Application of ionic liquid electrolytes in Dual-ion batteries

The poor safety, material shortage and low energy density of conventional commercial lithium-ion batteries have driven the development of next-generation energy storage devices. Dual-ion batteries (DIBs) have attracted extensive attention from researchers in recent years as a novel energy storage device with high energy density, environmental protection, low cost and novelty. Unlike conventional “rocking chair” lithium-ion batteries, the cations and anions in the electrolyte of Dual-ion batteries can simultaneously participate in the electrochemical reactions of the positive and negative electrodes during charging and discharging. During the charging process, the cations form alloys on the surface of the metal negative electrode material or embed into the graphite negative electrode material; at the same time, the anions are electrochemically combined with the free radical positive electrode material or embedded into the graphite positive electrode material. The discharge process is the opposite. This new energy storage mechanism can not only greatly reduce the mass, volume and manufacturing cost of the battery, but also significantly increase the operating voltage of the battery, thereby increasing the energy density of the battery. In addition, the cathode of the dual-ion battery uses carbonaceous materials, which is more environmentally friendly and cost-effective, without the need for expensive, complex and toxic metal compound-based cathodes. This dual-ion battery technology provides new opportunities for designing green and miniaturized energy storage devices with high operating voltage (>4.5V) and long cycle stability, making Dual-ion batteries a promising new energy storage device to replace lithium-ion batteries. At present, electrolytes composed of carbonate organic solvents and conventional lithium salts are still the main research direction of Dual-ion batteries, but this type of electrolyte limits the electrochemical stability of Dual-ion batteries to a certain extent, and there are certain safety risks when it is used in batteries.

1 Overview of Dual-Ion Batterie

Dual-ion batteries are mainly composed of three parts: positive electrode materials, electrolytes, and negative electrode materials, and also include auxiliary parts such as separators and current collectors. Dual-ion batteries have different systems, and the electrochemical reactions that occur during operation are diverse. Among them, the electrochemical reaction process at the positive electrode mainly includes: the electrochemical combination of anions with free radical positive electrode materials or the reversible extraction/embedding of anions on graphite positive electrode materials; and the electrochemical reaction process at the negative electrode mainly includes: the reversible extraction/embedding of cations on graphite negative electrode materials, the electrochemical combination of cations and organic negative electrode materials, or the reversible electrochemical deposition of cations on the surface of metal negative electrode materials. Figure 1 is a schematic diagram of the charging and discharging mechanism of traditional LIBs and DIBs. Assuming that the anode and cathode in DIBs are both composed of graphite materials, during the discharge process, positive and negative ions are simultaneously extracted from the graphite electrode material and returned to the electrolyte; during the charging process, positive and negative ions are respectively embedded in the positive and negative graphite materials. Through this back-and-forth extraction/embedding of positive and negative ions, energy conversion and storage are achieved. The main differences between Dual-ion batteries and lithium-ion Datteries in principle are as follows: First, anions are embedded in the cathode during the charging process, which leads to a different electrochemical energy storage mechanism. This mechanism enables Dual-ion batteries to have high discharge voltage and high energy density. However, the high discharge voltage also places high demands on the electrolyte, which needs to be able to withstand high pressure without decomposing. Secondly, since the anions participating in the reaction come from the electrolyte, the electrolyte is also considered to be the active substance of the dual-ion battery, so a high-efficiency and high-concentration electrolyte is required in the dual-ion battery. A large amount of active anions is conducive to reducing the weight of the battery and increasing the energy density. In addition, Dual-ion batteries require higher porosity and thicker separators. At present, the research on Dual-ion batteries is still in its infancy.

