Enhanced Electrochemical Performance of LaMnO₃ Nanoparticles by Ca/Sr Doping

Abstract

In this paper, the sol-gel method was used to synthesize powders of LaMnO₃ (LMO), La_0.85Ca_0.15MnO₃ (LCM), and La_0.85Sr_0.15MnO₃ (LSM). The effect of substituting Ca and Sr at the A-site on the perovskite crystal structure and electrochemical capabilities of LMO was investigated. LCM retained its orthogonal structure in comparison to the parent LMO components, whereas LSM transitioned to a rhombic structure. At 0.5 A/g, the specific capacitance of LCM and LSM electrodes is 185.5 F/g and 248 F/g, respectively. The specific capacitance of LCM was three times greater than that of the LMO electrode. Among the three samples (LMO, 22.25 m² g⁻¹; LSM, 31.56 m² g⁻¹), the LCM sample exhibited the highest specific surface area of 38.79 m² g⁻¹. The charge transfer resistances of the LMO, LCM, and LSM are 0.48 Ω, 0.36 Ω, and 0.38 Ω, respectively. The LCM electrode exhibits the greatest capacitance performance due to its more refined morphology, increased concentration of oxygen vacancy, and more complete utilization of the perovskite bulk structure. The above results demonstrate that Ca or Sr substitution of A-site compounds has great potential for supercapacitor applications.

1. Introduction

With the increasing depletion of traditional energy sources, it is urgent to find effective energy storage equipment [1]. Supercapacitors have gained increased attention in recent times because of their rapid charging, impressive power density, and extended lifespan. Nonetheless, the practical applications of supercapacitors are restricted due to their limited energy density. Consequently, numerous investigations were carried out to enhance the energy density of supercapacitors [2,3,4]. The energy storage performance of a supercapacitor depends on the electrode materials [1,5]. Usually, there are three categories of electrode materials, specifically carbon substances, conductive polymers, and transition metal oxides [5,6,7,8]. Transition metal oxides, in comparison to carbon materials, exhibit a greater energy density and superior cycling stability when compared to conductive polymers. Consequently, transition metal oxide materials are widely used as electrode materials for supercapacitors [8,9,10,11,12,13,14].

LaMnO₃ is a perovskite material with an ABO₃ structure, belonging to the space group Pm-3m. In this structure, La is located at the A site in the cubic cell center, while Mn is situated at the B site in the octahedral center. Mn is connected to six oxygen ions in a coordinated manner [7,13,15]. The perovskite compounds exhibit minimal B-site cation defects and interstitial oxygen [16], unlike perovskite materials containing B atoms (e.g., manganese or titanium), which possess a significant number of imperfections in the A-site ions while upholding a consistent structure [17]. Thus, A-site cation defects and oxygen anion vacancies are the main defects in perovskite materials. Pseudocapacitance relies on oxygen vacancies within perovskite as locations for storing charges [18]. The oxygen vacancies can be determined using neutron powder diffraction and X-ray photoelectron spectrometry [19,20,21,22]. According to the findings, it is primarily the pseudocapacitance resulting from the intercalation of oxygen anion that contributes significantly to the high specific capacitance of perovskite materials [1]. The performance of pseudocapacitance is not solely dependent on the presence of oxygen vacancy but also on the pathway of electron transfer or electron conductivity [23]. Hence, enhancing the concentration of oxygen vacancies can optimize the specific capacitance of perovskite materials [24,25].

