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Copper-diphosphide Composites: A Key Factor Evaluation and Capacity Enhancement Route for High Energy Li-Ion Storage Rasu Muruganantham, Ping-Chuan Chiang, and Wei-Ren Liu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00470 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Energy Materials

Copper-diphosphide Composites: A Key Factor Evaluation and Capacity Enhancement Route for High Energy Li-Ion Storage Rasu Muruganantham, Ping-Chuan Chiang, Wei-Ren Liu* Department of Chemical Engineering, R&D Center for Membrane Technology, Chung Yuan Christian University, Chung Yuan Christian University, 200 Chung Pei Road, Chung Li District, Taoyuan City, Taiwan 32023, R.O.C. KEYWORDS: CuP2, Li-ion batteries, anodes, capacities, rate performance, capacity behaviour ABSTRACT: Carbon-modified phosphide-rich CuP2 anode is prepared by mechanical ballmilling technique for Li-ion batteries. HRTEM image demonstrate carbon is fully and homogeneously wrapped on CuP2 core material. The bare and carbon coated CuP2 deliver an initial specific capacity of 1454 mAh/g and 1416 mAh/g with columbic efficiencies of 83 and 61 % at 100 mA/g, respectively. Conversely, carbon coated electrode exhibited superior cyclic stability than bare electrode. The CuP2/C lithiation capacity is 700 mAh/g over the 100 cycles at 100 mA/g. Furthermore, 1 wt. % of CNTs addition in CuP2/C slurry dramatically enhanced the Li-storage and rate performance than rest electrodes. The capacity nature is studied by cyclic voltammetry analysis and the result of bare electrode is based on diffusion process. Surprisingly, after carbon modified electrode, the capacity contribution nature is observed surface control

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behavior. Hence, the enhancement of electrochemical performance is predicted by accelerating of kinetics, buffer of volume changes by carbon coating, improvement of catalytic effect and surface control of capacity behaviour. 1. INTRODUCTION Advanced energy materials have engrossed fast growing and tremendous attention for various types of energy storage applications namely batteries, supercapacitors and so on.

[1-3]

Among them, batteries are a great solution for energy crisis and environmental issues. Past and present Li-ion batteries have rectified the energy crises for portable and electric vehicle applications. Nowadays, the development of heavy electric devices and scientists has become an important target to identify the high-energy storage electrode materials.[4-7] The electrode theoretical capacity is inversely proportion to the molecular weight of the compound and proportional to the electron transfer number. Essentially, the active materials with multiple electrons reaction and low molecular weight could achieve high specific capacity. At present, phosphides compounds are more favorable high potential negative electrode materials for Li/Na-ion storage due to the weaker electronegativity of phosphorus. Phosphorus exists in three main allotropes: white, red and black. White phosphorus consists of tetrahedral P4 molecules. It is highly reactive and toxic and therefore unsuitable as a battery material. Red and black phosphorus have been widely investigated previously as an anode material for second batteries because of their high theoretical specific capacity of 2,596 mAh g−1 compare to other metals like Cu, Co and etc. The drawbacks of P as anode materials, however, are poor electronic conductivity of ~10-14 S cm-1 and large volume expansion of ~300% during charge and discharge processes,[8-12] which lead to serve capacity fade and poor rate performance. To overlap these issues, one of the strategies is surface modification by using carboneous materials, namely


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graphene/graphene oxide or combination of other metals, to enhance structural stability and electronic conductivity.[13,14] Metal phosphides as an anode for batteries are classified into two different kinds, including metal-rich phosphides[15-17] and phosphide-rich metal compounds.[18-20] On the other hand, the capacity behaviour analysis of active materials is a key factor for the development of high-performance batteries.[21,

22]

It is worthy to identify the mechanism of

battery electrodes capacity behaviours based on diffusion-controlled intercalation process (DIP) or surface-induced capacitance process (SCP) namely pseudocapacitance.[23-29] The fabrication of active material at nanoscale and conductive coating are an effective method to enhance the kinetics of the electrode. In this work, we report the synthesis and evaluation of CuP2 and carbon modified CuP2 composites as anode materials for Li-ion batteries. The phase purity, structural and morphological performances were carried out by XRD, Raman spectra, XPS and SEM/HRTEM analyses. The Li-ion storage performance was improved by carbon coating of CuP2 active material and further addition of CNTs as a conductive additive of the electrode slurry. The pristine and carbon modified hybrid electrodes storage capability, cyclic, rate performance, capacity behaviour and kinetics were investigated via charge/discharge profiles, CV and EIS, respectively. The electrochemical mechanism was verified by ex-situ XRD analysis of CuP2 electrode at different voltage point of charge/discharge profile. 2. EXPERIMENTAL SECTION Synthesis of CuP2 and CuP2/C: Bare and carbon modified CuP2 composites were synthesized by high-energy mechanical ball-milling technique. The stoichiometric amounts of commercial copper (99%, 45 µm, Acros Organics) and red phosphorus (98+%, Alfa Aesar) were mixed with an atomic ratio of 1:2. Then, the mixture was put into a hardened steel vial (80 cm3) with


