Mg²⁺ and Cr³⁺ Co-Doped LiNi_0.5Mn_1.5O₄ Derived from Ni/Mn Bimetal Oxide as High-Performance Cathode for Lithium-Ion Batteries

Abstract

In this study, pure and Mg²⁺/Cr³⁺ co-doped Ni/Mn bimetallic oxides were used as precursors to synthesize pristine and doped LNMO samples. The LNMO samples exhibited the same crystal structure as the precursors. XRD analysis confirmed the successful synthesis of LNMO cathode materials using Ni/Mn bimetallic oxides as precursors. FTIR and Raman spectroscopy reveal that Mg²⁺/Cr³⁺ co-doping promotes the formation of the Fd3m disordered phase, effectively reducing electrochemical polarization and charge transfer resistance. Furthermore, co-doping significantly lowers the Mn³⁺ content on the LNMO surface, thereby mitigating Mn³⁺ dissolution. Significantly, Mg²⁺/Cr³⁺ co-doping induces the emergence of high-surface-energy {100} crystal facets in LNMO grains, which promote lithium-ion transport and, finally, enhance rate capability and cycling performance. Electrochemical analysis indicates that the initial discharge capacities of LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015 were 126.4, 125.3, 145.3, and 138.2 mAh·g⁻¹, respectively, with capacity retention rates of 82.45%, 82.93%, 83.32%, and 82.08% after 100 cycles. Furthermore, the impedance of LNMO-0.010 prior to cycling was 97.38 Ω, representing a 14.35% reduction compared to the pristine sample. After 100 cycles, its impedance was only 58.61% of that of the pristine sample, highlighting its superior rate capability and cycling stability. As far as we know, studies on the synthesis of LNMO cathode materials via the design of Ni/Mn bimetallic oxides remain limited. Accordingly, this work provides an innovative approach for the preparation and modification of LNMO cathode materials. The investigation of Ni/Mn bimetallic oxides as precursors, combined with co-doping by Mg²⁺ and Cr³⁺, for the synthesis of high-performance LiNi_0.5Mn_1.5O₄ (LNMO) aims to provide insights into improving rate capability, cycling stability, reducing impedance, and enhancing capacity retention.

1. Introduction

In recent years, lithium-ion batteries (LIBs) have found widespread application in portable smart devices, electric vehicles, plug-in hybrid vehicles, and energy storage systems due to their superior energy density and cycle life [1,2,3,4,5,6]. To meet the growing demands for enhanced energy density, extended cycle life, and high-rate performance in these applications, it is crucial to improve the performance of LIBs, which is primarily determined by the choice of cathode materials [7]. Among various lithium battery cathodes, the high-voltage spinel LNMO has emerged as one of the most promising candidates for next-generation high energy density LIBs. This is attributed to its high operating voltage (~4.7 V), substantial energy density (658 Wh∙kg⁻¹), and three-dimensional lithium-ion diffusion pathways [8,9,10].

Despite these advantages, the electrochemical performance of synthesized LNMO encounters several significant challenges. The manganese (Mn) within LNMO can dissolve in the electrolyte at elevated temperatures through the disproportionation reaction 2Mn³⁺→Mn²⁺ + Mn⁴⁺, leading to Jahn–Teller distortion that alters the crystal structure. This alteration compromises the structural integrity of the cathode material, thereby diminishing its stability [11,12,13,14,15]. Moreover, at the high operating voltage of 4.7 V for LNMO, side reactions such as electrolyte decomposition are likely to occur at the electrode/electrolyte interface [16]. These side reactions lead to the formation of a cathode/electrolyte interphase (CEI) layer, which in turn increases the interfacial membrane resistance [17]. Strategies such as ion doping and surface coating have been demonstrated to effectively mitigate these challenges [18,19,20]. However, studies on Mg²⁺/Cr³⁺ co-doping in LNMO are relatively scarce, and the synergistic effects of Mg²⁺ and Cr³⁺ remain unclear.

The arrangement of Ni and Mn atoms within the spinel structure differs, resulting in LNMO materials that consist of both the disordered Fd3m phase and the ordered P4₃32 phase with distinct spatial symmetry [21]. In the P4₃32 ordered structure, Mn ions exhibit a valence state of +4, while in the Fd3m disordered structure, both Mn³⁺ and Mn⁴⁺ coexist. The presence of Mn³⁺ facilitates ionic transport, leading to ionic conductivity in the Fd3m disordered phase that is two orders of magnitude higher than that of the P4₃32 ordered phase, thereby enhancing the rate performance of LNMO [22]. However, excessive Mn³⁺ presence can negatively impact the cycle life of the battery [23]. Moreover, during the battery’s charge/discharge process, compared to the two-phase transition in the P4₃32 ordered structure, the Fd3m disordered structure undergoes a single-phase transition, which experiences less mechanical stress and benefits the cycling stability of the material [24,25]. Consequently, synthesizing LNMO materials with a higher proportion of the Fd3m disordered phase and appropriate Mn³⁺ content can simultaneously enhance both rate capability and cycling performance.

