Enhancing coulombic efficiency and rate capability of high capacity lithium excess layered oxide cathode material by electrocatalysis of nanogold

Abstract

A Lithium-rich cathode material Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ modified with nanogold (Au@LMNCO) for lithium-ion (Li-ion) batteries was prepared using co-precipitation, solid-state reaction and surface treatment techniques. Au@LMNCO was prepared by thermally spraying gold on the surface of the lithium-rich cathode material (LMNCO). X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) results indicate that Au was successfully integrated into the surface of LMNCO. The cyclic voltammogram of Au@LMNCO shows a significant reduction in the reaction overpotential compared to that of LMNCO, which was a result of the nanogold formation. The stable reversible capacity of the Au@LMNCO electrode was 249 mA h g⁻¹, and it could be retained at 244 mA h g⁻¹ (98% retention) after 100 cycles at 0.5C. The coulombic efficiencies were over 98% except for the first five cycles. Moreover, Au@LMNCO also exhibited excellent rate capability. Even at a 5.0C rate, its discharge capacity was about 190 mA h g⁻¹. The superior electrochemical performance can be attributed to its unique nanoplate characteristics, its structural stability, and the electrocatalytic activity of nanogold.

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1 Introduction

Lithium excess layered oxide cathode materials (xLi₂MnO₃·(1 − x)LiMO₂) have been widely investigated because of their high discharge capacities (up to 250 mA h g⁻¹),^1–3 low cost, non-toxicity and environmental benignity compared with conventional commercial cathode materials such as LiCoO₂.⁴ However, several issues such as their poor rate performance, low first coulombic efficiency and voltage degradation during cycling hinder their application as a cathode material of lithium-ion (Li-ion) batteries.⁵ To improve the electrochemical performance of lithium excess layered oxide cathode materials, a great deal of research has been conducted over the past several years.^6–8

Lithium excess layered oxide cathode materials (denoted as xLi₂MnO₃·(1 − x)LiMO₂) were firstly applied as cathode materials for rechargeable Li-ion batteries by Thackeray et al.⁹ xLi₂MnO₃·(1 − x)LiMO₂ materials comprise a rhombohedral LiMO₂ structure (space group: R3̄m) and a monoclinic Li₂MnO₃ structure (space group: C2/m).¹⁰ It is generally accepted that the charging capacity in the LiMO₂ component of xLi₂MnO₃·(1 − x)LiMO₂ materials is limited by the number of transition metal ions in a tetravalent state. The Li₂MnO₃ component of xLi₂MnO₃·(1 − x)LiMO₂ materials consists of Mn⁴⁺, which is considered to be electrochemically inactive. However, charging xLi₂MnO₃·(1 − x)LiMO₂ materials above 4.5 V vs. Li/Li⁺ can apparently electrochemically activate the materials due to the extraction of Li and O, which in turn leads to the formation of the MnO₂ host structure in the compound. Subsequently lithium ions can then be reversibly intercalated into the host structure.¹¹ Michael M. Thackeray believed that, during the initial charge process of xLi₂MnO₃·(1 − x)LiMO₂ materials, electrochemical extraction of lithium ions from the Li₂MnO₃ component occurs in two steps.¹² The two-step reaction can be represented as follows.

Li₂MnO₃ → 2Li⁺ + Mn⁴⁺O₃⁴⁻ + 2e⁻ (1)

Mn⁴⁺O₃⁴⁺ → MnO₂ + ½O₂ (2)

The oxygen formation accompanied by lithium ion extraction and the structure rearrangement during this plateau have been confirmed by Armstrong using in situ differential electrochemical mass spectrometry.¹³ In this process, electrolyte degradation may also occur, forming gases such as CO and CO₂.¹⁴

