Conjugated Molecules and Polymers in Secondary Batteries: Comparison
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Intrinsically conducting polymers ICPs are oligo- or polymeric organic materials with numerous strikingly unusual properties like high electronic conductivity depending on their state of oxidation and pronounced electrochemical redox activity. Because a redox process is associated with electronic charge transfer ICPs have been proposed as charge storage materials in electrodes of secondary batteries or supercapacitors. In addition their use as binder in electrodes or as coating material has been suggested.  ICPs was briefly introduced and these various applications in batteries were highlighted here.

  • intrinsically conducting polymers
  • oligomers
  • conjugated molecules
  • secondary batteries
  • electrochemical energy conversion
  • electrochemical energy storage

1. Introduction

In electrochemical devices for energy storage electric energy is stored by electrochemi­cal transformation (electrode reaction) of electrode materials (active masses) from a state of lower energy (corresponding to a lower value of Gibbs energy or free reac­tion enthalpy with respect to the device containing two electrodes; frequently stated data with respect to only one material or electrode are meaningless) into a higher state (com­monly called charged state). The option of storing electric energy without such conver­sion in the electric field of a capacitor or the magnetic field of a coil has become rele­vant for the first option only recently with the advent of supercapacitors, the second op­tion is relevant only for storage on a small scale for a few special applications. Figure 1 illus­trates these options schematically.

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Figure 1. Basic modes of electric energy storage (top), cross section of a secondary battery, coin-type (bottom).

The associated electrode reactions should proceed fast and reversibly (in its various com­mon meanings: close to equilibrium conditions without excessive losses due to elec­trode overpotentials caused by slow electrode reactions, in both directions on the same reac­tion pathway), they should be possible in both directions as many times as desired by the user without significant deterioration of the participating materials [1][2]. Numerous mate­rials of mostly inorganic origin have been examined, many of them are commonly em­ployed in commercial products [3]. Organic materials have been considered only infre­quently, presumably because of concerns about insufficient stability and about infe­rior charge storage capability. An overview of typical data (Table 1) comparing storage capabilities of inorganic and organic materials corrects the latter concerns:

Table 1. Selected electrochemical data of some electrode materials.

Material Molecular Weight of Repeat Unit/g Oxidation Level */- Theor. Q # Measur. Q/F·g−1
PANI 93 0.5 750 F·g−1 240
PPy 67 0.33 620 F·g−1 530
PTh 84 0.33 485 F·g−1 -
PEDOT 142 0.33 210 F·g−1 92
Porphyrin

C20H14N4		 310.35 1 311 As·g−1 -
Quinone/HQ 108 2 1787 As·g−1 -
Ferrocene 185 1 522 As·g−1 -
Li 6.939 1 13,904 As·g−1 -
Al 26.98 3 10,728 As·g−1 -
PbO2 239 2 807 As·g−1 -

1 Data taken from [4]. PANI = polyaniline, PPy = polypyrrole (also poly (2,5-pyrrolylene)), PTh = Polythiophene (also poly (2,5-thienylene)), PEDOT = poly-3,4-ethylenedioxythiophene. * Oxidation level, also “dopant level”, reports the fraction of oxidized repeat units and the number of electrons transferred in the electrode reaction. # gravimetric charge density can be stated with respect to the electrode reaction in units As·g−1 or in case of a material where no clear electrode reactions can be stated as an amount of charge stored within a change of electrode potential in units of As·g−1 (i.e., F·V−1·g−1).

Numbers reported in the representative selection of Table 1 should be considered with care when referring to ICPs. In case of materials with well-defined discrete charge-storage sites like lead ions in PbO2 or aluminum atoms in an aluminum electrode in case of ICPs charge transfer may proceed at a given location of the polymer chain, but the created charge is delocalized along the molecular chain leaving the ques­tion along how many repeat units delocalization is effective. The answer is stated with the oxidation level. At a level equal to 1 all repeat units are oxidized (reduced), but real val­ues tend be much smaller as discussed below. Such a state may be reached with PANI when the pernigraniline state is established (for more details see below), but this state is chemically rather unsta­ble, it is commonly called overoxidized with respect to the application of ICPs pre­sented here. Overoxidation of PANI as well as other ICPs has been studied, but as de­plored elsewhere [5] no review has been provided so far. It appears safe to state at this point that increase of storage capability by applying lower/higher electrode potentials will most likely result in faster electrode and consequently battery degradation.

A further comparison of electrode materials as shown in a representative selection only in Table 1 must address its ionic and electronic conductivity. Both modes of charge transport depend on concentration and mobility of charge carriers. At first glance metallic electrodes (e.g. lead in the lead acid battery) appear to be almost ideal. Real electrodes even of lead or zinc are porous in order to provide a large surface area. The effective electrical conductance will be decreased. The majority of battery electrode materials – both organic and inorganic ones – are poor conductors, semiconductors or almost insulating. Whether the poor electronic conductance of a material is due to low concentration or mobility depends on many factors, in case of inorganic materials crystallinity, impurities, morphology are among the relevant factors. In case of organic materials details of molecular structure, degree of polymerization and molecular ordering can be relevant, an overview provides more insights [6]. Ionic mobility is relevant when chemical reactions and ion transport phenomena are connected to the electrochemical redox reaction as the central step in energy conversion and storage. Mobility of lithium atoms and ions in graphite is one of the factors limiting current generation capability of this negative electrode in lithium-ion batteries, the same applies to intercalation or insertion compounds serving both as negative or positive electrode in metal-ion batteries. With organic materials such connected reactions are less frequent, in reports on the performance of organic electrode materials including those presented below limitations related to ionic mobility are rarely – if at all – addressed. In a wider sense mobility of ions acting as counter ions for charge balancing may also be relevant. Ions moving in or out of the porous electrode material slower or faster may negatively affect the possible current. Studies addressing explicitly this issue are lacking. Even without exact knowledge of conductivity-related material parameters electrode design (for a scheme see below) attempts to take care of the related issues more or less intuitively.

