NiCo2Se4 as an anode material for sodium-ion batteries

NiCo2Se4 as an anode material for sodium-ion batteries

Journal Pre-proofs NiCo2Se4 as an anode material for sodium-ion batteries Li-Cheng Qiu, Qin-Chao Wang, Xin-Yang Yue, Qi-Qi Qiu, Xun-Lu Li, Dong Chen, ...

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Journal Pre-proofs NiCo2Se4 as an anode material for sodium-ion batteries Li-Cheng Qiu, Qin-Chao Wang, Xin-Yang Yue, Qi-Qi Qiu, Xun-Lu Li, Dong Chen, Xiao-Jing Wu, Yong-Ning Zhou PII: DOI: Reference:

S1388-2481(20)30035-7 ELECOM 106684

To appear in:

Electrochemistry Communications

Received Date: Revised Date: Accepted Date:

3 December 2019 4 February 2020 5 February 2020

Please cite this article as: L-C. Qiu, Q-C. Wang, X-Y. Yue, Q-Q. Qiu, X-L. Li, D. Chen, X-J. Wu, Y-N. Zhou, NiCo2Se4 as an anode material for sodium-ion batteries, Electrochemistry Communications (2020), doi: https://

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NiCo2Se4 as an anode material for sodium-ion batteries Li-Cheng Qiu, Qin-Chao Wang, Xin-Yang Yue, Qi-Qi Qiu, Xun-Lu Li, Dong Chen, XiaoJing Wu, Yong-Ning Zhou*

Department of Materials Science, Fudan University, Shanghai 200433, China. * Corresponding author. Email address: [email protected] (Y. N. Zhou) Abstract Exploring new anode materials is critical for the development of Sodium-ion batteries (SIBs). Herein, a binary-metal selenide NiCo2Se4 was synthesized and investigated as a new anode material for SIBs. After compositing with conductive carbon, the [email protected] composite delivers a reversible capacity of 603.2 mAh g-1 with a high initial coulombic efficiency of 85.79% at 0.5 A g-1. At an ultrahigh current density of 2 A g-1, a reversible capacity of 377.5 mAh g-1 can still be obtained after 600 cycles. The detailed Na storage mechanism for NiCo2Se4 is revealed. After discharge, Na2Se, Ni and Co nanoparticles are formed and highly dispersed in Na2Se matrix. After recharge, NiCo2Se4 phase can be regenerated with small amount of CoSe2 and NiSe phases. The multi-phase coexistence after recharge is responsible for the initial capacity loss and the excellent cycle performance in subsequent cycles. Keywords: sodium-ion batteries, anode, selenide, reaction mechanism


1. Introduction Sodium-ion batteries (SIBs) are considered as a promising alternative due to resource abundance and potential low cost [1]. To date, many kinds of anode materials for SIBs have been investigated, including carbon-based materials, metal oxides, phosphides, sulfides, etc. Carbon-based materials, such as graphite, show good cyclic performance in lithium-ion batteries, but it suffers from low electrochemical activity with Na. Hard carbon is proved to be electrochemically active in SIBs, but the reversible capacity is only about 300 mAh g-1, and it has the potential risk of sodium dendrite growth [2-4]. For metal oxides, they usually present poor reversibility in SIBs, due to the extreme difficulty for the decomposition of Na2O formed after discharge [5,6]. Phosphides have high theoretical capacity, but the instability and flammable properties of phosphides limit its practical application [7-9]. In recent years, transition metal sulfides and selenides show great potential as anode materials for SIBs due to their good cycle performance and high rate capability [10,11]. Compared to sulfides, selenides have narrower band gap, thus gain much better electronic conductivity. Recently, many metal selenides have been reported as anode materials for SIBs. MoSe2 and FeSe2 showed reversible capacities of 364 and 374 mAh g-1 at a high current rate of 1 A g-1 [12-16]. MoSe2 has a layered structure similar as MoS2, which are active for both conversion reaction and intercalation reaction. [email protected] exhibits a reversible capacity of 460 mAh g−1 at 0.5 A g−1, and Ni0.85Se/C exhibits a reversible capacity of 390 mAh g−1 at 0.2 A g-1, indicating that they have better sodium storage ability than iron and molybdenum selenides [17-21]. Fang et al. reported a CoSe2/ZnSe heterostructure and used as the anode material for SIBs. It exhibited promising cyclic


