Effect of calcining oxygen pressure gradient on properties of LiNi0.8Co0.15Al0.05O2 cathode materials for lithium ion batteries

Journal Pre-proof Effect of calcining oxygen pressure gradient on properties of LiNi0.8Co0.15Al0.05O2 cathode materials for lithium ion batteries Yan Nie, Wei Xiao, Chang Miao, Mingbiao Xu, Changjun Wang PII:

S0013-4686(20)30045-1

DOI:

https://doi.org/10.1016/j.electacta.2020.135654

Reference:

EA 135654

To appear in:

Electrochimica Acta

Received Date: 30 October 2019 Revised Date:

12 December 2019

Accepted Date: 6 January 2020

Please cite this article as: Y. Nie, W. Xiao, C. Miao, M. Xu, C. Wang, Effect of calcining oxygen pressure gradient on properties of LiNi0.8Co0.15Al0.05O2 cathode materials for lithium ion batteries, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135654. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Effect of calcining oxygen pressure gradient on properties of LiNi0.8Co0.15Al0.05O2 cathode materials for lithium ion batteries Yan Nie, Wei Xiao, Chang Miao, Mingbiao Xu, Changjun Wang College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou, 434023, P. R. China The synthesis schematic and working mechanism of NCA-0.1 and NCA-0.6

Effect of calcining oxygen pressure gradient on properties of LiNi0.8Co0.15Al0.05O2 cathode materials for lithium ion batteries Yan Nie, Wei Xiao∗, Chang Miao, Mingbiao Xu, Changjun Wang College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou, 434023, P. R. China Abstract: To understand the effects of the calcining pressure gradient on the structure, morphology, and electrochemical properties of LiNi0.8Co0.15Al0.05O2 (NCA), Ni0.8Co0.15Al0.05 (OH)2 precursors prepared using a coprecipitation method are calcined under gradually increasing oxygen pressures to fabricate different cathode materials. The results of characterization using X-ray diffraction and scanning electron microscopy suggest that the material (NCA-0.6) possesses the best comprehensive properties when the calcining oxygen pressure is 0.6 MPa, at which the values of I(003)/I(104) and D50 are appropriately 1.727 and 11.06 µm, respectively. A cell assembled with NCA-0.6 delivers a discharge specific capacity of 143.3 mAh g-1 with a capacity retention ratio of 80.3 % after 100 cycles at 1.0 C. Moreover, it presents a lower resistance of 327.5 Ω and redox peak difference of 0.11 V compared to the values of 1953.2 Ω and 0.26 V for NCA-0.1. These results suggest that appropriately enhancing the calcining oxygen pressure is an effective way to improve the cycle stability of NCA materials without any modified method. Key word: LiNi0.8Co0.15Al0.05O2; Cathode; Calcining oxygen pressure; Lithium ion batteries 1 Introduction With the rapid development of technology and science, an increasing amount of attention is being given to energy and environmental issues [1-4]. Therefore, there is an urgent need to develop green and safe power sources

[5-8]

. In the present market, lithium ion batteries (LIBs) represent one of the most

popular portable power sources because they are eco-friendly and have a high energy density, low self-discharge rate, and long cycle stability [9-12]. Moreover, the energy density of LIBs mainly depends on the cathode materials, suggesting that they play a vital role in LIBs system

[13-17]

. At present, the

most intensively investigated traditional cathode materials primarily consist of LiFePO4 [18, 19], LiCoO2 [20, 21]

, Ni-rich materials

[22-24]

, and so on. In addition, layered Ni-rich materials such as

LiNi1-x-yCoxAlyO2 (0< x ≤0.2, 0≤ y ≤0.2) are considered to be the most promising electrode materials for ∗

Corresponding author, Tel.: +86 716 8060984. E-mail address: [email protected] (W. Xiao)

1

LIBs because of their low cost, excellent safety performance, and high capacity. They have been widely applied in electric vehicles, mobile facilities, and other electronic equipments in daily life

[25-27]

. In

recent years, various methods for preparing LiNi1-x-yCoxAlyO2 (0< x ≤0.2, 0≤ y ≤0.2) have been established, such as spray pyrolysis, coprecipitation, and sol-gel method

[28, 29]

. However, the problems

of complicated processes, structural instability, and low thermal stability limit their further industrializations. A series of synthesis conditions from the precursor stage to LiNi1-x-yCoxAlyO2 (0< x ≤0.2, 0≤ y ≤0.2) cathode materials have also been detailedly explored by many researchers [30-32]. To be specific, more attention is being given to the effects of the lithium source, lithium compounds and precursors mixing ratio, calcining temperature, and calcining time in respect of the synthesis process. Controlling these factors has decisive effects on the Li/Ni mixing degree, alkalinity, particle size distribution, and other important parameters of materials, which can further influence the battery properties. In recent years, a number of modification methods have been explored to better solve these existing problems, which can be divided into doping and coating methods. In previous studies, cationic materials like Mo4+ [33,

34]

