The Effects of Ru4+ Doping on LiNi0.5Mn1.5O4 with Two Crystal Structures

Doping of Ru has been used to enhance the performance of LiNi0.5Mn1.5O4 cathode materials. However, the effects of Ru doping on the two types of LiNi0.5Mn1.5O4 are rarely studied. In this study, Ru4+ with a stoichiometric ratio of 0.05 is introduced into LiNi0.5Mn1.5O4 with different space groups (Fd3¯m, P4332). The influence of Ru doping on the properties of LiNi0.5Mn1.5O4 (Fd3¯m, P4332) is comprehensively studied using multiple techniques such as XRD, Raman, and SEM methods. Electrochemical tests show that Ru4+-doped LiNi0.5Mn1.5O4 (P4332) delivers the optimal electrochemical performance. Its initial specific capacity reaches 132.8 mAh g−1, and 97.7% of this is retained after 300 cycles at a 1 C rate at room temperature. Even at a rate of 10 C, the capacity of Ru4+-LiNi0.5Mn1.5O4 (P4332) is still 100.7 mAh g−1. Raman spectroscopy shows that the Ni/Mn arrangement of Ru4+-LiNi0.5Mn1.5O4 (Fd3¯m) is not significantly affected by Ru4+ doping. However, LiNi0.5Mn1.5O4 (P4332) is transformed to semi-ordered LiNi0.5Mn1.5O4 after the incorporation of Ru4+. Ru4+ doping hinders the ordering process of Ni/Mn during the heat treatment process, to an extent.


Introduction
The continuous development of lithium-ion batteries has driven the progress of the electric vehicle industry. However, the cruising mileage of electric vehicles still has a great deal of room for improvement. Energy density has always been one of the key factors limiting the range. From the formula W = Q • U, we know that increasing the specific capacity and discharge voltage of the cathode material is the main way to increase the power density of lithium-ion batteries (LIBs). Spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) is considered to be one of the most promising candidates for high-power lithium-ion battery systems, and this is attributed to its ultra-fast 3D Li + diffusion speed, good high-rate capability, excellent cyclic stability, low cost, and environmental friendliness. LiNi 0.5 Mn 1.5 O 4 has a high discharge platform (vs. Li/Li + ≈ 4.7 V) [1][2][3] and a theoretical discharge capacity of 146.7 mAh g −1 ; therefore, it has a fairly high theoretical energy density [4].
Built on the arrangement of Mn 4+ and Ni 2+ in the crystal lattice, LiNi 0.5 Mn 1.5 O 4 can have two crystal structures: a face-centered cubic structure (Fd3m) or a simple cubic structure (P4 3 32). The former is a disordered structure in which Mn 4+ and Ni 2+ ions are randomly distributed on the octahedral 16d sites. The latter has an ordered structure in which Mn 4+ ions occupy the 12d positions and Ni 2+ ions occupy the 4a sites [5,6]. In addition, almost no Mn 3+ ions are generated in the crystal structure. It is generally believed that disordered LiNi 0.5 Mn 1.5 O 4 containing Mn 3+ has higher electronic conductivity and lithium-ion conductivity, and thus has better high-rate performance [7][8][9][10]. However, the presence of too many Mn 3+ ions in LiNi 0.5 Mn 1.5 O 4 will damage the cyclic stability of the cathode material, because Mn 3+ is prone to the disproportionation reaction

Material Synthesis
LiNi 0.5 Mn 1.5 O 4 with different structures was synthesized via traditional solid-state reactions. A typical route was as follows: (1) Li 2 CO 3 , NiCO 3 , and MnCO 3 in a stoichiometric ratio of 0.525:0.5:1.5 were mixed in alcohol; (2) a process of ball milling was performed at a speed of 400 r/min for 4 h, and the obtained mixture was completely dried at 60 • C and subsequently pulverized; (3) the powder was heated to 900 • C in a tube furnace at a rate of 5 • C/min and held for 12 h, followed by cooling naturally to room temperature to obtain the LiNi 0.5 Mn 1.5 O 4 with space group of Fd3m, denoted LNMO (Fd3m). Alternatively, the heated material was insulated at 700 • C for 48 h and cooled naturally to room temperature to give the P4 3 32-structured LiNi 0.5 Mn 1.5 O 4 , denoted LNMO (P4 3 32). Li 2 CO 3 , NiCO 3 , MnCO 3 , and RuO 2 were mixed in a stoichiometric ratio of 0.525:0.45:1.5:0.05, and the above steps were repeated to obtain Ru-doped LiNi 0.5 Mn 1.5 O 4 with different structures, denoted Ru 4+ -LNMO (Fd3m) and Ru 4+ -LNMO (P4 3 32), respectively.

