High‐Performance, Activation‐Free Magnesium‐Ion Batteries Enabled by Ionic Liquid Electrolyte Additive

ABSTRACT

Magnesium‐ion batteries (MIBs), as a highly promising next‐generation energy storage technology, benefit from the high theoretical volumetric capacity (3833 mAh L⁻¹) of magnesium metal and its intrinsic safety. However, its commercialization is still hindered by sluggish de‐solvation kinetics during cycling, which prolongs the activation periods to reach maximum capacity and impairs rate performance. To overcome this bottleneck, we add 4‐ethyl‐4‐methylmorpholinium cation (EMM⁺) as an additive into the conventional all‐phenyl‐complex (APC) electrolyte. Density functional theory computations confirm that EMM⁺ shows a strong affinity for chloride ions (−0.513 eV), which weakens the Mg–Cl coordination and, thus promotes Mg²⁺ de‐solvation. In CuS‐based MIBs, the modified APC‐EMM electrolyte eliminates the activation cycles that are required with pure APC, and achieves a high specific capacity of 405.1 mAh g⁻¹ at 100 mA g⁻¹, while maintaining excellent rate performance (220.1 mAh g⁻¹ at 1 A g⁻¹). Notably, this electrolyte also shows significant improvements in capacity, activation kinetics, and cycling stability when applied to other cathode materials, including CuSe, Cu₇Te₄, Mo₆S₈, and perylene‐3,4,9,10‐tetracarboxylic dianhydride. This study establishes a de‐solvation‐accelerated electrolyte design concept as a universal paradigm for the development of high‐performance MIBs.

First‐principles calculations reveal that the 4‐ethyl‐4‐methylmorpholinium ion (EMM⁺) exhibits exceptionally strong affinity for Cl^–, which significantly accelerates the de‐solvation kinetics of Mg²⁺. By incorporating EMM⁺ into an all‐phenyl‐complex electrolyte, the cycle life of Mg symmetric batteries has been extended to 4000 h. When paired with a CuS cathode, this optimized electrolyte not only eliminates the activation process, but also enhances magnesium storage performance. Moreover, its beneficial effect extends to other cathode materials for Mg‐ion batteries.

1. Introduction

Lithium‐ion batteries (LIBs) have achieved mass production and widespread application due to their high specific energy [1, 2, 3]. However, the uneven distribution and scarcity of lithium resources, as well as safety concerns, have promoted extensive research into post‐lithium battery technologies [4, 5]. Among various alternatives to LIBs—such as potassium‐ion batteries [6, 7], sodium‐ion batteries [8, 9], zinc‐ion batteries [10, 11], and aluminum‐ion batteries [12, 13] and magnesium‐ion batteries (MIBs) have attracted significant attention because of their abundant magnesium resources [14], outstanding theoretical capacity (3833 mAh L⁻¹) [15], and the multi‐electron redox reactions of the magnesium metal anode [16]. Nevertheless, the development of MIBs remains hindered by challenges related to electrode–electrolyte incompatibility [17] and the strong Coulombic effect associated with multivalent Mg²⁺ ions [18].

Transition metal compounds, owing to their high capacity and abundant reserves, have long been regarded as the prime candidates for cathode materials in MIBs [19]. In particular, transition metal sulfides and selenides, represented by CuS [20], Cu₂S [21], and CuSe [22], have garnered wide attention due to their excellent specific capacity and voltage platforms. However, these cathode materials often suffer from rapid capacity fading, poor rate capability, limited cycling stability, and prolonged activation periods (the number of cycles required to achieve maximum capacity), which are primarily attributed to significant ionic polarization and volume changes during cycling. To address these issues, researchers have explored strategies, such as morphology control [23], element doping [24], and hybridization with functional materials, like carbon‐based materials [25] or MXene [26]. In addition, extensive efforts have been made to investigate novel magnesium storage materials including Bi‐based materials [27], functionalized metal organic frameworks [28, 29], and other transition metal compounds [30]. However, most of these studies have focused only on the electrode materials, neglecting the core issue of de‐solvation kinetics, resulting in unsatisfactory overall performance.

