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Truly quasi-solid-state lithium cells utilizing carbonate free polymer electrolytes on engineered LiFePO4
Electrochimica Acta 199 (2016) 172–179
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Truly quasi-solid-state lithium cells utilizing carbonate free polymer electrolytes on engineered LiFePO4 Jijeesh R. Naira,* , Daniel Cíntora-Juárezb , Carlos Pérez-Vicenteb , José L. Tiradob , Shahzada Ahmadc , Claudio Gerbaldia,* a b c
GAME Lab, CHENERGY Group, Department of Applied Science and Technology (DISAT), Politecnico di Torino, C.so Duca degli Abruzzi 24, Turin 10129, Italy Laboratorio de Química Inorgánica, Campus de Rabanales, Universidad de Córdoba, 14071, Spain Abengoa Research, Abengoa, C/Energía Solar n 1, Campus Palmas Altas, Sevilla 41014, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 January 2016 Received in revised form 23 March 2016 Accepted 26 March 2016 Available online 26 March 2016
Stable and safe functioning of a Li-ion battery is the demand of modern generation. Herein, we are demonstrating the application of an in-situ free radical polymerisation process (thermal curing) to fabricate a polymer electrolyte that possesses mechanical robustness, high thermal stability, improved interfacial and ion transport characteristics along with stable cycling at ambient conditions. The polymer electrolyte is obtained by direct polymerization over the electrode surface in one pot starting from a reactive mixture comprising an ethylene oxide-based dimethacrylic oligomer (BDM), dimethyl polyethylene glycol (DPG) and lithium salt. Furthermore, an engineered cathode is used, comprising a LiFePO4/PEDOT:PSS interface at the current collector that improves the material utilization at high rates and mitigates the corrosive effects of LiTFSI on aluminium current collector. The lithium cell resulting from the newly elaborated multiphase assembly of the composite cathode with the DPG-based carbonate-free polymer electrolyte film exhibits excellent reversibility upon prolonged cycling at ambient as well as elevated temperatures, which is found to be superior compared to previous reports on uncoated electrodes with polymer electrolytes. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: polymer electrolyte PEDOT lithium iron phosphate corrosion protection lithium battery
1. Introduction The ever increasing consumer demand [1] for energy-driven devices to perform better and better is creating lot of pressure on energy storage systems [2]. Lithium-ion batteries (Li-ion, LiBs) share this burden since their commercialisation in the 90’s by SONY [3]. Moreover, the clamour for moving towards storing energy from renewable resources is increasing along with the mandatory transition from petroleum-derived commutation to electricity-driven transportation. Even though the presence of LiBs is highly visible in the market, they cannot fulfil such a huge responsibility. Problems related to safety, cost and materials toxicity and limited energy density than the customer requisites are the major challenges of such systems to rule the market in a full-fledged manner. These disadvantages can be overcome by using new materials, which are superior to existing ones in terms of safety, eco compatibility and overall performance [4,5].
* Corresponding authors. Tel.: +39 011 090 4643; fax: +39 011 090 4629. E-mail addresses: [email protected] (J.R. Nair), [email protected] (C. Gerbaldi). http://dx.doi.org/10.1016/j.electacta.2016.03.156 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
Commercial LiBs are generally constructed using an anode based on graphitic carbon and a cathode based on transition metal oxide, both capable of reversibly intercalating/de-intercalating (inserting/de-inserting) Li+ ions, separated by a membrane soaked with an organic carbonate-based liquid electrolyte [6]. However, the usual problems such as leakage of liquid electrolytes from the pack, production of gas and flammability under abuse conditions are yet to be resolved, and they can be overcome only by replacing these inadequate components with safe, easy to manufacture and cost-effective polymer electrolytes [7]. Indeed, polymer electrolytes show advantages in terms of mechanical robustness, fabrication procedures and shape adaptability, with the unique possibility of fabricating an intimate electrode/electrolyte interface and lightweight/cost-effective packaging. In general, polymer electrolytes, especially gel or quasi-solid ones, encompass organic carbonate-based liquid electrolytes [8–10] or room temperature ionic liquids [11,12]. However, some attempts were also made to prepare polymer electrolytes with other components such as succinic nitrile, organic lactones, glymes of various lengths, etc. [13,14] to avoid the use of volatile organic carbonates. Another drawback is related to the preparation techniques that are employed to obtain self-standing polymer electrolytes. Typical
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2.2. Electrode preparation
example is the use of solvents (e.g., acetonitrile, acetone, dimethyl sulfoxide, dimethyl formamide, N-methyl-2-pyrrolidone, etc.) to obtain thin electrolyte films. Such low boiling solvents are the major players of environmental hazards (volatile organic components, VOC); add to it, one must consider the difficulty to absolutely remove the last traces from the resulting membranes. Such residual solvent components mainly induce stability issues and potential shoot-ups, which lead to thermal runaways by compromising the advantages offered by polymer electrolytes. Regarding the selection of an appropriate lithium battery cathode, LiFePO4 is a meritorious candidate [6] as it offers a safer operation window than LiCoO2 at elevated temperature, and the possibility to sustain higher charge/discharge rates in large format batteries. LiFePO4-based electrodes with optimized electronic conductivity through the electrode's interphases enable high rate capability, as it is known for carbon-coated particles [15] or for conducting polymer coatings [16], where the latter might be applied also for protecting the aluminium current collector from oxidation, especially when LiTFSI-based electrolytes are used. In the present study, we evaluate engineered cathodes, which is incorporated with PEDOT:PSS conducting polymer. PEDOT:PSS will act in two ways: first, as a conductive additive to increase the conductivity of LiFePO4, and secondly, as a protective coating against the aluminium current collector corrosion. Moreover, we are demonstrating that by appropriately implementing soft crosslinkers and high boiling plasticisers, we can prepare crosslinked polymer electrolytes without using any additives and hazardous solvents for film casting. The technique proposed here is a free radical based thermal polymerisation, which can be completed in about 60 mins. It is energy saving, cost effective, well established and ready to be industrially scaled-up. The reactive monomer mixture (liquid pre-polymer) is thermally crosslinked directly over the electrode surface in single step along with the lithium salt to retain the solid-like features and dimensional stability without hampering the ionic mobility. The performance of such system is then evaluated in lab-scale cells and optimum stability at high specific capacity is demonstrated, which is the highest reported so far for similar systems at ambient temperature.
The insoluble fraction (gel content) of the cross-linked polymer electrolyte was studied as follows: accurately weighed samples were enclosed in a stainless steel net, and then immersed in CHCl3 to extract the unreacted components from the crosslinked polymer electrolyte. The extraction step involved 18 h of residence time for the solvent to remove the soluble fractions from the sample at ambient temperature. The insoluble (cross-linked) fraction was then calculated by dividing the weight of the final dry sample left after the extraction by the original sample weight. The glass transition temperature (Tg) was evaluated by differential scanning calorimetry (DSC) using a DSC 204 F1 Phoenix1 (Netzsch) instrument. An accurately weighed polymer sample was placed in aluminium crucibles in the dry room. In a typical measurement, it was cooled from room temperature to 80 C and then heated at 10 C min1 up to 100 C under N2 flux. The Tg was calculated as the midpoint of the heat capacity change observed in the DSC profile during the transition from glassy state to rubbery state. The thermal stability was tested under N2 flux using thermogravimetric analysis (TGA) with a TG 209 F1 Libra1 instrument from Netzsch (25–600 C) at a heating rate of 10 C min1.
