Chemical bonding strategy on boosting superior Li⁺ diffusion kinetics towards long-stable lithium metal anode

Designing additives to form chemical bonding with “solvent” for fast Li⁺ diffusion

Generally, Li⁺ coordinates with solvent molecules and anions in three different modes [Figure 3A]: solvent-separated ion pairs (SSIPs) by coordinating with solvent molecules, contact ion pairs (CIPs) by coordinating with one anion, and aggregates (AGGs) by coordinating with multiple Li⁺ ions and anions^[24]. LCEs consist mainly of SSIPs and free solvent molecules, known as a salt-in-solvent system. As the concentration of lithium salt increases, the quantity of solvent molecules decreases, and anions gradually penetrate the Li⁺ ions solvation shell, resulting in a significant decrease of SSIPs, while the number of CIPs and AGGs increases, which is referred to as a solvent-in-salt electrolyte. Li⁺ ions diffuse in electrolytes in two modes: vehicular motion and structural motion [Figure 3B]. Vehicular motion refers to the diffusion of Li⁺ ions along with their tightly bound solvation shell as one entity (making it difficult to move), which is usually considered as the main ionic diffusion mechanism in dilute liquid electrolytes. In contrast, structural motion involves the diffusion of Li⁺ through successive ion dissociation/association exchange across different shells (where Li⁺ mobility is higher)^[25]. As shown in Figure 3C, the movement of Li⁺ ions can be metaphorically illustrated using these two diffusion mechanisms in battery systems: Vehicular motion resembles a 400-meter race; structural motion is akin to a 4 × 100-meter relay race. In vehicular mode, one person moves from back to front, while in structural mode, several people move from back to front. Apparently, the structural mode is more efficient for Li⁺ diffusion.

Figure 3. (A) Schematic of two kinds of Li⁺ diffusion mechanism from different solvation structures: vehicular and structural motion from t = 0 to t = Δt; (B) The change of Li⁺ solvation structure with salt concentration. Reproduced with permission^[24]. Copyright 2023, IOP Science; (C) A metaphor illustration on these two diffusion mechanisms in battery system: 400 meters race represents vehicular motion; 4 × 100 meters relay race represents structural motion.

Li⁺ with CIPs and AGGs diffuses along via the structural motion. This mechanism explains why high-concentrated electrolyte (HCE) systems always present a high Li⁺ transference number. As shown in Figure 4A, the electrolyte system composed of 7 M Li[CF₃SO₂)₂N] (LiTFSI) in 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) exhibits an unexpectedly high t₊ of 0.73. This is attributed to reduced solvation sheath around Li⁺ in HCE, which makes Li⁺ more mobile^[19]. This HCE concept can also be applied in solid-state lithium batteries (SSLBs). Liu et al. reported a poly (vinylidene fluoride-co-hexafluoro-propylene) (PVDF-HFP)-based polymer-in-salt electrolyte (PISSE)^[26]. With the increase of lithium salt concentration, PVHLi-1.1 (PVHLi-x, where x is the mass ratio of polymer to lithium salt) contains a large number of AGG structures, indicating the strengthened binding of Li⁺/anion at high salt concentration [Figure 4B]. Besides the transport path along polymer chains such as conventional solid polymer electrolytes (SPEs), PISSEs offer another transport channel due to these AGGs, enabling Li⁺ rapid migration, as evidenced by the high t₊ of 0.53 [Figure 4C and D].

Figure 4. (A) Viscosity, ionic conductivity and t₊ of electrolytes with different ratios. Reproduced with permission^[19]. Copyright 2013, Springer Nature; (B) Raman characterization of solvation structure of PVHLi-x. Reproduced with permission^[26]. Copyright 2021, WILEY-VCH; (C) The measurement of t₊ of PVHLi-1.1. Reproduced with permission^[26]. Copyright 2021, WILEY-VCH; (D) The schematic diagram of Li⁺ ion transport pathways in PISSEs. Reproduced with permission^[26]. Copyright 2021, WILEY-VCH. PISSEs: Polymer-in-salt electrolytes.

Apart from this high-concentrated approach for constructing CIP and AGG, researchers found that regulating the solvation structure of Li⁺ ions through the rational utilization of intermolecular interactions can also achieve AGG mode [Figure 5A]. Specifically, the LATP nanoparticles are embedded in a PVDF-HFP matrix with one-dimensional channel structure, and the interactions between these LATP particles and succinonitrile (SN) inside the channel affect the solvation structure of Li⁺ ions in the electrolyte, promoting the formation of AGG structures, which significantly enhance the transport rate of Li⁺ ions^[27]. With this concept, Li⁺ solvation structure can be easily tuned by designing chemical bonds between inorganic additives and “polymer solvent” rather than increasing the concentration of lithium salts. In recently reported BN-assisted polymer electrolyte^[11], BN not only affects the interaction between Li⁺ and -CF groups, allowing Li⁺ to readily hop along polymer segments, but it also shifts the Li⁺ solvation structure from polymer units to AGG ion pairs, accelerating Li⁺ diffusion [Figure 5B]. As shown in Figure 5C, lithium montmorillonite in a polymer matrix can also change Li⁺ solvation structure through solvent molecules to anion participated clusters, as evidenced by Raman spectra. These as-constructed AGGs transport channels efficiently make rapid Li⁺ ions transfer kinetics. Quasi-solid electrolytes can also affect the ion transport behavior and deposition pattern^[28]. Researchers reported that, in a quasi-solid electrolyte (SiO₂-SO₃Li/PVDF-HFP), the component interactions among the polymer network, liquid electrolyte, and sulfonated SiO₂ nanoparticles were helpful for enhanced Li⁺ conductivity and excellent liquid electrolyte retention without volatilization^[29]. An infiltrating structure of a quasi-solid electrolyte comprising a concentrated electrolyte in a poly(tri-acrylate) network resulted in an ion transfer number of 0.79^[30].

Figure 5. (A) Schematic illustration of the as-designed JSE structure. Reproduced with permission^[27]. Copyright 2022, WILEY-VCH; (B) Schematic representation of the Li⁺ transport channel of the PHLBH electrolyte. Reproduced with permission^[11]. Copyright 2023, Elsevier; (C) Schematic representation of dual Li⁺ transport pathways in the Li-MPSE. Reproduced with permission^[12]. Copyright 2023, Elsevier. JSE: Janus quasi-solid electrolyte; PHLBH: PVDF-HFP/LiTFSI/BN; MPSE: montmorillonite/PVDF-HFP solid electrolyte.