During the first discharge process of xLi₂MnO₃·(1 − x)LiMO₂ materials, lithium ions are pulled out of the anode and they will flow back to the cathode. The discharge compensation mechanism is considered as combined participation of the Ni^2+/4+ redox couple and the oxygen anion redox couple, and the partial participation of Mn^3+/4+.^15–17 Jihyun Hong identified the critical role of oxygen evolved from the lattice of layered lithium excess metal oxides in Li rechargeable batteries.¹⁸ In order to improve the utilization efficiency of oxygen, in our work attention was directed towards the reversible redox reaction between oxygen and its reaction products for lithium excess layered oxide cathode materials. In the coin-type electrochemical cells made with lithium excess layered oxide materials, partially reversible formation and decomposition of Li₂CO₃ occurred, which was similar to the mechanism known for Li–air batteries with a carbonate-based electrolyte. Li₂CO₃ can also electrochemically decompose with a voltage plateau at ca. 4.2 V under the existence of a catalyst, which is 0.2 V higher than that for Li₂O₂ as already reported by Ogasawara and co-workers.¹⁹ Yi-Chun Lu and co-workers proved that Au has catalytic activity for the oxygen reduction reaction in Li–O₂ cells.²⁰ Because of the similarity between the surface reaction mechanism of lithium excess layered oxide cathodes and that of the cathode for Li–O₂ cells, we propose that the surface-modified Au can catalyze the oxygen reduction for a lithium excess layered oxide electrode in Li-ion batteries.

In this study, Mn_0.7Ni_0.2Co_0.1(OH)₂ with a nanoarchitecture was first synthesized by a co-precipitation reaction. During the synthesis process, the hexagonal shaped nanoplates (“primary particles”) assembled into secondary particles with an average size of 4–6 μm. Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ was obtained after lithiation, and then the lithium-rich Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ cathode material (LMNCO) was fabricated. Subsequent thermal spraying of gold onto the LMNCO surface resulted in the formation of a new cathode material, lithium-rich Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ with modified nanogold, denoted as Au@LMNCO. Hexagonal shaped nanoplates cannot only render a large contact area between active materials and an electrolyte during an electrochemical reaction, but also can shorten the path lengths for Li ion transport. In particular, the surface modification of Au can enhance the electronic conductivity as well as can catalyze the reduction reaction of oxygen and the oxidation reactions of its reduction products. All the factors will contribute greatly to the high specific capacity, high rate capability and good cycling performance of Au@LMNCO. The enhancement mechanism will be discussed in the discussion section.

2 Experimental section

2.1 Preparation of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂

Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ was prepared by a solid-state reaction from lithium hydroxide and manganese-nickel-cobalt hydroxide Mn_0.7Ni_0.2Co_0.1(OH)₂. The manganese-nickel-cobalt hydroxide precursor was firstly prepared by co-precipitation from an aqueous mixture of MnCl₂·4H₂O, NiSO₄·7H₂O, and CoCl₂·6H₂O (Mn : Ni : Co = 7 : 2 : 1 molar ratio; the combined concentration was 2 mol L⁻¹) and a 2.0 mol L⁻¹ NaOH aqueous solution with a desired amount of NH₃·H₂O. The solutions were mixed slowly in a nitrogen filled reactor and the pH of the mixed solution was kept in the range of 10.4–10.6 during the precipitation process. Finally, the precipitated Mn_0.7Ni_0.2Co_0.1(OH)₂ particles were filtered, washed using deionized water, and then dried in a vacuum at 120 °C for 24 h. The obtained Mn_0.7Ni_0.2Co_0.1(OH)₂ and LiOH·H₂O were then mixed at a molar ratio of 1.00 : 1.55 by using a mortar and pestle, and pressed into pellets, to which 5% excess lithium was added to compensate for the lithium evaporation during the calcination process at a high temperature. The pellets were heated at 850 °C for 20 h in air and then quenched to room temperature.

2.2 Synthesis of LMNCO and Au@LMNCO cathode materials

The lithium-rich cathode material LMNCO was fabricated by mixing the cathode powder (Li_1.2Mn_0.56Ni_0.16Co_0.08O₂), Super P carbon, and polyvinylidene fluoride (PVDF) (at a mass ratio of 80 : 10 : 10) to form a slurry using N-methyl-2-pyrrolidone (NMP). The slurry was cast on Al foil, which was then dried in a vacuum oven overnight. The Au@LMNCO cathode material was fabricated by thermally spraying gold on the surface of LMNCO using a Precision Etching Coating System (Gatan 682).

2.3 Characterization

The crystal structures of the synthesized samples were determined by X-ray diffraction (XRD) using a D/max-γβ X′pert diffractometer (Rigaku, Japan) with Cu Kα radiation. The morphologies of the as-prepared samples were observed with a scanning electron microscope (SEM, HITACHI, S-4700). Elemental analysis was performed using energy dispersive spectrometry (EDS) under the SEM mode. The microstructural characteristics of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ nanoplates were observed using a high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2010) working at an accelerating voltage of 200 kV. The lattice structure was identified by the selected area electron diffraction (SAED) technique. The nano-gold content in the prepared sample was accurately quantified by inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Optima 5300DV, PerkinElmer, Waltham, Ma, USA).