, Al3+ [35-37], Zr4+ [38-40], Mg2+ [41-43], and Ti4+ [44-46] were added during the

sintering process to calcine into the lattice. It is found that incorporating these inactive cationic materials into the lattice could lower the Li/Ni mixing degree and enhance the stabilization of the crystal structure in the repeated charge/discharge process. Moreover, a thin film layer of Li3PO4 AlF3

[48]

, LiZrO3

[49]

[47]

,

, or some other substances was employed to coat the surface of the secondary

particles to relieve side reactions between active materials and liquid electrolytes. The results provide solid evidences that these methods could effectively improve the electrochemical properties. Although many factors have been investigated in previous studies, few detailed investigations have been conducted to detect the effects of the calcining pressure on the structure, morphology, and electrochemical properties of LiNi1-x-yCoxAlyO2 (0< x ≤0.2, 0≤ y ≤0.2) or other Ni-rich materials [50]. In the present work, LiNi0.8Co0.15Al0.05O2 materials are prepared from Ni0.8Co0.15Al0.05(OH)2 precursors obtained using a coprecipitation method under different calcining oxygen pressures, and the effects of these different calcining oxygen pressures on the crystal structure, lattice parameters, surface morphology, and electrochemical performance of the LiNi0.8Co0.15Al0.05O2 are investigated in detail. 2 Experimental 2.1 Preparation of LiNi0.8Co0.15Al0.05O2 cathode materials As the starting materials, an aqueous solution of NiSO4·7H2O, CoSO4·7H2O, and Al2(SO4)3 at a 2

molar ratio of 0.800:0.150:0.025 is used to synthesize spherical Ni0.8Co0.15Al0.05(OH)2 (NCAOH) precursors using a coprecipitation method. First, the starting solution and a solution mixture of NaOH and NH3·H2O are slowly pumped into a reactor under a N2 atmosphere at 50 °C with the pumping progress lasting for 2 h, and the pH value is controlled at approximately 11 using an NH3·H2O solution in real time. Then, the obtained matcha green sediment are placed into an oven at 120 °C for 12 h after being washed and dried to produce the NCAOH precursors. Finally, the different targeted LiNi0.8Co0.15Al0.05O2 (NCA) cathode materials are prepared by calcining the mixture of NCAOH and LiOH·H2O at total metal ions to lithium ions about 1.00:1.05 in a pressureproof tube furnace (purchased from Hefei Kejing Materials Technology Co., Ltd.), in which the mixture are firstly heated at 480 °C for 5 h and then calcined at 750 °C for 15 h under the different oxygen pressures of 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 MPa by precisely adjusting the intake and outlet rate of oxygen. The NCA samples calcined under these different oxygen pressures are labeled as NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) for convenience. All of the chemicals were purchased from Aladdin Co., Ltd. with analytical grade and were used without any further purification. 2.2 Materials characterization The ratio of Ni, Co, and Al in the precursor is detected using inductively coupled plasma optical emission spectrometry (ICP-OES, VARIAN) to determine the optimal mixing ratio of lithium hydroxide and precursor. Thermogravimetry analysis (TGA, LABSY-SEVO) is used to analyze the optimal calcining temperature in the range of 30-900 °C with a heating rate of 10 °C·min-1 under an air atmosphere. The crystal structure is recorded using powder X-ray diffraction (XRD, EMPYREAN) in the range of 5-120 ° with a step size of 0.01 °, where the instrument is operated at 45 KV and 40 mA with CuKα radiation. The surface morphologies of the samples are observed by field-emission scanning electron microscope (FE-SEM, MIRA3, TESCAN) and the high-resolution transmission electron microscope (HR-TEM, FEI Tecnai G20s) is employed to observe the microstructure of the samples. A laser particle sizer (Zhuhai, OMEC) is used to measure the particle size as well as the particle size distribution. N2 adsorption-desorption measurements are conducted by using a Micromeritics ASAP2020 analyzer to study the specific surface area of the as-prepared samples. Fourier transform infrared spectra (FT-IR) of the samples are recorded with a Nicolet iS50 FT-IR spectrometer in the frequency of 4000-400 cm-1. The valence of elements on surface of the samples is analyzed via an X-ray photoelectron spectroscopy (XPS, Thermo K-Alpha). 3

