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Enhanced electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathode materials via Li4P2O7 surface modification for Li-ion batteries
Ceramics International xxx (xxxx) xxx–xxx
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhanced electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathode materials via Li4P2O7 surface modification for Li-ion batteries Kaihua Hua,1, Xianyue Qia,1, Caifeng Lub, Ke Dua, Zhongdong Penga, Yanbing Caoa, ⁎ Guorong Hua, a b
School of Metallurgy and Environment, Central South University, Changsha City 410083, China Confucious Institute at University of Zagreb, Zagreb 10000, Croatia
A R T I C LE I N FO
A B S T R A C T
Keywords: Li-ion batteries LiNi0.8Co0.1Mn0.1O2 Li4P2O7 surface modification Cycle performance Rate properties
The performance of LiNi0.8Co0.1Mn0.1O2 cathode materials has been significantly improved by Li4P2O7 surface modification. Some electrochemical tests are carried out to investigate the relevant performances of LiH2PO4coated LiNi0.8Co0.1Mn0.1O2, such as inductively coupled plasma test (ICP), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results indicate that LiNi0.8Co0.1Mn0.1O2 is clad with a layer of average thickness about 30–40 nm. And the alkalinity on the surface of the modified materials has declined dramatically after titration measurement. The effect of surface modification can be credited to the two main factors: On the one hand, the weak acidity of LiH2PO4 neutralizes the lithium residues (LiOH/Li2CO3) on the surface of LiNi0.8Co0.1Mn0.1O2 cathode materials; on the other hand, LiH2PO4 continues to decompose and react with the unneutralized lithium residues to generate a coating layer of Li4P2O7 during the annealing process, which can prevent the active materials from contacting with the electrolyte directly and act as an outstanding Li+ conductor, thus inhibiting the interface reaction and improving the cycle performance as well as the rate properties of the host materials.
1. Introduction With the requirement for environmental protection, the governments of the world take great efforts to boost the development of new energy vehicles, which brings about the increasingly growing demand for lithium-ion batteries. As the most essential cathode materials in lithium-ion batteries, some of whom are more and more frequently on the list of the major domestic and foreign journals, which have been widely studied recently such as LiFePO4, LiMn2O4 and LiNi1-xyCoxMnyO2 materials, etc. [1–3]. The Ni-rich cathode materials are considered to be the most promising cathode materials for power batteries due to their excellent comprehensive performance [4,5]. However, the high lithium residues on the surface are the prominent problems in the process of commercialization of Ni-rich materials. Excessive Li element is added in order to make up for the loss in the sintering process, (which exists as Li2O at high temperature), and Li2O can absorb the H2O and CO2 in the air to convert into LiOH and Li2CO3 at room temperature. [6,7]. The high lithium residue on the surface is the most vulnerable weakness in its large-scale commercialization. First, lithium residues will absorb moisture easily in the abstrich
⁎
1
process, resulting in jelly in the homogenate process; and then, Li2CO3 will decompose to produce CO2 during the charging process, leading to expansion and lower safety performance of batteries. Next, the lithium residues also cause irreversible capacity loss during charging and discharging processes [8,9]. Therefore, it is extremely essential to reduce the lithium residues in the Ni-rich production. Surface modification is an effective method to enhance the cycling property and storage performance of cathode materials. It can make the active materials and the electrolyte isolated, which leading to reducing the occurrence of side reactions, and inhibit the dissolution of transition metalions in electrolyte coated by metal oxides [10,11], fluoride [12] or phosphates [13,14] with some proper thickness on the surface of cathode materials. However, above-mentioned cladding materials just play the role of physical isolation, cannot effectively reduce the lithium residues. Generally speaking, only few coating materials have ionic or electronic conductivity, and the coating materials hinder the diffusion of Li+ instead, which leading the materials to sacrifice some effective capacity for cycling performance. Hence, it is very indispensable to find a kind of material with ionic or electronic conductivity that can not only hinder the contact of the active materials with the electrolyte, but also effectively reduce lithium residues.
Corresponding author. E-mail address: [email protected] (G. Hu). These authors contributed equally to this work.
https://doi.org/10.1016/j.ceramint.2018.05.024 Received 20 April 2018; Received in revised form 3 May 2018; Accepted 4 May 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hu, K., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.05.024
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Fig. 1. XRD patterns of (a) bare LNCM and LNCM@LPO samples, and (b) the corresponding enlarged diffraction peaks.
designed to be 30%, the ratio of LiNi0.8Co0.1Mn0.1O2 and LiH2PO4 was 99:1 by weight. The obtained mixture was put into the vacuum oven at 120 ℃ for 12 h, and then tempered for 5 h at 450 ℃ in the air. At last, the Li4P2O7-coated LiNi0.8Co0.1Mn0.1O2 (abbreviated as LNCM@LPO) materials were obtained.
