E ﬀ ect of Combined Conductive Polymer Binder on the Electrochemical Performance of Electrode Materials for Lithium-Ion Batteries

: The electrodes of lithium-ion batteries (LIBs) are multicomponent systems and their electrochemical properties are inﬂuenced by each component, therefore the composition of electrodes should be properly balanced. At the beginning of lithium-ion battery research, most attention was paid to the nature, size, and morphology peculiarities of inorganic active components as the main components which determine the functional properties of electrode materials. Over the past decade, considerable attention has been paid to development of new binders, as the binders have shown great e ﬀ ect on the electrochemical performance of electrodes in LIBs. The study of new conductive binders, in particular water-based binders with enhanced electronic and ionic conductivity, has become a trend in the development of new electrode materials, especially the conversion / alloying-type anodes. This mini-review provides a summary on the progress of current research of the e ﬀ ects of binders on the electrochemical properties of intercalation electrodes, with particular attention to the mechanisms of binder e ﬀ ects. The comparative analysis of e ﬀ ects of three di ﬀ erent binders (PEDOT:PSS / CMC, CMC, and PVDF) for a number of oxide-based and phosphate-based positive and negative electrodes for lithium-ion batteries was performed based on literature and our own published research data. It reveals that the combined PEDOT:PSS / CMC binder can be considered as a versatile component of lithium-ion battery electrode materials (for both positive and negative electrodes), e ﬀ ective in the wide range of electrode potentials. capacity due to enhancement of ionic and electronic conductivity of the electrode; (ii) enhancement of the cycling stability of electrodes resulting from lower degradation of electroactive material grains with a polymeric protective layer; and (iii) green water-based material preparation route in substitution for processing of a ﬂuorine-containing polymer in toxic volatile solvent (for PVDF). Galvanostatic charge–discharge (GCD) tests were performed on an automatic GCD battery cell test the current density experimental 4 composite electrode with di ﬀ erent amounts of PEDOT:PSS (6–16 wt. %), which is accomplishing a dual role of a binder and conducting additive, are investigated. The optimal value of speciﬁc capacity 110 mAh · g − 1 (1.0 C) are obtained at 8 wt. %. The authors concluded that only a thin layer of binder between LFP grains is e ﬀ ective due to moderate conductivity of binder. The results presented in [67] also support the conclusion that it is the thin-layer surface modiﬁcation of active grains that plays a very important role in enhancement of the functional characteristics.


Introduction
It is well known that electrodes of lithium-ion batteries (LIBs) are multicomponent systems, consisting of active material and other components such as conductive additives and binder. Hence, we deal with so-called composite micro-heterogeneous electrodes, where active component is in mixture with others. The properties of such multicomponent electrode materials are dependent on all components, which influence each other and therefore should be properly balanced. Conductive additives (typically up to 10 wt. % carbon black) are necessary to ensure high electrical conductivity of the LIB electrode because the inherent electronic conductivity of inorganic electrode materials is very low. A polymer binder is commonly required to bind active material and conductive additive together and support good contact between particles and with current collector, the proper structure of electrodes, and their mechanical integrity [1][2][3][4]. Galvanostatic charge-discharge (GCD) tests were performed on an automatic GCD battery cell test instrument CT-3008W-5V10mA (Neware Co., Shenzhen, China) in the current density range from 0.2 to 30 C at room temperature (20 ± 2 • C). All experimental capacity values in figures were normalized to Energies 2020, 13,2163 5 of 24 the total mass of electrode excluding current collector, if not stated otherwise. Cyclic voltammetry was performed on Autolab PGSTAT 30 potentiostat/galvanostat (Eco-Chemie, Utrecht, The Netherlands).
The morphology of prepared composites was characterized by scanning electron microscopy (SEM, SUPRA 40VP Carl Zeiss, Oberkochen, Germany).

Morphology of Electrodes
It has been reported that the adhesion of the electrodes with CMC binder is much stronger than that of the electrodes with PVDF binder. This was explained by strong hydrogen bonding of the carboxyl and hydroxyl groups in CMC with the active material and the current collector [71].
This effect is also true for all active materials studied with PEDOT:PSS/CMC binder. Compared to PVDF, introduction of PEDOT:PSS/CMC binder resulted in improvement of electrode morphology, electrode layer integrity, and better adhesion of electrode layer to current collector. Figures 1-4 demonstrate different aspects of electrode morphology on the example of different active materials.
normalized to the total mass of electrode excluding current collector, if not stated otherwise. Cyclic voltammetry was performed on Autolab PGSTAT 30 potentiostat/galvanostat (Eco-Chemie, Utrecht, The Netherlands).
