Chloride Ions Tuning Organic Alkaline Electrolyte for Optimizing MnO₂ Cathodes in Aqueous Sodium Batteries

Abstract

The growing demand for efficient energy storage solutions has highlighted the potential of aqueous sodium-ion (Na⁺) batteries, known for their cost-effectiveness and environmental benefits. Despite their promise, challenges such as low specific capacities resulting from proton (H⁺) intercalation issues have limited their effectiveness. This study introduces a novel alkaline electrolyte environment using tetrabutylammonium hydroxide (TBAH) combined with chloride ions (Cl⁻) to improve the Na⁺ storage performance of manganese oxide (MnO₂) cathodes. The optimized electrolyte achieved a remarkable reversible capacity of 101 mAh g⁻¹ for γ-MnO₂ at a current density of 0.1 A g⁻¹, surpassing conventional aqueous solutions. The synergistic effect of TBAH and Cl⁻ not only suppresses H⁺ intercalation, but also prevents the formation of manganese hydroxide passivation layers during cycling. These advancements contribute to a better understanding of electrolyte design for high-performance Na⁺ storage electrodes, marking a significant step forward in aqueous sodium-ion battery technology.

1. Introduction

The increasing demand for clean energy solutions is driving the search for sustainable, cost-effective, and safe energy storage technologies. Among these options, aqueous batteries have emerged as a promising alternative due to their environmental advantages, lower costs, and improved safety compared to traditional organic electrolyte-based batteries [1,2]. By utilizing water as the solvent in their electrolytes, aqueous batteries significantly reduce the risk of flammability, a major concern associated with many organic lithium-ion and sodium-ion systems. Additionally, the availability of abundant, non-toxic, and inexpensive materials and electrolytes positions aqueous batteries as a viable choice for large-scale energy storage applications, such as grid stabilization and renewable energy integration [3,4,5]. Aqueous sodium ion (Na⁺) batteries are particularly attractive devices because they offer a balance of performance, cost-effectiveness, and environmental sustainability, making them suitable for next-generation energy storage systems [6].

Despite their advantages, aqueous batteries face significant challenges that limit their widespread implementation. Key issues include their low energy density, the narrow electrochemical stable window of aqueous electrolytes, and the corrosive reactions that adversely affect battery lifespan [4,7,8,9]. Furthermore, the neutral or acidic electrolytes commonly used in these batteries present specific challenges, particularly the tendency for proton (H⁻) intercalation during electrochemical processes involving cathode materials like vanadium oxide and manganese oxides [10,11,12,13]. This intercalation can lead to hydroxylation and structural degradation of the electrodes, resulting in reduced ion diffusion and accelerated capacity loss [10,14,15,16,17,18]. A summary of the performance of various manganese-based oxides in aqueous sodium-ion batteries is shown in Table S1. It is evident that the sodium storage capacity of manganese-based oxides in aqueous electrolytes is generally below 70 mAh g⁻¹, which is significantly lower than their performance in organic sodium-ion batteries [19]. Reports have indicated that proton intercalation can block sodium ion diffusion [20], which restricts the sodium storage capacity of manganese oxide electrodes. The use of alkaline electrolytes has been shown to mitigate proton intercalation and improve the specific capacity of these materials to some extent [20], and traditional NaOH electrolytes tend to be corrosive, leading to the formation of inactive MnOOH, Mn(OH)₂, and Mn₃O₄ within the material [21]. This also limits the specific capacity of the electrode materials. Consequently, minimizing H⁺ insertion while ensuring effective Na⁺ transport becomes crucial for optimizing aqueous Na⁺ ion battery systems.

In this study, we developed a suitable alkaline electrolyte environment for a manganese-based oxide cathode by introducing an organic alkaline agent, tetrabutylammonium hydroxide (TBAH), along with chloride ions (Cl⁻) to enhance aqueous sodium storage performance. The Cl⁻-tuned TBAH aqueous electrolyte improved the specific capacity of the electrode material. Notably, the γ-MnO₂ cathode achieved a reversible capacity of 101 mAh g⁻¹ at a current density of 0.1 A g⁻¹, significantly surpassing the capacities observed in other aqueous electrolytes. This electrolyte design not only creates a suitable alkaline environment to inhibit the embedding of H⁺, but also inhibits the formation of MnOOH species to ensure effective Na⁺ transport. Our findings provide valuable insights into the design of alkaline electrolytes for reversible cathodes in aqueous sodium batteries.

