The role of graphene in nano-layered structure and long-term cycling stability of Mn_xCo_yNi_zCO₃ as an anode material for lithium-ion batteries

Abstract

Transition metal carbonates with high energy density of lithium storage via conversion reactions as anode materials for lithium-ion batteries are a hot research focus. However, the large volume changes during the processes of insertion/extraction of Li⁺ ions and low electronic conductivity are great challenges. Herein, a nano-layered Mn_xCo_yNi_zCO₃/graphene composite and micro-spherical Mn_xCo_yNi_zCO₃ are synthesized via a facile hydrothermal route in the presence and absence of graphene, respectively. As an anode material, the prepared Mn_xCo_yNi_zCO₃/graphene composite delivers a final 500th reversible discharge specific capacity of 1046 mA h g⁻¹ at 100 mA g⁻¹ and furthermore presents a high value of 711 mA h g⁻¹ at 1000 mA g⁻¹ after 500 cycles, which shows prominent superiority in comparison with Mn_xCo_yNi_zCO₃ (517 mA h g⁻¹ at 100 mA g⁻¹ after 500 cycles) and other reported works about transition metal carbonates. The high reversible capacity, long-term cycling stability and perfect rate performance of the Mn_xCo_yNi_zCO₃/graphene composite should be ascribed to the existence of graphene, which plays an important role in keeping the nanostructure of the sample, enhancing the electronic conductivity of the sample, preventing the aggregation of particles and accommodating volume changes of the electrode during the cycling process. Hence, the prepared Mn_xCo_yNi_zCO₃/graphene composite is a promising anode material for lithium-ion batteries.

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Introduction

In recent years, the greenhouse effect and other irreversible environmental pollution associated with the abuse of fossil fuels have caused serious damage to the environment. Thus, the development of clean and sustainable energy sources has been recognized as an effective solution to the energy and environment crises. As one of the most promising energy storage devices, rechargeable lithium-ion batteries with attractive advantages of high energy density, long cycling life and environmental amity have been widely used in electric vehicles and portable electronic devices.^1–3 However, to satisfy the ever-increasing energy requirements, great efforts should be made to improve the holistic electrochemical performances of lithium-ion batteries.⁴ Considering the low theoretical capacity of graphite as a commercial anode material (372 mA h g⁻¹), the development of novel anode materials is extremely imperative.

Presently, transition metal carbonates,^5,6 transition metal oxides,^7,8 titanate^9–12 and alloys^13–15 as anode materials have attracted much attention owing to their much higher theoretical capacity than that of graphite.¹⁶ Among those materials, transition metal carbonates are a hot research focus based on the high energy density of lithium storage via conversion reactions, much higher practical capacity than theoretical capacity, durable cycling performance, facile synthesis and environmental amity.^17,18 Moreover, the practical capacities of transition metal carbonates are usually as high as or even much higher than the corresponding transition metal oxides.¹⁹ However, the serious capacity decline and poor rate performance caused by the low electronic conductivity and large volume changes of the electrode during the cycling process are great challenges.^20,21 Many methods have been adopted to solve these intractable problems, including reducing the particle size, carbon coating, doping and designing special structures. However, the improvements are still limited.^22–24

Graphene is an ideal carbon matrix for the preparation of composites, which shows a two-dimensional (2D) network structure and many attractive properties, such as outstanding electronic conductivity, large specific surface area, superior mechanical flexibility and excellent structural stability.^25–29 As for the graphene-based composites, the existence of graphene can effectively improve the electronic conductivity of the materials, prevent the possible aggregation of particles and accommodate the volume changes of the electrodes during the cycling process, which contributes to great improvements in electrochemical performance.^30–33 Compared with the materials without graphene, the corresponding graphene-based composites such as NiO/graphene,³⁴ SnO₂/graphene,³⁵ Si/graphene,³⁶ TiO₂/graphene³⁷ and Fe₃O₄/graphene³⁸ demonstrate much better electrochemical performance. Thus, it can be inferred that addition of graphene is an applicable way to solve the defects of transition metal carbonates. However, it is a pity that the present work about transition metal carbonates/graphene composites is very scarce.