The structural distortion of the perovskite occurs due to the partial compensation of the charge imbalance caused by the Mn³⁺ ion, as stated by the Jahn–Teller effect [26]. The arrangement of perovskite’s molecular structure was found to significantly affect the concentration of oxygen vacancies, the diffusion of O²⁻, and the resulting electrochemical capabilities. Wang et al. investigated LSM as an electrode material for supercapacitors and found that the specific capacitance of LSM electrodes was higher (205 F/g) than that of LMO (178 F/g) at the same scan rate [5]. The review article authored by Yatoo et al. highlighted the extensive research conducted on LSM in the context of solid-oxide batteries. One prominent issue identified in these batteries is the occurrence of Sr separation, which leads to a gradual deterioration in performance over the course of time [27]. According to the findings of Lang et al., the electrochemical performance of La_1−xSr_xMnO₃ was improved by the presence of a loosely organized granular structure, as reported in their study [25]. In their study, Mo et al. showed that the replacement of Ca²⁺ ions significantly increased the specific capacitance of LaMnO₃, with a maximum value of 170 F/g [7]. It is important to mention that the radius of Ca ion (r_Ca2+ = 0.99 Å) is slightly less than the radius of La ion (r_La3+ = 1.06 Å), whereas the radius of Sr ion (r_Sr2+ = 1.13 Å) is comparatively greater than that of La ion. Consequently, the incorporation of Ca and Sr elements has distinct impacts on both the composition and electrochemical properties of LaMnO₃. Several studies have reported the structural development and electronic behavior of La_1−xM_xMnO₃ (x = 0, 0.15; M = Ca, Sr) [28,29]. However, there is a lack of discussion regarding the electrochemical performance, which is closely associated with the assessment of the structure.

In this work, LaMnO₃, La_0.85Ca_0.15MnO₃, and La_0.85Sr_0.15MnO₃ (referred to as LMO, LCM, and LSM, respectively) samples were prepared by the sol-gel method. The effect of substituting Ca and Sr at the A-site on the perovskite crystal structure and electrochemical capabilities of LMO was investigated. At 0.5 A/g, the specific capacitance of LCM and LSM electrodes is 248 F/g and 185.5 F/g, respectively.

2. Experimental Section

2.1. Materials Preparation

LaMnO₃, La_0.85Ca_0.15MnO₃, and La_0.85Sr_0.15MnO₃ powders were synthesized using the sol-gel method. No additional purification was necessary, as the chemicals and solvents employed were of analytical quality. Initially, La(NO₃)₃·6H₂O, MnCl₂, Sr(NO₃)₂, and Ca(NO₃)₂·4H₂O were measured based on stoichiometry and combined with a suitable quantity of citric acid in 30 mL of deionized water. To acquire a transparent solution, the solution that had been prepared was placed in a magnetic mixer and agitated for a duration of 24 h. The transparent solution was dehydrated in a rapid oven at 80 °C for 72 h. Ultimately, the dehydrated specimens were positioned in a furnace and subjected to a temperature of 700 °C with a gradual increase of 5 °C per minute. The specimens were subsequently exposed to atmospheric conditions for a duration of 2 h, resulting in the production of a powdered sample exhibiting a darkened visual appearance.

2.2. Characterization

The samples that were prepared underwent analysis using X-ray powder diffraction (XRD, Bruker D8 Advance, Karlsruhe, Germany) to determine their phases and crystal structure. The analysis was conducted with the scanning range set at 20–60°, a scanning speed of 6°/min, and a step size of 0.02. The structure of the samples that were prepared was analyzed using field emission scanning electron microscopy (Merlin Compact, Carl Zeiss, Jena, Germany) and transmission electron microscopy (TEM, 2100F, JEOL, Shoshima City, Japan). The samples’ synthesis temperature was determined by employing a thermal analysis system called Diamond TG/DTA, manufactured by Perkin Elmer S.A. (Waltham, MA, USA). The specimens underwent testing with 5 mg and were subjected to heating up to 800 °C at a rate of 15 degrees Celsius per minute in the presence of air. X-ray photoelectron spectrometry (XPS) was used to analyze the elemental composition and chemical valence of the samples using Thermo Scientific NEXSA. Measurements of the specific surface area were conducted using a tester specifically designed for this purpose (BET, ASAP 2020, Micromeritics, Norcross, GA, USA).

2.3. Electrochemical Measurements

To create the electrodes, a mixture of the sample (80% by weight), acetylene black (10% by weight), and polyvinylidene fluoride (PVDF, 10% by weight) was combined with N-methylpyrrolidone (NMP) and stirred for a duration of 24 h. Subsequently, the homogeneous solution was applied to carbon paper and subjected to heating at 80 °C in a vacuum oven for 24 h, resulting in the formation of the electrodes. An electrochemical workstation (CHI660E) was used to test the electrodes, employing a three-electrode setup with a Hg/HgO reference electrode and a 3 M KOH mixture as the electrolyte. The testing included the examination of electrochemical impedance spectra (EIS), cyclic voltammetric curves (CV), and constant current charge/discharge point curves (GCD).

