English

• 0.   Introduction

  Lithium-ion batteries (LIBs) are believed to be the most competitive products in energy storage fields because of their high energy density^[1-3]. However, the active materials, especially cathodes of LIBs, still need to be further investigated to meet the increasing demands in emerging fields like electric vehicles (EVs) and advanced portable devices, where high power/energy densities are urgently needed^[4-8]. LiNi_0.5Mn_1.5O₄ (LNMO) has received a great deal of attention because of its high voltage (4.7 Ⅴ), high theoretical specific capacity (146.7 mAh·g^-1), high energy density (650 Wh·kg^-1), rapid Li⁺ diffusion channels, and low cost^{[7, 9-12]}.

  However, because the working voltage of the LNMO anode material is too high, it exceeds the voltage window of stable operation of the electrolyte (4.3 Ⅴ)^{[7, 13]}, resulting in its oxidation and decomposition, and the generated byproducts have side reactions with the contact of LNMO material^{[12, 14-15]}. In addition, the reaction between LNMO and HF will lead to the dissolution of transition metal, thus further destroying the crystal structure of the cathode material, reducing its discharge-specific capacity, and deteriorating the electrochemical performance of the cells^[15-18]. The existence of these problems has seriously affected the electrochemical performance of LNMO.

  Commonly employed modification methods, such as surface modification^{[13, 19-25]}, elemental doping^[26-31], and nanocrystallization^[32-33], can effectively enhance the electrochemical properties of LNMO to some extent. Among them, surface coating modification is one of the effective ways to improve the electrochemical properties of LNMO materials. As a widely recognized protective mechanism, these coating materials can inhibit the oxidation of electrolytes on the surface of cathode materials and the dissolution of transition metal cations, thereby slowing the occurrence of side reactions^[34].

  Herein, AlPO₄ (AP) coating modified LNMO (LNMO-AP) was prepared. The differences in morphology and electrochemical properties of LNMO cathode materials before and after AP coating were compared, and the effects of AP coating on its structure and electrochemical properties were studied. The results indicated that compared with the LNMO|Li cell, the LNMO-1%AP|Li cell exhibited an obvious improvement in the cycling stability and rate capability at room temperature. These performance improvements were attributed to the fact that AP coating made the surface of LNMO in contact with the electrolyte more stable, effectively promoted the Li⁺ transport, and reduced the polarization voltage of the electrode. The successful synthesis of LNMO-AP composite materials presents a novel approach for advancing LIBs materials.

  1.   Experimental

  1.1   Preparation of LNMO

  Pristine LNMO was synthesized by a facile high-temperature solid-phase method. Li₂CO₃ (Innochem), Mn₃O₄ (Aladdin), and NiO (Aladdin) with a molar ratio of 0.515∶0.5∶0.5 for the preparation of LNMO were dispersed in 100 mL of anhydrous ethanol. After ball-milling for 10 h and drying to get mixed powder, the mixture powder was pre-calcined at 500 ℃ for 5 h, then at 850 ℃ for 12 h, and finally annealed at 600 ℃ for 5 h in the air atmosphere to obtain the final pristine LNMO material.

  1.2   Preparation of LNMO-AP

  AlPO₄-coated spinel LNMO was synthesized by a wet chemical method. Al(NO₃)₃·9H₂O (Innochem), (NH₄)₂HPO₄ (Macklin), and LNMO were first dissolved in distilled water to form a mixed solution, then, the mixed sample was evaporated in the magnetic stirring heater (80 ℃) until the pure water basically evaporated, and in a vacuum drying oven for 12 h, subsequently sintered at 550 ℃ for 5 h to obtain a series of AlPO₄-coated LNMO materials. Based on the mass ratios of AlPO₄ to LNMO were 0, 1.0%, and 2.0%, the finally obtained powders were noted as LNMO, LNMO-1%AP, and LNMO-2%AP, respectively.

