During the plating process, the solvated lithium ions migrate from the electrolyte to the anode surface, where they are de-solvated, reduced, and heterogeneously nucleated (Li, 2007; Biswal et al., 2019). At the initial stage of deposition, one or two layers of lithium atoms are deposited, which greatly affects the subsequent deposition behavior. The structural mismatch between lithium metal and the crystal of the substrate would increase the interfacial energy, resulting in high nucleation barrier and one-dimensional dendrite growth of lithium. Researchers introduce the concept of lithiophilicity to describe the affinity to lithium of the substrate (Winand, 1991; Zhang R. et al., 2017). The nucleation overpotential, which represents the heterogeneous nucleation barrier, is directly related to the lithiophilicity of the substrate. Surface engineering of the current collector is a common method to improve its lithiophilicity, homogenize the lithium deposition, and improve cycle efficiency.

Various strategies have been employed to improve the lithiophilicity of the current collector, among which introduction of lithiophilic sites on the surface is a promising method. With studies on various metal elements, researchers have found that metals with a definite solubility in lithium, like Au and Ag, could effectively reduce the nucleation barrier and the overpotential (Li et al., 2017). The nanocapsule structure consisting of hollow carbon spheres with metal nanoparticles inside could successfully suppress dendrite formation. Hou et al. (2019) synthesized homogeneously distributed Ag nanoparticles on Cu foil via an electroless plating process for a lithiophilic current collector, which effectively reduced the nucleation overpotential from 240 to 50 mV, realizing uniform lithium nucleation and subsequently stable lithium plating/stripping. Kim et al. (2021) fabricated a 1D hollow carbon fiber incorporating with lithiophilic Au nanoparticles as lithiophilic nucleation sites on Cu foil (Au@HCF), reducing the current density and confining lithium to mitigate dendrite growth. The Au@HCF achieved CE of 99.9% under 1 mA/cm² with 2 mAh/cm² lithium plating/stripping (Figure 2A).

In addition to metals, the introduction of non-metal heteroatoms or corresponding composites is also effective to improve the lithiophilicity of the current collector. Non-metallic atoms, such as O/B/N, doped carbon is an electron-rich donor which acts as Lewis bases to attract Lewis acidic Li⁺ and induces uniform lithium deposition. Chen et al. (2019), Fang et al. (2021) reported the composite structure consisting of porous three-dimensional graphene (scaffold to suppress volumetric change) decorated with nitrogen-doped carbon nanotubes (lithiophilic sites for uniform lithium nucleation) to lead uniform lithium deposition and extend the long-term stability to 1,200 cycles at 10 mA/cm². He et al. (2019) introduced a functional polydopamine (PDA) layer to the surface of Cu foil, improving CE to >97% for 100 cycles at 0.5 mA/cm² (Figure 2B). Li⁺ reacted with the hydroxyl groups of the PDA layer, yielding uniformly distributed lithium complexing carbonyl groups which worked as nucleation sites and induced uniform and stable lithium plating/stripping on the current collector. Highly lithiophilic silver nanoparticles combined with polydopamine and graphene oxide have also been coated on Cu to facilitate stable lithium cycling (Wondimkun et al., 2021). In addition, other polymers like PVDF (Gao et al., 2019), PMMA (Zhou et al., 2020), and PEO (Assegie et al., 2018) have also been developed to coat Cu foil.

Surface engineering is one of the most widely studied methods for current collector engineering. Various materials, including lithiophilic elements, metal alloys, and polymers, and synthesis methods have been explored to effectively improve the lithiophilicity of the anode current collector. Despite these progresses, there are still many challenges for practical application. The working mechanism of the surface modifications, especially for polymer and composite materials, is still lacking. The evolution of the surface modification during cycling has not been well understood. In addition, most of the reported surface coatings are liable to be destroyed or pulverized. It is still challenging for current collector engineering to achieve stable surface coating with uniform lithiophilic sites as well as alleviate anode/electrolyte side reactions.

Although surface lithiophilic modification to induce uniform lithium deposition could stabilize lithium cycling to some extent, it cannot deal with the volume change and pulverization of Li during plating/stripping. Lowering the local current density along the anode surface with a 3D porous structured current collector could alleviate uneven Li⁺ flux (Li et al., 2016), retard the onset of dendrite nuclei, and reduce the dendrite growth (Xu et al., 2014; Yang et al., 2015). Meanwhile, the architecture could facilitate Li + migration and undermine the volume change of lithium (Ni et al., 2020).