Table 1 Lattice parameters of the bare LNCM and LNCM@LPO samples. Sample
a(Å)
c(Å)
I003/I104
Bare NCM LNCM@LPO
2.8702 2.8698
14.2253 14.2260
1.669 1.673
2.2. Characterization In this thesis, LiH2PO4 is found to do the surface treatment on LiNi0.8Co0.1Mn0.1O2 cathode materials because of its excellent performance. Compared with other coating materials, this substance is in possession of some incomparable advantages. First, the lithium residues can be effectively removed by chemical reaction of LiH2PO4 with lithium compound residues. Second, the generated Li4P2O7 coating layer is an excellent Li+ conductor after sintering process [15,16]. Meanwhile, it can stabilize the crystal structure, and hinder the contact of the active substances with the electrolyte, thus inhibiting the interface reaction; furthermore, it can improve the diffusion coefficient of Li+ in materials and significantly enhance the rate performance of the bare materials. The following is the study of the synthetic method and performance of the cladding materials.
Overall composition and the content of impurities in samples were tested by ICP, the physical phases and structures of the as- synthesized samples were analyzed by XRD, and the instrument was a RIGAKU D/ Max 2550 VB + 18 kW target X-ray diffractometer. The surface morphology and element distribution of the materials were observed by SEM and EDS (JSM6360LV SEM, JEOL, Japan), and the internal crystal structure was observed by TEM (JEM 2100 F transmission electron microscope, JEOL, Japan). The chemical composition on the surface of the materials was analyzed by XPS (XPS Spectroscopic Analysis Instrument Model Thermo Fisher-VG Scientific ESCALAB 250Xi). 2.3. Electrochemical performance test The modified materials were uniformly mixed with acetylene black and polyvinylidene fluoride (mass ratio 8:1:1) and quickly dispersed in NMP, and then the wet-grinded dispersion was coated on a current collector aluminum foil. Then it was dried in the vacuum oven at 150 °C for 12 h, finally cut to get a pole wafer with a diameter of 14 mm. A mixed solution of 1 mol L−1 LiPF6 EC/DEC/EMC (volume ratio 1:1:1) was used as the electrolyte. The lithium metal was used as anode, and the Celgard 2400 microporous polypropylene film was the diaphragm. The CR2032 button cells were assembled in a glove box with the argon content less than 0.5 ppm, and stood for 10 h before being tested for electrochemical performance on a Land electrochemical tester. The weight of active materials was approximately 3.5 mg and the amount of the electrolyte was approximately 0.2 ml. The test voltage range was from 3.0 V to 4.3 V, current density was 1 C= 180mAh/g, while the magnification rate were 0.2 C, 0.5 C, 1 C and 2 C, etc. And the test temperature was 25 °C or 55 °C as well. Electrochemical impedance spectroscopy (EIS) curve was tested on an electrochemical workstation.
2. Experimental 2.1. Preparation of LiNi0.8Co0.1Mn0.1O2 First of all, a concentration of 2.0 mol/L NiSO4, CoSO4, and MnSO4 mixed solution with molar ratio of 8:1:1, a concentration of 10 mol/L NaOH solution, and a concentration of 5 mol/L ammonia solution were fed into the 10 L continuously stirred tank reactor (CSTR) at a certain rate under nitrogen atmosphere. The pH value, temperature, and stirring speed were controlled at pH 11.5 ± 0.05, 50 ℃ and 750 rpm. Then Ni0.8Co0.1Mn0.1(OH)2 precipitate obtained after reaction was filtered, washed for 5 times with deionized water, and dried in drum wind drying oven at 120 ℃ for 12 h, then the Ni0.8Co0.1Mn.1(OH)2 precursor was obtained. Next, Ni.8Co.1Mn0.1(OH)2 precursor and LiOH were mixed to fed into the tube furnace with the molar ratio of 1:1. 05, and preheated at 500 ℃ for 4 h, then calcined at 800 ℃ for 12 h, finally LiNi0.8Co0.1Mn0.1O2 (abbreviated as LNCM) was obtained. As for Li4P2O7-coated LiNi0.8Co0.1Mn0.1O2, LiH2PO4 was dispersed in the ethanol solvent, and then LiNi0.8Co0.1Mn0.1O2 was injected into the aforementioned solution with magnetic stirring at 50 ℃ until the solvent was completely evaporated. The initial solid content was
3. Results and discussion Fig. 1 shows the XRD patterns of bare LNCM and LNCM@LPO samples. As it can be seen in Fig. 1(a), the characteristic peak of the 2
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Fig. 2. SEM images of the bare LNCM and LNCM@LPO samples: (a), (b), (c) bare LNCM; (d), (e), (f) LNCM@LPO.