2. Results and Discussion

Firstly, γ-MnO₂ powders were synthesized by a hydrothermal reaction for use as cathode materials for aqueous sodium batteries. The detailed procedure is described in the Section 4. The X-ray diffraction (XRD) pattern (Figure 1a) shows that the main three strongest diffraction peaks corresponding to the (120), (131), and (160) facets are well matched with the standard card (JCPDS No. 14-0644).The results confirm that the as-prepared manganese oxide powders can be assigned to γ-MnO₂ with an orthorhombic structure. Scanning electron microscopy (SEM) images of γ-MnO₂ were collected and are shown in Figure 1b. The γ-MnO₂ powders display tens of micrometer-sized spheres in their morphology, with interwoven microwire agglomerates (Figure S1). Moreover, transmission electron microscopy (TEM) images of the γ-MnO₂ powders further disclose their uniform morphology, with no particle- or segment-like samples, confirming their good crystalline and robust structure (Figure 1c and Figure S2). Then, the Raman scattering spectrum of the γ-MnO₂ powders was measured and is shown in Figure 1d. The Raman shift located at around 640 cm⁻¹ could be ascribed to Mn-O stretching modes. The Raman peaks in the 200–400 cm⁻¹ region correspond to the deformation modes of Mn–O–Mn chain vibration in the γ-MnO₂ lattice [22]. The above characterizations indicate that γ-MnO₂ powders with a good crystalline structure have been synthesized.

To select and design suitable alkaline aqueous electrolytes for our γ-MnO₂ cathode to be operated in aqueous sodium batteries, two different alkaline electrolytes were prepared by dissolving Na₂SO₄ or NaCl in tetramethylammonium hydroxide (TBAH). In comparison, three other types of aqueous electrolytes were prepared by simply dissolving TBAH+Na₂SO₄, NaCl, or NaOH in water. The charge/discharge curves of the γ-MnO₂ cathode in a half battery tested in different aqueous electrolyte systems at a current density of 0.1 A g⁻¹ were collected and are shown in Figure 2a. In Na₂SO₄ and NaCl, which are salt-based electrolytes, the tested capacities were lower than 40 mAh g⁻¹, implying that their features are incompatible with γ-MnO₂ cathodes for aqueous sodium storage. Specially, the TBAH+NaCl aqueous electrolyte was found to provide a high specific capacity of up to 101 mAh g⁻¹ for γ-MnO₂ cathodes. The CV curve (Figure S3) confirms that the cathode material underwent a redox reaction at the tested voltage range. It should be noted that Mn-based oxides have been widely investigated as a promising Na-ion battery cathode because of their high theoretical capacity. However, achieving a capacity higher than 70 mAh g⁻¹ for Mn-based oxides in aqueous sodium-ion batteries is challenging and rarely reported. Therefore, the introduction of Cl⁻ into alkaline TBAH can ensure the high specific capacity of γ-MnO₂ cathodes for aqueous sodium batteries.

Next, the rate performance of the γ-MnO₂ cathode in the TBAH+NaCl aqueous electrolyte at different current densities was evaluated and the corresponding charge/discharge curves were collected. As shown in Figure 2b, the γ-MnO₂ cathode shows a good rate performance, and reversible capacities of 101, 82, 71, 62, 58, 52, and 44 mAh g⁻¹ can be realized at corresponding current densities of 100, 200, 400, 600, 800, 1000, and 1500 mA g⁻¹. And even at a high current density of 2000 mA g⁻¹, a reversible capacity of 40 mAh g⁻¹ can be still retained, which confirms its good rate performance and fast sodium ion transport. The specific capacities of the γ-MnO₂ cathode in the TBAH+NaOH, NaOH, and TBAH+Na₂SO₄ electrolytes are relatively low across various specific currents (Figures S4 and S5). This comparative experiment shows that the coexistence of TBAH and Cl⁻ ions is important for enhancing the specific capacity. Notably, the high capacity of the γ-MnO₂ cathode in TBAH+NaCl is highly repeatable (Figure S6). The long-term stability of the γ-MnO₂ cathode was evaluated at a current density of 0.5 A g⁻¹, as shown in Figure 2c. In the TBAH+NaCl aqueous electrolyte system, the γ-MnO₂ cathode demonstrated a stable performance for 1500 cycles, although it exhibited some decay, with a capacity retention of 41.5% after 1500 cycles. Compared to the NaOH and TBAH+Na₂SO₄ systems, the TBAH+NaCl system showed a higher capacity loss in terms of percentage degradation. This may be attributed to the alkaline environment causing corrosion and passivation of the electrode materials over prolonged cycling. Therefore, although enhancing the specific capacity remains a key consideration, further improving the cycling stability of the γ-MnO₂ cathode will be an important focus for future research.

