Core–shell Cu1−x NCo3−y /a-CuFeCo antiperovskite as high-performance anode for Li-ion batteries

Currently, there is an emergent interest in the antiperovskite family of materials in the context of energy applications in view of their distinct and peculiar set of structural and electronic properties. This work examines the surface-modified antiperovskite nitride CuNCo3 as a high-performance anode material for Li-ion storage devices. The antiperovskite CuNCo3 was prepared by the hydrothermal method followed by calcination in the NH3 atmosphere. An amorphous layer on the surface of CuNCo3 (Cu1−x NCo3−y /a-CuFeCo) was also fabricated to enhance its performance as an anode material for Li-ion batteries. The surface-modified Cu1−x NCo3−y /a-CuFeCo material was noted to deliver an extraordinarily high reversible capacity of ∼1150 mAh g−1 at a current density of 0.1 A g−1, whereas the CuNCo3 showed a reversible capacity of ∼408 mAh g−1 at the same current density. The initial capacity of Cu1−x NCo3−y /a-CuFeCo exhibited excellent retention (>62%) even after 350 cycles. A ∼6 nm thin amorphous layer around the surface of pure CuNCo3 helped almost double the specific capacity as compared to the pure CuNCo3 due to the presence of a multi-redox center for Li-ion to react and also concomitantly improved electrical conductivity property. The cyclic stability of the Cu1−x NCo3−y /a-CuFeCo material at a higher current density (0.5 and 1.0 A g−1) was also noticeable. This work opens up new materials routes and promising processing strategies to develop high reversible capacity anodes for alkali ion batteries.


Introduction
Lithium-ion batteries (LIBs) are one of the most important energy storage systems that enable green energy utilization and storage in diverse applications from hand-held devices and wearables to e-mobility and grid storage [1].The most prominent application among these, which is being currently discussed worldwide is, of course, e-mobility.LIBs will continue to play a vital role in this arena for several years, as they afford several virtues of an established and tested robust technology.The excellent stability, ease of fabrication, and simple electrochemistry have led LIBs to attract great attention in the commercial market all over the world in diverse applications.Despite the huge initial success in the commercialization of this battery chemistry, intense research is continuously being done toward the search of new and alternate materials that could enhance all the battery qualifying factors such as very high energy and power density and excellent stability for both the anode and the cathode electrode materials [1].The need for seeking battery performance enhancements and even search for newer chemistries have arisen during the past several years because of the international drive for highly enhanced clean energy usage which necessarily requires good and robust storage options [2,3].
Any new anode or cathode materials are expected to deliver better than the existing ones in certain important aspects such as cyclic stability, voltage hysteresis, operating potential window, capacity, and current stability [1,[4][5][6].Since dendrite formation prevents the direct use of Li metal as an anode, recently there has been considerable progress on this front in the domain of LIBs.Several different anode materials have since been explored under three different categories, namely alloying, conversion, and intercalation [5].Unfortunately, all these types of anode materials suffer from one or more drawbacks such as pulverization due to volume expansion, low electronic conductivity, and/or rapid capacity fading.Various strategies have therefore been explored to improve the performance of all these anode materials by controlling capacity fading and volume expansion, although all the issues have only been partially resolved thus far [7].From the commercial perspective, all these anode materials are still not able to fulfill the required specifications except graphite, which continues to fuel further research.
Many 2D and 3D oxides, carbides, nitrides, chalcogenide, and perovskite family of materials of different structural/chemical properties and morphologies have been explored as new anode materials for LIBs electrodes.Nanomaterials have certain drawbacks in terms of applicability, such as limited tap density, poor electrical conductivity, a large surface area leading to high solid electrolyte interphase (SEI)-related initial capacity loss, and significant problems of surface side reactions.To overcome the disadvantages of nanomaterials the idea of core-shell nanostructure was introduced with some success [8].Promising core-shell anode materials with different crystal types [9,10] and different coating layers have been reported [11][12][13][14].The core is the most important component with different functional qualities, whereas the outer shell serves as a protective layer or adds new features to the functional core strengths.Core-shell structures often display superior physical and chemical properties over their single-component counterparts, and hence are extensively used in energy storage [15], magnetism [16], biomedicine [17], catalysis [18], energy conversion, and storage [19].
To achieve a suitable anode electrode material for LIBs, various materials have been explored which aim to compete with the commercial graphite anode material.One such family of materials is the perovskite system, and many oxides and halides perovskites have already been examined in this context [20][21][22].However, as anode materials, these systems are found to still suffer from poor electronic conductivity, cyclic stability, capacity fading, material degradation.Antiperovskites with their distinctly different chemical environments and electronic states are now beginning to attract more attention.These include materials such as lithium halide hydroxide, oxyhalide, cluster-based, hydride-based, and transition metal-based antiperovskites.These have been examined as solid-state electrolyte and cathode material for LIBs [23].Indeed, the Li-rich antiperovskites have also been proposed to serve as cathode materials by possible cation and/or anion manipulations [23,24].Antiperovskite carbides and nitrides have a very high density of state at the Fermi level (E F ) exhibiting metallicity and excellent conductivity [25].The highly conducting property of nitride antiperovskites has already led to promising applications in the field of electrocatalysis [26], and these properties also encourage explorations of their further applicability as anodes in LIBs.
Herein we have explored pure antiperovskite CuNCo 3 and surface-modified CuNCo 3 -based core-shell like material Cu 1−x NCo 3−y /a-CuFeCo for the LIB anode application.The material not only shows remarkably high capacity but also outstanding stability.The surface-modified CuNCo 3 (Cu 1−x NCo 3−y /a-CuFeCo) delivers an extremely impressive reversible capacity of ∼1150 mAh g −1 at a current density of 0.1 A g −1 whereas pure CuNCo 3 showed only ∼408 mAh g −1 at the same current density.Amorphous layer of metal oxyhydroxide shell on the surface of the CuNCo 3 in Cu 1−x NCo 3−y /a-CuFeCo material increased the electronic conductivity of the system and accelerated the chemical redox reaction of Li-ion resulting in almost doubling of the performance of the battery as compared to CuNCo 3 .