performance due to the multistep redox reactions in the heterostructure and rich phase boundaries [22]. Binary-metal selenides combined the features of two metal elements can sometimes yield better electrochemical performance than binary selenides, owing to richer redox sites, better electrochemical activity and improved electronic conductivity, if the two elements are chosen properly [23]. For example, Fe2CoSe4 could deliver a reversible capacity of 614.5 mAh g−1 at 1 A g−1 after 100 cycle [24,25], which is much superior than monometal selenides of iron and cobalt [14,17]. It was attributed to the enhanced intrinsic conductivity and larger crystal size than monometal selenides. Ge et al. developed a Ni0.67Fe0.33Se2 anode by using Ni-Fe Prussian blue analogs [26]. It could retain a reversible capacity of 375 mAh g−1 after 10000 cycles at 10 A g−1. Cho’s group reported a hierarchical yolk-shell-structured NiCoSe2/CNT [27]. It showed a reversible capacity of 366 mAh g-1 after 10000 cycles at a high current density of 3.0 A g-1. The promising rate capability is resulted from the synergistic effects of NiCoSe2 nanoparticles, CNTs walls with interstitial mesopores, and hierarchical yolk-shell structure. Although the binarymetal selenides exhibit promising sodium storage ability, the detailed reaction mechanism during discharge and charge is still not well understood. The competing reaction of two metal elements during the recharge process makes the electrochemical reaction mechanism more complicate than monometal selenides. Herein, a binary-metal selenide NiCo2Se4 was synthesized via a facile hydrothermal process and used as a new anode material for SIBs. The metallic nature of NiCo2Se4 and synergistic effect between Ni/Co atoms are expected to be beneficial for its reversible sodium storage and release [27,28]. After compositing with conductive carbon, [email protected] exhibits promising cycle stability and rate capability. By combining X-ray 3

diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS) techniques, the electrochemical reaction mechanism of NiCo2Se4 with sodium is revealed thoroughly.

2. Experimental 1 mmol Ni(NO3)2 6H2O and 2 mmol Co(NO3)2 6H2O were dissolved in 44 mL N, NDimethylformamide. Afterwards, 4 mmol selenium powder, 20 wt.% Super-P, 12 mmol urea and 16 mL hydrazine hydrate (80%) were added into the solution and the mixture was stirred continually for another 1 h to obtain a black solution. The solution was transferred to an autoclave, treated hydrothermally for 12 h at 180 oC. After that, the product was cooled down to room temperature, washed by the deionized water and ethanol several times, and collected by centrifugation. Finally, it was heated at 70 oC for 12 h in the vacuum oven to obtain [email protected] composites. The pure NiCo2Se4 sample was synthesized under the same condition. The only difference was the absence of Super-P additive. XRD was measured with Cu Kα (λ=1.54 Å) radiation using a Bruker D8 Advance diffractometer. SEM was characterized by field emission scanning electron microscope (Cambridge S-360). TEM and SAED were obtained on a JOEL JEM 2010 TEM. XPS were carried out with PHI-5000C ESCA system (Perkin Elmer). Raman spectra was tested on XploRA micro-Raman system (Horiba Jobin Yvon, France) with 532 nm laser. The electrodes were prepared by mixing the active materials, conductive agent (Super-P) and binder (carboxymethylcellulose sodium) in a weight ratio of 7:2:1. The homogenous slurry was pasted on copper foil and dried at 70 oC. The mass loading of the active materials in the electrode is about 3 mg cm-2. Sodium was pasted on current collector as the counter electrode, and glass fiber was used as a separator. The electrolyte was sodium tri-fluoro4

methane-sulfonate (NaCF3SO3) in ethylene carbonate (EC) and polycarbonate (PC) with a 1:1 volume ratio. 2032-type coin cells were assembled in argon-filled glove box. Galvanostatic cycling was carried out with a Land CT2001A battery test system. The cyclic voltammetry (CV) was performed on a CHI660A electrochemical workstation. 3. Results and discussion The XRD pattern of the [email protected] is shown in Fig.1a. The main peaks are located at 33.53°, 45.10° and 51.21°, corresponding to the (101), (102) and (110) crystal planes of monoclinic NiCo2Se4 phase (JCPDS No. 08-4821), respectively. The XRD patterns of Ni0.85Se (JCPDS No. 18-0888) and Co0.85Se (JCPDS No. 52-1008) phases are also shown in Fig. 1a. Although they are similar as NiCo2Se4 phase, the two peaks at 16.53o and 16.90o in NiCo2Se4 phase cannot been found in Co0.85Se or Ni0.85Se phases. Fig. 1b shows the morphology of the as-synthesized [email protected] It exhibits homogeneous distribution of irregular particles in a size of ~2 μm. After zooming in one particle (Fig. 1c), it can be seen that the particle is composed by nanoflakes. TEM image (Fig. 1d) shows porous structure of the particles. In high-resolution TEM image shown in Fig. 1e, clear lattice fringes can be observed. The spacing distances of 0.271 and 0.197 nm of the lattice fringes are attributed to (101) and (102) planes of the hexagonal NiCo2Se4 phase. The EDS mapping (Fig.1f) exhibits uniform distribution of Ni, Co and Se elements in [email protected] sample. Fig. S1 shows the C 1s XPS spectra of [email protected] The spectrum can be deconvoluted into three peaks. The peak at 285.8 eV is attributed to C-C, and the peaks at 286.8 and 289.9 eV are attributed to C-O and C=O, respectively [25]. The C-O and C=O bonding are originated from the inevitable surface oxidation. The Raman spectrum of the [email protected] is presented in Fig. S2. The peaks at 1340 and 1589 cm-1 are assigned to the D band (sp3