3. Results and Discussion

To determine the calcination temperature of the LMO, LCM, and LSM samples, the LaMnO₃ precursors underwent thermal gravity analysis (TG) in an air atmosphere. The analysis was conducted within a temperature range of 50–800 °C, with a heating rate of 15 °C/min. Figure 1a illustrates the process. At 250 °C, the precursor’s mass reduces by 20%, primarily because of the organic solvents volatilizing and gases and moisture evaporating from the specimen. A significant reduction in the mass of the precursor material was observed within the temperature range of 300 to 600 °C. Beyond 600 °C, the weight remained relatively stable. Consequently, a calcination temperature of 650 °C was determined to be appropriate for the LMO precursors. Subsequent XRD analysis indicated the absence of synthesized LMO, prompting an increase in the calcination temperature to 700 °C. This adjustment resulted in subsequent XRD patterns that confirmed the successful production of LMO.

Figure 1b displays the XRD patterns of the LMO, LCM, and LSM samples after being heated at 700 °C for 2 h. In comparison to the standard card (PDF 50-0297), the presence of peaks at 23.0°, 32.6°, 40.1°, 46.7°, 52.7°, and 58.2°, corresponding to the (110), (200), (022), (220), (130), and (024) crystal planes, respectively, indicates the successful synthesis of single-phase LMO and LSM samples. Similarly, when compared to the standard card (PDF 50-0299), the absence of any additional peaks in the diffraction pattern confirms the synthesis of single-phase LCM samples. The diffraction peak of LCM in Figure 1c exhibits a slight shift towards a greater angle, indicating a lattice distortion in the crystal structure. Doping causes a reduction in the average ionic radius at point A due to the larger ionic radius of La³⁺ (1.06 Å) compared to Ca²⁺ (0.99 Å). The presence of alternating long and short A-site cation radii can lead to a significant mismatch in the MnO₆ octahedra and a pronounced bending of the Mn-O-Mn bond angles [21,22,29]. This mismatch exerts a considerable influence on the orthorhombic structure, causing elongation along the c-axis due to the Jahn–Teller effect and amplifying the lattice distortion of the compounds [21,22,29]. It is clear that replacing La³⁺ with a smaller Ca²⁺ reduces the lattice volume and thus increases the diffraction angle. On the other hand, the small change in the diffraction peak position of the LSM towards a smaller angle indicates that increased Sr²⁺ (1.18Å) substitutions result in a rhombohedral structure characterized by a significant c-value. The normal chalcogenide structure implies a significant lattice distortion along the c-axis. The Mn³⁺ Jahn–Teller phenomenon results in a distortion of the lattice due to a mismatch in the oxygen octahedral structure [16,25]. Hence, the distinct structural evolution of Ca and Sr substitutions for LaMnO₃ may have varying impacts on the electrochemical performance [26,28].

The SEM and TEM results of the LMO, LCM, and LSM samples are displayed in Figure 2. Figure 2a–c shows that the LMO, LCM, and LSM samples exhibit consistent grain size and fine grains in their microstructures. The three specimens primarily contain clumped particles, with LMO exhibiting the most rugged structure and the biggest clusters. The clusters of LCM and LSM are significantly smaller in size when compared to the pure LMO nanoparticle sample. The TEM analysis in Figure 2d–f reveals additional intricate details regarding the morphology and structure of the LMO, LCM, and LSM samples. The results indicate that the agglomerates consist of layered stacks of nanoparticles exhibiting a fine and relatively uniform distribution of grain sizes. The TEM results demonstrate that the size of the pure LMO sample, approximately 50 nm, surpasses the grain size of the LSM (25 nm) and LCM (22 nm) samples. The analysis illustrates that the size of LMO nanoparticles diminishes with the introduction of Ca and Sr doping, while the LCM grain size remains the smallest. Moreover, the LCM exhibits a relatively smaller grain size and superior dispersion. Figure 2g–i displays the area electron diffraction patterns of LMO, LCM, and LSM. It is evident from these patterns that the LMO, LCM, and LSM samples exhibit concentric ring patterns, indicating the presence of a polycrystalline structure in all three samples.