In terms of electrolytes, the development of electrolyte systems capable of highly reversible magnesium plating/stripping and excellent stability is crucial for advancing MIBs. To this end, various electrolytes have been designed, including all‐phenyl‐complex (APC) [31], magnesium aluminum chloride complexes [32], and Mg(AlCl₂BuEt)₂/tetrahydrofuran (THF) [33]. Although these traditional electrolytes meet certain practical requirements, their high concentration of free chloride ions often leads to corrosion of the magnesium metal electrode [34]. Furthermore, strong Coulombic interactions cause Mg²⁺ ions to form strong associations with halide ions and solvent molecules, which not only slows down the de‐solvation process of magnesium ions, but also limits the cycle life. In recent years, innovative strategies—mainly by introducing additives such as alkali metal ions [35], functional ions [36], or organic molecules [37]—have significantly enhanced the electrochemical performance of MIBs. While the mechanisms by which ionic liquid additives improve magnesium storage properties in some intercalation‐type cathode materials have been investigated [38], their effects in conversion‐type cathode materials remain unexplored.

The key to addressing the challenges of rapid capacity fading, unsatisfactory rate performance, limited cycling life, and prolonged activation periods lies in electrolyte regulation. Herein, we devise a de‐solvation‐centered electrolyte strategy based on additive screening guided by density functional theory (DFT). We identify 4‐ethyl‐4‐methylmorpholinium (EMM⁺) as an ideal chloride‐docking agent for APC electrolytes. By weakening the Mg–Cl bonds and accelerating the de‐solvation process, this strategy enables activation‐free operation and delivers excellent performance in copper‐based conversion cathodes such as CuS, CuSe, and Cu₇Te₄, while also enhancing the performance of widely used Mo₆S₈ and an organic perylene‐3,4,9,10‐tetracarboxylic dianhydride (PTCDA) cathode. In addition to accelerating de‐solvation, the additive helps stabilize the reaction intermediates and induces the formation of a MgCl₂‐rich cathode electrolyte interphase (CEI) layer, both of which further boost the performance of CuS cathodes. This work provides a comprehensive perspective on the application of ionic liquid additives in APC electrolytes and presents a promising new approach for improving the performance of MIBs.

2. Results and Discussion

2.1. Accelerated De‐Solvation and Enhanced Compatibility with Mg Anode and CuS Cathode

In conventional electrolytes, the strong interaction between Mg and Cl leads to the generation of persistent [Mg–Cl]⁺ complexes, resulting in a high energy barrier for the de‐solvation of Mg²⁺ [39, 40, 41]. To address this issue, we employed DFT calculations to screen the ion‐docking abilities of various ionic liquid cations (Figures 1a and S1), with EMM⁺ exhibiting the best performance (−0.513 eV). In this analysis, we only considered ionic liquids with the shortest carbon chains to minimize the electron‐donating effect of alkyl groups (Figure S2). Due to the solubility limitation of 4‐methyl‐4‐morpholinium chloride (EMMCl, Figure S3), we introduced 0.1 M EMM⁺ ions into the traditional APC electrolyte, referring to it as the APC‐EMM electrolyte, aiming to optimize MIB performance. As illustrated in Figure 1b–d, to assess the impact of ionic liquid identity on electrolyte performance, we selected two representative cations: EMM⁺, which exhibits the strongest ion‐docking effect (−0.513 eV), and 1‐butyl‐1‐methylpiperidinium (PP₁₄ ⁺), a widely employed ionic liquid cation additive with a comparatively modest ion‐docking effect (−0.298 eV). By choosing cations with different ion‐docking abilities, we sought to explore how this effect influences the dichlorination capability within the first solvation shell of Mg²⁺, thereby influencing the electrochemical performance.

FIGURE 1.

Design of APC‐EMM electrolyte. (a) Adsorption energies of chloride ions for different ionic liquid cations (b–d) Schematic diagrams of the operating mechanism of APC‐ionic liquid electrolytes (yellow, green, red, blue, and white spheres represent Mg, Cl, O, N, and H atoms, respectively).