Characterization
To investigate the influence of the Ru 4+ doping on the crystal structure of LiNi 0.5 Mn 1.5 O 4 with different space groups, X-ray diffraction (XRD, Ultima IV, Tokyo, Japan) was carried out using Cu Kα radiation in the range 10 • ≤ 2θ ≤ 70 • . The morphologies of Rudoped LiNi 0.5 Mn 1.5 O 4 with different structures were recorded using scanning electron microscopy (SEM, FESEM Quanta TEG 450, Hillsboro, OR, USA). The phase structures of LiNi 0.5 Mn 1.5 O 4 with and without doping for the different structures were investigated using a Renishaw inVia plus-type micro-Raman spectrometer.

Preparation of Electrodes and Construction of Cells
LiNi 0.5 Mn 1.5 O 4 (80 wt%) was mixed with 10 wt% acetylene black and 10 wt% polyvinyli -dene fluoride (PVDF) in the appropriate amount of N-Methyl pyrrolidone (NMP). The obtained slurry was coated onto aluminum foil, then dried at 80 • C for 2 h in air and at 120 • C for 8 h in a vacuum oven. Finally, a Celgard 2400 argon-filled glove box was used as the separator to assemble coin-type LiNi 0.5 Mn 1.5 O 4 /Li cells.

Electrochemical Measurements
The electrochemical properties of LiNi 0.5 Mn 1.5 O 4 before and after Ru 4+ doping were measured with CR2025-type coin cells (HF-Kejing, Hefei, China). All the charge-discharge behaviors were evaluated at a rate of 1 C at room temperature, utilizing a LAND battery testing system. The rate of 1 C was set at 147 mA g −1 , and the current densities for testing were determined on the basis of the weight of cathode material. In the evaluation of cycle performance, the cells were charged and discharged in the voltage range of 3.5 V to 5.0 V for 300 cycles. The rate tests were conducted at rates of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C and then reversed successively back to 0.2 C.

Results and Discussion
The X-ray diffraction patterns of the LiNi 0.5 Mn 1.5 O 4 (Fd3m, P4 3 32), with and without Ru 4+ doping, are compared in Figure 1. Clearly, all peaks of the four as-prepared samples are in agreement with the XRD patterns of typical spinel LiNi 0.5 Mn 1.5 O 4 . Ru 4+ doping does not change the primary lattice framework of the LiNi 0.5 Mn 1.5 O 4 . Since the radius of Ru 4+ is comparable to that of Ni 2+ [30], a slight distortion of the lattice will be caused by the introduction of Ru 4+ . This is also reflected in the diffraction patterns, with the (111) peaks of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (Fd3m, P4 3 32) all shifting slightly towards the low-angle region. Furthermore, the larger lattice parameters a and c facilitate the diffusion of Li + and subsequently enhance the high-rate capability of the battery. The weak peaks (2θ at ≈37.5 • , 43.6 • , 47.5 • , and 63.5 • ) correspond to the rock-salt impurity phase Li x Ni 1−x O, which is mainly caused by oxygen loss [31,32]. In terms of traditional solid-state methods, insufficient mixing and high sintering temperatures inevitably contribute to the volatilization of Ni/Li and the formation of Li x Ni 1-x O components. Amplifying a partial area (2θ = 40 •~5 0 • ), it can be found that the weak peaks of the impurity basically disappear in the spectrum of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32), but tiny impurity peaks still emerge in the spectrum of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (Fd3m). This demonstrates that Ru 4+ doping has a noticeable effect in eliminating Li x Ni 1−x O-like impurity phases for LiNi 0.5 Mn 1.5 O 4 (both Fd3m and P4 3 32) and stabilizing the spinel crystal structure. tron microscopy (SEM, FESEM Quanta TEG 450, Hillsboro, OR, USA). The phase structures of LiNi0.5Mn1.5O4 with and without doping for the different structures were investigated using a Renishaw inVia plus-type micro-Raman spectrometer.

