With the advantages of abundant, nontoxic, and inexpensive, spinel lithium manganese oxide (LiMn₂O₄) is a promising candidate of the layered cathode materials such as LiCoO₂ [1] [2] . Especially, the good stability of LiMn₂O₄ may ensure its large-scale usage in the batteries for electric vehicle or energy storage [3] . However, LiMn₂O₄ shows obvious capacity fade when cycling at high temperature [4] -[6] . It was reported that the capacity fading mechanism at high temperature was related to the Jahn-Teller distortion and dissolution of Mn³⁺ ions [7] [8] . Mn dissolution is induced by HF acid, which is generated by secondary chemical reactions from temperature-enhanced electrolyte decomposition.

In order to solve this problem, earlier studies have been focused on a chemical modification of LiMn₂O₄ by a partial substitution of Mn with some metal ions to obtain LiM_xMn_2-xO₄ (M = Co, Mg, Cr, Ni, Fe, Al, Ti and Zn) [9] -[12] . These results indicated that the substitution of Mn with metal ions significantly improved the cycle performance of LiMn₂O₄. However, the partial substitutions decrease the capacity of LiMn₂O₄. Another effective way is surface coating on LiMn₂O₄ by oxide with high thermal and structural stability. ZrO₂ [13] , Al₂O₃ [14] , SiO₂ [15] and MgO [16] have been used to coat LiMn₂O₄ by some chemical processes. However, the aforementioned oxides with non-spinel structure are hard to grow on the surface of LiMn₂O₄. With a similar structure to LiMn₂O₄, LiNi_0.5Mn_1.5O₄ can grow more easily on LiMn₂O₄ than other alien phase and form a more close-contacted protective covering layer, which may suppress the dissolution of Mn. Therefore, it is expected that the modified LiMn₂O₄ will show an excellent cycle performance at elevated temperature. In this study, we proposed an approach to synthesize LiNi_0.5Mn_1.5O₄ on the surface of spinel LiMn₂O₄. The effect of LiNi_0.5Mn_1.5O₄ layer on the morphology and electrochemical performances of LiMn₂O₄ cathode materials were examined in detail.

LiMn₂O₄ powder was purchased from Heibei Strong-Power Li-ion Battery Tecnology Co. Ltd (D98, China). To coat LiMn₂O₄ with LiNi_0.5Mn_1.5O₄, LiCH₃COO∙2H₂O (0.558 g), Mn (CH₃COO)₂∙4H₂O (2.011 g) and Ni (CH₃COO)₂∙4H₂O (0.68 g) with a stoichiometric ratio (2:1:3) were dissolved in distilled water to form a clear solution. An aqueous solution of ethylene glycol and citric acid as a chelating agent was added to the mixtures. pH value at 6.5 - 7.0 was achieved by Ammonium hydroxide. Then the LiMn₂O₄ powders (50 g) were slowly added to the sol and vigorously stirred at 85˚C for 8 h. As the evaporation of water proceeding, the sol was turned into a viscous transparent gel. After drying and sieving, the powder was sintering in air at 400˚C for 10 h and 750˚C for 3 h to obtain LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄. For a comparison, pristine LiMn₂O₄ was also heat- treated in the same condition.

Figure 1 shows the XRD patterns of pristine and LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄. The peaks of both samples could be indexed to a cubic spinel structure with the space group Fd3m. There is no substantial difference between XRD patterns for pristine and modified LiMn₂O₄. The crystal lattice parameters were calculated by using the software of Jade, are 8.245 and 8.243 Å for the pristine and LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄, indicating that the bulk structure of LiMn₂O₄ unchange after surface modification. The characteristic peaks corresponding to LiNi_0.5Mn_1.5O₄ are not observed because of the low content (about 2.0 wt%).