Li4P2O7 was generated on the surface of the modified materials after tempering based on that, and the crystallinity is low. Table 1 shows the lattice parameters of LNCM cathode materials before and after Li4P2O7 surface treatment. It can be seen that the lattice parameters (a, c) of the samples have no obvious changes before and after coating, indicating that the Li4P2O7 surface modification basically doesn’t affect the structure of the bulk materials. Fig. 2 displays the SEM images of the bare LNCM and LNCM@LPO samples. From the SEM images, it can be seen that both the surface of bare LNCM and LNCM@LPO samples are relatively smooth, however, the primary particle on the surface of the modified materials is smaller than the bare materials at a large magnification after tempering, which is probably because the coating materials react with the lithium residues remaining on the surface of the original materials, generating finer primary particles and uniformly covering the surface of the materials. This phenomenon can be seen in Table 2, which is the residual alkalinity test result on the surface of the materials. The total residual alkalinity on the surface of the original materials is as high as
Table 2 The residual lithium data of bare LNCM and LNCM@LPO samples. Sample
LiOH(ppm)
Li2CO3(ppm)
Both(ppm)
LNCM LNCM@LPO
1541 609
2697 867
4238 1476
modified sample corresponds to the unmodified sample, which is exactly the same as the standard card PDF#85–1969, showing a typical aNaFeO2 structure, which belongs to the R3m space group. Comparing the 003 characteristic peaks of the two materials, it does not change substantially before and after cladding, which indicating that surface modification does not affect the main structure of the materials [17]. In addition, the XRD pattern of LNCM@LPO was under close magnification from 20° to 40°, it can be seen that there are four broad peaks in the modified materials which basically corresponding to the standard card PDF#13–0282 of Li4P2O7. Which can be preliminary estimated that
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Fig. 3. SEM image of (a) LNCM@LPO sample and the corresponding EDS elemental images of (b) Ni, (c) Co, (d) Mn and (e) P.
unmodified samples corresponds to each other and there is no significant change, which indicating that the overall structure of the materials is not affected after modification. This is consistent with the previous XRD results. However, in Fig. 5(a), the peak of the binding energy of Ni 2p3/2 in the modified sample increases to 855.49 eV (the original sample is 855.14 eV), indicating that the coating layer reduces the content of Ni2+ in the material, while the presence of Ni2+ leads to an increase in the irreversible capacity of the materials [18,19], which is consistent with the original intention of the experiment. Meanwhile, in Fig. 6(b), the intensity of the binding energy to 531 eV for O1s of LNCM@LPO is significantly smaller than that of the bare LNCM, indicating that the content of Li2CO3 and LiOH in the modified sample is smaller. At the same time, the intensity of the binding energy to 529 eV for O1s is obviously larger than the uncoated materials, which indicating that there are more O2+ in Li-M-O bond in modified materials [20–22]. In addition, Fig. 6(e) reflects the binding energy positions of element P before and after modification. The P2p peak was observed at the binding energy of 133.94 eV for the coated materials and that is not only consistent with the characteristic peak of Li4P2O7 reported in the literature, but also consistent with XRD test results. Hence, through the
4238 ppm, while the modified materials is only 1476 ppm, which further illustrates that LiH2PO4 neutralizes the excess compound residues on the surface of bare materials during the annealing process. The materials are tested by EDS for facilitating the study of the element distribution. Fig. 3 shows the surface scanned EDS images of the LNCM@LPO material. It can be observed that the P, Ni, Co, and Mn are uniformly distributed, which indicates that the coating layer on the surface is evenly distributed. The bare LNCM and LNCM@LPO samples were tested by TEM in order to further verify the above conjecture. From the TEM images in Fig. 4, it can be clearly seen that the surface of the original sample in Fig. 4(a, b) is smooth without cladding layer. However, Fig. 4(c, d) show that there is a uniform coating layer on the surface of LNCM@LPO sample with an average thickness about 30–40 nm. Through further observation, it can be found that the coating layer is in close contact with the active materials, and there is no gap, the transition at the interface is very smooth as well. Fig. 5 diaplays the XPS spectra of the bare LNCM and LNCM@LPO samples, which reflects the surface changes of the modified sample. In Fig. 6(a), (c) and (d), the XPS spectra of the modified and the
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Fig. 4. TEM images of (a, b) the bare LNCM and (c, d) LNCM@LPO samples.