The general formula for bimetal oxides is AB₂O₄, where A represents divalent cations such as Mg²⁺, Fe²⁺, or Ni²⁺, and B represents trivalent cations such as Mn³⁺, Cr³⁺, or Al³⁺. These oxides share the Fd3m space’s group spinel structure with LNMO [26,27]. Bimetal oxides exhibit excellent performance in various fields [28]. Utilizing Ni/Mn bimetal oxides as a doping template facilitates ion doping and mitigates the stress effects associated with crystal structure changes during the calcination process for LNMO synthesis. Furthermore, in NiMn₂O₄ (NMO), both tetrahedral and octahedral sites can be occupied by Ni and Mn cations, leading to cation disorder within the oxide [29,30]. Importantly, NiMn₂O₄ exhibits a porous composite structure that enhances the intercalation and deintercalation of lithium-ions, promoting rapid ion transport. Consequently, utilizing Ni/Mn bimetal oxides as precursors for the synthesis of both LNMO and doped LNMO constitutes an effective and feasible strategy. As far as we know, the preparation of LNMO cathodes using Ni/Mn bimetal oxide precursors has rarely been investigated.

In this study, NH₃·H₂O was utilized as a complexing agent and precipitant to achieve a uniform mixing of Ni and Mn metal ions in solution. Air was subsequently introduced into the system to oxidize the metal hydroxides into metal oxide precipitates, which were then pre-calcined to obtain Ni/Mn bimetal oxides. The resultant Ni/Mn bimetal oxides were mixed with Li₂CO₃ in stoichiometric ratios and calcined, leading to the transformation of the Ni/Mn bimetal oxides into LNMO. To further enhance the electrochemical performance of LNMO, Mg²⁺/Cr³⁺ co-doped Ni/Mn bimetal oxide precursors were synthesized, resulting in a series of co-doped LiNi_0.5-x/2Mg_x/2Cr_x/2Mn_1.5-2/xO₄ (x = 0, 0.005, 0.01, 0.015). The results confirm the successful synthesis of the LNMO cathode material, with ionic co-doping effectively facilitating the formation of the Fd3m disordered structure and suppressing Mn³⁺ content. The co-doped LNMO demonstrates significantly improved cycling stability and rate performance.

2. Experimental Section

2.1. Material Synthesis

The specific preparation process is detailed in Scheme 1, where the pristine and co-doped LiNi_0.5-x/2Mg_x/2Cr_x/2Mn_1.5-x/2O₄ (x = 0, 0.005, 0.010, 0.015) was synthesized through liquid-phase oxidation followed by two-stage calcination. Initially, (0.5-x/2) mol of NiSO₄·6H₂O, (1.5-x/2) mol of MnSO₄·H₂O, (x/2) mol of MgSO₄, and (x/2) mol of CrH₃O₁₂S₃ were mechanically stirred and dissolved in 1000 mL of deionized water, designated solution A. Subsequently, NH₃·H₂O was mixed with deionized water in a 1:1 ratio to serve as a complexing and precipitating agent, referred to as solution B. A total of 500 mL of deionized water was added to the reactor, which was heated in a water bath while continuously supplying air until the temperature reached 70 °C. Solutions A and B were introduced into the reactor at rates of 1.33 mL∙min⁻¹ and 2.71 mL∙min⁻¹, respectively, and the reaction was maintained for 14 h. Upon completion of the reaction, the black-brown precipitate was filtered, washed several times with deionized water, and dried at 100 °C for 3 h. The precursor was subjected to calcination at 500 °C in a muffle furnace for 10 h to synthesize Ni/Mn bimetallic oxide precursors, designated NMO-0, NMO-0.005, NMO-0.01, and NMO-0.015, corresponding to varying doping content. This precursor was uniformly mixed with stoichiometric Li₂CO₃ (with an excess of 5 wt% to compensate for lithium loss) and calcined at 850 °C for 20 h. After natural cooling to room temperature, the powder was collected to obtain the product LiNi_0.5-x/2Mg_x/2Cr_x/2Mn_1.5-x/2O₄ (x = 0, 0.005, 0.01, 0.015). The resulting products were designated according to the co-doping content: LNMO-0, LNMO-0.005, LNMO-0.01, and LNMO-0.015.

2.2. Material Characterization and Electrochemical Analysis

All detailed information for the material characterization, computational methods, and electrochemical measurements are shown in the Supporting Information.

3. Results and Discussion

The morphology of the pristine and Mg²⁺/Cr³⁺ co-doped LNMO during the preparation process was characterized using scanning electron microscopy (SEM). As illustrated in Figure 1a,e,i,m, the morphology of the pristine and lightly co-doped precursors consists of nanosheets that adhere to and grow on the surfaces of nanospheres, whereas the precursor with 0.015 Mg²⁺/Cr³⁺ co-doping is entirely composed of nanosheets. With an increasing Mg²⁺/Cr³⁺ content, the size of the nanospheres diminishes, suggesting that Ni and Mn in the precursor exist as more micro-aggregated nanosheets. LNMO precursors prepared via coprecipitation generally exhibit a more disordered spherical morphology [31]. In contrast, the morphology obtained through this method, characterized by the coexistence of nanosheets and nanospheres, is more organized and provides an increased specific surface area for enhanced reactivity. Energy-dispersive spectroscopy (EDS) was employed to assess the chemical composition and elemental distribution of both the pristine and co-doped precursors. Figure S1a–m and Figure 1q–u reveal the presence of Ni, Mn, and O in the pristine precursor, while the co-doped Mg²⁺/Cr³⁺ precursor includes Ni, Mn, Mg, Cr, and O. EDS mapping demonstrates that Mg and Cr are uniformly distributed throughout the doped precursor, confirming the successful incorporation of Mg²⁺ and Cr³⁺ ions into the precursor.