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hardened steel balls along with 20 wt % of carbon block under sealing in argon filled glove box. After sealing, ball milling was performed with a Retsch ball-miller for 20 h in 500 rpm. The total weight of the powder was adjusted to 2.0 g with a ball-to-powder mass ratio of 20:1. The assynthesized composite samples were ground and stored in an argon-filled glove box. For bare CuP2 sample was synthesized by the same processes without addition of carbon block. The carbon coated and bare CuP2 samples are labelled as CPC and CP, respectively. Characterizations:

The crystal structure and purity were determined by X-ray powder

diffraction (XRD) using a Bruker D8 diffractometer with monochromatic CuKα radiation. It was operated at 40 KV and 30 mA and the wavelength of λ = 1.54060 Å. The diffraction data was recorded in the range 2θ=10-80°. Raman spectra of the powder samples were taken on a Renishaw in Via plus Raman microscope with 613 nm wavelength laser excitation under 1 mV power. The morphology of as-synthesized CuP2 and C-modified CuP2 were observed by field emission scanning electron microscopy (FE-SEM, JSM-7600F, JEOL, Germany) with energy dispersive spectrometer/electron mapping (EDS, X-MAX) and high resolution transmission electron microscopic (HR-TEM) (Techni G2 S-TWIN, FEI, Netherlands) technique. The Chemical valence states of as-prepared materials elements were investigated using X-ray photoelectron/absorption spectroscopy techniques (XPS, PHI model 5802 and XAS). Electrode preparation: The preparation of working (negative) electrodes, slurries consisting of 70 wt.% active material (bare or C-modified CuP2), 15 wt.% carbon black as conductive additive (Super P, Timcal®), and 15 wt.% Polyvinylidene fluoride (PVdF) as a binder in Nmethylpyrrolidone (NMP) solvent to form a homogeneous slurry. Then, the mixed slurry was spread uniformly on a thin copper foil using doctor blade coating method and dried in vacuum at 120 °C for 6 h. Afterwards, the electrodes were punched into disc types (diameter in 1.2 cm)


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with a typical active mass loading of 1.8-2.0 mg cm−2 and a thickness of ~40 µm. Furthermore, for improving the electrochemical performances, 1 wt. % of CNTs were added into the CPC (CuP2/C) slurry. The active material, conductive additive and binder weight ratio were found to be 70: (14+1): 15 wt. %. The electrode was labelled as CPCM. Li-ion half-cell fabrication: The Li-ion half cell was fabricated using CR2032 coin-type cell with Li-metal as a reference electrode and prepared electrode as working electrode. Polypropylene (Celgard® 2500) was used as a separator between the working electrode and a lithium metal counter/reference electrode. The electrolyte was composed of 1M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC; 1:1 (v/v)) with 5 vol % vinyl carbonate (VC). The coin cell assembling procedures were performed using Ar-filled glove box (MBraun lab star model) by keeping both the oxygen and moisture levels less than 1 ppm. Electrochemical measurements: The galvanostatic discharge-charge measurements were performed using a constant current and voltage programmable AcuTech battery testing system (Taiwan, R.O.C, model 750B) in the potential range of 0.01-2.0 V (V vs. Li+/Li) at different currents under ambient temperature. The Cyclic voltammograms (CV) were measured by electrochemical workstation of CH Instruments Analyzer (CHI 6273E) at different scan rate between 0.05-5 mV/s. The AC impedance was carried out in the range of 0.01 Hz to 100 KHz at A.C voltage of 5 mV amplitude. All the electrochemical performance was tested in room temperature. For ex-situ measurements, the cycled electrodes were obtained by disassembling the coin cells, washing with DEC, and drying naturally under an argon atmosphere. 3. RESULTS AND DISCUSSION


