Graphene-supported binary active Mn_0.25Co_0.75O solid solution derived from a CoMn-layered double hydroxide precursor for highly improved lithium storage

Abstract

Dispersion of multiple chemically active components strongly affects the electrochemical performances of electrode nanomaterials for lithium-ion batteries. We herein describe highly improved lithium storage of a graphene-supported binary active solid solution (Mn_0.25Co_0.75O) derived from CoMn-layered double hydroxide/graphene oxide precursor (CoMn-LDH/GO). Ex situ X-ray diffraction characterization clarifies the topotactic transformation from the CoMn-LDH/GO precursor to the resulting Mn_0.25Co_0.75O solid solution with increasing temperatures. The electrochemical test shows that the Mn_0.25Co_0.75O solid-solution electrode is able to exhibit highly improved electrochemical performances, which are superior to those of the electrodes of individual CoO/G, MnO/G, and the mixture (mMnO + CoO/G). The reversible capacity of the Mn_0.25Co_0.75O electrode reaches 980 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹, and especially up to 1087 mA h g⁻¹ after 1300 cycles at a high current density of 2 A g⁻¹. TEM observations and Nyquist plots provide information on the morphological preservation of the solid-solution nanoplatelets consisting of small nanoparticles observed after the super-long cycling, and the low charge transfer resistance to underlie the improvements, respectively. Our LDH precursor-based protocol may be extended to prepare other multiple-component well-dispersed metal oxides or even sulfides, and thus provide a new strategy for construction of high-performance electrodes for energy storage.

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Introduction

Layered double hydroxides (LDHs), also known as hydrotalcite-like anionic clays,¹ have attracted increasing interest with various potential applications, including heterogeneous catalysts,² electrocatalysts for water splitting,³ and superconductive/magnetic devices.⁴ LDHs have a general formula expressed typically by [M^II_1−xM^III_x(OH)₂]^x+(Aⁿ⁻)_x/n·yH₂O, where the cations (M^II and M^III) are well-orderly arranged within a brucite-like layer, the anion (Aⁿ⁻) is intercalated between the hydrated interlayer galleries, and the value (x) of the M^II/M^III molar ratio ranges typically between 2 and 4. The unique versatility of LDHs lies in tuning the types of metal cations and the intercalated anions, the M^II/M^III molar ratios, and cation distribution over a wide range, which has especially enabled the LDHs with a great potential in designing and fabricating LDH-derived nanomaterials for lithium-ion batteries (LIBs).^5–7

Previous studies of LDHs have shown that with the aid of facilely tuning LDH layer metallic cations and intercalated anions, various transitional metallic oxides (TMOs) anode nanomaterials for LIBs were prepared by direct thermal decomposition of different LDH precursors. The TMOs studied range from an initial pure NiFe₂O₄ spinel derived from NiFe²⁺Fe³⁺-LDH precursor,⁸ to TMO/spinel composites, such as ZnO/ZnAl₂O₄ nanoarray film derived from ZnAl-LDH precursor,⁹ and ZnO/ZnFe₂O₄ nanoparticle from ZnFe-LDH precursor.¹⁰ In our previous study,¹¹ we have demonstrated that the CoO/CoFe₂O₄ prepared from CoFe-LDH single-source precursor exhibited the greatly enhanced electrochemical performances compared with the pristine individuals and their mixture, by virtue of the bi-active well-dispersed, crystal-phase heterostructured nanodomains. The above composites were able to exhibit highly enhanced reversible capacities compared with the individuals, however, with the rapid decay in reversible capacity observed commonly during cycling.

Further improvement strategy is to prepare the LDH precursor-derived TMO composites via introduction of conductive carbon supports.^7,12 Graphene-encapsulated bi-active CoO/CoFe₂O₄ composite,¹² obtained by thermal decomposition of graphene oxide-coated CoFe-LDH precursor, showed a reversible capacity of 600 mA h g⁻¹ at 100 mA g⁻¹ after 80 cycles, much higher than that of CoO/CoFe₂O₄. However, there are still a large space for further improving the reversible capacity and cycling performance. In addition, from previous studies it is well-known that the ratio and mutual dispersion of multicomponent TMOs strongly affect their electrochemical properties.^11,13,14 Inspired by these studies, we expect that the atomic-level-dispersion multiple-component TMOs could exhibit greatly boosted electrochemical performances compared with the individuals and their mixture, when the nanodomain-sized dispersion is downsized to the atomic scale.