Herein, an Mn_xCo_yNi_zCO₃/graphene composite with a nano-layered structure is synthesized by a facial hydrothermal method, which demonstrates much better electrochemical performance than those of the prepared micro-spherical Mn_xCo_yNi_zCO₃. The results reveal that graphene is a crucial factor to keep the nanostructure and enhance the electrochemical performance of Mn_xCo_yNi_zCO₃, which is the main topic of this work. So far, analogous works have not been reported.

Experimental

Synthesis

Graphene oxide (GO) was synthesized by the modified Hummers' method using natural graphite powder as the raw material.^39,40 To get the graphene, the prepared GO sheets were calcined in a tube furnace at 500 °C for 2 h under flowing gas of 5% H₂/Ar.⁴¹ To synthesize the Mn_xCo_yNi_zCO₃/graphene composite, 0.172 g of graphene was dispersed in 40 mL of distilled water by sonication for 3 h. Then, 0.331 g of MnSO₄·H₂O, 0.081 g of CoSO₄·7H₂O, 0162 g of NiSO₄·6H₂O (in the molar ratio of 1.00 : 0.24 : 0.48) and 2 g of urea were added into the graphene suspension liquid. After sonication for another 1 h, the mixed solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and then kept at 180 °C for 24 h. After cooling down to room temperature, the final product marked as Sample-R was collected by centrifugation with distilled water and ethanol. The pure Mn_xCo_yNi_zCO₃ marked as Sample-P was also prepared by the same method in the absence of graphene. Via the differences in structure and electrochemical performances between the two samples, the effects of graphene can be demonstrated clearly.

Characterization

The X-ray powder diffraction (XRD) measurements were performed on a Bruker D8 diffractometer with Ni-filtered Cu Kα radiation (40 kV, 40 mA) in the 2θ range of 5° to 80° at a scanning rate of 10° min⁻¹. The scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) spectra were obtained using an S-4800 field-emission scanning electron microscope operated at a voltage of 1.0 kV. The transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL JEM-2100F transmission electron microscope equipped with a postcolumn Gatan imaging filter (GIF-Tridium) at an acceleration voltage of 200 kV. The X-ray photoelectron spectrometer (XPS) spectra were acquired using an Escalab 250Xi X-ray photoelectron spectrometer.

Electrochemical measurements

The electrochemical performances of the samples were tested in a model test cell system. The electrodes were prepared by pressing a mixture of the active material (60 wt%), acetylene black (30 wt%) and polytetrafluoroethylene (PTFE) (10 wt%) onto a nickel grid. Prior to being used, the electrodes were dried at 120 °C in a vacuum furnace for 24 h. The electrolyte was a solution of l M LiPF₆ in ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 by volume). The separator was Celgard 2400 porous polypropylene. The counter and reference electrodes were lithium foil. The model test cells were assembled in an argon-filled glove box. Charge/discharge tests were performed at different current densities between 0.01 and 3.00 V (vs. Li/Li⁺). Electrochemical impedance spectroscopy (EIS) tests and cyclic voltammetry (CV) tests were conducted using a CHI 600A electrochemical workstation.

Results and discussion

Structure and morphology characterization

Fig. 1a shows the XRD patterns of Sample-R and Sample-P to identify their crystal structure, confirming that both Sample-R and Sample-P take rhodochrosite MnCO₃ as the pure phase. All the 2θ values and intensities of the diffraction peaks belonging to the two samples are in full accordance with the standard XRD pattern of rhodochrosite MnCO₃ (JCPDS File Card No. 83-1763). There are no excrescent peaks existing, indicating the high purity of the two samples. Via comparison, the peaks of Sample-R show much higher intensities than those of Sample-P, revealing that Sample-R has much better crystalline. In the XRD pattern of Sample-R, the peak with a 2θ numerical value of approximate 13° is extremely weak, suggesting that the GO has been mainly reduced to graphene. Moreover, owing to the relatively low content and amorphous state of graphene, no diffraction peaks ascribed to graphene are detected in the XRD pattern of Sample-R. However, TGA analysis demonstrates that the content of graphene in Sample-R is about 11 wt% (Fig. 1b).

Fig. 1 XRD patterns (a) and TGA curves (b) of Sample-R and Sample-P.

