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  0   Introduction

  Spinel LiMn₂O₄ is one of the most promising cathode for electric vehicles (EVs), hybrid electrical vehicles (HEVs), and plug-in hybrid vehicles (PHEVs) due to its adequate capacity, economical production, safety, low toxicity and high thermal stability ^[1]. Unfortunately, poor rate capability, cyclability and high-temperature performance limit its further applica-tion for power batteries due to the Jahn-Teller distortion^[2]. In addition, a further improvement in terms of cycling life and energy density is still required to fulfill the demands of these applications. As we know, the partial substitution by other metals for Mn in LiMn₂O₄ could stabilize the crystal structure and improve the cycling performance^[3-5]. Among all doped LiM_xMn_2-xO₄, the Ni-doped spinel LiNi_0.5Mn_1.5O₄ has attracted great interests for its good rate capability, high theoretical capacity (147 mAh·g^-1) and much high discharge voltage at around 4.7 V corresponding to the redox reactions of Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox couples ^[6]. However, LiNi_0.5Mn_1.5O₄ usually losses oxygen and disproportionates to a spinel and Li_xNi_1-xO or NiO when it is heated above 650 ℃^[7]. Hence, this LiNi_0.5Mn_1.5O₄ compound still has a non-negligible capacity fading during cycling due to the structural and chemical instabilities resulted from the presence of high spin Mn³⁺ ions. Hence, morphology controlling^[8], doping^[9-12] and surface coating^[13-17] were considered as effective ways to improve the electrochemical perfor-mance of LiNi_0.5Mn_1.5O₄ materials. Various morphologies of LiNi_0.5Mn_1.5O₄, such as nanoparticles^[18], nanorods^[19], and microspheres^[20], have been successfully fabricated to improve the electrochemical performance. However, nanomaterial frequently results in a low volumetric energy density of the cell. A variety of methods used to prepare LiNi_0.5Mn_1.5O₄ have been developed, including solid-state reaction^[21], sol-gel^[22], emulsion drying^[23], composite carbonate process^[24], hydrothermal method^[25] and co-precipitation^[26]. Among those routes to preparation of cathode materials, the co-precipitation is one of the most effective and conventional and inexpensive methods to synthesize the final product of LiNi_0.5Mn_1.5O₄^[27]. Li₃PO₄ is known to be a fast solid lithium ionic conductor^[28], and Li₃PO₄ coating has been used to improve the electrochemical performance of LiMn₂O₄^[29], LiCoO2^[30], LiFePO₄^[31] cathode materials. With this consideration, we have developed a novel ethanol-assisted co-precipitation method to synthesize spherical LiNi_0.5Mn_1.5O₄ and Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ composites. With this method, the Mn³⁺ in the LiNi_0.5Mn_1.5O₄ can be efficiently limited. Consequently, the overall electrochemical performance of the LiNi_0.5Mn_1.5O₄ can be obviously improved.

  1   Experimental

  1.1   Material preparation

  For the preparation of Li₃PO₄, a certain amount of LiOH and H₃PO₄ was dissolved in deionized water, and then heated in water bath with mechanical stirring at 80 ℃ for 6 h. Then the turbid liquid was filtered, and dried in vacuum drying oven for 12 h at 120 ℃, yielding Li₃PO₄ powders. LiNi_0.5Mn_1.5O₄ powders were prepared by ethanol-assisted oxalic acid co-precipitation method. The NiSO₄·6H₂O and MnSO₄·H₂O were dissolved in the mixed solution of deionized water and ethanol with a molar ratios of 1:2, and named solution 1. The NH₄HCO₃ was also dissolved in the deionized water, and the molar ratio between NH₄HCO₃ and sulphate (NiSO₄·6H₂O+MnSO₄·H₂O) is 1:1, and named solution 2. The solution 1 and solution 2 were mixed, and then the resulting precursor solution was transferred to a Teflon-lined stainless steel autoclave and heated at 200 ℃ for 10 h. The powder deposited at the bottom of the reactor was collected by centrifugation. The powder and appropriate Li₂CO₃ was mixed, then the precursors were heat treated at 800 ℃ for 12 h at ambient condition, and then treated at 600 ℃ for 6 h, and then air-cooled to the room temperature, yielding LiNi_0.5Mn_1.5O₄ dark powders.

