Evaluation of graphene-wrapped LiFePO₄ as novel cathode materials for Li-ion batteries

Abstract

Graphene-wrapped LiFePO₄ (LiFePO₄/G) is introduced as a cathode material for Li-ion batteries with an excellent rate capability. A straightforward solid-state reaction between graphene oxide-wrapped FePO₄ and a lithium precursor resulted in highly conducting LiFePO₄/G composites, which feature ∼70 nm-sized LiFePO₄ crystallites with robust connection to the external graphene network. This unique morphology enables all LiFePO₄ particles to be readily accessed by electrons during battery operation, leading to a remarkably enhanced rate capability. The in situ electrochemical impedance spectra were studied in detail throughout charge and discharge processes, by which enhanced electronic conductance and thereby reduced charge transfer resistance was confirmed as the origin of the superior performance in the novel LiFePO₄/G.

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Introduction

Since the first study by the Goodenough group in 1997, LiFePO₄ has been considered one of the most promising cathode materials for lithium ion batteries due to its high theoretical capacity (170 mA h g⁻¹), moderate operating voltage (3.4 V vs. Li⁺/Li), environmental benignity, long cycle life, superior safety, and low material cost.^1–3 However, there are several disadvantages in using LiFePO₄ for wider use in the market. The main obstacles are the poor electronic conductivity (∼10⁻⁹ S cm⁻¹) and low ion-transfer coefficient (∼10⁻¹⁵ cm² s⁻¹), which result from a small polaron hoping and the one-dimensional (1D) lithium-ion diffusion channel, respectively.^4–6 Aiming to solve these problems, many efforts have been made to improve the electrochemical performance of LiFePO₄, by modifying surface with electronically conductive layers,^7–14 reducing Li⁺-diffusion lengths,^15,16 or doping with supervalent ions.¹⁷ Of these methods, coating with carbonaceous materials is the most common and effective way to enhance the electrical conductivity of LiFePO₄.^7–12 Among derivatives of carbon, graphene has attracted tremendous interests due to its intrinsic characteristics (superior electronic conductivity, high mechanical strength, structural flexibility, and large surface area), and therefore graphene can offer an improved interfacial contact to LiFePO₄.^18–27 Of particular note is stacking graphene on the surface of target materials via electrostatic interaction to incorporate graphene.^18,20,23,24 In the case of directly attaching graphene on the surface of active materials via electrostatic interactions, LiFePO₄ nanoparticles easily becomes aggregated, which inhibits individual graphene from wrapping or coating around each LiFePO₄ nanoparticle. As a result, these aggregated active materials, which cannot be benefitted from the incorporated graphene, will be left electronically isolated, resulting in unsatisfactory electrochemical performances.^22–24

Herein, we report on an effective strategy to obtain graphene-wrapped LiFePO₄ from graphene oxide-wrapped metal phosphate and a Li precursor, instead of simple graphene wrapping on prepared LiFePO₄ particles. The subsequent solid-state reaction led to a composite of LiFePO₄ and three-dimensional (3D) conducting networks of graphene on the LiFePO₄ surface, and as such composite is electronically percolated via the graphene sheets. The method of precursor-incorporated graphene-wrapping proved to exhibit a faster electron transfer rate than the conventional graphene-wrapping.¹⁸ The graphene-wrapped LiFePO₄ achieved a high capacity of ∼165 mA h g⁻¹ (theoretical capacity: 170 mA h g⁻¹) at a discharge rate of 0.1C (1C = 170 mA g⁻¹) and ∼100 mA h g⁻¹ even at a rate of 10C. In addition, the origin of superior electrochemical performance of the graphene-wrapped LiFePO₄ was investigated by in-depth in situ analysis of the electrochemical impedance spectroscopy.

Experimental

Synthesis of graphene-wrapped LiFePO₄

FePO₄ nanoparticles were synthesized using FeCl₃ and NH₄H₂PO₄. 100 ml of a distilled water solution containing FeCl₃ (0.02 M) was slowly added to 200 ml of deionized (DI) water solution containing NH₄H₂PO₄ (0.02 M). The reaction mixture was stirred for 5 h at room temperature, then the resulting FePO₄ nanoparticles were centrifuged and washed several times with DI water. The obtained yellow precipitate was dried at 200 °C under vacuum overnight with its change of color to dark green.

