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Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4652−4663

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Enhanced Electrochemical Performances of Cu/CuxO‑CompositeDecorated LiFePO4 through a Facile Magnetron Sputtering Wenyu Yang,†,‡ Yue Chen,†,‡ Xihong Peng,†,‡ Yingbin Lin,†,‡ Jiaxin Li,†,§ Zhensheng Hong,†,§ Guigui Xu,*,†,∥ and Zhigao Huang*,†,§

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College of Physics and Energy, Fujian Normal University, Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, Fuzhou 350117, China ‡ Fujian Provincial Engineering Technical Research Centre of Solar-Energy Conversion and Stored Energy, Fuzhou 350117, China § Fujian Provincial Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Xiamen 361005, China ∥ Concord University College, Fujian Normal University, Fuzhou 350117, China S Supporting Information *

ABSTRACT: Through a facile magnetron sputtering technique, Cu/ CuxO composite nanoparticles were dispersed uniformly on the surface of the LiFePO4 electrode. Confirmed by X-ray photoelectron spectra, Raman spectroscopy, and high-resolution transmission electron microscopy, the Cu/CuxO composite particle possesses an eccentric core−shell structure with metallic copper as the core, whose partial superficies are surrounded by an oxidation composite consisting of cuprous oxide and cupric oxide. The deposition time of the Cu/CuxO composite on the pole piece is varied in the range from 20 to 120 s. The best results are attained for the sample prepared at sputtering time of 60 s. The electrochemical measured results indicate that LiFePO4 with appropriate composite decoration displays excellent rate performances and cycling stability under high current density. The enhanced performances are considered to be induced by the existence of metallic copper on the surface of the electrode, which contributes to the strengthening ion diffusivity and conductivity with moderate copper additive. The LiFePO4 with appropriate composite modification has a lower surface work function, which can verify this point well. Furthermore, it is observed that the LiFePO4 electrode modified by a moderate composite remains an intact lattice structure after many cycles at high C-rate, implying that homogeneous composite surface modification can effectively suppress the degeneration of the material crystal structure. This is attributed to the oxidation composite wrapping around the metallic copper surface which acts as a significant role in buffering the undesirable reaction between active material and electrolyte. Therefore, the surface modification of the electrode based on the facile magnetron sputtering technique has a great advantage for simplifying the preparation process while evidently enhancing the electrochemical performances of materials. KEYWORDS: lithium iron phosphate, surface modification, Cu/CuxO composite, electrochemical performances, work function, magnetron sputtering

1. INTRODUCTION Today, global warming, energy production, and energy storage are increasingly attracting people’s attention. To cope with the energy demand of the continuous development of the economy, many advanced renewable energy sources have already been exploited and utilized. These clean renewable energy resources include wind energy, solar energy, tidal energy, and so on. However, they are all inherently intermittent and generally dispersed. For the sake of enabling renewable energy to achieve full utilization, we urgently develop superior energy-storage systems.1,2 Fortunately, the lithium-ion battery, which has been used as electric energy storage of portable electronic devices during the past decade, now stands as a promising candidate for high-power and long© 2019 American Chemical Society

life battery application owing to the revolutionary extension of cycle life and substantial improvement of capacitance, typically for the power systems of transport vehicles (e.g., battery electric vehicle and hybrid electric vehicle) and the clean energy storage systems.3,4 To date, research into batteries has climbed sharply in popularity. There are a considerable amount of studies focusing on positive electrode material with regard to specific capacities and structural stability. Among the three commonly used cathode materials, olivine lithium iron phosphate (LFP) is more stable in thermodyReceived: January 1, 2019 Accepted: June 12, 2019 Published: June 12, 2019 4652

DOI: 10.1021/acsaem.9b00004 ACS Appl. Energy Mater. 2019, 2, 4652−4663

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ACS Applied Energy Materials

migration pathway for active material.24 As a result, the electrode polarization of LiFePO4 is alleviated, and specific capacity is promoted. It is not difficult to find that excellent electrochemical performances of the cell can be obtained through blending copper together with active powder. However, there still remain many issues for metallic copper involvement, such as difficulty in exactly controlling copper content and uncontrolled particle clusters. These inevitably lead to a more rigorous and complex preparation process. More importantly, metallic copper can be oxidized accompanied with the decomposition of electrolyte when charge voltage exceeds 4.0 V that is a requirement of lithium extraction from LiFePO4.26 Hence, designing an eccentric core−shell structure nanoparticle to coat the cathode surface will be attractive. Here the metallic copper is used as the core, and the oxidation composite formed on the partial surface of the copper core is used as the shell. The eccentric core−shell structure of the composite is easily fabricated via utilizing natural oxidation of metallic Cu, which is a highlight of the work. This core−shell surface modified layer could not only provide an effective surface conductive net but also play the protection role. At high cutoff voltage, the special oxide shell acts as a significant role in effectively preventing the irreversible reaction between the electrode surface, metallic copper, and electrolyte. Recently, the magnetron sputtering technique has been widely used in the surface modification engineering of electrodes originating from the benefit of its low cost and facile thickness control on the nanoscale.27,28 Accordingly, with a view to strengthen the electric contact between active materials and provide an effectively protective layer, composite particles with eccentric core−shell structure were successfully dispersed on the surface of the electrode fabricated with LiFePO4 via a facile magnetron sputtering. Electrochemical performances of the composite-decorated LiFePO4 electrode were investigated by a series of electrochemical characterization techniques including galvanostatic charge/discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). To seek the optimal modification parameters, the effect of deposition time of composite material (Cu/CuxO) on the performance of electrodes was systematically studied. Besides, the structural and surface work function changes of electrodes that experienced many cycles were analyzed in detail. At last, the role the mechanism of the Cu/ CuxO composite played in improving LiFePO4 performances was discussed.

