Recently developed strategies to restrain dendrite growth of Li metal anodes for rechargeable batteries

Abstract

Lithium metal has been regarded as one of the most promising anode materials for high-energy-density batteries due to its extremely high theoretical gravimetric capacity of 3860 mAh·g−1 along with its low electrochemical potential of − 3.04 V. Unfortunately, uncontrollable Li dendrite growth and repetitive destruction/formation of the solid electrolyte interphase layer lead to poor safety and low Coulombic efficiencies (CEs) for long-term utilization, which largely restricts the practical applications of lithium metal anode. In this review, we comprehensively summarized important progresses achieved to date in suppressing Li dendrite growth. Strategies for protection of Li metal anodes include designing porous structured hosts, fabricating artificial solid electrolyte interface (SEI) layers, introducing electrolyte additives, using solid-state electrolytes and applying external fields. The protection of Li metal anodes can be achieved by regulating the stripping and deposition behaviours of Li ions. Finally, the challenges remaining for lithium metal battery systems and future perspectives for Li metal anodes in practical applications are outlined, which are expected to shed light on future research in this field.

Introduction

Over the past few decades, the ever-growing demands on Li-ion batteries (LIBs) for high-energy-density, high power density and long cycle life have stimulated tremendous efforts in exploring and developing novel electrode materials [1,2,3,4]. The energy density of insertion-type LIBs has been raised to values that are close to the theoretical value by optimizing the battery’s components and construction. However, the performances of the insertion-type LIBs are still far from the requirements for large-scale energy storage and sustainable transport applications. Owing to its high theoretical capacity of 3860 mAh·g−1, low gravimetric density of 0.53 g·cm−2 as well as its low negative electrochemical potential (− 3.04 V vs. standard hydrogen electrode), Li metal serves as an ideal anode material [5,6,7,8,9,10]. By replacing the carbonaceous materials for Li metal as anode, the energy density of Li batteries might be largely increased. For example, energy densities as high as 2600 and 3500 Wh·kg−1 are possible for Li–S batteries and Li-air batteries, respectively [11,12,13]. However, Li dendrite growth from pure Li metal anode may cause short circuiting and poses serious safety issue. On the other hand, the low Coulombic efficiency (CE) due to the formation of an unstable solid electrolyte interface (SEI) layer may result in short battery lifetime. Therefore, strategies to restrict dendrite growth and the formation of unstable SEI layer have been proposed and studied [14,15,16].

As early as 1970s, Li metal was used as an anode at the beginning of Li secondary battery research, including in the prototype of Li secondary batteries designed by Whittingham [17]. Commercial Li metal batteries were successfully fabricated by pairing a MoS2 cathode with excess Li with hundreds of cycle life in the late 1980s. Unfortunately, safety concerns of fire risk led to the recall of all sold cells. Since late 1990s, Sony has developed reliable Li-ion cells by replacing Li metal with carbonaceous anodes. Although the energy densities of the traditional Li-ion cells have approached the theoretical values after 30 years of development, batteries with higher energy density are highly desired.

In recent years, diverse approaches have been investigated to restrict the dendrite growth of Li metal anodes. Figure 1 summarizes methods such as employing conductive structured hosts, creating artificial SEI layers, using solid electrolytes, introducing electrolyte additives and applying external fields [5,6,7,8,9,10,11,12,13,14,15,16]. In this work, an overview of the models of Li dendrite growth was first present. Then, we summarize the recent progress on protecting Li metal anodes, which can suppress Li dendrite growth and enable a high Columbic efficiency. Finally, the challenges remaining for Li metal battery systems are outlined and discussed.

Fig. 1
figure1

Challenges and strategies for Li metal anode used in rechargeable Li metal batteries

Mechanism of Li dendrite nucleation and SEI formation

General models of Li dendrite nucleation and growth

The stripping/deposition behaviour of Li ions in Li metal batteries (LMBs) is very different from the intercalation/extraction behaviour in LIBs. Metal dendrite formation is a common phenomenon in the field of electrodeposition, and many metals, such as Zn, Cu, Ag and Sn, exhibit dendrite morphologies [18]. The dendrite formation mechanisms that occur during the electrodeposition of Zn and Cu have been widely studied and reported [19]. The knowledge related to the dendrite nucleation and growth of Zn is very helpful for the understanding of Li dendrites. Several models have been proposed over the past four decades to describe Li deposition. Under polarization, the ionic concentration gradient (∂C/∂x) in the Li symmetric cell can be described with the following equation [20, 21]:

$$\frac{\partial C}{\partial x}(x) = \frac{{J\mu_{a} }}{{{\text{e}}D(\mu_{a} + \mu_{{\text{Li}}^{ + } } )}}$$
(1)

where J is the effective electrode current density, D is the ambipolar diffusion coefficient, e is the electronic charge, and μa and \(\mu_{{\text{Li}}^{ + } }\) are the anion and Li+ mobilities. The concentration gradient is proportional to the effective current density. Therefore, there should be a critical ionic concentration gradient for dendrite growth, which is determined by the inter-electrode distance (L) and initial Li salt concentration (C0). If \(\partial C/ \partial x\) < 2C0/L, the ionic concentration at the negative electrode maintains almost a steady state. If \(\partial C/ \partial x\) > 2C0/L, the ionic concentration drops to zero at the negative electrode at the “Sand’s time” (τ), which can be expressed as follows:

$$\tau = \uppi D\frac{{{\text{e}}^{2} C_{0}^{2} (\mu_{a} + \mu_{{\text{Li}}^{ + }} )^{2} }}{{4J^{2} \mu_{a}^{2} }}$$
(2)

In general, the “Sand’s time” is regarded as the starting time of Li dendrite growth, in which the anionic and Li+ concentrations exhibit different behaviours, leading to an excess in positive charges at the negative electrode. The excessive positive charges result in a large electric field and subsequent nucleation of Li dendrites. Thus, dendrite nucleation can be postponed by reducing the effective current density and total deposition capacity and increasing the anion mobilities and electrolyte concentration.

