Journal ofMaterials Chemistry A

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Polythiophene co

aKey Laboratory for Liquid-Solid Structura

Ministry of Education, School of Materia

University, Jinan, Shandong 250061, P. R

+86-531-88390236bChemical Sciences Division, Oak Ridge Nati

Oak Ridge, TN 37831, USA. E-mail: sunx@ocDepartment of Chemistry, University of TendIntegrated Composites Laboratory (ICL), D

Engineering, University of Tennessee, Knoxv

† This manuscript has been authored bDE-AC05-00OR22725 with the U.S. DepaGovernment retains and the publisher, backnowledges that the United Statespaid-up, irrevocable, world-wide licenseform of this manuscript, or allow otherspurposes. The Department of Energy willof federally sponsored research in accord(http://energy.gov/downloads/doe-public-a

‡ Electronic supplementary informa10.1039/c7ta07893e

Cite this: J. Mater. Chem. A, 2017, 5,24083

Received 7th September 2017Accepted 30th October 2017

DOI: 10.1039/c7ta07893e

rsc.li/materials-a

This journal is © The Royal Society of C

ated aromatic polyimide enabledultrafast and sustainable lithium ion batteries†‡

Hailong Lyu,ab Jiurong Liu, *a Shannon Mahurin,b Sheng Dai, bc Zhanhu Guo d

and Xiao-Guang Sun *b

Organic composite electrode materials based on an aromatic poly-

imide (PI) and electron conductive polythiophene (PT) have been

prepared by a facile in situ chemical oxidation polymerizationmethod.

The common aromatic structure possessed by both electroactive PI

and electron conductive PT allows intimate contacts, resulting in

conductive polymeric composites with highly reversible redox reac-

tions and good structural stability. It has been demonstrated that the PI

composite material with 30 wt% PT coating (PI30PT) has the optimal

combination of good electronic conductivity and fast lithium reaction

kinetics. The synergistic effect between PI and PT enables a high

reversible capacity of 216.8 mA h g�1 at a current rate of C/10, as well

as a high-rate cycling stability, that is, a high capacity of 89.6 mA h g�1

at a high current rate of 20C with a capacity retention of 94% after

1000 cycles. The elaborate combination of the high electronic

conductivity of the PT coating and the fabulous redox reaction

reversibility of the PI matrix offers an economic way to prepare high

performance lithium ion batteries for sustainable energy storage

applications.

l Evolution and Processing of Materials,

ls Science and Engineering, Shandong

. China. E-mail: jrliu@sdu.edu.cn; Tel:

onal Laboratory, One Bethel Valley Road,

rnl.gov; Tel: +1-865-241-8822

nessee, Knoxville, TN 37996, USA

epartment of Chemical & Biomolecular

ille, TN 37996, USA

y UT-Battelle, LLC under contract no.rtment of Energy. The United Statesy accepting the article for publication,Government retains a non-exclusive,to publish or reproduce the publishedto do so, for United States Governmentprovide public access to these resultsance with the DOE Public Access Planccess-plan).

tion (ESI) available. See DOI:

hemistry 2017

1. Introduction

Although conventional lithium-ion batteries (LIBs) based oninorganic cathodes have been utilized in a wide range ofapplications,1–6 they still have several issues restricting theirforward evolvement, such as safety, power density andsustainability.7 Therefore, efforts have been directed to look foralternative, greener and naturally abundant electrode materialsfor LIBs.8,9 As a result, renewable organic electrodes, such asorganosulfur compounds,10,11 organic radical polymers,12–16

conducting polymers17,18 and organic carbonyl compounds,19–22

have been intensively investigated.23 These organic electrodematerials have some unique properties that are generally notavailable in inorganic cathodematerials. For instance, safer andmore exible electrodes can be easily achieved due to the lightweight, exibility and chemical tunability of organic electrodematerials.3,24 More importantly, without toxic heavy metalelements they can be easily recycled as paper with a minimalenvironmental footprint.25 Nevertheless, serious obstacles stillexist for the practical application of organic electrodes, such aspoor rate capability due to their low electronic conductivity andrapid capacity fading during cycling due to the dissolution ofactive organic cathodes.26,27