Owing to the high Raman-active property of carbon, the existing form of graphene in Sample-R can be easily revealed by the Raman spectrum (633 nm laser excitation) as shown in Fig. 2. There are two strong peaks at 1320 and 1595 cm⁻¹, corresponding to the D band of disordered carbon and the G band of graphite carbon, respectively. The higher intensity of the D band (I_D/I_G = 1.202) reveals that amorphous carbon holds the dominant position, simultaneously proving that many defects exist in the graphene of Sample-R.⁴² Those defects might be brought by element O, which not only provides strong interaction between Mn_xCo_yNi_zCO₃ and graphene via physical adsorption but also enhances the electronic conductivity of the sample.⁴³ There is also a weak peak appearing at 2640 cm⁻¹ ascribed to the 2D band with width of approximately 100 cm⁻¹ at half height, revealing that three or more graphene layers stack together layer by layer.

Fig. 2 Raman spectrum of Sample-R.

Fig. 3 presents the XPS spectra of Sample-R and Sample-P fitted by Gaussian fitting method to analyze the oxidation states of the elements. As for Sample-R, the Mn 2p_3/2 peak at 642.1 eV and the Mn 2p_1/2 peak at 654.0 eV with a splitting width of 11.9 eV indicate the Mn(II) state (Fig. 3a), which is in agreement with the related literature.⁴⁴ The Co 2p_3/2 peak at 782.0 eV and the Co 2p_1/2 peak at 798.6 eV with a splitting width of 16.6 eV (Fig. 3c) confirm the Co(II) state. Meanwhile, the additional peaks of Co 2p at 785.0 and 802.7 eV are satellite shake-up peaks (marked as sat.) at the higher binding energy side.^45,46 The Ni(II) state is verified by the splitting width of 17.9 eV from the Ni 2p_3/2 peak at 856.8 eV and Ni 2p_1/2 at 874.7 eV accompanied by the satellite shake-up peaks at 861.7 and 881.2 eV (Fig. 3e).⁴⁷ In the XPS spectrum of O 1s (Fig. 3g), the peak at 532.1 eV is related to the O atoms bonded to graphene and metallic atoms simultaneously, suggesting that the Mn_xCo_yNi_zCO₃ particles are strongly fixed on the graphene layers.⁴⁸ The XPS spectrum of C 1s (Fig. 3i) is composed of three peaks appearing at 284.9 eV, 287.0 eV and 288.9 eV, corresponding to the carbon sp² bonding (C–C), epoxide/hydroxyl groups (C–O) and carbonyl/carboxyl groups (C=O), respectively. The peak at 284.9 eV associated with the C–C bond shows the highest intensity, indicating that a large amount of oxygen-containing functional groups have been removed and GO is mainly reduced to graphene.⁴⁹ Via comparison, the Mn 2p (Fig. 3b), Co 2p (Fig. 3d), Ni 2p (Fig. 3f) and O 1s (Fig. 3h) XPS spectra of Sample-P show no remarkable differences with those of Sample-R. However, as for the C 1s XPS spectrum of Sample-P (Fig. 3j), a new O–C=O peak appears at 289.6 eV accompanied by the disappearance of C–O and C=O peaks.

Fig. 3 XPS spectra of Sample-R (a, c, e, g and i) and Sample-P (b, d, f, h and j).

The specific surface area and pore size distribution of Sample-R and Sample-P were further investigated by N₂ absorption/desorption measurements at 77 K (Fig. 4). According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the adsorption/desorption isotherms of the two samples with the hysteresis loops in the relative pressure (P/P₀) range of 0.8–1.0 can be indexed to the typical Type-IV profile, which is the typical characteristic of mesoporous materials. Based on the Brunauer–Emmett–Teller (BET) model, the specific surface area, pore volume and average pore diameter of Sample-R are 27.09 m² g⁻¹, 0.09 cm³ g⁻¹ and 6.54 nm, respectively, while Sample-P shows lower numerical values of 13.50 m² g⁻¹, 0.06 cm³ g⁻¹ and 3.82 nm, respectively. The larger specific surface area of Sample-R reveals the smaller particle size of Sample-R. Such a large surface area can enlarge the electrode–electrolyte interface and shorten the transmission paths of ions and electrons, thus facilitating electrochemical reactions.⁵⁰ The insets in Fig. 4 present the derived pore size distribution curves obtained by the BJH method, confirming that the pore size of Sample-R and Sample-P mainly distributes in the range of 20–40 nm. Such a porous structure is beneficial by promoting the diffusion of the electrolyte into active materials and accommodating the volume variation of electrodes during the cycling process, thus contributing to more effective lithium storage.⁵¹

Fig. 4 N₂ adsorption/desorption isotherms and pore size distribution curves (inset) at 77 K of Sample-R (a) and Sample-P (b).

