Enhanced high-voltage cycling stability and rate capability of magnesium and titanium co-doped lithium cobalt oxides for lithium-ion batteries

Applied Surface Science 458 (2018) 111–118

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Full Length Article

Enhanced high-voltage cycling stability and rate capability of magnesium and titanium co-doped lithium cobalt oxides for lithium-ion batteries

T

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Meiling Zhanga,1, Ming Tana,1, Hongyuan Zhaoa,b, , Shanshan Liua, Xiaohui Shua, Youzuo Hua, ⁎ Jintao Liua, Qiwen Rana, Hao Lia, Xingquan Liua, a b

R&D Center for New Energy Materials and Devices, School of Materials & Energy, University of Electronic Science and Technology of China, Chengdu 610054, China Research Branch of Advanced Materials & Green Energy, Henan Institute of Science and Technology, Xinxiang 453003, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Lithium-ions battery Cathode material LiCoO2 Mg and Ti co-doping Synergistic effect

To improve the high-voltage cycling stability and rate capability, the Mg2+ and Ti4+ co-doping strategy is firstly proposed to modify the LiCoO2 cathode material. The synergistic effect of co-doping with Mg2+ and Ti4+ ions on the structure, morphology and high-voltage electrochemical performance of LiCoO2 is investigated. For the codoped sample, the introduction of Mg2+ and Ti4+ ions can efficiently optimize the particle size distribution and reduce the aggregation behavior. Compared with the undoped and single-doped samples, the Mg2+ and Ti4+ codoped LiCoO2 sample presents better high-voltage cycling stability and rate capability due to the fact that the Mg2+ and Ti4+ ions co-doping can make full use of the respective advantages of Mg2+-doping and Ti4+-doping. When cycled at 1.0 C, the co-doped sample exhibits an initial discharge capacity of 179.6 mAh g−1 in the voltage range of 2.75–4.5 V. After 100 cycles, the capacity retention of this sample can reach up to 82.6%. Moreover, the co-doped sample shows better rate performance with high discharge capacity of 151.4 mAh g−1 at 5.0 C. These outstanding results may be attributed to the suppressed phase transition, decreased charge transfer resistance, improved thermal stability, enhanced electrical conductivity and uniform particle size distribution of the Mg2+ and Ti4+ co-doped LiCoO2 sample.

1. Introduction

the structure of LiCoO2 at high voltage [8–18]. Among various modification methods, bulk doping has been widely considered as effective methods to enhance the structural stability. Dopants are usually mental ions (Al3+, Mg2+, Ni2+, Zn2+, Ti4+, Zr4+, etc.) and metalloid cation (B3+, Si4+, etc.) due to the expectation of occupying Co3+ sites [19–32]. Among these elements, magnesium is cheap, light and abundant. The previous research results show that the Mg-doping can not only enhance the electrical conductivity of LiCoO2, but also promote the insulator-to-conductor transition in early deintercalation process [20,33]. Moreover, compared with other cation doping, the capacity reduction caused by Mg-doping (−130 mAh g−1 when x = 1 in LiCo1xMgxO2) is much smaller [34]. However, it should be noted that the ability of Mg-doping to stabilize LiCoO2 structure is deficient [35]. For layered LiCoO2, doping with Mg2+ ions will raise the average oxidation state of cobalt, which is undesirable to enhance the high-voltage cycling performance. According to existing literature [36], such undesirable result can be solved by introducing equal amount of tetravalent cations in the crystal structure of LiCoO2. Yu et al. have found that the Ti-

Among commercialized cathode materials, layered LiCoO2 has been regarded as the most popular cathode material for lithium-ion batteries (LIBs). This material can demonstrate excellent cycling stability with practical discharge capacity of 140 mAh g−1 when charged to 4.2 V [1]. However, such capacity performance can hardly meet the increasing requirement in high energy density for next-generation lithium-ion batteries. For layered LiCoO2, the theoretical specific capacity is 274 mAh g−1, but the actual discharge capacity in commercial Li-ion batteries only reach up to approximately 60% of the theoretical value. In order to enhance the capacity performance, many researchers attempt to solve this problem by raising the charge voltage. Unfortunately, the high charge voltage always results in the rapid capacity fading upon cycling. This undesirable result can be attributed to many causes, such as phase transition, cobalt dissolution and particle cracking [2–7]. Up to now, many researchers have made many efforts to stabilize

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Corresponding authors at: R&D Center for New Energy Materials and Devices, School of Materials & Energy, University of Electronic Science and Technology of China, Chengdu 610054, China (H. Zhao). E-mail addresses: [email protected] (H. Zhao), [email protected] (X. Liu). 1 Meiling Zhang and Ming Tan contribute equally to the work. https://doi.org/10.1016/j.apsusc.2018.07.091 Received 19 March 2018; Received in revised form 5 June 2018; Accepted 11 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.