Among different organic electrodes, aromatic polyimides arevery promising candidates with a theoretical capacityapproaching 400 mA h g�1 and a working voltage of around 2.5V vs. Li/Li+.19 During the discharge process (lithium intake),aromatic polyimides can stepwise accept two electrons, result-ing in the formation of a delocalized radical anion and dianion(Scheme 1).3 Although two more electrons can be acceptedduring the discharge process, a lower redox potential coupledwith poor structural stability usually prevents such an effort.28,29

Different from lithium intercalation in inorganic cathodes, thelithium storage mechanism in aromatic polyimides is a simpleredox reaction, facilitating fast lithium reaction kinetics.8 Toovercome the intrinsic electrical insulation of aromatic poly-imides and obtain high rate performance, expensive carbonadditives such as graphene or carbon nanotubes have been used

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Scheme 1 Electrochemical redox reactions of PI.

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for electrode fabrication.30–32 For example, a polyimide deriva-tive containing a single walled carbon nanotube network(PMAQ–SWNT) exhibited a high capacity of 190 mA h g�1 ata current rate of 0.1C, as well as good rate performance.33

Alternatively, a composite electrode of a polyimide in situpolymerized on a three-dimensional graphene network (3D-RGO/PI) showed a high capacity of 175 mA h g�1 at a currentrate of 0.1C and a low capacity of 40 mA h g�1 at a high currentrate of 5C.34 Unfortunately, large amounts of conductive carbonadditives were used in those composites, i.e. 30 wt% of SWNTsin PMAQ–SWNT and 20 wt% of 3D-RGO in 3D-RGO/PI, whichlimited the loading of the active polyimide materials in thecathodes and signicantly increased the cost of the electrodes.

Besides conductive carbons, conducting polymers areattractive alternatives to enhance the electron conductivity ofpolymer composites due to their easy synthesis, high electronconductivity, environmental stability, cost effectiveness andunique electrochemical redox properties.35,36 Among differentconducting polymers, unmodied polythiophenes are insolublein most organic solvents, which makes them an ideal candidatefor the modication of the electronic conductivity of polymercathodes.37 Herein, a novel polymer composite based ona conductive polythiophene coated aromatic polyimide (PI@PT)is prepared by a facile in situ chemical oxidation polymerizationapproach. The effect of PT contents on the electrochemicalperformance of the PI@PT composite is investigated and opti-mized. Besides providing an outstanding electronic conduc-tivity for the polymer composite, the PT coating also acts as anionic adsorbent and protective shell for the composite materialbecause of its good electrochemical stability.38,39 Overall, thePI@PT composite demonstrates high capacity, long-termcycling stability and good rate performance in rechargeableLIBs.

2. Experimental2.1. Materials

1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA,$98%), hydrazine hydrate (reagent grade, N2H4 50–60%),thiophene (C4H4S, $99%) and iron(III) chloride anhydrous(FeCl3, 97%) were purchased from Sigma-Aldrich. 4-Chlor-ophenol (ClC6H4OH, 99%) was purchased from Alfa Aesar.Chloroform (CHCl3, 99.8%) was purchased from BDH.Conductive carbon black (Super C45) was purchased fromTIMCAL. Battery grade ethylene carbonate (EC), diethylcarbonate (DEC), dimethyl carbonate (DMC), and lithium

24084 | J. Mater. Chem. A, 2017, 5, 24083–24090

hexauorophosphate (LiPF6) were obtained from BASF. All thechemicals and solvents were used directly without furthertreatment.

2.2. Synthesis of N,N0-diamino-1,4,5,8-naphthalenetetracarboxylic bisimide (DANTCBI)

As the intermediate, DANTCBI was synthesized via the substi-tution reaction between NTCDA and hydrazine. NTCDA (10 g)and ethanol (200 ml) were added into an ice-cooled ask andstirred to achieve a homogeneous solution under a nitrogenatmosphere. Hydrazine hydrate (20 ml) was then added drop-wise into the cold reaction mixture. Aer stirring for 1 h, themixture was heated to reux for 1 h. The resulting dark yellowsolid was collected by ltration and dried in a vacuum to obtain10.6 g of product in �96% yield. 1H NMR (DMSO-d6, 400 MHz):d ¼ 5.86 ppm (s, 4H) and d ¼ 8.68 ppm (s, 4H).

