Enhanced Electrochemical Performances of Cobalt-Doped Li2MoO3 Cathode Materials

Co-doped Li2MoO3 was successfully synthesized via a solid phase method. The impacts of Co-doping on Li2MoO3 have been analyzed by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR) measurements. The results show that an appropriate amount of Co ions can be introduced into the Li2MoO3 lattices, and they can reduce the particle sizes of the cathode materials. Electrochemical tests reveal that Co-doping can significantly improve the electrochemical performances of the Li2MoO3 materials. Li2Mo0.90Co0.10O3 presents a first-discharge capacity of 220 mAh·g−1, with a capacity retention of 63.6% after 50 cycles at 5 mA·g−1, which is much better than the pristine samples (181 mAh·g−1, 47.5%). The enhanced electrochemical performances could be due to the enhancement of the structural stability, and the reduction in impedance, due to the Co-doping.


Introduction
Recently, the development of high-capacity cathode materials has become a hot topic in the field of Li-ion batteries. Mn-based Li-rich layer oxides xLi 2 MnO 3 ·(1 − x)LiMO 2 (0 < x < 1.0, M = Mn, Ni, Co, etc.) were proposed as potential cathode materials, due to their high discharge capacities of above 280 mAh·g −1 , and thus, the structure stability of the Li 2 MnO 3 component [1][2][3][4][5][6][7][8]. Unfortunately, numerous reports have indicated that the drawbacks of Li 2 MnO 3 -based composites, such as low initial Coulombic efficiency, a fast decline in capacity, and potential safety hazards, were difficult to overcome, which severely restricted their practice applications [9][10][11]. Thus, much attention has been paid to find other transition metals instead of Mn, to build new Li 2 MO 3 (M = Ru, Ir, Mo, etc.)-based materials for next generation Li-ion batteries in recent years [12][13][14][15]. Li 2 MoO 3 as a type of Li-rich layer cathode material with alternating Li layers and randomly distributed [Li 1/3 Mo 2/3 ] layers, has attracted much research interest [15][16][17][18][19][20][21]. The previous studies verified that Li 2 MoO 3 promised a high theoretical capacity of up to 339 mAh·g −1 , and a near-absence of oxygen evolution [17,18], which supported Li 2 MoO 3 as a candidate to replace Li 2 MnO 3 in constructing Li-rich cathode materials. However, the poor cycling stability and rate capability of the Li 2 MoO 3 material, owing to its low conductivity and irreversible phase transition, hinders its practical application. Hence, it is necessary to find a suitable modification method to improve the performance of the Li 2 MoO 3 material.
cycling stabilities over the 10 cycles. In our previous study, carbon-coated Li2MoO3 composites were successfully prepared, and they achieved much lower impedances and better electrochemical performances than bare Li2MoO3 [21]. Cobalt doping has been considered to be a facile and effective method in enhancing the electrochemical performances, since it can improve structure stability and reduce the impedance of cathode materials [22][23][24][25][26]. In this paper, cobalt was selected to improve the electrochemical performances of Li2MoO3 for the first time. The structural characteristics and electrochemical performances of Li2Mo1-xCoxO3 are presented here.

Electrochemical Tests
The electrochemical performances were tested through CR2032-type coin cells (HF-Kejing, Hefei, China). Cathodes were prepared by mixing 70 wt % Li2Mo1-xCoxO3, 20 wt % acetylene carbon black and 10 wt % polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solution. The slurry was cast evenly onto a stainless steel sheet, and dried in a vacuum oven (Suopuyiqi, Shanghai, China) at 120 °C for 12 hr. Lithium metal and Celgard 2400 were used as the anode and separator, respectively. A concentration of 1 mol·L −1 LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (volume ratio: 1:1) solution was adopted as the electrolyte. The coin cells were assembled in an argon-filled glove box, and measured on a Land CT2001A system (LANHE, Wuhan, China) in

Electrochemical Tests
The electrochemical performances were tested through CR2032-type coin cells (HF-Kejing, Hefei, China). Cathodes were prepared by mixing 70 wt % Li 2 Mo 1−x Co x O 3 , 20 wt % acetylene carbon black and 10 wt % polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solution. The slurry was cast evenly onto a stainless steel sheet, and dried in a vacuum oven (Suopuyiqi, Shanghai, China) at 120 • C for 12 h. Lithium metal and Celgard 2400 were used as the anode and separator, respectively. A concentration of 1 mol·L −1 LiPF 6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (volume ratio: 1:1) solution was adopted as the electrolyte. The coin cells were assembled in an argon-filled glove box, and measured on a Land CT2001A system (LANHE, Wuhan, China) in galvanostatic mode at 30 • C. Electrochemical impedance spectroscopy (EIS) was tested using an electrochemical station (CHI660B) (Chenhua, Shanghai, China) in 10 −2 Hz-10 MHz, with a voltage amplitude of 5 mV. Cycliation at a scanning speed of 10 −1 mVc voltammetry (CV) was performed with the same electrochemical st s −1 .