The morphology of prepared composites was characterized by scanning electron microscopy (SEM, SUPRA 40VP Carl Zeiss, Oberkochen, Germany).

Morphology of Electrodes
It has been reported that the adhesion of the electrodes with CMC binder is much stronger than that of the electrodes with PVDF binder. This was explained by strong hydrogen bonding of the carboxyl and hydroxyl groups in CMC with the active material and the current collector [71].
This effect is also true for all active materials studied with PEDOT:PSS/CMC binder. Compared to PVDF, introduction of PEDOT:PSS/CMC binder resulted in improvement of electrode morphology, electrode layer integrity, and better adhesion of electrode layer to current collector. Figures 1-4 demonstrate different aspects of electrode morphology on the example of different active materials.
To show the difference in the morphological properties of electrode materials with PVDF and PEDOT:PSS/CMC binders, typical scanning electron microscopy (SEM) images of LMO-electrodes with the same amount of carbon black are presented in Figure 1. The electrode layer with PEDOT:PSS/CMC binder demonstrates denser and smoother surface than the electrode with PVDF binder. The polymer coating is visible on the surface of LMO grains and efficiently fills the gaps between them and carbon black.  To show the difference in the morphological properties of electrode materials with PVDF and PEDOT:PSS/CMC binders, typical scanning electron microscopy (SEM) images of LMO-electrodes with the same amount of carbon black are presented in Figure 1. The electrode layer with PEDOT:PSS/CMC binder demonstrates denser and smoother surface than the electrode with PVDF binder. The polymer coating is visible on the surface of LMO grains and efficiently fills the gaps between them and carbon black.
Sectional SEM images confirm good density and integrity of the electrode layers with PEDOT:PSS/CMC binder. For instance, Figure 2a,b shows the electrode layers of LTO PEDOT:PSS/CMC and LFMP PEDOT:PSS/CMC , which perfectly adhere to the current collector. The electrode layers with PEDOT:PSS/CMC binder successfully maintained integrity after long galvanostatic charge-discharge (GCD) tests [64,66,68,69].
As the conductive PEDOT:PSS/CMC binder besides binding function should improve the electrical contact between the particles in the composite material, its distribution should be uniform to maintain conductivity. The distribution of the main elements mapped using locally-resolved EDX analysis is shown in Figure 2c on the example of LTO PEDOT:PSS/CMC . Relatively uniform distribution of Ti and C over the electrode suggests that the solid particles in the electrode slurry were thoroughly mixed, while S mapping confirms uniform distribution of the sulfur-containing PEDOT:PSS component of the binder. Figure 3 shows typical SEM images of LFP PEDOT:PSS/CMC electrode that demonstrate the smooth surface of LFP grains wrapped with conducting polymer binder. We can see the high completeness of LFP grain wrapping that remains intact after charge-discharge cycling of the material in coin cell (Figure 3b). It can be supposed that the wrapping of active grains with conducting polymer Energies 2020, 13, 2163 6 of 24 maintains good electric contacts in the electrode material, and could be one of the factors providing high electrochemical performance of the electrode material.         Figure 4, the active layer of PVDF-bound electrode had cracked and partially delaminated from Al current collector, while PEDOT:PSS/CMC-bound electrode maintains electrode layer integrity. Our observations are in consistence with the results of comparative studies of LMO electrodes with different binders in [72], where LMO electrodes with CMC binder had more compact active layer with higher adhesion to substrate after long-term cycling than the electrodes with PVDF binder. Compared with conventional PVDF-based electrodes, the CMC/LiNi 0.4 Co 0.2 Mn 0.4 O 2 cathodes also displayed more uniform distribution of active grains and carbon particles together with strong adhesion among the particles and with the current collector, preventing the delamination of the electrode layer [62].