The modified Hummers' method was used to synthesize graphene oxide (GO), and the details are described elsewhere.^11,18,28 Prior to GO wrapping, the surface of FePO₄ was modified to be favourable for assembly with GO. In a typical process, the dried FePO₄ powder was dispersed in 100 ml of anhydrous ethanol by ultrasonication for 1 h. Then, aminopropyltriethoxysilane (C₉H₂₃NO₃Si: APTES, 1 ml) was injected into a FePO₄ ethanol dispersion, followed by being kept under constant stirring overnight. The APTES induces positive charges on the surface of the FePO₄ particles. The fabrication of graphene-oxide-wrapped FePO₄ was rendered by electrostatic interactions between positively-charged APTES-modified FePO₄ and negatively-charged graphene oxide.^29–33 For the self-assembly, an aqueous graphene-oxide suspension (100 ml, 1 mg ml⁻¹) was added into the APTES-modified FePO₄ dispersion (500 ml, 1 mg ml⁻¹) under stirring for 1 h. The FePO₄/GO powder was collected by centrifuge, and dried for 12 h. The resulting powder was mixed thoroughly with lithium acetate (C₂H₃LiO₂) as a lithium precursor at a Li : Fe molar ratio of 1.05 : 1, followed by annealing at 600 °C for 10 h under H₂/Ar (5 vol% H₂ in argon) atmosphere. After the pyrolysis, the graphene oxide turned into graphene (reduced graphene oxide), and the graphene-wrapped LiFePO₄ (LiFePO₄/G) could be obtained. The terminology “graphene” was used throughout this article for the convenience's sake, although it is actually reduced graphene oxide.

For comparison, LiFePO₄ without graphene (bare LiFePO₄) and graphene-wrapped commercial LiFePO₄ (c-LiFePO₄/G) were also prepared in a similar manner. Bare LiFePO₄ was synthesized by the same method as above, except the use of graphene oxide. As for the c-LiFePO₄/G, the surface modification of commercial LiFePO₄ powder (Sigma-Aldrich) was performed by mixing APTES with commercial LiFePO₄ in ethanol dispersion for 12 h. The resulting well-dispersed solution was slowly added to GO solution, and gently stirred for 1 h. The obtained graphene-wrapped commercial LiFePO₄ powder was centrifuged, and dried for 12 h. Thermal reduction of GO was carried out under H₂/Ar (5 vol% H₂ in argon) at 600 °C for 3 h.

Characterization

The crystal structures of the prepared samples were characterized by X-ray diffraction (XRD, D8 Advance: Bruker). The morphologies of synthesized materials were analyzed using a field-emission scanning electron microscopy (FE-SEM, Merlin Compact: Zeiss) and high-resolution transmission electron microscopy (HRTEM, JEM-3000F: JEOL, Japan). The graphene amount in the composites was measured using a CHNS analyzer (Flash EA 1112: Thermo Electron Corp.).

For the electrochemical characterization, the active materials were tested by using coin-type half cells (2016 type) with a Li counter electrode. The composition of the electrode was set to be the same for all samples, which consisted of an active material, super P carbon black, and a polyvinylidene fluoride (PVDF) binder with a weight ratio of 7 : 2 : 1. Loading level of active materials was ∼1 mg cm⁻². The electrolyte contained 1 M LiPF₆ in ethylene carbonate and diethyl carbonate (1/1 vol%) (Panax Etec). The cells were charged and discharged between 4.3 and 2 V by applying various current densities. The cycling tests were performed at a constant current density of 170 mA g⁻¹ (1C rate based on the theoretical capacity of LiFePO₄). The specific capacity was calculated based on the mass of only LiFePO₄. Electrochemical impedance spectra (EIS) were measured using a potentiostat (CHI 608C: CH Instrumental Inc.) from 10 mHz to 100 kHz with an AC amplitude of 5 mV.