Figure 1 Schematic diagram of the charge and discharge mechanism of (a) conventional LIBs and (b) DIBs (assuming that both the anode and cathode in DIBs are made of graphite materials)

In dual-ion batteries, the anion intercalation mechanism of the positive electrode material is the most important reaction. Graphite materials can accommodate anions due to their special layered structure. At the same time, the intercalation layers of graphite have a relatively large interlayer spacing, resulting in weak interlayer interactions. Therefore, the complex electrostatic and elastic interactions between anions will affect the subsequent anion intercalation kinetics. At present, there have been many studies on the intercalation mechanism of anions in graphite cathodes. These studies involve different anions, such as: tetrachloroaluminate (A1CI^–₄),hexafluorophosphate (PE), bis (trifluoromethylsulfonyl) imide (TFSI^–), difluoro (oxalate) borate (DFOB^–), tetrafluoroaluminate (AIF^–₄), fluorosulfonyl-(trifluoromethylsulfonyl) imide (FTFSI^–) and superhalogen (LiCh^–₂) . Among them, the first three are the most common intercalation anions. The negative electrode materials of dual-ion batteries have also been widely studied, and their reaction kinetics are also of great significance to the electrochemical performance of dual-ion batteries. The reaction mechanism of DIBs negative electrode materials is similar to that of lithium-ion battery negative electrodes, including deintercalation, alloying, conversion, and stripping/plating. At present, the two reaction mechanisms of deintercalation and alloying are more studied.

1.2 Commonly used electrolytes in dual-ion batteries

The electrolyte is one of the key components of dual-ion batteries. The anions and cations in the electrolyte not only participate in the redox reaction of the positive and negative electrodes as electrochemically active substances, but also play the role of transferring charges, thereby realizing energy conversion and storage. Therefore, the electrolyte plays a very important role in the electrochemical performance of dual-ion batteries. So far, there are many different types of electrolytes used in dual-ion batteries, such as: traditional carbonate-based electrolytes, ether-based electrolytes, sulfone-based electrolytes, gel polymer-based electrolytes and ionic liquid-based electrolytes. The ideal electrolyte for dual-ion batteries should have the following characteristics: ① safe and environmentally friendly, low cost; ② able to be used in a wide temperature range; ③ good stability, not easy to burn and decompose; ④ high ionic conductivity (>1.0mS·cm^-1); ⑤ a wide electrochemical window, so that the battery can operate at a higher operating voltage.

At present, the reported dual-ion battery electrolytes are mainly three categories: aqueous electrolytes, organic electrolytes and ionic liquid electrolytes.

1. Organic electrolytes

Organic solvent electrolytes are the most widely used type in LIBs, and their potential applications in DIBs have also been studied. Organic electrolytes mainly refer to electrolytes made by dissolving sodium salts, lithium salts or potassium salts as solutes in carbonate organic solvents such as ethylene carbonate (BC) or dimethyl carbonate (DMC), and adding vinylene carbonate (VC) or vinyl sulfite (ES) as additives. In addition, there are organic electrolyte systems with ethers, nitriles or sulfones as solvents.

Santhanam and Noel used polypropylene-graphite as an electrode to study the effect of different organic solvents on the intercalation/deintercalation efficiency of anions and cations in graphite electrodes. The solvents used in this study included N.N-dimethylacetamide (DMF), acetonitrile (AN), propylene carbonate (PC), dimethyl sulfoxide (DMSO) and methanol (MeOH), cations included lithium ions, sodium ions, potassium ions and tetrabutylammonium (TBA⁺), and anions included chlorine tetroxide (C1O^–₄) and boron tetrafluoride (BF^–₄). By analyzing the cyclic voltammetry (CV) data of the intercalation/deintercalation process of cations and anions in polypropylene-graphite (Table 1 and Table 2), it was shown that solvents with a high donor number (such as DMSO) are good solvents for monovalent cations, while solvents with a high acceptor number (such as PC) are conducive to the deintercalation of CIO^–₄ and BF^–₄ anions.