Cyclic Voltammetry
Cyclic voltammograms of electrodes with different active materials (LFP, LFMP, LMO, and LTO) and two types of binder (PEDOT:PSS/CMC and PVDF) are shown in Figure 5. For proper comparison, the currents in cyclic voltammograms are normalized by the mass of active material. As shown in Figure 5, in most cases, the voltammetric responses of electrodes with conductive binder are more reversible and PEDOT:PSS/CMC-bound electrodes show higher capacities than PVDF-bound ones. This is especially evident from CVs of LTO and LFP electrodes, where the significant differences in the shapes of voltammetric responses indirectly indicate different conditions of charge transfer during the charge-discharge cycles. Both LTO and LFP electrodes with combined conductive binder demonstrated higher peak currents and lower peak-to-peak separation compared to PVDF-bound materials at the same scan rate. The obtained data indicate increased reversibility of lithium intercalation processes in LTO and LFP electrodes and lowering of internal resistance, when a conductive polymer additive is introduced. The peak currents (I p ) were almost linear with square root of the scan rates, demonstrating that the electrochemical kinetics was diffusion-controlled with slow intercalation of lithium ions. The difference in the shape of cyclic voltammograms for LiFe0.4Mn0.6PO4 and LiMn2O4 was less expressed, probably due to higher intrinsic conductivity of these compounds, whereas the normalized currents were also higher in the case of conductive polymer binder. It indicates on the enhanced specific capacities of electrodes with conductive binder in comparison with PVDF-bound materials.  The difference in the shape of cyclic voltammograms for LiFe 0.4 Mn 0.6 PO 4 and LiMn 2 O 4 was less expressed, probably due to higher intrinsic conductivity of these compounds, whereas the normalized currents were also higher in the case of conductive polymer binder. It indicates on the enhanced specific capacities of electrodes with conductive binder in comparison with PVDF-bound materials.

Galvanostatic Charge-Discharge
The comparison of GCD tests of cells with electrodes with different active materials LFP, LFMP, LMO, and LTO and three types of binder PEDOT:PSS/CMC, CMC, and PVDF, (except for LFMP, where data with only two binders are available) at 0.2 C current is presented in Figure 6, and the values of specific capacities of the electrodes both normalized by the total mass of electrodes excluding current collector (Q electrode ) and by mass of electroactive material (Q electroactive material ) are given in Table 2. The electrochemical performance tests were carried out on an automatic galvanostatic charge-discharge battery cell test instrument for the series of 3-4 batteries of given composition. The results on the specific capacities were well reproducible.    It can be seen that the highest values of capacities at 0.2 C normalized by electrode mass (Q electrode ) were obtained for PEDOT:PSS/CMC-bound electrodes. These values are higher than those for PVDF-bound electrodes by 14%, 14%, 15%, and 13% for LFP, LFMP, LMO, and LTO, respectively. In the case of CMC-bound electrodes, we mainly see intermediate values of specific capacities (Q electrode ). However, after normalization by mass of electroactive material (Q electroactive material ), the lowest values were obtained for LMO CMC material, whereas for LTO CMC we can see comparable values with those of PEDOT:PSS/CMC-bound electrodes at low current densities (0.2C). Thus, in all cases, the employment of PEDOT:PSS/CMC conductive polymer binder (4 wt. % total) along with small amount of carbon black improved the specific capacity of electrodes, both Q electrode and Q electroactive material . Normalization of the capacities by the mass of the electrode excluding current collector is important for assessing the practical capacity. The increase of specific capacity (Q electrode ) at low discharge currents (0.2 C) is primarily due to higher mass fraction of the active material in PEDOT:PSS/CMC-bound electrodes (extra 6-10 wt. %, see Table 1) and to improved ionic and electronic conductivity of material, facilitating its full utilization.
The advantages of proposed PEDOT:PSS/CMC binder are most noticeable at the high charge-discharge rates. The C-rate performance of electrode materials with different binders is compared in Figure 7. The most effective impact of conductive binder PEDOT:PSS/CMC was observed for LFP (at 5 C) and LTO (at 20 and 30 C) materials, whereas active materials containing manganese show less significant increase of the specific capacity that may be due to higher intrinsic conductivity of these materials (both electronic and ionic). In the case of CMC-bound electrodes, the values of specific capacities at low current densities were lower or comparable with those of PEDOT:PSS/CMC bound electrodes, whereas high C-rate capability of PEDOT:PSS/CMC bound electrodes was superior, especially for LMO.
At high currents, the capacities of PVDF-bound electrodes were lower than the capacities of PEDOT:PSS/CMC-bound electrodes. Such a difference in the behavior of electrodes with PEDOT:PSS/CMC and PVDF binders at high discharge rates may be due to lower kinetic and diffusion limitations when using the PEDOT:PSS/CMC binder. Thus, with an increase in the current (or discharge rate), the capacities of electrodes modified with a combined conductive binder are higher than for electrodes of conventional composition, which indicates a more efficient and complete recharging process at high currents.
It should be noted that the conductivity of PEDOT:PSS component is not sufficient to produce carbon black-free electrode materials without the loss of power characteristics, as shown in several our works [64,67]. It is also shown in [64] that extremely high carbon black content (20 wt. %) does not improve the specific capacity of LTO electrodes at high discharge rates, as one would expect.