Results and Discussion
3.1. Characteristics of the as-Prepared Li 2 Mo 1−x Co x O 3 Figure 2a and b shows the XPS spectra of Co 2p 3/2 in Li 2 Mo 0.90 Co 0.10 O 3 , and the corresponding precursor. The peak of Co 2p 3/2 in the precursor is at 779.8 eV (Figure 2a) with a weak satellite at 789.9 eV. This satellite peak is 10.1 eV above Co 2p 3/2 , corresponding diamagnetic Co 3+ (S = 0), which is very consistent with the XPS result reported for LiCoO 2 [27]. After the reduction by hydrogen at a high temperature, the original satellite disappears, and a new intense satellite appears at 784.4 eV, as shown in Figure 2b. Compared with the original satellite, the new satellite is 4.1 eV higher than the core level line (780.3 eV), corresponding to high-spin Co 2+ (S = 3/2) compounds, indicating that the valence state of the Co element will be reduced from Co 3+ to Co 2+ after reduction processing under hydrogen [28]. Figure 2c and d show that the XPS spectra of Mo in Li 2 MoO 3 and Li 2 Mo 0.90 Co 0.10 O 3 . The peak at around 230.1 eV is assigned to Mo 4+ 3d 5/2 , and the peak at around 232.6 eV is assigned to Mo 6+ 3d 5/2 . Obviously, after cobalt doping, the peak intensity of Mo 6+ 3d 5/2 rises, and the peak intensity of Mo 4+ 3d 5/2 decreases, indicating an increased amount of Mo 6+ . galvanostatic mode at 30 °C. Electrochemical impedance spectroscopy (EIS) was tested using an electrochemical station (CHI660B) (Chenhua, Shanghai, China) in 10 −2 Hz-10 MHz, with a voltage amplitude of 5 mV. Cyclic voltammetry (CV) was performed with the same electrochemical station at a scanning speed of 10 −1 mV s −1 . Figure 2a and b shows the XPS spectra of Co 2p3/2 in Li2Mo0.90Co0.10O3, and the corresponding precursor. The peak of Co 2p3/2 in the precursor is at 779.8 eV (Figure 2a) with a weak satellite at 789.9 eV. This satellite peak is 10.1 eV above Co 2p3/2, corresponding diamagnetic Co 3+ (S = 0), which is very consistent with the XPS result reported for LiCoO2 [27]. After the reduction by hydrogen at a high temperature, the original satellite disappears, and a new intense satellite appears at 784.4 eV, as shown in Figure 2b. Compared with the original satellite, the new satellite is 4.1 eV higher than the core level line (780.3 eV), corresponding to high-spin Co 2+ (S = 3/2) compounds, indicating that the valence state of the Co element will be reduced from Co 3+ to Co 2+ after reduction processing under hydrogen [28]. Figure 2c and d show that the XPS spectra of Mo in Li2MoO3 and Li2Mo0.90Co0.10O3. The peak at around 230.1 eV is assigned to Mo 4+ 3d5/2, and the peak at around 232.6 eV is assigned to Mo 6+ 3d5/2. Obviously, after cobalt doping, the peak intensity of Mo 6+ 3d5/2 rises, and the peak intensity of Mo 4+ 3d5/2 decreases, indicating an increased amount of Mo 6+ .  Figure 3a shows the XRD patterns of the synthesized Li2Mo1-xCoxO3 (x = 0, 0.05, 0.10, 0.15). Except the sample with x = 0.15, all samples match well with the α-NaFeO2 structure, which could be indexed to Li2MoO3 (see Figure 3a). When the Co content adds up to 0.15, the characteristic peaks of the impurity phase Li4MoO5 and Co appear. The splitting of the (006)/(101) peaks at 36°, reflecting that the layer structure weakens with the increase of Co content, indicating that Co-doping increases the disorder of the cations. Rietveld refinements for Li2Mo1-xCoxO3 were carried out, to obtain more information from XRD (see Figure 3b,c). The a(b)-parameters are increased, and c-parameters are decreased, with a rise of Co-doping content. Notably, the variations of cell parameters are negligible while x is higher than 0.10 considering the fitting error, which indicates that the solubility limit of Co is around x = 0.10. Moreover, the values of c/a drop with the increase of the Co-doping content, which  Except the sample with x = 0.15, all