Further advantages and also limitations of organic materials, in particular ICPs, as battery electrode materials will be addressed in detail below.

2. The Mmaterials: Intrinsically Cconducting Ppolymers

The term intrinsically conducting polymer designates a class of mostly organic oli­gomeric or polymeric materials which show electronic conductivity because of mobile charge carriers capable of moving along conjugated segments and hopping between such seg­ments. Materials afforded with electronic conductivity by addition of an electronically con­ducting material like graphite powder or metal fibers to an insulating material are sim­ply and unfortunately somewhat misleadingly called conducting poly­mers, their other designation as filled polymers is not better and as imprecise because it ad­dresses the addition of a second material (forming a composite) which may be insulat­ing and/or may provide some other functionality.

Because of this amazing merger of typical properties of an organic material with those of a metal, for example, a very inorganic material, ICPs have sometimes been called synthetic met­als. Unfortunately the latter term has also been applied to a class of crystalline materi­als: charge transfer complexes (for example, N,N'-dicyano­naphtha­quinone­di­imi­ne (DCNNI) and tetrathiafulvalene (TTF). This has finally resulted in some confusion and frus­trated readers guided to reports on such materials [7][8] when expecting materials for en­ergy storage in ICPs.

ICPs are composed mostly of carbon and hydrogen; sulfur, nitrogen and oxygen may be present as heteroatoms. A selection of typical examples with simplified molecular structures, systematic names and generally accepted acronyms and typical values of elec­tronic conductivities is provided in Figure 2. More examples can be found in mono­graphs and handbooks [9][10][11][12][13]. Because of their fascinating combination: organic matter with metal-like properties ICPs have attracted tremendous research activities once in particu­lar the electronic conductivity was noticed; but actually they have been known much longer (for brief historic overviews see [14][15]. Among the numerous applications of these oligo- and polymeric materials showing conjugation already in their repeat units and even more extended conjugation along the molecular chains their use as active materi­als in devices for electrochemical energy storage and conversion was suggested early. Initially this meant use in primary and secondary batteries, with the advent of superca­pacitors their use in these systems was examined also. It was fo­cused on secondary batteries, but given the ongoing merger of batteries and supercapaci­tors noticed before [16] and reviewed elsewhere [17] considerations and arguments apply fre­quently also to supercapacitors; materials may turn up in both types of devices.

For overviews on selected aspects of the application of ICPs in supercapacitors see [18][19][20][21][22][23][24][25][26] their use in flexible devices has been discussed in [27][28], in stretchable devices in [29].

A different class of polymers with a pronounced electrochemical redox activity is called redox-polymers [13]. In these polymers localized redox-active functional entities, mostly substituents (or pendant groups) at a molecular backbone showing neither own electro­chemical activity nor conjugation enabling electronic charge transport along the back­bone, provide an electrochemical response which might indeed also be of practical rele­vance for electrochemical energy conversion and storage. In case these moieties are at­tached to the backbone of an ICP they will possibly disturb conjugation, diminish the ICPs capability to stabilize mobile charge carriers and thus turn the ICP into a poorly conduct­ing polymer [30]. This can possibly be avoided by substitution with ionogenic moie­ties showing no redox functionality like sulfonate groups [31], for examples see be­low. Because they differ chemically very much from ICPs and because their mode of opera­tion is very different also they are not treated here, reviews are available [32][33][34]. Some­times these polymers show electric conductivity enabled by charge transfer be­tween these redox centers by charge hopping. Such polymers are called re­dox-conducting polymers [35], they are also beyond the scope here.

ICPs can be prepared by a variety of polymerization methods with widely varying fea­tures more or less suitable for a particular application, for an overview see [36]. Prepara­tion conditions and experimental parameters can be adjusted yielding specific mor­phologies [37], for more examples and details see below.

 

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Figure 2. Examples of intrinsically conducting polymers.

3. The Aapplication: Intrinsically Cconducting Ppolymers in Ssecondary Bbatteries

ICPs have been suggested as constituent in secondary batteries, in particular in electrodes, in various functions. These are briefly addressed below, selected applied ICPs will be presented thereafter.

3.1 Use as Aactive Mmass

Initial optimism regarding real applications of ICPs in numerous fields including the subject of this entry here has been marred by either observed or at least expected insuffi­cient lack of long-term stability resulting in electrochemical applications in turn in decreasing stor­age and current capability. In general terms this has been addressed for ICPs in secondary batter­ies in [1] and for supercapacitors in [38]. The sometimes sloping discharge cell volt­ages were sometimes claimed as a further drawback. The characteristic property of every ICP, its electronic conductivity, changes as a function of the state of oxida­tion/reduction of the ICP across several orders of magnitude (for example, from about 107 S·m-1 for doped, oxidized trans-PA to 10-2 S·m-1 for neutral, undoped trans-PA). Sometimes (for example, PANI) protonation plays an additional role (at pH = - 0.3 the change covers four orders of magnitude, at pH = 7 only three orders, in organic electrolyte solutions even less). This fascinating property is an unwel­come one in the application discussed here because low conductance tends to limit cur­rent capability of an electrode material and to increase the Ohmic resistance of elec­trode material and complete cell. The rather uninspired and fairly traditional approach to­wards a compensation of this is the addition of conducting materials like carbon black or graphite. Unfortunately, this adds dead weight and decreases specific storage capabil­ity. Another approach addressed below is the use of thin films on highly conducting sub­strates or 3D-architectures.

