Perylenetetracarboxylic Diimide as Diffusion‐Less Electrode Material for High‐Rate Organic Na‐Ion Batteries

Abstract In this work 3,4,9,10‐perylenetetracarboxylic diimide (PTCDI) is investigated as electrode material for organic Na‐ion batteries. Since PTCDI is a widely used industrial pigment, it may turn out to be a cost‐effective, abundant, and environmentally benign cathode material for secondary Na‐ion batteries. Among other carbonyl pigments, PTCDI is especially interesting due to its high Na‐storage capacity in combination with remarkable high rate capabilities. The detailed analysis of cyclic voltammetry measurements reveals a diffusion‐less mechanism, suggesting that Na‐ion storage in the PTCDI film allows for exceptionally fast charging/discharging rates. This finding is further corroborated by galvanostatic sodiation measurements at high rates of 17 C (2.3 A g−1), showing that 57 % of the theoretically possible capacity of PTCDI, or 78 mAh g−1, are attained in only 3.5 min charging time.


Introduction
Growing costs and limited resources of Li has triggered substantials cientific interestt of ind alternatives to the Li-ion systems. In this respect, the former research on Na-ion batteries (SIBs) has lately been intensely revived, mostly motivated by the high naturala bundance of Na. [1,2] Therefore,a nimportant effort is being made in developing high capacity materials for Na-ion batteries, [3,4] whicha re inherently eco-efficient and environmentally friendly, [5] in combination with feasible approaches to low-cost production and recyclability. [6] Organics emiconductingm aterials, either based on conjugated polymers, [7] or small molecules as the core semiconductor element, [8] hold the promise of delivering low-cost and energy-efficient "green" electrodes for Na-ion batteries. [9,10] Quinones are af ascinatingg roup of these organic battery materials comprising ah igh theoretical Na-storage capacity, fast reaction kinetics,a nd al arge structurald iversity. [11] These excellent qualities are mainly due to their 1,4-benzoquinone units.D ifferent typeso fq uinones have been considered for nonaqueous and aqueous organic Na-ion batteries. [12][13][14] These compounds store charge via an ion-coordination mechanism wheret he Na ions associate to the negatively charged oxygen atomsu pon electrochemical reductiono ft he carbonyl groups, and dissociater eversibly during the reverse oxidation. Perylene diimidesa nd its derivatives have been previouslyi nvestigated as cathode material for organic SIBs. In 2014, Luo et al. [15] reportedo n3 ,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA), ac ommercially available organic pigment, to work efficientlya sN ai on battery cathode. In their work ah igh capacity of 145 mAh g À1 ,h igh rate capability up to 1000 mA g À1 ,a nd stable cycling performance over 200 cycles has been reported. [15] In as ubsequent work in 2015,a lso the perylene-diimide derivative, 3,4,9,10-perylenetetracarboxylic diimide( PTCDI), which has ah ydrogen residual on the nitrogen atom of the imide moiety,h as been investigated towards its capability as cathode materiali nS IBs by Deng et al. [16] Similar to the previously reportedP TCDA molecule, the PTCDImolecule is capable of at wo-electron redox reaction with an association/disassociation of two Na ions to the negatively charged oxygen atom of two dicarboximide groups in the potentialw indowo f1 -3 V vs. Na/Na + .H owever,d uring the first cycleo facyclic voltammetry( CV) measuremento nly one reductionp eak at the potentialo f1 .6 Vw as reported.T his suggestst hat al arge polarizationi sn eededf or the first reduction of the PTCDIm olecule. In subsequentc ycles three peak couples in the potential range of 3-1.5 Va re formed and remaina lmostc onstant regarding their potentials and intensities,i ndicating ar eversible, multielectron redox reaction. Galvanostatic cycling showed as table charge/discharge capacity of 138.7/138.6 mAh g À1 (constant current of 10 mA g À1 )a fter the first cycle, which correlatest oa two-electron redox reaction per PTCDIm olecule (giving at heoreticalcapacity of 137 mAh g À1 ). [16] Organic cathode materials are known for their poor cycle life performance, due to their structurali nstability particularly at high oxidation potentials and their tendencyt od issolve in organic battery electrolytes. [17] In contrast, the long-term cycling stabilityo fP TCDI is exceptionally enhanced, possibly due to the larger p-conjugated structure and therefore increased p-p interactions and strong intermolecularH -bondingo fP TCDI molecules. [8] This improves the structurals tability of the PTCDI skeletons during repeated sodiation and desodiation cycling. [18,19] In addition to its high storagec apacity,t he PTCDI cathodec an also accommodate ar emarkable high rate capability,w hichh as only been sparsely investigated and is by now not fully understood. In 1980, Laviron et al. [20] developed amultilayer model for space distributed, redox modified electrodes, which is found to describe the PTCDI-carbon paper (Cp) composite electrode system well. PTCDI on Cp is semiconducting but, due to its morphologya nd structure,e asily accessible for ions to diffuse into.