  For the preparation of LiNi_0.5Mn_1.5O₄@Li₃PO₄, the Li₃PO₄ was added to the mixture of solution 1 and solution 2, and transferred to a Teflon-lined stainless steel autoclave and heated at 200 ℃ for 10 h. The following synthesis process is the same for LiNi_0.5Mn_1.5O₄ powders. The predetermined amounts of Li₃PO₄ in LiNi_0.5Mn_1.5O₄ are 5% and 10% (mass percent), respectively.

  1.2   Material characterization

  The crystal structure was characterized by X-ray diffractometry (XRD) measurements performed on a Rigaku instrument with Cu Kα₁ radiation (45 kV, 50 mA, step size=0.02°, 10° < 2θ < 90°). The morphology and the microstructure of the products were examined by a scanning electron microscopy (SEM, SU8000). FT-IR spectroscopy of the samples was performed using a Nicolet Nexus 6700 FT-IR spectrophotometer with a resolution of 4 cm^-1. A total of 1.5 mg sample dried at 120 ℃ was thoroughly mixed with 200 mg KBr and pressed into pellets and the scans were performed immediately to avoid water absorption. The frequency range was 800~400 cm^-1.

  1.3   Electrochemical analysis

  The electrochemical characterizations were performed using CR2025 coin-type cell. The working electrode was prepared by mixing 80% (mass percent) active material, 10% (mass percent) conductive super P carbon and 10% (mass percent) polyvinylidene fluoride (PVDF) as binder in N-methyl pyrrolidinone (NMP). After being uniformly coated onto a copper foil, the slurry was dried in a vacuum at 120 ℃ for 10 h. A solution of 1 mol·L^-1 LiPF₆ dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1, in volume) was used as the electrolyte and porous polypropylene Celgard 2300 was used as separator. The charge-discharge measurements were recorded on multichannel Land Battery Test System (Wuhan Jinnuo, China) at room temperature between 3.5 and 4.95 V (vs Li/Li⁺) carried out at different charge-discharge rates. Cyclic voltammetry (CV) test was carried out on a CHI 1000C electrochemical workstation with a voltage between 3.5 and 4.95 V at a scanning rate of 0.05 mV·s^-1. Electrochemical impedance spectroscopy (EIS) of OCV (open circuit voltage, before cycle) is measured by a Princeton P4000 electrochemical working station over a frequency range from 0.01 Hz to 10 kHz at a potentiostatic signal amplitude of 5 mV. The open circuit voltage is about 3.2 V.

  2   Results and discussion

  Fig. 1 shows the X-ray diffraction patterns of the pristine and Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ powders. All the sharp diffraction peaks can be attributed to the well-defined cubic spinel structure of LiNi_0.5Mn_1.5O₄. This indicates that the Li₃PO₄ coating does not change the spinel structure of LiNi_0.5Mn_1.5O₄. In addition, no trace of impurity phase (such as Li_xNi_1-xO or NiO) is detected. However, the characteristic Li₃PO₄ diffraction peak is not obvious, indicating that the Li₃PO₄ in the LiNi_0.5Mn_1.5O₄/Li₃PO₄ composites are amorphous during the calcination process. The strong reflections located at 19.1°, 36.8°, 38.5°, 44.5°, 48.8°, 58.9°, 64.8°, 68.1°, 76.6° and 77.6° can be indexed to the (111), (311), (222), (400), (331), (511), (440), (531), (533) and (620) diffractions, respectively. It is well known that LiNi_0.5Mn_1.5O₄ has two different space groups, Fd3m or P4₃32 depending on Ni ordering in the lattice. In the Fd3m structure, Li⁺ ions occupy the tetrahedral (8a) sites; Mn or Ni ions reside at the octahedral (16d) sites random; and O2-ions are located at (32e) sites. In P4₃32 structure the Li atoms are located at 8c sites, Ni atoms at 4a sites, Mn atoms at 12d sites, and O atoms at 8c and 24e sites^[32-33]. In LiNi_0.5Mn_1.5O₄ with P4₃32 space group, it can be found a decrease of lattice parameter and symmetry caused by cation ordering, and weak superstructure reflections around 2θ≈15°, 24°, 35°, 40°, 46°, 47°, 57°, and 75° can be found ^[34]. However, the scanning rate in this work was too fast for us to detect them.