2.3 Electrochemical performance The electrochemical performance of the NCA samples is measured by analyzing assembled CR2025 coin cells. First, the NCA cathode material is stirred with acetylene black and polyvinylidene fluoride at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidinone to obtain the desired slurry. Then, aluminum foil is cast with the slurry and dried at 120 °C under vacuum for 12 h, after which it is cut into circular pieces of 12 mm in diameter to use as working electrodes. Finally, the working electrodes are assembled into CR2025 coin cells with 1.0 mol/L LiPF6-EC/DEC/DMC (1:1:1, in volume, purchased from Dongguan Shanshan Battery Materials Co., Ltd.) under an argon atmosphere. The assembled cells are aged at 25 °C for 24 h before the electrochemical test. A Land-CT 2001A system (Wuhan Land Co., Ltd.) is used to investigate the charge/discharge properties in an voltage range of 2.8-4.3 V at 25 °C. It is worth noting that all of the cells are first activated at 0.1 C for 5 cycles followed by cycling at 1.0 C for 100 cycles, in which 1.0 C is defined as 200 mA g-1. Electrochemical impedance spectroscopy (EIS) tests are carried out on a CHI600E electrochemistry workstation (Shanghai Chenhua Co., Ltd.) from 105 to 0.01 Hz with an amplitude of 5 mV. 3 Results and discussion Fig. 1(A) displays the XRD patterns of the NCAOH precursor prepared by a coprecipitation method in a range of 5-120 °. As can be seen from Fig. 1(A), all of the diffraction peaks are sharp and match well with the peaks of Ni(OH)2 (JCPDS#73-1520). Moreover, the peaks appear some irregular shifts to some degree as a result of the doping of Co and Al atoms, which may suggest that the precursor is successfully synthesized by coprecipitation processes. SEM images of the NCAOH precursor are shown in Fig. 1(B), in which all of the primary particles display a sphere-like shape with a diameter of approximately 15 µm. To precisely calculate the mixed weight of LiOH·H2O, ICP-OES analysis results indicate that the main chemical composition of the as-prepared precursor includes approximate 86.56 % Ni, 10.19 % Co, and 1.78 % Al in molar ratio. These results could be employed to determine the appropriate added content of LiOH·H2O to prepare targeted NCA materials. TGA is used to determine the appropriate calcining temperature, and the results are demonstrated in Fig. 2. It can be obviously observed from Fig. 2 that there are four major weight-loss plateaus in the TG curve, as well as their corresponding endothermic peaks in the DSC curve, in a temperature range of 30-900 °C under a flowing air atmosphere. The first weight-loss plateau of approximately 17.1 % in the range of 60-110 °C can be ascribed to the loss of the crystal water from LiOH·H2O and the 4

absorbed water from the mixture, which corresponds to the endothermic peak at approximately 98.7 °C in the DSC curve. The second weight-loss plateau, appearing between 200 and 270 °C, can be assigned to the loss of water molecules from the decomposition of the NCAOH precursor, which corresponds to an endothermic peak at approximately 257.7 °C in the DSC curve. The third plateau is found between 270 and 550 °C, with an approximately 5.1 % weight-loss, and could be due to the decomposition of LiOH, corresponding to the endothermic peak at approximately 468.8 °C. After heating over 550 °C, a slightly continued weight-loss process can be ascribed to the volatilization of Li salts, together with the appearance of the solid-melt phase, which leads to an increasing endothermic phenomenon. Therefore, under the flowing oxygen atmosphere, the first calcining temperature can be set as 480 °C to ensure the thorough removal of the absorbed water and crystal water. Moreover, a second calcining temperature of 750 °C is selected to guarantee the formation of the targeted NCA materials, which costs lower energy consumption and is consistent with the previous reports [51-55]. Fig. 3(A) presents the XRD patterns of the NCA materials synthesized under different oxygen pressures from 0.1 to 1.0 MPa in the 2θ range of 5-120 °. It is clearly observed from Fig. 3(A) that all of the diffraction peaks can be well indexed to the layered hexagonal-type α-NaFeO2 structure with the —

space group R3m, and no impurity phase is detected in any of the samples. Generally, the value of I(003)/I(104) is regarded as an important parameter to characterize the Li/Ni mixing degree that means a degree of Li ions at 3b replaced by Ni2+ ions with the same ionic radius, where a value greater than 1.2 indicates a lower Li/Ni mixing degree. The values of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) are 1.561, 1.604, 1.698, 1.727, 1.755, and 1.688, respectively, and it is obviously found that an appropriate calcining pressure can effectively reduce the Li/Ni mixing degree. It is not difficult to observe from Fig. 3(B) that the peaks of (006)/(102) split more perfectly in NCA-0.6 compared to ones in other materials, indicating that a more perfect layer structure forms under 0.6 MPa. Additionally, the XRD survey peaks shift to a higher angle, from 38.39 ° for NCA-0.1 to 38.49 ° for NCA-0.6, which means that the Li ions extraction/insertion process may be hindered by decreasing the interlayer spacing and crystal cell. Moreover, NCA-0.6 shows the same tendency of (018)/(110) peaks demonstrated in Fig. 3(C), in which the peaks present a better splitting degree and shift to a higher angle of 65.16 ° compared to the other samples. The XRD plots of the as-prepared NCA materials calcined under different oxygen pressures are refined with the GSAS software, and the corresponding refined patterns are displayed in Fig. 4, in 5