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Scheme1 represents the synthesis and coin cell fabrication of bare and carbon modified CuP2. The bare and C-coated CuP2 were prepared by solid state reaction using high energy ball-milling process. Initially, copper, red phosphorus and carbon black were mixed in 250 ml of agate coated ball-milling jar in Ar atmosphere. Afterwards, the mixed elements were ball-milled at a rotation speed of 500 rpm for 20 h. Finally, it was observed CuP2/C (CPC) powder material. The bare CuP2 (CP) was prepared by except of carbon black in the same reaction. The general ball-milling procedure steps and formation stages of CuP2 are described in supporting information. The assynthesized powders were fabricated as anode for Li-ion batteries via two approaches of electrode preparation such as with/without CNTs (1 wt. %) addition in the slurry and studied their storage capability. The as-prepared samples powder X-ray diffraction with standard pattern is shown in Figure 1(a). All the diffraction peaks are corresponding to the standard data of CuP2 (ICSD No. 35282, monoclinic phase structure, Space group of P21/c). It represents the successful formation of CuP2 and without any residual precursors. Besides, no peaks related to carbon are observed in the carbon coated CuP2 sample, signifying that carbon remained in its amorphous nature. This result is good agreement of earlier reports. [18, 19] Furthermore, the carbon presence was analyzed by Raman spectra and the result is shown in Figure 1(b). It can be clearly seen that carbon peaks of D and G band at 1367, 1605 cm-1 (ID/IG ratio of 0.98) for carbon treated CuP2 sample. The pronounced carbon peaks were not observed in bare CuP2 sample. The Raman spectra of low frequency regions in 200-500 cm-1 are represented the characteristic vibrational modes of Cu and P in CuP2 samples.

[30, 31]

In P21/c phase system of all atoms in spodumene display C1 site

symmetries.[32] The analysis of Raman spectrum of spodumene in P21/c phase system has not yet reported. Hence, The Cui ions occupy a Ci site symmetry which predicts 6 infrared and 3 Raman


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active modes described by 2A2 + 2B1u + 2B2u + 3B3u. These vibrations are attributed at 495 (Ag), 455 (Ag), 400 (B3u), 345 (B2u), 305 (B1g), 194 (B1u), 170 (B1u), and 240 (B1u + B2u) cm-1, respectively. [33] Moreover, three bands from 300 to 500 cm-1 could be assigned to B1, A1 and E1 modes.

[34]

These band peaks intensities are weaker in carbon modified CuP2 sample, which is

attributed to the confinement of particle size reduction. Thus, the confirmation of Cu and P2 compounds are formed in single compound (CuP2) via high-energy ball-milling process. The absolute intensity of Cu and P signals of XRD and Raman spectra of CuP2/C sample was attenuated compare with pure CuP2, which is probably due to the surface coating of carbon on CuP2 and reduction of particle size. The high-resolution XPS Cu 2p spectra of bare and C-modified CuP2 are shown in Figure 1(c). The binding energies of bare and carbon coated samples are similar. The binding energy peaks at 932.7, 935.2, 944.1, 952.8, 955.2 and 962.9 eV are attributed to the Cu and satellite peaks of Cu with P bonding of CuP2. [35] The high resolution XPS P spectra of both samples are observed 2P peaks corresponds to phosphide (~129.7 eV) and phosphate (~134 eV) type from rep P source (Figure 1(d)). The core XPS spectrum for bare CuP2 sample, P 2p peaks were presented at 134.2, 129.8, and 129.3 eV, respectively. The broad peak at 134.2 eV was assigned to phosphate for bare sample.

[36, 37]

The peaks located at 129.8 and 129.3 eV are corresponding to the 2p1/2 and

2p3/2 of P-P bond in both samples from phosphides.

[37, 38]

Noticeably, the peak intensity of

phosphate was reduced and shifted to lower binding energies (133.6 and 133.3 eV) with increases of 2p1/2 and 2p3/2 phosphide bond intensity. It is due to the formation of P-O-C bond [36] and reduction of oxidation during the high energy ball-milling process in C-modified CuP2 sample. The wide-range core spectra of bare and carbon modified CuP2 are shown in Figure S1 (a). All the elements presence has been confirmed and the results are good agreement of as


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earlier reports.

[19, 37]

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Figure S1 (b) and (c) illustrate the X-ray absorption spectra (XAS) of bare

and carbon modified CuP2 at Cu K-edge and P K-edge, respectively. The Cu K-edge spectra of both samples intense band between 8994-9002 eV are attributed to the existence of Cu species (Figure S1 (b). The bare and CPC samples P K-edge spectra are shown in Figure S1 (c). The band energies at 2145 and 2154 eV are attributed to P-species of CuP2. The carbon coated CuP2 intensity was varied, which is due to the C bonded with O wrapped on CuP2. The intensity of the CuP2 spectra line was slightly higher than that of CuP2/C, while that of its pre-edge shoulder peak was slightly lower, indicating that Cu-species in CuP2 were more electron deficient than that in CuP2/C.