Synthesis of CuNCo 3
A hydrothermal method was used to synthesize antiperovskite nitride CuNCo 3 .In a typical synthesis, 1.5 mmol cobalt acetate, 0.5 mmol copper acetate, and 4 mmol HMT were dissolved in 50 ml deionized water and mixed properly.The mixture was then transferred into a 100 ml Teflon-lined autoclave and heated at 120 • C for 12 h.After cooling down to room temperature, the precipitate was collected through centrifuging (1000 rpm for 15 min) and washed with deionized water and ethanol 3-4 times.The collected product was dried in a vacuum oven at 80 • C overnight.The dried product was then ground well and calcined at 420 • C for 4 h under an NH 3 atmosphere in a tube furnace.After cooling down to room temperature, the furnace atmosphere was changed to N 2 flow [27].The final product was collected for further characterization.

Synthesis of Cu excess CuNCo 3 (CuNCo 3 + Cu)
CuNCo 3 + Cu was synthesized using a similar protocol as for CuNCo 3 but by using excess copper acetate during the hydrothermal process.

Synthesis of core-shell Cu 1−x NCo 3−y /a-CuFeCo
A solid product of CuNCo 3 + Cu was used as a starting material to synthesize Cu 1−x NCo 3−y /a-CuFeCo.Desired amounts of CuNCo 3 + Cu were mixed in an aqueous solution of the appropriate amount of FeCl 3 .6H 2 O and the resulting solution was stirred for 30 min at room temperature.Finally, after 30 min, the solid sample was collected through centrifugation (1000 rpm for 15 min) followed by washing with deionized water and ethanol 3-4 times.It was then kept in the oven at 80 • C overnight.