Figure 1b,f,j,n illustrates the morphologies of the pristine and co-doped NMO precursors. All four samples consist of numerous small octahedral structures that aggregate into spherical shapes. As the Mg²⁺/Cr³⁺ content increases, the porosity of the spherical octahedral aggregates also increases, leading to a higher specific surface area. Figure S2 and Table S1 indicate that increasing the content of Mg²⁺/Cr³⁺ would enhance the BET surface area, pore volume, and average pore diameter of the NMO precursor, which is consistent with the SEM results. However, the data for NMO-0.010 show a downward trend, which is due to the fact that the voids in the morphology of NMO-0.005 are concentrated and relatively large, while the octahedral structure size of NMO-0.015 becomes smaller. The morphology of NMO-0.010 exhibits a uniform octahedral structure size and an even distribution of voids within the spherical aggregates, which can provide more active sites and diffusion pathways, potentially enhancing the capacity of LNMO. The structures of both pristine and co-doped NMO, as well as LNMO, were analyzed using X-ray diffraction (XRD), and the results for LNMO were subjected to Rietveld refinement. As shown in Figure 2a, the main diffraction peaks of all samples align perfectly with the spinel-type structures of NiMn₂O₄ (PDF#97-002-7813) and Mn₃O₄ (PDF#00-013-0162) in the Fd3m space group. It is observed that as the Mg²⁺/Cr³⁺ co-doping content increases, the XRD signals gradually shift toward higher angles, indicating the successful incorporation of Mg²⁺ and Cr³⁺ into the NMO precursor. Given that the ionic radius of Cr³⁺ (0.61 Å) is smaller than that of Mn³⁺ (0.65 Å), the crystal volume decreases [32]. The emergence of the Mn₃O₄ phase results from the stoichiometric ratio of Ni to Mn in solution A being 1:3, while in the bimetallic oxide NiMn₂O₄, the ratio is 1:2. This discrepancy leads to the calcination of excess Mn that cannot bond with Ni, resulting in the formation of Mn₃O₄.

Figure 1c,g,k,o illustrates the morphologies of the pristine and co-doped LNMO samples. All four samples consist of numerous small octahedral crystals that aggregate into spherical shapes, resembling the spherical octahedral aggregates of the NMO precursor; however, they exhibit fewer voids and a more compact arrangement of the octahedral crystals. The spherical aggregation in LNMO-0 and LNMO-0.005 is less pronounced, with the octahedral crystals appearing fragmented and disordered, accompanied by a significant amount of surface impurities. In contrast, LNMO-0.010 and LNMO-0.015 display more pronounced spherical aggregation and cleaner surfaces, free from other impurities. The smaller sizes of the spherical aggregates and octahedral crystals in LNMO-0.015 are attributed to the micro-aggregated nanosheet precursors and the smaller octahedral crystal size observed in the NMO-0.015 precursor. LNMO-0.010 exhibits a favorable morphology with clear spherical aggregation and larger, uniformly distributed octahedral crystals. The larger particles may reduce the contact area between the cathode material and the electrolyte, enhancing structural stability [33,34]. Figure 1d,h,l,p presents the crystal morphologies of LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015. Notably, the morphology of the primary particles exhibits intriguing variations with increasing Mg²⁺/Cr³⁺ co-doping content. The crystal surface orientations correspond to the established {111} plane orientation of the octahedral face-centered cubic framework and their truncated surface orientations [35,36]. The primary particles of LNMO-0 are composed of {111} crystal facets, while the grains of LNMO-0.005, LNMO-0.010, and LNMO-0.015 form new {100} crystal facets at the truncated corners of the octahedral structure. Previous studies indicate that when doping with Cr³⁺ and calcining at temperatures above 800 °C, the {100} crystal planes, which possess higher surface energy, tend to grow preferentially over the {111} planes [37,38,39]. This suggests that during the calcination of NMO-0, the low-energy {111} planes preferentially grow, resulting in the sacrifice of the {100} and other higher-energy planes [40,41,42]. In contrast, after co-doping with Mg²⁺/Cr³⁺, the lattice structure becomes more stable, allowing for greater exposure of high-energy crystal planes during calcination. Among these, the LNMO-0.010 sample exhibits the highest exposure of high-energy crystal planes, which may facilitate the lithium-ion insertion/extraction process, thereby ensuring improved specific capacity and rate performance.