samples match well with the α-NaFeO 2 structure, which could be indexed to Li 2 MoO 3 (see Figure 3a). When the Co content adds up to 0.15, the characteristic peaks of the impurity phase Li 4 MoO 5 and Co appear. The splitting of the (006)/(101) peaks at 36 • , reflecting that the layer structure weakens with the increase of Co content, indicating that Co-doping increases the disorder of the cations. Rietveld refinements for Li 2 Mo 1−x Co x O 3 were carried out, to obtain more information from XRD (see Figure 3b,c). The a(b)-parameters are increased, and c-parameters are decreased, with a rise of Co-doping content. Notably, the variations of cell parameters are negligible while x is higher than 0.10 considering the fitting error, which indicates that the solubility limit of Co is around x = 0.10. Moreover, the values of c/a drop with the increase of the Co-doping content, which indicates that Co-doping increases the disorder of Li 2 MoO 3 materials. As indicated in the above XPS results, the valence of cobalt should be +2 in the samples. Its radius (0.745 Å) is very similar to that of Li + (0.76 Å), which may result in the increase of disorder. Figure 3d exhibits the unit cell volume of the pristine and the Co-doped Li 2 MoO 3 . Clearly, the unit cell volume increases with the rise of the Co-doping contents, which could be related to the replacement of Mo 4+ (0.65 Å) by Co 2+ (0.745 Å). indicates that Co-doping increases the disorder of Li2MoO3 materials. As indicated in the above XPS results, the valence of cobalt should be +2 in the samples. Its radius (0.745 Å) is very similar to that of Li + (0.76 Å), which may result in the increase of disorder. Figure 3d exhibits the unit cell volume of the pristine and the Co-doped Li2MoO3. Clearly, the unit cell volume increases with the rise of the Co-doping contents, which could be related to the replacement of Mo 4+ (0.65 Å) by Co 2+ (0.745 Å). In order to observe the impacts of Co-doping on the particle morphologies of the samples, the SEM images of Li2Mo1-xCoxO3 (x = 0, 0.05, 0.10) were examined (see Figure 4). The particle sizes of the pristine Li2MoO3 present a wide distribution range from 1 to 3 µm (see Figure 4a), whereas the doped samples show smaller particles with more uniform distributions in the range of 200-300 nm (see Figure 4b,c). The results suggest that the addition of Co affects the morphology, and decreases the particle size of cathode materials. Particle growth may be restricted by lattice distortion of Li2MoO3 due to the replacement of Mo by Co. Similar phenomena have also been observed by some other groups [29,30]. Ma, J. et al. studied the stability of Li2MoO3 in air [16]. Their results verified that Li2MoO3 easily adsorbed O2 and thus was partially oxidized to Li2MoO4. Meanwhile, the CO2 in air also reacted with Li2MoO3 to produce Li2CO3, which consumed the Li ions near the surface and produced MoO3 [16]. In order to investigate the effects of Co-doping on the stability of Li2Mo1-xCoxO3 in air, FTIR spectra of Li2MoO3 and Li2Mo0.90Co0.10O3 were carried out (see Figure 5). The samples were stored in the air for 7 days before the FTIR test. Obviously, both samples present a similar FTIR spectra. The peaks at 446, 497, 559 and 698 cm −1 are assigned to Li2MoO3, which is consistent with the previous study [16]. While the peaks at 1480, 1420, 830 and 817 cm −1 could be attributed to Li2CO3 and Li2MoO4, respectively. These species are believed to be the reaction products between of Li2MoO3, CO2 and O2, revealing that both samples are partially decomposed in air. No peaks related MoO3 are detected in the FTIR spectra, which may contribute to the shorter storage time compared with the previous report [16]. In order to observe the impacts of Co-doping on the particle morphologies of the samples, the SEM images of Li 2 Mo 1−x Co x O 3 (x = 0, 0.05, 0.10) were examined (see Figure 4). The particle sizes of the pristine Li 2 MoO 3 present a wide distribution range from 1 to 3 µm (see Figure 4a), whereas the doped samples show smaller particles with more uniform distributions in the range of 200-300 nm (see Figure 4b,c). The results suggest that the addition of Co affects the morphology, and decreases the particle size of cathode materials. Particle growth may be restricted by lattice distortion of Li 2 MoO 3 due to the replacement of Mo by Co. Similar phenomena have also been observed by some other groups [29,30]. Ma, J. et al. studied the stability of Li 2 MoO 3 in air [16]. Their results verified that Li 2 MoO 3 easily adsorbed O 2 and thus was partially oxidized to Li 2 MoO 4 . Meanwhile, the CO 2 in air also reacted with Li 2 MoO 3 to produce Li 2 CO 3 , which consumed the Li ions near the surface and produced MoO 3 [16]. In order to investigate the effects of Co-doping on the stability of Li 2 Mo 1−x Co x O 3 in air, FTIR spectra of Li 2 MoO 3 and Li 2 Mo 0.90 Co 0.10 O 3 were carried out (see Figure 5). The samples were stored in the air for 7 days before the FTIR test. Obviously, both samples present a similar FTIR spectra. The peaks at 446, 497, 559 and 698 cm −1 are assigned to Li 2 MoO 3 , which is consistent with the previous study [16]. While the peaks at 1480, 1420, 830 and 817 cm −1 could be attributed to Li 2 CO 3 and Li 2 MoO 4 , respectively. These species are believed to be the reaction products between of Li 2 MoO 3 , CO 2 and O 2 , revealing that both samples are partially decomposed in air. No peaks related MoO 3 are detected in the FTIR spectra, which may contribute to the shorter storage time compared with the previous report [16].    Figure 6 compares the initial charge-discharge profiles of pristine and Co-doped samples in the voltage range of 1.5-4.5 V at 5 mA·g −1 . The samples present initial discharge capacities of 181, 213, 220 and 165 mAh·g −1 , respectively. When the doping content is 0.05 and 0.10, the first discharge capacities of Co-doped samples are higher than that of the pristine sample. It also can be found the first discharge capacity decreases obviously while x = 0.15. What's more, voltage difference between charge and discharge profiles of Li2Mo0.90Co0.10O3 is much smaller than that of pristine Li2MoO3, indicating that Co-doping can effectively suppress the polarization and enhance reversibility of Li2MoO3 materials. Notable that the charge behaviors with two regions for Co-doped Li2MoO3 cathode materials are similar to that for the pristine sample in the first charge-discharge process, which may relate to the delithiation reaction corresponding to the oxidation of the Mo ions in the 1.5-3.7 V region and a Li2MoO3-Li0.91MoO3 two phase reaction in the 3.7-4.5 V region [17].    Figure 6 compares the initial charge-discharge profiles of pristine and Co-doped samples in the voltage range of 1.5-4.5 V at 5 mA·g −1 . The samples present initial discharge capacities of 181, 213, 220 and 165 mAh·g −1 , respectively. When the doping content is 0.05 and 0.10, the first discharge capacities of Co-doped samples are higher than that of the pristine sample. It also can be found the first discharge capacity decreases obviously while x = 0.15. What's more, voltage difference between charge and discharge profiles of Li2Mo0.90Co0.10O3 is much smaller than that of pristine Li2MoO3, indicating that Co-doping can effectively suppress the polarization and enhance reversibility of Li2MoO3 materials. Notable that the charge behaviors with two regions for Co-doped Li2MoO3 cathode materials are similar to that for the pristine sample in the first charge-discharge process, which may relate to the delithiation reaction corresponding to the oxidation of the Mo ions in the 1.5-3.7 V region and a Li2MoO3-Li0.91MoO3 two phase reaction in the 3.7-4.5 V region [17].  