[39, 40]

Thus, the XPS and XAS analyses of CPC sample are noticed that the

carbon chemically strongly bonded with P (C-O-P) in CuP2/C, it will promote the higher electrochemical performance. The bare and carbon coated CuP2 SEM images are shown in Figure 2(a) and Figure 3(a). It was observed irregular shapes with agglomerated particles, owing to the mechanical ball-milling process. The high-resolution transmission electron microscopy (HR-TEM) images of both samples are presented in Figure 2(b) and Figure 3(b). The particles size of CuP2 reduced comparing with bare material. It was observed the diameter of bare sample ~100 nm and carbon coated sample diameter was ~20 nm, respectively. The corresponding lattice image of bare CuP2 can be clearly seen in the lattice at (-112) plan (Figure 2c). In addition, the CuP2/C sample was randomly wrapped by carbon in the whole CuP2 core material shown in Figure 3(c). The SAED image of CuP2/C sample is presented at Figure S2. It indicates the carbon region was amorphous and CuP2 selected region shows diffraction spot to represents the crystalline form of assynthesized CuP2 material. Figure 2(d) shows the SEM selected image for elemental distribution analysis and the corresponding EDX mapping of Cu and P for bare CuP2 sample are shown in


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Figure 2(e) and (f), respectively. The SEM selected image for elemental distribution analysis of CuP2/C sample SEM is illustrated in Figure 3(d) and the corresponding Cu, P and C element distribution E-map images are shown in Figure 3(e-g). The presence of all elements are homogenously distributed on the prepared both products. The atomic ratio was estimated from EDX spectra (Figure S3 (a) and S3 (b)). The calculated atomic ratio of Cu/P was 1:2 for both samples. The carbon wrapped on the core material will helps to enhance the electrical conductivity of bare material, also will serves as a stifle to buffer the volume change of electrode material during electrochemical lithiation/de-lithiation process. Figure S4 (a) and S4 (b) show the SEM images of surface view and cross-section inside view of CPCM electrode and the corresponding TEM results are shown in Figure S4 (c) and S4 (d), respectively. The Li-ion storage of CP, CPC and CPCM electrodes were evaluated by charge/discharge profile in room temperature. The initial three cycles of bare CuP2 electrode lithiation/delithiation capacities were 1454/1202, 1256/1222 and 1228/1208 mAh/g at 100 mA/g in the potential range of 0.01-2 V, as shown in Figure 4(a). The corresponding columbic efficiencies were found to be 83, 97 and 98 %, respectively. Initial lower columbic efficiency might be ascribed to the irreversible initial reaction of CuP2 electrode afterward increase the cycles gradually enhanced the coulombic efficiency. As discussed to the earlier reports, the main drawbacks of P-based anode materials are large volume expansion during lithiation/delithiation process, which results in cracking and significant capacity loss.

[19, 41]

For improving the cyclability of CuP2, we use

carbon-based materials to modify it. The corresponding electrochemical properties of modifiedCuP2 in the first three cycles are shown in Figure 4(b). The specific capacities and current densities of CPC and CPCM electrodes were calculated on the basis of the total weight including carbon (20 wt %) without addition of CNTs. It delivered 1416/867, 868/790 and 796/755 mAh/g


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at the 1st, 2nd and 3rd cyclic with coulombic efficiencies of 61, 91 and 95 % at 100 mA/g, respectively. Furthermore, improvement of capacity and rate capability could be achieved by introducing 1 wt. % carbon nanotubes (CNTs) as conductive additives during slurry making process. The charge/discharge profiles of CNTs-modified CuP2/C (CPCM) composites in the first three cycles are displayed in Figure 4(c) and the delivered specific capacities of 2145/1508, 1492/1441 and 1431/1401 mAh/g at 100 mA/ g, respectively. The corresponding columbic efficiencies are 70, 97 and 98 % for the initial three cycles of CPCM cell. Lower columbic efficiency of CPCM electrode in the first cycle might be due to the formation of SEI layer during the initial lithiation process and heterogeneous interfacial storage mechanism. [42, 43] The carbon modified electrodes initial columbic efficiency was decreased compared with bare electrode and the results are consent with previously reports of P-based carbon modified electrodes for LIBs/SIBs anode.[19, 44] The initial cyclic columbic efficiency is based on several factors and some of the following factors are (i) particle size of surface area between the electrode and electrolyte, (ii) formation of SEI and their thickness, (iii) choosing of electrolyte and additive and so on. Kim et al. [19] reported that by using carbon as a conductive additive, reversible capacity of CuP2/C composite would unavoidably decrease compared to that of pure CuP2. The decreases of columbic efficiency in carbon modified P-based metal electrodes, possibly due to the reduced particle size after ball milling process. The increase of specific surface area for milled-active material result in larger irreversible capacity loss derived not only from solid electrolyte interface formation onto the surface of electrode, but also from creating structural defects which irreversibly trap lithium ions during charge and discharge processes. The differential capacity plots of CP, CPC and CPCM electrodes are shown in Figure S5 (ac). Surprisingly, the voltage plateau profile of bare CuP2 electrode was altered after carbon