Except polymer electrolytes, this inorganic additive strategy can also be applied in liquid electrolyte systems to boost the Li⁺ diffusion kinetics. Recently, researchers reported the successful development of a weakly solvating environment using nano-suspended additives such as Li₂O, Li₃N, and LiF which accelerated the migration of Li⁺ ions in the electrolyte^[31-33]. They first reported a suspended electrolyte composed of Li₂O nanoparticles, which altered the solvation environment of Li⁺ and produced a LiF-rich SEI^[31]. In traditional electrolytes [1 M LiPF₆ in ethylene carbonate (EC)/diethyl carbonate (DEC) with 10% fluoroethylene carbonate (FEC)], strong solvation interactions between Li⁺ ions and solvent molecules form SSIPs, making desolvation difficult. Furthermore, decomposition of solvent species results in organic-rich SEI. Li₂O suspended electrolyte can modify the Li⁺ solvation environment through its strong surface adsorption, causing Li⁺ ions to become weakly solvated by solvents and thus creating a weak solvation environment. In addition to Li₂O nanoparticles, the suspended electrolytes containing Li₃N have also been systematically studied due to their outstanding thermodynamic stability, high Li⁺ conductivity, and ability to suppress lithium dendrite growth in previous reports. Li₃N is lithiophilic, which can guide Li⁺ ions to deposit around Li₃N particles and avoid dendrite growth. Additionally, similar to the action of Li₂O, Li₃N can also create a weakly solvated environment by reducing the interaction between Li⁺ ions and solvents in the electrolyte, which accelerates Li⁺ ion transport [Figure 6A]^[32]. The Raman spectrum also indicates that in the Li₃N suspended electrolyte, the interactions between Li⁺ ions and the oxygen atoms in the C=O and C-O groups of EC are both reduced, suggesting that Li₃N weakens the coordination between Li⁺ ions and the solvent [Figure 6B]. Nano LiF particles have also been utilized recently as a model to capture free solvent molecules and build a low-activity solvent system [Figure 6C and D]^[33].

Figure 6. (A) The schematic illustration of Li₃N suspension electrolyte impacting the solvation structure and SEI composition. Reproduced with permission^[32]. Copyright 2023, American Chemical Society; (B) Raman peak area ratios of RE and Li₃N SE. Reproduced with permission^[32]. Copyright 2023, American Chemical Society; Schematic representation of the Li⁺ solvation environment and electrode interface chemistry in (C) control and (D) LiF-based suspension electrolyte. Reproduced with permission^[33]. Copyright 2024, American Chemical Society. SEI: Solid electrolyte interphase; RE: reference electrolyte; SE: suspension electrolyte.

Rational design of solvent for weakening the interaction between Li⁺ and its solvation shell

Ion transport is highly dependent on the solvation structure, which can be optimized by regulating the complex interactions between Li⁺ and the solvent. To enhance the kinetics of Li⁺ ion transport, one approach is to select appropriate solvent components, while another approach is to reduce the solvation energy of Li⁺ through rational design of solvent molecules. Huang et al. designed a hybrid solvent electrolyte 7FEC/3AN [1 M LiPF₆ in FEC/acetonitrile (AN), 7/3 by vol.]^[34]. The FEC-dominant solvation shell can form a stable SEI, and the AN constructs the ion hopping channel for fast Li⁺ migration. The solvation frequency was also proposed as the efficiency of Li⁺ ions hopping between solvents. The 4.4 V graphite (Gr)/NMC811 cell keeps a capacity retention of 70% after 1,000 cycles at 15 C [Figure 7A]. As illustrated in Figure 7B, achieving fast Li⁺ ion transport in batteries depends on high Li⁺ conductivity, low desolvation energy, and low energy barrier of Li⁺ ions crossing the SEI^[35]. A 1.8 M lithium bis(fluorosulfonyl)imide (LiFSI)-DOL electrolyte was also proposed, which facilitated Li⁺ ion transport and the Li/Gr cell delivered a remarkable capacity of 320 mAh·g^-1 at 4 C rate. In comparison, the conventional 1.0 M LiPF₆ EC/dimethyl carbonate (DMC) electrolyte exhibited a much lower capacity of only 20 mAh·g^-1.

Figure 7. (A) The schematic illustration of the solvation structure in different ratios of FEC/AN. Reproduced with permission^[34]. Copyright 2022, American Chemical Society; (B) All steps that affect kinetics of Li⁺ ions from cathode to anode during charging. Reproduced with permission^[35]. Copyright 2022, WILEY-VCH; (C) The summary of non-fluorinated weakly coordinating solvents. Reproduced with permission^[23,36-39]; (D) The summary of fluorinated weakly coordinating solvents. Reproduced with permission^[16,40-45]. FEC: Fluoroethylene carbonate; AN: acetonitrile.

Recent studies have focused on rational design of solvent molecules at the molecular level to reduce the solvation energy of Li⁺ ions and solvent molecules. This approach can be divided into two types based on whether a solvent is fluorinated. As shown in Figure 7C, the solvent solvation ability can be adjusted through the steric hindrance effect of solvent molecules. For example, Chen et al. designed a new solvent 1,2-diethoxyethane (DEE) by replacing the methoxy groups on the DME molecule with larger ethoxy groups^[36]. The increased steric hindrance of DEE resulted in lower solvation ability of Li⁺ ions compared to DME. Similarly, Park et al. introduced a methyl group into the middle segment of the DME molecule to design a low dielectric constant solvent 1,2-dimethoxypropane (DMP), which accelerates the kinetics of Li⁺ ion transport by reducing its solvation energy through increased steric hindrance effect^[37]. These bulky functional groups of solvents occupy space, thereby restricting the movement or reactions of other molecules or groups, which, in turn, reduces the solvation energy and accelerates Li-ion transport kinetics. In addition, the stability of the coordination structure between Li⁺ ions and solvent molecules is also one of the criteria for evaluating the solvent solvation ability. Ma et al. calculated the solvation ability of dimethoxymethane (DMM) and DME using density functional theory (DFT) calculations, and the DMM had weaker solvation ability for Li⁺ ions due to the poorer stability of the Li⁺-DMM four-membered compared to the Li⁺-DME five-membered structure^[23]. Compared to linear ethers, cyclic ethers have smaller dielectric constants. Yao et al. reported the non-polar solvent 1,4-dioxane (1,4-DX) as a weakly solvating electrolyte to accelerate the desolvation of Li⁺ ions and achieve fast charging in batteries^[38]. Li et al. found that the monodentate ether DPE had a weaker chelation ability than the tridentate ether DIG. Using highly non-polar DPE as a solvent, low-concentration (1.8 M LiFSI) ether-based electrolytes can be used in 4.3 V high-voltage LMBs^[39]. The high electronegativity of fluorine (F) can weaken the electron cloud density of oxygen atoms in ether compounds, resulting in their weak solvation ability. Based on the fundamental structure of DEE molecules, Yu et al. designed a series of fluorinated-DEE molecules by introducing -CF₃ or -CHF₂ groups at the β-position of DEE [Figure 7D], and found that CHF₂ groups exhibit superior electrical conductivity and performance compared to the fully fluorinated -CF₃ group^[40]. The weakly solvating solvent can promote the formation of anion-derived SEI, but solvents that are excessively weak can hinder ion dissociation and conduction. Thus, it is important to find a balance between electrode stability and ion transport. Based on the molecular structure of fluorinated 1,4-dimethoxybutane (FDMB), Wang et al. also increased the number of -CF₂ units in backbones to enhance the oxidative stability of the solvent molecules. The designed fluorinated 1,6-dimethoxyhexane (FDMH) was used as the main solvent, with DME as the co-solvent^[41]. By reasonable optimization of the electrolyte formulation, the anode-free Cu/NCM811 pouch cell keeps a 75% capacity retention after 120 cycles. It has been reported that 1,2-bis(2-fluoroethoxy) ethane (FDEE) solvent exhibits better salt solubility and oxidation resistance than 1,2-bis(2-chloroethoxy)-ethyl ether (ClDEE) solvent^[42,43]. To address the low conductivity of Li⁺ ions in fluorinated solvents, Shi et al. designed an amphiphilic fluorinated ether molecule [1,1,2,2-tetrafluoro-3-methoxypropane (TFMP)]. The highly fluorinated segment ensures the oxidative stability of the electrolyte (-CF₂CHF₂), while the other segment (-CH₂OCH₃) retains a methoxy group to enable weak coordination with Li⁺ ions^[44]. The TFMP solvents can form a core-shell-like solvation structure with Li⁺ ions in the electrolyte. The design strategy of combining the high oxidative stability of fluorinated ethers with the high solvation ability of ether solvents for Li⁺ ions has also been reported by Zhao et al., such as 2,2-dimethoxy-4-(trifluoromethyl)-1,3-dioxolane (DTDL) and 2-ethoxy-4-(trifluoromethyl)-1,3-dioxolane (cFTOF)^[45]. Recently, Xu et al. developed a series of fluorinated ester solvents as electrolytes for Li-ion batteries under extreme operating conditions by balancing the donor number (DN) value, dielectric constants (ε), and Li⁺-solvent binding energies^[16]. The methyl difluoroacetate (MDFA) was chosen as the soft solvent, methyl 2,2-difluoro-2 (fluorosulfonyl)acetate (MDFSA) as the co-solvent, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) as the anti-solvent. Additionally, 1 M LiTFSI in MDFA/MDFSA-TTE was used in the Gr/NCM811 battery and achieved a capacity retention of 93.9% at -30 °C after 260 cycles. The molecular design of weakly solvating electrolytes has received widespread attention and rapid development, as it fundamentally reduces the energy barrier of desolvation and enables fast Li⁺ transport kinetics.

