Enhanced Cycling Stability through Erbium Doping of LiMn2O4 Cathode Material Synthesized by Sol-Gel Technique

In this work, LiMn2−xErxO4 (x ≤ 0.05) samples were obtained by sol-gel processing with erbium nitrate as the erbium source. XRD measurements showed that the Er-doping had no substantial impact on the crystalline structure of the sample. The optimal LiMn1.97Er0.03O4 sample exhibited an intrinsic spinel structure and a narrow particle size distribution. The introduction of Er3+ ions reduced the content of Mn3+ ions, which seemed to efficiently suppress the Jahn–Teller distortion. Moreover, the decreased lattice parameters suggested that a more stable spinel structure was obtained, because the Er3+ ions in a ErO6 octahedra have stronger bonding energy (615 kJ/mol) than that of the Mn3+ ions in a MnO6 octahedra (402 kJ/mol). The present results suggest that the excellent cycling life of the optimal LiMn1.97Er0.03O4 sample is because of the inhibition of the Jahn-Teller distortion and the improvement of the structural stability. When cycled at 0.5 C, the optimal LiMn1.97Er0.03O4 sample exhibited a high initial capacity of 130.2 mAh g−1 with an excellent retention of 95.2% after 100 cycles. More significantly, this sample showed 83.1 mAh g−1 at 10 C, while the undoped sample showed a much lower capacity. Additionally, when cycled at 55 °C, a satisfactory retention of 91.4% could be achieved at 0.5 C after 100 cycles with a first reversible capacity of 130.1 mAh g−1.


Introduction
With increasing environmental awareness, many people have realized the importance of green travel, which is very useful for reducing environmental pollution and protecting human health. As an optimal choice for green travel, electric vehicles with rechargeable batteries have become very popular all over the world. Meanwhile, lithium-ion batteries, as the power source, have been developed quickly in recent years [1][2][3][4][5][6][7][8]. It is generally known that there are four major classes of mature cathode materials, namely LiCoO 2 [9,10], LiFePO 4 [11,12], LiNi 1−x−y Co x M y O 2 (M = Mn, Al) [13,14], and LiMn 2 O 4 [15,16], for batteries. Among these materials, LiMn 2 O 4 shows many virtues such as mature production The crystal structures of the obtained erbium-doped spinels were studied by X-ray diffraction (XRD, Bruker DX-1000, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15406 nm). The lattice parameters of these erbium-doped samples were obtained by using MDI Jade 5.0 software. The surface morphologies and microstructures were determined by using scanning electron microscopy (SEM, JEOL JSM-6360LV, Tokyo, Japan) with an energy dispersive X-ray spectrometer (EDX, EDAX Inc., Mahwah, NJ, USA). X-ray photoelectron spectroscopy (XPS) was obtained by using a Thermo ESCALAB 250XI instrument (Thermo Fisher Scientific, Waltham, MA, USA) with a monochromatic Al Ka (1486.6 eV) X-ray source.
The active electrode consisted of the obtained erbium-doped spinels, conductive acetylene black, and polyvinylidene fluoride (weight ratio = 85:10:5). The anode material and diaphragm were lithium foil and Celgard 2400 polymer (Celgard, Charlotte, NA, USA), respectively. A mixture of 1 M of LiPF6, ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and ethylene carbonate (EC) was used as the electrolyte (VEMC:VDMC:VEC = 1:1:1, (Guangzhou Tinci Materials Technology Co., Ltd., Guagnzhou, China)). The electrochemical measurements were carried out on a NEWARE battery testing system (NEWARE, Shenzhen, China). The electrochemical impedance spectroscopy (EIS) was carried out by using a CS-350 electrochemical workstation (Wuhan Corrtest Instruments Crop., Ltd., Wuhan, China). The impedance plots were recorded by applying an AC (alternating current) voltage of 5 mV amplitude in the frequency range of 0.1-100 kHz.  , suggesting that the introduction of a small amount of erbium ions did not have detectable influence on the material's structure [35,42]. All the LiMn2−xErxO4 (x = 0.01, 0.03, 0.05) samples maintained the inherent spinel structure of LiMn2O4. According to the previously reported results [21,45] and according to the reported references [21,48], the (220) peak of LiMn2O4 is particularly sensitive to the other cations at tetrahedral sites (8a). If the doped ions inhabit the tetrahedral sites, the (220) peak should appear in the corresponding XRD pattern. However, the (220) peak cannot be observed in the XRD patterns in all the LiMn2−xErxO4 (x = 0, 0.01, 0.03, 0.05) samples. This indicates that the erbium ions replaced the manganese ions at the octahedral sites in the Er-doped LiMn2O4 samples. The crystal structures of the obtained erbium-doped spinels were studied by X-ray diffraction (XRD, Bruker DX-1000, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15406 nm). The lattice parameters of these erbium-doped samples were obtained by using MDI Jade 5.0 software. The surface morphologies and microstructures were determined by using scanning electron microscopy (SEM, JEOL JSM-6360LV, Tokyo, Japan) with an energy dispersive X-ray spectrometer (EDX, EDAX Inc., Mahwah, NJ, USA). X-ray photoelectron spectroscopy (XPS) was obtained by using a Thermo ESCALAB 250XI instrument (Thermo Fisher Scientific, Waltham, MA, USA) with a monochromatic Al Ka (1486.6 eV) X-ray source.