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modification and the discharge curves changes to a steady sloping voltage profile. The main cathodic and anodic peaks of three electrodes are all located at the similar potentials of pure CuP2. However, the discharge differential plot at below ~0.75 V of C-modified electrodes can be clearly seen in peak with increased capacity than bare CuP2. It notices the sufficient conductivity to increase the initial conversion to decompose of CuP2 to form of alloys in initial lithiation process. However, the insufficient conductivity of CP electrode within SEI could not fully react within CuP2 material in initial lithiation. The discharge capacity differential plots of C-modified electrodes are manifest as pseudocapacitive nature.

[45]

The increasing capacity of C-modified

electrodes are generally ascribed to the kinetically activation of smaller size of active materials with boosting of both ionic/electronic conductive via modification of carbon matrices and SEI formation of the electrodes. Thus, the C-modified electrodes potential plateaus were smaller than slower diffusion kinetics originating from the larger size of active material with lower conductivity of CuP2 electrode. During the de-lithiation process, the lower intensity peak at ~0.9 V and higher intensity peaks at ~1.1 and ~1.3 V can be observed in CP electrode. It can be described in insufficient of reaction with rapid decomposition of Li ions to generate poor cyclic and rate performance of bare CP electrode. Besides, the C-modified electrodes the peak intensity ~0.9 V were increased and other potential peaks (~1.1 and ~1.3 V) intensities were reduced comparably bare CuP2. It represents the adequate conductivity to fast up the kinetics and slower decompose of Li ion in carbon modified CuP2 to promote the higher rate and remarkable cyclic stability. Subsequently, the capacity contribution behaviour of the electrodes was described in forthcoming section. According to the early report, it can be described due to the reversible formation and decomposition of polymeric gel like layer during discharge/charge cycles, which could provide


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interfacial storage for excess Li-ion via the pseudocapacitive nature of behaviour. [24]

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[13, 45]

Li et al.

reported ramp type discharge profile curve based on diffusion controlled process and surface

controlled process was linear line type. Overall these reasons, the small voltage plateaus of cycles, higher irreversible capacity and lower columbic efficiency of initial cycle were observed in carbon modified CuP2 electrodes. The information about controlling of SEI thickness and their nature of swot still are quit challenging task of novel electrode materials design for batteries applications. The cyclic stability of the CP, CPC and CPCM electrodes are shown in Figure 4(d). At the end of 100 cycles, the discharge capacity of CP, CPC and CPCM were found to be 257, 475 and 933 mAh/g at 100 mA/g. The capacity of bare CuP2 was gradually decreased with increases cycles, which represents the P-based electrode common issue of volume expansion. However, the carbon coated CuP2 electrode exhibits remarkable more stability and CPCM electrode reveals superior enhanced capacity with good stability. The average columbic efficiency of CP and CPCM are 99.1 and 99.4%, respectively. At higher current densities CPCM electrode lithiation capacities were 814 and 579 mAh/g at 1000 mA/g and 2000 mA/g over the 50 cycles (Figure S6(a)). Figure 4(e) illustrates the rate capability of CP, CPC and CPCM samples with gradually increasing current densities. The capacity retention was estimated by low current rate at high capacity. All the electrodes exhibited ~99 % columbic efficiency even at higher C rates (Figure S6 (b)). For CP electrode, the average discharge capacities were measured to be 1090, 628, 482, 415 and 380 mAh/g at 100, 200, 500, 1500 and 2500 mA/g, respectively. Conversely, the CPC electrode exhibited average discharge capacities of 890, 736, 688, 566 and 483 mAh/g at 100, 200, 500, 1500 and 2500 mA/g for each 10 cycles, respectively. The CPCM sample demonstrates