2. Experimental
2.4. Electrochemical characterisation techniques
2.1. Materials and polymer electrolyte preparation
Unless otherwise stated separately, cell-assembly procedures were performed in the dry room. The ionic conductivity of the cross-linked polymer electrolyte was determined by electrochemical impedance spectroscopy (EIS) analysis. Two-electrode test cells (2.54 cm2, ECC-Std, EL-Cell, Germany) were assembled by sandwiching the polymer electrolyte film between two stainless steel blocking electrodes. A potentiostat (PARSTAT-2273, Princeton Applied Research, USA) with inbuilt Frequency Response Analyser was used for EIS measurements at various temperatures between 1 Hz and 100 kHz at the open circuit potential (OCV). Cells were kept overnight at 80 C, and tested between 0 and 100 C; the bulk resistance was measured at every 10 C. Cells were housed in a climatic chamber (BINDER model MK-53, temperature control 1 C) and kept for 1 h between every 10 C ramp to attain the thermal equilibrium. The ionic conductivity was calculated using the equation: s ¼ l ðARb Þ (1)where s is the ionic conductivity (S cm1), Rb the bulk resistance, ‘ and A are the thickness and area of the test sample, respectively. Rb was given by the high frequency intercept of the Nyquist plot by analysing the impedance profile using the Electrochemistry Power Suite fitting software, version 2.58 provided by Princeton Applied Research. The compatibility of the polymer electrolyte film with the lithium metal anode was tested by monitoring the evolution of the
The polymer electrolyte preparation began by formulating a pre-polymer reactive mixture. The pre-polymer contained a dimethacrylate-based soft crosslinker (bisphenol A ethoxylate (15 EO/phenol) dimethacrylate BDM, Mn = 1700 Da), a noncarbonate based high boiling ionic conductivity enhancer (dimethyl polyethylene glycol DPG, Mn = 250 Da), a source of lithium ions (bis (trifluoromethylsulfonylimide) lithium salt LiTFSI, battery grade, Solvionic), and a suitable temperature sensitive free radical initiator (2,20 -Azobis(2-methylpropionitrile) AIBN). Unless otherwise stated separately, materials were purchased from Sigma Aldrich and used after treatment in vacuum. DPG was dried under high vacuum (50 C for 3 hours), and later stored for several days in a glass bottle that contains molecular sieves. The polymer electrolytes were synthesized by thoroughly mixing in appropriate proportions BDM and DPG with LiTFSI; 3 wt. % of AIBN was added to initiate the polymerisation reaction. The resulting liquid pre-polymer was casted in a Teflon petri dish. Later, the set up was taken to an oven and kept at 80 C for 1 hour to obtain a thin, flexible and self-standing film of about 100 5 mm thickness. The sample preparation procedure was carried out in the environmentally controlled atmosphere of a dry room (10 m2, RH < 2 1% at 20 C, Soimar Italy).
LiFePO4 electrodes with PEDOT:PSS coating over the current collector were prepared as described previously [17,18]. A certain amount of PEDOT:PSS (Clevios PH-1000) aqueous dispersion doped with ethylene glycol (5% v/v) was deposited over a 1 cm2 aluminium disc by drop casting. A PEDOT:PSS-coated aluminium current collector was obtained after evaporation of the solvents under vacuum at 100 C. A dispersion of LiFePO4 active material, carbon black and PVDF (85:8:7) in N-methyl-2-pyrrolidone was deposited over the PEDOT:PSS-coated current collector and dried at 80 C for at least 12 hours and, then, stored in the dry room. The average load of active material in the electrodes was ca. 1–2 mg cm2. 2.3. Characterisation techniques
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impedance response of a non-blocking symmetrical cell (Li/polymer electrolyte/Li) under OCV at ambient temperature. In this case, the thickness of the polymer electrolyte was around 200 mm. The lithium ion transference number (tLi+) was measured at 25 C, by employing a combined AC impedance and DC polarization measurement using a symmetric Li/polymer electrolyte/Li cell as explained by Evans and Watanabe [19,20]. Before test cell assembly, the surface of lithium metal was refreshed using a scalpel in the dry box [21]. A DC potential (DV = 10 mV) was applied (2–3 hours) until a steady current flow was obtained, and the initial (I0) and steady state (Iss) current that flow through the cell were measured. Simultaneously, the impedance spectra of the cell were recorded between 100 KHz and 0.1 Hz, with an oscillating potential of 10 mV, before and after DC polarization. Subsequently, the initial (R0) and final (Rss) bulk resistances of the electrolyte, and the initial (RC0) and final (RCss) charge transfer resistances (V) of the electrode/electrolyte interface were derived. Using these values, the tLi+ was calculated by the following equation: Rss ðDVI0 RC0 Þ (2) tLiþ ¼ IIss0 R 0 ðDVI ss Rcss Þ The salt diffusion coefficient of the polymer electrolyte was estimated using the method proposed by Ma et al. [22]. In this case, cells were polarized at 10 mV before the potential was interrupted. Then, cells were kept at OCV until a steady state was achieved. Later, curves were plotted as the natural log of potential (V) versus time (t). DLi+ values were calculated from the slope of the linear curves using the following equation, where L is the thickness of the polymer electrolyte membrane:
Slope ¼ p D2 Liþ (3) L The anodic breakdown voltage was evaluated by running a linear sweep voltammetry in a 3-electrodes EL-Ref cell of area 2.54 cm2. Al was used as the working electrode and Li metal as both the counter and the reference electrodes, and the given polymer membrane as the electrolyte separator (potential scan range from 3 to 6 V vs. Li; potential scan rate 0.500 mV s1). Under these conditions, the onset of the current was assumed to indicate the decomposition potential of the electrolyte. The cathodic breakdown voltage was evaluated by running a linear sweep voltammetry in a 3-electrodes EL-Ref cell with the following configuration: Cu as the working electrode, Li metal as the counter 2
and the reference electrodes, and the given polymer film as the electrolyte (potential scan range from 3 to 0.5 V vs. Li; potential scan rate 0.500 mV s1). 2.5. Fabrication of multiphase electrode/electrolyte composites and electrochemical characterization in Lab-scale cells Laboratory-scale cells were tested at 25 C in terms of galvanostatic charge/discharge cycling at different current rates using an Arbin Instrument Testing System model BT-2000. Cells were assembled by combining a lithium metal anode with a multiphase quasi-solid-state electrode/electrolyte composite. In particular, the liquid pre-polymer was directly deposited over the composite PEDOT:PSS-based LiFePO4 cathode film and kept in an oven at 80 C for 1 hr. After the polymerisation process, the electrode/electrolyte multiphase composite was dried under high vacuum and contacted to a lithium metal foil to envisage a lab scale polymer cell. Assembly of the cell prototypes was performed in an environmentally controlled Ar filled dry glove box (MBraun Labstar, [O2] and [H2O] 0.1 ppm). 3. Results and discussion 3.1. Polymer electrolyte characterisation DPG is an organic solvent having an evaporation temperature >200 C; it contains an average of five -EO- repeating units. It was selected as the plasticiser as it can conduct Li+-ions [13] and, most importantly, it can exhibit superior thermal properties than the classical carbonate-based organic solvents. Bisphenol A ethoxylate (15 EO/phenol) dimethacrylate (BDM) is a soft crosslinking agent that exhibits low glass transition temperature (below 40 C) [23]. It can be easily polymerised by free radical polymerisation technique to obtain a cross-linked polymer membrane with the complete conversion of methacrylic double bonds [24]. Fig. 1 illustrates the materials and the process used for the electrolyte preparation, while Scheme 1 describes the thermally induced free radical polymerisation reaction including the initiation step (1), propagation step and network formation reactions (2).
