Facile synthesis of nanostructured MoO₃ coated Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ materials with good electrochemical properties

Abstract

Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ cathode materials were synthesized by a co-precipitation method. The secondary particles consisted of mainly nanorods and nanoplates that expose electrochemically active crystal surfaces. The rate performance and cycling stability of the cathode materials were remarkably enhanced after MoO₃ coating using the molten salt method. Particularly, the 1 wt%-MoO₃ coated materials deliver the highest discharge capacity of 217 mA h g⁻¹ at 1C rate and a retention higher than 91.5% after 100 cycles. The structure of the material was investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy. The results show remarkable phase transformation from a layered structure to spinel structure at the surface of the cathode material after MoO₃ coating, while the bulk structure remained unchanged. The improved cycling stability and the rate performance are ascribed to the formation of the spinel structure at the surface.

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Introduction

Lithium-ion batteries have been widely applied to portable electronic devices and electrical vehicles because of their advantages in terms of high energy density, long service life, low self discharge, no memory effect and high security.^1,2 In the last decade, many studies have focused on the development of high-energy and low-cost lithium-ion materials.³ In this regard, lithium-rich layered cathode materials have been considered the most promising next generation cathode materials, owing to their higher capacity (>250 mA h g⁻¹) and significantly reduced cost compared to conventional cathode materials represented by LiFePO₄, LiCoO₂ and LiMn_1/3Ni_1/3Co_1/3O₂, among others.⁴

Despite these advantages, Li-rich materials suffer from several critical drawbacks which have limited their practical applications, including intrinsically poor cycling, rate capabilities, initial coulombic efficiency and irreversible side reactions with electrolyte at high operating voltages.⁵ Firstly, structure transition from layered structure to spinel of the Li-rich cathode materials was observed during the cycling.⁶ Such structural changes are believed to be associated with transition metal/Li ions migration caused by deintercalation of lithium ions during the charge and discharge processes,^7,8 finally leading to the deterioration of cycling performance of battery cells. Secondly, the irreversible side reactions between electrode surface and the electrolyte at high operating voltages were also observed, causing a thickening of the SEI layer and dissolution of the transition metals and consequently further deterioration of cycling performance of battery cells.⁹ Furthermore, Li₂MnO₃ phase was activated upon the initial charging process where a significant amount of Li ions were exacted and cannot be inserted back to the cathode material during discharging process, leading to a high irreversible capacity loss and low initial coulombic efficiency.¹⁰ The poor rate capabilities of Li-rich materials are also associated with the low electronic conductivity of the Li₂MnO₃ component.

Surface coating and bulk doping modifications are common approaches applied to the Li-rich materials to improve their cyclic stability. The surface coating layers suppresses the transition metal dissolution to the electrolyte and alleviates side reactions. Meanwhile, bulk doping technique enhances stability of the resulting layered structure.^11–13 The main surface coating materials include metal oxides, such as Al₂O₃,¹⁴ MgO,^15–17 Sm₂O₃,¹⁸ TiO₂,¹⁹ ZrO₂,²⁰ metal fluorides, such as AlF₃,²¹ CeF₃,²² FeF₃ (ref. 23) and lithium compounds, such as LiAlO₂,^24,25 Li₃PO₄,²⁶ etc. Compared with these materials, surface coating compound MoO₃ can provide additional lithium ion insertion sites owing to its high theoretical capacity up to 1117 mA h g⁻¹ and therefore reduce irreversible capacity loss of the Li-rich materials.²⁷ MoO₃ has a two-dimensional layered structure where each layer is connected with an adjacent layer by van der Waal forces. Each layer consists of MoO₆ octahedra with Mo in the center and O in the edge where two MoO₆ octahedra share O–O edges.²⁸ Such unique layered structure exhibits large cavities which are suitable for insertion and extraction of lithium ions. MoO₃ has been reported to be used as surface modification materials for Li-rich materials. Wu et al. reported synthesis of MoO₃ coated Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ composite where ammonium molybdate and Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ were mixed using high energy ball milling method and then calcined in the air at 600 °C for 6 h.²⁹ The 5 wt% MoO₃ coated composite exhibits a good cyclic stability, and when MoO₃ content increases to 20 wt% the first-cycle irreversible capacity loss of the composite decreases to 1.2 mA h g⁻¹ with discharging capacity of 273.9 mA h g⁻¹. However, the morphology of the particles will be damaged during high energy ball milling process using this method. Another strategy is to synthesize MoO₃ coated Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ composite via a wet coating process using ammonium molybdate as Mo sources reported by Wang et al.³⁰ The 3 wt% MoO₃ coated composite exhibits the best rate performance and cyclic stability, with capacity retention of 90.8% after 100 cycles. The morphology of the particles remained almost unchanged. However, the synthesis process requires complicated reaction condition controls including PH value, reactant concentration, temperature, etc.