  1.3   Preparation of LNMO and LNMO-AP cathodes

  The cathode was prepared from the slurry, which comprised active materials, Super P, and polyvinylidene fluoride (PVDF) (Canrd) binder in a mass ratio of 8∶1∶1 in N-methyl pyrrolidinone (NMP) (Canrd) under sufficient stirring to form a homogenous slurry. The slurry was applied onto the aluminum foil using a coater and then dried at 60 ℃ for 12 h under a vacuum. Finally, a slicing machine was used to cut the material into small circular pieces of about 12 mm, and rectangular pieces of about 40 mm×55 mm. The areal mass loading of active materials was about 1.8 mg·cm^-2.

  1.4   Assembly of cells

  LNMO|Li and LNMO-AP|Li coin cells: The CR2032-type coin cells were meticulously assembled in the glove box following the sequence of lithium disc, PP (porous polypropylene, Celgard 2500) film, LNMO or LNMO-AP cathodes sheet, and steel disc.

  LNMO||Li and LNMO-AP||Li pouch cells: lithium foil anode, LNMO or LNMO-AP cathode, and PP were assembled in the glove box, and the anode and cathode were connected to copper foil and aluminum foil, respectively. Finally, it was encapsulated with aluminium-plastic film.

  The glove box environment was strictly regulated to maintain oxygen and moisture levels (volume fraction) below 10^-8. The electrolyte was 1 mol·L^-1 LiPF₆ solution in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (1∶1∶1, V/V).

  1.5   Characterizations

  The X-ray diffraction (XRD, Bruker D8 Advance, Germany) with Cu Kα radiation source (λ=0.154 nm) was performed to identify the crystalline structure of the as-prepared samples in the 2θ range of 10°-90° with a scanning rate of 5 (°)·min^-1 at room temperature (40 kV, 100 mA). Raman spectrum was acquired by using a laser Raman tester (LabRAM Odyssey, Japan) at 514 nm excitation with a wavenumber range of 200 to 800 cm^-1. Fourier transformation infrared spectrometer (FTIR, Thermo Scientific Nicolet iS5, Ever-Glo^TM, America) was used to identify the Ni/Mn ordering distribution in samples with a wavenumber range of 400 to 700 cm^-1. The surface morphology and microstructures of the samples were characterized by scanning electron microscope (SEM, Zeiss Gemini300, 10 kV, Germany) and transmission electron microscope (TEM, JEOL JEM-F200, 100 kV, Japan). The X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha Plus, hν=1 486.6 eV, America) analysis was performed to examine the surface information of the prepared samples. The nitrogen adsorption-desorption isotherms of the samples were recorded on a gas adsorption apparatus (BELSORP-mini Ⅱ, Japan) to produce the specific surface area.

  1.6   Electrochemical characterizations

  Galvanostatic charge/discharge tests and rates performance tests were measured using the Neware batteries testing system with the working over a voltage range of 3.5-4.9 Ⅴ. Electrochemical impedance spectrum (EIS) measurements of the cells were performed on an electrochemical workstation (CHI604E, China) in a range of 1 MHz to 0.01 Hz and the amplitude of 0.1 mV. The cyclic voltammetry (CV) curves were tested by the voltage range of 3.5-4.9 Ⅴ at 0.1 mV·s^-1. The galvanostatic intermittent titration technique (GITT) experiments were measured using the Neware batteries testing system at working voltages of 3.5-4.9 Ⅴ for charge/discharge analysis at a rate of 0.1C (1C=146.7 mA·g^-1), with a charge/discharge time of 10 min and a rest time of 1 h.

  2.   Results and discussion

  2.1   Preparation process

  LNMO was first prepared by the high-temperature solid phase method, and then AP was coated on the LNMO surface by the wet chemical method (Fig. 1).

  图 1

  

  Figure 1.  Schematic diagram of LNMO and LNMO-AP preparation

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  2.2   Characterization and analysis

  To further determine the crystal structure of the samples, XRD tests were performed (Fig. 2a). By analyzing the shape of diffraction peaks, it could be seen that LNMO had a disordered Fd3m space group and a standard LNMO crystal structure (PDF No.80-2162)^[35]. In addition, no obvious impurity diffraction peak was found in the XRD patterns of LNMO‑1%AP and LNMO-2%AP. Moreover, the amount of AP coating was relatively small, only 1% and 2%, indicating that the AP coating did not affect the crystal structure of LNMO or the spatial group structure of Fd3m^{[9, 36]}.