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explained by the bigger radius of dopant ions (rMg2+ = 0.72 Å, rTi4+ = 0.61 Å, rCo3+ = 0.54 Å) [20,38]. These results indicate that the introduction of Mg2+ and Ti4+ ions can extend the two-dimensional channel of lithium ions, which is beneficial to the intercalation and deintercalation process of lithium ions. As a result, the Mg2+ and Ti4+ ions doped LiCoO2 samples may show good electrochemical performance. Fig. 2 shows the SEM images of the LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples. As we can see from Fig. 2a, the undoped LiCoO2 sample consists of irregular particles with uneven particle size distribution. The smallest particle size is only 1.0 μm, while the biggest particle size can reach up to about 10 μm. Such result of undoped sample is distinct when compared with those modified samples which demonstrate more regular particles with reduced particle size in Fig. 2b–d. But visible difference can be found as the Mg-doped sample still contains large particle and the Ti-doped sample exhibit severe aggregation behavior. By contrast, the co-doped sample presents most uniform particle size and reduces the particle aggregation. These results indicates that the substitution of Mg2+ and Ti4+ ions for partial cobalt ions efficiently optimize the particle size distribution and reduce the aggregation behavior to some extent, which agree with the previous research results [28,39]. Regular and smaller-sized particles shorten the diffusion distance of Li+ and thus reduce the electrochemical polarization, which contributes to its electrochemical properties. In order to characterize the chemical and oxidation state of each element in the modified material, X-ray photoelectron spectroscopy (XPS) is carried out. Fig. 3 shows the XPS spectra of Li1s, Co2p, Mg1s, Ti2p and O1s in the LiCo0.98Mg0.01Ti0.01O2 sample. As shown here, the spectrum of Li1s, Co2p and O1s presents well-defined characteristic binding energy peaks, which matches completely with reported studies [40,41]. It is noticeable that the Co2p spectrum is split to two components (Co2p3/2 and Co2p1/2) due to spin orbital interactions, and two satellite peaks resulted from ligand-to-metal charge transfer are clearly observed [42,43]. As shown in Fig. 3b, the characteristic peak of Co2p3/ 2 component is observed at 779.8 eV with a satellite peak at 789.8 eV, and the characteristic peak of Co2p1/2 component is observed at 794.9 eV with a satellite peak at 804.9 eV. This result indicates that Co element is in trivalent state which is in good agreement with previous works [40,42–45]. In Fig. 3c the Mg1s peak is assigned to 1303.2 eV, which corresponds to the divalent state of Mg ions. And in Fig. 3d the Ti2p peaks for Ti4+ions reveal a Ti2p3/2 peak at 457.98 eV and a Ti2p1/ 2 peak at 463.68 eV. These binding energy positions are in good agreement with previous research results and further confirm that introducing equal amount of Mg and Ti ions does not alter the oxidation of cobalt [46–49]. The thermal behavior of the LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) is investigated by thermogravimetric analysis (TGA). According to previous study, the weight loss of LiCoO2 material mainly comes from the evaporation of absorbed species and loss of oxygen at elevated temperature [50,51]. As shown in Fig. 4, all samples demonstrate weight loss rate less than 1% with increasing temperature. However, the co-doped sample exhibiting more slowly quality decreasing rate, indicating the improved thermal stability caused by codoping with Mg2+ and Ti4+ ions. Four-point probe method is carried out on the electrodes of all samples and the results are shown in Fig. 5 [52]. It can be obviously found that all modified samples demonstrate apparently improved electrical conductivity due to the introduction of Mg2+ or Ti4+, or both. But compared with other modified samples, Mg-doped sample exhibits the highest electrical conductivity, which indicates the apparent improvement on electrical conduction ability caused by Mgdoping. In particular, although Ti-doping cannot bring as much improvement as Mg-doping dose, the co-doped sample exhibits similar electrical conductivity to Mg-doped sample. This result suggests that Mg2+ as dopants effectively enhance the electrical conduction ability of LiCoO2, and such visible enhancement is also brought by co-doping