hybridization) and G band (sp2 hybridization) of carbon, respectively. The intensity ratio of D to G band (ID/IG) is 0.997, indicating the proportion of sp3 and sp2 hybridization is comparable in [email protected]

Fig. 1. (a) XRD pattern, (b, c) SEM images, (d, e) TEM images and (f) EDS mapping of [email protected] The electrochemical performance of [email protected] as the anode material for SIBs are presented in Fig. 2. NaCF3SO3 is selected as the salt of electrolyte [10]. The first three CV 6

curves are shown in Fig. 2a between 0.01 and 3 V at the scan rate of 0.1 mV s-1. In the first discharge, the reduction peak at 0.90 V should be associated with the electrochemical reaction between NiCo2Se4 and Na, and the formation of solid-electrolyte-interface (SEI) layer. In the charge process, the oxidation peak located at 1.78 V is attributed to the desodiation reactions. In the second discharge, three reduction peaks at 0.68, 1.16 and 1.38 V can be observed, associating with multistep sodiation processes. The difference between the first and second discharge suggests unidentical reaction pathways of the two discharge processes, which are responsible for the irreversible capacity in the first cycle. Fig. 2b presents the charge-discharge curves of the first, second, fifth and tenth cycle of [email protected] at the current density of 0.5 A g-1. In the first discharge, a high specific capacity of 702.7 mA h g−1 can be obtained with a flat plateau at around 0.9 V followed by a slope. In the first charge, a capacity of 602.9 mA g-1 is retained with a plateau at 1.8 V. The Coulombic efficiency of the initial discharge and charge is about 85.8 %, which is very high compared with many reported anode materials for SIBs [5-11]. In the second discharge, the voltage curves become slopy and fluctuant, indicating complicated reaction process, but the capacity (603.2 mA g-1) is nearly same as the charge capacity. The chargedischarge curves are in good agreement with the CV results in Fig. 2a. The rate performance of [email protected] is shown in Fig. 2c. The discharge capacities of 632.9, 596.9, 562.7, 509.5 and 463.4 mAh g-1 are obtained at the current densities of 0.5, 1.0, 2.0, 5.0 and 10.0 A g-1, respectively. After that, the capacity of 552.4 mAh g-1 can still be achieved when the current rate is set back to 0.5 A g-1. The cycle performance of [email protected] at a current density of 0.1 A g-1 is presented in Fig. S3. It shows a rapid capacity fading in the first 40 cycles from 650 to 350 mAh g-1. Afterwards, the capacity increases until 120 cycles,


then decreases up to 200 cycles. The capacity fluctuation could be attributed to the unstable SEI layers formed under low current density. Fig. 2d presents the cycle performance of [email protected] and pure NiCo2Se4 at a high current density of 2 A g-1. After 600 cycle, the discharge capacity of 377.5 mAh g-1 can be retained, about 79% of the first discharge capacity. For comparison, the pure NiCo2Se4 presents severe capacity fading after 10 cycles. Only 60.9 mAh g-1 is retained after 600 cycles. It indicates that the composition with carbon is essential for enhancing the long-term cycle performance of NiCo2Se4.