  图 2

  

  Figure 2.  (a) XRD patterns of all samples; (b) Raman and (c) FTIR spectra of LNMO and LNMO-1%AP

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  To study the structural characteristics of the samples further, Raman and FTIR spectra were performed. Raman spectra (Fig. 2b) showed two main peaks at 490 and 630 cm^-1, exhibiting the Ni—O and Mn—O stretching modes, respectively. The peaks located at 580-630 cm^-1 had no obvious splitting, which is characteristic of the typical disordered Fd3m structure^{[11, 24, 37-38]}. FTIR spectra (Fig. 2c) showed the characteristic Mn—O and Ni—O bonds at 400 to 700 cm^-1. These bonds were utilized to assess the order of cations in the spinel lattice′s 16d sites. Five absorption peaks at 622, 580, 556, 502, and 468 cm^-1 can be seen in the spectrum. The absorption peaks at 622 and 556 cm^-1 are related to the vibration of Mn—O, while the absorption peaks at 580 and 502 cm^-1 are related to the vibration of Ni—O^{[2, 39]}. In addition, the peak intensity of the Mn—O band at 622 cm^-1 was higher than that of the Ni—O band at 580 cm^-1 for the two samples, which proves the existence of a disordered Fd3m structure^{[36, 39]}. Therefore, based on the FTIR and Raman spectra data, all materials belong to the disordered Fd3m structure.

  The specific surface area of a material is crucial to its practical application. For the cathode material of LIBs, the larger specific surface area is favorable to improve the Li⁺ transport rate, thus improving the performance of batteries. This is because the larger specific surface area can increase the contact area between the cathode material and electrolyte and improve the electrochemical reaction rate and ion transport rate. The test data showed that the specific surface area of LNMO-1%AP was up to 4 m²·g^-1, which was greater than LNMO (2 m²·g^-1) and LNMO-2%AP (3 m²·g^-1).

  The SEM image of the original LNMO (Fig. 3a) with sharp edges and octahedral shapes indicated that the LNMO crystals were well-developed. LNMO particles were morphologically intact with a smooth and flat surface, and complete spinel-type characteristics could be observed^[37]. Compared with the original LNMO, there were obvious differences on the surface of LNMO-1%AP crystal particles. It could be seen that the surface of the LNMO-1%AP crystal particles covered a coating (Fig. 3b). We could see clearly that the Mn, Ni, O, P, and Al (Fig. 3c-3g) elements were distributed uniformly in the LNMO-1%AP, indicating that the coating was successful. The TEM image of LNMO displayed octahedral morphology (Fig. 3h), which was consistent with the results of SEM. To investigate the crystal particles furtherly, we used high-resolution TEM (HRTEM) to test the samples, and the lattice spacing of about 0.47 nm belongs to the (111) planes of spinel LNMO (Fig. 3i)^{[7, 27]}. There were uniform AP coatings on the LNMO-1%AP (Fig. 3j) and LNMO-2%AP (Fig. 3k) surfaces; those were also consistent with the energy dispersive X-ray spectroscopy (EDS) results, and the coating layer thickness of the former was 12 nm and that of the latter was 22 nm.

  图 3

  

  Figure 3.  SEM images of (a) LNMO and (b) LNMO-1%AP; EDS mappings of (c) Mn, (d) Ni, (e) O, (f) P, and (g) Al of LNMO-1%AP; (h) TEM image of LNMO; HRTEM images of (i) LNMO, (j) LNMO-1%AP, and (k) LNMO-2%AP

  The right inset of i: Enlarged view of lattice stripe.