Characterization
The powder X-ray diffraction (PXRD) measurement was carried out using Bruker D8 advanced x-ray diffractometer (Cu-K α λ = 1.541 78 Å).The morphology of the obtained solid products was studied via field-effect scanning electron microscopy (FESEM, ZEISS-Ultra Plus-4095).High-resolution transmission electron microscopy (HRTEM) images were obtained on a transmission electron microscope (a Jeol, JEM 2200FS at 200 keV) with a selected area electron diffraction (SAED) pattern.X-ray photoelectron spectroscopy (XPS) study was carried out using a Kα x-ray photoelectron spectrometer (Thermo-Fisher Scientific Instrument, UK).

Electrochemical characterizations 2.6.1. Fabrication of electrode and coin cell
The electrodes for both the CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo materials were made by mixing the active material, conducting carbon (acetylene black) and polyvinylidene difluoride as a binder (Sigma-Aldrich; weight ratios were maintained as 7:2:1) in N-methyl-2-pyrrolidone solvent and coating the slurry onto a Cu foil.The electrodes were dried in an oven at 80 • C overnight.The dried electrodes were then cut into round electrodes by using a disc cutter.We made 2032-type coin cells in an Ar-filled glovebox.For all the studies, LiPF 6 electrolyte was used in a solvent combination of 1:1 ethylene carbonate (EC) and dimethyl carbonate (DMC) along with 2 wt.% fluoroethylene carbonate (FEC; Sigma Aldrich) as an additive.Celgard 2500 was used as the separator.2 mg cm −2 material loading was used in the construction of the coin cell.

Electrochemical characterizations
The galvanostatic charge/discharge tests were performed on a coin cell and tested at different current densities in a voltage range of 0.01-3.00V in a Neware battery tester.The cyclic voltammetry (CV) and electrochemical impedance spectroscopy were recorded using electrochemical workstation (Parstat).The CV data for CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo were recorded at 0.5 mV s −1 scan rate to elucidate the electrochemistry of the anode material.To draw the relationship between the relative contributions of the diffusion and capacitive-controlled charge storage of Cu 1−x NCo 3−y /a-CuFeCo, CV data were recorded at different scan rates.The electrochemical impedance measurements were carried out in the frequency range of 1 MHz-0.01Hz after cell fabrication and after five cycles of CV (scan rate 0.5 mV s −1 ) for the CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo materials.

Formation mechanism of CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo
The procedures for the synthesis of CuNCo 3 and surface-modified Cu 1−x NCo 3−y /a-CuFeCo are schematically illustrated in scheme 1.The core-shell formation mechanism of Cu 1−x NCo 3−y /a-CuFeCo is similar to the reported mechanism of the core-shell of CuNNi 3 [28].Initially, copper is dissolved according to the chemical reaction 1 [28].After the addition of FeCl 3 .6H 2 O into an aqueous solution of Cu excess CuNCo 3 (CuNCo 3 + Cu), Co and Cu in the antiperovskite structure are partially etched by Fe +3 according to reactions 1 and 2. Significantly, Fe 3+ , Cu 2+ , and Co 2+ ions are hydrolyzed to form a CoFeCu(oxy)ydroxide colloid and H + (reaction 3) [29,30].
Hydrolysis is promoted due to the consumption of H + .As a result, CuFeCo is deposited on the surface of the antiperovskite, which leads to the formation of core-shell like Cu 1−x NCo 3−y /a-CuFeCo material Cu + 2Fe 3 + = Cu 2 + + 2Fe 2 +  (1)

Structural characterization
The PXRD pattern of CuNCo 3 is presented in figure 1 represents the amorphous nature of the shell region [32] which will be further discussed in the morphological characterizations section.The Raman spectra of the CuNCo 3 was recorded to understand the presence of different vibrational modes in the system.Figure S1 shows the Raman spectrum of CuNCo 3 where the vibrational modes of E g , F 2g and A 1g are clearly observed for the Co-N bonds [33].