which the refined patterns are well matched with their XRD data. Table 1 lists the Rietveld refinement results of XRD patterns for NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0). It is obviously observed from Table 1 that the values of parameters a and c first present declining trends, from 2.8750 and 14.2024 Å for NCA-0.1 to 2.8672 and 14.1884 Å for NCA-0.6, respectively, and then increase to 2.8709 and 14.1974 Å for NCA-1.0. It is well-known that the value of parameter a reflects the spatial distance between metal atoms, while the value of parameter c mirrors the thickness of the layer structure. The crystal will tend to grow along the c-axis direction with an increasing c/a value, which is conducive to forming a more ordered layer structure. Obviously, the c/a values of the samples calcined under increasing pressures increase from 4.9399 for NCA-0.1 to 4.9485 for NCA-0.6 and then decrease to 4.9455 for NCA-1.0, in which the c/a value for NCA-0.6 is higher than those for the other samples, suggesting that the calcining pressure of approximately 0.6 MPa could be beneficial in forming a more ordered hexagonal structure for NCA materials. In addition, the cell volume presents a variation trend similar to those for the values of parameters a and c, in which the value of the cell volume decreases from 101.655 Å3 for NCA-0.1 to 101.014 Å3 for NCA-0.6 and then increases to 101.340 Å3 for NCA-1.0. These results correspond well with the shifting peaks in the XRD patterns. A decrease in the cell volume could hinder the Li ions extraction/insertion process, which could further weaken the battery performance. Moreover, it can be obviously observed from Table 1 that the atomic site occupation values of Ni2+ in Li+ layer of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) are 5.00, 4. 36, 4.08, 4.05 3.97 and 4.34, indicating that a proper calcining pressure can effectively lower the Li/Ni mixing degree to maintain the layer structure stability. From the above analyses, as for the influence of the oxygen pressure on the crystal growth, it can be concluded that an appropriate pressure of approximately 0.6 MPa could construct a more ordered crystal structure with a smaller crystal volume. Fig. 5 shows the SEM images of the samples calcined under different oxygen pressures. It can be clearly observed from Fig. 5 that the secondary particles of the samples present a good sphere-like shape, but some pronounced differences also exist in terms of the surfaces of these samples. When calcined under a relatively low pressure, many small primary particles are randomly distributed to form loose secondary particles on the surfaces of NCA-0.1 and NCA-0.2, as seen in Fig. 5(A-B), in which the diameter of the primary particles of NCA-0.2 is larger than that of those of NCA-0.1. Compared to NCA-0.2, more primary particles are gradually enlarged and uniformly distributed on the surfaces of the secondary particles of NCA-0.4, which causes the morphology to appear a spherical mulberry. With 6

an increase in the pressure, these large particles easily embed into the surfaces of the NCA-0.2 and NCA-0.4 and present an appearance of wooden wedges, which causes their structures to be closer. When the pressure increases to 0.6 MPa, these wooden wedges surrounded by small particles are tightly inset into the sphere, as shown in Fig. 5(D), which generates a smoother surface, as well as a tighter structure. However, the morphology tends to change when the pressure increases to 0.8 MPa, as displayed in Fig. 5(E), in which the surface of NCA-0.8 is not as smooth as that of NCA-0.6 and gets as loose as that of NCA-0.1. It appears that some small particles of NCA-0.8 have been squeezed out as a result of the excessively increasing pressure. When the pressure further increases, the morphology of NCA-1.0 presented in Fig. 5(F) is similar to that of NCA-0.8, and more small primary particles have been squeezed out. Enlarged views are placed in the right top corner of each image displayed in Fig. 5. It is clearly observed from these enlarged views that the microstructure of NCA-0.6 is tighter and smoother than those of the others. It is well-known that the surface morphologies of NCA materials have an important correlation with their electrochemical performance. Materials with a loose and rough surface like NCA-0.1 and NCA-0.2 will be convenient for the infiltration of liquid electrolytes and the Li ions de-intercalation process, but they will easily collapse in the repeated charge/discharge process, which may lead to a degenerative cycle property. Moreover, NCA materials calcined under excessively high pressures, such as NCA-0.8 and NCA-1.0, easily yield many small primary particles that are squeezed out, which may result in unsatisfactory electrochemical performance. Therefore, it is reasonable to deduce that materials with homogeneous particle distributions like NCA-0.6 can maintain their material integrity during the charge/discharge process and deliver excellent electrochemical performance. The surface microstructures of the as-prepared NCA materials present significant differences as a result of the increasing pressure. To better understand the reasons, the particle size distribution data of the samples are collected and listed in Table 2, in which the values of D50 and the average particle size (Dav) are defined as important parameters to evaluate the particle size of the Ni-rich cathode materials. It is easily found that the D50 value decreases from 15.41 µm for NCA-0.1 to 11.06 µm for NCA-0.6 and then increases to 13.10 µm for NCA-1.0, which suggests that the appropriate pressure could decrease the particle size by orderly packing. Furthermore, the span values calculated from (D90-D10)/D50 are used to describe the particle size distribution width, which is the opposite to the trend of the particle size variation. The span value changes from 1.23 for NCA-0.2 to 1.33 for NCA-0.6 and 7