Scanning electron microscopy reveals that the pristine and modified samples present a uniform particle distribution, ranging from 2 to 7 μm. The pristine spinel crystals are smooth with well-defined facets, as observed in Figure 2(a) and Figure 2(c). It can be seen that the morphology and particle diameter of the LiNi_0.5Mn_1.5O₄-

coated LiMn₂O₄ powders in Figure 2(b) and Figure 2(d), are similar to the pristine sample. No LiNi_0.5Mn_1.5O₄ agglomerations and obscured facets of spinel LiMn₂O₄ are observed.

The element composition is determined by EDS analysis. The element mapping of Ni and Mn is displayed in Figure 3. The dense accumulation of Mn spots is attributed to the host material of LiMn₂O₄ and there is no significant agglomeration of Ni. This indicates that LiNi_0.5Mn_1.5O₄ is homogeneously dispersed on the surface of the LiMn₂O₄ particles.

TEM micrographs are displayed in Figure 4. Compared to the pristine sample (Figure 4(a)), about 4 - 5 nm

thick layer of LiNi_0.5Mn_1.5O₄ is uniformly formed on the surface of the LiMn₂O₄ (Figure 4(b)). The coating layer is clearly distinguishable from the crystalline LiMn₂O₄. The TEM analysis demonstrates that the sol-gel method is an effective way to coat the LiNi_0.5Mn_1.5O₄ layer on the surface of LiMn₂O₄.

The oxidation state of manganese ions at the surface was determined from XPS data by the curve fitting of Mn 2p spectral peaks. The experimental peak shape for Mn 2p_3/2 was modeled by employing multiple-splitting patterns derived for Mn³⁺ and Mn⁴⁺ at binding energies of 641.6, and 642.8 eV from the standard compounds Mn₂O₃ and MnO₂. Figure 5 shows the fit of the models to the experimental spectra for pristine LiMn₂O₄ and LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ respectively. The surface of the pristine LiMn₂O₄ sample consists of almost equal amounts of Mn⁴⁺ and Mn³⁺. By contrast, LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ exhibited a Mn⁴⁺:Mn³⁺ ratio of 59.4:40.6. The difference of Mn⁴⁺:Mn³⁺ ratio on the surface is due to the formation of LiNi_0.5Mn_1.5O₄, because it has higher valence state of manganese ions. This result is good agreement with the observation of HR-TEM.

As shown in Figure 6, the galvanostatic charge-discharge curves under a current rate of 2C were conducted at elevated temperatures (55˚C). It shows two discharge plateaus, which should be attributed to orderly intercalating of lithium ions in the tetrahedral (8a) sites at 4.1 V and disorderly intercalating lithium ions at 3.9 V which substantially maintains the intercalation feature of LiMn₂O₄ substrate [17] , indicating LiNi_0.5Mn_1.5O₄ surface layer rather than Ni-doped LiMn₂O₄ because LiMn₂O₄ with Ni-doped spinel surface showed two ambiguously resolved discharging plateaus. LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ shows a lower discharge capacity (104.2 mAh/g) compares to the pristine sample (106.3 mAh/g). The reason of low initial discharge capacity is that LiNi_0.5Mn_1.5O₄ shows no capacity at this voltage range due to its high discharge voltage over 4.35V [18] .

Figure 7 shows the cycling performance of electrodes with and without LiNi_0.5Mn_1.5O₄ coating. After 100 cycles, the discharge capacity of the pristine LiMn₂O₄ drops from 106.3 to 89.2 mAh/g. In contrast, the discharge capacity of modified sample changes from 104.2 to 98 mAh/g. The capacity retention increases from 84.0% to 94.4% after LiNi_0.5Mn_1.5O₄ coating.

To further verify the effects of surface coating on manganese ions dissolution, the quality of the manganese element was directly determined by using ICP-AES. Li metal anode was washed by dilute hydrochloric acid after 100th cycle at 55˚C ± 2˚C. It can be seen in Table 1, the dissolved quality of Mn²⁺ ions of the pristine and LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ electrode was 20.54 and 12.17 μg/cm², respectively. It can be concluded that after coating by LiNi_0.5Mn_1.5O₄ layer, the dissolution of the manganese ions was significantly reduced. Therefore, LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ electrode had better cycle stability at elevated temperature.