Results and Discussion
The active electrode consisted of the obtained erbium-doped spinels, conductive acetylene black, and polyvinylidene fluoride (weight ratio = 85:10:5). The anode material and diaphragm were lithium foil and Celgard 2400 polymer (Celgard, Charlotte, NA, USA), respectively. A mixture of 1 M of LiPF 6 , ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and ethylene carbonate (EC) was used as the electrolyte (V EMC :V DMC :V EC = 1:1:1, (Guangzhou Tinci Materials Technology Co., Ltd., Guagnzhou, China)). The electrochemical measurements were carried out on a NEWARE battery testing system (NEWARE, Shenzhen, China). The electrochemical impedance spectroscopy (EIS) was carried out by using a CS-350 electrochemical workstation (Wuhan Corrtest Instruments Crop., Ltd., Wuhan, China). The impedance plots were recorded by applying an AC (alternating current) voltage of 5 mV amplitude in the frequency range of 0.1-100 kHz. suggesting that the introduction of a small amount of erbium ions did not have detectable influence on the material's structure [35,42]. All the LiMn 2−x Er x O 4 (x = 0.01, 0.03, 0.05) samples maintained the inherent spinel structure of LiMn 2 O 4 . According to the previously reported results [21,45] and according to the reported references [21,48], the (220) peak of LiMn 2 O 4 is particularly sensitive to the other cations at tetrahedral sites (8a). If the doped ions inhabit the tetrahedral sites, the (220) peak should appear in the corresponding XRD pattern. However, the (220) peak cannot be observed in the XRD patterns in all the LiMn 2−x Er x O 4 (x = 0, 0.01, 0.03, 0.05) samples. This indicates that the erbium ions replaced the manganese ions at the octahedral sites in the Er-doped LiMn 2 O 4 samples. Table 1 lists the corresponding crystal parameters of these samples. The lattice parameters of these erbium-doped samples were obtained by using MDI Jade 5.0 software. It is obvious from these data that all the Er-doped LiMn 2 O 4 samples possessed a Fd-3m space group. As the Er-doping content increased, the LiMn 2−x Er x O 4 (x = 0.01, 0.03, 0.05) samples showed smaller lattice parameters and cell volumes. Figure 2b shows the magnified map of the (111), (311), and (400) peaks. It can be clearly seen that the introduction of erbium ions caused a shift toward the higher angle, which further indicated the decrease of the crystal parameters. These results suggest the formation of a more stable spinel structure [20,49]. This is principally because the Er 3+ ions in the ErO 6 octahedra showed stronger bonding energy (615 kJ/mol) than that of the Mn 3+ ions in the MnO 6 octahedra (402 kJ/mol) [50]. In addition, it should be noted that the Er-doped LiMn 2 O 4 samples showed higher (311)/(400) peak intensity ratios, which have much to do with the cycling life of LiMn 2 O 4 [21,51]. An analysis of the previously published results indicated that the introduction of erbium ions may play a constructive role in enhancing the electrochemical properties.  Table 1 lists the corresponding crystal parameters of these samples. The lattice parameters of these erbium-doped samples were obtained by using MDI Jade 5.0 software. It is obvious from these data that all the Er-doped LiMn2O4 samples possessed a Fd-3m space group. As the Er-doping content increased, the LiMn2−xErxO4 (x = 0.01, 0.03, 0.05) samples showed smaller lattice parameters and cell volumes. Figure 2b shows the magnified map of the (111), (311), and (400) peaks. It can be clearly seen that the introduction of erbium ions caused a shift toward the higher angle, which further indicated the decrease of the crystal parameters. These results suggest the formation of a more stable spinel structure [20,49]. This is principally because the Er 3+ ions in the ErO6 octahedra showed stronger bonding energy (615 kJ/mol) than that of the Mn 3+ ions in the MnO6 octahedra (402 kJ/mol) [50]. In addition, it should be noted that the Er-doped LiMn2O4 samples showed higher (311)/(400) peak intensity ratios, which have much to do with the cycling life of LiMn2O4 [21,51]. An analysis of the previously published results indicated that the introduction of erbium ions may play a constructive role in enhancing the electrochemical properties.  