The recent surge in interest in ICPs for such applications is pre­sumably related to the interest in materials with practically unlimited resources (which is a growing concern with many current battery materials), to the possibility of rather simple handling of used/worn out materials not requiring the specific procedures re­quired for handling heavy metal-containing batteries and devices, to the simple redox reac­tions not encumbered by intercalation or other possibly slow reaction steps, and to mostly smaller energy usage in preparing these materials under environmentally accept­able conditions (for a broader overview with regard to metal-ion batteries see [39]). Changes in materials properties can be afforded in most cases using the tools of well-established organic synthesis [30]. Finally these materials may provide a bridge to the use of renewable materials like lignin [40][41][42][43][44][45] or other materials derived from natu­ral resources [46][47][48]. Lastly these materials may enable or at least simplify construc­tion of flexible and even stretchable devices. The pronounced conjugation in ICPs, which even changes as a function of the degree of oxidation/reduction, stabilizes charges on the chain, it also results in strong interac­tions be­tween sites where oxidation/reduction proceeds, this is one of the causes of sometimes poorly developed current peaks in CVs [49].

Lacking compatibility with established systems in terms of cell voltage as another bar­rier initially observed does not appear to be a problem anymore with batteries in­stalled in electronic devices without an option of simply exchanging them like with stan­dard batteries before. Non-standard cell voltages and other specific features of a given cell thus can be easily integrated into the device. The current level of interchangeability of de­vices and batteries (with cell voltages of mostly aqueous systems around 1.5 V) is thus lost, but this appears to be no major concern anymore in many cases. Instead, perspectives of all-polymer or even paper-based batteries appear to be more attractive or even realistic [50].

Charge storage in every material proceeds by redox reactions. Another possibility – charge accumulation in the electrochemical double as employed in electrochemical dou­ble layer capacitors EDLC [51][52] – is also of considerable interest but not in the focus of this contribution, it is not related to conjugation in molecules and the associated elec­tronic conductivity. Because an electrode with an ICP as active mass will establish a dou­ble layer with the associated double layer capacity when brought into contact with an electro­lyte solution capacitive contributions to charge storage and with regard to the full cell to energy storage may be important nevertheless. How to separate these contribu­tions is the subject of an ongoing heated debate [53], specific aspects with regard to PANI have been reviewed recently [54].

In case of an ICP charge transfer proceeds via removal (oxidation, most frequently) or addition of an electron (reduction, less frequently observed because of the relatively low electrode potentials needed in most cases and the poor stability of the created spe­cies) yielding a radical cation in the former and a radical anion in the latter case. The radi­cal property is caused by the fact, that removal from an electron in a HOMO leaves an unpaired electron; addition of an electron into a LUMO also yields an unpaired elec­tron. This radical property has been extensively probed with electron paramagnetic reso­nance spectroscopy [55]. The term doping, p-doping in case of the oxidation and n-doping in case of a reduction, is frequently used. Different from the quite common mean­ing in semiconductor physics where doping means substitution of atoms (e.g. sili­con or germanium) by very small quantities of (dopant) atoms with different valencies (e.g. boron, gallium, phosphorus causing hole or p-conduction in case of the group 3 ele­ments and n-conduction in case of the group 5 element) with ICPs apparently refers to the type conduction at first glance: hole conduction upon oxidation, electron conduction in case of reduction. Whether hole conduction actually means electron movement into the opposite direction is perhaps a more philosophical question in the present context. In any case the numbers (correctly number densities or concentrations) of generated charge carri­ers is larger by orders of magnitude as compared to semiconductor doping of e.g. sili­con. Adding to the confusion is the further use of dopant ion to designate counter ions mov­ing in or out of the ICP for charge compensation. In this entry neither generation of fur­ther confusion nor imprecision are intended, terms related to doping are avoided.

Because of conjugation in the studied oligo- or polymer this electron (or hole) is not located, the observed EPR-spectra show single lines and no hyperfine structure. This delocaliza­tion provides also the electronic conductivity along the molecular chain as a typical fea­ture of an ICP. Movement of charges between molecular chains is provided by electron hop­ping. Removal of more than one electron is possible, accordingly there may be no un­paired electrons anymore, the radical property is lost as evidenced again with EPR-spectroscopy [55]. As a consequence of the delocalization of the generated charge(s) differ­ent from redox processes with metal ions no well-defined redox potential is created, the electrode potential and consequently the cell voltage change more or less continu­ously as a function of transferred charged, this is generally called sloping discharge volt­age.

These processes can be envisaged with PANI as shown in Figure 3.Molecules 27 00546 g003 550

Figure 3. Redox processes of PANI.
Upon closer inspection of the oxidation reaction and the associated structural changes of the molecular chain, the changing extent of conjugation, as highlighted in Figure 4, becomes apparent:
Molecules 27 00546 g004 550

 

Figure 4. Changing extent of conjugation upon the oxidation of the emeraldine form of PANI.

