Bottom-Up Electrochemical Deposition of Poly(styrene sulfonate) on

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Bottom-up electrochemical deposition of poly(styrene sulfonate) on nano-architectured electrodes Michele Braglia, Ivan Vito Ferrari, Thierry Djenizian, Saulius Kaciulis, Peiman Soltani, Maria Luisa Di Vona, and Philippe C Knauth ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04335 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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ACS Applied Materials & Interfaces

Bottom-up electrochemical deposition of poly(styrene sulfonate) on nano-architectured electrodes Michele Braglia1,2,3, Ivan Vito Ferrari2,1,3, Thierry Djenizian4, Saulius Kaciulis5, Peiman Soltani5 , Maria Luisa Di Vona2,3*, Philippe Knauth1,3* 1

Aix Marseille Univ (AMU), CNRS, Madirel (UMR 7246), Electrochemistry of Materials Group, site St Jérôme, 13397 Marseille, France

2

University of Rome Tor Vergata (URoma2), Department of Industrial Engineering, Via del Politecnico 1, 00133 Rome, Italy

3

International Associated Laboratory (L.I.A.), Ionomer Materials for Energy (AMU, CNRS, Uroma2), France, Italy 4

Ecole Nationale Supérieure des Mines de Saint-Etienne, Flexible Electronics Department, Center Microelectronics Provence, 13541 Gardanne, France 5

Institute for the Study of Nanostructured Materials, ISMN – CNR, P.O. Box 10, 00015 Monterotondo Stazione, Rome, Italy

*Coordinating authors: [email protected] , [email protected]

Abstract The cathodic deposition of poly(styrene sulfonate) on nano-architectured TiO2 electrodes is explored by cyclic voltammetry and potentiostatic and galvanostatic experiments, showing a diffusion-controlled deposition described by Cottrell’s law. The structure and composition of the polymer is evidenced by various spectroscopic techniques, including NMR, FTIR and XPS and its morphology is studied by SEM. The average chain length can be estimated from the NMR spectra. The electropolymerisation mechanism initiates by radical anion formation. The cycling behaviour in half-cell batteries against Li metal is excellent, especially at high rates explored up to 10 C. The areal insertion capacity is above recent literature results, up to 80 𝛍Ah cm-2. The combination of normalized areal power density and areal energy density is one of the best reported in literature. Keywords: Li battery, TiO2 nanotubes, solid electrolyte, aromatic polymer, PSS, insertion capacity

ACS Paragon Plus Environment


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Introduction Recent safety incidents of mobile telephones underline the need for solid electrolytes in high performance batteries. In principle, the advantages offered by solid electrolytes

1-3

include i)

simpler device design and fabrication (facile shaping, patterning and integration); ii) easier miniaturization and no need for liquid containment; iii) better resistance to changes in conditions (e.g. shock vibration or temperature-pressure variation); iv) stability (non-volatile) and safety (non-flammable). Among the requirements that must be reached for high performance battery operation, one can mention a high ionic conductivity (∼10-3 S/cm at room temperature), negligible electronic conductivity (negligible internal short-circuit) and sufficient mechanical stability (ceramics are generally fragile) 2. The interface compatibility with electrode materials 1 (e.g. large contact area and similar thermal expansion coefficients to avoid delamination) and a high chemical and electrochemical stability (wide potential window) to avoid reductive or oxidative decomposition are further necessities. Solid Electrolyte Interphase (SEI) passivation layers can protect against decomposition, but they need certain electrolyte compositions to form spontaneously. Among the possible solid electrolytes, inorganic solids include perovskites 4-5, NaSICON phases 6