Following this model, substantial work has been done to further elucidate dendrite formation. Chazalviel [18] suggested that the dendrite growth rate (v) followed the equation below:

$$V = - \mu_{a} E$$
(3)

where E is the electric field strength. However, Chazalviel’s model did not take Li-ion diffusion into account. Monroe and Newman [22] demonstrated that the rate of dendrite growth depended on the current density, and the electric field provides the main driven force at the beginning stage of dendrite growth. Bai and co-workers [23] revealed that Li deposition was a two-step process, since the growth of a mossy-like root was observed on the surface of the Li foil, which was followed by tip-induced dendrite growth.

SEI layer formation on surface of Li metal

SEI layer is another important factor influencing the formation of Li dendrites. With the most negative electrochemical potential of Li+/Li, all electrolytes can be reduced at the Li surface. It is well accepted that the SEI layer is electronically insulating but ionically conductive, thus preventing further side reactions but transferring Li+. As reported by Archer’s group, mixtures of Li salts give rise to a thin passivation layer that can suppress the formation of Li dendrites [24, 25]. Li metal can react with most of the Li salts and organic electrolytes to form an SEI layer on the surface. The inorganic species on Li surfaces mainly consist of Li2O, LiOH, LiF, LiI, Li3N and Li2CO3, and the major organic species are ROLi, RCOOLi, ROCOLi, RCOO2Li and ROCO2Li (R = alkyl groups). A stable passivation SEI layer can protect the Li metal from further reactions with the electrolyte components [26]. Such a layer is elastic and can stretch and contract to accommodate the changes of the lithium volumes and aid in the redeposition of Li underneath this layer, thus reducing the exposure of fresh Li metal to the electrolyte.

Strategies to suppress formation of Li dendrites

Porous and conductive host materials

According to the Sand’s time model, reducing the local current density of the Li metal anode will effectively suppress the growth of Li dendrites. In this regard, the use of porous and conductive host materials is viable to regulate the deposition of Li+ flux. An ideal host material should have the following features [27,28,29]: (1) relatively uniform electric field distribution to ensure the homogenous deposition of Li on the surface; (2) high electronic conductivity; (3) high shear modulus and elastic strength, which can physically block lithium dendrite growth; (4) lithiophilic nature, thus accommodating the thermally infused Li or electrochemically deposited Li; and (5) appropriate pore size, which is related to the stripping/plating behaviour of the Li metal.

Carbon-based materials are the most frequently used host materials for stabilizing Li metal anodes due to their facile preparation process, variable morphologies and good electronic conductivity. Wang et al. [30] synthesized three-dimensional (3D) interconnected lithiophilic carbon nanotubes (CNTs) on a porous carbon cloth, which largely reduced the polarization of the electrode, thus inducing the homogenous nucleation of Li and ultimately leading to the continuous smooth plating of Li. Peng et al. [31] successfully obtained a thin lithiophilic LiC6 layer between carbon fibres (CFs) and metallic Li by a one-step rolling method. The lithiophilic LiC6 layers effectively alleviated volume expansion and retarded dendritic growth upon Li deposition. The assembled Li/CF|Li/CF batteries could operate for > 90 h with a small polarization voltage of 120 mV at a 50% depth of discharge (DOD). With mesoporous carbon nanofibres, Niu et al. [32] prepared a self-smoothing lithium-carbon anode structure, which exhibited a cell-level energy density of 350–380 Wh·kg−1 (counting all the active and inactive components) and a stable cycling lifetime up to 200 cycles while coupled with a Li–Ni–Mn–Co–O cathode (NCM622 and NCM811). Copious functional groups on the carbon nanofibres improved the Li wettability of the carbon host, changing the surface from non-wetting to superwetting. The pore channels or cavities on the surface worked as the initial nucleation sites upon Li plating. During cycling, Li was uniformly deposited on the surface of the carbon fibres to form a smooth and stable Li layer along with the SEI layer.

A nanostructured trilayer TiC/C/Li anode was fabricated by filling molten Li into CVD-grown TiC/C core/shell nanowire arrays, which exhibited a remarkably improved cycling stability and CE with respect to pristine Li foil anodes [33]. Li et al. [34] designed a hierarchical Co3O4 nanofibre/carbon sheet skeleton with an improved wettability towards molten Li. Here, the carbon sheet serves as a primary framework by providing adequate lithium nucleation sites and sufficient electrolyte/electrode contact for fast charge transfer, while the in situ formed Co/Li2O nanofibres provide the physical confinement of the deposited Li and further redistribute the Li+ flux on the carbon fibres. When paired with LiFePO4 cathodes, the Li/Co–CS cell delivered an 88.4% capacity retention after 200 cycles at 2C. By incorporating microscale Li metal inside a cellular graphene scaffold, the thin Li metal layers anchoring on the graphene sheets enhanced the electrochemical reversibility of Li and inhibited lithium dendrite growth. A Li symmetrical cell prepared using such an electrode could operate at a current density of 20 mA·cm−2 [35]. Wang et al. [36] fabricated a unique wrinkled graphene cage (WGC) host for Li metal. Upon Li decomposition, the graphene shell first became thicker, and the cage was gradually filled with Li metal, which remarkably reduced the exposure of Li metal to electrolyte (Fig. 2). Such a wrinkled graphene cage structure exhibited a CE of ~ 98.0% at 0.5 mA·cm−2 in commercial carbonate electrolytes and 99.1% with high-concentration electrolytes. Liang et al. [37] mixed acetylene black (AB) and N-doped carbon spheres (NCSs) with metallic Li. Since AB had a good electron conductivity and NCS had a lithiophilic surface, they worked together to enhance the electrochemical performances, with CE of 98.4% over 800 cycles at 1 mA·cm−2 for 1 mAh·cm−2. A modified MXene was also used as host for Li metal batteries with obvious advantages in guiding the uniform nucleation and horizontal growth of lithium on the surface [38]. The parallelly aligned MXene-Li composite delivered a long cycle life up to 900 h and deep stripping-plating capabilities up to 35 mAh·cm−2.