In this work, by evaluating CV measurements at different scan rates and implementing the theoretical model of Laviron, [20] am echanistic understanding of the underlying electrochemicalp rocess of Na-ion storage in PTCDI-Cp composite electrodes is gained. Based thereon,adiffusion-less mechanism can be inferred, suggesting that the diffusion of Na ions into the organic film is extremelyf ast. It suggests that the transport of the Na counter ion is not the rate-limiting factor but the electron transferfrom the Cp substrate throughout the organic film. Therefore, by utilizing an anostructured Cp support in combination with at hin PTCDI-film coverage, very fast sodiation rates can be sustained. As ap roof of concept, we show, that at fast sodiation rates of 17 C( 2.3 Ag À1 ), still 57 %o ft he theoretical capacity,o r7 8mAh g À1 ,o ft he PTCDI covered Cp electrodes is attained. Full sodiation of aP TCDI-Cp composite electrode in about 3.5min (17 C) is, to the best of our knowledge, the highest sodiation rate reported for PTCDI by now.

Results and Discussion
The Cp substrates are coated with a2 50 nm thick PTCDI film via at hermal evaporation process. Ad etailed spectroscopic characterization of the PTCDI film on Cp, by meanso fR aman spectroscopy,i sp resented in the SupportingI nformation (Figure S1).
For am orphological characterization, scanning electron micrograph (SEM) images of PTCDI-coated( 250 nm) Cp electrodes are showni nF igure 1f or two different electrode spots. The low magnification SEM images (Figure 1a,d)r eveal the interconnected network of the carbon fibers forming the supporting Cp substrate. The magnified SEM images in Figure 1b,e and Figure 1c,f,f ocus more on individual carbon fibers.F or both electrode spots, the organic film of PTCDI active materialc overing the subjacent fiber is observed. The thermally evaporated PTCDI film is approximately 250 nm thick and clearlyv isible as ac losed hull, covering the subjacent carbon fiber homogeneously.T he organic PTCDI film follows the broad irregularities of the subjacent carbon fiber network. This is in line with previous reports showing that PTCDIa nd other aromatic ring containing molecules tend to stack one-dimensionally by p-p interactions to form column structures and 2D stacked layers (Figure 2b). [19,[21][22][23]  As reported in Refs. [22] and [24],t he H-terminated PTCDI molecule (H 2 PTCDI) undergoes ar eversible two-electron redox reaction( Figure 2a)i nt he potential range from 1t o3Vm easured versus Na/Na + .T herefore, two reduction peaks and two back-oxidation peaks are expected in the CV,c orresponding to the reduction of the neutral H 2 PTCDI molecule to the radical anion, the reduction of the radical anion to the dianion, and both the corresponding back-oxidation reactions (Figure 2a). The H 2 PTCDI-Cp composite electrode in aN a-ion battery halfcell can be thought of as at hree-component system composed of the liquid electrolyte, the solid H 2 PTCDI film, and the solid Cp substrate.