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remarkable capacity retention than bare ones and the average discharge capacities of 1455, 1154, 965, 776 and 616 mAh/g at 100, 200, 500, 1500 and 2500 mA/g for each 10 cycles (Figure 4(e)). Moreover, as-synthesized CPCM samples deliver average capacity can readily return to 1094 mAh/g when decreases the current density to 100 mA/g over the 20 cycles. Figure S6(c) depicts the comparison of rate capability of as-synthesized CPCM sample and P-rich anodes reported by previous group. To understand the lithiation/delithiation process, differential capacity plots from charge/discharge profile and cyclic voltammetry (CV) were performed. The initial two cycle CV curves of CP and CPCM electrodes are shown in Figure S7 (a) and S7 (b) at a scan rate of 0.05 mV/s in the potential range of 2-0.001 V. During the initial cathodic scan, there was a broad reduction peak in the potential range of 1.5-0.5 V, which corresponds to the common irreversible decomposition of electrolyte and the formation of solid electrolyte interface (SEI) on fresh electrode surface. [44, 46] When swept negative to 0.5 V, the cathodic current density further rose continuously, symptomatic of electrochemical conversion and lithiation of CuP2 both electrodes in this low potential regime. In the consequent anodic sweep, two represented anodic waves centered at 0.67 and 0.88 V were observed as a result of multi-step de-lithiation process. The CV results are good agreement with differential capacity plots. The total stored charge can be separated into two components: (i) the diffusion contribution from intercalation/conversion/alloying process from the charge transfers with surface/subsurface atoms and (ii) the capacitive contribution from the electric double layer effect. The capacitive effect of CP and CPCM samples were investigated by measuring of current (i) changed with the scan rate (V) at a fixed potential window (0.001-2V), as shown in Figure 5(a) and 5(d). The variations of the cathodic/anodic currents were observed to study the capacity nature. It is noted


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that the peak current increased with increasing the scan rates. Hence, it was assumed that the scan rate and peak current following a power-law equation: [6, 24] i= aVb Whereas, i is the measured peak current, V is the voltage sweep rate, a and b are adjustable parameters. The b value was determined from the slope of the plot of log i vs log V. For b= 0.5, the current is proportional to the square root of the scan rate. It indicates a current response in diffusion-controlled reaction namely a faradaic intercalation process. While for b=1.0, is capacitive response because the capacitive current is proportional to the scanning rate v. [47, 48] Figure 5(b) shows the CP electrode cathodic scans at 0.7 and 0.21 V of current variation versus square root of scan rate plot. The b value of cathodic scan at 0.7 V was found to be 0.30, which represents the diffusion controlled reaction. Conversely, at peak potential of 0.21 V in cathodic scan, the b value was 1.34, indicating that the capacitive response reaction of this region, it feasibly originate from decomposes /conversion of CuP2 to formation of metallic alloys and SEI.[19] Figure 5 (c) illustrates the anodic scans at 1.20 and 1.32 V of current variation versus square root of scan rate plot. The b values were 0.52, 0.66 at both of the anodic scan potential, it indicates the whole charge process of bare CuP2 electrode based on diffusion controlled reaction. The CPCM electrodes b values were 0.88 and 0.80 for cathodic potential at 0.7 and 0.21 V, respectively (Figure 5e). The b value of anodic potential at 1.20 and 1.32 V were observed 0.83 and 0.82, respectively (Figure 5d). It can be clearly observed that the capacity based on surface controlled reaction in terms of pseudocapacitance behavior of carbon modified electrode. The nature of capacity contribution could be described by the following equations via the relationship between current and sweep rate at a fixed potential: [21, 45] i (V)=k1v+k2v0.5


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i (V)/v0.5=k1 v0.5+k2 In this two equations, both k1 and k2 are constant. k1v and k2v0.5 are the current contribution terms resulted from surface capacitive effects and diffusion-controlled intercalation/extraction processes, respectively.

[4]

We would like to quantitatively distinguish for CuP2/C composite

anode, which phenomena dominates the whole charge/discharge processes. The corresponding plots of the plot i (V)/v0.5 vs. v0.5 and the slope and y-axis intercept of the plot correspond to k1 and k2 are shown in Figure S8a and S8b. We can distinguishably see in Figure S8(a) of CP electrode potential point ~0.21V discharge process based on capacitive contribution, moreover, discharge at ~0.7V and de-lithiation capacity originate from diffusion controlled reaction process (Figure S8c). The CPCM electrode all the discharge/charge potential points are exhibited pseudocapacitance nature of the capacity storage (Figure S8b and S8d). The contribution proportion of surface capacitive to the total capacity could be determined by integrating the area of CV from the k1v vs. potential plot. Figure S9a and S9b demonstrate the diffusion controlled and surface capacitive controlled process (greenish shadow) at a scan rate of 4 mV/s CV curves of CP and CPCM electrodes. The CP electrode diffusion contribution process was observably higher and CPCM electrode the diffusion contribution was reduced and increased the capacitance contribution and behaviour of pseudocapacitance nature. Commonly, the diffusion controlled process conceivably difficulties to achieve high rate capacity for batteries applications. For the reasons, the utilization of higher surface capacitance of CPCM electrode exhibited remarkable rate capability due to the fast capacitive reactions. The electrochemical kinetics of fresh and rate performed coin cells were studied by Electrochemical Impedance Spectroscopy (EIS). Figure 6(a-c) shows the Nyquist impedance plots of the prepared fresh and rate-performed electrodes. In high frequency region, an intercept