Figure 3 presents the SEM images of the LiMn2−xErxO4 (x = 0, 0.01, 0.03, 0.05) samples obtained by the sol-gel technology. It can be clearly observed that the introduction of the erbium ions had a certain influence on the surface morphology of the powders. For the undoped LiMn2O4 particles shown in Figure 3a, the particle size distribution was unsatisfactory because of severe particle agglomeration. By contrast, all the Er-doped LiMn2O4 particles (Figure 3b-d) showed relatively good surface morphology with relatively little particle agglomeration. When the Er-doping content increased, the mean diameter of the LiMn2−xErxO4 (x = 0.01, 0.03, 0.05) samples showed a decreasing    Figure 3a, the particle size distribution was unsatisfactory because of severe particle agglomeration. By contrast, all the Er-doped LiMn 2 O 4 particles (Figure 3b-d) showed relatively good surface morphology with relatively little particle agglomeration. When the Er-doping content increased, the mean diameter of the LiMn 2−x Er x O 4 (x = 0.01, 0.03, 0.05) samples showed a decreasing tendency. In particular, the LiMn 1.97 Er 0.03 O 4 particles shown in Figure 3c presented the most uniform size distribution, which is conducive to the enhancement of cycling life [21,28,32]. These results indicate that the introduction of erbium ions can effectively optimize the size distribution, which contributes to the improvement of the cycling stability. Figure Figure 5 shows the XPS spectra of Li1s, Mn2p, Er4d, and O1s in the LiMn1.97Er0.03O4 sample, which was selected as a representative sample of the Er-doped LiMn2O4 samples. The binding energy peaks of the Li1s, Mn2p, and O1s are well shown in Figure 5a,b,d and coincide with the previous reported literature [21]. It is important to note that the Mn2p3/2 binding energy of the manganese element was at 642.4 eV. However, according to the existing literature [36,52], the   Figure 5a,b,d and coincide with the previous reported literature [21]. It is important to note that the Mn2p 3/2 binding energy of the manganese element was at 642.4 eV. However, according to the existing literature [36,52], the Mn2p 3/2 binding energies of the trivalent and tetravalent manganese ions are at 641.7 eV and 643.1 eV, respectively. Thus, it can be inferred that the manganese element in the LiMn 1.97 Er 0.03 O 4 sample corresponded to the coexistence state of the trivalent and tetravalent manganese ions. As for the erbium element, the binding energy peak shown in Figure 5c corresponded to the oxidation states for Er4d, which was assigned to Er 3+ at 168.8 eV, which agrees with the previous result [53].  Figure 6a presents the first discharge curves of these samples, which were tested at 0.5 C. All the Er-doped LiMn2O4 samples showed similar characteristic discharge curves to that of the undoped spinel. There were two distinct voltage platforms around 4.15 V and 4.00 V, suggesting that the introduction of the erbium ions did not change the electrochemical redox reaction mechanism, as all the LiMn2−xErxO4 samples had two extraction/insertion steps of Li + ions [29,43]. Figure 6b presents the cycling life of the LiMn2−xErxO4 samples. The cycling life of the LiMn2−xErxO4 (x = 0.01, 0.03, 0.05) samples was significantly improved as the erbium-doping amount increased because of the inhibition of the Jahn-Teller distortion and the improvement of the structural stability. Note, however, that the introduction of more erbium ions had a harmful effect on the reversible capacity of the LiMn1.95Er0.05O4 sample because of the reduction of the trivalent manganese ions. Figure 6c shows the comparison plots of the initial discharge capacities and capacity retentions of these samples. We can clearly observe the positive influence on the capacity retention and the adverse effect on the discharge capacity. These results indicate that introducing an appropriate amount of erbium ions can play an active role in enhancing the cycling life of a sample. Figure 6d presents the long cycling life of the undoped LiMn2O4 and LiMn1.97Er0.03O4 samples. For the optimal LiMn1.97Er0.03O4 sample, the initial reversible capacity could exhibit 130.2 mAh g −1 . After 100 cycles, this sample exhibited   Figure 6a presents the first discharge curves of these samples, which were tested at 0.5 C. All the Er-doped LiMn 2 O 4 samples showed similar characteristic discharge curves to that of the undoped spinel. There were two distinct voltage platforms around 4.15 V and 4.00 V, suggesting