Ultrafast Li+ Diffusion Kinetics of 2D Oxidized Phosphorous for Quasi

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Ultrafast Li Diffusion Kinetics of 2D Oxidized Phosphorous for Quasisolid-state Bendable Batteries with Exceptional Energy Densities Jiang Cui, Shanshan Yao, Woon Gie Chong, Junxiong Wu, Muhammad Ihsan-Ul-Haq, Lianbo Ma, Ming Zhao, Yewu Wang, and Jang-Kyo Kim Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Chemistry of Materials

Ultrafast Li+ Diffusion Kinetics of 2D Oxidized Phosphorous for Quasisolid-state Bendable Batteries with Exceptional Energy Densities Jiang Cui,a Shanshan Yao,a Woon Gie Chong,a Junxiong Wu,a Muhammad Ihsan-Ul-Haq,a Lianbo Ma,a Ming Zhao,b Yewu Wangb and Jang-Kyo Kim*a aDepartment

of Mechanical and Aerospace Engineering, The Hong Kong University of Science

and Technology, Clear Water Bay, Hong Kong, P. R. China. bDepartment

of Physics & State Key Laboratory of Silicon Materials, Zhejiang University,

Hangzhou 310027, P. R. China.

ABSTRACT

Phosphorous has drawn much attention for energy storage applications due to its high theoretical capacity while its surface is prone to oxidization, causing alterations of its physicochemical properties. Herein, we report a previously overlooked Li storage mechanism in the oxidized 2D black phosphorous/graphene oxide (BP/GO) heterostructure, where Li+ ions transport at an ultralow diffusion barrier of 80 meV and an ultrafast diffusion kinetics of 2.5×10-6 cm2 s-1 according to the ab initio molecular dynamics simulations. Furthermore, significant synergy arises when the 2D BP sheets chemically bind with GO layers, giving rise to an exceptional mechanical strength and flexibility of the BP/GO paper. The BP/GO composite anode sustains 500 stable cycles with Coulombic efficiencies as high as 99.6 % in a Li ion half cell. A quasi-solid-state, bendable Li-ion full-cell battery is assembled for the first time using the BP/GO anode, a V2O5/CNT cathode and a gel polymer electrolyte. It delivers simultaneously high gravimetric and

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volumetric energy densities of 389 Wh kg-1 and 498 Wh L-1, respectively, with a high retention rate of 92.3 % even after 100 cycles of repeated folding and unfolding. The foregoing discovery makes the current flexible battery ideally suited for powering wearable electronics that require both high energy densities and mechanical robustness.

INTRODUCTION

The rapid, widespread emergence of flexible and wearable electronics requires the development of flexible energy storage devices having high energy densities. Previous research on flexible energy storage is mainly focused on aqueous supercapacitors because the electrode and electrolyte of supercapacitors can be easily made flexible, but their relatively low energy densities severely hinder wide applications

1,2.

Emerging flexible Li ion batteries (LIBs) are considered the most

promising alternative to flexible supercapacitors owning to their high energy densities and mature technology with well-established working principles 3. Although the fabrication of flexible LIBs using convectional electrode materials can be realized by incorporating highly porous 3D current collectors

4,5

or thick polymer electrolyte/protective films 6, they deliver poor electrochemical

performance, especially with very low volumetric energy densities. Therefore, a key challenge to the development of advanced flexible LIBs is to identify proper electrode materials with both high energy densities and flexibility. Recent widespread exploration of 2D materials may offer a new opportunity to address the above challenges 7,8. Many 2D materials, including graphene, MXene and few-layer black phosphorus (BP), can be dispersed in a solvent and assembled into flexible papers having excellent mechanical strengths 9,10. Apart from flexibility, they also possess high Li storage capacities.


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2D phosphorus sheets, in particular, have an excellent theoretical specific capacity of 2596 mAh g-1 by alloying with Li, which is the highest among all 2D materials. Unlike graphene or MXene which usually requires pre-expansion or chemical etching to achieve effective exfoliation, the weak van der Waals forces between the adjacent phosphorene layers enable facile and high-yield liquid exfoliation of bulk BP crystals into 2D phosphorus sheet without any pre-treatment 11. The exfoliated phosphorus sheets are chemically stable in organic solvents for days, facilitating easy fabrication of phosphorus electrodes in ambient temperature

12.

Besides, BP is much more

conductive than other 2D semiconductors, e.g. MoS2 and SnS2, owing to its high carrier mobility as high as 1000 cm2/V⋅s, which in turn gives rise to an excellent rate capability of LIBs made from it 7. Along with the maturing large-scale production of BP crystals from the cheap red phosphorous precursor, the low-cost synthesis of 2D phosphorus electrodes is highly foreseeable. All of the above merits offer 2D phosphorus promising future as an advanced electrode material for flexible batteries. Owing to its high theoretical capacity, significant research efforts have been devoted to developing high capacity anodes by either P-doping or direct incorporation of P allotropes

13,14.

