Li-ion batteries (LIBs) demonstrate high specific energy densities, low self-discharge, and high coulombic efficiencies, making them the most widely used commercial rechargeable batteries [1–5]. Graphite is currently the most widely used anode material in LIBs owing to its low cost, low reduction potential, and long cycle life. However, the low theoretical capacity of graphite (372 mAh g⁻¹) limits its application in batteries requiring higher energy densities, such as the batteries used in electric vehicles (EVs) and energy storage systems (ESSs) [6–11]. Consequently, extensive research has been conducted to develop high-capacity anodes to replace graphite. These studies have focused primarily on two categories of materials: alloy materials (such as Si and Sn) [12–15] and conversion-type materials based on transition-metal oxides (such as CoO, CuO, MnO, Fe₃O₄, and Mn₃O₄) [16–18].

Conversion-type transition metal oxides were first reported as promising high-capacity anode materials by Tarascon group [19], and this finding has been corroborated in several subsequently published studies. These conversion-type materials exhibit higher theoretical capacities than commercial graphite because they can incorporate more than one Li ion per metal. Moreover, the safety concerns associated with the use of graphite related to Li plating because of its low potential (which is similar to the potential for Li deposition) limit the application of the material. In contrast, transition-metal oxides, with their higher lithiation reaction voltages, can mitigate Li plating issues [20–22]. Although they exhibit lower capacities than alloying materials, their smaller volume changes during the lithiation/delithiation process prove advantageous in terms of cycling performance.

MnO is considered an ideal anode material because of its high theoretical capacity (756 mAh g⁻¹), availability, low cost, environmental compatibility, and lower electrochemical motivation force (1.032 V vs. Li⁺/Li) than other transition-metal oxides [23–25]. However, the practical application of MnO is hindered by the significant volume changes exhibited by the material during lithiation/delithiation and its poor electrical conductivity. These volume changes lead to electrode cracking and pulverization, resulting in the loss of physical contact. Increased contact with the electrolyte owing to volume expansion promotes continuous side reactions. Furthermore, the poor conductivity inherent to transition-metal oxides impedes the kinetics of the electrode [26–30]. These impediments lead to poor cyclic stability and poor rate performance of MnO electrodes during cycling.

Various strategies have been developed to address these challenges, including the use of nanosized particles [31–34], surface modifications such as carbon and nitride coatings [35–37], and the development of MnO composite electrodes [38–41]. Among these strategies, downsizing particle size shortens the diffusion paths of Li⁺ ions and electrons, thereby improving the electrode kinetics. However, the increased surface area leads to more side reactions with the electrolyte, which limits the improvement in Li storage performance. Considering these side reaction issues, surface modifications such as coating offer significant advantages over particle downsizing methods. Surface modifications enhance cycle life by mitigating volume changes during lithiation/delithiation and preventing direct contact with the electrolyte, thus preventing a reduction in the reversible capacity because of side reactions. Additionally, the high conductivities of carbon and manganese nitride coatings improve the kinetics of the MnO electrode [42–44]. However, the practical application of surface modification is hindered by impediments identified in previous studies arising from the complexity of the solvent-involved processes such as hydrothermal process at high pressure and temperature and alcoholysis, which increase costs and leave residual impurities. Additionally, the use of toxic chemicals such as benzoic acid further complicates handling during the process [28,42].

Herein, we propose a simple method for synthesizing surface-nitrided MnO by heat-treating MnO powder with urea at 800°C in an autogenic reactor that was isolated from the outer atmosphere. MnO powder and the nitrogen precursor urea were combined in a homogeneous mixture without the use of a solvent, thus avoiding additional expensive processing steps. The omission of ammonia gas during the process also reduces the associated processing costs significantly. Ammonia generated from the decomposition of urea reacts with the surface of the MnO powder to form manganese nitride under autogenic pressure. The nitride formed on the MnO surface is robust and electrochemically inert, providing a stable coating layer. Therefore, the simple and efficient heat-treatment process produces a surface nitride layer that significantly enhances the electrochemical performance of conversion-type negative electrodes (such as MnO) for application in high-capacity LIBs.

Fig. 1 shows the XRD patterns of the prepared samples, including those of bare MnO, urea 16.6 wt%, urea 25.0 wt%, and urea 50.0 wt% urea. The XRD pattern of the bare MnO corresponds to a pure face-centered cubic MnO phase (JCPDS 75-1090). The XRD patterns of the samples heat-treated with urea in the autogenic reactor exhibited the same MnO diffraction peaks, suggesting that this synthesis method did not significantly alter the crystal structure of MnO. The XRD pattern of the urea 16.6 wt% sample closely resembles that of bare MnO, whereas a new diffraction peak emerges in the urea 25.0 wt% sample. This peak is more pronounced in the urea 50.0 wt% urea sample, which has the highest mass ratio of urea and corresponds to the Mn₂N_0.86 phase (JCPDS 71-0200). This suggests that during the heat-treatment process, the ammonia derived from urea reacts with the MnO powder, forming Mn₂N_0.86 on the surface of the MnO particles. Therefore, it seems that using urea as a nitrogen-containing precursor in the heat-treatment method enables the facile introduction of nitrides on the surface of the MnO powder without causing phase transition.

