Boosting the electrochemical performance of 3D composite lithium metal anodes through synergistic structure and interface engineering

Journal Pre-proof Boosting the Electrochemical Performance of 3D Composite Lithium Metal Anodes through Synergistic Structure and Interface Engineering Yuanmao Chen, Xi Ke, Yifeng Cheng, Mouping Fan, Wenli Wu, Xinyue Huang, Yaohua Liang, Yicheng Zhong, Zhimin Ao, Yanqing Lai, Guoxiu Wang, Zhicong Shi PII:

S2405-8297(19)31101-8

DOI:

https://doi.org/10.1016/j.ensm.2019.12.023

Reference:

ENSM 1029

To appear in:

Energy Storage Materials

Received Date: 27 September 2019 Revised Date:

8 December 2019

Accepted Date: 12 December 2019

Please cite this article as: Y. Chen, X. Ke, Y. Cheng, M. Fan, W. Wu, X. Huang, Y. Liang, Y. Zhong, Z. Ao, Y. Lai, G. Wang, Z. Shi, Boosting the Electrochemical Performance of 3D Composite Lithium Metal Anodes through Synergistic Structure and Interface Engineering, Energy Storage Materials, https:// doi.org/10.1016/j.ensm.2019.12.023. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Boosting the Electrochemical Performance of 3D Composite Lithium Metal Anodes through Synergistic Structure and Interface Engineering Yuanmao Chen1,2, Xi Ke1,2,*, Yifeng Cheng1,2, Mouping Fan1,2, Wenli Wu1,2, Xinyue Huang1,2, Yaohua Liang1,2, Yicheng Zhong1,2, Zhimin Ao3, Yanqing Lai4, Guoxiu Wang5,* and Zhicong Shi1,2,* 1

Smart Energy Research Centre, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China 2

Guangdong Engineering Technology Research Center for New Energy Materials and Devices, Guangzhou 510006, China 3

Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, Institute of Environmental Health and Pollution Control, School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China

4

School of Metallurgy and Environment, Central South University, Changsha, 410083, China

5

Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, University of Technology Sydney, Sydney NSW 2007, Australia * Corresponding author: E-mail: [email protected] (X. Ke), [email protected] (G. Wang), [email protected] (Z. Shi)

Author Contribution Statement Yuanmao Chen: Conceptualization, Investigation, Validation, Data curation, Writing-Original

draft

preparation.

Xi

Ke:

Supervision,

Conceptualization,

Methodology, Data Curation, Writing-Reviewing and Editing. Yifeng Cheng: Investigation. Mouping Fan: Investigation. Wenli Wu: Data curation. Xinyue Huang: Data curation. Yaohua Liang: Visualization. Yicheng Zhong: Visualization. Zhimin Ao: Software. Yanqing Lai: Validation. Guoxiu Wang: Writing-Review and Editing. Zhicong Shi: Conceptualization, Validation, Supervision.

1

Boosting the Electrochemical Performance of 3D Composite

2

Lithium Metal Anodes through Synergistic Structure and

3

Interface Engineering

4 5

Abstract: Construction of three-dimensional (3D) composite lithium metal anodes (LMAs)

6

based on Li melt-infusion into a 3D porous scaffold has been demonstrated to be effective for

7

solving the issue of the considerable relative volume change of LMAs during Li

8

plating/stripping. However, little attention has been paid to controllable regulation of the

9

structure and interface of 3D composite LMAs. In this study, 3D composite LMAs, namely

10

Li-AuLi3@CF electrodes, are firstly fabricated by infusion of molten Li into carbon fiber (CF)

11

paper modified with nanoporous gold (NPG) which is converted to AuLi3 after infusion. We

12

herein demonstrate a synergistic structure and interface engineering strategy realized by a

13

simple and effective pre-stripping protocol to initially expose a portion of the 3D AuLi3@CF

14

scaffold to create “PS-Li-AuLi3@CF” electrodes, which greatly boosted the electrochemical

15

performance. Symmetrical Li|Li cells with PS-Li-AuLi3@CF electrodes show an overpotential

16

of 111 mV after cycling at a current density of 0.5 mA cm-2 for 1800 h. Additionally, Li|LiFePO4

17

(LFP) and Li|sulfurized polyacrylonitrile (SPAN) full cells with PS-Li-AuLi3@CF electrodes

18

exhibit a high capacity retention of 96.1% with a Coulombic efficiency (CE) of 99.2% after

19

1000 cycles at 5C, and a capacity retention of 70.6% with a CE of 99.8% after 1000 cycles at 2C,

20

respectively. This work provides a simple and highly effective method for engineering the

21

structure and interface of 3D composite LMAs to boost their electrochemical performance for

22

high-energy-density rechargeable lithium metal batteries (LMBs). 1

23

Keywords: lithium metal anodes, pre-stripping, structure engineering, interface engineering,

24

nanoporous gold

25 26

1. Introduction

27

Traditional lithium ion batteries (LIBs) based on reversible intercalation/decalation of lithium

28

ions are approaching their energy density limits and not meeting the ever-increasing demand of

29

today’s electric power and energy storage applications including electric vehicles and portable

30

devices[1-3]. In pursuing next-generation batteries with significantly higher energy density,

31

alternative systems beyond lithium intercalation chemistry have been developed such as

32

Li-S[4] and Li-O2[5, 6] systems based on Li metal plating/stripping electrochemistry. In these

33

systems, Li metal, which possesses fascinating advantages of an ultrahigh specific capacity

34

(3860 mAh g-1), a very low redox potential (-3.040 V versus standard hydrogen electrode) and a

35

small gravimetric density (0.534 g cm-3), has been widely considered to be the ultimate anode