Fig. 2. (a) CV curves of [email protected] for the first three cycles; (b) galvanostatic charge– discharge voltage profiles of [email protected] for the 1st, 2nd, 5th and 10th cycles; (c) Rate performance of [email protected] (d) Cycle performance of [email protected] at a current density of 2.0 A g-1. (e) CV curves of [email protected] at scan rates from 0.2 to 0.8 mV s-1; (f) log i vs log v plots of [email protected]

In order to understand the Na-ion storage process of [email protected] in detail, CV measurements were carried out at different scan rates from 0.2 to 0.8 mV s-1 (Fig. 2e). The relationship between the Log (scan rate, mV s-1) and Log (peak current, mA) can be described in the formula below: i = avb


Log(i) = bLog(v) + Log(a)


Herein, parameter b describes the charge storage behavior of the electrochemical system. When b is 0.5, the process is controlled by ion diffusion. While b is 1, the process is controlled by a capacitive effect (extrinsic pseudocapacitance effect). When 0.5 < b < 1, both ion diffusion and capacitive effect are involved. After mathematical manipulation, the value of b is illustrated in Fig. 2f. The b values of the two peaks are about 0.91 and 0.83 for










pseudocapacitance effect has more contribution than Na-ion diffusion. The high contribution of pseudocapacitance could be one of the reason for the superior rate capability of [email protected] [29]. To study the electrochemical reaction mechanism of [email protected] with Na in details, XRD measurements were employed. Fig. 3a present the ex-situ XRD patterns of


[email protected] electrode in the first cycle. Corresponding voltage curve is presented in Fig. 3b. At the pristine and the half-discharged state (D0.6 V), the (101), (102) and (110) diffraction peaks of NiCo2Se4 at 33.53°, 45.10° and 51.21° can be observed. When the cell is full discharged to 0.01 V, the three diffraction peaks vanish and a peak at 37.10° appears, corresponding to the (220) crystal plane of Na2Se phase [19], indicating the decomposition of NiCo2Se4 and the formation of Na2Se after discharge. However, no other phases can be identified, possibly due to that the size of the other phases is less than X-ray coherence length (6 nm), which could not be detected by X-ray diffraction [30], In the charge process, the peaks of Na2Se fade continuously, and diminish over 2.5 V. However, no new peak can be observed even after charging to 3.0 V. It is probably due to the nanocrystalline or amorphous structure of the charged products.

Fig. 3. (a) Ex-situ XRD of [email protected] at various voltages and (b) corresponding voltage profile during the first cycle. In order to further confirm the discharge and charge products, TEM was performed on the samples at discharged and charged states. Fig. 4a and b show the HRTEM and SAED


images of the pristine [email protected] The lattices and SAED patterns are in good agreement with the XRD pattern of NiCo2Se4 shown in Fig. 1a. Besides, a hazy ring corresponding to (002) plane of the coated carbon can also be observed. Fig. 4c and d show the HRTEM image and SAED pattern of [email protected] at the discharged state, respectively. Several nanocrystallines with different lattices can be observed. After measuring, these lattices can be assigned to Na2Se, Co and Ni phases. It should be noted that the metallic Co and Ni particles are less than 5 nm and highly dispersed. It is why we cannot find the diffraction peaks of Co and Ni in the XRD pattern of discharged sample. In the SAED pattern shown in Fig. 4d, a group of hazy rings can be observed, which are attributed to Na2Se and carbon phases, in accordance with the XRD pattern shown in Fig. 3. After recharging to 3.0 V (Fig. 4e and f), NiCo2Se4 phase was regenerated. Interestingly, new phases of CoSe2 and NiSe with relative smaller particle size compared with NiCo2Se4 particles can also be observed in HRTEM images, suggesting the formation of CoSe2 and NiSe compounds in the recharge process, associated with NiCo2Se4. It should be responsible for the different voltage curves of [email protected] in the 1st and 2nd discharge processes.


Fig. 4. HRTEM images and corresponding SAED patterns of [email protected] at the pristine state (a, b), after first discharging to 0.01 V (c, d) and recharging to 3.0 V (e, f), respectively.

Fig. 5. Co and Ni 2p XPS spectra of [email protected] at the pristine state (a, b), after first discharging to 0.01 V (c, d) and recharging to 3.0 V (e, f), respectively. To monitor the valence changes of Co, Ni in [email protected] during cycling, XPS measurement was conducted and the results are shown in Fig. 5. For the pristine [email protected] (Fig. 5a, b), the main peaks at 782.6 and 797.6 eV in Co XPS spectrum correspond to the binding energy of Co 2p3/2 and 2p1/2 orbitals, respectively. After deconvolution, each peak can be divided into three components [31]. The peaks at 779.4 and 793.4 eV are attributed to Co3+, and the peaks at 782.6 and 797.6 eV are attributed to Co2+ [15,19,32,33]. The wide peaks at around 788 and 803 eV are satellite peaks. The results suggest that the main valence state of Co in the pristine [email protected] is 2+. For Ni 2p XPS spectrum, the peaks at 853.4 and 872.6 eV are attributed to Ni2+, and the peaks at 857.0 and 874.2 eV are attributed to Ni3+ [20,34]. It indicates that Ni in the pristine