Lu et al. (2016) designed a free-standing Cu nanowire network to accommodate lithium and limit dendrites/expansion (Figure 2C), resulting in enhanced cycling stability with average CE of 98.6% over 200 cycles and outstanding rate performance owing to the high conductivity of the network. Wang L.-M. et al. (2019) fabricated a Cu current collector consisting of finger-like pores as microchannels for electrolyte and Li⁺ diffusion, and random small pores as cages to accommodate lithium deposits. The CE of Li||Cu cell maintains 97.6% for 200 cycles. Introducing the lithiophilic components into the 3D structure could further enhance uniform lithium plating/stripping, prevent dendrite formation, and improve the cycling. Synthesis of a lithiophilic 3D porous Cu–Zn current collector with a lithiophilic residual Cu–Zn alloy by chemical dealloying brass has been reported (Zhang D. et al., 2019). Similarly, Cu decorated with the aligned LiZn/Li₂O nanorod arrays (Figure 2D) was used to regulate the lithium plating/stripping behavior, reduce the local current density, and alleviate the volume change during cycling. With the suppression of dendrites growth on the current collector, it exhibits high CE up to 93.3% after 200 cycles (Wang G. et al., 2019). Hierarchically porous Cu with various types of lithiophilic components, including Li-Sb interphase (Wang J. et al., 2021) and Cu_xO (Wang Y. et al., 2021) have been reported to simultaneously lower the local current density and reduce the lithium nucleation barrier.

Lithium preferentially deposits on top of the 3D structure, which easily causes blockage and short-circuit. Novel gradient structure has been developed to induce homogeneous instead of top deposition. Zheng et al. (2020) fabricated a 3D porous lithiophilic–lithiophobic–lithiophilic dual-gradient Cu-Au-ZnO-PAN-ZnO (CAZPZ) current collector. The Au and ZnO at the bottom were more lithiophilic, while the ZnO-PAN-ZnO skeleton provided space to accommodate deposited lithium and the lithiated ZnO (Li₂O/Li_xZn) layer regulated Li⁺ flux. As a result, long-term stabilization for 1200 h at 0.5 mA/cm² and a low overpotential of 22 mV at 3 mA/cm² were achieved in symmetric cells. 3D Si@carbon nanofibers (CNFs)@ZnO-ZnO-Cu skeleton (SCZ) (Zhang et al., 2021) for homogeneous bottom-growth of lithium was reported, in which the top-growth of lithium was successfully avoided by the conductivity and overpotential gradient induced by the top Li_xSi@CNF and bottom Li_yZn@CNF layers.

Internal resistance and electroactive surface area increase often accompany with the 3D structures, which may reduce the effective current density, and facilitate electrolyte decomposition at the anode/electrolyte interface. The combination of architecture design with other strategies, like surface engineering and artificial SEI, is a hot topic in the investigation for anode current collector engineering. However, the complicated manufacturing process of these nanostructured materials restricts their practical application. The mechanical stability of the designed architecture is another concern regarding to practical application. It is essential to develop facile strategies for the fabrication of anode current collector with carefully designed architectures, which could simultaneously achieve excellent electrochemical performance, cycling stability and cost effectiveness.

SEI is a Li⁺ conductive and electron insulative interphase passivation layer forming on the surface of the electrode material by interfacial reaction between the electrode and electrolyte during initial cycle, and its stability greatly affects the batteries’ cyclability. The fragility of SEI and deformability of Li would cause SEI rupture and continuous consumption of electrolyte and lithium due to lithium/electrolyte side reactions, impeding the long-term cycling. Hence, the strategy of constructing stable artificial SEI is proposed to suppress the growth of lithium dendrites and improve cyclability (Peled and Menkin, 2017; Shi et al., 2018).