Herein, we propose synthesis of MoO₃ coated Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ composite via a molten salt method benefiting from the low melting point of MoO₃. The synthesis method is very simple and easy to control compared to previously reported methods, therefore more suitable for industrialization. Negligible damage to the morphology of the precursor was observed. The required coating amount of MoO₃ is as low as 1 wt% to deliver high rate performance and cyclic stability. The results show remarkable phase transformation from layered structure to spinel structure at the surface of the Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ materials. In agreement with previous report,³¹ the spinel-layered core shell structure efficiently enhanced the performance of the materials.

Experimental

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

The precursor for lithium-rich layered cathode material Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ was synthesized via a co-precipitation method. MnSO₄·H₂O, NiSO₄·6H₂O and CoSO₄·7H₂O were used as the starting materials. An aqueous mixed solution of MnSO₄, NiSO₄, and CoSO₄ (7 : 2 : 1 in molar ratio) with total metal ion concentration of 2 mol L⁻¹ was pumped into a continuously stirred tank reactor with capacity of 50 L under a nitrogen atmosphere. At the same time, a 5 mol L⁻¹ NaOH solution (aq) as precipitant and the desired amount of NH₄OH solution (aq) as chelating agent were also separately fed into the reactor. The concentration and pH value (10.25) of the solution, the operation temperature (60 °C) and the stirring speed (450 rpm) in the reactor were carefully controlled. The obtained [Mn_0.7Ni_0.2Co_0.1](OH)₂ particles were then filtered, washed, and dried at 90 °C. The as-prepared precursor were thoroughly mixed with proper amount of lithium carbonate (3% excess in mol) and preheated at 500 °C for 5 h, and finally calcined at 870 °C for 12 h in air.

The synthetic procedure for MoO₃ coated Li-rich layered Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ via a molten salt method is illustrated in Fig. 1. MoO₃ was firstly ball milled until the particle size is smaller than 500 nm. Then the as-prepared layered Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ was thoroughly mixed with different amount of MoO₃ (0.5 wt%, 1 wt% and 2 wt%). The mixture was gradually heated to 790 °C at 2 °C min⁻¹, and then calcined at 790 °C for 5 h in air. The temperature was chosen to ensure that MoO₃ had melted. The surface modified samples were named as 0.5% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ and 2% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ for short.

Fig. 1 Schematic illustration of the synthetic procedure for MoO₃ coated Li_1.2Mn_0.56Ni_0.16Co_0.08O₂.

2.2 Material characterization

The compositions of the materials were measured using inductively coupled plasma atomic emission spectrometer (ICP, Atomscan Advantage). The crystal structure of the material was characterized by powder X-ray diffraction (PXD, Philips X'Pert PRO MPD) using a Cu Kα (wavelength: 0.15418 nm) radiation source with operation voltage and operation current of 40 kV and 40 mA, respectively. X-ray photoelectron spectroscopy (XPS, Escalab 250) measurements were performed using monochromatized Al Kα (hν = 1486.7 eV) as the excitation source. The likely charging of samples was corrected by setting the binding energy of the adventitious carbon (C_1s) to 284.8 eV. The morphology of the powders was determined by scanning electron microscopy (SEM, FEI NOVA NANOSEM 450) equipped with energy dispersive spectrometry (EDS). EDS and elemental mapping were used to demonstrate the existence and distribution of Mo, Ni, Co and Mn elements. High resolution transmission electron microscopy (HRTEM, JEOL-2100F) experiments were carried out and operated at 200 kV. Raman spectroscopic studies (Renishaw inVia Reflex) were carried out using a 532 nm excitation line with a laser power of 0.1 mW on the sample surface. The thermal stability of the materials were studied by thermogravimetric analysis (TGA, DTG-60H). Firstly, cell was charged to 4.8 V at 0.1C, then the cell was disassembled to get electrode materials film in an Ar-filled glove box. Secondly, the film was washed by dimethyl carbonate (DMC) and naturally dried. Thirdly, the electrode materials were scraped from the film and electrolyte and tested from room temperature to 500 °C at 5 °C min⁻¹ under N₂.