In this present study, we describe a simple preparation of graphene-supported Mn_0.25Co_0.75O solid solution (Mn_0.25Co_0.75O/G) derived from CoMn-LDH/GO precursor obtained readily by a scalable method (Scheme 1). The CoMn-LDH/GO is chosen as a precursor, considering the comparable size and chemical nature of the Mn²⁺ and Co²⁺ cations and especially the ease of miscibility of the MnO/CoO solid solution. The as-prepared Mn_0.25Co_0.75O/G may bundle the following merits: (i) excellent distribution of bi-active CoO and MnO at the atomic scale to synergistically enhance lithium storage, (ii) the minimized cost and toxicity of CoO by partially replacing CoO with MnO, and (iii) the greatly improved conductivity contributed from the supporting graphene, as well as (iv) the simple synthesis protocol of the scalably prepared CoMn-LDH/GO precursor and subsequent direct thermal decomposition. The Mn_0.25Co_0.75O/G solid solution is expected to exhibit a highly improved reversible capacity, good cycling stability, and rate capability.

Scheme 1 Schematic illustration of Mn_0.25Co_0.75O/G derived from CoMn-LDH/GO precursor, and the post-cycled morphology.

Experimental section

Preparation of Mn_0.25Co_0.75O/G

All the chemicals were analytical grade, and used without further purification. The graphite oxide (GO) support was prepared by chemical oxidation of crystalline flake graphite according to a modified Hummers' method.¹⁵

CoMn-LDH/GO precursor was prepared according to our previous study of a scalable method involving separate nucleation and aging step (SNAS).¹⁴ In brief, a salt solution was obtained by dissolving Co(NO₃)₂·6H₂O and Mn(NO₃)₂·4H₂O in a Co/Mn molar ratio of 3 : 1 in freshly deionized water with a total cation concentration of 0.2 M, and then the synthesized GO was dispersed in the above salt solution with a Co(NO₃)₂·6H₂O/GO mass ratio of 40 : 1. An aqueous base solution of NaOH and Na₂CO₃ was prepared with the concentrations of [CO₃²⁻] = 2.0[Mn²⁺], [OH⁻] = 1.6([Co²⁺] + [Mn²⁺]). Equal volumes of the salt/GO mixture and the base solution were simultaneously added to a colloid mill reactor with a rotor speed of 3000 rpm, and mixed for 3 min. The resulting suspension was collected and aged at 30 °C for 5 h, then washed thoroughly with deionized water by centrifugation, and finally dried in vacuum at 40 °C overnight to obtain the CoMn-LDH/GO precursor. In the above aging process, air was bubbled throughout the entire addition period for the oxidation of Mn²⁺.

The resulting Mn_0.25Co_0.75O/G composite was prepared by thermal decomposition of CoMn-LDH/GO precursor in a quartz tube furnace at 600 °C for 2 h with a temperature ramping rate of 2 °C min⁻¹ in Ar. The ex situ XRD patterns were recorded for the phase-transformation process of the CoMn-LDH/GO precursors with increasing temperatures from 350 °C to 600 °C.

For comparison, MnO/G, CoO/G, and their mixture (denoted as mMnO + CoO/G) were prepared by physically mixing commercial MnO, CoO, MnO + CoO (with the identical amounts and in a Co/Mn molar ratio of 3 : 1) and graphene in the mass ratio of 40 : 1, respectively.

