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Dec 18, 2015 - Nanogenerator in Li3V2(PO4)3/C Cathode Li-Ion Batteries. Xihui Nan,. † .... good cyclic stability. It demonstrated the best pulse power.

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Highly Efficient Storage of Pulse Energy Produced by Triboelectric Nanogenerator in Li3V2(PO4)3/C Cathode Li-Ion Batteries Xihui Nan,† Changkun Zhang,† Chaofeng Liu,† Mengmeng Liu,† Zhong Lin Wang,*,†,§ and Guozhong Cao*,†,‡ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P.R. China Department of Materials and Engineering, University of Washington, Seattle, Washington 98195-2120, United States § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States ‡

S Supporting Information *

ABSTRACT: Triboelectric nanogenerator (TENG) has been considered as a new type of energy harvesting technology, which employs the coupling effects of triboelectrification and electrostatic induction. One key factor having limited its application is the energy storage. In this work, a high performance Li3V2(PO4)3/C material synthesized by lowcost hydrothermal method followed with subsequent annealing treatment was studied to efficiently store the power generated by a radial-arrayed rotary TENG. Not only does the Li3V2(PO4)3/C exhibit a discharge capacity of 128 mAh g−1 at 1 C with excellent cyclic stability (capacity retention is 90% after 1000 cycles at a rate of 5 C) in Li-ion battery, but also shows outstanding energy conversion efficiency (83.4%) compared with the most popular cathodic materials: LiFePO4 (74.4%), LiCoO2 (66.1%), and LiMn2O4 (73.6%) when it was charged by high frequency and large current electricity directly from by TENG. KEYWORDS: Li3V2(PO4)3, cathode material, cyclic stability, energy conversion efficiency, Li-ion battery, triboelectric nanogenerator

1. INTRODUCTION Electrification and related technologies are indispensable parts of today’s human beings’ activities.1,2 As the electronics and smart devices advance rapidly with increasing functionalities in recent years without any sign of slowing down, it is imperative to develop materials for new and better power generation and storage technologies.3 On the other hand, the increased concern over the environmental pollution and better and healthy life has imposed a great pressure on research community and industry to find to at least partially replace or supplement the conventional energy largely based on the consumption of fossil fuels.4,5 In the past few years, triboelectric nanogenerator (TENG), a new type of energy harvesting technology, which employs the coupling effects of triboelectrification and electrostatic induction,6−8 emerged to harvest various kind of mechanical energy.9−11 The electricity collected and converted from mechanical energy by TENG can be used to power microelectromechanical systems,12,13 environmental sensors,14 and even personal electronics immediately.15 It can also be stored by lithium-ion batteries (LIBs)16,17 to provide more stable power output subsequently. Although it has been demonstrated that LIBs can store the electrical energy generated by TENGs, the power storage efficiency remains low, and the impacts of high frequency pulse power on the electrochemical properties of the storage materials and on the energy storage performance of LIBs remain an unexplored © 2015 American Chemical Society

territory. The storage of power from TENG, particularly when TENG is integrated with other electronic devices such as biomedical and sensory instruments, imposes different and extra challenges to the storage materials and the devices. For example, for imbedded integrated TENG and biomedical devices, the electrical energy storage batteries need to have combined properties or performance including at least (1) a very high power density for high frequency pulse power inputs, (2) a very high energy storage efficiency (a small energy loss at each charge−discharge cycle), and (3) a very small loss of energy during charge and discharge processes, since the energy loss would generate heat, which would cause a big challenge to dissipate the heat from an imbedded biomedical device. LiCoO2 is one of the common commercial cathode materials; however, the instability, high cost, and toxicity limit its application.18 Spinel LiMn2O4 is another commercial cathode material but suffers from severe capacity fading caused by the formation of tetragonal phase upon 3 V cycling,19 loss of oxygen during charging,20 and the dissolution of manganese into electrolyte.21,22 Polyanion compounds including LiFePO4,23 Li3V2(PO4)3,24,25 and LiFeBO3,26 possessing high thermal and structural stability, show a great potential in high Received: October 26, 2015 Accepted: December 18, 2015 Published: December 18, 2015 862