1.16 for NCA-1.0, indicating that the distribution width of the particles slightly changes while the pressure makes the particles smaller. It is well known that NCA materials with relative smaller particle sizes will present larger specific surface areas that can provide larger contact areas between active materials and liquid electrolytes and lower the Li ions diffusion distance, which will be conducive to enhancing the battery properties. It can be clearly observed from Table 2 that the values of BET specific surface area of the NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) are 0.253, 0.274, 0.303, 0.330, 0.312 and 0.291 m2 /g-1, respectively. The variation trends of span values and BET specific surface areas are in accord with the trend for the surface morphology changes for all samples, which demonstrates that only using an appropriate calcining pressure can compact particles and decrease their size in the preparation process. In order to understand the influence of the different oxygen pressures on the structure of NCA cathode materials, the valence and chemical environments of Ni, O and C on the surface of NCA-0.1 and NCA-0.6 are detected by XPS and the corresponding results are displayed in Fig. 6. As shown in Fig. 6(A-B), both NCA-0.1 and NCA-0.6 present two Ni 2p3/2 and Ni 2p1/2 main peaks at around 855.8 and 873.5 eV with the corresponding two satellite peaks at 861.8 and 879.5 eV, respectively. According to the fitting curves, the high resolution spectra of Ni 2p3/2 can be divided into two peaks at around 855.5 and 856.8 eV, corresponding to Ni2+ and Ni3+, respectively. Moreover, the content of Ni3+ in NCA-0.6 is evaluated at 50.6 %, which is obviously higher than the one of 46.2 % in NCA-0.1. Clearly, NCA-0.6 possesses a higher content of Ni3+ that can present better cycling performance by lowering the Li/Ni mixing degree. As for O 1s spectra demonstrated in Fig. 6(C-D), the peaks of lower binding energy at around 529.4 eV can be attributed to the lattice oxygen (Olattice) derived from the crystal materials and the peaks of higher binding energy at around 531.9 eV can be assigned to the absorbed oxygen (Oabsorbed) originated from the active oxygen species (LiOH and Li2CO3), respectively. It is apparent to detect that NCA-0.6 presents more Olattice of 9.3 % than that of NCA-0.1 of 7.7 %, suggesting that higher oxygen pressure can provide more Olattice to form the stronger metal-O bonding to stabilize the structure of materials. In addition, NCA-0.1 shows more Oabsorbed of 92.3 % than that of NCA-0.6 of 90.7 %, suggesting that the surface of NCA-0.1 may exist more LiOH and Li2CO3 phases by reactions between Oabsorbed and Li+. Moreover, the high resolution spectra of C 1s presented in Fig. 6(E-F) are simulated into two binding energy peaks at around 284.8 and 290.0 eV, which well correspond to the hydrocarbon and carbonate, respectively. These results are confirmed and expounded 8

in detail by the literature [56, 57]. To be specific, NCA-0.1 shows a higher amount of 23.7 % than that of NCA-0.6 of 13.1 % at 290.0 eV, which also indicates that there are more Li2CO3 phases on the surface of NCA-0.1. Moreover, the existence of LiOH/Li2CO3 originated from residual lithium with H2O and CO2 from the air confirmed by FT-IR spectra (seen in Fig. S1) would trigger a series of side reactions with electrolytes to decrease the discharge specific capacity and initial columbic efficiency by consuming extra lithium ions, which are be explained in the following experiment. Fig. 7 displays the electrochemical properties of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) between 2.8 and 4.3 V at 25 °C. As can be seen from Fig. 7(A), all of the samples display a typical charge voltage plateau at approximately 3.75 V, which corresponds to the Ni2+/Ni4+ redox process. Specifically, the initial discharge specific capacity presents a dramatic decrease from 199.0 mAh g-1 of NCA-0.1 to 171.2 mAh g-1 of NCA-1.0, suggesting that an increasingly tight structure compacted by gradually increasing pressures would weaken the discharge specific capacity by blocking the extraction of Li ions. The rate performance curves of the samples at 0.1, 0.2, 0.5, 1.0, and 0.1 C are shown in Fig. 7(B). It can be obviously seen from Fig. 7(B) that a boundary exists between NCA-0.4 and NCA-0.6, in which NCA materials with calcining pressures of less than 0.4 MPa possess a higher discharge capacity because of their loose surface and the close contact between electrolytes and cathode materials. However, when the pressure is greater than 0.4 MPa, the NCA-x materials undergo an obvious activating process in the first ten cycles as a result of their tight structures. Furthermore, when the current density is set back to 0.1 C after 20 cycles at different rates, NCA-0.6 delivers a discharge capacity of approximately 188.2 mAh g-1. To explore the detail reasons for poor rate capability of NCA-0.6, the HRTEM and GITT tests are performed and the corresponding results are demonstrated in Fig. S2 and S3. As shown in Fig. S2, the NCA-0.6 presents a lower interplanar distance of (104) plane of 0.198 nm than one of 0.202 nm for NCA-0.1, which is corresponding to a lower lattice volume of NCA-0.6 than NCA-0.1 and will hinder Li ions transfer. The suppositions are proved by the GITT data shown in Fig. S3, in which NCA-0.6 presents a lower DLi+ of about 2.97 E-10 cm2 s-1 than 5.41 E-10 cm2 s-1 of NCA-0.1. Thus, both of these disadvantages lead to a poor rate capability for NCA-0.6. Fig. 7(C) presents the cycle and coulombic efficiency curves of the samples at 1.0 C at 25 °C, in which the initial discharge specific capacities of the samples gradually decrease with the increasing calcining pressure and the coulombic efficiency almost retains at 100 % except for the first two cycles. When the pressure increases, the discharge specific capacity increases from 120.1 mAh g-1 for NCA-0.1 to 143.3 9