improved. Fig. 8 displays the SEM images of the two samples after 100 cycles. It can be seen that the microstructure of LNCM@LPO is preserved in good condition, which strongly supports the above conclusions. Fig. 6(d) shows the rate performance of the bare LNCM and LNCM@LPO samples from 0.2 C to 10 C and then back to 1 C. The discharge specific capacities of the two materials are comparable below 1 C rate; however, as the magnification rate increases sharply, the difference of the discharge specific capacity between the two materials becomes larger and larger. After returning to the 1 C rate, the uncoated materials cannot return to its previous discharge specific capacity, but the coated material basically can. This is because of a series of side reactions (LiPF6→PF5 +LiF, PF5 +H2O→POF3 +HF, POF3 +Li2O→ LixPOFy +LiF, Li2O/LiOH+HF→H2O+2LiF) that leading the surface of the uncoated materials to be more easily crushable during high-power charge and discharge. Furthermore, the high alkalinity of the original sample can also cause outward release of Li+ within the crystal lattice, resulting in an increase in the irreversible capacity [24]. To research the kinetic process of the bare LNCM and LNCM@LPO samples, EIS test was adopted. Fig. 7 displays the EIS plots of the bare LNCM and LNCM@LPO samples before and after cycle, the EIS data is exhibited in Table 3. As observed from Fig. 7a and Table 3, the impedance of the two samples is roughly the same before cycle, however, the impedance of LNCM@LPO is slightly smaller. The EIS curves can be decomposed into three parts: the semicircle in the high frequency region corresponds to the surface film impedance Rf, the semicircle in the medium frequency region corresponds to the charge transfer impedance Rct, and the straight line in the low frequency region corresponds to the ionic migration impedance Zw [25,26]. As seen from Fig. 7b and Table 3, the value of the surface film impedance Rf for the LNCM@LPO
XPS spectra, it can be concluded that the LiH2PO4 transforms into Li4P2O7 eventually, and this coating layer can prevent the transition from Ni3+ to Ni2+, making the materials less susceptible to water absorption and effectively reducing the impurities on the surface, such as Li2CO3 and LiOH. Li4P2O7 is an excellent Li+ conductor with a stable structure at high temperature [23]. Therefore, the temperature performance tests were carried on the samples at 25 ℃ and 55 ℃, respectively, and the rate performance test was carried at a large magnification of 10 C rate. As observed from Fig. 6(a), that the initial discharge curves of the two materials are similar, and the discharge platform and the initial discharge specific capacity of the LNCM@LPO material are slightly higher than the bare LNCM. Fig. 6(b) and (c) show the cycling performance of the bare LNCM and LNCM@LPO samples at 1 C rate at room and high temperature respectively. The modified materials show excellent cycling performance at both two temperatures. After 100 cycles, the discharge specific capacity of the original materials is reduced from the initial 183.0 mAh g−1 to current 159.9 mAh g−1, and 87.4% of capacity retention rate maintains at room temperature; while reducing from 196.6 mAh g−1 to 134.4 mAh g−1, and 68.4% capacity retention rate maintains at high temperature. Whereas for the modified materials, from 185.9 mAh g−1 to 174.6 mAh g−1, and 93.9% maintains at room temperature; while from 199.7 mAh g−1 to 176.3 mAh g−1, 88.3% maintains at high temperature. One of the reasons for the decrease of capacity retention rate is that the active materials on the surface of the original materials are exposed to the electrolyte and continuously react with it. On the contrary, the coating layer of the modified materials effectively blocks the contact of the active substances with the electrolyte, thus the cycling performance of modified materials has been
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Fig. 5. XPS spectra of the bare LNCM and LNCM@LPO samples: (a) Ni, (b) O, (c) Co, (d) Mn, (e) P.
was lower than that of the bare, which meant that the LNCM@LPO materials had a smaller polarization reaction and decreased the continuous formation of the SEI layer. Besides, the Rct of the bare NCM is significantly greater than that of LNCM@LPO measured under the same conditions after 100 cycles, which can be concluded that Li4P2O7
coating layer has lower charge transfer impedance than the surface of uncoated materials (which mixed with more LiOH and Li2CO3 impurity phase). The lower charge transfer impedance ensures the excellent rate and cycling performances of LNCM@LPO.
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Fig. 6. (a) Initial charge-discharge curves, (b) Cycling performance at 1 C and 25 °C, (c) Cycling performance at 1 C and 50 °C, (d) Rate capability at 25 °C of the bare LNCM and LNCM@LPO samples.
Fig. 7. EIS plots of (a) before cycle and (b) after 100 cycles of the bare LNCM and LNCM@LPO samples.
4. Conclusion
the surface of the modified materials. (2) The Li4P2O7 coating layer protects the integrity of the secondary particles during the cycle process, making them unbreakable. (3) The coating layer effectively stops the side reactions caused by the direct contact of the lithium compound residues on the surface with the electrolyte as well, which letting the materials have a lower charge transfer impedance. (4) Furthermore, the excellent Li+ conductivity of Li4P2O7 also facilitates the lithium-ion diffusion of the host materials.
In summary, it can decrease lithium compound residues on the surface of LiNi0.8Co0.1Mn0.1O2 with Li4P2O7 surface modification, and the modified materials show excellent cycling and rate performances at both room and high temperature. Through the analysis and research, we can see that: (1) these excellent properties are due to the reduction of lithium compounds on 7
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Fig. 8. SEM images of (a) the bare LNCM and (b) LNCM@LPO after cycling. Table 3 The EIS data of the bare LNCM and LNCM@LPO samples.
[11]
Sample
Cycle
Rf(Ω)
Rct(Ω)
Bare LNCM
0 100 0 100
– 133.7 – 114.5
28.81 571.4 23.75 316.9
LNCM@LPO
[12]
[13]
[14]
Therefore, it is worth promoting to do the surface modification on LiNi0.8Co0.1Mn0.1O2 and even other cathode materials with Li4P2O7.
[15]
Acknowledgments
[16]
We gratefully acknowledge the Nature Science Foundation of China (Grant No. 51602352).