DOI: 10.1021/acsami.5b10262 ACS Appl. Mater. Interfaces 2016, 8, 862−870

Research Article

ACS Applied Materials & Interfaces

calorimetry (TG/DSC) through Simultaneous Thermal Analyzer (STA 449F3, NETZSCH, Germany) from 25−900 °C with a heating rate of 10 °C min−1 in air atmosphere. The Brunauer−Emmett−Teller (BET) surface area was analyzed by nitrogen adsorption measurements using a Micromeritics surface area and porosity analyzer (ASAP 2020 HD88, USA). The conductivity of samples was studied by ST2722 four-point probe method under a pressure of 4 MPa. The electrochemical properties were tested with 2032 coin-type cells assembled in a glovebox filled with pure argon gas. The lithium foil was used as the anode of the cells and the electrolyte was composed of 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (the weight ratio of EC/DMC is 1:1). Celgard polypropylene was used as the separator of the cells. The cathode electrode slurry was mixed by 85% active materials, 10% acetylene black, and 5% sodium carboxymethyl cellulose (CMC-Na). The slurry was coated on a pure Al foil and dried in oven at 80 °C for 4 h before it was transferred into vacuum to further dry for another 12 h at 120 °C. The mass loadings of active material each electrodes for galvanostatic and pulse charge/discharge measurement ranged from 1.5−2.0 mg cm−2 and 3.0−3.4 mg cm−2, respectively. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) performance of the cells were characterized on a Solartron Instrument. Also, the frequency of the EIS was arranged from 100 kHz to 0.01 Hz. The galvanostatic charge/discharge experiment was performed on multichannel battery test system (LAND CT2001A). All the electrochemical measurements were carried out in the potential range of 3.0−4.3 V versus Li/Li+. The cells were connected to the electrometer (Keithley 6514) in series and electrometer (Keithley 6517) in parallel to detect the instantaneous voltage and current of cells during the charging process on TENG.

power density applications. Among them, Li3V2(PO4)3 has distinct advantages over other commercial LIBs cathode candidates for its high redox potential (4 V vs Li/Li+) compared with LiFePO4 (3.4 V vs Li/Li+), very stable crystal structure, thermal stability,27 and relatively high theoretical specific capacity (197 mAh g−1 and 133 mAh g−1 when cycled at potential windows of 3.0−4.8 V and 3.0−4.3 V, respectively.).28 In particular, its sodium superionic conductor (NASICON) structure endows monoclinic Li3V2(PO4)3 threedimensional pathways for Li ion insertion/extraction to provide a very high ion diffusion coefficient (from 10−9 to 10−10 cm2 s−1).29 However, it suffers from low electronic conductivity (2.4 × 10−7 S cm−1)30 due to separated VO6 octahedrons in its structure.29 Three common strategies have been studied to improve it: doping with other elements,31,32 reducing particle size,33 and coating carbon.34 Among these three strategies, coating a homogeneous layer of carbon is the most convenient way to improve electronic conductivity and suppress agglomeration.35 Radial-arrayed rotary TENG is one kind of TENGs that can collect energy produced by rotary surfaces to produce high frequency and high stability current and voltage pulse compared with other kinds of TENGs.36 Here, it was used to charge the cathode materials rapidly and efficiently. In this study, we report the design and synthesis of Li3V2(PO4)3 nanoparticles coated with a few nanometer thick carbon films (LVP/C core−shell nanocomposite) via a simple hydrothermal method subsequently followed with annealing treatment. The synthesis condition and heat treatment parameters have been studied to obtain desired nanostructured LVP/C nanocomposites with excellent lithium ion intercalation/deintercalation properties, particularly at high rates with exceedingly good cyclic stability. It demonstrated the best pulse power storage with a high energy storage efficiency (83.4%) as compared with LiFePO4 (74.4%), LiCoO2 (66.1%), and LiMn2O4 (73.6%) when charged at very high frequency (1300 Hz) and large current (5 mA, 20 C) produced by a Radial-arrayed rotary TENG.

