LiFePO4 Anchored on Pristine Graphene for Ultrafast Lithium Battery

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LiFePO Anchored on Pristine Graphene for Ultrafast Lithium Battery Haijian Tian, Xiaoli Zhao, Jiajia Zhang, Mengxiong Li, and Hongbin Lu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00721 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Applied Energy Materials

LiFePO4 Anchored on Pristine Graphene for Ultrafast Lithium Battery Haijian Tian,‡ Xiaoli Zhao,‡ Jiajia Zhang, Mengxiong Li, Hongbin Lu*

State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite, Materials and Department of Macromolecular Science, Fudan University, Shanghai 200433, China

Keywords: lithium iron phosphates, graphene, cathode material, lithium ion batteries, defect free, coupling

Abstract Defects make a difference in the performance of graphene or other carbonaceous materials when used as conductive additives in electrodes. Reduced graphene oxide (rGO) has been widely used in lithium iron phosphate cathode (LiFePO4) to promote electron transport and improve lithium storage. Though the defects in rGO or GO induce strong coupling between LiFePO4 nanoparticles and conductive sheets, they inevitably impair the in-plane carrier mobility and thus the conductivity throughout the electrode. In this work, LiFePO4 was robustly anchored on pristine graphene with the assistance of branched polyethyleneimine. The pristine graphene used here features with high crystallinity and anti-restacking merit. The resulting composite

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electrodes reveal excellent rate performance: a specific capacity of 143 mAh g-1 at 10 C (1.7 A g-1) and 137 mAh g-1 at 20 C (3.4 A g-1).

1. Introduction Electric vehicles (EVs) have been intensively explored for decades for their advantages with respect to reducing the fossil fuel consumption and the air pollution caused by vehicle exhaust. An EV uses electric energy storage devices. In this regard, lithium ion batteries (LIBs) are deemed among the best options because of the high energy density, good cyclic stability, and low toxicity.1 For LIBs, high-performance cathode materials are still one of the greatest challenges.2 Lithium iron phosphates (LiFePO4) has proved to be ideal cathode materials for LIBs, especially due to its low toxicity, low cost, environmentally benignity, and safety performance.3-9 However, limited rate capability or power density remains its prominent drawback. In particular, hour-long charging time is far inferior to that of gasoline, which largely limits the commercial utilization of LiFePO4. The low rate performance of LiFePO4 primarily arises from its inefficient lithium ion diffusivity and electron migration.10, 11 To deal with these issues, many efforts have been paid to three strategies: scaling down the LiFePO4 particles, metal doping and carbon coating.12, 13

Graphene has manifested great potential in boosting the electrochemical performance of LiFePO4 cathodes, owing to the high specific surface area and superior electrical conductivity.14-18 In the previous studies, reduced graphene oxide (rGO) was usually used,19-29 due to its feasibility in a scalable production of graphene derivatives.30 Zhou 2 ACS Paragon Plus Environment

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et al. reported a scalable spray-drying strategy to prepare graphene-coated LiFePO4.15 Nevertheless, defects in rGO considerably impair its role as conductive additives. Hu et al. observed the cathode capability from electrochemically exfoliated graphene in LiFePO4/graphene composite and ascribed the beyond the theoretical capability to the redox active site induced in the electrochemical exfoliation process.31 However, the rate performance of the composite was also impaired by the reaction on defects of graphene as well as the decreased electrical conductivity, for example, delivering a capacity of 125 mAh g-1 at 10 C, only 60% of that at 0.1 C.

By comparison, pristine graphene contains fewer defects, promising for better electrical conductive additives than rGO or other graphene derivatives.32-34 In the previous study, we have proposed a non-dispersible strategy for production of few-defect graphene in a facile and scalable manner.32 The resulting graphene exhibits both anti-restacking merit and high lattice integrity, beneficial for the optimization of LiFePO4 cathodes and other active nanoparticles.

Here we report a kind of LFP/G composites in which LiFePO4 nanoparticles are in-situ grown on pristine graphene. The interaction between LiFePO4 precursors and graphene are effectively enhanced by branched polyethyleneimine (PEI). To further increase the electrical conductivity of the cathode, a carbon layer was coated through a simple mixing-pyrolysis method. The composite shows superior rate performance, with 143 mAh g−1 at 10 C (1.7 A g-1) and 137 mAh g−1 at 20 C (3.4 A g-1). In another



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word, it can be charged within 5.5 min with a capacity efficiency of 93%, and 172 s with ∼ 89%.