namics aspects relative to layer type and spinel electrode materials. Moreover, LFP has many advantages in cost, environmental compatibility, and source of raw materials.5,6 For this material, the major issues are low intrinsic conductivity and sluggish diffusion rate of the Li ion.7,8 The solution strategies include carbon coating,9,10 metal oxide coating,11,12 metal fluoride coating,13,14 metal doping,15,16 as well as nanocrystallization of size.17,18 The multitudinous ameliorated methods of the materials are impressive, and the main goals of modification are to increase the material-specific capacity and to improve the cycle life. In this article, we hope to develop a novel electrode surface modification approach, taking advantage of this opportunity to further enrich the decoration methods. In our previous research, semiconductor silicon nanoparticles have been dispersed evenly on the composite electrode surface of LFP 18650 cylindrical batteries by a novel ultrasonic spraying technique.19 The batteries with silicon decoration demonstrate excellent rate performances and cycling stability, which are attributed to the introduction of a semiconductor silicon forming an effective isolating protective layer between the electrode and the electrolyte. Nanoscale silicon shields the active material within the composite electrode from hydrofluoric acid (HF) attack by the absorption of H+ in the electrolyte. Meanwhile, Zhou et al. have utilized an amorphous Li 3 PO 4 (LPO) possessing good ionic conductivity as the surface modification layer of a lithium cobalt oxide (LCO) composite electrode. 20 With an appropriate LPO coating thickness, the decorated LCO shows significant improvement of electrochemical performances, especially at elevated temperature. It is found that LPO provides an efficient pathway to enhance Li-ion transport. Obviously, the direct surface modification for the composite electrode is feasible and promising in ameliorating battery electrochemical activity, especially for the lithium-ion intercalation and deintercalation.19−21 On one hand, it can provide the cell with an artificial buffer layer to effectively isolate the side reaction between the electrode and the electrolyte without sacrificing battery-specific energy. On the other hand, selecting ion-conductive materials as a physicochemical layer of the electrode surface would be beneficial for enhancing the lithium-ion diffusion coefficient, resulting in significant improvement for battery rate performances. Up to now, most of the surface modification materials play only an isolation layer role in batteries, while they do not generally improve the surface electric conductivity of cathode. Metallic copper, as we know, can supply good electronic conductivity. The studies about introducing metallic copper to ameliorate the electrochemical performance of LiFePO4 have been reported.22−25 For example, Croce et al. have successfully synthesized LiFePO4/metal (e.g., copper) composite material through involving metal nanoparticles with a concentration of 1 wt % during the sol−gel preparation process.22 The metal addition considerably enhances Li-ion transfer kinetics. In addition, Feng et al. have prepared the optimal performance of the LiFePO4/Cu composite containing 6.18% nanoscale copper by a wet chemical process.23 The composite can deliver more capacity than pure one, which is ascribed to the decrease of charge transfer resistance resulting from the enhancement of electric conductivity between the particles via the nanoscale copper introduction. Employing the microwaveassisted method, Hsieh et al. have fabricated the Cu network onto LiFePO4 powders, which can provide an efficient electric

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. Through traditional solid reaction, iron oxalate dehydrate (FeC2O4·2H2O, 99% purity), lithium acetate (LiC2H3O2, 99% purity), and ammonium dihydrogen phosphate (NH4H2PO4, 99% purity) were selected as raw material and mixed in an agate jar containing enough ethyl alcohol (about 30 mL) with stoichiometric ratio of n(Fe):n(Li):n(P) = 1:1:1. The weight of FeC2O4·2H2O, LiC2H3O2, and NH4H2PO4 was 4.4974, 1.6500, and 2.8758 g, respectively. Moreover, the 5 wt % sucrose was added as reducing agent and carbon source. Chemicals are analytical grade and purchased from Tianjin Yongda. The mixture was rotated with 220 rad/min, continuing 24 h in the ball mill. The obtained LiFePO4 precursor was dried at 80 °C in vacuum. Then the precursor was placed in the tube furnace and underwent heat treatments at 350 °C for 6 h and 700 °C for 12 h under argon atmosphere, respectively. The argon gas flow rate of 20 mL/min was chosen. 4653