[email protected] is dominated by Ni3+. After fully discharge (Fig. 5c, d), the peaks of Ni and Co show distinct movement towards low energy, suggesting the reduction of transition metal ions. The peaks at 780.6 and 795.4 eV in Co spectrum are ascribed to Co0 in metallic Co, and the peaks at 855.6 and 872.8 eV in Ni spectrum are ascribed to Ni0 in metallic Ni [35,36]. These results further confirm the formation of metallic Co and Ni after discharge. After recharging to 3.0 V (Fig. 5e, f), all the peaks in Co and Ni XPS spectrum shift back towards higher energy, which can be assigned to divalent and trivalent ions. These results further confirm the reversible redox of Co and Ni in [email protected] during discharge and charge.

Fig. 6. Schematic for the electrochemical reaction mechanism of NiCo2Se4 with sodium during the initial discharge and charge processes.

Based on the results above, the electrochemical reaction mechanism of [email protected] with Na can be summarized as follows and illustrated in Fig 6. 13

Initial discharge: NiCo2Se4 + 2Na+ + 8e- → Na2Se + Ni + 2Co


Subsequent cycles: 4Na2Se + Ni + 2Co - 8e- → NiCo2Se4 + 8Na+ (2) 2Na2Se + Co - 4e- → CoSe2 + 4Na+


Na2Se + Ni - 4e- → NiSe + 2Na+ (4) In the first discharge process, NiCo2Se4 reacts with sodium to form Na2Se, Ni and Co (Reaction 1). The Co and Ni nanoparticles (<5 nm) are highly dispersed in the Na2Se matrix. The ultrafine Co and Ni nanoparticles play a vital role in catalyzing full decomposition of Na2Se during charge, responsible for the high initial coulombic efficiency. In the charge process, most of the discharge products convert back to NiCo2Se4 phase (Reaction 2), but small amount of CoSe2 and NiSe phases are also formed (Reaction 3 and 4). This should be responsible for the difference between the first and second discharge curves. It is very interesting that multi-phase coexistence (NiCo2Se4, CoSe2 and NiSe) is revealed after recharge. Phase separation has been known to be beneficial for stabilizing the (de)sodiation reactions during cycling [37,38]. It is because that competition between the reactions of different phases is favorable to enhancing cyclability of the electrode, and the excess heterogeneous phase boundaries can facilitate ion transportation. As the result, the [email protected] electrode exhibits promising cycle stability and superior rate capability in SIBs. 4. Conclusions In summary, NiCo2Se4 was synthesized via a facile hydrothermal method successfully, and investigated as a new anode material of SIBs. After composite with carbon, [email protected] delivers a large reversible capacity of 603.2 mAh g-1 with a high initial coulombic efficiency of 85.79% at 0.5 A g-1. At an ultrahigh current density of 2 A g-1, a


reversible capacity of 377.5 mAh g-1 can still be obtained after 600 cycles, indicating its superior rate capability. The reaction mechanism of [email protected] with Na during charge and discharge is revealed thoroughly. In the discharge process, NiCo2Se4 decomposes and Na2Se, Ni and Co are formed. The Co and Ni nanoparticles (<5 nm) are highly dispersed in the Na2Se matrix. In the charge process, most of the discharge products convert back to NiCo2Se4 phase, and small amount of CoSe2 and NiSe phases are also formed. The multiphase coexistence after recharge is responsible for the initial capacity loss and the excellent cycle performance in subsequent cycles. This work opens new opportunities in designing high-performance anode materials for SIBs.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502039 and 51902058) and Science & Technology Commission of Shanghai Municipality (No. 19ZR1404200).

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Credit author statement Li-Cheng Qiu: conceive the concept and carry out the experiment. Qin-Chao Wang: contributed to the materials characterization.


Xin-Yang Yue: contribute to the materials synthesis Qi-Qi Qiu: contribute to the XPS data analysis. Xun-Lu Li: contribute to the XRD and TEM data analysis. Dong Chen: contribute to mechanism analysis. Xiao-Jing Wu: Writing and Editing. Yong-Ning Zhou: conceive the concept and supervise the research. 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.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Highlights 1. NiCo2Se4 was studied as an anode material for SIBs for the first time. 2. [email protected] exhibits high capacity of 401.2 mAh g-1 at 2A g-1 after 200 cycles. 3. Phase separation was revealed in the recharge process of [email protected]