  Figure 1.  XRD pattern of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  In fact, the structural difference between these two space groups is hardly to be clearly distinguished by X-ray diffraction because of the similar scattering factors of Ni and Mn. FT-IR spectroscopy has proved to be an effective technique in qualitatively resolving the cation ordering (Fig. 2). It has been reported that the peak at about 623 cm^-1 in Fd3m phase are more intensive than those at 593 cm^-1, which is contrary to the P4₃32 phase^[35]. In addition, three new peaks at about 650, 470 and 432 cm^-1 are absent in Fd3m structure. Hence, it can be concluded that pristine LiNi_0.5Mn_1.5O₄ and 10% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ has a space P4₃32 groups. However, the peak of 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ at about 623 cm^-1 are weaker than that at 593 cm^-1, revealing that 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ has a P4₃32 and Fd3m mixed phase. This indicates that 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ has the biggest degree of disorder among all samples. It has been reported that the crystal with disordered space groups have the better transmission path of electronic and Li^{+ [36]}. Therefore, it can be concluded that the 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ has the better electrochemical performance than ordered LiMn_1.5Ni_0.5O₄ and 10% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄.

  Figure 2.  FT-IR of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Fig. 3 shows the SEM images of the pristine and Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ powders. It can be found that the particles of all samples exist as homogeneous sphere-like particles. The diameters of the spheres distribute within the range of 1~1.5 μm. The sphere-like particles are composed of nano particle LiNi_0.5Mn_1.5O₄ at about 100 nm. Very small particles of coated LiNi_0.5Mn_1.5O₄ powders were found to be highly dispersed on the coated Li₃PO₄ particles as shown in Fig. 3b and Fig. 3c. In addition, the surface morphology of pristine LiNi_0.5Mn_1.5O₄ is extremely smooth. From a comparison of this three powders surface morphology, it can be speculated that the surface of the prepared LiNi_0.5Mn_1.5O₄ is covered with small Li₃PO₄. This indicates that the surface modification leads to formation of uniform coating.

  Figure 3.  SEM images of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Fig. 4 examines the initial charge and discharge behaviors of a series of Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ materials at room temperature in 3.5~4.95 V range, at a current density of 0.2C. The initial discharge capa-cities of pristine and 5, 10% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ cathode material are 118.1, 131.6 and 132.9 mAh·g^-1, respectively. Obviously, Li₃PO₄ coating increases the initial discharge capacity of LiNi_0.5Mn_1.5O₄ cathode.

  Figure 4.  Initial charge-discharge curves of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Fig. 5 shows the rate capabilities for three samples. The cells were discharged at increasingly higher currents from 0.2C to 0.5C rates at room temperature. It can be found that the capacity difference between the pristine LiNi_0.5Mn_1.5O₄ electrode and 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ electrode progressively increased. At the 0.5C charge-discharge rate after 20 cycles, the discharge capacity was 107.4 mAh·g^-1 for the pristine LiNi_0.5Mn_1.5O₄ material (90.9% of the capacity at 0.2C), and 120 mAh·g^-1 for the 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ material (91.2% at 0.2C). This result indicates that the Li₃PO₄ coating improves the rate capability of LiNi_0.5Mn_1.5O₄ as well as its capability to store Li ions. The reason may be suggested as follows.