SEM images were used to assess the grain size and surface morphology of the bare MnO and the three heat-treated samples (Fig. 2). As shown in Fig. 2a, the bare MnO particles have sizes greater than 1 μm and feature flat, smooth surfaces. SEM analysis confirmed that the heat-treated samples have smaller particle sizes and feature rougher surface morphologies than bare MnO (Fig. 2b–d). These changes are attributed to the density difference between MnO (crystallographic density: 5.37 g cm⁻³) and Mn₂N_0.86 (crystallographic density: 6.56 g cm⁻³). During the nitriding process, oxygen desorbs from the surface of the MnO particles and combines with nitrogen to form Mn₂N_0.86, which has a 22% higher density, thereby reducing the volume. This volume reduction induced cracks in the particles, causing them to fragment into smaller particles. The surface morphology changes caused by this density difference have been reported in previous studies related to the nitridation of transition metal oxide surfaces through heat treatment using urea [30,35]. These surface changes become more pronounced as the urea mass ratio increases during the synthesis process, which is consistent with the XRD results. SEM analysis confirmed the formation of Mn₂N_0.86 on the surface of the urea 16.6 wt% sample; however, the Mn₂N_0.86 peak was not observed because of the small amount produced. In contrast, the two heat-treated samples with more pronounced surface changes exhibited XRD patterns in which the Mn₂N_0.86 peak began to appear in the urea 25.0 wt% urea sample and was clearly observed in the urea 50.0 wt% urea sample. Therefore, heat treatment using urea as an ammonia precursor effectively nitrides the surface of MnO, resulting in a reduction in particle size and a roughened surface. This improvement is attributed to the presence of the Mn₂N₀.₈₆ layer on the surface, which is anticipated to prevent side reactions by blocking direct contact with the electrolyte

Fig. 3 shows the initial voltage profiles of Li/MnO cells with the prepared bare MnO and heat-treated MnO electrodes under a constant current density of 25 mA g⁻¹ within the voltage range of 0.01–2.5 V (vs. Li/Li⁺). The bare MnO exhibits a lithiation capacity of 980 mAh g⁻¹ and a delithiation capacity of 628 mAh g⁻¹. The delithiation capacity is slightly lower than its theoretical capacity of 756 mAh g⁻¹, which might be attributed to its irreversible reaction [45]. In the lithiation curve, a plateau region around 0.25 V indicates the two-phase conversion reaction of MnO + 2Li⁺ + 2e^– → Mn + Li₂O. During the subsequent delithiation process, a plateau around 1.2 V corresponds to the reversible conversion reaction of Mn nanoparticles and amorphous Li₂O matrix back to MnO [46–48].

Compared with the heat-treated and bare MnO samples, the capacity decreased as the mass ratio of urea increased. This is attributed to the conversion of the active material, MnO, into electrochemically inert Mn₂N_0.86 by reaction with ammonia during heat treatment in the autogenic reactor. The sample with the lowest mass ratio of urea (16.6 wt%) showed a delithiation capacity of 610 mAh g⁻¹, similar to that of bare MnO, indicating that the amount of MnO consumed during surface nitridation was small. In contrast, the samples with higher urea mass ratios (25.0 wt% and 50.0 wt%) exhibited reduced delithiation capacities (588 mAh g⁻¹ and 563 mAh g⁻¹, respectively). This capacity reduction because of MnO consumption during heat treatment is consistent with the XRD patterns shown in Fig. 1, in which the Mn₂N_0.86 peak is clearly observed as the mass ratio of urea increases. As shown in Fig. 2, SEM observations confirm that the particle size of the heat-treated samples decrease because of the density difference between MnO and Mn₂N_0.86. This reduction in particle size increased the contact area with the electrolyte, leading to increase side reactions during lithiation. As noted previously, the nitride layer formed on the surface suppresses side reactions by preventing direct contact with the electrolyte. The 16.6 wt% sample, with insufficient nitride coverage, showed a lithiation capacity similar to that of bare MnO owing to increased side reactions, despite the loss of MnO that occurs during heat treatment. In contrast, the more extensively nitrided 25.0 wt% and 50.0 wt% samples show a further decrease in lithiation capacity depending on the degree of nitridation. This reduction in the lithiation capacity is attributed to the higher coverage of the formed nitride layer on the surface suppressing side reactions, resulting in the loss of MnO during heat treatment, which has a more pronounced effect on the lithiation capacity.