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Figure 1. (a) XRD pattern, (b) SEM image, and (c) the thermogravimetric analysis curve of LiFePO4 powder synthesized via the solid-state method with sucrose as the carbon source and the (d) corresponding HR-TEM image showing the thickness of amorphous carbon on the particle surface. The N-methy-1−2-pyrrolidone-based battery’s slurry consisting of 80 wt % active material (LiFePO4 powder), 10 wt % super-P, and 10 wt % polyvinylidene fluoride (PVDF) was ground evenly using an agate mortar, followed by pasting the formed slurry on an aluminum foil and drying in vacuum under 110 °C for 12 h. For fabrication of the cell, the cathode electrode was punched into circular disks with 12.5 mm diameter. The mass loading of the electrode is about 2.3− 2.5 mg/cm2. The coin cells (R2025) were assembled in an argon-filled glovebox. Then, lithium metal was used as counter electrode, and the Celgard 2300 microporous polyethylene membrane served as the separator. The solution of 1 M LiPF6 in a mixture of ethyl carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) (1:1:1 in vol. ratio) was chosen as a battery electrolyte. 2.2. Magnetron Sputtering Deposition. Prior to sputtering using a commercial copper target, the prepared LiFePO4 electrode was placed in the sputtering sample cavity, and the copper target surface was treated with dilute hydrochloric acid to remove the oxidation layer. Later, the base pressure of the sample cavity was evacuated to 7.0 × 10−4 Pa, and then the pure argon gas was transmitted into the sputtering system. During the sputtering process, the work power was performed at 80 W, and the work pressure was fixed at 0.6 Pa. Through controlling sputtering time, the diverse content of the Cu/CuxO composite on the electrode surface can be achieved. The modified electrode would be placed in a constant temperature and humidity box for 48 h after sputtering, utilizing natural oxidation of metallic Cu in order to acquire the eccentric core−shell structure of the composite. To identify simplistically, the electrodes sputtered with 0, 20, 60, and 120 s were marked as samples A, B, C, and D, respectively. 2.3. Characterization. The LiFePO4 powder and prepared electrodes were characterized by X-ray diffraction (XRD, Rigaku MiniFlex II) with Cu Kα radiation (λ = 0.15405 nm). The

determination of practical carbon amount in the powder sample was achieved by a thermogravimetric analyzer (Netzsch STA409PC TA Instruments) at a scanning rate of 5 °C min−1 with air flow of 40 mL min−1 from room temperature to 800 °C. The morphologies of the electrode surface and powder were detected by a scanning electron microscope (SEM, Hitachi SU8010). The element identification and distribution were realized by energy-dispersive Xray spectroscopy (EDS) (SEM attachment, Hitachi SU8010). The high-resolution TEM images about crystallographic structure were obtained through an FEI Tecnai G2 F20 microscope. The Raman spectra were acquired via HORIBA Jobin Yvon Evolution with laser excitation at 532 nm, and the laser intensity was controlled below 5 mW. The chemical valence states of the Cu/CuxO composite were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi) with Al Kα (1486.6 eV) excitation source. Kelvin probe force microscopy (KPFM) was conducted by means of a commercial atomic force microscope (Bruker Dimension Icon) in pure Ar flow glovebox. The surface height was recorded first using tipping mode with a Pt/Ir-coated cantilever (resonance frequency of 75 kHz, force constant of 3 N/m) before the acquisition of surface potential for material. Afterward, the tip was lifted from the sample surface with a constant distance of 100 nm to trace surface topography. Through applying an offset direct current voltage to equilibrate contact potential between the tip and sample surface, the surface potential of samples can be obtained during the scanning process. The work function of samples can be calculated based on the following eq 129−31

Vsp = 4654

ϕtip − ϕsample e

(1) DOI: 10.1021/acsaem.9b00004 ACS Appl. Energy Mater. 2019, 2, 4652−4663

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Figure 2. Surface morphology images of (a) pure LiFePO4 electrode, (b) sample B, (c) sample C, and (d) sample D, respectively. where Vsp denotes surface potential; φtip and φsample represent work functions of the tip and sample, respectively; and e is elementary charge. The electrochemical performance evaluation was conducted using a cell test instrument (LAND electronic Co.) in a voltage range of 2.5− 4.3 V in the form of galvanostatic charge/discharge at room temperature (25 °C). The cyclic voltammetry (CV) test was measured using an electrochemical workstation (CHI660C) at sweeping rate of 0.1 mV/s. Then electrochemical impedance spectroscopy (EIS) was recorded by an electrochemical workstation (CHI660C). Its frequency ranges from 10 mHz to 100 kHz with an amplitude of 5 mV. In addition, the disassembly of cells was carried out in a glovebox filled with argon gas at the discharged state. Electrodes dismantled were washed several times with pure dimethyl carbonate (DMC) and then dried under vacuum.