Design of sieving layer with functional groups for desolvation

Introduction of a sieving layer seems a feasible way to actively peel off the solvation shell. Thankfully, the emergence of porous materials, such as covalent organic frameworks (COFs) and metal-organic frameworks (MOFs), offers new possibilities for formulating ion sieves not only by separator modification but also by direct electrolyte modifications and as Li⁺ transporting porous layers/coatings on an electrode [Figure 8A]. Their pore structure acts as a sieve to filter out extraneous solvent molecules inside the Li⁺ solvation layer. Because of their tunable chemical characteristics, long-lasting electrochemical/thermal stability, abundant cavity construction/porosity, open metal sites or functional groups, and vast surface area, COFs/MOFs have steadily been employed in LMBs for Li⁺ transport regulation^[49,50]. In these confinement channels formed by uniform nanopores of porous structures, Li⁺ can be partially or completely desolvated [Figure 8B] as long as the pore size is smaller than solvent molecular size^[51]. This sieving layer dramatically reduces the desolvation energy and boosts Li⁺ fast migration at interphase, hence effectively suppressing dendrites formation.

Figure 8. (A) Schematic representation of using porous membrane structures such as MOF, ZIF, COF, and others to favor desolvation and improve the Li⁺ transport kinetics at interface; (B) The selective Li⁺ transport while locking up the other species within the pores caused by the typical MOF channel is depicted graphically. Reproduced with permission^[50]. Copyright 2018, Elsevier. MOF: Metal-organic framework; ZIF: zeolitic imidazolate framework; COF: covalent organic framework.

Based on the above concepts, a series of ionic conductors based on sieving layer have been developed. For example, Chang et al. reported MOF films soaked with 1 M propylene carbonate (PC)-LiTFSI solution as ionic conductors to replace conventional separators and electrolytes^[51]. This ionic conductor exclusively contains the solvent-depleted CIP ion pairs since the solvent molecules have been peeled off before entering into MOF film. However, only PC molecules accompany Li⁺ penetrating into MOF pores due to the similar size (0.65 nm). The developed electrolyte displayed a significantly increased electrochemical stability window (4.5-5.4 V vs. Li/Li⁺), as well as improved stability to NCM-811 cathodes^[51]. Certainly, MOFs can also be used to modify separators for binding these anions and solvent molecules through their unique pore size, thus resulting in a high Li⁺ transference and uniform flow^[52]. For example, Li et al. designed a MOF coating layer on conventional separators, but different from the above MOF modification, these MOF particles are bridged with nitrogen-doped carbon (MOF@CC). As large-sized solvated Li⁺ ions pass through the small channels of this MOF@CC layer, the outer solvation sheath of the complex can be sieved, and the active C-N bridging bond in MOF@CC functioned as a catalytic site to drive the desolvation of inner Li(solvent)x⁺ with the aid of quick electronic injection facilitating the fast generation of naked Li⁺ ions in L/S batteries. To cope with the slow sulfur/sulfide reaction rates in Li/S batteries, the as-prepared MOF with catalytic functionalities is sought to also improve the lithium polysulfide (LPS) conversion kinetics. In bulk solution, Li⁺ ions surrounded by organic ether solvents impede the redox conversion between sulfur species. The solvation structure of Li⁺ transforms into anion-derived modes such as AGGs and CIPs. These AGGs and CIPs increased the rate of Li⁺ diffusion, preventing the formation of LPSs. The MOF@CC@PP modified cell shows higher capacities compared to the PP-based cell. Remarkably, the large areal pouch cell could maintain a capacity of 855 mAh·g^-1 after 70 cycles^[53].

As shown schematically in Figure 9A, the confining electrolyte within the UiO-66-MOF channels induces partial desolvation of Li⁺, lowering the activation energy barrier for charge transfer^[54]. Moreover, benefiting from the chemical bonding interaction (confirmed by binding energy calculation by DFT) between Lewis acidic sites of MOF and anions, this MOF-modified electrolyte results in a high diffusion coefficient (3.23 × 10^-7 cm²·s^-1) and a high t₊ (0.59), substantially suppressing the uneven Li deposition. Additionally, these MOF coatings on cathodes can also be beneficial in optimizing the Li⁺ solvation structure. Chang et al. Designed a novel “Li⁺ de-solvated ether electrolyte” by mechanically coating a ZIF-7 MOF layer on NCM811 cathode^[55]. After cycling, the “Li⁺ de-solvated electrolyte” has a unique CEI layer on NCM811 surface. Li et al. prepared strong anion-shielding MOF-based composite interlayers on the anode in combination with polyvinylidene fluoride-hexafluoropropylene copolymer to attain high Li⁺ transference number and homogeneous Li deposition due to the facile desolvation of Li⁺ [Figure 9B]^[56].