Figure 6 compares the initial charge-discharge profiles of pristine and Co-doped samples in the voltage range of 1.5-4.5 V at 5 mA·g −1 . The samples present initial discharge capacities of 181, 213, 220 and 165 mAh·g −1 , respectively. When the doping content is 0.05 and 0.10, the first discharge capacities of Co-doped samples are higher than that of the pristine sample. It also can be found the first discharge capacity decreases obviously while x = 0.15. What's more, voltage difference between charge and discharge profiles of Li 2 Mo 0.90 Co 0.10 O 3 is much smaller than that of pristine Li 2 MoO 3 , indicating that Co-doping can effectively suppress the polarization and enhance reversibility of Li 2 MoO 3 materials. Notable that the charge behaviors with two regions for Co-doped Li 2 MoO 3 cathode materials are similar to that for the pristine sample in the first charge-discharge process, which may relate to the delithiation reaction corresponding to the oxidation of the Mo ions in the 1.5-3.7 V region and a Li 2 MoO 3 -Li 0.91 MoO 3 two phase reaction in the 3.7-4.5 V region [17]. Figure 7 shows the cycling performances of pristine and Co-doped samples between 1.5 and 4.5 V at 5 mA·g −1 . It is clearly seen that Li 2 Mo 0.95 Co 0.05 O 3 and Li 2 Mo 0.90 Co 0.10 O 3 deliver much higher discharge capacity than that of pristine sample. After 50 cycles, pristine and Co-doped sample present the discharge capacities of 86, 121, 140 and 52 mAh·g −1 with the capacity retentions of 47.5%, 56.8%, 63.6% and 31.5%, respectively. Clearly, the cycling stability of pristine is poor. However, while increasing cobalt content to 0.05 and 0.10, the discharge capacity and cycling stability are significantly improved. With a further increase of Co content, the discharge capacity and cycling stability reduce, which may be attributed to the rise of inert impurities of Li 4 MoO 5 and Co. Li 4 MoO 5 delivers poor electrochemical performances because of its low electron conduction, low Coulombic efficiency and critical irreversible phase transition [31]. In addition, Co shows ignorable specific capacity above 1.5 V [32]. Therefore, the appearance of Co and Li 4 MoO 5 in the sample has negative effects on the electrochemical performance of Li 2 MoO 3 . Li 2 Mo 0.90 Co 0.10 O 3 possesses the highest discharge capacity and the best capacity retention, which indicates the amounts of dopant will be important for the electrochemical performances of Li 2 MoO 3 .  Figure 7 shows the cycling performances of pristine and Co-doped samples between 1.5 and 4.5 V at 5 mA·g −1 . It is clearly seen that Li2Mo0.95Co0.05O3 and Li2Mo0.90Co0.10O3 deliver much higher discharge capacity than that of pristine sample. After 50 cycles, pristine and Co-doped sample present the discharge capacities of 86, 121, 140 and 52 mAh·g −1 with the capacity retentions of 47.5%, 56.8%, 63.6% and 31.5%, respectively. Clearly, the cycling stability of pristine is poor. However, while increasing cobalt content to 0.05 and 0.10, the discharge capacity and cycling stability are significantly improved. With a further increase of Co content, the discharge capacity and cycling stability reduce, which may be attributed to the rise of inert impurities of Li 4 MoO 5 and Co. Li4MoO5 delivers poor electrochemical performances because of its low electron conduction, low Coulombic efficiency and critical irreversible phase transition [31]. In addition, Co shows ignorable specific capacity above 1.5 V [32]. Therefore, the appearance of Co and Li4MoO5 in the sample has negative effects on the electrochemical performance of Li2MoO3. Li2Mo0.90Co0.10O3 possesses the highest discharge capacity and the best capacity retention, which indicates the amounts of dopant will be important for the electrochemical performances of Li2MoO3.    