2. Experimental Section 2.1 Materials

H3PO4 (85 wt % aqueous solution, AR), LiOH⋅H2O (AR), FeSO4⋅7H2O (AR), glucose (AR), ethylene glycol (EG, AR), N-methyl-2-pyrrolidone (NMP, AR), and L-ascorbic acid (AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. PEI (branched, M = 600 Da, 99 %) was obtained from Alfa Aesar.

2.2 Synthesis of LFP/G composite Graphene was prepared through a sonication method as previously reported,32 which consists mainly of single-layered graphene nanosheets and can be well suspended in NMP for months. The Graphene/NMP dispersion was first substituted with the EG-water binary solvent by cycles of centrifugation, decanting and redispersion. Then PEI was put into the graphene dispersion under magnetic stirring, to obtain the PEI-modified graphene. The weight ratio of graphene and PEI was 3:10.

The preparation of LFP/G starts from a solvothermal method using LiOH⋅H2O, H3PO4 and FeSO4⋅7H2O with a molar ratio of 2.7:1:1 in the presence of PEI-modified graphene. An EG-water binary solvent (the volume ratio of EG to water was 3:1) was used as the solvent. H3PO4 and LiOH⋅H2O were dissolved into the EG-water binary solvent subsequently with an interval time of half an hour. After stirring for an hour, 4 ACS Paragon Plus Environment

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FeSO4⋅7H2O and L-ascorbic acid (with a Fe/ascorbic acid molar ratio of 1:0.18) were added to the suspension quickly. The suspension was sealed and stirred for 10 min before transferred into a Teflon-lined autoclave, and then kept in 200 oC oven for 10 hours. The precipitate was obtained by centrifugation and washed for five times with water and ethanol, followed by drying at 60 oC in a vacuum oven for 2 hours. The dried powder was mixed with glucose (glucose/LiFePO4: 0.1/1 (w/w)). The mixture was annealed at 700 °C for 5 hours in N2 atmosphere.

As a control experiment, the electrode material was prepared without graphene. Other synthesis conditions were kept. The obtained electrode material was denoted as LFP.

2.3 Characterization

The crystalline structure of LFP/G composite and LFP was analyzed by powder X-ray diffraction (XRD) on a Rigaku D/max−2550 V XRD system over a 2θ range of 10– 70° at 8 kW (40 kV, 200 mA). The carbon contents of LFP and LFP/G were determined by thermal gravimetric analysis (TGA) on a Mettler Toledo TGA instrument from 100 to 700 Celsius degree at a heating rate of 10 Celsius degree per minute in air. The morphologies of LFP/G composite and LFP were observed by field-emission scanning electron microscopy (FESEM) on a Zeiss-Ultra55 microscope and high-resolution transmission electron microscopy (HRTEM) on a JEM-2100F microscope.

2.4 Electrochemical Tests



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LFP/G and LFP were loaded in coin cells (CR2016) for electrochemical testing. Active material, acetylene black, and polyvinylidene fluoride with a mass ratio of 8:1:1 were added to NMP to form a slurry and then spread uniformly on a piece of Al foil with the loading density of 1 mg cm-2 (dry mass). The test cell used a Celgard 2400 membrane as the separator and lithium metal foil as the counter electrode. The electrolyte solution was 1M LiPF6 in a mixed solvent of ethylene carbonate, dimethyl carbonate, and diethyl carbonate (1:1:1 by volume). The cell was assembled in an Ar-filled M Braun glove box with both water moisture and oxygen contents kept ≤1 ppm.

The galvanostatic charge-discharge measurement was performed at different currents within a potential range of 2.3–3.8 V (vs. Li+/Li) on a LAND CT2001A battery test system

(LANHE,

electrochemical

Wuhan,

impedance

China). Cyclic voltammetry (CV) curves and spectra

(EIS)

were

collected

on

a

CHI660E

electrochemical working station (Chenhua, Shanghai, China). CV was scanned over a potential range of 2.5–4.2 V. The scan rates were 0.1, 0.2, 0.5, 1.0, and 2.0 mV s-1, respectively. EIS was tested in the frequency range of 105 to 10−2 Hz with an amplitude potential of 5 mV at open circuit voltage.


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3. Results and Discussion Scheme 1. Synthesis of LFP/G.