Several recent studies have also attempted to obtain large-size exfoliated BP sheets and achieve high electrochemical performance of the electrodes made therefrom in LIBs using a convectional coin cell configuration 15,16. Some hybrid structures were also prepared as electrodes by assembling BP sheets with graphene oxide (GO) as conductive matrix and mechanical buffer to accommodate large volume changes during repeated cycles 17–19. Despite the abovementioned pioneering studies on flexible batteries prepared from 2D phosphorus anodes, their practical application has yet to be realized owing to several remaining challenges, as follows. (i) The lithiation mechanisms of BP/GO electrodes remain largely unexplored and little attention has thus far been paid to the oxidization of BP layers by GO, hindering further


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optimization of BP/GO for flexible LIBs. (ii) Little is known about the mechanical properties of BP/GO composite structure, which is crucial to the bendability of flexible batteries. (iii) There is lack of proper cathode materials and difficulty to maintain sufficient contact between the 2D phosphorus anode and the electrolyte upon lithiation. (iv) It is still challenging to assemble the active components into a battery case while maintaining both excellent bendability and complete sealing. Herein, we addressed all the above issues by a combination of theoretical calculations and experiments. It is surprising to reveal that the oxidation of BP surface by GO was found beneficial for enhanced electrochemical performance according to the density functional theory calculations. The underlying mechanisms responsible for superior Li storage were probed and the exceptional mechanical strength of the 2D phosphorus/GO paper was proven by both experiments and molecular dynamics simulations. These findings shed lights on further optimizing the BP/GO anode structure to construct a novel flexible LIB possessing an ultrahigh energy density. Finally, a prototype mechanically-robust quasi-solid-state flexible full cell was assembled for the first time using the optimized BP/GO paper anodes, a V2O5/CNT cathode and a gel polymer electrolyte to deliver an ultrahigh energy density of 389 Wh kg-1, demonstrating an excellent potential of 2D phosphorus electrodes for advanced flexible energy storage applications.

RESULTS AND DISCUSSION

Li storage mechanisms in the BP/GO heterostructure

Although phosphorous has a high theoretical specific capacity of 2596 mAh g-1 in LIBs, it cannot alone form an electrode due to its poor electronic conductivity and large volume expansion upon lithiation. Nanocarbon, one of the most common additives for anodes thanks to its high electrical conductivity and its capability to accommodate the volume expansion, is often added to form a


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composite with phosphorous to achieve enhanced electrochemical performance. Several pioneering studies have adopted this strategy to synthesize composite anodes consisting of BP and graphene, exhibiting enhanced electrochemical performance when testing in a half cell

17–19.

However, the BP/graphene electrode structure was not fully optimized for flexible battery applications due partly to the lack of understanding of Li storage mechanisms. In particular, the composite assembled with oxidized 2D BP and GO could have physicochemical properties completely different from their pristine forms. The heterostructure assembled from these oxidized components strongly correlates to the mechanical stability and Li diffusion kinetics of the electrodes, which has rarely been explored previously. In order to fully optimize the BP/GO electrode for flexible LIB applications, we carried out the Car-Parrinello MD simulations (CPMDS), revealing its Li storage mechanisms. Unlike conventional density functional theory (DFT) calculations, the CPMDS can effectively predict possible chemical reactions in the presence of oxygen. The initial setup of the CPMDS is shown in Figure 1a and the details of calculation are given in the supporting information. After the relaxation and production run for 4 ps, the layered structures of both GO and BP sheets remained intact without any phase transformation (Figure 1b). However, the P and C atoms presented significant displacements from their original positions (Figure 1a), which is largely different from the BP/neat graphene heterostructure without any oxygen

20.

The former composite displayed a lattice distortion arising from the chemical

interactions through the oxygenated functional groups (Figure 1c). The epoxy and hydroxyl groups on GO reacted with the P atoms of BP layer to form C-O-P bonds while C-P bonds were directly formed between some of the unbound C and P atoms. These atoms had to move from their original positions to balance the total force of the heterostructure so as to accommodate the chemical bond length differences, causing the distortion of crystal lattice. To further illustrate the electronic


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interactions between GO and BP, the charge density on a 2D cut-plane comprising the P-O-C bonds is plotted (Figure 1d). The atomic orbitals of C, P and O overlapped with each other like red P did

21,

generating a strong binding force that held the GO and BP layers together during the

lithiation/delithiation cycles of BP. Similar to the reaction between GO and S to form C-O-S bonds in lithium sulfur batteries (LSBs) 22, the chemical reaction taking place between the BP reductants and GO oxidants was responsible for the formation of P-O-C bonds where the bond-dissociation energy of P-O (2.76 eV) was much higher than that of C-O (0.54 eV) (Figure 1e). The abundant oxygenated functional groups on GO were easily attracted by the surface P atoms of BP due to the higher bond-dissociation energy of P-O, leading to the surface redox reaction to simultaneously form the P-O-C chemical bonds and partially reduce GO. The abovementioned chemical reactions can be represented by Equation 1 and 2

23

where the subscript p/r stands for partial reduction.

Thanks to the strong chemical bonds between them, the electrically conductive network can be well maintained despite the large volume change of BP during cycles, contributing to the superior cyclic performance of the BP/GO electrode 24,25. (1)

GO + BP→GOp/r + BP(OaHb) GOp/r + BP(OaHb) + nOH ― →BP/GO + cH2O

(2)


6

(h)

(i)

GO

HI reduced BP/GO

BP/GO

C-O-C C-OH 1049 1223

C=C 1622

1800

C-O-C 1078

Intensity (a.u.)

C=C 1637

P-O-C 1016

P-O 845/793

GO

Absorbance (a.u.)

