Electrochemical energy technologies, such as lithium-ion batteries, metal-air batteries, fuel cell, and water electrolysis, etc., have been recognized as one type of the most reliable and efficient options for clean and sustainable electricity energy storage and conversion (Khan et al., 2018; Wang et al., 2018; Zhang H. et al., 2018; Zhang L. et al., 2018). For relatively mature lithium-ion batteries (LIBs), the cathode materials play a dominating role in the performance of LIBs. In order to further increase energy densities of the devices, high-nickel NCM-based materials such as LiNi_xCo_yMn_zO₂ (here x+y+z = 1 with x>y+z) family have been explored and used in LIBs. It seems that one of the most promising LIB cathode materials is LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811, where x = 0.8, y = 0.1, and z = 0.1), which shows higher specific capacity than other family members (Cao et al., 2018; Li Q. et al., 2018).

In general, for high-nickel NCM-based materials, there are several shortcomings which have been identified. (1) Li⁺/Ni²⁺ mixing when Ni²⁺ content is increased. In high-nickel NCM-based material, part of Ni²⁺ may occupy the Li⁺ position, forming a cation mixing and then reducing the specific capacity of material. Furthermore, during the charging process, the low-valence nickel ion in the transition metal (Co, Mn) layer can migrate to the lithium layer, occupying the lithium vacancies, affecting the diffusion of Li⁺. The voltage stability of the material is also relatively weak, which is easy to decline rapidly (Noh et al., 2013; Li et al., 2019; Zhang et al., 2019). (2) Poor thermal stability. Studies have shown that the higher the nickel content in the ternary material, the worse the thermal stability, causing safety issue (Aurbach, 2000; Ma et al., 2019). (3) Unstable surface structure. The excessive delithiation occurs in the surface structure with charging, and the layered structure of the high-nickel NCM-based material changes to the spinel structure and the inert rock salt structure with a hicker inert NiO layer. Furthermore, the high-valent active transition metal ions on the surface layer can have side reactions with the electrolyte, causing the increase of polarization and then the decay of the capacity (Noh et al., 2006). (4) Too high surface alkali content. The nickel element in the NCM-based material is alkaline, which is easy to absorb water and CO₂ when exposed to air and reacts with residual lithium in the surface layer to form LiOH and Li₂CO₃, which can further increase the pH value of the material and seriously affect the electrochemical performance of the NCM-based material (Bi et al., 2016). In recent years, many improvements have been carried out for high-nickel NCM-based materials of LIB cathodes, including innovating the synthesis processes, doping modification, and matching of microstructure with electrolyte. There are mainly two methods to improve the performance of high-nickel NCM-based materials. (1) Element doping for improving the cationic mixing and lattice stability, such as doping Mg, Al, Zr, and other components into materials to improve the cycling performance (Nobili et al., 2012; Min et al., 2017). (2) Coating the material particles with electrochemically inert materials to prevent or suppress the side reactions between the cathode surface and electrolyte, and improve the particles' structural stability for enhancing the cycle-life of the corresponding LIBs. The commonly used physical protective coating materials are Al₂O₃ (Chen et al., 2019), MgO (Yoon et al., 2012), CeO₂ (Wu et al., 2019), ZrO₂ (Li G. et al., 2018), Co₃O₄ (Hao et al., 2019), etc. The common feature of these coatings is that their metal ions have only one stable valence state with high chemical stability under the electrochemical window conditions. Of course, the coating also have some shortcomings, because as the insulators of ions and electrons, they may have a negative impact on the transport of lithium ions, thereby affecting or reducing the Coulombic efficiency, power output, and other electrochemical properties of LIBs.

In addition to the above lithium-free metal oxides used as coating materials for cathode materials, some researchers have employed lithium-containing Li₂ZrO₃ (Dong et al., 2015), Li₃PO₄ (Song et al., 2011), Li₂O-2B₂O₃ (LBO) (Chan et al., 2005; Lim et al., 2014; Song et al., 2019) to coat cathode materials. These lithium-containing compounds, in particular, conductive LBO can maintain stable electrochemical properties at high oxidation potential. LBO coating has shown a good ionic conductivity, which makes the coated cathode materials more conducive to lithium ions, thus leading to better electrochemical properties. It has been reported in the literature that the surface coatings of LBO glass on LiNi_0.5Mn_1.5O₄ (Lim et al., 2014), LiNi_0.5Co_0.2Mn_0.3O₂ (Song et al., 2019), and LiMn₂O₄ (Chan et al., 2005) cathodes can make remarkable improvements in charge-discharge behavior, cycle-life and other electrochemical properties. The influence of coating on the electrochemical properties of cathode materials is shown in Table 1. However, there are a few of researches reporting about the coating of LBO on LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode material.