These results suggest the crucial importance of balance of different components of electrode materials. Special attention should be paid to ionic and electronic conductivity of the intergranular medium (polymer binder, carbon black) wherein the electrolyte is located and good wettability of the active grains by electrolyte. observed for LFP (at 5 С) and LTO (at 20 and 30 С) materials, whereas active materials containing manganese show less significant increase of the specific capacity that may be due to higher intrinsic conductivity of these materials (both electronic and ionic). In the case of CMC-bound electrodes, the values of specific capacities at low current densities were lower or comparable with those of PEDOT:PSS/CMC bound electrodes, whereas high C-rate capability of PEDOT:PSS/CMC bound electrodes was superior, especially for LMO.   At high currents, the capacities of PVDF-bound electrodes were lower than the capacities of PEDOT:PSS/CMC-bound electrodes. Such a difference in the behavior of electrodes with PEDOT:PSS/CMC and PVDF binders at high discharge rates may be due to lower kinetic and diffusion limitations when using the PEDOT:PSS/CMC binder. Thus, with an increase in the current (or discharge rate), the capacities of electrodes modified with a combined conductive binder are higher than for electrodes of conventional composition, which indicates a more efficient and complete recharging process at high currents.
It should be noted that the conductivity of PEDOT:PSS component is not sufficient to produce carbon black-free electrode materials without the loss of power characteristics, as shown in several

Cycling Performance
Cycle life stability of tested batteries with different electrode materials is shown in Figure 8. The cycling stability tests showed satisfactory capacity retention for materials with PEDOT:PSS/CMC binder-it is superior to that of the PVDF-bound electrodes or at least on the same level. The most effective impact of PEDOT:PSS/CMC binder on the cycling stability was observed in the case of LTO material. Based on the results of [71] and our data [56], we assume that PEDOT:PSS and CMC polymers suppress the changes in the surface structure of the LTO material, since polymers wrap the LTO grains and form a protective layer that effectively inhibits side reactions and protects the active material from interaction with the electrolyte, without slowing down lithium transport.
We also tested the full battery cell with a LFMP PEDOT:PSS/CMC cathode and a LTO PEDOT:PSS/CMC anode. The mass loading ratio of LFMP PEDOT:PSS/CMC and LTO PEDOT:PSS/CMC electrodes was 0.879. The discharge capacity of the battery cell is presented in Figure 9 normalized to the mass of LFMP PEDOT:PSS/CMC . It has shown 98-99% Coulombic efficiency and good cycling performance with ca. 16% capacity decay over 1000 charge-discharge cycles, these values matching commercial standards.
binder-it is superior to that of the PVDF-bound electrodes or at least on the same level. The most effective impact of PEDOT:PSS/CMC binder on the cycling stability was observed in the case of LTO material. Based on the results of [71] and our data [56], we assume that PEDOT:PSS and CMC polymers suppress the changes in the surface structure of the LTO material, since polymers wrap the LTO grains and form a protective layer that effectively inhibits side reactions and protects the active material from interaction with the electrolyte, without slowing down lithium transport. We also tested the full battery cell with a LFMPPEDOT:PSS/CMC cathode and a LTOPEDOT:PSS/CMC anode. The mass loading ratio of LFMPPEDOT:PSS/CMC and LTOPEDOT:PSS/CMC electrodes was 0.879. The discharge capacity of the battery cell is presented in Figure 9 normalized to the mass of LFMPPEDOT:PSS/CMC. It has shown 98-99% Coulombic efficiency and good cycling performance with ca. 16% capacity decay over 1000 charge-discharge cycles, these values matching commercial standards.

Discussion
In this section, we present a short summary and discussion of specific capacity values for different electrode materials with CMC and PVDF taken from the literature (see Table 3) compared to the data for materials with PEDOT:PSS/CMC binder.

Discussion
In this section, we present a short summary and discussion of specific capacity values for different electrode materials with CMC and PVDF taken from the literature (see Table 3) compared to the data for materials with PEDOT:PSS/CMC binder.  In general, as follows from the analysis of results presented in Tables 2 and 3, the electrodes with aqueous binders (CMC and PEDOT:PSS/CMC) as a rule exhibited superior C-rate capability and long-term cycling stability than those with PVDF binder. However, the comparison of electrodes with CMC binder with electrodes with combined PEDOT:PSS/CMC binder showed better electrochemical performance of most electrodes with PEDOT:PSS/CMC binder, which is attributed to faster lithium ion diffusion kinetics and lower electrode polarizations.