mAh g-1 for NCA-0.6 and then decreases to 131.8 mAh g-1 for NCA-1.0 after 100 cycles, which is consistent with the previously discussed structure and morphology variation tendencies. This general variation trend of the discharge specific capacity after 100 cycles is similar to the capacity retention ratio, in which the values of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0) are approximately 60.4, 64.7, 73.3, 80.3, 76.9 and 77.0 %, respectively. In other words, with an appropriate calcining pressure of 0.6 MPa, the as-prepared materials can present excellent cycle, which can be partly attributed to the decreasing Li/Ni mixing degree that can evidently enhance the integrality of the ordered hexagonal structure, and partly to the improved surface properties that can distinctly lower side reactions between liquid electrolytes and active electrodes. SEM images of NCA-0.1 and NCA-0.6 after 100 cycles at 1.0 C between 2.8 and 4.3 V are shown in Fig. 8. Although both the fresh NCA-0.1 and NCA-0.6 display regular sphere-like morphologies, as displayed in Fig. 5, their morphologies present significant differences after 100 cycles, which may explain why NCA materials calcined under an appropriate pressure can exhibit better stabilities in the cycling process. The secondary particle volume of the NCA materials expands in the charge/discharge process, and the volume expansion ratio becomes larger when the Ni content increases, in which cracks may generate among the primary particles and more exposed surfaces would contact liquid electrolytes, leading to the consumption of more active materials and the production of more solid electrolyte interphase (SEI) film. The original morphology of NCA-0.1 is loose, and it is easy to find that numerous secondary particles fracture after 100 cycles, as shown in Fig. 8(A). In the enlarged views of Fig. 8(B-C), the surfaces of the secondary particles are heavily damaged and become looser because of the repeated volume changes, with many holes appearing between the primary particles, which is conductive to the easy permeation of liquid electrolytes into the active materials. Moreover, the primary particles on the fractured surface are coated by a thick layer of SEI film, which is a by-product produced by the extreme consumption of liquid electrolytes and active materials, leading to the serious deterioration of cycle performance of NCA-0.1. In contrast, the morphology of NCA-0.6 after 100 cycles displays a good sphere distribution in Fig. 8(D). Because of the tightly original morphology, its secondary particles can preserve a good spherical shape and are coated by a homogeneous SEI film, as seen in Fig. 8(E-F). The tight packing mode of NCA-0.6 effectively restricts the collapse of the structure during the charge/discharge process, which can maintain the structural stability and enhance the cycling performance of NCA-0.6. 10

Fig. 9 presents the dQ/dV curves of NCA-0.1 and NCA-0.6 after 1 cycle and 100 cycles at 1.0 C. It is well known that the difference in the redox peaks (△E) is used to mirror the degree of polarization and irreversibility of the electrode material during the charge/discharge process. At the first cycle, NCA-0.1 and NCA-0.6 have similar oxide peaks of approximately 3.74 V originated from the transition process from hexagonal to monoclinic phase of NCA materials, while the reduction peak of NCA-0.6 (3.68 V) is slightly higher than that of NCA-0.1 (3.62 V), which suggests that NCA-0.6 has a lower △E value of approximately 0.06 V than the value of approximately 0.12 V for NCA-0.1. After 100 cycles, the redox peaks of both NCA-0.1 and NCA-0.6 shift to a higher potential. It is worth noting that the redox peaks of NCA-0.1 and NCA-0.6 shift to 4.00 and 3.81 V, while their reduction peaks shift to 3.74 and 3.70 V, respectively, suggesting that the △E value of NCA-0.1 (0.26 V) is much larger than that of NCA-0.6 (0.11 V). These results are in good accord with the cycle stabilities of the two electrodes and confirm that NCA-0.6 delivers an excellent electrochemical reversibility. To further investigate the structure stability changes during cycles, the XRD patterns of NCA-0.1 and NCA-0.6 before and after 100 cycles are obtained and described in Fig. 10. It can be obviously found that the diffraction peak intensities of both samples decrease after 100 cycles, as displayed in Fig. 10(A), suggesting that the structure of the electrode suffers some irreversible phase transformation during the charge/discharge process. However, NCA-0.6 can still preserve quite sharp peaks that show a representative layer structure compared to NCA-0.1. Moreover, the width of the (003) peak can be attributed to the generation of the spinel phase, in which NCA-0.1 shows a broader (003) peak than NCA-0.6. As demonstrated in Fig. 10(B), the (003) diffraction peaks of both samples shift to higher angles, with a difference of 0.31 ° for NCA-0.1 compared to 0.25 ° for NCA-0.6 after being cycled, indicating that NCA-0.6 can maintain a better structural stability. To understand the effects of different calcining pressures on the battery properties, EIS measurements are recorded and the Nyquist plots of the assembled cells charged to 4.1 V after 1 cycle and 100 cycles are demonstrated in Fig. 11. It is obviously observed from Fig. 11 that each impedance spectra consist of two condensed semicircles in the high- and medium-frequency regions and an oblique line in the low-frequency region, where the two semicircles reflect the resistance (Rsf) from Li ions passing through a SEI film, along with the charge transfer resistance (Rct) in the chemical reaction, and the oblique line represents the Li ions diffusion in the bulk. Table 3 displays the fitted Rsf and Rct values of the samples. After 1 cycle, the Rsf value gradually increases from 10.5 Ω for NCA-0.1 to 56.4 11