To gain deeper insights into the discrepancies in solvation structures among different electrolytes, we conducted molecular dynamics (MD) simulations (green, light green, red, blue, and white spheres represent Mg, Cl, O, N, and H atoms, respectively). The solvation architecture plays a pivotal role in determining the electrolyte's ionic conductivity, stability, and its interactions with electrode materials. As shown in Figure 2a–c, in all three electrolytes, magnesium ions are coordinated with Cl^–, Ph^–, and THF molecules, while the ionic liquid additives are located slightly farther from the magnesium ions. The calculated radial distribution functions (RDF) in Figure 2d further show that as the ion‐docking effect strength increases, the peak distance between magnesium and chloride gradually increases (see Videos S1, S3, in which yellow and green spheres represent Mg and Cl ions, respectively), and the coordination number of chloride in the first solvation shell of magnesium ions decreases considerably. Figure S4 depicts the coordination structures of magnesium ions in different electrolytes. By measuring the distance between Mg and Cl, it can be observed that the distance increases from 2.664 Å in the APC electrolyte to 2.698 Å in APC‐PP₁₄, and finally to 2.837 Å in APC‐EMM. Raman spectra in Figure S5 further reveal that with the enhancement of the ion‐docking effect, the Raman peak intensity of the Mg–Cl bond progressively diminishes, while that of the solvated Mg–Cl species gradually elevates [42]. This indicates that a stronger ion‐docking effect can effectively reduce the bond energy of the magnesium–chloride bond, thereby facilitating the de‐solvation process of magnesium ions.

FIGURE 2.

Characterization of the solvated structure of electrolytes and their compatibility with magnesium metal anode. Snapshots of MD simulation boxes for (a) APC, (b) APC‐PP₁₄, and (c) APC‐EMM electrolytes (green, light green, red, blue, and white spheres represent Mg, Cl, O, N, and H atoms, respectively). (d) RDF of Mg²⁺ to Cl^– interactions in the three electrolytes. (e–g) SEM images (scale bar: 10 µm) and the corresponding cross‐sectional SEM images (scale bar: 15 µm) of magnesium anodes using the three electrolytes. (h) Tafel curves of the three electrolytes. DRT results of Mg||Mg symmetric batteries with (i) APC, (j) APC‐PP₁₄, and (k) APC‐EMM electrolytes. (l) Long‐term cycling performance of Mg||Mg symmetric cells in three different electrolytes. (m) Comparison of the running time of Mg||Mg cells using APC‐EMM and various reported electrolytes.

According to the above calculations, the introduction of ionic liquid additives with strong ion‐docking effects lead to a looser coordination between magnesium ions and chloride ions, thereby resulting in faster reaction kinetics. Electrochemical tests of the three electrolytes demonstrate that APC‐EMM exhibits superior performance. Linear sweep voltammetry (Figure S6) confirms that EMM ions undergo no decomposition within the tested voltage window, demonstrating their compatibility with the APC electrolyte. To further evaluate magnesium deposition/stripping efficiency across the three electrolytes, Coulombic efficiency (CE) was measured using the Aurbach method on both copper and stainless‐steel foil substrates. As shown in Figures S7 and S8, APC‐EMM delivers the lowest overpotential and the highest CE among all three electrolytes, indicative of suppressed side reactions at the electrode–electrolyte interface. As observed from the SEM images (Figures 2e–g and S9), the morphology of magnesium deposits and the extent of corrosion differ significantly among the three electrolytes after 200 h of cycling. In the APC electrolyte, substantial cracks and non‐uniform deposition appear on the magnesium surface, and cross‐sectional views reveal severe corrosion‐induced fissures. With the APC‐PP₁₄ electrolyte, the deposited layer is relatively smoother, but multiple cracks remain, and the cross‐section shows a corrosion depth of about 25 µm. In contrast, the APC‐EMM electrolyte results in a uniform, crack‐free deposit and reduces the corrosion layer thickness to approximately 4 µm. In Figure 2 h, APC‐EMM shows a higher exchange current density (increasing from 2.6 × 10⁻⁷ mA cm⁻² to 1.0 × 10⁻⁶, and 3.8 × 10⁻⁶ mA cm⁻²) in Mg||Mg cells, indicating much improved magnesium‐ion kinetics. Additionally, the ionic conductivity of APC‐EMM (1215 µS cm⁻¹) is also higher than that of APC (904 µS cm⁻¹) and APC‐PP₁₄ (1068 µS cm⁻¹), promoting faster reaction kinetics (Figure S10).