Fig. 2
figure2

Wrinkled graphene cages as hosts for Li metal anode: a, c, g schematic; bf, hk SEM observations of WGC electrode during Li metal plating under different current densities [36]. Copyright American Chemical Society. All rights reserved

Moreover, the introduction of N atoms (e.g. pyridinic and pyrrolic nitrogen) into the carbon matrix may enrich the lithiophilic functional groups, thereby strengthening the affinity of Li with the host materials. Usually, N doping is achieved by a heating process performed at 600–1000 °C under an ammonia atmosphere. Low N content (less than 4%) in the resultant carbon host materials is a key issue. Using commercially available melamine foam (MF) as a precursor, a graphitic sponge composite electrode with an N content as high as 9.87% was prepared and delivered a specific capacity of ~ 3175 mAh·g−1 [39]. A self-supporting lithiophilic N-doped carbon rod array with N content of 9.43%and large surface area effectively inhibited the dendritic growth of Li and reduced the mass loss of Li from 0.063 to 0.01 mg·cm−2 in each cycle [40]. Using N-doped composite graphene (NCG) as Li plating host, a new kind of sandwich-type composite lithium metal (STCL) electrode was designed and developed, which showed overpotential less than 40 mV after cycled for more than 4500 h [41]. Similarly, the oxygen and nitrogen co-doped porous carbon granules (ONPCGs) made from polyacrylonitrile powder led to uniform Li nucleation and deposition [42], and exhibited high CEs (> 99%) for over 350 cycles at a current of 2 mA·cm−2. A 3D fluorine-doped graphene supported Li anode delivered an average CE as high as 99% over 300 cycles [43].

Compared with two-dimensional (2D) foil current collectors, 3D metallic hosts can largely increase the contacting surface and hence reduce the local current density of the anode [44]. Since Cu foils are the commonest current collectors in the battery industry, recent efforts have been focused on the modification of Cu foils for application in LIBs. The nanoporous/macroporous Cu current collectors with large surface area can effectively reduce the local current density, which facilitates the uniform deposition of Li on their surface [45,46,47,48,49]. In 2015, Yang et al. [45] designed and synthesized a 3D Cu current collector with a submicron skeleton, which significantly improved the electrochemical deposition behaviour of Li because of high electroactive surface area. Here, numerous protuberant tips on the submicron fibres work as the charge centres and nucleation sites, which contribute to a roughly uniform electric field and give rise to the nucleation and growth of nanosized lumps of Li on the submicron Cu fibres, and consequently resulting in a relatively uniform Li surface. As a result, a Li anode in the 3D Cu current collector runs for 600 h without short circuit. Li et al. [46] fabricated a 3D porous Cu current collector/Li metal (3D Cu/Li) composite anode by mechanically pressing Cu mesh into Li metal. The porous structure of the composite anode offered a large number of cages for the deposition of hostless Li and successfully accommodated the volume expansion of electrode. No short-circuit was observed for the composite anode after 1280-h measurements. Similarly, Liu et al. [47] used nanoporous/macroporous Cu current collector as the skeletons to promote uniform Li metal electrodeposition. The modified Cu current collector exhibited an enhanced cycling stability with a high average CE of 98% for 400 h without dendrite growth. A voltage hysteresis of 10 mV was obtained after 400 h at a current density of 0.5 mA·cm−2 and areal capacity of 0.5 mAh·cm−2. Qiu et al. [48] fabricated 3D porous Cu by a simple time-saving hydrogen bubble dynamic template method. The porous Cu current collector maintained a stable performance and low voltage hysteresis for over 2000 h in symmetric cells. A unique biporous and bicontinuous structured Cu current collector was fabricated with a phase inversion tape casting method [49]. Such 3D Cu current collector delivered a CE of 97.6% for 200 cycles in half cells.

Alternatively, modification of Cu current collector by compositing with other high-conductivity species has been widely studied. Yang et al. [50] obtained 3D graphene anchored on a copper foam current collector, which exhibited a CE of 97.4% for 150 cycles at a current density of 2 mA·cm−2 in half cells and an area capacity of 6 mAh·cm−2 at 6 mA·cm−2 in symmetric cells. A 3D CuO@Cu matrix with a LiF enriched surface displayed a high areal capacity (5 mAh·cm−2) and an excellent stability for 350 h at 5 mA·cm−2 [51]. Owing to its large surface area, high Li content and stable protective layer, the 3D framework effectively reduced the local current density, promoted ion diffusion and decreased the interfacial impedance. Dendrite growth was significantly suppressed as well with a negligible volume change during Li stripping/plating process. Chen et al. [52] found that a Cu current collector co-modified with rGO and Cu2O (rGO–Cu2O/Cu) could guide a uniform nucleation of Li and suppress the formation of Li dendrite. With the synergistic effects, rGO–Cu2O/Cu was fully discharged with an extended cycling life up to 300 h in the symmetrical cell. A Li anode deposited on an AlN-modified Cu substrate showed a cycling life that was 5 times longer than the Li anode deposited on bare Cu [53]. A Cu foil-supported Cu3P nanowires (Cu3P@Cu) was fabricated [54], which converted to Li3P and Cu–Li alloy nanocomposite by possible chemical reaction between Cu3P and molten Li. A mixed ion/electron-conducting skeleton (MIECS) containing Li3P and Cu–Li alloy phase was thus constructed successfully. The symmetrical Li@MIECS cells displayed very stable cycling performance for nearly 1000 h at 1 mA·cm−2. By directly growing CNT sponge on Cu substrate, Shen et al. [55] obtained an integrated C-host@Cu electrode, which achieved stable cycling with CE of 99% at 1 mA·cm−2 over 250 cycles. More encouragingly, Cui’s group proposed a synthetic rubber connected 3D-patterned Li metal anode [56]. A 2D Cu coil was first fabricated and then attached to tape, which was followed by casting a poly(styrene-ethylene-butylene-styrene) (SEBS) rubber solution. Li metal was then electrochemically deposited onto the polished side of the Cu coil, resulting in a stretchable Li metal anode. The 2D Cu coil exhibited an enhanced extendibility due to the hierarchical spiral structures upon stretching. Once the external stress was released, the extended SEBS rubber exerted a contraction force onto the small microdomains, allowing the whole electrode to recover its original shape and dimension. When cycled at 1 mA·cm−2 in an ether-based electrolyte, the average CE was determined to be 97.5% for the first 176 cycles. This new structure of Li anode not only improved the cycling stability of the Li anode, but also solved the problem with mechanical stretch ability of batteries.