Fig. 1. Materials and processes used to obtain the solid polymer electrolyte along with the appearance of the self-standing polymer electrolyte (right side) after thermal polymerisation reaction. Inset: the multiphase electrode/electrolyte composite in which the pre-polymer reactive mixture was deposited over the electrode surface and in situ polymerised (note that the non-homogeneous-like appearance of the coated surface comes from the shining of light on top of the reflective surface of the multiphase electrode/electrolyte composite).
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Scheme 1. The reaction mechanism that describes the thermally induced free radical polymerisation reaction including the initiation step (1), propagation and network formation reactions (2).
The finally obtained polymer electrolyte film was thin, flexible, transparent and easy to manage. A highly cross-linked soft thermoset polymer membrane can efficiently hold DPG molecules as it contains many ethoxy groups. The aspect of the polymer electrolyte is shown in Fig. 1. A LiTFSI salt concentration of 15 wt. % was used after thorough analysis on several different compositions: it contains an -EO- to Li ratio of 18, which we found to be more favoured than extremely higher or lower concentrations. Indeed, these latter may either deliver insufficient source of alkali ions or create ionic aggregates or precipitates. Such observations are common in LiTFSI-based polymer systems. Moreover, the crosslinker (BDM) to DPG ratio was fixed to 40:60 as it guarantees sufficient mechanical integrity for the resulting polymer electrolyte to tolerate some bending/stretching activities. Such data are not included being out of scope of the present study. The liquid prepolymer formulations described in Section 2.1 are obtained by incorporating the oligomer BDM, DPG and LiTFSI salt with AIBN initiator. After keeping the liquid pre-polymer in an oven at 80 C for 1 hour under argon atmosphere we obtain a highly transparent, self-standing, flexible and non-sticky polymer electrolyte as shown in Fig. 1.
The characterisation of the thermally crosslinked polymer electrolyte included the evaluation of the conversion of the methacrylate double bonds to polymeric network by means of gel content study. The insoluble fraction value of 95% with respect to the BDM content was obtained after the extraction process in chloroform. This indicates that the methacrylate bonds were converted to polymer upon forming the three dimensional network as the only polymerising functionality is the methacrylate group of BDM, which can hold both DPG and LiTFSI. This suggests that the polymerisation was complete and the reaction time used for the polymerisation was sufficient to form a complete and stable network. The thermal characteristics were then investigated, such as the evaluation of the glass transition temperature (Tg) by differential scanning calorimetry (DSC) and thermal stability by means of thermo-gravimetric analysis (TGA). Fig. 2A shows the differential scanning calorimetry curves, obtained in the 80 to 100 C range, for the polymer electrolyte. The value of Tg was found to be around58 C, indicating that at ambient conditions (>80 C above Tg) the polymer electrolyte is in a rubbery state. This is certainly welcome to obtain a soft polymer that can facilitate the ionic mobility by segmental motion. Even though the Tg was far below the ambient temperature, the polymer electrolyte samples
Fig. 2. A) Differential scanning calorimetry (DSC) curves of the polymer electrolyte that contains 15 wt. % of LiTFSI, 51 wt. % of DPG, 34 wt. % of BDM and 3 wt. % of AIBN; B) Thermogravimetric analysis (TGA) along with related differential profiles (blue line). Taking into account the experimental errors, the weight loss pattern is consistent with the polymer electrolyte composition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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were self-standing, transparent and easy to handle. The low Tg guarantees a higher flexibility of the polymer chains, which will ultimately give more free path for Li+ ions transport. The linear nature of the DSC curve confirms that the resulting polymer is amorphous without any noticeable phase separation or leaking of the DPG plasticiser. The results of TGA analysis under N2 flux are shown in Fig. 2B. In our previously published article [24], we have shown that the thermal stability of methacrylic-based membranes is reasonably high, the weight loss being less than 10 wt. % up to 250 C. Indeed, in the present system, the presence of DPG as plasticiser slightly reduced the thermal stability to 110 C. According to K.M. Abraham et al. [13], DPG is slightly more volatile than its high molecular weight homologous, and first signs of decomposition may occur around this temperature. These results indicate that the proposed electrolyte system is a potential candidate to be safely used in polymeric LiBs at reasonably high temperatures without noticeable safety hazards. 3.2. Conductivity, compatibility, electrochemical testing To demonstrate the possible application of the newly elaborated polymer membrane as a quasi-solid-state electrolyte separator in LiBs, tests in terms of its ionic conductivity and compatibility (interfacial stability) with the lithium metal electrode were performed by means of electrochemical impedance spectroscopy (EIS) analysis. Furthermore, cyclic voltammetry, electrochemical impedance spectroscopy, and chrono-amperometry were performed either separately or together with other tests to evaluate [25] the ion transport and ion diffusion characteristics of the polymer electrolyte. The quasi-solid-state polymer electrolyte under study showed a remarkable ionic conductivity value of about 0.14 mS cm1 at 20 C (noteworthy, it approached 0.1 mS cm1 at 0 C), which is ascribed to the presence of DPG in the sample (Fig. 3). The ionic conductivity increased with temperature, finally settling at a value of 1.4 mS cm1 at 100 C, indicating that it is temperature dependent. It is worth mentioning that all impedance profiles (see Nyquist plots in the inset) obtained between 0 and 100 C were linear without any signs of high-frequency semicircles; it accounts for the highly homogeneous nature of the polymer matrix with no signs of phase separation. The ionic conductivity behaviour [26] was fitted using the Vogel–Tamman–Fulcher (VTF) equation, which describes that the mechanism of ion conduction inside the polymer electrolyte is largely contributed by the DPG molecules. The VTF fitting provided a value of activation energy (Ea) of 6.04 kJ mol1, which is surely
Fig. 4. Nyquist plot representing the evolution of the interfacial resistance with time for a symmetric Li/polymer electrolyte (thickness: 200 mm)/Li cell; inset: evolution of the bulk resistance (at high frequencies) as a function of the time at ambient temperature.
