Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ as the high performance cathode materials for lithium-ion batteries

Abstract

The preparation of Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials by sintering the SiO₂-coated nickel-manganese oxides with lithium salts using abundant and low-cost sodium silicate as the silicon source was reported. The samples were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was found that a uniform and complete SiO₂ coating layer could be obtained at a suitable pH value of 10, which transformed to a good Li₄SiO₄ coating layer afterwards. When used as the cathode materials for lithium-ion batteries, the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ samples deliver a better electrochemical performance in terms of the discharge capacity, rate capability, and cycling stability than that of the pristine material. It can still deliver 111.1 mAh/g at 20 C after 300 cycles, with a retention ratio of 93.1% of the stable capacity, which is far beyond that of the pristine material (101.3 mAh/g, 85.6%).

Keywords

lithium-ion batteries / cathode material / LiNi_0.5Mn_1.5O₄ / lithium-ion conductor / coating

Introduction

Since its first introduction into the market in 1991, lithium-ion batteries (LIBs) have been rapidly developed and extensively applied in portable electronic devices, electric vehicles and grid storage. However, the present LIBs cannot fulfill the increasing demands of these applications for high energy and power density. Thus, development of high-potential materials and high-capacity cathode materials are imperative. Among the existing cathode materials used in LIBs, the spinel LiNi_0.5Mn_1.5O₄ material has a stable discharge potential plateau as high as ~4.7 V (vs. Li/Li⁺) and a theoretical specific capacity of 147 mAh/g, as well as good rate capability benefiting from its three dimensional channels for Li⁺ transportation [1,2], which could make LIBs have a high energy density and a high power density.

Although possessing these advantages, the LiNi_0.5Mn_1.5O₄ material suffers from severe capacity fading, particularly at elevated temperatures (50°C–60°C), which is triggered by the side reactions at the electrode/electrolyte interface [3,4] and the presence of Mn³⁺ in the crystal structure, leading to structural deterioration and dissolution of Mn [5,6]. In this regard, surface coating [7–9], lattice doping [10,11], the novel synthesis method [12–14] and novel electrolyte additives [15,16] are effective methods to improve the cycling performance. Of these strategies, surface coating is regarded as the most effective route to overcome the above drawbacks by achieving a relatively stable interface. Various coating materials of metal oxides including Al₂O₃ [17], Bi₂O₃ [17,18], ZnAl₂O₄ [7] and La_0.7Sr_0.3MnO₃ [19] have been proposed to improve the electrochemical performance. However, most of these coating materials are insulators to lithium ions, which have a detrimental influence on the high-rate performance due to their poor Li⁺ transport ability at the interface. Therefore, an excellent coating material should facilitate the transfer of lithium ions or electrons. Recently, lithium-ion conductors [9,20–22], especially the lithium-based silicate with a good ionic conductivity [23,24], have been of increasing interest as a coating material for the purpose of stabilizing the electrode/electrolyte interface and facilitating fast transfer of lithium ions at the interface, thus enhancing the cycling performance and rate capability. For examples, Deng et al. reported the LiNi_0.5Mn_1.5O₄ material coated with 3%(wt) LiAlSiO₄ was prepared by the sol-gel method, which exhibited a high capacity retention ratio (94.3%) of the initial capacity after 150 cycles at 55°C higher than that of the uncoated material (76.5%) [9]. Zhang’s group synthesized the LiFePO₄ material coated with Li₄SiO₄, which showed a specific capacity of 98 mAh/g at 5 C larger than that of pristine material because of the increased Li⁺ conductivity [24]. The above results suggest that lithium-based silicate coating layers not only enhance the capacity retention, but also improve the discharge capacity and rate performance. However, the conventional coating methods have similar issues of complex process and high cost [9,25]. Thus, it is imperative to develop an efficient and low-cost coating method for the LiNi_0.5Mn_1.5O₄ materials with Li₄SiO₄ coating layers.