Results and discussion

A schematic depicts how graphene-wrapped LiFePO₄ (LiFePO₄/G) and graphene-wrapped commercial LiFePO₄ (c-LiFePO₄/G) are synthesized, as shown in Fig. 1 and S1.† In the case of LiFePO₄/G (Fig. 1), the positively-charged FePO₄ nanoparticles (Fig. S2,† prepared through the surface modification by APTES) attracted the negatively-charged graphene oxide (GO) to assemble themselves into GO-wrapped FePO₄ (GO/FePO₄) nanocomposites. After the electrostatic self-assembly, the GO/FePO₄ nanoparticles were thoroughly mixed with lithium acetate followed by annealing under a reducing atmosphere to obtain LiFePO₄/G. In this solid-state reaction, the graphene sheets became crimped, and connected to form a conducting 3D network, while the LiFePO₄ nanoparticles were confined, and wrapped by this graphene matrix. These well-connected graphene sheets not only act as a conductive network for LiFePO₄ particles, but also constrain the size of LiFePO₄ from getting larger during the heat treatment. For c-LiFePO₄/G, on the other hand, the likelihood that each LiFePO₄ particle is confined and connected by GO sheets is relatively small Fig. S1.† Because of the non-uniform contact between LiFePO₄ and graphene, the electrons could reach only the LiFePO₄ particles that are directly attached to graphene during electrochemical cycling. As a result, the transport of electrons in c-LiFePO₄/G is limited, leading to low utilization of LiFePO₄ at a given charge/discharge rate.

Fig. 1 Schematic illustration showing the synthetic process for the graphene-wrapped LiFePO₄.

Transmission electron microscopy (TEM) images in Fig. 2a and b revealed that the FePO₄ nanoparticles were fully wrapped by the flexible GO. While the SEM images (Fig. 2c and d) could not visualize the crinkled textures associated with the presence of flexible graphene sheets, high magnification TEM clearly exhibited that most of the LiFePO₄ nanoparticles were well-wrapped by the 3D network of flexible graphene sheets (Fig. 2e–g). The HRTEM image in Fig. 2h clearly shows lattice fringes, corresponding to the (101) plane of LiFePO₄. On the contrary, for c-LiFePO₄/G, only a few LiFePO₄ particles were attached to the graphene sheets (Fig. S3†).

Fig. 2 (a and b) TEM images of the graphene oxide-wrapped FePO₄. (c and d) SEM images of the graphene-wrapped LiFePO₄. (e and f) TEM images of the graphene-wrapped LiFePO₄. (g and h) HRTEM images of local areas in the graphene-wrapped LiFePO₄ as marked in (f).

The graphene content in LiFePO₄/G composite was 1.7 wt% (from CHN analysis, Table S1†), which is lower than that of the conventional carbon-coated and/or graphene-wrapped active material having ∼10 wt%.^11,15 The average grain size of graphene-wrapped LiFePO₄ particles estimated from the Scherrer equation (Fig. S4†) was ∼80 nm,^11–13 which is consistent with the size distribution of graphene-wrapped LiFePO₄ grains from the SEM images, as shown in Fig. S5.† When compared with LiFePO₄ particles synthesized without GO (∼400 nm), the size of graphene-wrapped LiFePO₄ particles was smaller (Fig. S3 and Table S1†). These results confirm that even small amount of graphene layer (∼1.7 wt%) successfully prevent the growth of LiFePO₄ nanoparticles during the annealing process.

