A high-performance chloride-ion battery based on MnO₂@V₂O₅@C cathode synergy

Abstract

Chloride-ion batteries (CIBs) possess high theoretical energy densities and are promising successors to lithium ion batteries. In this study, we present a novel MnO₂@V₂O₅@C cathode design. We introduced a water-in-salt system into the battery, which effectively solved the safety problem. This battery offered the advantages of high performance, low weight, low cost, high flexibility and enhanced safety. Furthermore, it achieved a discharge platform of up to 2.6 V, a maximum discharge capacity of 457.4 mA h g⁻¹, and a cycle life of 1000. The cathode was lightweight, weighing only 0.02 g, and the battery used fewer materials, making it less expensive. It could be arbitrarily folded, punctured and burned (1300 °C) without explosion, and it could be operated normally before complete destruction. Thus, aqueous CIBs exhibit high safety. These advantages make this new type of chloride-ion battery a potential candidate for applications in automotive, portable devices, aerospace and other fields, and it opens a new path for the development of batteries.

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1. Introduction

Energy resource shortage and environmental issues are among the major challenges currently faced by humanity. These challenges urge us to reform and adjust our energy structure.^1,2 A key issue while solving energy-related problems is energy storage, and researchers have conducted a series of studies in this area.^3–5 Currently, lithium-ion batteries continue to dominate the energy storage field and are widely used in transportation, portable devices, and other areas.^6–8 However, lithium batteries have issues related to cost and safety. Thus, researchers have shifted their attention to metal-ion batteries. Na⁺,^9–11 Mg²⁺,^12–14 Ca²⁺,^15–17 and Zn²⁺ (ref. 18–20) batteries are developing rapidly, but they too face problems, such as limited cycle life.

Non-metallic elements have also been introduced into the energy storage field. Halogen elements (F, Cl, Br, and I) offer advantages such as abundant resources, low cost, and high theoretical specific capacity.^21,22 In particular, CIBs stand out for their easy resource availability (e.g., seawater), low cost, long cycle life, and high energy density.^23–26 We introduced a water-in-salt system into CIBs, addressing the safety issues. Currently, CIBs primarily use cathode materials, such as metal chlorides, metal oxychlorides, and layered double hydroxides (LDHs),^27–31 while anode materials include lithium, magnesium, and graphite.^32,33 Electrolytes mainly consist of binary ionic liquids, solid-state polymers, and water-in-salt systems.^28,34–37

In our previous research, we successfully addressed the safety issue of chlorine-ion batteries by introducing a water-in-salt system. The addition of LiCl to the electrolyte improved the performance by combining with the cathode ions. The use of VOC cathodes significantly increased the discharge capacity of batteries.³⁸ Through our research, we found that the synergistic effect of MnO₂ and V₂O₅ significantly enhanced the discharge capacity and cycling stability of these batteries. When using Zn as the anode and MnO₂@V₂O₅@C as the cathode, the maximum discharge capacity of aqueous chlorine-ion batteries increased to 457.4 mA h g⁻¹ (compared to 200 mA h g⁻¹ with a graphite sheet cathode), and the cycle life was greatly enhanced (600 cycles), with a discharge platform around 2.6 V. This exceptional performance was primarily attributed to the combined action of MnO₂@V₂O₅@C with chlorine and lithium ions. The equation is as follows:

Cathode:

2Li⁺ + MnO₂ + 2V₂O₅ + H₂O + 2e⁻ ↔ LiMnO₂ + LiV₂O₅ + 2VO₂ + 2OH⁻

CCl + e⁻ ↔ C + Cl⁻

Anode:

Zn − 2e⁻ ↔ Zn²⁺

Overall:

2Zn + 2Li⁺ + MnO₂ + 2V₂O₅ + 2CCl + H₂O ↔ LiMnO₂ + LiV₂O₅ + 2VO₂ + 2C + 2Cl⁻ + 2Zn²⁺ + OH⁻