In situ electrochemical impedance spectroscopy (EIS) measurements were conducted during the stripping/plating process of Mg||Mg symmetric cells (Figure S11). As shown in Figure 2i–k, the relaxation time distribution functions fitted from these measurements indicate that, in symmetric cells, our focus is on Mg²⁺ de‐solvation and solid electrolyte interphase (SEI) formation behavior. The distribution of relaxation time (DRT) results (10⁻⁶ to 0.05 s) clearly show that the resistances associated with the characteristic time constants τ₁ (electrolyte contact), τ₂ (de‐solvation), and τ₃ (SEI formation) exhibit distinct electrochemical behaviors [43]. With the enhancement of the ion‐docking effect, the resistances for de‐solvation and SEI formation diminish significantly, indicating greatly accelerated de‐solvation kinetics and the formation of a thinner and faster‐forming SEI (Tables S2 and S3). At the same time, the number of tests required to reach a stable phase is also reduced. This further demonstrates that the reinforced de‐solvation structure significantly improves the compatibility of conventional APC electrolytes with magnesium metal anodes. During symmetric cell cycling, the nucleation potential decreases with stronger ion‐docking effects. As displayed in Figure 2l, significant polarization is observed for APC after around 300 h of cycling. The APC‐PP₁₄ electrolyte extends the cycle life to about 2200 h, while the APC‐EMM electrolyte surpasses 4000 h. In Mg||Mg cells, the electrochemical performance of APC‐EMM far exceeds that of other reported electrolytes. We compared the cycling stability of magnesium metal symmetric batteries using different electrolytes (Figure 2m and Table S1) and found that APC‐EMM exhibits much higher cycling stability than other reported electrolytes [44, 45, 46, 47, 48, 49, 50].

We further assembled full cells with all the three electrolytes to evaluate the performance in the MIBs, by pairing them with synthesized CuS nanosheets as the cathode due to the high theoretical capacity and high electronic conductivity. (Figures S12, S13 and Tables S4, S5). In the CuS nanosheets||Mg coin cell, the use of APC‐EMM electrolyte results in a significant improvement in battery performance. In Figures 3a, b and S14, analysis of the galvanostatic charge/discharge profiles and cyclic voltammetry (CV) indicates that all three electrolytes exhibit a two‐step electrochemical reaction mechanism. Notably, the APC‐EMM electrolyte lowers the oxidation peak potential from above 2.0 V (observed with APC) to approximately 1.7 V, while the reduction peak shifts upward from 0.7 to 0.8 V. In contrast, the APC‐PP₁₄ electrolyte exhibits only a slight reduction in polarization. CV analysis further demonstrates that the APC‐EMM electrolyte possesses lower polarization due to its faster de‐solvation kinetics, which are beneficial for achieving consistent long‐term high performance in the cell [20, 23]. Due to the accelerated de‐solvation, the APC‐EMM system reaches a stable state in CV curves within only two cycles, whereas APC system requires more than 15 cycles. APC‐PP₁₄ shortens the activation period in the CV curves to five cycles, which corresponds to the dechlorination kinetics of Mg²⁺ in the first solvation shell among the three electrolytes (APC‐EMM > APC‐PP₁₄> APC), suggesting that the reduction in activation period is related to the dechlorination process. As shown in Figure 3c, the modified electrolyte results in substantial improvements in both battery capacity (405.1 mAh g⁻¹ vs. 308.7 and 190.5 mAh g⁻¹) and cycling stability (capacity retention rate of 98.3% vs. 86.2% and 68.5%). Remarkably, with the CuS nanosheet cathode, the APC‐EMM system requires no activation period for capacity growth, whereas the APC electrolyte needs approximately 20 cycles to reach its maximum capacity. Consistent with these observations, APC‐PP₁₄ reduces the activation cycle to five cycles, further corroborating the positive influence of improved de‐solvation kinetics on the activation process. To rule out any interference from the introduced chloride ions, EMMPF₆ salt was incorporated as a control. The results demonstrate that all electrolytes containing EMM⁺ ions consistently show no activation phenomenon, further confirming that EMM⁺ is the primary cause of the suppressed activation (Figure S15).

FIGURE 3.

Electrochemical performance of CuS nanosheets||APC‐EMM||Mg coin cells for magnesium storage. (a) Galvanostatic charge/discharge profiles at 0.1 A g⁻¹. (b) CV curves of CuS nanosheet cathodes at a scan rate of 0.1 mV s⁻¹. (c) Comparative cycling performance at 0.1 A g⁻¹. (d) Galvanostatic charge/discharge curves at various current densities. (e) Rate performance of CuS nanosheet cathodes. (f) Comparison of electrochemical performance between this work and previously reported electrolyte‐modulation studies. (g) Long‐term cycling stability of CuS nanosheet cathodes.

