Polydiphenylamine Composites with Significantly Improved

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LiV3O8/Polydiphenylamine Composites with Significantly Improved Electrochemical Behavior as Cathode Materials for Rechargeable Lithium Batteries Limin Zhu, Lingling Xie, and Xiao-Yu Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00364 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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ACS Applied Materials & Interfaces

LiV3O8/Polydiphenylamine Composites with Significantly Improved Electrochemical Behavior as Cathode Materials for Rechargeable Lithium Batteries Limin Zhu,†,‡ Lingling Xie,†,‡ Xiaoyu Cao*,†,‡ †

College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology, Zhengzhou 450001, PR China ‡

Key Laboratory of High Specific Energy Materials for Electrochemical Power

Sources of Zhengzhou City, Henan University of Technology, Zhengzhou 450001, PR China *Corresponding author. Tel.: +86 371 67756193; fax: +86 371 67756718. E-mail address: [email protected] (X. Cao) ABSTRACT: Although LiV3O8 is regarded as a potential cathode candidate for rechargeable lithium batteries, it has restricted by its weak dissolution and lattice structure change. Here, the polydiphenylamine is successfully introduced to trigger the evolution of LiV3O8 material through an in-situ oxidative polymerization method, significantly improving electrochemical properties and inhibiting the adverse reaction. Expectedly, the 10 wt.% LiV3O8/polydiphenylamine composite delivers an high initial specific discharge capacity of 311 mAh g−1, remaining 272 mAh g−1 after 50 cycles at the current density of 60 mA g−1. Even at high current density of 2000 mA g−1, it still exhibited the reversible specific capacity of 125 mAh g−1 after 50 cycles. Quantitative kinetics analysis confirms the fundamental reasons of the enhanced rate capacity. The ex-situ XRD and SEM results suggest that 10 wt.% LiV3O8/polydiphenylamine composite own an ultrahigh structural stability during cycling. 1

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KEYWORDS:

rechargeable

lithium

batteries,

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cathode

materials,

LiV3O8/polydiphenylamine composite, in-situ oxidative polymerization, high rate capability

1. INTRODUCTION Among the studied vanadates-based cathode materials, lithium trivanadate (LiV3O8, LVO) has obtained remarkable attention because of its low cost, facile synthesis and high capacity.1-4 In spite of this, LVO still suffers from inferior cycling stability and low rate capability, hindering its practical application in rechargeable lithium batteries. According to the previous reports,5-8 the inferior cycling stability is primarily ascribed to irreversible phase transformation and dissolution of nonhomogeneous vanadium in the electrolyte. Moreover, the poor rate capability is chalked up to the intrinsically low electronic conductivity and the slow Li+ diffusion kinetics.9-12 To address these issues, various research groups have proposed different solutions, such as novel synthesis method,13, 14 morphology control,15-17 surface modification,18-20 and ion-doping.21 The reported electrochemical performance of the LVO has been summarized in Table 1. Although different synthesis methods and surface modifications affect the electrochemical properties of LVO, the more efficient way is the microstructural improvement, which reduces the length of Li+ ion diffusion path, enhances the diffusion kinetics and results in improved rate capability of LVO. However, the nanostructure largely enhances electrode/electrolyte interface, which 2


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speeds up the adverse reactions on the LVO surface and accelerates the solubility of vanadium in the electrolyte and leads to the capacity decay during charge-discharge cycling. From a practical viewpoint, both rate capability and cyclic stability should be improved at the same time. Among these modification methods, conductive layer coating based on nanostructure is a simple and efficient way.22-24 For example, Sun et al.25, 26 have reported a graphene/LVO nanoparticles composite, which showed excellent cyclic performance (237 mA h g−1 after 200 cycles) and rate capability (as high as 59 mAh g−1 at current density of 6 A g−1). However, the synthesis method is relatively complicated and parameters are difficult to control. Comparatively, coating a conducting layer of polymer to stabilize LVO and prepare LVO/polymer composite is relatively simple. 20, 27 Different from the inorganic compositing materials, conducting polymers do not only sustain volumetric changes effectively stop vanadium dissolution without hindering Li+ diffusion to improve the cycle stability and rate capability of LVO, but also participate in the electrochemical reaction and contribute towards the specific capacity of rechargeable lithium batteries. Polydiphenylamine (PDPA) is one of the most common conducting polymer