C-OH 1259

(j) BP (002)

BP/GO

GO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1500

1200

Wavenumber (cm-1)

900

P2p3/2

P2p

P2p1/2

Intensity (a.u.)

Page 7 of 36

POx

HI reduced HI reducedBP/GO BP/GO

GO

5

10

15

20

25

o

 ( )

30

BP

135

132

129

126

123

Binding Energy (eV)

Figure 1. (a) Initial configuration and (b) optimized atomistic structure of BP/GO composite generated from CPMDS; (c) enlarged view of the interactions between BP and GO leading to the formation of P-O-C and P-C bonds; (d) map of electron density on a 2D cut-plane comprising the P-O-C bonds; (e) comparison between bond-dissociation energies of P-O and C-O; (f) calculated MSD and (g) the Arrhenius plot of Li atoms in the BP/GO structure at different temperatures. (h) FTIR and (i) XRD spectra of mildly reduced BP/GO, BP/GO and GO; and (j) deconvoluted XPS spectra of P2p peaks;


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Although densely packed graphene-based electrodes promise a high volumetric energy density, their practical electrochemical performance is limited by the sluggish Li diffusion through the electrode material 26–28. Surprisingly, the addition of BP into GO to form BP/GO composites with a firmly bonded interface significantly enhanced the Li diffusion kinetics. The diffusion coefficient of Li, DLi, through the BP/GO interface was calculated based on the Einstein relation of mean squared displacement (MSD) described by Equation 3, where r(t) is the displacement of Li atoms at time t 29. The diffusion barrier was also obtained using the Arrhenius equation given by Equation 4, where D0 is a pre-exponential factor, E is the diffusion barrier, KB is the Boltzmann constant and T is the temperature 30. DLi = lim { < |r(t)Li ― r(t = 0)Li|2 > /(6t)}

(3)

t→∞

―E

DLi = D0e

KBT

(4)

Four groups of MSDs at different temperatures were obtained by CPMD runs of the optimized BP/GO structure for 8 ps (Figure 1f). An ultralow diffusion barrier of 80 meV was determined by linear fitting of Equation 3, as shown in Figure 1g, which is known to be among the lowest for all anode materials for LIBs reported thus far (Table S2) and even lower than that (120 meV) of the BP/neat graphene without any oxygen

20.

Consequently, the Li ions experienced extremely fast

transmission through the BP/GO interface with an ultrahigh diffusion coefficient of 2.5×10-6 cm2 s-1 at room temperature, at least four orders of magnitude faster than DLi = 1.12 x 10-10 to 6.51 x 10-11 cm2 s-1 in graphite, typical anode material for LIBs 31. The chemical interactions between the BP sheets and the GO layers through the oxygenated functional groups were revealed in the above CPMDS. To verify the CPMDS results experimentally, the Fourier-transform infrared (FTIR) spectroscopy was carried out on the BP/GO composite and GO, as shown in Figure 1h. Their spectra revealed significant differences: new


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peaks corresponding to the P-O and P-O-C bonds emerged while all other peak positions for the C-O-C, C-OH and C=C bonds up-shifted in the BP/GO spectrum. The P-O-C bonds located at 1016 cm-1 played a key role in maintaining the structural stability of the BP/GO electrode during cycles, as proven previously for similar red P/carbon anodes

21,24,25,32.

The up-shifts of peak

positions are associated with the interactions between GO and BP 33,34. The peak located at 1622 cm-1 is attributed to the vibration of aromatic C=C bonds of GO, which up-shifted to1637 cm-1 for BP/GO as a result of the formation of chemical bonds between BP and GO changing the vibrational frequency of C=C bonds 35. The effect of interactions became more obvious for the hydroxyl (peak at 1223 cm-1) and epoxy groups (peak at 1049 cm-1) attached on the GO surface, making them easier to vibrate than the aromatic C=C bonds to trigger up-shifts of these peaks for the same reasons. The formation of P-C bonds was also revealed by the Raman spectrum (Figure S1). A broad peak emerged between 600 and 700 cm-1, while the G band up-shifted to 1600 cm-1, both indicating the formation of P-C bonds 36. In addition to the formation of chemical bonds, the expansion of interlayer spacing of GO was also confirmed by the X-ray diffraction (XRD) measurement, as shown in Figure 1i. The characteristic peak of GO paper centered at 9.6 down-shifted to 8.1 after the addition of BP. 2θ = 9.6 corresponds to an interlayer d-spacing of 0.92 nm 37, which was expanded to 1.09 nm because of the interaction between GO and BP with increased disorders in the GO layers 38. The BP/GO paper was further reduced under a mild condition using hydrogen iodide (HI) to enhance its electrical conductivity while preserving the original morphology for flexible battery applications. After the reduction, the paper exhibited a shiny silver color, indicating successful reduction of GO 39. The d-spacing of BP/GO paper after reduction shrank back to 0.40 nm with an accompanied peak shift to 22.3. This observation arose from the removal of both residual water molecules and part of


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oxygenated functional groups from the GO paper