36

material to replace graphite anodes[7-10]. However, practical usage of lithium metal anodes

37

(LMAs) has been challenged by the issues of low Coulombic efficiency (CE) and Li dendrite

38

growth, originating from the highly reactive nature of Li metal in organic electrolyte and an

39

unstable solid electrolyte interphase (SEI), as well as nontrivial volume change during Li metal

40

plating/stripping[11-13]. In order to solve these problems, a large number of strategies, for

41

example, adding electrolyte additives to facilitate SEI formation[14, 15], constructing artificial

42

SEI films to protect LMAs from electrolyte attack[16-19], modifying separators to homogenize

43

Li ion flux[20, 21], improving lithiophilicity to reduce lithium nucleation overpotential[22-26],

44

have been developed and effectively prolonged the lifespan of LMAs. However, these 2

45

strategies are usually effective at relatively low current densities and cycling capacities, and

46

need further improvements for high-rate battery applications. Structure-engineered composite

47

electrodes with embedded hosts which can stabilize Li plating/stripping process are demanded

48

for high-rate, high-capacity and long-lifespan LMAs[27-33].

49

Three-dimensional (3D), highly porous current collectors and skeletons have been

50

introduced into LMAs as 3D hosts to improve electrochemical performance. These 3D hosts

51

can effectively suppress lithium dendrite growth not only by accommodating huge volumetric

52

change but also by reducing the effective current density during Li plating/stripping

53

processes[12, 34]. However, most of these structured LMAs are initially Li-free, and require

54

pairing with Li-containing cathodes, which results in a lack of Li to offset irreversible

55

consumption of lithium during SEI formation and later cycling. In order to match Li availability

56

at both ends of the cell when including Li-free cathodes, such as in high-energy-density S and

57

O2 cathodes, electrochemical plating of lithium into these 3D hosts prior to cell assembly is

58

necessary[35, 36], which usually causes uneven Li deposition, and also makes this process

59

infeasible for battery manufacturing industry. Therefore, pre-storing of Li metal into 3D hosts is

60

important for obtaining a composite lithium metal electrode. In this regard, Cui et al. are

61

pioneers. They developed a facile and effective method to fabricate composite LMAs by

62

infiltrating molten Li metal into 3D hosts[29]. Using this method, a variety of 3D highly porous

63

scaffolds, such

64

wood/polymer[28], have been employed to produce 3D composite LMAs. Compared with

65

hostless Li metal electrodes, such 3D composite LMAs are capable of confining Li metal within

66

3D matrices, and maintaining a relatively constant electrode dimension, thus addressing the

as nickel foam[37], carbon cloth/paper[30, 38] and carbonized

3

67

issue of large volume change associated with Li plating/striping, and realizing dendrite

68

suppression and stable cycling. Nevertheless, most of reported 3D hosts have lithiophobic

69

surfaces that need lithiophilicity conversion, and therefore, an extra phase, such as ZnO[39, 40],

70

SnO2[41] and Si[29], have frequently been introduced to enhance lithiophilicity. The

71

modification process always demands complicated facilities, e.g., chemical vapor deposition

72

and atomic layer deposition, which are often labour-intensive, time-consuming and highly

73

expensive. More importantly, such semiconducting phases lower electronic conductivity at the

74

host-Li interface, which was usually overlooked in previous studies. In this manner,

75

development of novel and simple strategies for constructing a lithiophilic and highly

76

conductive interface on 3D hosts is of tremendous importance for fabricating 3D composite

77

LMAs. In addition to a lithiophilic interface, the electrode structure of 3D composite LMAs is

78

another important factor that contributes significantly to overall performance. However, it is a

79

great challenge to control the electrode structure of 3D composite LMAs during the fabrication

80

procedure, since the infusion process of molten Li metal is completed in a spontaneous manner

81

within a short time period. Thus, in order to boost the electrochemical performance of 3D

82

composite LMAs, there is a need for an alternative way to regulate the electrode structure after

83

the melt-infusion process. So far, very little attention has been paid to the pretreatment of 3D

84

composite LMAs.

85

In this study, we propose a synergistic structure and interface engineering strategy, which is

86

simple and easy to conduct without the use of complicated instruments, to construct a newly

87

designed 3D composite LMA for boosting its electrochemical performance. Specifically, a

88

lithiophilic nanoporous gold (NPG) film with hierarchical micro/nano-porosity was initially 4

89

coated on the surface of carbon fibers (CF), which reacts with molten Li and converts to a

90

AuLi3 film during molten Li infusion process. Due to the fact that the AuLi3@CF scaffold was

91

completely buried in Li metal layer after the melt-infusion process, a fraction of Li metal was

92

then stripped from the 3D composite LMA (Li-AuLi3@CF) in order to expose the upper porous

93

AuLi3@CF structure. We term this treatment process as “pre-stripping” (PS) of 3D composite

94

LMAs. The as-designed 3D composite LMA (PS-Li-AuLi3@CF) possesses some remarkable

95

advantages: (i) the pre-stripped AuLi3@CF scaffold is highly porous and conductive, which

96

promotes rapid and homogeneous electron/ion transport at the PS-Li-AuLi3@CF/electrolyte

97

interface, allowing fast electrode kinetics; (ii) the surface of the pre-stripped scaffold, namely

98

the AuLi3 phase, is highly lithiophilic, which can bring about a much lower Li nucleation

99

overpotential, enabling suppression of Li dendrite growth; (iii) the pre-stripped scaffold has a

100

relatively large surface area, which can significantly reduce local current densities during

101

cycling, thus enlarging the Sand’s time[42] and retarding Li dendrite formation; (iv) the

102

released void space of pre-stripped scaffold provides extra room to accommodate Li deposition,

103

alleviating the huge volume change during Li plating/stripping. Owing to the above mentioned

104

merits, the PS-Li-AuLi3@CF based 3D composite LMAs exhibit outstanding electrochemical

105

performance. In Li|Li symmetric cells, PS-Li-AuLi3@CF based 3D composite LMAs can run

106

for 1800 h without cell failure at a current density of 0.5 mA cm-2. PS-Li-AuLi3@CF|LiFePO4

107

(LFP) cells show an excellent capacity retention of 96.1% with a CE of 99.2% after 1000 cycles

108

at 5C. Furthermore, PS-Li-AuLi3@CF|sulfur/polyacrylonitrile (SPAN) cells deliver a capacity

109

retention of 70.6% with a high CE of 99.8% after 1000 cycles at 2C.