Hence, in this work, a facile and economic approach was designed and developed for the preparation of the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials. First, SiO₂ was synthesized and covered on the nickel-manganese oxides (Ni-Mn-O) by hydrolysis and condensation of sodium silicate. Then, the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials were obtained through a synchronous lithiation into the SiO₂ and Ni-Mn-O materials. During the synchronous lithiation, part of Si⁴⁺ can migrate and incorporate into the crystal structure, which will improve the structural stability of the host LiNi_0.5Mn_1.5O₄ phase and the binding of Li₄SiO₄ surface layer with the host structure. The electrochemical results reveal that the Li₄SiO₄ coating layer can effectively reduce side reactions and charge transfer resistances at the electrode/electrolyte interface, thereby contributing to the improved cycling performance and rate capability.

Experimental methods

Material preparation

Synthesis of NMO@SiO₂-n (n=09, 10 and 11) materials

First, nickel-manganese oxides (Ni-Mn-O) were synthesized after the thermal treatment of the mixture containing Mn(CH₃COO)₂·4H₂O and Ni(CH₃COO)₂·4H₂O with a molar ratio of 3:1 at 450°C for 5 h. Then, 0.5 g of Ni-Mn-O was dispersed and stirred vigorously in the solution (with a water and ethanol ratio of 7:1 in volume) under 85°C. 20 mL of 0.025 mol/L sodium silicate (Na₂SiO₃·9H₂O) solution was added dropwise to the Ni-Mn-O suspension and the pH value of the reaction system was controlled at 09, 10 and 11, respectively, by adjusting the flow rate of ammonia solution. The 3% (wt) SiO₂-coated Ni-Mn-O samples obtained were denoted as NMO@SiO₂-n (n=09, 10 and 11), where n represents the pH of the reaction system.

Synthesis of HV@LS-n (n=09, 10 and 11) materials

The Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials were synthesized by the solid-state reaction of the NMO@SiO₂ materials and the excess 10% of LiCH₃COO·2H₂O using a step procedure: at 500°C for 5 h, then at 900°C for 10 h and at last at 700°C for 10 h in air. The Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials derived from NMO@SiO₂-n were denoted as HV@LS-n (n=09, 10 and 11), where n represents the pH value of the reaction system. For comparisons, as is reported [26], the pristine LiNi_0.5Mn_1.5O₄ material (denoted as Pristine) was synthesized by a two-step solid-state method with lithium acetate, manganese acetate and nickel acetate.

Physical characterization

The crystal structure and phase analyses of the prepared materials were conducted by an X-ray diffractometer (PANalytical, X’pert Pro) employing Cu Ka radiation in the range of 10^o–80^o. The morphology, microstructure and elemental mapping of the prepared materials were characterized using a scanning electron microscopy (SEM, JEOL JSM-7800F) and a transmission electron microscopy (TEM, JEOLJEM-2100).

Electrochemical performance tests

Cell assembly

The electrochemical performances of the Pristine and HV@LS-n cathode materials were evaluated with standard CR2016 coin cells. The cathode was fabricated by blending the as-prepared cathode materials, acetylene black, and polyvinylidene fluoride with a weight ratio of 80:10:10 in the solvent of N-methyl-2-pyrrolidone. Then, the slurry obtained was cast onto an aluminum foil and dried overnight at 58°C in a vacuum. The cathode was used as the working electrode, and the lithium electrode was employed as both the counter and reference electrodes. With the Celgard 2400 membrane used to separate the LNMO-R cathode and the lithium metal anode, CR2016 type coin cells were assembled in an argon-filled glove box with the LB315 electrolyte (1 mol/L LiPF₆ solution in a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate with a weight ratio of 1:1:1, Guotaihuarong Co., Zhangjiagang, China).

Electrochemical measurements

The charge-discharge tests were performed by galvanostatically charging and discharging in the voltage range of 3.5–5.0 V on a battery test system (LANHE, CT2001A). The rate capability was assessed with the cell charged at 1 C and discharged at various rates from 1 C to 40 C, and then back to 1 C, where 1 C corresponds to 147 mA/g, in which the active material loadings of all electrodes are 1.60–2.40 mg/cm². The high-temperature (55°C) cycling performance at 1 C and the high-rate cycling performance charged at 1 C and discharged at 20 C were also evaluated. The electrochemical impedance spectra (EIS) were conducted on Princeton MC with AC amplitude of 5 mV in the frequency range of 100 kHz–10 mHz. All the measurements were performed at room temperature.