Preparation of Electrodes and Construction of Cells
LiNi0.5Mn1.5O4 (80 wt%) was mixed with 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) in the appropriate amount of N-Methyl pyrrolidone (NMP). The obtained slurry was coated onto aluminum foil, then dried at 80 °C for 2 h in air and at 120 °C for 8 h in a vacuum oven. Finally, a Celgard 2400 argon-filled glove box was used as the separator to assemble coin-type LiNi0.5Mn1.5O4/Li cells.

Electrochemical Measurements
The electrochemical properties of LiNi0.5Mn1.5O4 before and after Ru 4+ doping were measured with CR2025-type coin cells (HF-Kejing, Hefei, China). All the charge-discharge behaviors were evaluated at a rate of 1 C at room temperature, utilizing a LAND battery testing system. The rate of 1 C was set at 147 mA g −1 , and the current densities for testing were determined on the basis of the weight of cathode material. In the evaluation of cycle performance, the cells were charged and discharged in the voltage range of 3.5 V to 5.0 V for 300 cycles. The rate tests were conducted at rates of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C and then reversed successively back to 0.2 C.

Results and Discussion
The X-ray diffraction patterns of the LiNi0.5Mn1.5O4 (Fd3 m, P4332), with and without Ru 4+ doping, are compared in Figure 1. Clearly, all peaks of the four as-prepared samples are in agreement with the XRD patterns of typical spinel LiNi0.5Mn1.5O4. Ru 4+ doping does not change the primary lattice framework of the LiNi0.5Mn1.5O4. Since the radius of Ru 4+ is comparable to that of Ni 2+ [30], a slight distortion of the lattice will be caused by the introduction of Ru 4+ . This is also reflected in the diffraction patterns, with the (111) peaks of Ru 4+ -LiNi0.5Mn1.5O4 (Fd3 m, P4332) all shifting slightly towards the low-angle region. Furthermore, the larger lattice parameters a and c facilitate the diffusion of Li + and subsequently enhance the high-rate capability of the battery. The weak peaks (2θ at ≈37.5°, 43.6°, 47.5°, and 63.5°) correspond to the rock-salt impurity phase LixNi1−xO, which is mainly caused by oxygen loss [31,32]. In terms of traditional solid-state methods, insufficient mixing and high sintering temperatures inevitably contribute to the volatilization of Ni/Li and the formation of LixNi1-xO components. Amplifying a partial area (2θ = 40°~50°), it can be found that the weak peaks of the impurity basically disappear in the spectrum of Ru 4+ -LiNi0.5Mn1.5O4 (P4332), but tiny impurity peaks still emerge in the spectrum of Ru 4+ -LiNi0.5Mn1.5O4 (Fd3 m). This demonstrates that Ru 4+ doping has a noticeable effect in eliminating LixNi1−xO-like impurity phases for LiNi0.5Mn1.5O4 (both Fd3 m and P4332) and stabilizing the spinel crystal structure.   are shown in Figure 2. The peaks at around 630 cm are assigned to the symm stretching vibration, and the peaks at around 482 cm −1 correspond to the Nimode. The splitting of the F2g (1) vibration mode near 580-600 cm −1 is clear evid ordered structure, while a lack of F2g (1) splitting corresponds to the disordere generally accepted that a higher degree of disorder indicates a higher Mn 3+ co be observed that Ru 4+ -LiNi0.5Mn1.5O4 (Fd3 m) and LiNi0.5Mn1.5O4 (Fd3 m) have lar F2g (1)   It is generally believed that the crystal plane in contact with the electro particle size both have an important influence on the electrochemical per LiNi0.5Mn1.5O4 cathode material [33][34][35][36]. The dissolution of the transition me linked to the stability of the interface. For example, Mn 2+ derived from the di ation reaction is most easily dissolved from the {110} crystal plane into the Therefore, inhibiting the growth of the {110} crystal plane can effectively red solution of the transition metal and thus improve the cyclic stability [37]. Com It is generally believed that the crystal plane in contact with the electrolyte and the particle size both have an important influence on