Nanosized Monometallic Selenides Heterostructures Implanted into Metal Organic Frameworks-Derived Carbon for Ecient Lithium Storage

In this work, monometallic selenide heterostructure are synthesized by controlled selenylation of MOFs, which demonstrate enhanced lithium storage properties to its counterparts. The superiority of monometallic selenide heterostructure with in-depth investigation endow it with great potentials in other energy-related applications. Thus, we think this research is particularly well suited for the broad readerships of Journal of Advanced Ceramics . any We look to we to ABSTRACT Two-phase heterostructure with rich phase boundaries holds great potential in engineering advanced electrode materials. However, current heterostructures are largely generated by introducing exotic cations or anions, complicating synthetic procedures and disturbing real insights into the intrinsic effect of heterostructure. Herein, nanosized monometallic selenides heterostructures are developed by precisely controlled selenylation of metal organic frameworks, which are implanted into in-situ formed carbon (NiSe/NiSe 2 @C, CoSe/CoSe 2 @C). The disordered atoms arrangement at two-phase boundary leads to the redistribution of interfacial charge and generation of lattice distortions, promoting easy adsorption and swift transfer of Li + , and providing extra active sites. As a proof of concept, the NiSe/NiSe 2 @C exhibits far surpassing lithium storage properties to single-phase counterparts (NiSe@C and NiSe 2 @C), including higher reversible capacity of 1015.5 mAh g -1 , better rate capability (500.8 mAh g -1 at 4 A g -1 ), and superior cyclic performance. As expected, the NiSe/NiSe 2 @C manifests lower charge transfer resistance, higher Li + diffusion coefficient, and accelerated capacitive kinetics. Ex-situ X-ray diffraction, high-resolution transmission electron microscopy, and selected area electron diffraction combined with differential capacity versus voltage plots reveal multi-step redox mechanism of NiSe/NiSe 2 @C and the reason of conspicuous capacity enhancement. This work demonstrates the enormous potential of monometallic monoanionic heterostructure in energy-related field.


Dear editor,
We would like to submit the enclosed manuscript entitled "Nanosized Monometallic Selenides Heterostructures Implanted into Metal Organic Frameworks-Derived Carbon for Efficient Lithium Storage" by Zhichao Liu, Dong Wang,* Hongliang Mu, Chunjie Zhang, Liqing Wu, Liu Feng,* Xiuyu Sun, Guangshuai Zhang, Jie Wu, Guangwu Wen, which we wish to be considered for publication as a "Research Article" in Journal of Advanced Ceramics. All authors have read and agreed with the submission.
Metal selenides stand out from conversion-type electrodes owing to the weak metal-selenium bond and narrow energy gap, which are kinetically favorable for Li + ions insertion/extraction, compared with metal oxides and sulfides. Despite this, metal selenides still suffer from severe volume variation and sluggish Li + diffusion kinetics, resulting in capacity fading and inferior rate capabilities. Modulation of electronic structure by engineering heterostructure demonstrates great effectiveness on improving the electrochemical performance of metal selenides. Benefitting from the interfacial charge distribution and lattice distortion at hetero-phase boundaries, the Li + ions could be adsorbed easily and transferred swiftly, facilitating reaction kinetics. However, the current selenides heterostructures are largely formed by introducing foreign cations or anions, which makes synthetic process more complex. Moreover, in these heterostructures, heteroatom doping inevitably happen for single selenide component, which could disturb the real insight into the intrinsic mechanism of the performance improvement by two-phase heterostructure. Therefore, it is of great significance to design monometallic selenides heterostructure without any exotic elements and give an in-depth insight into its electrochemical behaviors. Herein, we report the monometallic selenide heterostructures by precisely controlled selenylation of metal-organic frameworks (MOFs) with enhanced lithium storage properties. The main results are highlighted as follows: (1) Nanosized monometallic selenide heterostructures that uniformly implanted into porous carbon (i.e., NiSe/NiSe2@C, CoSe/CoSe2@C) are successfully synthesized via a controlled selenylation of MOFs. Appropriate ratio of MOFs to Se is critical in this synthetic strategy.