low [27] and in agreement with the low glass transition temperature and the high ionic conductivity achieved in the present case (Fig. 3). The interfacial stability of the polymer electrolyte towards the lithium metal electrode was evaluated at ambient temperature by recording the impedance response of the Li/polymer electrolyte/Li cell at progressively longer contact periods (20 days) under open circuit potential. The impedance response, shown in Fig. 4, evolved as large semicircles comprising the charge transfer process and the passivation layer formation. Indeed, it was observed that in the initial hours, especially within the first 48 hours, there was a change, both in the bulk resistance and the charge transfer resistance. This behaviour possibly indicates some interfacial issues between the polymer electrolyte and the lithium metal electrode [28,29]. However, this behaviour was stabilised within few days, and a stable interfacial behaviour was obtained, which settled around 900 V cm2 for a continuous storage of at least 20 days. The real component of the impedance at the lowest frequency also represents the Li/electrolyte interfacial resistance, which is a combination of electrolyte, charge transfer and passivation layer resistances. The observed interfacial resistance variations upon time, particularly those at low frequency are due to the polymer electrolyte under study: it can assure the formation of a homogeneous interfacial layer at the surface of the lithium metal
Fig. 3. A) Arrhenius plot showing the ionic conductivity behaviour vs. temperature of the solid polymer electrolyte (in the inset: high frequency magnification of the Nyquist plots at various temperatures); B) fitting by the VTF equation of the ionic conductivity as a function of ln (s) vs. 1/(T-T0).
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electrode, which remains also stable for extended duration. The results shown in Fig. 4 further confirm that the diameter of the semicircle is stable, even if there is a slight fluctuation in high (inset) or low frequency resistance values. It is worth noting that the resistance value corresponding to high frequency is also stabilised around 100 V cm2 after 20 days. This further confirms that the polymer electrolyte is stable upon prolonged storage and can effectively retain DPG for a long period. In addition to high ionic conductivity, the newly designed polymer electrolyte demonstrated a wide electrochemical stability window, which is an appealing property in view of practical battery application. Thus, cathodic (Fig. 5a) and anodic (Fig. 5b) breakdown voltages were evaluated by linear sweep voltammetry to complete the overall electrochemical testing of the chosen polymer electrolyte. This is illustrated in Fig. 5a and b, which shows the current–voltage response of the polymer electrolyte. In the case of cathodic stability (Fig. 5a), it was observed that the polymer electrolyte was able to appropriately plate and strip lithium around 0 V vs. Li [30,31]. In the anodic scan (Fig. 5b), the plot was found to be very flat and straight up to 4.7 V [28] vs. Li with very low residual current prior to the current onset that indicates the breakdown voltage. The absence of peaks around 4 V vs. Li confirms the high purity of both the prepared polymer electrolyte and the synthesis procedure adopted. However, it is interesting to note that the real marked and rapid current increase corresponding to the electrolyte decomposition took place above 5 V vs. Li. A polymer electrolyte with such an extended oxidation stability is very important if considered for practical application in
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high working potential batteries. Overall, the polymer electrolyte showed efficient lithium plating/stripping processes indicating that the matrix is stable in the potential range between 0 and 4.7 V vs. Li. The value of transference number was calculated using the method suggested by Abraham et al. [13], which is a modified version of the one proposed by Evans and Bruce [19]. The test (Fig. 6 top and bottom) is a combination of EIS measurements and chrono-amperometry analysis. The obtained tLi+ value of 0.45 is definitely acceptable for a truly quasi-solid-state polymer electrolyte and the result is in agreement with both the high ionic conductivity delivered at 20 C and the acceptable activation energy. Diffusion measurements were also performed using the method proposed by Ma et al. [22], which demonstrated that lithium ions inside the crosslinked matrix exhibit a diffusion coefficient of 2.16 108 cm2 s1. 3.3. Electrochemical behaviour in quasi-solid-state Lab-scale cell
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Finally, the polymer electrolyte was assembled in a lab-scale quasi-solid-state polymer cell and galvanostatically cycled at ambient temperature as well as at 70 C. The cell was assembled by combining a lithium electrode with an electrode-electrolyte composite prepared by thermally induced crosslinking of a viscous pre-polymer reactive mixture directly over the surface of the composite PEDOT:PSS-based LiFePO4 cathode (details in Section 2). We used direct thermal polymerisation in order to overcome the difficulties arising from the insufficient contact between the active electrode materials and the polymer matrix, particularly when polymer electrolytes are used. The process enabled us to achieve stable, thin (40 10 mm) and homogeneous polymer electrolyte films reaching out to major portions of the electrode active material particles. We already demonstrated a similar procedure with UV-induced photopolymerization in a previous article [32], which resulted in improved active area at the interface between the electrode and the polymer electrolyte, correspondingly improving both specific energy and power of the quasi-solid-state cell. It also helped in retaining the shape and structure of the active electrode material. The electrochemical response of the cell is shown in Fig. 7 in terms of galvanostatic charge/discharge profiles, and specific capacity versus cycle number for prolonged cycling of more than
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Fig. 7. Electrochemical response of the composite PEDOT:PSS-based LiFePO4 cathode/electrolyte lithium cell in terms of galvanostatic charge/discharge profiles and specific capacity versus cycle number at various current rates and for prolonged cycling. Cycling behaviour at ambient temperature: a) charge/discharge cycling profiles, b) specific capacity vs. cycle number at different current rates and c) long term cyclability of the same cell at C/10 current rate for more than 300 cycles at ambient temperature. Cycling behaviour at 70 C: d) charge/discharge cycling profiles, e) specific capacity vs. cycle number at different current rates and f) long term cyclability at C/10 current rate for more than 300 cycles.