110 5

111

2. Experimental Section

112

2.1. Fabrication of NPG@CF scaffolds

113

The fabrication procedure of NPG@CF scaffolds consists of two steps. Firstly, a

114

commercial CF (Toray Carbon Fiber Paper, Japan) paper was cut into rectangular pieces with

115

a size of 1×6 cm2 and successively washed with acetone, dilute hydrochloric acid solution,

116

ethanol, and distilled water in an ultrasonic bath for 20 min to remove impurities on the CF

117

surface, and then dried in a vacuum oven at 60 °C. The AuSn alloy film was electrodeposited

118

onto the pre-treated CF with an area of 1 cm2 exposed in the electrolyte at a current density of

119

5 mA cm-2 for 20 min at 45 °C, using the CF and the Pt-modified Ti mesh as the working and

120

counter electrodes, respectively. Secondly, the AuSn alloy modified CF (AuSn@CF) scaffolds

121

were immersed into a solution containing 5 mol L-1 NaOH and 1 mol L-1 H2O2 for one week

122

at room temperature for fully dealloying the Sn component to obtain NPG@CF scaffolds.

123 124

2.2. Fabrication of PS-Li-AuLi3@CF electrodes

125

PS-Li-AuLi3@CF electrodes were fabricated based on a combination of Li melt-infusion

126

into NPG@CF scaffolds and pre-stripping of Li metal. The whole fabrication procedure was

127

accomplished in an argon-filled glove box with an oxygen and water level below 0.1 ppm.

128

Firstly, both sides of Li foils were scratched and polished with a sharp scalpel until the

129

surfaces were exceedingly shiny, then the polished Li foils were put onto a heating station

130

padded with a piece of Ni foil at 400 °C. NPG@CF scaffolds were laid on the molten Li

131

surface to absorb molten Li. Secondly, Li-AuLi3@CF electrodes were assembled in CR2032

132

coin cells with Li foils as the counter/reference electrodes, and a specific amount of Li metal 6

133

was stripped from Li-AuLi3@CF electrodes to obtain PS-Li-AuLi3@CF electrodes. For

134

comparison, pristine CF scaffolds were also put on molten Li surface to absorb molten Li into

135

the bottom part of scaffolds to obtain Li@CF composite electrodes (Fig. S1).

136 137

2.3. Computational details

138

Calculations were performed using density functional theory (DFT) as implemented in the

139

Vienna ab initio simulation package (VASP)[43]. The interaction between ions and electrons

140

is described using ultrasoft pseudopotentials[44]. The generalized gradient approximation

141

(GGA)

142

exchange-correlation functional[45]. The cell structure was fully optimized using the

143

Broyden-Fletcher-Goldfarb-Shanno (BFGS) minimization technique[46]. The cut-off energy

144

of the plane wave was set to 400 eV, and the Brillouin zone sampling was performed using a

145

7×7×1 set of the Monkhorst–Pack mesh for the slab model of AuLi3(111) and graphite(0001).

146

The convergence thresholds of geometry optimization were set to be 1×10-6 eV/atom for the

147

energy change and 0.015 eV/Å for the maximum force. The optimization would stop when

148

these criteria were satisfied. Adsorption energies Eads of Li atom on the surface of AuLi3(111)

149

and graphite(0001) was calculated as follows:

with

the

Perdew-Burke-Ernzerhof

(PBE)

functional

was

used

as

the

150

Eads = Etotal – Eslab – ELi

151

where Etotal and Eslab are the total energies of the relaxed slab model with and without Li atom,

152

respectively, and ELi is the energy of isolated Li atom.

153 154

2.4. Characterization 7

155

X-ray diffraction (XRD, Rigaku D/max-2200/PC) using a Cu Kα radiation source at a

156

wavelength of 0.1541 nm was conducted to confirm the crystal structures of scaffolds and

157

composite Li metal electrodes. Brunauer-Emmett-Teller (BET) surface area measurements of

158

electrodes were carried out using a specific surface analyzer (SSA, BeiShiDe, 3H-2000).

159

Field-emission scanning electron microscopy (FESEM, JEOL, JSM-6700F) was employed to

160

characterize surface morphology of electrodes. Specifically, before characterization, coin cells

161

were firstly disassembled in a glovebox to obtain electrodes, which were immersed into

162

1,3-dioxolane/dimethoxyethane (DOL/DME) solvents to remove residual electrolyte and Li

163

salt, and then dried for minutes. An anaerobic sealed box was used to transfer the samples

164

from the glovebox to the SEM chamber.