(2) Redistribution of interfacial charge at the two-phase boundaries could promote adsorption and diffusion of Li + ions, while the lattice distortions could provide extra active sites. As a result, the monometallic selenide heterostructures showcase improved electron/ions transfer rates and reaction kinetics than single-phase counterparts. As a proof of concept, the NiSe/NiSe2@C manifest excellent electrochemical performance in view of high reversible capacity (1015.5 mAh g -1 at 0.1 A g -1 ), good rate capability (500.8 mAh g -1 at 4 A g -1 ), and long-term cyclic stability.
(3) Ex-situ XRD, HRTEM, and SAED accompanied with differential capacity versus voltage plots reveal multi-step redox mechanism of the obtained monometallic selenide heterostructures and the reason of conspicuous capacity enhancement.
In this work, monometallic selenide heterostructure are synthesized by controlled selenylation of MOFs, which demonstrate enhanced lithium storage properties to its counterparts. The superiority of monometallic selenide heterostructure with in-depth investigation endow it with great potentials in other energy-related applications. Thus, we think this research is particularly well suited for the broad readerships of Journal of Advanced Ceramics.
Finally, this work is original and has not been submitted to or published in any journal. We appreciate your time in reading this article and look forward to hearing from you. Should you have any questions, we will be happy to answer them.

Introduction
Lithium-ion batteries (LIBs) are currently dominant energy storage devices towards commercial requirements owing to the advantages of high energy density, long lifespan, high safety [1,2]. However, the low theoretical capacity of commercial graphite anode limits the further applications of LIBs in electric vehicles and portable electronics [3]. Conversion-type anode materials represented by metal oxides, sulfides, and selenides, have been capturing broad attentions due to their high theoretical capacity and rich reserves [4][5][6]. Particularly, compared with metal oxides and sulfides, the metal-selenium bonds in selenides are easier to break and interplanar spacing is larger due to the larger radius of selenium atom, which could remarkably promote the kinetics of conversion reaction [7]. Moreover, selenides with narrower energy gap also possess superior electrical conductivity, thus accelerating electrons transportation. Up to now, various selenides including ZnSe [8], NiSe [9], CoSe [10], NiSe2 [11], have been used as anodes of LIBs. However, the metal selenides still suffer from huge volume variation and sluggish Li + diffusion kinetics during lithiation/delithiation processes, which result in severe structural degradation, capacity fading, and unpromising rate capability [12].
Tremendous efforts have been devoted to circumvent these obstacles of metal selenides, mainly including hybridizing with conductive carbon, geometrical structural regulation, and modulating electronic structure [13]. Notably, modulation of electronic structure by engineering heterogeneous structures demonstrates great effectiveness on improving the electrochemical performance [14,15]. Compared with single-phase metal selenide, the charge redistribution at the two-phase boundaries in the heterostructure are favorable for the adsorption of ions, thus accelerating reaction kinetics. Moreover, the misaligned arrangement at the heterointerface with rich dislocations, defects, lattice distortions, could improve thermodynamic stability and reaction activity, resulting in improved electrochemical performance. For example, Fang et al. reported that the heterogeneous interface in the CoSe2/ZnSe could dramatically improve the ions adsorption ability through theoretical calculations and experimental results [16]. Chen et al. demonstrated that SnSe/SnO2@graphite possesses higher electron mobility and reversible capacity than SnO2@graphite, benefitting from the built-in electric field of the SnSe/SnO2 heterostructure, which delivers a high reversible capacity of 810 mAh g -1 after 200 cycles [17]. However, previously reported selenides-based heterostructures were generally formed through introducing exotic anions or cations to generate two distinct phases, which complicated the preparation processes with high cost. Moreover, the introduction of foreign atoms always gives rise to element-doping into single selenide component, which could disturb the real insight into the intrinsic mechanism of the performance improvement by the two-phase heterostructure. In this regard, an in-depth research on the monometallic selenides heterostructure could provide straightforward insight into the role of two-phase heterojunction on the electrochemical performance, and also advance better exploitation and practical application of other heterostructures. In addition, the preparation of monometallic selenide heterostructures using metal organic framework (MOF) as the precursor could inherit its porous characteristics, thus providing rich channels for electrolyte infiltration and Li + ions diffusion. Moreover, the in-situ formed carbon skeleton also improve the conductivity of the material and restrict the agglomeration of active nanocrystals [18].