Figure 9. The Li⁺ desolvation is depicted schematically by channeled/porous layers on anode for fast Li⁺ transport and uniform deposition mechanism. (A) Li⁺ desolvation in liquid electrolyte and MOF-based SSI developed on LMA with immobilized anions interacting with Lewis acidic sites of pores and ionic channels for rapid Li⁺ transport. Li⁺ flow (arrows) and concentration (grayscale) along the LMA surface, as well as Li deposition morphology with and without MOF-based SSI, are shown. Reproduced with permission^[54]. Copyright 2020, Elsevier; (B) The behavior of Li plating/stripping on naked Cu foil and MOF-interlayer@Cu foil. MOF is responsible for desolvation, the rapid Li⁺ transfer and uniform flow, resulting in a smooth Li deposition. Reproduced with permission^[56]. Copyright 2022, WILEY-VCH; (C) A pure lithium-metal anode has poor desolvation and a lithium anode with an AMMT layer has a low desolvation barrier resulting in simplified desolvation. Reproduced with permission^[48]. Copyright 2023, WILEY-VCH. MOF: Metal-organic framework; SSI: semi-solid interphase; LMA: lithium metal anode; AMMT: Ag-montmorillonite.

COF materials with -OH functional groups may form hydrogen bonds with electrolyte solvents, which may regulate the desolvation process. For example, Yang et al. developed 2,4,6-trihydroxy-1,3,5-benzenetricarboxaldehyde (TPB)-4,4’-diamino-[1,1’-biphenyl]-3,3’-dicarboxylicacid COF membrane [TPB-BD(OH)₂-COF]-modified separators to create plenty of hydrogen bonds, such as OH...F (in OH/PF₆), OH...O (in OH/EC), and OH...O [in OH/ethyl methyl carbonate (EMC)]. Benefiting from these hydrogen bonds, the overwhelming number of free solvent molecules in the Li⁺ solvation sheath is reduced, hence leading to the easy desolvation process, while forming more aggregative structure for fast Li⁺ migration in bulk electrolyte. The full cell built with a COF-modified separator demonstrates superior cycling stability and even works functionally under extreme conditions (-60 °C)^[57]. A cationic COF comprised of positively charged organic moieties and weakly linked fluoride anions (F^-) has been reported to modify the commercial polypropylene separator to form (COFF@PP). It was discovered that the organic units have an abundance of nanopores which homogenized the Li⁺ flux, and interaction between the solvent molecules and organic units of COFs reduced the degree of Li⁺ solvation structure resulting in a simpler desolvation of Li⁺ thus increasing the mobility rate. COF-DEC has a roughly threefold greater interaction energy than Li⁺ and DEC, indicating that the COF units would compete with Li⁺ for solvent molecules and bind them around the separator to simplify the Li⁺ desolvation structure. Furthermore, the F^- inside the nanopores has been shown to aid in the construction of a strong LiF-rich SEI to avoid parasitic reactions between LMA and electrolyte^[49]. The chemical bonding strategy can also be applied in a cationic COF with positively charged organic moieties, which can form weak interaction with fluoride anions (F^-). The anchored anions result in a simpler desolvation of Li⁺ thus increasing the mobility rate. From an absorption perspective, it has been calculated that COF-DEC has a roughly threefold greater interaction energy than Li⁺ and DEC, indicating that the COF units would compete with Li⁺ to interact with solvent molecules and bind them around the separator to simplify the Li⁺ desolvation structure^[52].

Development of catalytic modulation layer for desolvation

Some catalytic protection layers or artificial SEIs have recently been noticed to encourage the desolvation process, too. Multiple lithiophilic hotspots in artificially produced LiNO₃-rich layers, for example, readily promoted desolvation resulting in well-distributed surface adsorption prior to deposition. The underlying process is the enhanced capacitive adsorption of Li^+[58]. The electrical interaction between the solvated Li⁺ and the defect-based catalysts can significantly reduce the Li⁺-solvent chemical binding, allowing for quicker desolvation^[13].

Pure Li foils can absorb organic solvents from electrolytes raising the energy barrier for desolvation and nucleation. According to extensive spectroscopic, simulation, and electrochemical analyses, lithophilic or catalytic artificial layers could also reduce the LMA surface affinity towards solvent molecules and significantly homogenize the lithium atoms flow for lateral deposition lowering the desolvation and nucleation barriers^[59,60]. For the artificial SEI, Feng et al. constructed an AMMT-based inherent channel with a negative charge layer on nanosheets. Combined with lithiophilic cations (such as Mg²⁺ and Ag⁺) exchange and exfoliation, the prepared layer acted as an ion-rectifying pump, offering intrinsic rapid Li⁺ transfer pathways. Directly dissociating solvated Li⁺ from the solvent molecules due to the strong coulombic force considering the negatively charged layers of AMMT-SEI results in a lower desolvation barrier [Figure 9C]^[48].

Wang et al. reported a catalytic kinetic layer that acted as a promoter to bind atomic iron to cationic vacancy-rich 3D porous carbon^[61]. Free Li⁺ ions are electrocatalytically dissociated from the Li⁺ solvation structure for uniform lateral diffusion by lowering desolvation and diffusion barriers via a catalytic layer, resulting in smooth dendrite-free Li plating [Figure 10A and B]. In another study, Wang et al. reported highly active single metal cobalt anchored in a vacancy on hierarchical porous nanocarbon^[62]. Several atomic sites, serving as an artificial protective modulation layer on the lithium surface, demonstrate superiority in modifying Li⁺ transport behaviors and smoothing the lithium deposition [Figure 10C and D].

Figure 10. Schematic representation of (A) Li plating on pristine LMA with sluggish desolvation and (B) Li plating on SAFe-based catalytic surface with facile interfacial Li⁺ desolvation. Reproduced with permission^[61]. Copyright 2023, WILEY-VCH; Schematic representation of (C) plated Li on pristine Li and (D) Plated Li on SACo/ADFS@HPSC catalytic layer with fast Li⁺ transport. Reproduced with permission^[62]. Copyright 2021, American Chemical Society. LMA: Lithium metal anode.

Clamp-like coordination structure of polyethylene oxide (PEO)-based electrolytes causes a crowding of the solvation sheath and excessively strong chemical bonding interaction between electrolytes and Li⁺, making it difficult to dissociate free Li⁺. Wen et al. investigated their weakly solvating properties via the control of coordinating geometry. The unique distorted asymmetry helical solvation sheath efficiently reduced Li⁺-electrolyte binding, resulting in accelerated Li⁺ desolvation kinetics, changed interfacial chemistry, and improved the performance at room temperature^[63]. To truly comprehend the charge-transfer behavior, a more sophisticated understanding of the chemical bonding interaction between solvation structure, interphase content, and corresponding ion dynamics, along with modern experimental analytical techniques at the interphase, is required.