165 166

2.5. Electrochemical measurements

167

CR2032-type symmetrical Li|Li coin cells with two identical Li foil, Li-AuLi3@CF and

168

PS-Li-AuLi3@CF electrodes, respectively, were assembled in an argon-filled glovebox with a

169

Celgard separator and an electrolyte of 1 M LiPF6 in EC/DEC (1:1 vol%) without any

170

additives. The amount of electrolyte used in assembling a cell was about 25 µL. The Li|Li

171

cells were galvanostatically cycled at current densities of 0.5, 1 and 3 mA cm-2 with a capacity

172

of 1 mAh cm-2 using LAND battery testing system at room temperature. Full cells were

173

assembled with Li foil, Li-AuLi3@CF and PS-Li-AuLi3@CF electrodes as the anodes, and

174

LiFePO4 (LFP) with an areal loading of 2.15 mg cm-2 and sulfurized polyacrylonitrile (SPAN)

175

with an areal S loading of 0.49 mg cm-2 as the cathodes, respectively, employing the same

176

separator and electrolyte as those in Li|Li cells. For full-cell cycling, the cells were firstly 8

177

cycled at 0.2 C for 2 cycles for cell activation and SEI formation, and Li|LFP and Li|SPAN

178

cells were cycled in voltage windows of 2.4-4.2 V and 1.0-3.0 V (vs Li+/Li), respectively.

179

Electrochemical impedance spectra (EIS) measurements were conducted using an

180

electrochemical workstation (ZIVE, SP1) over a frequency range from 100 kHz to 10 mHz.

181 182

3. Results and discussion

183

Fig. 1a illustrates the fabrication process of PS-Li-AuLi3@CF electrodes. CF paper was

184

chosen in this study as a 3D host due to its lightweight as well as high electrical conductivity

185

and porosity. However, the pristine CF surface is intrinsically lithiophobic, which makes it

186

difficult to spontaneously wet with molten Li metal. Therefore, in order to enhance the surface

187

lithiophilicity of CF paper, we firstly adopted a facile method developed previously in our

188

group to coat a nanoporous gold (NPG) film on the CF skeleton, which was accomplished by a

189

combination of AuSn alloy electrodeposition and chemical dealloying of Sn[47]. It is worth

190

noting that NPG, rather than gold nanoparticles[22, 25] used in previous studies, was employed

191

as the lithiophilic phase herein because the 3D hierarchical micro- and nanoporosity of the NPG

192

film could bring multiple advantages. Details will be published elsewhere. Then, the NPG@CF

193

scaffold was put into contact with molten Li metal. NPG reacted with molten Li to form AuLi3

194

and a significant quantity of molten Li metal was infused into the scaffold, giving a fully

195

Li-infused CF electrode (Li-AuLi3@CF). Finally, a portion of Li metal was stripped via a

196

pre-stripping treatment of the Li-AuLi3@CF electrode (PS-Li-AuLi3@CF), to expose the top

197

part of the AuLi3@CF scaffold. Fig. S2 displays optical images of a CF-based electrode at

198

different fabrication stages. The color appearance of the CF electrode turns sequentially from 9

199 200

Figure 1. Schematic illustration (a) and SEM images under different magnifications of pristine

201

CF (b, g), AuSn alloy coated CF (AuSn@CF) (c, h), NPG coated CF (NPG@CF) (d, i),

202

NPG@CF after molten Li infusion (Li-AuLi3@CF) (e, j), and Li-AuLi3@CF after pre-stripping

203

(PS-Li-AuLi3@CF) (f, k).

204 205

gray-black to grey, gold, silvery, and then yellow-white after AuSn alloy electrodeposition,

206

chemical dealloying of Sn, melt-infusion of Li metal, and pre-stripping of Li metal, respectively.

207

The color change of the electrode is very uniform, which indicates the structure and surface of

208

the electrode have been homogeneously modified by this protocol, which was confirmed by 10

209

scanning electron microscopy (SEM) as shown in Fig. 1b-k. The pristine CF paper electrode

210

shows a highly porous 3D structure (Fig. 1b) consisting of an interconnected network of smooth

211

fibers with a diameter of about 8 µm (Fig. 1g). After electrodeposition of AuSn alloy, the AuSn

212

alloy film was conformally wrapped on fiber surfaces (Fig. 1c). The higher magnification SEM

213

images (Fig. 1h and Fig. S3a) reveal that the alloy film was composed of densely stacked

214

microscale polygonal bricks with a thickness of 150~200 nm. After chemical dealloying of Sn

215

component, the scaffold displays a uniformly rough and porous surface (Fig. 1d). In line with

216

our previous study[47], the dealloying process transforms the solid AuSn bricks into

217

bicontinuous porous NPG sheets (Fig. 1i and Fig. S3b), as confirmed by XRD result shown in

218

Fig. S4a. Meanwhile, energy dispersive spectroscopy (EDS) mapping results reveal that C, Au

219

and Sn components in AuSn@CF, and C, Au components in NPG@CF samples are uniformly

220

distributed (Fig. S5). Such a lithiophilic NPG film with hierarchical porosity (microscale

221

porosity among NPG sheets and nanoscale porosity inside NPG sheets) can not only provide a

222

large chemical driving force through Au-Li alloy reaction, but also form a considerable

223

physical drawing force by capillary absorption. Besides these effects, this hierarchical

224

micro/nano-porosity could also help to anchor the deposited Li metal, thus significantly

225

improving the stability of CF-Li interface. As a result, the NPG@CF electrode exhibits a very

226

fast uptake of molten Li metal (Fig. S6a and Movie S1), while the pristine CF electrode is

227

relatively difficult to fully wet by molten Li (Fig. S6b and Movie S2). Owing to superior

228

lithiophilicity of NPG@CF electrodes, it is noted that molten Li was fully infused into

229

electrodes within two seconds and thus it is not convenient to control the amount of infused

230

molten Li during such a short period, which is different to previous studies where infusion 11

