Kinetic understanding of lithium metal electrodeposition for lithium anodes

Rong Fang ^a, Yu-Xi Li ^a, Wei-Wei Wang ^ab, Yu Gu *^ab and Bing-Wei Mao *^ab
aState Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. E-mail: ygu@xmu.edu.cn; bwmao@xmu.edu.cn
^bInnovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen 361005, China

First published on 7th August 2024

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Rong Fang

Rong Fang is currently pursuing her MSc in College of Chemistry and Chemical Engineering at Xiamen University. Her current research focuses on electrolyte engineering for lithium metal batteries.

Yu-Xi Li

Yu-Xi Li is currently pursuing her MSc in College of Chemistry and Chemical Engineering at Xiamen University. Her current research focuses on electrochemical energy storage systems, especially lithium/sodium-based batteries.

Wei-Wei Wang

Wei-Wei Wang obtained her PhD in Physical Chemistry from Xiamen University in 2020. She is currently a research assistant at Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM). Her research interests focus on the operando characterization of lithium-based batteries.

Yu Gu

Yu Gu received his PhD in Physical Chemistry from Xiamen University in 2019. He is currently an associated research fellow at Xiamen University. His main research interests include the interfacial electrochemistry, chemistry of alkali metal anodes, and in situ/operando characterization of interfacial processes in lithium-based batteries.

Bing-Wei Mao

Bing-Wei Mao is a Professor of Chemistry at Xiamen University. She graduated from Fudan University in 1982 and obtained her PhD degree from the University of Southampton in 1987. She then worked as a post-doctor in Fudan University and Xiamen University, respectively. After then she worked at Xiamen University and became a Professor in 1996. Her main research interests include electrochemical scanning probe microscopies (ECSPM), interfacial electrochemistry of various aspects including ionic liquids and energy-related electrochemistry, and ECSPM-based molecular electronics.

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1. Introduction

Amidst a backdrop marked by substantial demand and the swift evolution of new energy sources, the imperative for efficient energy conversion and storage stands out as one of the foremost global challenges. Electrochemical energy storage technology, facilitating energy storage and release through the reversible conversion of electrical and chemical energy, has garnered widespread adoption worldwide.^1,2 Among the array of electrochemical energy storage devices, lithium-ion batteries (LIBs), which entered commercialization in 1991, reign supreme with the highest market share and remain the subject of fervent research endeavors.^3,4 However, traditional LIBs encounter constraints due to the lower theoretical specific capacity (372 mA h g⁻¹) and higher voltage plateau of the graphite anode, hindering substantial breakthroughs in specific energy necessary to meet the escalating demands for enhanced energy/power densities.⁵ From the perspective of anodes in batteries, Li metal emerges as a promising contender, boasting the lowest mass density (6.94 g mol⁻¹) and bulk density (0.534 g cm⁻³), thereby yielding a higher theoretical specific capacity (3860 mA h g⁻¹, 2060 mA h cm⁻³). Meanwhile, Li metal exhibits an exceptionally low redox potential (−3.04 V vs. SHE). These features suggest that replacing graphite with Li metal could yield a significant upsurge in battery energy density.⁶ Moreover, Li metal electrodeposition stands out as the foundational process for the Li anodes. While as the first metallic element in the periodic table, the electrodeposition behavior of Li metal is profoundly characteristic, making it a pivotal system for expanding and deepening our understanding of metal electrodeposition processes. Therefore, the study of Li metal electrodeposition holds significant importance both in terms of fundamental understanding and practical applications.

In general, metal electrodeposition involves the transportation of metal-ions from solution to the electrode/solution interface, where they undergo discharge to form metal elements. This process encompasses liquid-phase mass transfer, electrode/electrolyte interfacial charge transfer, and subsequent nucleation–growth processes, influenced by kinetic and thermodynamic factors such as overpotential (or current density), metal-ion concentration, interfacial electrical double layer, and substrate surface microstructure. A notable distinction between Li metal and common metals lies in its extremely high (electro)chemical reactivity,⁷ necessitating its reaction with the electrolyte. This reaction results in the formation of a solid–electrolyte interface (SEI) on the Li metal surface, characterized by its Li⁺ conductive and electron-insulating properties. While Li electrodeposition adheres to the general principles of metal electrodeposition, it also possesses unique characteristics due to the presence of SEI (Fig. 1): On one hand, Li⁺ must transport through both the electrolyte and SEI to reach the electrode surface for discharge. The conductivity of Li⁺ within the SEI is notably lower than in the electrolyte, potentially rendering the Li⁺ transport in the SEI bulk as the primary rate-limiting step of the mass transfer process. On the other hand, the SEI, in conjunction with the electrode and electrolyte, forms the intricate electrochemical interface of electrode/SEI/electrolyte, where solvated Li⁺ must undergo desolvation at the SEI/electrolyte interface to penetrate the SEI, and desolvated Li⁺ undergo discharging and subsequent nucleation processes at the electrode/SEI interface. Consequently, Li deposition is influenced by both the Li⁺ transport in SEI bulk and the charge transfer kinetics at the SEI coupled interfaces. The thermodynamics and kinetics involved in Li electrodeposition strongly hinge on the composition, structure, and properties of the SEI. These factors directly impact lithium deposition characteristics, rendering the Li electrodeposition intricate and posing a series of significant challenges for Li metal anodes.^8–10

First, in the cycling process, Li metal is prone to uneven deposition, leading to the formation of dendrites on the surface. These dendrites pose a risk of piercing through the separator and reaching the cathode, potentially causing a short circuit in the battery. Meanwhile, during Li dissolution, dendrites may detach from their roots or break in the middle, resulting in the accumulation of electrochemically inert “dead lithium” near the electrodes. This accumulation leads to the loss of active substances and hinders ion transport, ultimately reducing the cycling efficiency of the battery.^11,12

Secondly, SEIs with poor properties tend to induce dendrite growth or become penetrated by dendrites. This scenario leads to the exposure of fresh Li metal, which reacts with the electrolyte. Furthermore, ruptured SEIs exacerbate dendrite growth, further impacting the reversibility of the Li deposition–dissolution cycling process and the overall efficiency of the batteries.