2.3 Evaluation of electrochemical performance

Cathode electrodes were prepared by pasting a thoroughly mixed slurry containing the active material, polyvinylidene fluoride, and Super P in a weight ratio of 80 : 10 : 10 in N-methyl-2-pyrrolidone (NMP) onto aluminum foils. The coin-type cell of CR2016 was assembled in an argon-filled glove box (H₂O and O₂ < 0.1 ppm) using a metal lithium foil as the counter electrode. The electrolyte solution was 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a 1 : 1 : 1 volume ratio. The charge–discharge experiments were performed between 2.0 and 4.8 V at room temperature on a Land battery tester (Land CT2001A, Wuhan, China). The current density of 1C, in our definition, was based on a capacity of 200 mA g⁻¹. EIS measurements were conducted with an impedance analyzer (Solartron 1260A) in the frequency range of 100 kHz to 0.01 Hz.

Results and discussion

The XRD patterns of the pristine and MoO₃ coated Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ materials are shown in Fig. 2. Most of the diffraction peaks can be indexed according to the layered α-NaFeO₂ structure space group R3̄m. However, there is no detectable phase due to the introduction of the MoO₃ coating which indicates the high dispersion of the MoO₃ particles. This could be likely caused by the low MoO₃ loadings or the amorphous structure of MoO₃ particles. Some weak super lattices reflections between 20° and 25° (2θ value) are ascribed to the hexagonal LiMn₆ super-ordering in Li₂MnO₃ monoclinic phase with C2/m symmetry.³² The splitting of the (006)/(012) and (108)/(110) peaks of the XRD patterns of sample indicates the formation of a well-layered structure.³³

Fig. 2 XRD patterns of the pristine and MoO₃ coated samples (a) pristine, (b) 0.5% MoO₃/Li_1.2Mn_0.56Ni_0.16Co, (c) 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co, and (d) 2% MoO₃/Li_1.2Mn_0.56Ni_0.16Co.

Raman spectroscopy was used to investigate the micro-structural changes of these lithium-rich layered oxides. As shown in Fig. 3, three main Raman bands are observed at 602, 490, and 433 cm⁻¹, respectively. The result agrees well with those reported previously³⁴ and is consistent with the theoretical prediction given for a hexagonal (R3̄m) and a monoclinic (C2/m) crystal, respectively.³⁵ The 602 cm⁻¹ and 490 cm⁻¹ bands are corresponds to A_1g with the symmetrical stretching of M–O and E_g with the symmetrical deformation of ideal layered lithium transition metal oxide with (R3̄m) symmetry. However, the bands at 465 cm⁻¹ and 588 cm⁻¹ are also ascribed to the blending E_g and stretching A_1g modes of M–O (M = Ni, Mn) by others,¹¹ the difference of the peak positions may due to the different chemical environment of metal ions which may affected by the synthesis method, the exactly composition, the oxygen vacancy etc. The band at 433 cm⁻¹ originates from the Li₂MnO₃-like structure.³⁶ Besides, a weak shoulder band at around 650 cm⁻¹ is correlated with the spinel-phase band.^31,37 No obvious change was observed of all the peaks which indicate the bulk structure unchanged after MoO₃ coating.

Fig. 3 Raman spectra of the pristine and MoO₃ coated materials (a) pristine, (b) 0.5% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, (c) 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, and (d) 2% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, the inset shows enlarger of the main peak.