231

processes can take as long as tens of seconds[48-50]. After melt-infusion of Li metal, the

232

composite electrode displays a compact structure with the CF scaffold fully embedded in Li

233

metal (Fig. 1e, j and Fig. S7a). Meanwhile, XRD results reveal that gold phase was converted to

234

AuLi3 phase as shown in Fig. S4b. It can be inferred that, when paired with a Li-containing

235

cathode, such a fully-infused composite LMA would behave like a hostless Li electrode owing

236

to the ineffectiveness of buried AuLi3@CF scaffold. When matched with a Li-free cathode, the

237

first discharge process unmasks the outermost extent of the AuLi3@CF scaffold, but the effect

238

of the AuLi3@CF scaffold could be significantly attenuated during the Li stripping process

239

(especially during the initial period of the discharge), since the exposure of the AuLi3@CF

240

scaffold takes place gradually throughout the discharge process. In this context, it is reasonable

241

to conduct a pre-stripping process to pre-expose the 3D AuLi3@CF scaffold to fully activate its

242

multifunctional capabilities as mentioned above. Moreover, removal of excessive Li metal will

243

achieve a higher energy density for a cell. After 10 mAh cm-2 of Li metal is stripped out, the

244

PS-Li-AuLi3@CF electrode displays a highly porous structure with unformly stripped fibers

245

(Fig. 1f and Fig. S7b). A higher magnification SEM image (Fig. 1k) shows that the fiber surface

246

exhibits a different morphology compared with the initial stage as shown in Fig. 1i, and XRD

247

results reveal that the surface layer is composed mainly of AuLi3 phase (Fig. S4c). It should be

248

noted that although a high surface area of 3D current collectors can reduce the effective current

249

density during Li plating/stripping process and retard Li dendrite growth[12, 34], it may also

250

lead to additional side reactions and severe SEI formation. The BET measurement shows that

251

the surface area of PS-Li-AuLi3@CF electrodes is about 2.3 m2 g-1. This value is relatively

252

small compared with that of other previously reported 3D scaffolds with a stable SEI layer 12

253

during cycling[51, 52]. Hence, the exposed surface area of PS-Li-AuLi3@CF electrodes would

254

not cause severe SEI formation.

255 256

Figure 2. Calculated binding energies of a Li atom on a AuLi3 slab with (a) a Au-terminated

257

surface or (b) a Li-terminated surface, and (c) a pristine CF slab for comparison. (d) Voltage

258

profiles of galvanostatic Li deposition on a PS-Li-AuLi3@CF and a pristine CF electrodes at a

259

current density of 0.5 mA cm-2 with the inset showing comparison of Li nucleation

260

overpotentials on the two electrodes.

261 262

As reported in our previous study, the AuLi3 phase was found to be effective in suppressing

263

the growth of Li dendrites and prolonging the lifespan of LMAs[25]. Herein, we further

264

employed density functional theory (DFT)[53] to investigate the origin of this effect by

265

calculating the binding energies between a Li atom and the AuLi3 or the carbon fibre (Fig. 2a-c). 13

266

It should be noted here that the structural model of the AuLi3 alloy slab presents two different

267

cleaved surfaces, namely the Au-terminated and the Li-terminated surfaces, respectively. The

268

theoretical calculations reveal that both the Au-terminated and the Li-terminated AuLi3(111)

269

surfaces exhibit much larger binding energies of -2.29 and -1.81 eV, respectively, compared

270

with -1.42 eV for the pristine CF, indicating that the coated AuLi3 phase is much more

271

lithiophilic than the pristine CF surface. As has been well demonstrated by previous

272

studies[22-26], a lithiophilic modification of the host surface is capable of reducing nucleation

273

overpotential of Li metal and guiding uniform Li plating/stripping, thus enabling suppression of

274

Li dendrite growth and prolonging the lifespan of LMAs. Herein, thanks to the excellent

275

lithiophilicity of the AuLi3 layer, the nucleation overpotential of Li metal on the

276

PS-Li-AuLi3@CF electrode (30 mV) was much smaller than that on the pristine CF electrode

277

(55.8 mV) (Fig. 2d). Compared with previous studies using discretely distributed metallic

278

nanoparticles[22, 50] or semiconductive thin films[29, 39] as lithiophilic species, the

279

continuous AuLi3 film with superior electrical conductivity employed in the present study

280

could not only provide homogeneously dispersed lithiophilic sites on the host surface, but also

281

supply a highly conductive and continuous interface between the host and the Li metal. Owing

282

to these characteristics, the PS-Li-AuLi3@CF electrode is reasonably expected to exhibit stable

283

Li plating/stripping behavior and excellent electrochemical performance in lithium metal

284

batteries (LMBs).

285

In order to reveal the effect of AuLi3 film on Li plating/stripping behavior, CR2032-type coin

286

cells were assembled with PS-Li-AuLi3@CF and Li@CF (see Experimental Section for details)

287

electrodes as the working electrode, and a Li foil as the counter electrode. An amount of 5 mAh 14

288 289

Figure 3. Representative voltage-time profiles of a galvanostatic Li plating/stripping cycle at a

290

current density of 0.5 mA cm-2 with a plating/stripping capacity of 5 mAh cm-2 (a), and the

291

corresponding SEM images of PS-Li-AuLi3@CF (b-d, h-j) and Li@CF (e-g, k-m) electrodes at

292

different stages. 15

293

cm-2 Li was plated on anodes at a current density of 0.5 mA cm-2 and then stripped away. In

294

comparison with Li@CF electrodes, PS-Li-AuLi3@CF electrodes display a more stable voltage

295

plateau, which indicates a better interfacial stability during cycling. Meanwhile,

