From Liquid to Solid-State Lithium Metal Batteries: Fundamental Issues and Recent Developments

Abstract

The widespread adoption of lithium-ion batteries has been driven by the proliferation of portable electronic devices and electric vehicles, which have increasingly stringent energy density requirements. Lithium metal batteries (LMBs), with their ultralow reduction potential and high theoretical capacity, are widely regarded as the most promising technical pathway for achieving high energy density batteries. In this review, we provide a comprehensive overview of fundamental issues related to high reactivity and migrated interfaces in LMBs. Furthermore, we propose improved strategies involving interface engineering, 3D current collector design, electrolyte optimization, separator modification, application of alloyed anodes, and external field regulation to address these challenges. The utilization of solid-state electrolytes can significantly enhance the safety of LMBs and represents the only viable approach for advancing them. This review also encompasses the variation in fundamental issues and design strategies for the transition from liquid to solid electrolytes. Particularly noteworthy is that the introduction of SSEs will exacerbate differences in electrochemical and mechanical properties at the interface, leading to increased interface inhomogeneity-a critical factor contributing to failure in all-solid-state lithium metal batteries. Based on recent research works, this perspective highlights the current status of research on developing high-performance LMBs.

Keywords: All-solid-state lithium metal battery; Interface; Li dendrite; Lithium metal batteries; Solid electrolyte.

Fig. 1

Schematic illustration of fundamental issues and corresponding strategies for lithium metal batteries (LMBs)

Fig. 2

Schematics of a SEI formation mechanism and b revised mechanism in nonaqueous electrolytes. Reproduced with permission from [29] Copyright 2010, American Chemical Society. Reproduced with permission [31] Copyright 2018, Royal Society of Chemistry. c Schematics of double-layer structure of SEI and corresponding Li⁺ transport mechanism. Reproduced with permission from [36] Copyright 2012, American Chemical Society. d Schematic diagram of SEI layer formed on LMAs in three Li salts. Reproduced with permission from [41] Copyright 2018, American Chemical Society. e Schematic diagram of Li⁺ transport process and energy profile for Li⁺ desolvation. Reproduced with permission from [43] Copyright 2010, American Chemical Society

Fig. 3

a Schematic illustration of Li-ion mass-transfer process with concentration gradient profiles and strategies affiliated by corresponding mechanism near the surface of lithium metal anode. The Li-ion concentration and potential profiles under two conditions: b < and c < when Li-ion depletion occurs. Reproduced with permission from [46] Copyright 1999, Elsevier. $dC/dx$ < $2C_0/L$ and c $dC/dx$ < $2C_0/L$ when Li-ion depletion occurs [46]

Fig. 4

Interface engineering for LMAs in nonaqueous electrolytes. a Schematic diagrams of direct membrane coating by two-dimensional atomic crystal layers composed of hexagonal boron nitride (h-BN) and graphene. Reproduced with permission from [62] Copyright 2014, American Chemical Society. b Schematic illustration of mitigation mechanism of high-dielectric-constant artificial SEI and corresponding electrochemical performance. Reproduced with permission from [75] Copyright 2021, American Chemical Society. c Schematic diagrams of the construction of the carbon-based hybrid (ECH) interface on LMA surface via electrochemical pretreatment and corresponding TEM characterizations. Reproduced with permission from [81] Copyright 2022, American Chemical Society. d Schematic diagrams of surface LiF coating layer through in-situ chemical reactions. Reproduced with permission from [84] Copyright 2017, American Chemical Society. e Schematic illustration of the fabrication process of the robust biphasic surface layer (BSLs) and its inhibition in shuttling of Li–S batteries. Reproduced with permission from [85] Copyright 2021, Wiley–VCH. f Reaction mechanism for the formation of the PTMEG–Li/Sn alloy hybrid layer. Reproduced with permission from [93] Copyright 2019, Wiley–VCH.