The H 2 PTCDI film, having at hickness of 250 nm, is sandwiched between al iquid/solid interface of the electrolyte and the film and as olid/solid interface of the Cp substratea nd the H 2 PTCDI film. As the H 2 PTCDI film is conducting for both, the Na ions and the electrons, the materiali sc onsidered as a mixed electron-ion conductor.T he liquid/solidi nterface be-tween the electrolyte andt he film is acting as ab locking boundaryf or the electrons and the solid/solid interfaceo ft he film and the carbon paper is acting as blockingb oundaryf or the Na ions ( Figure 2c). For further investigation of the electrochemicalc haracteristics of this system, the battery half-cell is cycled in aN acontaining electrolyte with several different scan rates. Figure 3s hows CV measurements for different scan rates from 200 to 0.05 mV s À1 .T wo broad reduction peaks and one back-oxidation peak are visible. Figure 3b showsamagnification of the CVs with slow scan rates from 5t o0 .05 mV s À1 .T he two reduction peaks denoted as (A) and (B) and the back-oxidation peaks (B' &A ')b ecome very sharp, with peak widths (full width at half maximum)o fo nly 15 (A), 45 (B), and 60 mV (B' &A ')a ts low scan rates below 10 mV s À1 .W hile in the first cycle only one reduction peak is observable at 1.81 Vand one back-oxidation peak at 2.22 V( Figure S2), in the consecutive cycle the reduction peak splits into two peaks at 1.99 (Fig-Figure 2. a) Chemical structure of PTCDIa nd the corresponding two-electron redox reactionsofH 2 PTCDI with the reduction (sodiation)o ft he neutral molecule to the radical anion and the furtherreduction( sodiation) to the dianion and bothofi ts back-oxidations (desodiations) in as odium electrolyte system. b) Schematic representationo fP TCDI bulk crystals tructurew ith white:H ,t urquoise:C,r ed:O,a nd blue:N;reproducedw ith permission form Ref. [23]. c) Schematic representation of the processes takingp lace at the working electrode, diffusionofs odium ions, and electron-transferr eactions.  ( Figure 3b,B ), corresponding to two one electron reductionso ft he H 2 PTCDIm olecule. Theb ack-oxidation peak appearsa t2 .20 V ( Figure 3b,B ' &A '). Comparingt he first and the second cycle, it can be seen that the very first reductiono ft he molecule hasapolarization overpotential of about 180 mV ( Figure S2) and initially,o ne reduction reaction comprising two electrons takes place.Apossible reasonf or this polarization overpotential is an initial rearrangemento f the H 2 PTCDI molecules to activate the electron transfer and/or an insertion of Na ions into the H 2 PTCDI lattice. [25] The back-oxidation peak also shows am arginal polarization of 20 mV and the current density of the oxidation peak increases from 5.5 mA cm À2 for the first cycle to 6.7 mA cm À2 for the second cycle. This suggests that not all H 2 PTCDI molecules are electrochemically active in the first CV cycle. One has to point out that the redox reactions of the H 2 PTCDI molecules in the PTCDI-Cp composite electrode are surprisingly well resolved, even at high scan rates of 200 mV s À1 ,w hich is not expected when compared to most electrodes tested forN a-ion batteries.
For as low scan rate of 1mVs À1 ,t he first very sharp reduction peak (Figure 3b,Aand Figure 4a)a ppears at 2.05 Vf ollowed by as econd slightly broader reduction peak (Figure 3b, B) at 1.91 V. Interestingly,f ollowing the reduction peak (A), a small post-peak appears ( Figure S3). The back-oxidation peak (Figure 3b,B ' &A ')a tapotential of 2.13 Vi st ailing.I ts eems as if the back-oxidation of the H 2 PTCDI molecule only shows one peak. Either the peak (A')i sasuperposition of two peaks (A' and B')o r, the back-oxidation is at wo-electron reactiona t the potential of 2.13 V. Af urther,q ualitative evolution of the CV curvesw ith decreasing scan rate shows, that the back-oxidation peak (B' &A )s tarts tailing and splits into two separate back-oxidation peaks (B')a nd (A')w ith decreasing scan rates. The reduction peaks (A) and (B) also show somet ailing for the small scan rates and almost vanish for the slowests can rate of 0.05 mV s À1 .T he observed behavior,t hat is, the tailing at small scan rates,m ay be explained by the kinetic limitation of the electron exchange reaction throughoutt he film. Laviron et al. [20] mentioned in at heoretical description of am ultilayer surfacec onfined electrode, that the kinetic limitation of the electrode exchange reaction throughout the layers is initially observable for slow scan rates, whereas the kinetic limitation of the electrochemical reaction of the first layer with the substrate follows for faster scan rates. Physically this could be interpreted as the H 2 PCTDI molecules in different layersn eeding as lightly different energy (potential) to be reduced/oxidized, hence the broadening of the peaks. This phenomenon seems to be more significant with decreasing scan rate.