38,40,

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while the BP-GO interactions persisted

even after the mild reduction to maintain a slightly enlarged d-spacing. The complete disappearance of GO peak indicates both the surface and the interior of GO film have been reduced by HI vapor, which agrees well with a previous report 41. The conclusions drawn from the FTIR and XRD analyses were verified by the X-ray photoelectron spectroscopy (XPS) which is very sensitive to the possible interactions even after the reduction of GO. Figure 1j shows the deconvoluted P2p spectra that had almost identical P2p1/2 and P2p3/2 peaks in both the reduced BP/GO and BP. However, a new peak POx centered at 133.6 eV emerged in the reduced BP/GO, a reflection of the formation of P-O-C bonds 24, consistent with the result from FTIR. It is worth noting that although GO was mildly reduced by HI vapor, its interactions with BP through the oxygenated functional group remained active as confirmed by the XPS results. As such, unless otherwise specified, the mildly reduced electrode is labelled as BP/GO in the following. Mechanical properties of the BP/GO paper Both high tensile strength and high modulus of freestanding electrodes are vital for stable cyclic performance of flexible batteries under mechanical deformation 42. The mechanical properties of GO papers were evaluated in our previous study 37. However, the incorporation of BP layers into the GO papers with chemical bonds between them may significantly alter the mechanical properties of the composites, necessating to probe the strengthen or softening mechanisms. A BP/GO model was built using the optimized geometry obtained from the CPMDs while a vacuum slab was added in the simulation box to avoid self-interaction (Figure 2a). The left edge of the box was fixed while a constant strain was applied to the right edge, followed by full relaxation of the structure under the canonical ensemble at 300 K (Figure 2b). The simulated stress-strain curves of the BP/GO and GO papers were obtained by calculating the virial stress, σij, similar to previous


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studies 37,43, generating relatively accurate results. Both the BP/GO and GO papers presented an approximately linear elastic stress-strain relationship, as shown in Figure 2c, where the modulus of the former paper was eight times the latter paper, i.e. 107.7 vs 13.2 GPa. It should be noted that the moduli of GO papers normally decreased upon introducing a second phase, such as MoS2 and carbon nanofibers

44,

42

partly because of lack of strong interactions between them unable to

tighten the interlayer spacing. In sharp contrast, a much enhanced modulus was achieved when the GO paper was interpolated by 2D BP sheets thanks to the strong P-O-C bonds formed between them, as schematically illustrated in Figure 2d.

Unload

(a)

(b)

(c)

7

GO

6

BP/GO

5

Load

Stress (GPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 % Strain

4 3 2 1

Applied Force

0 0

2

4

6

Strain (%)

(d)

GO sheets O

O

H

H

O

O

O

O BP sheets

O

COOH

HOOC O

O

O

H O

O BP sheets

O

COOH

HOOC O

O H O

Figure. 2 (a) Top views of the BP/GO model under 0 and 5 % strain before production run; (b) top and cross-sectional views of the optimized BP/GO model under 5 % strain by MD simulations; (c) simulated stress-strain curves of GO and BP/GO papers. (d) schematic showing crosslinking of GO and BP sheets through P-O-C chemical bonds.


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To verify the MD simulation results, the mechanical properties of BP/GO and GO papers were measured in uniaxial tension of 25 mm long x 8 mm wide rectangular samples

37.

The BP/GO

composite paper was fabricated by mixing the exfoliated BP sheets sorted by centrifugation with GO dispersion, followed by sonication and vacuum filtration, as shown in Figure 3a. More details of the synthesis of BP/GO and the characterization are described in the supporting information (Figure S2-6). The stress-strain curves of the papers prepared from BP/GO, GO and BP are plotted in Figure 3b. It should be noted that the thickness of BP/GO paper was slightly larger than the GO paper, which had a negligible influence on measured stress 45. The BP paper had a very low tensile strength and Young’s modulus representing weak van der Waals interactions between the BP nanosheets. The presence of hydrogen bonds among GO sheets endowed the GO paper with a higher tensile strength and Young’s modulus than the BP counterpart. The composite paper consisting of BP and GO sheets exhibited a strong synergistic effect giving rise to significant increases in both strength and modulus. It is worth noting that the stress-strain curves of both BP/GO and GO were comprised of bi-linear regions (Figure 3c and d): the first linearity (E1) is ascribed to flattening of wrinkles in the GO sheets and overcoming the interactions between residual solvent molecules and GO, while the second linearity (E2) arises mainly from the elastic deformation of the paper 43. Both the moduli of the BP/GO paper were much higher than those of the GO paper, i.e. E1 = 9.7 vs 5.3 GPa and E2 = 19.2 vs 12.1 GPa, respectively. The abovementioned measurements were conducted on the pristine BP/GO papers. A marginal decrease in modulus (19.2 GPa vs 18.3 GPa) was noted after reduction, as shown in Figure S7, indicating the interlocking structure of GO being maintained during the mild reduction by HI. It is worth noting that the models used in MD simulations contained BP sheets of the same size and uniformly placed between the GO layers. As a result, the modulus enhancement predicted by the


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MD simulations tended to be greater than that from the experimental measurements. Moreover, the MD simulations showed approximately linear stress-strain curves due to the absence of solvent molecules between the GO layers, consistent with the experimental measurement of the vacuum dried and reduced BP/GO (Figure S7). Nevertheless, both the experiments and simulations revealed a consistent result with much enhanced mechanical properties of BP/GO papers compared to GO papers. Apart from the tensile strength and modulus, the durability of BP/GO electrode upon bending cycles was also measured to evaluate its long-term flexibility in service as flexible battery electrode. The normalized electrical resistance change, R/R0, presents a negligible variation for 200 cycles (Figure 3e) of 120 folding and unfolding (Figure 3f), a reflection of excellent mechanical flexibility and durability while maintaining the conductive pathway intact.