The surface composition and the oxidation states of the elements were analyzed using XPS. Fig. 4 shows the XPS spectra of the pristine Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ and 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂. The peaks at the binding energies of 235.5 eV and 232.3 eV are assigned to Mo⁶⁺.^38,39 A small amount of Li₂MoO₃ or Li₂MoO₄ may exist at the interface between the cathode materials and the MoO₃ layer which could not be detected by XPS. The Mn_2p, Co_2p and Ni_2p peaks for the MoO₃-coated Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ shows no obvious chemical shift of binding energy, indicating that the Co, Ni, and Mn ion environment in the structure remained unchanged. The observed binding energies for Ni_2p3/2, Co_2p3/2 and Mn_2p3/2 coincide well with those for the standard Ni²⁺ (854.2 and 854.6 eV), Co³⁺ (779.9 eV) and Mn⁴⁺ (642.4 eV), respectively.⁴⁰ The peak intensities of these peaks decreased due to MoO₃ coating. The dominating peak of O_1s spectral is located at 529.6 eV, which shift to 529.9 eV after MoO₃ coating. This peak can be assigned to the lattice oxygen in the Li-rich compound. A small peak is located at the higher binding energy of 531.4 eV for the bare Li_1.2Mn_0.56Ni_0.16Co_0.08O₂. This peak can be assigned to the active oxygen species of the surface or the impurity of adsorbed species, such as Li₂CO₃ and LiOH.^41,42 However, in other reports, peaks at 528.9, 529.9 and 531.3 eV are assigned as the contribution from oxygen linked as Mn–O, Co–O and Ni–O respectively.⁴² After MoO₃ coating, intensity of peak at 531.4 eV decreased. As referred to the C_1s spectral, intensities of peaks at 288.2 eV and 289.6 eV also decreased after MoO₃ coating, these peaks are likely ascribed to carbonate species. The results indicated that surface residual alkali such as Li₂CO₃ and LiOH decreased after MoO₃ coating. Consequently, the water absorption resistance of the material can be improved.

Fig. 4 XPS spectra of the pristine and MoO₃ coated materials: pristine (black line), 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ coated (red line).

Fig. 5 shows the typical SEM images of the lithium-rich materials. It can be seen clearly that the secondary particles show uniform monodispersed spherical-like shape. The magnified image reveals the existence of the hierarchical structures built up from a mixture of mainly plate-like subunits and a few rod-like subunits. The plate-like structure mainly exposed [010] planes and therefore this 3D structure is especially beneficial for the Li⁺ transition as reported.⁴³ At the same time, the rod-like structure is also beneficial for the transition of Li⁺ compared with nanoparticle-shaped structure.⁴⁴ Therefore, the above mentioned hierarchical structures result in excellent electrochemical performance. Materials comprising 1D micro- and nanostructured materials also exhibit outstanding electrochemical performances owing to the shortening of electron and lithium diffusion paths, an appropriate contact area between active materials and electrolyte.⁴⁵

Fig. 5 Typical SEM images of the pristine Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ samples.

The HRTEM images of the pristine and MoO₃ coated materials are shown in Fig. 6. For the pristine materials, the surface shows typical layered structure with a spacing of 0.43 nm which indexes with the [020] planes of C/2m symmetry.⁴³ Electron diffraction patterns collected along the [001] and [1̄10] hexagonal zone axis are presented in Fig. 6e and f respectively, and the corresponding HRTEM are shown in Fig. S2.† The SAED patterns present of a rhombohedral LiCoO₂ patterns and three monoclinic Li₂MnO₃ patterns. The spinel phase observed by Raman spectra may exists in the bulk. As for the MoO₃ coated materials, the interplanar spacings of the surface are measured to be 0.146 nm, which is well indexed to [440] planes of cubic-spinel structure. The d-spacing at 0.221 nm is likely related to the [012] planes of layered structure. The results show remarkable phase transformation from layered structure to spinel structure at the surface of the cathode material after MoO₃ coating while the bulk structure remained unchanged. Such layered-to-spinel structural changes can be ascribed to the fact that the Li⁺ and O²⁻ at the surface may reacted with the MoO₃ layer, resulting in the formation of Li₂MoO₃ or Li₂MoO₄ phase.³⁰ In addition, a small volume of amorphous bulges were observed at the surface after MoO₃ coating which is believed to be generated by melted MoO₃. EDS mapping (Fig. S3†) shows that the dispersion of MoO₃ was very uniform. Such surface changes can not only decrease the gas formation but also promote the initial discharge/charge efficiency.