the electrochemical performance of LiNi 0.5 Mn 1.5 O 4 cathode material [33][34][35][36]. The dissolution of the transition metal is closely linked to the stability of the interface. For example, Mn 2+ derived from the disproportionation reaction is most easily dissolved from the {110} crystal plane into the electrolyte. Therefore, inhibiting the growth of the {110} crystal plane can effectively reduce the dissolution of the transition metal and thus improve the cyclic stability [37]. Compared with the {110} planes, the {100} crystal planes have a positive effect on the electrochemical performance [38,39]. Particle size is another important factor affecting stability. Nanoscale LiNi 0.5 Mn 1.5 O 4 has shorter Li + diffusion paths, but at the same time, the larger specific surface area also leads to more serious side reactions at the interface and instability of the battery system [40,41]. Large (micron level) LiNi 0.5 Mn 1.5 O 4 particles have a smaller specific surface area, which effectively reduces the degree of side reactions to enhance cycling performance.
The micro-morphology of the LiNi 0.5 Mn 1.5 O 4 (Fd3m, P4 3 32) before and after Ru 4+ doping is shown in Figure 3. Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (Fd3m) particles have a truncated octahedral morphology, and Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) particles have a spherical truncated polyhedron morphology. Additionally, both of them have a certain particle size distribution. Some particle diameters are about 2 µm, and a large number of particle diameters are about 1 µm. After doping, grains of LiNi 0.5 Mn 1.5 O 4 with different structures have no obvious distortion or morphological changes. However, strictly speaking, the particle growth of the active material seems to be repressed by Ru 4+ cooperation during the calcination process, which reduces the final particle size to some extent.  [38,39]. Particle size is another important factor affecting stability. Nanoscale LiNi0.5Mn1.5O4 has shorter Li + diffusion paths, but at the same time, the larger specific surface area also leads to more serious side reactions at the interface and instability of the battery system [40,41]. Large (micron level) LiNi0.5Mn1.5O4 particles have a smaller specific surface area, which effectively reduces the degree of side reactions to enhance cycling performance.
The micro-morphology of the LiNi0.5Mn1.5O4 (Fd3 m, P4332) before and after Ru 4+ doping is shown in Figure 3. Ru 4+ -LiNi0.5Mn1.5O4 (Fd3 m) particles have a truncated octahedral morphology, and Ru 4+ -LiNi0.5Mn1.5O4 (P4332) particles have a spherical truncated polyhedron morphology. Additionally, both of them have a certain particle size distribution. Some particle diameters are about 2 μm, and a large number of particle diameters are about 1 μm. After doping, grains of LiNi0.5Mn1.5O4 with different structures have no obvious distortion or morphological changes. However, strictly speaking, the particle growth of the active material seems to be repressed by Ru 4+ cooperation during the calcination process, which reduces the final particle size to some extent. The initial charge-discharge curves of the LiNi0.5Mn1.5O4 (Fd3 m, P4332) before and after doping in the voltage range of 3.5~5.0 V at a rate of 0.2 C are shown in Figure 4. Compared with the discharge capacity (123.0 mAh g −1 ) of LiNi0.5Mn1.5O4 (Fd3 m), the capacity of Ru 4+ -LiNi0.5Mn1.5O4 (Fd3 m) is increased to 127.7 mAh g −1 . Meanwhile, the 4.1 V platform capacity and the capacity ratio are reduced from 18.9 mAh g −1 /15.4% to 14.1 mAh g −1 /11.0%, respectively. Compared to LiNi0.5Mn1.5O4 (P4332), the 4.1V platform capacity (9.3 mAh g −1 , 7.07%) of Ru 4+ -LiNi0.5Mn1.5O4 (P4332) has more than doubled, and the discharge capacity has reached the maximum value. In addition, the average valence of the transition metal nickel ions may decrease with the introduction of Ru 4+ , resulting in an increase in the 4.7 V