doping can enormously improve the stability of LiCoO2 [28,29]. The Ti4+-doped LiCoO2 sample demonstrates excellent cycling stability and outstanding rate performance at 4.4 V voltage. By comprehensive considering of the respective improvement of Mg-doping and Ti-doping, we attempt to combine the advantages of Mg-doping and Ti-doping to more effectively enhance the high-voltage cycling performance of LiCoO2. In this work, we successfully synthesized the layered LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples by citric acid-assisted sol-gel method. The synergistic effect of co-doping with Mg2+ and Ti4+ ions on the structure, morphology and high-voltage electrochemical performance of LiCoO2 was investigated in detail. 2. Experimental The Mg2+ and Ti4+ ions co-doped LiCo0.98Mg0.01Ti0.01O2 sample has been synthesized by sol-gel method. Firstly, stoichiometric lithium hydroxide monohydrate and citric acid monohydrate were added into deionized water to form a solution. The mixed solution of cobalt acetate and magnesium nitrate was added dropwise into the above solution under constantly stirring at 55 °C. And then, the ethanol solution of butyl titanate was added dropwise into the mixed solution. Next, adding ammonia water to manipulate the pH of the obtained solution to about 7.0. The achieved solution was evaporated at 85 °C to get a wet purple gel. After dried at 105 °C for 24 h in oven, the purple gel was sintered at 450 °C for 6 h in air. Subsequently, the decomposed gel precursor was ground in mortar and sintered at 900 °C for 12 h in air to get the black product. For comparation, the un-doped LiCoO2, LiCo0.98Mg0.02O2 and LiCo0.98Ti0.02O2 samples were synthesized by the same sol-gel process. In order to confirm the crystal structures, X-ray diffraction (XRD, Bruker DX-1000, Cu Ka radiation) was carried out. The surface morphology of bare and modified samples were identified by scanning electron microscopy (SEM, JEOL JSM-6360LV) and the chemical or oxidation state was measured by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI). The thermal stability of all samples was investigated by the thermogravimetric analysis (TGA, Q500) under air atmosphere at a heating rate of 10 °C per min. For electrochemical evaluation, the working electrodes were obtained by using obtained samples as active materials. Lithium foil and polypropylene membrane were used as anode material and diaphragm, respectively. 1 M LiPF6 in a mixture (VDMC:VEMC:VEC = 1:1:1) was used as the electrolyte. Electrochemical measurements were carried out by using LAND CT2001A. Electrochemical impedance spectra (EIS) and cyclic voltammogram (CV) were studied by CS-350 electrochemical workstation. The electron conductivity of all samples was investigated by Four-point probe device (RTS-9). 3. Results and discussion To identify the influence of doping with Mg2+ and Ti4+ ions on the structure of LiCoO2, XRD characterization is carried out on the LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples, and the corresponding XRD patterns are shown in Fig. 1a. The characteristic diffraction peaks of these samples are consistent with the standard diffraction peaks of LiCoO2 (JCPDS No. 75-0532), indicating that all these samples possess the typical α-NaFeO2 structure with R-3 m space group. Moreover, it can be seen that the separation of (0 0 6)/(0 1 2) peaks and (0 1 8)/(1 1 0) peaks is very obvious, which suggests the formation of well-ordered layered structure of all these samples [37]. Fig. 1b shows the magnified (0 0 3) peak of these samples. Compared with the undoped sample, the (0 0 3) peaks of all the doped samples shift slightly to lower angle, indicating that doping with Mg2+ or Ti4+ ions can enlarge the lattice parameter. Table 1 lists the accurate values of lattice parameters calculated from the XRD patterns. The values of lattice parameter a and c varies with doping with Mg2+ or Ti4+ ions, which agrees with the analysis result from Fig. 1b. This effect can be 112

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Fig. 1. (a) XRD patterns and (b) magnified (0 0 3) peaks of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01).