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  We used XPS in the binding energy range of 0- 1 000 eV to investigate the surface chemical elements on the LNMO-1%AP cathode materials. The survey spectrum revealed the presence of several significant elements in the samples, including Ni, Mn, O, P, Li, and Al (Fig. 4a). The signal of Mn2p had two major significant characteristic peaks at 641.9 and 654.1 eV (Fig. 4b), representing Mn2p_3/2 and Mn2p_1/2, respectively^{[26, 39-40]}, indicated that the oxidation states of Mn were +3 and +4, confirmed the presence of Mn. The signal of Ni2p also had two major significant characteristic peaks at 854.7 and 872.2 eV (Fig. 4c), representing Ni2p_3/2 and Ni2p_1/2, respectively, indicating that the oxidation states of Ni were +2 and +3, confirming the presence of Ni^[38]. P2p (133.4 eV, PO₄^3-) spectrum (Fig. 4d) and Al2p (74.6 eV, Al—O) spectrum (Fig. 4e) confirmed the presence of P and Al, indicating that the AP layer was successfully modified on the LNMO surface^[41]. The deconvoluted O1s spectrum showed an M—O (M: metal) bond at 529.6 eV and a P—O bond at 531.5 eV (Fig. 4f).

  图 4

  

  Figure 4.  (a) Survey, (b) Mn2p, (c) Ni2p, (d) Al2p, (e) P2p, and (f) O1s XPS spectra for LNMO-1%AP

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  2.3   Electrochemical performance

  To elaborate the electrochemical kinetics between the electrolyte and the LNMO or LNMO-AP electrodes, the EIS measurements were performed, and the Nyquist plots can be obtained by analyzing theobtained data after the impedance test of the cells (Fig. 5a). It could be observed that these resultant Nyquist plots were composed of a semicircle and a sloping straight line. Where the intercepts at the high-frequency x-axis were the electrolyte resistance (R_s) of the cells, and the semicircles in the intermediate‑ frequency ranges correspond to the electrode charge-transfer resistance (R_ct), the constant phase element (CPE) is used to describe the behavior of actual electrode surface capacitance effects deviating from the ideal state, and the slope line in the low-frequency range corresponds to the Warburg impedance (Z_W)^{[9, 27, 42]}. The charge transfer resistance of the LNMO-1%AP|Li cell was the smallest (about 102 Ω), and that of the LNMO-2%AP|Li and LNMO|Li cells were about 113 and 150 Ω, respectively. Excessive AP layers could lead to a decrease in the efficiency of the layer, thus increasing the resistance to charge transfer.

  图 5

  

  Figure 5.  (a) Nyquist plots, (b) rate performance, and (c) the 20th cycle charge/discharge curves at 5C of the samples; Charge/discharge curves at different rates of (d) LNMO|Li, (e) LNMO-1%AP|Li, and (f) LNMO-2%AP|Li cells

  Inset: Equivalent circuit diagram.

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  We could see that the LNMO-1%AP|Li cell had the best performance (Fig. 5b), especially at the high rate of 5C and 10C, followed by the LNMO-2%AP|Li cell, and the LNMO|Li cell was the worst. However, at 0.1C, the discharge specific capacity of the LNMO-2%AP|Li cell was lower than the LNMO|Li cell, which may be because the coating layer was too thick. This indicates that a suitable thickness of the protective layer has a better Li⁺ diffusion path, which accelerates the Li⁺ diffusion rate and improves the rate performance of the cells.

  In the 20th cycle charge/discharge curves at 5C (Fig. 5c), it could be seen that the LNMO-1%AP|Li cell still showed the best performance. All cells had a platform at ca. 4.7 Ⅴ during the discharged process, corresponding to the Ni²⁺/Ni⁴⁺ redox reaction, and at ca. 3.8-4.0 Ⅴ, corresponding to the redox couple of Mn⁴⁺/Mn^{3+ [6, 11]}.

  The charge/discharge curves of the cells at different rates (Fig. 5d-5f), with the increasing of the current density, LNMO-1%AP|Li cell showed the highest discharge specific capacity, implying the excellent redox reaction kinetics, which is attributed to the high Li⁺ diffusion rate endowed by AP coating.

  To explore the impact of AP coating, we studied the long cycling performance of the cells at 1C. It can be seen that the discharge-specific capacity of the cells decreased with the increased cycle times (Fig. 6a). The discharge‑specific capacity of the LNMO|Li cell decreased more and more rapidly, while the decreases of the LNMO-1%AP|Li and LNMO-2%AP|Li cells were not apparent. Especially in the 450th cycle, the discharge‑specific capacity of the LNMO|Li cell decreased to 86.04 mAh·g^-1, and that of the LNMO-1%AP|Li cell decreased to 108.78 mAh·g^-1. It can be seen that the appropriate amount of AP coating could effectively improve the cycling performance of the LNMO|Li cell, however, excessive AP coating could also weaken the cycling performance of the cells.