Characterization

Powder X-ray diffraction (XRD) data were measured on a Rigaku XRD-6000 powder diffractometer with Cu-Kα radiation (40 kV, 30 mA, λ = 0.1542 nm). High-resolution transmission electron microscopy (HRTEM) characterization was performed through a JEOL JEM-2100 electron microscope equipped with EDX at an accelerating voltage of 200 kV. Samples were dispersed in ethanol and then deposited onto a copper microgrid. Raman spectroscopy was recorded on a confocal Raman spectrometer (Renishaw RM2000) with a 514 nm excitation laser (laser spot size of 0.5 μm) operated at a low power level (∼2 mW). Samples were drop-cast onto a silicon substrate for Raman measurements. The X-ray photoelectron spectroscopy (XPS) analyses were conducted through a Kratos Axis ULTRA X-ray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer. Elemental analysis was employed for metal ions using a Shimadzu inductively coupled plasma emission spectrometer (ICP-ES).

Electrochemical measurements

Electrochemical measurement was performed with Swagelok-type cells, which were assembled in an argon-filled glove box. In the case of preparing working electrodes, a mixture of active material, super-P acetylene black, and poly(vinyl difluoride) (PVDF) was prepared in a weight ratio of 70 : 20 : 10, and then pasted on a Cu foil. Lithium foil was used as the counter electrode. A glass fiber (GF/D) from Whatman was used as a separator. The electrolyte consisted of a solution of 1 M LiPF₆ salt in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) (1 : 1 : 1 in wt%) plus 2 wt% vinylene carbonate (VC). Galvanostatic cycling of the assembled cells was applied with LAND CT2100A cell-testing system in the voltage range between 0.01 V and 3.0 V (vs. Li⁺/Li), as reported previously.¹¹ After 1300 cycles at 2 A g⁻¹, the cells were disassembled in the glove box, and the working electrodes were taken out, washed with anhydrous N-methyl-2-pyrrolidone (NMP), and dried in vacuum for 6 h for HRTEM visualization. Cyclic voltammograms (CVs) were recorded on an electrochemistry work station (CHI 760E) at a scan rate of 0.1 mV s⁻¹ in the voltage range between 0 and 3.0 V (vs. Li⁺/Li). Electrochemical impedance spectra (EIS) were measured over the frequency range from 100 kHz to 0.1 Hz at an open circuit potential, with a Parstat 2273 advanced electrochemical system.

Results and discussions

The CoMn-LDH/GO precursor was prepared using a readily scalable SNAS method,¹⁶ owing to the availability of LDH nanoplatelets with narrow size distributions. As expected, the CoMn-LDH/GO precursor shows the characteristic hydrotalcite-like XRD patterns (Fig. S1a in the ESI†), including the basal reflection peaks of (003), (006), and (009)/(012), as well as the non-basal reflection peaks of (110) and (113).¹⁷

Thermal decomposition of the CoMn-LDH/GO precursor was carried out under the Ar atmosphere, and the ex situ XRD patterns were recorded for the phase-transformation process of the CoMn-LDH/GO precursor from 350 °C to 600 °C. Fig. 1a shows that after reaching 325 °C, the hydrotalcite-like feature disappears due to the loss of interlayer water of LDHs and the collapse of the brucite-like layers.¹⁸ Upon further heating to 350 °C, the prominent reflection peaks of CoO phase appears. The preferred formation of CoO phase is indeed consistent with our previous studies showing the preferential nucleation of CoO phase derived from CoFe-LDH precursor,¹¹ and ZnO phase from ZnAl-LDH precursor.¹⁹ With the temperature further increasing to 450 and 550 °C, the intensity of (111) reflection peak is found to increase remarkably, accompanied with the appearance of the (200) and (220) planes of the face-centered cubic (fcc) CoO. Perusal of the above XRD patterns observed over the rang from 350, through 400, 450, 500, to 550 °C, shows that no characteristic diffraction peaks of MnO phase are observed (Fig. 1a). Especially, no departure is observed for all the reflection peaks, with respect to the counterparts of the pristine CoO (JCPDS 43-1004). However, we do note that a discriminable shift is observed for all the reflection peaks of the product calcined at 600 °C when compared with those of the pristine CoO (Fig. 1a). We interpret this departure resolved at this stage to reflect the Mn doping into the CoO phase, resulting in the formation of the Mn-doped CoO solid solution during the carbothermal process in Ar atmosphere. This transformation process, i.e., from the preferentially formed CoO to the resulting Mn-doped CoO, is well consistent with our previous report¹⁹ of the conversion mechanism from ZnAl-LDH precursor to ZnO/ZnAl₂O₄ albeit under the different calcination condition in air. The latter study has unambiguously clarified the initial formation of ZnO at a low temperature, the subsequent doping of Al into ZnO to yield ZnAl₂O₄ embedded in ZnO matrix, and eventual aggregation of the ZnO/ZnAl₂O₄ phases with the increasing temperature.¹⁹ In our case, the GO-supported CoMn-LDH was chosen as precursor to utilize the carbothermal process in the presence of GO at the moderate temperature (600 °C) under the Ar atmosphere and thereby to avoid the formation of CoMn₂O₄. Indeed, no CoMn₂O₄ is found in our case. However, pervious studies have shown that the CoMn₂O₄ phase was typically formed by calcining CoMn-containing hydroxide precursors under the inert atmosphere.^20,21