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of samples LVP/C-A, LVP/ C-B, and LVP/C-C. All the peaks in samples LVP/C-B and

2. EXPERIMENTAL SECTION Li3V2(PO4)3/C was synthesized by hydrothermal method. Ascorbic was used as reductant and ethylene glycol (EG) was introduced as both carbon source and dispersant. Stoichiometric amounts of lithium carbonate (Li2CO3), vanadium oxide (V2O5), and ammonium dihydrogen phosphate (NH4H2PO4) were dissolved into distilled water with stirring at room temperature. After the formation of a yellow transparent solution, a different amount of ascorbic was added into the solution (molar ratio of V2O5/C6H8O6 = 4:1, 4:3, 4:5), and 3 mL of EG was added into the transparent solution. Then the solution was stirred for 30 min before being transferred into 100 mL Teflonlined autoclave. The solution in the sealed autoclave was kept at 180 °C for 40 h, then cooled to room temperature naturally. The resulting brown gel-like residue was oscillated in an ultrasonic cleaning chamber for 30 min and then dried at 120 °C overnight. The precursor was preheated at 350 °C for 4 h in argon atmosphere to get the intermediate powder. Finally, the powder was sintered at 750 °C for 8 h in argon atmosphere to yield the final LVP/C material. The products were denoted as LVP/C-A, LVP/C-B, and LVP/C-C, corresponding to the molar ratios of V2O5/C6H8O6 = 4:1, 4:3, and 4:5, respectively. The crystal structure was analyzed by X-ray diffraction (XRD) with monochromatic Cu Kα radiation. The field emission scanning electron microscope (FE-SEM, SU8020) and transmission electronic microscope (TEM) studies were carried out to invest the morphology and nanostructure of synthesized product. The carbon contents of the materials were measured by thermogravimetry/differential scanning

Figure 1. XRD patterns of the samples LVP/C-A, LVP/C-B, and LVP/C-C made with different molar ratios of LVP and carbon precursors.

LVP/C-C can be well indexed to monoclinic structure (JCPDS NO. 96−223−7424) with space group P21/n, and sharp diffraction peaks suggest good crystallinity of the materials. There is no diffraction peak of carbon, indicating that the residual carbon is amorphous, which agrees well with literature.37 In addition to Li3V2(PO4)3, there are two secondary phases in sample LVP/C-A, including VPO4 and LiVP2O7, which might be attributed to the insufficient amount of ascorbic leads to incomplete reduction. The average crystallite sizes of LVP/C nanoparticles were determined from XRD (111) broadening using the Scherer equation,38 D = 0.89λ/β cos θ, where λ is the wavelength of X-ray radiation, 863

DOI: 10.1021/acsami.5b10262 ACS Appl. Mater. Interfaces 2016, 8, 862−870

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ACS Applied Materials & Interfaces and β is the full width at half-maximum of the peak at diffraction angle θ. The particle size reduced with the increasing amount of ascorbic (Table 1). There was no significant particle

respectively. When the content of ascorbic is low, carbon would not be able to form a complete coating on LVP particles in LVP/C-A. With a moderate amount of carbon source in LVP/ C-B, a conformal carbon layer was formed, which stabilizes Li3V2(PO4)3 crystals from excessive growth or agglomeration and also provides a conductive network for electron transport. For LVP/C-C, the molar ratio of V2O5 to C6H8O6 is 4:5, and the excessive ascorbic formed a thick carbon coating. Figure S2 shows the nitrogen sorption isotherms of samples LVP/C-A, LVP/C-B, and LVP/C-C and their pore size distribution calculated from corresponding nitrogen sorption isotherms using BHJ method. The pore volume, specific surface area, and pore size determined by and calculated from nitrogen sorption isotherms indicate the appreciable impacts of the amount of ascorbic added to the system (Table 1). It can be seen that the pore size distributions of LVP/C-A and LVP/C-C are in the range of 10−50 nm and 2−50 nm, respectively. LVP/ C-B possesses a BET surface area of 27.1 m2 g−1 and bimodal pore distribution with peak sizes at 2−3 nm and 6−15 nm. The pore volume increased with the increasing amount of ascorbic added to the system. The relatively high surface area and mesoporous structure can promote the interfacial contact between the electrode and electrolyte. Thermogravimetric analysis (TGA) and DSC were adopted to estimate the carbon content in these three samples, and the results are presented in Figure S3. All the experiments were carried out in a temperature range of 25−900 °C at a heating rate of 10 °C/min in air with a flow rate of 50 mL/min. The mass loss between room temperature to 300 °C is attributed to the evaporation of moisture and volatile organic components. The distinct weight loss between 300 and 450 °C was found in samples LVP/C-B and LVP/C-C and is attributed to the gasification of carbon during its oxidation process. However, there was no distinct weight loss in sample LVP/C-A at the same temperature range. It is hypothesized that part of V3+ ions might be oxidized to higher valence state during the gasification of carbon precursor, leading to a weight gain. Hence, it is difficult to get the carbon content directly from the TG-DSC curves for there was simultaneous weight loss due to the oxidation of carbon and weight gain from the oxidation of