Our pouch cell manufacturing method is extremely simple and easy to operate. Each cathode sheet uses only 10 mg of the mixed materials, resulting in low material consumption and low cost. The performance is excellent. At present, the maximum discharge capacity of CIBs with good performance is about 200–300 mA h g⁻¹, such as the CIB composed of FeOCl@MCF cathode, the maximum discharge capacity is 235 mA h g⁻¹;²³ The aqueous CIB composed of MoS₂ cathode has a maximum discharge capacity of 258.1 mA h g⁻¹,³⁹etc. Our battery has a first discharge capacity of up to 460 mA h g⁻¹ and still has a capacity of 237 mA h g⁻¹ after 600 cycles, which exceeds the maximum discharge capacity of most CIBs. As shown in Fig. 1, our soft-pack battery, even if punctured, crushed or burned, will not explode or result in an accident, and it is very safe. The high safety and performance of these batteries make the application of aqueous chlorine-ion batteries in military and aerospace fields possible.

Fig. 1 (a) Image of the working of the folded battery. (b) Safety testing under fire.

2. Experimental methods

2.1. Material

Graphite paper (thickness: 50 μm) was purchased from Sinopharm Chemical Reagent Co., Ltd (CH₃)₄NCl was obtained from Tianjin Damao Chemical Reagent Factory. Graphite powder was purchased from Shenzhen Jinda Power Technology Co., Ltd LiCl was purchased from Shanghai Aladdin Biochemical Co., Ltd V₂O₅ and MnO₂ were purchased from Tianjin Damao Chemical Reagent Factory.

2.2. Preparation of the electrode and electrolyte

The MnO₂, V₂O₅, and powdered graphite were mixed in a mass ratio of 7 : 3 : 2.5, followed by adding 10% PVDF (of the total mass) as a binder. The NMP solvent was then introduced to form a homogeneous slurry. The slurry was uniformly coated onto graphite paper (2.5 × 2.5 cm²) and dried at 60 °C under vacuum for 1 h to complete the cathode preparation.

The anode is made of zinc foil of 2.5 cm × 2.5 cm.

(CH₃)₄NCl (5.48 g) was dissolved in 10 mL deionized water and stirred for 10 min to obtain 5 M (CH₃)₄NCl. Then, 1.06 g of LiCl was added to the above solution and stirred for 30 min to obtain 5 M (CH₃)₄NCl + 2.5 M LiCl electrolyte.

2.3. Electrochemical measurements

A polyolefin microporous membrane (100 μm) was used as the diaphragm and was assembled with positive and negative electrodes and electrolyte to form a soft-pack battery. A battery test system with constant voltage charge and constant discharge (Neware BTS 4000) was used for the test. The charging process was conducted at a constant voltage of 3.1 V, and the discharge process was conducted at a constant current of 0.33 A g⁻¹.

2.4. Material characterization

X-ray diffraction (XRD) spectra of the electrode were collected using an X-ray diffractometer (D/Max 2400, Japan). Raman spectra were obtained using a DXR microscope. XPS was tested on a Thermo ESCALAB XI + equipped with a hemispherical analyzer. Cyclic voltammetry (CV) curve testing was conducted using an electrochemical workstation (RST5201F).

3. Results and discussion

Our aqueous CIB uses an MnO₂@V₂O₅@C composite material as the cathode, Zn foil as the anode, and (CH₃)₄NCl + LiCl as the electrolyte. The battery assembly process is illustrated in Fig. 2. During discharge, the cathode composite material MnO₂@V₂O₅@C is reduced to MnO₂⁻, V₂O₅⁻, and VO₂. The Cl in the composite material is reduced to Cl⁻ and released from the graphite.^37,40 MnO₂⁻ and V₂O₅⁻ combine with Li⁺ in the electrolyte to form LiMnO₂ and LiV₂O₅. The anode Zn is oxidized to Zn²⁺. During charging, MnO₂⁻, V₂O₅⁻, and VO₂ are oxidized to MnO₂ and V₂O₅, which deposit on the cathode surface. Cl⁻ is oxidized to Cl0 and embedded in graphite, stored in the graphite interlayer.^37,40 The anode Zn²⁺ gains electrons to form Zn, which is deposited on the anode surface.