2.4 Electrochemical evaluation

Two-electrode CR2032-type coin cells were assembled with LMNCO and Au@LMNCO as the cathodes, respectively, metallic lithium foil as the anode and Celgard-2320 membrane as the separator. The electrolyte used comprised 1 M LiPF₆ dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) at a volumetric ratio of 1 : 1 : 1. Galvanostatic charge and discharge were performed at different current densities in a voltage range of 2.0–4.75 V using an 8-channel battery analyzer (Neware, China). Theoretical capacities of LMNCO and Au@LMNCO cathode materials were set to 250 mA h g⁻¹, i.e., a current density of 250 mA g⁻¹ corresponding to 1C. Electrochemical storage capacities of the working electrodes were calculated based on the mass of active cathode materials. Cyclic voltammograms (CV) of different cathodes were recorded at a scanning rate of 0.1 mV s⁻¹ between 2.0 and 4.8 V using an electrochemical analyzer CHI630b (Chenhua, China). All experiments were carried out at a temperature of 25 ± 0.5 °C.

3 Results and discussion

The crystal structure of the hydroxide precursor Mn_0.7Co_0.1Ni_0.2(OH)₂ was determined by using XRD, as shown in Fig. 1. The main peaks of the XRD pattern were indexed based on β-Ni(OH)₂ (P3̄m) which are the most probable structures that can form during a co-precipitation process of manganese, nickel and cobalt. The peak at 19.1° (2θ) could be indexed to (001) for a Ni(OH)₂-like phase, which has the highest intensity due to the preferential growth of the crystallites perpendicular to the c-axis.²¹ The remaining X-ray peaks at 18°, 31°, 44.2° and 58.3° correspond to the Bragg positions of the Mn₃O₄ tetragonal phase (I41/amd).

Fig. 1 XRD pattern of hydroxide precursor Mn_0.7Co_0.1Ni_0.2(OH)₂.

Fig. 2 shows the SEM images of the precursor Mn_0.7Co_0.1Ni_0.2(OH)₂ at different magnifications. The photographs show that the precursor particles were large agglomerates (Fig. 2(a)) composed of several secondary particles, which were typically 4 to 8 μm in size. Each secondary particles were formed by hexagonal-shaped nanoplates with a thickness of about 50 nm and lateral dimensions ranging from 500 to 800 nm (Fig. 2(b)). The hexagonal shape of the primary particles reflects the three-fold symmetry of the hydroxide precursor's crystal structure, in agreement with the powder XRD results. These nanoplates were expected to be preserved after lithium addition, thereby accelerating lithium ion diffusion through the primary nanoplate particles. Moreover, it was expected that an electrolyte would have better wettability as a result of the existence of macropores between the hexagonal plates. Another important advantage is that the material loading density at the level of a cell will not be sacrificed, as it would be if these nanoplates were entirely isolated.

Fig. 2 SEM images of hydroxide precursors at different magnifications.

From Fig. 3(a) it can be seen that the as-prepared Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ had a nanoflower morphology and all the secondary particles had similar sizes (the average particle size was ca. 6 μm). As shown in Fig. 3(b), a single particle comprised primary particles, which were nanoplate particles with diameters in a range of 500–800 nm and a thickness of about 80 nm. After calcination at 850 °C and the addition of Li ions into the structure, the nanoplates expanded in thickness.

Fig. 3 (a and b) SEM images of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂; (c) TEM image and (d) HRTEM image of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂.