The nitrogen adsorption and desorption isotherm curves in Figure 3a are Type IV curves, indicating that the mesoporous region of the LMO, LCM, and LSM samples because of N₂ uptake in the P/P₀ region is up to 0.8 [30]. The specific surface areas of the LMO, LCM, and LSM samples are 24.82 m² g⁻¹, 31.56 m² g⁻¹, and 38.79 m² g⁻¹, respectively. Figure 3b shows the pore size distribution of the LMO, LCM, and LSM samples. It can be seen that the average pore sizes of the LMO, LCM, and LSM samples were 3.048 nm, 2.539 nm, and 2.806 nm, respectively. The results indicate that the LSM samples exhibit a higher quantity of mesopores compared to the LMO samples, while the LCM samples demonstrate the highest abundance of mesopores.

The XPS analysis in Figure 4 shows the changes in the surface oxidation state due to different A-site substitutions. The full XPS spectra for LMO, LSM, and LCM samples in the binding energy from 0 to 1300 eV indicate no presence of impurities (Figure 4a). Figure 4b shows that the La 3d_5/2 spin-orbital peaks are located at 834.3 and 838.1 eV, while the peaks at 850.6 and 855.6 eV correspond to the La 3d_3/2 spin–orbital peaks. The above results indicate that the La element in the LMO, LCM, and LSM samples is present in the +3 valence state [31]. The split peak fit analysis of the Mn 2p spectra of the three samples is shown in Figure 4c. The two main peaks are shown: Mn 2p_1/2 with a high binding energy and Mn 2p_3/2 with a low binding energy, with the difference remaining at around 11.7 eV. The Mn 2p spectrum of LMO shows a wide emission line width and a clear maximum range, which indicates the presence of Mn in different oxidation states. The results of the split-peak fit indicate that Mn has three oxidation states: Mn²⁺ (3d⁵), Mn³⁺ (3d⁴), and Mn⁴⁺ (3d³). The Mn 2p_3/2 peaks of the LMO samples were 644.41 eV, 642.31 eV, and 641.25 eV, corresponding to the Mn²⁺, Mn³⁺, and Mn⁴⁺ oxidation states, respectively. The percentages of the three oxidation states in the LMO, LCM, and LSM of the element Mn are listed in

Table 1. Proportions of Mn²⁺, Mn³⁺, and Mn⁴⁺ ions in Manganese and relative concentrations of the three oxygen of O 1s in LMO, LCM, and LSM.

	LaMnO3	La0.85Ca0.15MnO3	La0.85Sr0.15MnO3
Mn2+(%)	17.84	14.53	14.82
Mn3+(%)	65.22	58.63	56.03
Mn4+(%)	16.94	26.83	29.14
O 1(%)	44.8	37.9	39.9
O 2(%)	47.6	55.6	53.6
O 3(%)	7.6	6.5	6.5

. The Mn⁴⁺ content of the LMO sample is the lowest. The increment in Mn⁴⁺ content in the LCM sample is not enough to change its crystal structure; therefore, both LCM and LMO remain orthorhombic. In addition, the LSM sample has the highest Mn⁴⁺ content. The excessive Mn⁴⁺ ions in the LSM sample impels the transition of the LSM from an orthorhombic to a rhombic structure, which reduces the energy tendency of the Jahn–Teller effect [32]. It is hypothesized that the smaller size and higher charge density of the Mn⁴⁺ cation favor the contraction of the Mn-O bond, leading to the rhombic structure [33]. The high-resolution spectra of O 1s fitted by three peaks are shown in Figure 4d. The three peaks at 529.5, 531.6, and 533.8 eV correspond to the O1, O2, and O3 components, respectively [31]. O1, O2, and O3 denote lattice oxygen (O²⁻), surface adsorbed oxygen (O⁻, O²⁻, and O₂²⁻), and oxygen-containing groups (OH), respectively. The relative concentrations of the three types of oxygen are shown in Table 1. The oxygen vacancy concentration of the electrode material is closely related to its electrochemical performance. The higher the concentration of the O2 substance, the easier the adsorption of OH⁻, which accelerates the surface redox reaction and will improve the electrochemical performance [34]. The oxygen vacancy concentration is expressed as the O2/O1 molar ratio. It is calculated that the magnitude of oxygen vacancy concentration in LMO, LCM, and LSM samples is in the following order: LCM (1.47) > LSM (1.34) > LMO (1.06). Thus, LCM has better electrochemical performance compared to LSM and LMO. Figure 4e shows the peak difference analysis of the Ca 2p spectrum of the LCM sample, showing the two main peaks formed by the low binding of 2p_1/2 and 2p_3/2, with the difference remaining around 5.2 eV and the Ca ion present in the sample in the +2 valence state [35]. Figure 4f shows the peak difference analysis of the Sr 3d spectrum of the LSM sample, showing the two main peaks formed by the low binding of 3d_3/2 and 2p_5/2, with the difference remaining around 2.2 eV, demonstrating that the Sr in the system is +2 valence [36].