To investigate the structural changes in the γ-MnO₂ cathode during charge and discharge cycles, we performed ex situ TEM and Raman spectroscopy on cycled cathode electrodes. As illustrated in Figure 3a, after 100 cycles in the NaCl+TBAH electrolyte, the γ-MnO₂ cathode exhibits a distinct (120) crystal plane with 0.40 nm of spacing. In contrast, the high-resolution transmission electron microscopy (TEM) image of the cycled cathode in the Na₂SO₄ + TBAH electrolyte reveals 0.30 nm of notable interlayer spacing corresponding to the (100) planes of Mn(OH)₂ (Figure 3b), indicating structural evolution and phase transformation during cycling.

Furthermore, ex situ Raman measurements corroborate the inhibitory effect of Cl⁻ on the formation of Mn(OH)₂ throughout the electrochemical cycle. As shown in Figure 3c, the Raman shift at around 640 cm⁻¹ corresponds to the vibrational modes of functional groups in the γ-MnO₂ cathode after 100 cycles in the NaCl+TBAH electrolyte. In contrast, the Raman spectrum of the cathode cycled without Cl⁻ displays a new peak at 579 cm⁻¹, attributed to the Mn-O bond in Mn(OH)₂ (Figure 3d). These findings highlight the critical role of Cl⁻ in suppressing the formation of Mn(OH)₂. In the electrolyte system free of Cl⁻ ions, the material surface exhibits a characteristic distribution of active sites with hydroxyl adsorption capability. The low binding energy between active sites and hydroxyl groups leads to the facile formation of a stable surface passivation layer through chemisorption of hydroxyl ions. This passivation layer negatively impacts sodium ion transport, hindering sodium ion intercalation/deintercalation reactions. Within the chemical microenvironment of Cl⁻ ions, the repulsive effect exerted by water molecules significantly kinetically hinders the adsorption process of hydroxyl groups (OH⁻). Although crystallographic analysis reveals theoretically available hydroxyl adsorption sites in chloride ion-coordination regions, the elevated binding energy barriers at these potential sites prevent effective coordination binding in TBAH+NaCl electrolytes. This dual inhibition mechanism, incorporating both thermodynamic and kinetic obstacles into surface passivation layer formation, ensures the maintenance of rapid and stable transport channels for sodium ions (Na⁺) at the electrode–electrolyte interface [23].

To further disclose the structure evolution and the reconstruction process of the γ-MnO₂ cathode, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was conducted on the cycled cathode. Technically, TOF-SIMS allows us to obtain isotopic, elemental, and molecular information from the surfaces of electrode samples; therefore, this technique is powerful enough to identify the intercalation species within electrodes operating under an aqueous TBAH+NaCl electrolyte environment. The depth-profiled TOF-SIMS data shown in Figure 4,ab demonstrate that Na⁺ ion fragments are distributed throughout the whole electrode, which confirms the reversible sodium intercalation during charge/discharge. Notably, H⁺ is virtually undetectable inside the cycled γ-MnO₂ cathodes, demonstrating that proton intercalation is prohibited in the TBAH+NaCl electrolyte environment.

Figure 5 illustrates the design strategy of this work and the mechanism for the improvement in electrode performance. In acidic and neutral aqueous electrolytes (Figure 5a), the intercalation of protons in manganese oxide materials occurs easily, leading to the formation of inactive phases such as MnOOH, Mn(OH)₂, and Mn₃O₄. These inactive phases reduce sodium ion storage capacity, which is why manganese oxide electrode materials have previously shown low capacities of less than 40 mAh g⁻¹ in studies of aqueous sodium-ion batteries. By switching the electrolyte to an alkaline solution (Figure 5b), proton intercalation can be suppressed, resulting in a significant increase in initial capacity. However, as cycling progresses, a strongly alkaline environment can easily lead to the formation of a passivation layer of Mn(OH)₂ on the electrode’s surface, hindering the intercalation and deintercalation of Na⁺ and causing rapid capacity degradation of the electrode. If Cl⁻ ions are introduced into an aqueous solution with an organic alkaline (Figure 5c), they can not only suppress proton intercalation, but also prevent the formation of the surface passivation layer. This allows for an enhancement of the specific capacity of the MnO₂ electrode material. In summary, a suitable alkaline environment, in conjunction with additive Cl⁻ ions, can optimize the performance of manganese oxide electrode materials in aqueous sodium-ion battery systems. Although our designed electrolyte system suppresses the generation of inert species through weak alkaline environment design and Cl⁻ ions, thereby improving specific capacity during the initial stages of battery cycling, it fails to fully mitigate the long-term effects of alkaline corrosion as the process progresses. This results in a gradual decrease in specific capacity over cycles.