To further examine the architecture of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, the samples were investigated using TEM and HRTEM and the TEM and HRTEM images are shown in Fig. 3(c) and (d), respectively. It was observed that Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ comprised hexagonal nanoplates and the diameter of a single nanoplate was estimated to be 500–600 nm, which is in agreement with the powder SEM images results. A high-resolution TEM micrograph from a representative particle is shown in Fig. 3(d). Distinct lattice fringes confirm good crystallinity of the particles. The distance between two lattice fringes was calculated to be ca. 0.47 nm, which corresponds to (003) planes from the rhombohedral phase and/or (001) planes from the monoclinic phase.²² The selected area electron diffraction (SAED) pattern of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ as shown insets in Fig. 3(d) reveals the material is highly crystalline in nature and indexed with LiMO₂ structure. The lattice pattern was indexed to electron beam direction of LiMO₂ for Li_1.2Mn_0.56Ni_0.16Co_0.08O₂. The Li₂MnO₃ and LiMO₂ phases were distinguished from SAED pattern of the layered composite and well indexed with different structures. The highly crystallized layered phase can greatly improve the crystallographic structure stability and charge–discharge capacity of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂.²³

The phase structures of the as-synthesized LMNCO and Au@LMNCO samples characterized using power XRD are shown in Fig. 4. All the peaks in the XRD patterns can be indexed as an integrated structure comprising two components: a rhombohedral phase LiNiO₂ (space group R3̄m) and a monoclinic phase Li₂MnO₃ described by the space group C2/m. The XRD patterns also show that the pair reflections (006)/(012) and (018)/(110) for LMNCO were well separated, and the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ was 1.4. All these results indicate that the samples possessed a high degree of crystallinity, a good hexagonal order, and a layered structure.²⁴ Compared with that of LMNCO, the XRD pattern of Au@LMNCO shows an additional peak that can be attributed to Au, indicating that Au was successfully thermal-sprayed on the surface of LMNCO. The EDS analysis (the spectra are shown in Fig. 4(b)) also confirmed the existence of Au on the surface of LMNCO, which is in agreement with the XRD results. According to the ICP result, the average amount of nano-gold in the synthesized sample was ca. 0.428 g m⁻². The HRTEM images of the Au@LMNCO sample and corresponding Au element mapping are shown in Fig. S1 and S2 (ESI†), from which it can be observed that nano-gold particles with a diameter of 5 nm were distributed on the surface of the bulk particles. Additionally, the mapping images of the transition metal elements (Ni, Co, Mn) and Au by scanning TEM show that the distribution of the nano-gold on the Au@LMNCO sample surface was homogeneous.

Fig. 4 (a) XRD patterns of LMNCO and Au@LMNCO; (b) SEM image and EDS spectrum of Au@LMNCO.

Fig. 5(a) shows the initial cyclic voltammograms (CV) of LMNCO and Au@LMNCO in a potential range of 2.0–4.8 V at a scan rate of 0.1 mV s⁻¹. In comparison with that of LMNCO, the current intensities of oxidation and reduction peaks of Au@LMNCO were obviously higher. The potential difference between these two peaks was smaller for the Au@LMNCO electrode, indicating that the polarization of the electrode had been significantly reduced under existence of nanogold. The irreversible peak at ca. 4.64 V vs. Li due to the oxygen release reaction was observed. In the first cycle for each voltammogram an irreversible peak at ca. 4.8 V vs. Li was observed, which was caused by the electrolyte decomposition. In order to reduce electrolyte decomposition, the charged and discharged upper and lower voltage limits were set at 4.75 V and 2.0 V, respectively.

Fig. 5 (a) The cyclic voltammograms of LMNCO and the Au@LMNCO electrodes in a potential range of 2.0–4.8 V at a scan rate of 0.1 mV s⁻¹; (b) the CV voltammograms of Au@LMNCO electrode for the 1st, 2nd, 5th and 100th cycles.

The CV curves of the obtained Au@LMNCO electrode for the 1st, 2nd, 5th and 100th cycles are shown in Fig. 5(b). It can be seen that the CV curve of the initial charge process displays three oxidation peaks. The initial CV curve is accompanied by an irreversible oxidation peak at 4.6 V, which was beyond the formal oxidation peaks of Ni²⁺ to Ni⁴⁺. This oxidation peak can be assigned to an irreversible loss of oxygen from the lattice.^25,26 For the initial discharge process, the CV curve exhibits a large broad hump in a voltage range of 3.2 to 4.2 V. In contrast to the initial charge process, one oxidation peak was observed at ca. 4.0 V in the subsequent cycling process. A new reduction peak appeared at ca. 3.2 V in the 2nd and 5th cycles, suggesting the phase transformation from a layered one to a spinel one during the initial discharge process,²⁷ which could lead to a layered-spinel intergrowth structure and a redox reaction at ca. 3.2 V.²⁸ Moreover, as the cycle number increased, all the oxidation and reduction peaks moved towards lower voltages. In other words, the CV curves show obvious voltage decay with the increase in cycle numbers, revealing that the contents of the spinel-like structure components in the electrode increased with cycling.