Results and discussion

The XRD patterns of the as-prepared Ni-Mn-O and NMO@SiO₂-n (n=09, 10 and 11) materials are shown in Fig. 1. After thermal-treatment of nickel acetate and manganese acetate, the mixtures of Ni_xMn_yO_z phases [27] are obtained for the Ni-Mn-O sample. The NMO@SiO₂-n (n=09, 10 and 11) samples have diffraction peaks similar to those of the Ni-Mn-O sample but have no distinct peak of SiO₂ [28,29], probably because of its low content or amorphous phase. The ICP results of Ni-Mn-O, NMO@SiO₂-09, NMO@SiO₂-10, and NMO@SiO₂-11 materials show that the Mn/Ni molar ratio is 3.01:1, 2.82:1, 2.93:1 and 2.80:1, respectively, indicating the dissolution of Ni-Mn-O, especially the Mn element into the ammonia solution. Compared to the pristine Ni-Mn-O material, the NMO@SiO₂-09 and NMO@SiO₂-11 samples have broaden and weak diffraction peaks, which may be caused by the structural degradation derived from the dissolution of Ni-Mn-O in the ammonia solution. The sharp and strong peaks of the NMO@SiO₂-10 sample are indicative of well-developed crystal structures, probably because the formation of a complete and dense SiO₂ coating layer protects Ni-Mn-O from dissolving in the ammonia solution.

To the best of the authors’ knowledge, pH plays an important role in the formation of the NMO@SiO₂-n materials during the reaction processes. As indicated in Fig. 2(a), the pristine Ni-Mn-O material is composed of well-dispersed nanoparticles with smooth surfaces and clear outlines. The particle sizes of NMO@SiO₂-09 and NMO@SiO₂-11 samples (Fig. 2(b) and Fig. 2(d)) prepared under basic conditions (pH=09 and 11) are smaller than those of the uncoated sample. Besides, a large amount of small particles with the size of 30 nm are obtained even in the NMO@SiO₂-11 sample, which is caused by the dissolution of Ni-Mn-O in ammonia solution. Compared with NMO@SiO₂-09 and NMO@SiO₂-11 samples, the morphology of the NMO@SiO₂-10 sample (Fig. 2(c)) is similar to that of the pristine Ni-Mn-O material, probably because the formation of a dense and complete silica layer can effectively avoid the dissolution of Ni-Mn-O. It can be concluded that pH plays an important role in the formation of silica coating. The optimal pH value is of great benefit to construct a uniform and complete silica coating on the Ni-Mn-O materials, which in turn, avoids the structural degradation derived from the dissolution of Ni-Mn-O particles.

Figure 3 presents the SEM image of NMO@SiO₂-10 and the corresponding elemental mappings of Mn, Ni, and Si. As is observed from Fig. 3(b)–(d), the elements of Mn, Ni, and Si are distributed uniformly within NMO@SiO₂-10. Combined with the SEM images (Fig. 3(e) and (f)) with high resolution, it can be inferred that a uniform and complete SiO₂ coating layer is formed on the surface of Ni-Mn-O. Therefore, it is feasible to prepare the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials with uniform distributions of elements via the SiO₂-coated Ni-Mn-O materials.

To form a complete and homogeneous SiO₂ coating, good dispersion of Ni-Mn-O nanoparticles is one of the vital prerequisite conditions. At a suitable pH value [30], the Ni-Mn-O nanoparticles can be well-dispersed in the aqueous solution via the electrostatic stabilization. The Ni-Mn-O suspension shows a negative zeta potential of ‒26.2 mV when measured at pH 10.7, which indicates that a number of OH ions accumulate on the surface and form an electric double layer [31]. Within the pH range of 10–11, the Ni-Mn-O suspension is considered to be well dispersed, which is beneficial for the fabrication of a complete and uniform SiO₂ coating.

