexed to be (101), (102) and (110) planes of NiSe (JCPDS No. 02-0892) rather than Ni0.85Se, indicating a phase change along with cycling. The phenomenon of phase change is common for chalcogenides to be used as conversation-type cathodes for Mg batteries [14,45]. When other selenides such like NiSe2 were reacted with Mg2+, similar phenomena were also observed that the electrochemical reaction was mainly based on NiSe [44]. When charged to 2.4 V, the peaks shifted to high-angle region. Such a shift is significant at 30th cycle, but it is not obvious at 60th cycle. This
indicates an irreversible phase transformation with cycling, and the Mg-storage reaction is a conversion reaction rather than intercalation. In the discharge states, MgSe and metallic Ni0 peaks are not observed due to the poor crystallinity, which is usually observed for conversion materials. MgSe and metallic Ni0 form nanograins, which would lead to broad XRD peaks difficult to identify. The Mg-storage mechanism could also be illustrated by XPS results. As seen in Fig. 5(b), the shape of Ni 2p peaks changes to some extent upon mix with carbon, but the peak position does not shift. The peaks emerged at 852.5 and 869.8 eV of the fully discharged state (0.01V) correspond to Ni0 2p3/2 and Ni0 2p1/2 [32], respectively, demonstrating the existence of metallic Ni0. In the charge state (2.4 V), the weak Ni0 peaks indicate a small part of metallic Ni0 is remained, serving as a conductive framework favors the reaction reversibility. In Fig. 5(c), the Mg 2p band at 50.1 eV originates from discharge product of MgSe [46,47]. Furthermore, TEM observation was also carried out for the conformation. As shown in Fig. 5(d,e), D-Ni0.85Se could maintain the hexagonal nanosheets morphology after cycling. In a HRTEM image at the discharged state (Fig. 5f), the (111) plane of cubic metallic Ni0 (JCPDS No. 88-2326) could be clearly observed with a spacing of 0.20 nm.
Fig. 5. (a) Ex-situ XRD patterns at different states after 30 and 60 cycles. XPS spectra of (b) Ni 2p and (c) Se 3d at different states after 60 cycles. (d, e) TEM and (f) HRTEM images of D-Ni0.85Se at discharge state after 60 cycles. According to the XRD, XPS and TEM results above and reported observations [32,33,44], the Mg-storage mechanism of Ni0.85Se is as follows. During the activation process, an irreversible phase transformation from Ni0.85Se to NiSe occurred, providing a low theoretical capacity of about 29 mAh g‒1. Meanwhile, NiSe underwent reversible conversion with Mg2+ ions, which became prominent as the cycling proceeded. Mg ions react with Ni2+ to form Ni0 during the magnesiation process (discharge), and reverse reaction occurs in the demagnesiation process (charge). As a small portion of metallic Ni0 does not participate in the reaction, the reversible capacity is slightly lower than the theoretical capacity (390 mAh g‒1). Further kinetic analyses were also conducted to understand the different performance of the Ni0.85Se cathodes. Fig. 6(a and b) describe CV profiles at different scan rates for D-Ni0.85Se and F-Ni0.85Se cathodes, respectively. As shown in Fig. 6(c
and d), a relationship of i = avb could be simulated between peak current (i) and scan rate (v) [48,49]. In Fig. 6(c and d), D-Ni0.85Se exhibits relatively higher b values than F-Ni0.85Se, indicating the enhanced pseudo-capacitive behavior of hexagonal nanosheet morphology. Fig. 6(e and f) compares the pseudo-capacitive contribution of D-Ni0.85Se and F-Ni0.85Se [50,51]. D-Ni0.85Se exhibits higher pseudo-capacitive contributions than F-Ni0.85Se at all scan rates, which is in good accordance with the b value analysis. The hexagonal nanosheet structure favors faradic reaction at the surface or near-surface, thus leading to a better rate performance for D-Ni0.85Se.
Fig. 6. CV profiles at various scan rates for (a) D-Ni0.85Se and (b) F-Ni0.85Se. Log (peak current, i) vs. log (scan rate, v) for (c) D-Ni0.85Se and (d) F-Ni0.85Se. (e) Pseudo-capacitive contribution of D-Ni0.85Se at 1.0 mV s‒1. (f) Pseudo-capacitive contributions for D-Ni0.85Se and F-Ni0.85Se at various scan rates. Soild-state Mg2+ diffusion kinetics was also determined using the CV results above (see Table S1 for the details) [52,53]. The apparent Mg2+ diffusion coefficients (DMg2+) of peaks C and A are 4.8×10‒11 and 7.9×10‒11 cm2 s‒1 for D-Ni0.85Se, respectively, which are much higher than those of F-Ni0.85Se (1.2×10‒11 and 2.1×10‒11 cm2 s‒1). The hexagonal nanosheets of D-Ni0.85Se establish short-range Mg2+ diffusion
path, thus enhancing the solid-state Mg2+ diffusion kinetics. Moreover, EIS results in Fig. S6 reveal that the resistance of D-Ni0.85Se is much lower than those of F-Ni0.85Se at fully discharge state after 60 cycles, suggesting a faster near-surface charge transfer reaction in the 2D nano-structure [54].
4. Conclusions In summary, a facile one-step solvothermal synthesis is used to prepare two Ni0.85Se nano-materials including dispersive hexagonal nanosheets and flower-like assembled nanosheets, whose morphologies exhibit large difference owing to the different reaction rates. The electrochemical performance as Mg-storage materials are systematically
investigated
for
a
comprehensive
understanding
of
the
structure-property relationship. D-Ni0.85Se deliveries high reversible capacities of 168 and 92 mAh g‒1 at 50 and 500 mA g‒1, respectively, which is benefited from the 2D structure facilitating the pseudo-capacitive behavior and solid-state Mg2+ diffusion. Moreover, D-Ni0.85Se exhibits a remarkable long-term cycling stability for 500 cycles. The Ni0.85Se cathodes experience a phase transformation toward NiSe during initial cycles, and the Mg-storage reaction is mainly between NiSe and metallic Ni0. This work not only explores new Mg-storage materials, but also provides significant understanding for the development of advanced conversion Mg battery cathodes.
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. Declaration of interests
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 This work was financially supported by Intergovernmental International Science and Technology Innovation Cooperation Project (2019YFE010186), the Hubei Provincial Natural Science Foundation (2019CFB452 and 2019CFB620) and the Fundamental Research Funds for the Central Universities.
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TOC
Ni0.85Se with different structures is used as Mg secondary battery cathode, exhibiting high capacity, remarkable long-term cycleability and fast solid-state Mg2+ diffusion kinetics.