Table 1 CV data of cation insertion/deinsertion processes on polypropylene-graphite electrodes using different solvents

Note: DPP, Etn, QdivQin and QaQim are the deintercalation peak potential, insertion potential, deintercalation charge, insertion charge and insertion-deintercalation efficiency, respectively; the electrolyte concentration is 0.25 mol·L^-1, and the scan rate is 40 mV·s^-1

Table 2 CV data of anion insertion/deinsertion processes on polypropylene-graphite electrodes using different solvents

a. For lithium salt, the concentration of electrolyte is 0.25 mol·L^-1 and the scan rate is 40mV·s^-1

Seel et al. studied the electrochemical phenomena of PF^–₆ intercalation into graphite with different concentrations of organic solvents [such as ethyl methyl sulfone (EMS) and ethylene carbonate/diethyl carbonate (EC/DEC)] as electrolytes (Figure 2). The results show that the peaks with differential capacity [Figure 2 (a), (b) 1 are related to the coexistence of two phases during the intercalation of PF^–₆ into graphite (corresponding to the voltage plateau in the charge and discharge curves). The peak at 5.13V corresponds to the transition from stage 3 to stage 2, and the valley between the peaks indicates the formation of various pure layered (PF₆)C phases. In addition, in Figures 2 (a) and (b), the positions of the peaks and valleys appear at the same voltage, indicating that EMS is quite stable during anion intercalation even at high cut-off voltages. However, the dq/dV values in Figure 2 (c) are inconsistent with those in Figure 2 (d), indicating that EC and DEC may decompose or that the graphite structure is destroyed during PE intercalation. The lithium/graphite battery obtained a specific capacity of 95mAh·g^-1 at a discharge rate of C/7.

Figure 2 dq/dV performance

Graphite cycled in 2 mol·L^-1LiPF. /EMS to 5.3 V (early cycle) (a) and 5.4 V (late cycle) (b); graphite cycled in 1 mol·L^-1LiPF_6/EC and DEC to 4.9 V (early cycle) (c) and 5.1 V (early cycle) (d)

Fluorinated solvents and additives can generally make DIBs show better oxidation stability. Read et al. reported a DIBs that can operate stably and efficiently at a high cutoff voltage of up to 5.2V. By using LiPF₆ ethyl methyl carbonate-fluoroethylene carbonate (EMC: FEC = 6: 4) solution as the electrolyte, tris (hexafluoroisopropyl) phosphate (HFIP) as an additive is conducive to the embedding of PF anions into the graphite cathode, so that the battery shows good cycle stability. Figure 3 (a) shows the cycling performance of two different types of electrolytes, FEC and EC-EMC, in lithium/graphite batteries. In non-fluorinated carbonates, the cycling stability of lithium/graphite batteries is very poor, and the capacity of the battery drops rapidly after only 10 cycles, while the FEC-based electrolyte can enable the battery to operate stably for 200 weeks. The higher oxidation potential of the fluorinated component in the electrolyte helps to improve stability. Figure 3(b) shows the cycling performance of the dual-graphite full cell at C/7, with an initial capacity of about 60 mAh g^-1. This capacity gradually decreases in subsequent cycles (0.75% per cycle), which is worse than that in the semi-graphite cell (Figure 3(a)).

Figure 3 (a) Cycling behavior of lithium/graphite half-cell in FEC-based electrolyte: (1) specific capacity, (2) charge/discharge efficiency and
3) specific capacity in 1.2 mol·L^-1LiPF6EC-EMC (3:7) electrolyte: (b) Cycling performance of dual graphite battery:
(1) specific capacity and (2) charge/discharge efficiency

Although organic electrolytes are widely used, they cannot always exist stably under the high oxidation potential of the graphite positive electrode. The electrolyte will continue to degrade during the cycle, and the organic solvent will usually decompose, resulting in problems such as a shorter cycle life of the battery system and a lower Coulombic efficiency. In the application of large-scale energy storage devices, the flammability of organic electrolytes poses a safety hazard. Therefore, it is crucial to suppress the flammability of organic electrolytes. On the other hand, when the organic solvent undergoes a co-intercalation reaction in the layered structure of graphite, the solvent molecules in the graphite interlayer will decompose to produce gas, causing the graphite structure to expand, resulting in the destruction of the electrode structure and thus causing the graphite to fall off.

