Rational design of high performance lithium iron phosphate positive electrodes with conducting polymer for lithium-ion batteries

Resumen

RESUMEN DE LA TESIS DOCTORAL DE D./Dª Daniel Cíntora Juárez 1. Introducción o motivación de la tesis LiFePO4 has been pointed out as a next-generation active material for lithium-ion (Li-ion) batteries and its performance has been progressively improved mostly by optimizing the synthesis conditions of this cathode material in order to obtain carbon‑coated nano-particles of controlled purity. Nevertheless, there is still the need for improving the composition and the preparation methods of LiFePO4-based electrodes, which is paramount for maximizing the capacity at high charge/discharge rates. In this regard, this thesis was focused on the improvement of the charge transport throughout the different electrode interphases in order to increase the conductivity and to enable higher energy and power output as compared to conventional electrode formulations. The general strategy followed in this thesis consisted in forming composite electrodes of LiFePO4 with intrinsically conducting polymers of the type poly(3,4‑alkylenedioxythiophene) [PXDOT] by electrochemical polymerization, by blending and by coating methods. Although composite battery electrodes with conducting polymers have been described previously,[1] this thesis presents effective means for obtaining composite electrodes by the electrochemical polymerization of monomers directly over LiFePO4-based electrodes, or by combining the cathode material with ready available conducting polymer. 2. Contenido de la investigación The first approach for the electrochemical preparation of the composite electrodes consisted in the formation of a poly(3,4‑ethylenedioxythiophene) [PEDOT] coating by electrodeposition over an electrode based either on LiFePO4 (uncoated) or LiFePO4/C (carbon‑coated).[1] The experimental conditions of the potentiostatic electropolymerization enabled forming a conducting polymer coating over the active material particles, resulting in mechanically stable electrodes of better electrochemical performance than the electrode based on LiFePO4 without conducting polymer. This improvement was attributed to the lower electrical resistance of the composite electrode with conducting polymer, as estimated by means of electrochemical impedance spectroscopy and manifested as a low charge/discharge polarization. The second approach for preparing composite electrodes with conducting polymer by electrochemical polymerization consisted in the galvanostatic oxidation of EDOT or 3,4‑propylenedioxythiophene (ProDOT) monomers over a LiFePO4-based electrode during the initial charging cycles in a test battery.[2] This novel approach was designated as in battery because the polymerization is carried out inside the battery. The in battery methods are based on the oxidative polymerization of monomers over de‑lithited LiFePO4 (Li1‑xFePO4, 0 ≤ x ≤ 1), which is formed during the battery charging. By the end of the battery charging, the conducting polymer coating covers the delithiated LiFePO4 and the surface of the electrode, without the need of adding any oxidizing compound to carry out the polymerization. The Fe2+ to Fe3+ oxidation in Li1‑xFePO4 was monitored by Mössbauer spectroscopy, which revealed that this oxidation process is more efficient upon the formation of the conducting polymer coating. Upon discharge of the battery, the reduction of Fe3+ to Fe2+ and the lithium reinsertion are facilitated by the conducting polymer coating. The in battery electrochemical polymerization can be carried out either in one or in two battery charging steps. Both of these variations produced cathodes with higher initial capacity and superior charge/discharge rate performance, as well as a more extended cycleability than the uncoated LiFePO4-based electrode. The superior electrochemical performance of the composite electrodes with conducting polymer was attributed to the lower resistance of the electrode due to the improvement of the ionic and electronic connectivity of the active material particles, promoted by the conducting polymer coating. The preparation of LiFePO4/PEDOT electrodes by blending was implemented for PEDOT obtained from two different sources: i) PEDOT synthesized by electrochemical polymerization and ii) PEDOT:PSS (PSS: polystyrene sulfonate), which is commercially available and is produced by chemical polymerization. PEDOT synthesized electrochemically was prepared by a potentiostatic polymerization method, previously reported,[3] over a platinum electrode in an H2O/CH2Cl2 medium with tetraethylammonium tetrafluoroborate. These conditions allowed obtaining a porous polymer film of PEDOT. Blending