thickness of about 3 nm. It is suggested that the percentage carbon of 3.18 wt % is insufficient to facilitate a completely uniform particle surface coating. Obviously, an incomplete passivation layer and disconnected carbon net would not be beneficial for the enhancement of LiFePO4 electrochemical characteristics.34,35 Figure S1 presents the diffraction diagram of sample C. From the figure, it is observed that there is no appearance of diffraction peak belonging to copper oxide or copper. It is attributed to the reason that the detection of a little composite substance is beyond the range of sensitivity of an X-ray diffraction instrument. To verify the existence of the Cu/CuxO composite, the morphologies of the electrode surface were detected by a scanning electron microscope. Figures 2(a)−(d) show the surface morphology images of samples A, B, C, and D, respectively. Compared to the images of the pure electrode, it can be observed that nanoparticles are homogeneously dispersed on the surface of active materials for samples B and C. As deposition time increases, a phenomenon of nanoparticle accumulation appears on the surface of sample D. Moreover, Figure S2 shows the energy-dispersive X-ray spectroscopy for the Cu/CuxO-composite-decorated LiFePO4 electrode and the distribution mapping of the copper element on the surface of the LiFePO4 electrode. From the figure, the uniform distribution of the copper element on the electrode surface is found, which implies that the homogeneous modification of the Cu/CuxO composite is realized. In order to further confirm the component of the Cu/CuxO composite, its chemical valence state was affirmed by X-ray photoelectron spectroscopy. Figure 3(a) presents XPS spectra of the Cu/CuxO composite, which consisted of the Cu 2p3/2 peaks accompanied with a series of satellites and Cu 2p1/2 peaks. For the sake of clear identification of Cu 2p3/2 peaks, satellites, and Cu 2p1/2 peaks, the red vertical lines are employed. The fitted results reveal that the peaks at 932.7 and 934.8 eV correspond successively to metallic copper and Cu+ followed by two satellites on the higher binding energy side at 942.8 and 944.3 eV belonging to Cu2+.36−38 Three satellites peaks of Cu 2p1/2 are, respectively, located at 954.3, 956.1, and 962.8 eV, which indicates the presence of CuO at the surface of the metallic particle.39 Through the contrast of the binding

3. RESULTS AND DISCUSSIONS Figure 1(a) shows the XRD diffraction pattern of LiFePO4 powder synthesized by traditional solid reaction. No impurity reflection is found except for characteristic reflection signifying olivine-type phase structure with a space group of Pmnb (JCPDS NO.40-1499). In addition, well-defined diffraction peaks for LiFePO4 indicate the benign crystallinity of powder. Figure 1(b) shows the SEM image of LiFePO4 powder. From the figure, it is found that the grain size for LiFePO4 is about or less than 1 μm. The carbon content in the LiFePO4 composites was quantified through a thermogravimetric analyzer, as shown in Figure 1(c). The observed weight gain at temperature ranges between 300 and 500 °C is due to the oxidation of Fe2+ to Fe3+. When the pure LiFePO4 achieves the complete oxidation of Fe2+ to Fe3+ in air, the theoretical value of weight increase is about 4.8 wt %.32,33 Hence, the weight loss difference calculated by comparing the total weight gain of asprepared LiFePO4 composites and the pure one is 3.18 wt %. To observe carbon formation of the as-synthesized sample surface, HR-TEM investigation was conducted, and images were demonstrated in Figure 1(d). The dark gray region corresponds to an olivine-type phase which has crystalline fringe spacing of 0.43 nm indexed to the (011) planes of LiFePO4. The light gray region indicates amorphous carbon on the surface, which forms an uneven coating layer with a 4655


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speculated that an eccentric spherical core−shell structure Cu/CuxO is formed on the surface of the electrode. To pursue definite proof, HR-TEM investigation was carried out, and the image is shown in Figure 3(c). The Cu/CuxO composite nanoparticle is mainly composed by two crystallites. The values of the interplanar spacing d in the crystallite have been measured. The (200) plane of CuO with d = 0.212 nm and the (111) plane of Cu d = 0.208 nm are defined. It can be observed intuitively that a crystalline copper core is partially surrounded by a mixture of cupric oxide and amorphous substance, which forms an eccentric core−shell structure. The amorphous film is probably the cupric oxide according to the lattice fringe that can be indexed in the shell or cuprous oxide. The formation of the surface oxide layer on the metallic copper nanoparticle originates from the fact that the exposed copper particle surface would be subject to aging and become even more oxidized over time in the ambient environment.46,47 In addition, the generation of the copper oxide mixed phase may also be attributed to violent collision between copper particles under low vacuum, which has been reported in a previous study.48 The amorphous oxide observed here appears in others’ work as well.49,50 To investigate the effect of the Cu/CuxO composite amount on the electrochemical performance of LiFePO4, the contents of the copper element for samples A, B, C, and D were affirmed by means of EDS and are listed in Table 1. Figure Table 1. Copper Element Content on the Electrode Surface As a Function of Sputtering Time element content

A

B

C

D

at %

0

0.34

1.17

3.61

4(a) shows the discharge capacity for samples A, B, C, and D at 0.5 C, 1 C, 3 C, 5 C, and 10 C (1 C = 170 mAg−1) in the voltage range from 2.5 to 4.3 V at 25 °C. From the figure, it is found that, with increasing current density, the sample C always demonstrates more specific discharge capacity than the pure one, especially for 5 C and 10 C, which means that the moderate Cu/CuxO composite modification is beneficial for the enhancement of LiFePO4 rate performance, resulting from the improvement of electric contact between active materials due to the existence of metallic copper in the composite. Nevertheless, it is obvious that for sample D superabundant Cu/CuxO composite decoration has a negative effect on the battery’s performance, which gives rise to the sharp decrease of discharge-specific capacity. It should result from the fact that the superabundant Cu/CuxO composite blocks lithium-ion transportation. When excessive copper surface is exposed, it may introduce a drastic side reaction causing much active lithium loss.26 In addition, the cycle performances of samples A, B, C, and D at 2 C under 25 °C are shown in Figure 4(b). It can be seen clearly that sample A demonstrates unsatisfactory cycling performance, due to insufficient carbon content establishing uniform conductive film over the entire particle surface.51 In stark contrast, sample C not only exhibits high Coulomb efficiency during the first charge−discharge but also presents more stable cycle performances. Clearly, while promoting the electron transfer, the introduction of the copper core has the potential to enhance the diffusion kinetics of lithium ions. The existence of oxide of copper on the copper metal surface can form an integrated protective layer to suppress the degeneration of battery performance, especially