Figure 7 shows the cycling performances of pristine and Co-doped samples between 1.5 and 4.5 V at 5 mA·g −1 . It is clearly seen that Li2Mo0.95Co0.05O3 and Li2Mo0.90Co0.10O3 deliver much higher discharge capacity than that of pristine sample. After 50 cycles, pristine and Co-doped sample present the discharge capacities of 86, 121, 140 and 52 mAh·g −1 with the capacity retentions of 47.5%, 56.8%, 63.6% and 31.5%, respectively. Clearly, the cycling stability of pristine is poor. However, while increasing cobalt content to 0.05 and 0.10, the discharge capacity and cycling stability are significantly improved. With a further increase of Co content, the discharge capacity and cycling stability reduce, which may be attributed to the rise of inert impurities of Li 4 MoO 5 and Co. Li4MoO5 delivers poor electrochemical performances because of its low electron conduction, low Coulombic efficiency and critical irreversible phase transition [31]. In addition, Co shows ignorable specific capacity above 1.5 V [32]. Therefore, the appearance of Co and Li4MoO5 in the sample has negative effects on the electrochemical performance of Li2MoO3. Li2Mo0.90Co0.10O3 possesses the highest discharge capacity and the best capacity retention, which indicates the amounts of dopant will be important for the electrochemical performances of Li2MoO3. The comparison of rate capabilities between the pristine and Co-doped Li2MoO3 with current density from 5 mA·g −1 to 20 mA·g −1 is evaluated in Figure 8. The Li2Mo0.90Co0.10O3 possesses a higher The comparison of rate capabilities between the pristine and Co-doped Li 2 MoO 3 with current density from 5 mA·g −1 to 20 mA·g −1 is evaluated in Figure 8 material (180 mAh·g at 5 mA·g and 71 mAh·g at 20 mA·g ). The difference of discharge capacities between Li2MoO3 and Li2Mo0.90Co0.10O3 increases from 38 mAh·g −1 to 66 mAh·g −1 with a rise of current density from 5 mA·g −1 to 20 mA·g −1 . When the current density returns to 5 mA·g −1 , the discharge capacity of Li2Mo0.90Co0.10O3 could reach up 155 mAh·g −1 , while that of pristine Li2MoO3 only lefts 104 mAh·g −1 . The above results suggest that Co-doping significantly enhances the rate capability of Li2MoO3.  Figure 9 illustrates CV curves of Li2MoO3 and Li2Mo0.90Co0.10O3 between 1.5 and 4.5 V. The redox peaks of Li2MoO3 in CV curve are located at 2.965 V and 1.978 V, which are respectively related to the delithiation/lithiation processes corresponding to the oxidation/reduction of Mo 4+ /Mo 6+ couple [18]. The oxidation peak of the Li2Mo0.90Co0.10O3 at 2.863 V is lower than that of Li2MoO3 and the reduction peak of the Li2Mo0.90Co0.10O3 at 2.136 V is above that of the pristine sample. Therefore, Li2Mo0.90Co0.10O3 possesses a smaller difference of the redox peak potential (∆E, 0.727 V) than Li2MoO3 (0.987 V). It is well known that difference of the redox peaks potential is highly correlated with electrode polarization. Hence, the conclusion can be drawn that Co-doping can reduce the polarization of Li2MoO3, which are coincident with the improvement in electrochemical performances.    [18]. The oxidation peak of the Li 2 Mo 0.90 Co 0.10 O 3 at 2.863 V is lower than that of Li 2 MoO 3 and the reduction peak of the Li 2 Mo 0.90 Co 0.10 O 3 at 2.136 V is above that of the pristine sample. Therefore, Li 2 Mo 0.90 Co 0.10 O 3 possesses a smaller difference of the redox peak potential (∆E, 0.727 V) than Li 2 MoO 3 (0.987 V). It is well known that difference of the redox peaks potential is highly correlated with electrode polarization. Hence, the conclusion can be drawn that Co-doping can reduce the polarization of Li 2 MoO 3 , which are coincident with the improvement in electrochemical performances. density from 5 mA·g −1 to 20 mA·g −1 . When the current density returns to 5 mA·g −1 , the discharge capacity of Li2Mo0.90Co0.10O3 could reach up 155 mAh·g −1 , while that of pristine Li2MoO3 only lefts 104 mAh·g −1 . The above results suggest that Co-doping significantly enhances the rate capability of Li2MoO3.  