Figure 2b presents the XRD patterns for the synthesis of LiNi_0.5-x/2Mg_x/2Cr_x/2Mn_1.5-x/2O₄ (x = 0, 0.005, 0.01, 0.015). The main diffraction peaks of all samples align with the LiNi_0.5Mn_1.5O₄ structure in the Fd3m space group (PDF#97-009-4762). Remarkably, the diffraction peaks for LNMO-0 and LNMO-0.010 exhibit enhanced intensity and sharpness, indicating a higher degree of crystallinity, which is advantageous for improving charge/discharge capacity and cycling performance [43,44]. As the co-doping content of Mg²⁺ and Cr³⁺ increases, a gradual shift of XRD signals toward higher angles is observed, confirming the successful incorporation of these ions into the NMO precursor, leading to a reduction in crystal volume. The LNMO-0.015 sample shows weaker diffraction peaks at 2θ = 35°, 43.5°, and 63.5°, which can be attributed to the presence of rock salt impurity phases (Ni_xO, Ni₆MnO₈, or Li_1-xNi_xO, marked with *) [45,46]. This phase indicates that, during high-temperature calcination, some Mn in LNMO-0.015 is reduced from Mn⁴⁺ to Mn³⁺ due to lithium volatilization, maintaining charge balance and leading to the formation of disordered phases with an increased Mn³⁺ content [47]. Figure 2c,d,e,f depicts the Rietveld refinement results for the pristine and Mg²⁺/Cr³⁺ co-doped LNMO samples based on the Fd3m space group, where the 8a site is occupied by Li, the 16d sites are occupied by Ni and Mn, and the 32e sites are occupied by O. The lower R_wp values obtained in all refinements indicate a high level of agreement between the measured and fitted values.

Table 1. Rietveld refinement results for the pristine and Mg²⁺/Cr³⁺ co-doped LNMO.

Samples	a (Å)	V (Å3)	I311/I400	Rwp
LNMO-0	8.174959	546.33	0.97	8.10
LNMO-0.005	8.173774	546.09	0.95	9.68
LNMO-0.010	8.168736	545.09	0.87	10.73
LNMO-0.015	8.168580	545.05	0.86	13.75

summarizes the refined structural parameters, showing a decreasing trend in crystal parameters, which is consistent with the SEM results. It can be inferred that Mg²⁺/Cr³⁺ co-doping contributes to the reduction of crystal structure parameters; specifically, the ionic radius of Cr³⁺ (0.61 Å) is smaller than that of Ni²⁺ (0.69 Å), leading to a decrease in crystal volume. Furthermore, this is mainly ascribed to fewer oxygen deficiencies caused by the higher bond strengths of Mg/O (389 kJ∙mol⁻¹) and Cr/O (461 kJ∙mol⁻¹) compared with Ni/O (382 kJ∙mol⁻¹) and Mn/O (402 kJ∙mol⁻¹), in view of the larger ionic radius of Mn³⁺ than that of Mn⁴⁺ [48]. Additionally, the stronger Mg/O and Cr/O bonds contribute to the decrease in crystal volume, thereby reducing the lattice parameters. The I₃₁₁/I₄₀₀ ratio serves as an indicator of the distortion degree within the cubic spinel structure and reflects the stability of the crystal structure [49,50]. A smaller I₃₁₁/I₄₀₀ ratio signifies a more stable structure and enhanced cycling performance for the LNMO materials. As indicated in Table 1, the I₃₁₁/I₄₀₀ ratio decreases with Mg²⁺/Cr³⁺ co-doping, suggesting that the crystal structures of the co-doped samples exhibit greater stability, which supports the selection of more high-energy crystal planes during the calcination process of the samples with Mg²⁺/Cr³⁺ co-doping.

LNMO materials are generally a mixture of the ordered P4₃32 and disordered Fd3m structures, with the proportion of the Fd3m space group playing a key role in achieving superior electrochemical performance. Studies indicate that the Fd3m disordered structure offers enhanced electrochemical properties due to its greater charge transport capacity [24]. Since XRD analysis alone cannot reliably distinguish between the Fd3m and P4₃32 space groups in synthesized LNMO samples [51], FTIR and Raman spectra were employed to assess the Ni/Mn ordering in both pristine and Mg²⁺/Cr³⁺ co-doped LNMO, as shown in Figure 3. According to Amatucci et al. [52], the P4₃32 ordered structure presents eight infrared absorption bands in the range of 400–700 cm⁻¹, whereas the Fd3m disordered structure, due to peak broadening, exhibits only five absorption bands. The FTIR for LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015 in Figure 3a reveals five absorption bands at 620, 582, 555, 493, and 465 cm⁻¹, indicative of the Fd3m disordered structure. Peaks observed at 620, 582, 555, and 493 cm⁻¹ correspond to the Fd3m disordered structure, while the 465 cm⁻¹ peak is associated with the P4₃32 ordered structure [53]. Additionally, the signals at 620 cm⁻¹ and 582 cm⁻¹ correspond to symmetric stretching vibrations of Mn/O and Ni/O [10]. The FTIR intensity ratio at 620 cm⁻¹ and 582 cm⁻¹ (I₆₂₀/I₅₈₂) qualitatively reflects the disorder level in LNMO, with a higher I₆₂₀/I₅₈₂ ratio suggesting increased Ni/Mn disorder. From Figure 3a, the I₆₂₀/I₅₈₂ values for LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015 are 1.43, 1.45, 1.56, and 2.80, respectively, indicating that Mg²⁺/Cr³⁺ co-doping partially substitutes Ni²⁺ and Mn⁴⁺, enhancing Ni/Mn disorder at the octahedral 16d sites and increasing the proportion of the Fd3m disordered structure. This trend aligns with the improved electrochemical performance observed upon Mg²⁺/Cr³⁺ co-doping. The Raman spectra of LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015 are shown in Figure 3b. Mn/O stretching is observed at 634 cm⁻¹ [54], while Ni²⁺/O stretching modes appear at 394 and 494 cm⁻¹ [55]. Compared to the doped LNMO samples, LNMO-0 displays a pronounced peak at 162 cm⁻¹, a feature of the P4₃32 space group attributed to the reduced symmetry from Ni/Mn ordering [56] and the P4₃32 polymorph [57,58]. This peak’s intensity diminishes significantly following Mg²⁺/Cr³⁺ co-doping, suggesting an increased disorder in LNMO. Additionally, the absence of distinct peak splitting in the 589–634 cm⁻¹ range aligns with the Fd3m disordered structure. Thus, FTIR and Raman analyses confirm that all samples primarily exhibit the Fd3m disordered space group, with only a minor presence of the P4₃32 ordered structure.