300 cycles at various current rates. Two cells were cycled at different temperatures of 25 and 70 C (left and right hand-sided images of Fig. 7, respectively). The typical cell was assembled by simply contacting a lithium metal foil at the polymer side of the multiphase electrode/electrolyte composite films that comprise polymer electrolyte and cathode materials along with Al current collector. The constant current charge/discharge profiles shown in
plots (a) and (d) reflect the good properties of the newly elaborated system; even at low temperature, they show rather flat potential plateaus both on charge and discharge related to the typical Li+ extraction/insertion mechanism out of/in LiFePO4, with a steep potential increase/decay at its end. The polarization is rather limited particularly at elevated temperatures, which accounts for efficient redox reaction kinetics,
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due to the limited internal resistance at the electrode/electrolyte interface as well as the limited cell overpotential contributions. In general, a high Coulombic efficiency (percentage of charge capacity recovered in the following discharge capacity) very close to 100% is obtained, as for the good overlapping of the charge/discharge curves. When cycled at 70 C, our novel multiphase system is able to reversibly insert/extract lithium ions even at rather high 1C current regime, as shown in plot Fig. 7(e), which is definitely remarkable for a truly quasi-solid-state system. Additionally, the cycling stability is very competitive and the specific capacity obtained is only slightly lower than the one obtained for the same LiFePO4 electrode material at same current density in liquid electrolyte [17,18]. Moreover, the all polymer cell shows good capacity retention exceeding 70% after prolonged cycling, as shown in Fig. 7(f). This is a direct indication of the formation of an intimate interfacial contact between the electrodes and the electrolyte separator and the intrinsic stability and robustness of both the composite electrode material particles and the polymer electrolyte matrix. 4. Conclusions In the present work we have demonstrated a unique preparation and optimization of a newly designed carbonate free polymer electrolyte using a solvent-free and cost-effective in situ polymerisation technique. In spite of being quasi-solid-state, the polymer electrolyte demonstrated an ionic conductivity higher than 0.1 mS cm1. The key point of the present work is the demonstration of the promising electrochemical properties and the practical performance of the elaborated polymeric materials when directly deposited (in situ) over the composite cathode, comprising a LiFePO4/PEDOT:PSS interphase at the current collector that improved the material utilization at high rates and mitigated the corrosive effects of LiTFSI on aluminium. The resulting multifunctional electrode/electrolyte quasi-solid-state composite films were then incorporated in lab-scale polymer cells, showing stable charge/discharge characteristics (specific capacity 100 mAh g1) at different current regimes and temperatures with very limited capacity fading upon prolonged cycling. The approach can be extended to other energy related device applications like Na-ion batteries, dye-sensitized solar cells and supercapacitors, owing to its simple, scalable, economic and eco-friendly preparation procedure. Acknowledgements J.R.N. gratefully acknowledges financial support from MARS-EV project (FP7/2007-2013, under grant agreement n 609201). References [1] X. Luo, J. Wang, M. Dooner, J. Clarke, Overview of current development in electrical energy storage technologies and the application potential in power system operation, Applied Energy 137 (2015) 511–536. [2] H. Chen, T.N. Cong, W. Yang, C. Tan, Y. Li, Y. Ding, Progress in electrical energy storage system: A critical review, Progress in Natural Science 19 (2009) 291– 312. [3] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652– 657. [4] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, Z. Zhang, A review of recent developments in membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci. 7 (2014) 3857–3886. [5] L. Jin, P.C. Howlett, J.M. Pringle, J. Janikowski, M. Armand, D.R. MacFarlane, M. Forsyth, An organic ionic plastic crystal electrolyte for rate capability and stability of ambient temperature lithium batteries, Energy Environ. Sci. 7 (2014) 3352–3361. [6] G. Meligrana, C. Gerbaldi, A. Tuel, S. Bodoardo, N. Penazzi, Hydrothermal synthesis of high surface LiFePO4 powders as cathode for Li-ion cells, J. Power Sources 160 (2006) 516–522.
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Passerini, Ionic Liquid-Based Electrolytes for High Energy, Safer Lithium Batteries, Ionic Liquids: Science and Applications, Chapter 4, ACS Symposium Series 1117 (2012) 67–128. [12] A. Lewandowski, A. Swiderska-Mocek, Ionic liquids as electrolytes for Li-ion batteries—An overview of electrochemical studies, J. Power Sources 194 (2009) 601–609. [13] K.M. Abraham, Z. Jiang, B. Carroll, Highly Conductive PEO-like Polymer Electrolytes, Chem. Mater. 9 (1997) 1978–1988. [14] D. Zhou, Y.-B. He, R. Liu, M. Liu, H. Du, B. Li, Q. Cai, Q.-H. Yang, F. Kang, In Situ Synthesis of a Hierarchical All-Solid-State Electrolyte Based on Nitrile Materials for High-Performance Lithium-Ion Batteries, Adv. Energy. Mater. 5 (2015), doi:http://dx.doi.org/10.1002/aenm.201500353. [15] J. Wang, X. Sun, Olivine LiFePO4: the remaining challenges for future energy storage, Energy Environ. Sci. 00 (2015) 1–29. [16] V. Srinivasan, J. Newman, Discharge Model for the Lithium Iron-Phosphate Electrode, J. Electrochem. Soc. 151 (2004) A1517–A1529. [17] D. Cíntora-Juárez, C. Pérez-Vicente, S. Ahmad, J.L. Tirado, Electrochemical in battery polymerization of poly(alkylene dioxythiophene) over lithium iron phosphate for high-performance cathodes, Phys. Chem. Chem. Phys. 16 (2014) 20724–20730. [18] D. Cíntora-Juarez, C. Perez-Vicente, S. Kazim, S. Ahmad, J.L. Tirado, Judicious design of lithium iron phosphate electrodes using poly(3 4ethylenedioxythiophene) for high performance batteries, J. Mater. Chem. A 3 (2015) 14254–14262. [19] J. Evans, C.A. Vincent, P.G. Bruce, Electrochemical measurement of transference numbers in polymer electrolytes, Polymer 28 (1987) 2324–2328. [20] M. Watanabe, S. Nagano, K. Sanui, N. Ogata, Estimation of Li+ transport number in polymer electrolytes by the combination of complex impedance and potentiostatic polarization measurements, Solid State Ionics 28–30 (1988) 911–917. [21] H. Zhang, C. Liu, L. Zheng, F. Xu, W. Feng, H. Li, X. Huang, M. Armand, J. Nie, Z. 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Nair, Super Soft All-Ethylene Oxide PolymerElectrolyte for Safe All-Solid Lithium Batteries, Scientific Reports 6 (2016) 19892. [26] Jijeesh R. Nair, M. Destro, F. Bella, G.B. Appetecchi, C. Gerbaldi, Thermally cured semi-interpenetrating electrolyte networks (s-IPN) for safe and agingresistant secondary lithium polymer batteries, J. Power Source 306 (2016) 258–267. [27] H. Wang, M. Matsui, Y. Takeda, O. Yamamoto, D. Im, D. Lee, N. Imanishi, Interface Properties between Lithium Metal and a Composite Polymer Electrolyte of PEO18Li(CF3SO2)2N-Tetraethylene Glycol Dimethyl Ether, Membranes 3 (2013) 298–310. [28] S. Yamaguchi, M. Yoshizawa-Fujita, H. Zhu, M. Forsyth, Y. Takeoka, M. Rikukawa, Improvement of charge/discharge properties of oligoether electrolytes by zwitterions with an attached cyano group for use in lithium-ion secondary batteries, Electrochim. Acta 186 (2015) 471–477. [29] B. Sun, C. Xu, J. Mindemark, T. Gustafsson, K. Edström, D. 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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Truly quasi-solid-state lithium cells utilizing carbonate free polymer electrolytes on engineered LiFePO4 Jijeesh R. Naira,* , Daniel Cíntora-Juárezb , Carlos Pérez-Vicenteb , José L. Tiradob , Shahzada Ahmadc , Claudio Gerbaldia,* a b c
GAME Lab, CHENERGY Group, Department of Applied Science and Technology (DISAT), Politecnico di Torino, C.so Duca degli Abruzzi 24, Turin 10129, Italy Laboratorio de Química Inorgánica, Campus de Rabanales, Universidad de Córdoba, 14071, Spain Abengoa Research, Abengoa, C/Energía Solar n 1, Campus Palmas Altas, Sevilla 41014, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 January 2016 Received in revised form 23 March 2016 Accepted 26 March 2016 Available online 26 March 2016
Stable and safe functioning of a Li-ion battery is the demand of modern generation. Herein, we are demonstrating the application of an in-situ free radical polymerisation process (thermal curing) to fabricate a polymer electrolyte that possesses mechanical robustness, high thermal stability, improved interfacial and ion transport characteristics along with stable cycling at ambient conditions. The polymer electrolyte is obtained by direct polymerization over the electrode surface in one pot starting from a reactive mixture comprising an ethylene oxide-based dimethacrylic oligomer (BDM), dimethyl polyethylene glycol (DPG) and lithium salt. Furthermore, an engineered cathode is used, comprising a LiFePO4/PEDOT:PSS interface at the current collector that improves the material utilization at high rates and mitigates the corrosive effects of LiTFSI on aluminium current collector. The lithium cell resulting from the newly elaborated multiphase assembly of the composite cathode with the DPG-based carbonate-free polymer electrolyte film exhibits excellent reversibility upon prolonged cycling at ambient as well as elevated temperatures, which is found to be superior compared to previous reports on uncoated electrodes with polymer electrolytes. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: polymer electrolyte PEDOT lithium iron phosphate corrosion protection lithium battery
1. Introduction The ever increasing consumer demand [1] for energy-driven devices to perform better and better is creating lot of pressure on energy storage systems [2]. Lithium-ion batteries (Li-ion, LiBs) share this burden since their commercialisation in the 90’s by SONY [3]. Moreover, the clamour for moving towards storing energy from renewable resources is increasing along with the mandatory transition from petroleum-derived commutation to electricity-driven transportation. Even though the presence of LiBs is highly visible in the market, they cannot fulfil such a huge responsibility. Problems related to safety, cost and materials toxicity and limited energy density than the customer requisites are the major challenges of such systems to rule the market in a full-fledged manner. These disadvantages can be overcome by using new materials, which are superior to existing ones in terms of safety, eco compatibility and overall performance [4,5].