Fig. 6 HRTEM images of (a) and (b) pristine Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, (c) and (d) 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, (e) and (f) SAED patterns of 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂.

The initial charge and discharge curves of the pristine and MoO₃ coated samples at 0.1C rate (20 mA g⁻¹) are presented in Fig. 7. The initial discharge capacity of the pristine sample is 274 mA h g⁻¹ and the coulombic efficiency is 81%. For the 0.5% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ sample, the initial discharge capacity is the same with the pristine sample but the coulombic efficiency is 83%. As for the 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, the initial discharge capacity and coulombic efficiency are 278 mA h g⁻¹ and 84%, respectively. But when the content of MoO₃ content further increased to 2 wt%, the initial capacity decreased to 259 mA h g⁻¹ and the coulombic efficiency also decreased to 81%. Excessive amount of MoO₃ leads to the formation of large MoO₃ particles at the surface of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ which inhibit the transformation of Li⁺. Therefore, we mainly chose the 1 wt% MoO₃ coated sample as the research object (Fig. 8).

Fig. 7 Initial charge and discharge profiles of the pristine and MoO₃ coated materials in the voltage range of 2.0–4.8 V at 0.1C rate.

Fig. 8 Charge/discharge curves of the samples at different rates (a) pristine Li_1.2Mn_0.56Ni_0.16Co_0.08O₂, and (b) 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂.

The rate performance of the pristine and MoO₃ coated samples are presented in Fig. 9. The cells were cycled between 2.0 and 4.8 V with the same charge and discharge current density at 0.1C, 0.2C, 0.5C, 1C, and 2C-rates. The MoO₃ coated sample delivers better discharge capacities and the difference in capacity retention between the 1 wt% MoO₃ coated sample and the pristine sample progressively increases with increasing C-rate. In particular, the 1 wt% MoO₃ coated sample delivers discharge capacities of 218 and 188 mA h g⁻¹ at 1C and 2C-rate, respectively. In comparison, the discharge capacities of the pristine sample are 202 and 172 mA h g⁻¹ at 1C and 2C-rate, respectively. The rate performance of our material is similar to that reported previously,³⁰ but the initial efficiency is better. As discussed above, spinel structure formed at the surface of 1 wt% MoO₃ coated sample. The spinel structure exhibits three-dimensional transition channels for fast Li ion-diffusion and therefore enhanced the rate performance of this sample. The capacity was recovered when returning to the 0.5C-rate after the 2C-rate capability test. The reversibility demonstrates that the capacity degradation at large current densities is caused by diffusion-limited end-of-life polarization, rather than by irreversible structural changes.⁴⁶

Fig. 9 Cycle performance of the pristine and 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂.

The capacity retention of 1 wt% MoO₃ coated sample is 91.5% after 100 cycles while that of the pristine sample is only 83%, as shown in Fig. 9. There are some fluctuations for the curve which may due to the fluctuations of temperature. The cycle performance was retested. The trend of cyclic performance was nearly the same with that tested before. So the trend of cyclic performance influenced little by slight fluctuations of temperature. The cycle performance is better than the 5 wt% MoO₃ coated materials reported previously.²⁹ The enhanced cyclic stability is ascribed to less side reactions between the active materials and the electrolyte after MoO₃ coating. Besides, the surface spinel structure may inhibit the transition of the crystal phase of the MoO₃ coated materials.