Herein, silica is formed by the hydrolysis of sodium silicate and the following condensation process. The mechanism of silica coating in alkaline solution is as follows. As Eq. (1) describes, sodium silicate is hydrolyzed to form small-sized H₂SiO₄²⁻ and H₃SiO₄⁻ [32] micelles, which can be anchored on the surface of Ni-Mn-O particles by interacting with OH⁻ groups. Meanwhile, the condensation reaction (Eq. (2)) also occurs [32–34]. As a result, silica sol is formed and coated on Ni-Mn-O surfaces. Afterwards, the silica sol loses water molecule and condenses via Si-O-Si bonding (Eq. (3)), leading to the formation of a SiO₂ layer covered on the Ni-Mn-O material. To the authors’ knowledge, pH has a great influence on the hydrolysis and condensation of sodium silicate. An appropriate value of pH is also essential to form a uniform and complete SiO₂ coating. At a low pH value of 9, sodium silicate can be rapidly hydrolyzed and a number of siliceous micelles are quickly formed and anchored on Ni-Mn-O particles. The formed silica sol is usually aggregated at some regions of Ni-Mn-O particles and transforms to an island-like coating layer. As pH is increased from 9 to 11, the hydrolysis and condensation rates of sodium silicate are much lowered, resulting in the formation of less siliceous micelles and silica sol on the surface of the Ni-Mn-O particles. Moreover, the silica framework has a significantly increased solubility and could partially dissolve back into the strong alkali solution [35]. Therefore, it is also difficult to make a homogenous and complete coating under a strong basic condition of pH 11. The pH value of 10 is the appropriate condition for preparing Ni-Mn-O materials with a homogenous and condensed SiO₂ coating, which has been confirmed by the results from Figs. 1 and 2.

The XRD patterns of the as-prepared Pristine and HV@LS-n (n=09, 10 and 11) materials are depicted in Fig. 4. All the XRD patterns can be indexed as the spinel LiNi_0.5Mn_1.5O₄ phase with typical intensive peaks, such as (111), (311), and (400). There are no distinct reflections of Li₄SiO₄ visible in the patterns of HV@LS-n (n=09, 10 and 11) samples, perhaps due to its low coating content or amorphous phase. The peaks slightly shift to larger degrees, indicating that some Si⁴⁺ with a smaller ion radius may be doped into the spinel structures [9], which is beneficial for improving the structural stability of LiNi_0.5Mn_1.5O₄. It can be clearly observed that the HV@LS-09 and HV@LS-11 samples have weak peaks at ~37^o and 43.7^o ascribed to the common impurity phase Li_xNi_1-xO (x ≈ 0.2) in LiNi_0.5Mn_1.5O₄ [36], which may be related to two issues—degradation of the Ni-Mn-O particles and oxygen release from the crystal lattice during the process of high temperature synthesis. The less Li_xNi_1-xO impurities in the Pristine and HV@LS-10 sample are considered to be only related with the oxygen release.

Figure 5 compares the SEM images of the Pristine and HV@LS-n (n=09, 10 and 11) samples. As shown in Fig. 5(a), the Pristine sample possesses the morphology of polyhedral blocks with a smooth surface and a particle size between 0.8mm and 1.5 mm. The HV@LS-09 and HV@LS-11 sample (Fig. 5(b) and (d)) are both composed of irregular blocks with the size of 1–3mm. It can be observed that the HV@LS-09 sample has a tendency to evolve into the octahedral shape of spinel materials. A number of small fragments (100–400 nm) are aggregated around the micron particles of the HV@LS-11 sample, perhaps resulting from the degraded crystal structure and destroyed morphologies of Ni-Mn-O substrates (as the EDS results shown in Fig.A1). The fragments formed have a negative effect on the electrochemical performance of the HV@LS-11 material. Figure 5(c) reveals that the HV@LS-10 sample has an octahedral morphology with the largest particle size of 1.5–2.5mm, indicating its well-developed crystal structure and good crystallinity. Compared to the Pristine material, the HV@LS-n (n=09, 10 and 11) samples have rough surfaces, indicating the presence of Li₄SiO₄ coating layer. However, the HV@LS-10 sample has a compact surface, suggesting that a good Li₄SiO₄ coating layer may be formed. Thus, the HV@LS-10 sample is expected to obtain an excellent electrochemical performance.