Table 1. Pore Volume (VBJH), BET Surface Area (SBET), Pore Size (Wpore), Particle Size (Calculated from XRD Patterns, D (111)), and Carbon Coating Thickness (Wcarbon) of Different LVP/C Samples sample

VBJH (cm3 g−1)

SBET (cm2 g−1)

Wpore (nm)

D (111) (nm)

Wcarbon (nm)


0.07 0.15 0.18

8.9 27.1 29.0

10−50 2−3, 6−15 2−50

48.2 35.2 34.9

0−4 3−4 9−14

size reduction in LVP/C-C, which indicates that continuing to increase ascorbic is not conducive to obtain smaller particle size to get better following performance. Figure 2, panels a−c show that LVP/C-B with a moderate particle size compared with the large particles of LVP/C-A and the particles of LVP/C-C with varying sizes were embedded into the carbon matrix. It shows that the ascorbic, with a low carbonization temperature, has a great influence on the morphology of the final product, and a moderate amount of ascorbic will contribute to the formation of nanoparticles. It should also be noted that SEM pictures (Figure S1) reveal the particle size of the sample heated at 350 °C is smaller than that calcined at 750 °C, and there exist varying degrees of agglomeration of nanoparticles in the samples sintered at high temperatures. The high-resolution transmission electron microscopy (HRTEM) images (Figure 2d−f) clearly show that the lattice fringes with interplanar distance of 0.362, 0.352, and 0.204 nm correspond to the d-spacing of respective (104)̅ , (200), and (311̅) planes of monoclinic Li3V2(PO4)3, which is also consistence with the d-spacing obtained from FFT analysis. As shown in the insets of Figure 2, panels d−f, d-spacings of the crystalline nanodomains were measured from FFT analysis and correspond to (434̅), (200), and (123̅) planes of Li3V2(PO4)3. The carbon coatings on LVP/C-A, LVP/C-B, and LVP/C-C are estimated to be 0−4 nm, 3−4 nm, and 9−14 nm,

Figure 2. SEM images of (a) LVP/C-A, (b) LVP/C-B, (c) LVP/C-C, which were calcined at 750 °C for 8 h. TEM images and the corresponding FFT patterns (insets) of (d) LVP/C-A, (e) LVP/C-B, and (f) LVP/C-C, respectively. 864

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Figure 3. (a) The second charge−discharge curves of LVP/C-B at various rates from 0.1−20C; (b) rate performance of LVP/C-A, LVP/C-B, and LVP/C-C in the potential range of 3.0−4.3 V; (c) charge−discharge cycling test of LVP/C-B at a rate of 5C for 1000 cycles.