The actual extent of conjugation may differ. Determination of molecular weights of ICPs is notoriously difficult; in addition actual values may differ wildly for a given ICP de­pending on preparation conditions. This extent should not be confused with the effec­tive conjugation length introduced by Zerbi et al. [56].

The associated electrochemical response of a film of PANI is shown in Figure 5.

Molecules 27 00546 g005 550

 

Figure 5. CV of a polyaniline-coated (electropolymerization by 300 electrode potential cycles within the potential range as indicated) stainless steel grid electrode (1 cm2) in an aqueous electrolyte solution of 0.1 M aniline + 1 M HClO4, dE/dt = 100 mV·s−1, nitrogen purged.

In the positive-going electrode potential scan in a cyclic voltammogram the first re­dox transition from leucoemeraldine to emeraldine is observed as the current peak around ERHE = 0. 5 V, the second peak indicating the further transition to pernigraniline around ERHE = 1 V is not very pronounced because electrode excursion into this region (overoxidation) causes diminished chemical stability of the polymer resulting in subsequent degradation. In the subsequent negative-going scan both processes are reversed. The distinctly differ­ent peak shapes are due to the different electrochemical and structural situations: When start­ing at ERHE = 0. 0 V PANI is in its poorly conducting (semi-conducting possibly depend­ing on the definition of this term [6]) state providing hardly electronic communica­tion between redox sites where electron transfer proceeds resulting in a well-developed peak [49]. The second peak recorded with PANI already in its highly conduct­ing emeraldine state is sometimes much broader (as observed here) because of the extensive electronic interactions between redox sites supported by the extended conjuga­tion along the molecular chain. Depending on preparation conditions [57] , film thick­ness, morphology and so on. different results can be observed as shown e.g. in [58] with a thin film on a gold electrode providing a well-developed second peak. In the negative go­ing return scan the oxidation processes are reversed, the same arguments as made be­fore can be applied to peak shape. In addition to the more or less pronounced current peaks a rather large residual current between the peaks can be observed. Assignment to a dou­ble layer charging current or to a redox process has been the subject of extensive re­search and discussion; some considerations have been collected in [49]. Because ICPs show frequently an irregular morphology suggesting a large surface area of the material in contact with the electrolyte solution considerable values of a double layer capacitance can be expected, this is of particular interest for high-current applications (like in supercapaci­tors), where the fast charge/discharge of the double layer sustains much higher currents than the relatively slow redox processes. Because of the relatively fast dis­charge of EDLC supercapacitors due to dissipation of the accumulated charges in the dou­ble layer [59] this contribution is less relevant for secondary batteries considered here. The frequently claimed high fraction of capacitive/pseudocapacitive contributions in battery electrodes is most likely based on a wrong model as briefly discussed before [53] and pointed out initially elsewhere [60]. Electrochemical impedance measurements may provide an approximation to the desired separation [61][62].

Closely associated with these redox processes are further changes possibly resulting in deterioration of the ICP and in case of an electrode with an ICP as active mass in elec­trode performance.

Shape change

The redox processes as shown in Fig. 2 are associated with ion movements. For charge balancing upon oxidation either negative (an)ions have to move into the film or cations (e.g. protons) have to move out. The latter is possible only with polymers having mo­lecular sites which can be protonated/deprotonated. Whether ion ingress and/or egress proceed and which process possibly dominates has been studied extensively, for an introduction see [63]. These processes are associated with volume changes (swell­ing/shrinking, also: shape change) of the polymer because the ions move with some solva­tion shell. This may negatively affect stability and performance. These changes may re­sult in fragmentation of the polymer, loss of contact between ICP particles, added conduct­ing carbon and the current collector. Various options to mitigate this shape change have been proposed and examined:

Malinauskas has suggested self-doped ICPs, e.g. self-doped PANI [64]. In such polymers the presence of fixed negative charges located on anionic groups changes the mechanism of charge compensation during the redox processes of PANI: in­stead of ingress of anions during oxidation possibly associated with the problems dis­cussed above release of cations (e.g. protons or lithium ions) bound at the anionic sites suf­fices for charge compensation with less detrimental effects. Such self-doped PANI can be obtained in various ways:

  • Sulfonation of PANI by chemical post-treatment.
  • Polymerization of suitable substituted monomers.
  • Copolymerization of aniline and a second suitably substituted comonomer (e.g. co­polymerization of aniline and e.g. N-methylaniline and N(3-sulpho­pro­pyl)aniline [65]] (see Figure 6) or aniline and o-aminobenzene sulfonic acid [66] or m-aminobenzoic acid and aniline [67][68]).
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Figure 6. Structural formula of N(3-sulpho¬propyl)aniline.

A copolymer prepared from aniline and N(3-sulpho­propyl)aniline (for examples see [65][69]) has been studied. In the former study with N-methylaniline as the comonomer high­est electrochemical activity was observed when a comonomer ratio of 1:1 was estab­lished in the electropolymerization solution. This agrees quite well with the degree of dop­ing 0.5 listed above for PANI in Table 1 with half of the repeat units formally participat­ing in the redox reaction. Somewhat surprisingly this approach has not been ex­ploited in reported research elsewhere. In an investigation employing a copolymer of ani­line and m-aminobenzoic acid as positive electrode 30 % capacitance loss after 1000 cy­cles for a complete supercapacitor cell with a PPy negative electrode were noticed, unfortu­nately no comparison with a similar system without a self-doped ICP was at­tempted [70]. The copolymer obtained from aniline and metanilic acid by electropolymeriza­tion showed significant electrochemical activity in a neutral aqueous electro­lyte solution of 0.5 M Na2SO4 with about 50 % capacitance retention when assem­bled into a symmetric supercapacitor [71].