[3,17,28,53]. By contrast, the LiCo0.98MO2 (M = Mg0.02, Ti0.02, Mg0.01Ti0.01) samples can demonstrate excellent cycling performance. In particular, the co-doped LiCoO2 sample still achieves more than 148.4 mAh g−1 after 100 cycles and the capacity retention of this sample reach up to 82.6%, which is higher than that of the LiCo0.98MO2 (M = Mg0.02, Ti0.02) samples in Table 2. Fig. 6b shows the typical charge-discharge curves of the co-doped LiCoO2 sample. As shown here, the charge-discharge capacities of the 1st, 50th and 100th cycles present decreased trend, the typical charge-discharge curves do not show distinct difference, suggesting that the co-doped sample has good reversibility and structure stability [54]. Such excellent high-voltage cycling performance mainly benefits from the synergistic effect of the Mg2+ and Ti4+ ions co-doping. According to the research results [20,28,29], the Mg2+-doping can improve the electrical conductivity and partly stabilize the structure of LiCoO2, and the Ti4+-doping can effectively enhance the structure stability of LiCoO2. Therefore, it is not difficult to understand the best high-voltage cycling performance of the Mg2+ and Ti4+ ions co-doped sample. To further understand the influence of Mg2+ and Ti4+ ions doping on the structure of LiCoO2 during charge and discharge cycling, the cyclic voltammogram (CV) is carried out on the LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples. Both cells are scanned between 3.3 and 4.5 V at 0.1 mV s−1 scanning rate and the results are shown in Fig. 7. As we can see from Fig. 7a, the cyclic voltammogram curve of the undoped LiCoO2 sample presents three pairs of symmetrical oxidation and reduction peaks. The oxidation peaks are observed at 4.03, 4.09 and 4.20 V whereas the corresponding reduction peaks are observed at 3.83, 4.04 and 4.16 V, respectively. According to existing literatures [2,55], the oxidation peak at 4.03 V corresponds to the oxidation of Co3+/Co4+ ions and the other two oxidation peaks (4.09 and 4.20 V) correspond to the transition from hexagonal phase to monoclinic phase and then to hexagonal phase again during the deintercalation process of Li+ ions. The converting from monoclinic phase to hexagonal phase results in large change in c-axis of LiCoO2 [3]. And the significant change in lattice parameters during intercalation and deintercalation can lead to poor capacity retention with cycling [56]. By contrast, as shown in Fig. 7b and c, the two oxidation peaks (4.09 and 4.20 V) of the LiCo0.98Mg0.02O2 sample and LiCo0.98Ti0.02O2 samples are inhibited to some extent. In particularly, the cyclic voltammogram curve of the LiCo0.98Mg0.01Ti0.01O2 sample only exhibits one pair of obvious oxidation and reduction peaks. The oxidation peaks at 4.09 and 4.20 V shown in Fig. 7a are not observed in Fig. 7d. These results suggests that the phase transitions of LiCoO2 at 4.09 and 4.20 V are totally suppressed by co-doping with Mg2+ and Ti4+ ions and the codoped LiCoO2 sample undergoes less lattice change during the intercalation and de-intercalation process of Li+ ions when cycled at high

Table 1 Crystal parameters of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01). Samples

a (Å)

c (Å)

c/a

I(0 0 3)/I(1 0 4)

LiCoO2 LiCo0.98 Mg0.02O2 LiCo0.98 Ti0.02O2 LiCo0.98Mg0.01Ti0.01O2

2.80651 2.82284 2.81612 2.81839

13.98622 14.09512 14.05228 14.06859

4.98349 4.99324 4.989944 4.99171

2.47 1.69 1.44 1.48

Fig. 2. SEM images of (a) LiCoO2, (b) LiCo0.98Mg0.02O2, (c) LiCo0.98Ti0.02O2 and (d) LiCo0.98Mg0.01Ti0.01O2.

with Mg2+ and Ti4+, which contributes significantly to its electrochemical performance. Fig. 6 shows the cycling performance and typical charge-discharge curves of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples. All cells are charged to 4.5 V and discharged to 2.75 V at 1.0 C rate. As shown in Fig. 6a, a decrease in initial discharge capacity can be observed due to the introduction of inactive ions. For the undoped LiCoO2 sample, the discharge capacity fades rapidly upon cycling. Although the initial discharge capacity of this material can reach up to 189.2 mAh g−1, the discharge capacity rapidly decreases to 14.0 mAh g−1 with very bad capacity retention of 7.4% after 100 cycles. This tremendous degeneration at high voltage has been reported in previous work and badly limits the practical application of LiCoO2 113

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Fig. 3. XPS spectra of Li1s, Co2p, Mg1s, Ti2p and O1s in the LiCo0.98Mg0.01Ti0.01O2 sample.