When considering the data in Table 3 it should be taken into account that a comparison of quantitative parameters is sensitive to initial nature and morphology of active materials, their grain size, mass loading, and some other properties. The difference in specific capacity values of formally similar in composition electrodes also reflects the impact of the structure and properties of initial active material grains (size, morphology, core-shell structures, different carbon additives, etc.). Thus, a proper comparison of the results given in the literature should account for peculiarities of the experimental preparation of electrode mass and in this sense the comparison of data from different works is less Energies 2020, 13, 2163 14 of 24 reliable than the results of a comparative study of different binders obtained in similar conditions. Therefore, in our opinion, only works representing data on a comparative study of different electrode compositions produced from the same active material can be the answer about the advantages or disadvantages of binder components.
In this sense, the articles with comparative study of effect of binders are of interest. For instance, in [72], the electrochemical properties of LiMn 2 O 4 cathodes with four different binders based on PVDF, CMC, and polyacrylic acid (PAA) with NMP and water (10 wt%. by mass) are investigated. These results show that the LiMn 2 O 4 cathode with PAA/CMC binder displays the best cycle performance among these four cathodes. The highest values of capacity (131 mAh·g −1 at 0.2 C) are found for composition with CMC and PAA (in NMP) binders, while for the composition with PVDF a lower value (122 mAh·g −1 at 0.2 C) is obtained. However, with increase of current density, the capacity loss is more expressed for CMC-bound electrodes (112 mAh·g −1 at 1.0 C) compared to PVDF (115 mAh·g −1 at 1.0 C), and at 3.0 C the capacities are 85 mAh·g −1 for CMC and 92 mAh·g −1 for PVDF. These results showing higher capacity loss of CMC-bound LMO electrodes with increase of charge-discharge rate are in agreement with those shown in Figure 7 (Section 3.3).
On the other hand, the observations for LiNi 1/3 Mn 1/3 Co 1/3 O 2 cathode material [107] are quite the opposite. PVDF-bound electrode exhibits the highest discharge capacity at 0.1 C rate, but in the range of 0.2-5 C rates CMC-bound electrode shows outstanding performance.
The comparison of data for LMO-based materials with different aqueous binders shows that LMO with PEDOT:PSS/CMC binder has higher capacities at 0.2 and 1 C than LMO with sodium alginate [49], CMC [72], and polyacrylonitrile [97] binders. No data were found in the literature for LMO materials with aqueous binders for comparison at 10 C current density. Using complicated synthetic approaches, creating complex materials [13], or using additives such as LiNbO 3 [92] naturally have led to superior performance of electrodes, but in our opinion these approaches are more difficult than tuning electrode performance by introduction of a conductive binder.
It should also be noted that exceptional results have been achieved for the cathodes based on commercially available lithium iron phosphate (LFP) and CMC-Li as a water-soluble binder [61]. Compared with PVDF binder, the battery with CMC-Li as a binder retains 97.8% of initial reversible capacity after 200 cycles (176 mAh·g −1 )-a value which is even beyond the theoretical specific capacity of LFP. The authors of [61] concluded that CMC-Li as a ion-conductive polymer can increase the contents of freely moving lithium ions in lithium-ion batteries and shorten the diffusion pathway to the cathode particle surface, which leads to enhancement of the charge-discharge processes.
In [10,70], higher specific capacity values are found for the LFP-based electrodes with graphene additives and PVDF binder than for the LFP-based material with PEDOT:PSS/CMC binder (at 0.2 and 1.0 C). This effect is assigned to efficient structure of materials and contribution to capacity from graphene additives. At high currents (5 C), where data are available, the highest specific capacity value is observed for LFP with PEDOT:PSS/CMC binder in comparison with alternative water-based binders.
In other cases presented in Table 3, the LFP electrodes with PEDOT:PSS/CMC binder exhibited, as a rule, higher specific capacities than those with CMC and PVDF binders, especially at increased current densities.
In [55], the electrochemical properties of carbon black-free LiFePO 4 composite electrode with different amounts of PEDOT:PSS (6-16 wt. %), which is accomplishing a dual role of a binder and conducting additive, are investigated. The optimal value of specific capacity 110 mAh·g −1 (1.0 C) are obtained at 8 wt. %. The authors concluded that only a thin layer of binder between LFP grains is effective due to moderate conductivity of binder. The results presented in [67] also support the conclusion that it is the thin-layer surface modification of active grains that plays a very important role in enhancement of the functional characteristics. The electrochemical performance of LFP cathode with CMC binder reported in [40,108] also shows higher capacities and lower irreversible capacity losses compared to PVDF-bound cathodes.
In the case of LFMP-based materials, similar trends are observed with PEDOT:PSS/CMC binder: the highest specific capacity at high C-rates, moderate values at 1 C in comparison with carbon-modified grains and high values at low current density. It should be noted here that electrochemical properties of LFMP have a strong dependence on the ratio of Fe to Mn; different ratios may complicate the comparison of data.