  图 6

  

  Figure 6.  (a) Cycling performances of the samples at 1C; Charge/discharge curves of (b) LNMO, (c) LNMO-1%AP|Li, and (d) LNMO-2%AP|Li cell at 1C under different cycle numbers; (e) CV curves at 0.1 mV•s-1 of the samples

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  With the increasing cycling number (Fig. 6b-6d), the LNMO-1%AP|Li cell also maintained the highest discharge specific capacity and a longer discharge platform at around 4.7 Ⅴ. This was due to the AP protective layer, which ensured the structural stability of the LNMO material, as well as making the surface of LNMO in contact with the electrolyte more stable, resulting in better capacity retention.

  The CV curves of the first cycle measured at 0.1 mV·s^-1 in the voltage range between 3.5 and 4.9 Ⅴ (Fig. 6e). Compared with the LNMO|Li cell, the LNMO-1%AP|Li cell showed a better CV curve in the first cycle, not only with a higher peak current but also with a relatively small polarization voltage of 0.17 Ⅴ.

  In the Voltage-time diagram (Fig. 7a), the LNMO-1%AP|Li cell exhibited the longest discharge time and the lowest polarization voltage under the same conditions. We used the GITT to measure the polarization voltage and the diffusion coefficients of Li⁺ (D_Li⁺)^[37](Fig. 7b). The LNMO-1%AP|Li cell exhibited the highest specific capacity and lowest polarization voltage. Based on the electrochemical results, the LNMO-1%AP|Li cell promoted higher specific capacity and improved stability than the LNMO|Li cell^[34]. We calculated the values of the D_Li⁺ from Fick′s second law and plotted the logarithms of the values of D_Li⁺concerning voltage (Working voltage: 4.7 Ⅴ) (Fig. 7c)^{[37, 43]}. The D_Li⁺value of the LNMO-1%AP|Li cell was higher than that of the LNMO|Li cell, suggesting that Li⁺ could diffuse rapidly in the AP coating layer. These results showed that surface modification with AP coating on cathode materials contributed to the conductive transport of Li⁺, thereby enhancing the electrochemical performance.

  图 7

  

  Figure 7.  (a) Voltage-time curves, (b) GITT curves at 0.1C, and (c) Li+ diffusion coefficients at different voltages of the samples

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  To evaluate the application of LNMO and LNMO-1%AP, we assembled an LNMO||Li pouch cell and an LNMO-1%AP||Li pouch cell. The open circuit voltage (OCV) of the LNMO||Li pouch cell maintained at 3.596 Ⅴ, which could successfully light up the light-emitting diode (LED) lamp with letters “Li-S” (Fig. 8a). The OCV of the LNMO-1%AP||Li pouch cell maintained at 3.348 Ⅴ, which also could successfully light up the LED lamp (Fig. 8b). The cycling performance of LNMO||Li and LNMO-1%AP||Li pouch cells were compared (Fig. 8c). The LNMO-1%AP||Li pouch cell had more stable cycling performance and high discharge specific capacity throughout the cycles, but the LNMO||Li pouch cell unstable and failed at around 50 cycles. We also analyzed the 1st, 5th, 50th, and 100th cycle performances of both pouch cells (Fig. 8d and 8e). The discharge-specific capacities of both cells were relatively low in the 1st cycle, however, the LNMO-1%AP||Li pouch cell improved more obviously in the 5th cycle. LNMO-1%AP||Li pouch cell could still have normal and stable charge/discharge curves in the 50th and 100th cycles, but the LNMO||Li pouch cell was almost invalid in the 50th cycle. It could be assumed that the AP coating played an important role in the application of pouch cells under high-rate charge/discharge conditions.