Fig. 5

The construction of 3D current collector for LMAs. Schematic illustration of the effect of tortuosity on the structure evolution of LMAs during the cycling: a high tortuosity, b low tortuosity. Reproduced with permission from [111] Copyright 2020, Elsevier. c Schematic illustration of the transition from 2D planar to 3D current collector. Reproduced with permission from [96] Copyright 2015, The Authors, published by Springer Nature. d Overpotentials of galvanostatic Li deposition on Cu and Au substrate. Reproduced with permission from [117] Copyright 2016, The Authors, published by Springer Nature. e Schematic diagram and calculated binding energy of a Li atom with Cu, graphene, and different functional groups of N-doped graphene. Reproduced with permission from [121] Copyright 2017, Wiley–VCH. f Electronegativities of various elements in the periodic table and related Gibbs free energy () of elements or compounds reacted with molten Li. Reproduced with permission from [124] Copyright 2019, The Authors, published by Springer Nature

Fig. 6

The electrolyte modification for LMAs. Illustration of Li deposition process based on self-healing electrostatic shield (SHES) mechanism: a without and b with 0.01 M CsPF6 addition. Reproduced with permission from [135] Copyright 2013, American Chemical Society. c Schematic illustration of a modification of NO3− in stabilizing the SEI. Reproduced with permission from [157] Copyright 2022, Wiley–VCH. d Chemical structures of additives used for passivating LMAs. Reproduced with permission from [140] Copyright 2018, Wiley–VCH. Electrolyte structures of e conventional dilute electrolyte, f high concentration electrolyte (HCE) and g localized high concentration electrolytes (LHCE). Reproduced with permission from [176] Copyright 2021, IOP Science. h Comparison of the properties and performances of dilute electrolyte, HCE and LHCE. Reproduced with permission from [182] Copyright 2019, The Authors, published by Springer Nature. i Schematic illustration of the effect of the reactive F-content in the carbonate-based HCE on LMA and Ni-rich cathode. Reproduced with permission from [178] Copyright 2018, Elsevier.

Fig. 7

The separator modification for LMAs. a Schematic illustration of nano-shield design for separators to resist Li dendrite formation. Reproduced with permission from [188] Copyright 2020, Wiley–VCH. b Fabrication process and characterization of a PBO nano-porous separator membrane. Reproduced with permission from [191] Copyright 2016, American Chemical Society. c SEM images of LiF-fiber-woven interlayer. Reproduced with permission from [195] Copyright 2022, Oxford University Press. d Schematic illustrations of Li deposition behaviors through PP separator and PP-PVDF-LLZTO composite separator. Reproduced with permission from [194] Copyright 2019, Elsevier. Schematic illustrations of Li deposition and corresponding mechanisms e without and f with GCN layer modification. Reproduced with permission from [198] Copyright 2019, Wiley–VCH.

Fig. 8

The alloyed anode design for LMAs. Schematic illustration of a bare Li and b Li-Mg alloy anodes during Li stripping/plating process. SEM images of c bare Li and d Li-Mg alloy anodes after Li plating at 0.5 mA cm⁻² for 24 h. Reproduced with permission from [207] Copyright 2019, Wiley–VCH. Schematic illustration of Li deposition behavior on e Cu current collector and f Ga-In-Sn liquid metal (LM). Reproduced with permission from [209] Copyright 2021, Wiley–VCH.

Fig. 9

The external field regulation for LMAs. a SEM images of Li nuclei under different temperature conditions. b Comparison of Li nuclei size, nucleation density, nucleation overpotentials and mass-transfer overpotentials under different temperature conditions. Reproduced with permission from [51] Copyright 2019, Wiley–VCH. Schematic illustration c without and d with external magnetic field to show the elimination effect of tip Li dendrite growth by Lorentz force. Reproduced with permission from [213] Copyright 2019, Wiley–VCH. e The variation of surface morphology and electrochemical performance under different magnetic fields. Reproduced with permission from [214] Copyright 2019, Wiley–VCH. Schematic illustration of the morphology of Li dendrites f without and g with external pressure to show the impact of external pressure on LMAs. h The analysis results of LMAs under 0–14 MPa external pressure. Reproduced with permission from [216] Copyright 2021, Wiley–VCH

