Mechanism and solutions of lithium dendrite growth in lithium metal batteries

Abstract

Lithium metal has traditionally been regarded as an ideal anode material for high energy density batteries owing to its ultra-high theoretical specific capacity (3862 mA h g⁻¹), extremely low redox potential and low density. Developing lithium metal electrodes is of great significance for developing solid-state batteries. However, the safety issues caused by lithium dendrite growth during the cycling process of lithium metal batteries seriously hinder their commercial applications. Numerous works on how to suppress lithium dendrite growth and construct safe lithium metal batteries have been reported. This review focuses on the internal environment of lithium metal batteries (LMBs) and mainly discusses five possible mechanisms for lithium dendrite growth and three important strategies to suppress lithium dendrites. The effects of factors such as electrolyte composition, current density, metal valence states, and electric fields on the formation of lithium dendrites are revealed. Significant considerations for lithium metal anode development including appropriate electrolyte components, electrode interfaces, SEIs, separators, electrode fabrication strategies, and practical device engineering, and suggestions for future development are proposed. This work will provide a reference for the rational design of lithium metal batteries.

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Yafei Huang

Yafei Huang is studying for master in the Department of Chemistry at the School of Science, Shanghai University, China, under the supervision of Professor Guorong Chen. She obtained a Bachelor in 2021. Her research is focused on the efficient regulation of lithium dendrites in lithium metal batteries.

Haotian Yang

Haotian Yang is studying for master in the Department of Chemistry at the School of Science, Shanghai University, China, under the supervision of Professor Guorong Chen. He received his Bachelor in 2021. His research interests are focused on solid state electrolytes for all solid-state batteries.

Guorong Chen

Guorong Chen, an Associate Researcher at the Nanoscience and Technology Research Center of the School of Science, Shanghai University. He mainly engaged in research on the positive and negative electrode materials, surface interface structure and mechanism of lithium-ion batteries.

1. Introduction

Lithium-ion batteries (LIBs) hold an important position in the global second energy storage field because of their good cycle life and high specific energy density. In recent years, with the developing mobile energy storage field, the requirements for lithium-ion batteries have become higher and higher,^1,2 and the issues such as energy density, cycle life and safety have attracted much more attention. The energy density and safe reliability of traditional LIBs with graphite as an anode already cannot meet the current high demand for advanced electric devices, and researchers need to find alternative materials with high specific capacity for anodes. Lithium metal has traditionally been regarded as an ideal anode material for high energy density batteries owing to its ultra-high theoretical specific capacity (3862 mA h g⁻¹), extremely low redox potential and low density.^3,4 However, there are still several issues that need to be addressed before applying lithium metals to lithium metal batteries: (1) the extra irreversible reaction between lithium electrodes and organic electrolytes will produce a thick passivation film (SEI) on the surface of lithium electrodes, consume a large amount of lithium and electrolyte, and also increase impedance and reduce the cycle life.^5,6 (2) Uncontrolled growth of lithium dendrites in repeated plating/stripping results in the formation of “dead” lithium, which is easy to pierce the separator and cause an inner short circuit, bringing great safety hazards. (3) The volume and morphology changes of lithium metal batteries (LMBs) caused by repeated plating/stripping processes of lithium metal anodes affect their safety use.⁷ The uneven SEI layer on the surface of the lithium electrode and different current densities inside the battery can lead to the uneven deposition of lithium. The lithium dendrites formed after long-term cycling are prone to puncture the separator, causing a short circuit in the battery and affecting its electrochemical performance. Therefore, it is urgent to solve the problem of lithium dendrite growth. This work introduces five kinds of formation mechanisms of lithium dendrites, summaries current solutions from three perspectives of electrolyte, interface and separator engineering, and proposes several viewpoints on the possible direction that may arise in the further development of lithium metal batteries (Fig. 1).

Fig. 1 The mechanisms and solving strategies of lithium dendrite growth. The problem of lithium dendrite growth is one of the problems that must be solved on the commercialization path of lithium metal batteries. This article briefly introduces five mechanisms of lithium dendrite growth and summarizes strategies to suppress lithium dendrite growth from three perspectives.

2. The formation mechanism of lithium dendrites

The problem of lithium dendrites has always been the biggest obstacle to the practical development of lithium electrodes. The growth of lithium dendrites often causes problems such as lower coulombic efficiency, short cycle life and poor capacity retention in lithium batteries, as well as the safety hazards of cell short circuit and volume expansion. The study of the lithium dendrite growth mechanism is an indispensable part of inhibiting and preventing lithium dendrite growth.

2.1. Space charge growth mechanism

As early as 1990, Chazalviel first discovered the deposition of Cu²⁺ ions on a Cu electrode in Cu symmetric batteries with electrolyte of dilute CuSO₄ (aq) and proposed the mechanism of metal dendrite growth caused by the uneven distribution of space charges inside batteries.⁸ Chazalviel divided the interior of the cell into two regions: the region away from the electrodes named region I, where ion migration is controlled by ion diffusion and occupies most of the cell, and the region near the electrodes named region II, where ion migration is controlled by an electric field (Fig. 2). Initially, anions and cations were evenly distributed inside the battery (Fig. 2a). When applying an voltage, the cations and anions will undergo directional movement, with the cations moving towards the cathode and the anions moving towards the anode (Fig. 2b). Due to the different migration rates of ions in the two regions, an anion-rich region is generated near the anode and a cation-rich region is generated near the cathode (Fig. 2c); thus, an additional local electric field is formed, resulting in a space charge field (Fig. 2d). The space charge generated at the cathode leads to an inhomogeneous electric field within zone II, which induces the growth of dendrites (Fig. 2e and f). In addition, Chazalviel demonstrated the growth rate of dendrites, which is also related to anion mobility and the electric field.

Fig. 2 Schematic diagram of dendrite growth caused by space charge inside the symmetric cell of Cu/CuSO₄/Cu. The growth of lithium metal dendrites is related to the uneven distribution of space charges inside the battery. Due to the different ion migration rates in different regions, the formation of lithium dendrites is ultimately induced by an uneven electric field.

On this basis, Brissot and Rosso studied symmetrical lithium metal batteries of Li/PEO-salt/Li, and an in situ observed lithium dendrite growth process at both high and low current densities, respectively.^9,10 The growth process and rate of lithium dendrites at high current densities are consistent with Chazalviel’ predictions. At high current densities, the Li⁺ ions transfer faster and rapidly deposited on the electrodes, because the lithium ion concentration of the electrode vicinity becomes zero at a certain moment (this time is also called Sand's time¹¹), generating a lithium ion depletion zone that leads to form a space charge, resulting in the inhomogeneous electric field and promoting the formation of lithium dendrites; as the number of cycles increases, lithium dendrite growth is faster.

2.2. SEI defect deposition mechanism

SEI films on lithium electrodes are inevitably formed after first charging and discharging processes due to electrolyte reduction. However, the SEI morphology, conductivity and mechanical strength have a great influence on lithium ion deposition. Yamaki and Cohen proposed that lithium ions are preferentially deposited at the location with higher lithium ion conductivity or SEI defects.^12,13 At low current densities, the Li⁺ ions have enough time to evenly diffuse and deposit under the SEI, and more uniform deposition and dissolution of lithium ions on lithium metal electrodes are almost balanced, which hard to form lithium dendrites (left side of Fig. 3). As the current density increases, the deposition of lithium ions becomes uneven, lithium ions will preferentially deposit in areas with weak mechanical strength and high conductivity of the SEI film, more and more lithium ions continue to be deposited at this nonuniform distribution of mechanical stresses on the SEI, and the volume of the lithium metal electrode has rapid expanded, eventually leading to cracking of the SEI. And lithium dendrites are formed at the SEI crack and extrude out of the SEI, and followed by gradual growth (right side of Fig. 3).