Fig. 2 Fabrication process of the novel aqueous chloride-ion battery.

As shown in Fig. 3(a and d), by comparing the typical constant current curves of batteries with MnO₂@V₂O₅@C as the cathode, 5 M (CH₃)₄NCl as the electrolyte, and Zn as the anode, we found that the addition of LiCl to the electrolyte can improve battery performance. The addition of LiCl increased the concentrations of Cl⁻ and Li⁺ in the electrolyte, thereby promoting the Cl/Cl⁻ conversion and the formation of LiMnO₂ and LiV₂O₅. The increased amounts of LiMnO₂ and LiV₂O₅ also provide more adsorption sites for Cl⁻, consequently enhancing the battery capacity.

Fig. 3 Typical constant-current charge–discharge curves of batteries at a current density of 0.33 A g⁻¹: (a) MnO₂@V₂O₅@C cathode, 5 M (CH₃)₄NCl electrolyte. (b) V₂O₅@C cathode, 5 M (CH₃)₄NCl + 2.5 M LiCl electrolyte. (c) MnO₂@C cathode, 5 M (CH₃)₄NCl + 2.5 M LiCl electrolyte. (d) MnO₂@V₂O₅@C cathode, 5 M (CH₃)₄NCl + 2.5 M LiCl electrolyte. (e) Cycle life of the battery with MnO₂@V₂O₅@C as the cathode and 5 M (CH₃)₄NCl + 2.5 M LiCl as the electrolyte. (f) Charge–discharge voltage curves of the battery with MnO₂@V₂O₅@C as the cathode and 5 M (CH₃)₄NCl + 2.5 M LiCl as the electrolyte at the 20th, 150th, 400th, and 600th cycles.

As shown in Fig. 3(b and d), compared with the typical constant current curves of batteries with a V₂O₅@C cathode and 5 M (CH₃)₄NCl + LiCl as the electrolyte, the maximum discharge capacity of the battery can be increased from 226 mA h g⁻¹ to 457.4 mA h g⁻¹ when MnO₂ is added to the cathode to synthesize the MnO₂@V₂O₅@C composite cathode. After 600 cycles, the capacity reached 237 mA h g⁻¹, which is a significant improvement. This enhancement is primarily attributed to the involvement of MnO₂ in the reaction. MnO₂⁻ and Li + react in the electrolyte to form LiMnO₂ with a layered structure. The layered LiMnO₂ exhibits an orthorhombic or monoclinic structure, and its synergy with LiV₂O₅, which exhibits n-layer folds, provides more space for Cl⁻ attachment. This increases the contact area between the cathode and the electrolyte, making the reaction more stable and comprehensive, thereby improving the discharge capacity and lifespan of the battery.⁴¹ Thus far, the discharge capacity of this battery exceeds that of previous aqueous chloride-ion batteries.

To prove the synergy of LiV₂O₅ and LiMnO₂, as shown in Fig. 3(c), the capacity is 264 mA h g⁻¹ when MnO₂@C is used as the anode. Although the battery performance was improved, it was still significantly lower than that of MnO₂@V₂O₅@C. This result was mainly attributed to the fact that the reaction only produced LiMnO₂. As shown in Fig. 3(e), approximately 20 activation cycles are required for the battery reaction to stabilize. This is mainly because the initial reaction produces a small amount of VO₂, and as its quantity increases, the battery gradually stabilizes. As shown in Fig. 3(f), the capacity retention rate of the battery reached 99% in the first 60 cycles, indicating that the initial reaction was thorough. The subsequent capacity decay is mainly due to the partial accumulation of VO₂, which hinders Cl⁻ adsorption. After about 200 cycles, the various reactants and products reach equilibrium, and cycling stabilizes up to 600 cycles, with little fluctuation in capacity. However, since our battery is not completely sealed, the water in the water-in-salt system gradually decreases, reducing conductivity and affecting performance.