Fig. 10

Advantages of the transition from liquid to solid electrolyte in LMBs. a Merits of solid-state electrolyte (SSE) over organic liquid electrolyte for battery thermal safety. Reproduced with permission from [222] Copyright 2017, Elsevier. b Comparison of onset temperature of exothermal behaviors for different SSEs. Reproduced with permission from [224] Copyright 2020, American Chemical Society. c Schematic illustration of packed structure reformation in bipolar-stacked battery for solid-state lithium metal batteries (ASSLMBs). Reproduced with permission [17]. Copyright 2022, Royal Society of Chemistry

Fig. 11

Schematic illustration of evolution of fundamental issues and corresponding strategies in lithium metal batteries (LMBs) from liquid to solid-state electrolyte

Fig. 12

Fundamental issues for LMA in ASSLMBs. a Electrochemical window and ionic conductivity for different SSEs. Reproduced with permission from [224] Copyright 2020, American Chemical Society. b Schematic illustration of three types of interfaces between SSEs and LMA. Reproduced with permission from [267] Copyright 2015, Elsevier. c Schematic illustration of current constriction phenomenon due to macroscopic insufficient contact, low active site concentration and loss of atomic contact during cycling. Reproduced with permission from [100] Copyright 2019, Wiley–VCH. The distribution of stress response in polycrystalline LLZO electrolyte for d hydrostatic stress and e von Mises stress. Reproduced with permission from [289] Copyright 2022, The Authors, published by Springer Nature. f The initialization and propagation of cracks and the propagation of Li dendrites. Reproduced with permission from [291] Copyright 2021, The Authors, published by Springer Nature. g Schematic illustration of stress distribution and Li filament growth. Reproduced with permission from [294] Copyright 2019, IOP Science

Fig. 13

Interface engineering for LMA in ASSLMBs. a Schematic illustration of the formation of LiF-rich SEI layer between LPS electrolyte and LMA, and the corresponding evaluation from the perspective of critical Li dendrite length. Reproduced with permission from [302] Copyright 2018, The American Association for the Advancement of Science. b Schematics of sustained release effect of PPC between LPSC electrolyte and LMA. Reproduced with permission from [306] Copyright 2020, Wiley–VCH. c Voltage profiles for PEO-PPC double-layer electrolyte in NCM622/Li cell. Reproduced with permission from [307] Copyright 2022, American Chemical Society. d Schematics of the wetting behavior of garnet surface with molten Li after Al2O3 coating. Reproduced with permission from [315] Copyright 2016, The Authors, published by Springer Nature. e Wetting behaviors to molten Li of a series of different surface morphologies LLZTO pellets obtained by the grinding of 400, 800, 2000 and 4000 grit sandpapers. Reproduced with permission from [318] Copyright 2023, Wiley–VCH. f Schematic illustration of the formation process and roles of the Li3PO4 modification layer between the garnet SSE and LMA. Reproduced with permission [320]. Copyright 2019, Royal Society of Chemistry. g Schematic illustration of ASSLMBs with garnet LLZT and carbon-treated LLZT and EIS spectra for Li/garnet/Li and LFP/garnet/Li cells. Reproduced with permission from [321] Copyright 2018, American Chemical Society

Fig. 14

Selection of fillers for solid composite electrolytes (SCEs). a Schematic illustration of fluorinated PEO-based electrolyte by Lewis-acid AlF3 filler. Reproduced with permission from [338] Copyright 2022, The Authors, published by Springer Nature. b The selection of Li-ion conducting fillers and the varying performance under different content of LLZTO nanoparticles. Reproduced with permission from [327] Copyright 2018, Elsevier. c Schematic illustration of Li-ion diffusion pathways in PEO-based SCEs with different amount of nanofillers. Reproduced with permission from [341] Copyright 2022, American Chemical Society. d Li-ion conduction pathways in SCEs with nanoparticles, random nanowires and aligned nanowires. Reproduced with permission from [343] Copyright 2017, The Authors, published by Springer Nature. e SEM images of ice-templated LAGP framework. Reproduced with permission from [345] Copyright 2019, Elsevier. f Schematic illustration of the Li salt state in SCE composed of PVDF matrix and BaTiO3-LLTO nanowires coupling the ceramic dielectric and inorganic SSE. Reproduced with permission from [349] Copyright 2023, The Authors, published by Springer Nature. g Schematics illustration of the electric field distribution of conductive filler in PEO electrolyte. Reproduced with permission from [350] Copyright 2023, Wiley–VCH