Fig. 1 (a) Ex situ XRD patterns of calcined products obtained by calcining CoMn-LDH/GO precursors from 350 °C to 600 °C, (b) comparison of XRD patterns between the mMnO + CoO/G mixture (black line) and the Mn_0.25Co_0.75O/G composite obtained at 600 °C (red line), as well as (c) TEM and (d) HRTEM images of Mn_0.25Co_0.75O/G composite.

We also compared the XRD patterns between the Mn_0.25Co_0.75O/G composite and the mMnO + CoO/G mixture. Obviously, no shift of all the reflection peaks is visible to the mMnO + CoO/G mixture, with respect to the individual CoO and MnO (Fig. 1b). This clearly manifests that the XRD patterns of the mMnO + CoO/G mixture is actually a combination of those of both individuals. By contrast, all the reflection peaks of the Mn_0.25Co_0.75O/G composite are clearly resolved to shift slightly to lower angels compared with pure CoO. This shift is in good agreement with the previous studies of Mn_xCo_1−xO and Fe_1.7Mn_0.3O₃ solid solutions,^22,23 which is attributed to the doping of Mn²⁺ with a slightly larger ionic radius (0.083 nm) than that of Co²⁺ (0.0745 nm). The agreement strongly justifies the formation of the Mn_0.25Co_0.75O solid solution in our case, and also clearly rules out the possibility of XRD instrumental error.

Element analysis, obtained by using ICP-ES, gives a Co/Mn molar ratio of 2.96, quite close to that (3 : 1) of the starting materials and also yielding the very formula of the Mn_0.25Co_0.75O solid solution. In addition, a weak and broad peak in the range between 20° and 28° is assigned to the (002) plane refraction of graphitic structure.²⁴ Therefore, we can think that the graphene-supported Mn_0.25Co_0.75O solid solution is successfully transformed from the CoMn-LDH/GO precursor in the presence of GO at the moderate calcination temperature of 600 °C under the Ar atmosphere.

The morphology of the Mn_0.25Co_0.75O/G composite was visualized by TEM. Fig. 1c shows that the Mn_0.25Co_0.75O nanoparticles are anchored on the underlying graphene substrate. Elemental mappings of TEM/EDS images reveal the co-existence and uniform distribution of Co, Mn, and O, as well as C elements (Fig. S2 in the ESI†). HRTEM visualization reveals that the spacing of the well-defined lattice fringes is determined to be 0.25 nm (Fig. 1d), corresponding to the (111) crystalline planes of both CoO and MnO revealed by the above XRD results.