Based on SEM (Fig. 5a–d) and TEM (Fig. 5k and n) observations, the primary units of Sample-R and Sample-P are square nanosheets with side length between 400 and 500 nm. In order to reduce the interfacial energy of the reaction system and improve the structural stability of product, the nanosheets furthermore stack layer by layer into irregular nano-layered nanocubes. As for Sample-R, the nanocubes are fixed on graphene layers uniformly. A single nanocube shows two approximately smooth surfaces, which are parallel to each other, and four vertical zigzag surfaces. Obviously, it is the smooth surface with larger superficial area interacting with graphene layers rather than the jagged surface, thus contributing to much stronger interaction between the nanocubes and the graphene layers. The nanostructure of Sample-R contributes to the large electrode–electrolyte interface and effectively shortens the transmission paths of ions and electrons, thus facilitating electrochemical reactions during the cycling process.⁵⁰ However, in the absence of graphene, the nanocubes of Sample-P aggregate into microspheres triggered by high surface energy. Therefore, the comparison suggests that graphene acts as a buffer to prevent the aggregation of Mn_xCo_yNi_zCO₃ nanocubes and furthermore maintains the nanostructure of Sample-R. The involved formation processes of Sample-R and Sample-P are illustrated in Fig. 6.

Fig. 5 (a and b) Low and high magnification SEM images of Sample-R. (c and d) Low and high magnification SEM images of Sample-P. (e–g) EDX spectra of Sample-R. (h–j) EDX spectra of Sample-P. (k–m) TEM image, HRTEM image and selected area FFT pattern of Sample-R. (n–p) TEM image, HRTEM image and selected area FFT pattern of Sample-P.

Fig. 6 Schematic illustration of the formation mechanism of Sample-R and Sample-P.

The elemental analysis mappings of Sample-R (Fig. 5e–g) and Sample-P (Fig. 5h–j) indicate that the Mn, Co and Ni elements distribute uniformly over the entire region. Energy dispersive X-ray (EDX) spectroscopy analysis reveals that the molar ratios of Mn, Co and Ni elements contained in Sample-R and Sample-P are about 1.00 : 0.24 : 0.25 and 1.00 : 0.24 : 0.24, respectively, which are quite different from the theoretical value of 1.00 : 0.24 : 0.48. This phenomenon reveals that Ni element is lost considerably during the synthesis process, which can be reasonably explained by the different solubility product constant (K_sp) values of NiCO₃ (1.4 × 10⁻⁷), MnCO₃ (2.2 × 10⁻¹¹) and CoCO₃ (1.4 × 10⁻¹³). Owing to the much higher K_sp value of NiCO₃, the Ni²⁺ ions cannot precipitate completely in solution.

The high-resolution transmission electron microscopy (HRTEM) images and the corresponding selected area fast Fourier transform (FFT) patterns of Sample-R (Fig. 5l and m) and Sample-P (Fig. 5o and p) reveal the terrific monocrystalline nature and perfect crystallinity of the samples. As for Sample-R, the [421̄] zone axis can be confirmed based on the (104) and (012) crystal planes with layer spacings of 0.28 and 0.37 nm, respectively. The interface between Mn_xCo_yNi_zCO₃ nanocubes and graphene layers can be clearly observed as marked in Fig. 5l. However, as for Sample-P, the (012) crystal plane cannot be clearly detected, which may be caused by the much lower diffraction intensity of the peak ascribed to the (012) crystal plane of Sample-P than that of Sample-R, as shown in the XRD patterns (Fig. 1a).