, garnets 7, LiSICON

8

and thio-LiSICON 9, argyrodites 10, complex hydrides 11, composites 12,

glass ceramics 13, and glasses 14-15. The idea of polymer electrolytes was originally proposed by Wright and Armand 16. The initial system, which is still very much investigated, relied on aliphatic polyethers, especially poly(ethylene oxide) (PEO 17). These polymers form gels, which guarantee high ion conductivity, helped by flexible chain motions, and good adaptation to the electrode interface. However, they present two significant disadvantages: i) the soft polymer structure does not impede the formation of Li dendrites on recharge, which is a significant safety issue, ii) anions have generally a higher transference number than Li cations, due to the strong affinity of Li+ to ether groups, which leads to unwanted polarization phenomena on charge. Crystalline PEO shows instead a better mechanical resistance, but is unable to reach the required conductivity 18. Two important improvements are thus to guarantee a higher mechanical stability by using a stiffer polymer backbone and to immobilize the anions by grafting them on the macromolecule. Some efforts have been made in these directions. Bouchet and coworkers proposed block copolymers where polystyrene blocks reinforced the PEO-based chains; bis(sulfonyl)imide


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anions were grafted on the polystyrene blocks 19. Another approach was based on the grafting of sulfonimide groups directly on relatively stiff and strong aromatic polymers 20. In microbatteries, the requirements are even more stringent, because the thin electrolyte layer must present a sufficient mechanical stability to resist Li dendrite growth. We have developed in recent years microbatteries based on titanium dioxide nanotubes (TiO2nt) that present an excellent insertion capacity and cycling performance

21-22

. We have prepared a thin solid

electrolyte layer of PEO reinforced with PMMA using a PEO-MMA precursor 23-24. In this work, we show that electrodeposition of polymer electrolytes used to cover planar electrode surfaces

25-26

can be applied to form a continuous layer on nanotubular titania

electrodes. The electropolymerisation of styrene

27

and styrene sulphide

28

have been studied

before. The one-pot deposition of poly(styrene sulfonate) (PSS) directly on a complicated electrode morphology opens new perspectives for the realization of intricate electrode architectures. We study the electrochemical deposition by cyclic voltammetry and by galvanostatic or potentiostatic experiments. The composition of the polymer layer is investigated by various spectroscopic techniques, including Nuclear Magnetic Resonance (NMR), Infrared (FTIR), X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoemission Spectroscopy (UPS); its morphology was observed by Scanning Electron Microscopy (SEM). The cycling of microbattery half-cells containing TiO2nt and PSS polymer electrolyte vs Li metal was investigated at various current densities. Experimental 2.1. Fabrication of TiO2 nanotubes TiO2nt were fabricated by anodization of a commercial Ti foil (Goodfellow) in a glycerol solution containing 96.7 wt% Glycerol, 1.3 wt% NH4F and 2 wt% water. The Ti foil was previously cleaned and sonicated in acetone, propanol and methanol for ten minutes each. The anodization process was carried out by means of a generator (ISO-TECH IPS-603) which applied 60 V for three hours on a two-electrode electrochemical cell, consisting of a Pt foil as counter and the Ti foil as working electrode. During the whole process, the glycerol solution was slightly stirred (100 rpm) to promote a better morphology and overall quality of the nanotubes. After the anodization, the self-supported TiO2nt were abundantly rinsed with deionized water and dried at 80 °C under vacuum for two hours. Finally, the nanotubes were annealed at 450 °C for three hours to get the crystalline anatase phase 22.