Fig. 15

Effect of additives for solid composite electrolytes (SCEs). a Electrochemical reaction evolutions of LiN3 in ASSLMBs. Reproduced with permission from [358] Copyright 2017, Wiley–VCH. b Cross-sectional synchrotron X-ray tomography images of symmetric Li/Li cells with and without Mg(ClO4)2 additive. Reproduced with permission from [360] Copyright 2021, American Chemical Society. Cryo-TEM characterization of PEO/Li interface using c PEO-LiTFSI electrolyte and d PEO-LiTFSI-Li2S electrolyte. Reproduced with permission from [361] Copyright 2020, Wiley–VCH. [361]. e Ab initio molecular dynamics (AIMD) simulations of LiTFSI structural configuration changes without and with LiNO3 addition at 333 K adjacent to LMA surface. Reproduced with permission from [362] Copyright 2021, Elsevier

Fig. 16

Li-ion conductive anode structure design for LMA in ASSLMBs. a Schematic illustration of patterned LMA fabricated by micro-template imprint method. Reproduced with permission from [368] Copyright 2020, Elsevier. b Schematic for the manufacturing process, SEM images and COMSOL simulation for a layered-stacking anode structure. Reproduced with permission from [369] Copyright 2022, Wiley–VCH. c Schematic illustration of sintering garnet porous structure. Reproduced with permission from [371] Copyright 2018, Elsevier. d Schematics and SEM characterizations of acid etching process on a garnet SSE by HCl. Reproduced with permission from [373] Copyright 2020, American Chemical Society. e Fabrication and characterization of 3D-SSE via laser micro-sculpture method. Reproduced with permission from [375] Copyright 2021, Wiley–VCH

Fig. 17

Alloyed anode design for LMA in ASSLMBs. a Wettability comparison of Li-Sn alloys on garnet substrates. Reproduced with permission from [376] Copyright 2017, Wiley–VCH. b Schematics and SEM image with elemental mapping within Li-Mg alloy anode during the Li stripping/plating process. Reproduced with permission from [377] Copyright 2018, Wiley–VCH. c Schematic illustration of Li dendrites propagation along the LPSC/Li and LPSC/Li-In interface. Reproduced with permission from [386] Copyright 2021, The Authors, published by Springer Nature. d Schematic illustration of Ag-C nanocomposite layer during Li stripping/plating process and corresponding electrochemical performance of NCM/Li cells. Reproduced with permission from [387] Copyright 2020, The Authors, published by Springer Nature

Fig. 18

External field regulation for LMA in ASSLMBs. a CCD tests for Li/LLZO/Li cells at 30, 70, 100, 130 and 160 °C. Reproduced with permission from [390] Copyright 2016, Elsevier. b Voltage profiles for Li/LPSC/Li cells at different pressures and current densities. Reproduced with permission from [391] Copyright 2019, The Authors, published by Springer Nature. c Design of pressure control device as well as electrochemical performance for symmetric Li/Li cells and full cells under various external pressure. Reproduced with permission from [392] Copyright 2019, Wiley–VCH. d Schematic illustration of the effect of voltage pulse on stabilizing interface as well as the EIS spectra and polarization curves for symmetric Li/Li cells. Reproduced with permission from [395] Copyright 2022, American Chemical Society

Fig. 19

Summary of fundamental issues for the transition from LIBs to LMBs and ASSLMBs