Because of the pH-sensitivity of a zinc electrode, which is hardly compatible with an acidic aqueous electrolyte solution preferred for plain PANI, self-doped PANI has been suggested as a form of PANI without the need for an acidic electrolyte solution for a rechargeable zinc-ion PANI bat­tery [72]. As a compromise a mildly acidic electrolyte solution (pH = 4.5) has been sug­gested for a PANI/zinc cell [73]. Deposition of PANI as a thin film on an electronically highly conductive 3D-substrate further assisted in ameliorating the problem of moderate elec­trochemical activity at this pH [74].

The concept of self-doping has rarely been explored beyond the particularly pH-sensitive PANI. An attempt to prepare an oligomeric bis[3,4-ethylene­dioxy­thio­phene]3thiophene butyric acid has been reported [75], its use in “green energy” applica­tion has been proposed but not explored.

Another approach examined more frequently is the formation of micro- and nanostruc­tures allowing shape and volume change to an extent large enough to keep the opera­tion capability of the ICP at still acceptable performance of the material. A compari­son of various morphologies (2D: thin film; 3D: microsphere, microtube, microparti­cles, nanowires networks, nanowire arrays) in particular for supercapacitor electrode applica­tions has been provided [24], nanowire arrays have been suggested as the most promis­ing approach. A similar overview focused on secondary batteries is available [76]. A broader overview focused on 1-D structures of ICPs was provided [77]. In addition to pro­viding more stable electrode structures (morphologies, architectures) nanostructuring may also help to improve performance in terms of current capability. A growing current across the elec­trode/electrolyte solution interface at a constant interfacial area results in larger devia­tion of the electrode potential from its equilibrium value, a larger overpotential. This unwelcome effect can be limited by increasing the interfacial surface area using a po­rous electrode or another 3D-structure. Bottom-up strategies starting with molecular enti­ties assembled in a controlled way into suitable 3D-structures are available as well as top-down strategies starting with bulk materials transformed by milling, evapora­tion-deposition and so on into suitable 3D-structures. For ICPs obviously the former approach is appropriate, details and examples are presented below. A further concern comes into play: For increased energy of a cell and larger storage capability of an electrode more mate­rial, for example, a thicker electrode, is required. As illustrated schematically in Figure 7 more mass and current capability limited by Ohmic resistance of the electrode material and the elec­trolyte solution must be taken into account.

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Figure 7. Sketches of battery electrode architectures. Bottom right: black: electronically conducting support, red: active mass with ion (→) and electron (→) pathway).

A thin electrode (Figure 7a) may not provide enough mass, a thicker non-porous elec­trode (Figure 7b) has longer electronic pathways when the electrode reaction still takes place at the ICP/solution interface, an electronically conducting 3D-support (Figure 7c) provides lar­ger interfacial area to be covered subsequently with active mass (Figure 7d), the actual coat­ing must finally balance electronic and ionic conduction pathways and their respec­tive lengths and contribution to Ohmic resistance (Figure 7e and insert). 3D-supports (in­stead of smooth metal foils) can be metal grids or meshes, carbon or graphite paper or car­bon structures prepared by pyrolysis of natural materials from biological sources. Fur­ther structuring on an even finer level can be afforded by controlled and directed ICP depo­sition. These considerations also apply to materials and electrodes for supercapaci­tors and they have been highlighted before [78]. Possible options for various ICPs will be ad­dressed below.

Two further modes of ICP and thus electrode deterioration have been noticed and exam­ined in detail:

Peeling off

Closely related and particularly relevant for electrodes where the ICP has been di­rectly deposited onto the cur­rent collector (for example, no binder is used) is peeling off from the cur­rent collector leaving the removed particles lost for charge/discharge. Because it is closely related to shape-change, actually may be a result of it, remedies against the former may help against peeling off.

Overoxidation

Overoxidation is closely associated with charge/discharge of an ICP in a battery electrode. When the electrode potential of an ICP is moved into positive direc­tion (see Figure 5) electrooxidation forms cations on the polymer chain, chemically speak­ing they are radicals (polarons is another designation borrowed from solid state physics). These species can be subject to chemical transformation by reac­tion with electrolyte solution species. When sufficiently positive electrode potentials are reached additional oxidation of water may yield radicalic intermediates which in turn chemically attack and degrade the ICP. The anions moving into the ICP in most cases for charge compensation may affect these reactions, stud­ies of anion-specific effects show such influ­ences for PANI [79]. Reaction mechanisms and effects of overoxida­tion vary, and in most studies decreases in possible electronic conductance, changes of molecular struc­ture, decrease of possible charge storage and in the extreme case complete loss of electrochemi­cal redox re­sponse are reported (for examples see [80][81]). Unfortunately, a wider review on this subject seems to be missing, only the degradation of some forms of PEDOT has been inspected more closely in an overview [82]. At first glance a simplistic view would suggest an easy solution by limiting the potential excursion with suitable elec­tronic circuitry providing limitation of maximum charge voltage. Unfortunately, practi­cal execution is more complicated. Although the operating voltage limits of batteries have always required appro­priate cir­cuit design considerations the rather low operating voltage of a secon­dary battery or a supercapacitor with an aqueous electrolyte solution and the sensitivity to­wards overoxida­tion require more precise voltage limitation making the use of batteries and supercapacitors with these mate­rials less attractive. In addition, potential distribution inside of an elec­trode may leave parts of the material ex­posed to electrode potentials already too high causing overoxi­dation. Consequently, other options have been examined (for example see [24]). Selec­tion of suitable solvents and electrolytes may help to avoid oxidative formation of chemi­cally reactive species (like hydroxyl radicals when water is used as a solvent).