2.3. Preparation of PI@PT composites

The precursor PI was synthesized by a simple condensationpolymerization method. Equimolar NTCDA and DANTCBI weredissolved in warm 4-chlorophenol solvent, followed by heatingto reux under nitrogen with stirring for 6 h. The product wasltered, thoroughly washed with methanol, and nally dried at120 �C under vacuum for 12 h. The pristine PI was obtained byheat treatment at 350 �C under nitrogen for 8 h. The PI@PTcomposites were synthesized by a typical in situ chemicaloxidation polymerization approach. Well-ground PI (1.8 g) andFeCl3 (0.8 g) were uniformly dispersed in CHCl3 by sonicationand stirring for 1 h, and then a solution of thiophene (0.19 ml,equal to 10 wt% in the nal product) and CHCl3 (30 ml) wasadded slowly. The reaction mixture was stirred for 10 h at 0 �Cunder nitrogen. The product was washed several times withmethanol and collected by ltration. Finally, a red-brownpowder was obtained by drying at 80 �C under vacuum anddenoted as PI10PT. Similarly, the amount of thiophene wasincreased to 30 wt% and 50 wt%, while maintaining the weightratio of thiophene to FeCl3 as 1 : 4, to obtain the products ofPI30PT and PI50PT, respectively. As a baseline, pure PT was alsosynthesized following the same procedure without PI.

2.4. Characterization1H nuclear magnetic resonance (NMR) spectra were obtained ona Bruker Advance 400 MHz spectrometer using DMSO-d6 as thesolvent. The chemical structures of the polymer products werecharacterized by Fourier transform infrared (FTIR)

This journal is © The Royal Society of Chemistry 2017

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spectroscopy. The morphologies and microstructures of thesamples were observed on a Hitachi HD-2000 scanning trans-mission electron microscope (STEM) operating at 200 kV. Theelemental compositions of the polymer composites wereanalyzed by energy dispersive X-ray spectroscopy (EDS) ona SEM instrument with an EDAX accessory.

2.5. Electrochemical measurements

Polymer composite electrodes were fabricated by casting wellhomogenized slurries of active material, C45, and poly-vinylidene uoride (PVDF) binder with a weight ratio of 6 : 3 : 1in N-methylpyrrolidone (NMP) on aluminum foils. Aer solventevaporation, the electrodes were cut into discs with a diameterof 12.7 mm and dried thoroughly under vacuum at 120 �C for 12h. Half-cells for electrochemical measurement were assembledwith the polymer composites as the cathode, lithiummetal foilsas both counter and reference electrodes, Celgard 2320 as theseparator and 1 M LiPF6 dissolved in EC, DEC, and DMC(1 : 1 : 1 vol) as the electrolyte. Coin cells were assembled in anargon lled glove-box with the oxygen and moisture contentsbelow 0.5 ppm. The coin cells were cycled galvanostatically atvarious current rates on a LAND CT2001A battery test system ina voltage range from 1.8 to 3.2 V. Cyclic voltammetry (CV) wascarried out on a Biologic VSP instrument with a scan rate of 0.05mV s�1 in a voltage range of 1.8–3.2 V. Electrochemicalimpedance spectra (EIS) were measured on the same instru-ment with a 10mV AC bias in a frequency range from 200 kHz to10 mHz.

3. Results and discussion

The PI@PT composites were synthesized by an in situ oxidationpolymerization method. To ensure the homogeneous coating ofPT on the surface of PI, the latter was ground into a ne powderand well dispersed in chloroform in which the monomer thio-phene was soluble. The surface coating was proceeded slowlyunder the catalysis of FeCl3 at a low temperature of 0 �C. With

Fig. 1 Photographs of (a) PI, (b) PI10PT, (c) PI30PT, (d) PI50PT and (e)PT powders; (f) FTIR spectra of PI, PI10PT, PI30PT, PI50PT and PT.