In this work, NCM811 cathode material is prepared by a co-precipitation method. In order to improve the electrochemical properties, several materials such as Al₂O₃, ZrO₂, and LBO are used, respectively, to coat NCM811 for LIB cathode materials. The microstructures and morphologies of the coated NCM811 materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The coated NCM811 materials as the cathodes are assembled in coin LIBs for performance validation. The effects of different coatings on the electrochemical properties of NCM811 are studied by testing the charge/discharge curves and Coulomb efficiencies, cycle performance and DC resistances of the corresponding coin LIBs.

Figure 2 Shows the XRD spectra of uncoated and differently coated NCM811 materials. It can be seen that the pure phased and differently coated NCM811 materials have good a hexagonal α-NaFeO₂ layered structure and a R-3m space point group. The main peaks in the XRD spectra of the coated materials are identical, which indicates that the coated materials do not change the layered structure. In addition, the peaks of 006/102 and 108/110 can be observed from XRD spectra, indicating that the pure NCM811 and the coated NCM811 samples both have a good crystallinity and an ordered layered structure (Li et al., 2014). No characteristic peaks of the coated materials of Al₂O₃, ZrO₂, and LBO can be found in the XRD spectra of the coated NCM811 materials. There were no obvious differences in the location and intensity of the diffraction peaks between pure phase material and the coated materials, and no new diffraction peaks can be observed. These can be explained by that the coated material layer does not change the original lattice structure of the bulk material, and that the coated materials of Al₂O₃, ZrO₂, and LBO are amorphous, and only adhere to the surface of the bulk materials as the coating layers.

From the SEM images shown in Figure 3, it can be seen that the appearances of uncoated and the coated NCM811 samples are basically similar, and they are spheroids bound by the secondary solid particles. The particle size range of the secondary sphere is 3–16 um.

Figure 4 shows the TEM images of NCM811 and the coated NCM811 samples. Figure 4A is a NCM811 sample without any coating. The surface of the uncoated material is smooth and there is no coating layer that can be observed. Figure 4B is a TEM image of the NCM811 coated with Al₂O₃. The surface of NCM811 has a coating layer with a thickness of 20–80 nm. Figure 4C is a TEM image of the NCM811 coated with ZrO₂. The thickness of the coating layer is 20–120 nm. Figure 4D is a TEM image of the NCM811 coated with LBO. The coating layer is 50–200 nm thick. For the layered cathode material, the coating layer can reduce the undesired reaction between cathode material and electrolyte and suppress lattice distortion caused by the dissolution of transition metal ions into electrolyte during charging (Zuo et al., 2017). It is expected when NCM811 is coated by a layer, the cathode could be protected, benifiting batttery performance.

In order to further confirm the composition of NCM811 and NCM811 coated by Al₂O₃, ZrO₂, and LBO, the prepared samples were characterized by XPS, as shown in Figure 5. The characteristic binding energies of Co2p_3/2, Mn2P_3/2, and Ni2P_3/2 can be seen from Figures 5A–C, which are 780.04, 641.99, and 854.69 eV, indicating that the valence states of Co, Ni and Mn are +2, +3, and +4, respectively (Tan et al., 2015). The characteristic peak of 73.8 eV can be found only for NCM811 coated with Al₂O₃ (Sample B) (Figure 5D), which corresponds to the characteristic binding energy of Al2p_3/2 of Al₂O₃ (Wen et al., 2018). The XPS spectrum of Zr3d include two peaks: Zr3d_5/2 (181.75 ev) and Zr3d_3/2 (184.25 ev), which are in good agreement with the values in literature (Huang et al., 2009). Boron can only be detected on sample of NCM 811 coated with LBO, as shown in Figure 5F. The characteristic binding energy of B1S is 191.83 eV. This is consistent with the binding energy of LBO (Li₂O-2B₂O₃) in literature (Wang et al., 2015). XPS results show that Al₂O₃, ZrO₂, and LBO have been successfully coated on the surface of MCM811, respectively.