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(a) Sonication Filtration

5 µm

40 20

0.1

0.2

0.3

0.4

0.5

E2 = 19.2 GPa

60 40 20

E1 = 9.7 GPa

0 0.0

0.6

0.1

0.2

Strain (%)

(e)

60

GO paper

80

60

0 0.0

(d)

BP/GO paper

Stress (MPa)

Stress (MPa)

80

(c)100

BP GO BP/GO

Stress (MPa)

(b)100

0.3

0.4

0.5

Strain (%)

20

0 0.0

0.6

E2 = 12.1 GPa

40

E1 = 5.3 GPa

0.1

0.2

0.3

0.4

0.5

0.6

Strain (%)

(f)

Flat

12

Bent

9 6

R/R0 %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

BP/GO Paper

0 -3

120 o

-6 -9 -12

BP/GO paper 0

20

40

60

80

100

120

140

160

180

200

Bending Cycle

Figure. 3 (a) Schematic representation of the synthesis of flexible BP/GO electrodes; (b) Stressstrain curves of BP/GO, GO and BP papers obtained from uniaxial tensile tests; moduli of (c) BP/GO and (d) GO papers; (e) Normalized electrical resistance change, R/Ro, of BP/GO electrode measured by folding/unfolding cycles. R and Ro refer to the resistances measured after and before bending, respectively, and the resistance change, ΔR = R − Ro; (f) Schematic of straight and bent BP/GO paper electrodes. Electrochemical analyses Highly flexible BP papers fabricated in a scalable manner using the exfoliated, few-layer BP sheets are ideally suited for high-energy flexible anodes. In order to optimize the electrochemical performance of BP/GO papers, three different BP to GO precursor ratios of 1:1, 2:3 and 1:2 were studied, and the results are shown in Figure 4d-e and Figure S7. The pristine BP/GO suffered from


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an extremely poor electrical conductivity, causing almost negligible Li storage capacities (Figure S8). After reduction by HI, the galvanostatic charge/discharge (GCD) curves of the freestanding BP/GO electrode against the Li counter electrode present two plateaus at ~ 0.7 and 0.4 V (Figure 4d) during lithiation, which are attributed to two-step alloying reactions between P and Li to form LiP and Li3P, respectively.16 The plateaus became less evident with increasing current density due to the inevitable concentration polarization. A high fraction of graphene is crucial to maintaining structural stability and a high electrical conductivity of the composite electrode during cycles. However, a low BP fraction at a BP/GO ratio of 1:2 sacrificed the specific capacity of the anode because the BP sheets were the predominant source of lithium storage capacity. An optimized electrochemical performance was achieved at a BP/GO ratio of 2:3 (Figure S9), which is equivalent to  50 wt% BP in the reduced BP/GO composite according to the TGA results of BP, BP/GO (Figure S10) and GO 37, where GO lost ~ 33 % of its weight upon reduction to graphene. The optimized BP/GO electrode delivered a high reversible capacity of 737 mAh g-1 at a current density of 0.5 A g-1, while it retained an excellent capacity of 477 mAh g-1 after 500 cycles with Coulombic efficiencies as high as 99.6 % (Figure 4e). In sharp contrast, the electrode with a high BP/GO ratio of 1:1 (Figure S9b) exhibited drastic capacity fade with a remaining capacity of 189 mAh g-1 after 500 cycles because the high BP content caused a large volume change and potential structural failures during the prolonged cycles 46, offsetting the possible capacity gains from the extra active materials.


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(b)1200 2

3.0

1

0.5

-1

0.2 0.1 A g

2.5 2.0 1.5 1.0 BP/GO = 2:3

0.5 0.0

0

200

400

600

800

100

0.2 A g-1

1000

BP/GO = 2:3 BP/G = 2:3

800

60

600 40

400

20

200 0.5 A g-1

0

1000

0

100

200

-1

Specific capacity (mAh g )

(c)

80

Efficiency (%)

3.5

Dischage Capacity (mAh/g)

(a)

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cycle

300

400

0 500

(e)

(d) A C

B

A C B

Figure. 4 (a) GCD curves of the BP/GO anode in a Li-ion half-cell measured at different current densities; (b) comparison of cyclic performance between the optimized BP/GO and BP/neat graphene anodes; (c) EIS of the BP/GO and BP/neat graphene electrodes; (d) dx/dV curve derived from the GCD curve of the BP/GO electrode at 0.1 A g-1; (e) CV curve of the BP/GO electrode.