Figure 8 shows the CV profiles of pristine and LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ electrodes in the 10th and 100th cycles at the scan rate of 0.1 mV∙s⁻¹. The CV peaks of LiNi_0.5Mn_1.5O₄-coated sample show two symmetrical couples of redox peaks at around 3.98 and 4.1 V respectively (Figure 8(b)), indicating that electrochemical insertion and extraction reactions of Li⁺ ions are two step processes. It is in agreement with the two plateaus in Figure 6 and demonstrates that LiNi_0.5Mn_1.5O₄ coating does not change the electrochemical reaction mechanism of LiMn₂O₄. After 10 cycles, two narrow and separate redox peaks appear around at 3.96 and 4.11 V, as shown in Figure 8(a). However, after 100th cycles, due to the dissolution of Mn³⁺ ions into the electrolyte

(Jahn-Teller distortion), both anodic and cathodic peaks become much broader and lower in peaks current. In contrast, the oxidation and reduction peaks relate to LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ are much steadier after 100 cycles (Figure 8(b)), which indicated that modified LiMn₂O₄ has better reversibility and stability than the pristine LiMn₂O₄.

Electrochemical impedance spectra (EIS) and equivalent circuits are shown in Figure 9. The measurements were carried out with fully charged state (4.35 V). An intercept in the high frequency region of the Z_rel axis indicates the ohmic resistance (Rs), the combined resistance of the electrolyte and the contacts of the cell [19] . The semicircle in high-middle frequency region corresponds to the charge transfer (Rct) process on the electrode interface, revealing the lithium transfer rate parameters and the capacitance of the SEI (solid eleteolyte interface) [20] . The inclined line in the lower frequency region represents the Warburg impedance (Zw), which is corresponded to the diffusion of Li⁺ in LiMn₂O₄ particles [21] . The plots are fitted and listed in Table 2. As shown in

Figure 9. EIS of pristine and LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ at the end of 10th and 100th cycle.

the table, after 10 cycles, the Rs for LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ is slightly larger than the pristine sample, because that the coating layer may slightly increase the elecrolyte and contact resistance. The charge transfer resistance of both samples is approximately the similar (26.82 and 24.4 Ω∙cm²). After 100 cycles, the change in R_s is negligible. However, the R_ct value of the LiNi_0.5Mn_1.5O₄-coated electrode (34.8 Ω∙cm²) is two times smaller than that the value pristine electrode (73.8 Ω∙cm²). It attributes to the restraint of structural instability caused by the subsequent Mn dissolution and vacancies formation. This result is also in accordance with the enhanced cycling performance of LiNi_0.5Mn_1.5O₄-coated electrodes.

In summary, the surface of LiMn₂O₄ sample was modified by LiNi_0.5Mn_1.5O₄ using a sol-gel method. TEM and XPS results confirm the existence of LiNi_0.5Mn_1.5O₄ layer. A uniform and dense layer about 4 - 5 nm was coating on the surface of pristine LiMn₂O₄. The LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ sample exhibits much better cycling stability at elevated temperature (55˚C) compared with the pristine sample. The CV tests indicated that the LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ electrode has better reversibility and stability. Meanwhile, the charge transfer resistance of the LiNi_0.5Mn_1.5O₄-coated LiMn₂O₄ was much less than that of pristine sample after 100 cycles, which is ascribed to the better structural stability and restraint of Mn dissolution. These results demonstrated that this is an effective way to improve the high-temperature cyclic performance of spinel LiMn₂O₄.

This work was supported by National Science Foundation of China (No. 50672026). This work was also supported by Shanghai Nano Technology Promotion (No. 12ZR1448800).