28

which can be synthesized from cheap diphenylamine (DPA) with the conductive polyparaphenylene backbone and offers electroactive polyaniline structure. In this work, PDPA was utilized to enhance the electrochemical performance of LVO. To the best of our knowledge, this is the first report on the synthesis and electrochemical characterization of LVO/PDPA nanocomposites for rechargeable lithium batteries 3



applications.

2. EXPERIMENTAL LVO samples were synthesized through rheological phase reaction method.29 The stoichiometrical LiOH·H2O, NH4VO3, and C6H8O7·6H2O were thoroughly milled and the mixture was transferred into a Teflon-lined autoclave. Few drops of deionized water were added under continuous energetic stirring. As soon as the solid-liquid rheological state appeared, the Teflon-lined was transferred to a stainless autoclave and placed in an oven at 80 oC for 8 h. After cooling to room temperature, the precursor material was desiccated again at 100 oC for 12 h and then calcined in a muffle furnace at 350 oC for 10 h. The resulting calcined powder was washed with deionized water for several cycles and dried under vacuum at 60 oC for 10 h. To synthesize LVO/PDPA nanocomposites, as shown in Figure S1, LVO and ferric trichloride (FeCl3) (4:1, FeCl3/DPA mole ratio) were dispersed into 10 mL chloroform (CHCl3) and the suspension was magnetically stirred at room temperature for 10 min in N2. The DPA monomer was dissolved in CHCl3 and added into the solution mentioned above drop by drop. The mixture was kept under constant stirring at 50 oC for 12 h in N2. The obtained suspension was poured into methanol to precipitate and washed with ethanol. The resulting product was dried under vacuum at 50 oC for 6 h to obtain a black powder. The LVO/PDPA nanocomposites with different weight percentages of PDPA (5, 10, 20, and 30 wt.% PDPA) were synthesized and referred as 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.% LVO/PDPA, respectively. 4


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The structure and morphology of the nanocomposites were investigated by XRD (Rigaku

MiniFlex

600),

NICOLETAVATAR360),

Fourier SEM

transform

infrared

(FEI-Quanta

250

spectroscopy FEG)

and

(FT-IR, TEM

(HITACHI-HT7700). The cathodes films were fabricated by mixing LVO/PDPA nanocomposites powder, Ketjen Black (KB) and PTFE binder in the weight ratio of 7:2:1. CR2016 type testing cells were assembled in an argon-filled glove box with oxygen and water content of less than 0.1 ppm. Pure Li metal disc served as anode electrode, commercial polyethylene paper was used to separate anodic and cathodic compartments and 1 mol L−1 LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (1:1:1 v/v/v) , was used as an electrolyte for Li+ ions movement. Charge and discharge measurements were completed by using LAND multichannel battery tester (CT2001A). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (CHI 660D). The amplitude of EIS signal was ±5 mV and the frequency ranged from 100 kHz to 10 mHz. The EIS was carried out at open-circuit voltage. The specific capacities and current densities were calculated based on the mass of the LVO/PDPA composites.