The chemical states of the species of the Mn_0.25Co_0.75O/G composite were examined by using XPS technique. The survey XPS spectrum corroborates the co-existence of Co, Mn, and O, as well as C (Fig. 2a). The XPS peaks centered at 780.8 and 786.4, as well as 796.6 eV and 803.6 eV, can be attributed to Co 2p_3/2 and 2p_1/2, respectively (Fig. 2b), indicative of the Co²⁺ oxidation state of the solid solution.^24,25 The signals centred at 641.3 eV and 653.0 eV can be ascribed to Mn 2p_3/2 and Mn 2p_1/2, respectively (Fig. 2c), conforming the Mn²⁺ oxidation state of the solid solution.²⁶ Indeed, the co-bivalent oxidation states of Co²⁺ and Mn²⁺ support the doping of Mn²⁺ into the CoO phase, as revealed by the above XRD results. Two O 1s peaks are observed at 530.1 and 532.2 eV (Fig. 2d), which indicate the surface lattice oxygen (O²⁻) of both the Mn_0.25Co_0.75O and the residual O²⁻ species bonded with C atoms of the supporting graphene.²⁷ In addition, the strong C 1s peak centred at 284.9 eV corresponds to the graphitic carbon of the graphene support, and the co-existence of the weak peaks at 286.2, 287.9, and 288.7 eV are assigned to the oxygenated carbons of C–OH, C–O–C, and C=O, respectively (Fig. 2e), indicative of the de-oxygenation process from the initial GO to the resulting graphene with the aid of the carbothermal reduction.¹⁸

Fig. 2 XPS spectra for the Mn_0.25Co_0.75O/G composite: (a) full scan survey, (b) Co 2p, and (c) Mn 2p, (d) O 1s, and (e) C 1s spectra, as well as (f) Raman spectra of CoMn-LDH/GO (black line) and Mn_0.25Co_0.75O/G hybrid (red line).

Raman spectroscopy was utilized to examine the graphitic structure of the Mn_0.25Co_0.75O/G composite. The intensity ratio (I_D/I_G) of Mn_0.25Co_0.75O/G is determined to be 1.13, much higher than that (0.94) of the CoMn-LDH/GO precursor (Fig. 2f). The high I_D/I_G value strongly suggests the decrease in the average size of the sp² domains and the effective removal of the oxygen functional groups upon reduction of the GO,²⁴ well consistent with the above XPS result.

By combining the above-mentioned results of ex situ XRD patterns, HRTEM observations, and XPS technique, as well as Raman spectra, we therefore conclude that the transformation is achieved from the pre-designed CoMn-LDH/GO precursor to the resulting Mn_0.25Co_0.75O/G solid solution. From previous studies of solid solutions with the high electrochemical performances as anode nanomaterials for LIBs,^23,28 our bi-active Mn_0.25Co_0.75O/G solid solution with the unique dispersion at the atomic scale could be able to synergistically enhance the reversible specific capacity and cycling stability.

We first recorded CV curves on the electrodes of the Mn_0.25Co_0.75O/G composite, as well as the mMnO + CoO/G mixture, pure CoO and MnO for comparison. In the case of the mMnO + CoO/G electrode, two cathodic peaks were observed at 0.6 V and 1.0 V in the first cathodic scan (Fig. 3a). The former can be assigned to the conversion reaction²⁶ from Mn²⁺ to Mn⁰, and the latter to the pulverization of the CoO nanoparticles and the formation of solid electrolyte interface (SEI) film.²⁴ During the anodic scan, a weak anodic peak at 1.3 V and a strong anodic peak at 2.1 V are observed, which can be attributed to the oxidization reactions of Mn⁰ to MnO and Co⁰ to CoO, respectively. After the second and third cycles, two cathodic peaks at 1.03 and 1.35 V are quite close to those (1.1 and 1.35 V) of the pure CoO electrode (Fig. S3a†), which can be ascribed to the conversion reaction^25,29 from CoO to Co. It is interesting to note that the initial reduction peak at 0.6 V is found to shift to 0.42 V, which can be assigned to the redox conversion reaction³⁰ of MnO. Such a shift is also observed for the pure MnO/G after the second and third cycles (Fig. S3b in the ESI†). Apparently, this similarity suggests that the CV behaviour of the mMnO + CoO/G electrode is actually a superimposition of those of the pristine CoO/G and MnO/G electrodes, similar to the above-observed combination of the individual XRD patterns. In the case of the Mn_0.25Co_0.75O/G electrode, however, we note that two strong reduction peaks at 0.65 and 0.92 V are visible in the first cathodic scan, both of which are similar to those of the CoO/G electrode. After the second and third cycles, those two reduction peaks of the Mn_0.25Co_0.75O/G electrode are found to shift to 0.9 and 1.3 V, respectively (Fig. 3a). Although the values are quite close to the counterparts (1.1 and 1.35 V) of the CoO/G electrode, the Mn_0.25Co_0.75O/G electrode indeed shows distinguishable shifts to lower potential values compared with the pristine CoO/G electrode, owing to the doping^23,31 of Mn²⁺. Both the similarity and difference between the Mn_0.25Co_0.75O/G and CoO/G electrodes clearly reveal that the CV behaviour of Mn_0.25Co_0.75O/G electrode is distinctly different from the superimposed individual behaviours of the mMnO + CoO mixture. In addition, a good overlap was resolved for the Mn_0.25Co_0.75O/G electrode through the subsequent two cycles, which is also observed for the CoO electrode (Fig. S3a†). However, this overlap is not the case for the mMnO + CoO mixture electrode, possibly implying the improved cycling stability of the Mn_0.25Co_0.75O solid solution used as anode nanomaterial for LIBs.