Figure 9 illustrates CV curves of Li2MoO3 and Li2Mo0.90Co0.10O3 between 1.5 and 4.5 V. The redox peaks of Li2MoO3 in CV curve are located at 2.965 V and 1.978 V, which are respectively related to the delithiation/lithiation processes corresponding to the oxidation/reduction of Mo 4+ /Mo 6+ couple [18]. The oxidation peak of the Li2Mo0.90Co0.10O3 at 2.863 V is lower than that of Li2MoO3 and the reduction peak of the Li2Mo0.90Co0.10O3 at 2.136 V is above that of the pristine sample. Therefore, Li2Mo0.90Co0.10O3 possesses a smaller difference of the redox peak potential (∆E, 0.727 V) than Li2MoO3 (0.987 V). It is well known that difference of the redox peaks potential is highly correlated with electrode polarization. Hence, the conclusion can be drawn that Co-doping can reduce the polarization of Li2MoO3, which are coincident with the improvement in electrochemical performances.  To further analyze the kinetic behaviors of the pristine and Co-doped samples, EIS measurements of Li 2 MoO 3 and Li 2 Mo 0.90 Co 0.10 O 3 were carried out and the results are presented in Figure 10. Both EIS plots display similar shapes (see Figure 10a), which are fitted through the equivalent circuit (see Figure 10c) and the fitting results are listed in Table 1. In the equivalent circuit, R f is related to Li + diffusion in the SEI film, R ct is corresponding to the charge transfer resistance at electrolyte-electrode interface and R s is considered to be ohmic resistance. As can be seen from Table 1, the R s and R f of both samples change slightly. In contrast, the R ct of Li 2 MoO 3 is significantly reduced duo to the Co-doping. The Li 2 Mo 0.90 Co 0.10 O 3 presents the R ct of 105.70 Ω, which is far below the pristine Li 2 MoO 3 (478.75 Ω). The R ct is major part to the total electrode impedance, and its reduction reveals that Co-doping is very beneficial to enhance the kinetic behaviors of Li 2 MoO 3 . In addition, The Li + ion diffusion coefficients (D Li + ) were estimated by the following formula:

Electrochemical Performances
R, T, A, F and C Li + are the gas constant, the absolute temperature, the area of the electrode surface, the Faraday's constant and the molar concentration of Li ions, respectively [7]. The σ corresponding to the Warburg factor could be calculated by the Z /ω −0.5 (see Figure 10b) and the following formula: measurements of Li2MoO3 and Li2Mo0.90Co0.10O3 were carried out and the results are presented in Figure 10. Both EIS plots display similar shapes (see Figure 10a), which are fitted through the equivalent circuit (see Figure 10c) and the fitting results are listed in Table 1. In the equivalent circuit, Rf is related to Li + diffusion in the SEI film, Rct is corresponding to the charge transfer resistance at electrolyte-electrode interface and Rs is considered to be ohmic resistance. As can be seen from Table  1, the Rs and Rf of both samples change slightly. In contrast, the Rct of Li2MoO3 is significantly reduced duo to the Co-doping. The Li2Mo0.90Co0.10O3 presents the Rct of 105.70 Ω, which is far below the pristine Li2MoO3 (478.75 Ω). The Rct is major part to the total electrode impedance, and its reduction reveals that Co-doping is very beneficial to enhance the kinetic behaviors of Li2MoO3. In addition, The Li + ion diffusion coefficients (DLi + ) were estimated by the following formula: R, T, A, F and CLi + are the gas constant, the absolute temperature, the area of the electrode surface, the Faraday's constant and the molar concentration of Li ions, respectively [7]. The σ corresponding to the Warburg factor could be calculated by the Z'/ω −0.5 (see Figure 10b) and the following formula: The improving trend in the values of DLi + is very similar to the reducing trend in the values of Rct. The pristine and Co-doped Li2MoO3 deliver the Li + ion diffusion coefficients of 3.89 × 10 −17 and 1.94 × 10 −16 cm 2 ·s −1 , respectively. As we can see, the Li + ion diffusion coefficients of Li2MoO3 have an obvious growth due to the Co-doping. These results clearly indicate that Co-doping significantly improves the kinetics behavior of Li + and reduces the impedance of Li2MoO3, which is responsible for the better rate capability and cycling stability of Li2Mo0.90Co0.10O3.  