Organic SEI layer

The use of EC in electrolytes has successfully opened the door to commercial LIBs. Understanding the decomposition products of EC molecules is the starting point for researchers to realize the organic component of the SEI. Fong et al. identified EC as a key solvent in the formation of SEI, but its reductive decomposition products have not been confirmed^[66]. Previously, the organic component in SEI was believed to be lithium ethylene di-carbonate (LEDC), which was formed via a single-electron charge transfer reaction of EC at around 0.8 V [Figure 11A]. Wang et al. synthesized lithium ethylene mono-carbonate (LEMC) and lithium methyl carbonate (LMC) through a simple and high-yield way, and measured ionic conductivity of LEMC as 0.006 mS·cm^-1, which is a good ionic conductor [Figure 11B and C]^[67]. In contrast, LEDC and LMC are almost ionic insulators, and LEDC is highly reactive and unstable in batteries. As the main organic component in SEI, LEMC ensures the rapid transfer of Li⁺ ions.

Figure 11. (A) The EC reduction via a single-electron pathway. Reproduced with permission^[67]; (B) Chemical synthesis of LEMC, LMC and LEDC·2DMSO. Reproduced with permission^[67]; (C) The ionic conductivity measurement of LEMC. Reproduced with permission^[67]. Copyright 2019, Springer Nature; (D) The schematic illustration of SEI layer with organic species. Reproduced with permission^[21]. Copyright 2023, Elsevier; (E) Raman spectra of electrolytes with different salt concentrations (LiTFSI in DOL/DME) and the schematic illustration of SEI layer with organic species. Reproduced with permission^[21]; (F) XPS analysis spectra of C 1s, F 1s, and Li 1s of the SEI. Reproduced with permission^[68]; (G) Li/Li symmetrical cell cycled under a current density of 5 mA·cm^-2 and an areal capacity of 10 mA·h·cm^-2. Reproduced with permission^[68]. Copyright 2018, American Chemical Society. EC: Ethylene carbonate; LEMC: lithium ethylene mono-carbonate; LMC: lithium methyl carbonate; LEDC: lithium ethylene di-carbonate; DMSO: dimethyl sulfoxide; SEI: solid electrolyte interphase; LiTFSI: Li[CF₃SO₂)₂N]; DOL/DME: 1,3-dioxolane/1,2-dimethoxyethane; XPS: X-ray photoelectron spectroscopy.

Chu et al. conducted systematic research of the relationship between SEI and the solvation structure at different concentrations of 0.1-1.0 M LiTFSI in DOL/DME [Figure 11D]^[21]. In low-concentration systems, Raman spectra [Figure 11E] showed that there are many free solvent molecules, and the preferential decomposition of the solvent results in the presence of a large number of organic species in the SEI components, leading to poor cycle performance of Li/Li and Li/Cu cells at different current densities.

High elastic modulus design is an effective method to improve the compatibility of the organic SEI layer and the volume effect of electrode materials during the charge and discharge process of LMAs. Hu et al. designed the 1 M LiTFSI in vinylene carbonate (VC) electrolyte that generates a stable and highly conductive SEI on the LMAs^[68]. X-ray photoelectron spectroscopy (XPS) analysis revealed that the SEI layer has a high carbon content on the surface and the F element remains relatively stable throughout the SEI. This indicates that VC plays a crucial role in the initial stage of reduction polymerization to form the organic SEI, which stabilizes the LMAs [Figure 11F]. Li/Li symmetric cells can maintain stable plating and stripping for 4,000 h, even at a high current density of 5 mA·cm^-2 and an areal capacity of 10 mAh·cm^-2 [Figure 11G]. However, most organic SEI components still have drawbacks such as low Li⁺ ions conductivity and poor interfacial stability. Overall, the single organic SEI component is still not the optimal choice for stable LMBs.

Inorganic SEI layer

Compared with the organic SEI layer, the inorganic-rich SEI layer has excellent ionic conductivity and electronic insulation^[27]. Moreover, the high Young’s modulus of the inorganic SEI layer can also suppress the growth of lithium dendrites. This section will discuss the construction strategies of inorganic SEI components from a coordination chemistry perspective, including LiF, LiCl, Li₃N, and hybrid inorganic components. With LiF as the main component, the fluorinated SEI can improve the cyclic stability of LMAs and facilitate the uniform plating/stripping^[69]. LiF-rich SEI can be produced in high-concentration electrolytes containing abundant FSI^- or bis(trifluoromethanesulfonyl)imide (TFSI^-) anions. Additives such as FEC and di-fluoroethylene carbonate (DFEC) are also commonly used to construct fluorinated SEI. Xie et al. designed fluorinated molecules to promote the formation of LiF to construct stable and conductive SEI. The C–F bond on a perfluoro chain (-CF₂-CF₂-) has a high binding energy and is difficult to break during electrochemical reactions^[70]. The β-position of 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (DFA) molecules contains an inert methoxy group (H₃CO-), which cannot undergo a defluorination progress. By introducing NO₃^- functional groups at the β-position, all the F atoms can be effectively released into 2,2,3,3-tetraflurobutane-1,4-diol dinitrate (AFA) molecules. The elimination reaction at the β-position can activate the C–F bond and further induce the removal of the F atoms [Figure 12A]. In addition, the hydrofluoroether (HFE) has been shown to be an effective diluent in previous localized high-concentration electrolyte (LHCE) designs. As demonstrated in Zhang’s work, fluorinated alkyls (-CF₂-CF₂-) are difficult to release F and construct LiF-rich SEI in LMAs. Tu et al. proposed the synergistic effect of diluents and anions in regulating the solvation structures and constructing SEI^[5]. The aromatic compound 1,2-dfBen was introduced as a diluent into the Pyr₁₃FSI ionic liquid electrolytes (ILEs) dissolving LiFSI. This 1,2-dfBen in ILE not only intensifies the interaction between Li⁺ and FSI^-, achieving an ultra-high proportion of AGGs in the solvation structure, but also participates in constructing LiF-rich SEI. The diluent reduces the viscosity of the ILE system and greatly improves Li⁺ transport kinetics in the bulk electrolyte and the interface. It was revealed that the specific coordination mode is the enhancement of the coordination between Li⁺ and the O atom on the side of the N atom. This extends the concept of LHCE and rational optimization of diluents [Figure 12B]. Similarly, Zhu et al. proposed two other aromatic compound diluents, trifluoromethoxybenzene (TFMB) and benzotrifluoride (BZTF), which have a synergistic effect with anions to promote the formation of uniform and stable LiF-rich SEI [Figure 12C]^[71]. Different from LiF, the chlorine-doped double halide SEI (LiF_1-xCl_x) has faster Li⁺ ions conductivity^[72]. As shown in Figure 12D, the energy barrier for Li⁺ ions to migrate in LiF_1-xCl_x is significantly lower than that in LiF under the same migration pathway^[73]. The Cu/NCM523 pouch cell with 1.3 M LiFSI in DME/DCE (1,2-dichloroethane) provides 70% capacity retention after 125 cycles.