LTO-based anode material with PEDOT:PSS/CMC binder has achieved the theoretical specific capacity value at 0.2 C current, while another PEDOT-containing electrode composition [34] shows a bit lower value; comparable values are observed for most electrodes at 1-10 C current densities. Extremely high capacity value beyond the theoretical one (188 mAh·g −1 at 1C) is reported in [98] and explained by Zr-doping of LTO. Higher capacity value (174 mAh·g −1 ) is also obtained in [37] for LTO/PEDOT:PSS composites with PVDF binder at low charge rate (0.2 C) compared to LTO/PVDF (166 mAh·g −1 ). Li 4 Ti 5 O 12 with CMC binder [71] has good rate capability at 0.2 and 1 C, but the highest capacity value achieved for this composition at high current density (10 C) is extremely low (60 mAh·g −1 ).
The enhanced performance of CMC-bound electrodes is usually assigned to strong hydrogen bonds resulting from the interaction of the -OH groups with surface oxygen ions of active materials and lithium ions, which facilitate to transport of lithium ions along molecular chains [61,71,104,105]. It is also true in the case of combined PEDOT:PSS/CMC binder, where a similar conclusion on the mechanism of binder influence could be drawn.
As follows from the analysis of data presented, water-based binders such as CMC and PEDOT:PSS/CMC are more efficient than PVDF for the active materials with poor ionic conductivity. PVDF hinders the diffusion of lithium in the charge-discharge process of the battery, and reduces the electrochemical activity of electrode material. However, as presented results clearly show, the CMC binder often limits the rate capability of the electrode.
Concerning the mechanistic insights on the effects of binder's nature on the specific capacities, C-rate capability and cycling stability of the electrodes, the role of such components as conducting polymer PEDOT:PSS and carboxymethyl cellulose is highlighted as follows: (1) PEDOT:PSS acts as an electronic/ionic conductive component, providing more conductive media between active grains with tight electrical contacts. (2) CMC additive acts as a thickening agent with good ionic conductivity, improving material by adjustable porosity and wettability by electrolyte, which facilitate Li + ion movement. (3) In addition, the presence of ionogenic groups in the binder components (such as CMC and PEDOT:PSS) can create an increased concentration of lithium ions around the active material particles, which can also contribute to more efficient mass transfer during the discharge process.
The advantages of the combined PEDOT:PSS/CMC binder are additionally supported by charge transfer kinetic aspects, which are investigated in many of the papers cited in Table 3. In particular, systematic impedance measurements are performed in [64][65][66][67] for the three discussed binders, and the data on the interfacial resistances and diffusion coefficients of Li + ions are evaluated. The comparison of kinetic parameters obtained for all investigated electrodes (LFP, LTO, LMO, and LFMP) with different binders reveals two main factors which are responsible for significant improvement of electrochemical performance of PEDOT:PSS/CMC bound electrodes. It is found that in the row of binders PVDF-CMC-PEDOT:PSS/CMC, a gradual decrease of interfacial charge transfer resistance and the increase of Li + apparent diffusion coefficient takes place [64][65][66][67].
For LFP electrodes, the decrease of interfacial charge transfer resistance and the increase of Li + effective diffusion coefficient are observed in the row PEDOT:PSS/CMC < CMC < PVDF. For example, for LFP electrodes at fully discharged state, the values of interfacial resistance R int (the sum of R SEI (the resistance in the solid electrolyte interphase) and R ct (the charge transfer resistance)) are 715 (PVDF), 180 (CMC), and 56 Ohm (PEDOT:PSS/CMC). The decrease of Warburg constants σ w , which determine the values of the Warburg diffusion resistance, is also observed in this row: 203 (PVDF), 72 (CMC), and 44 Ohm s −1/2 (PEDOT:PSS/CMC). A similar tendency is observed for other electrode materials, such as LFMP [65], LTO [64] and LMO [66].
The mechanism of influence of conductive binder on the interfacial resistance and Li + ion transport is somewhat similar to the influence of carbon coating of active grains. As reported in many papers [64][65][66][67], the surface electronic conductivity of electrode materials is greatly enhanced by carbonization, which results in the lowering of interfacial resistance. The wrapping of active grains by conducting polymer PEDOT:PSS provides more reliable electrical contact between neighboring particles. In combination with ionic conductivity of both components of binder, it would greatly enhance the coupled electron and ion transfer at interface and Li + ion transport in pores around active grains.