In addition to the above discussion, we analyzed the effects of different electrolyte concentrations and various ratios of composite cathodes on the battery performance. As shown in Fig. 4(a–c), with the increase in LiCl concentration in the electrolyte, the battery performance is improved. Fig. 4(d) intuitively shows that the optimal LiCl concentration is 2.5 M. This is mainly due to the increased concentration of Cl⁻ in the electrolyte from the addition of LiCl, which enhances the number of chlorine ions shuttling. Additionally, more Li⁺ combines with V₂O₅⁻ and MnO₂⁻, causing greater morphological changes and providing more space for Cl adsorption and Cl⁻ shuttling. Through experiments, we found that the maximum solubility of LiCl in a 5 M (CH₃)₄NCl solution is 2.5 M, which is saturated. Therefore, the battery performance is optimal when the LiCl concentration is 2.5 M.

Fig. 4 Typical constant-current charge–discharge curves of batteries with MnO₂@V₂O₅@C as the cathode, zinc foil as the anode, 5 M (CH₃)₄NCl and LiCl of different concentrations as the electrolyte: (a) 1 M LiCl, (b) 1.5 M LiCl, and (c) 2 M LiCl. (d) Typical constant-current charge–discharge curves of 5 M (CH₃)₄NCl + 0 M, 1 M, 1.5 M, 2 M, 2.5 M LiCl batteries. Typical constant-current charge–discharge curves of batteries with zinc foil as the anode, 5 M (CH₃)₄NCl + 2.5 M LiCl as the electrolyte, and different proportions of MnO₂ and V₂O₅ as the cathode: (e) 3 : 7, (f) 1 : 1, (g) 4 : 1, and (h) 9 : 1. (i) Typical constant current charge and discharge curves of the battery with MnO₂: V₂O₅ = 0 : 1, 3 : 7, 1 : 1, 7 : 3, 4 : 1, 9 : 1, and 1 : 0.

Furthermore, we found that the battery performs best when the ratio of MnO₂ to V₂O₅ in the MnO₂@V₂O₅@C composite material is 7 : 3. As shown in Fig. 4(e and f), using 5 M (CH₃)₄NCl + 2.5 M LiCl as the electrolyte and Zn as the anode, the performance decreases when the ratio of MnO₂ to V₂O₅ is reduced compared to the 7 : 3 ratio. This is because an increase in V₂O₅ results in more VO₂ adhering to the cathode during the reaction, hindering contact between the cathode and the electrolyte, which affects Cl adsorption by C and thus impacts battery performance. As shown in Fig. 4(g and h), when the ratio of MnO₂ to V₂O₅ continues to increase, the performance is also worse than that of the 7 : 3 ratio. Although there are many layers of LiMnO₂, the reduction in V₂O₅ leads to less formation of LiV₂O₅. The folded structure on the cathode side is reduced compared to the 7 : 3 ratio, decreasing the surface area involved in the reaction and reducing the contact area with Cl⁻, thereby decreasing Cl adsorption by the cathode and thus reducing capacity. Fig. 4(i) shows that when the ratio of MnO₂ to V₂O₅ is 7 : 3, the performance is optimal.