3. RESULTS and DISCUSSION Figure 1 shows the XRD patterns of pure LVO and LVO/PDPA composites. It can be readily observed that all the samples exhibit similar XRD patterns, which are 5



in good agreement with the layered monoclinic LVO structure (JCPDS 72-1193). A small amount of impurity phase, Li0.3V2O5, can be found in all samples, exhibited by XRD peak at 12.5o.30-32 Besides, the LVO/PDPA composites display a impure phases of Li0.04V2O5 which is a probable way to accomplish better electrochemical performance.33 Moreover, the composites exhibit a broader X-ray diffraction pattern, which suggests a poor crystallization. Remarkably, the relative intensity of the (100) _

peak is much higher than that of (111), which indicates that the (100) planes may be the major growth direction in these samples. FT-IR spectra of the pure LVO, PDPA and 10 wt.% LVO/PDPA composite are shown in Figure 2a. The FT-IR spectrum of the pure LVO shows three characteristic peaks at 953, 745 and 598 cm−1, which are attributed to V=O stretching vibration, the symmetric V-O-V stretching vibration and the asymmetric V-O-V stretching vibration, respectively.34-36 The FT-IR spectrum of PDPA shows five characteristic peaks at 1600, 1494, 1307, 1230 and 1178 cm−1. The peak at around 1600 cm−1 is assigned to the stretching vibration of C=N, the peak at approximately 1494 cm−1 is attributed to C=C stretching vibration, the peaks at about 1307 and 1230 cm−1 are credited to the stretching vibration of C-N and the peak at around 1178 cm−1 is assigned to C–H bending in benzene ring 28. Furthermore, the FT-IR spectrum of 10 wt.% LVO/PDPA composite shows the characteristic peaks of LVO and PDPA, which suggests that the composite of LVO and PDPA has been successfully achieved in this work. Raman spectrum is more sensitive to the change of material structure, and the Raman spectra of LVO, PDPA and LVO/PDPA composites are shown in Figure 2b. In 6


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the spectrum of the PDPA, the bands at 1196, 1310 and 1561 cm−1 were attributed to C-H angular deformation, inter-ring C-C stretching and C-N stretching, the band at 1604 cm−1 is characteristic of C-C stretching of the aromatic ring.37 It can be seen that LVO and LVO/PDPA composites samples display the typical bands of monoclinic LVO, which shows that the addition of PDPA does not change the main structure of LVO. However, the LVO/PDPA composites samples show an obvious broadening of the peaks, further indicating an increased number of oxygen vacancies, which provide more open voids for easy lithium-ion diffusion.3, 38, 39 The morphology and particle size of LVO and 10 wt.% LVO/PDPA composites were observed by SEM and results are shown in Figure 3a and b. The pure LVO mainly consists of nanorods with the slippy surface, whereas the surface of LVO became sloppy after mixing with PDPA. The 10 wt.% LVO/PDPA composite showed uniform particles with a minimum size and slightly jagged edges (SEM micrographs of other LVO/PDPA composites are shown in Figure S2) which is very important to both the cycle performance and rate capability

2, 4

. TEM and EDS of 10 wt.%

LVO/PDPA composite is displayed in 3c, d and e. As shown in Figure 3c and d, the surfaces of the LVO were coated with a layer of PDPA and the average thickness of the polymer was around 20 nm. EDS result indicates the presence of C and N elements which further proved that PDPA is successfully composited with LVO. The initial discharge curves of LVO and LVO/PDPA composites, at the current density of 60 mA g−1, are shown in Figure 4a. LVO displayed multiple discharge plateaus, consistent with the CV results which will be presented later and the 5 wt.% 7