In this work, we reported the MOF-engaged synthesis of nanosized monometallic selenide heterostructure embedded into in-situ formed carbonaceous matrix (i.e., NiSe/NiSe2@C and CoSe/CoSe2@C), by accurate ratio regulation of MOF to Se. Taken NiSe/NiSe2@C as an illustrative example, abundant phase boundaries with lattice dislocations and defects between NiSe and NiSe2 domains were verified, while the Xray photoelectron spectroscopy (XPS) and Raman proved the interfacial charge redistribution inside the heterostructure, which could promote rapid adsorption and swift transfer of Li + ions at the heterointerfaces. As expected, the as-synthesized NiSe/NiSe2@C shows enhanced lithium storage properties than the NiSe@C and NiSe2@C in view of higher reversible capacity (1015.5 mAh g − 1 after 100 cycles at 0.1 A g − 1 ; NiSe@C: 591.3 mAh g − 1 ; NiSe2@C: 529.7 mAh g − 1 ), better rate capability (500.8 mAh g − 1 at 4 A g − 1 ; NiSe@C: 333.3 mAh g − 1 ; NiSe2@C: 277.9 mAh g − 1 ), and stable cyclic performance (540.3 mAh g − 1 after 1000 cycles). Electrochemical analyses demonstrated that the NiSe/NiSe2@C manifests accelerated diffusion kinetics of Li + ions and more favored pseudocapacitance contribution. Ex-situ X-ray diffraction (XRD) together with ex-situ high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) reveal the multi-step reaction mechanism and structural evolution of the NiSe/NiSe2@C during lithiation/delithiation processes. Furthermore, the differential capacity versus voltage plots discloses the increased utilization of selenides heterostructure upon cycling, and reversible generation and decomposition of polymeric-gel film owing to gradual electrode activation. As another proof of concept, the CoSe/CoSe2@C also showcase superior reversible capacity, rate capability, and cyclic performance to its counterparts.

Materials preparation
Preparation of Ni-MOF: All chemicals are of analytical grade and were directly used without further purification. Typically, Ni(CH3COO)2·4H2O (0.3732g, 1.5 mmol) was dissolved in deionized water (20 ml) and ethanol (20 ml), and then PVP (Mw=10000, Preparation of NiSe/NiSe 2 @C: The Ni-MOF precursor and Se powder were thoroughly ground and mixed in an agate mortar with mass ratio of 3:2. Then, the mixture was annealed at 450 °C for 2 h in Ar atmosphere with a heating rate of 1 °C min -1 . After cooling down to room temperature, the NiSe/NiSe2@C product was obtained. The preparation processes of the NiSe@C and NiSe2@C were identical to that of NiSe/NiSe2@C except for the mass ratio of Ni-MOF precursor to Se powder.
For the NiSe@C, the mass ratio of Ni-MOF: Se was 3:1, while for the NiSe2@C, the mass ratio of Ni-MOF: Se was 3:3.