Fig. 5. Electrical conductivity data of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01). Fig. 4. TGA plots of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01).

exhibit decreased discharge capacity with increasing charge-discharge rate due to the effect of cell polarization and internal resistance [45]. Compared with the undoped LiCoO2 sample, the Mg2+ and Ti4+ doped samples present superior rate performance. Especially, at 1.0 C rate, the co-doped sample can deliver discharge capacity of 178.5 mAh g−1. When the charge-discharge rate increases to 5.0 C after 20 cycles, the co-doped sample can achieve the discharge capacity of 150.9 mAh g−1,

voltage. As a result, the Mg2+ and Ti4+ co-doped LiCoO2 sample present more stable structure stability and excellent high-voltage cycling performance. Fig. 8a shows the rate performance of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples, which are cycled at different rates in the voltage range of 2.75–4.5 V. As shown here, these two samples 114

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Fig. 6. (a) Cycle performance of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) and (b) typical charge-discharge curves of LiCo0.98Mg0.01Ti0.01O2. Table 2 Cycle performance Mg0.01Ti0.01).

data

of

LiCo0.98MO2

(M = Co0.02,

Mg0.02,

which is much higher than that of the undoped LiCoO2 sample. In addition, when the charge-discharge rate restores to 1.0 C after 25 cycles, the co-doped sample still presents high discharge capacity of 169.0 mAh g−1 with good capacity recovery rate of 94.7%, compared to the recovery rate of 82.4% for the undoped LiCoO2 sample. Furthermore, the co-doped sample presents both superior discharge capacity and cycling stability when cycled at high rate of 5.0 C, as shown in Fig. 8b. For the co-doped sample, the initial discharge capacity can reach up to 151.4 mAh g−1 with good capacity retention of 93.7% after 100 cycles. By contrast, the undoped LiCoO2 sample only show lower initial discharge capacity and worse capacity retention. These results

Ti0.02,

Samples

Initial discharge capacity (mAh g−1)

100th discharge capacity (mAh g−1)

Capacity retention after 100 cycles (%)

LiCoO2 LiCo0.98 Mg0.02O2 LiCo0.98 Ti0.02O2 LiCo0.98Mg0.01Ti0.01O2

189.2 181.1 176.9 179.6

14.0 112.8 136.8 148.4

7.4 62.3 77.3 82.6

Fig. 7. Cyclic voltammogram curves of (a) LiCoO2, (b) LiCo0.98Mg0.02O2, (c) LiCo0.98Ti0.02O2 and (d) LiCo0.98Mg0.01Ti0.01O2. 115

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Fig. 8. (a) Rate cycling performance of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) and (b) 5 C cycling performance of LiCoO2 and LiCo0.98 Mg0.01Ti0.01O2.

Fig. 9. Charge-discharge curves at different rates of (a) LiCoO2, (b) LiCo0.98Mg0.02O2, (c) LiCo0.98Ti0.02O2 and (d) LiCo0.98Mg0.01Ti0.01O2.

rates [57]. By contrast, the discharge curves of the Mg2+ and Ti4+ ions co-doped sample exhibits highest consistency at different charge-discharge rates. Moreover, the voltage plateau remains clear and flat with increasing charge-discharge rate. These results indicate that the increasing effect of polarization at high charge-discharge rate is effectively suppressed by co-doping with Mg2+ and Ti4+ ions, which further explains good reversibility and superior rate performance of the codoped sample. To understand the kinetics of the electrode reaction, electrochemical impedance spectroscopy (EIS) is carried out on the obtained samples. The EIS experiments are performed in the discharged state of 2.75 V and all cells are measured after 1 cycle, 50 cycles and 100 cycles in the frequency range of 100 kHz to 0.01 Hz, respectively. Fig. 10