Fig. 3 An illustration of the morphological phenomena developed on Li electrodes during Li deposition. Reproduced with permission. Copyright 2000, American Chemical Society. The uneven deposition of lithium ions in areas with weak mechanical strength and high conductivity of the SEI film leads to the rupture of the SEI layer, followed by the continued growth of lithium dendrites.

2.3. Non-homogeneous phase nucleation mechanism

Lithium dendrites have been observed by a non-homogeneous phase nucleation process. Ely studied the non-homogeneous phase nucleation process of lithium dendrites by theoretical derivation from both thermodynamic and kinetic aspects, respectively.¹⁴ The results show that the system with less negative of bulk free energy of transformation has a higher energy barrier for lithium nucleation, and lithium nucleation is formed only at higher over potentials. Instead, when the bulk free energy of transformation is more negative, the lithium nucleation energy barrier of the system is lower, with ability to form lithium nucleation at smaller over potentials, and non-homogeneous lithium nucleation may also form when the bulk free energy of transformation is very negative or when the contact angle is very small at positive over potentials.

Ely divided the lithium dendrite growth process into five stages: nucleation suppression (stage I), long incubation time (stage II), short incubation time (stage III), early growth (stage IV), and late growth stage (stage V).¹⁴ In stage I, lithium nucleation is thermodynamically unstable and may dissolve again in the electrolyte solution. During stage II, although the Gibbs free energy of the lithium nucleation exceeds the critical energy barrier, the driving force for further growth of the lithium nucleation is still comparable to the thermal fluctuations; thus the lithium nucleation is in a metastable state in the long incubation time regime. In stage III, the critical nucleation thermodynamic radius and kinetic critical radius values are similar, and the local interactions are smaller, which is favorable for the accelerated growth of lithium nucleation. In stage IV, thermodynamics and kinetics contribute to the steady growth of lithium nucleation and the growth rate gradually increases. Finally, at stage V, the appearance of lithium dendrites is mainly controlled by the local electric field.

2.4. Surface nucleation mechanism

Jäckle calculated the fundamental properties of lithium and magnesium during electrochemical deposition by using periodic density functional theory (DFT).¹⁵ The results show that compared to lithium, magnesium has lower diffusion barriers and a high coordination configuration, and has a trend to grow towards smooth surfaces; in other words, magnesium tends to be uniformly deposited on the electrode surface.¹⁶

Ling studied the morphology of deposited lithium and magnesium by thermodynamics and kinetics.¹⁷ However, Mg–Mg bonds are stronger than Li–Li bonds. Thus, the difference in free energy between the high-dimensional and low-dimensional phases of magnesium is higher than that of lithium. This result indicates that Mg is more tended to form two-dimensional layered crystals, while lithium is more tended to form one-dimensional dendrites during electrochemical deposition.

2.5. Solid electrolyte promoting growth mechanism

The application of solid electrolytes (SEs) in inhibiting lithium dendrite growth has also received great attention.^18,19 In contrast to previous lithium dendrite growth mechanisms, Han reveals that high ion conductivity promotes the formation of lithium dendrites in solid electrolytes (SEs). They selected three common SEs (LiPON, LLZO and Li₃PS₄) for comparative experiments. After comparing the three SEs, it is evident that Li₂S–P₂S₅ and LLZO have conductivities that are several orders of magnitude higher than that of LiPON. The dynamic evolution of lithium concentration profiles in SEs is monitored by time-resolved operando neutron depth profiling (NDP) to visualize the growth of lithium dendrites in real time (Fig. 4a–e). The results show that lithium dendrites were directly deposited on LLZO and Li₃PS₄, while dendrites cannot be observed in the system of LiPON. In contrast to the conventional dendrite growth theory, lithium dendrites grow directly nucleated in most regions of the SEs and the lithium dendrite content increases uniformly (Fig. 4f–h). Han suggests that the high conductivity allows lithium to bind to electrons, forming lithium dendrites directly inside the SEs when the potential reaches the lithium deposition potential.²⁰ Therefore, how to reduce the conductivity of solid electrolytes at high current densities is of great significance to inhibit the formation of lithium dendrites in solid-state electrolytes.

Fig. 4 (a) Schematic of the experimental set-up for operando NDP. (b) Schematic structures of LiCoO₂/LiPON/Cu, Li/LLZO/Cu and Li/Li₃PS₄/Pt cells. Li is plated on Cu or Pt, and the depth profiles are measured from the top surfaces of the cells during plating. Time-resolved lithium concentration profiles of LiCoO₂/LiPON/Cu (c), Li/LLZO/Cu (d) and Li/Li₃PS₄/Pt (e) cells. The lithium concentration profiles in d and e are obtained from cells tested at 100 °C. The grey arrows indicate the continuous plating of Li. Visualization of the depth distribution of dendrites in SEs. Lithium concentration profiles of LiPON-25 °C (f), LLZO-25 °C (g) and Li₃PS₄-25 °C (h) at different times during lithium plating. (a)–(h) Reproduced with permission. Copyright 2019, Springer Nature. Based on the data of three common SEs (LiPON, LLZO, and Li₃PS₄), it is shown that reducing the conductivity of solid electrolytes at high current densities is of great significance in inhibiting the formation of lithium dendrites.

3. Controlling dendrite growth by optimizing electrolyte components

The electrolyte in lithium-ion batteries, like human blood, transports lithium ions, providing a healthy working environment for lithium-ion batteries. Therefore, the components of the electrolyte are crucial for extending the service life of batteries.²¹ However, owing to the strong reducibility of lithium metals, during the first charging process, the organic carbonate electrolyte is first reduced on the surface of lithium metal to create an electronic insulated solid electrolyte interface film (SEI), which not only insulates the association between the lithium metal and the electrolyte but also prevents the continuous reaction of the electrolyte. The composition and stability of the SEI film both are significantly correlated with the cycling life of LMBs. A stable SEI film can extend the lifespan of LMBs; otherwise, it will accelerate battery capacity decay and cause serious lithium deposition, lithium dendrites, and even cause safety accidents such as internal short circuits and fires in the battery. Therefore, the electrolyte composite is very important for forming the stable SEI and ensuring the reliability of LMBs.²²

In the current classic carbonate electrolyte system (non-aqueous electrolyte), ethylene carbonate (EC) is a very important component with a high dielectric constant (ε: 89.6) and has high solvation ability for lithium salts, forming (ECLi) solvated lithium ions and free anions of PF₆⁻. The solvation sheath is the precursor of the SEI, due to the fact that the reduction potential of carbonate solvents is ahead of the deposition potential of lithium; when solvated lithium ions flit to the surface of the lithium metal, an organic solvent preferentially receives electrons and is reduced, forming organic lithium salts or polymer lithium salts that deposit on the outside of lithium, forming an SEI film that can transfer lithium ions. This allows lithium ions in solvated lithium ions to pass across the SEI and deposit on the lithium metal.^23–25 By optimizing the components of the electrolyte and modifying the solvent structure of solvated lithium, the composition and stability of the SEI can be controlled. The research results have shown that SEIs with high organic contents are usually unstable and will rupture and continue to react during subsequent cycles. The SEI film with LiF as the main component has been proven to be a stable SEI film, which will provide longer circulate life and excellent rate characteristics to the battery. Optimizing the electrolyte composition and constructing the SEI membrane with a large amount of component LiF on the lithium metal have become an important research direction for improving the cycle life of lithium metal batteries.^26,27

Scientists have conducted many studies for overcoming the lithium dendrite growth, among which electrolyte engineering has received much attention. Optimizing the composition of the electrolyte and forming the SEI layer with grand uniformity and stability in situ is the simplest, most efficient, and low-cost ways to restrain the growth of lithium dendrite in the industry. Table 1 summarizes the research progress in electrolyte modification to inhibit the growth of lithium dendrites in the past decade. We can see that the optimization strategy is mainly divided into three aspects: electrolyte additives, high salt concentration electrolytes (HCEs), and solid electrolytes.