Fig. 3 (a) The first three cyclic voltammogram curves of Mn_0.25Co_0.75O/G (solid line) and mMnO + CoO/G (dot line) electrodes between 0 and 3.0 V at the scan rate of 0.1 mV s⁻¹. (b) Rate capabilities of the electrode of Mn_0.25Co_0.75O/G, mMnO + CoO/G, CoO/G, and MnO/G from 100, through 200, 400, 800, 1000 and 2000, to 100 mA g⁻¹. (c) Cycling performance at the current of 100 mA g⁻¹ for those four electrodes. (d) Super-long cycling behaviour of Mn_0.25Co_0.75O/G electrode at the current of 2 A g⁻¹.

We then carried out galvanostatic discharge/charge test on those four electrodes between 0.01 and 3 V at 100 mA g⁻¹. The initial discharge and charge capacities of the Mn_0.25Co_0.75O/G electrode are 1938 and 960 mA h g⁻¹, respectively, giving rise to a low coulombic efficiency of 49.5% (Fig. S4a in the ESI†). By comparing all the electrodes, we can see that the initial charge capacity of the Mn_0.25Co_0.75O/G electrode is much higher than those of the other three electrodes, which are 860, 867, and 751 mA h g⁻¹ for the mMnO + CoO/G, CoO/G, and MnO/G, respectively (Fig. S4 in the ESI†). After the subsequent discharge/charge cycles from the 2nd, 5th, and 10th cycles, a good overlap is clearly visible for the Mn_0.25Co_0.75O/G electrode (Fig. S4a†). This overlap is well consistent with the above-observed CV behaviours, reflecting a possibility of possessing good cycling stability.

We further examined the cycling stability of those four electrodes at the same current density of 100 mA g⁻¹ (Fig. 3c). The CoO/G and MnO/G electrodes show approximate reversible capacities of 607 and 581 mA h g⁻¹ for after 100 cycles, respectively. The mMnO + CoO/G electrode exhibits a reversible capacity of 787 mA h g⁻¹ for after 90 cycles. The slightly improved reversible capacity might be ascribed to the synergistic lithium storage of bi-active components of the mMnO + CoO/G mixture. The Mn_0.25Co_0.75O/G electrode, however, is distinctly different in reversible capacity from those other tree electrodes. The reversible capacity of the former is 980 mA h g⁻¹ after completion of the 100^th cycling, greatly higher than those of the other three electrodes.