* Corresponding authors. Tel.: +39 011 090 4643; fax: +39 011 090 4629. E-mail addresses: [email protected] (J.R. Nair), [email protected] (C. Gerbaldi). http://dx.doi.org/10.1016/j.electacta.2016.03.156 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
Commercial LiBs are generally constructed using an anode based on graphitic carbon and a cathode based on transition metal oxide, both capable of reversibly intercalating/de-intercalating (inserting/de-inserting) Li+ ions, separated by a membrane soaked with an organic carbonate-based liquid electrolyte [6]. However, the usual problems such as leakage of liquid electrolytes from the pack, production of gas and flammability under abuse conditions are yet to be resolved, and they can be overcome only by replacing these inadequate components with safe, easy to manufacture and cost-effective polymer electrolytes [7]. Indeed, polymer electrolytes show advantages in terms of mechanical robustness, fabrication procedures and shape adaptability, with the unique possibility of fabricating an intimate electrode/electrolyte interface and lightweight/cost-effective packaging. In general, polymer electrolytes, especially gel or quasi-solid ones, encompass organic carbonate-based liquid electrolytes [8–10] or room temperature ionic liquids [11,12]. However, some attempts were also made to prepare polymer electrolytes with other components such as succinic nitrile, organic lactones, glymes of various lengths, etc. [13,14] to avoid the use of volatile organic carbonates. Another drawback is related to the preparation techniques that are employed to obtain self-standing polymer electrolytes. Typical
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2.2. Electrode preparation
example is the use of solvents (e.g., acetonitrile, acetone, dimethyl sulfoxide, dimethyl formamide, N-methyl-2-pyrrolidone, etc.) to obtain thin electrolyte films. Such low boiling solvents are the major players of environmental hazards (volatile organic components, VOC); add to it, one must consider the difficulty to absolutely remove the last traces from the resulting membranes. Such residual solvent components mainly induce stability issues and potential shoot-ups, which lead to thermal runaways by compromising the advantages offered by polymer electrolytes. Regarding the selection of an appropriate lithium battery cathode, LiFePO4 is a meritorious candidate [6] as it offers a safer operation window than LiCoO2 at elevated temperature, and the possibility to sustain higher charge/discharge rates in large format batteries. LiFePO4-based electrodes with optimized electronic conductivity through the electrode's interphases enable high rate capability, as it is known for carbon-coated particles [15] or for conducting polymer coatings [16], where the latter might be applied also for protecting the aluminium current collector from oxidation, especially when LiTFSI-based electrolytes are used. In the present study, we evaluate engineered cathodes, which is incorporated with PEDOT:PSS conducting polymer. PEDOT:PSS will act in two ways: first, as a conductive additive to increase the conductivity of LiFePO4, and secondly, as a protective coating against the aluminium current collector corrosion. Moreover, we are demonstrating that by appropriately implementing soft crosslinkers and high boiling plasticisers, we can prepare crosslinked polymer electrolytes without using any additives and hazardous solvents for film casting. The technique proposed here is a free radical based thermal polymerisation, which can be completed in about 60 mins. It is energy saving, cost effective, well established and ready to be industrially scaled-up. The reactive monomer mixture (liquid pre-polymer) is thermally crosslinked directly over the electrode surface in single step along with the lithium salt to retain the solid-like features and dimensional stability without hampering the ionic mobility. The performance of such system is then evaluated in lab-scale cells and optimum stability at high specific capacity is demonstrated, which is the highest reported so far for similar systems at ambient temperature.
The insoluble fraction (gel content) of the cross-linked polymer electrolyte was studied as follows: accurately weighed samples were enclosed in a stainless steel net, and then immersed in CHCl3 to extract the unreacted components from the crosslinked polymer electrolyte. The extraction step involved 18 h of residence time for the solvent to remove the soluble fractions from the sample at ambient temperature. The insoluble (cross-linked) fraction was then calculated by dividing the weight of the final dry sample left after the extraction by the original sample weight. The glass transition temperature (Tg) was evaluated by differential scanning calorimetry (DSC) using a DSC 204 F1 Phoenix1 (Netzsch) instrument. An accurately weighed polymer sample was placed in aluminium crucibles in the dry room. In a typical measurement, it was cooled from room temperature to 80 C and then heated at 10 C min1 up to 100 C under N2 flux. The Tg was calculated as the midpoint of the heat capacity change observed in the DSC profile during the transition from glassy state to rubbery state. The thermal stability was tested under N2 flux using thermogravimetric analysis (TGA) with a TG 209 F1 Libra1 instrument from Netzsch (25–600 C) at a heating rate of 10 C min1.