In order to investigate the impact of Mg²⁺/Cr³⁺ co-doping on the surface elemental distribution, X-ray Photoelectron Spectroscopy (XPS) was employed to analyze the valence states of elements in both bare and Mg²⁺/Cr³⁺ co-doped LNMO. Figure 4a shows the full XPS spectrum of LNMO-0, revealing the presence of Mn, Ni, O, and C on the surface. Figure 4b,d,g,j depicts the Mn2p spectra for LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015, respectively, all of which can be deconvoluted into two peaks. The characteristic peaks at approximately ~642.0 eV and ~653.7 eV correspond to Mn³⁺, while those at around ~643.2 eV and ~654.5 eV indicate Mn⁴⁺, confirming the simultaneous presence of both Mn³⁺ and Mn⁴⁺ ions on the surfaces of all four samples [59]. Figure 4c and Figure S3a–c display the Ni2p spectra for LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015, where four characteristic peaks can be observed. The peaks at approximately ~854.7 eV and ~872.7 eV correspond to the main peaks of Ni²⁺ in the Ni 2p_3/2 and Ni 2p_1/2 states, respectively. The observed characteristic peaks at approximately ~861.2 eV and ~879.5 eV correspond to the satellite peaks of Ni2p_3/2 and Ni2p_1/2, respectively, attributed to the multiple splitting of energy levels in nickel oxide (NiO) [59]. These features confirm that Ni in all four samples exists in the +2 oxidation state. Figure 4e,h,k presents the Mg1s spectra for LNMO-0.005, LNMO-0.010, and LNMO-0.015. Remarkably, no Mg1s characteristic peak is observed in LNMO-0.005, likely due to the low doping content of Mg²⁺ being below the detection limit of XPS. However, the presence of Mg²⁺ is confirmed in LNMO-0.010 and LNMO-0.015, with Mg1s characteristic peaks appearing at approximately ~1303.5 eV [10,60]. Figure 4f,i,l illustrates the Cr2p spectra for LNMO-0.005, LNMO-0.010, and LNMO-0.015. The absence of Cr2p peaks in LNMO-0.005 is attributed to the Cr³⁺ doping content being below the detection threshold. In LNMO-0.010 and LNMO-0.015, the characteristic peaks for Cr2p_3/2 and Cr2p_1/2 appear at approximately ~575.1 eV and ~585.2 eV, respectively, confirming the presence of Cr³⁺ [2,61]. The characteristic peaks of Mg1s and Cr2p validate the successful doping of Mg²⁺/Cr³⁺. Furthermore, the peak areas of Mg1s and Cr2p in LNMO-0.015 are greater than those in LNMO-0.010, indirectly indicating a higher co-doping content of Mg²⁺/Cr³⁺ in LNMO-0.015. Since the dissolution of Mn²⁺ and Mn⁴⁺ is generally associated with the disproportionation of Mn³⁺, a higher Mn³⁺ content can negatively impact the cycling performance of LNMO materials. Therefore, reducing the Mn³⁺ content in the surface region may alleviate the dissolution of Mn²⁺ and Mn⁴⁺, thus enhancing the cycling performance of LNMO. The relative content of Mn³⁺ can be quantified based on the peak area ratio of Mn2p_3/2 [62]. As observed in Figure 4b,d,g,j, the Mn³⁺ content on the particle surface decreases with 0.010 Mg²⁺/Cr³⁺ co-doping, whereas it exceeds that of the undoped sample at a doping content of 0.015.