Table 1 The recent studies on suppressing lithium dendrite growth by optimizing electrolytes

Electrolyte	Method	SEI layer	Performance (rate, cycle number, retention)	Ref.
PFPE-MC/LiTFSI	PFPE and LiTFSI as the additives, “anion–solvent” interaction	LiF-rich	0.1C, 120, 95%	28
FEC/LiPF6-EC-DEC	FEC as the additive	LiF-rich	1C, 100, 65%	29
H2O/LiPF6-PC	Trace water as the additive	LiF-rich	—	30
LiNO3/DIOX-DME	LiNO3 as the additive	LiNxOy-rich	—	31
LiTFSI/LiNO3 -SL/HFE	LiNO3 and HFE as the additives	LiNxOy-LiF-rich	0.5C, 200, 99.5%	32
FEC-LiNO3/DME	LiNO3 and FEC as the additives	LiF-rich	1C, 1000, 80.8%	33
LiDFBOP-FEC/LiPF6-EMC	LiDFBOP and FEC as the additives	LiF-LixPOyFz-rich	1C, 600, (127.5 mA h g^−1)	34
LiAsF6-VC/LiPF6-PC	LiAsF6 and VC as the additives	LixAs alloy-LiF-rich	—	35
TTE-FEC/LiTFSI-BL	TTE and FEC as the additives	LiF-rich	1C, 650, 89%	23
LiTFSI-LiNO3-LiFSI/DME-DOL	LiTFSI, LiNO3 and LiFSI as the additives	Li2O–LiF-rich	0.5C, 500, 92%	36
LiFSI-DMC-BTFE	Locally high concentration electrolyte (LHCE)	LiF-rich	2C, 700, >80%	37
LiFSI-1.2 M DME-3 M TTE	Locally high concentration electrolyte (LHCE)	LiF-rich	C/3, 155, 80%	38
Triple-DMC/1,2-dfBen	Locally high concentration electrolyte (LHCE)	LiF-rich	0.5C, 100, 69.6%	27
LiFSI-BDE/DME (5/1 v/v)	Locally high concentration electrolyte (LHCE)	LiF-rich	1C, 500, 80%	39
LiFSI-TEP/BTFE	Solid electrolyte	LiF-rich	C/3, 50, 78%	40
SbF3-PEO-LLZTO	Solid electrolyte and SbF3 as the additives	LiF-Li3Sb-rich	1C, 200, 91.3%	41
Amorphous 3D carbon-PEO	Novel self-healing solid electrolyte	—	1C, 850, >80%	42
Rod-shaped alumina and graphite coating	Polymer solid electrolytes	Li–Al–O and LiC6 layer	0.5C, 100, 90.9%	43
I2-PEO-LiTFSI/Li6.4La3Zr2Al0.2O12-PEO-LiTFSI (IP/LLP)	Janus-structured composite solid electrolyte	Stable ionic-conductive SEI	0.2C, 500, 87.8%	44
LiI-doped Li3PS4	Solid electrolyte	High ionic-conductive	—	45
LiCPON/LiNbON/LiCPON	Sandwich structure solid electrolyte	—	0.5C, 120, 89.8%	46
Ta-doped LLZO	Solid electrolyte	High ionic-conductive	—	47
0.6Li2S + 0.4[xSiS2 + 1.5(1 − x) PS5/2]	Glassy solid electrolytes	Highest conductivity	—	48
LiFSI/DMC/TTE	Counter solvent electrolytes	LiF-rich	2C, 200, 80.1%	49

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3.1. Electrolyte additives

With regard to the composition of the SEI layer, lithium fluoride (LiF) is found to be an ideal SEI component to inhibit lithium dendrite growth on account of its strong mechanical strength and electrochemical stability around a wide potential range.^50,51 The in situ fabrication of a dense and uniform SEI layer of LiF rich can effectively inhibit dendrite growth. With an eye to form the LiF-rich SEI layer on the lithium metal anode, the use of a variety of electrolyte additives is an effective path, such as vinyl carbonate (VC),⁵² fluoroethylene carbonate (FEC),^53,54 lithium bisoxalato-difluorophosphate (LiDFBOP),³⁴etc. As shown in Fig. 5a, Zhang Q. et al. added fluoroethylene carbonate (FEC) into the electrolyte as an additive agent, and induced the formation of the dense and uniform LiF-rich SEI layer to hold back the evolution of dendrites. The fluorinated SEI layer derived from FEC exhibits high Young's modulus and weak electronic conductivity,^28,34 particularly conducive to achieving dendrite-free lithium deposition in LMBs. During battery charging, the −0.87 eV of the lowest unoccupied molecular orbital (LUMO) of the lithium anode surface with a lower FEC level is significantly lower than that the −0.38 eV of EC and 0 eV of DEC; so FEC is preferentially reduced to form the SEI with LiF-rich (Fig. 5b). Therefore, FEC induces a LiF-rich SEI layer to protect the lithium metal, effectively inhibiting the parasitic reaction between lithium and electrolyte solvents. Fig. 5c and d show the lithium deposition morphologies of Cu foil in the Li|Cu half-cell both after 50 cycles. The Cu surface with lithium deposition without the addition of the FEC electrolyte is very uneven, indicating multiple dead lithium and porous lithium. After preparing the electrolyte containing 5% FEC, an equable and smooth Cu surface with lithium deposition was obtained, without any dendrite or porous structures. This intuitively indicates that the LiF-rich SEI attributed to FEC additives is beneficial for uniform lithium deposition, reducing the formation of dead lithium, and obtaining a dense, uniform, and low resistance lithium deposition layer, effectively extending the cycle life of LMBs.^29,39

Fig. 5 (a) Schematic illustration of the effect of FEC additives on a Li metal anode. The electrolyte is 1.0 m LiPF₆ in EC/DEC (1 : 1 by volume) with/without FEC additives. Reproduced with permission. Copyright 2017, Wiley. (b) Electrochemical performance and SEM images of Li|Cu cells. SEM images of Li depositing morphology on Cu foils after 50 cycles with 0% and 5% FEC. Reproduced with permission. Copyright 2017, Wiley. (c) and (d) High-temperature performance of Li|LiFePO₄ coin cells with either the EC/DEC or FEC/LiNO₃ electrolyte at 60 °C and corresponding CE at 1.0C. Reproduced with permission. Copyright 2017, Wiley. Using FEC as an electrolyte additive can induce the preferential formation of LiF-rich SEI layers and inhibit the growth of lithium dendrites.