platform discharge capacity. On the other hand, Ru 4+ doping can also prevent the sample from reacting with oxygen during the heat treatment process, which is manifested by the presence of a certain amount of Mn 3+ . The presence of Mn 3+ can further increase the discharge capacity of the material. Therefore, the discharge capacity of Ru 4+ -LiNi0.5Mn1.5O4 (P4332) reaches the maximum value among the four samples. The initial charge-discharge curves of the LiNi 0.5 Mn 1.5 O 4 (Fd3m, P4 3 32) before and after doping in the voltage range of 3.5~5.0 V at a rate of 0.2 C are shown in Figure 4. Compared with the discharge capacity (123.0 mAh g −1 ) of LiNi 0.5 Mn 1.5 O 4 (Fd3m), the capacity of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (Fd3m) is increased to 127.7 mAh g −1 . Meanwhile, the 4.1 V platform capacity and the capacity ratio are reduced from 18.9 mAh g −1 /15.4% to 14.1 mAh g −1 /11.0%, respectively. Compared to LiNi 0.5 Mn 1.5 O 4 (P4 3 32), the 4.1V platform capacity (9.3 mAh g −1 , 7.07%) of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) has more than doubled, and the discharge capacity has reached the maximum value. In addition, the average valence of the transition metal nickel ions may decrease with the introduction of Ru 4+ , resulting in an increase in the 4.7 V platform discharge capacity. On the other hand, Ru 4+ doping can also prevent the sample from reacting with oxygen during the heat treatment process, which is manifested by the presence of a certain amount of Mn 3+ . The presence of Mn 3+ can further increase the discharge capacity of the material. Therefore, the discharge capacity of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3   The LiNi0.5Mn1.5O4/Li half cells were all pre-cycled at 0.2 C for three cycles. Figure 5 shows the cycling performance of cells at the 1 C rate (147.0 mA g −1 ) in the voltage range of 3.5 V to 5.0 V (vs. Li/Li + ) at room temperature. It can be observed that the Ru 4+ -LiNi0.5Mn1.5O4 (Fd3 m) delivers a higher discharge capacity than LiNi0.5Mn1.5O4 (Fd3 m), with an initial specific capacity increase from 121.7 mAh g −1 to 124.8 mAh g −1 . In addition, the capacity retention after 300 cycles is also increased from 97.3% to 98.5%. On the other hand, the LiNi0.5Mn1.5O4 (P4332) sample shows an initial discharge capacity of 125.4 mAh g −1 but delivers a lower retention of 92.0%. With Ru 4+ doping, the initial capacity of Ru 4+ -LiNi0.5Mn1.5O4 (P4332) reaches the highest value of 132.8 mAh g −1 , and the capacity retention after 300 cycles also recovers to 97.8%. The discharge voltage curves in the 1st and 300th cycles of the four samples at the 1C rate (147.0 mAh g −1 ) at room temperature are displayed in Figure 6. Before and after 300 cycles, the curves for LiNi0.5Mn1.5O4 (Fd3 m) have a high degree of coincidence, but careful observation shows that both the 4.7 V and 4.1 V platforms are shortened slightly. Ni 2+ and Mn 3+ dissolve into the electrolyte, causing a loss of battery capacity. The introduction of Ru 4+ inhibits the dissolution of Ni 2+ and Mn 3+ during the charge-discharge process by stabilizing the crystal structure. Thus, the plateau-shortening degree of Ru 4+ -LiNi0.5Mn1.5O4 (Fd3 m) is slightly less than that of LiNi0.5Mn1.5O4 (Fd3 m). In addition, LiNi0.5Mn1.5O4 (P4332) as a cathode material suffers severe capacity attenuation. The discharge voltage curves in the 1st and 300th cycles are significantly dissimilar, reflecting the observable shortening of the 4.7 V platform. Compared to LiNi0.5Mn1.5O4 (P4332), Ru 4+ -LiNi0.5Mn1.5O4  Figure 5 shows the cycling performance of cells at the 1 C rate (147.0 mA g −1 ) in the voltage range of 3.5 V to 5.0 V (vs. Li/Li + ) at room temperature. It can be observed that the Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (Fd3m) delivers a higher discharge capacity than LiNi 0.5 Mn 1.5 O 4 (Fd3m), with an initial specific capacity increase from 121.7 mAh g −1 to 124.8 mAh g −1 .