The diffusion kinetics of magnesium ions were quantitatively analyzed using galvanostatic intermittent titration technique. Compared to APC and APC‐PP₁₄ electrolytes, the APC‐EMM exhibits a significantly enhanced Mg²⁺ diffusion coefficient (D _Mg2+) (Figure S16), which is directly attributed to faster solid‐state ion transport and, consequently, superior rate performance. Notably, this enhancement is mainly correlated with accelerated de‐solvation kinetics, which promote rapid ion dissociation at the electrode–electrolyte interface. EIS measurements (Figure S17) further corroborate the mechanistic advantage of APC‐EMM, as evidenced by a markedly reduced interfacial energy barrier relative to both APC and APC‐PP₁₄ electrolytes. This decrease highlights the important role of efficient de‐solvation in accelerating battery reaction kinetics. The kinetic advantage thus leads to excellent rate capability. In Figures 3d–f, APC‐EMM delivers capacities of 389.2, 287.4, 274.2, and 221.6 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 A g^{− 1}, respectively, outperforming previous electrolyte‐modification systems and demonstrating the outstanding rate stability provided by optimized de‐solvation [44, 46, 47, 48]. More importantly, at a current density of 1 A g⁻¹, APC‐EMM maintains 83.8% of its capacity after 1500 cycles, with an initial capacity of 220.1 mAh g^{− 1} (Figure 3g). Pseudocapacitive redox analysis (Figures S18 and S19) further provides mechanistic insights into the capacity enhancement associated with de‐solvation kinetics. The b values derived from CV analysis are 0.656 (oxidation peak 1), 0.991 (oxidation peak 2), and 0.774 (reduction peak), indicating a hybrid charge storage mechanism in which the accelerated de‐solvation process enhances the accessibility of magnesium ions in the bulk phase [20]. In the APC electrolyte, the b values similarly suggest a hybrid mechanism; however, the lower capacitive contribution is consistent with the inferior de‐solvation capability of APC relative to APC‐EMM. The presence of EMM⁺ may moderately influence the electrical double‐layer capacitance at the cathode interface. However, DFT calculations (Figure 4c) reveal that, the adsorption energy of EMM⁺ on CuS (−2.38 eV) is substantially weaker than that of MgCl⁺ (−4.09 eV) or Mg²⁺ (−4.50 eV), indicating minimal direct contribution from EMM⁺ to the double‐layer capacitance. Instead, the dominant source of enhanced capacitive behavior is the EMM⁺‐accelerated Mg²⁺ de‐solvation. The strong chloride‐docking ability of EMM⁺ weakens Mg–Cl coordination, reduces the de‐solvation barrier, and provides partially desolvated Mg²⁺ species for rapid, surface‐confined redox reactions, thereby amplifying the pseudocapacitive response (Figures S18 and S19). Control experiments with APC‐EMMPF₆ (Figure S15) further corroborate this interpretation. Therefore, while the ionic liquid additive modestly modifies the double‐layer capacitance, the accelerated de‐solvation kinetics serve as the primary driver of the improved pseudocapacitive contribution.

FIGURE 4.

The magnesium storage reaction mechanism of the CuS nanosheet cathode. (a) Ex situ XRD patterns of the CuS nanosheet cathode using APC‐EMM electrolyte. (b) Absorption models of possible cations from the APC‐EMM electrolyte on CuS. (c) The relevant calculated absorption energies. (d) Binary phase diagram between S and Cu. (e) Schematic diagram of the reaction mechanism.