In addition, unlike LIBs that rely on the insertion and extraction of Li⁺ within the host material, Li metal anodes operate as a host-less transformed anode. Its charge–discharge cycling directly involves the deposition/dissolution reaction of Li⁺/Li⁰ on the Li metal or the current collectors. This process leads to unrestricted volume expansion and contraction during cycling, resulting in the formation of a porous and loose structure upon Li metal deposition. Consequently, this phenomenon increases the occurrence of side reactions associated with SEI formation.

The three aforementioned aspects of the problem are inherently intertwined and mutually reinforcing. Their combined effect ultimately leads to poor interfacial stability of Li metal anodes, thereby affecting battery performance and cycle stability. Therefore, achieving uniform and reversible Li plating/stripping, as well as ensuring stable interfacial behavior, becomes imperative for Li metal batteries to advance to a higher level. An in-depth understanding of these factors and their influence mechanisms on Li deposition, leading to their precise regulation, is essential for achieving uniform Li deposition.

In recent years, there has been a proliferation of review articles focusing on Li metal anodes.^8,9,13–18 The majority of these reviews tend to summarize strategies for inhibiting dendrite growth, such as electrolyte modulation, artificial SEI construction, and collector structure design, primarily from the standpoint of materials and methods. Alternatively, they may discuss the impact of interfacial energy, temperature, and other thermodynamic factors on Li deposition. However, there remains a paucity of reports offering in-depth and systematic analyses of Li deposition behaviors specifically from the perspective of Li deposition kinetics. Diverging from existing review articles, this review centers on the kinetic aspect of Li electrodeposition. It conducts an analysis of various kinetic factors influencing the uniform deposition of Li, delving into charge and mass transfer aspects, while also scrutinizing the influence of the SEI on the mass and charge transfer processes. Additionally, this review compiles methods and strategies aimed at inhibiting dendrite growth and fostering the uniform Li deposition through kinetic modulation of the electrolyte and SEI. Lastly, it deliberates on the challenges and outlines future directions for promoting uniform Li deposition from a kinetic perspective.

2. Kinetics of lithium metal deposition

2.1 Charge transfer process and influencing factors

Lithium electrodeposition involves a one-electron electrode reaction step, whose reaction rate can be significantly influenced by the electrode potential. The phenomenological Butler–Volmer (B–V) equation (eqn (1)) may be utilized to quantitatively describe the kinetics of the electrode reaction

	(1)

where j is the reaction current density at the electrode, j₀ is the exchange current density, η is the overpotential, α is the cathodic transfer coefficient, and F, T and R have their usual meanings. The exchange current density is influenced by the activation energy of the electron transfer step, and therefore serves as a pivotal parameter reflecting the intrinsic kinetics of the electron transfer activity at the electrode. Accordingly, it affects the overpotential at a certain current density or the achievable current density under an applied overpotential, and therefore profoundly influences the kinetics of electrodeposition and the morphology of the deposited metal layer, particularly at low overpotentials or small current densities. Liu et al.¹⁹ developed a phase-field model to simulate the kinetic behavior of electrochemical processes based on the B–V equation. They observed that when using [Li(G4)][TFSI] electrolyte, the exchange current density was low, resulting in the deposited Li exhibiting a short and thick columnar morphology. Conversely, when [Li(G4)₈][TFSI] electrolyte was used, a high exchange current density was observed, leading to the deposited Li displaying a sparse dendritic morphology. Boyle et al.²⁰ conducted measurements of the platting and stripping kinetics of Li metal using transient voltammetry (Fig. 2(a)). They advocated for the utilization of the Marcus–Hush–Chidsey (MHC) model^21–23 that offers a more detailed molecular-level description, along with the Marcus–Hush model^21,22 derived from the low overpotential approximation of MHC, to determine the rate constant and to analyze Li deposition behavior instead of relying on the B–V model. This preference stems from the incorporation of key concepts such as reorganization energy, electronic coupling, and reaction-related free energy changes in the Marcus–Hush model, which disclose the influence of solvent polarity on the electrode reaction rate, enabling a more accurate depiction of electron transfer kinetics. Shashank and co-workers²⁴ argued against the efficacy of the low overpotential approximation of the MHC model in representing non-homogeneous electron transfer kinetics under fast charging and discharging conditions. They highlighted that the reorganization energy and limiting current densities obtained by the MHC model are more precise in such scenarios. These controversies reflect the complexity of the Li deposition process, which has proven challenging to measure and understand.

Generally, the applied overpotential or current density serves as an external factor determining the rate of electrode reaction and directly impacting the size and number of nuclei during Li deposition. Therefore, any description of the Li nucleation and growth process must account for the effects of applied overpotential and current. Pei et al.²⁶ investigated the nucleation and growth behavior of Li deposition on copper substrates at constant potential (Fig. 2(c)). They found that constant current deposition of Li exhibits instantaneous nucleation with the size of the nuclei being inversely proportional to the overpotential, and the surface density of the nuclei being proportional to the cubic power of the overpotential, aligning with classical nucleation and growth theory.^29,30 Biswal and co-workers also elucidated the relationship between nuclei size and current density under constant current conditions.³¹ They demonstrated a square–cubic inverse dependency of nuclei radius on current density involving the synergistic effects of electrolyte and SEI.