approximtes the theoretical capacity of Li3V2(PO4)3 (133 mAh g−1) when the first two lithiums were extracted from it (as shown in Figure 3). However, using the same discharge current density of 1C, the capacities of LVP/C-A and LVP/C-C are only 85 mAh g−1 and 68 mAh g−1, respectivly. In spite of the distinct difference in lithium ion storage capacity, all three samples demonstrated excellent rate performance and cyclic stability; for example, all three samples have no capacity loss compared with the initial discharge capacity at a rate of 0.5 C (Figure 3b). Figure 3, panel c revealed that sample LVP/C-B exhibits an initial capacity of 125 mAh g−1, and retains a discharge capacity of 112 mAh g−1 after 1000 cycles at a high rate of 5 C, with a capacity retention up to 90%, showing both excellent rate performance and cyclic stability. To have a better understanding of the enhanced eletrochemical performance of LVP/C composites, EIS and CV were measured, analyzed, and compared. The Nyquist plots of samples LVP/C-A, LVP/C-B, and LVP/C-C are shown in Figure 4, panel a. Rs represents the combined Ohmic resistance of the cell, including the resistance of eletrolyte, separator, and the metal electrodes, corresponding to the first intercept of the semicircle at Z′ axis at the high frequency. As shown in Table 2, the values of Rs for three samples are almost identical, suggesting the cell assembly was good and the experiments are solid. The charge transifer resistance (Rct) is determined by the distance between the two intercepts of the semicircle at Z′ axis, and the Rct of three samples is distinctly different, varying from 166.2 Ω for LVP/C-A, to 39.7 Ω for LVP/C-B, to 63.8 Ω for samples LVP/C-C. The smaller Rct value in LVP/C-B indicates

anhydrous Li3V2(PO4)3. When the temperature was increased to 900 °C, the mass of the sample approached a fixed value.39 The total mass change of LVP/C from room temperature to 900 °C includes three parts:40 evaporation of moisture (mw), gasification of carbon (mc), and oxidation of Li3V2(PO4)3 (mo). The relationship of them is shown as follows: Δm = − m w − mc + mo


Δm = −m w − mc + K (M − mc − m w )


The oxidation coefficient (K = 0.0784) was obtained by Kuang et al.39 The contents of amorphous carbon in samples LVP/C-A, LVP/C-B, and LVP/C-C are calculated based on eqs 1 and 2 to be 4.6%, 7.5%, and 10.1%, respectively. Figure 3, panel a shows the charge/discharge curves of LVP/ C-B at various current rates in the potential window from 3.0− 4.3 V versus Li/Li+. For the charge curves at 0.1 C, three plateaus at 3.59 V, 3.67 V, and 4.08 V were clealy observed, corresponding to the three phase tansition processes between Li3V2(PO4)3 and Li2.5V2(PO4)3, Li2.5V2(PO4)3 and Li2V2(PO4)3, and Li2V2(PO4)3 and LiV2(PO4)3, respectivly. The corresponding discharge plateaus are located at respective 3.58 V, 3.66 V, and 4.06 V. The narrow gaps between charge and discharge plateaus illustrate the high reversibility and suggest the high energy conversion efficiency. The corresponding charge/ discharge curves of LVP/C-A and LVP/C-C are shown in Figure S4. With continued increase of charge−discharge current density, the plateaus become shorter and blurred, gradually resulting in a smaller lithium ion storage capacity. The discharge capacity of LVP/C-B is 128 mAh g−1 at 1 C, which 865

DOI: 10.1021/acsami.5b10262 ACS Appl. Mater. Interfaces 2016, 8, 862−870

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Figure 4. (a) The Nyquist plots at 3.6 V of samples LVP/C-A, LVP/C-B, and LVP/C-C; (b) the relationship curves between Z′ and ω−1/2 in low frequencies calculated from the EIS data presented in panel a; (c) the second-cycle CV curves of samples LVP/C-A, LVP/C-B, and LVP/C-C at a potential range of 3.0−4.3 V at a scan rate of 0.1 mV/s; and (d) the charge/discharge curves of samples LVP/C-A, LVP/C-B, and LVP/C-C at a rate of 0.5 C.

warburg factor, which is obtained from the slope of the fitting lines of EIS data at low frequencies in Figure 4, panel b. In eq 4, R is the gas constant, T is the absolut temperature, A is the active surface area of electrode, n is the number of electrons tansferred per mole of active material during oxidization, F is Faraday constant, and C is the concentration of lithium ions in battery (C = (2Li/6.02 × 1023) mol/(900.927 × 10−24) cm−3 = 3.687 × 10−3 mol cm−3). The lithium diffusion coeffcients of LVP/C-A, LVP/C-B, and LVP/C-C calculated using eq 3 and (4) from the EIS data from Figure 4, panel a are 7.76 × 10−11 cm2 s−1, 8.07 × 10−10 cm2 s−1, and 3.30 × 10−10 cm2 s−1, respectively (as listed in Table 2). The lithium diffusion coeffcient, DLi+, of sample LVP/C-B is the highest among three samples, indicating its fastest lithium-ion diffusivity. To further research the effect of carbon coating, the conductivity of three samples was studied by ST2722 four-point probe method under a pressure of 4 MPa. As is shown in Table 2, high carbon content contributes to improvement of conductivity of materials. Especially for LVP/C-B, the conductivity reached 4.81 × 10−3 S cm−1, being improved about four orders of magnitude. However, the conductiviy of LVP/C-C, which has the highest carbon content, dropped to 6.29 × 10−4 S cm−1. This phenomenon might due to the agglomeration of carbon particles and carbon coated active materials. Therefore, carbon particle segregation exists under excessive amount of carbon circumstances, which hinders the electron transport and decreased the electrical conductivity of the material.42 That might also be the reason why LVP/C-C has a larger Rct value