The preparation process of LFP/G is illustrated in Scheme 1. Compared with GO or rGO, the graphene used here has fewer oxygen-containing groups with an oxygen content of 5.9 atom% with hydroxyl (-OH) as the primary oxygen-containing group. The single layer graphene percentage is >90% and lateral size ranges from 0.5-5 µm.32 Branched PEI, with abundant amine end groups, is an ideal crosslinking agent to the graphene and the precursors and negatively charged nanoparticles.35, 36 PEI associates with the graphene through the hydrogen bonding between amine groups and hydroxyl groups. The protonated PEI attracts phosphate anion groups around. This helps the deposition of Li3PO4 and Fe3(PO4)2 on the graphene. Following a “dissolution–nucleation–crystallization” mechanism,37 LiFePO4 nanoplatelets were formed in-situ on PEI modified graphene during the solvothermal process. Note that the restacking of graphene is well inhibited due to the electrostatic repulsion of the



adsorbed cationic PEI. It is noteworthy that the hydroxyl density on the graphene is too low to absorb the precursor or the LiFePO4 nanoparticle, blocking the direct growth of LiFePO4 on graphene. As shown in Figure S1a, for the sample synthesized without PEI, the white LiFePO4 nanoplatelets were separated out from the graphene suspension after centrifugation. In contrast, the presence of PEI enables LiFePO4 and graphene to effectively couple together, giving a clean supernatant after centrifugation (Figure S1b).

Impurities, like Fe2P and Li+−Fe2+ anti-site defects, strongly affect the LiFePO4 performance.38, 39 The crystalline structure of the synthesized LFP/G and LFP were checked by XRD (Figure 1). All the peaks in the two XRD profiles were assigned to those of the standard olivine LiFePO4 with the space group Pnma (JCPDS 83-2092), indicating that PEI and graphene do not influence the phase purity of LiFePO4.

Figure 1. XRD profiles of LFP/G (a) and LFP (b). The standard diffraction profile (c) is indexed from the space group Pnma.

The morphologies of LFP/G and LFP were examined by SEM and TEM (Figure 2). The synthesized LiFePO4 nanoplatelets are mainly rectangular in shape, with about 8 ACS Paragon Plus Environment

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150~200 nm in length, 50~100 nm in width, and 20~30 nm in height. The nanoplatelets severely aggregated, which hinders an efficient ion diffusion and conductive additives incorporation. In contrast, for LFP/G, LiFePO4 nanoplatelets are well dispersed on graphene, demonstrating the advantage of graphene as a growing substrate. Acidic environment favors the developing of (100)-oriented LiFePO4 nanoplatelets.37, 40-43

He et al. proposed that at low pH value, LiFePO4 crystal prefers to absorb anions

which favors the developing of (100) facet; at high pH value, LiFePO4 prefers to absorb ethylene glycol or water molecules, which favors the developing of (010) facet.40 In this work, the acidic environment was generated by the H3PO4 excess to LiOH (i.e., molar ratio LiOH / H3PO4=2.7:1) and the addition of ascorbic acid. HRTEM was used to analyze the oriented growth of the LiFePO4 nanocrystal. Revealed by the fringe spacing (Figure 3), LiFePO4 nanoplatelets in LFP and LFP/G share the predominant (100) face exposed. Carbon content was calculated from TGA,44 as shown in Figure S2. LFP/G contained 3.5 wt% carbon and LFP 2 wt%. The graphene content here was about 1.5 wt%, rather small compared with those reported in the literatures.19, 45



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Figure 2. SEM (a, c) and TEM (b, d) images of LFP (a, b) and LFP/G (c, d).

Figure 3. HRTEM and their corresponding FFT (right) images of (a) LFP and (b) LFP/G composite.


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Electrochemical performances of LFP/G and LFP were evaluated by galvanostatic charge-discharge measurements. Figure 4a shows typical charge and discharge profiles of LFP/G and LFP at a low rate of 0.2 C (0.034 A g-1). The discharge capacity of LFP/G (∼154 mAh g−1) is slightly smaller than that of LFP (∼156 mAh g−1). This is attributed to that the carbon content in LFP/G (3.5 wt%) is higher than that in LFP (2 wt%), as the specific capacities here were calculated based on the total mass of LFP or LFP/G.