Materials characterization
The crystal structure of the as-prepared samples was characterized by XRD on a D/MAX-IIIC with Cu Kα radiation at 30 kV and 20 mA. For the semi-quantitative analysis, the scanning range is 10-90°, the scanning step is 0.02°, and the dwell time is 3 s. Scanning electron microscopy (SEM; Sirion 200 FEI Netherlands) was used to characterize the surface morphology of the sample. Transmission electron microscope (TEM), HRTEM, and energy dispersive X-ray spectroscopy (EDS) analysis were carried out on a JEM-2100 microscope to analyze the microstructure of the products. XPS (ESCALAB 250XI) was performed to examine the surface electronic state and chemical valences of the samples. Raman spectra were collected using a Renishaw Invia Raman microscope. The specific surface area was calculated by using Brunauer-Emmett-Teller (BET) and pore diameter distribution plot were obtained by the N2 adsorption/desorption isotherms on a JWGB, JW-BK200C tester.

Electrochemical measurement
The electrochemical performance of the obtained samples was assessed based on CR2032-type coin cells. To prepare the working electrode, 70 wt % active material, 20 wt % acetylene black, and 10 wt % polyvinylidene fluoride (PVDF) were mixed and stirred in N-methyl pyrrolidone (NMP) to form a homogeneous slurry. Then, the slurry was coated onto a copper foil and dried in a vacuum oven at 110 °C for 24 h. The mass loading of the active material was ~1.0 mg cm 2 . The coin cells were assembled in an Ar-filled glove box with lithium foil as the reference and counter electrodes, Celgard  Taken NiSe/NiSe2@C as an example, the synthetic process of the monometallic selenides heterostructure is schematically illustrated in Fig. 1. Rod-like Ni-MOF precursor (Fig. S1, Supporting Information) was first synthesized through an incubation process at ambient temperature, during which the co-precipitation between Ni 2+ ions and trimesic acid occurred. Then, the Ni-MOF was converted into nanosized selenides implanted into porous carbon skeleton via a facile selenylation process with Se. Note that the amount of Se powder plays a critical role on obtaining the heterojunction NiSe/NiSe2. Specifically, the NiSe/NiSe2@C was obtained with Ni-MOF: Se of 3:2 by weight. On the other hand, when the mass ratios were 3:1 and 3:3, the NiSe@C and NiSe2@C were generated, respectively.  Notably, the peak for the NiSe/NiSe2 at 508.4 cm −1 slightly shift to higher wavenumbers compared with that for pure NiSe, which should be owing to an opposite charge transfer induced by the heterogeneous interfaces between NiSe2 and NiSe phases [23,24]. In addition, the two peaks at about 1337.4 and 1565.6 cm -1 belong to D-band (disordered graphitic carbon) and G-band (sp 2 -bonded carbon) of the MOFs-derived carbon, respectively [25]. The elemental composition and surface electronic states of the NiSe/NiSe2@C, NiSe@C, and NiSe2@C were further analyzed by XPS. It can be seen that these materials all contain anticipated elements, i.e., C, N, O, Ni, and Se (Fig. 3b), consistent with the elemental mapping results. For the NiSe/NiSe2@C, the highresolution Ni 2p spectrum can be deconvoluted into two core-level peaks at 852.66 eV (2p3/2) and 869.94 eV (2p1/2), accompanied with their satellite peaks located at 859.89 and 879.16 eV, corresponding to Ni 2+ , while the peaks located at 854.86 (2p3/2) and 872.76 (2p1/2) are assigned to Ni 3+ (Table S1, Supporting Information) [26]. Notably, it can be observed that the peaks of Ni 2p in NiSe/NiSe2@C are positively shifted (~1.04 eV) compared with that in NiSe@C, which are negatively (~0.46 eV) shifted compared with that in NiSe2@C, suggesting strong electronic interaction between NiSe2 and NiSe phases induced by the electrons transfer from NiSe to NiSe2 (Fig. 3c) [27,28]. Fig. 3d display the high-resolution Se 3d spectra of the three samples. For the NiSe/NiSe2@C, the two peaks located at 53.56 eV and 54.36 eV should be attributed to Se 2-3d3/2 and Se 2-3d5/2, respectively, while the two peaks at 55.45 eV and 56.25 eV respectively correspond to Se -3d3/2 and Se -3d5/2 (Table S2, Supporting Information) [29]. Obviously, the Se 3d spectra in NiSe/NiSe2@C exhibit significantly positive shift (~0.30 eV for Se 2and ~0.88 eV for Se -) compared with NiSe@C and slightly negative shift (~0.26 eV for Se 2and 0.20 eV for Se -) compared with NiSe2@C, again indicating interfacial interaction at the heterogeneous boundary and the establishment of strong coupled interface [30]. The electrons transfer from NiSe to NiSe2 is induced by the lattice defect and distortions at two-phase boundary and could cause interfacial charge redistribution, which are beneficial for Li + ions transportation and improved reaction kinetics [16,31].