To investigate the Li⁺ migration kinetics in pristine and Mg²⁺/Cr³⁺ co-doped LNMO, cyclic voltammetry (CV) measurements were performed on LNMO/Li half-cells over a voltage range of 3.5–5.0 V at scan rates of 0.05, 0.075, 0.10, 0.125, and 0.150 mV/s. As shown in Figure 5a–d, at a scan rate of 0.1 mV/s, all samples exhibit cathodic and anodic peaks near 4.6 V and 4.8 V, respectively. The redox peaks are split into two distinct peaks, corresponding to the Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox processes [63], with LNMO-0.010 displaying the most pronounced peak splitting. G.B. Zhong et al. reported that the potential difference between Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox peaks is greater in the Fd3m disordered phase than in the P4₃32 ordered phase [64]. These findings suggest that the relative content of the Fd3m disordered phase among the samples follows the order LNMO-0.015 > LNMO-0.010 > LNMO-0.005 > LNMO-0, consistent with FTIR and Raman spectroscopy results. With the increase in scan rate, the anodic peak (de-lithiation) potentials of the four samples shift to higher values, while the cathodic peak (lithiation) potentials shift to lower values in Figure 5a–d. The potential difference between the anodic and cathodic peaks serves as a reliable indicator of the electrode material’s polarization degree.

Table 2. Potential differences (ΔV, V) between anodic peaks (φ_pa, V) and cathodic peaks (φ_pc, V).

ν (mV∙s−1)	LNMO-0			LNMO-0.005			LNMO-0.010			LNMO-0.015
	φpa	φpc	ΔV	φpa	φpc	ΔV	φpa	φpc	ΔV	φpa	φpc	ΔV
0.05	4.806	4.622	0.184	4.816	4.654	0.162	4.791	4.645	0.146	4.806	4.627	0.179
0.075	4.816	4.608	0.208	4.790	4.683	0.107	4.798	4.640	0.158	4.828	4.607	0.221
0.1	4.839	4.586	0.253	4.825	4.642	0.183	4.819	4.628	0.191	4.853	4.584	0.269
0.125	4.852	4.595	0.257	4.833	4.630	0.203	4.820	4.621	0.199	4.867	4.58	0.287
0.150	4.868	4.577	0.291	4.838	4.621	0.217	4.834	4.609	0.225	4.886	4.569	0.317

compares the potential differences between the anodic and cathodic peaks of the four samples at different scan rates. At scan rates of 0.05, 0.125, and 0.150 mV/s, LNMO-0.010 exhibits the smallest potential difference, indicating the lowest electrode polarization among all samples. Figure 5e illustrates the linear relationship between the peak current (i_p) and the square root of the scan rate (ν^1/2). The anodic peak current increases with ν^1/2, while the cathodic peak current decreases correspondingly, demonstrating a well-defined linear trend. This relationship is commonly employed to calculate the Li⁺ diffusion coefficient (D_Li) using the Randles–Sevcik equation [65]:

i_p = 2.69 × 10⁵An^3/2D_LiCν^1/2

(1)

where A represents the surface area of the electrode (1.1304 cm²), n is the number of electrons transferred per reaction, and C is the volumetric concentration of LNMO (0.02378 mol∙cm⁻³).

Table 3. The diffusion coefficients of Li⁺ in all samples.

Samples	Slopepa	Slopepc	Li-Deintercalation DLi (cm2∙s−1)	Li-Intercalation DLi (cm2∙s−1)
LNMO-0	1.976 × 10−2	−2.308 × 10−2	7.468 × 10−12	1.019 × 10−11
LNMO-0.005	2.636 × 10−2	−2.641 × 10−2	1.329 × 10−11	1.333 × 10−11
LNMO-0.010	3.180 × 10−2	−3.122 × 10−2	1.934 × 10−11	1.864 × 10−11
LNMO-0.015	2.458 × 10−2	−2.956 × 10−2	1.156 × 10−11	1.671 × 10−11

presents the slopes of i_p versus ν^1/2 and the corresponding D_Li values for Li⁺ extraction/insertion in the four samples. Mg²⁺/Cr³⁺ co-doping increases the Li⁺ extraction rate by an order of magnitude and also enhances the Li⁺ insertion rate, indicating that co-doping promotes faster Li⁺ diffusion. Among the co-doped samples, LNMO-0.010 exhibits the highest D_Li value, suggesting that a Mg²⁺/Cr³⁺ co-doping content of 0.010 improves the electronic conductivity and structural stability of LNMO, leading to superior electrochemical performance. A minor redox peak near 4.0 V, attributed to the Mn³⁺/Mn⁴⁺ redox process, is observed in all samples. Studies have shown that a reduced Mn³⁺ content mitigates the Jahn–Teller effect and decreases Mn dissolution in the electrolyte [54]. Figure 5f provides a magnified view of the Mn³⁺/Mn⁴⁺ redox peaks at 0.1 mV/s. At doping contents of 0.005 and 0.010, the peak areas at 4.0 V decrease, indicating a reduction in Mn³⁺ content. However, at a doping content of 0.015, the Mn³⁺ content surpasses that of the pristine sample, consistent with the Mn³⁺ relative content obtained from the XPS peak area ratios. This increase can be attributed to the charge compensation mechanism, wherein Mg²⁺/Cr³⁺ substitution for Mn⁴⁺ leads to an increase in Mn³⁺ content.