that the introduction of the erbium ions did not change the electrochemical redox reaction mechanism, as all the LiMn 2−x Er x O 4 samples had two extraction/insertion steps of Li + ions [29,43]. Figure 6b Figure 6c shows the comparison plots of the initial discharge capacities and capacity retentions of these samples. We can clearly observe the positive influence on the capacity retention and the adverse effect on the discharge capacity. These results indicate that introducing an appropriate amount of erbium ions can play an active role in enhancing the cycling life of a sample. Figure 6d presents the long cycling life of the undoped LiMn 2 O 4 and LiMn 1.97 Er 0.03 O 4 samples. For the optimal LiMn 1.97 Er 0.03 O 4 sample, the initial reversible capacity could exhibit 130.2 mAh g −1 . After 100 cycles, this sample exhibited 123.9 mAh g −1 with an outstanding retention of 95.2%. However, the undoped LiMn 2 O 4 sample showed a poor cycling life with low reversible capacity after the 100th cycle. In particular, the undoped LiMn 2 O 4 sample only delivered 93.7 mAh g −1 with a lower retention of 67.8% after 100 cycles. In addition, we compared the cycling performance of the LiMn 1.97 Er 0.03 O 4 sample with that of the other doped samples, as shown in Table 2. It can be found that the erbium-doped LiMn 2 O 4 sample show good cycling performance. These analyses further confirm the improvement of the cyclic stability by introducing some appropriate erbium ions into the spinel structure. For the practical application of LiMn2O4, the rate performance is an important factor. The undoped LiMn2O4 and Er-doped LiMn1.97Er0.03O4 samples were tested successively at different rates. Figure 7a shows the corresponding discharge curves of the LiMn1.97Er0.03O4 samples. It can be seen that there were two voltage platforms, which were obvious at 0.2 C (the red color) and 0.5 C, suggesting the diffusion process of the lithium ions [20,33]. When the rate was further increased, these two potential plateaus gradually showed ambiguous boundaries and shifted toward the lower voltage when the cycling rate increased. This result has a lot to do with the polarization effect and ohmic drop [45,54]. Furthermore, when the cycling rate recovered to 0.2 C (the saffron yellow color), it was found that the LiMn1.97Er0.03O4 sample could show similar discharge capacity compared with the initial discharge capacity at 0.2 C (the red color), suggesting the excellent restorative performance of the LiMn1.97Er0.03O4 sample. Figure 7b shows the cycling stability of the undoped LiMn2O4 and the optimal LiMn1.97Er0.03O4 samples at varying rates. When cycled at 0.2 C, the capacities of these two samples reached up to 140.5 and 133.2 mAh g −1 , respectively. However, what is important to pay attention to is the reversible capacity of the Er-doped LiMn2O4 sample. With the increasing of the cycling rate, these two samples can show much more different results. In particular, For the practical application of LiMn 2 O 4 , the rate performance is an important factor. The undoped LiMn 2 O 4 and Er-doped LiMn 1.97 Er 0.03 O 4 samples were tested successively at different rates. Figure 7a shows the corresponding discharge curves of the LiMn 1.97 Er 0.03 O 4 samples. It can be seen that there were two voltage platforms, which were obvious at 0.2 C (the red color) and 0.5 C, suggesting the diffusion process of the lithium ions [20,33]. When the rate was further increased, these two potential plateaus gradually showed ambiguous boundaries and shifted toward the lower voltage when the cycling rate increased. This result has a lot to do with the polarization effect and ohmic drop [45,54]. Furthermore, when the cycling rate recovered to 0.2 C (the saffron yellow color), it was found that the sample. Figure 7b shows the cycling stability of the undoped LiMn 2 O 4 and the optimal LiMn 1.97 Er 0.03 O 4 samples at varying rates. When cycled at 0.2 C, the capacities of these two samples reached up to 140.5 and 133.2 mAh g −1 , respectively. However, what is important to pay attention to is the reversible capacity of the Er-doped LiMn 2 O 4 sample. With the increasing of the cycling rate, these two samples can show much more different results. In particular, when cycled at 10 C, the LiMn 1.97 Er 0.03 O 4 showed 80.7 mAh g −1 , while the LiMn 2 O 4 samples only showed 20.7 mAh g −1 .