296

PS-Li-AuLi3@CF electrodes show a smaller voltage hysteresis which is defined as sum of

297

overpotential for Li plating and Li stripping (Fig. 3a). The evolution of surface morphology of

298

PS-Li-AuLi3@CF and Li@CF electrodes during a lithium plating/stripping cycle was studied

299

by SEM after cell disassembly. The SEM images of the electrodes at different stages of cycling

300

are presented as Fig. 3b-m. It is shown that, during the Li plating stages marked with Ⅰ, Ⅰ and

301

Ⅰ, for the PS-Li-AuLi3@CF electrode, Li metal selectively deposits on the AuLi3@CF skeleton,

302

and thus the diameter of the AuLi3@CF is enlarged continuously from about 8 to 12 µm by Li

303

plating (Fig. 3b-d), indicating that the lithiophilic AuLi3 film is able to effectively guide Li

304

metal nucleation and growth. In comparison, Li metal randomly deposits into the interspaces

305

between CFs with a rather dendritic appearance (Fig. 3e-g). Furthermore, during the Li

306

stripping stages marked with Ⅰ, Ⅰ and Ⅰ, for the PS-Li-AuLi3@CF electrodes, the diameter of

307

the AuLi3@CF skeleton gradually reduces (Fig. 3h-j), which indicates that the Li deposit on the

308

AuLi3@CF surface is smoothly eroded. At the final stage of the stripping, the AuLi3@CF

309

surface recovers nearly to its original state as shown in Fig. 1k (Fig. 3j). In contrast, for Li@CF

310

electrodes, although the size of Li dendrites seems to decrease with the progression of stripping

311

(Fig. 3k-m), there is irreversible Li deposition accumulated in the CF matrix. It is noted that

312

there is no growth of Li dendrites on PS-Li-AuLi3@CF electrodes throughout plating/stripping

313

cycles. For Li@CF electrodes, the poor lithiophilicity of the upper part of Li@CF electrodes

314

results in inhomogeneous Li metal nucleation and growth, and a large Li nucleation 16

315

overpotential which can induce Li dendrite growth during Li plating/stripping cycles. These

316

results reveal the significant impact of the lithiophilic AuLi3 modification on Li

317

plating/stripping behavior and suppression of Li dendrite growth, which is in agreement with

318

the previous reports that have utilized other lithiophilic species such as Ag[50], ZnO[39, 40]

319

and SnO2[41] to improve electrochemical performance of LMAs.

320 321

Figure 4. SEM images of (a-c) Li foil, (d-f) Li-AuLi3@CF and (g-l) PS-Li-AuLi3@CF

322

electrodes after cycling for 10 cycles (a, d, g, j), 100 cycles (b, e, h, k) and 150 cycles (c, f, i, l),

323

respectively, at a current density of 0.5 mA cm-2 with a capacity of 1 mAh cm-2 Li.

17

324

In order to investigate the impact of electrode structure on the cycling performance of LMAs,

325

symmetrical coin cells with two identical Li metal electrodes, including Li foils, Li-AuLi3@CF

326

and PS-Li-AuLi3@CF electrodes, were assembled using an electrolyte of 1 mol L-1 LiPF6 in

327

EC/DEC (1:1 vol%) without any additives. Fig. 4 shows SEM images of these Li metal

328

electrodes after cycling for 10 cycles, 100 cycles and 150 cycles, respectively, at a current

329

density of 0.5 mA cm-2 with a capacity of 1 mAh cm-2 Li. The Li foil electrode forms cracks

330

after only 10 cycles (Fig. 4a), and the size of cracks enlarges with the progression of cycling

331

(Fig. 4b, c). Without a host scaffold to minimize the whole-electrode level volume change, the

332

Li plating/stripping process on the Li foil electrode may be constantly mechanically constrained

333

by other inactive cell components such as separator, thus inducing stress fields in Li metal

334

electrode[54], which could cause the electrode to generate cracks. These cracks would

335

destabilize the anode/electrolyte interface, and increase side reactions between Li metal and

336

electrolyte, leading to an unstable and thick SEI which greatly degrades electrochemical

337

performance of LMAs. Although Li deposition shows a denser morphology in the initial stage

338

for Li-AuLi3@CF electrodes without pre-stripping (Fig. 4d), cracks with a smaller size than

339

that of Li foil electrodes still come out after long cycling (Fig. 4e, f), since most part of the

340

AuLi3@CF scaffold is buried inside bulk Li metal at most of the time during Li

341

plating/stripping cycling, which severely restricts the extent of the AuLi3@CF function. In

342

comparison, for PS-Li-AuLi3@CF electrodes (Fig. 4g-i), Li deposit exhibits a uniform, smooth

343

and dense morphology even after cycling for 150 cycles. The higher magnification SEM

344

images reveal that Li metal closely surrounds the AuLi3@CF skeletons (Fig. 4j-l), which is in

345

line with the Li deposition behavior as depicted in Fig. 3. In addition, as shown in Fig. S8, 18

346

morphology evolution of these electrodes after cycling at higher current densities of 1 and 3 mA

347

cm-2 further revealed the structural advantages of PS-Li-AuLi3@CF composites in maintaining

348

the electrode integrity. These results futher verify the effectiveness of the pre-stripping strategy,

349

which functionalizes the exposed portion of AuLi3@CF scaffold and confers cycling stability

350

of Li-AuLi3@CF electrode. It is worth mentioning that a recent study reveals the different

351

cycling behaviors of thin Li foil electrodes with initial plating and stripping. Initial surface

352

morphology, reactive sites and distribution of dead Li are found to govern the following

353

morphology evolution trend during Li plating/stripping process, and subsequently determine

354

the electrochemical performance of LMAs[55]. Hence, further investigation is needed to

355

clarify the difference of cycling behaviors between initially plated and stripped 3D composite

356

LMAs.