  Figure 5.  Rate performance of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Trace water impurity in the electrolyte would cause the liberation of acid HF through the decom-position of LiPF₆-based electrolyte. The chemical reactions were proposed as follows^[37-38]:

  HF will dissolve LiNi_0.5Mn_1.5O₄ proposed as follows^[39]:

  Therefore, it can be concluded that a uniform Li₃PO₄ coating on the surface of the LiNi_0.5Mn_1.5O₄ not only can act as an ion-conductive layer, but also acts to suppress the decomposition of Mn and Ni during cycling, as demonstrated in Fig. 6. However, 10% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ electrode shows an unsati-sfactory rate performance, indicating that the contents of coated Li₃PO₄ have strong impact on the rate capability of LiNi_0.5Mn_1.5O₄ electrode. Therefore, it is important to optimize the coated Li₃PO₄ content in order achieve a good cell performance.

  Figure 6.  Schematic illustration of how the Li₃PO₄ layer acts as a conductive and protective layer to suppress the direct contact between electrolyte and LNMO and decomposition of electrolyte

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  Fig. 7 presents typical cyclic voltammograms (CVs) of pristine and Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄. The intense and sharp reduction/oxidation peaks of Ni²⁺/Ni⁴⁺ are observed at around 4.7 V in pristine and Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄, with trace amount of the couple of Mn⁴⁺/Mn³⁺ that usually appears at around 4.0 V shown in inset. The appearance of 4 V peak was due to Mn³⁺ which was formed by the oxygen loss during high temperature calcinations. It can be found that the CV curve of pristine LiNi_0.5Mn_1.5O₄ presents a much more obvious redox peaks in the potential region around 4.0 V (from the redox couples Mn⁴⁺/Mn³⁺), which means that the oxygen deficiency is more severe from the oxygen loss due to the Li₃PO₄ coating^[40]. This indicates that Li₃PO₄ coating destroys the ordering of Ni and Mn ions, and the proportion of Fd3m spinel increase with the rise of Li₃PO₄ content.

  Figure 7.  CV curves of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  The potential differences between anodic and cathodic peaks reflect the polarization degree of the electrode^[41]. The potential difference of the pristine and Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ electrodes between oxidation and reduction peaks is listed in Table 1. It can be found that the potential difference (Δφ_p=φ_pa-φ_pc) of pristine LiNi_0.5Mn_1.5O₄ is 393 mV, obviously much larger than those for the pristine and Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ electrodes. 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ sample shows the lowest potential interval between anodic and cathodic peak (353 mV), which indicate that the right amount of Li₃PO₄ coating is favorable for reducing the electrode polarization. This means that 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ has the excellent electrochemical reversibility and faster lithium insertion/extraction kinetics. This observation confirms that right amount of Li₃PO₄ coating enhances the reversibility of the LiNi_0.5Mn_1.5O₄, and then exhibits reversibility and good rate capability.

  Table 1.  Peak potential differences of CV test for pristine and coated LiNi_0.5Mn_1.5O₄ material

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  Sample	φpa/V	φpc/V	△φp / mV
  LiNi0.5Mn1.5O4	4.924	4.531	393
  5% Li3PO4-coated LiNi0.5Mn1.5O4	4.920	4.567	353
  10% Li3PO4-coated LiNi0.5Mn1.5O4	4.855	4.473	382

  Table 1.  Peak potential differences of CV test for pristine and coated LiNi_0.5Mn_1.5O₄ material

  The kinetics of lithium ion extraction and insertion of the pristine and Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ electrodes were further investigated by EIS. Fig. 8 shows the Nyquist plots of all samples, and the inset is the equivalent circuit used to fit impedance spectra. The circuit consists of R_s (ohmic resistance), R_f (the resistance of a solid electrolyte interphase film), C_f (the capacitance of a solid electrolyte interphase film), R_ct (charge-transfer resistance), C_dl (double layer capacitance for lithium-ion intercalation) and W (Warburg impedance of solid phase diffusion)^[42-43]. The fitted results are summarized in Table 2.