Figure 5 demonstrates the characterization of the cyclic voltammetric (CV) curves of LMO, LCM, and LSM electrodes at a scan rate of 10–100 mV/s using 3 M KOH as the electrolyte in order to examine the effect of Ca/Sr-doping on the electrochemical performance of LaMnO₃. The results indicate that the form of the CV curves remained largely unaltered as the scanning speed increased, suggesting favorable reversibility. In the meantime, the configuration of the CV curves demonstrates the pseudocapacitance characteristics of the LMO, LCM, and LSM electrodes. Additionally, the redox peaks, which correspond to the alteration of Mn²⁺ and Mn³⁺ valence states, are distinctly visible, with two peaks at 0.1 and −0.1 V, respectively. Mefford et al. proposed an anion−intercalation mechanism, which suggests that the charge storage mechanism for LaMnO3 depends on the presence of oxygen excess or oxygen deficiency during the charge/discharge process [37]. Equations (1) and (2) [29] provide comprehensive redox reaction equations.

$$La\left[M{n}_{2\delta }^{2+}; M{n}_{1-2\delta }^{3+}\right]O_{3-\delta }+2\delta O{H}^{{}^{-}}\leftrightarrow LaM{n}^{3+}{O}_3+2\delta e^{-}+\delta H_{2}O$$

(1)

$$LaM{n}^{3+}{O}_3+2\delta O{H}^{{}^{-}}\leftrightarrow La\left[M{n}_{2\delta }^{4+}; M{n}_{1-2\delta }^{3+}\right]O_{3+\delta }+2\delta e^{-}+\delta H_{2}O$$

(2)

During the initial stage of the oxygen intercalation procedure in the LaMnO₃ electrode, hydroxide ions (OH⁻) are taken in by oxygen imperfections to produce superoxide ions (O²⁻) and water (H₂O) in the alkaline electrolyte. Subsequently, O²⁻ ions are transported across the octahedral lattice to maintain the structural stability of oxygen vacancies. In the process of the reaction, the nearby Mn²⁺ changes into Mn³⁺ while releasing an electron, resulting in the formation of the neutral LaMn³⁺O₃ (refer to Equation (1)). In Equation (2), the subsequent stage involves the emergence of manganese from the core of the oxygen octahedron. This leads to the incorporation of surplus oxygen into the outer region, causing the valence of the Mn cation to rise from Mn³⁺ to Mn⁴⁺, resulting in an oxygen excess. Because there are additional oxygen vacancies, the divalent element (Ca²⁺/Sr²⁺) takes the place of some La³⁺ in the A site. Subsequently, the response transforms into the subsequent.

$$L{a}_{0.85}{M}_{0.15}\left[M{n}_{2\delta }^{2+}; M{n}_{1-2\delta }^{3+}\right]O_{2.925-\delta }+2\delta O{H}^{{}^{-}}\leftrightarrow L{a}_{0.85}{M}_{0.15}M{n}^{3+}{O}_{2.925}+2\delta e^{-}+\delta H_{2}O$$

(3)

$$L{a}_{0.85}{M}_{0.15}M{n}_{}^{3+}{O}_{2.925}+2\delta O{H}^{{}^{-}}\leftrightarrow L{a}_{0.85}{M}_{0.15}[M{n}_{2\delta }^{4+};M{n}_{1-2\delta }^{3+}]O_{2.925+\delta }+2\delta e^{-}+\delta H_{2}O$$