Morphological characterizations
To further understand the morphology of CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo, FESEM and HRTEM data were recorded.The FESEM image shown in figure 3(a) indicates the nanowire morphology of CuNCo 3 which is the self-assembly of 50 nm nanoparticles into nanowires [27].The HRTEM images shown in figure 3(b) also shows similar morphological features, as expected.Figure 3(c) illustrates the lattice fringes of the CuNCo 3 which correspond to the (111) plane with an interplanar lattice spacing of 0.22 nm and the corresponding lattice constant of the unit cell of CuNCo 3 is 3.75 Å which nicely corroborates with our theoretically obtained optimized lattice constant of 3.74 Å (figure 3(d)) using density functional theory (DFT).The crystal unit cell of CuNCo 3 is presented in figure 3(d).The stoichiometry of the CuNCo 3 is confirmed by EDS as shown in figure S3(a 4(c) confirms the core-shell-like structure while the white dotted line discriminates the core and shell regions.The Fast Fourier Transform (FFT) image of the core-shell was recorded to analyze the detailed nature of the core and shell which is illustrated in figure 4(c), where the core is crystalline and the shell is amorphous in nature.Figure 4(c) (i) shows the interface region between the core and shell whereas figure 4(c) (ii) shows the cubic crystalline region of the CuNCo 3 .Figure 4(d) is the schematic illustration of the core-shell of Cu 1−x NCo 3−y /a-CuFeCo.The SAED pattern of Cu 1−x NCo 3−y /a-CuFeCo is shown in figure S5 where the main diffractive rings from the center to the edge are (111), ( 200) and (220) of Cu 1−x NCo 3−y /a-CuFeCo.The EDS data of the Cu 1−x NCo 3−y /a-CuFeCo is presented in figure S6(a).Elemental mapping and line-scan spectra shown in figures S6(b) and (c), confirm the presence of Co, Cu, Fe, and N. It may be noted that the nitrogen signal (pink curve) is absent outside the core region suggesting that the shell does not have a nitride and Fe (yellow signal) is present through the whole region.