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2.2. Electropolymerization Commercial 4-styrenesulfonic acid sodium salt hydrate (Sigma-Aldrich) was used as precursor without further purification. The electropolymerization was achieved at room temperature in a three-electrode set-up made up of the bare TiO2nt (working electrode, A = 0.5 cm2) facing a Pt counter electrode and an Ag/AgCl reference electrode. During the process the three electrodes were immersed into a solution containing 0.1 M of the precursor, 0.5 M of lithium bis(trifluoromethane)sulfonimide (LiTFSI, Sigma-Aldrich) as supporting electrolyte and dimethylsulfoxide (DMSO) as solvent. The polymerization process was extensively investigated by Cyclic Voltammetry (CV), exploring various potential ranges between -0.9 and -2.1 V vs Ag/AgCl, and by potentiostatic and galvanostatic measurements. For CV, the working electrode was polarized at various constant potentials with the best results obtained at -1.4 V vs. Ag/AgCl. The potentiostatic and galvanostatic experiments were conducted at a constant current of 0.1 mA. After the electropolymerization, the TiO2nt/PSS samples were dried under vacuum at 80 °C for 24 h to remove traces of any residual solvent. A potentiostat/galvanostat (BioLogic VP300) was used for all electrochemical tests and for electrochemical impedance spectroscopy (EIS). The impedance spectra were recorded at open circuit potential between 1 Hz to 1 MHz with an oscillating voltage amplitude of 20 mV. All experiments were carried out at room temperature and atmosphere. 2.3. Spectroscopic and electron microscopic analysis 1

H NMR spectra were recorded with a Bruker Avance 300 spectrometer operating at 300.13

MHz. Chemical shifts (ppm) are referenced to tetramethylsilane (TMS). The measurements were performed on solutions made by direct immersion of the TiO2nt/PSS samples in D2O. The experiments were conducted for 16 h to get a sufficient signal/noise ratio; the acquisition time was 10 s. FTIR spectra were recorded directly on TiO2nt / PSS in reflectance mode from 4000 to 400 cm

1

with a Bruker Equinox 55 spectrometer. A background spectrum was run and sample

spectra were normalized against the background spectrum. The surface analyses (XPS and UPS) were carried out by using an electronic spectrometer Escalab 250Xi (Thermo Fisher Scientific, East Grinstead, UK) with monochromatic Al K source and a charge compensation system composed of ion and electron flood sources. XPS


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spectra were collected at 20 eV pass energy of the analyzer in standard mode of electromagnetic lenses, which corresponds to about 0.9 mm diameter of analyzed surface area. UPS measurements of the valence band were carried out by using the He source tuned to He I (21.2 eV) or He II (40.8 eV) excitation lines. All experimental data were processed by using the software Avantage v.5 (Thermo Fisher Scientific). The scale of the binding energy (BE) was calibrated with a precision of ± 0.05 eV, whereas the relative errors in XPS quantification of elemental concentrations were below 10 % of the determined values. The Scanning Electron Microscope images were recorded using a Philips FEG XL30 operated at 5 kV. For the cross-sectional observations, the samples were cut prior to observation under liquid nitrogen cooling. 2.4. Battery tests Half-cell batteries using samples of TiO2nt / PSS were assembled against a Lithium foil in a Swagelok cell inside a glove box filled with Ar. A Whatman paper separator was added between the two electrodes. The cycling test was run at 25 °C at various C-rates between C/5 and 10 C (a discharge rate of 1 C corresponds to a current density of 56 µA cm-2). 3. Results and discussion 3.1. Electropolymerization The electropolymerization of styrene derivatives is possible both in anodic and cathodic conditions

28

; electron-withdrawing substituents, such as sulfonic acid, facilitate the cathodic

process. Electropolymerization of sulfonated styrene on TiO2nt, which have n-type semiconducting properties 29, is possible only cathodically. The process was investigated by different techniques. In the case of cyclic voltammetry (Figure 1) the potential was swept between -0.9 V and -1.7 V vs Ag/AgCl for 15 cycles at a scan rate of 20 mV/s. One may notice how the current density decreases with increasing cycle number, due to the formation of an electronically insulating layer of PSS onto the TiO2nt surface. The highest decrease in current takes place between the first and the second cycle in which the polymer formation rate is maximum. Apart from the electrochemical polymer formation, a significant contribution to the current density during the forward/reverse scans is due to Li insertion/de-



insertion into TiO2nt 26, as evident from the anodic peak around -1.2 V. At cathodic potentials below -1.7 V, some hydrogen evolution can be observed that is detrimental to the PSS quality.