However, at high current densities, the lower ion conductivity of LiF has a certain impact on the transport ability of lithium ions in the SEI. Therefore, many people have begun to study the simultaneous formation of LiF, LiN₃, LiN_xO_y and other substances in the SEI.³² Zhang Q et al. also studied the combination of carbonate solvents and anions, which both can help to enhance the stability of the SEI (Fig. 6a and b). The NO₃⁻ in LiNO₃ can take part in the solvation of lithium ions in ether electrolytes, generating LiN_xO_y in the SEI, consequently improving the uniformly distributed lithium plating. They introduced LiNO₃ and FEC into the electrolyte simultaneously, resulting in abundant LiF and LiN_xO_y in the SEI owing to NO₃⁻ and FEC introduced into the solvated of lithium ions, promoting the dense and stable SEI and obtaining a dendrite free lithium metal anode.³¹ By comparing EC/DEC electrolytes, it is found that the existence of FEC and LiNO₃ indeed changes the solvation sheath of lithium ions. As shown in Fig. 6c and d, the rate performance of coin cells using FEC/LiNO₃ as the electrolyte has significantly improved, and when tested under harsh conditions at a high temperature of 60 °C, the battery has a longer cycle life and higher coulombic efficiency (CE).³³ It is evident that the use of electrolyte additives such as FEC and LiNO₃ to construct LiF rich SEI layers has significant effects in inhibiting lithium dendrite growth.

Fig. 6 Schematics of the solvation sheath of lithium ions and the SEI formed in (a) FEC/LiNO₃ and (b) EC/DEC electrolytes, in which PF₆⁻ is not shown for clear comparison. Reproduced with permission. Copyright 2018, Wiley. (c) Rate capacity and CE of Li|LiFePO₄ coin cells at −10 °C at 1.0C. Reproduced with permission. Copyright 2018, Wiley. (d) High-temperature performance of Li|LiFePO₄ coin cells with either the EC/DEC or FEC/LiNO₃ electrolyte at 60 °C and corresponding CE at 1.0C. Reproduced with permission. Copyright 2018, Wiley. (e) Physical properties of BDE and conventional solvents. Reproduced with permission. Copyright 2022, Elsevier. (f) The proposed unique solvation structure of the BDE/DME electrolyte. Reproduced with permission. Copyright 2022, Elsevier. The optimization of electrolytes through electrolyte additives, high concentration electrolytes, and solid electrolytes essentially involves constructing a more stable SEI layer and uniformly depositing lithium ions to suppress dendritic growth.

3.2. High salt concentration electrolytes (HCEs)

Optimizing electrolytes by preparing high concentration electrolytes (HCEs), developing new lithium salts,^55,56 and adding additives^30,57 are all simple and effective ways to control lithium dendrite growth. The salt content in high concentration electrolytes is usually higher than 3 M. Due to the lack of solvents and the abundance of anions, anions inevitably appear in the main solvation sheath of Li⁺, forming ion pairs or aggregates.⁵⁸ This solvation structure leads to the anion derived SEI and determines the trend of the SEI layer and subsequent lithium deposition. Therefore, the SEI layer and lithium plating can also be improved by adjusting solvated lithium ions in HCEs. For example, high concentrations of FSI⁻, which are good for forming the stable SEI with LiF-rich on lithium metals, exhibit the ability to hamper the lithium dendrite formation. Especially, in the high concentration ether-based electrolytes, most of the free solvent molecules are confined to the solvated structure of Li⁺ ions, and form the durable SEI and avoid excessive consumption of lithium metals and electrolytes.³⁸ However, high salt electrolytes also have obvious drawbacks, such as high viscosity making it difficult to soak electrodes and separators, resulting in insufficient electrolyte infiltration of the separators and electrodes. In addition, poor electrochemical behavior under low-temperature conditions and huge costs are also significant challenges that HCE applications must face.^59–61 To address these issues, the introduction of inert diluting solvents in HCEs can reduce the electrolyte viscosity and help to form “localized high concentration electrolytes (LHCEs)” or “pseudo concentrated electrolytes” with low stickiness, great wettability, and high conductivity of lithium ions.^36,37 Some perfluorinated diluent molecules are used as cosolvents to help maintain the Li⁺ ion solvation structure similar to that in HCEs, and such LHCEs can be developed.⁴⁰ However, most of the inert dilution molecules used in LHCEs do not interact with lithium salts and make lithium salts to dissociate, which limits improving ion conductivity. Therefore, diluent cosolvent is needed to support lithium salt dissociation, improve its solvation ability, and benefit the ionic conductivity.^49,62

Wang et al. designed and synthesized a new partially fluorinated cosolvent of bis (2,2-difluoroethyl ether) (BDE), which interacted with dimethoxy ethane (DME) to obtain a bifunctional cosolvent, forming new LHCEs. Fig. 6e compares the physical performance of BDE with other solvents. As the data show that the energy level of the highest occupied molecular orbital (HOMO) of BDE is slightly lower than that of bis (2,2,2-trifluorethyl) ether (BTE), nevertheless BDE compared to ethyl ether (EE) and DME is much lower, indicating that BDE has the lowest electrochemical oxidation p potential. BTE has previously been used as a diluent for LHCEs in LMBs, but its boiling point is only 64 °C, which is relatively low and unable to meet the expected wide temperature window for electrolytes. Compared with BTE, BDE is more suitable for use as an electrolyte, especially in electrolytes with a wide temperature range, as it has more F⋯H hydrogen bonds and a boiling point of up to 108 °C. As the electronic static potential calculated by DFT is shown, the electron absorption ability of difluoroethyl is weaker than trifluoroethyl, and the electron cloud density around oxygen in BTE is not as high as that around BDE, which means that the interaction between BDE and Li⁺ is stronger. They proposed a possible solvation structure model through calculation and analysis; as shown in Fig. 6f, dimethyl ether and FSI ⁻ strongly coordinate with Li⁺ ions, a portion of BDE molecules coordinate with Li⁺ ions through slight interactions in the first solvated sheath, while another portion of the BDE molecule acts as diluents surrounding the outer shell. This peculiar solvent structure leads to comparatively high ion conductivity and the transfer number of Li⁺ ions in LHCEs, and affects the composition of the SEI, thereby affecting the interfacial and electrochemical properties of LMBs. In a nutshell to say, this new partially fluorinated cosolvent BDE molecule maintains parts of an electron cloud around the polar group, enabling it to coordinate well with Li⁺ ions. It remarkably improves the ionic conductivity of the electrolyte and helps to construct a better SEI layer on the lithium surface, ultimately achieving the long-term dendrite-free growth of lithium metal batteries.³⁹

3.3. Solid state electrolytes

When lithium metal batteries undergo volume expansion during plating and stripping, cracks often inevitably appear at the solid electrolyte interface layer, leading to fresh lithium metal exposure and a series of side reactions in contact with the liquid electrolyte, resulting in excessive consumption of the electrolyte and lithium metal, as well as the creation of lithium dendrite. Therefore, some researchers have begun to attempt to replace liquid electrolytes with solid electrolytes. Although traditional inorganic electrolytes can solve the growth problem of lithium dendrite, they are relatively brittle and prone to fracture, so there are certain problems in the practical application of LMBs. Polymer electrolytes are greatly limited in practical applications for the reason of their bad ionic conductivity and slender electrochemical window at room temperature.^63,64 The researchers envisage using multi-functional polymers to form a rigid and flexible cross-linked network structure to improve the ionic conductivity of gel electrolytes.^65–67 The gel polymer electrolyte (GPE) thus obtained has high ionic conductivity, which helps to solve the above problems and becomes a potential electrolyte material conducive to the uniform deposition of the lithium metal. As opposed to the irregular lithium deposition of liquid electrolytes, the GPE is dense and the lithium ion distribution is uniform, ensuring the equal deposition of lithium and thus inhibiting the generation of lithium dendrite.