Figure 12. (A) Molecular structure of AFA and DFA. Reproduced with permission^[70]. Copyright 2022, WILEY-VCH; (B) The schematic illustration and Raman spectra of M-ILE and three types of Li-FSI^- coordination. Reproduced with permission^[5]. Copyright 2022, American Chemical Society; (C) The schematic diagram of synergistic function between diluents and anions. Reproduced with permission^[71]. Copyright 2022, American Chemical Society; (D) The energy barrier of migration pathway of LiF and LiF_1-xCl_x. Reproduced with permission^[73]. Copyright 2022, Springer Nature. AFA: 2,2,3,3-tetraflurobutane-1,4-diol dinitrate; DFA: 2,2,3,3-tetrafluoro-1,4-dimethoxybutane; ILE: ionic liquid electrolyte.

The Li₃N is a common component of inorganic SEI and exhibits superior ionic conductivity compared to LiF. Adding LiNO₃ to carbonate or ether electrolyte systems is the simplest and most widely used way to construct Li₃N-rich SEI. However, NO₃^- has a high DN value (22) and the strong interaction between Li⁺ and NO₃^-, resulting in a low solubility. Liu et al. dissolved large amounts of LiNO₃ using dimethyl sulfoxide (DMSO) (DN value of 30), and then added the complex (LiNO₃-S) into LiPF₆ FEC/DMC electrolyte^[74]. After the addition of LiNO₃-S, the solvation sheath structure of Li⁺ ions transforms from FEC and DMC molecules to NO₃^-, forming a Li₃N-rich SEI. The morphology of lithium deposition also changes from a loose dendritic structure to a dense block shape [Figure 13A]. In addition to using solvents with high DN values to dissolve LiNO₃, Zhang et al. reported Sn²⁺ and In³⁺ as the Lewis acids to increase the solubility of LiNO₃ in carbonate electrolyte, greatly improving the cycling stability of LMBs^[75]. However, the influence of these multivalent metal cations on the SEI may be related to specific adsorption of inner Helmholtz plane (IHP). Yan et al. found that trace amounts of Cu²⁺ reached the lithium metal interface and then NO₃^- anions, as the main solvation sheath of Cu²⁺, were carried to the IHP, inducing the formation of LiN_xO_y-rich SEI and promoting the spherical lithium metal deposition [Figure 13B]^[76]. Wang et al. designed a novel amide-based electrolyte 1 M LiTFSI in 2,2,2-Trifluoro-N,N-dimethylacetamide (FDMA)/FEC (1/1 by vol.), which also formed Li₃N-rich SEI through solvent optimization. The N-CH₃ bond in FDMA is attacked by two Li⁺ and two electrons, eventually forming Li₃N-rich SEI^[77]. Research shows that the hybrid inorganic SEI components often exhibit superior performance. By combining the rapid Li⁺ ion transport kinetics of Li₂CO₃ with the excellent electron insulation properties of LiF, Wu et al. developed tris(4-fluorophenyl)phosphine (TFPP) as an electrolyte additive to construct a Li₂CO₃/LiF heterostructured SEI^[78]. The heterostructured SEI exhibits the highest Li⁺ ion affinity (0.66 eV), which facilitates uniform Li⁺ ion flow and enables rapid diffusion of Li⁺ ions in the SEI [Figure 13C]. Recently, Mao et al. reported an anion-enrichment interface, which is achieved by using weakly dissociated lithium salts [LiBF₄ and lithium difluoro (oxalate) borate (LiDFOB)] and ethyl 3,3,3-trifluoropropanoate (tFEP). Anion enrichment at the interface promotes further decomposition of anions, forming an SEI layer rich in various inorganic substances [Figure 13D]. In the designed electrolyte, the energy barrier for Li⁺ ion migration through the SEI is 25.89 kJ·mol^-1, which is lower than 36.58 kJ·mol^-1 of the commercial electrolyte^[79]. The impact of Li₂O-rich SEI on CE has been recently revealed, demonstrating the possibility of achieving CE higher than 99% using oxygenated rather than fluorinated solvents and salts to obtain highly competitive SEI^[80]. Therefore, the combination of multiple inorganic substances in the inorganic SEI layer has greater potential to stabilize LMAs and promote fast Li⁺ ion transport kinetics.

Figure 13. (A) Li morphology deposited on Cu foil with LiNO₃-free and LiNO₃-S electrolyte. Reproduced with permission^[74]. Copyright 2020, WILEY-VCH; (B) Schematic illustration of adsorption characteristics with different ions in IHP and OHP. Reproduced with permission^[75]; (C) The heterostructured SEI exhibiting the highest Li⁺ ions affinity. Reproduced with permission^[78]. Copyright 2022, WILEY-VCH; (D) Schematic illustration of anion-enrichment interface. Reproduced with permission^[79]. Copyright 2023, Springer Nature. IHP: Inner Helmholtz plane; OHP: outer Helmholtz plane; SEI: solid electrolyte interphase.

Robust hybrid organic/inorganic SEI layer

The inorganic SEI layer with high Young’s modulus shows the ability to inhibit the growth of lithium dendrites, but its rigid characteristics are not compatible with the volume effect of the electrode material, and repeated rupture and reconstruction accelerate the failure of the battery. The hybrid organic-inorganic composition of SEI not only preserves the rapid Li-ion conductivity of the inorganic component but also possesses the excellent flexibility of the organic component, significantly improving the cycling stability of LMBs. Therefore, a polymer-in-salt electrolyte (PISE_0.7-TTE₉₀) has been developed using a highly concentrated LiTFSI as the salt, polyethylene glycol dimethyl ether (PEGDME) as the solvent, and TTE as the cosolvent^[65]. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) results provide further insight into the dual-layer structure of the SEI formed in the PISE_0.7-TTE₉₀ system. The maximum intensity of the C₂HO^- signal is observed at the SEI surface, followed by rapid decay. Conversely, the maximum intensity of the LiF^- signal appears at 10 s, followed by a smooth decay, indicating the formation of LiF under the C₂HO^- layer. The 3D reconstruction images clearly demonstrate that the SEI is composed of organic compounds on the top layer and inorganic compounds on the bottom layer. The formation mechanism of dual-layered SEI has also been thoroughly investigated through frontier orbital theory. In the PISE_0.7-TTE₉₀ system, TFSI^- has a reduced lowest unoccupied molecular orbital (LUMO) than PEGDME molecules, resulting in its preferential decomposition into inorganic components on the LMAs. Then, PEGDME molecules are attacked by electrons to form radicals, which get self-polymerization to form a polyether network.