800

400 j / µAcm-‐2

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0

Cycle number

-‐400

-‐800

-‐1200 -‐1.8

-‐1.7

-‐1.6

-‐1.5

Poten/al / V vs. Ag/AgCl -‐1.4 -‐1.3 -‐1.2 -‐1.1

-‐1

-‐0.9

-‐0.8

Figure 1. Typical cyclovoltammograms of the electropolymerization of PSS on TiO2 nanotubes. Potential range -(0.9 - 1.7) V; scanning rate: 20 mV/s. Figure 2 shows a typical chronoamperometric curve for the same electrochemical system, where a constant potential of -1.4 V was applied to the system for 1 h. The absolute value of the cathodic potential was kept relatively low to reduce hydrogen formation, which may interfere with the formation of a continuous and homogeneous layer of polymer 26.


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a) b)

Figure 2. a) Chronoamperogram for deposition of PSS at -1.4 V vs. Ag/AgCl electrode. b) Typical square root time dependence of the current showing a diffusion-controlled process. As in the Cyclic Voltammetry tests, the absolute value of the current density decreases over time, which is coherent with the formation of an electronically non-conductive polymer onto the nanotubes surface. The current density j can be fitted by a square root law (Figure 2), which corresponds to a diffusion-controlled process. j can be described by the Cottrell equation, where F is the Faraday constant 30: 1/ 2

⎛ ρDc ⎞ −1/ 2 j = F ⎜ ⎟ t ⎝ M ⎠

(1)

Given the molar mass of the precursor M = 189 g mol-1, its concentration c and the density of PSS (ρ = 1.2 g cm-3), the slope of the straight line can be used to calculate the diffusion coefficient of the precursor through the polymer formed onto TiO2nt. The value of D is equal to (1.3±0.2) 10-10 cm2 s-1. Figure 3 shows typical impedance spectra before and after the electropolymerization process (dots) corresponding to the Cyclic Voltammetry test reported in Figure 1. The experimental impedance spectra are well-fitted (continuous lines) by the equivalent circuit shown in Figure 3 in which Q represents a constant phase element (CPE). The impedance of a CPE can be written 31

: Q = Y° (iω)-n

(2)



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where i is the imaginary unit, ω the angular frequency, Y° the CPE value and n is the CPE exponent, indicating the physical nature of the element: a value near 1 indicates an imperfect capacitance. Non-linear least-square fitting of the spectra gave the values reported in Table 1. R1 is the resistance corresponding to the liquid electrolyte solution and the TiO2nt

29

. The

increase of R1 after electrodeposition can be attributed to the resistance of the PSS layer 25. The corresponding constant phase element (Q1) is typical of the imperfect capacitance of a thin layer (in the order of 10-10-10-9 F cm-2). The second part of the circuit describes the electrode interface. The interfacial resistance decreases extremely when PSS is present; the electrode behavior changes from purely blocking to ion transfer. Q2 describes the imperfect electrode capacitance. Consistently, its value is typical of a metallic electrode before formation of PSS (in the order of µμF cm-2), but increases very strongly, when the polymer is present. The very high value (in the order of mF cm-2) corresponds to a so-called “chemical” capacitance 32; this pseudo-capacitance is related to Li insertion at the electrode/polymer interface.


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Figure 3. EIS spectra before and after CV from -(0.9 - 1.7) V. The points are the experimental data and the lines correspond to the non-linear least square fit using the indicated equivalent circuit.