Results and discussion
Apart from Se and Ni, the C 1s, N 1s, and O 1s spectra were also scanned in highresolution (Fig. S6, Supporting Information). The corresponding results unveil that the MOF-derived carbon matrix is codoped by N and O species, which are beneficial for improving the conductivity and electrochemical activity of carbon. The lithium storage properties of these three samples were evaluated in half-cell configuration. Fig. 4a and S7 (Supporting Information) show the initial five CV curves of the NiSe/NiSe2@C, NiSe@C, and NiSe2@C with a voltage range of 0.01-3.0 V and a scan rate of 0.1 mV s −1 . In the first scan of NiSe/NiSe2@C, two cathodic peaks appear at 1.47 and 1.13 V, which correspond to the orderly reduction of NiSe2 and NiSe to metallic Ni 0 as well as the formation of Li2Se [32]. In addition, the cathodic peak at 0.72 V, which disappears in subsequent cycles, belongs to the irreversible formation of solid-electrolyte interface (SEI) film [33]. During the anodic process, the two peaks at 1.95 and 2.02 V should be assigned to the regeneration of NiSe and NiSe2, respectively.
In the subsequent scans, the cathodic peaks shift from 1.13 and 1.47 V to 1.38 and 1.71 V, respectively, while the anodic peaks deviate slightly to 2.11 V, which should be ascribed to microstructural rearrangement [34]. The rate capabilities of the three samples at various current densities are demonstrated in Fig. 4b. It can be seen that the NiSe/NiSe2@C manifest much higher reversible capacity than the two counterparts, delivering capacities of 624.1, 649.1, 681.2, 649.9, 588.6, and 500.8 mAh g -1 as the current rates increase from 0.1 to 4 A g -1 . When the current density returns back to 0.1 A g -1 , the capacity recovers to 829.2 mAh g -1 , indicating the superb electrochemical reversibility and excellent rate capability of the NiSe/NiSe2@C. The lower capacities of the NiSe/NiSe2@C in the initial several cycles than that of NiSe@C might be ascribed to smaller specific surface area of NiSe/NiSe2@C (Fig. S8, Supporting Information), since larger surface area could provide more surface-active sites for Li + storage. As shown in Fig. 4c, an impressive reversible capacity of 1015.5 mAh g -1 is obtained after 100 cycles for NiSe/NiSe2@C, far exceeding those of NiSe@C (591.3 mAh g -1 ) and NiSe2@C (529.7 mAh g -1 ). The superior Li + storage properties of NiSe/NiSe2@C could be attributed to the abundant phase boundaries with distortions and defects inside the NiSe/NiSe2 heterostructure, which could facilitate the adsorption and transfer of Li + ions and thus improve reaction kinetics. The long-term cyclic performance of NiSe/NiSe2@C at 1 A g -1 is presented in Fig. 4d with the initial four cycles tested at 0.1 A g -1 to activate the electrode material. Obviously, the NiSe/NiSe2@C surpass other two materials in terms of reversible capacity and cyclic stability, delivering a substantial value of 540.3 mAh g -1 after 1000 cycles. More significantly, the cyclic performance of NiSe/NiSe2@C is also superior to most of previously reported selenide-based electrodes (Table S3, Supporting Information). To further investigate the effects of NiSe/NiSe2 heterostructure on reaction kinetics and charge storage mechanism, CV measurements at various scan rates from 0.1 to 1.2 mV s -1 were carried out for these three electrodes after several charge/discharge cycles ( Fig. 5d and Fig. S11a, c, Supporting Information). As the scan rate increases, the CVs of NiSe/NiSe2@C present similar shapes and slightly shifted peaks. Generally, the total charge storage of electrode consists of surface-induced capacitive contribution and diffusion-dominated faradaic region, while the peak current (i) and scan rate (ν) comply with the formulas below [36] i= (2) log( ) = log( ) + log( ) where a and b are two variable parameters. The b value could be calculated by the slope of the log(i)-log(v) plot, while b=0.5 represents the diffusion-controlled process and b=1.0 suggest capacitive behaviors [37]. There are six major redox peaks during the charge/discharge processes, and their corresponding b values are 0.94, 0.83, 0.89, 0.82, 0.74, 0.79, telling us that the total Li + storage capacity of NiSe/NiSe2@C is contributed by the capacitive-dominated and diffusion-controlled process simultaneously (Fig. 5e).