Li et al. designed a double salt (LiTFSI-LiPF₆) gel polymer electrolyte (GPE) with a three-dimensional cross-linked polymer network, which can reduce lithium dendrite growth and build a stable SEI layer. They prepared a three-dimensional polymer crosslinked with poly(ethylene glycol) diacrylate (PEGDA) and ethoxylated trimethylolpropane triacrylate (ETPTA), and introduced a double salt electrolyte (LiTFSI-LiPF₆) into the three-dimensional structure. Lithium salts can completely dissolve in the electrolytes since the gel electrolytes are absolutely amorphous. This is conducive to the transfer of lithium ions and additionally improves the ionic conductivity of GPEs. The introduction of double salts strengthens the ionic conductivity of the SEI, enhances the thermal stability of the gel electrolyte, advances the ion migration, and thus strengthens the stability of the SEI. By utilizing these benefits, the GPE exhibits good performance in inhibiting lithium dendrite growth. The capacity retention rate of LiFePO₄|PE|Li batteries after 300 cycles at 0.5C can reach 87.93%, providing an ideal reference for the electrolyte design of energy storage devices.⁶⁸

Recently, optimizing electrolytes to inhibit the generation of lithium dendrites has been widely applied in research on the effects of lithium capacity utilization and charging current density on the stability of lithium metal anodes and the performance of lithium metal batteries. It is worth noting that the fiber structure morphology of lithium will increase the adjoined area between the electrolyte and the lithium metal, exacerbating side reactions. Therefore, the coulombic efficiency of the first cycle will be lower than that of lithium anodes with lower surface areas. If the SEI layer formed is porous, rigid, or insulated, it not only fails to effectively protect the lithium metal, but may also lead to a series of problems such as low coulombic efficiency and short battery cycle life due to continuous parasitic reactions between the electrolyte and the lithium metal. In contrast, if the SEI layer formed during the initial cycle is dense, uniform, and has stronger ionic conductivity, then this high-quality SEI will effectively protect the lithium metal, greatly reducing further reactions between the electrolyte and the lithium metal, greatly improving the coulombic efficiency and the cycle life of LMBs.

4. Inhibiting dendrite growth through interface protection

When there is a drastic concentration change at the interface between the lithium metal anode and the electrolyte, lithium dendrite will be generated and attempts have been made to induce lateral deposition of lithium by accelerating the Li⁺ ion diffusion rate, reducing the concentration gradient, or changing the collector, with a view to refrain the problem of lithium dendrite growth resulting from vertical deposition of lithium. Interface protection engineering that can directly regulate the deposition of lithium metal is considered an economical and effective optimization strategy.⁶⁹ It mainly regulates the diffusion rate of lithium ions through a series of surface treatment strategies to assist in the even deposition of lithium. Below is a brief explanation of the existing strategies for suppressing lithium dendrite growth through interface engineering treatment, which can be divided into an interface protection layer and a pre-treatment SEI layer.

4.1. Interface protection layer

Previous studies have found that the surface of lithium metal anodes modified with liquid metals can seriously passivate the lithium metal, delivering better lithium affinity and improving Li⁺ ion diffusion coefficients.⁷⁰ In spite of the fact that in commercial carbonate-based electrolytes, the electrochemical performance of lithium metal anodes is significantly enhanced.⁷¹ Feng's research group used the room temperature in situ dissolution method to prepare Ba–Li alloy as a surface modifying layer to suppress lithium metal anode dendrites. A slim alloy protective layer is formed on a lithium metal anode using barium salt (CF₃SO₃)₂Ba as the solute and tetrahydrofuran as the solvent. Atoms of Ba are reduced on the lithium electrode during the charging–discharging process and formed alloys of Li–Ba (as shown in Fig. 7a). Ba atoms change the position of lithium atoms, making the denser surficial layer. The Ba–Li alloy layer has a faster diffusion rate of lithium ions, leading to even deposition of lithium. What's more, the divalent properties of Ba²⁺ also play a role in the generation of a more stable SEI, thereby inhibiting the growth of dendrites in multiple ways. The results show that any dendrite cannot be found on the Ba–Li alloy anode even at a high area capacity of 4 mA h cm⁻², and the battery still has excellent cycling and rate performance.⁷²

Fig. 7 (a) Schematic illustration of the Ba-modified surface. Reproduced with permission. Copyright 2021, Elsevier. (b) Schematic diagram of the lateral growth of Li dendrites in Ni grids. Reproduced with permission. Copyright 2020, Wiley. (c) The top-view SEM images of the Li/Ni anode with plating capacities of 0.5, 1, 1.5 and 2 mA h cm⁻². Reproduced with permission. Copyright 2020, Wiley. (d) The top-view SEM images of Li/Ni anodes with stripping capacities of 0.5, 1, 1.5 and 2 mA h cm⁻². Reproduced with permission. Copyright 2020, Wiley. Interface engineering modification is convenient and efficient, and some strategies can change the direction of lithium metal deposition by adjusting the deposition preference of lithium, achieving the uniform deposition of the lithium metal.

Over and above this, research has found that appropriate structural design can help adjust the morphology of lithium plating.⁷³ For example, three-dimensional matrices such as porous copper and copper nanowires with large specific surface areas can reduce the current density and effectively suppress the lithium dendrite. Commercial foam nickel (NF) is widely used in the past research as a three-dimensional material carrying lithium metal. However, due to the disorder of pores and the non-uniformity of lithium ion flux, dendrites in this composite electrode often grow in a disordered and uncontrollable manner. Chen et al. proposed an interface engineering technique for modifying lithium metal anodes using a periodic nickel mesh based on micrometer grids. They combined the traditional lithium metal with a micrometer sized Ni mesh and developed a new type of composite anode. The Ni network reported as an intermediate layer, rather than a 3D scaffold, achieves the lateral growth of dendrite by adjusting the interface electric field between the lithium metal anode and the electrolyte (Fig. 7b), avoiding lithium dendrites piercing the separator and causing other safety accidents. After plating/stripping a certain amount of lithium at the same current density of 0.5 mA cm⁻², the lithium battery was disassembled for SEM testing. When the plating capacity is 0.5 mA h cm⁻², lithium preferentially deposits around the nickel mesh. As the plating capacity increases, lithium fills from the edge of the Ni mesh towards the center. Ultimately, when the plating capacity reaches 2 mA h cm⁻², the surface of Li/Ni is absolutely concealed by deposited lithium, forming a smooth and dendrite free lithium surface (Fig. 7c), which strongly proves that the lithium metal has a strong deposition preference on the nickel mesh. This can be attributed to the high electric field intensity around the nickel skeleton, while the nucleation potential of the Ni skeleton is low. The SEM images of the subsequent peeling process also demonstrate the strong reversibility of the Li/Ni anode. Fig. 7d shows that during the stripping process the deposited lithium metal begins to dissolve from the center of each grid. When the stripping capacity is 2 mA h cm⁻², the deposited lithium metal is entirely stripped from the Li/Ni anode, demonstrating the outstanding reversibility of the Li/Ni electrode. Compared with commercial foam nickel, the honeycomb nickel mesh can not only adapt to the drastic volume change of lithium electrodes during the charging and discharging process, but as well effectively adjust the electric field on the lithium surface, consequently inhibiting lithium dendrite growth, extremely diminishing opportunity of short circuits, and strengthening the durability of LMBs. The experimental results show that the lithium/nickel composite anode has an extremely low overpotential of 6–8 mV and an ultra-stable cycle performance of more than 1000 h in the symmetrical battery test. The full battery is assembled with Li/Ni as the anode and LiFePO₄ as the cathode that delivers an initial capacity of up to 133 mA hg⁻¹ and cycles over 160 cycles.⁷⁴ He's group used a simple manufacturing method to prepare lithium expanded graphite composite anodes. They press a mixture of expanded graphite (EG) and Li powder together, and then embed EG into the Li metal through heating, obtained the lithium expanded graphite composite anode (Li-EG) with uniform internal dispersion. EG uniformly accommodates the lithium metal in intergranular and interlayer voids, benefit from its light weight, excellent electrochemical stability, abundant porosity and excellent mechanical flexibility. LiC₆ is spontaneously generated, which provides sufficient nucleation sites during the cycling process, reducing part of current density, and thus hindering the nucleation and creation of lithium dendrite. The half-cell of the Li-EG composite anode performs excellent capacity retention after more than 1500 cycles at a current density of 10 mA cm⁻², exhibiting outstanding electrochemical performance.⁷⁵