Table 1. Equivalent circuit elements from EIS fitting Q1/Fcm-2

R1/ Without PSS

-10

389±1

With PSS

n±0.1

(4±2) 10

0.91

-10

404±1

(3±1) 10

0.93

Q2/Fcm-2

R2/ 16

4 10

n±0.1

-5

0.83

-3

0.83

(8±2) 10

118±2

(6±3) 10

3.2. Polymer structure Scheme 1 presents the electropolymerization mechanism in cathodic conditions. The electron transfer step leads to the formation of a radical anion, stabilized by the phenyl group. In literature, the living anionic polymerization mechanism is also discussed, but it should occur only in perfectly anhydrous conditions 27. In our case, considering the hygroscopic nature of the solvent DMSO and the absence of nitrogen flux, the mechanism is different, because the radical anion is quenched by the medium with proton addition, leading to a terminal methyl group. The radical is localized on the benzylic carbon that is more stable than the primary carbon. The polymerization continues with formation of a linear aliphatic chain. The relative amount of methyl groups can be used to estimate the average chain length. CH2

CH2

.

-

.

e-

O S O

H+

-

O S O

O

-

CH2

CH3

+ O S O

-

O S O O

O

O

-

H3C

H3C

. O O

-

O

S

O

O O S O

-

O


n

-

S O O S O O

-


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Scheme 1. Electropolymerization mechanism for poly(styrene sulfonate) (PSS).

*

a)

H a c H

H

b

d e e d b

O S O

a

-

O c

ppm

c H3C a'

*

b)

O O

a

b n

-

d

S

e

O

O S O e

-

O b

d

a, a’ c

ppm

Figure 4. 1H NMR spectra of a) 4-styrene sulfonate and b) poly(styrene sulfonate) in D2O. The signal * corresponds to protons due to the solvent.


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Figure 4 shows the

1

H NMR spectrum of sulfonated styrene precursor (4a) and

electropolymerized PSS (4b)33. In Figure 4a, a typical system of a para-substituted phenyl ring is present with signals due to the hydrogen in ortho to sulfonic acid groups (7.7 ppm), downshielded with respect to the hydrogen in ortho to the vinyl moiety (7.5 ppm). The two terminal hydrogens of the vinyl group (a,b) are chemically not equivalent due to the presence of the double bond; the hydrogen b appear at 5.9, because of the long range effect of the phenyl group, while the hydrogen a is at higher field (5.4 ppm). The remaining c hydrogen, in α to the phenyl ring, is at 6.5 ppm and is present as a double doublet. In the spectrum of PSS (Figure 4b), the main difference is the absence of vinyl protons; while the aliphatic system appears around 1.5 (b, secondary hydrogen) and 1.8 (a, a’, tertiary hydrogen) ppm. The aromatic signals are upfield with respect to signals of the precursor. Especially hydrogens d in PSS do not feel the deshielding effect of the vinyl moiety and appear at 6.5 ppm. The hydrogens e, in ortho to the sulfonic acid group, are instead only a little shifted by the presence of the aliphatic chain (7.4 ppm). The signal around 0.9 ppm can be ascribed to the terminal methyl group, as shown in the Scheme 1. Considering that each CH3 signal correspond to 3 hydrogens, one can estimate that the average chain length is of about 100 repeat units. The a’ signal, related to the presence of the terminal CH3 group, is difficult to distinguish due to the very similar chemical environment and the low intensity compared to the signal a. The FTIR spectrum of PSS on TiO2nt is reported in Figure 5. Table 2 shows the peak assignments of FTIR spectra

34-35

. The presence of 1,4-substituted phenyl and sulfonic acid

groups confirm the nature of PSS.


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Reﬂectance / a.u.


3800 3400 3000 2600 2200 1800 1400 1000 Wavenumber / cm-‐1

600

Figure 5. FTIR reflectance spectrum of TiO2nt/PSS (made by CV down to -1.7 V). The peak at 2350 cm-1 is due to atmospheric CO2.


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Table 2: FTIR peak assignments Wavenumber (cm-1) 670

Assignment C-H out of plane bending in 1,4-substituted benzene ring C-H wagging C-H bending Aromatic ring in-plane deformation S=O stretching O=S=O stretching C-C aromatic stretching H2O liquid C-H stretching OH stretching of water and sulfonic acid

770 740, 800, 870 1010, 1130 1040, 1230 1020, 1070, 1160 1310, 1370, 1450, 1490, 1540 1650 2920 3430

The XPS spectra of constituent elements are presented in Fig. 6a-g.