Organic-inorganic SEI can also be constructed using room-temperature ionic liquids (RTILs). Wang et al. immersed the LMAs in a reactive ionic liquid (Pyr₁₃FSI), where the highly reactive FSI^- anions react with LMAs to form an inorganic layer of LiF, and the organic segments were chemically adsorbed on the inorganic layer^[81]. Thus, a well-ordered organic-inorganic SEI layer can be obtained through a simple self-assembly approach [Figure 14A]. The atomic force microscopy (AFM) shows that the average roughness of the LMA surface was only 19 nm, and the Young’s modulus of the organic and inorganic layers was further calculated from the force curve to be 2.1 and 25.19 MPa, respectively. Ionic liquids can effectively construct organic-inorganic dual-layer SEI due to their unique molecular structure. Wang et al. dissolved 0.1 M LiDFOB in N-methyl-N-methoxyethyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([MEMP][TFSI]) ionic liquid with TTE as a co-solvent, developing an ultralow-concentration electrolyte (ULCE). Due to the lower LUMO value, Li⁺-DFOB^- and Li⁺-TFSI^- will preferentially decompose at the interface, ensuring the cycling stability of LMAs. Additionally, the surface protrusions of LMAs possess a higher potential, resulting in the migration of MEMP⁺ towards them under an electric field [Figure 14B]. The electrostatic repulsion effect between Li⁺ and MEMP⁺ will promote the formation of uniform Li⁺ ion flux and suppress the growth of lithium dendrites^[82].

Figure 14. (A) The schematic synthesis of ordered SAHL SEI layer. Reproduced with permission^[81]; (B) Schematic illustrations of the solvation structure of ULCE. Reproduced with permission^[82]; (C) Organic/inorganic SEI dual-layer derived from decomposing FEC solvent. Reproduced with permission^[83]; (D) Schematic representation of the dual-layered SEI formed on the lithium metal electrode. Reproduced with permission^[84]. SAHL: self-assembled ordered organic/inorganic hybrid dual layers; SEI: solid electrolyte interphase; ULCE: ultralow-concentration electrolyte; FEC: fluoroethylene carbonate.

Yan et al. have demonstrated that FEC can spontaneously form a dual-layered film on the surface of LMAs [Figure 14C]^[83]. The ab initio molecular dynamics (AIMD) analysis reveals that positive-charged Li atoms and negative-charged F atoms can break the C–F bonds in FEC through electrostatic attraction. As a result, organic compositions such as CH₂CHOCO₂Li and CH₂CHOLi deposit on the electrode surface, forming a dual-layered interface structure with an organic outer layer and an inorganic inner layer. The average Coulombic efficiency (CE) of Li/Cu cells can reach 98.3%, and the capacity retention of anode-free Cu/NCM523 cells remains at 67.7% after 20 cycles, and the morphology of the LMA is dendrite-free. Li et al. developed a quasi-solid-state polymer electrolyte with an ionic conductivity of 2.2 × 10^-4 S·cm^-1 at -20 °C by utilizing the self-polymerization of 1,3,5-trioxane (TXE). The co-decomposition of DFOB^- anions and FDMA produced a dual-layer SEI with a dominating outer layer of Li_xBO_yF_z and a LiF-rich inner layer [Figure 14D], which ensured the superior low-temperature performance of the polymer electrolyte^[84]. The activation energy for Li⁺ ion migration is calculated to be 0.33 eV, and the Li⁺ ion transference number is as high as 0.8 at -20 °C. In summary, regulating solvent structure can be used to obtain SEI with different compositions. However, single-component SEI still has limitations in battery applications, such as poor interfacial compatibility, component fragility, and electrochemical instability. The organic-inorganic hybrid SEI layers have the advantages of high Li⁺ ion conductivity and robust mechanical properties. Understanding the intrinsic connection between solvent structure and SEI composition provides new opportunities for high-performance LMB design.

Host conductivity enhancement

Porous hosts with high conductivity and high surface area are expected not only to lower and uniformize the deposition current density through efficient coupling between electrons and Li⁺, but also to backstop the volume changes of platted Li, thus improving the deposition kinetics^[87]. Among various conducting materials, metal hosts, due to their superior conductivity, have been initially reported. These porous metal hosts (Ni, Cu, Ti, etc.) successfully bond with Li to reduce the deposition energy barrier. For example, Li et al. developed a copper mesh, which provides abundant deposition sites for Li plating. Compared to Cu foil as a current collector, this Cu mesh with large surface boosts electronic exchange, and simultaneously decreases the local current density; hence, superior plating stability can be observed^[88]. Although these porous metal hosts make some progress on improving Li cycle stability, it is still far away from the expectation of commercial application. The main reason is probably caused by the weak affinity between Li and metal hosts, which cannot support long lifespan of Li anodes^[89]. Moreover, the heavy weight of metal hosts is another negative factor for practical application in terms of cost and energy density. So, addressing these issues is still a big challenge.

Compared to heavy metal hosts, light carbon and its derivatives seem much more appealing due to their good conductivity^[90]. Nevertheless, because pure carbon materials are lithiophobic, Li dendrite development on carbon structures could be hampered by a large barrier to Li nucleation and uneven Li⁺ distribution^[91]. Graphene, for example, has a low affinity for Li deposition/stripping (high overpotential). A lithiophilic medium is required to spatially guide and confine the Li deposition (bonding with Li⁺) within the interior voids of porous carbon matrix^[92]. As a consequence, development of light hosts with lithiophilic properties seems necessary.

Lithiophilic modification of host

Lithiophilic modification enhances the affinity between Li and host material, and this increased bonding energy effectively reduces deposition energy barrier, thus suppressing dendrite formation. The larger binding energy between Li and the nucleation sites in the host, the easier lithium metal nucleation occurs on the host sites, well demonstrating Li deposition is a competitive process between lithium-lithium bonding and lithium-host bonding. Dopants, functional groups, and affinity coating layers have been normally introduced to a host for enhancing its lithiophilicity^[93].

The lithiophobic carbon host normally introduces dopants to modify its affinity with Li. For example, Wang et al. reported a graphene-based aerogel host with a Ag dopant (Ag@Graphene), which shows lower lithium deposition energy compared to a host without Ag, as evidenced by the lower Li nucleation overpotential^[92]. These Ag dopants dramatically increase the CE of Li/host in both carbonate and ether-based electrolytes with various capacities at different current densities. The reversible Li plating/stripping behavior can be attributed to the strong Li–Ag bonds, thus deriving a lower overpotential of Li deposition. In addition, heteroatom dopants also contribute to uniformly Li plating through the strong polar/polar interaction^[89]. For example, Br-doped graphene hosts boot Li transport kinetics through the strong Li–Br chemical bonds, which tremendously reduces Li nucleation energy barrier, acting as lithiophilic sites that facilitate rapid and uniform deposition [Figure 16]^[94].