  Figure 8.  Nyquist plots of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Table 2.  Some fitting parameters obtained by EIS

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  Sample	Rs/Ω	Rct/Ω	i0 / (mA·cm-2)	DLi/ (cm2·s-1)
  LiNi0.5Mn1.5O4	9.488	1 855	0.009	7.89×10-17
  5% Li3PO4 coated-LiNi0.5Mn1.5O4	6.684	545	0.031	2.72×10-16
  10% Li3PO4 coated-LiNi0.5Mn1.5O4	7.158	802	0.02!	6.11×lO-17

  Table 2.  Some fitting parameters obtained by EIS

  The R_s reflects electric conductivity of the electrolyte, separator, and electrodes. It can be found that 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ has the smallest ohmic resistance among all samples, indicating a high conductivity between electrolyte and electrodes. Table 2 shows that the charge transfer resistance of Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ electrode is much lower than that of the pristine one. This reveals that Li₃PO₄ modifica-tion is favorable to improve upon the electronic cond-uctivity. In addition, 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ has the smallest charge transfer resistance among all samples. It is reasonable to infer that the lowest charge transfer resistance of 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ electrode corresponds with the smallest electrochem-ical polarization, and then lead to the best electro-chemical performance. Afterwards, the exchange current density, i₀, can be calculated by means of the charge transfer resistance,

  where R is the gas constant (8.314 5 J·mol^-1·K^-1); T is the absolute temperature (298.15 K); F is the Faraday′s constant (96 485 C·mol^-1), and A is the area of the electrode surface (1.54 cm²). The calculated results are given in Table 2. Obviously, 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄ has the biggest c exchange current density among all samples, revealing the lowest intercalation/deintercalation resistance and highest electrochemical activity.

  As we know, lithium ion diffusion rate also plays an important role in accelerating lithium ion insertion/extraction during the charge/discharge process^[44]. The diffusion coefficient of lithium ion (D_Li) can be calculated from the plots in the low frequency region, and can be obtained according to the following equations^[45]:

  where ω is the angular frequency in the low frequency region; R is the gas constant (8.314 5 J·mol^-1·K^-1); T is the absolute temperature (298.15 K); n is the number of electrons transferred in the half-reaction for the redox couple (n=1); F is the Faraday′s constant (96 485 C·mol^-1); A is the area of the electrode surface (1.54 cm²); C_Li is the molar concentration of Li⁺ ions calculated by the molar volume (2.37×10^-2 mol·cm^-3)^[46], and σ is the Warburg impedance coefficient, which is relative to Z_re-σ can be obtained from the slope of the lines in Fig. 9.

  Figure 9.  Graph of Z_re plotted against ω^-1/2 in the low-frequency region for the pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  The calculated diffusion coefficient of lithium ion is given in Table 2. For the 5% Li₃PO₄-coated LiNi_0.5 Mn_1.5O₄, lithium-ion diffusion coefficient is estimated to be 2.72×10^-16 cm²·s^-1, which is larger than that of 7.89×10^-17 cm²·s^-1 for pristine LiNi_0.5Mn_1.5O₄ cathode. Considering the similar particle sizes and morpho-logies of three samples, it can be concluded that the improved lithium-diffusivity might be attributed to the modification of Li₃PO₄. Based on the above calcula-tion, the charge transfer resistance and lithium-ion diffusion coefficient indicate that Li₃PO₄ coating can enhance the conductivity of LiNi_0.5Mn_1.5O₄, enabling much easier charge transfer at the interface between the electrode and the electrolyte.