Li1s

I (arb. units)

I (arb. units)

S2p3/2 B

S2p3/2 A

S2p1/2 B S2p1/2 A

50

52

54

56

58

60

164

62

166

168

170

172

174

176

Binding Energy (eV)

Binding Energy (eV)

Figure 6b: Peak fitting of S2p region.

Figure 6a: Li 1s region.

N1s

I (arb. units)

C1s A

C1s C

I (arb. units)

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


C1s D C1s B

280

282

284

286

288

290

292

294

296

396

398

400

402

Binding Energy (eV)

Binding Energy (eV)

Figure 6c: Peak fitting of C1s region.


Figure 6d: N 1s region.

404


O1s A Ti2p3/2

452

456

460

464

I (arb. units)

I (arb. units)

Ti2p1/2

468

472

O1s B

528

530

532

Binding Energy (eV)

534

536

538

Binding Energy (eV)

Figure 6e: Ti 2p region.

Figure 6f: Peak fitting of O 1s region.

F1s

I (arb. units)

I (arb. units)

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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682

684

686

688

690

692

694

696

-2

0

Binding Energy (eV)

Figure 6g: F 1s region.

2

4

6

8

10

12

14

16

18

Binding Energy (eV)

Figure 6h: XP spectrum of the valence band.

The binding energy scale was calibrated by using BE = (285.0 ± 0.05) eV value for aliphatic carbon. The results of XPS elemental quantification are presented in the Table 3. The photoemission spectra were deconvoluted according to the references 36, 37. The signal of Ti 2p3/2 at BE = (458.7 ± 0.05) eV is attributed to titania36-c and is very weak testifying that the TiO2nt are well-covered by the polymer. The different species of carbon in the C 1s spectrum correspond mainly to the aliphatic chain of PSS (component A at BE = (285.0 ± 0.05) eV) and pendent aromatic side groups (component C at BE = (290.3± 0.05) eV). Some oxidation by atmospheric oxygen leads to a small amount of C-O bonds (component B at BE = (286.7 ± 0.05) eV)

36-b

and some CF3 (component D at BE = (293.1 ± 0.05) eV) from remaining TFSI.

The signals of N 1s and F 1s correspond also to supporting electrolyte LiTFSI remaining on the sample surface 37; the relative amount is consistent with the TFSI composition. The major part of Li is present in the PSS with a minor part as LiTFSI. Two chemical species in the S 2p


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spectrum are attributed to the SO2 groups from TFSI (component A at BE = (168.8 ± 0.05) eV) 36

) and sulfonic acid groups of PSS (component B at BE = (169.5 ± 0.05) eV). 35 Table 3. XPS results: chemical state identification and quantification.

Name

Peak BE/ FWHM/ ± 0.05 eV

Atomic /%

Chemical State

± 0.05 eV ΔA/A≤10%

C1s A

285.0

1.2

13.0

Caliphatic 37, 38

C1s B

286.7

1.2

0.5

C O 36, 38

C1s C

290.3

1.2

7.5

Caromatic (PSS) 36

C1s D

293.1

1.2

3.2

CF3 (LiN(SO2CF3)2) 37

F1s

689.0

1.8

13.8

CF3 (LiN(SO2CF3)2) 37

Li1s

55.5

1.5

20.7

-SO3Li (PSS); LiN(SO2CF3)2 37

N1s

399.9

1.2

1.8

LiN(SO2CF3)2 37

O1s A

532.2

1.4

25.1

SO3- (PSS); SO2 (TFSI) 37

O1s B

533.2

1.4

8.9

H2O 39

S2p3/2 A

168.8

1.2

1.7

SO2 (LiN(SO2CF3)2) 37

S2p3/2 B

169.5

1.2

3.6

SO3- (PSS) 36

Ti2p3/2

458.7

0.6

0.3

TiO2 40

Ultraviolet photoemission spectra acquired by using the excitation lines of He I and II are presented in Figure 7. The value of the work function