For the galvanostatic measurement at 0.2 C, LFP/G shows less polarization, revealed by a smaller potential difference between the charge and discharge plateaus (24 mV) than LFP (38 mV) (Figure 4a, the inset). This indicates the electrochemical kinetics of LFP/G is better than that of LFP.

The electrochemical performances of LFP are highly related to the exposed crystal facet.17-19, 21, 23, 26, 45-47 (100) facet-oriented LiFePO4 nanoplatelets were reported to be of good rate performance, delivering a discharge capacity of about 122 mAh g−1 at 20 C.43 Here LFP shows comparable rate performance, with a discharge capacity of 129 mAh g−1 at 10 C, and 120 mAh g−1 at 20 C (Figure 4b). With the assistant of highly conductive pristine graphene network, the rate performance is greatly enhanced. LFP/G delivers a discharge capacity of 143 mAh g-1 at 10 C (10.8% higher than LFP), and 137 mAh g−1 at 20 C (14% higher than LFP). The rate performance of LFP/G cathode (10 C/ 0.2 C=93%) is better than those using rGO (68-88%).14, 15, 18, 19, 21-23, 45, 47, 48, 49

The capacity at 10 C of LFP/G is comparable with the reported best results 11 ACS Paragon Plus Environment


with complex preparation processing (147-162 mAh g-1).25,

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50, 51

A comparison is

given in Table S1.

Good rate performance asks for shorter charging time while keeping a good capacity retention. For LFP/G, it took 5.0 minutes to complete a galvanostatic charge process at 10 C, and another 0.5 minutes in the subsequent constant voltage charging process (Figure S3). The capacity retention reaches ∼ 93% (based on the capacity at 0.2 C) under a total charging time of 5.5 minutes. Moreover, when the charge rate was increased to 20 C, the capacity retention can still reach ~89% with a charging time of 172 s (Figure S4). Also, the cycling stability of LFP/G is obviously better than LFP. Figure 4c shows their cycling performances at 10 C (1.7 A g-1). LFP/G retains 90% of the initial capacity after 500 cycles, while LFP only 80%.


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Figure 4. Electrochemical properties of LFP/G and LFP. (a) Galvanostatic charge and discharge profiles at 0.2 C. (b) Rate performance at current densities from 0.2 C to 20 C. (c) Cycling performance and Coulombic efficiency at 10 C in 500 cycles.

CV scan was implemented to further study their electrochemical properties (Figure 5). Figure 5a shows the CV curves at a low scan rate of 0.1 mV s−1 over a potential range of 2.5–4.2 V (versus Li+/Li). For LFP/G and LFP, there are only a pair of 13 ACS Paragon Plus Environment


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oxidation/reduction peaks, indicating the two-phase charge/discharge reaction of LiFePO4 and FePO4.52 Compared with LFP, LFP/G shows a higher peak current intensity and a smaller potential difference between the charging and discharging peaks (146 mV), demonstrating the improved electrode kinetics and reversibility.53 Figure 5b and 5c show the CV curves at scan rates from 0.1 to 2.0 mV s−1. The potential separation between the oxidation and reduction peaks increases with the scan rate, indicating the polarization increases with charge/discharge current densities. For both cells, it was found that the peak current densities (Ip/mass, in the unit of A g-1) were proportional to the square root of scan rate v (Figure 5b and 5c, the inset). This indicates the oxidation/reduction reaction is reversible.54 Randles−Sevcik equation describes the relationship between Ip/mass and v as follows:53, 55 Ip/mass = 2.69×105n3/2AD1/2v1/2C

(1)

where n is the number of electrons transferred per molecule (here n=1), and A is the active surface area per unit mass of the electrode (cm2 g-1), and C is the initial concentration of lithium ions in LiFePO4 crystal (mol cm-3) which is deemed to be the same for LFP and LFP/G, and D is the Li+ diffusion coefficient (cm2 s-1).

From Eq. 1, we can conclude that the electrochemical reaction in the cathode is more efficient in the electrode when the slope of the plot of Ip and v1/2 is larger.22, 45 We analyzed the Ip at the discharge process (Figure 5). LFP/G shows a slope of 4.0, larger than that of LFP (3.3) (Figure 5b and 5c, inset), which agrees with the rate performance in Figure 4b. 14 ACS Paragon Plus Environment

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Figure 5. Normalized CV curves. (a) Comparison of the LFP and LFP/G at the low scan rate of 0.1 mV s−1. (b, c) CV curves of LFP (b) and LFP/G composite (c) at different scan rates (v) from 0.1 to 2.0 mV s−1. The insets are the corresponding plots of extracted Ip/mass and v1/2.