further confirm the improvement in rate performance of LiCoO2 modified by co-doping with Mg2+ and Ti4+ ions at high-voltage. Fig. 9 shows the charge-discharge curves of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples at different rates. As shown here, we can find that the charge-discharge curves of the Mg2+ and Ti4+ doped LiCoO2 samples show a relatively smaller change compared with the undoped sample. At low charge-discharge rate of 1.0 C, the discharge curves of these two samples show little difference with a clear voltage plateau around 3.8 V. However, when the charge-discharge rate increases to 5.0 C, the discharge curve of the undoped LiCoO2 sample becomes severely sloped and the voltage plateau becomes unobvious, suggesting the severe effect of polarization on the high-voltage electrochemical performance at high charge-discharge 116

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Fig. 10. Nyquist plots of (a) LiCoO2, (b) LiCo0.98Mg0.02O2, (c) LiCo0.98Ti0.02O2 and (d) LiCo0.98Mg0.01Ti0.01O2.

indicates the better structure stability of the co-doped sample. This result presents good agreement with our analysis mentioned above and further proves the enhancement caused by introducing equal amount of magnesium and titanium.

Table 3 Fitted values of Rsei and Rct of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01). Samples

LiCoO2 LiCo0.98Mg0.02O2 LiCo0.98Ti0.02O2 LiCo0.98Mg0.01Ti0.01O2

Rsei (Ω)

Rct (Ω)

After 1 cycle

After 50 cycles

After 100 cycles

After 1 cycle

After 50 cycles

After 100 cycles

141.6 138.7 127.9 116.5

85.7 102.6 98.1 80.3

78.8 98.4 91.7 72.5

35.3 40.5 27.6 23.0

1005.1 296.8 140.5 104.2

1578.3 413.1 257.9 170.6

4. Conclusion To summarize, the Mg2+ and Ti4+ co-doping strategy is firstly proposed to improve the high-voltage cycling stability and rate capability of LiCoO2 cathode material. The introduction of Mg2+ and Ti4+ ions not only retains the inherent crystal structure, but also reduces the aggregation behavior to some extent. For single-cation doping, the Mg2+-doping can improve the electrical conductivity and partly stabilize the structure of LiCoO2, and the Ti4+-doping can effectively enhance the structure stability of LiCoO2. By contrast, the Mg2+ and Ti4+ ions co-doping shows the anticipated synergetic effect, which fully utilizes their respective advantages to enhance the high-voltage electrochemical performance of LiCoO2. The obtained LiCo0.98Mg0.01Ti0.01O2 sample demonstrates excellent high-voltage electrochemical properties. When cycled at 1.0 C, it could show the high initial discharge capacity of 179.6 mAh g−1 at 1.0 C in the voltage range of 2.75–4.5 V. After 100 cycles, the capacity retention of this sample can reach up to 82.6%. Moreover, the co-doped sample also present superior rate performance and decreased charge transfer resistance. All these results indicate that the substitution of Mg2+ and Ti4+ ions for partial cobalt ions may be an effective method to enhance the high-voltage electrochemical performance of LiCoO2.

shows the Nyquist plots of LiCo0.98MO2 (M = Co0.02, Mg0.02, Ti0.02, Mg0.01Ti0.01) samples. It can be observed that all Nyquist plots consist of three parts. The high-frequency semicircle relates to the SEI films (Rsei). The medium-to-low-frequency semicircle is concerned with charge transfer resistance (Rct) and the low-frequency oblique line is associated with Li+ diffusion in material [33]. The equivalent electrical circuit is shown in Fig. 10b and the fitting values of Rsei and Rct are listed in Table 3. As shown here, the charge transfer resistance of bare LiCoO2 presents rapidly increasing with cycling. After the first cycle the Rct value of un-doped sample is only 35.3 Ω, which raises to 1005.1 Ω after 50 cycles and finally reaches up to 1578.3 Ω after 100 cycles. This considerable increase of Rct is attributed to the severe interface passivation and serious microcrack defects, and consequently indicates the degradation of electrochemical properties of bare LiCoO2 charged to high voltage [10,35,58]. By contrast, the Rct value of the Mg2+ and Ti4+ ions co-doped sample is the most stable with cycling, which 117

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Acknowledgments

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