The CV peaks were furthera nalyzed by plottingt he logarithm of the peak current I P versus the logarithm of the scan rate (Figure 4b-d). The values of the peak currents I P (Figure 4b-d) are the absolutev alues of the respective CV measurements, having the capacitive current for the pure carbon paper substrate without active material( Figure S4) subtracted. Interestingly,t he linear fit fort he first reduction peak (A) is not uniform over all scan rates but has as lope of about 1f or the scan rates from 0.05-10 mV s À1 (Figure 4d,r ed dashed line) and as lope of about 0.5 for the faster scan rates from 20-200 mV s À1 (Figure 4d,b lue dashed line). The linear fit for the second reduction peak (B) has as lope of about 1t hroughout all scan rates (Figure 4c), and the linear fit of the back-oxidation peak (B' &A ')i sa lso characterized by as lope of about 1 for all scan rates (Figure4b). As lope of 1i ndicates that the current is proportional to the scan rate, representative of ac apacitiver esponse, [26] while as lope of 0.5 shows ad ependence of the currentw ith the square root of the scan rate. Furthermore the dependenceo ft he peak current with the square root of the scan rate is indicative for as emi-infinite diffusion behavior, [27] for example, if the diffusion of the Na ions into the H 2 PTCDI film (Figure 2b)i sl imiting the peak current. In the H 2 PTCDI system the peak current is mainly proportional to the scan rate. This behaviori ndicates that the peak current in this system is not limited by the diffusion of the Na ions but rather by either the electrochemical reaction of the first layer of the H 2 PTCDI molecules with the Cp substrate or the electrone xchange reaction in the following PTCDI layers (compare Figure 2c). To further analyze the CV measurements, the peak potentials of the Peaks (A), (B), and (B'&A')a re plotted against the natural logarithm (ln) of the scan rate (Figure 5a).
The peak potentialv alues are IR drop corrected. The resistance (R)o ft he cell was determined by electrochemical impedance spectroscopy( EIS)a nd is about 3 W (compareF igure S5 in the Supporting Information). The peak potentials for the peaks stay constant overawide range of scan rates (0.05-50 mV s À1 ), with potentials of 2.05, 1.91, and 2.13 Vf or the peaks (A), (B), and (B'&A'), respectively.F or the faster scan rates (100-200mVs À1 ), as ignificant deviation from the constant potential is observable, as the reduction peak potentials shift to more cathodic potentialsa nd the oxidation peak potential shifts to am ore anodic potential. The constant peak potentials over this wide range of scan rates indicate the reversibility of the redoxr eaction under these conditions, butu nlike the ideal behavior of ar eversible surface confined systemst he peak potentials of the reduction and oxidation reactions are not the same even for slow scan rates. [28] The differenceb etween the peak potentials, denoted as DE (A)/(B'&A') and DE (B)/(B'&A') ,a re 88 and 223 mV,r espectively. One possible reason for this nonideal behaviorm ay be an attractive intermolecular interaction between the H 2 PTCDI molecules. [20,28,29] Neutral, radicala nion,a nd dianion states of the H 2 PTCDI film therefore have different interaction energies. These interaction energies can be investigated by analyzing the half-height width of the peaks (d)i nt he CV measurements. [20,29] Figure 5b shows that d decreases initially with decreasing scan rates, stays nearly constantf or the scan rates from 10 to 2mVs 1 ,a nd increasesa gain for smaller scan rates. The data showingaconstant d can be used to calculate the interaction parameters fort he H 2 PTCDI film. For all scan rates where no data points are given, the half-height width was not obtainable from the CVs due to as ubstantial broadening of the peaks. For ar eversible reactiono fasurface-confined electrode, Daifuku et al. [29] gave at heoretical equation that links d with the interaction parameter X,Equation (1). and q T denotes the surfacec overagea nd is assumed to be 1f or all three different states of the film. R is the ideal gas constant The interaction energies W are found to be positive for all three redox states of the H 2 PTCDI-film-neutral (Table 1, grey), radical-anion (Table 1, red), and dianion (Table1,b lue). Furthermore, the neutralf ilm has the strongest interactions and the radical-anion film the weakest, resulting in as harp peak spike of the first reduction wave of the H 2 PTCDI-film on Cp (indicated by Ai nF igure 4a). [20] The half-height width increases on increasingscan rates because someirreversibility of the reactions is occurring, as mentioned before.I nterestingly,t he same seems to be true for small scan rates.