To determine and verify the composition of the cathode products during the reaction process, we conducted X-ray diffraction (XRD) tests on the cathode in three states: original, charged, and discharged. As shown in Fig. 5(a), by comparing the XRD patterns of the charged and discharged states, we can see that a significant amount of LiMnO₂, LiV₂O₅, and VO₂ are generated during the discharge process. When the battery was in the charging process, we observed a reduction in these compounds, which fully confirmed the composition of the cathode products. To further validate our findings, we performed cyclic voltammetry (CV) tests on the battery. We tested the battery at sweep rates of 10, 20, and 50 mV s⁻¹. As shown in Fig. 5(b), a reduction peak is observed around 2.6 V. This peak is primarily caused by two combined reactions: one due to conversion between MnO₂/MnO₂⁻, V₂O₅/VO₂, and V₂O₅/V₂O₅⁻, and the other due to the Cl/Cl⁻ conversions. The theoretical platform for the first reaction is E = E_MnO2/MnO2⁻ + E_{V⁵⁺/V⁴⁺} – E_Zn/Zn²⁺ = 0.95 V + 0.991 V – (−0.76 V) = 2.7 V. The second theoretical platform is E = E_Cl/Cl⁻ – E_Zn/Zn²⁺ = 1.358 V – (−0.76 V) = 2.118 V. The peaks observed at 2.1 V and 2.6 V during the test corresponded well with the calculated theoretical discharge platform, matching the measured voltage closely.

Fig. 5 (a) XRD test diagram of the MnO₂@V₂O₅@C cathode. (b) CV curve of the battery at different scanning rates of 10, 20 and 50 mV s⁻¹.

To further determine the composition of the cathode, we conducted XPS testing on the fully charged cathode, and the results are shown in Fig. 6(a–e). As shown in Fig. 6(b), the Mn 2p spectrum consists of two peaks corresponding to the Mn 2p_1/2 and Mn 2p_3/2 spin–orbit doublets. The deconvolution peaks of Mn 2p_3/2 at 641.5 eV and 642.6 eV correspond to Mn³⁺ and Mn⁴⁺, respectively, indicating the presence of mixed valence states and proving the existence of Mn–O bonds. As shown in Fig. 6(c), we analyzed the V 2p spectrum and found two peaks due to spin–orbit splitting. The V 2p_3/2 spectrum shows two peaks at 516.9 eV and 517.4 eV, corresponding to V⁴⁺ and V⁵⁺, respectively, also indicating the presence of V–O bonds. The shuttle of Cl atoms is attributed to the adsorption of C, as evidenced in Fig. 6(d). The peak at 196.35 eV strongly indicates the presence of C–Cl bonds. Furthermore, we analyzed the O 1s spectrum. There are five peaks in the spectrum: the peaks at 529 eV, 529.9 eV, and 530.0 eV correspond to MnO₂, V₂O₅, and VO₂, respectively. The peak at 530.5 eV is due to O–H bonds in water molecules, and the peak at 531.3 eV confirms the existence of Li–O bonds.

Fig. 6 (a) X-ray photoelectron spectra (XPS) of the cathode MnO₂@V₂O₅@C: (b) Mn 2p, (c) V 2p, (d) Cl 2p, and (e) O 1s.

4. Conclusion

In this study, we propose a novel aqueous chloride-ion battery consisting of an MnO₂@V₂O₅@C cathode, a Zn anode, and a (CH₃)₄NCl + LiCl electrolyte. This new type of battery exhibits high discharge capacity, with a maximum discharge capacity of 457.4 mA h g⁻¹ and a discharge plateau of approximately 2.6 V. Even after 600 cycles, the discharge capacity remained at 237 mA h g⁻¹, surpassing the maximum discharge capacity of most aqueous chloride-ion batteries. We validated and explained the reaction mechanism of the battery through a series of tests. This outstanding performance is primarily due to the synergistic effect between MnO₂ and V₂O₅, which enhances the shuttling of Cl⁻, along with the adsorption effect of C on Cl, leading to excellent performance. Moreover, we conducted various safety tests on the battery, including burning, puncturing, folding, and rolling. As long as the battery is not completely damaged, it can continue to operate. Notably, the battery does not explode or cause other safety incidents even when exposed to flames at 1300 °C. This significantly improves the capacity of aqueous CIBs and increases their potential for replacing lithium batteries in portable devices, electric vehicles, military applications, and other fields, providing a new direction for the development of CIBs.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format, they are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Dalian University of Technology.

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