Electrochemical characterization
The electrochemical performance of CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo materials was tested in a LiPF 6 medium in EC/DMC (1:1 v/v) was used as the electrolyte with 2 vol.% of FEC additive.A total amount of 200 µl of electrolyte was used for a coin cell to analyze the LIB performance.It was observed that CuNCo 3 shows a high first discharge capacity of ∼950 mAhg −1 as presented in figure 5(a).The conducting nature due to the overlap of valance and conduction bands at the Fermi level (E F ) (figure S7) and the presence of a high amount (60%) of cobalt (Co) nitrogen (N) could lead to the enhancement of the capacity since the capacity contribution mostly due to the Co oxidation and reduction.However, from the second cycle onward, the reversible capacity obtained is only 412 mAhg −1 .CuNCo 3 showed a reversible capacity of only 210 mAhg −1 after 350 cycles with 50% retention, as presented in figure 5(c).The rate performance was measured at different current densities of 100, 200, 300, 400, and 500 mAg −1 and CuNCo 3 showed a specific capacity of 225 mAh g −1 at a high current density of 500 mAg −1 (figure S8(a)).The decay observed in the case of CuNCo 3 with the increasing number of cycles could be attributed to the volume expansion of the cell as evident from the theoretical study shown in figure S9 where with successive lithiation (1-5 Li) inside the octahedral and tetrahedral interstitial void space(s) a significant volume change was noted.This may lead to the pulverization within the electrode material which could detach the electrode particle and may reduce the conductivity and the capacity fade observed in figure S8(a).
To improve the performance further, the surface of CuNCo 3 was modified to realize a core-shell configuration.Several such strategies have been implemented to control the volume expansion of the materials during the charging and discharging [8].The strategy of core-shell nanostructure appears quite promising to increase the structural stability and storage capacity of the Li-ion electrode.It has been reported that to enhance the electrochemical performance of the family of alkali ion batteries amorphization engineering is an interesting option due to the unique functionalities of amorphous phases [34,[38][39][40][41].Many important factors influence the electrochemical performance of crystalline host materials such as the available energetically equivalent sites for guest-ion occupation/transport, crystal orientation, structural stability, phase transition, the spatial dimension of ion migration, defects in crystal, and the stoichiometric limitation of ion insertion [42][43][44].Compared to those crystalline electrodes, the amorphous counterparts could deliver much improved specific capacities and long-term cyclability over a wide potential window [39,40].This is because amorphous phases exhibit several advantages, such as the existence of abundant percolation pathways, improved ionic intercalation/deintercalation kinetics, a larger free volume and a higher specific surface area to accommodate system lattice distortions [45].A thin amorphous layer (∼6 nm) around the surface of pure CuNCo 3 was therefore designed and employed to improve the battery performance of CuNCo 3 .The performance of the Cu 1−x NCo 3−y /a-CuFeCo showed very impressive enhancement by a factor of two in the specific capacity as compared to the pure CuNCo 3 (figure 5(b)) which is much higher than the earlier reported study based on the 3D carbide antiperovskite Fe 3 SnC by Roy et al [46] and Co 3 ZnC by Chen et al [47] for antiperovskite systems.The first discharge capacity of Cu 1−x NCo 3−y /a-CuFeCo was found to be exceptionally high ∼2200 mAhg −1 , at a current density of 100 mAg −1 .After 350 cycles, the reversible capacity dropped down to ∼710 mAhg −1 which is also very high compared to the pure CuNCo 3 .Moreover, it also showed excellent stability (59% retention) as shown in figure 5(d) even after 350 cycles.The rate performance of Cu 1−x NCo 3−y /a-CuFeCo was also examined at different current densities of 100, 200, 300, 400, and 500 mAg −1 as shown in figure S8(b) and it showed the capacity decay with increasing the current densities.A capacity of ∼465 mAhg −1 was observed at high current density of 500 mAg −1 , which is very high as compared to that of the CuNCo 3 (225 mAh g −1 ). Figure S10 shows the cyclic stability of Cu 1−x NCo 3−y /a-CuFeCo at 0.5 Ag −1 and 1 Ag −1 .At the current density of 0.5 Ag −1 , initial specific discharge capacity obtained is 529.3 mAhg −1 which is reduced to 448.36 mAhg −1 after initial 25 cycles.This gets stabilized later and shows a specific capacity of 470.20 mAhg −1 at 250th cycles.At a higher current density of 1Ag −1 , the initial specific capacity obtained is 430.27 mAhg −1 and retains a value of 265.91 mAhg −1 after 100 cycles.The specific capacity of a commercial intercalating carbon-based graphite anode at current density 0.5 Ag −1 is 350 mAhg −1 [48].Compared to this, the capacity exhibited by our core-shell material is 28% more at the same current density (0.5 Ag −1 ).