  Beaulieu et al reported that lithium ions can react with the grain boundary phase in polycrystalline materials or the liquid electrolyte at the solid/liquid interface^[47]. According to the model of the LiNi_0.5Mn_1.5O₄-Li₃PO₄ composites shown in Fig. 10, in situ coated Li₃PO₄ is tightly combined with LiNi_0.5Mn_1.5O₄, and then many LiNi_0.5Mn_1.5O₄-Li₃PO₄ phase interfaces can be formed. Li₃PO₄ is a super ionic conductors, and the Li ionic conductivity of Li₃PO₄ (about 10^-6 S·m^-1) facilitates the charge transfer reactions on the electrode/electrolyte interface^[48]. The combination of in situ coated Li₃PO₄ can improve the Li diffusion coefficient and reduce the charge transfer resistance. The LiNi_0.5Mn_1.5O₄-Li₃PO₄ phase interfaces can also store electrolyte and provide more places for the insertion/extraction reactions of lithium ions, and then improve the reaction kinetics and reduce electro-chemical polarization during cycling. Thus it may be a reason for the superior high rate capability of Li₃PO₄-coated LiNi_0.5Mn_1.5O₄. Hence, Li₃PO₄ in situ modifica-tion is an effective way to improve the electrochemical performance of LiNi_0.5Mn_1.5O₄.

  Figure 10.  odel of the LiNi_0.5Mn_1.5O₄-Li₃PO₄ composites

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  3   Conclusions

  Surface modification of the spherical LiNi_0.5Mn_1.5O₄ is successfully done by Li₃PO₄ coating by the precipitation method. The combination of in situ coated Li₃PO₄ can improve the Li diffusion coefficient and reduce the charge transfer resistance of LiNi_0.5Mn_1.5O₄, and then provides more places for the insertion/extraction reactions of lithium ions, leading to the improvement of the reaction kinetics. 5% Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ exhibits the lowest charge-transfer resistance and the highest lithium diffusion coefficient among all samples, and it thus shows higher discharge capacities and better rate capability than the pristine material. The improved electrochemical properties also can be attributed that the Li₃PO₄ coating layer retards the side reactions of the active material with electrolyte. Hence, it is reasonable to infer that the Li₃PO₄ coating would be an effective way to improve the electrochemical properties of LiNi_0.5Mn_1.5O₄ cathode materials.

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  Figure 1  XRD pattern of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 2  FT-IR of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 3  SEM images of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 4  Initial charge-discharge curves of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 5  Rate performance of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 6  Schematic illustration of how the Li₃PO₄ layer acts as a conductive and protective layer to suppress the direct contact between electrolyte and LNMO and decomposition of electrolyte

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  Figure 7  CV curves of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 8  Nyquist plots of pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 9  Graph of Z_re plotted against ω^-1/2 in the low-frequency region for the pristine and coated LiNi_0.5Mn_1.5O₄ (a) pristine LiNi_0.5Mn_1.5O₄; (b) 5% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄; (c) 10% Li₃PO₄ coated-LiNi_0.5Mn_1.5O₄

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  Figure 10  odel of the LiNi_0.5Mn_1.5O₄-Li₃PO₄ composites

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  Table 1.  Peak potential differences of CV test for pristine and coated LiNi_0.5Mn_1.5O₄ material

  Sample	φpa/V	φpc/V	△φp / mV
  LiNi0.5Mn1.5O4	4.924	4.531	393
  5% Li3PO4-coated LiNi0.5Mn1.5O4	4.920	4.567	353
  10% Li3PO4-coated LiNi0.5Mn1.5O4	4.855	4.473	382

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  Table 2.  Some fitting parameters obtained by EIS

  Sample	Rs/Ω	Rct/Ω	i0 / (mA·cm-2)	DLi/ (cm2·s-1)
  LiNi0.5Mn1.5O4	9.488	1 855	0.009	7.89×10-17
  5% Li3PO4 coated-LiNi0.5Mn1.5O4	6.684	545	0.031	2.72×10-16
  10% Li3PO4 coated-LiNi0.5Mn1.5O4	7.158	802	0.02!	6.11×lO-17

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