We also evaluated the rate capabilities of all the electrodes. We notice that in the case of those three electrodes of mMnO + CoO/G mixture, CoO/G, and MnO/G, all exhibit much lower rate capacities in comparison with the Mn_0.25Co_0.75O/G electrode at the corresponding current densities (Fig. 3b). When the current density returns to 100 mA g⁻¹, the rate capacities of those three electrodes are only close to or even much lower than their own initial counterparts at the same current density of 100 mA g⁻¹. However, the Mn_0.25Co_0.75O/G electrode is able to exhibit the rate capabilities of 1080, 985, 889, 800, 712, 601 mA h g⁻¹ at the current densities of 100, 200, 400, 800, 1000, and 2000 mA g⁻¹, respectively. When the current density recovers to 100 mA g⁻¹, the reversible specific capacity is able to return to 1080 mA h g⁻¹, clearly indicating that the Mn_0.25Co_0.75O/G electrode remains very stable during the extended rate cycling processes.

We also examined the long-term cycling performance of the Mn_0.25Co_0.75O/G electrode. Fig. 3d shows that the Mn_0.25Co_0.75O/G electrode delivers an initial charge capacity of 743 mA h g⁻¹ at a high current density of 2 A g⁻¹, followed by the fading of reversible capacities before the 214^th cycling, and then by gradually increasing reversible capacities. The reversible capacity eventually reaches 1087 mA h g⁻¹ after completion of 1300 cycles. Such a trend of decrease-then-increase reversible capacities was also reported to the carbon-supported metal oxides during long-term cycles.^32–34 Typically, the trend of electrochemical behaviors has been recognized to correlate well to the following possibilities: (i) the formation of stable SEI film^32,33, (ii) the pulverization of active components,^32,34 and (iii) the good preservation of the overall morphology of active components. To support the possibilities, we carried out TEM visualization on the Mn_0.25Co_0.75O electrode after the 1300^th charge process at 2 A g⁻¹. Fig. 4a shows that the post-cycled Mn_0.25Co_0.75O nanoplatelets are pulverized, which are composed of much smaller and uniformly dispersed nanoparticles than the Mn_0.25Co_0.75O nanoplatelets (as marked by yellow dotted circles in Fig. 4b, and also illustrated in Scheme 1). However, it is interesting to note that the overall morphology of the nanoplatelet-like nanostructure is still preserved. From the early studies,^35,36 it is well-known that the small nanoparticles upon pulverization could increase the surface area of the electrode and release more active sites for lithium storage, and further can increasingly improve the reversible capacity and cycling stability. Quantitative analysis shows that the mean dimensional size of the small nanoparticles is determined to be 3.5 nm by Gaussian fitting the numbers obtained by manually measuring the dimension sizes of >70 nanoparticles from HRTEM images, much smaller than that (ca.54.5 nm) of the pre-cycled Mn_0.25Co_0.75O nanoparticles (Fig. 4c and d). HRTEM further reveals that the small nanoplatelets are crystalline. The spacing of the well-defined lattice fringes is determined to be 0.25 nm, corresponding to the (111) crystalline plane of the Mn_0.25Co_0.75O phase, well consistent with the charge process, viz., the conversion reaction from Co⁰ to CoO and Mn⁰ to MnO after the charge process.

Fig. 4 (a and b) HRTEM images of the Mn_0.25Co_0.75O/G nanoparticles after 1300 cycles at a current of 2 A g⁻¹, and comparison of particle dimensional sizes between (c) the Mn_0.25Co_0.75O/G nanoplatelets before the cycling, and (d) the small nanoparticles observed after 1300 cycles.

By perusal of the above-mentioned remarkable differences in CV behavior, and especially reversible capacity, cycling stability, as well as rate capability, we can conclude that the Mn_0.25Co_0.75O/G electrode, with the binary active components well dispersed at the atomic level, is indeed able to exhibit the greatly enhanced electrochemical performances in comparison with the electrodes of CoO/G, MnO/G, and their physical mixture. In addition, we compared the reversible capacities between our Mn_0.25Co_0.75O/G and CoO- or CoMn-containing TMOs reported previously.^23,37–41 Table S1 (in the ESI†) shows that the specific capacities of our Mn_0.25Co_0.75O/G electrode are comparable to those of the electrodes reported early.