Figure 6 shows the CV data for CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo over the potential range of 0.01-3 V at the scan rate of 0.5 mV s −1 to elucidate the electrochemistry of the anode material.For the material CuNCo 3 , reduction peaks were observed in the first cycle which occurred from the conversion reaction of the material.The broad peaks at 0.73 V and ∼1.46 V are attributed to the multistep reduction of cobalt to the cobalt metal and lithium compound formation [49][50][51].The broad reduction peak shifts slightly to ∼0.8 V from 0.73 V in the 5th cycle.This shift could be due to the changes in the structure after the multiple lithiation/de-lithiation process.The anodic broad peaks centered at ∼1.16 V and 2.18 V correspond to the oxidation of the cobalt [49][50][51].The electrochemical behavior changes with the material morphology, crystallinity, and microstructure [49].The same we can see in the surface modified CuNCo 3 (Cu 1−x NCo 3−y /a-CuFeCo), after the system stabilizes, the 5th cycle cobalt reduction peaks occurred slightly at lower potentials of 0.64 V and 1.31 V as compared to CuNCo 3 .Similar to CuNCo 3 , the broad oxidation peak observed at 2.18 V in the case of Cu 1−x NCo 3−y /a-CuFeCo signifies cobalt oxidation.Other minor peaks in both the materials in the first cycle vanish from the second cycle, and are thus attributed to the SEI formation and electrolyte reduction.The observed anodic and cathodic current for the Cu 1−x NCo 3−y / a-CuFeCo material is more than that for the CuNCo 3 case at the same scan rate.This indicates the greater amount of reaction taking place due to the higher electrochemical surface area of the Cu 1−x NCo 3−y / a-CuFeCo material (figure 6).The total charge in Farad has been calculated for all curves at different scan rates.Figure S11(c) shows the region B (CV curve at 1 mV s −1 ), which is considered for the calculation of Faradaic contribution at all the different scan rates.The charges calculated for all the curve has been tabulated in figure S11(d).The average of all has been calculated and compared with the charge obtained from region A. In conclusion, the ratio of the capacitive and diffusion-controlled charge obtained was 0.58:0.77.
The charge transfer was investigated by electrochemical impedance analysis and the corresponding measurement was carried out in the frequency range of 1 MHz-0.01Hz after cell fabrication and after five cycles of CV (scan rate 0.5 mV s −1 ) for both materials.Figure 7 represents the Nyquist plots for the CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo samples.The diameter of the semicircle at the mid-frequency region is attributed to the charge transfer process, which is less for the core-shell material even after the five cycles of CV, suggesting the lower contact and charge transfer resistance [51].Another small semicircle is observed on the 5th cycle impedance obtained from the SEI layer.The values for the resistance associated with the SEI and charge transfer were modeled with the modified Randle's equivalent circuit which is listed in table S2 for all the cycles.This AC impedance study brings out the enhanced electronic conductivity and electrochemical properties of the core-shell material (Cu 1−x NCo 3−y /a-CuFeCo).
The diffusion properties of the materials were calculated by the galvanostatic intermittent titration technique .The huge difference in the particle size and the surface chemical composition of the materials does not make a fair comparison together.However, the lithium-ion diffusion coefficient for both materials was found to be in the order of 10 −9 cm 2 s −1 as shown in figure S12.
The significant capacity increase of the core-shell system suggests a major role of the amorphous nature of the shell consisting of multimetal oxyhydroxide (figure 2(d)) of CuFeCo deposited on the crystalline surface of the CuNCo 3 .The presence of the porosity, large surface area (figure S13), and the multi-redox center for Li-ion to react help in allowing more Li-ions to pass into the core-shell structure which could be the reason for the improvement in capacity and structural stability of Cu 1−x NCo 3−y /a-CuFeCo.It is a well-known fact that metal oxyhydroxides have very good electrical conducting properties [38,52,53].After forming the metal oxyhydroxide shell the surface of CuNCo 3 , the core and shell together increased the electronic conductivity [12] of the system and accelerated the chemical redox reaction of Li-ion with the electrode; thereby increasing the performance of the battery.The electrochemical performance of the antiperovskite Cu 1−x NCo 3−y /a-CuFeCo was compared with the literature reports presented in tables S3 and S4 which indicate that our material shows the best performance among any perovskite and anti-perovskite based materials and also shows quite promising performance based on recently reported core-shell structures.