LVO/PDPA and 10 wt.% LVO/PDPA composites showed similar discharge curves except for the disappearance of voltage plateau at around 2.2 V. This hints about the improved cyclic performance of composite electrode.19 The pure LVO and x wt.% LVO/PDPA composites (x=5, 10, 20, and 30) display the first specific discharge capacity of 286, 305, 311, 229, and 193 mAh g−1, respectively. The 10 wt.% LVO/PDPA composite exhibits the highest initial discharge capacity, which shows that the appropriate PDPA amount is very important to improve the electrochemical properties of LVO/PDPA composite The cyclic performance of LVO and LVO/PDPA composites at the current density of 60 mA g−1, between 4.0 and 1.8 V, is shown in Figure 4b. It can be observed that the capacity of LVO faded quickly in the first few cycles and pure LVO delivered the specific capacity of 169 mAh g−1 at the 50th cycle, which corresponds to the capacity retention of 59% after 50 cycles. On the other hand, it is worth mentioning that the specific capacity of LVO/PDPA composites increase gradually in the first several cycles and then remain stable throughout with excellent cycle life. These results demonstrate that the PDPA significantly improved the cyclic performance of LVO. Among the composites, the 10 wt.% LVO/PDPA composite delivered the specific discharge capacity of 272 mAh g−1 at the 50th cycle, which might be related to the smaller particle size of the composite, providing short Li+ ion diffusion path. Figure 4c and d show the charge-discharge curves of pure LVO and 10 wt.% LVO/ PDPA composite at different cycle numbers, measured at the current density of 8


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60 mA g−1. It is evident that the 10 wt.% LVO/PDPA composite display more clear and symmetrical voltage plateaus than LVO and other LVO/PDPA composites from the beginning (charge/discharge curves of other LVO/PDPA composites are shown in Figure S3), which indicates the lower electrochemical polarization of the composite electrode. Furthermore, the discharge and charge curves for 10 wt.% LVO/PDPA composite are almost overlapped in the local voltage regions during 50 cycles, signifying an outstanding structural stability and charge-discharge reversibility for LVO/PDPA composite. Figure 4e and f displays the differential capacity curves of LVO and 10 wt.% LVO/PDPA composite under the same conditions. The differential capacity curves of other LVO/PDPA composites are shown in Figure S4. As shown in the Figure 4e, the differential capacity curves of pure LVO in every cycle displayed noncoincidence, which corresponds to an unstable chemical reaction. However, LVO/PDPA composites revealed a stable chemical reaction. These results indicate that the electrochemical performances of LVO significantly improved when composited with PDPA. Also, 10 wt.% LVO/PDPA composite showed a higher peak intensity than pure LVO in each cycle, suggesting that the PDPA compositing assisted in achieving higher charge-discharge capacity and faster kinetics. Moreover, the 10 wt.% LVO/PDPA composite also displays remarkable improvement in the rate capability. As shown in Figure 5a, the discharge capacity of 10 wt.% LVO/PDPA composite at the current densities of 30, 60, 120, 180, and 240 mA g−1 is 311, 300, 290, 280, and 271 mAh g−1, respectively. However, for the pure 9



LVO electrode, the discharge capacities are only 286, 240, 205, 179, and 153 mAh g−1 at the same current densities. Evidently, the discharge capacity of both electrodes decreased as the current density increases, but the composite exhibited slowed capacity decay. More importantly, when the current density resumed to 30 mA g−1, after 40 charge-discharge cycles, the 10 wt.% LVO/PDPA composite returned back to its initial capacity, indicating an excellent rate performance. Even at a high current density of 2000 mA g−1, as shown in Figure 5b, the first discharge capacity of 10 wt.% LVO/PDPA composite was found to be 244 mAh g−1 and the composite electrode delivered the specific capacity of 125 mAh g−1 at the 50th cycle. Whereas the capacity of the pure LVO gradually decreased to 58 mAh g−1 after 50 cycles, which can be due to the irreversible phase transition coupled with the deterioration of the crystal structure and the low electrical conductivity.38 Figure 6a shows the CV curves of pure LVO and 10 wt.% LVO/PDPA composite at a scan rate of 0.1 mV s−1, in the potential range of 1.8-4.0 V (vs. Li+/Li), at the second cycle. There are several cathodic and anodic peaks for pure LVO, which can be assigned to the Li+ intercalation and deintercalation processes. However, only one broad pair of cathodic-anodic peaks are observed for 10 wt.% LVO/PDPA composite, which indicates that the PDPA effectively inhibits the phase transition of LVO during charge and discharge process. Besides, the Li+ diffusion coefficient (DLi+) can be calculated from the given equation: Ip = 0.4463(F3/RT)1/2n3/2AD1/2CLi+ν1/2 Where Ip is the peak current, F is Faraday's constant, R is the gas constant, T is 10