In order to get insights into the intrinsic effects of MoO₃ coating on the electrochemical performance of pristine materials, electrochemical impedance spectroscopy (EIS) analysis was performed. The measurements were carried out after three cycles before charging. As shown in Fig. 10, the Nyquist plots for both electrodes are composed of a depressed semi-circle from the high to medium frequency region and a quasi-straight line in the low frequency region. An equivalent circuit model is utilized to fit the experimental results and the fitting impedance parameters are shown in Table 1. Generally, the high to medium frequency semi-circle is assigned to the charge transfer resistance (R_ct) in the electrode/electrolyte interface.¹³ The low frequency tail is related to the Warburg impedance which signifies the impedance of lithium ion diffusion in bulk electrode material. Compared to the pristine samples, the 1 wt% MoO₃ coated sample has a much smaller R_ct value. Materials coated with electrochemical inactive MgO can also decrease the impedance.¹⁷ These results indicate that MoO₃ coating could increase the surface electron migration rate and consequently the surface conductivity.

Fig. 10 Electrochemical impedance spectroscopy (EIS) and the equivalent circuit used (inset) for the pristine and 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ after 3 cycles before charging, the inset shows the equivalent circuit model.

Table 1 EIS parameters of the pristine and 1% MoO₃/Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ samples

Sample	Rs (Ω)	Rct (Ω)	CPE	Wo
Pristine	2.32	167.6	0.77	0.30
1% MoO3/Li1.2Mn0.56Ni0.16Co0.08O2	2.76	145.8	0.76	0.62

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The differential capacity (dQ/dV) plots of the charge and discharge curves (Fig. 11) are used to show the effects of the MoO₃ coating. In the 1st charge process, there are two distinct peaks for the pristine and 1 wt% MoO₃ coated materials. The peak at approximately 4 V was attributed to the reversible extraction of lithium ion from the LiNi_1/3Co_1/3Mn_1/3O₂ content. The other peak near 4.5 V was attributed to the removal of Li₂O from the Li₂MnO₃ component. This sharp oxidation peak disappears in the subsequent test which indicates the process is irreversible. In the 1st discharge process, the peak below 4.1 V belonged to the reduction of cobalt and nickel ions, which occurred essentially at the same potential for both pristine and 1 wt% MoO₃ coated materials.^10,13,40,47 The evolution of a redox reaction in 3.0 V region represents the layer-to-spinel transition.^25,48 The peak of the discharge process shifts to a lower voltage when the rate increases, indicating structural change of the materials. When the rate increased to 2C, peaks observed at 2.8 V can be seen for the pristine materials which means more spinel structure was formed on this material. In other words, the MoO₃ coating could improve the stability of the materials especially at high rates. The better stability of the MoO₃ coated materials mainly due to that the side reactions between the electrolyte and the cathode were suppressed after MoO₃ coating. MoO₃ can also restrict the thickening of SEI film during the charge–discharge process. Besides, spinel structure was formed at the surface of the Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ which may also enhance the stability. Although spinel structure also formed on the pristine materials after cycles, the spinel structure should mainly exist in the bulk. The TGA curves of the pristine and MoO₃ coated materials are shown in Fig. S1.† It can be seen that the temperature beginning to lose weight is 196.8 °C for the pristine but 208.9 °C for the MoO₃ coated materials. We analyzed the deriv weight curve of TGA profiles and found that the first peak position of the pristine material was located at 272 °C, however, the first peak position of the 1% MoO₃ coated material was located at about 329 °C, which indirectly inferred that the 1% MoO₃ coated material with higher thermal stability.

Fig. 11 The dQ/dV plots of the charge/discharge curves of the electrodes at different rates (a) 0.1C, (b) 0.5C, (c) 1C, and (d) 2C.

Conclusions

Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ materials was synthesised by a co-precipitation method, with the secondary particles mainly composed of nanorods and nanoplates that expose electrochemically active crystal surfaces. A novel method for preparation of MoO₃ coated layered cathode material for high energy and high-power Li-ion batteries were demonstrated. The MoO₃ coating could induce the formation of spinel structure at the surface of the materials without obvious change of the bulk structure. The spinel structure formed on the surface of the materials as well as the layered MoO₃ are responsible for the high capacity, excellent cycling ability, outstanding rate capability and high initial efficiency. Our results presented here provide a simple and economic synthesis strategy paving the way to commercialization of lithium-rich layered cathode materials.

Acknowledgements

This work was financially supported by the National High Technology Research and Development Program of China (2015AA034601).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21637d

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