Fig. 6 shows the initial charge–discharge profiles of LMNCO and Au@LMNCO at a low rate of 0.1C (1.0C = 250 mA g⁻¹) between 2.0 and 4.75 V. A slope region of the first charging curve was observed for both the electrodes, which is attributed to the extraction of Li⁺ from the lithium layer. The Au@LMNCO composite delivered a higher discharge capacity (ca. 310 mA h g⁻¹) than that of LMNCO cathode (ca. 280 mA h g⁻¹). The initial coulombic efficiency of the Au@LMNCO (ca. 82%) was also higher than that of the LMNCO (ca. 75%). It is known that the theoretical capacity of LMNCO can be calculated as 250 mA h g⁻¹ when assuming all Mn⁴⁺ could be reduced to Mn³⁺ in the first charging process and one electron reduction based on the transition metals (i.e., Co³⁺/Co⁴⁺, Ni³⁺/Ni⁴⁺, and Mn³⁺/Mn⁴⁺). Surprisingly, it was found that the specific discharge capacity of the LMNCO material exceeded 250 mA h g⁻¹ at 0.1C, which may be attributed to the fact that oxygen works as a reversible redox species on an electrode surface without release of oxygen gas beside the transition metal redox reactions.²⁹ In the present work, the nanogold was expected to promote the transformation reactions at the ca. 4.5 V plateau during the first charging process of LMNCO, in which oxygen atoms were removed from the crystal lattice and Mn⁴⁺ ions were reduced to Mn³⁺ simultaneously.³⁰ In addition, irreversible side reactions for the Au@LMNCO cathode might be suppressed in the initial charging process due to the significantly reduced polarization by adding nanogold to LMNCO, as shown in the CV curves (Fig. 5(a)). Since Mn³⁺ ions are reversible in the following Li insertion/extraction process, the higher degree of Mn⁴⁺/Mn³⁺ transformation and the less irreversible side reaction could be attributed to the increased discharge capacity and coulombic efficiency of Au@LMNCO.

Fig. 6 The initial charge–discharge profiles of LMNCO and Au@LMNCO at a low rate of 0.1C between 2.0 and 4.75 V.

Fig. 7 shows the cycling behaviors and coulombic efficiency of the LMNCO and the Au@LMNCO electrodes cycled at 0.5C. Typically, the capacity and coulombic efficiency of a cell increase initially with the progression of the cycling and reach the optimum at 5 cycles at 0.1C. The capacity retention was above 98% after 100 cycles for the Au@LMNCO composite, which was slightly higher than that for LMNCO (95%), indicating their good cycling behaviors. However, the average coulombic efficiency of the Au@LMNCO was 5% higher compared to that of LMNCO, which might be attributed to the catalytic effect of gold nanoparticles in the reduction reactions of the O₂, CO₂ and CO during the discharge process.

Fig. 7 Discharge capacities as a function of cycle numbers at 2.0–4.75 V (left) and coulombic efficiency for each cycle (right).

To evaluate the rate performance of LMNCO and the Au@LMNCO composite, both of them were cycled from 0.1 to 5.0C (i.e., 25.0 mA g⁻¹ to 1250 mA g⁻¹) between a voltage range of 2.0–4.75 V vs. Li⁺/Li (Fig. 8) and 10 cycles for each were performed. It seems that the rate capability of the Au@LMNCO was also improved compared to that of LMNCO. Although the performance of these two materials was similar at low rates, Au@LMNCO retained much higher capacity at high rates than LMNCO. Even at a 5.0C rate, Au@LMNCO delivered a discharge capacity of about 190 mA h g⁻¹, while the LMNCO electrode only exhibited a capacity of ca. 100 mA h g⁻¹. Compared with the electrochemical data published for xLi₂MnO₃·(1 − x)LiMO₂ series materials so far (135.6 mA h g⁻¹ at a 5.0C rate),³¹ it can be concluded that we have developed an Au@LMNCO cathode material with substantially improved performance.

Fig. 8 The rate capabilities of LMNCO and Au@LMNCO measured at a series of current rates.