In addition, the capacity retention after 300 cycles is also increased from 97.3% to 98.5%. On the other hand, the LiNi 0.5 Mn 1.5 O 4 (P4 3 32) sample shows an initial discharge capacity of 125.4 mAh g −1 but delivers a lower retention of 92.0%. With Ru 4+ doping, the initial capacity of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) reaches the highest value of 132.8 mAh g −1 , and the capacity retention after 300 cycles also recovers to 97.8%. The LiNi0.5Mn1.5O4/Li half cells were all pre-cycled at 0.2 C for three cyc shows the cycling performance of cells at the 1 C rate (147.0 mA g −1 ) in the v of 3.5 V to 5.0 V (vs. Li/Li + ) at room temperature. It can be observed th LiNi0.5Mn1.5O4 (Fd3 m) delivers a higher discharge capacity than LiNi0.5Mn1 with an initial specific capacity increase from 121.7 mAh g −1 to 124.8 mAh g −1 the capacity retention after 300 cycles is also increased from 97.3% to 98.5%. hand, the LiNi0.5Mn1.5O4 (P4332) sample shows an initial discharge capacity o g −1 but delivers a lower retention of 92.0%. With Ru 4+ doping, the initial capa LiNi0.5Mn1.5O4 (P4332) reaches the highest value of 132.8 mAh g −1 , and the ca tion after 300 cycles also recovers to 97.8%. The discharge voltage curves in the 1st and 300th cycles of the four samp rate (147.0 mAh g −1 ) at room temperature are displayed in Figure 6. Before a cycles, the curves for LiNi0.5Mn1.5O4 (Fd3 m) have a high degree of coincidence observation shows that both the 4.7 V and 4.1 V platforms are shortened sligh Mn 3+ dissolve into the electrolyte, causing a loss of battery capacity. The int Ru 4+ inhibits the dissolution of Ni 2+ and Mn 3+ during the charge-discharge pr bilizing the crystal structure. Thus, the plateau-shortening degree of Ru 4+ -L (Fd3 m) is slightly less than that of LiNi0.5Mn1.5O4 (Fd3 m). In addition, L (P4332) as a cathode material suffers severe capacity attenuation. The disch curves in the 1st and 300th cycles are significantly dissimilar, reflecting th The discharge voltage curves in the 1st and 300th cycles of the four samples at the 1C rate (147.0 mAh g −1 ) at room temperature are displayed in Figure 6. Before and after 300 cycles, the curves for LiNi 0.5 Mn 1.5 O 4 (Fd3m) have a high degree of coincidence, but careful observation shows that both the 4.7 V and 4.1 V platforms are shortened slightly. Ni 2+ and Mn 3+ dissolve into the electrolyte, causing a loss of battery capacity. The introduction of Ru 4+ inhibits the dissolution of Ni 2+ and Mn 3+ during the charge-discharge process by stabilizing the crystal structure. Thus, the plateau-shortening degree of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (Fd3m) is slightly less than that of LiNi 0.5 Mn 1.5 O 4 (Fd3m). In addition, LiNi 0.5 Mn 1.5 O 4 (P4 3 32) as a cathode material suffers severe capacity attenuation. The discharge voltage curves in the 1st and 300th cycles are significantly dissimilar, reflecting the observable shortening of the 4.7 V platform. Compared to LiNi 0.5 Mn 1.5 O 4 (P4 3 32), Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) delivers excellent cyclic stability after 300 cycles. Ru 4+ plays a major role in improving the cyclic stability. The Ru-O bond energy is higher than those of the Ni-O bond and the Mn-O bond, which can stabilize the crystal structure to reduce the lattice damage. Based on the analysis of the Raman spectroscopy results, Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) transforms to semi-ordered LiNi 0.5 Mn 1.5 O 4 and contains a certain amount of Mn 3+ . The octahedral distortion caused by Mn 3+ promotes the generation of more Li + diffusion channels in active materials, which is beneficial to the cycling stability of the electrode. Therefore, Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) not only has superior capacity but also has considerable cyclic stability.