Figure 3. (a) X-ray photoelectron spectroscopy of the Cu/CuxO composite. (b) The Raman spectrum of sample C. (c) HR-TEM image of copper nanoparticle presenting an eccentric core−shell structure with a crystalline copper core partially surrounded by a cupric oxide shell.

energy peak area, it is found that the copper oxide area ratio is about 73.8%, which is higher than that of metallic copper of 26.2%. These results ascertain that, compared to zero valence copper, the oxide of copper comprised of cupric oxide and cuprous oxide holds the dominant position. Moreover, the Raman spectrum was simultaneously applied because it is wellknown that the surface information for nanometer materials can be provided via a Raman spectrum.40 Figure 3(b) shows the Raman spectrum of sample C. From the figure, it is observed that there are three other peaks located at 219, 348, and 636 cm−1, respectively, except for the observation of a peak around 950 cm−1 corresponding to the LiFePO4 phase.41 The most intense peak located at 348 cm−1 and the weak scattering peak at 636 cm−1 are assigned to B1g and B2g modes of CuO, respectively.42−44 Besides, the peak at 219 cm−1 indicates the appearance of Cu2O.45 Obviously, the Raman spectrum result indicates the copper nanoparticle is perfectly covered by CuO and Cu2O mixed phases in the Cu/CuxO composite. On the basis of the above discussion, it is 4656


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Figure 4. (a) Rate performance for samples A, B, C, and D at 0.5 C, 1 C, 3 C, 5 C, and 10 C (1 C = 170 mAg−1) in the voltage range from 2.5 to 4.3 V at 25 °C, respectively. (b) The cycle performances of samples A, B, C, and D under 25 °C at 2 C between 2.5 and 4.3 V. (c) The typical impedance spectra of samples A, B, C, and D experienced 150 cycles at 25 °C at 2 C in the fully discharged state. (d) The Z′ vs ω−1/2 plots in the low frequency for samples A, B, C, and D, respectively. (e) The Li-ion diffusion coefficient as a function of Cu/CuxO-composite-modified content. (f) The cyclic voltammogram curves of samples A, B, C, and D at sweeping rate of 0.1 mV/s in the voltage range from 2.5 to 4.3 V.

could effectively suppress enlargement in the electrochemical polarization of the battery. In order to further confirm the fact that the decrease of electrochemical polarization for battery is due to the effective conductive network supplied by the Cu/CuxO composite, the typical impedance spectra of samples A, B, C, and D that experienced 150 cycles at 25 °C at 2 C in the fully discharged state were obtained via EIS, as shown in Figure 4(c). The Nyquist plot is mainly comprised of a depressed semicircle and a straight line. In the high-frequency region, the intercept of depressed semicircle on the real axis is indicative of the resistance related to lithium-ion migration through the SEI film (Rsf), and during the medium frequency region, the depressed semicircle intercept on the real axis refers to the charge transfer resistance at the solid−film interface (Rct). The straight line at the low-frequency region indicates Warburg impedance. Later, the equivalent circuit present in the inset of Figure 4(c) was applied to simulate the impedance spectra of the samples. The fitted data about Re (solution resistance), Rsf, and Rct are listed in Table 2. Remarkably, Rsf and Rct values for sample C have a

under high current density. In comparison with the previous literature,12 the Cu/CuxO-composite-modified LiFePO4 in Figures 4(a),(b) displays more excellent performance at high C rate. Moreover, sample D has lower Coulomb efficiency during the first charge−discharge, meaning that side reaction such as electrolyte solvent decomposition is promoted due to some metallic copper surface overexposure. As a result, more active lithium is lost. It is speculated that the blockage of the lithiumion diffusion path should be the main reason for the poor performance of sample D during the following cycles. Figure S3 shows the discharge profiles of samples A, B, C, and D at different discharge rate (0.5 C, 1 C, 3 C, 5 C, and 10 C) between 2.5 and 4.3 V. The discharge plateaus of samples all exhibit very flat and around 3.4 V under 0.5 C discharge rate, which means Fe2+/Fe3+ redox reaction.52 The sample C remains a flat discharge plate as current density gradually increases in comparison with others. Even though the discharge rate is up to 10 C, it still has high discharge voltage, implying that moderate Cu/CuxO composite modification 4657


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ACS Applied Energy Materials Table 2. Simulation Value of Re, Rsf, and Rct for Samples A, B, C, and D after 150 Cycles at 2 C at 25°C, Respectively room temperature

A, 0 at %

B, 0.34 at %

C, 1.17 at %

D, 3.61 at %

Re(Ω) Rsf(Ω) Rct(Ω)