Electrochemical properties

The discharge/charge curves of Sample-R and Sample-P for the 1st, 2nd and 500th cycles at a constant current density of 100 mA g⁻¹ in the voltage range of 0.01–3.0 V (vs. Li/Li⁺) are presented in Fig. 7a and b, respectively. Sample-R and Sample-P deliver initial discharge/charge specific capacities of 1576/1304 mA h g⁻¹ and 1529/944 mA h g⁻¹ with initial coulombic efficiencies of 82.7% and 61.7%, respectively, which are about three times higher than the theoretical capacities based on the molar ratio of MnCO₃ (466 mA h g⁻¹), CoCO₃ (451 mA h g⁻¹) and NiCO₃ (451 mA h g⁻¹). This should be attributed to the formation of a solid electrolyte interphase (SEI) layer on the electrode caused by the decomposition of the electrolyte and reduction of C⁴⁺ to C⁰ or other lower valences triggered by the electrochemical catalysis effect based on the newly formed metal M (M = Mn, Co and Ni) nanoparticles.^19,52 The corresponding reactions are listed as follows:

MCO₃ + 2Li ↔ Li₂CO₃ + M (1)

Fig. 7 Discharge/charge curves of Sample-R (a) and Sample-P (b) for the 1st, 2nd and 500th cycles at a constant current density of 100 mA g⁻¹ between 0.01 and 3.0 V. (c) Discharge cycling performances of Sample-R and Sample-P at the current density of 100 mA g⁻¹ between 0.01 and 3.0 V. (d) Rate performances of Sample-R and Sample-P at current densities of 100, 300, 500, 1000 and 2000 mA g⁻¹ between 0.01 and 3.0 V. (e) Discharge cycling performance of Sample-R at the current density of 1000 mA g⁻¹ between 0.01 and 3.0 V. (f) Cyclic voltammograms for the first 5 cycles of Sample-R at the rate of 0.1 mV s⁻¹.

The 2nd discharge specific capacities of Sample-R and Sample-P are 1423 mA h g⁻¹ and 1004 mA h g⁻¹, corresponding to serious initial capacity losses of 9.7% and 34.3%, respectively, which is in relation to the formation of SEI layer during the initial cycling process. Eventually, Sample-R presents a 500th reversible discharge specific capacity of 1046 mA h g⁻¹ with a capacity retain rate of 73.5% from the 2nd cycle to the 500th cycle, demonstrating prominent superiority to Sample-P (517 mA h g⁻¹ after 500 cycles) and other related works (Table 1).^52–56

Table 1 Comparison of final discharge specific capacities between previous literature and this work

	Anode material	Current density (mA g^−1)	Voltage range (V)	Cycle times	Final discharge specific capacity (mA h g^−1)
J. Mater. Chem. A, 2014, 2, 14947–14956	Mn0.54Ni0.13Co0.13(CO3)0.8	250	0.01–3	100	450
J. Power Sources, 2011, 196, 2863–2866	MnCO3	—	0.01–3	25	450
Mater. Lett., 2013, 111, 165–168	MnCO3	100	0.01–3	100	647
ACS Appl. Mater. Interfaces, 2014, 6, 12346–12352	CoCO3	500	0.01–3	40	469
Inorg. Chem., 2012, 51, 5554–5560	MnCO3	466	0.01–3	80	500
Our work	MnxCoyNizCO3/graphene	100	0.01–3	500	1046

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Cycling performances of Sample-R and Sample-P at a constant current density of 100 mA g⁻¹ in the voltage range of 0.01–3.0 V (vs. Li/Li⁺) are shown in Fig. 7c. Gratifyingly, Sample-R demonstrates long-term cycling stability with the discharge specific capacity decreasing slowly from 1576 mA h g⁻¹ in the 1st cycle to 1046 mA h g⁻¹ in the 500th cycle. However, for Sample-P, the discharge specific capacity decays sharply from 1529 mA h g⁻¹ in the 1st cycle to 516 mA h g⁻¹ in the 29th cycle and then increases gradually to 851 mA h g⁻¹ in the 142th cycle. The phenomenon of capacity increase during the cycling process is related to the reversible formation/dissolution of the gel-like polymeric layer during deep discharge reactions and the larger specific surface area caused by the structural fragmentation during the cycling process.⁵⁷ Eventually, the 500th reversible discharge specific capacity of Sample-P exhibits a low value of 517 mA h g⁻¹, which should be attributed to the poor electronic conductivity of the sample and the large volume changes of the electrode during the cycling process.^19–21