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the temperature, n is the number of transferred electrons, A is the effective contact area between the electrode and the electrolyte, CLi+ is the concentration of the Li+ ions in the cathode calculated according to the crystallographic cell parameter of LVO and ν is the scan rate. The Doxidation and Dreducation value of the pure LVO and 10 wt.% LVO/ PDPA composite were calculated and found to be Doxidation (2.88 V)= 3.70×10−8 cm2 s−1，Dreducation (2.52 V)= 2.32×10−8 cm2 s−1 and Doxidation (2.77 V)= 5.03×10−8 cm2 s−1，Dreducation (2.51 V)= 5.19×10−8 cm2 s−1, respectively. The diffusion coefficient values demonstrate that the introduction of PDPA improves the Li+ ions diffusion ability of LVO. Figure 6b displays the CV curves of 10 wt.% LVO/PDPA composite at different scan rates. As the scan rate increased, the shape of the peak is almost identical, but the height of the redox peaks increased. According to the relationship between the measured current (i) and the scan rate (v): i=avb, the value of b can be determined from the slope of the log i-log v plot (Figure 6c). As is known to all that for a diffusion-controlled process the value of b approaches 0.5,40, 41 and the b-value for both cathodic and anodic peaks can be calculated at scan rates from 0.1 to 0.4 mV s−1, indicating the diffusion-limited charge and discharge process for the composite. In Figure 6d the percentage of capacitive contribution at a scan rate of 0.1 mV s−1 could be quantified determined by separating the current response i from the diffusion-controlled and capacitive contribution at a fixed voltage. We can see that 37% of the total capacity is identified as the capacitive contribution for the 10 wt.% LVO/PDPA composite electrode, indicating that the diffusion process is feasible at 11



this region. To further comprehend the electrochemical performance of 10 wt.% LVO/PDPA composite, the EIS was carried out and Nyquist plots are shown in Figure 7. Figure 7a shows the Nyquist plots of the pure LVO and 10 wt.% LVO/PDPA composite electrodes, measured at open-circuit voltage. Both of the Nyquist plots mainly consist of two semicircles in the high to medium frequency range, corresponding to the resistance of the SEI layer and the charge-transfer resistance (Rct), and a sloping line in the low-frequency region, relating to the Li+ ion diffusion resistance in the electrodes (Zω). The values of Rct for the pure LVO and 10 wt.% LVO/PDPA composite electrodes are 962 and 366 Ω, respectively. The Rct of pure LVO is three times higher than that of 10 wt.% LVO/PDPA composite electrode, which indicates that PDPA facilitates faster Li+ and reduces charge transfer resistance. The values of exchange-current density (i0) can be calculated from the following equation: i0=RT/nFRct where n is the electron-transfer number.42 The i0 values of the pure LVO and 10 wt.% LVO/PDPA composite were found to be 11.2×10−3 and 40.6×10−3 mA cm−2, respectively. These results represent that the LVO/PDPA composite has better electrodynamic features and lower electrode polarization, which are beneficial for the rapid intercalation/deintercalation of Li+ ions during charge and discharge processes. Also, the Li+ ion diffusion coefficient (DLi+) can be obtained from the following equation: 38 ZRe = Rs + Rct + σω-1/2 12