To investigate the structural transformation of the samples during charge-discharge process, XRD patterns of Li2MoO3 and Li2Mo0.90Co0.10O3 after 20 cycles are exhibited in Figure 11. There are notable differences in the structure of Li2MoO3 before and after cycling. For pristine Li2MoO3 material, the strongest diffraction peak transfers from (003) to (104) and ratio of (003)/(104) drops to 0.65 after cycling, which could be indexed into the Li-insufficient structure [17]. This Li-insufficient structure contributes to the partially reversible migration of the Mo ions and the partial recovery of the Mo3O13  To investigate the structural transformation of the samples during charge-discharge process, XRD patterns of Li 2 MoO 3 and Li 2 Mo 0.90 Co 0.10 O 3 after 20 cycles are exhibited in Figure 11. There are notable differences in the structure of Li 2 MoO 3 before and after cycling. For pristine Li 2 MoO 3 material, the strongest diffraction peak transfers from (003) to (104) and ratio of (003)/(104) drops to 0.65 after cycling, which could be indexed into the Li-insufficient structure [17]. This Li-insufficient structure contributes to the partially reversible migration of the Mo ions and the partial recovery of the Mo 3 O 13 clusters during charge-discharge processes, leading to irreversible capacity loss and poor cycling stability. In contrast, ratio of (003)/(104) of Li 2 Mo 0.90 Co 0.10 O 3 after cycling can maintain at 0.83, which is much better than the pristine Li 2 MoO 3 after cycling. It indicates that Co-doping can effectively enhance the structural stability during charge-discharge process. clusters during charge-discharge processes, leading to irreversible capacity loss and poor cycling stability. In contrast, ratio of (003)/(104) of Li2Mo0.90Co0.10O3 after cycling can maintain at 0.83, which is much better than the pristine Li2MoO3 after cycling. It indicates that Co-doping can effectively enhance the structural stability during charge-discharge process.

Conclusions
Co-doped Li2MoO3 was successfully synthesized via a solid phase method. The influences of Codoping on the structural and electrochemical characteristics of Li2MoO3 are analyzed. The results show that the addition of Co affects the morphology and decreases the particle size of cathode materials. Electrochemical measurements confirm that Co-doping can effectively improve the electrochemical performances of Li2MoO3 materials. The Li2Mo0.90Co0.10O3 presents an initial discharge capacity of 220 mAh·g −1 with the capacity retention of 63.6% after 50 cycles at 5 mA·g −1 , which is much better than the pristine samples (181 mAh·g −1 , 47.5%). Additionally, the rate capability of Li2MoO3 is also enhanced by Co-doping. It is found that the Li2Mo0.90Co0.10O3 delivers a much lower Rct and a higher Li + ion diffusion coefficients than pristine Li2MoO3. What is more, the irreversible structural transformation is also suppressed by Co-doping. The enhanced electrochemical performances could be attributed to the improvement in structural stability and reduction in impedance due to the Co-doping. Our work reveals that doping modification will be a promising method for improving the electrochemical performances of Li2MoO3 material, and thus benefit for its application.