The homogeneity of current density distribution on the substrate surface is crucial for achieving uniformity in deposited Li layers.³² Protrusions, impurities, and crystal defects on the substrate can influence the energy barrier of Li nucleation and may serve as favorable nucleation sites. Consequently, on Cu foils with uneven surfaces, the fluctuating Li nucleation energy barrier can result in disordered Li nucleation.³³ Moreover, when the substrate surface is uneven, charge tends to accumulate at the protrusions, causing the local current density to exceed the applied current density. This phenomenon makes Li⁺ more likely to be deposited at the tips, exacerbating the unevenness of the deposited Li layer and even promoting dendrite growth. Therefore, providing a flat and homogeneous substrate is crucial to achieving uniform Li deposition.³⁴

In most cases, there exists a dynamic competition between the mass and charge transfer processes of Li⁺, which can result in diverse growth patterns.^25,35,36 The Damköhler number (D_a = i₀/i_L) quantifies this competition by comparing the rate of electrochemical reaction at the electrode (i₀) to the rate of ion transport in the electrolyte (i_L). This dimensionless ratio helps determine whether the metal is deposited in a predominantly planar/compact or non-planar/non-compact form (Fig. 2(b)). Under conditions of slow charge and mass transfer, characterized by a low D_a, Li⁺ are initially drawn to the tips of dendritic deposits but diffuse out before the reaction occurs. This results in the formation of a uniformly dense and non-dendritic Li deposition layer. Conversely, increasing the D_a leads to a higher deposition rate at the tips of the Li nuclei, promoting dendrite growth.²⁵ Additionally, there is a dynamic competition between the diffusion of Li atoms along the substrate surface and the electrochemical reaction. When the lateral diffusion rate is significantly slower than the deposition rate perpendicular to the electrode surface, Li atoms tend to form dendrites. The diffusion-limited current density (j_lim) during Li deposition often serves as a critical value of current density in determining the formation and growth of Li dendrites. It is widely acknowledged that Li deposition proceeds smoothly at current densities below j_lim, while current densities exceeding j_lim tend to expedite dendrite growth. Wang et al.³⁷ employed cryo-transmission electron microscopy to unveil the connection of the disordered–ordered phase transition during Li nucleation and growth with the applied current density and deposition time. They interpreted that high current density accelerates Li⁺ aggregation, thereby promoting nucleation but hindering the formation of electrochemically reversible Li deposit in the glassy state. However, the irregular structure of the deposited Li layer may not develop exclusively at the high current density regime (above j_lim), as it can be influenced by a combination of factors.^27,38–40 Bai et al.²⁷ demonstrated that anisotropic dendritic morphology may still be observed during Li deposition even below the j_lim (Fig. 2(d)). This phenomenon could be attributed to various factors, such as volume expansion and inhomogeneous Li⁺ transport within the electrolyte and SEI. Cheng et al.³⁹ discovered that mossy Li deposits also occur at current densities below j_lim, albeit at a slower pace. Their study, which aimed to visualize the distribution of Li⁺ concentration on the electrode surface during Li deposition using stimulated Raman scattering microscopy, revealed a mutual feedback mechanism between the inhomogeneous Li⁺ distribution on the electrode surface and the growth of Li dendrites. At low current densities, the Li⁺ is not completely depleted. However, the uneven distribution of ions induces sluggish growth of mossy Li. Upon increasing the current density, the local Li⁺ depletion accelerates, and the uneven Li⁺ flux on the electrode surface fosters the uneven growth of Li. The newly formed Li protrusions migrate to regions with higher Li⁺ concentration, thereby amplifying the unevenness of Li⁺ flux and hastening the failure process. The group of Zhang concluded that both current density and cycling capacity influence the Li deposition behavior.⁴¹ They identified two distinct failure modes of the Li metal anode under varying cycling conditions: polarization and short circuit failure modes. Through morphological characterization of the Li metal anodes and analysis of the corresponding electrochemical polarization curves, they observed that polarization failure predominantly occurs at lower current density and cycling capacity conditions (<4 mA cm⁻², 4 mA h cm⁻²), attributed to SEI thickening and dead Li accumulation. In contrast, short circuit failure is more prevalent at higher current densities and cycling capacities (>7 mA cm⁻², 7 mA h cm⁻²), primarily caused by the dendrite growth.

Currently, most reported research tends to overlook the influence of the SEI on the charge transfer process. Experimental strategies in electrochemistry aiming to bypass SEI formation have also been implemented. For instance, the group of Cui²⁸ and the group of Li⁴² obtained the intrinsic kinetic characteristics of Li deposition without SEI influence under fast charging conditions using ultra-microelectrode technology combined with ultra-fast cyclic voltammetry scanning (Fig. 2(e)). This approach allows the deposition process to complete before SEI formation. It was observed that the Li deposition morphology resembles a rhombic dodecahedron, governed by thermodynamic surface energy in the absence of SEI influence or during SEI breakdown. However, as highlighted by Archer and co-workers, Li⁺ diffusion within the bulk electrolyte during Li electroplating is equally crucial as surface diffusion through the SEI.³¹ The presence of SEI renders the Li nucleation–growth behavior more intricate, and methods for regulating SEI and interfacial properties to achieve uniform deposition layers are yet to be fully explored.

2.2 Mass transfer process and influencing factors

2.3 Charge and mass transfer of Li⁺ in the presence of SEI

Generally, SEI formation is primarily attributed to electrolyte reduction, which occurs in two stages:⁶² Initially, the electrolyte decomposition yields a porous SEI layer on the electrode surface characterized by a mosaic structure comprising an inorganic inner layer and an organic outer layer. Subsequently, electrolyte permeates the pore space of the SEI, leading to ongoing decomposition and continuous SEI growth until the inner layer becomes homogeneous and dense, effectively impeding further electrolyte decomposition. SEIs are ideally electronic insulating and Li⁺ conducting. Regrettably, native SEIs in conventional electrolytes appear unstable and experience continual rupture and restructuring during Li deposition–dissolution, which exacerbates dendrite growth and electrolyte consumption and meanwhile complicates the composition and structure of the SEIs. SEIs profoundly impact the charge and mass transfer processes of Li⁺ during Li electrodeposition, while their coupled interfaces constituted by the substrate/SEI/electrolyte also play roles in Li electrodeposition.