Apart from the lithium metal alloying treatment and lithium metal composite anode strategy mentioned earlier, there are also artificial protective layers for the interfacial protection.^76,77 Lithium easily react with any non-aqueous liquid electrolyte, instantly forming a SEI with fragile heterogeneous in thermodynamics,³ which is the root causes of Li uneven deposition and lithium dendrite generation. Previously, scientists proposed using substances such as polymers and inorganic ceramics^78,79 as non-in situ coatings for lithium metals, which can provide a certain degree of mechanical strength and overcome the problem of the SEI being too brittle and fragile to some extent. However, lower ion conductivity and poor interface contact still have a significant impact. Huang and his colleagues are committed to studying an artificial protective layer for lithium metal anodes with the rapid diffusion of lithium ions and high Young's modulus. They reasonably hybridized LiF and polyvinylidene fluoride hexafluoropropylene (PVDF-HFP) to obtain a film, and used as the artificial protective layer (APL) of the lithium anode. The addition of rigid LiF results in a high Young's modulus (6.72 GPa) for this APL, far exceeding the original SEI of 150 MPa and the PVDF-HFP film of 0.8 GPa. This high Young's modulus enables the APL to provide a strong mechanical barrier to lithium dendrite growth (Fig. 8a–c). In addition, PVDF-HFP is gentle and sticky, and after embedding rigid LiF particles with strong toughness, the synergistic effect between the structural units of the two makes the APL have good ion conductivity, helping to achieve more uniform lithium deposition. Due to the stable interface, there is no lithium dendrite dense deposition, and the interface resistance is greatly reduced after cycling, greatly extending the cycling life of LMBs. Subsequent experiments found that when using APL, the lithium anode protected by APL had an enormous capacity of 150.6 mA h g⁻¹ and a CE of over 99% in LiFePO₄ cathode batteries, greatly enhancing cycling stability. The cycle life of lithium metal batteries protected with the APL is 2.5 times that of batteries without the APL. It can be seen that the organic inorganic composite SEI can provide a practical and feasible method for achieving and improving lithium batteries.⁸⁰

Fig. 8 Schematic illustrations of Li deposition (a) without protection, lithium metal dendrites and dead Li forms after cycling. Reproduced with permission. Copyright 2018, Wiley; (b) with a pure PVDF-HFP layer that is of poor mechanical modulus, interfacial fluctuation with dendrites piercing the PVDF-HFP layer occur after cycling; and (c) with APL composed of organic PVDF-HFP and inorganic LiF that is conformal and mechanically strong to suppress Li dendrite penetration and stabilize the Li metal surface. Reproduced with permission. Copyright 2018, Wiley. (d) Schematic of the designed full cells using the ALD-coated LLCZN, Li metal anode, LFMO/carbon black/PVDF composite cathode. (e) and (f) Schematic of the wetting behavior of the garnet surface with molten Li. Directly adding an artificial interface layer to the surface of the lithium metal can significantly regulate the uniform distribution of lithium ions.

The traditional liquid lithium batteries have many obstacles, just like poor safety and short cycling life, limited voltage, and unstable interfaces between phases. The emergence of solid-state lithium batteries provides potential solutions to these issues. The electrochemical window of garnet type solid electrolytes is very stable up to 6 V, making the most stable interface on the lithium electrode. It also has excellent ion conductivity (close to 1 mS cm⁻¹) and excellent environmental stability, which has attracted great attention from researchers.^81,82 However, there are few successful cases of using these special materials to develop high-performance solid-state batteries, and the high solid–solid interface impedance between garnet electrolytes and electrode materials is still a huge challenge. Hu and his colleagues introduced ultra-thin aluminum oxide (Al₂O₃) coating on garnet like Li₇La_2.75Ca_0.25Zr_1.75Nb_0.25O₁₂ (LLCZN) by atomic layer deposition (ALD) (Fig. 8d), which reduced the interface impedance from 1710 Ω cm² reduced to 1 Ω cm², effectively improved the interface impedance. As shown in Fig. 8e and f, the ultra-thin Al₂O₃ coating obtained by the atomic layer deposition (ALD) that is helpful for the conformal coating of the molten lithium metal on the garnet surface, so that there is almost no gap when the lithium metal contacts the surface of the garnet electrolyte, and the Li–AL₂O₃ interface is helpful for the effective transmission of lithium ions between the garnet electrolyte and the lithium metal anode. They also explored the mechanism of ALD-Al₂O₃ coating improving the interface through experiments and computation. The results showed that the ALD-Al₂O₃ coating formed an interface on the surface of the garnet electrolyte, and the stronger binding energy of lithium and lithium alumina further enhanced interface bonding. In addition, the ultra-thin Al₂O₃ coating can prevent the decomposition of the garnet electrolyte in touch with the lithium metal, maintain the stability of the interface between the garnet electrolyte and the lithium metal, and ultimately achieve excellent electrochemical performance. This work improved the solid–solid interface impedance at a certain extent, and successfully designed a solid-state lithium battery with a high-voltage cathode, and it is a major breakthrough in the practical application progress of solid-state batteries with ultra-high energy density.⁸¹

4.2. Artificial SEI

An excellent SEI can help to facilitate forming uniform lithium plating; therefore, the “artificial SEI” directly deposited on the lithium surface through pre-treatment is also a feasible method. Zhou and his colleagues immersed the lithium metal in the EC/DEC electrolyte with Mn(NO₃)₂ added, and reduced the electrolyte and Mn(NO₃)₂ through chemical reactions, obtaining a black organic/inorganic SEI on the lithium surface. The additive Mn(NO₃)₂ used is a common and inexpensive reagent, and the pre-treatment operation is simple and efficient. The black SEI protective layer is mainly composed of nanotube shaped substances generated by the reaction between lithium and the electrolyte, which are arranged to form nanotube arrays. The nanotube array has abundant lithium nucleation sites, which can assist in the uniform deposition of lithium, and its arrangement framework significantly reduces the local current density, helping to slow down the generation of lithium dendrite. The free space in nanotubes can also effectively alleviate the volume expansion caused by electrochemical cycling. In addition, these SEIs composed of unique nanotube arrays can not only effectively hinder the creation of dendrite, but also help to improve the stability of the interface, thus significantly reducing the overpotential. In the symmetrical battery performance test, the pretreated lithium battery has an overpotential of only ∼60 mV at a very high current of 20 mA cm⁻², which is in sharp contrast to the bare lithium battery. Moreover, the pre-treated Li‖LiMn₂O₄ battery still exhibits good capacity and coulombic efficiency at high rates and temperatures, indicating that the pre-treatment strategy for metallic lithium is very effective.²