2.2. Excellent Reaction Reversibility and EMM⁺‐Stabilized New Intermediate Phase Cu₇S₄

Ex situ x‐ray photoelectron spectroscopy (XPS) reveals the battery's operational mechanism by tracking the valence state evolution of cathode reactants during the reaction cycle. As shown in Figure S20, the CuS nanosheet cathode initially exhibits a pure Cu²⁺ signal (Cu 2p). Upon discharging to 1.2 V, a newly emerged Cu⁺ signal indicates partial reduction. As discharge progresses to 0.7 V, the Cu⁺ signal intensifies while the Cu²⁺ signal diminishes; at 0.1 V, Cu⁰ becomes the dominant species, with only a small amount of residual Cu²⁺. After recharging to 2.0 V, the pure Cu²⁺ signal is restored, confirming its excellent electrochemical reversibility. In situ EIS (Figures S21 and S22) show that the reversibility of CuS in APC‐EMM electrolyte is superior, as evidenced by minimal changes in Nyquist plots, whereas substantial variations are observed with the APC system. Notably, DRT analysis confirms that the de‐solvation kinetics in the APC‐EMM system is accelerated, with resistance in the 10⁻⁴–10⁻³ s interval (representing the de‐solvation regime) being significantly lower than that of APC. This aligns with the results from Mg||Mg symmetric cells and indicates markedly enhanced interfacial kinetics. In the APC‐EMM system, the SEI layer is thinner (as indicated by lower resistance in the 10⁻³–10^{− 2} s range), and the CEI layer remains stable (with suppressed peak shifts in the 10^{− 2}–10^{− 1} s interval), which further demonstrates effective suppression of electrode side reactions and greatly improved electrode/electrolyte compatibility. More importantly, the APC‐EMM system exhibits much lower solid diffusion resistance in the 10⁰–10² s interval (Figure S23) [43], implying rapid ion transport within the cathode and consequently enhanced electrochemical performance when using the APC‐EMM electrolyte.

To identify the Cu⁺‐containing intermediate phase, ex situ x‐ray diffraction (XRD) (Figures 4a and S24) detected phase transitions during discharge: CuS→Cu₇S₄→Cu⁰, accompanied by progressively intensified formation of MgS. During charging, these phases are reversibly recovered, highlighting the excellent reaction reversibility. Ex situ high‐resolution transmission electron microscopy (HRTEM, Figure S25) further confirms this evolution: At 1.2 V, facets of CuS, Cu₇S₄, and MgS coexist; At 0.7 V, Cu₇S₄ and MgS become the dominant phases; and at 0.1 V, pure Cu⁰ is observed. After charging, the CuS facets reappear. Elemental distribution mapping via energy dispersive x‐ray energy spectroscopy (EDS, Figure S26) under different charge/discharge states verifies the reversible storage and release Mg²⁺. Notably, EDS analysis shows that no nitrogen signal is detected in the CuS nanosheet cathode during cycling (Table S7), indicating that EMM⁺ ions do not act as charge carriers—unlike previously reported ionic liquid additives [38, 51]. The calculated absorption energies presented in Figure 4b,c show that EMM⁺ exhibits a less negative absorption energy on the CuS cathode compared with the Mg‐containing cations. Therefore, these computational results suggest that under open‐circuit or zero applied voltage conditions, EMM⁺ is more stable in the electrolyte bulk and is less likely to participate in direct reactions at the cathode interface. Moreover, phase diagram and formation energy analyses further rationalize the presence of the Cu₇S₄ intermediate: Its thermodynamically favorable formation (ΔG < 0) is further stabilized by the interfacial coordinative or electrostatic stabilization of EMM⁺ in the discharge state (Figure 4d). Compared to the Cu₂S intermediate formed in the APC system (Figure S27), the unique Cu₇S₄ intermediate formed in the APC‐EMM system is more favorable for the complete conversion to Cu⁰. This is mainly because the ΔG for the transformation from Cu₇S₄ to Cu⁰ is larger than that from Cu₂S to Cu⁰, thus enabling full utilization of the capacity (Figure 4e).

2.3. EMM⁺ Induced MgCl₂‐Dominated Inorganic‐Rich CEI

Comprehensive interfacial analysis using cryogenic‐TEM (cryo‐TEM), XPS depth profiling, and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) reveal that EMM⁺ fundamentally restructures the CEI [52]. Cryo‐TEM imaging (Figure 5a, b) shows a 66.7% reduction in CEI thickness (from 15 nm for APC to only 5 nm for APC‐EMM), demonstrating that parasitic reactions are effectively suppressed, which agrees well with the previous DRT analysis. Fast Fourier transform patterns further show that in the APC electrolyte, the CEI exhibits a mixed structure dominated by organic components and accompanied by minor inorganic species (Figures 5a1,a2,b1–b4). In striking contrast, the CEI formed in the APC‐EMM electrolyte is rich in inorganic MgCl₂ components. This structural transformation, not only enhances interfacial stability, but also accelerates de‐solvation kinetics by minimizing ion diffusion barriers [37]. In Figure S28a,c, the APC‐EMM‐derived CEI shows an inorganic Cl 2p peak at a higher binding energy, in contrast to the organic chlorine peak at lower binding energy in the APC‑derived CEI. Meanwhile, the Mg 2p spectra (Figure S28b,d,e) indicate that the content of MgO—associated with slow Mg²⁺ migration—is reduced in the APC‐EMM‐derived CEI, while the contents of MgCl₂ and Mg₃N₂—associated with faster Mg²⁺ migration—are increased [17, 53]. The higher proportions of electroactive components (MgCl₂ and Mg₃N₂) coupled with the reduced content of inert MgO indicate that EMM⁺ promotes the formation of electroactive species, thereby enhancing the stability of the CEI derived from the APC‐EMM electrolyte and ultimately improving the long‐term cycling performance of the CuS nanosheets||APC‐EMM||Mg battery.