Firstly, the properties of an SEI are intrinsically related to the structure of SEI. Often, the thickness of the SEI progressively increases during regular charge/discharge cycles, suggesting that the SEI is not a perfect electronic insulator and continuous electrolyte depletion may take place. Meanwhile, as the SEI thickens, the ionic conduction length increases, leading to elevated cell resistance and compromised mass transfer kinetics. Xu et al.⁶³ employed a combination of in situ transmission electron microscopy and scanning tunneling microscopy to directly assess the electrical properties of SEIs formed on Cu and Li substrates in various electrolyte systems. Their findings revealed that anion-derived components in the SEI contribute to lower electronic conductivity of the SEI, leading to the formation of large and uniformly distributed Li particles. Conversely, solvent-derived components in the SEI lead to higher electronic conductivity of the SEIs, which promotes the formation of thicker SEI and “dead” Li during charge/discharge cycle, resulting in reduced Coulombic efficiencies in Li||Cu and Li||NCM811 batteries and poor cycling stability. On the other hand, Liu et al.⁶⁴ developed a coupled electrochemical–mechanical model to simulate the influence of the ionic conductivity of SEI on the uniformity of Li deposition. It was found that low ionic conductivity in the SEI leads to decreased deposition probability and results in a tooth-like morphology. Conversely, higher ionic conductivity of SEI significantly enhances deposition probability, yielding a spherical morphology. Overall, since the conductivity of Li⁺ in the SEI is typically lower than that in the electrolyte bulk, Li⁺ transport within the SEI becomes a rate-determining step under mass-transfer-controlled condition. These studies suggest that SEIs with higher ionic conductivity facilitate improved mass transfer kinetics, thereby promoting the uniform Li deposition. Interestingly, however, there can be a transition in the rate-determining step of the mass-transfer-controlled Li deposition under certain conditions. Gu et al.⁶⁵ found that in a SEI system with high Li⁺ transference number and conductivity, the obtained i–t curves does not conform to the S–H model under diffusion control, suggesting dominance of electromigration, rather than diffusion, in the mass transport of Li⁺ within the SEI (Fig. 4(a)). Conversely, in a SEI system with low Li⁺ transference number and conductivity, the curves partially adhered to the S–H model, but exhibited a unique transition in the trend of current–time curve. Such a phenomenon occurs when the electromigration current of Li⁺ through SEI fails to meet the total current requirement for Li electrodeposition so that diffusion process sets in, which enables the formation of a SCL at the electrode/SEI interface and eventually leads to dendrite embryo growth. Subsequently, the dendrites continue growing until they breach the SEI, transforming Li deposition into an electrolyte-controlled mass-transfer kinetic process. Therefore, the conformity of i–t curves with the S–H model can serve as an indicator of SEI properties, while the potential stepping method can be utilized for rapid identification of initial dendrite growth.

Second, the SEI/electrolyte interface and substrate/SEI interface are important sites for Li⁺ desolvation and Li⁺ discharge, respectively. The composition and structure of the SEI directly affect the two associated charge transfer processes (Fig. 4(b)). Studies have demonstrated that SEI rich in inorganic components can lower the energy barriers for Li⁺ desolvation and Li⁺ transport within the SEI, thereby facilitating faster charge transfer kinetics.⁶⁶ He et al.⁶⁹ utilized quantum chemical calculations to investigate the desolvation process of Li⁺ in the presence and absence of SEI. They discovered that the accelerated desolvation of Li⁺ is primarily attributed to the structure of the SEI rather than the solvated structure of Li⁺. Their findings indicate that inorganic SEIs promote the desolvation process more effectively. Wang et al.⁷⁰ employed in situ AFM to investigate how the composition and structure of SEI, particularly the inner layer composition, impact Li nucleation–growth behavior. In the PC-based electrolyte system, SEI displayed typical characteristics of inner inorganic and outer organic layers, resulting in three-dimensional nucleation and growth of Li and spherical stacking morphology of Li deposits. Conversely, in the PC-based electrolyte containing LiI additive, the introduction of LiI facilitated solvent reduction, leading to an organic–inorganic mixed SEI structure. This alteration transformed Li deposition into three-dimensional nucleation and growth combined with two-dimensional growth, resulting in Li deposition characterized by planar layers.