It can be inferred that the presence of Na ions gives rise to a competition between hydrogen bonding in the PTCDIf ilm and the classical ion-dipole interaction, suggesting al essening in the interaction energieso ft he neutral molecules. Unfortunately,t his lessening in the interaction energies may be addingt o the potential material instability upon reduction and the dissolution of active material upon repeateds odiation/desodiation cycling.
The experimental response is characterized by as loping plateau-like region around the peak potentials A, B, and A' &B ' that are known from the CV measurements ( Figure 3). The most pronouncedp lateau was measureda tt he oxidation peak potentialA ' &B ' (Figure6a). The first cycle has as odiation capacity of 14.1 mAh and ad esodiation capacity of 13.8 mAh, which corresponds to as pecificg ravimetric capacity of 156 and 152 mAh g À1 .T his is highert han the theoretical capacity of 137 mAh g À1 (Figure 6b,b lack dashed line) expected for at woelectron reduction/back-oxidation of the H 2 PTCDIm olecule. The additional capacity can be attributed to the contribution of the Cp paper substrate, which itselfh as ac apacity contribution of 2.6 mAh for an appliedc onstant current of 5 mA( compare Supporting Information, Figure S6 a, dashedb lack line). The 12 th cycle shows ad istinct decreaseo ft he plateau-like regions,w hich can be attributed to the loss of active material, probablyd ue to dissolution. This is further corroborated by UV/Vis measurements of the battery electrolyte solutiona fter 115G CPL measurements shown in Figure 6, revealing an absorptionm aximum at around 570 nm and consequently ad istinct color change of the battery electrolyte towards violet ( Figure S7). The capacity values for the 12 th cycle are 12.4/ 12.0 mAh or 136/133 mAh g À1 for the sodiation/desodiation, respectively (Figure 6b and in more detail in Figure S6 in the Supporting Information). This corresponds to an active material loss of about 12 %i nt he first 12 cycles, under the assumption that the other contributions to the capacity remainc onstant.  The 100 th cycle (depicted as dashedb lack line in Figure 6a)h as as pecific capacity of 115mAh g À1 ,w hich corresponds to al oss of 26 %o fa ctive material. In Figure 6b the obtained values of the specific gravimetric capacity and the corresponding coulombic efficiency over 115c ycles with different appliedc onstant currents of 5(% 0.4 C), 70 ( % 6C), 210 ( % 17 C) and 35 mA ( % 3C)are depicted. The coulombic efficiency for the evolution of the specific gravimetric capacity for the first 12 cycles stays nearly constant with av alue of 97 %. For the next 15 cycles a constantc urrent of 70 mA( % 6C)i sa pplied. The specific capacity in the first cycle at 70 mAi sm easured with 100/ 97 mAh g À1 for sodiation/desodiation, respectively.A fter 15 cycles at 70 mA, the composite electrode loses about 7% of its capacityr esulting in ac oulombic efficiency of 98 %. When the appliedc onstant current is furtherr aised to 210 mA( % 17 C), the measured specific capacity drops to 78/77 mAh g À1 for the first cycle and shows al oss in capacity of 5% after 15 cycles. The coulombic efficiency is thereby maintained above 99 %.