Table 2. Rs, Rct, and DLi+ Values of Different LVP/C Samples sample LVP/C-A LVP/C-B LVP/C-C

Rs (Ω cm−2)

Rct (Ω cm−2)

1.0 1.5 1.3

166.2 39.7 63.8

DLi+ (cm2 s−1) −11

7.76 × 10 8.07 × 10−10 3.30 × 10−10

κ (S cm−1) 7.35 × 10−5 4.81 × 10−3 6.29 × 10−4

its high electronic conductivity and rapid eletrochemical reaction, ready attributing to the conformal carbon coating on small sized LVP particles. The carbon layer in sample LVP/ C-A is inhomogeneous and incomplete and thus may not form a percolated network for electron transport, while it is a little puzzling that the thick carbon layer in sample LVP/C-C presents a large resistance. Although more experiments are needed to verify the hypotheses, it is possible that the formation of thick carbon film and the particle agglomeration might have prevented the complete removal of undesired components and retarded the carbon graphitization. From the same sets of EIS data, the lithium ion diffusion coefficients for samples LVP/C-A, LVP/C-B, and LVP/C-C were calculated using the following equations.41 Z′ = R e + R ct + σω + ω−1/2 DLi + =

R2T 2 2A n F C σω 2 2 4 4 2



In eq 3, Re is the resistance between electrode and electrolyte, Rct is the charge transfer resistance, and σω is the 866

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Figure 5. (a) Charging current and (b) voltage of different electrode materials.

Figure 6. (a) The discharge curves of the four cathode materials after they are charged by the triboelectric nanogenerator for 30 s (the solid lines are discharge curves of the cathodes with the largest output energy density; the dash lines are the discharge curves of the cathodes that charged for another four times with a input energy density fluctuating in a definitive range). (b) The energy conversion efficiencies of different cathode materials for each charge−discharge process.

discharge platforms of different samples were consistent with three pairs of cathodic and anodic peaks locals in CV curves. The identical peak positions and plateau levels in Figures 4, panels c and d also suggested that the parasitic secondary phase in sample LVP/C-A is not electrochemical active during lithium ion intercalation/decalation processes. LVP/C-B was studied to store electrical energy converted from mechanical energy by a radial-arrayed rotary TENG. LiFePO4 (LFP), LiMn2O4 (LMO), and LiCoO2 (LCO) as three kinds of commen cathode materials were also charged by TENG for comparison. The structure of TENG was shown in Figure S5. The electricity-generation process was illustrated by Figures S6 and S7, and the equivalent circuit of battery charging by the TENG was shown in Figure S8.43 Figure 5, panel a shows the pulse charge current produced by TENG with peak current at 5 mA. The charging rates are both kept around 20 C by adjusting the mass loading of the active material on the electrodes, and there is not much difference on the thickness of the electrodes since the theoretical capacities of the four cathode materials, LVP, LCO, LMO, and LFP are not far from one another. The voltage is kept below 5 V, and most pulse of voltage is around 4.5 V, which will not lead to much decomposition of electrolyte (as shown in Figure 5b). The frequency of the current and voltage is 1300 Hz. These four electrodes were all charged-discharged three cycles before being tested on TENG. To keep the materials at