To elucidate the beneficial effects of Mg²⁺/Cr³⁺ co-doping on LNMO, electrochemical impedance spectroscopy (EIS) was utilized to assess the impedance and interfacial behavior during the cycling process, activated by running five cycles before the test. Figure 6a,c displays the Nyquist plots obtained before cycling and after 100 cycles at 0.5C. All samples exhibit similar profiles, comprising a semicircle in the high-frequency region and an inclined line in the low-frequency region. The intercept in the high-frequency region corresponds to the ohmic resistance (R_s), which is attributed to the contributions from the electrolyte, electrode material, and separator. The semicircle represents the charge transfer resistance (R_ct) at the electrode/electrolyte interface, while the inclined line reflects the Warburg diffusion resistance (Z_w), indicative of lithium-ion solid-state diffusion within the electrode material [66]. The Nyquist plots were fitted using software, achieving a high degree of correlation between the experimental data and fitted results. The equivalent circuit model employed for fitting and the derived R_ct values are shown in Figure 6a–d. Before cycling, the pristine LNMO (LNMO-0) exhibited a high impedance of 113.7 Ω, while the R_ct values of LNMO-0.005 (118.7 Ω) and LNMO-0.015 (109.4 Ω) were comparable to that of LNMO-0. In contrast, LNMO-0.010 demonstrated a significantly lower impedance of 97.38 Ω, representing a reduction of 14.35% relative to the pristine sample. The reduced impedance of LNMO-0.010 can be attributed to the introduction of Mg²⁺/Cr³⁺, which enhances conductivity and promotes a disordered structure. After 100 cycles at 0.5C, all samples exhibited increased R_ct values. The impedances of LNMO-0, LNMO-0.005, and LNMO-0.015 were approximately 500 Ω. However, LNMO-0.010 exhibited a markedly lower impedance of 279.5 Ω, which is only 58.61% of that of the pristine sample, indicating a substantial reduction. The inability of LNMO-0.005 and LNMO-0.015 to achieve similar impedance reductions is likely due to different factors. The low doping level in LNMO-0.005 failed to effectively substitute Ni and Mn sites, while the higher doping content in LNMO-0.015 led to an increased Mn³⁺ content, exacerbating Mn dissolution and interfacial side reactions. These findings highlight that Mg²⁺/Cr³⁺ co-doping at an optimal content of 0.010 effectively stabilizes the LNMO structure and the cathode/electrolyte interface, thereby enhancing the cycling performance.

The charge/discharge profile serves as a critical metric for assessing the electrochemical performance of LNMO materials. Figure 7a presents the initial charge/discharge curves of the four samples. All samples exhibit two distinct voltage plateaus at approximately 4.7 V, corresponding to the Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox couples. Additionally, a short voltage plateau appears at ~4.0 V in all four samples, attributed to the Mn³⁺/Mn⁴⁺ redox couple, indicating that the Fd3m disordered structure is predominant in all samples. This result is consistent with the findings from XRD, FTIR, Raman, and CV analyses. The Mn³⁺ content was calculated by dividing the discharge capacity within the 3.8~4.25 V range by the total discharge capacity [67]. Based on Figure 7a, the Mn³⁺ contents for LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015 were determined to be 9.18%, 8.81%, 7.21%, and 9.33%, respectively. These values are in good agreement with the results obtained from XPS and CV. The reduced Mn³⁺ content in LNMO-0.010 effectively suppresses the Jahn–Teller effect and minimizes Mn dissolution, thereby achieving enhanced storage capacity.

The cycling performance of all samples was assessed through galvanostatic charge/discharge testing at 0.5C, as shown in Figure 7b. Prior to testing, each cell underwent 10 activation cycles at 0.2C to stabilize the electrodes. As presented in Figure 7b, the initial discharge capacities of LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015 were 126.4, 125.3, 145.3, and 138.2 mAh∙g⁻¹, respectively. After 100 cycles, the corresponding discharge capacities were 103.3, 104.84, 121.1, and 113.4 mAh∙g⁻¹, with capacity retention rates of 82.45%, 82.93%, 83.32%, and 82.09%, respectively. The observed capacity degradation during cycling is primarily attributed to the progressive increase in charge transfer resistance (R_ct) caused by interfacial reactions at the electrode/electrolyte interface. As shown in Figure 7b, Mg²⁺/Cr³⁺ co-doping notably enhances both the initial discharge capacity and cycling stability of the samples. This improvement is largely attributed to the higher binding energy of Mg/O and Cr/O bonds, which reinforces the structural stability of LNMO. Additionally, the three electrons in the third orbital of Cr³⁺ effectively mitigate the Jahn–Teller effect. Compared to LNMO-0.010, the performance of LNMO-0.005 is hindered by its insufficient doping level, which fails to fully capitalize on the advantages of Mg²⁺/Cr³⁺ co-doping. Conversely, LNMO-0.015 exhibits reduced cycling stability due to excessive doping, which increases Mn³⁺ content through charge compensation during Mg²⁺/Cr³⁺ substitution of Mn⁴⁺, thereby exacerbating side reactions and capacity fade. LNMO-0.010 exhibits better cycling performance, primarily due to the reduced Mn³⁺ content induced by doping and the increase in {100} crystal facets, which consequently reduce the proportion of {111} crystal facets. Previous studies have demonstrated that the cycling stability of LNMO is strongly influenced by surface orientation exposed to the electrolyte [35], and the exposed {111} crystal facets increase Mn dissolution and induce unstable interfacial behavior at higher voltages. This insight explains the rapid capacity decay observed in LNMO-0, whose particles predominantly expose {111} planes. Therefore, Mg²⁺/Cr³⁺ co-doping at an optimal concentration of 0.010 provides the most significant enhancement in both initial capacity and cycling stability, establishing LNMO-0.010 as the best-performing material. Figure 7b further depicts the Coulombic efficiency trends during cycling. The initial Coulombic efficiency is relatively low, which can be attributed to inevitable electrolyte decomposition and side reactions at the electrode/electrolyte interface under high voltage conditions [68]. However, a stable interfacial layer forms during cycling, leading to the subsequent stabilization of Coulombic efficiency in later cycles.