Recently, a facet selective nucleation and growth of Li on Cu substrate was disclosed [57], which was reasonably attributed to the different Li adsorption energy among the Cu facets. Analyses by electrochemistry and electron-backscatter diffraction revealed that deposition of Li on the Cu(100) plane is preferential. Differing from the conventional Cu foil with randomly oriented surface facets, a majorly (100) plane-orientated Cu foil showed much more uniform nucleation of Li with a 10-fold higher nuclei density, which resulted in a two-fold increase in the cycling stability of Li metal anode. It was therefore believed that controlling the facet structure provides a new design principle for the thin-film Li metal anode for lithium metal batteries. Cao et al. [58] produced vertically oriented Li–Cu–Li arrays by traditional rolling or repeated stacking approaches. Such vertically oriented arrays well regulated the Li-ion flux and guided the regular plating of Li, consequently inducing a remarkable deep stripping and plating capability (50 mAh·cm−2).

Moreover, by growing NiCo2O4 nanorods on Ni foam, Huang et al. [59] observed a voltage hysteresis of 16 mV and CE of 98.7% for over 500 cycles (1000 h) at 1 mA·cm−2. Peng and co-workers reported a Li metal anode prepared with Li9Al4–Li3N–AlN as the host materials, which presented a CE of 94.1% at 4 mA·cm−2 in carbonate-based electrolytes, thanks to the intrinsic lithophilicity of the Li9Al4 sites and attraction of Li+ by the adjacent Li3N groups for fast migration and uniform plating/stripping [60]. A current collector of highly lithiophilic cobalt nitride nanobrush on a Ni foam (Co3N/NF) was also constructed [61], which worked as a stable host to modulate the deposition behaviour of Li by forming highly Li-ion-conductive Li3N. The Co3N/NF electrode afforded ~ 98.3% CE over 200 cycles. An excellent cycling stability (> 900 cycles) was observed for the single-cluster Au, with almost 100% CE and deep cycling behaviour at a high areal capacity (up to 20 mAh·cm−2) [62]. Moreover, the formation of dead Li was effectively suppressed by plating metallic Li on a prelithiated LixSi alloy anode (Fig. 3) [63]. Since the delithiation of LixSi is low event at potential higher than the stripping potential of deposited Li on LixSi, stripping of the plated Li is completed before delithiation of LixSi. Thus, the CE of Li plating/stripping was enhanced to 99.7% that of the Li plating/stripping capacity at 1.0 mAh·cm−2. The improved Coulombic efficiencies (CEs) of Li metal anodes fabricated with carbon-based and metal-based host materials are also listed in Tables 1, 2, respectively [28, 31, 33, 35, 36, 39, 40, 42, 43, 50, 51, 53, 56, 57, 59, 60, 62, 64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88].

Fig. 3
figure3

Schematic diagrams of different Li metal anode host structures: a pristine Li metal and b LixSi [63]. Copyright Cell Press. All rights reserved

Table 1 Conclusion of CEs for Li/Cu half-cell of carbon-based hosts in Li metal anodes
Table 2 Conclusion of CEs for Li/Cu half-cell of metallic hosts in Li metal anodes

Creating an artificial SEI layer

A protective layer on the particle surface of Li metal was found to be helpful in preventing Li metal from participating in a side reaction with the corrosive electrolyte, and at the same time enabling fast Li-ion diffusion and suppressing Li dendrite growth (Fig. 4) [89]. An ideal interfacial layer should possess a Young’s modulus as high as 6 GPa, good chemical and mechanical stability, high ionic conductivity but low electronic conductivity, high electrolyte affinity, and ability to smooth the growth of Li dendrites [90].

Fig. 4
figure4

Ultrathin bilayer of graphite/SiO2 as solid interface for Li metal anode: a, b SEM topography; c, d AFM topography; and e, f corresponding Young’s modulus mapping of bare Li and graphite-SiO2 [89]. Copyright WILEY–VCH. All rights reserved