The NiSe@C and NiSe2@C electrode also showcase similar charge storage behaviors ( Fig. S11b, d, Supporting Information). However, the NiSe/NiSe2@C manifests higher b values than the NiSe@C and NiSe2@C (Fig. 5f), suggesting a more favored capacitive kinetics of NiSe/NiSe2@C. The surface-induced capacitive contribution could be further quantified by the following formula [38] i(V)= 1 + 2 0.5 (4) where i(V) is the total current response at a given potential V and v represents the scan rate. By determining the k1 values, the fraction of the capacitive current (k1v) can be distinguished from the Li + diffusion contribution (k2v 0.5 ). It is worth noting that more capacitive behavior benefits for fast reaction kinetics and stable cyclic performance especially at high rate [39]. Hence, the high capacitive contribution for the NiSe/NiSe2@C electrode is consistent with its superior rate capability and cyclic stability. In order to unveil the electrochemical reaction mechanism of the NiSe/NiSe2@C, ex-situ XRD together with ex-situ TEM, HRTEM and SAED measurements were performed to examine the evolution of crystalline structure during charge/discharge.
According to the slow-sweep CV curve at 0.01 mV s -1 , the initial lithiation/delithiation processes of the NiSe/NiSe2@C is divided based on eight stages (stages A-H in Fig.   S12, Supporting Information). During the first discharge process, the diffraction peaks of the original electrode (Fig. 6a, stage A) belong to NiSe (32.8°) and NiSe2 (33.5° and 36.8°). As the potential decreases to 1.10 V (stage B), the diffraction peaks of NiSe at 32.8° and NiSe2 at 36.8° disappear due to the conversion reaction of NiSe and partial NiSe2 with Li + ions as depicted in equations 5 and 6. As the discharge continues to 0.01 V (stage D), new diffraction peak around 25.7° is detected with enhanced intensities, which is indexed to the discharge product of cubic Li2Se. In addition, a weak peak at 33.5° belonging to NiSe2 is saved probably due to incomplete utilization. No diffraction peaks of metallic Ni 0 are observed at the fully discharged state, which might be ascribed to its ultrafine or amorphous feature [40]. To confirm the aforementioned structural evolution of the electrode, HRTEM and SAED patterns were collected at stages C and D (Fig. 6b, c). and metallic Ni 0 . The regeneration of NiSe and NiSe2 at stage H is also confirmed by HRTEM and SAED (Fig. 6d, e). In addition, the postmortem TEM characterizations show that the NiSe/NiSe2 domains are consistently anchored into the carbon framework during the whole charge/discharge processes, indicating excellent structural stability of the NiSe/NiSe2@C (Fig. S13, Supporting Information). Based on the above results, the electrochemical reaction process of the NiSe/NiSe2@C should be summarized as follows Discharge: Charge: 2Li2Se+Ni → NiSe2+4Li + +4e - Li2Se+Ni → NiSe+2Li + +2e - The galvanostatic charge/discharge tests indicate that the NiSe/NiSe2@C electrode delivers gradually increased reversible capacities at various current densities (Fig. 4).