This journal is © The Royal Society of Chemistry 2017

increasing the amount of PT, the color of the nal productbecomes darker, as shown in Fig. 1a–e, suggesting thesuccessful coating of PI by PT. Fig. 1f shows the FTIR spectra ofpristine PT and PI, and their composites, PI10PT, PI30PT andPI50PT. For the pristine PT, the strongest absorption peak at782 cm�1 is attributed to the Cb–H out-of-plane stretchingvibration of the thiophene ring,40 indicating the Ca–Ca

connection of the thiophene rings in PT.39 The Ca–Ca connec-tion is crucial to achieve high electron conductivity and goodelectrochemical performance of the composites, as it has beenproved that the Ca–Ca connected polythiophene exhibited thehighest electron conductivity among different polythiophenestructures.11 The weaker absorption peaks at 1490 and 694 cm�1

of the PT can be assigned to the C–H and C–S stretching of thethiophene ring, respectively.17,41 As for the pure PI and PI@PTcomposites, the characteristic vibration absorptions of C]Oand C–N at 1703 and 1318 cm�1 are observed and fullyconsistent with the previously reported spectra, suggesting thesuccessful synthesis of the target aromatic polyimide compos-ites.34,42 Meanwhile, the intensities of the aforementioned PTcharacteristic peaks increase with increasing the PT amount,from PI10PT to PI50PT. Furthermore, the spectra of the PI@PTcomposites are only superposition of the characteristicabsorption peaks of PI and PT without generating additionalnew peaks, demonstrating the non-covalent interactionbetween the PI and PT coating, thus preserving the integrity ofthe redox-active carbonyl groups in PI or the electrochemicalreactivity of the composite electrode.

Themorphologies of PI, PI10PT, PI30PT, PI50PT and pure PTwere characterized by SEM, as shown in Fig. 2. The pristine PIparticle shows an apparent porous surface structure (Fig. 2a). Incontrast, the surface of the pure PT particle is compact andsmooth, without any porous structure (Fig. 2e). For the PI@PTcomposites, the porous surface structure of the PI particlegradually disappears with increasing the amount of PT coating,as shown in Fig. 2b–d. This is consistent with the BETmeasurement, as shown in Fig. S1.‡ The surface areas of PI,PI10PT, PI30PT, PI50PT and PT are 74.3, 32.6, 24.2, 13.9 and13.7 m2 g�1, respectively. It is noted that the surface coating ofPT in the PI50PT sample is so thick that it covers the entire PIparticle (Fig. 2d), similar to pure PT (Fig. 2e). This compact andnon-porous morphology of PI50PT will impede the penetrationof the liquid electrolyte, affecting the diffusion of lithium ionsas well as rate capability as will be shown later.

To further investigate the polymeric composites, EDSelemental analysis was also carried out. Fig. 3a shows a typicalEDS mapping of the PI30PT sample. As can be seen from thegure, a uniform distribution of carbon and sulfur whereasalmost no nitrogen and oxygen can be found within the samplearea, conrming that PT was indeed coated on the surface of PI.Fig. 3b shows the EDS spectra of PT, PI, PI10PT, PI30PT andPI50PT. The peaks of nitrogen and oxygen can be clearly seen inthe PI sample, while they are hardly observed in the othercomposite samples. In contrast, the sulfur peak in the PI@PTcomposites gradually increases with increasing the content ofPT. It is noted that the spectrum of the PI50PT sample is almostidentical to that of pure PT, suggesting that the PI particles in

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Fig. 2 SEM images of (a) PI, (b) PI10PT, (c) PI30PT, (d) PI50PT, and (e) PT.

Fig. 3 (a) SEM image of PI30PT, its total as well as individual EDSmappings of elements C, N, O and S; (b) EDS spectra of PT, PI, PI10PT,PI30PT and PI50PT.

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PI50PT are totally covered by PT. These results further conrmthe successful synthesis of the target polymer composites ofPI@PT.