[17]
[18]
Notes The authors declare no competing financial interest.
[19]
References
[20]
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhanced electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathode materials via Li4P2O7 surface modification for Li-ion batteries Kaihua Hua,1, Xianyue Qia,1, Caifeng Lub, Ke Dua, Zhongdong Penga, Yanbing Caoa, ⁎ Guorong Hua, a b
School of Metallurgy and Environment, Central South University, Changsha City 410083, China Confucious Institute at University of Zagreb, Zagreb 10000, Croatia
A R T I C LE I N FO
A B S T R A C T
Keywords: Li-ion batteries LiNi0.8Co0.1Mn0.1O2 Li4P2O7 surface modification Cycle performance Rate properties
The performance of LiNi0.8Co0.1Mn0.1O2 cathode materials has been significantly improved by Li4P2O7 surface modification. Some electrochemical tests are carried out to investigate the relevant performances of LiH2PO4coated LiNi0.8Co0.1Mn0.1O2, such as inductively coupled plasma test (ICP), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results indicate that LiNi0.8Co0.1Mn0.1O2 is clad with a layer of average thickness about 30–40 nm. And the alkalinity on the surface of the modified materials has declined dramatically after titration measurement. The effect of surface modification can be credited to the two main factors: On the one hand, the weak acidity of LiH2PO4 neutralizes the lithium residues (LiOH/Li2CO3) on the surface of LiNi0.8Co0.1Mn0.1O2 cathode materials; on the other hand, LiH2PO4 continues to decompose and react with the unneutralized lithium residues to generate a coating layer of Li4P2O7 during the annealing process, which can prevent the active materials from contacting with the electrolyte directly and act as an outstanding Li+ conductor, thus inhibiting the interface reaction and improving the cycle performance as well as the rate properties of the host materials.
1. Introduction With the requirement for environmental protection, the governments of the world take great efforts to boost the development of new energy vehicles, which brings about the increasingly growing demand for lithium-ion batteries. As the most essential cathode materials in lithium-ion batteries, some of whom are more and more frequently on the list of the major domestic and foreign journals, which have been widely studied recently such as LiFePO4, LiMn2O4 and LiNi1-xyCoxMnyO2 materials, etc. [1–3]. The Ni-rich cathode materials are considered to be the most promising cathode materials for power batteries due to their excellent comprehensive performance [4,5]. However, the high lithium residues on the surface are the prominent problems in the process of commercialization of Ni-rich materials. Excessive Li element is added in order to make up for the loss in the sintering process, (which exists as Li2O at high temperature), and Li2O can absorb the H2O and CO2 in the air to convert into LiOH and Li2CO3 at room temperature. [6,7]. The high lithium residue on the surface is the most vulnerable weakness in its large-scale commercialization. First, lithium residues will absorb moisture easily in the abstrich
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1
process, resulting in jelly in the homogenate process; and then, Li2CO3 will decompose to produce CO2 during the charging process, leading to expansion and lower safety performance of batteries. Next, the lithium residues also cause irreversible capacity loss during charging and discharging processes [8,9]. Therefore, it is extremely essential to reduce the lithium residues in the Ni-rich production. Surface modification is an effective method to enhance the cycling property and storage performance of cathode materials. It can make the active materials and the electrolyte isolated, which leading to reducing the occurrence of side reactions, and inhibit the dissolution of transition metalions in electrolyte coated by metal oxides [10,11], fluoride [12] or phosphates [13,14] with some proper thickness on the surface of cathode materials. However, above-mentioned cladding materials just play the role of physical isolation, cannot effectively reduce the lithium residues. Generally speaking, only few coating materials have ionic or electronic conductivity, and the coating materials hinder the diffusion of Li+ instead, which leading the materials to sacrifice some effective capacity for cycling performance. Hence, it is very indispensable to find a kind of material with ionic or electronic conductivity that can not only hinder the contact of the active materials with the electrolyte, but also effectively reduce lithium residues.
Corresponding author. E-mail address: [email protected] (G. Hu). These authors contributed equally to this work.
https://doi.org/10.1016/j.ceramint.2018.05.024 Received 20 April 2018; Received in revised form 3 May 2018; Accepted 4 May 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hu, K., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.05.024
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Fig. 1. XRD patterns of (a) bare LNCM and LNCM@LPO samples, and (b) the corresponding enlarged diffraction peaks.
designed to be 30%, the ratio of LiNi0.8Co0.1Mn0.1O2 and LiH2PO4 was 99:1 by weight. The obtained mixture was put into the vacuum oven at 120 ℃ for 12 h, and then tempered for 5 h at 450 ℃ in the air. At last, the Li4P2O7-coated LiNi0.8Co0.1Mn0.1O2 (abbreviated as LNCM@LPO) materials were obtained.