We attribute the great enhancement predominantly to the excellent dispersion of bi-active synergistic components of the solid solution at the atomic scale. From a previous study of NMR spectroscopy, it is well recognized that metal cations are well-orderedly arranged within the LDH crystalline layer.⁴² High-temperature thermal decomposition of the LDH precursors enables the resulting TMOs with good dispersion. Such a good dispersion in the form of nanodomain, as reported in the previous studies of our and others' laboratories,^11,43–45 has been demonstrated to facilitate the enhancement of the electrochemical performances of the multiple-component compared with each individual and their mixtures, owing to the synergistic lithium storage of crystal-phase heterojunction nanostructure.⁴⁵ Similar to the nanodomain-sized dispersion, the Mn_0.25Co_0.75O solid solution with the excellent dispersion at the atomic level is indeed able to greatly boost the electrochemical performances compared with the individuals and their mixture. This enhancement could be similar to the lithium-rich layered oxides with the homogeneously distributed multiple transitional metal cations in the lattice.⁴⁶ In addition, from the early studies,^22,47 Mn_xCo_1−xO solid solutions were prepared by calcining either the mixture of CoO and MnO in appropriate stoichiometric quantities at 1200 K for approximately one week via a solid-state reaction,²² or the pre-prepared solid mixture of MnCO₃/CoCO₃ precursors in the presence of agar-gel matrices for two days in H₂/Ar gas at 400 °C or 600 °C for 4 h.⁴⁷ Our CoMn-LDH precursor-based synthesis route might thus be a simple alternative method to prepare graphene-supported Mn_0.25Co_0.75O/G solid solution, especially with the highly enhanced electrochemical performances used as anode nanomaterial for LIBs.

On the other hand, we ascribe the good rate capability to the improved electronic conductivity. Electrochemical impedance spectra (EIS) were recorded for all electrodes. Fig. 5 shows that the diameter of depressed semicircle for the Mn_0.25Co_0.75O/G electrode appears to be much smaller than those of the other three electrodes. This clearly reveals that the Mn_0.25Co_0.75O/G electrode is capable of facilitating charge-transfer kinetics and fast electron transport, and thereby improving the rate capability. Quantitatively, the value of the charge transfer resistance of the semicircle diameter of the Mn_0.25Co_0.75O/G composites is determined to be 26.4 Ω, which is much lower than those of CoO/G (53.0 Ω), MnO/G (66.8 Ω), and mMnO + CoO/G (42.4 Ω). Apparently, the reason of the low resistance value is attributed to the improved electronic conductivity of Mn-doped CoO electrode.

Fig. 5 Comparison of Nyquist plots between the electrodes of Mn_0.25Co_0.75O/G, mMnO + CoO/G, MnO/G, and CoO/G.

Conclusions

In summary, we have explored the greatly improved electrochemical performances of the graphene-supported Mn_0.25Co_0.75O solid solution compared with the individuals and their mixture. It is possible to design the miscible metallic cations of Co/Mn-containing LDH precursor to prepare binary active Mn_0.25Co_0.75O solid solution at the moderate temperature (600 °C) under the Ar atmosphere. With the aid of the excellent dispersion of bi-active phases at the atomic scale, the Mn_0.25Co_0.75O solid solution is able to exhibit the highly improved electrochemical performances, which are distinctly different from those of the individuals and their mixture. The reversible capacity reaches up to capacity of 980 mA h g⁻¹ after completion of 100 cycles at 100 mA g⁻¹, and especially can be maintained to 1087 mA h g⁻¹ after 1300 cycles at the high current density of 2 A g⁻¹. Considering the well-ordered arrangements of metal cations of the diverse LDH precursors, the doping of metal cations might be extended to be a facile and effective alternative to improve the cycling performance, even for the Li-rich layered metal oxides for Li-ion battery applications.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (973 Program, 2014CB932102), the National Natural Science Foundation of China.

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns, HRTEM images, EDX spectra, cyclic voltammogram, galvanostatic discharge/charge test. See DOI: 10.1039/c6ra01500j

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