2. Aqueous electrolytes

Considering the cost and safety, it is very necessary to replace organic solvent electrolytes with aqueous electrolytes. Aqueous solvents are feasible in DIBs systems because they allow the insertion of anions at relatively low voltages and have advantages such as high conductivity, sustainability and low cost. However, one of the fatal drawbacks of aqueous electrolytes is their narrow electrochemical stability window. At present, there are relatively few reports on the use of aqueous electrolytes in dual-ion batteries. The Beck team in Germany carried out a series of studies in this regard in the 1990s. They mainly reported acidic aqueous solutions such as HBF4 and HF. During the charge and discharge process, H undergoes redox reactions in negative electrode materials such as lead, polypyrrole, carbon black, anthraquinone and its derivatives. In recent years, there have also been reports on dual-ion batteries based on new high-concentration dual salt-in-water electrolytes such as MgClz aqueous solution [SI, Mg(TFSI)2 aqueous solution, KFSI salt-in-water electrolyte, LiFSI and LiTFSI. The operating voltage of dual-ion batteries using aqueous electrolytes is lower than that of organic solvents, mainly because the decomposition voltage of water is lower. Sun et al. developed an environmentally friendly PVA-gelatin hydrogel-based salt-in-water electrolyte with transient degradation advantages for high-performance dual-ion batteries. PVA and gelatin together confine the water coordinated with ZnClz in the polymer network, which in turn enhances the reaction kinetics of the battery due to the reduced radius of the hydrated Zn ion. DIBs based on this electrolyte have an operating voltage of 2.0V and excellent cycling stability (capacity retention rate of 96.2% after 8000 cycles).

In DIBs, graphite-substituted cathode materials [e.g., metal-organic framework (MOF) materials, polycyclic aromatic hydrocarbon compounds (PAH), polydiphenylamine (PDPA), dihydrophenazine copolymers, p-type organic materials, and graphite-modified materials] all show lower working potentials than graphite. In addition, new anion carrier materials [i.e., BiF₄, Fe(C₂H₅), Mn₃O₄:], 1.4-bis(dibenzamido)benzene (BDB), and poly(2.2.6.6-tetramethylpiperidin-4-methacrylate-1-heloxy radical) (PTMA)] have sufficiently low working potentials to allow aqueous electrolytes to operate safely. For example, manganese (II, III) oxide (Mn₃O₄) [see Figure 4(a)] can reversibly intercalate/deintercalate NO in 1 mol·L^-1 NH₄NO₃ aqueous electrolyte. When the size of the cation (i.e., K, Nat) is small enough to be embedded in the Mn₃O₄ structure, it replaces the anion and is embedded in the Mn₃O₄. When cycled at a current density of 1 A·g^-1between 0 and 1 V (vs. Ag/AgCI), the battery achieves a discharge capacity of 150 mAh·g^-1. Although the capacity decays rapidly afterwards, it can be stabilized at around 50 mAh·g^-1 with a Coulombic efficiency of 99%, indicating that the reaction is highly reversible [Figure 4(b)]. In ferrocene [Fe(C₂H₅)₂], the Fe” ion is sandwiched between two cyclopentadienyl rings to form a monoclinic structure [Figure 4 (c)]. This electrode material can be used for reversible insertion/extraction of anions at low pressure. In order to inhibit its decomposition in aqueous electrolytes, ferrocene can be infiltrated into nanoporous activated carbon by melt diffusion. When the ferrocene-based half-cell is cycled between 0.4 and 1.2 V, in an electrolyte containing 30 mol·L^-1 ZnCl2, the reversible capacity is 106 mAh·g^-1 after 100 cycles at 100°C, and the capacity retention rate is 80%. The charge and discharge curves of the battery in the first week are shown in Figure 4 (d).

Figure 4 (a) Structure of MnsO4; (b) Charge and discharge test at a current density of 1 A·g^-1; (c) Schematic diagram of the monoclinic structure of ferrocene [Fe(C₂H₅)₂]; (d) Constant current charge and discharge curves of the ferrocene-based half-cell at a rate of 1C for the 1st to 3rd cycles in an electrolyte containing 30 mol·L^-1 ZnCl₂ aqueous solution.