Ω for NCA-1.0, which may be attributed to the unstable SEI film originated from the activation process. In addition, the Rct values show a similar trend, and NCA-1.0 shows a higher value of 60.8 Ω, compared to 27.9 Ω for NCA-0.1. It is obviously found that an increasing growth of the Rsf and Rct values corresponds well with the gradual decrease of the initial discharge specific capacities of the NCA materials calcined under increasing pressures. However, after 100 cycles, the Rsf values show some irregular changes, while the Rct values dramatically change, in which the Rct value of NCA-0.6 shows the lowest growth from 28.1 to 282.8 Ω, while that of NCA-0.1 presents the fastest growth from 27.9 to 1901.3 Ω. The enormous resistance changes for NCA-0.1 can be attributed to the fracture of the secondary particles and huge growth of the polarization degree in the above analyses, which further leads to poor cycle performance. In contrast, NCA-0.6, with its tight-heaping spherical structure, can preserve the integrity of the secondary particles in the charge/discharge process to decrease the side reaction and polarization degree, which is conducive to improving the battery performance by decreasing the resistance growth. 4 Conclusions The effects of the calcining oxygen pressure gradient on the physicochemical and electrochemical properties of NCA materials are carefully investigated, and the results indicate that the material calcined under an oxygen pressure of 0.6 MPa (NCA-0.6) presents the lowest Li/Ni mixing degree and smoothest surface. Although the cell assembled with NCA-0.6 delivers a decreasing initial charge/discharge specific capacity, a remarkably improved cycling performance could be gained when the NCA-0.6 is used as the cathode material in lithium ion batteries. Therefore, employing an appropriate calcining oxygen pressure can be regarded as a simple and economical way to effectively enhance the electrochemical properties of LIBs for practical applications. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51874046 and 51404038), the Project of Scientific Research of Jingzhou (No. 2018029) and the Yangtze Youth Talents Fund (No. 2016cqr05). References [1] Y. Li, X. Han, T. Yi, Y. He, X. Li, Journal of Energy Chemistry, 31 (2019) 54-78. [2] B. Xiao, B. Zhang, J.-c. Zheng, L.-b. Tang, C.-s. An, Z.-j. He, H. Tong, W.-j. Yu, Ceramics International, 44 (2018) 13113-13121. 12

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16

Figure captions Fig. 1 XRD patterns (A) and SEM images (B) of NCAOH precursor Fig. 2 TG and DSC curves of NCAOH precursor mixed with LiOH·H2O Fig. 3 XRD patterns (A) of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) as well as their partial enlarged details (B and C) Fig. 4 Rietveld refinement patterns of NCA-0.1 (A), NCA-0.2 (B), NCA-0.4 (C), NCA-0.6 (D), NCA-0.8 (E), and NCA-1.0 (F) Fig. 5 SEM images of NCA-0.1 (A), NCA-0.2 (B), NCA-0.4 (C), NCA-0.6 (D), NCA-0.8 (E), and NCA-1.0 (F), and the corresponding enlarged SEM image in each insert Fig. 6 XPS spectra of Ni 2p, O 1s and C 1s of NCA-0.1 (A, C and E) and NCA-0.6 (B, D and F) Fig. 7 Initial charge/discharge (A), rate (B), cycle and coulombic efficiency (C) curves of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) in the voltage range of 2.8-4.3 V at 25 °C Fig. 8 SEM images of NCA-0.1 (A, B, C) and NCA-0.6 (D, E, F) after 100 cycles at 1.0 C between 2.8 and 4.3 V Fig. 9 dQ/dV curves of NCA-0.1 and NCA-0.6 after 1cycle (A) and 100 cycles (B) at 1.0 C Fig. 10 XRD patterns of as-prepared NCA-0.1 and NCA-0.6 before and after 100 cycles (A), as well as partial enlarged views (B) Fig. 11 Nyquist and the corresponding fitting plots of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) after 1 cycle (A) and 100 cycles (B) Table 1 Rietveld refinement results of XRD patterns for NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) Table 2 Particle size distribution and specific surface area analysis results for NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) Table 3 Fitted values of corresponding parameters of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0)