  图 8

  

  Figure 8.  (a) Photo of the LNMO||Li pouch cell showing an OCV of 3.596 Ⅴ and photo of the pouch cell lighting up LED lamps; (b) Photo of the LNMO-1%AP||Li pouch cell showing an OCV of 3.348 Ⅴ and photo of the pouch cell lighting up LED lamps; (c) Long cycling performance of pouch cells at 5C; Charge/discharge curves of (d) LNMO-1%AP||Li and (e) LNMO-1%AP||Li pouch cells at 5C under different cycle numbers

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  3.   Conclusions

  LNMO was prepared by using the traditional high-temperature solid phase method; in addition, AP was coated on the surface of the LNMO sample by the wet chemical method. The various electrochemical properties of LNMO|Li, LNMO‑1%AP|Li, and LNMO‑ 2%AP|Li cells were analyzed and compared. The LNMO-1%AP|Li cell had the highest specific capacity and the lowest capacity decay rate at different cycles. After 450 cycles at 1C, the discharge specific capacity of the LNMO-1%AP|Li cell decreased to 108.78 mAh·g^-1, however, the LNMO|Li cell decreased to 86.04 mAh·g^-1. Not only that, but the AP-coating was also helpful in improving the rate performance. This is attributed to the fact that the AP coating makes the surface of LNMO in contact with the electrolyte more stable, effectively promotes Li⁺ transport, and reduces the polarization voltage of the electrode. Therefore, coating AP with a protective layer is an effective way to improve the electrochemical performance of the LIBs cathode electrode material. This work is expected to drive the realization of low-cost, high-energy density, and high-performance LIBs.

  
  Acknowledgments: This work was financially supported by National Natural Science Foundation of China (Grants No.52372185, 52062004), Guizhou Provincial High Level Innovative Talents Project (Grant No.QKHPTRC-GCC[2022]013-1), Innovation Team for Advanced Electrochemical Energy Storage Devices and Key Materials of Guizhou Provincial Higher Education Institutions (Grant No.QianJiaoJi[2023]054), Advanced Electrochemical Energy Storage Devices and Key Materials Technology Innovation Talent Team Construction of Guizhou Province (Grant No.QKHPTRC-CXTD[2023]016).
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  Figure 1  Schematic diagram of LNMO and LNMO-AP preparation

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  Figure 2  (a) XRD patterns of all samples; (b) Raman and (c) FTIR spectra of LNMO and LNMO-1%AP

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  Figure 3  SEM images of (a) LNMO and (b) LNMO-1%AP; EDS mappings of (c) Mn, (d) Ni, (e) O, (f) P, and (g) Al of LNMO-1%AP; (h) TEM image of LNMO; HRTEM images of (i) LNMO, (j) LNMO-1%AP, and (k) LNMO-2%AP

  The right inset of i: Enlarged view of lattice stripe.

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  Figure 4  (a) Survey, (b) Mn2p, (c) Ni2p, (d) Al2p, (e) P2p, and (f) O1s XPS spectra for LNMO-1%AP

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  Figure 5  (a) Nyquist plots, (b) rate performance, and (c) the 20th cycle charge/discharge curves at 5C of the samples; Charge/discharge curves at different rates of (d) LNMO|Li, (e) LNMO-1%AP|Li, and (f) LNMO-2%AP|Li cells

  Inset: Equivalent circuit diagram.

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  Figure 6  (a) Cycling performances of the samples at 1C; Charge/discharge curves of (b) LNMO, (c) LNMO-1%AP|Li, and (d) LNMO-2%AP|Li cell at 1C under different cycle numbers; (e) CV curves at 0.1 mV•s-1 of the samples

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  Figure 7  (a) Voltage-time curves, (b) GITT curves at 0.1C, and (c) Li+ diffusion coefficients at different voltages of the samples

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  Figure 8  (a) Photo of the LNMO||Li pouch cell showing an OCV of 3.596 Ⅴ and photo of the pouch cell lighting up LED lamps; (b) Photo of the LNMO-1%AP||Li pouch cell showing an OCV of 3.348 Ⅴ and photo of the pouch cell lighting up LED lamps; (c) Long cycling performance of pouch cells at 5C; Charge/discharge curves of (d) LNMO-1%AP||Li and (e) LNMO-1%AP||Li pouch cells at 5C under different cycle numbers

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