The redox processes discussed above with PANI as an example are one option of charge storage in addition to double layer storage. The associated ion movement is basically not related to any redox process at the other electrode, there is even no need that any participating ion is involved in reac­tions at both, i.e. the negative and the positive electrodes (Frequently the former elec­trode is called the anode, the latter one the cathode. Obviously these terms are correct only during discharge; during charge the anode turns into a cathode with considerable possi­bilities of confusion [83]. Consequently, this terminology should be avoided). In case of the very popular metal-ion batteries (for example, lithium-ion or sodium-ion bat­tery) ions, mostly the metal cations, move between the electrodes. This has caused the nick-name “rocking-chair battery”. Their major advantage is the almost completely con­stant concentration of ions, i.e. cations, in the electrolyte solution keeping the ionic conductiv­ity and thus the Ohmic internal resistance of the battery constant. Accordingly the ICP operating in an electrode in such a cell should provide sites for metal ion storage. At this point the more or less conjugated system of an ICP is not helpful for providing stor­age sites by itself. Instead ICPs with attached functional groups have been proposed. An overview is provided in [5], and representative examples will be given below.

It is about various aspects of ICPs as applied in electrochemi­cal energy conversion and storage are available [19][30][78][83][84][85]. This includes the particularly frequently studied use of ICPs as host material for sulfur in metal-ion sulfur batteries confining soluble polysul­fide species and suppressing the shuttle mechanism, for overviews on this subject see [5][86].

3.2 Use as Bbinder

With very few exceptions (lead or lead dioxide electrode) addition of a binder to the active mass for an electrode is required to provide cohesion between electrode particles and to provide adhesion to the electrode support and current collector. Conventionally the use of synthetic polymers added with several percents of weight is the standard. These binders are electrochemical inactive and electrically insulating, they decrease the specific storage capability of the electrode masse and negatively affect the conductivity of the electrode and in turn the current capability. ICPs have been proposed as substitutes which form in additional coatings on active particles (core-shell structures) supporting improved performance. In particular PEDOT has been studied as a binder for electrodes in lithium-ion batteries. An overview describes advantages and already achieved performance [87]. The adhesive properties of this ICP provide enough cohesion to maintain integrity of the active material, in addition their electronic conductivity may enhance current capability. The additional transport pathways are schematically illustrated in Figure 8.



Figure 8: Electron transport in a positive electrode of a lithium ion battery with added carbon black and a polymeric binder (left) and with PEDOT:PSS as a binder (right) [87].

3.3 Use as Ccoating

Some electrode materials show attractive storage capacities but insufficient stability. This may be due to volume changes during redox processes resulting in mechanical disintegration, this may also be due to solubility of the material at least in one state of charge. Coating with ICPs has been tested successfully. A polypyrrole-coated LiV3O8-nanocomposite for use in a lithium-ion battery with an aqueous electrolyte solution schematically illustrated in Figure 9 has significantly improved long-term cycling performance [88].



Figure 9: Proposed mode of action of a PPy-coating on a LiV3O8-nanocomposite [89].

3.4 Miscellaneous Uuses

Coating of metal foils as employed in many metal-ion batteries as mechanical support and current collectors with various ICPs to improve material adherence and to afford corrosion protection has been proposed [87].

4. Selected Ppolymers

Here are some representative ICPs proposed and/or actually studied for application in secondary batteries are briefly presented and typical application examples and obtained re­sults are included. For overviews see also [90][91][92].

4.1 Polyaniline

Possibly the first secondary battery with a PANI-based positive electrode has been re­ported in 1990 [93][94]. In the coin-type cell with a nominal cell voltage of 3 V the nega­tive electrode was prepared by electroplating lithium on aluminium, the positive elec­trode was PANI electropolymerized on a metal mesh, a mixture of propylene carbon­ate and 1,2-dimethoxyethane with LIBF4 served as electrolyte solution. Reasons for their rather disappointing marketing have not been reported, it might be speculated that – like with PPy (see below) – lacking interchangeability and risks inherent with a metallic lith­ium electrode in a secondary battery may have been factors.

To improve performance of PANI in a metal-ion battery (e.g. sodium-ion) sul­fonated PANI (see Figure 10) has been proposed as sodium storage material [31][95], as dis­cussed above this might also be a route to stabilization of the ICP by reducing the effects of shape change associated with counter-ion movement.

 
Molecules 27 00546 g008 550
Figure 10. Poly(aniline-co-aminobenzene-sodium-sulfonate).

Instead of self-doping by suitable substituents the use of e.g. PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) can introduce the same effect: avoid deprotonation of PANI and loss of electrochemical activity [96]. A nitro-group intro­duced by chemical copolymerization of aniline and nitroaniline has been proposed as a further option [97].