357

Based on the above results, the morphology evolution of PS-Li-AuLi3@CF electrodes

358

compared with Li foil and Li-AuLi3@CF electrodes can be schematically illustrated as shown

359

in Fig. 5. For hostless Li foil electrodes, due to large relative volume changes during Li

360

plating/stripping, stress fields exist in the electrode which finally results in formation of cracks.

361

In Li-AuLi3@CF electrodes, since the AuLi3@CF scaffold is buried well inside bulk Li metal,

362

its function should degrade gradually, and the electrode may behave like a Li foil electrode after

363

especially long cycling. In contrast, for PS-Li-AuLi3@CF electrodes, because a portion

364

of AuLi3@CF scaffold has been initially exposed by the pre-stripping protocol, the as-exposed

365

AuLi3@CF scaffold can provide enough void space to accommodate Li plating/stripping. Thus,

366

structural stability of the electrode at a whole-electrode level is maintained, which can

367

significantly weaken the internal stresses in the electrode, and also supply a highly lithiophilic 19

368

AuLi3 interface layer to reduce Li nucleation overpotential and guide uniform Li

369

plating/stripping process, thus suppressing Li dendrite growth.

370 371

Figure 5. Schematic diagram of the morphology evolution on Li foil (left), Li-AuLi3@CF

372

(middle), and PS-Li-AuLi3@CF (right) electrodes.

373 374

In order to evaluate electrochemical performance of LMAs, symmetrical coin cells were

375

assembled with Li foil, Li-AuLi3@CF and PS-Li-AuLi3@CF electrodes, respectively, and

376

cycled at current densities of 0.5, 1 and 3 mA cm-2 with a capacity of 1 mAh cm-2 Li. Fig. 6a

377

compares voltage profiles of the three types of electrodes during cycling at a current density of

20

378 379

Figure 6. Cycling stability of symmetrical coin cells with Li foil, Li-AuLi3@CF and

380

PS-Li-AuLi3@CF electrodes at current densities of 0.5 (a), 1 (b) and 3 (c) mA cm-2 with a

381

capacity of 1 mAh cm-2, respectively. Nyquist plots of impedance spectra of symmetrical

382

PS-Li-AuLi3@CF, Li-AuLi3@CF and Li foil cells after 10 cycles at a current density of 0.5 mA

383

cm-2 (d). Li stripping curve of the PS-Li-AuLi3@CF electrode (e). 21

384

0.5 mA cm-2. The plain Li foil electrodes exhibit a significant increase of Li plating/stripping

385

overpotential until a sudden voltage drop followed by voltage fluctuations appears after cycling

386

for 500 h, which can be attributed respectively to continuous reformation of SEI layer and

387

partial short-circuiting owing to Li dendrite penetration. The Li-AuLi3@CF electrodes show

388

relatively stable cycling for 900 h, which is a great improvement over the Li foil electrodes.

389

After that, however, the Li-AuLi3@CF electrodes exhibit a gradual increase in overpotential.

390

The overpotential of the Li-AuLi3@CF electrodes is ~300 mV after cycling for 1800 h. This is

391

consistent with the SEM observation which reveals that the Li-AuLi3@CF electrode behaves

392

like a Li foil electrode after long cycling. In contrast, the PS-Li-AuLi3@CF electrodes display a

393

much smaller overpotential with a much slower increasing trend than that of Li-AuLi3@CF

394

electrodes during all the cycling. The overpotential of PS-Li-AuLi3@CF electrodes was ~111

395

mV after cycling for 1800 h. At higher current densities of 1 and 3 mA cm-2, PS-Li-AuLi3@CF

396

electrodes also show a more stable cycling behavior with smaller polarizations (Fig. 6b, c).

397

These results clearly demonstrate the advantages from synergistic structure and interface

398

engineering of PS-Li-AuLi3@CF electrodes. In terms of symmetrical cells cycling in an

399

electrolyte of 1 M LiPF6 in EC/ DEC (1:1 vol%) without any additives, PS-Li-AuLi3@CF

400

electrodes deliver one of the best electrochemical performances among the reported 3D

401

composite LMAs (Table. S1). The highly suppressed polarization and more stable cycling are

402

also evidenced by electrochemical impedance spectroscopy (EIS). The corresponding Nyquist

403

plots show that PS-Li-AuLi3@CF electrodes possess the smallest interfacial resistance after 10

404

cycles compared with the plain Li electrodes (Fig. 6d), indicating that the engineered structure

405

and interface of PS-Li-AuLi3@CF electrodes can enable a faster and more stable Li ion 22

406

transport. Besides excellent cycling stability, PS-Li-AuLi3@CF electrodes also maintain their

407

capacity well. Fig. 6e shows that a capacity of about 3041 mAh g-1 (calculated based on the

408

whole electrode weight) can be stripped from a PS-Li-AuLi3@CF electrode when charged to

409

0.5 V, which is equal to about 78.8% of the theoretical capacity of a pure Li metal electrode.

410

Meanwhile, the areal capacity values were also calculated, which were about 30 mAh cm-2 (Fig.

411

S9). The high specific capacity can be ascribed to both light weight of the CF scaffold and

412

pre-stripping of the infused Li metal. Compared with other studies using heavy metallic

413

scaffolds as hosts[36, 56, 57], the AuLi3@CF-based host material presents more promising for

414

fabricating high-performance 3D composite LMAs without significant influence of host weight

415

on the electrode capacity.