From Figure 6 and Table 2 above, it can be seen that the first charge specific capacity of coin LIB made of the uncoated NCM811 sample (A) is 229.2 mAh/g, the first discharge specific capacity is 201.7 mAh/g and the first charge-discharge Coulomb efficiency is 88.00%, respectively. The first charge specific capacity of coin LIB made of the NCM811 coated by Al₂O₃ (B) is 226.3 mAh/g and the first discharge specific capacity is 199.2 mAh/g. The first charge-discharge Coulomb efficiency is 88.02%. These data indicate that the first charge-discharge specific capacity of NCM811 is negatively affected by Al₂O₃ coating, but the first charge-discharge Coulomb efficiency is slightly increased. For the coin LIB made of NCM811 coated with ZrO₂ (C), the first charge specific capacity is 225.7 mAh/g, the first discharge specific capacity was 198.7 mAh/g, and the first charge-discharge Coulomb efficiency was 88.04%, respectively. The first charge and discharge capacities of NCM811 coated with ZrO₂ is slightly lower than that of NCM811 without any coating, but the first charge and discharge Coulomb efficiency is increased slightly, which is consistent with that of Al₂O₃ coating. The first charge specific capacity of coin LIB made of NCM811 coated with LBO (D) is 230.0 mAh/g, the first discharge specific capacity is 204.3 mAh/g, and the first charge-discharge efficiency is 88.83%, respectively. These results are higher than those of both coated by Al₂O₃ and ZrO₂. It is presumed that this may be due to the lack of electrochemical activity of both Al₂O₃ and ZrO₂ coating, which may hinder the insertion/removal of some Li⁺ ions, resulting in a slight decrease in the specific capacities of charge/discharge. However, the surface interface structure of NCM811 can be stabilized by Al₂O₃ or ZrO₂ coating layer, and the reaction between the electrode material and the electrolyte can be restrained to some extent. As a result, the Coulomb efficiencies of charge/discharge are improved. In addition to the advantages of normal surface coatinging layer, LBO as a fast ionic conductor has a good ionic conductivity and certain electrochemical activity when it is coated on the surface of NCM811. It is conducive to the transport of lithium ions during charging and discharging. This is an important reason why LBO coating layer has a better electrochemical performance than Al₂O₃ and ZrO₂ coatings. The first charge capacity, discharge capacity and Coulomb efficiency of NCM811 coated with LBO (D) are slightly higher than those of NCM811 without any coating, and also better than those of NCM811 coated with Al₂O₃ and ZrO₂.

From Figure 7 and Table 3, it can be seen that the water values of the electrode made of NCM811 (A) without any coating rises from 215.1 ppm to 670.2 ppm when it is stored at room temperature for 72 h at a relative humidity of 10% RH. It can be found that the water contents of the uncoated, Al₂O₃, ZrO₂, and LBO-coated NCM811 electrodes are increased by 455.1, 274.5, 312.0, and 255.1 ppm after 72 h of storage, respectively. The water absorption of all the coated NCM811 cathodes during storage is less than that of the uncoated one. The lower water content in the cathode is beneficial to both the better discharge capacity and Coulomb efficiency of LIB (Li et al., 2016). The NCM811 cathode coated with LBO has the smallest moisture increment and has the best storage stability.

From Figure 8, it can be seen that after 100 cycles, the LIB capacity retentions of pure NCM811, NCM811 coated by Al₂O₃, ZrO₂, and LBO are decreased to 98.89, 99.61, 99.66, and 99.78% of the initial capacity, respectively, demonstrating that the coated NCM811 cathode materials can give better capacity retentions. In Comparison of the data of capacity retention rate after 100 charge/discharge cycles in Figure 8 with the data in the literature listed in Table 1, it can be found that the cathode material in this work shows a higher capacity retention rate after the same number of cycles, thus demonstrating a better cycle performance. Compared with Al₂O₃-coated and ZrO₂-coated NCM811 materials, NCM811 coated with LBO has a better cycling performance. The improved electrochemical performances of the coated samples are attributed to the protective function of the coating layer. The improved cycling performance of the coated cathode materials is attributed to the reduced electrolyte decomposition reactions and the alleviation of the impedance (see Figure 9) due to the existence of coating layers (Wang et al., 2013). The existence of coating layer is also conducive to preventing the excessive expansion of the active material particles, thus making the structure of the electrode stable and conducive to the stability of the charging and discharging processes (Zhou et al., 2019).