= (4.5 ± 0.05) eV was determined from

the cut-off in the He I spectrum. The spectral features A and B correspond to the hybrid convolution of S 3p, O 2p and C 2p states 41, whereas the feature C corresponds to the convolution of carbon 2s and 2p states with a shoulder D at about BE = 12.5 eV which can be attributed to F 2p band 42. The feature E at higher BE can be explained by the contribution of S 3s level from sulfonate. It should be noted that the same bands C, D and E are visible also in the XPS valence band spectrum, only their relative intensities are different. Having in mind that the UPS is more surface sensitive than XPS, it is possible to deduce that a higher intensity of S 3s level indicates a higher amount of sulfonate on the surface, whereas a lower intensity of F 2p band is due to the deeper distribution of residual TFSI electrolyte in the sample.



C

b) He II

C a) He I

Cut-off

D I (arb. units)

B I (arb. units)

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E B

A -4

-2

0

2

4

6

8

10

12

14

16

-2

0

2

4

6

8

10

12

14

16

18

20

Binding Energy (eV)

18

Binding Energy (eV)

Figure 7. a) UP spectrum with He I source: work function spectrum with He II source.

= 21.2 – 16.7 = 4.5 eV; b) UP

Figure 8 shows the typical microstructure of TiO2nt with deposited PSS layer in top-view (left) and in cross-section (right). From this and other micrographs, the average length of the nanotubes is 1.5

m and the thickness of the PSS layer about 0.3

m. One can observe that the

contact between oxide and polymer is very intimate. The homogeneous surface morphology of PSS is an important parameter for the battery performance (see below).


Page 17 of 25

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Figure 8. SEM micrographs of TiO2nt with electrochemically deposited PSS layer. Left: top view, right: cross-sectional view. 3.3. Microbattery tests The half-cell microbattery cycling curves are reported in Figure 9 at a rate of C/5. Figure 9a and 9b present respectively the voltage as function of time during consecutive discharge and charge cycles and the potential dependence on the amount of stored charge. The plateaus at around 1.75 and 1.9 V vs. Li metal correspond to Li+ insertion/deinsertion into crystalline anatase 43.



Poten/al / V vs Li

a)

3.5

C/5

3 2.5 2 1.5 1 0.5 0 0

b)

50000

100000 150000 t / s

200000

250000

3.5 Cycle 1 Cycle 2 Cycle 3 Cycle 4

3 Poten/al / V vs Li

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 18 of 25

2.5 2 1.5 1 0.5 0 0

20

40

60 80 Q / µAhcm-‐2

100

120

Figure 9. Cycling at 25 °C of the cell formed by TiO2nts covered with PSS against Li metal at C/5. a) Time dependence of cell potential during discharge/charge cycles, b) Potential dependence on discharge/charge capacity.


Page 19 of 25

140 TiO2 nt/PSS -‐ I const TiO2 nt/PSS -‐ CV PrisAne TiO2 nt

100

95%

C/5

80 60

2C

40

4C

C/2 1C

20

C/5 C/3

3C

10

20

85%

6C

5C

0 0

90%

1C

Coulombic Eﬁciency

100%

120

Qdis / µAhcm-‐2

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


10 C

30 40 Cycle number

50

80% 60

Figure 10. Microbattery areal capacity values for various C-rates at 25 °C and Coulombic efficiency (x) for PSS made by galvanostatic deposition (I const). The TiO2nt length is ~1.5 m. The areal capacity values for various charge/discharge rates vs. cycle number of the microbattery are shown in Figure 10 for PSS made by CV and by galvanostatic deposition (I const). The large capacity decrease in the first cycles is due to irreversible reactions with surface OH groups of TiO2nt and residual humidity of the polymer layer