The rate performances of Sample-R and Sample-P were tested with the current density increasing from 100 mA g⁻¹ to 2000 mA g⁻¹ in the voltage range of 0.01–3.0 V (vs. Li/Li⁺) to further analyze their cycling performance at higher current densities. As shown in Fig. 7d, Sample-R demonstrates excellent rate performance with high reversible discharge specific capacities of 1166, 1090, 1009, 803 and 508 mA h g⁻¹ at current densities of 100, 300, 500, 1000 and 2000 mA g⁻¹, respectively. Subsequently, when the current density declines to 100 mA g⁻¹ after an additional 20 cycles, the 70th reversible discharge specific capacity shows a much higher value of 1288 mA h g⁻¹ than the initial 10th capacity at 100 mA g⁻¹ (1166 mA h g⁻¹), which is caused by the activation of the electrode at higher current densities. Moreover, even at a continuous high current density of 1000 mA h g⁻¹, Sample-R delivers a satisfying 500th reversible discharge specific capacity of 711 mA h g⁻¹ (Fig. 7e). However, for Sample-P, the reversible discharge specific capacities are only 709, 528, 460, 245 and 131 mA h g⁻¹ at current densities of 100, 300, 500, 1000 and 2000 mA g⁻¹, respectively, indicating that the rate performance of Sample-P lags far behind that of Sample-R.

Obviously, graphene dominantly contributes to the perfect long-term cycling stability and excellent rate performance of Sample-R, which plays an important role in keeping the nanostructure of the sample, increasing the electronic conductivity of the sample, accommodating the volume changes of the electrode and preventing aggregation of particles in the electrode during the cycling process, thus offsetting the defects of transition metal carbonates.^25–33 A schematic illustrating the synergistic effect between the existence of graphene in Sample-R and the electrochemical performance of Sample-R is shown in Fig. 8.

Fig. 8 Schematic illustration of the synergistic effect between the existence of graphene in Sample-R and the electrochemical performances of Sample-R.

Fig. 7f presents the initial 5 cyclic voltammograms (CVs) curves of Sample-R at a scan rate of 0.1 mV s⁻¹ in the potential range of 0.01–3.0 V vs. Li/Li⁺ to investigate the lithium storage mechanism during the cycling process. During the first cathodic scan, the strong cathodic peak at 0.40 V is caused by the reduction of M²⁺ (M = Mn, Co and Ni) to metallic state M⁰. In the subsequent anodic scan, there are three broad peaks appearing at 1.28 V, 1.63 V and 2.12 V, corresponding to the oxidation of Mn⁰ to Mn²⁺, Co⁰ to Co²⁺ and Ni⁰ to Ni²⁺, respectively. After the first cathodic and anodic scan, the initial Mn_xCo_yNi_zCO₃ is separated into individual MnCO₃, CoCO₃ and NiCO₃, thus resulting in obvious differences between the subsequent 2nd–5th and the 1st CV curves based on the different potentials of the Mn²⁺/Mn⁰, Co²⁺/Co⁰ and Ni²⁺/Ni⁰ redox pairs. Moreover, the subsequent 2nd-5th curves are almost identical, confirming the perfect electrochemical reversibility of Sample-R.⁵² In the 2nd–5th curves, the cathodic peak at 0.39 V is ascribed to the reduction of Mn²⁺ to Mn⁰ and the other one at 1.53 V is in relation to the synchronous reduction of Ni²⁺ to Ni⁰ and Co²⁺ to Co⁰ via a combined effect. Owing to the activation of the electrode, the anodic peaks shift slightly towards the right appearing at 1.31 V, 1.70 V and 2.21 V, respectively.

Fig. 9a is the EIS spectra of Sample-R and Sample-P electrodes before cycling with an inset showing the Randles equivalent circuit to analyze impedance data. The ohmic resistance corresponding to the electrolyte resistance R_s in the equivalent circuit leads to the intercept of the semicircle in the high frequency range. The semicircle in the medium frequency range reflects the charge transfer between the electrode and the electrolyte, according to the parallel circuit elements of the double layer capacitance C_dl and charge transfer resistance R_ct in the equivalent circuit. The inclined line in the low frequency range is caused by the diffusion of lithium ions in electrode, which equals the Warburg impedance Z_w in the equivalent circuit.⁵⁰ Based on Fig. 9a and the non-linear least squares method, the R_ct numerical values of Sample-R and Sample-P are 78 Ω and 113 Ω, respectively, indicating that the existence of graphene successfully improves the electronic conductivity of Sample-R. This result is in accordance with the electrochemical performances of the samples.