To demonstrate the difference of non-conducting and conductive binders, the possible mechanism of electron and ion pathways in conductive binder is schematically presented in Figure 10. The conductive polymer coating of active particles increases the electronic conductivity on the surface of an individual particle and reduces the transfer resistance of ionic and electronic charge. In the case of non-conducting PVDF binder, charge transfer occurs only in the region of point contact of active grains with carbon black particles ( Figure 10).
In the case of a conductive polymer binder, charge transfer occurs over all the surface area of an active material particle that is in contact with the polymer. This contributes to a better electrical contact, uniform charge distribution and, accordingly, a more efficient and fast charge-discharge process. This effect is very similar in nature to the effect of a conductive carbon coating, which improves the surface conductivity of materials and significantly reduces polarization [19].
The employment of conductive binder formally enlarges the electric contact area and Li + diffusion cross-section area (Figure 11), thus decreasing diffusion resistance. Larger effective contact area means a greater probability to insert or drain Li + ions to/from the electrode. In addition, ionically conductive polymer coating also may facilitate ion transport around the active grains. It can be supposed that partly de-solvated Li + ions from polymer wrapping layer can be easily transmitted into the grain, providing an additional pathway for lithium ion insertion. The conductive polymer coating of active particles increases the electronic conductivity on the surface of an individual particle and reduces the transfer resistance of ionic and electronic charge. In the case of non-conducting PVDF binder, charge transfer occurs only in the region of point contact of active grains with carbon black particles ( Figure 10).
In the case of a conductive polymer binder, charge transfer occurs over all the surface area of an active material particle that is in contact with the polymer. This contributes to a better electrical contact, uniform charge distribution and, accordingly, a more efficient and fast charge-discharge process. This effect is very similar in nature to the effect of a conductive carbon coating, which improves the surface conductivity of materials and significantly reduces polarization [19].
The employment of conductive binder formally enlarges the electric contact area and Li + diffusion cross-section area (Figure 11), thus decreasing diffusion resistance. Larger effective contact area means a greater probability to insert or drain Li + ions to/from the electrode. In addition, ionically conductive polymer coating also may facilitate ion transport around the active grains. It can be supposed that partly de-solvated Li + ions from polymer wrapping layer can be easily transmitted into the grain, providing an additional pathway for lithium ion insertion.
In this case, the polymer conductivity is described by the dynamic percolation model of ion transport, according to which lithium ions can diffuse through the polymer medium using segment motions of -SO 3 groups that are associated with lithium ions (relay mechanism of lithium ion transfer) [109]. However, there is additional complexity, since this kind of conductivity is most often considered for systems where only the polymer is an ionic conductor. In the case, when the polymer is in contact with a liquid electrolyte, the ion transport can proceed not only due to the ion hopping from one ionic group to another, but also due to dissociation, solvation, and diffusion of the ion to the next group and association with it. Thus, in [110], it is shown for CMC, in contact with water, that the second mechanism of ion transport through dissociation and association is realized exactly the same. The nature of ion transport for the system considered in this paper is not yet fully understood and its clarification needs further research.
the case of non-conducting PVDF binder, charge transfer occurs only in the region of point contact of active grains with carbon black particles ( Figure 10).
In the case of a conductive polymer binder, charge transfer occurs over all the surface area of an active material particle that is in contact with the polymer. This contributes to a better electrical contact, uniform charge distribution and, accordingly, a more efficient and fast charge-discharge process. This effect is very similar in nature to the effect of a conductive carbon coating, which improves the surface conductivity of materials and significantly reduces polarization [19].
The employment of conductive binder formally enlarges the electric contact area and Li + diffusion cross-section area (Figure 11), thus decreasing diffusion resistance. Larger effective contact area means a greater probability to insert or drain Li + ions to/from the electrode. In addition, ionically conductive polymer coating also may facilitate ion transport around the active grains. It can be supposed that partly de-solvated Li + ions from polymer wrapping layer can be easily transmitted into the grain, providing an additional pathway for lithium ion insertion. Figure 11. Diagram of ionic transport in an electrode material using a PEDOT:PSS/CMC binder. 1, ionic transport in polymer pores; 2, ionic transport in the polymer.
In this case, the polymer conductivity is described by the dynamic percolation model of ion transport, according to which lithium ions can diffuse through the polymer medium using segment motions of -SO3 groups that are associated with lithium ions (relay mechanism of lithium ion transfer) [109]. However, there is additional complexity, since this kind of conductivity is most often considered for systems where only the polymer is an ionic conductor. In the case, when the polymer is in contact with a liquid electrolyte, the ion transport can proceed not only due to the ion hopping from one ionic group to another, but also due to dissociation, solvation, and diffusion of the ion to the next group and association with it. Thus, in [110], it is shown for CMC, in contact with water, that the second mechanism of ion transport through dissociation and association is realized exactly Figure 11. Diagram of ionic transport in an electrode material using a PEDOT:PSS/CMC binder. 1, ionic transport in polymer pores; 2, ionic transport in the polymer.