FIGURE 5.

In‐depth investigation of the interaction mechanism between the electrolyte and the CuS nanosheet cathode. Cyro‐TEM images of the CuS nanosheet cathode after 100 cycles in (a) APC and (b) APC‐EMM electrolytes. ToF‐SIMS depth profiles of (c) Cl^–, (d) Mg^–, and (e) MgCl₃ ^–, along with (f–h) the corresponding three‐dimensional renderings of the CuS nanosheet cathode after 100 cycles.

ToF‐SIMS analysis further reveals the positive impact of EMM⁺ introduction on the bulk phase of the electrode during cycling. Compared to the CuS nanosheet cathode using APC electrolyte, the signal intensities of Cl^– (Figures 5c–f), Mg^– (Figures 5d–g), and MgCl₃ ^– (Figures 5e–h) inside the CuS nanosheet cathode using APC‐EMM electrolyte are all significantly reduced. This indicates that the introduction of EMM⁺ promotes the de‐solvation of Mg²⁺ and reduces the carrier size, thereby lowering the irreversible side reactions between Mg²⁺ and the electrode material, and enabling a more complete insertion/extraction of magnesium ions.

2.4. Versatility of APC‑EMM in Other Typical MIB Cathodes and Pouch Cells

Apart from investigating the CuS nanosheet cathode, we have also conducted electrochemical evaluations on a series of copper‐based cathode materials. After incorporating of the EMM⁺ ionic liquid cation additive into the APC electrolyte, the capacity, and rate performance, and cycling stability of CuSe and Cu₇Te₄ cathodes are significantly improved (Figures 6a,b, S29–S33, and Tables S8–S10). Specifically, at 100 mA g⁻¹, the CuSe cathode achieves a maximum capacity of 230.4 mAh g⁻¹ after only seven activation cycles, which is a substantial improvement compared to the APC electrolyte alone, where 60 cycles are required to reach a capacity of 160.5 mAh g⁻¹ (Figure S34a). Similarly, the Cu₇Te₄ cathode attains a maximum capacity of 115.6 mAh g⁻¹ after 16 activation cycles, while with the APC electrolyte, only 27.1 mAh g⁻¹ is achieved after 48 cycles (Figure S34b). This electrolyte modification significantly reduces the number of activation cycles required to achieve optimal high‐capacity performance.

FIGURE 6.

Universal compatibility of APC‐EMM in other typical MIB cathodes and the performance of CuS nanosheet||Mg pouch cells. (a) Galvanostatic charge/discharge curves and (b) rate performance of CuSe. (c) Galvanostatic charge/discharge profiles and (d) rate capability of PTCDA. (e) Galvanostatic charge/discharge curves and (f) rate performance of Mo₆S₈. (g) Capacity comparison after 200 cycles in APC‐EMM and APC electrolytes for CuSe, Cu₇Te₄, PTCDA, and Mo₆S₈. (h) Cycling performance of CuS nanosheet||Mg pouch cell at 500 mA g⁻¹ and (i) the concerned charge/discharge profiles of selected cycles. (j) Comparison with previously reported pouch cell performance. Inset in (h): digital photograph of a three‐layer CuS nanosheet||Mg pouch cell.