To explain the surface electrochemical or chemical reaction processes in the presence of nanogold during the first and following charge/discharge cycles of this Au@LMNCO cathode material, we propose a surface catalytic mechanism of gold nanoparticles for a lithium excess layered oxide electrode that occurs during charging and discharging cycles for the electrode surface components based on the results of the previous studies (Fig. 9).

Fig. 9 Scheme of the proposed surface catalytic mechanism of gold nanoparticles for a lithium excess layered oxide electrode.

For the LMNCO electrode, the Li₂MnO₃ component can be considered as a nanometer complex of Li₂O and MnO₂. Subsequently Li₂O can decompose into Li⁺ and O₂ in the presence of MnO₂ and Au, where Au acts as a catalyst when the electrochemical potential of the Li/LMNCO cell increases from 4.4 to 4.8 V during the first charge process. This electrochemical process at the surface of the Li₂MnO₃ component can be represented by the following reactions ((3) and (4)):

Li₂MnO₃ → Li₂O + MnO₂ (3)

in which the oxygen production is accompanied by lithium ions extraction. During the plateau, electrolyte degradation may also occur, forming gases such as CO and CO₂. During the first discharge process, drawing the current pulls the lithium ions out of the anode and back to the cathode. The discharge compensation mechanism is proposed to be combined participation of the Ni^2+/4+ redox couple and the oxygen anion redox couple, and partial participation of Mn^3+/4+. In this process, Au exhibits catalytic activity for reduction reactions of oxygen and carbon oxides. Therefore, the coulombic efficiency of the Au@LMNCO was higher than that of LMNCO. Towards the end of the discharging process, Li₂CO₃ and Li₂O₂ were formed on the surface of the cathode. During the subsequent cycling process, partially reversible formation and decomposition of Li₂O₂ and Li₂CO₃ occurred under the existence of nanogold as an electrocatalyst. These reactions can be represented as follows:

Therefore, the improved rate capability for the Au@LMNCO may result from the fact that the surface nanogold layer can restrain the formation of an undesired solid-electrolyte interphase (SEI) film, leading to a higher Li⁺ transport rate in the surface region by reducing the charge transfer resistance. This is similar to the result in the work reported by Yabuuchi ​ et al.,³⁰ who suggested that suppression of the lithium carbonate formation is necessary to improve the cycle ability.

4 Conclusions

In summary, we have designed and synthesized an Au@LMNCO nanoplate cathode material with superior electrochemical properties. A nanoplate hydroxide precursor for these cathodes with a thickness of ca. 50 nm and lateral dimensions ranging from 500 to 800 nm was obtained using a co-precipitation method. A subsequent solid phase reaction and surface treatment produced the Au@LMNCO cathode material. XRD and EDS results confirm that Au was decorated on the surface of the LMNCO cathode material. The stable reversible capacity of Au@LMNCO electrode was 249 mA h g⁻¹, and could be retained at 244 mA h g⁻¹ (98% retention) after 100 cycles at 0.5C. The coulombic efficiencies were over 98% except for the first five cycles. Moreover, excellent rate-capability performance was observed for this Au@LMNCO cathode material. The superior electrochemical performance can be attributed to its unique nanoplate characteristics, its structural stability, and the catalytic activity of nanogold for the reduction reaction of oxygen and the oxidation reactions of its reduction products in the cathode material. These results demonstrate that Au@LMNCO is a promising cathode material for Li-ion batteries.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51274075), the National Environmental Technology Special Project (No. 201009028), and Guangdong Province-department University-industry Collaboration Project (Grant No. 2012B091100315).