3. Conclusions

In this work, we designed an alkaline aqueous electrolyte system by introducing chloride ions (Cl⁻) into a tetramethylammonium hydroxide (TBAH) solution to greatly optimize the electrochemical performance of a γ-MnO₂ cathode for aqueous sodium batteries. Specifically, the γ-MnO₂ cathode delivers a high reversible capacity of 101 mAh g⁻¹ at 0.1 A g⁻¹ in an TBAH+NaCl aqueous electrolyte and good rate performance even at high current densities of up to 2000 mA g⁻¹, exceeding the performance of other reported systems. In situ TEM and Raman results indicate that the TBAH+NaCl aqueous electrolyte inhibits the generation of Mn(OH)₂, thus enabling highly reversible Na⁺ diffusion of the electrode material. TOF-SIMS results confirm that H⁺ intercalation is prohibited in such an environment, which ensures high-capacity sodium ion storage. Our work highlights the potential of engineered alkaline electrolytes to optimize the performance of aqueous sodium-ion batteries, offering insights into the design of more efficient and stable energy storage systems.

4. Methods

4.1. Synthesis of γ-MnO₂ Cathode Materials

MnSO₄·H₂O, (NH₄)₂S₂O₈, NaCl, tetrabutylammonium hydroxide (TBAH), and NaOH were purchased from Aladdin Co., Ltd. Graphene was purchased from Nanjing XFNANO Materials Tech Co., Ltd., Nanjing, China. The nanoparticles were synthesized via a hydrothermal approach. The experimental procedure was carried out as follows: First, 3.375 g of MnSO₄·H₂O and 4.575 g of (NH₄)₂S₂O₈ were individually dissolved in 40 mL of deionized water. When these solutions were completely dissolved, they were mixed. The mixture was then subjected to magnetic stirring for approximately 20 min. Subsequently, the well-stirred mixed solution was transferred into a 100 mL Teflon-lined autoclave. The autoclave was tightly sealed and subsequently heated in an oven at a constant temperature of 90 °C for a duration of 24 h. After the heating process, the solution was permitted to cool to room temperature. The resulting solid products were then centrifuged. These products were subsequently washed with distilled water and absolute ethanol to ensure purity. Finally, the washed solids were dried in an air atmosphere at 50 °C for 5 h.

4.2. Material and Electrode Characterization

The crystallographic structures of the samples were characterized by X-ray diffraction (XRD) using the D8 Advance, Bruker AXS GmbH (Karlsruhe, Germany) with a Cu Kα X-ray source (λ = 1.5406 Å). The scanning electron microscopy (SEM) morphology and compositions of the electrode materials were characterized by a JEOL-7100Ft. High-resolution transmission electron microscopy (HRTEM) was conducted on a Talos F200S. The thin films were directly deposited on Si wafers or TEM grids for SEM and TEM measurements. TOF-SIMS spectrometry was performed on a PHI nano TOFII Time-of-Flight SIMS instrument.

4.3. Electrochemical Measurements

GCD and EIS tests were conducted using a CHI760E electrochemistry workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). Cycle stability tests were performed at 20 °C using the LAND battery test system (CT3001A, Wuhan, China) and Neware battery test system (CT-4008, Shenzhen, China). The cathode electrode was made using a mixture of MnO₂, acetylene black, and PVDF, with mass ratio of 7:2:1. The mass loading of the electrode was around 1 mg⁻². The electrolyte was prepared by the dissolution of different salts (e.g., 3 M NaCl+4.5 M TBAH) in DI water. The half battery was assembled in a PTFE Swagelok-type three-electrode system, in which a Hg/HgO electrode served as the reference electrode. In this work, all potential values are converted into values versus standard hydrogen electrodes (SHE).