Post-cycling characterization of Cu 1−x NCo 3−y /a-CuFeCo
To check the morphology evolution of Cu 1−x NCo 3−y /a-CuFeCo due to cycling, we performed ex-situ post-cycling FESEM imaging of the electrodes.FESEM images of fresh, after 1st cycle, and after 100 cycles of Cu 1−x NCo 3−y /CuFeCo are shown in figure 8 and they reveal the SEI layer formation on the electrode surface, which leads to stable anodic performance.Furthermore, the morphology of the material was largely intact after 100 cycles which establishes its robustness.

Conclusion
Designing a new, effective, and robust high-capacity anode material for alkali ion battery continues to be an exciting research opportunity with technological implications.A new strategy is utilized herein to boost the reversible capacity via surface engineering of an antiperovskite CuNCo 3 that delivers highly superior performance.While the 3D nitride antiperovskite CuNCo 3 as an anode material provides a reversible capacity of ∼408 mAhg −1 at 0.1 Ag −1 current density, the surface-modified core-shell like Cu 1−x NCo 3−y /a-CuFeCo delivers a very high reversible capacity of ∼1150 mAhg −1 at the same current density, which is double the value obtained with the base material.The specific discharge capacity of Cu 1−x NCo 3−y /a-CuFeCo is ∼1.33 times more (at 0.5 Ag −1 ) and 1.1 times more (at 1 Ag −1 ) as compared to the specific capacity of a commercial intercalating carbon-based graphite anode.Thus, the surface-modified CuNCo 3 exhibits excellent and robust Li-storage performance.We believe that this work will open up various new possibilities to focus on 3D antiperovskite nitride or carbide systems to be explored for energy storage.
(a) and the sharp peaks in the PXRD pattern reveal the formation of pure phase of the CuNCo 3 with good crystallinity.All the peak positions and corresponding planes in figure1(a) match well with the standard JCPDS pattern (#PDF: 00-053-0435)[27,31].The PXRD pattern of the CuNCo 3 + Cu and the surface-modified CuNCo 3 (Cu 1−x NCo 3−y /a-CuFeCo) have been presented in figure1(b).In the case of CuNCo 3 + Cu, two small peaks at 2θ = 43.93• and 51.04 • correspond to metallic Cu, and the remaining three strong peaks at 42.3 • , 48.9 • , and 71.4 • correspond to CuNCo 3 .After etching by Fe 3+ , the peaks associated with metallic Cu are not observed in the case of Cu 1−x NCo 3−y / a-CuFeCo, suggesting that metallic copper is successfully dissolved.Additionally, the three main peaks of CuNCo 3 almost remain the same except that there is a slight shift of the peak position toward the lower 2θ due to surface amorphous phase induced strain.The broad region between the 2θ range from 20 • to 35• )-(d) represent the
) which suggests the near exact atomic ratio of 3:1:1 of the sample.The elemental mapping shown in figure S3(b) further confirms homogeneous distribution of the elements in CuNCo 3 SEM image and corresponding energy dispersive spectroscopy (EDS) of the CuNCo 3 + Cu are presented in figure S4 which confirm the excess Cu in CuNCo 3 + Cu.The HRTEM image of Cu 1−x NCo 3−y /a-CuFeCo shown in figure 4(a) shows rod like morphology.The zoomed image presented in figure 4(b) confirms that the nanorods are made of nanoparticles similar morphology of CuNCo 3 .The zoomed image shown in figure

Figure 7 .
Figure 7. (a) and (b) Represents the impedance spectra of the CuNCo3 and Cu 1−x NCo 3−y /a-CuFeCo at two different cycles.

Figure
FigureS11(a) shows the CV curve obtained for the core-shell material when subjected to the scan rates of 0.1, 0.2, 0.5 and 1 mV s −1 .The different charge contributions have been marked as region A and region B. Corresponding total charge in Farad has been calculated for both regions.FigureS11(b)shows the linear fit between the peak current (anodic side) and the scan rate of region A. The slope of the fit is the total charge in Farad which is 0.58 F. To calculate the total contribution from region B (only the anodic side), the charge in Farad has been calculated by the method stated below.Total Coulomb of charge (C) = area under curve/scan rate Total charge in Farad (F) = total Coulomb of charge (C)/voltage tange The total charge in Farad has been calculated for all curves at different scan rates.FigureS11(c) shows the region B (CV curve at 1 mV s −1 ), which is considered for the calculation of Faradaic contribution at all the different scan rates.The charges calculated for all the curve has been tabulated in figureS11(d).The average of all has been calculated and compared with the charge obtained from region A. In conclusion, the ratio of the capacitive and diffusion-controlled charge obtained was 0.58:0.77.The charge transfer was investigated by electrochemical impedance analysis and the corresponding measurement was carried out in the frequency range of 1 MHz-0.01Hz after cell fabrication and after five cycles of CV (scan rate 0.5 mV s −1 ) for both materials.Figure7represents the Nyquist plots for the CuNCo 3 and Cu 1−x NCo 3−y /a-CuFeCo samples.The diameter of the semicircle at the mid-frequency region is attributed to the charge transfer process, which is less for the core-shell material even after the five cycles of

Figure 8 .
Figure 8. Post-cycling ex-situ FESEM images of (a) fresh electrodes; (b) after 1st cycle; and (c) after the 100th cycle of Cu 1−x NCo 3−y /a-CuFeCo electrodes in the half cell configuration.