Interface layers containing lithium salts have been studied and explored for their flexibility and hardness. An inorganic–organic hybrid layer composed of LiCl was coated on the surface of Li metal through a surface restraint dehalogenation reaction, which enabled Li|Li symmetric cells to maintain a stable overpotential of 20 mV at 1 mA·cm−2 after cycling for over 3000 h [91]. Kim et al. [92] obtained a phosphorene-derived Li3P protective layer with a high mechanical strength (34.3 GPa) and fast ionic conductivity (1 × 10−4 S·cm−1), which successfully suppressed Li dendrite growth. A thin (≈ 4 μm) β-PVDF coated Cu collector enabled uniform Li deposition/stripping at high current densities up to 5 mA·cm−2, and the Li plating capacity reached 4 mAh·cm−2 [93]. The presence of LiF was regarded as one of the most important reasons for the improved electrochemical properties. Xu et al. [94] constructed a poly(vinylidene-co-hexafluoropropylene) (PVDF-HFP) and LiF hybrid layer, which facilitated a long-term cyclability due to the high interfacial stability. The Young’s modulus of the artificial protective layer was characterized to be 6.72 GPa, which significantly exceeds that of pristine SEI (~ 150 MPa). As a result, the hybrid layer-protected Li anode exhibited a large capacity of 150.6 mAh·g−1, a high CE of > 99%, and an extended lifespan (80% capacity retention after 250 cycles). Similarly, polyvinyl alcohol (PVA) was also proved to be a favourable protection layer for Li metal anodes in both ether and carbonate-based electrolytes due to the formation of a robust SEI layer. The average CE was calculated to be 98.3% over 630 cycles. Recently, we reported a high-melting point ionic liquid [P4444]TFSI as protective layer for Li nanoparticles, prepared by cryo-milling Li foil with the ionic liquid (Fig. 5) [95]. The ionic liquid-protected Li powder anode delivered the electrochemical properties of a low overpotential and an ultrahigh areal capacity, and Li dendrite growth was effectively suppressed or prevented.

Fig. 5
figure5

[P4444]TFSI ionic liquid-protected Li metal nanoparticles: a XRD patterns; b, c XPS spectra of prepared Li powders; d, e SEM images; and f, g TEM images [95]. Copyright WILEY–VCH. All rights reserved

In principle, the chemical stability of SEI components correlates highly with the energies of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). Zhang et al. [96] introduced a highly stable organic interface (HSOI) with a well-tailored LUMO energy to improve the anti-reduction ability of the SEI components against Li metal. The HSOI layer was synthesized via in situ interfacial reactions of hexafluoro-iso-propanol (HFIP) and ethylene carbonate (EC) with Li metal. The cycling lifespan of the HSOI-protected Li metal anode extended up to 1000 h at 2 mA·cm−2, which is almost 5 times long with respect to that of pristine Li metal anode. A Li polyacrylic acid SEI layer with a high elasticity was also used to address the Li plating/stripping processes and accommodate the volume change [97]. Qi et al. [98] found that the polymer of intrinsic microporosity (PIM) with strong adhesion and mechanical flexibility relieved the stress of the structural changes upon reversible lithiation. Alternatively, various ceramics, such as SiO2, TiO2, SnO2, Al2O3 and V3O7, were very promising interfacial layers that can be used to buffer the volumetric expansion of the anode materials [99,100,101]. Pathek et al. [89] prepared a graphite-SiO2 ultrathin bilayer on a Li metal chip with a radio frequency sputter method to achieve homogeneous Li deposition. The graphite-SiO2 bilayer effectively protected the bare Li from side reactions. The average roughness characterized by the surface root mean square was reduced from 520.1 to 324.5 nm, while the Young’s modulus value was increased from 0.23 to 10.7 GPa. After covering with an ultrathin indium sheet, the deposition of Li became more uniform. The indium sheet functioned here as a passivation layer and a current collector [102]. Mo6S8/carbon films coated with Li foils have nearly four times higher exchange current densities than bare Li anodes [103]. With a roll-to-roll mechanical approach, Li et al. [104] produced a lamellar structure of Ti3C2 MXene (graphene, BN)-metallic lithium film, which exhibited a very small overpotential of 32 mV at 1.0 mA·cm−2, and increased by only 1.5% in 200 cycles. Moreover, the artificial SEI layers developed so far are summarized in Table 3 [89, 92,93,94, 96, 97, 99, 100, 102, 105,106,107,108,109,110,111,112,113].

Table 3 Conclusion of CEs for Li/Cu half-cell and overpotential for Li/Li symmetric cell of artificial SEI layer in Li metal anode

Additives for liquid-state electrolytes

It is well known that the growth of Li dendrites is closely related to the chemical constituents of the liquid electrolytes in Li metal batteries. Metallic Li can react with nearly all the components in liquid-state electrolytes, leading to low CE and dendrite growth. Electrolyte additives are used to form a stable passivation layer on the surface of Li metal and regulate the Li+ deposition behaviour. A variety of electrolyte additives have been developed to enhance the electrochemical performance of Li metal anodes (Table 4) [114,115,116,117,118,119,120].

Table 4 Conclusion of CEs of Li metal anodes with electrolyte additives

A LiF-rich SEI layer is highly desirable for the uniform nucleation of Li deposition [121, 122]. A reaction between metallic Li and a polyvinylidene fluoride (PVDF)-dimethyl formamide (DMF) solution was proposed to fabricate the LiF coating layer [123]. The resultant full cell delivered a capacity retention of 85.7% after 200 cycles. When using poly(sulphur-random-1,3-diisopropenylbenzene) as an additive, the aromatic-based organic components with a planar backbone conformation and \(\uppi{\text{-}}\uppi\) interaction facilitated the formation of a morphology with low roughness for Li metal anode and enhanced the toughness and flexibility [124], which remarkably increased Li deposition/dissolution efficiency (99.1% over 420 cycles). The ionic liquid 1-dodecyl-1-methylpyrrolidinium (Pyr1(12)+) bis (fluorosulfonyl)imide (FSI-) effectively mitigated dendrite growth via the joint effects of electrostatic shielding and lithiophobicity [114]. Moreover, nitrate anions as an “electrolyte-agnostic” additive was also studied, because they were preferentially reduced during the formation of an SEI to afford a conductive interfacial environment, which facilitated the controllable deposition of Li [115]. The authors observed improved ion transport and faster charge transfer kinetics with the presence of N-containing species. A non-flammable LiFSI-TEP electrolyte exhibited a CE of > 99.3% with up to 350 cycles and a high cycling stability in a wide temperature range [116]. Dong et al. [117] reported an electrolyte consisting of 2.8 mol·L−1 lithium bis(trifluoro-methanesulfonyl) imide in triethyl phosphate with 10 vol% fluoroethylene carbonate (FEC) for high-performance LMBs. The presence of FEC in the TEP-based electrolyte enabled a stable LiF-rich SEI layer, consequently suppressing the parasitic reactions between Li and the TEP solvent. A novel fluorescent probing strategy was recently developed to visualize the active Li distribution, based on the principle that the reaction between Li and 9,10-dimethylanthracene (DMA) gives rise to fluorescence quenching [125].