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at the Zre axis indicates Ohmic resistance (Rs) of the electrolyte. A semicircle in the middle frequency is owing to the charge transfer resistance (Rct) applied to the electrolyte/active particles interface. The slope at the low frequency was endorsed to the Li-ion diffusion in the prepared electrodes, which indicates the Warburg impedance (Zw). The charge transfer resistance of CP, CPC and CPCM fresh and after rate electrodes are shown in Table S1. The Li-ion diffusion coefficient can be calculated according to the following equation (1): [49] D=R2 T2 /2A2 n4 F4 C2σ2

(1)

R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule during oxidization (n=6), F is Faraday constant, C is lithium concentration in electrode (0.001 mol. cm-3), σ is the Warburg factor which is relative with Zre, shown as equation (2): Zre = Rs+ Rct + σω−1/2

(2)

Where, ω is the angular frequency in the low frequency region. An exchanged current density (i0) is one of the important factors to affect the kinetic process, which can be used to measure the catalytic activity of electrodes. The following equation (3) describes the calculation of Exchange current density (i0): i0 = RT/ nFRct

(3)

Figure 6 (d) and (e) show the linear fitting relationship plot between Zre and the reciprocal root square of the lower angular frequencies for the fresh and after rate performed cells. The prepared fresh and after different current densities cycled electrode cells were estimated the diffusivity (DLi+) and Exchange current density (i0), as shown in Table S1. The CPCM cell displayed obvious catalytic effect and diffusivity than that of rest electrode cells. It represents the enhanced electrochemical performance of the electrodes.


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To describe the reaction mechanism of CuP2, we disassembled the coin cells at various charge/discharge stages of initial two cycles and carry out ex-situ XRD measurements (Figure S 10 (a) and S10(b)). From the result of XRD pattern at the initial two full lithiation state (0.01V), it can be clearly seen that there exist pronounced peaks locating at ca. 23.3º (002) and 42.4 º (210) for Li3P phase (ICSD # 642223) and 2θ=36.2º (2-12) for Cu3P phase (ICSD # 15056), respectively. However, the former peaks disappeared from the XRD pattern of the both fully delithiation state (2 V). Furthermore, to verify the structural stability of CP, CPC and CPCM cells were performed after 100 cycles of lithiation/delithiation process at 100 mA/g and the results as shown in Figure S10(c). All cycled-electrodes exhibit amorphous-like diffraction patterns, which indicated that the amorphous phosphorus with mixing of Cu3P phase was formed during lithiation process. Thus, it can be concluded that CuP2 electrode exhibits conversion and alloy-typed Li-ion storage mechanism. [18, 19, 50] Overall, the aforementioned CPCM electrode cell exhibited superior electrochemical performance than that of bare CuP2 electrode. The possible explanation may be owing to the following reasons: (i) the strong chemical bond formation of P-O-C in active CuP2 materials carboneous materials modification could act as a conductor matrix to maintain the good electrical conduction pathway and a cushion to accommodate the severe volume change during cycling;[50] (ii) C and CNTs co-modification could tolerate the Li+ ion and e- mobility; (iii) the carboneous modification to altered the capacity contribution of diffisuion controlled to surface controlled capacitive process and improve the catalytic effect. 4. CONCLUSION In summary, CuP2 and carbon-wrapped CuP2 were successfully prepared by mechanical ballmilling technique without calcination process. X-ray diffraction (XRD), X-ray photoelectron