Summary
Improved electrode materials for LIBs are in demand for different applications in power technologies. Enhanced specific capacities, C-rate capability, and cycle life of LIBs have been traditionally achieved via synthetic routes, e.g., synthesis of electrode materials with new structures or their modifications directed at improved charge-discharge properties. Recently, the wide choice of binders has offered the opportunity to improve the performance of energy storage materials, and design of binder compositions has become a trend in the development of LIB electrode materials.
In this review, the comparative analysis of effects of three different binders (PEDOT:PSS/CMC, CMC, and PVDF) for a number of oxide-based and phosphate-based positive and negative electrodes for lithium-ion batteries is performed based on literature and our own published research data. It reveals that the combined PEDOT:PSS/CMC binder can be considered as a versatile component of lithium-ion battery electrode materials (for both positive and negative electrodes), effective in the wide range of electrode potentials. Special attention is paid to the comparison of the electrochemical properties of the electrodes with similar composition, fabricated with the use of the same in the origin active material powders and different binders. In this case, when the binder is the only variable component, the most reliable comparison of results can be performed.
Among the electrodes produced from four active materials (LFP, LFMP, LMO, and LTO) and three different binders, in most cases, all the electrodes with PEDOT:PSS/CMC conductive binder have shown superior properties in comparison with CMC-bound and PVDF-bound electrodes, in particular, increased specific capacity and good capacity retention. In particular, the use of PEDOT:PSS/CMC binder allows reducing the amount of inactive components, thus increasing the practical specific capacity. The advantages of PEDOT:PSS/CMC conductive binder were most expressed for high rate performance of LiFe 0.4 Mn 0.6 PO 4 and Li 4 Ti 5 O 12 . Full battery test of electrodes with the same conductive binder composition and content (2 wt. % of PEDOT:PSS and 2 wt. % of CMC) produced from these materials demonstrated good cycling performance with capacity decay of only 16% over 1000 cycles. For most CMC-bound electrodes, the values of specific capacities at low current densities were higher than those for PVDF-bound electrodes and slightly worse or comparable with those of PEDOT:PSS/CMC bound electrodes, whereas both the high C-rate capability and the long-term cycling stability of PEDOT:PSS/CMC-bound electrodes were superior.
Concerning the mechanistic insights on the effects of binder's nature on the specific capacities, C-rate capability and cycling stability of the electrodes, the role of such components as conducting polymer PEDOT:PSS and carboxymethyl cellulose is highlighted as follows: (i) PEDOT:PSS acts as an electronic/ionic conductive component, providing more conductive media between active grains with tight electrical contacts; (ii) CMC additive acts as a thickening agent with good ionic conductivity, improving material by adjustable porosity and wettability by electrolyte, which facilitate Li + ion movement; and (iii) the presence of ionogenic groups in the binder components (such as CMC and PEDOT:PSS) can create an increased concentration of lithium ions around the active material particles, which can also contribute to more efficient mass transfer during the discharge process.
In addition, the advantages of combined PEDOT:PSS/CMC binder are confirmed by improved kinetics of charge transfer in the electrodes. In particular, the results of systematic impedance measurements revealed two main factors which are responsible for significant improvement of electrochemical performance of PEDOT:PSS/CMC-bound electrodes. In the row of binders PVDF-CMC-PEDOT:PSS/CMC, a gradual decrease of interfacial charge transfer resistance and the increase of Li + apparent diffusion coefficient, which facilitate to fast charge-discharge processes, are observed.
The combined conductive PEDOT:PSS/CMC binder can be considered as an efficient alternative to both the aqueous ion-conductive CMC binder and the non-conductive PVDF binder for commercial lithium-ion batteries. Easy one-step electrode preparation procedure makes composites of electroactive materials with PEDOT:PSS/CMC promising as industrial materials for lithium-ion batteries. In this review, we show a smart cost-effective approach for fabrication of LFP, LFMP, and LMO cathodes and LTO anodes from commercially available active materials for production of commercial LIBs with improved performance.
We believe that this mini-review, which summarizes the results of recent progress in application of PEDOT:PSS/CMC blend as a binder and conductive additive, will stimulate the researchers for search of new effective aqueous binder compositions and induce some new ideas in this field of research.