The TEM image of the HV@LS-10 material is displayed in Fig. 6. It can be clearly observed that a coating layer (S1) is well coated on the surface of LiNi_0.5Mn_1.5O₄ particles (S2), which indicates that the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials are successfully prepared by the SiO₂-coated Ni-Mn-O intermediates. The Li₄SiO₄ coating layer is expected to improve the cycling performance and rate capability of LiNi_0.5Mn_1.5O₄ materials.

The charge and discharge profiles of the Pristine and HV@LS-n (n=09, 10 and 11) samples are presented in Fig. 7. In the charging process, all the four cathode samples exhibit a short plateau around 4.0 V, which can be easily identified in the discharge profiles, then followed by two long and flat plateaus attributed to the oxidation reactions of Ni²⁺ to Ni⁴⁺ [37] in the region of 4.60‒4.80 V. The potential plateau at 4.0 V ascribed to the redox reactions of Mn³⁺/Mn⁴⁺ indicates that the HV@LS-n (n=09, 10 and 11) materials are of Fd–3m structure, which is in good agreement with the results of XRD patterns. From the small voltage difference between charge and discharge plateaus around 4.7 V, it can be inferred that the HV@LS-10 sample has a small electrochemical polarization and an excellent electrochemical reversibility. The large polarizations for the HV@LS-09 and HV@LS-11 samples could have been originated from the negative inherent properties stemming from the NMO@SiO₂ materials with a low crystallinity and a degraded crystal structure. The Pristine, HV@LS-09, HV@LS-10 and HV@LS-11 samples delivered discharge capacities of 121.3, 112.4, 122.4 and 83.7 mAh/g, respectively. Electrochemical inactive components and possible crystal defects resulting from the degraded Ni-Mn-O particles are believed to be the main causes for their lowest capacities of the HV@LS-09 and HV@LS-11 samples. As the ICP results mentioned above, serious dissolutions occur in the NMO@SiO₂-09 and NMO@SiO₂-11 precursors, which would result in much electrochemical inactive Li_xNi_1-xO impurities, lattice distortion, and crystal defects as well as the low accessible capacity and poor rate capability for the HV@LS materials. It should be noted that the HV@LS-10 sample has similar specific capacities to those of the pristine LiNi_0.5Mn_1.5O₄ material without being influenced by the Li_xNi_1-xO impurities and the inactive Li₄SiO₄ coating.

The high-rate cycling performance at 20 C of the prepared cathode materials are investigated and presented in Fig. 8. To optimize the utilization of the active materials, the half cells are charged and discharged at 1 C in the first three cycles, and then charged at 1 C and discharged at 20 C in the following cycles. It can be observed that both of the Pristine and HV@LS-11 samples deliver relatively low specific capacities at the beginning, which gradually increase to a stable and maximum value afterwards. The low accessible capacities in the initial cycles are supposed to be caused by the agglomerated structure (Fig. 5(a)) and high contents of inactive components, which lead to large obstacles for Li⁺ transfer and thereby make insufficient active materials participate in the redox reactions. In contrast, the highly reactive HV@LS-09 and HV@LS-10 samples deliver maximum capacities in the initial cycles. The Pristine, HV@LS-09, HV@LS-10, and HV@LS-11 samples deliver specific capacities of 118.4, 92.3, 119.3, and 80.8 mAh/g at the 20th cycle discharged at 20 C, respectively. After 300 charge-discharge cycles, the discharge capacities are reduced to 101.3, 74.3, 111.1 and 75.1 mAh/g, maintaining 85.6%, 80.5%, 93.1% and 92.9% of that at the 20th cycle, respectively. Derived from partially dissolved Ni-Mn-O material, the incompletely developed HV@LS-09 particles may have latent structural instability, which leads to the fast capacity fading. The lowest accessible capacity of the HV@LS-11 sample may be caused by the existence of inactive phases. In order to further investigate the role of the Li₄SiO₄ coating layer in improving the cycling stability of LiNi_0.5Mn_1.5O₄, the cycling property of the Pristine and HV@LS-10 materials at an elevated temperature of 55^oC were also investigated. As shown in Fig. 8(b), at 55^oC, the HV@LS-10 material delivers a stable specific capacity of 135.6 mAh/g and maintains a capacity ratio of 91.2% after 150 cycles at 1C rate, higher than that of the Pristine sample (130.7 mAh/g, 89.7%). Of all the samples, the HV@LS-10 sample with a good Li₄SiO₄ coating exhibits largest capacities and best cycling performance. Although the Li₄SiO₄ coating reduces the theoretical capacity of the cathode sample, Figs. 7 and 8 demonstrate that the HV@LS-10 sample obtains larger specific capacities than the Pristine material, perhaps benefiting from the enhanced conductivity and interface stability.