Notes and references

1. M. Gu, I. Belharouak and A. Genc, Nano Lett., 2012, 12, 5186.
2. H. Yu and H. Zhou, J. Phys. Chem. Lett., 2013, 4, 1268.
3. S. X. Liao, C. H. Shen, Y. J. Zhong, W. H. Yan, X. X. Shi, S. S. Pei, X. D. Guo, B. H. Zhong, X. L. Wang and H. Liu, RSC Adv., 2014, 4, 56273–56278.
4. X. J. Guo, Y. X. Li, M. Zheng, J. M. Zheng, J. Li, Z. L. Gong and Y. Yang, J. Power Sources, 2008, 184, 414.
5. M. Gu, I. Belharouak, J. Zheng, H. Wu, J. Xiao, A. Genc and K. Amine, ACS Nano, 2013, 7, 760.
6. S. Hy, J. H. Cheng, J. Y. Liu, C. J. Pan, J. Rick, J. F. Lee, J. M. Chen and B. J. Hwang, Chem. Mater., 2014, 26, 6919.
7. Y. Li, J. Bareño, M. Bettge and D. P. Abraham, J. Electrochem. Soc., 2015, 162, A155.
8. L. Li, S. Song, X. Zhang, R. Chen, J. Lu, F. Wu and K. Amine, J. Power Sources, 2014, 272, 922.
9. M. M. Thackeray, C. S. Johnson, J. T. Vaughey, N. Li and S. A. Hackney, J. Mater. Chem., 2005, 15, 2257.
10. M. M. Thackeray, S. H. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek and S. A. Hackney, J. Mater. Chem., 2007, 17, 3112.
11. Z. Lu and J. R. Dahn, J. Electrochem. Soc., 2002, 149, A815.
12. J. S. Kim, C. S. Johnson, J. T. Vaughey and M. M. Thackeray, Chem. Mater., 2004, 16, 1996.
13. A. R. Armstrong, M. Holzapfel, P. Novak, C. S. Johnson, S. H. Kang, M. M. Thackeray and P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 8694.
14. S. Hy, F. Felix, J. Rick, W. N. Su and B. J. Hwang, J. Am. Chem. Soc., 2014, 136, 999.
15. D. Buchholz, J. Li, S. Passerini, G. Aquilanti, D. Wang and M. Giorgetti, ChemElectroChem, 2015, 2, 85.
16. N. Tran, L. Croguennec, M. Menetrier, F. Weill, Ph. Biensan, C. Jordy and C. Delmas, Chem. Mater., 2008, 20, 4815.
17. Z. Zheng, Z. G. Wu, Y. J. Zhong, C. H. Shen, W. B. Hua, B. B. Xu, C. Yu, B. H. Zhong and X. D. Guo, RSC Adv., 2015, 5, 37330–37339.
18. J. Hong, H. D. Lim, M. Lee, S. W. Kim, H. Kim, S. T. Oh, G. C. Chung and K. Kang, Chem. Mater., 2012, 24, 2692.
19. T. Ogasawara, A. Débart, M. Holzapfel, P. Novák and P. G. Bruce, J. Am. Chem. Soc., 2006, 128, 1390.
20. Y. C. Lu, Z. Xu, H. A. Gasteiger and S. Chen, J. Am. Chem. Soc., 2010, 132, 12170.
21. D. Wang, I. Belharouak, G. Zhou and K. Amine, Adv. Funct. Mater., 2013, 23, 1070.
22. D. Mohanty, S. Kalnaus, R. A. Meisner, K. J. Rhodes and J. Li, J. Power Sources, 2013, 229, 239.
23. F. Amalraj, M. Talianker, B. Markovsky, D. Sharon, L. Burlaka and G. Shafir, J. Electrochem. Soc., 2013, 160, A324.
24. G. Z. Wei, X. Lu, F. S. Ke, L. Huang, J. T. Li, Z. X. Wang, Z. Y. Zhou and S. G. Sun, Adv. Mater., 2010, 22, 4364.
25. M. Jiang, B. Key, Y. S. Meng and C. P. Grey, Chem. Mater., 2009, 21, 2733.
26. J. Zheng, X. Wu and Y. Yang, Electrochim. Acta, 2013, 105, 200.
27. D. Wang, I. Belharouak, X. Zhang, Y. Ren, G. Meng and C. Wang, J. Electrochem. Soc., 2014, 161, A1.
28. C. S. Johnson, N. Li, C. Lefief, J. T. Vaughey and M. M. Thackeray, Chem. Mater., 2008, 20, 6095.
29. T. Ohzuku, M. Nagayama, K. Tsuji and K. Ariyoshi, J. Mater. Chem., 2011, 21, 10179.
30. N. Yabuuchi, K. Yoshii, S. T. Myung, I. Nakai and S. Komaba, J. Am. Chem. Soc., 2011, 133, 4404.
31. K. Du, F. Yang, G. Hu, Z. Peng, Y. Cao and S. R. Kwang, J. Power Sources, 2013, 244, 29.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26667j

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