Hu et al. proposed an aqueous dual-ion battery with defective Prussian blue (PB) as cathode, Ti₃C₂Tx-type MXene as anode, and mixed low-concentration KNO₃ and HNO₃ solution as aqueous electrolyte. Both the PB cathode and the MXene anode contain abundant structural water molecules, which can promote K intercalation and provide a continuous hydrogen bond network for H+ co-intercalation. The synergistic effect of electrolyte and electrode enables this unconventional aqueous dual-ion battery to show significantly improved energy density (41.5Wh·kg^-1), rate performance and cycle stability, and can achieve a capacity retention rate of 74% after 3000 cycles.

Wrogemann et al. designed a new type of DIBs based on aqueous/non-aqueous electrolytes, which achieved high cathode stability by forming a protective layer on the negative electrode (non-aqueous solvent) and improved its sustainability by using graphite-based positive electrode materials. The researchers used ex situ X-ray diffraction to confirm that the dual-ion battery can form a secondary acceptor-type graphite intercalation compound (GIC) composed of TFSI- anions in an aqueous electrolyte. The study also found that the selection of negative electrode materials has an important influence on the electrochemical performance of dual-ion batteries: the use of graphite negative electrodes can release a specific capacity of up to 40 mAh g at a current density of 200 mA g^-1. However, the selection of LiTi₂(PO₄)₃as the negative electrode material can also show some advantages, such as the initial discharge specific capacity of the battery at a current density of 200 mA g^-1 is about 42 mAh g^-1, and the capacity retention rate after 500 cycles is 71%.

2 Application of ionic liquids in dual-ion batterie

Trifluoromethanesulfonic acid, hexafluorophosphate, tetrafluoroboric acid and chloroaluminate ionic liquids can all be used as electrolytes in dual-ion batteries.

The study of ionic liquids in DIBs can be traced back to 1994. At that time, Carlin et al. reported the successful application of various room temperature ionic liquids as electrolytes in dual graphite batteries. By changing the types of cations [such as 1,2-dimethyl-3-propylimidazole (DMPI⁺) and 1-ethyl-3-methylimidazole (EMI⁺)] and anion types (such as AICI^–₄, BF^–₄, PF^–₆, CF₃SO^–₃ and CH₆, CO^–₂), different batteries based on dual ion insertion/extraction were constructed. By using [DMPI][AIC₄] as the electrolyte, the battery achieved an open circuit voltage of 3.5V and a cycle efficiency of 85%.

Sutto et al. used ionic liquids as electrolytes and studied the intercalation and deintercalation reactions of various cations and anions in graphite by combining electrochemical characterization and X-ray diffraction spectroscopy. They studied two different types of imidazole cations, especially disubstituted and trisubstituted imidazoles: EMI, 1,2-dimethyl-3-n-propyl imidazole (MMPI⁺) and 1-butyl-3-methyl imidazole (BMI⁺). These cations can be paired with the following anions: BE^–₄, PE^–₆, TFS-, bis(perfluoroethanesulfonyl)imide (PFESI-), NO^–₃ and HSO^–₄. Table 3 lists the charge and discharge efficiencies of different ionic liquids. The results show that trisubstituted imidazoles have stronger intercalation chemistry, while most anions show poor charge and discharge efficiencies, indicating that these anions are difficult to embed into graphite. Only imide-based anions show both high charge and discharge efficiencies and can form clear graphite embedding stages.

Table 3 Charge and discharge efficiency of cations and anions in different ionic liquids

Rothermel et al. used a mixture of N-butyl-N-methylpyrrolidine bis(trifluoromethanesulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide (Pyr1.4TFSI-LiTFSI) as the electrolyte of the dual graphite battery, and added ethylene sulfite (ES), an additive to promote the formation of a solid electrolyte interface (SEI), to the solution.