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spectra (XPS) and X-ray absorption spectra (XAS) epitomized the without any phase impurity, and presences of Cu, P and C species nature. The carbon wrapped of CuP2 was not only improved the conductivity of the material, but also buffered the volume change of active materials. Besides, an addition of CNTs helps to additional boost of conductivity improvement to enhance the cyclic stability as well as rate capability and reduce the dislocated cracked particles via connection of CNTs to accelerating the Li-ion diffusivity. Interestingly, the capacity contribution was increased from diffisuion controlled behaviour to surface controlled capacitive nature via surface modification of carboneous materials. The observed CPCM electrode exhibits pseudocapacitance behaviour of lithiation/delithiation reaction. The CPCM electrode deficts outstanding electrochemcial performance than bare and C-modifed CuP2. It delivered initial capacity of 2145/1508 mAh/g and after 100th cycles it was maintained about 933 mAh/g at 100 mA/g. The CNTs added carbon wrapped CuP2 electrode showed excellent rate capability and higher current cyclic stability. The kinetic parameters of diffusivity and catalytic effects were enhanced by modification of carboneous materials, which represent the simple superior route of accelerate the Li-ion oxidation/reduction reaction. Thus, the work provides a feasible capacity behaviour analysis and simple approach to enhance the capacity, which would benefit the development of new advanced electrode materials for next generation high-energy batteries. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.


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Additional information of wide-range XPS spectra, XAS spectra of prepared samples, HRTEM with SAED image and EDX spectra of CuP2 and CuP2/C, SEM/TEM image of CPCM electrode, electrochemical performance of long-term cyclic stability at high current density, estimation of coulombic efficiency, capacity retention of different rates, comparison of rate capability with earlier reports, differential capacity plots, CV curves, capacity contribution analysis, ex-situ XRD results and table of the electrode kinetic parameters of prepared samples from EIS data are shown in Supporting Information (PDF) AUTHOR INFORMATION Corresponding Author *Wei-Ren Liu ([email protected]; Tel:+ 886-3-2654140) ORCID Wei-Ren Liu: 0000-0003-0468-895X Rasu Muruganantham: 0000-0003-3946-2774 Author Contributions R.M. wrote the manuscript, performed the experiments, characterizations, scheme and Figures. P.C.C. contributed to experiment. W.R.L supervised the research and contributed to editing of manuscript. All authors discussed and commented on the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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The authors gratefully acknowledge for the financial support to carry out this work by Ministry of Science and Technology (MOST 104-2628-E-033-002-MY3, MOST103-2739-M-213-001MY3 and MOST 105-2622-E-033-003-CC2).

Figure Captions Scheme 1. Schematic illustration of synthetic process of CuP2 composites and fabrication of Liion storage half cell. Figure. 1 (a) XRD patterns (inset crytal phase structure), (b) Raman spectra, (c) XPS Cu2P core spectra and (d) XPS P2p-core spectra of as-synthesized bare and carbon coated CuP2 materials. Figure 2. Surface morphology of bare CuP2:(a) SEM image; (b) TEM image; (c) High reslution TEM image; (d) Selected surface of EDX mapping image of (e) Cu and (f) P, respectively. Figure 3. Surface morphology of carbon coated CuP2:(a) SEM image; (b) TEM image; (c) High reslution TEM image;(d) Selected surface of EDX mapping image of (e) Cu, (f) P and (g) Carbon, respectively. Figure 4. Li-ion storage performance: (a) Charge/discharge tests of CuP2 (CP); (b) charge/discharge tests of CuP2/C (CPC); (c) Charge/discharge tests of CNTs incorborated


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CuP2/C (CPCM); (d) Cyclic stability of CP, CPC and CPCM, and (e) Rate capability of CP, CPC and CPCM. Figure 5. Capacity behavior analyses: (a) CV curves at different scan rates of CP electrode, (b, c) the corresponding linear relationships between logarithum currents logi and logrithum scan rate log v of cathodic and anodic process of CP, (d) CV curves at different scan rates of CPCM electrode (e, f) the corresponding cathodic and anodic scans of linear relationships between the Log i vs log scan rate (the cathodic scan at 0.7, 0.21 V and anodic scan at 1.2 and 1.32 V for CP and CPCM), respectively. Figure 6. Kinetic behavior analysis via electrochemical impedance spectra: Nyquist plots of fresh and cycled electrodes: (a) CuP2, (b) CuP2/C, (c) CNTs incorborated CuP2/C, and the correspond inset enlarge of Rct parts. Nyquist plots liner fitting of the real parts (Z′) of the impedance versus ω−1/2 (d) fresh and (e) after rate performed cells of CP, CPC

and

CPCM

electrodes, respectively.


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Scheme. 1


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Figure. 1


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Figure. 2


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Figure. 3


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Figure.4


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Figure. 5


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Figure. 6 ACS Paragon Plus Environment

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GRAPHICAL ABSTRACT


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REFERENCES


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Copper-diphosphide Composites: A Key Factor ... - ACS Publications

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