Following, ac onstant current of 35 mA(% 3C)w as applied for the next 15 cycles.T he obtained specific capacity valuesf or its first cycle are 99/95 mAh g À1 ,a nd al oss of 3% after 15 cycles at 35 mAi sr ecorded with ac oulombic efficiencyo f9 6%.W hen the applied currenti sl owered again to the initial current of 5 mA(% 0.4 C), the sodiation/desodiation capacity is measured with as pecific capacity of 138/112 mAh g À1 ,r espectively.T his capacityi sv ery close to the capacity after the initial 12 cycles at 5 mA, revealing the good cyclability of the H 2 PTCDI-Cp composite electrodes at elevated sodiation rates. The capacity, however, drops significantly to 110/96 mAh g À1 after an additional 50 cycles at the applied lowc onstant current of 5 mA, which is al oss of 20 %f or the sodiation capacity and of 14 % for the desodiation capacity.T his impinges clearly on the coulombic efficiency,w hich shows av alue of only 81 %a ti ts first cycle with 5 mAa nd of 87 %a ti ts 50 th cycle. This severe drop in efficiency is rooted in an irreversible reduction reactionn ear 1V in the sodiation cycle,w hich is best seen in ad ifferential capacityp lot for the GCPL data( Figure S8). This irreversible side reactioni sm ost dominantf or smallc urrents, for two main reasons. First, its overpotential increasesw ith increasing Cr ate (beyond1 V) and secondly,t he reduction reaction is most likely linked to an electrolyte decomposition reaction as it can be also seen on the pure Cp substrate. This electrolyte decompositioni sf ound to increase the lower the potential is (the more reductively biased) and the longer the electrode is kept at this low potential. Ad ischarge rate of 0.4 Cc orresponds to a discharge time of about 2.5 h, while adischarge rate of 6C corresponds to ad ischarge time of about 0.17 ho r1 0min. The higher the rate, the less time the electrode is kept at low potentials and the less the electrode is sufferingf rom side reactions and material dissolution,l eading to higher coulombic efficiencies (Figure 6b). The capacity resulting from the active materialg ets lower during cycling while the capacity of the irreversible side reactiond oes not drop correspondingly;h ence, this results in adrop in efficiency with prolonged cycling. Overall, the loss of activematerialcapacity during the whole experiment is 37 %( loss of desodiationcapacity).
The observed cycle-life performances are relatively good compared to low-molecular-weight organic active materials previously reported for lithium or sodium systems, which typically suffer from poor cycle performance due to the dissolution of the redox-activem olecules into the electrolyte solutions. [14,32] While there are numerousp ublicationso no rganic cathode materials for Li-and Na-ion batteries, there is still a manageable amount of literature on perylene-based cathode materials for Na-ion batteries. To put the results obtained for our PTCDI-Cp composite electrodes in perspective, Figure7 showsacomparison of the charging time over the specific capacity reached for various perylene-based cathode materials that have been reported for their application in Na-ion batteries. Among them are 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA), [15] PTCDI for Na- [16] and Li [25] -based electrolytes, N,N'-bis(n-propylacetyl)-perylene-3,4,9,10-tetracarboxylic diimide( PDI), [24] N,N'-diamino-3,4,9,10-perylenetetracarboxylic poly-imide (PI), [30] and polyimide (PI)/multi-walled carbon nanotube (MWCNT) composite electrode. [31] In comparison, it can be seen that our PTCDI-Cp composite electrodes allow for both, high specific capacity and exceptionally fast charging/ discharging rates (Figure 7).

Conclusions
We demonstrate that Na-ion storage in the H-terminatedp erylenetetracarboxylic diimide (H 2 PTCDI) film allows for remarkably fast charging/dischargingr ates, governed by ad iffusionless mechanism, where the transport of the Na counter ioni s not al imiting factor.C onsequently,e xceptionally high sodiation rates of 17 C( 2.3 Ag À1 )a re possible. Since the PTCDI film is best described by am ultilayer surface-confined electrode, the kinetic limitation of the electrode seems to be governed by the electron exchange reaction throughout the stacked PTCDI layers. All three redox states of the H 2 PTCDI molecules Figure 7. Comparison of the charging time over the specific capacity reachedfor various perylene-based cathode materials for Na-ion batteries. Definitions of the shownmaterials are:3,4,9,10-perylene-tetracarboxylicaciddianhydride (PTCDA), 3,4,9,10-perylene-bis(dicarboximide) (PTCDI), N,N'bis(n-propylacetyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI), N,N'-diamino-3,4,9,10-perylenetetracarboxylic polyimide (PI) and multiwalled carbon nanotube (MWCNT). The data has been reproduced from several literature references. [15,16,24,25,30,31] (neutral, radical-anion,a nd dianion) are found to have positive interaction energies, explaining the non-idealb ehavior,t hat is, the hysteresis effect and the observed sharp peaks upon cyclic voltammetry (CV) measurements, compared to ar eversible surface confined system. While H 2 PTCDI on carbon paper (Cp) demonstrates reasonably good cycle-life performances, its dissolution in the battery electrolyte is still as erious issue that has to be addressed in future studies. Similar to inorganic, carbon-based materials, such as reduced graphene oxide or expanded carbon, [33,34] quinone-based organic composites like H 2 PTCDI on Cp may offer ac ost-effective,a bundant,a nd also environmentally benign cathode materialf or fast rechargeable Na ion batteries.