Nanostructuring of PANI already during formation by mostly chemical polymeriza­tion into e.g. nanofibers, nanotubes, nanorods and their arrays on suitable sup­ports (see above) has been addressed, for overviews and examples see [84][85][97][98][99]. Effects of template materials (both hard and soft) on obtained nanostructures are illus­trated in the following microscope images (Figure 11).

Molecules 27 00546 g009 550
Figure 11. SEM photographs of the nanostructures. (a) PANI + oxalic acid (OA), (b) PANI + OA + α-alumina, (c) PANI + camphoric acid (CA), (d) PANI + CA + α-alumina (for further details, see[100] ).

PANI and its composites as active mass in electrodes for further metal-ion batteries have been evaluated [101]. Not suffering from the mismatch of suitable pH-values for PANI and a zinc electrode in an aqueous electrolyte solution addressed above combination of PANI as negative electrode with a positive PbO2-electrode yields a type of rocking-chair bat­tery with anions (sulfate) moving between the electrodes [102].

PANI in supercapacitors has been reviewed earlier [103][104]. Further aspects of PANI in electrochemical energy conversion and storage have been discussed before [105].

Vanadium oxygen hydrate intercalated and exfoliated with PANI has been exam­ined as positive electrode material for an aqueous zinc-ion battery has been studied [106]. Elec­tronic conjugation of the PANI backbone and the conceivable electrostatic interaction of the conjugated π-system with Zn2+-ions and O2—-ions of the V-O-layers have been sug­gested as cause for the growth of the measured effective diffusion coefficient of Zn2+-ions by a fac­tor of 10 to 100.

4.2 Polypyrrole

The redox processes in a PPy-electrode schematically shown in Figure 12 lack the noticeable pH-dependency observed with PANI in aqueous electrolyte solution.

Molecules 27 00546 g010 550
Figure 12. Redox processes of PPy.

A first cell with two PPy electrode has been described [107]. Although high Colum­bic efficiencies were noticed poor charge retention – an essential for secondary batteries – was a major drawback, the high Ohmic resistance of PPy in the neutral state caused a large internal resistance of the cell: a further drawback. Presumably the first secondary metal-ion battery with a PPy-film 50 μm thick as positive electrode, a lithium metal foil 100 μm thick and an electrolyte solution of 0.5 M LiClO4 in propylene carbonate has been re­ported in 1987 [108][109][110]. The open-circuit cell voltage when fully charged was 3.6 V, opera­tional cell voltages between 2.0 and 4 V were recommended. At the latter value presuma­bly overoxidation of the positive electrode might have become significant; in addi­tion safety problems related to dendritic lithium deposition may have been one more ar­gument preventing a commercial success of this system. At the time of development the already mentioned problems of cell voltage incompatibility might have contributed to the commercial failure of this attempt. In a comparative study PPy as positive elec­trode with a metallic lithium negative electrode has been identified as relatively more sta­ble than PANI and PTh [111]. The same conclusion has been reported elsewhere [112].

Polypyrrole nanopipes have been proposed as positive electrode materials for lith­ium-ion batteries [113]. They combine high rate capability because of the established struc­ture combined with improved storage capability in comparison to e.g. activated car­bon used for high rate lithium-ion systems (batteries and capacitors) and mechanical stability.

Electrochemical procedures towards some PPy nanostructures have been developed [114], further aspects of nanostructures of PPy as obtained by electrodeposition have been reviewed elsewhere [115]. PPy nanotubes have shown particularly high electronic con­ductances [116][117], this in turn is advantageous when they are applied as battery elec­trode material.

4.3 Polythiophene

First attempts to use electropolymerized PTh as electrode material for secondary batter­ies have been reported by several groups starting in 1982 [118][119][120]. The redox proc­ess of the oxidation process is schematically shown in Figure 11, the reduction process yield­ing a radical anion instead of the cation as shown in Figure 13 is analogous.

Molecules 27 00546 g011 550
Figure 13. Redox processes of PTh.

In case of PTh the ICPs obtained via electropolymerization (as the ones discussed first) and by chemical polymerization showed striking differences [121]. It was suggested a different polymer sequence in another report. (Figure 14).

Molecules 27 00546 g012 550
Figure 14. Suggested structure of PTh prepared by chemical polymerization [121].

Strictly speaking this polymer is not a conjugated one. As a consequence it was sug­gested a sulfur-localized redox behavior also being the possible cause of the less sloping cell voltage. The reaction is schematically illustrated in Figure 15 [121]:

Molecules 27 00546 g013 550
Figure 15. Proposed redox reactions of the PTh illustrated in Figure 14.

For further details on chemically prepared PTh and its use as a battery electrode see [122]. An all-PTh battery has been reported [123][124]. Slightly higher doping levels than 0.33 reported for plain PTh (see Table 1) were observed with polydithienothiophene (Figure 16) [125][126][127].

Molecules 27 00546 g014 550
Figure 16. Polydithieno(3,2-b:2′,3′d)thiophene [125][126].