44

. After this initial decrease, the

microbatteries exhibit a good capacity retention over more than 50 cycles as well as good cycling performances, especially for large C rates. The storage capacity (~ 58 µAh cm-2 at 1 C) is more than doubled vs. previous experiments with PEO-PMMA coating (25 µAh cm-2 at 1 C 44). The areal capacity can be transformed into the gravimetric capacity using the density of crystalline anatase (4 g cm-3) and taking into account the nanotube length (~1.5 µμm) and porosity. Assuming (50±5) % porosity, the obtained value (195±20) mAh/g is actually above the gravimetric capacity of crystalline anatase (168 mAh/g 43), indicating that a part of the nanotubes might be amorphous, in agreement with previous findings on nanoporous anatase 45. The volumetric energy density (780±50 Wh/L) can be calculated from the length of the nanotubes and an average microbattery voltage of 2 V (see Figure 9b). The combination of normalized areal energy density (78±5

Wh/(cm2* m)) and normalized areal power density (for



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

a discharge rate of 1 C: 75±5

W/(cm2* m)) are among the best in literature, much above the

data reported by Plylahan et al. using different cathodes (60 W/(cm2* m) 46, 30

Wh/(cm2* m) and 20

areal energy density (15

Wh/(cm2* m)

nanostructured microbattery with at best 45 density below 10

Page 20 of 25

47

Wh/(cm2* m) and 6

W/(cm2* m) 44). Pikul et al. reported a lower ). The same group has recently published a Wh/(cm2* m) as energy density, but a power

W/(cm2* m) 48. Ye et al. obtained a higher areal power density, but a low

areal energy density (150

W/(cm2* m) and 1.5

Wh/(cm2* m) 49). A further increase of the

energy and power density is possible by optimization of a micro-battery with a high voltage cathode, such as LiNi0.5 Mn0.5 O4 50. Conclusions Electropolymerization is a powerful technique for the conformal deposition of thin layer of functional materials. In this work, we explore the electrochemical deposition of single Li+-ion conducting solid polymer electrolyte, polystyrene sulfonate (PSS), directly on nanotubular TiO2 electrodes. The successful deposition of a thin and homogeneous layer allows the realization of microbatteries with high areal insertion capacities and very good capacity retention especially at high cycling rates, studied up to 10 C. The areal energy and power densities are among the best reported in the literature. The electrochemical bottom-up deposition opens wide opportunities for the coating of complex nano-architectured electrodes. Acknowledgments This work was supported by a grant from the Aix Marseille Excellence Initiative (A*MIDEX) “International Mediterranée 2014” program. M. B. and I. V. F. gratefully acknowledge co-tutela grants by the Franco-Italian University (Vinci program). References (1) Janek, J.; Zeier, W. G. A Solid Future for Battery Development. Nat. Energy 2016, 1, 16141. (2) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587-603. (3)

Knauth, P. Inorganic Solid Li Ion Conductors: An Overview. Solid State Ionics 2009, 180,

911-916.


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(4) Uhlmann, C.; Braun, P.; Illig, J.; Weber, A.; Ivers-Tiffee, E. Interface and Grain Boundary Resistance of a Lithium Lanthanum Titanate (Li3xLa2/3-XTiO3, LLTO) Solid Electrolyte. J. Power Sources 2016, 307, 578-586. (5) Adachi, G. Y.; Imanaka, N.; Aono, H. Fast Li⊕ Conducting Ceramic Electrolytes. Adv. Mater. 1996, 8, 127-135. (6)

Rettenwander, D.; Welzl, A.; Pristat, S.; Tietz, F.; Taibl, S.; Redhammer, G. J.; Fleig, J. A

Microcontact Impedance Study on Nasicon-Type Li1+XAlxTi2-X(PO4)(3) (0

Bottom-Up Electrochemical Deposition of Poly(styrene sulfonate) on

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