Dual-salt electrolyte systems have been proposed to enhance the electrochemical performances of LIBs and LMBs [126]. Studies were mainly focused on thermally and chemically stable salts, such as lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI), which afforded a high ionic conductivity over a wide temperature range [127, 128]. A mixture of LiTFSI and lithium bis(oxalato)borate (LiBOB) in carbonate solvents of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with a trace amount of lithium hexafluorophosphate (LiPF6) endowed the Li||NMC full cells with a dramatic improvement in the cycling stability [129]. The positive effects of the lithium difluorophosphate (LiPO2F2) additive was also observed in carbonate solvents combined with LiTFSI and lithium trifluoro(perfluoro-tert-butyloxyl) borate (LiTFPFB) (Fig. 6) [130]. The NMC/Li full cell with the LiPO2F2 additive showed 72.5% of capacity retention (113.2 mAh·g−1/156.2 mAh·g−1) when cycled at 25 °C at a 0.2 C rate for 100 cycles. The addition of LiPO2F2 facilitated the formation of LiF, Li2O and P–O species on the surface of the Li metal anode, which resulted in a compact SEI layer and suppressed the growth of Li dendrites. In addition, Li2CO3 and P–O species amounts on the surface of the NMC cathode were increased, which prevented the structural degradation and the decomposition of electrolytes.

Fig. 6
figure6

Schematic illustration of positive effects of LiPO2F2 as electrolyte additive for Li metal anode and NMC cathode upon cycling in dual-salt electrolyte [130]. Copyright WILEY–VCH. All rights reserved

In addition, high-concentration electrolytes (HCEs, > 3 mol·L−1) have attracted much attention because of their unusual functionalities [131,132,133]. Qiu et al. [118] designed a ternary salt electrolyte composed of LiTFSI, LiNO3 and LiFSI for highly stable Li metal full cells. Here, LiNO3 and LiFSI contribute to the formation of stable Li2O-LiF-rich solid electrolyte interface layers, whereas LiTFSI helps to stabilize the electrolyte at high concentrations. As a result, a CE of 99.4% was obtained only with a slight excess of Li metal. Furthermore, a novel localized HCE (1.2 mol·L−1 lithium bis(fluorosulfonyl)imide in a mixture of dimethyl carbonate/bis(2,2,2-trifluoroethyl) ether at a molar ratio of 1:2) was reported. Such electrolyte enabled the dendrite-free cycling of the Li metal anodes and the high CE (99.5%) and excellent capacity retention (> 80% after 700 cycles) of Li||LiNi1/3Mn1/3Co1/3O2 batteries [119].

Solid-state electrolyte

Using solid-state electrolytes to replace liquid-state electrolytes is considered to be the most feasible way to address the safety issues of Li-ion rechargeable batteries. Solid-state electrolytes mainly include inorganic ceramic electrolytes and solid polymer electrolytes. Table 5 lists the electrochemical properties of the presently reported solid-state electrolytes [134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150]. It is apparent that there are still gaps between the practical utilization and laboratory performance of solid-state electrolytes.

Table 5 Conclusion of reported short-circuit time of Li metal anodes with solid-state electrolytes

The main problems with solid-state electrolytes are their low ionic conductivities and the high interface contact resistance between the electrolyte and electrode [151, 152]. Many studies have been devoted to improving the Li-electrolyte interface by surface chemistry strategies [153,154,155]. Nevertheless, the existence of grain boundaries inevitably induces formation and penetration of Li dendrites in the solid-state electrolyte, and ultimately resulting in short circuits [134]. Moreover, the poor solid interface contact and infinite volume change of Li metal anodes during cycling considerably limited the available capacity of Li metal anodes with ceramic electrolytes [135, 156]. However, Li dendrites are also formed in solid-state electrolytes, including Li7La3Zr2O12 (LLZO) and Li2S–P2S5 [157]. Actually, Li dendrites formed more easily in LLZO and Li2S–P2S5 than in the liquid electrolytes of lithium batteries [15]. Time-resolved operando neutron depth profiling (NDP) was used to illustrate the origin of dendrite formation by monitoring the dynamic evolution of Li concentration profiles in three popular but representative SEs (LiPON, LLZO and amorphous Li3PS4) during lithium plating (Fig. 7). The real-time visualization revealed that Li dendrites nucleated and grew directly inside LLZO and Li3PS4, and the high electronic conductivities of these materials were responsible for the formation of dendrites in these SEs. Hence, in addition to a high ionic conductivity of > 1×10−4 S·cm−1, a low electronic conductivity should also be another critical criterion for SEs in practical application in Li metal batteries. For dendrite-free Li plating performed at 1 and 10 mA·cm−2, the electronic conductivities of the SE should be lower than 1 × 10−10 and 1 × 10−12 S·cm−1, respectively.