Fig. 4 shows the cycle performance and normalized capacity of Li/MnO cells with bare MnO and heattreated MnO electrodes over 100 cycles at a current density of 25 mA g⁻¹. As shown in Fig. 4a, bare MnO electrode exhibits the highest initial capacity of 523 mAh g⁻¹, but its reversible capacity decreases sharply to 206 mAh g⁻¹. Although the heat-treated samples show lower initial capacities than bare MnO, owing to the MnO consumption during their synthesis, they demonstrate distinctively superior cyclability. This improvement is attributed to the nitride coating layer acting as a buffer to mitigate the significant volume changes of MnO during the lithiation/delithiation cycles and prevent continuous side reactions with the electrolyte, thereby mitigating the loss of reversible capacity. Furthermore, the high conductivity of the nitrided layer enhances the kinetics of the electrode by reducing the surface charge transfer resistance, thereby improving the cyclability.

A comparison of the normalized capacity of each cell further highlights the differences in cyclability (Fig. 4b). The bare MnO sample, with its rapid capacity decline, exhibits an extremely low capacity retention of approximately 40% after 100 cycles. The urea 16.6 wt% sample, with the lowest degree of nitridation among the heat-treated samples, maintained a slightly higher capacity retention than bare MnO after 95 cycles. In contrast, the samples with higher mass ratios of urea (25.0 wt% and 50.0 wt%) exhibit significantly better cyclability, with capacity retentions of about 70% and 80%, respectively, after 100 cycles. The urea 50.0 wt% sample showed a region of increasing capacity owing to the reversible return of a portion of the polymeric gel-like film during delithiation. This polymeric gel-like film is formed from the degradation of kinetically activated electrolytes during lithiation [49].

The rate capability of Li/MnO cells with bare MnO and heat-treated MnO electrodes at different current densities of 25 mA g⁻¹, 50 mA g⁻¹, 100 mA g⁻¹, 250 mA g⁻¹, 500 mA g⁻¹, and 1000 mA g⁻¹ is shown in Fig. 5a. Although the urea 25.0 wt% and 50.0 wt% samples exhibit lower initial specific capacities than the bare MnO sample, they demonstrate higher specific capacities at high current densities of 500 mA g⁻¹ and 1000 mA g⁻¹. Notably, the urea 50.0 wt% sample, which shows the lowest initial discharge capacity of 563 mAh g⁻¹, achieves a capacity of 186 mAh g⁻¹ at a current density of 1000 mA g⁻¹, indicating the highest rate capability among the samples. Normalization of capacity for Li/MnO cells at various current densities clearly reveals the superior rate capability of the urea 25.0 wt% and 50.0 wt% samples (Fig. 5b). These two samples demonstrate improved capacity retention compared with the bare MnO and the urea 16.6 wt% sample throughout the range from low to high current densities. When the current density is reverted from 1000 mA g^–1 to a lower density of 25 mA g⁻¹, the bare MnO sample retains 83% of its initial capacity, whereas the urea 50.0 wt% sample retains 85%, demonstrating better capacity retention. This superior rate capability is attributed to the high conductivity of the Mn₂N_0.86 layer on the surface, which enhances the kinetics of the MnO electrode.

In conclusion, the heat-treated samples, particularly the urea 50.0 wt% sample, exhibit significantly improved cycle performance and rate capability, owing to the formation of a highly conductive Mn₂N_0.86 layer on the MnO surface, which enhances electrode surface stability and improves the kinetics of the electrode.

Surface-nitrided MnO electrodes were developed by mixing MnO powder with urea in different mass ratios and subjecting the mixture to heat treatment in an autogenic reactor. During heat treatment, ammonia generated from the decomposition of urea reacts with the surface of the microsized MnO, resulting in nitridation. The XRD analysis and SEM observations confirmed the formation of Mn₂N_0.86 on the surface. In Li/MnO halfcell tests, the surface-nitrided 50.0 wt% sample exhibited superior cycle performance and rate capability compared with the bare MnO sample. This improvement is attributed to the Mn₂N_0.86 layer acting as a buffer to accommodate volume changes, thereby preventing electrode pulverization and blocking direct contact with the electrolyte. Additionally, the high electronic conductivity of the material reduces the charge-transfer resistance, which enhances the kinetics of the MnO electrode and results in an excellent cycle performance and rate capability. We expect the results of this study to provide insights into surface nitriding methods using simple heat treatment and facilitate the application of conversion-type negative electrodes (such as MnO) in high-capacity LIBs.