Closely related to PTh is the ICP PEDOT (see Figure 2). It has been examined with respect to its suitability as an electrode material [128] revealing exceptional stability of this ICP in its oxidized form even in aqueous electrolyte solutions [129], more frequently in electrochemi­cal energy storage appears to be its use as a binder (for an introduction see [5]). With further substitution by N,N,N’,N’-tetraalkylated-p-phenylenediamine (see Figure 17) an ICP showing a charge transfer of up to 2.6 e- per repeat unit could be obtained with electro­polymerized material [130]. This corresponds to two redox waves observed in CVs. Possi­bly the majority of transferred electrons go into the substituent, following Table 1 a minor fraction might go into the polymer PTh backbone.

Molecules 27 00546 g015 550
Figure 17. N-((2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methyl)-N-ethyl-N’,N’-dimethyl-p-phenylenediamine [130].

4.5 Further ICPs

Beyond the three ICPs considered above in detail as representative examples and par­ent molecules for whole families of substituted molecules (sometimes misleadingly called derivatives) further ICPs have been suggested, and some of them like polyindole [131] have been reviewed with respect to their use in secondary batteries. More ICPs and fur­ther aspects have been addressed on the use of ICPs in secondary bat­teries and so on [5][25][132][133][134][135]. Polymers including ICPs to be used as active masses in metal-ion batteries have been compared [136]. Systems with polyacetylene – sometimes consid­ered as the most basic ICP and among the first to be suggested in an all-polymer bat­tery[137] – have been examined, results appear to go barely beyond academic inter­est. For an overview see [83].

5. General Aaspects and Cconsiderations

With essentially organic polymeric materials as active masses in the electrode(s) an all-polymer battery appears to be feasible, an example of an all-PTh battery has been men­tioned above. As pointed out reduction of an ICP – this would be necessary to gener­ate the charged state of the negative electrode – is of limited practical value when possi­ble at all. Frequently the reduced state containing radical anions is not sufficiently sta­ble chemically; whether their claimed [30][138] “large impedance” is a further limita­tion remains unclear. At first glance already a device having the same material in both elec­trodes with one electrode (the positive one) in the oxidized state and the other one (the negative) in the neutral state (frequently this state is erroneously called the reduced state, possibly because it is created by reduction of the oxidized state) should provide a cell voltage and be capable of storing energy. Because the electrode potentials will be rather close even in the charged state such a cell would not be very attractive as has been pointed out earlier [139][140][141]. Nevertheless such arrangements are still pursued as criti­cally noticed [78]. A second concept employing different ICPs with sufficiently differ­ent redox potentials of the neutral-to-oxidized state transition has been considered [139], in a third one possible only with few ICPs mostly from the thiophene family oxida­tion and reduction of the same ICP present in both electrodes is used. The CV of a polythiophene prepared by electropolymerization of ethyl-p-(3-thienyl)-benzoate (see in­set of figure) displayed in Figure 18 shows a typical example.

 
Molecules 27 00546 g016 550
Figure 18. CV of poly(ethyl-p-(3-thienyl)-benzoate) coated on a platinum electrode immersed in 0.2 M Et4NBF4 acetonitrile solution at a scan rate of 100 mV/s, for further details see [142][143][144][145][146].

Reduction of substituted PTys can be further supported by suitable substituents like fluo­rine [147][148][149][150][151] (for an example see Fig. 19) or phenyl units in poly(3-phenyl-thio­phene) [140], for further examples see [30], for general considerations [152].

Reduction of substituted PTys can be further supported by suitable substituents, such as fluorine (for an example see Figure 19) or phenyl units in poly(3-phenyl-thiophene) . For further examples, see ; for general considerations, see .
Molecules 27 00546 g017 550
Figure 19. 1,4-bis-(2-thienyl)-2,5-difluorophenylene .

The term all-polymer battery refers to the active masses except for the electrolyte solu­tion. In an all-solid-state battery this concept is developed further by inserting a solid elec­trolyte or at least a gelled-polymer electrolyte instead of the liquid electrolyte solu­tion commonly used. Some concepts beyond the scope of this report have been presented else­where [30].

A frequently encountered drawback of many ICPs is their insolubility in most sol­vents and their generally poor processeability. Notable exceptions are some polymers pre­pared from substituted monomers and oligomers having molecular weights (i.e. chain lengths) sufficiently large to show typical properties of an ICP and short enough to en­able solubility or at least preparation of stable dispersions like with PEDOT.

Modeling and theoretical tools have been applied to conjugated oligo- and polymers for secondary batteries and to full devices, for an early example see [153].

Nanostructured ICPs in particular with nitrogen as heteroatom have been used as start­ing material for pyrolytic generation of nanostructured carbon as electrode material or as supporting material. The nanostructure of the starting material frequently serves as tem­plate for the obtained carbon material. In addition the nitrogen heteroatoms support high electronic conductance [113].

6. Perspectives

The use of intrinsically conducting polymers in secondary batter­ies is attractive with respect to availability of the materials, their environmental compati­bility and their sustainability. Selection of monomers and ICPs should take into account complexity of their synthesis, and ICPs requiring complex synthesis procedures may be attractive from a scientific point of view but may turn out to be economically disappointing. Stability of the materials in their various functions is still insufficient or at least disappointing in many cases. Accordingly, more extensive lifetime studies under realistic conditions (in terms of minimum cycle number, depth of discharge, currents) are recommended. If possible actual usage settings and operating parameters (like electronic supervision circuits suppressing overcharge and overoxidation) should be considered. A deeper understanding of their behavior in particular with respect to degradation processes based on intensified research yielding insights into op­tions of enhancing stability by avoiding degrading operating conditions might help along this way.

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