Table 1 Lattice parameters of the bare LNCM and LNCM@LPO samples. Sample
a(Å)
c(Å)
I003/I104
Bare NCM LNCM@LPO
2.8702 2.8698
14.2253 14.2260
1.669 1.673
2.2. Characterization In this thesis, LiH2PO4 is found to do the surface treatment on LiNi0.8Co0.1Mn0.1O2 cathode materials because of its excellent performance. Compared with other coating materials, this substance is in possession of some incomparable advantages. First, the lithium residues can be effectively removed by chemical reaction of LiH2PO4 with lithium compound residues. Second, the generated Li4P2O7 coating layer is an excellent Li+ conductor after sintering process [15,16]. Meanwhile, it can stabilize the crystal structure, and hinder the contact of the active substances with the electrolyte, thus inhibiting the interface reaction; furthermore, it can improve the diffusion coefficient of Li+ in materials and significantly enhance the rate performance of the bare materials. The following is the study of the synthetic method and performance of the cladding materials.
Overall composition and the content of impurities in samples were tested by ICP, the physical phases and structures of the as- synthesized samples were analyzed by XRD, and the instrument was a RIGAKU D/ Max 2550 VB + 18 kW target X-ray diffractometer. The surface morphology and element distribution of the materials were observed by SEM and EDS (JSM6360LV SEM, JEOL, Japan), and the internal crystal structure was observed by TEM (JEM 2100 F transmission electron microscope, JEOL, Japan). The chemical composition on the surface of the materials was analyzed by XPS (XPS Spectroscopic Analysis Instrument Model Thermo Fisher-VG Scientific ESCALAB 250Xi). 2.3. Electrochemical performance test The modified materials were uniformly mixed with acetylene black and polyvinylidene fluoride (mass ratio 8:1:1) and quickly dispersed in NMP, and then the wet-grinded dispersion was coated on a current collector aluminum foil. Then it was dried in the vacuum oven at 150 °C for 12 h, finally cut to get a pole wafer with a diameter of 14 mm. A mixed solution of 1 mol L−1 LiPF6 EC/DEC/EMC (volume ratio 1:1:1) was used as the electrolyte. The lithium metal was used as anode, and the Celgard 2400 microporous polypropylene film was the diaphragm. The CR2032 button cells were assembled in a glove box with the argon content less than 0.5 ppm, and stood for 10 h before being tested for electrochemical performance on a Land electrochemical tester. The weight of active materials was approximately 3.5 mg and the amount of the electrolyte was approximately 0.2 ml. The test voltage range was from 3.0 V to 4.3 V, current density was 1 C= 180mAh/g, while the magnification rate were 0.2 C, 0.5 C, 1 C and 2 C, etc. And the test temperature was 25 °C or 55 °C as well. Electrochemical impedance spectroscopy (EIS) curve was tested on an electrochemical workstation.
2. Experimental 2.1. Preparation of LiNi0.8Co0.1Mn0.1O2 First of all, a concentration of 2.0 mol/L NiSO4, CoSO4, and MnSO4 mixed solution with molar ratio of 8:1:1, a concentration of 10 mol/L NaOH solution, and a concentration of 5 mol/L ammonia solution were fed into the 10 L continuously stirred tank reactor (CSTR) at a certain rate under nitrogen atmosphere. The pH value, temperature, and stirring speed were controlled at pH 11.5 ± 0.05, 50 ℃ and 750 rpm. Then Ni0.8Co0.1Mn0.1(OH)2 precipitate obtained after reaction was filtered, washed for 5 times with deionized water, and dried in drum wind drying oven at 120 ℃ for 12 h, then the Ni0.8Co0.1Mn.1(OH)2 precursor was obtained. Next, Ni.8Co.1Mn0.1(OH)2 precursor and LiOH were mixed to fed into the tube furnace with the molar ratio of 1:1. 05, and preheated at 500 ℃ for 4 h, then calcined at 800 ℃ for 12 h, finally LiNi0.8Co0.1Mn0.1O2 (abbreviated as LNCM) was obtained. As for Li4P2O7-coated LiNi0.8Co0.1Mn0.1O2, LiH2PO4 was dispersed in the ethanol solvent, and then LiNi0.8Co0.1Mn0.1O2 was injected into the aforementioned solution with magnetic stirring at 50 ℃ until the solvent was completely evaporated. The initial solid content was
3. Results and discussion Fig. 1 shows the XRD patterns of bare LNCM and LNCM@LPO samples. As it can be seen in Fig. 1(a), the characteristic peak of the 2
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Fig. 2. SEM images of the bare LNCM and LNCM@LPO samples: (a), (b), (c) bare LNCM; (d), (e), (f) LNCM@LPO.