416

As shown in Fig. 7, the electrochemical performance of PS-Li-AuLi3@CF electrodes

417

compared with Li foil and Li-AuLi3@CF electrodes was further investigated in full cells

418

employing LiFePO4 (LFP) and sulfurized polyacrylonitrile (SPAN) cathodes, respectively. The

419

Li|LFP and Li|SPAN full cells were cycled at 5C (Fig. 7a) and 2C (Fig. 7b), respectively, for

420

1000 cycles at room temperature. The specific capacity of PS-Li-AuLi3@CF|LFP is about 122

421

mAh g-1 in the first cycle and retains 117 mAh g-1 after 1000 cycles, which is 96.1% of its

422

original capacity, while the specific capacities of Li-AuLi3@CF|LFP and Li|LFP full cells are

423

initially 119 and 118 mAh g-1, and decrease to 90 and 60 mAh g-1 after 1000 cycles,

424

corresponding to a capacity retention of 75.5% and 50.8%, respectively. Meanwhile,

425

PS-Li-AuLi3@CF|SPAN full cell delivers an initial specific capacity of 637 mAh g-1 with a

426

capacity retention of 70.6% after 1000 cycles at 2C, while Li-AuLi3@CF|SPAN and Li|SPAN

427

full cells exhibit an original specific capacity of 683 and 638 mAh g-1 with a capacity retention 23

428 429

Figure 7. Cycling performance of Li foil, Li-AuLi3@CF and PS-Li-AuLi3@CF electrodes in

430

full cells with (a) LiFePO4 (LFP) cathode at 5C, and (b) SPAN cathode at 2C, respectively. (c)

431

Rate capability of Li foil, Li-AuLi3@CF and PS-Li-AuLi3@CF electrodes in full cells with LFP

432

at different rates from 1 to 10 C and (d) the corresponding areal current density with cycling.

433 434

of 23.4% and 27.8% after 1000 cycles at 2C, respectively. It is noted that Li-AuLi3@CF|SPAN

435

and Li|SPAN full cells show comparable specific capacities and a similar degrading trend after

436

200 cycles, suggesting that the AuLi3@CF scaffold has been buried deeply in bulk Li metal,

437

and the Li-AuLi3@CF electrodes begin to behave like Li foil electrodes at that time. Compared 24

438

to Li|LFP full cells with Li foil and Li-AuLi3@CF electrodes, Li|LFP full cell with

439

PS-Li-AuLi3@CF electrodes displays a better rate capability (Fig. 7c). Especially at a higher

440

areal current density of 3.7 mA cm-2 (equal to 10C) (Fig. 7d), a higher specific capacity is

441

attained by the PS-Li-AuLi3@CF|LFP full cell. The above results further demonstrate that the

442

synergistic structure and interface engineering strategy with a pre-stripping protocol to expose a

443

portion of porous scaffold with a lithiophilic surface is of significant importance for boosting

444

the electrochemical performance of 3D composite LMAs fabricated by Li melt-infusion.

445 446

4. Conclusion

447

In summary, we have demonstrated a synergistic structure and interface engineering strategy

448

for 3D composite LMAs, namely “Li-AuLi3@CF” electrodes, which are fabricated by Li

449

melt-infusion into 3D porous CF paper scaffold modified with NPG that converts to AuLi3 after

450

molten Li infusion, through a simple and effective pre-stripping (PS) protocol to initially

451

expose a portion of AuLi3@CF scaffold, yielding “PS-Li-AuLi3@CF” electrodes. Compared

452

with Li foil and Li-AuLi3@CF electrodes, the as-designed PS-Li-AuLi3@CF electrodes

453

maintained structural stability at a whole-electrode level to accommodate huge volume change

454

during Li plating/stripping, and also provided a highly lithiophilic AuLi3@CF interface to

455

guide uniform Li nucleation/growth and suppress Li dendrite growth. These structural and

456

interfacial advantages lead to significantly improved electrochemical performance of

457

PS-Li-AuLi3@CF electrodes. Symmetrical Li|Li cells with PS-Li-AuLi3@CF electrodes can

458

run for 1800 h without cell failure at a current density of 0.5 mA cm-2 with a capacity of 1 mAh

459

cm-2 Li. Furthermore, Li|LFP and Li|SPAN full cells with PS-Li-AuLi3@CF electrodes exhibit 25

460

an excellent capacity retention of 96.1% with a CE of 99.2% at 5C and a capacity retention of

461

70.6% with a CE of 99.8% at 2C for 1000 cycles, respectively. Besides these characteristics,

462

PS-Li-AuLi3@CF electrodes can deliver a relatively high capacity of about 3041 mAh g-1,

463

minimizing the influences of host on the specific capacity. This work provides a new strategy to

464

rationally design and fabricate high-performance 3D composite LMAs based on Li

465

melt-infusion, which shows new avenues for the development of next-generation high energy

466

density rechargeable LMBs.

467 468

Supporting Information

469

Additional optical and SEM images, XRD characterizations, and additional electrochemical

470

measurements.

471 472

Notes

473

The authors declare no competing financial interests.

474 475

Acknowledgments

476

The research was supported by the National Key R&D Program of China (2018YFB0104200).

477 478

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Highlights Nanoporous gold is coated on carbon fibers to improve lithiophilicity. Pre-stripping is conducted for synergistic structure and interface engineering of 3D composite lithium metal anodes. PS-Li-AuLi3@CF electrodes show a specific capacity of about 3041 mAh g-1. Symmetrical Li|Li cells with PS-Li-AuLi3@CF electrodes can run for 1800 h without cell failure at a current density of 0.5 mA cm-2. Boosted electrochemical performance is achieved in Li|LFP and Li|SPAN full cells with PS-Li-AuLi3@CF electrodes.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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