Various materials have been used to establish artificial SEI and facilitate stable Li plating/stripping. Inorganic materials such as metal chloride perovskite and carbon can shield Li from liquid electrolyte and allow fast Li⁺ shuttle (Yin et al., 2020). For example, Wu Q. et al. (2021) proposed an oxygen defect-rich carbon with MgO_x domains as a 3D monolithic host and artificial SEI film simultaneously. However, inorganic artificial SEI is prone to fracture due to volume variation of lithium. Polymers with superior flexibility and high elasticity well offset this disadvantage (Ma et al., 2020). However, polymer-based artificial SEI is difficult to inhibit the dendrite growth during long-term cycling due to its low mechanical modulus. Therefore, organic–inorganic composite materials are used for artificial SEI. Zhong et al. (2019) fabricated a lithium alginate-based artificial SEI layer that allows fast Li⁺ transport and stable operation. Zeng et al. (2021) proposed a superstructured single-ion conducting polymer brush (CNF-g-PSSLi) as artificial SEI layer (Figure 2E), which contained robust cellulose nanofibril (CNF) backbone nanonetwork to ensure excellent mechanical properties with 5.3 GPa elasticity modulus. Poly (lithium p-styrenesulfonate) (PSSLi) side chains with rich -SO₃ ⁻ functional groups coordinating with lithium ions provided fast ion transport channels to enable excellent rate performance and avoid undesirable detachment.

Artificial SEI is an ingenious method for anode current collector engineering, which can purposefully manipulate the interfacial composition and stability. Organic–inorganic composite materials as artificial SEI films effectively address the limitations of organic polymer or inorganic SEI films. However, appropriate methods to incorporate inorganic components into polymer and develop multifunctional SEI are still highly desired. With much attention on the artificial SEI study, constructing artificial SEI films with high mechanical stability, good elasticity, excellent electrochemical compatibility, and high current density endurance is still urgently needed for anode current collector engineering.

The traditional copper current collector is lithiophobic, which easily induces inhomogeneous lithium deposition, dendrite formation, and poor lithium cycling efficiency (Pande and Viswanathan, 2019). Novel materials, including carbon-based and polymer-based materials, have emerged for the fabrication of anode current collector to overcome the defect of traditional Cu.

Carbon-based material exhibits great promise as alternative current collector for lithium metal due to its excellent mechanical strength, light weight, easily-manipulated architecture, and excellent chemical stability. Matsuda et al. (2017) fabricated the free-standing polyacrylonitrile-based insulative microfiber matrix to enable stable Li cycling at 10 mAh/cm². In addition, the lithiophilic modification of carbon-based current collector is also important. Deng et al. (2018) found carbon fiber cloth, which confined lithium in micro-channels, improved the reversibility of lithium. Furthermore, graphene sheets in micro-channels and lithiophilic ZnO nanoparticles on fibers were constructed to simultaneously provide powerful conductive network, abundant deposition sites and strong surface lithiophilicity. Inspired by the structure of the lotus root, Yu et al. (2020) reported 3D multichannel carbon fibers decorated with lithiophilic Ag nanoparticles (Figure 2F). The produced multi-channels provided sufficient space for volume changes of lithium, while Ag nanoparticles ensured homogeneous nucleation and deposition. Kwon et al. (2021) synthesized an atomically defective carbon current collector with multivacancy defects using the surface-oxidized carbon fiber paper to induce homogeneous SEI formation and uniform lithium growth. Polymers usually contain a number of polar groups, such as hydroxyl, nitro, and carboxyl, which possess excellent lithiophilicity. Therefore, the polymer materials are also promising as novel current collector. However, polymer is hardly to be used alone. Wang et al. (2018) used a 3D polyvinyl nanofiber network with sufficient polar functional groups (O-H, C-H, C-O) to reinforce Li⁺–polymer interaction and regulate the uniform lithium deposition in the voids between the nanofibers. Ju et al. (2020) made a biomacromolecule matrix by trifluoroethanol-modified natural eggshell membrane to control lithium growth and achieve an ultralong cycling life of over 1200 h at 5 mA/cm². Weldeyohannes et al. (2021) designed the gold sputter perforated polyimide film (PI@Au) as the anode current collector to guide the lithium plating/stripping with an average CE of 98.7%.

Novel carbon-based and polymer-based materials are attracting increasing attention on the development of current collectors for lithium anode due to their light weight, flexibility, and accessibility for structural/compositional regulation. Despite these studies, the electronic conductivity, electrochemical performance, mechanical strength, and stability of polymer- or carbon-based materials are still not satisfying to fabricate free-standing current collectors. In addition, low-cost fabrication approaches which are comparable to Cu foil are also essential.