Li4P2O7 was generated on the surface of the modified materials after tempering based on that, and the crystallinity is low. Table 1 shows the lattice parameters of LNCM cathode materials before and after Li4P2O7 surface treatment. It can be seen that the lattice parameters (a, c) of the samples have no obvious changes before and after coating, indicating that the Li4P2O7 surface modification basically doesn’t affect the structure of the bulk materials. Fig. 2 displays the SEM images of the bare LNCM and LNCM@LPO samples. From the SEM images, it can be seen that both the surface of bare LNCM and LNCM@LPO samples are relatively smooth, however, the primary particle on the surface of the modified materials is smaller than the bare materials at a large magnification after tempering, which is probably because the coating materials react with the lithium residues remaining on the surface of the original materials, generating finer primary particles and uniformly covering the surface of the materials. This phenomenon can be seen in Table 2, which is the residual alkalinity test result on the surface of the materials. The total residual alkalinity on the surface of the original materials is as high as
Table 2 The residual lithium data of bare LNCM and LNCM@LPO samples. Sample
LiOH(ppm)
Li2CO3(ppm)
Both(ppm)
LNCM LNCM@LPO
1541 609
2697 867
4238 1476
modified sample corresponds to the unmodified sample, which is exactly the same as the standard card PDF#85–1969, showing a typical aNaFeO2 structure, which belongs to the R3m space group. Comparing the 003 characteristic peaks of the two materials, it does not change substantially before and after cladding, which indicating that surface modification does not affect the main structure of the materials [17]. In addition, the XRD pattern of LNCM@LPO was under close magnification from 20° to 40°, it can be seen that there are four broad peaks in the modified materials which basically corresponding to the standard card PDF#13–0282 of Li4P2O7. Which can be preliminary estimated that
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Fig. 3. SEM image of (a) LNCM@LPO sample and the corresponding EDS elemental images of (b) Ni, (c) Co, (d) Mn and (e) P.
unmodified samples corresponds to each other and there is no significant change, which indicating that the overall structure of the materials is not affected after modification. This is consistent with the previous XRD results. However, in Fig. 5(a), the peak of the binding energy of Ni 2p3/2 in the modified sample increases to 855.49 eV (the original sample is 855.14 eV), indicating that the coating layer reduces the content of Ni2+ in the material, while the presence of Ni2+ leads to an increase in the irreversible capacity of the materials [18,19], which is consistent with the original intention of the experiment. Meanwhile, in Fig. 6(b), the intensity of the binding energy to 531 eV for O1s of LNCM@LPO is significantly smaller than that of the bare LNCM, indicating that the content of Li2CO3 and LiOH in the modified sample is smaller. At the same time, the intensity of the binding energy to 529 eV for O1s is obviously larger than the uncoated materials, which indicating that there are more O2+ in Li-M-O bond in modified materials [20–22]. In addition, Fig. 6(e) reflects the binding energy positions of element P before and after modification. The P2p peak was observed at the binding energy of 133.94 eV for the coated materials and that is not only consistent with the characteristic peak of Li4P2O7 reported in the literature, but also consistent with XRD test results. Hence, through the
4238 ppm, while the modified materials is only 1476 ppm, which further illustrates that LiH2PO4 neutralizes the excess compound residues on the surface of bare materials during the annealing process. The materials are tested by EDS for facilitating the study of the element distribution. Fig. 3 shows the surface scanned EDS images of the LNCM@LPO material. It can be observed that the P, Ni, Co, and Mn are uniformly distributed, which indicates that the coating layer on the surface is evenly distributed. The bare LNCM and LNCM@LPO samples were tested by TEM in order to further verify the above conjecture. From the TEM images in Fig. 4, it can be clearly seen that the surface of the original sample in Fig. 4(a, b) is smooth without cladding layer. However, Fig. 4(c, d) show that there is a uniform coating layer on the surface of LNCM@LPO sample with an average thickness about 30–40 nm. Through further observation, it can be found that the coating layer is in close contact with the active materials, and there is no gap, the transition at the interface is very smooth as well. Fig. 5 diaplays the XPS spectra of the bare LNCM and LNCM@LPO samples, which reflects the surface changes of the modified sample. In Fig. 6(a), (c) and (d), the XPS spectra of the modified and the
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Fig. 4. TEM images of (a, b) the bare LNCM and (c, d) LNCM@LPO samples.