Figure 9 displays the rate capability of the as-prepared cathode materials. The HV@LS-09 and HV@LS-11 samples exhibit lower accessible capacities and a fast capacity decay at high rates, which are perhaps related to the unstable crystal structure and electrochemical inactive phases. At moderate rates (<10 C), the Pristine and HV@LS-10 samples deliver similar specific capacities. As the discharge rates (>10 C) increase, the HV@LS-10 sample exhibits a superior rate capability to the Pristine material. The HV@LS-10 sample achieves 114.8 mAh/g at 20 C, which is 93.7% of the accessible capacity at 1 C. The excellent rate capability can be attributed to its well-developed crystal structure and good Li₄SiO₄ coating, which facilitate the fast transportation of Li⁺. It can be concluded that a good Li₄SiO₄ lithium-conducting coating is effective to enhance the rate performance of the LiNi_0.5Mn_1.5O₄ material.

The as-prepared HV@LS-10 material exhibits excellent electrochemical performance, which is comparable to most results reported in previous literature. To get a further understanding of the mechanism of Li₄SiO₄ coating, electrochemical impedance spectra were performed on the half-cells of the Pristine and HV@LS-10 cathodes after 100 and 300 cycles. As shown in Fig. 10(a) and (b), each of the impedance spectra includes three parts: a semicircle in the high-frequency region reflecting Li⁺ migration process through the surface layer; a semicircle in the medium-to-low frequency region referring to the charge-transfer process, and a sloping line at very low frequencies associated with Li⁺ diffusion in the bulk material [38], which matches the equivalent circuit model shown in Fig. 10(c). It can be noted in Fig. 10(c) that the HV@LS-10 sample has smallerR_f values and a smaller variation from the 100th to 300th cycle than those of the Pristine material. This indicates that the side reactions at the LiNi_0.5Mn_1.5O₄ electrode/electrolyte interface have been effectively inhibited by the Li₄SiO₄ coating layer in the HV@LS-10 cathode, which accounts for the improved cycling performance. Similar toR_f values, the R_ct values of the HV@LS-10 sample are approximately equal during the past 200 cycles and significantly smaller than those of the Pristine sample. The stable interface structure and excellent charge-transfer property of the HV@LS-10 sample are favorable for insertion/extraction of Li⁺, thereby contributing to the better rate performance than the pristine one [39].

Conclusions

In summary, the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials were successfully prepared by the SiO₂-coated Ni-Mn-O precursors using abundant and low-cost sodium silicate as the silicon source. The pH value plays a significant role in the formation of SiO₂ coating layer on the Ni-Mn-O surfaces, and thereby exerts great effects on the physical and electrochemical properties of the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ materials. A uniform and complete SiO₂ coating layer can be formed on the surface of Ni-Mn-O at pH 10, which transforms to a good Li₄SiO₄ layer coated on the LiNi_0.5Mn_1.5O₄ material afterwards. Impedance analysis indicates that the Li₄SiO₄ coating dramatically reduces side reactions and the charge-transfer resistances at the electrolyte/electrode interface. When used as cathode materials of lithium-ion batteries, the Li₄SiO₄-coated LiNi_0.5Mn_1.5O₄ exhibits enhanced electrochemical performance in terms of the discharge capacity, rate capability and cycling stability. It can still deliver 111.1 mAh/g at 20 C after 300 cycles, with a retention ratio of 93.1% of the stable capacity, which is far beyond 101.3 mAh/g (85.6%) of the pristine LiNi_0.5Mn_1.5O₄ material.

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