5.025 525.4 889

6.982 419.3 652.1

8.825 378.8 299.1

4.13 612.1 992.1

significant decrease. It verifies that the copper core in the Cu/ CuxO composite can provide the effective conductive network for the electrode, thereby reducing charge transfer resistance. On the other hand, sample D possesses the largest Rsf value, meaning severe electrolyte solvent decomposition reaction occurs on the electrode surface. It is significantly associated with metallic copper surface overexposure, whose overexposed surface would promote a drastic side reaction. Figure 4(d) presents the relationship between Z′ and ω−1/2 in the low frequency under room temperature for samples A, B, C, and D, respectively. The lithium-ion diffusion coefficient (DLi) can be obtained based on the following theoretical eqs 2 and 353,54 Z′ = R ct + R e + σω−1/2 R2T 2 DLi = 2 4 4 2 2 2A n F C Liσ

(2) Figure 5. (a) XRD diffraction patterns of samples A, C, and D after 150 cycles at 2 C at 25 °C, respectively. (b) The amplification drawing for XRD diffraction pattern in the range from 16.5° to 18.5°. (c) The Raman spectra of fresh LFPC and samples A, C, and D after 150 cycles at 2 C at 25 °C. (d) The local map for the Raman spectrum in the range from 930 to 970 cm−1.

(3)

where Z′ refers to the resistance containing charge transfer resistance (Rct) and solution resistance (Re); σ stands for the Warburg factor; ω is the angular frequency; R represents the gas constant; T signifies the absolute temperature; n indicates the number of electrons per molecule during redox process; A denotes the surface area of electrode; F refers to the Faraday constant; and CLi is the concentration of the lithium ion. The value of DLi as a function of Cu/CuxO composite content is shown in Figure 4(e). As can be observed, sample C demonstrates the highest Li diffusion coefficient, revealing the involvement of the Cu/CuxO composite indeed contributes to the enhancement of lithium-ion transfer kinetics in the electrode. However, sample D with low Li+ diffusion coefficient suggests that superabundant Cu/CuxO composite decoration can block the Li+ transfer. Thus, the excess Cu/CuxO composite modification not only holds back Li+ to get through the solid−liquid interphase but also exacerbates electrolyte solvent decomposition due to metallic copper surface overexposure, which gives rise to the poor electrochemical performances of sample D.26 Figure 4(f) shows the cyclic voltammogram curves of samples A, B, C, and D at a sweeping rate of 0.1 mV/s in the voltage range from 2.5 to 4.3 V. The redox peaks for sample A are located at 3.599/3.289 V, which corresponds to Fe3+/Fe2+ redox couple transformation along with lithium-ion insertion/desertion behavior.55 Compared to the pure one, sample C demonstrates relatively narrow difference of redox voltage, which appears at 3.530/3.330 V. It implies that the Cu/CuxO composite modification of LiFePO4 displays the lower electrode polarization and more superior lithium-ion kinetic behavior, which is consistent with the above electrochemical testing results. In order to shed light on the role of oxide of copper in improving the performance of LiFePO4, the aged samples A, C, and D were chosen to conduct XRD characterization. Figures 5(a) and (b) show the XRD diffraction patterns of samples A, C, and D after 150 cycles, respectively. In contrast to sample C without any impurity signal except for diffraction peaks corresponding to the LiFePO4 phase, for sample A after 150

cycles at 2 C, it is evidently observed that the weak diffraction peak belonging to the FePO4 phase appears at 18.1°. Simultaneously, it is accompanied by the left shift of the diffraction reflection corresponding to the (020) crystal face. It is affirmed that oxide of copper acting as a significant artificial isolation layer can efficiently ameliorate the interface condition between electrode and electrolyte. This result can also give a good explanation for the improvement of electrochemical performance for sample C. It is worth mentioning that sample D still keeps relatively good crystal structure. However, sample D is subject to plenty of capacity loss after experiencing many cycles. It is considered that superabundant metallic copper without copper oxide coating not only accelerates the electrolyte oxidation decomposition but also hinders the extraction and insertion of lithium ion. Otherwise, it has been reported that valuable information about the material surface can be assessed by peak broadening and scattering peak shift of the Raman peak profile.44 When the LiFePO4 electrode is modified by the appropriate Cu/CuxO composite, the peak width at half height of the Raman characteristic peak located at 950 cm−1 corresponding to P−O symmetry stretching vibration of LiFePO4 internal modes was well maintained after aging, as shown in Figures 5(c) and (d). It reveals that the surface crystal structure of LiFePO4 is preserved perfectly due to appropriate Cu/CuxO composite involvement. As we all know, the electrolyte inevitably contains a little HF which can attack the cathode material during the charge−discharge process.56,57 The above results indicate that homogeneous distribution of copper oxide acting as an important protective layer role could defend the electrode active material against corrosion from the acid in the electrolyte. To clarify the effects of Cu additive on the electronic conductivity and Li-ion diffusivity, we performed a firstprinciples calculation for the electronic and Li-ion diffusion properties of the LiFePO4 (010) surface adsorbed with copper. 4658


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Figure 6. Surface potential distribution map related to samples (a) A, (b) C, and (c) D experienced 50 cycles, at 2 C at room temperature. (d) The work function curves of aged samples A, C, and D. (e) Schematic diagram of work function (WF) change for the aged samples A, C, and D. Here, Esb, Ecp, ED, EVAC, and EF signify surface barrier, chemical potential, the surface dipole barrier, vacuum level, and Fermi level, respectively (LFPC/ Cu/CuxO60 and LFPC/Cu/CuxO120 signify the samples C and D).