Materials 2022, 15, x FOR PEER REVIEW 7 of 10 (P4332) delivers excellent cyclic stability after 300 cycles. Ru 4+ plays a major role in improving the cyclic stability. The Ru-O bond energy is higher than those of the Ni-O bond and the Mn-O bond, which can stabilize the crystal structure to reduce the lattice damage.

Conclusions
With the introduction of Ru 4+ , the samples all deliver a higher disch greater cycling stability, and better rate performance (especially at high cha current). Compared with LiNi0.5Mn1.5O4 (Fd3 m), the P4332-structured cat shows the most obvious improvement in electrochemical performance after 300 cycles at a 1 C rate, the capacity retention of Ru 4+ -LiNi0.5Mn1.5O4 (P4332) i (at 1st cycle ≈132.8 mAh g −1 ; at 300th cycle ≈129.8 mAh g −1 ), which is large doped samples. Intriguingly, the specific capacity of Ru 4+ -LiNi0.5Mn1.5O4 (P at 100 mAh g −1 even at the extreme charge-discharge rate of 10 C (1470 m introduction of Ru 4+ hinders the ordering process of nickel/manganese io nealing treatment to suppress the lattice damage in the charge-discharge stronger Ru-O bonding in the Ru 4+ -doped samples stabilizes the cathode further improves the cycling stability. The greater number of O-Ru/Ni-O-R movement paths in the Ru 4+ -doped LiNi0.5Mn1.5O4 contribute to increasin Thirdly, compared with Ni (3d 8 , 2 vacancies), Ru (4d 4 , 6 vacancies) has more outer vacancies and has a wider conduction band overlapping with the O 2p orbitals, which both contribute to enhancing the movement of electrons and lithium ions. It is worth noting that the LiNi 0.5 Mn 1.5 O 4 cathode material has a tendency to be disordered as a result of Ru 4+ doping. Disordered LiNi 0.5 Mn 1.5 O 4 remains in a disordered state, and ordered LiNi 0.5 Mn 1.5 O 4 transforms to semi-ordered LiNi 0.5 Mn 1.5 O 4 . Therefore, Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) without an impurity phase has better high-rate properties than Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (Fd3m).

Conclusions
With the introduction of Ru 4+ , the samples all deliver a higher discharge capacity, greater cycling stability, and better rate performance (especially at high charge-discharge current). Compared with LiNi 0.5 Mn 1.5 O 4 (Fd3m), the P4 3 32-structured cathode material shows the most obvious improvement in electrochemical performance after doping. After 300 cycles at a 1 C rate, the capacity retention of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) is still at 97.7% (at 1st cycle ≈132.8 mAh g −1 ; at 300th cycle ≈129.8 mAh g −1 ), which is larger than for undoped samples. Intriguingly, the specific capacity of Ru 4+ -LiNi 0.5 Mn 1.5 O 4 (P4 3 32) remains at 100 mAh g −1 even at the extreme charge-discharge rate of 10 C (1470 mAh g −1 ). The introduction of Ru 4+ hinders the ordering process of nickel/manganese ions during annealing treatment to suppress the lattice damage in the charge-discharge process. The