Third, the uniformity and mechanical strength of the SEI are also critical for ensuring the uniform deposition of Li and preventing SEI fracture. Structural inhomogeneity within the SEI, such as variations in thickness and chemical composition, can disrupt the distribution of Li⁺ flux on the electrode surface, resulting in localized Li deposition at regions with higher ionic conductivity. Non-uniform electrodeposited Li can generate considerable stress, causing the SEI to rupture. The breach in the SEI exposes fresh Li to the electrolyte, triggering further side reactions.⁷¹ Meanwhile, any irregularity on the SEI surface may induce a spherical diffusion flux of Li⁺ and an increase in local charge density, causing accelerated dendrite growth. The spatial distribution of initial Li nucleation sites is largely influenced by the homogeneity of the SEI, with uniform and smooth SEIs promoting dense nucleation sites.⁴⁰ On the other hand, the mechanical strength of the SEI plays a crucial role in ensuring its stability and preventing fracture. SEIs with low mechanical strength are susceptible to fracturing when the electrode surface smoothness fluctuates. This occurs because Li metal deposition at the interface between the SEI and the substrate causes deformation of the SEI, resulting in localized stresses of the SEI. Young's modulus, a physical property describing material stiffness and elasticity, can be measured for SEI at the nanoscale using atomic force microscopy.^34,67 Wang et al.⁶⁷ explored the nanomechanical properties of three monolayer SEIs differing in mechanical stiffness—strong, medium, and soft (Fig. 4(c)). They established a criterion for distinguishing and rapid assessing the structural and mechanical characteristics of SEIs. Based on their findings, they proposed that SEIs featuring alternating inorganic–organic–inorganic composite structures exhibit superior inhibition of dendritic growth. The simulations conducted with the coupled electrochemical–mechanical model illustrate that SEI films with lower Young's modulus tend to deposit Li metal preferentially in the neck region of the dentate morphology, leading to significant stress concentration, which is the primary area of SEI rupture.⁶⁴ As the mechanical strength of the SEI increases, the deposition morphology of Li metal gradually transitions to a spherical shape, and the region stress concentration shifts to the top corner of the columnar feature. When the Young's modulus of the SEI is higher, Li metal is uniformly electrodeposited, with minimal occurrence of stress concentration phenomenon.

Additionally, the formation process of SEI may be regarded as a critical factor determining its composition and structure, as well as influencing Li deposition behavior. Wang et al.^68,72 investigated the formation sequence of SEIs and their impacts on the Li deposition–dissolution process through in situ AFM imaging and nanoindentation measurements (Fig. 4(d)). They discovered two distinctly structured SEIs shells, i.e. segmented-SEI and integrated-SEI shells, during Li deposition, depending on the initial formation of SEI on bare substrate surface or on the surface of Li nuclei. During Li dissolution, segmented-SEIs were prone to rupture at their tops due to uneven forces, resulting in an open structured SEI shell. Consequently, the open SEI shell could not be reused upon Li redeposition, leading to the formation of a dead SEI, which hindered the reversibility of the Li deposition–dissolution process. Conversely, the integrated-SEI exhibited uniform stress distribution during Li dissolution, forming a wrinkled structure. This configuration allowed for its reusability upon Li redeposition and effectively improved the reversibility of the Li deposition–dissolution cycling. Gu et al.⁷³ develop a depth-sensitive plasmon-enhanced Raman spectroscopy method to realize in situ, nondestructive, and real-time detection of the interfacial processes at the Li metal anodes. It is found that during conventional battery operation, an unstable primary Cu-SEI layer dominated by high-oxidation-state components always forms on the surface of Cu current collector. After Li deposition, the Cu-SEI undergoes chemical reconstruction with the participation of metallic Li, and eventually forms a relatively stable Li-SEI dominated by low-oxidation-state components. This result updates the conventional knowledge that SEIs do not form on Cu collectors, or that Cu-SEIs are the same as Li-SEIs. Meanwhile, the chemical reaction involving Li⁰ has an important effect on the composition and properties of SEIs. Based on this, if the formation of unstable primary SEI on Cu is inhibited by manipulating the potential or current in practical applications, more stable Li-SEI with better properties can be formed directly on Li metal, which can significantly improve the lifetime and cycling stability of Li anodes.

3 Kinetic regulations for homogeneous Li deposition

Refining the kinetics of Li deposition to attain uniformity is pivotal for enhancing the performance and safety of Li-based batteries. Various strategies are employed to achieve this, primarily centered on controlling the electrolyte composition and modulating the SEI formation.

3.1 Electrolyte engineering

The modification of the electrolyte involves consideration of a combination of factors such as ionic concentration, solvation structure, and their impact on SEI formation. These aspects are directly linked to dendrite growth time, ionic mobility, and desolvation barriers.

The concentration of Li⁺ significantly influences the properties of the electrolyte, consequently affecting the Li deposition process. According to Sand's time model, dendrite growth correlates with the initial concentration of Li⁺ in the electrolyte, and increasing this concentration can delay dendrite growth. Therefore, emphasis has been placed on high-concentration electrolyte systems. These systems not only exhibit high Li⁺ mobility and a certain viscosity but also show greatly enhanced interfacial stability, effectively inhibiting dendrite growth. Suo et al.⁶⁰ designed an ultra-high Li salt concentration (up to 7 mol L⁻¹) electrolyte known as “Solvent-in-Salt” for the LiTFSI/DME-DOL system, which effectively inhibits dendrite growth. Additionally, they observed that applying such an electrolyte to Li–S battery systems reduces polysulfide solubility, mitigating their diffusion and migration to the Li surface, thereby reducing corrosion. Similarly, the group of Zhang developed a 4 mol L⁻¹ LiFSI/DME high-concentration electrolyte.⁶¹ They observed that this electrolyte facilitated the formation of a dense and stable SEI film on the Li surface, effectively suppressing dendrite growth (Fig. 5(a)). As a result, they achieved reversible cycling for 1000 cycles at a current density of 4 mA cm⁻¹, with an average coulometric efficiency of 98.4%. However, while high-concentration electrolytes enhance the stability of the Li anode, they can also lead to decreased electrolyte conductivity and increased costs, which may not be conducive to practical application.^74,75 To address the challenges associated with high-concentration electrolyte systems, researchers have ingeniously introduced “inert” fluorinated ether dilution solvents to these systems. These dilution solvents, which have weaker coordination with Li⁺, create a localized high-concentration electrolyte. This approach preserves the solvation structure of the high-concentration system while effectively mitigating its drawbacks. This innovation has garnered significant attention in recent years.^76,77 For example, Chen et al.⁷⁸ diluted the high-concentration electrolyte containing 5.5 M LiFSI/DMC by incorporating BTFE, an inert diluent that does not dissolve Li salts but is compatible with DMC. This resulted in a localized high-concentration electrolyte comprising 1.2 M LiFSI/DMC/BTFE. This approach preserved the benefits of the high-concentration electrolyte in stabilizing the electrode/electrolyte interface while achieving a lower Li salt concentration, reduced viscosity, higher ionic conductivity, and improved wettability. Compared to high-concentration electrolytes, relatively few studies have explored low-concentration electrolytes, primarily due to the decline in properties such as ionic conductivity when reducing the Li salt concentration (<0.8 M). However, this conclusion is specific to electrolytes used in Li-based batteries. In the case of sodium-ion battery electrolytes, where the Stokes radius and desolvation energy of Na⁺ are lower than those of Li⁺, adequate kinetic performance can theoretically be achieved with lower concentration electrolytes. For instance, the group of Hu reported an ultra-low-concentration Na salt electrolyte (0.3 M),⁷⁹ which not only reduces costs effectively but also notably enhances the high and low-temperature performance of sodium-ion batteries.