Fig. 7
figure7

Characterization of Li dendrite formation in solid-state electrolyte: a, b schematic of experimental set-up for operando and structures of all-solid-state batteries; time-resolved lithium concentration profiles for c LiCoO2/LiPON/Cu, d Li/LLZO/Cu and e Li/Li3PS4/Pt all-solid-state batteries [15]. Copyright Nature Publishing Group. All rights reserved

Garnet-type compounds such as LLZO exhibit a good compatibility with metallic Li [135]. The interfacial resistance was estimated to be only 11 Ω cm2 for an all-solid-state battery made of a lithium-graphite (Li–C) composite anode and an LLZTO electrolyte [136]. In this case, a closely interconnected Li–C/garnet interface can be obtained by casting an Li–C composite onto a garnet-type solid-state electrolyte. However, the growth of Li dendrites via grain boundaries and interconnected pores was still observed inside the LLZO electrolyte. To address this issue, a Ta-doped LLZO electrolyte was proposed, which survived the Li striping/plating test under a unidirectional current polarization of 0.5 mA·cm−2 applied for more than 8 h [134]. Recent studies revealed that introducing 10 at% Mg into the Li metal anode effectively improved the contact between Li metal and the LLZO electrolyte [158]. The use of a chemically inert and mechanically robust BN film as the protective interface substantially retarded the reduction of the Li1.3Al0.3Ti1.7(PO4)3 (LATP) solid electrolyte by Li et al. [137]. The thin BN layer is electronically insulating but still allows Li ions to permeate. All the solid-state batteries in the configuration of LFP/PEO/LATP/BN/PEO/Li showed nearly no capacity fading over the 500 cycles performed in 70 days. Additionally, an LAGP–based composite protective layer was found to be favourable for reducing interfacial polarization and restricting the growth of Li dendrites [138]. Studies on the electrochemical–mechanical behaviour of Li metal anodes with the Li6.25Al0.25La3Zr2O12 garnet electrolyte revealed that a stable morphology of the interface maintained when the anodic loading did not exceed a critical value of approximately 100 μA·cm−2 [159]. A 3D mixed electron/ion conducting framework (3D-MCF) based on a porous-dense-porous trilayer garnet electrolyte imparted a 3D solid-state lithium metal anode with a resistance of 25 Ω·cm2 [139]. Upon repeatedly Li stripping/plating, the 3D-MCF cell could operate at 1 mA·cm−2 over 180 h.

Polymer solid-state electrolytes can be obtained by combining a polymer electrolyte with a Li salt or other insulating oxides and show great potential in meeting the demand of Li metal anodes. Using a LiNO3-gel polymer electrolyte, the Li metal anode exhibited an excellent cycling stability and a low voltage polarization (∼ 30 mV at 0.5 mA·cm−2) for 300 h [140]. Electrochemical performance of the Li–S batteries can be enhanced with this protected Li anode. A composite polymer electrolyte is prepared by dispersing ceramic filler grains in the polymer matrix. Ceramic fillers not only effectively enhance ionic conductivity but also improve mechanical and thermal stabilities of polymer electrolytes. A thermoresponsive solid polymer electrolyte of high ionic conductivity synthesized by the copolymerization of poly(1,3-dioxolane) (PDOL) and poly(lithium allyl-sulphide) (PLAS) provide good protection for LMBs, which realizes autonomic shutdown by efficiently inhibiting the ionic conduction between electrodes that occurs beyond an unsafe temperature (70 °C) [141]. When a dual polymer network of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(ethylene oxide) (PEO) of high toughness, stretchability, modulus and ion conductivity was used as the framework of a gel electrolyte, the Li metal anodes could cycle 400 times with a CE of 96.3% [142]. Recently, a unique solid polymer electrolyte containing MXene-based mesoporous SiO2 nanosheets (MXene-mSiO2) was fabricated through controllable hydrolysis of tetraethyl orthosilicate around the surface of MXene-Ti3C2 under the direction of cationic surfactants [160]. With the MXene-mSiO2 containing electrolyte, the Li symmetric cells delivered an ultralong and stable stripping/platting cycling up to 2000 h at 0.05 mA·cm−2 with low overpotential (39.6 mV). The performance of solid-state Li–O2 batteries using the integrated GPE–Co3O4@CC electrolyte assembly pressed on the lithium foil was greatly enhanced [161].

A combination of ceramic and polymer electrolytes results in a solid-state electrolyte with high ionic conductivity and good mechanical flexibility simultaneously. The composite electrolyte of PEO–LiTFSI–Li7La3Zr2O12 polymer-ceramic showed a high ionic conductivity of 1.7 × 10−4 S·cm−1 at 70 °C and low activation energy of 0.35 eV above 53 °C [143]. The cells assembled with a Li metal anode and a LiFePO4 cathode delivered average energy densities of up to 185 Wh·kg−1 and 345 Wh L−1. An all-solid-state Li battery composed of Li anode modified with Li phosphorous oxynitride (LiPON) and a composite solid electrolyte of LAGP–PEO(LiTFSI) provided an initial discharge capacity of 152.4 mAh·g−1 with a good cycling stability and rate performance at 50 °C [162]. The LiPON film with a thickness of 500 nm exhibited a favourable interface property between the Li metal anode and the LAGP–PEO(LiTFSI) solid electrolyte. LiPON itself possesses a high chemical stability against Li metal and a high electrochemical stability up to 5.5 V (vs. Li/Li+), and there is a relatively uniform transference of Li+ throughout the electrolyte with LiPON modified Li metal anode. A composite of PAN@LAGP (80 wt%) was used as an intermediate layer to inhibit dendrite penetration and ensure a compact interface [144]. The electrochemical window of the solid-state electrolytes was expanded to 0–5 V, and the symmetric Li battery retained a low voltage polarization at 2 mA·cm−2.