improved. Fig. 8 displays the SEM images of the two samples after 100 cycles. It can be seen that the microstructure of LNCM@LPO is preserved in good condition, which strongly supports the above conclusions. Fig. 6(d) shows the rate performance of the bare LNCM and LNCM@LPO samples from 0.2 C to 10 C and then back to 1 C. The discharge specific capacities of the two materials are comparable below 1 C rate; however, as the magnification rate increases sharply, the difference of the discharge specific capacity between the two materials becomes larger and larger. After returning to the 1 C rate, the uncoated materials cannot return to its previous discharge specific capacity, but the coated material basically can. This is because of a series of side reactions (LiPF6→PF5 +LiF, PF5 +H2O→POF3 +HF, POF3 +Li2O→ LixPOFy +LiF, Li2O/LiOH+HF→H2O+2LiF) that leading the surface of the uncoated materials to be more easily crushable during high-power charge and discharge. Furthermore, the high alkalinity of the original sample can also cause outward release of Li+ within the crystal lattice, resulting in an increase in the irreversible capacity [24]. To research the kinetic process of the bare LNCM and LNCM@LPO samples, EIS test was adopted. Fig. 7 displays the EIS plots of the bare LNCM and LNCM@LPO samples before and after cycle, the EIS data is exhibited in Table 3. As observed from Fig. 7a and Table 3, the impedance of the two samples is roughly the same before cycle, however, the impedance of LNCM@LPO is slightly smaller. The EIS curves can be decomposed into three parts: the semicircle in the high frequency region corresponds to the surface film impedance Rf, the semicircle in the medium frequency region corresponds to the charge transfer impedance Rct, and the straight line in the low frequency region corresponds to the ionic migration impedance Zw [25,26]. As seen from Fig. 7b and Table 3, the value of the surface film impedance Rf for the LNCM@LPO
XPS spectra, it can be concluded that the LiH2PO4 transforms into Li4P2O7 eventually, and this coating layer can prevent the transition from Ni3+ to Ni2+, making the materials less susceptible to water absorption and effectively reducing the impurities on the surface, such as Li2CO3 and LiOH. Li4P2O7 is an excellent Li+ conductor with a stable structure at high temperature [23]. Therefore, the temperature performance tests were carried on the samples at 25 ℃ and 55 ℃, respectively, and the rate performance test was carried at a large magnification of 10 C rate. As observed from Fig. 6(a), that the initial discharge curves of the two materials are similar, and the discharge platform and the initial discharge specific capacity of the LNCM@LPO material are slightly higher than the bare LNCM. Fig. 6(b) and (c) show the cycling performance of the bare LNCM and LNCM@LPO samples at 1 C rate at room and high temperature respectively. The modified materials show excellent cycling performance at both two temperatures. After 100 cycles, the discharge specific capacity of the original materials is reduced from the initial 183.0 mAh g−1 to current 159.9 mAh g−1, and 87.4% of capacity retention rate maintains at room temperature; while reducing from 196.6 mAh g−1 to 134.4 mAh g−1, and 68.4% capacity retention rate maintains at high temperature. Whereas for the modified materials, from 185.9 mAh g−1 to 174.6 mAh g−1, and 93.9% maintains at room temperature; while from 199.7 mAh g−1 to 176.3 mAh g−1, 88.3% maintains at high temperature. One of the reasons for the decrease of capacity retention rate is that the active materials on the surface of the original materials are exposed to the electrolyte and continuously react with it. On the contrary, the coating layer of the modified materials effectively blocks the contact of the active substances with the electrolyte, thus the cycling performance of modified materials has been
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Fig. 5. XPS spectra of the bare LNCM and LNCM@LPO samples: (a) Ni, (b) O, (c) Co, (d) Mn, (e) P.
was lower than that of the bare, which meant that the LNCM@LPO materials had a smaller polarization reaction and decreased the continuous formation of the SEI layer. Besides, the Rct of the bare NCM is significantly greater than that of LNCM@LPO measured under the same conditions after 100 cycles, which can be concluded that Li4P2O7
coating layer has lower charge transfer impedance than the surface of uncoated materials (which mixed with more LiOH and Li2CO3 impurity phase). The lower charge transfer impedance ensures the excellent rate and cycling performances of LNCM@LPO.
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Fig. 6. (a) Initial charge-discharge curves, (b) Cycling performance at 1 C and 25 °C, (c) Cycling performance at 1 C and 50 °C, (d) Rate capability at 25 °C of the bare LNCM and LNCM@LPO samples.
Fig. 7. EIS plots of (a) before cycle and (b) after 100 cycles of the bare LNCM and LNCM@LPO samples.
4. Conclusion
the surface of the modified materials. (2) The Li4P2O7 coating layer protects the integrity of the secondary particles during the cycle process, making them unbreakable. (3) The coating layer effectively stops the side reactions caused by the direct contact of the lithium compound residues on the surface with the electrolyte as well, which letting the materials have a lower charge transfer impedance. (4) Furthermore, the excellent Li+ conductivity of Li4P2O7 also facilitates the lithium-ion diffusion of the host materials.
In summary, it can decrease lithium compound residues on the surface of LiNi0.8Co0.1Mn0.1O2 with Li4P2O7 surface modification, and the modified materials show excellent cycling and rate performances at both room and high temperature. Through the analysis and research, we can see that: (1) these excellent properties are due to the reduction of lithium compounds on 7
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Fig. 8. SEM images of (a) the bare LNCM and (b) LNCM@LPO after cycling. Table 3 The EIS data of the bare LNCM and LNCM@LPO samples.
[11]
Sample
Cycle
Rf(Ω)
Rct(Ω)
Bare LNCM
0 100 0 100
– 133.7 – 114.5
28.81 571.4 23.75 316.9
LNCM@LPO
[12]
[13]
[14]
Therefore, it is worth promoting to do the surface modification on LiNi0.8Co0.1Mn0.1O2 and even other cathode materials with Li4P2O7.
[15]
Acknowledgments
[16]
We gratefully acknowledge the Nature Science Foundation of China (Grant No. 51602352).
[17]
[18]
Notes The authors declare no competing financial interest.
[19]
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