and conductivity. It can be expected that a moderate copper additive could encourage Li-ion transfer kinetics to be improved as a whole. To further reveal the internal mechanism under microscopic scale, the Kelvin probe force microscope (KPFM) with simplicity and nondestructibility in the nanoscale field was applied to measure the work function (WF) of the material surface in order to acquire surface physical and chemical properties.58,59 The WF is dependent on the surface dipole barrier (ED) and Fermi level, reflecting the electron’s ability to get rid of constraint from material superficies.60,61 Generally, the WF is made up of two parts: chemical potential (Ecp) and surface barrier (Esp), among which the value of surface barrier is deemed to be strongly related with the surface microstructure.62 Therefore, the WF that is highly sensitive to surface microstructure change can be available to assess the surface electronic behavior of the material. Figures 6(a), (b), and (c) exhibit the surface potential distribution map for samples A, C, and D after 50 cycles, at 2 C at room temperature. No significant differences are observed. Figure 6(d) shows the work functions for samples A, C, and D after 50 cycles. From the figure, it can be found that sample C owns lower WF compared to others, signifying that electrons should only conquer small potential barrier to flee from the material surface. Based on low intrinsic WF values of cupric oxide and metallic copper,62−66 the composite electrode’s overall WF would be reduced, which is of benefit to the fast transport of electrons. Figure 6(e) intuitively shows the work function (WF) change for the aged samples A, C, and D, respectively. Moreover, sample D demonstrates the highest WF compared

Figures S3(a) and S3(b) show the relaxed surface structures of the pure LiFePO4 (010) surface and the surface adsorbed with Cu atom, respectively. Figures S3(c) and S3(d) show the total density of states (DOSs) of the pure LiFePO4 (010) surface and the (010) surface adsorbed with Cu atom, respectively. From Figure S3(c), it is found that a band gap of 1.43 eV is obtained for the pure LiFePO4 (010) surface. By contrast, the DOS for the Cu surface-modified LiFePO4 exhibits nonzero electron density at the Fermi level, thereby reflecting metallic behavior, as shown in Figure S3(d). Therefore, it can be expected that the LiFePO4 (010) surface with Cu additive will exhibit better electronic conductivity. To further analyze the impact of Cu additive on the Li-ion diffusivity, the nudged elastic band (NEB) method is used to calculate the activation energies for Li-ion diffusion along the b-channels of the LiFePO4 (010) surface adsorbed with copper. Here we consider three Li-ion surface diffusion paths illustrated in Figure S5(a), which are denoted as “path1”, “path2”, and “path3”, respectively, and the corresponding activation energies are shown in Figures S5(b)−S5(d). From Figure S5(b), it can be seen that, compared to the pure LiFePO4 (010) surface, Cu adsorption increased activation energy of about 0.21 eV for Liion diffusion along the “path1”, implying that Li-ion diffusion would be impeded obviously along the Cu surface-adsorbed channel (e.g., path1). However, as shown in Figures S5(c) and S5(d), Cu adsorption decreased activation energies by about 0.03 eV for Li-ion diffusion along the “path2” and 0.06 eV for Li-ion diffusion along the “path3”. That is, Cu adsorption will enhance Li-ion transfer kinetics along the neighboring channels. Therefore, copper additive affects both ion diffusivity 4659


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irregular black matter, presenting inhomogeneous distribution (marked by red circle) after many cycles at high current density under room temperature. It can further prove that copper surface overexposure intensifies electrolyte oxidation decomposition, which is responsible for the deterioration of battery performance. Figure S6(a) demonstrates the Cu and O elemental intensity distribution curve corresponding to metallic copper film along the cross-sectional direction of the film. The film was prepared by the magnetron sputtering method on the silicon substrate with sputtering time of 120 s. The oxygen element is mainly concentrated near on the film surface accompanied with most copper elements being buried in the bottom of the membrane. It signifies that a considerable amount of particles inside the film are pure copper metal and did not develop into distinctive eccentric core−shell structure. Hence, the electrolyte can directly react with inner metallic copper particles through penetration, which would boost electrolyte oxidation decomposition. The green and yellow dot pictures represent Cu and O element distribution on the film, respectively, as shown in Figures S6(c),(d). Figure 8(a) shows the schematic illustration of the Cu/ CuxO-composite-decorated LiFePO4 electrode. It can be obtained from Figures 2(b),(c) and Figures 3(a)−(c) that Cu/CuxO composites with the eccentric core−shell structure are homogeneously dispersed on the surface of active material. Due to the presence of metallic copper in the Cu/CuxO composite, the effective electric conductive pathway can be provided between active particles, which are beneficial for the increase of battery-specific capacity under high current density. It is verified by the fact that sample C demonstrates lower charge transfer resistance and excellent rate performances. On the other hand, oxidation composite wrapping around the metallic copper surface acts as a significant artificial isolation layer role in suppressing undesirable side reactions between active material and electrolyte. The fact that sample C keeps relatively good crystal structure could also confirm this point. Moreover, it has been reported that metal oxide is capable of scavenging HF to form a stable physical barrier, further protecting active material from being corroded.68 Unfortunately, a superabundant Cu/CuxO composite deposited on the electrode surface has a negative influence on the material electrochemical performance. It could be reflected by sample D with low specific capacity at high current density. It is attributed to the oxidation of electrolyte containing EC and DMC, which can be intensified due to excess metallic copper surface exposure. Furthermore, the lithium-ion diffusion path hindered by the superabundant Cu/CuxO composite makes also contributions to the poor electrochemical performance. Figures 8(b) and (c) exhibit the phenomenological resistance model images for samples C and A, respectively. The RLFP, RC, RS, RCu, RCuO, and RCu2O are indicative of LiFePO4 particle resistance, carbon resistance, surface resistance, copper resistance, cupric oxide resistance, and cuprous oxide resistance, respectively. Based on the parallel circuit model, the total resistance is dependent on the low resistance component. Thus, the total interfacial resistance of LiFePO4 electrode is reduced by the metallic copper presence. This can also give a reasonable explanation for the improvement of sample C.