External field technologies

In general, the growth of Li dendrites is related to the inhomogeneous ion distribution and diffusion caused by electric driving force [22, 54]. Therefore, introducing external fields including electric field and magnetic field to rearrange the Li+ concentration on the anode surface could also be effective to obtain homogenous deposition of Li. In 2012, Mayers et al. [163] studied the effects of different pulse charging waveforms on Li deposition morphology by using a coarse-grained simulation model. The results indicated that pulse charging can effectively suppress the dendrite formation of Li at high overpotentials. This was further confirmed by molecular simulations to study the diffusion and solvation structure of lithium cations (Li+) in bulk electrolyte [164]. Furthermore, the effectiveness of pulse current charging on dendrite suppression was optimized by choosing proper pulse duty cycle because the ratio between current-on time and current-off time (Ton/Toff) was of critical importance for stabilizing the cycling behaviour. The optimal conditions were determined to be Ton/Toff = 1:5 or Ton/Toff = 1:3 for pulse charging. Yang et al. [165] pointed out that the pulse plating waveforms with short and widely spaced pulses remarkably improved Li deposition morphology and cycling efficiency. Aryanfar et al. [166] experimentally demonstrated ~ 2.5 times shortening for the average dendrite length while charging a scaled coin-cell prototype with 1 ms pulses followed by 3 ms rest periods, compared with those grown under continuous charging. Li et al. [167] designed and fabricated a triboelectric nanogenerator (TENG)-based pulse output with a novel waveform and frequency. By regulating the waveform and frequency of the TENG-based pulse output, the CE of Li plating/stripping was enhanced and the homogenous Li plating was realized in Li metal batteries.

In addition to optimization of charging and discharging, a uniform deposition of Li could also be achieved by applying external magnetic field to rearrange the Li+ concentration on the anode surface. Shen et al. [168] proposed a magnetohydrodynamic (MHD) effect for the Li ions while imposing a magnetic field to Li metal anodes (Fig. 8). Li ions are put into spiral motion by Lorentz force in electromagnetic fields, which promotes mass transfer and uniform distribution of Li ions, and consequently favouring the suppression of the dendrite growth. Based on 3D Cu/NiCo current collector, the cycling stability and rate capacity of Li metal anode were largely improved under magnetic field as the voltage was extremely stable with overpotential of only 38 mV at 350 h.

Fig. 8
figure8

Magnetic field-suppressed Li dendrite growth: a schematic illustrating effects of magnetic field on Li+ deposition; b CE profiles of 3D Cu/NiCo current collector within or without magnetic field during Li plating/stripping at different current densities [168]. Copyright WILEY–VCH. All rights reserved

Conclusion

Li metal is an ideal anode material for rechargeable batteries, especially for Li–O2 and Li–S batteries. However, Li dendrite growth and the low CE during Li plating/stripping processes severely restrict the commercialization of these types of rechargeable batteries. To suppress Li dendrite growth, roughly five different categories of strategies are applied. These are Li metal anodes with host materials, creation of artificial SEI layers, introduction of additives in electrolytes, use of solid-state electrolytes and application of external fields. Carbon-based materials are the most frequently used host materials for stabilization of Li metal anodes. Doping of N, O and F further enhances the lithiophilicity of the carbon matrix. The porous carbon granules co-doped with O and N may achieve stable Li plating/stripping and high CEs. Artificial SEI layers, especially Li salt-containing interface layers, can effectively protect Li metal from side reactions with corrosive electrolytes. An inorganic–organic hybrid layer containing LiCl enables Li|Li symmetric cells to maintain a very stable overpotential with prolonged cycling periods. Electrolyte additives facilitate the formation of a stable passivation layer and modulate the Li+ deposition behaviour. Lithium bis(fluorosulfonyl)imide as an electrolyte additive enabled a dendrite-free cycling of Li metal anodes and the high CE and capacity retention. In particular, replacing liquid-state electrolytes with solid-state electrolytes is regarded as the most feasible way to address the safety issues of the Li-ion rechargeable batteries. The all-solid-state battery with the configuration of LFP/PEO/LATP/BN/PEO/Li operated over 500 cycles in 70 days nearly without capacity fading. For practical applications, however, the average CE of Li metal anodes must be higher than 99.9%. Therefore, progress is still needed to realize the practical application of Li metal anodes.

Although various ex situ techniques have been adopted to characterize the morphology, components and structures of Li metal surfaces, these techniques might destroy the original morphology and structure. In order to acquire fundamental understanding about the growth of Li dendrites and low CEs, innovative strategies need to be developed. In situ or operando techniques can provide more details about the local concentration gradient, Li dynamic nucleation and dendritic growth, which can be utilized to simultaneously observe the formation of dendrites and SEI layers. In addition, the chemical reactions between Li metal and electrolytes or electrolyte constituents should be modelled to better understand the formation mechanism of the SEI layer as well as its chemical stability, ionic conductivity and mechanical strength. SEI layers with certain flexibility will tolerate dendrite growth on the surface of the Li metal anode, which can contribute to increasing the CE. On the other hand, enhancement in the conductivity of electrolytes can effectively reduce the internal voltage drop under high current density conditions. Since metallic Li is chemically incompatible with nearly all organic solvents, inorganic solid-state Li+ conductors with a good mechanical strength, stability and a high Li ionic conductivity as well as excellent compatibility with Li metal are desirable for rechargeable Li metal batteries. Furthermore, morphology and growth of Li dendrites strongly depend on the applied cell pattern and test conditions. As such, criteria for assessment of the electrochemical performance of Li metal anodes should be developed and standardized.

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Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (51831009), the National Materials Genome Project (2016YFB0700600) and the National Youth Top-Notch Talent Support Program.

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Correspondence to Yong-Feng Liu.

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Pu, K., Zhang, X., Qu, X. et al. Recently developed strategies to restrain dendrite growth of Li metal anodes for rechargeable batteries. Rare Met. 39, 616–635 (2020). https://doi.org/10.1007/s12598-020-01432-2

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Keywords

  • Rechargeable batteries
  • Anode materials
  • Lithium metal
  • Dendrite growth
  • Coulombic efficiency