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DLi+= 0.5R2T2/S2n4F4C2σ2 In these equations, S is the effective contact area between the electrode and the electrolyte, F is Faraday's constant, and C is the concentration of the Li+ ions in the cathode calculated based on the crystallographic cell parameter of LVO. The slope (σω) can be obtained from Figure 7b. The DLi+ value of the 10 wt.% LVO/PDPA composite is calculated to be 1.04×10−14 cm2 s−1, which is approximately 2 orders of magnitude higher than that of pure LVO (2.58×10−16 cm2 s−1). In general, DLi+ calculated by EIS is smaller than that of deduced from CV curves. These results demonstrate that the PDPA is beneficial for the faster Li+ ions diffusion. On the basis of the above results, the schematic illustration of this study is given in Scheme 1. PDPA played an essential role in the LVO/PDPA nanocomposites by ensuring the presence of conductive network for efficient and faster electronic transport, and improving the Li+ ion diffusion kinetics. To confirm the role of the PDPA in preventing the phase transition, ex-situ XRD of pure LVO and 10 wt.% LVO/PDPA composite electrodes were carried out after 50 charge/discharge cycles and results are presented in Figure 8. It can be readily observed that the XRD pattern of LVO cycled electrode changed a lot. The peak at about 14o disappeared, which indicates the distortion of the LVO crystal structure during charge and discharge process. Contrary to the LVO, the XRD pattern of 10 wt.% LVO/PDPA composite cycled electrode remained unchanged after 50 charge/discharge cycles in addition to the new diffraction peaks of the aluminum network and Ketjen black (KB), signifying worthy structural stability. 13



Figure 9 represents the morphological changes, observed by ex-situ SEM, of LVO and 10 wt.% LVO/PDPA composite electrodes after 50 charge-discharge cycles. Both the pure LVO and LVO/PDPA composite electrode displayed a smooth surface before charge/discharge process, however, after 50 charge/discharge cycles, the difference between the two electrodes is remarkable. As shown in Figure 9c, the surface of the pure LVO electrode appeared to be large cracked with pulverized and agglomerated structure, which caused partial LVO inactivation, resulting in a rapid decay in electrode capacity. However, the LVO/PDPA composite electrode maintained the microstructure and displayed a smooth surface without significant cracks. This can be ascribed to the ability of PDPA in mitigating the damage done by the volumetric changes during the LVO during the charge-discharge process. Based on all these factors, the LVO/PDPA composite electrode attained much improved reversible capacity, cycling life and rate performance.

4. CONCLUSIONS A novel LVO/PDPA composite cathode material for rechargeable lithium batteries has been synthesized by a simple in-situ oxidative polymerization method. The 10 wt.% LVO/PDPA composite demonstrated excellent electrochemical performance with superior cycle life and enhanced rate capability. At the current density of 60 mA g−1, the composite electrode delivered a reversible specific capacity of 272 mAh g−1 at the 50th cycle. The same electrode maintained the specific capacity of 125 mAh g−1 at a high current density (2000 mA g−1), indicating excellent rate 14


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performance. The significantly improved electrochemical performances of the LVO/PDPA composite electrode can be ascribed to the introduction of PDPA, which acts as a self-protective layer to protect active material during the electrochemical process. The vanadium dissolution from LVO can be minimized by avoiding the direct contact with the liquid electrolyte, which results in retention of crystal structure during charge/discharge process. The PDPA layer is also capable of acting as a buffer layer to accommodate volumetric changes. Moreover, the conductive nature of PDPA reduces the charge transfer resistance and increases the Li+ ion diffusion coefficient. Based on our results, the 10 wt.% LVO/PDPA composite shown promise as a cathode material for next-generation rechargeable lithium batteries. This work opens up an avenue towards the design of protecting cathode materials and broadens application of conducting polymer in more filed.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx. Schematic illustration of synthesis process of LVO/PDPA; SEM images of LVO/PDPA; Charge/discharge profiles of LVO/PDPA; Differential capacity curves of LVO/PDPA (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +86-371-67756193, Fax: +86-371-67756718 (X. Cao). 15