(4)

The oxidation process from Mn²⁺ to Mn³⁺ in La_0.85M_0.15MnO₃ samples exhibited similarities to the process described by Mefford et al. [1]. The oxygen vacancy is filled by O²⁻ intercalation. Despite the fact that all Mn²⁺ is oxidized to Mn³⁺, La_0.85M_0.15Mn³⁺O_2.925 still maintains a hypoxic state. As a result, the process of Mn³⁺ oxidation to Mn⁴⁺ consists of two distinct stages. During the initial phase, when δ is less than or equal to 0.075, the oxidation of Mn³⁺ to Mn⁴⁺ occurs by continuously removing O²⁻ to fill the remaining oxygen vacancies. Several oxygen vacancies migrate towards the surface of the material to form $L{a}_{0.85}{M}_{0.15}[M{n}_{0.15}^{4+};M{n}_{0.85}^{3+}]O_3\left(\delta =0.075\right)$. The second stage involves the same process of Mn³⁺ oxidation to Mn⁴⁺, resulting in the formation of $L{a}_{0.85}{M}_{0.15}[M{n}_{2\delta }^{4+};M{n}_{1-2\delta }^{3+}]O_{2.925+\delta }$.

Figure 6a–c illustrates the variations in galvanostatic charge-discharge (GCD) profiles for LMO, LCM, and LSM under varying current densities. As the current density increases, the GCD curves of LMO, LCM, and LSM show a decrease. This is due to the rise in internal resistance at higher current density, resulting in a decline in specific capacitance. The LCM and LSM exhibit extended discharge durations compared to the LMO, suggesting that the substitution of the A-site greatly enhances the capacitive performance. The charge–discharge curves exhibit asymmetrical triangular shapes, suggesting that the predominant capacitance in the samples is primarily pseudocapacitance. The impedance diagram can usually be divided into two parts, as shown in Figure 6d. The AC impedance consists of a semi-circular, curved area that represents the high-frequency region and indicates the redox reactions happening at the electrode. The remaining portion is the linear segment, denoted as the low-frequency range of the AC impedance, characterized by the capacitance produced on the electrode’s surface. The semi-circular arc portion in the impedance diagram represents the magnitude of the charge transfer resistance, which is equal to the diameter of the charge transfer resistance. The impedance spectrum was analyzed using an equivalent circuit, as depicted in the inset, which consisted of the internal resistance (Rs), charge transfer resistance (Rct), Warburg Element (open) (Wo), and Constant Phase Element (CPE). The values of Rs for the LMO, LCM, and LSM electrodes were determined to be 1.45 Ω, 1.54 Ω, and 1.01 Ω, respectively, while the corresponding Rct values were found to be 0.48 Ω, 0.36 Ω, and 0.38 Ω, respectively. It is worth noting that the LCM sample exhibited the lowest Rct, indicating superior electrochemical performance. Ca and Sr belong to the same main group, and the atomic radius of Ca is less than that of Sr. Doping Ca at the La site, as opposed to doping Sr, can enhance the formation of additional oxygen vacancies, thereby favorably enhancing the electrochemical characteristics of La-based perovskite structure materials [29].

4. Conclusions

In this work, the crystal structure, surface morphology, and surface oxidation states of LaMnO₃ perovskite with different A-site substitutions by Ca and Sr have been investigated, and the impact on the electrochemical capabilities is also revealed. After substitution, the LSM undergoes a transition to a rhombohedral configuration due to the rise in Mn⁴⁺ ions, while the LCM retains its original orthorhombic structure with the parent LMO composition. Samples of LCM and LSM that exhibit a higher number of inherent oxygen vacancies demonstrate superior capacitance properties compared to LMO. Additionally, they have the ability to store a greater amount of energy due to the redox pseudocapacitance that is specifically modified by the presence of oxygen vacancies. At a current density of 0.5 A/g, the capacitances for LMO, LSM, and LCM are 67 F/g, 185.5 F/g, and 248 F/g, respectively. The results indicate that Ca substitution at the A-site for LaMnO₃ holds significant promise for utilization in supercapacitors, and further optimization in its structure and electrochemical performance are necessary.