Phosphorous is notorious for its large volume expansion, leading to fast capacity fade upon lithiation and delithiation cycles 47,48. Assuming full lithiation of phosphorous to form Li3P (Figure S11), the theoretical volume expansion of neat BP was estimated to be 179 % according to a simple calculation using the lattice parameters of the components obtained from the materials genome approach (Table S1). The volume expansion of the BP/GO electrode with an optimized BP/GO ratio of 2:3 upon full lithiation was also estimated using the ex situ SEM images (Figure S12). The electrode thickness was enlarged by ~ 52 % due to both the formation of SEI layer and the volume expansion of BP, whereas there were negligible size changes of the electrode in the plane directions


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(inset of Figure S12). The much smaller volume change of the BP/GO electrode than the neat BP signifies the importance of GO sheets in accommodating the volume expansion, an ameliorating attribute essential for enhanced cyclic performance of the battery. In particular, the oxygenated functional groups inherent on GO played a vital role in maintaining stable cycles of the composite electrode. To study the effect of oxygenated functional groups, BP/neat graphene electrodes were prepared by replacing GO with pristine graphene having no oxygen groups while the other processing conditions were kept unchanged. The BP/neat graphene papers presented a much rougher surface with a more loosely-packed structure (Figure S13) than the BP/GO counterpart because of the lack of hydrogen bonds between the composite components 49. More importantly, the BP/neat graphene electrode experienced rapid capacity degradation within 50 cycles (Figure 4e) whereas the BP/GO electrode showed very stable capacity performance for 500 cycles. To verify the enhanced Li+ diffusion kinetics by oxidation of 2D phosphorus, the electrical impedance spectroscopy (EIS) of BP/GO was measured. For a thin film electrode such as BP/GO, the diffusion coefficient of Li+, DLi, was calculated based on the Fick’s second law according to Equation 5 50, where Rw is the Warburg resistance obtained from the curve fitting of EIS results with an equivalent circuit, VM is the molar volume of BP, a is the active area of the electrode, z is the charge transfer number of lithium, F is the Faraday constant, and L is the thickness of the BP/GO film. (dV/dx) is the derivative of voltage curve, which usually has a similar shape as the CV curve. We calculated the (dV/dx) curve based on the GCD curve of BP/GO (Figure 4a) and the result is plotted in Figure 4d. As a reference, the CV test was also carried out of the BP/GO electrode and the resultant curve is shown in Figure 4e. Three prominent peaks were identified in both the (dV/dx) and CV curves which are labelled as peaks A, B and C. The potential values corresponding to these peaks in both the dV/dx and CV curves agree well with each other, where


17


Page 18 of 36

the peaks A and B represent the two-step alloying reaction between BP and Li and the peak C is ascribed to the Li intercalation into GO. 𝑉𝑀 𝑑𝑉

𝐿

𝑅𝑤 = 𝑧𝐹𝑎( 𝑑𝑥 )(3𝐷𝐿𝑖)

(5)

The DLi values of the BP/GO and BP/neat graphene electrodes were calculated to be 6.6110-11 and 8.1910-12 cm2 s-1, respectively. It is noted that DLi of BP/GO was one to two orders of magnitude higher than that of nano silicon (~ 10-12 cm2 s-1)

51,

signifying its exceptional Li

diffusion kinetics. It is worth noting that the DLi values measured by EIS is likely much lower than that obtained by the CPMDS. This is because the former DLi value by EIS experiments takes into account the interfacial resistance between BP/GO and the electrolyte and the defects formed during the fabrication of practical BP/GO electrodes containing microscale GO sheets, whereas the latter DLi value is solely based on the diffusion through the defect-free BP/GO model containing nanoscale GO sheets. As a result, the overall DLi value measured by the EIS is usually several orders of magnitude lower than the theoretical calculation or the direct measurement using a nanobattery 52. It is also noted, however, that the measured DLi value of the BP/GO electrode was ~ 8 times higher than the BP/neat graphene electrode, indicating the positive role of oxygen on enhancing the diffusion kinetics of BP.

Practical application for bendable battery Thanks to its flexibility and high specific capacity, the BP/GO composite was able to make an excellent anode for flexible batteries requiring high energy densities. A prototype flexible battery was assembled using the BP/GO anode, a V2O5/CNT composite as the cathode and poly (vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) soaked in standard ester-based electrolyte as the gel polymer electrolyte. The working mechanism of the flexible battery was


18

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essentially identical to ordinary LIBs with the exception of all three components being flexible and the ion transportation being maintained by the quasi-solid-state electrolyte. It was very difficult to identify an appropriate cathode material which possessed both a high energy density and excellent flexibility. After extensive trial and error experiments, V2O5 was chosen as the cathode material in this study. The bulk V2O5 powder was dissolved and re-crystalized to grow long V2O5 nanowires 53, which were intertwined to form a freestanding and flexible web, as shown in Figure 5a. Multi-walled CNTs were evenly distributed within the V2O5 web to enhance the electrical conductivity of the electrode (Figure 5b). The single-crystal V2O5 nanowires had a diameter ranging 50-150 nm (Figure 5b and Figure S14), facilitating short diffusion paths for Li+ ion transfer for enhanced rate performance of the cathode. The electrochemical performance of the V2O5/CNT cathode was evaluated using lithium foil as the counter electrode. Multiple pairs of peaks emerged in the CV curves (Figure S15), which are attributed to the multi-stage intercalation of Li+ ions into the lattice of V2O5 crystals 54. All these peaks were approximately symmetrical and the CV curves overlapped with each other for all three initial cycles, a testament to superior cyclic stability of the electrode 55. The positions of CV peaks agreed well with the plateaus of GCD curves (Figure S16a). The V2O5/CNT cathode delivered an excellent specific capacity of 332 mAh g-1 at 0.1 A g-1 as well as high capacity retention of 94.1 % after 100 stable cycles with a Coulombic efficiency of nearly 100 % (Figure 5c and Figure S16b). In addition to the V2O5/CNT cathode and the BP/GO anode, a polymer electrolyte was chosen to ensure the required flexibility and stable Li+ ion diffusion pathways while avoiding the leakage of electrolyte during cycles. A facile and scalable electrospinning method was used to produce PVDF-HFP nanofibers 56, to which common liquid electrolyte was fully permeated to allow both good mechanical strength and flexibility


19


Page 20 of 36

(Figure 5d, e). The porous structure with a large surface area of fiber mats (Figure S17) was aimed at ensuring the formation of gel polymer by taking up a large amount of liquid electrolyte.