Fig. 4a shows the cyclic voltammograms (CVs) of the PI,PI10PT, PI30PT and PI50PT composite electrodes in the voltagerange of 1.8–3.2 V at a scan rate of 0.05 mV s�1. Two pairs ofwell-resolved redox peaks can be observed for the PI30PT elec-trode, involving two reduction peaks at 2.39 and 2.58 V and twocorresponding oxidation peaks at 2.52 and 2.77 V, respectively.A reversible two-electron redox reaction is conrmed by thedoublets during both lithiation and de-lithiation processes,

24086 | J. Mater. Chem. A, 2017, 5, 24083–24090

corresponding to a step-wise formation of a radical anion anddianion (Scheme 1).19 In comparison, the correspondingdoublets for both PI and PI10PT are not well dened, which canbe attributed to their low electronic conductivity due to no orless content of highly conductive PT coating, resulting in slowcharge transfer kinetics between the radical anion and thedianion.43 It is noticed that the peak currents of PI50PT aremuch lower than those of PI30PT, attributed to poor Li-iondiffusivity because of a too thick PT coating in PI50PT.Furthermore, the onset potentials of the rst reduction peakand the rst oxidation peak during the discharge/chargeprocess are 2.41 and 2.75 V, 2.29 and 2.78 V, 2.21 and 2.83 V,and 2.28 and 2.80 V for the PI, PI10PT, PI30PT and PI50PTelectrodes, respectively. The higher potential during dischargeand lower potential during charge for the PI30PT electrodesuggest that it has the lowest polarization among these poly-meric electrodes, resulting in high utilization efficiency of thepolymer cathode and high specic capacity as will be demon-strated later. Moreover, the CV of pristine PT shows no peaks inthe voltage range between 1.8 V and 3.2 V (Fig. S2‡), conrmingthat the PT contribution in the PI@PT composites in the testedvoltage range is only electron conductivity.

To further understand the effect of the polythiophenecoating on the Li-ion diffusion in the electrodes, CVs of the PI,PI10PT, PI30PT and PI50PT composite electrode were obtainedat various scan rates in the range of 0.05–2.0 mV s�1, as shownin Fig. S3a–d.‡ The Li-ion diffusion coefficients are calculatedusing the Randles–Sevcik equation (eqn (1)):44,45

Ip ¼ 269000n3/2AD1/2Cn1/2 (1)

where Ip is the peak current (A), n is the electron concentrationper molecule during the redox reactions, A is the surface area ofthe electrodes (cm2), D is the diffusion coefficient of lithiumions (cm2 s�1), C is the bulk concentration of lithium ions in theelectrodes (mol cm�3) and n is the scan rate (V s�1). A linear

This journal is © The Royal Society of Chemistry 2017

Fig. 4 Cyclic voltammograms (CVs) of (a) PI, PI10PT, PI30PT and PI50PT at a scan rate of 0.05 mV s�1; charge–discharge profiles of (b) PI,PI10PT, PI30PT and PI50PT at a current rate of C/10.

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relationship between Ip and n1/2 was obtained as shown inFig. S3e.‡ The diffusion coefficients of lithium ions werecalculated according to the slopes. It is estimated that thediffusion coefficients of lithium ions in PI, PI10PT, PI30PT andPI50PT are 6.39 � 10�13 and 8.35 � 10�13 cm2 s�1, 4.73 � 10�11

and 4.79 � 10�11 cm2 s�1, 3.14 � 10�10 and 2.92 � 10�10 cm2

s�1, and 9.60 � 10�11 and 7.08 � 10�11 cm2 s�1 for oxidationand reduction, respectively. The lithium ion diffusion coeffi-cient in the PI10PT electrode is almost two orders magnitudehigher than that in the PI electrode, but it is still one ordermagnitude lower than that in the PI30PT electrode. Compara-tively, the Li-ion diffusion coefficient in the PI30PT electrode ismore than triple that in the PI50PT electrode, which is doublethat in the PI10PT electrode. These data conrm that thePI30PT electrode has the highest lithium diffusion coefficients,which is consistent with the rate performance shown in Fig. 5.