In addition to Li⁺ concentration, the solvation structure of Li⁺ in the electrolyte has become a focal point of research in recent years.^82–84 Actually, Li⁺ concentration and its solvation structure mutually influence each other. Altering the transference number of Li⁺, reducing the desolvation energy of Li⁺, and forming an ideal SEI to promote the uniform Li deposition by modulating the solvation structure of Li⁺ represent emerging strategies in electrolyte regulation. The solvation energy of the solvent indicates the strength of the Li⁺–solvent interaction, while the solvation structure results from the competition between Li⁺–solvent and Li⁺–anion interactions. The salt concentration (or the ratio of solvent to Li salt) is considered a primary determinant of the fundamental solvation structure of the electrolyte. In conventional concentration electrolyte systems, strong solvation between Li⁺ and solvent molecules predominates, leading to SSIP as the dominant solvation structure. This not only restricts Li⁺ conductivity in the electrolyte, shifting charge conduction in solution towards anions and resulting in a low Li⁺ transference number, but also exacerbates concentration polarization during charging and discharging due to the high ionic desolvation energy barrier. Moreover, organic-rich SEIs formed by preferential solvent reduction promote electron leakage, increasing Li nucleation and growth overpotentials, ultimately leading to non-uniform Li deposition and dendrite formation. Creating high-concentration or localized high-concentration electrolytes by increasing Li salt concentration facilitates anionic access to the first solvated sheath layer of Li⁺. This leads to the formation of an electric double layer characterized by anion adsorption on the electrode surface, resulting in anion-derived SEIs rich in inorganic components such as LiF and Li₃N. These SEIs effectively inhibit dendrite growth. In addition, the weakly solvating electrolyte directly reduces the desolvation energy by weakening the solvent's ability to solvate, allowing both the solvent and the anions to coordinate with Li⁺. This strategy has also garnered significant attention.^85,86 Investigations have also been conducted into methods to mitigate the instability caused by excessive weak solvents in the weak solvation strategy, employing spatial steric hindrance effects.⁸⁷ Certainly, leveraging the inherent coordination ability of anions with Li⁺ has emerged as a strategy to enhance the transference number of Li⁺ and bolster the cycling stability of Li metal batteries. In addition to directly altering the solvation structure of Li⁺ in the bulk electrolyte to form a stable SEI, Zheng et al.⁸⁸ found that controlling the rate of Li stripping during high-rate discharge led to a highly aggregated Li⁺ concentration on the anode surface. This reduced the presence of free solvent molecules, thereby inhibiting sustained reactions between electrolyte and Li anode. Simultaneously, this transient high concentration transition layer on the electrode surface facilitated the formation of a stable SEI. Consequently, the asymmetric charging and discharging strategy developed as a result significantly enhanced the cycling stability of Li metal batteries.

Thus far, research has predominantly focused on understanding the coordination between solvents, anions, and Li⁺. However, the interaction between solvent molecules and anions has been described in vague terms. Exploring the impact of weak interactions between anions and solvents on interfacial kinetic processes, particularly when anions serve as coordination cluster centers, offers insights into elucidating complex behaviors in electrolyte systems.⁸⁹ Building upon Lewis acid–base interactions^90,91 or hydrogen-bond-like interactions,⁹² the design of additives or solvent molecules capable of directly promoting the derivatization of anions into inorganic-rich SEIs represents an untapped avenue for the rational design of electrolytes for Li metal batteries.

Notably, alongside enhancing mass transfer kinetics, slowing down the charge transfer kinetics of the Li deposition process can also optimize Li deposition behavior. Ma et al.⁹³ introduced a fluorocarbon surfactant into the ether-based electrolyte to create a lithiophobic adsorption layer on the Li metal surface, effectively inhibiting the charge transfer process of Li⁺. This slowed deposition kinetics significantly mitigated the concentration gradient at the electrode/electrolyte interface, promoting uniform Li deposition. Moreover, the self-healing electrostatic shielding mechanism was employed, where Cs⁺ and K⁺ additives in the electrolyte system were preferentially adsorbed at the tip of the Li anode without undergoing reduction.^94,95 Instead, they formed a positively charged shielding layer, preventing Li⁺ deposition on protrusions, thus suppressing dendrite growth due to localized high current density and facilitating homogeneous Li deposition.

3.2 SEI construction

Currently, there are two primary approaches to optimize SEI formation. The first involves adding additives to the electrolyte or altering its composition to enhance the natural forming process of SEI. The second method entails coating the Li metal surface with various exotic materials to fabricate artificial SEIs.