Fig. 9 (a) Electrochemical impedance spectra (EIS) of Sample-R and Sample-P with the inset of the equivalent circuit. (b) Linear fitting of Warburg impedance of Sample-R and Sample-P.

The linear fitting of Warburg impedance is shown in Fig. 9b. The Warburg coefficient A_w related to Z′ can be estimated from the following equation:

Z′ = R_s + R_ct + A_wω^−1/2 (3)

In this equation, A_w is the slope of ω^−1/2 versus the Z′ line at low frequency and ω is the angular frequency of the alternating current. The Li⁺ diffusion coefficient D_Li⁺ of the electrode can be obtained from the following equation:In this equation, V_m is the molar volume of the material, F is the Faraday constant, S is the surface area of the electrode and (dE)/(dx) is the slope of open-circuit potential E versus mobile Li⁺ concentration x at each x value.⁵⁰ For the model test cells, the above-mentioned parameters in eqn (4) are constant and thus the D_Li⁺ is in direct proportion to (1/A_w)². Via calculating, the A_w numerical values of Sample-R and Sample-P are 50.0 Ω s^−1/2 and 107.2 Ω s^−1/2, respectively, revealing the much higher D_Li⁺ numerical value and better ionic conductivity of Sample-R based on the existence of graphene. This result is consistent with the electrochemical performances of the samples.

Fig. 10 presents the XRD patterns and SEM images of the prepared Sample-R and Sample-P electrodes after 10 cycles at a constant current density of 100 mA g⁻¹ in the voltage range of 0.01–3.0 V (vs. Li/Li⁺) to further analyze the crystal structure and morphology changes of the samples after the cycling process. Based on Fig. 10a and b, the intensities of the diffraction peaks and nano-layered structure of Sample-R are well retained after cycling. However, the intensities of the diffraction peaks belonging to Sample-P become obviously lower after cycling, and especially the diffraction intensities of the peaks belonging to (104) and (024) crystal planes decline seriously. Meanwhile, the structure of the microspheres composing Sample-P is severely damaged after cycling (Fig. 10c), which should be ascribed to the large volume changes of the electrode caused by the insertion and extraction of Li⁺ ions during the cycling process. Those results further indicate that the existence of graphene acts as a buffer to effectively keep the crystal structure and morphology of the Mn_xCo_yNi_zCO₃ particles during the cycling process.

Fig. 10 XRD patterns (a) and SEM images (b and c) of the prepared Sample-R and Sample-P electrodes after 10 cycles at the constant current density of 100 mA g⁻¹ in the voltage range of 0.01–3.0 V (vs. Li/Li⁺).

Conclusion

In summary, the nano-layered Mn_xCo_yNi_zCO₃/graphene composite and micro-spherical Mn_xCo_yNi_zCO₃ are synthesized by a facile hydrothermal method in the presence and absence of graphene, respectively. On the one hand, the existence of graphene keeps the nanostructure of Mn_xCo_yNi_zCO₃, contributing to the large electrode–electrolyte interface and effectively shortening the transmission paths of the ions and electrons. On the other hand, graphene enhances the electronic conductivity of Mn_xCo_yNi_zCO₃, prevents the aggregation of particles and accommodates the volume changes of the electrode during the cycling process. Thus, the prepared Mn_xCo_yNi_zCO₃/graphene composite demonstrates much better cycling stability and rate performance than those of Mn_xCo_yNi_zCO₃. In a word, addition of graphene is an effective way to solve the defects of transition metal carbonates as anode materials for lithium-ion batteries.

Acknowledgements

This work was supported by the Ministry of Science and Technology of China (973 Project No. 2013CB932901) and the National Natural Science Foundation of China (No. 51672050, 11274066, 51172047, 51102050, U1330118). This project was sponsored by the Shanghai Pujiang Program and the “Shu Guang” project of the Shanghai Municipal Education Commission and the Shanghai Education Development Foundation (09SG01).

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