than LVP/C-B. The combination of low charge transfer resistance, high conductivity, and high lithium ion diffusion coefficient contributes to reduced voltage polarization and concentration polarization, which means that the lithium ion intercalation/deintercalation reactions in LVP/C-B take place at the electrode potential close to thermodynamic equilibrium potential, and the active material would be fully involved in the lithium ion intercalation process. Consequently, LVP/C-B reached the capacity close to its theoretical limit with excellent rate performance as revealed in Figure 3, panel b. Figure 4, panel c displayed and compared the CV curves of three different samples at a scan rate of 0.1 mV s−1. All samples have three pairs of cathodic and anodic peaks; each peak has an identical position for all three samples. In the charge process, the first lithium was extracted out in two steps at 3.61 and 3.70 V to form an ordered phase of Li2.5V3(PO4)3 and Li2V2(PO4)3, respectively. The second lithium was extracted in one step at 4.11 V to form the final LiV2(PO4)3 phase. In the discharge process, the two lithiums were reversibly inserted into the structure at 4.05 V, 3.65 V, and 3.58 V, respectively. Compared with LVP/C-A and LVP/C-C, the CV curve of LVP/C-B has a higher redox current value and largest area, indicating its large charge/discharge capacity. This is due to the fast kinetics and high reversibility of LVP/C-B during its electrochemical reaction. The charge−discharge curves at 0.5 C of the three samples are shown in Figure 4, panel d. Three charge− 867

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performance. However, the hostile charge condition may led to some problems, such as John-Teller distortion at the particle surface,44,45 producing of microstrain and destroy of crystal structure. For LCO, the intrinsic structure is instable during the charge process at a large high frequency current. The destruction of electrode structure and side reactions would lead to polarization of the cell, and the voltage will go up more rapidly than that in the conventional charge process. For the given charge energy, the charge current will be reduced when the voltage goes up quickly, limiting the deintercalation process in a given period (30 s in the present study). That may be the reason why LCO and LMO have relatively low energy conversion efficiencies. To further investigate the charge−discharge characteristics of different cathode materials, LCO, LMO, LFP, and LVP/C-B were used as cathodes in LIBs and were charged for 3 min with the high frequency pulse electricity generated by TENG under the same condition for comparison. Figure S10 shows the voltage profiles, which all went up rapidly when they were charged by the high frequency and large current produced by TENG. Also, the average voltages (or the center lines) of the charge profiles were located and corresponded around charge− discharge plateaus of the cathode material in question. The corresponding discharge curves (Figure 7) showed that LVP/

the initial state with all the lithium ions in the corresponding structures, LFP, LMO, LFP, and LCO were all discharged to 3 V before they were charged by the TENG. Then the four electrodes were all charged for 30 s on the TENG. The charge energy density ranged from 35 mWh g−1 to 42 mWh g−1, and discharge curves were shown in Figure 6, panel a. The discharge capacity of different materials fluctuated in a relative small range. The discharge energy increased with the increased charge energy. The solid discharge curves in Figure 6, panel a represent the four samples with a largest output energy density. The input energy density is shown in the Supporting Information Figure S9. The energy conversion efficiency (η) is determined by the output energy density divided by the input Table 3. Comparison of Energy Conversion Efficiency in LiIon Batteries with LFP, LMO, LCO, and LVP/C-B as Cathode Materials electrode LFP LMO LCO LVP/C-B

average input energy density (mWh/g) 39.1 37.5 36.9 37.3

± ± ± ±

1.3 0.9 0.7 1.3

average discharge energy density (mWh/g) 29.1 27.6 24.4 31.1

± ± ± ±

1.3 1.2 1.5 1.5

average energy conversion efficiency (%) 74.4 73.6 66.1 83.4

± ± ± ±

2.8 1.5 3.0 2.2

energy density, which is summarized in Table 3 including both individual and average values with standard deviation: η = Woutput energy density /Winput energy density