2. Experimental
2.4. Electrochemical characterisation techniques
2.1. Materials and polymer electrolyte preparation
Unless otherwise stated separately, cell-assembly procedures were performed in the dry room. The ionic conductivity of the cross-linked polymer electrolyte was determined by electrochemical impedance spectroscopy (EIS) analysis. Two-electrode test cells (2.54 cm2, ECC-Std, EL-Cell, Germany) were assembled by sandwiching the polymer electrolyte film between two stainless steel blocking electrodes. A potentiostat (PARSTAT-2273, Princeton Applied Research, USA) with inbuilt Frequency Response Analyser was used for EIS measurements at various temperatures between 1 Hz and 100 kHz at the open circuit potential (OCV). Cells were kept overnight at 80 C, and tested between 0 and 100 C; the bulk resistance was measured at every 10 C. Cells were housed in a climatic chamber (BINDER model MK-53, temperature control 1 C) and kept for 1 h between every 10 C ramp to attain the thermal equilibrium. The ionic conductivity was calculated using the equation: s ¼ l ðARb Þ (1)where s is the ionic conductivity (S cm1), Rb the bulk resistance, ‘ and A are the thickness and area of the test sample, respectively. Rb was given by the high frequency intercept of the Nyquist plot by analysing the impedance profile using the Electrochemistry Power Suite fitting software, version 2.58 provided by Princeton Applied Research. The compatibility of the polymer electrolyte film with the lithium metal anode was tested by monitoring the evolution of the
The polymer electrolyte preparation began by formulating a pre-polymer reactive mixture. The pre-polymer contained a dimethacrylate-based soft crosslinker (bisphenol A ethoxylate (15 EO/phenol) dimethacrylate BDM, Mn = 1700 Da), a noncarbonate based high boiling ionic conductivity enhancer (dimethyl polyethylene glycol DPG, Mn = 250 Da), a source of lithium ions (bis (trifluoromethylsulfonylimide) lithium salt LiTFSI, battery grade, Solvionic), and a suitable temperature sensitive free radical initiator (2,20 -Azobis(2-methylpropionitrile) AIBN). Unless otherwise stated separately, materials were purchased from Sigma Aldrich and used after treatment in vacuum. DPG was dried under high vacuum (50 C for 3 hours), and later stored for several days in a glass bottle that contains molecular sieves. The polymer electrolytes were synthesized by thoroughly mixing in appropriate proportions BDM and DPG with LiTFSI; 3 wt. % of AIBN was added to initiate the polymerisation reaction. The resulting liquid pre-polymer was casted in a Teflon petri dish. Later, the set up was taken to an oven and kept at 80 C for 1 hour to obtain a thin, flexible and self-standing film of about 100 5 mm thickness. The sample preparation procedure was carried out in the environmentally controlled atmosphere of a dry room (10 m2, RH < 2 1% at 20 C, Soimar Italy).
LiFePO4 electrodes with PEDOT:PSS coating over the current collector were prepared as described previously [17,18]. A certain amount of PEDOT:PSS (Clevios PH-1000) aqueous dispersion doped with ethylene glycol (5% v/v) was deposited over a 1 cm2 aluminium disc by drop casting. A PEDOT:PSS-coated aluminium current collector was obtained after evaporation of the solvents under vacuum at 100 C. A dispersion of LiFePO4 active material, carbon black and PVDF (85:8:7) in N-methyl-2-pyrrolidone was deposited over the PEDOT:PSS-coated current collector and dried at 80 C for at least 12 hours and, then, stored in the dry room. The average load of active material in the electrodes was ca. 1–2 mg cm2. 2.3. Characterisation techniques
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impedance response of a non-blocking symmetrical cell (Li/polymer electrolyte/Li) under OCV at ambient temperature. In this case, the thickness of the polymer electrolyte was around 200 mm. The lithium ion transference number (tLi+) was measured at 25 C, by employing a combined AC impedance and DC polarization measurement using a symmetric Li/polymer electrolyte/Li cell as explained by Evans and Watanabe [19,20]. Before test cell assembly, the surface of lithium metal was refreshed using a scalpel in the dry box [21]. A DC potential (DV = 10 mV) was applied (2–3 hours) until a steady current flow was obtained, and the initial (I0) and steady state (Iss) current that flow through the cell were measured. Simultaneously, the impedance spectra of the cell were recorded between 100 KHz and 0.1 Hz, with an oscillating potential of 10 mV, before and after DC polarization. Subsequently, the initial (R0) and final (Rss) bulk resistances of the electrolyte, and the initial (RC0) and final (RCss) charge transfer resistances (V) of the electrode/electrolyte interface were derived. Using these values, the tLi+ was calculated by the following equation: Rss ðDVI0 RC0 Þ (2) tLiþ ¼ IIss0 R 0 ðDVI ss Rcss Þ The salt diffusion coefficient of the polymer electrolyte was estimated using the method proposed by Ma et al. [22]. In this case, cells were polarized at 10 mV before the potential was interrupted. Then, cells were kept at OCV until a steady state was achieved. Later, curves were plotted as the natural log of potential (V) versus time (t). DLi+ values were calculated from the slope of the linear curves using the following equation, where L is the thickness of the polymer electrolyte membrane:
Slope ¼ p D2 Liþ (3) L The anodic breakdown voltage was evaluated by running a linear sweep voltammetry in a 3-electrodes EL-Ref cell of area 2.54 cm2. Al was used as the working electrode and Li metal as both the counter and the reference electrodes, and the given polymer membrane as the electrolyte separator (potential scan range from 3 to 6 V vs. Li; potential scan rate 0.500 mV s1). Under these conditions, the onset of the current was assumed to indicate the decomposition potential of the electrolyte. The cathodic breakdown voltage was evaluated by running a linear sweep voltammetry in a 3-electrodes EL-Ref cell with the following configuration: Cu as the working electrode, Li metal as the counter 2
and the reference electrodes, and the given polymer film as the electrolyte (potential scan range from 3 to 0.5 V vs. Li; potential scan rate 0.500 mV s1). 2.5. Fabrication of multiphase electrode/electrolyte composites and electrochemical characterization in Lab-scale cells Laboratory-scale cells were tested at 25 C in terms of galvanostatic charge/discharge cycling at different current rates using an Arbin Instrument Testing System model BT-2000. Cells were assembled by combining a lithium metal anode with a multiphase quasi-solid-state electrode/electrolyte composite. In particular, the liquid pre-polymer was directly deposited over the composite PEDOT:PSS-based LiFePO4 cathode film and kept in an oven at 80 C for 1 hr. After the polymerisation process, the electrode/electrolyte multiphase composite was dried under high vacuum and contacted to a lithium metal foil to envisage a lab scale polymer cell. Assembly of the cell prototypes was performed in an environmentally controlled Ar filled dry glove box (MBraun Labstar, [O2] and [H2O] 0.1 ppm). 3. Results and discussion 3.1. Polymer electrolyte characterisation DPG is an organic solvent having an evaporation temperature >200 C; it contains an average of five -EO- repeating units. It was selected as the plasticiser as it can conduct Li+-ions [13] and, most importantly, it can exhibit superior thermal properties than the classical carbonate-based organic solvents. Bisphenol A ethoxylate (15 EO/phenol) dimethacrylate (BDM) is a soft crosslinking agent that exhibits low glass transition temperature (below 40 C) [23]. It can be easily polymerised by free radical polymerisation technique to obtain a cross-linked polymer membrane with the complete conversion of methacrylic double bonds [24]. Fig. 1 illustrates the materials and the process used for the electrolyte preparation, while Scheme 1 describes the thermally induced free radical polymerisation reaction including the initiation step (1), propagation step and network formation reactions (2).
Fig. 1. Materials and processes used to obtain the solid polymer electrolyte along with the appearance of the self-standing polymer electrolyte (right side) after thermal polymerisation reaction. Inset: the multiphase electrode/electrolyte composite in which the pre-polymer reactive mixture was deposited over the electrode surface and in situ polymerised (note that the non-homogeneous-like appearance of the coated surface comes from the shining of light on top of the reflective surface of the multiphase electrode/electrolyte composite).
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Scheme 1. The reaction mechanism that describes the thermally induced free radical polymerisation reaction including the initiation step (1), propagation and network formation reactions (2).