Figure 5(a) and (b) show the discharge capacity and coulombic efficiency of the dual graphite battery using the new ionic liquid electrolyte and ES additive in the first 50 cycles. In the first few weeks of cycling, the coulombic efficiency is relatively low, which can be attributed to the “formation reaction”. Figure 5(c) shows the relationship between the cell voltage, anode and cathode potential and charge and discharge time at different cycles (1, 2, 49 and 50 cycles) of the dual graphite battery. Compared with the subsequent cycles, the first cycle has a relatively high anode potential, that is, stage 2 (LiC₁₂) is observed, which indicates that the graphite anode is not fully intercalated by lithium ions. According to the behavior of the anode potential, the cathode potential rises to 5.21 V (vs. Li/Li⁺) in the first cycle. At the increase in rate, the cathode potential drops to 5.14 V (vs. Li/Li⁺), which is most likely due to the increase in the resistance of the battery. In summary, in Pyr1.4TFSI containing LiTFSI salt and ES additive, the insertion/deintercalation of lithium ions and TFSI- anions are reversible. The team then investigated the electrochemical intercalation/deintercalation properties of TFSI- in graphite in electrolytes containing sodium salts, and the results showed that its capacity was inferior compared to that of ionic liquids containing lithium ions.

Figure 5 Discharge capacity of lithium-graphite half-cell during the first 50 cycles without grid voltage: (b) Coulombic efficiency (cut-off voltage is 3.0 V and 5.0 V (vs. Li/Li⁺), current is 50 mA g^-1); (c) Relationship between the voltage curve (black, dotted line) and the anode and cathode potential curves (red line) of the dual graphite battery and time during the charge and discharge cycle

Since the radius of most anions is larger than the interlayer spacing of graphite, some researchers have proposed that anions with smaller radius are more easily reversibly embedded in graphite. Meister et al. studied the electrochemical embedding process of asymmetric fluorosulfonyl (trifluoromethanesulfonyl) imide (FTFSI^–) in graphite cathodes for DIBs. The radius of FTFSI^– anion (0.65 nm) is smaller than that of TFSI^– (0.8 nm), and the discharge capacity of FTFSI^–based batteries is higher than that of TFSI^–based electrolytes at the same charging potential. This study used in situ XRD characterization to study the embedding/de-embedding behavior of FTFSI^– in graphite in detail. The study showed that the parameters of the electrolyte (such as the formation and self-aggregation of ion pairs, rather than the size of the anions) are the main reasons affecting the anion absorption capacity. In this work, the intercalation behavior of imide-based ionic liquids in graphite cathodes was also studied [Figure 6(a)], including bis(pentafluoroethanesulfonyl)imide (BETI^–), FSI^–, FTFS^–, TFSI^–, FSI^–/TFSI^– (molar ratio = 11:1) and TFSI^–/FSI^– (molar ratio = 10:1). As shown in Figure 6(b), except for BETI^–, several current peaks corresponding to anion intercalation and deintercalation from graphite were observed in all imide-based ionic liquids. The second largest anion TFSI^– exhibited the lowest initial potential, while the third largest anion FTFSI^– exhibited a higher initial potential of 4.48 V. However, the smallest FSI^–anion showed the highest anion intercalation potential (4.53 V). The mixture of two different anions can also affect the intercalation/deintercalation behavior [Figure 6(c)]. In addition, different electrolyte systems also cause obvious changes in the anion intercalation and deintercalation current peaks [Figure 6(b)1, indicating the role of different anions in their intercalation and deintercalation behaviors.

Lv et al. used 1,2-dimethyl-3-propyl imidazole chloroaluminate (DMPI⁺) (A1C1^–₄) as an ionic liquid electrolyte and graphite rods as electrodes to prepare DIBs with excellent electrochemical performance. The developed DIBs have high discharge voltage stability of 4.2~4.0V and 3.6~3.1V, a reversible specific capacity of about 80mAh·g^-1 at a current density of 300mA·g^-1, and a coulombic efficiency of 97% after 300 cycles. At present, the non-flammability and electrochemical stability of the electrolyte used have certain improvements on battery safety and electrochemical performance, so that the fabricated DIBs can be applied to various critical conditions, such as high-rate charge and discharge, continuous bending and even combustion conditions. Figure 7 is the working principle of DIBs based on ILs electrolyte containing (DMPI⁺) (AICI^–₄)

Figure 6 (a) Schematic diagram of the size of a series of imide-based ionic liquids; (b) CV curves of DIBs using different imide-based ionic liquid electrolytes (BETI^– , TFSI^–, FTFSI^–, and FSI^–anions); (c) CV curves of DIBs using a series of ternary (TFS^– and FSI^–) and quaternary (TFSI^–/FSI^–and FSI^–/TFSI^–) imide-based ionic liquid electrolytes