Li(Al0.06Mn1.94)O4
Co-precipitation method 117.4 mAh g −1 , 97.0% after 100 cycles at 1.0 C (55 °C) [44] Li(Cr0.05Mn1.95)O4 Citric acid-assisted combustion method 117.0 mAh g −1 , 81.2% after 100 cycles at 0.   Figure 8 shows the cycling performance of the LiMn2O4 and LiMn1.97Er0.03O4 samples at 10 C. As shown in Figure 8a, the high rate shows a greater negative impact on the characteristic voltage plateaus at around 4.15 and 4.0 V, respectively. For the LiMn1.97Er0.03O4 sample, these two voltage plateaus become blurred to a certain extent. What is worse, the LiMn2O4 sample presented a lower voltage plateau, and the capacity of the LiMn2O4 sample showed severe degradation. Figure 8b presents the cycling life of these two samples at 10 C. It can be found that the initial discharge capacity of the undoped LiMn2O4 sample only reached to 32.5 mAh g −1 with a poor capacity retention of 81.5%. By contrast, the optimal LiMn1.97Er0.03O4 sample displayed a higher discharge capacity of 83.1 mAh g −1 . The discharge capacity still showed 78.0 mAh g −1 with an excellent capacity   plateaus become blurred to a certain extent. What is worse, the LiMn 2 O 4 sample presented a lower voltage plateau, and the capacity of the LiMn 2 O 4 sample showed severe degradation. Figure 8b presents the cycling life of these two samples at 10 C. It can be found that the initial discharge capacity of the undoped LiMn 2 O 4 sample only reached to 32.5 mAh g −1 with a poor capacity retention of 81.5%. By contrast, the optimal LiMn 1.97 Er 0.03 O 4 sample displayed a higher discharge capacity of 83.1 mAh g −1 . The discharge capacity still showed 78.0 mAh g −1 with an excellent capacity retention of 93.9%. These results suggest that the high-rate performance of LiMn 2 O 4 can be enhance by doping manganese ions with erbium ions in the spinel structure.   Figure 9a presents the cycling stability of the undoped LiMn2O4 and LiMn1.97Er0.03O4 samples at 55 °C. It can be seen from Figure 7a that the initial capacity of the LiMn1.97Er0.03O4 sample could reach up to 130.1 mAh g −1 at 0.5 C. Moreover, this sample still maintained a high capacity of 118.9 mAh g −1 with an excellent retention of 91.4% after 100 cycles. Unfortunately, the undoped LiMn2O4 sample showed very poor high-temperature cycling performance. After 100 cycles, the undoped sample only showed a lower capacity of 62.5 mAh g −1 with a low-capacity retention of 45.3%. These results suggest that introducing erbium ions can be favorable for enhancing the high-temperature performance of such a sample. Figure 9b shows the rate capability of these two samples at 55 °C. As shown here, the undoped LiMn2O4 and LiMn1.97Er0.03O4 samples showed similar capacities at low rates. However, these two samples presented obvious differences with the increasing of the rates. When cycled at 10 C, the LiMn1.97Er0.03O4 sample could exhibit 78.2 mAh g −1 , while the LiMn2O4 sample only showed 18.3 mAh g −1 . Based on these results, it can be concluded that the introduction of erbium ions can improve the high-temperature rate performance of LiMn2O4.    showed very poor high-temperature cycling performance. After 100 cycles, the undoped sample only showed a lower capacity of 62.5 mAh g −1 with a low-capacity retention of 45.3%. These results suggest that introducing erbium ions can be favorable for enhancing the high-temperature performance of such a sample. Figure 9b shows the rate capability of these two samples at 55 • C. As shown here, the undoped LiMn 2 O 4 and LiMn 1.97 Er 0.03 O 4 samples showed similar capacities at low rates. However, these two samples presented obvious differences with the increasing of the rates. When cycled at 10 C, the LiMn 1.97 Er 0.03 O 4 sample could exhibit 78.2 mAh g −1 , while the LiMn 2 O 4 sample only showed 18.3 mAh g −1 . Based on these results, it can be concluded that the introduction of erbium ions can improve the high-temperature rate performance of LiMn 2 O 4 . Figure 10a,b show the EIS results of the undoped LiMn 2 O 4 and LiMn 1.97 Er 0.03 O 4 samples. As shown here, the high-frequency semicircle represents the charge transfer resistance (R 2 ), which is closely related to the cycling life [21,48]. Thus, the effect of doping manganese ions with erbium ions on the cycling stability was mainly studied. The fitting values of R 2 are listed in Table 3. For the LiMn 1.97 Er 0.03 O 4 sample, the original R 2 value only reached 73.4 Ω cm 2 but increased to 115.1 Ω cm 2 after 100 cycles. The R 2 value increase was relatively small with a low growth rate of 56.8%. However, the undoped sample only showed the unsatisfactory R 2 value. It can be seen that the undoped spinel showed a higher original R 2 value (118.3 Ω cm 2 ). After 100 cycles, the high growth rate reached up to 149.5% with the 100th R 2 value of 295.2 Ω cm 2 . These results indicate that the addition of erbium ions in the spinel structure can have a positive role in decreasing the R 2 value and enhancing the diffusion of lithium ions, which is conducive to the improvement of cycling stability [29,32].