17

Table 1 Rietveld refinement results of XRD patterns for NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) Pressure / a/Å

c/Å

c/a

Ni2+ /Li+ site

Rwp

Rp

occupation rate /%

/%

/%

3

V/Å

MPa 0.1

2.8750

14.2024

4.9399

101.655

5.00

9.20

6.87

0.2

2.8743

14.2019

4.9410

101.613

4.36

7.32

5.02

0.4

2.8674

14.1891

4.9484

101.035

4.08

9.43

6.98

0.6

2.8672

14.1884

4.9485

101.014

4.05

9.85

7.39

0.8

2.8686

14.1915

4.9472

101.134

3.97

8.34

6.32

1.0

2.8709

14.1974

4.9452

101.340

4.34

9.72

7.08

Table 2 Particle size distribution and specific surface area analysis results for NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) Sample

D10 / µm

D50 / µm

D90 / µm

Dav / µm

Span value

Specific surface area / m2 g-1

NCA-0.1

7.50

15.41

26.34

18.25

1.22

0.253

NCA-0.2

7.31

14.56

25.16

17.52

1.23

0.274

NCA-0.4

6.58

12.76

23.22

15.73

1.30

0.303

NCA-0.6

5.29

11.06

20.03

13.11

1.33

0.330

NCA-0.8

6.25

12.46

22.18

15.11

1.28

0.312

NCA-1.0

7.02

13.10

22.14

15.59

1.16

0.291

Table 3 Fitted values of corresponding parameters of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) 1st cycle

100th cycle

Sample Rsf / Ω

Rct / Ω

Rsf / Ω

Rct / Ω

NCA-0.1

10.5

27.9

50.2

1901.3

NCA-0.2

13.1

25.4

72.8

1343.1

NCA-0.4

22.0

28.0

92.3

320.2

NCA-0.6

28.4

28.1

44.7

282.8

NCA-0.8

52.9

53.7

75.3

348.6

NCA-1.0

56.4

60.8

60.0

513.0

Fig. 1 XRD patterns (A) and SEM images (B) of NCAOH precursor

Fig. 10 XRD patterns of as-prepared NCA-0.1 and NCA-0.6 before and after 100 cycles (A), as well as partial enlarged views (B)

Fig. 11 Nyquist and the corresponding fitting plots of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) after 1 cycle (A) and 100 cycles (B)

Fig. 2 TG and DSC curves of NCAOH precursor mixed with LiOH·H2O

Fig. 3 XRD patterns (A) of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) as well as their partial enlarged details (B and C)

Fig. 4 Rietveld refinement patterns of NCA-0.1 (A), NCA-0.2 (B), NCA-0.4 (C), NCA-0.6 (D), NCA-0.8 (E), and NCA-1.0 (F)

Fig. 5 SEM images of NCA-0.1 (A), NCA-0.2 (B), NCA-0.4 (C), NCA-0.6 (D), NCA-0.8 (E), an NCA-1.0 (F), and the corresponding enlarged SEM image in each insert

Fig. 6 XPS spectra of Ni 2p, O 1s and C 1s of NCA-0.1 (A, C and E) and NCA-0.6 (B, D and F)

Fig. 7 Initial charge/discharge (A), rate (B), cycle and coulombic efficiency (C) curves of NCA-x (x = 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) in the voltage range of 2.8-4.3 V at 25 °C

Fig. 8 SEM images of NCA-0.1 (A, B, C) and NCA-0.6 (D, E, F) after 100 cycles at 1.0 C between 2.8 and 4.3 V

Fig. 9 dQ/dV curves of NCA-0.1 and NCA-0.6 after 1cycle (A) and 100 cycles (B) at 1.0 C

〉Appropriate calcining oxygen pressure can availably decline the Li/Ni mixing degree. 〉NCA-0.6 shows a smooth surface of secondary particles with a lower particle size. 〉The cells with NCA-0.6 can remain 143.3 mAh g-1 after 100 cycles at 1.0 C. 〉NCA-0.6 can preserve integrity of the secondary particles after 100 cycles.

Author Contributions Section: Wei Xiao contributed conception and design of the study. Yan Nie and Wei Xiao organized the database. Yan Nie and Chang Miao performed the statistical analysis. Yan Nie and Chang Miao wrote the first draft of the manuscript. Mingbiao Xu and Changjun Wang wrote sections of the manuscript.

Declaration of Interest Statement: No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. All authors: Yan Nie, Wei Xiao, Chang Miao, Mingbiao Xu, Changjun Wang Corresponding author: Wei Xiao