There are also many similar works about artificial SEI films for protecting lithium electrodes and inhibiting lithium dendrites. Li et al. reported a 3D carbon framework seamlessly connected to lithium foil (A-3DCF@Li) with composite artificial solid electrolyte interface functionalization can adjust the Li⁺ ion flux. They used graphene (G) and carbon nanotubes (CNTs) as precursors to construct A-3DCF connected by ethyl carbide cellulose (EC). In the carbonate electrolyte, A-3DCF comes into a direct contact with the lithium metal and undergoes the in situ functionalization reaction, forming the A-SEI (LiF, Li₂CO₃, Li₂O, ROCO₂Li) and LiC_x. This new A-SEI connects A-3DCF with lithium foil tightly junction, and finally gets A-3DCF@Li, inhibiting the lithium dendrite. A-SEI and LiC_x can not only help to uniformly deposit Li⁺ ions by adjusting the Li⁺ ion flux, but also help to reduce the lithium deposition overpotential. The battery with the A-3DCF protected lithium anode proposes better cycling stability, and the specific capacity attenuation is not significant after 200 cycles. However, the specific capacity of LMBs without surface protection sharply decreases after 50 cycles, and the stability is significantly lower than that of lithium batteries protected by A-3DCF. At the same time, the rate performance of LMBs with A-3DCF protected has also been significantly improved. It can be seen that their proposed design concepts and methods provide excellent ideas for preparing high-performance lithium anodes.⁸³

5. Separator engineering

The functionalization and safety issues of commercial lithium-ion batteries are very important, and it is necessary to develop intelligent batteries with advantages such as versatility, self-protection, and adaptability.^84–86 The problems such as low coulomb efficiency, short cycling life, various interface reaction, and potential safety hazard related to lithium dendrites have greatly hindered the commercial development of lithium batteries.^58,87 Developing functional separators is a very effective strategy because it can suppress the generation of lithium dendrites through mechanical barriers or by regulating the transportation of Li⁺ ions, and this strategy does not significantly decrease the battery energy density. As shown in Fig. 9, the strategy of suppressing lithium dendrites through separator engineering mainly involves three points: (1) It is possible to increase the mechanical strength of the separator and hinder the growth of lithium dendrites through physical action; (2) redistribution of Li⁺ ion flux through controlling separator voids; (3) induce consistent plating of Li⁺ ions through increasing nucleation sites on separators.

Fig. 9 Schematic diagram of the separator engineering strategy. The strategy of suppressing lithium dendrites through membrane engineering can be simply divided into three aspects: physical inhibition of lithium dendrite growth, redistribution of Li⁺ ion flux, and regulation of lithium nucleation sites.

The blocking growth of lithium dendrites is the most direct and primitive method to address the problem of short circuits caused by lithium dendrite piercing the interior of LMBs. However, membranes such as commercial PE or PP films are easily punctured by lithium dendrites.⁸⁸ The hard layer coating can significantly improve the anti-permeability performance of commercial separators. As shown in Fig. 10a, rigid separators are often easily punctured when facing the growth of lithium dendrites, leading to battery short circuits. When modifying the separator, if its puncture resistance is strong enough, the tip of the lithium dendrite can be bent to avoid short circuits in lithium metal batteries. Zhu's team found that the interface stress of sharp points is dispersed by the curved surface that resists piercing. They utilized a nanoshield layer similar to a SiO₂ sphere layer on the separator, which increases the mechanical strength of the separator, inhibits lithium dendrites, and significantly improves the circulate life of batteries.⁸⁹ The hard modified coating effectively inhibited lithium dendrites to puncture the separator, but there are still issues that need to be noted. If there is a significant volume change during the cycling process, the hard coating may fall off. More than this, this strategy increases the weight and volume of the separator, which has a crucial negative impact on the energy density of the battery.

Fig. 10 (a) Schematic illustration of lithium dendrite blocking behavior by flat and curved surfaces. Reproduced with permission. Copyright 2020, Elsevier. (b) Schematic illustration of the piercing processes of LMBs. After being pierced using a syringe needle, the LMO/GO/Li battery can still illuminate a LED. But the conventional LMO/Li battery after being pierced cannot lighten the LED. Reproduced with permission. Copyright 2018, Elsevier. (c) Schematic of the LMO/GO/Li battery and LMO/Li battery etching Li dendrites. In the LMO/GO/Li battery, once Li dendrites grew through the separator, they could be efficiently eliminated by GO based on the spontaneous redox reaction, thus extending the lifetime of the battery. Reproduced with permission. Copyright 2018, Elsevier. (d) Schematic diagram showing the internal electron and Li⁺ transfer pathways in the direct contact lithiation process between SiO@PAA coating and Li foil under encapsulation pressure. Reproduced with permission. Copyright 2021, Wiley. Several different ways and effects of separator engineering to suppress dendrite growth were briefly demonstrated through schematic diagrams, which is a direct and efficient strategy.

The forming lithium deposition in traditional lithium ion batteries usually follows the basic electrochemical principles, including ion diffusion, adsorption, nucleation, and growth four stages. Nucleation is frequently considered as a key step in determining the morphology of lithium deposition during subsequent plating/stripping cycles. Regulating Li⁺ uniform nucleation by constructing more nucleation sites has significant advantages for suppressing lithium dendrites at the beginning.⁹⁰ Inorganic/heteroatom doped carbon materials have been used to aid in increasing Li⁺ nucleation sites,^91,92 which require polar groups as conductive sites for improving the Li⁺ ion flux. Graphite oxide (GO) is a two-dimensional (2D), monatomic thickness, hydrophilic material, which is rich in hydroxyl, carboxyl and other functional groups. In lithium sulfur batteries, graphite oxide modified membranes have high Li⁺ ion selectivity, which can effectively hinder the shuttle effect and prolong the cycle life of Li–S batteries. Qu et al. used GO to prevent the lithium dendrite formation. They sandwiched the GO film as an intermediate layer between two polypropylene (PP) microporous films to obtain a three-layer film, and then used commercial lithium manganate (LMO) as the cathode, the three-layer film as the separator, and lithium foil as the anode to obtain LMO/GO/Li batteries. They have demonstrated that the modified separator effectively eliminates the lithium dendrite in the LMBs; even if the battery is punctured using a syringe needle, it still works normally, and the LED remains on (Fig. 10b). In contrast, traditional LMO/Li batteries are punctured, causing direct contact between the lithium metal and LMO, resulting in a short circuit in the battery (Fig. 10b). The LMO/GO/Li batteries not only withstand breakdown, but also eliminate Li dendrites (Fig. 10c) because of the GO redox character. In LMO/GO/Li batteries, lithium dendrites are still formed, but before they reach the LMO cathode, GO can timely intercept and corrode them via redox reactions. It can be seen that the GO has effectively eliminated the dendrite growth. The LMO/GO/Li battery maintains a high cycling efficiency at 2C and underwent 6000 reversible charges and discharges, which was about 48 times longer than the LMO/Li battery. In conclusion, on the basis of spontaneous redox, GO can chemically etch the lithium metal and eliminate lithium dendrites. The sandwich LMO/GO/Li battery has also helped us take a big step towards safer and longer lifespan lithium metal batteries.⁹³ Although membranes modified with special carbon materials can effectively regulate the Li⁺ deposition, but Li⁺ ions may not completely deposit on the anode substrate as expected. If the electronic conductivity of the substrate is weaker than that of the modified coating, lithium ions are easy to deposit on the modified coating instead of the substrate. Therefore, when selecting this modification strategy, it is necessary to adjust the electronic conductivity of the anode substrate and coating modification well.