η is the energy conversation efficiency, which represents the percentage of input energy that can be released out. Woutput energy density values of different cathode materials charged by TENG were obtained from the integration of their galvanostatic discharge curves at 1C. Winput energy density values were all determined by integrating the area of the P−t curves, that is, Winput energy density = (1/m)∫ t0P(t) dt = (1/m)∫ t0V(t)I(t) dt. LVP/C-B possesses the highest average energy conversion efficiency (83.4%) compared with LFP (74.4%), LMO (73.6%), and LCO (66.1%). The input of LFP (the theoretic capacity is 170 mAhg−1) was relatively high because of its low mass loading, which kept the charge−discharge rate of LFP close to that of the other three materials. When the electrodes were charged by an almost identical input energy density (Winput energy density = 37.3 ± 0.2 mWh g−1), the energy conversion efficiencies of LFP, LMO, LCO, and LVP/C-B are 73.2%, 72.3%, 64.6%, and 81.2%, respectively. All of the energy conversion efficiency values for different cathode materials are shown in Figure 6, panel b; LVP/C-B has the highest value in each charge−discharge process. This is because of the conductive three-dimension network channels for both lithium ions and electrons promotes fast intercalation/ deintercalation process. In addition, high thermal stability and three-dimension Li-ion diffusion path enabled by its polyanion structure may also be key issues for the excellent performance of LVP/C-B. LFP has a similar structure with LVP/C-B; however, the Li-ion diffusion coefficient of LFP is relatively low compared with LVP. Hence, LFP showed relative low energy conversion efficiency under high frequency and large current charge condition. LMO shows spinel structure and a threedimension Li-ion diffusion path, which ensures its good rate

Figure 7. Discharge curves at a rate of 1C of different cathode materials, which have been charged with the pulse electricity generated by TENG for 3 min (voltage profiles are illustrated in Figure S10).

C-B provided both the largest discharge capacity and the discharge energy density among all samples, although its discharge plateau did not reach the highest 4.1 V, reinforcing the earlier results that LVP/C-B possesses excellent energy storage performance with high energy density and high energy conversion efficiency as well as excellent rate performance when it is charged by the radial-arrayed rotary TENG.

4. CONCLUSIONS Li3V2(PO4)3/C nancomposites synthesized by low-cost hydrothermal method following with annealing treatment demonstrated excellent energy conversion efficiency when used as a cathode in LIBs to store high frequency pulse electrical power generated by triboelectric nanogenerators. Li3V2(PO4)3/C nancomposites possess a discharge capacity of 128 mAh g−1 at 1 C, which is very close to the theoretical capacity of Li3V2(PO4)3 (133 mAh g−1 in the potential range of 3.0−4.3 V vs Li/Li+). The capacity retention is up to 90% after cycling for 1000 times at a rate of 5C, corresponding to a capacity fading of 0.01% per cycle. Li3V2(PO4)3/C composite shows outstanding 868

DOI: 10.1021/acsami.5b10262 ACS Appl. Mater. Interfaces 2016, 8, 862−870

Research Article

ACS Applied Materials & Interfaces

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energy conversion efficiency when it was charged by high frequency large current produced by TENG, far better than any known cathode materials including LiMn2O4, LiCoO2, and LiFePO4. This outstanding performance may contribute to the high thermal stability of the material, and the conductive threedimension network channels for both lithium ions and electronic promote fast charge−discharge process.


S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10262. SEM images of LVP/C-A, LVP/C-B, LVP/C-C; N2 adsorption−desorption isotherms and corresponding pore size distribution of the LVP/C samples; TG-DSC curves of LVP/C-A, LVP/C-B, and LVP/C-C in air atmosphere; charge/discharge curves of LVP/C-A and LVP/C-C at various rates from 0.1C−20C; schematic diagram of the TENG with a disk structure; skech illustrating the positions of a copper grid and electrodes during the rotate process; schematic diagram of the electricity-generation process of the TENG; equivalent circuit of battery charging by the TENG with a transformer and a rectifier; input power profiles of the four cathode materials; voltage profiles of high frequency pulse electricity generated by TENG used to charge LVP/C-B, LFP, LMO, and LCO cathode materials for 3 min (PDF)


Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the “thousands talents” program for pioneer researcher and his innovation team, China. This work was also supported by the National Science Foundation of China (51374029, 91433102), Program for New Century Excellent Talents in University (NCET-13-0668), Fundamental Research Funds for the Central Universities (FRF-TP-14008C1), and China Postdoctoral Science Foundation (2015M570988).


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DOI: 10.1021/acsami.5b10262 ACS Appl. Mater. Interfaces 2016, 8, 862−870

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