In order to investigate the battery performance of the graphene-wrapped LiFePO₄ and the effect of the graphene layers, the bare LiFePO₄, graphene-wrapped commercial LiFePO₄, (c-LiFePO₄/G), and graphene-wrapped LiFePO₄ (LiFePO₄/G) were cycled at various current densities (sequentially from 0.1C to 10C) (Fig. 3a–c). The battery performance of the bare LiFePO₄ was very poor due to the low electronic conductance, the large grain size (∼400 nm), and the agglomeration of the active particles leading to reduction of the exposed surfaces to the liquid electrolyte (Fig. S3a and b†). Even at a discharge rate of 0.1C, the bare LiFePO₄ showed a low capacity less than 50 mA h g⁻¹ (Fig. 3a), which is consistent with previous studies.^34–36 On the contrary, the formation of the surface-conducting layer (graphene) dramatically improved the capacity of both the commercial and synthesized LiFePO₄. Particularly, the LiFePO₄/G samples showed much better rate capability than that of the c-LiFePO₄/G with distinguishable features between them, as seen in Fig. 3d. The discharge capacity of LiFePO₄/G at a discharge rate of 0.1C was ∼165 mA h g⁻¹ (∼97% of the theoretical capacity), and a specific capacity of ∼100 mA h g⁻¹ could be retained at a 10C rate (∼60% of the capacity at a rate of 0.1C). In comparison with the excellent performance of LiFePO₄/G, the discharge capacity of c-LiFePO₄/G at a rate of 10C was ∼30 mA h g⁻¹ (∼30% of the initial capacity at a rate of 0.1C). The degree of polarization in the voltage profile is much less significant for LiFePO₄/G whereas c-LiFePO₄/G shows high polarization resistance with a limited flat-potential region (Fig. 3b and c). When the cycling performance of the LiFePO₄/G was investigated at a rate of 1C (Fig. 3e), the LiFePO₄/G still delivered a discharge capacity of ∼115 mA h g⁻¹ after 500 cycles which is about 86% of the initial capacity. The LiFePO₄/G has high specific capacity and excellent rate capability under fast discharging conditions, and this remarkable performance is attributed to the well-connected electron percolation among the LiFePO₄ nanoparticles via the graphene sheets. Comparison of this superior rate capability of the LiFePO₄/G with those of the previously-reported graphene-wrapped LiFePO₄ (Fig. 3f)^21–26 indicates that the LiFePO₄/G outperforms the others, or at least, it is competitive amongst other graphene-wrapped LiFePO₄.

Fig. 3 Charge–discharge curves of the (a) bare LiFePO₄, (b) graphene-wrapped commercial LiFePO₄, and (c) graphene-wrapped LiFePO₄. (d) Rate capability of the bare LiFePO₄, graphene-wrapped commercial LiFePO₄, and graphene-wrapped LiFePO₄ (1C = 170 mA g⁻¹). (e) Cycling performance of graphene-wrapped LiFePO₄ at a rate of 1C. (f) Comparison of the rate capability between this study and typical graphene-wrapped LiFePO₄.

The origin of the enhanced electrochemical properties for the LiFePO₄/G was identified by in situ electrochemical impedance spectroscopy (EIS). The EIS spectra were measured point by point at a rate of 0.1C (Fig. 4a and b). During lithiation/delithiation, the cell was switched off every hour to observe the EIS spectra at each open-circuit potential (OCP), in correspondence to the points marked by circles in the voltage profiles of Fig. 4a and b. By fitting trace of the spectra, the charge-transfer resistances and apparent Li⁺ diffusivities were derived, and plotted in Fig. 5a and b.

Fig. 4 The in situ impedance analysis results of (a) graphene-wrapped LiFePO₄ and (b) graphene-wrapped commercial LiFePO₄ at different state of charge (delithiation) and discharge (lithiation). It should be noted that different scales are used on both axes of the Nyquist plots for graphene-wrapped LiFePO₄ and graphene-wrapped commercial LiFePO₄. The circles in the voltage profiles indicate the intervals of impedance analysis.

Fig. 5 (a) Charge-transfer resistance and (b) apparent Li⁺ diffusivity of graphene-wrapped LiFePO₄ and graphene-wrapped commercial LiFePO₄ at different state of charge (delithiation) and discharge (lithiation). Different scales on the axis should be noted for charge-transfer resistance. The red and green solid lines represent the voltage profiles of the graphene-wrapped LiFePO₄ and graphene-wrapped commercial LiFePO₄, respectively.