Figure 5. (a) Low and (b) high magnification SEM images and inset (a) digital photograph of the flexible V2O5/CNT cathode; (c) rate performance of the V2O5/CNT cathode in a Li-ion half-cell; (d) SEM image showing the porous structure of electrospun PVDF-HFP fiber mat; (e) digital photographs of PVDF-HFP in dry and wet states (soaked in common liquid electrolyte).

The structure of the assembled flexible battery is schematically shown in Figure 6a. The gel polymer electrolyte was sandwiched between the V2O5/CNT cathode and the BP/GO anode, and the whole assembly was fully masked by polypropylene/polydimethylsiloxane (PP/PDMS) films as the protective cover to prevent the moisture and oxygen from getting into the battery. It should be mentioned that both the cathode and anode were lithium-free in their initial states and thus a facile pre-lithiation method was used to introduce lithium into the BP/GO anode 57. The battery


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was assembled and sealed in an Ar-filled glove box and operated in the ambient conditions. The battery exhibited excellent functional stability with almost no voltage drop upon folding to 90 degrees (Figure 6b), while it maintained uncompromised electrochemical performance with an almost constant output power to light a red light-emitting diode (LED) in bending (Figure 6c). Furthermore, the battery can be integrated into flexible circuits for specialty applications. In this study, four red LED lights were coupled with resistors and soldered onto a flexible circuit to demonstrate a flexible display with which the flexible battery was integrated to serve as the power source (Figure S18). The output power from the flexible battery successfully lit four LED lights both under flat and bent conditions, as shown in Figure 6d. There was no visible degradation of the intensity of the LED lights upon bending, indicating excellent mechanical and functional stability of the flexible battery. To quantitatively evaluate the electrochemical performance of the flexible battery, the GCD test was carried out under flat and bent conditions for 50 cycles each at a current density of 0.2 A g-1. The gravimetric energy density of the battery was calculated using the total mass of cathode and anode 54 and the GCD curve of the flexible battery is shown in Figure S19. The battery delivered an initial gravimetric energy density of 389 Wh kg-1 with an energy retention of 92.3 % after 100 cycles, which is among the highest for various film-type flexible energy storage devices reported in the literature (Figure 6e and Table S3). The ultrahigh energy density of the flexible battery makes it a promising candidate for powering electronics that require a continuously stable and high power supply, such as flexible display 55. Thanks to the flexibility of the BP/GO electrode, the current battery did not suffer performance reduction when the battery was tested in bending as well as under the flat condition. Both the V2O5/CNT cathode and the BP/GO anode had considerably high densities, thus ensuring a high volumetric energy density of the battery which


21


is seldom achieved by other types of electrode materials. The volumetric energy density was estimated using the bulk densities of both cathode and anode without taking into account their volume expansion after lithiation. The flexible battery developed in this work delivered a remarkable volumetric energy density of 498 Wh L-1, which is the highest among all the flexible energy storage devices reported thus far.

(a)

4.0

PDMS & PP

3.5 3.0

Al tap

2.5

Voltage (V)

V2O5/CNT PVDF-HFP BP/GO

2.0 1.5 1.0

Ni tap

PDMS & PP

0.5 0.0 0

Flat: 3.485 V

Fold: 3.484 V

-1

-1

-1

0.2 A g

750 400 600

Flat

300

Bent

200

0

450 300

100

150

0

25

50

75

Volumetric energy density (Wh L )

(d) 500

1

(c)

0 100

Gravimetric Energy Density (Wh kg-1)

(b)

Gravimetric Energy Density (Wh kg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

Cycle Number

2

3

Time (h)

4

5

(e) 400

Flexible Display

300

200

Current study Ref S15 Ref S16 Ref S17 Ref S18 Ref S19 Ref S20 Ref S21

100

0 10

100

Wearable Sensor 1000

10000

Cycle Number

Figure 6. (a) Schematic of the flexible battery structure consisting of a V2O5/CNT cathode, a BP/GO anode and a PVDF-HFP electrolyte; (b) photographs showing the negligible change in open-circuit voltage of the assembled flexible battery before and after folding; (c) flexible battery powering a LED before and after bending; (d) cyclic gravimetric energy density performance of


22

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the flexible battery at a current density of 0.2 A g-1 and the flexible displays consisting of an array of red LED lights powered by the flexible battery before and after bending (inset); and (e) comparison of electrochemical performance among various film-type flexible energy storage devices.