performance of such a sample. Figure 9b shows the rate capability of these two samples at 55 °C. As shown here, the undoped LiMn2O4 and LiMn1.97Er0.03O4 samples showed similar capacities at low rates. However, these two samples presented obvious differences with the increasing of the rates. When cycled at 10 C, the LiMn1.97Er0.03O4 sample could exhibit 78.2 mAh g −1 , while the LiMn2O4 sample only showed 18.3 mAh g −1 . Based on these results, it can be concluded that the introduction of erbium ions can improve the high-temperature rate performance of LiMn2O4.  Figure 10a,b show the EIS results of the undoped LiMn2O4 and LiMn1.97Er0.03O4 samples. As shown here, the high-frequency semicircle represents the charge transfer resistance (R2), which is closely related to the cycling life [21,48]. Thus, the effect of doping manganese ions with erbium ions on the cycling stability was mainly studied. The fitting values of R2 are listed in Table 3. For the LiMn1.97Er0.03O4 sample, the original R2 value only reached 73.4 Ω cm 2 but increased to 115.1 Ω cm 2 after 100 cycles. The R2 value increase was relatively small with a low growth rate of 56.8%. However, the undoped sample only showed the unsatisfactory R2 value. It can be seen that the undoped spinel showed a higher original R2 value (118.3 Ω cm 2 ). After 100 cycles, the high growth rate reached up to 149.5% with the 100th R2 value of 295.2 Ω cm 2 . These results indicate that the addition of erbium ions in the spinel structure can have a positive role in decreasing the R2 value and enhancing the diffusion of lithium ions, which is conducive to the improvement of cycling stability [29,32].

Conclusions
In summary, we have successfully used the sol-gel technology to prepare the Er-doped LiMn2O4 samples. All these samples maintained the spinel structure of LiMn2O4 and showed relatively even particle size distribution. The optimal LiMn1.97Er0.03O4 sample showed a better cycling performance. When tested at 0.5 C, this sample delivered a reversible capacity of 130.2 mAh g −1 with an excellent retention of 95.2% after 100 cycles. At higher rate of 10 C, the reversible capacity of the LiMn1.97Er0.03O4 sample peaked at 83.1 mAh g −1 , which is far higher than that of the undoped spinel. Moreover, this sample showed outstanding cycling stability at higher temperatures. All of these results indicate that the introduction of erbium ions could enhance the cycling stability of LiMn2O4.

Conclusions
In summary, we have successfully used the sol-gel technology to prepare the Er-doped LiMn 2 O 4 samples. All these samples maintained the spinel structure of LiMn 2 O 4 and showed relatively even particle size distribution. The optimal LiMn 1.97 Er 0.03 O 4 sample showed a better cycling performance. When tested at 0.5 C, this sample delivered a reversible capacity of 130.2 mAh g −1 with an excellent retention of 95.2% after 100 cycles. At higher rate of 10 C, the reversible capacity of the LiMn 1.97 Er 0.03 O 4 sample peaked at 83.1 mAh g −1 , which is far higher than that of the undoped spinel. Moreover, this sample showed outstanding cycling stability at higher temperatures. All of these results indicate that the introduction of erbium ions could enhance the cycling stability of LiMn 2 O 4 .

Conflicts of Interest:
The authors declare no conflicts of interest.