This phenomenon of capacity enhancement is generally attributed to the reversible generation and decomposition of polymeric-gel film [41]. However, the charge/discharge profiles at 0.1 A g -1 clearly show that the reversible capacity after 70 cycles is almost three times than that at third cycle (Fig. 6f). Such conspicuous enhancement of reversible capacity for the NiSe/NiSe2@C draws our attention to investigate the origin. The differential capacity versus voltage plots from 2 to 70 cycles are depicted in Fig. 6g and h, from which it could be seen that the redox peaks at 1.44 V (cathodic region) and 1.97 V (anodic region) become more and more obvious, and meanwhile a new pair of peaks at 1.67 V/2.21 V emerges with gradually improved intensities upon cycling. This demonstrates that more and more selenides are involved in the electrochemical lithiation/delithiation due to the electrode activation. Moreover, another new pair of peaks appears at 0.75 V (cathodic region) and 1.34 V (anodic region), which should be ascribed to the reversible formation and decomposition of polymeric-gel film on the surface of active materials [9,42]. The enhanced peak intensities should be due to gradual activation of the electrode that continually generates fresh sites, indicating that large amount of Li + ions are stored in this way upon repeated cycling. To further demonstrate the superiority of monometallic selenides heterostructure to single-phase selenides, we also prepared CoSe/CoSe2@C as well as CoSe@C and CoSe2@C samples under the identical synthetic process (Fig. 7, S14 and S15, Supporting Information). Fig. 7a shows that the as-prepared CoSe/CoSe2@C display shuttle-like morphology with numerous heterostructure nanocrystals uniformly embedded into the carbon matrix. HRTEM image reveals the two-phase property of the product with clear heterointerface (Fig. 7b), while the lattice fringes with interplanar spacings of 2.04 and 2.72 Å can be labeled by the (102)  and CoSe2@C (700.3 mAh g − 1 ). Moreover, the CoSe/CoSe2@C also manifests much superior rate capabilities, delivering reversible capacities of 622.6, 674.9, 529.9, 466.8, 363.5, and 271.3 mAh g − 1 at 0.1, 0.2, 0.5, 1.0, 2.0, 4.0 A g − 1 , respectively (Fig. 7f).
When the current density is restored to 0.1 A g − 1 , the average capacity reaches back to 1076.4 mAh g − 1 . The above results once again prove the advantages of monometallic heterostructure over single-phase materials towards LIBs application.

Conclusions
In summary, the monometallic selenides heterostructures surrounded by conductive carbon matrix (NiSe/NiSe2@C, CoSe/CoSe2@C) were synthesized, by adjusting the ratio of MOF precursor to selenium powder during calcination. The heterostructure materials manifest superior electrochemical performance to their singlephase counterparts as anodes for LIBs. Specially, the NiSe/NiSe2@C delivers high reversible capacity (1015.5 at 0.1 A g -1 ), excellent rate capability (500.8 at 4 A g -1 ), and long-term cyclic stability (540.3 mAh g − 1 after 1000 cycles at 1 A g -1 ). This enhanced lithium storage properties should be due to the heterogeneous interface between NiSe and NiSe2 domains with electrons rearrangement, which promotes rapid charge transfer and ion adsorption, as well as increases the electrochemically active sites, thus improving the reaction kinetics. EIS, GITT, and kinetics analysis further demonstrate that the NiSe/NiSe2@C possesses lower charge transfer resistance, faster diffusion coefficient of Li + ions, and more favored capacitive energy. Besides, the porous carbon skeleton in-situ converted from Ni-MOF not only increases the electrical conductivity of the material, but also provides abundant channel for the infiltration of electrolyte.
We anticipate the present strategy for engineering two-phase heterostructure with single cations and single anions could be extended to other compounds for various applications.