Internal resistance of a battery is an importance parameter that can have a significant effect on its electrochemical performance such as cycling and power characteristics. Different cathode materials can give different internal resistances, leading to different electrochemical performances of the batteries (Shao et al., 2010). From Figure 9, it can be seen that the initial DC internal resistances of the coin LIBs composed of uncoated NCM811 (A) is 62.4 mΩ, and the resistance increases to 92.3 mΩ after 100 cycles. The initial DC internal resistance of the coin LIBs made of NCM811 coated with Al₂O₃ (B) is 63.6 mΩ, and the resistance value increases to 86.6 mΩ after 100 cycles. The initial DC resistance of the coin LIB made of NCM811 coated with ZrO₂ (C) is 66.2 mΩ, which increases to 89.1 mΩ after 100 cycles. The initial DC resistance of the coin LIB made of NCM811 coated with LBO (D) is 64.9 mΩ, which increases to 82.5 mΩ after 100 cycles. Although the initial DC resistances of the coated NCM811 materials are slightly higher than that of uncoated one, the increasing trends of the resistances are slower than that of uncoated one. These slow increases can be beneficial to the stability of the electrochemical performance during the cycling. In comparison, NCM811 coated with LBO has the best resistance stability. Compared with Al₂O₃ and ZrO₂ coating materials without electrochemical activity, LBO coating with NCM811 has obvious electrochemical performance advantages. LBO is a fast ionic conductor, which not only makes NCM811 higher capacity, but also the LBO coating is conducive to the migration and transfer of lithium ions between the electrode interfaces, reducing the irreversible loss of lithium ions in the process of charging and discharging, so that the lithium-ion battery has a better capacity retention and a smaller resistance increase in the process of cycling.

In this paper, the precursor Ni_0.8Co_0.1Mn_0.1(OH)₂ is prepared by a co-precipitation method. Using this precursor, a cathode material of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) for lithium ion battery (LIB) is synthesized by mixing Ni_0.8Co_0.1Mn_0.1(OH)₂ and LiOH and sintering twice. Al₂O₃, ZrO₂, and LBO are used to coat on NCM811 to form the coated cathode materials by a wet chemical method. The effects of Al₂O₃, ZrO₂, and LBO coatings on the morphologies, structures, compositions, and electrochemical properties of NCM811 are studied by both the physical characterization and electrochemical measurements. XRD results show that the coating layers do not change the original lattice structure of bulk NCM811. The materials of NCM811 without any coating and coated with Al₂O₃, ZrO₂, and LBO all have a good hexagonal α-NaFeO₂ layered structure. The results of SEM, TEM, and XPS show that the prepared NCM811 with particle sizes of 3–16 μm can be successfully coated by Al₂O₃, ZrO₂, and LBO with the coating-layers thicknesses of 20–200 nm.

To validate the effects of coating layers on the material properties, coin LIBs are assembled with the uncoated NCM811, Al₂O₃-coated, ZrO₂-coated, and LBO-coated NCM811 materials as the cathodes. Experiment results show that although the first charge-discharge specific capacities of NCM811 coated with Al₂O₃ and ZrO₂ are slightly smaller than those of uncoated NCM811, their Coulomb efficiencies of the first charge-discharge are increased after the coating. However, the first charge capacity (230.0 mAh/g), the first discharge capacity (204.3 mAh/g) and coulomb efficiency (88.83%) of NCM811 coated with LBO show the higher performance than those of uncoated one.

Regarding the water adsorption, the cathodes made of materials of pure NCM811, coated with Al₂O₃, ZrO₂, and LBO, respectively, are stored at room temperature with 10% RH humidity for 72 h, and the changes of water contents of the cathodes during storage are measured, and the results show that the water contents of the uncoated NCM811, Al₂O₃, ZrO₂, and LBO-coated NCM811 cathodes are increased by 455.1, 274.5, 312.0, and 255.1 ppm, respectively. The water absorption performances of the coated NCM811 cathodes during storage are better than that of the uncoated NCM811 one. The NCM811 cathode coated with LBO has the smallest moisture increment, suggesting that it has the best storage stability.

For studying cycling performances, the capacity retentions of the pure NCM811, coated with Al₂O₃, ZrO₂, and LBO based LIBs are tested to be 99.50, 99.77, 99.84, and 99.91% of the initial capacities after 50 charge/discharge cycles, and the corresponding cathode resistances are increased by 19.6, 14.0, 13.6, and 12.2 mΩ, respectively. These results demonstrate that the cycle performance and resistance stability of the NCM811 material can be significantly improved by the coating. The cycle performance and resistance stability of NCM811 coated with LBO are better than those of NCM811 coated with Al₂O₃ and ZrO₂.

Based on the experiment observation and data analysis, it can be concluded that the charge-discharge specific capacity, Coulomb efficiency, water absorption stability, cycle characteristics and resistance stability of the NCM811 cathode material can be significantly improved by coating it using LBO particularly. Considering that the amount of coating material is small, the coating process is relatively simple, and the improved electrochemical performance of NCM811, the cost would not be increases significantly. So in conclusion, the surface coating to the particles of cathode materials using LBO is an effective and practical modification method to improve the electrochemical performance of LIBs. In future study of the coating modification of NCM811, the performance stability at high temperature and the performance at long cycle should be further improved.

The datasets generated for this study are available on request to the corresponding author.

HZ designed experiments, carried out experiments, and analyzed experimental results. JX analyzed experimental results. JZ designed experiments, carried out experiments, and wrote the manuscript.