Modulating the natural formation process of SEI through electrolyte manipulation is one of the simple and effective methods for SEI construction. The electric double layer (EDL) structure at the electrode/electrolyte interface is believed to be crucial in determining the competitive reactions for SEI formation on Li metal anodes. Yan et al.⁸⁰ found that the initial competitive adsorption of electrolyte components in the inner Helmholtz layer of the EDL on the electrode surface, along with its dynamic evolution with potential, determines the composition and structure of the SEI (Fig. 5(b)). This directly affects the desolvation energy of Li⁺ and the energy barriers for Li⁺ to traverse the SEIs. Zhou et al.⁹⁶ explored the formation mechanism of SEI using in situ liquid-phase secondary ion mass spectrometry. They found that prior to the initiation of interphasial chemistry, an EDL was established at the metal/electrolyte interface as a result of the self-assembly of solvent molecules, influencing the composition of the final SEI. Simultaneously, the negatively charged electrode surface repels anions from the inner layer, creating a thin and dense fluorine-free inorganic inner SEI layer that facilitates Li⁺ conduction and insulates electrons. This leads to the emergence of an outer SEI layer enriched with organic components and permeable to the electrolyte. In highly concentrated or fluoride-rich electrolytes, the LiF concentration in the inner SEI layer increases when anions are present in the EDL.⁹⁷ Xu et al.⁹⁸ investigated the thermodynamic and EDL factors influencing the competitive SEI formation reaction. Their findings suggest that thermodynamic stability alone inadequately explains the preferential decomposition of the electrolyte on the Li metal anode. Instead, the negatively charged nature of the Li metal surface leads to a significant enrichment of cations in the EDL. Only when anions from the solvated shell of cations are extensively drawn into the EDL can they be preferentially reduced to establish a sustainable SEI.

The reduction process of anions and solvents during SEI formation is highly potential-dependent, and the resulting composition of inorganic–organic components within the SEI can be directly regulated by a rational control of the potential. Gu et al.³⁴ achieved electrochemical polishing of Li metal surfaces and in situ construction of SEIs in a TFSI-DOL-based ether electrolyte system by developing an electrochemical modulation strategy of potentiostatic stripping–galvanostatic plating (Fig. 5(c)). This approach not only yielded a large-scale atomically flat Li surfaces but also facilitated the construction of multilayer ultrathin SEIs with alternating inorganic–organic phases, possessing both rigidity and elasticity. Consequently, it inhibited the growth of Li dendrites. Moreover, the current density also impacts the formation of SEIs.⁹⁹ SEIs on amorphous Li spheres deposited at low current density exhibit different structures and compositions from those on Li whiskers deposited at high current density. The former comprises a thin, LiF-dominated heterogeneous SEI layer, while the latter consists of a thick, organically-rich SEI.

Due to the heterogeneous composition, structure, and properties of the natural SEI produced by electrolyte decomposition, both horizontally and vertically, it leads to uneven Li⁺ flux and constant breakdown of the native SEI layer. In contrast, the physicochemical properties of artificial SEI can be precisely regulated. This includes improving mechanical stability to reduce SEI cracking, constructing uniform and well-defined structures and compositions to ensure high and uniform ion-conducting ability, and reducing or even blocking the parasitic reaction between the active Li anode and electrolyte. Ultimately, this facilitates the homogeneous Li deposition. The components for constructing artificial SEI can be broadly categorized into organic and inorganic materials.

Bulk LiF crystalline materials possess fascinating physical properties, including high mechanical strength, low solubility, a wide band gap (rendering it a good electronic insulator), and a high electrochemical window (up to 6.4 V compared to Li/Li⁺).¹⁰⁰ While theoretically limiting Li⁺ transport with an ionic conductivity of 10⁻¹³–10⁻¹⁴ S cm⁻¹,¹⁰¹ LiF plays a crucial role in regulating Li⁺ flux for uniform deposition. Moreover, studies have indicated that LiF exhibits higher ionic conductivity and higher surface energy when interacting with other components (such as Li₂O, Li₂CO₃, LiOH) at the nanoscale level.^102,103 For now, LiF is extensively studied as a crucial component of SEI. Gong et al.⁸¹ demonstrated through in situ imaging that fluoride-rich SEI promotes denser Li deposition, which is easier to strip and leaves less dead Li compared to dendritic Li deposited on fluoride-poor SEI (Fig. 5(d)). Sun et al.¹⁰⁴ utilized a co-sputtering method to prepare composite artificial SEIs with LiF and lithium phosphorus oxynitride (LiPON) heterostructures, achieving high ionic conductivity and fracture toughness, thereby suppressing dendrite formation and enabling uniform lithium deposition. Li₃N with an ionic conductivity of 10⁻³ S cm⁻¹ and Li₂S with an ionic conductivity of 10⁻⁵ S cm⁻¹ have also garnered significant attention due to their high ionic conductivity.^105,106 Ni et al.¹⁰⁷ synthesized porous graphene oxide thin films doped with sulfur and nitrogen atoms to form inorganic SEIs enriched with Li₂S and Li₃N, resulting in a stable anode morphology without dendrite growth.

Organic polymers, with their diverse functional groups and adaptable structures, are advantageous for ensuring a uniform distribution of Li⁺ flux and mitigating volume expansion due to dendrite growth. Their flexibility allows them to accommodate the volume changes during Li deposition and dissolution, maintaining robust interfacial contact and thereby inhibiting dendrite growth while enhancing cycling stability.¹⁰⁸ Taking the extensively researched polar group polymers as an example, these organic polymers exhibit dipole–dipole interactions with the electrolyte, which restrain the activity of free solvent molecules. Nie et al.¹⁰⁹ proposed a reactive substitution polymer featuring carboxylic acid and cyclic ether segments polymerized in situ with Li, thus fostering uniform lithium deposition.