Figure 16. Illustration of lithium metal deposition on Br-doped graphene host. (A) Li⁺ flow is homogeneous above Br-doped graphene-like sheet and CuBr; (B) CuBr was converted into LiBr after Li plating. The selective and targeted deposition of lithium metal on Br-doping sites, as well as the surrounding LiBr, govern lithium metal nucleation into Li seeds with a pancake-like shape. Gray, blue, red, and orange denote C, Br, Cu, and Li, respectively; (C) The nucleation and development of lithium metal on modified carbon host. Reproduced with permission^[94]. Copyright 2018, American Chemical Society.

Apart from dopants, affinity coating layers, such as SnSe, SnF₂, CuO, and so on, can also enhance the deposition performance of lithium. As illustrated in Figure 17A, SnSe-C affinity coating shows uniform Li plating behavior compared to a Sn-C layer due to the strong bonding energy in Li-Se. The Li/SnSe-C anode has an ultralong lifespan (over 1,100 h) and a low overpotential (18 mV)^[91]. The excellent performance can be contributed by the in-situ constructed Li₂Se with high ionic conductivity, which boots Li⁺ transport kinetics, enabling uniform Li⁺ deposition. Similarly, the CuO coating layer on Ti mesh also exhibits excellent ability on homogenization of Li⁺ flux by forming strong Li–O bonds, thus prolonging the Li anode lifespan [Figure 17B]^[95].

Figure 17. Schematics of Li deposition behaviors on lithiophilic modified scaffolds to regulate lithium nucleation and fast transport (A) on alloy SnSe-C/MGF scaffolds and Sn-C/MGF scaffolds. Reproduced with permission^[91]. Copyright 2013, Royal Society of Chemistry; (B) the Li plating behavior at bare Li wafer and a CuO@Ti-mesh CTM-based anode. Reproduced with permission^[95]. Copyright 2019, WILEY-VCH; (C) Li host of low lithiophilicity and high conductivity generated Li dendrites; (D) Li hosts with synergistic lithiophilicity, and conductivity showed homogeneous Li deposition and restricted Li dendrite development; (E) Host of high lithiophilicity and poor conductivity having high electrical resistance and inadequate for dendrite suppression. Reproduced with permission^[96]. Copyright 2020, WILEY-VCH. MGF: Modified glass fiber film; CTM: CuO@Ti-mesh.

Based on the above results, it can be concluded that lithiophilicity is an important factor as conductivity for a host; therefore, balancing both two features in one host seems quite significant^[96]. Figure 17C-E depicts the influence of hosts with varying lithiophilicity and conductivity. A novel Cu current collector patterned with MoO₂-Mo₃N₂ heterojunction nanobelts mounted onto reductive graphene oxide is reported. The collector possesses synergistic functions of exceptional lithiophilicity, an integrated electric field generated by superb interfacial contact, and a thick lithiophilic SEI layer, which enables robust charge transfer and Li⁺ diffusion^[97]. All these results well demonstrate that a high-quality host should possess both conductive and lithiophilic features.

Kinetic modulation by catalysts on hosts

The lateral diffusion of Li is much slower than the vertical localized deposition rate. The major cause of this problem is the higher surface diffusion barrier and poor surface activity. How to efficiently reduce the surface diffusion barrier attracts the attention of researchers. Recently, researchers found that catalysts behave efficiently on modulating the Li diffusion kinetics at Li anodes^[89], especially single atom catalysts (SACs)^[98,99]. The active sites can be completely exposed when the size of these metallic modulators is reduced to atomic level, reaching 100% atomic efficiency^[62]. Controlling surface activation aids in increasing diffusion kinetics and achieving uniform deposition. In comparison to pure lithium metal with large diffusion barriers, adding SAC activators into the matrices results in a considerable drop of Li diffusion barrier down to 0.09 eV, thus increasing lateral diffusion for uniform plating^[89]. Single-atom Zn sites have been shown to drive high-dimensional lithium deposition kinetics, a lower migration barrier, a larger surface binding energy [Figure 18A], and rapid nucleation behavior, allowing for easy Li deposition. Li plating/stripping on Zn-single atoms (SAs) achieved low overpotentials of around 12 mV and a high CE of up to 100% [Figure 18B and C]^[99].

Figure 18. (A) Difference in electron density and surface binding energy between graphene, N-graphene, and Zn-SAs; Lithium deposition behavior (B) on bare Li and (C) single-atom Zn sites. Reproduced with permission^[99]. Copyright 2019, Elsevier; (D) a mechanism of lithium atom diffusion on bare Li resulting in low dimensional vertical deposition (dendrites) and high dimensional lateral diffusion on SAC-modulated surface. Reproduced with permission^[101]. Copyright 2022, American Chemical Society. SAs: Single atoms; SAC: single atom catalyst.

To reduce the Li nucleation barrier, SACs can be transplanted into several carbon matrices (graphene, MXene) to provide effective adsorbing and catalyzing lithophilic sites. Several metal atoms supported onto nitrogen-doped graphene nanostructured matrices have been explored to govern their Li nucleation and deposition behaviors. By expanding the confined region, the higher concentration of SACs attached by nitrogen sites boosted the localized adsorption of Li atoms. Six kinds of SACs (M = Co, Mn, Zr, Ni, Zn, Cu) over N-doped graphene matrices have been examined for their lithiophilicity^[100]. Although single-atom zirconium (Zr) supported on a N-doped graphene matrix exhibited a higher predicted binding energy than other metals, it did not perform better in experiments. This means that the host requires a modest lithiophilicity for the Li to diffuse easily. The catalyst used to accelerate the Li⁺ diffusion kinetics should have a modest affinity for Li⁺/Li.

To effectively promote the Li growth and nucleation, single zinc atoms have been immobilized on MXene (Ti₃C₂Cl_x) layers. Owing to the excessive number of Zn atoms on the surface of Zn-MXene layers during the initial plating, Li tended to nucleate homogeneously and then expanded vertically along the nucleation sites, yielding bowl-like Li without dendrites. In contrast to graphene’s converging electron density within chemical bonds, the addition of Zn-SAs assisted in delocalizing the electronic density throughout the whole plane, improving the electronic conductivity^[98]. Wang et al. proposed a conductive network encapsulating the most active SA cobalt on an anodic defect-containing iron sulfide in a hierarchically porous sulfur-doped nanocarbon, which acts as a kinetic modulation layer on the Li surface. The improved Li electrode reduces the nucleation barrier (15 mV) and greatly accelerates the Li horizonal deposition^[62]. Previous research has shown that pristine defect-containing or single atomic catalysts can provide catalytic sites to increase Li⁺ kinetics.

The nucleation potential barrier for a Li atom has been reported to decrease from 225 to 29 mV at 1 mA·cm^-2 upon experiencing the binary Ni and Fe metal atoms in ultralight N-doped carbon nanofiber paper (SAFeNi@LNCP) modulation layer. The smallest charge transfer resistance of the SAFeNi@LNCP-Li indicates the rapid electrochemical kinetics accelerated by the SACs^[101]. Figure 18D shows the mechanism of Li diffusion at a host with and without the SAC modulation layer on LMA. All these results well suggest the efficiency of SACs in regulating Li plating/stripping behavior.