this electrochemically synthesized PEDOT with LiFePO4 or LiFePO4/C resulted in an easy and effective way to prepare stable and active composite electrodes; without the need of using extra conducting or agglomerating additives. Although this type of composite electrodes showed a better charge/discharge performance, as compared to previously reported composites with electrochemically synthesized PEDOT, the potentiostatic method to synthesize the polymer has a low yield. The blending method using PEDOT:PSS consisted in incorporating the conducting polymer as an additive for LiFePO4-based electrodes. PEDOT:PSS was incorporated to the composite electrode by blending it in different proportions with a mixture of LiFePO4, carbon black and PVDF binder.[4] It was found that the presence of 1% w/w of PEDOT:PSS within the bulk of the electrode resulted in a two-fold increase of the capacity and in an increase of the discharge plateau voltage in ca. 0.5 V at 5C, as compared to the electrode without conducting polymer at the same charge/discharge rate. In order to further increase the conductivity of PEDOT:PSS, a minute amount of ethylene glycol or dimethyl sulfoxide secondary dopants was dissolved in the polymer dispersion. The initial discharge profiles of the electrodes with doped PEDOT:PSS showed that only ethylene glycol had a small effect on the charge/discharge voltage, whereas dimethyl sulfoxide had no effect. The Mössbauer spectroscopy analysis showed that the oxidation of Fe2+ to Fe3+ upon battery discharge at C/10 is up to 10 % w/w more efficient in electrodes that contain PEDOT:PSS, undoped or doped with ethylene glycol, as compared to the uncoated LiFePO4 electrode. This effect was attributed to a higher proportion of the PEDOT phase (electronic conductor) relative to the PSS phase (ionic conductor), both determined by quantitative analysis of XPS spectra. Additionally, approximated resistance values of the electrodes in charged and in discharged states were obtained from the fitting of impedance spectra. These analyses provided evidence on the more effective lithium extraction promoted by the presence of 1% w/w of PEDOT:PSS mixed conductor, both in undoped state and when treated with ethylene glycol. Thus, these electrodes provide almost 50 % w/w of the theoretical capacity of LiFePO4 in only 6 minutes with a low charge/discharge polarization and showed excellent capacity retention at 2C during 50 charge/discharge cycles. PEDOT:PSS was also tested as a conductive coating for the aluminium current collector of LiFePO4‑based electrodes. This strategy was intended to improve the electric contact and the active material utilization. This coating was realized by drop casting PEDOT:PSS over the current collector.[4] Conductivity enhancement by treatment with secondary dopants, particularly with ethylene glycol, resulted in a higher capacity and a lower charge/discharge polarization at high charge/discharge rates. The correlation of the initial impedance, the direct current load resistance and the capacity at moderate and high rates of the electrodes containing PEDOT:PSS, showed that the interphase between the electrode layer and the current collector has the highest impact on the performance in LiFePO4-based electrodes. 3. Conclusión Composite electrodes based on LiFePO4 and poly(alkylenedioxithiophene) [PXDOT] were successfully fabricated by electropolymerization, by blending or by coating methods. Different variations of these methods were devised and applied for obtaining cathodes that are active for the reversible insertion/extraction of lithium in test batteries. The effectiveness of the prepared electrodes depends on the synthesis conditions of the polymer, and on how this is combined with LiFePO4 to form the composite electrode. The preparation conditions of the composite electrodes with LiFePO4 and PXDOT affect their electrical resistance, their morphology and their texture. Besides, the way in which the conducting polymer interacts with LiFePO4 particles and with the other components of the electrode influences the charge transport through the different interphases of the electrode. The strategies presented in this thesis for preparing and analyzing LiFePO4/PXDOT composite electrodes highlight the relevance of charge transport through interphases in battery electrodes. In general, the synthesis and preparation methods presented here could be implemented with slight modifications to the present methods for preparing LiFePO4 based electrodes. Furthermore, these strategies could be applied for other families of active materials and conducting polymers that present redox compatibility and stability when combined in order to form composite electrodes for alkali-ion batteries.