to others, which indicates that greater energy is needed for the electron to escape from the Fermi level, being harmful to rapid charge transport. It may be due to metallic copper surface overexposure causing violent irreversible reaction between copper and electrolyte during the charge−discharge process. The generated electrical isolation oxidation decomposition product could be attached to the active material surface just as a contamination layer, which can create more extra surface barrier for electrons escaping from the material surface, leading to an increase of WF. According to previous studies, the absorbed layer, oxide layer, as well as surface contamination have a great effect on the contact potential difference associated with WF.65,67 This is probably responsible for sample D possessing a considerably bad cycle performance. In order to ensure the viewpoint that the occurrence of drastic electrolyte oxidation decomposition on the electrode interface is caused by copper surface overexposure, the cyclic voltammogram curve of the Cu/CuxO composite sputtered on the aluminum foil for 120 s is shown in Figure 7(a). The

Figure 7. (a) Cyclic voltammogram curves of the Cu/CuxO composite which is sputtered on the aluminum foil for 120 s at sweeping rate of 0.1 mV/s in the range voltage from 2.5 to 4.3 V. The SEM images of (b) aged sample D and (c) aged sample C.

intensity oxidation peak at 3.62 V is evidently found, which is indicative of Cu oxidation to Cu2+ vs Li/Li+,26 and the redox peak is seriously asymmetric for this sample during the first cycle, meaning that active lithium consumption is promoted by superabundant copper surface exposure. Figures 7(b) and (c) show SEM images of samples D and C after 150 cycles at 2 C and at room temperature, respectively. In comparison with the spotless surface of sample C, it can be seen that sample D has

4. CONCLUSION The Cu/CuxO composite with metallic copper as the core and oxidation composite consisting of cuprous oxide and cupric 4660


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Figure 8. (a) Schematic illustration of Cu/CuxO-composite-decorated LiFePO4 electrode. The phenomenological resistance model images for (b) sample C and (c) sample A, respectively (LFPC/Cu/CuxO60 and LFPC/Cu/CuxO120 signify the samples C and D).

oxide as the shell was evenly dispersed on the surface of the LiFePO4 electrode via a facile magnetron sputtering technique. The results show that moderate composite-decorated LiFePO4 demonstrates more excellent rate performance and cyclic stability under high current density, which is ascribed to the enhancement of conductivity between active materials due to the existence of metallic copper in the Cu/CuxO composite. Based on a series of characteristic technologies, it could be found that LiFePO4 decorated by the Cu/CuxO composite remains in an intact crystal structure after undergoing many cycles at high current density, indicating that cupric oxide and cuprous oxide formed on the copper surface offer a significant support for serving as a physical barrier. Moreover, superabundant Cu/CuxO composite decoration on the surface of the electrode can deteriorate the electrochemical performance of the battery. This is due to excess copper surface exposure which intensifies the oxidation decomposition of the electrolyte, causing lots of capacity consumption.

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C, and D at different discharge rates (0.5 C, 1 C, 3 C, 5 C, and 10 C) at 25 °C, respectively; total DOS of the LiFePO4 (010) surface and LiFePO4 (010) surface adsorbed with the Cu atom; Li-ion surface diffusion paths and corresponding activation energies for Li-ion diffusion along the b-channels of the LiFePO4 (010) surface adsorbed with copper; and the Cu and O elemental intensity distribution curve corresponding to metallic copper film grown on the silicon substrate along the cross-sectional direction of the film (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z. Huang). *E-mail: [email protected] (G. Xu). ORCID

Zhensheng Hong: 0000-0002-2567-4955 Zhigao Huang: 0000-0002-8157-3550

ASSOCIATED CONTENT

Notes

S Supporting Information *

The authors declare no competing financial interest.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00004.

ACKNOWLEDGMENTS The authors wish to acknowledge the financial support by the Natural Science Foundations of China (No. 61574037,

XRD diagram and the energy-dispersive X-ray spectroscopy of sample C; the discharge profiles of samples A, B, 4661


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21203025, 11344008, 11204038, 61804030, 11874113) and the Natural Science Foundations of Fujian Province of China (2016J01011, 2016J05151). G. Xu would like to thank the support from Fujian Provincial College Funds for Distinguished Young Scientists of year 2015.

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