Fig. 4b shows the galvanostatic charge/discharge proles ofthe half-cells based on PI, PI10PT, PI30PT and PI50PT ata current rate of C/10. As expected from the CVs in Fig. 4a, a two-step charge/discharge process can be clearly observed for all thePI@PT composite materials. However, only a slope is observedfor the PI based cell during the charge process, which isattributed to the severe polarization due to its low electronicconductivity. These results are highly consistent with the CVanalysis in Fig. 4a, further conrming that the PI30PT electrodedelivers low cell polarization and fast lithium reaction kinetics.

Fig. 5 compares the rate and cycling performance of the half-cells based on PI, PI10PT, PI30PT and PI50PT, as well as the EISdata of the cells before and aer cycling. A common feature forall the polyimide based materials is the initial gradual increaseof capacity, which can be attributed to the activation processbecause of the gradual penetration of the liquid electrolyte intothe polymeric composite cathodes.46,47 As shown in Fig. 5a, thePI30PT based cell exhibits reversible capacities of ca. 216.8,193.6, 169.1, 151.1, 132.2 and 104.7 mA h g�1 at current rates ofC/10, C/5, C/2, 1C, 2C and 5C, respectively, with a highcoulombic efficiency of 100% except for the rate changingcycles. The capacity recovers to 205.8 mA h g�1 when the currentrate is switched back to C/10, which is equivalent to a 95%capacity retention. As a comparison, the reversible capacities of

This journal is © The Royal Society of Chemistry 2017

the half-cells based on PI and PI10PT are 116.0, 67.0, 41.9, 32.0,23.2, 11.4, 90.2 mA h g�1 and 192.5, 174.6, 143.1, 111.1, 71.1,18.4, 183.4 mA h g�1 at current rates of C/10, C/5, C/2, 1C, 2C,5C, and C/10, respectively. The comparison of rate performancein Fig. 5a suggests that the PI30PT cathode has the bestcombination of electron conductivity and lithium reactionkinetics, consistent with its highest lithium diffusion coeffi-cient. The reversible capacities of the PI50PT based cell arealways lower than those of the PI10PT based cell except at 5C,although the former has a higher lithium diffusion coefficientthan the latter. This is closely related to the thickness of thesurface coating and the different surface morphologies of theprimary composite particles, as shown in Fig. 2. Although thelithium diffusion coefficient in PI50PT is double that of PI10PT,on average the thickness of the PT coating in PI50PT is aboutve times that of PI10PT; therefore, at the same current ratelithium ions will reach the electroactive PI center faster inPI10PT than in PI50PT, resulting in higher capacity in theformer.

Besides good rate performance, the PI30PT based half-cellalso exhibits better cycling stability, as shown in Fig. 5b. Aera gradual increase within the initial few cycles, the reversiblecapacity of the PI30PT based cell reaches a high value of 186.6mA h g�1 at a current rate of C/2. Then it decreases slowly withcycling and is still as high as 170.9 mA h g�1 aer 300 cycles,resulting in a high capacity retention of 92%. In contrast, thereversible capacities of the PI10PT and PI50PT based cells are140.1 and 106.6 mA h g�1 aer the rst few activation cycles,and then decrease gradually to 101.5 and 71.6 mA h g�1 aer300 cycles, exhibiting capacity retentions of 72% and 67%,respectively. The fabulous cycling stability of the PI30PT basedcell is also supported by the EIS data before and aer cycling asshown in Fig. 5c and d, respectively. The total cell impedancebefore cycling continually decreases with increasing the PTcontent, consistent with the excellent electronic conductivity ofPT. Aer 300 cycles, the total cell impedance of all the cellsincreases. However, it only increases from 40.8 U before cyclingto 57.8 U aer 300 cycles for the PI30PT based cell, whereas itincreases from 15.2, 107.3, and 188.8 U before cycling to 58.8,286.7, and 417.4U aer 300 cycles for the PI50PT, PI10PT and PI

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Fig. 5 Charge–discharge capacities and coulombic efficiencies of the half-cells based on PI, PI10PT, PI30PT and PI50PT at (a) different currentrates and (b) a current rate of C/2; electrochemical impedance spectra of the half-cells based on PI, PI10PT, PI30PT and PI50PT (c) before cyclingand (d) after 300 cycles at C/2; (e) charge–discharge capacities and coulombic efficiencies of the half-cells based on PI30PT at different highcurrent rates (hollow circles and solid spheres represent charge and discharge capacities, respectively, while hollow stars represent coulombicefficiencies in (a), (b) and (e)).