The differential capacity curve (dQ/dV) is a valuable tool for investigating the capacity fading mechanisms in LIBs, as the characteristic peaks on the curve correspond to the redox peaks observed in CV curves [69]. Figure 7c illustrates the dQ/dV curves of the four samples during the first and 100th cycles at 0.5C, revealing three pairs of redox peaks. The short plateau at approximately 4.0 V is associated with the Mn³⁺/Mn⁴⁺ redox couple, while the two redox peaks near 4.7 V are attributed to the Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox processes, respectively [70]. As shown in Figure 7c, after 100 cycles, the anodic peaks of all four samples shift to higher potentials while the cathodic peaks shift to lower potentials, and the potential difference (ΔV) increases, indicating an increased degree of electrode polarization after cycling. When considered alongside the observed increase in impedance after 100 cycles, it is evident that side reactions at the electrode/electrolyte interface and electrolyte decomposition play significant roles in this process. The deposition of decomposition products on the electrode surface obstructs Li⁺ transport and intensifies polarization, thereby contributing to capacity degradation in the LNMO samples.

To assess the rate performance of the prepared samples, the cells were first cycled at 0.2C for several cycles to activate the electrodes, followed by charge/discharge testing at sequential rates of 0.2C, 0.5C, 1C, 5C, and 10C, and finally returned to 0.2C. The corresponding rate performance curves are shown in Figure 7d. After the rate performance tests, the capacities of all samples nearly returned to their initial values, demonstrating the structural stability of the materials during rapid lithium insertion/extraction processes. The introduction of Cr³⁺ dopants extends the Li/O bond length, facilitating the separation of Li⁺ from O²⁻ and enhancing lithium-ion migration within the lattice [71]. Furthermore, spinel-type materials with high surface energy provide advantageous conditions for Li⁺ diffusion during charge/discharge processes [35]. As a result, LNMO-0.010, characterized by a lower Mn³⁺ content, fewer impurity phases, a higher proportion of Fd3m disordered structure, and prominent {100} crystal planes, exhibited superior capacities across different rates. At low rates of 0.2C and 0.5C, all samples displayed excellent performance. However, at higher rates of 5C and 10C, the capacities of the materials declined significantly. The rate capability of the materials is primarily governed by their morphology, particle size, and the availability of three-dimensional Li⁺ diffusion pathways. As shown in Figure 1c,g,k,o, LNMO-0, LNMO-0.005, LNMO-0.010, and LNMO-0.015 are composed of spherical aggregates formed by densely packed micro-octahedral structures. This compact porous morphology slows Li⁺ migration, leading to reduced capacities at high charge/discharge rates.

4. Conclusions

Based on the principle of crystal structure consistency, we successfully synthesized pristine disordered-phase LNMO samples using Ni/Mn bimetallic oxides as precursors. Structural characterizations (XRD, FTIR, Raman) and electrochemical performance (initial discharge capacity of 126.4 mAh·g⁻¹) indicate the significant application potential of this approach. Furthermore, Mg²⁺/Cr³⁺ co-doped Ni/Mn bimetallic oxides were employed to prepare co-doped LNMO samples. Compared with the pristine LNMO, Mg²⁺/Cr³⁺ co-doping enhanced the degree of Ni/Mn disorder in the LNMO cathode material, reduced the surface Mn³⁺ content, and decreased the charge transfer resistance, thereby improving both the rate capability and cycling stability. Additionally, SEM analysis further revealed that Mg²⁺/Cr³⁺ co-doping facilitated the formation of high-surface-energy {100} crystal facets in LNMO grains, which promote lithium-ion migration. The 0.010 Mg²⁺/Cr³⁺ co-doped sample (LNMO-0.010) exhibited a significantly improved initial discharge capacity of 145.3 mAh·g⁻¹ at 0.5C. Additionally, due to reduced electrode polarization and suppressed Mn³⁺ dissolution, this sample exhibited less capacity degradation compared to the pristine LNMO, achieving capacity retention of 83.32% after 100 cycles, thus demonstrating superior rate capability and cycling performance. This study offers an innovative and effective strategy for the synthesis and modification of LNMO cathode materials.