Fig. 7 Working principle of DIBs based on ILs electrolyte containing (DMPI⁺)(A1C1^–₄)

Zhu et al. designed a metal-ion-free graphite NTCDA (1,4,5,8-naphthalenetetracarboxylic acid dihydrogenate) battery with pure Pyry4TFSI ionic liquid as electrolyte. The introduction of organic materials changes the storage mechanism of cations at the anode and weakens the self-discharge. The battery has a wide operating voltage range, a self-discharge rate of 6.12%·h^-1, and a discharge capacity of 114mAh·g^-1 at a current density of 100mA·g^-1 Due to the strong redox reaction between Pyn and the -CO- group of NTCDA, the battery voltage only dropped by 0.14V after standing for 15h. At the same time, the double-layer filter paper system with low porosity was used to improve the battery’s cycling performance. Figure 8 shows the working principle of the natural graphite dual-ion battery (NGB). During the charging process, Pyr⁺₁₄ combines with NTCDA and TFSI^– is inserted into the graphite layer. During the discharge process, both negative ions and cations can return to the electrolyte. The experimental results show that the redox reaction of the organic electrode can effectively reduce the self-discharge effect of the dual-ion battery with pure ionic liquid as the electrolyte, providing a feasible idea for designing better, more environmentally friendly and lower self-discharge rate new batteries.

Beltrop et al. added different conductive salt additives (including LiPF₆, LiBF₄ and LiDFOB) and fluorinated solvent additive methyl difluoroacetate (MDFA) to bis(fluorosulfonyl)imide (FSI)-based ionic liquids [i.e., ILs electrolytes (Pyr₁₄FSI/LiFSI)] to protect the A1 current collector (ACC) from anodic dissolution, thereby achieving reversible charge and discharge cycles in high-performance dual-ion batteries. The results show that the addition of a small amount of LiPF₆, LiBF₄ and MDFA (0.5%, mass fraction) to the FSI-based ILs electrolyte significantly improves the battery performance, while the addition of LiDFOB deteriorates the performance of the dual-ion battery. In addition, the dual-ion battery has excellent cycling performance in the Pyr₁₄FSI electrolyte with 5% LiPF₆, showing an average reversible capacity of 40mAh·g^-1 a coulombic efficiency of more than 98% and a capacity retention of 91%. Figure 9 shows the surface SEM image of the A1 current collector obtained after chronocoulometry (CC) measurements in electrolytes based on different ILs.

Xiong et al. developed a dual-ion battery-supercapacitor hybrid device (DIB-SCHD) using N-doped microporous carbon (N-MPC) as an electrode and LiCI mixed room temperature ionic liquid as an electrolyte. Both electrodes of this symmetrical energy storage device have battery-type and capacitive charge storage mechanisms. Combining ex situ characterization and theoretical calculations, DIB-SCHD simultaneously involves a synergistic energy storage mechanism of ion insertion/deinsertion and adsorption/desorption processes, achieving excellent electrochemical performance by promoting reaction kinetics.

Organic electrode materials have high theoretical capacity and flexible structure and are considered to be promising options in the field of rechargeable batteries. Wu et al. combined the advantages of DIBs and sodium-ion batteries to design a sodium-based dual-ion battery based on 3.4.9.10-tetracarboxylic dianhydride (PTCDA) organic anode, KS6 (a synthetic graphite) cathode and ILs electrolyte. Figure 10 is a working principle diagram of a sodium-based dual-ion battery. Due to the use of ILs, PTCDA can be dissolved in organic solvents. Combined with the stable crystal structure of PTCDA, the discharge capacity of the sodium-based dual-ion battery at a rate of 0.5 C can still reach 177 mAh·g^-1, and even at a rate of 20C, it has a discharge capacity of 60 mAh·g^-1, showing excellent cycle stability and rate performance. In addition, the sodium-based dual-ion battery also has an extremely low self-discharge rate (0.18%·h^-1at a rate of 0.5 C) and excellent fast charge-slow discharge performance.