Overall, inorganic materials exhibit better chemical stability, but their rigidity makes it challenging to accommodate the significant volume changes during the Li deposition and dissolution processes. On the other hand, organic materials offer better flexibility but lower mechanical strength, making them less effective in inhibiting dendrite formation and exhibiting relatively poorer chemical stability. Therefore, researchers have explored the synergistic effect of combining these two components. For instance, Cao et al.¹¹⁰ dissolved LiDFOB into poly(ethylene glycol) diacrylate (PEGDA) to create a homogeneous precursor solution. Triggered by azodiisobutyronitrile, LiDFOB decomposed near the Li surface, forming an inorganic LiF-rich component. Meanwhile, PEGDA reacted with Li to produce a lithiated polymer. This in situ construction of an organic–inorganic composite artificial SEI resulted in more uniform Li deposition in subsequent processes.

It is worth noting that the thickness of the artificial SEI film is an important measure in its design. Zhang et al.¹¹¹ discovered significant differences in ionic conductivity among inorganic layers of different thicknesses at the nanoscale, with thicker inorganic layers severely limiting ionic conduction. Zhai et al.¹¹² synthesized two-dimensional g-C₃N₄ layers with varying thicknesses on Cu foil. They observed that thin g-C₃N₄ layers (∼2 nm) rapidly decomposed and ruptured under Li⁺ flux, while thick g-C₃N₄ layers (∼50 nm) hindered Li⁺ transport. Only medium-thickness (∼10 nm) g-C₃N₄ layers promoted the in situ formation of stable artificial SEIs, enabling uniform Li deposition.

4 Conclusions and perspectives

We have extensively analyzed the influences of mass and charge transfer processes in Li deposition from a kinetic perspective, with a keen focus on the distinct impacts of SEI and electrolyte on these mechanisms. Moreover, we have explored both the commonalities and unique attributes of Li electrodeposition in comparison to conventional metal electrodeposition, while discussing recent research strategies aimed at fostering uniform Li deposition behavior. However, dendrite growth and unstable SEI persist as the foremost challenges hindering the practical application of Li anodes. To comprehensively understand the principles and laws governing Li electrodeposition kinetics and to devise strategies for inhibiting dendrite growth at its source, we propose the following directions for advancing Li metal anodes towards practicality and market viability:

(1) The existing kinetics theories for metal deposition have not considered the presence of SEI. Developing a more comprehensive kinetics theoretical framework for Li deposition that takes into account the presence of SEI and the characteristics of combined diffusion–electromigration of Li⁺ transport within SEI is highly desirable. Integrating the roles of electrolyte and substrate in Li deposition can facilitate a specific analysis of the challenges hindering uniform Li deposition under varying experimental conditions in battery systems.

(2) Complement theoretical simulations by experimental investigations, leveraging the capabilities of artificial intelligence for advanced data analysis. This integrated approach allows for a comprehensive and detailed consideration of factors influencing the Li deposition process. These factors include the properties of the SEI (such as thickness, electrical conductivity, and mechanical strength), electrolyte composition, substrate material surface condition, current density, charge/discharge rate, as well as environmental variables like temperature and pressure. By systematically eliminating uncontrollable factors, precise and efficient control of Li deposition uniformity can be achieved.

(3) The current research efforts in improving Li deposition process primarily focus on the electrolyte formulation and the SEI bulk properties with use of additives, which all aimed at enhancing the mass transfer kinetics of Li⁺. However, it is important to note that the structure and physicochemical properties at the substrate/SEI and SEI/electrolyte interfaces also play crucial roles in Li deposition. Precise regulation and modification of these interfaces can enhance Li nucleation–growth and desolvation kinetics, which is beneficial to achieving smooth Li deposition–dissolution.

(4) For a Li metal anode, the study of Li deposition must also consider the dissolution process, as the dissolution behavior significantly impacts subsequent Li deposition and morphology, and battery cycling stability. However, the mechanisms and factors affecting the Li dissolution process have yet to be systematically and thoroughly studied.

(5) In pursuit of cost-effective and high-energy-density Li metal batteries, the shift from Li anode to Li-free anode is an unavoidable trend. The nucleation–growth behavior of Li on a current collector (i.e., heterogeneous substrate), along with its deposition characteristics, plays a pivotal role in defining the Li-free anode. Therefore, optimizing the properties of the current collector to establish a stable interface significantly impacts the uniformity of Li deposition, battery lifetime, and remains a pivotal focus for the future advancement of high-energy-density Li metal batteries.

(6) By integrating instruments with spatial resolution (e.g., AFM) and chemical resolution (e.g., Raman and IR), one can not only track the evolution of electrode surface morphology during charging and discharging but also analyze real-time changes in its components. This approach provides novel insights into the formation and evolution mechanism of the SEI and its influence on Li deposition and dissolution behavior. Moreover, it offers a basis for targeted optimization of Li deposition–dissolution, leading to enhanced battery performance.

(7) Apart from the anode itself, which directly impacts the cycling stability of the Li metal anode, the cathode also plays a role in influencing the Li plating/stripping process within the practical battery systems. For instance, in batteries utilizing LiNi_xMn_yCo_1−x−yO₂ as the cathode, the presence of transition metal ions can dissolve and migrate to the anode, disrupting the kinetics of Li deposition. Moreover, in Li–sulfur and Li–oxygen batteries, the cathode undergoes a complex multi-step conversion reaction where soluble intermediates travel to the anode, further influencing the behavior of the Li metal anode. Therefore, a thorough investigation of the Li deposition process in practical battery systems is essential.

(8) Finally, as Li metal batteries transition from laboratory research to large-scale commercialization, the factors influencing the battery system become increasingly intricate. The behavior of Li plating and stripping is significantly impacted by various factors such as the loading of active substances, dosage of electrolyte, assembly pressure, and more. The Li anode encounters heightened challenges in this transition, necessitating a comprehensive and thorough investigation.

Data availability

This review article does not contain experimental data.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (22102137, 21972119, 22002129), Industry-University-Research Joint Innovation Project of Fujian Province (2023H6029).

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