Many groups have attempted to modify commercial separators through physical or chemical methods. For example, Feng and colleagues studied a simple separator modification optimization strategy by bonding and mixing amorphous SiO particles with polyacrylic acid (PAA), and then coating them on polyethylene (PE) separators using a spray method, resulting in a single-sided SiO@PAA-PE, which effectively suppresses the emergence of dendrites and improving the cycle stability of LMBs. When the SiO@PAA-PE separator was used in stainless steel (SS)‖Li batteries, the closer contact between the coatings of SiO@PAA and lithium due to the flexible characteristics of the composite separator and lithium, which accelerates lithiation and forms the uniform and continuous Li-SiO/PAA layer (as shown in Fig. 10d) having good interface stability. The Li-SiO/PAA layer not only reduces the interfacial side reactions, but also helps to homogenize the Li⁺ flux, ultimately effectively hindering the formation of lithium dendrites. When the SiO@PAA-PE separator is used in LiNi_0.8Mn_0.1Co_0.1O₂‖Li batteries, it can be observed that the battery exhibits a high CE of 99% at 5C, greatly improving cycle stability.⁹⁴

The use of porous membranes to deal the Li⁺ distribution on lithium electrodes is also a good idea. For example, the combination of the oxidized polyacrylonitrile nanofiber membrane and collector put on the anode surface with glass fiber cloth, the polar group and hydrophilic group work together to benefit the Li⁺ uniform distribution. Nevertheless, this combination cannot adjust the Li⁺ uniformity in a nanoscale.⁹⁵ Graphdiyne thin films can also promote the Li⁺ uniform distribution under highly non-uniform electric fields, inducing the uniform deposition of lithium.⁹⁶ However, the studies found that the Fermi energy level of the lithium metal is higher than the minimum value of the conduction band of graphdiyne, and electrons are easily transferred to graphdiyne, leading to the lithium metallization of graphdiyne. This lithium metallization will make lithium ions directly reduce on graphdiyne, leading to cell failure. Inspired by this, Luo and his colleagues attempted to use modified chlorinated graphdiyne films as stable “nanosieve” to suppress the dendrite growth. They modified graphdiyne thin films and found that chlorinated graphdiyne thin films can significantly increase their metallization resistance and delay the progress of lithium metallization. In addition, the conductivity of chlorinated graphdiyne to lithium ions is also enhanced. These advantages enable chlorinated graphdiyne thin films to serve as stable separators in lithium metal batteries to restrict the lithium dendrite, enhancing the electrochemical performance of LMBs.⁹⁷ However, there are still certain limitations and uncertainties in the application of separator engineering, such as regulating the conductivity difference of the anode substrate and coating to avoid lithium metal deposition on the surface of coating. Therefore, there are still many issues that can be explored in the inhibition of lithium dendrite growth in separator engineering.

6. Conclusions and prospects

With the rapid development of green energy, the requirements for high performance rechargeable batteries in the market have become increasingly urgent. Solid lithium batteries have regained research attention owing to their high specific energy density, which has also promoted the commercialization process of lithium metal electrodes. This work summarizes the five kinds of mechanisms and three major inhibiting strategies of lithium dendrite growth in LMBs. After more decades of research, many good results have been achieved in inhibiting lithium dendrite growth. However, many key issues and future research directions of high energy density lithium metal batteries still need to be cautiously studied.

(1) There can be a deeper understanding of the SEI films in LMBs. The SEI is an inevitable interfacial film formed during the first charging process of LMBs, and its composition and structure are closely related to the electrolyte and positive electrode materials. However, the SEI can stably determine the creation of lithium dendrites and the cycle life of LMBs. The current understanding of the stable SEI is limited to the presence of LiF as the main component; due to the complexity of SEI formation, a deeper understanding of the SEI is required. A large number of works have been conducted on the development of electrolyte additives, electrochemical engineering, artificial SEI films, and the production of LiF, and good results have been achieved. There are fewer research studies on the effects of the composition and structure of positive electrode materials, current density, and other environments on the formation of the SEI. It is hoped that more works will focus on the internal environment of LMBs to study the SEI formation in the future, comprehensively understand the formation mechanism of the SEI from multiple perspectives, analyze more components that are conducive to SEI stability, and provide the reference for the design of LMBs.

(2) The volume changes of lithium metal electrodes that occur during the cycling process also require significant attention. The working process of lithium metal batteries is also the dissolution and deposition processes of the lithium metal, which inevitably leads to lithium loss and a decrease in the volume of the lithium metal electrode, creating a gap between the lithium metal and the separator, increasing the internal interfacial impedance of the LMBs and affecting the reversible transport of lithium ions. This is also another important challenge faced by lithium metal batteries. The existence of such voids can lead to uneven current density inside the LMBs, affecting the deposition of lithium ions, and ultimately leading to the growth of lithium dendrites. In addition, the side reaction between the lithium metal and the electrolyte may also produce some combustible gases, which can easily lead to serious safety issues such as fire and explosion due to energy accumulation inside the battery. Recently, self-healing materials have been widely studied as solid-state electrolytes, binders or protective coating for electrode materials owing to their strong resistance to deformation. Inspired by this, the self-healing materials could be used to fill the gaps and block lithium dendrites, maintaining the safety use of Li metal anodes.

(3) Developing inorganic solid-state electrolytes with organic layers. Solid electrolytes include inorganic solid electrolytes and polymer based solid electrolytes. Solid electrolytes benefit from their good mechanical strength to efficiently prevent the growth of lithium dendrites, and their stable chemical properties can also avoid safety issues such as leakage and combustion. PSEs have lower ionic conductivity and mechanical strength than ISEs, but assembling batteries also requires consideration of the electrode interface contact and ease of assembly. There is still great exploration space for materials that can inhibit lithium dendrite growth through surface protection. The emergence of lithium dendrites mainly stems from the uneven distribution of lithium ions on the electrode surface. Some organic materials have repetitive structural units, which enable them to have uniform lithium transport sites and uniform surface potential density, directly inhibiting the emergence of lithium dendrites. Composite solid electrolytes (CSEs) composed of inorganic solid electrolytes and organic polymers can simultaneously exhibit high hardness to inhibit dendrite growth and ensure high ion conductivity, making them promising solid electrolytes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Plan of the Ministry of Science and Technology of China (2022YFE0122400) and the Science and Technology Commission of Shanghai Municipality (20520711500, 19DZ2293100, and 21DZ2280600), Engineering Research Center of Material Composition and Advanced Dispersion Technology, Ministry of Education. Yafei Huang and Haotian Yang collected and organized the literature, wrote the article content, and responded to the reviewer comments together. Guorong Chen proposed a review topic direction, wrote a revised manuscript, and responded to the reviewer comments. Yan Gao collaborated to organize the literature content, draw TOC diagrams, etc. Yan Li, Liyi Shi and Dengsong Zhang discussed the review structure and revised comments on the revised manuscript.

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Footnote

† Yafei Huang and Haotian Yang contribute equally to the work.

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