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based cells, respectively. It is noted that, although the imped-ance of PI50PT is lower than that of PI30PT before cycling, thecharge transfer resistance associated with the semi-arc inPI50PT increases more quickly than that in PI30PT, and it iseven higher than that in PI30PT aer 300 cycles. Thisphenomenon is closely related to the thickness of the PTcoating on the PI@PT composites. Although more PT contrib-utes to higher electron conductivity and lower resistance beforecycling, unfortunately, a too thick PT coating impedes lithiumreaching the electroactive center of PI during cycling. Asdemonstrated earlier, the lithium diffusion coefficient inPI30PT is more than triple that in PI50PT whereas the thicknessof the PT coating in the former is nearly half that in the latter;therefore, it is not surprising that the increase of the charge

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transfer resistance in PI50PT is faster than that in PI30PT at thesame current rate. This also underlines the importance ofbalancing electron conductivity with lithium diffusion in thesynthesis of composite electrode materials.

To further investigate the potential practical application ofthe PI30PT composite material, its long-term cycling perfor-mance at high current rates was also evaluated, as shown inFig. 5e. The reversible capacities are 116.3, 107.1 and 89.6 mA hg�1 at high current rates of 5C, 10C and 20C, respectively, withcoulombic efficiencies all near 99.8%. Aer 1000 cycles,reversible capacities are still as high as 102.5, 90.8 and 84.3 mAh g�1 at 5C, 10C and 20C, leading to high capacity retentions of88%, 85% and 94%, respectively. The superior cycling perfor-mance of the PI30PT based cells can be attributed to the

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optimal content of PT which not only provides good electronconductivity but also successfully acts as a protection shell tomaintain the structural integrity of the polyimide composite,preventing it from decomposing and dissolving in the liquidelectrolyte during cycling via the strong p–p interactionsbetween the polyimide matrix and the PT coating. Table S1‡further compares the battery performance of PI30PT with thoseof reported PI composite materials. As can be seen from thetable, PI30PT possesses high specic capacity at both low andhigh current rates, especially the long-term cycling stability athigh current rates is unprecedented, suggesting the greatpotential of PI30PT for practical application. In addition, withelectron conductive polymer PT replacing expensive carbonmaterials such as single-wall carbon nanotubes and 3D reducedgraphene oxide, the total polymeric composite PI30PT repre-sents a promising low-cost cathode material for “green andsustainable” lithium ion batteries.

4. Conclusion

Novel polymeric composites based on coating aromatic poly-imides with an electron conducting polythiophene have beenprepared by a facile in situ chemical oxidation polymerizationapproach for application in rechargeable lithium ion batteries.The optimal PT coating, 30 wt% (PI30PT), enables high electronconductivity and fast lithium reaction kinetics. Therefore, thePI30PT composite electrode delivers not only a reversible speciccapacity of 216.8 mA h g�1 at a low current rate of C/10 but alsoa remarkable high-rate cyclability, achieving a high capacity of89.6 mA h g�1 at 20C with a capacity retention of 94% aer 1000cycles. These superior electrochemical properties result from theelaborate synergy of the stable redox reversibility of PI and thehigh electronic conductivity of PT. Overall, PI30PT is proven to bea promising cathode material candidate for practical applicationin “green and sustainable” lithium ion batteries.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was supported by the ORNL laboratory-directedresearch and development (LDRD) program. The electronmicroscopy work was performed through a user project supportedby the ORNL's Center for Nanophase Materials Sciences, which issponsored by the US Department of Energy, Office of Science,Scientic User Facility Division. J. Liu and H. Lyu acknowledge thenancial support from the National Natural Science Foundationof China (No. 51572157). H. Lyu also acknowledges support fromthe China Scholarship Council (CSC) program.

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