As shown in Fig. 5a, the charge-transfer resistance (R_ct) of both the LiFePO₄/G and the c-LiFePO₄/G progressively increased with the depth of charge (delithiation), as reported by Delacourt et al.³⁷ The differences in values of R_ct were noticeable between the two which are prepared by different wrapping methods. Provided that electrolyte/electrode interfaces of the two electrodes are assumed to be the same (electrolyte/graphene), the enhanced charge transfer of LiFePO₄/G can be attributed to the well-established electronic percolation of the electrode by the in situ confinement of LiFePO₄ within graphene. On the contrary, the possible contact resistance between c-LiFePO₄/G particles, as already described in Fig. S1,† impedes the transport of electrons, the result of which is reflected in the greater R_ct value. Upon lithiation, the charge transfer resistance of LiFePO₄/G decreases down to R_ct value at the pristine state, while that of the c-LiFePO₄/G could not recover to its original R_ct value (Table S2†). It is possible that some of the c-LiFePO₄/G loses contact with graphene when the volume of the LiFePO₄ lattice shrinks by ∼6.8% on delithiation.² Obviously, the reversibility of R_ct for LiFePO₄/G might come from the well-defined 3D graphene network that tightly encloses each nanoparticle.

In order to identify whether enhanced electronic conductance was the main contributor to the superior electrochemical properties of LiFePO₄/G, the Li⁺ diffusivity (D_EIS) within the olivine structure of the LiFePO₄/G and c-LiFePO₄/G was estimated according to the Warburg equation (eqn (S1)†).³⁸ As seen in Fig. 5b, a trend was apparent in the D_EIS value profile along the delithiation depth for both LiFePO₄/G and c-LiFePO₄/G: the trend shows that the D_EIS value initially decreases, but rises back at the last stage of delithiation.³⁹ Since the diffusion of Li in LiFePO₄, which occurs through the 〈010〉 direction, is hindered when the volume of LiFePO₄ shrinks by ∼6.8% upon delithiation, the changes over the state of charge could be attributed to the coupling of the Li⁺ diffusion and the movement of two-phase boundary.³² In addition, the last stage of (de)lithiation that occurs through the solid solution reaction accounts for the abrupt increase in Li⁺ diffusivity at deep (dis)charged state.^2,40

The absolute D_EIS values are approximately two orders of magnitude higher for c-LiFePO₄/G than that for LiFePO₄/G. Even though the LiFePO₄/G has a shorter Li⁺ diffusion path because of its relatively-small grain size (Table S1†), overall higher Li⁺ diffusivity of c-LiFePO₄/G shall be rooted from the higher crystallinity of the commercial LiFePO₄. Since the solid state of Li⁺ diffusion in LiFePO₄ lattice is governed by t = L²D⁻¹, crystallinity of the commercial LiFePO₄ compromises the Li diffusion time to be comparable to that of the LiFePO₄/G. Hence, this comparative study indicates that the influence of solid-state diffusion of Li⁺ on the rate capabilities of the two samples is less significant. Instead, it can be concluded that the superior electrochemical performance of LiFePO₄/G to c-LiFePO₄/G mainly results from the boosted charge (especially electron) transfer that is rendered by the well-percolated conductive graphene network.

Conclusions

The graphene-wrapped LiFePO₄ (LiFePO₄/G) composites were synthesized by a simple solid-state reaction from graphene-wrapped FePO₄ and lithium acetate. The LiFePO₄/G had a specific capacity of ∼100 mA h g⁻¹ even at a rate of 10C. The enhanced electrochemical performance arose from the graphene sheets' well-established framework, percolating through all of the LiFePO₄ nanoparticles, which is confirmed by the reduced charge-transfer resistance. This effective cathode design could also be extended to other electrode materials, which promises to promote the development of next-generation Li-ion batteries.

Acknowledgements

This research was supported by the National Research Foundation of Korea (NRF: 2015K2A1A2070386 and 2016R1A2B4012938) and by the Office of Naval Research Global (ONRG: N62909-16-1-2083). C. Kim acknowledges the support from the NRF grant (2015R1D1A1A01056874).

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Footnotes

† Electronic supplementary information (ESI) available: Schematic illustration, size distribution, SEM images, XRD pattern, and impedance parameters of the LiFePO₄ electrodes. See DOI: 10.1039/c6ra24514e

‡ These authors equally contributed to this work.

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