To gain an intuitive understanding of the lithium ion transport kinetics and the involved resistances, EIS measurement was carried out (Figure 6). The charging or 15 ACS Paragon Plus Environment


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discharging state affects the EIS results.10, 56 Here EIS was implemented at 50% of discharge state, which is in the discharge plateaus region.57 The obtained Nyquist plots consisted of a depressed semicircle in the high-middle frequency region and a straight line with a slope of ~1 in the low-frequency region. The semicircle reflects the charge transfer resistance (Rct) at the electrolyte-electrode interface. The straight line is related to the Warburg diffusion element (Zw) of lithium ions in the solid electrode.10 Rct for LFP and LFP/G were found to be 62.7 and 52.4 Ω, analyzed by ZView program based on the equivalent circuit in Figure 6a, respectively.58, 59 The decreased Rct value indicates that the charge transfer is improved in LFP/G composite. LFP/G also displays a smaller ohm resistance Rs (3.43Ω) than LFP (3.82 Ω). Besides, the lithium ion diffusion coefficient D can be calculated as follows:60

D = R2T2/(2A2n4F4C2σw2)

(2)

in which R is the gas constant, and T is the absolute temperature (300 K here), and F is the Faraday constant, and A, n, and C are given in Eq. 1, and σw is the Warburg impedance coefficient (ohm Hz1/2) related to real part of impedance Z'. In the low-frequency range, Z' can be written as,57, 58

Z' = Rs + Rct +(2π)-1/2σwf (−1/2)

(3)

where f is the frequency and Rs is the ohm resistance. Thus σw can be calculated from the plot of Z' and f

(-1/2)

. Figure 6b shows that the slope of the plot of LFP was 2.8

times that of LFP/G. Thus AD1/2 of LFP/G is 2.8 times that of LFP. Note that in the 16 ACS Paragon Plus Environment

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CV results analyzed by Randles−Sevcik equation, the fitted AD1/2 is 1.2 times that of LFP (Figure 5b,c). Both results indicate a superior electrochemical kinetics of LFP/G to LFP. This can be attributed to that the high-speed electron pathway of the graphene network, which also effectively reduces the aggregation of LiFePO4 nanoplatelets and thus increases their accessible surface area and improves the ion diffusion.

Figure 6. (a) Nyquist plots and the corresponding fitting plots. The inset is the applied equivalent circuit. (b) The plots of Z' and f (-1/2). The slopes of the linearly fitted curves are 6.61 and 2.36 for LFP and LFP/G, respectively.

4. Conclusions



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In summary, the strongly coupled LiFePO4-pristine-graphene composite has been prepared through a solvothermal method. With the assistance of branched PEI, (100) facet-oriented LiFePO4 nanoplatelets in situ grow on pristine graphene with high crystalline integrity. The 3D graphene conductive network pronouncedly decreases the internal resistance of the electrode and enhance its electrochemical kinetics. As a result, LFP/G delivers a specific capacity of 143 mAh g−1 at 10 C, and 137 mAh g−1 at 20 C. So far as we know, LFP/G holds the highest capacity retention of 93% (10 C / 0.2 C). The composite can be charged within 5.5 minutes with a capacity efficiency of 93%, and 172 s of ∼ 89%. LFP/G promises for industrial production of cathode materials in EVs.

ASSOCIATED CONTENT

Supporting Information.

SI 1. Centrifugation results showing the association between LiFePO4 and graphene

SI 2. TGA profiles

SI 3,4. Fast charging profiles. The following file is available free of charge. (pdf)

AUTHOR INFORMATION


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Corresponding Author *Email: [email protected]

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGEMENT This work was supported by the 973 project (2011CB605702), Shanghai key basic research project (14JC1400600) and the National Science Foundation of China (51173027).

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TOC

LiFePO4 anchored on pristine graphene was prepared with the assistance of branched polyethyleneimine. The composite delivers a specific capacity of 143 mAh g−1 at 10 C (1.7 A g−1), and 137 mAh g−1 at 20 C (3.4 A g−1). It retains a capacity retention of ∼ 89% when charged in 172 s.


LiFePO4 Anchored on Pristine Graphene for Ultrafast Lithium Battery

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