CONCLUSION In summary, the chemical structure, Li+ ion diffusion kinetics, mechanical properties and electrochemical performance of the 2D phosphorus anode in the presence of GO were systemically probed by both theoretical calculations and experiments. It is revealed that the Li+ ions diffused through the interface between oxidized 2D phosphorus and GO at an extremely high rate of 2.5×106

cm2 s-1 and an ultralow energy barrier of 80 meV, which is surprisingly much faster than in the

non-oxidized phosphorus/neat graphene structure. Furthermore, the mechanical strength and modulus were greatly enhanced due to the C-O-P chemical bonds taking place between the phosphorous and GO in the presence of oxygen, which endowed the composite electrode with excellent structural and performance stability. A quasi-solid-state, flexible battery was assembled using the 2D BP/GO anode, the V2O5/CNT cathode and the custom-made, porous polymer fiber web electrolyte. It delivered remarkable gravimetric and volumetric energy densities of 389 Wh kg-1 and 498 Wh L-1, respectively, demonstrating practical application of oxidized 2D phosphorus for flexible energy storage requiring high energy densities. We believe that the findings presented here have significant implications in developing high-performance anodes using 2D phosphorus for flexible battery applications.


23


Page 24 of 36

EXPERIMENTAL SECTION Assembly of flexible battery The flexible battery was assembled using a V2O5/CNT cathode, a BP/GO anode and a gel polymer fiber web as both the electrolyte and separator. The V2O5/CNT and BP/GO papers were cut into 20 mm × 20 mm square pieces. Before assembling the flexible battery, the BP/GO anode was prelithiated by short circuiting BP/GO with electrolyte-wetted Li foil. The battery components were stacked together and sealed between the two half-cured PDMS coated PP films in an Ar-filled glove box. The battery was charge/discharge tested for one cycle to activate the electrodes and release any generated gases. After discharge, the flexible battery was stored inside the glove box for 24 h to fully cure the PDMS films. Finally, the edges of the PP films were sealed by plastic welding before removing the flexible battery from the glove box. Materials characterization The morphologies and atomic structures of BP, BP/GO paper, V2O5/CNT paper and PVDF-HFP nanofibers were characterized by optical microscopy (OM, BH2-MJLT, Olympus), scanning electron microscopy (SEM, JSM-7100F, JEOL) and transmission electron microscopy (TEM, JEM-2010, JEOL). The interlayer d-spacings of GO and BP/GO papers were examined by thin film X-ray diffraction analysis (XRD, PANanalytical) with a Cu Kα radiation from 5° to 30°. Their surface chemistries were studied by X-ray photoelectron spectroscopy (XPS, Surface analysis PHI5600, Physical Electronics). The chemical bonds in these materials were identified by attenuated total reflection-Fourier transform infra-red spectroscopy (ATR-FTIR, Vertex 70 Hyperion 1000) and Raman spectroscopy (InVia, Renishaw). The nitrogen adsorption/desorption isotherm curves were measured at 77 K on an automated adsorption apparatus (Micromeritics, ASAP 2020). The surface area was determined using the Brunauer–Emmett–Teller (BET) method.


24

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The thickness of few layer BP sheets was measured by atomic force microscopy (AFM, Dimension 3100, Digital Instruments). The BP content in the HI reduced BP/GO was determined by thermogravimetry analysis (TGA, Q5000) in argon from 25 to 600 °C at a heating rate of 2 °C min-1. The BP/GO electrode and the flexible battery were bent on a universal testing machine (MTS Alliance RT/10) at a crosshead speed of 5.0 mm min−1 while their electrical resistance was continuously monitored using a digital multimeter (34970A Data Acquisition/Data Logger Switch Unit, Agilent). Electrochemical tests The electrochemical performance of the BP/GO anode was evaluated using CR2032 half-cells which were assembled with Li foil as the counter electrode, Celgard 2400 as the separator and 1 M LiPF6 dissolved in ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) (1:1:1 vol %) with 1 wt % of vinylene carbonate (VC) as the electrolyte. The freestanding BP/GO paper was directly used as the working electrode without a binder or current collector. The same method was used to test the electrochemical performance of the V2O5/CNT cathode. The GCD tests of the half-cells were conducted on a battery testing system (Land 2001CT) at potentials ranging 0.005-3.0 V for the BP/GO anode and 1.5-4.0 V for the V2O5/CNT cathode at different current densities. The CV tests were performed at a scan rate of 0.1 mV s-1 with a voltage range of 1.5-4 V for V2O5 and 0-3 V for BP/GO on an electrochemical workstation (CHI 660C). The EIS measurements were carried out on the same workstation with a frequency range of 10-2-105 Hz. The GCD tests were conducted to evaluate the electrochemical performance of flexible batteries at potentials ranging 1.0-4.0 V under both the flat and bent conditions. The energy densities (E) were calculated by integrating the galvanostatic curves according to Equation 6, where I refers to


25


Page 26 of 36

the current density, V is the voltage, t is the time for a charge/discharge cycle. The gravimetric energy density was determined by the ratio of energy density to the total weight of cathode and anode while the volumetric energy density was obtained by multiplying the gravimetric energy density with the average density of anode and cathode. t2

E=

∫ IVdt

(6)

t1

ASSOCIATED CONTENT Supporting Information The experimental methods, XRD patterns, TEM images, and additional electrochemical data are reported in Supporting Information. This file is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *J.-K. Kim. E-mail address: [email protected]. Tel: +852 2358 7207, Fax: +852 2358 1543.

ACKNOWLEDGEMENTS This project was financially supported by the Innovation and Technology Commission (ITS/001/17) and the Research Grants Council (GRF Projects: 16212814, 16208718) of Hong Kong SAR. The authors also appreciate the technical assistance from the Materials


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Ultrafast Li+ Diffusion Kinetics of 2D Oxidized Phosphorous for Quasi

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