ORCID Xiaoyu Cao: 0000-0002-6740-4937 Author Contributions Limin Zhu performed the experiments and analyzed the data. Limin Zhu and Lingling Xie wrote the initial manuscript draft. Prof. Xiaoyu Cao designed and oversaw the experiments, revised and finalized the manuscript for submission. All authors have discussed the experimental results and commented on the manuscript. Notes The authors declare no conflicts of interest.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation, China (No. 21403057, 21773057, and U1704142), Program for Innovative Team (in Science and Technology) in University of Henan Province, China (No. 17IRTSTHN003), Program for Science and Technology Innovation Talents in Universities of Henan Province, China (No. 18HASTIT008), Postdoctoral Science Foundation, China (No. 2017M621833), Cultivation Plan for Young Core Teachers in Universities of Henan Province, China (No. 2016GGJS-068), Natural Science Foundation of Henan Province, China (No. 162300410050), Key Science and Technology Project of Henan Province, China (No. 162102210187), and Program for Henan Science and Technology Open and Cooperation Project, China (No. 172106000060).

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Figure captions Figure 1. XRD patterns of LVO and LVO/PDPA composites. Figure 2. The FT-IR spectra of LVO, PDPA and10 wt.% LVO/PDPA composite (a), and Raman spectra of the LVO, PDPA and LVO/PDPA composites (b). Figure 3. SEM micrographs of LVO (a), 10 wt.% LVO/PDPA composite (b), TEM micrographs of 10 wt.% LVO/ PDPA composite (c, d), and EDX of 10 wt.% LVO/PDPA composite(e). Figure 4. Initial discharge curves of LVO and LVO/PDPA composites (a), cycle performance of LVO and LVO/PDPA composites (b), charge/discharge curves of LVO at different cycles (c), charge/discharge curves of 10 wt.% LVO/ PDPA at different cycles (current density: 60 mA g−1) (d), the differential capacity curves of

LVO (e), and the differential

capacity curves of 10 wt.% LVO/PDPA composite (f). Figure 5. Rate capability of LVO and 10 wt.% LVO/PDPA composite at various current densities (a), and cycling performance of LVO and 10 wt.% LVO/PDPA composite at a current density of 2000 mA g−1 (b). Figure 6. CV curves of LVO and 10 wt.% LVO/ PDPA composite, at a scan rate of 0.1 mV s−1 (a), at different sweep rates between 4.0 and 1.8 V (vs. Li+/Li) (b), the relationship between logarithm cathodic and anodic peak current and logarithm scan rates (c), and capacitive charge storage contributions at a scan rate of 0.1 mV s−1 (d) . Figure 7. EIS of the pure LVO sample and10 wt.% LVO/PDPA composite (a), and the relationship curves between ZRe and ω−1/2 in the low frequency range (b). Scheme 1. Schematic illustration of Li+ and electron transfer pathway for LVO and LVO/PDPA composites. 21



Figure 8. Ex-situ XRD patterns of LVO powder and electrode at the 50th cycle (a), 10 wt.% LVO/PDPA composite powder and electrode at the 50th cycle (b), and XRD patterns of Al net and KB (c, d). Figure 9. SEM images of electrode surfaces of LVO before cycled (a), LVO/PDPA composite before cycled (b), LVO after 50 cycles (c), and LVO/PDPA composite after 50 cycles (d).

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Table 1. The electrochemical performances of the previously reported LVO.

charge/discharge capacity(mAh g-1) voltage range (cycle number)

samples

current density

microwave synthesis13

0.2 mA cm−2

1.8−4.0 V

335 (1), 230 (8)

sol-gel synthesis14

100 mA g−1

1.5−4.0 V

240(1), 260(100)

nanowires15

100 mA g−1

1.5−4.0 V

320(1), 271.7(100)

nanosheets16

130 mA g−1

1.5−4.0 V

260(1), 170(100)

nanorod17

300 mA g−1

1.5−4.0 V

226(1), 197(100)

LVO/polypyrrole20

0.2 mA cm−2

-0.6−0.2 V (vs. SCE)

Polydiphenylamine Composites with Significantly Improved

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