Beyond copper‐based materials, we also conducted universality tests on other widely used typical cathodes, such as PTCDA and Mo₆S₈ (Figures S35–S37 and Tables S8, S11). Figures 6c–f and S37 show clear improvements in reversible capacity, rate capability, and cycling performance, along with reduced voltage polarization. For example, the voltage polarization for Mo₆S₈ decreases from 0.57 V in the APC electrolyte to 0.18 V in the APC‐EMM electrolyte (Figure 6e). These results further demonstrate the broad applicability of the APC‐EMM electrolyte in the magnesium metal battery system, signifying that the fast de‐solvation kinetics in this battery system are universal (Figures 6 g, S34, and S38). As shown in the inset of Figure 6h, to evaluate the practical application potential of the APC‐EMM electrolyte, we assembled a three‐layer 4 × 6 cm magnesium metal pouch full cell using CuS as the cathode and a glass‐fiber separator; this pouch cell can be used to light LEDs and to power a mobile phone (Videos S4 and S5). This full cell likewise exhibits an activation‐free behavior and delivers a capacity of 185.4 mAh g⁻¹ at 0.5 A g⁻¹ (Figure 6i). When calculated based on the full cell mass, the energy density of the CuS nanosheets||Mg pouch cell is 134.5 Wh kg⁻¹, and the areal capacity is 4.3 Ah m⁻². In addition, it retains 92.7% of its capacity after 500 cycles (Figure 6h). In terms of both cycling stability and capacity retention, this work significantly outperforms previously reported magnesium metal pouch cells (Figure 6j and Table S12) [15, 54, 55, 56, 57, 58, 59], indicating that APC‐EMM maintains excellent electrochemical performance even under scaled‐up conditions, thereby providing strong support for its practical application in large‐scale energy storage.

3. Conclusions

Guided by DFT screening of chloride‐docking additives, we develop a multifunctional ionic liquid additive and construct an APC‐EMM electrolyte system. This design significantly accelerates the de‐solvation kinetics of Mg²⁺, endowing Mg||Mg symmetric cells with outstanding reaction dynamics and cycling stability, with cycle life exceeding 4000 h. In CuS nanosheets||Mg metal batteries, the electrolyte eliminates the conventional activation phase and achieves breakthrough electrochemical performance: a specific capacity of 405.1 mAh g⁻¹ at 0.1 A g⁻¹, and a capacity retention of 83.8% after 1500 cycles at 1 A g⁻¹. Furthermore, the introduction of EMM⁺ stabilizes the Cu₇S₄ intermediate, markedly enhancing the extent of the Mg‐storage conversion reaction in the CuS cathode. Characterization of the CEI reveals that rapid de‐solvation favors the formation of a MgCl₂‐rich interfacial layer, thereby improving its structural robustness and electrochemical stability. ToF‐SIMS analysis further shows the signals of Cl^–, Mg^–, and MgCl₃ ^– in the electrode are decreased, highlighting that tailored de‐solvation promotes efficient Mg‐ion uptake/release. This electrolyte modification strategy demonstrates broad applicability: For copper‐based cathodes, it shortens the activation period and boost capacity; For Mo₆S₈ and PTCDA cathodes, it simultaneously improves capacity, rate capability, and cycling stability. Collectively, this electrolyte engineering strategy provides an efficient and universal pathway for constructing high‐performance MIB systems.

Author Contributions

Renke Li: conceptualization, methodology, software, data curation, investigation, validation, formal analysis, writing – original draft, visualization. Yichen Du: conceptualization, methodology, data curation, project administration, visualization, resources, writing – review editing, supervision, formal analysis. Yaojie Lei: supervision, project administration, writing – review editing. Lili Song: conceptualization, writing – review editing, investigation, software. Jianlu Sun: conceptualization, methodology, software, data curation, investigation, validation, formal analysis. Yuehua Man: conceptualization, methodology, software, data curation, investigation, validation, formal analysis. Guoxiu Wang: supervision, project administration, writing – review editing, resources, validation, formal analysis. Xiaosi Zhou: resources, funding acquisition, supervision, project administration, writing – review editing, conceptualization, methodology, validation, formal analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie73131‐sup‐0001‐SuppMat.pdf.

Supporting File 2: anie73131‐sup‐0002‐VideoS1.mp4.

Supporting File 3: anie73131‐sup‐0003‐VideoS2.mp4.

Supporting File 4: anie73131‐sup‐0004‐VideoS3.mp4.

Supporting File 5: anie73131‐sup‐0005‐VideoS4.mp4.

Supporting File 6: anie73131‐sup‐0006‐VideoS5.mp4.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.