The finally obtained polymer electrolyte film was thin, flexible, transparent and easy to manage. A highly cross-linked soft thermoset polymer membrane can efficiently hold DPG molecules as it contains many ethoxy groups. The aspect of the polymer electrolyte is shown in Fig. 1. A LiTFSI salt concentration of 15 wt. % was used after thorough analysis on several different compositions: it contains an -EO- to Li ratio of 18, which we found to be more favoured than extremely higher or lower concentrations. Indeed, these latter may either deliver insufficient source of alkali ions or create ionic aggregates or precipitates. Such observations are common in LiTFSI-based polymer systems. Moreover, the crosslinker (BDM) to DPG ratio was fixed to 40:60 as it guarantees sufficient mechanical integrity for the resulting polymer electrolyte to tolerate some bending/stretching activities. Such data are not included being out of scope of the present study. The liquid prepolymer formulations described in Section 2.1 are obtained by incorporating the oligomer BDM, DPG and LiTFSI salt with AIBN initiator. After keeping the liquid pre-polymer in an oven at 80 C for 1 hour under argon atmosphere we obtain a highly transparent, self-standing, flexible and non-sticky polymer electrolyte as shown in Fig. 1.
The characterisation of the thermally crosslinked polymer electrolyte included the evaluation of the conversion of the methacrylate double bonds to polymeric network by means of gel content study. The insoluble fraction value of 95% with respect to the BDM content was obtained after the extraction process in chloroform. This indicates that the methacrylate bonds were converted to polymer upon forming the three dimensional network as the only polymerising functionality is the methacrylate group of BDM, which can hold both DPG and LiTFSI. This suggests that the polymerisation was complete and the reaction time used for the polymerisation was sufficient to form a complete and stable network. The thermal characteristics were then investigated, such as the evaluation of the glass transition temperature (Tg) by differential scanning calorimetry (DSC) and thermal stability by means of thermo-gravimetric analysis (TGA). Fig. 2A shows the differential scanning calorimetry curves, obtained in the 80 to 100 C range, for the polymer electrolyte. The value of Tg was found to be around58 C, indicating that at ambient conditions (>80 C above Tg) the polymer electrolyte is in a rubbery state. This is certainly welcome to obtain a soft polymer that can facilitate the ionic mobility by segmental motion. Even though the Tg was far below the ambient temperature, the polymer electrolyte samples
Fig. 2. A) Differential scanning calorimetry (DSC) curves of the polymer electrolyte that contains 15 wt. % of LiTFSI, 51 wt. % of DPG, 34 wt. % of BDM and 3 wt. % of AIBN; B) Thermogravimetric analysis (TGA) along with related differential profiles (blue line). Taking into account the experimental errors, the weight loss pattern is consistent with the polymer electrolyte composition. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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were self-standing, transparent and easy to handle. The low Tg guarantees a higher flexibility of the polymer chains, which will ultimately give more free path for Li+ ions transport. The linear nature of the DSC curve confirms that the resulting polymer is amorphous without any noticeable phase separation or leaking of the DPG plasticiser. The results of TGA analysis under N2 flux are shown in Fig. 2B. In our previously published article [24], we have shown that the thermal stability of methacrylic-based membranes is reasonably high, the weight loss being less than 10 wt. % up to 250 C. Indeed, in the present system, the presence of DPG as plasticiser slightly reduced the thermal stability to 110 C. According to K.M. Abraham et al. [13], DPG is slightly more volatile than its high molecular weight homologous, and first signs of decomposition may occur around this temperature. These results indicate that the proposed electrolyte system is a potential candidate to be safely used in polymeric LiBs at reasonably high temperatures without noticeable safety hazards. 3.2. Conductivity, compatibility, electrochemical testing To demonstrate the possible application of the newly elaborated polymer membrane as a quasi-solid-state electrolyte separator in LiBs, tests in terms of its ionic conductivity and compatibility (interfacial stability) with the lithium metal electrode were performed by means of electrochemical impedance spectroscopy (EIS) analysis. Furthermore, cyclic voltammetry, electrochemical impedance spectroscopy, and chrono-amperometry were performed either separately or together with other tests to evaluate [25] the ion transport and ion diffusion characteristics of the polymer electrolyte. The quasi-solid-state polymer electrolyte under study showed a remarkable ionic conductivity value of about 0.14 mS cm1 at 20 C (noteworthy, it approached 0.1 mS cm1 at 0 C), which is ascribed to the presence of DPG in the sample (Fig. 3). The ionic conductivity increased with temperature, finally settling at a value of 1.4 mS cm1 at 100 C, indicating that it is temperature dependent. It is worth mentioning that all impedance profiles (see Nyquist plots in the inset) obtained between 0 and 100 C were linear without any signs of high-frequency semicircles; it accounts for the highly homogeneous nature of the polymer matrix with no signs of phase separation. The ionic conductivity behaviour [26] was fitted using the Vogel–Tamman–Fulcher (VTF) equation, which describes that the mechanism of ion conduction inside the polymer electrolyte is largely contributed by the DPG molecules. The VTF fitting provided a value of activation energy (Ea) of 6.04 kJ mol1, which is surely
Fig. 4. Nyquist plot representing the evolution of the interfacial resistance with time for a symmetric Li/polymer electrolyte (thickness: 200 mm)/Li cell; inset: evolution of the bulk resistance (at high frequencies) as a function of the time at ambient temperature.
low [27] and in agreement with the low glass transition temperature and the high ionic conductivity achieved in the present case (Fig. 3). The interfacial stability of the polymer electrolyte towards the lithium metal electrode was evaluated at ambient temperature by recording the impedance response of the Li/polymer electrolyte/Li cell at progressively longer contact periods (20 days) under open circuit potential. The impedance response, shown in Fig. 4, evolved as large semicircles comprising the charge transfer process and the passivation layer formation. Indeed, it was observed that in the initial hours, especially within the first 48 hours, there was a change, both in the bulk resistance and the charge transfer resistance. This behaviour possibly indicates some interfacial issues between the polymer electrolyte and the lithium metal electrode [28,29]. However, this behaviour was stabilised within few days, and a stable interfacial behaviour was obtained, which settled around 900 V cm2 for a continuous storage of at least 20 days. The real component of the impedance at the lowest frequency also represents the Li/electrolyte interfacial resistance, which is a combination of electrolyte, charge transfer and passivation layer resistances. The observed interfacial resistance variations upon time, particularly those at low frequency are due to the polymer electrolyte under study: it can assure the formation of a homogeneous interfacial layer at the surface of the lithium metal
Fig. 3. A) Arrhenius plot showing the ionic conductivity behaviour vs. temperature of the solid polymer electrolyte (in the inset: high frequency magnification of the Nyquist plots at various temperatures); B) fitting by the VTF equation of the ionic conductivity as a function of ln (s) vs. 1/(T-T0).
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electrode, which remains also stable for extended duration. The results shown in Fig. 4 further confirm that the diameter of the semicircle is stable, even if there is a slight fluctuation in high (inset) or low frequency resistance values. It is worth noting that the resistance value corresponding to high frequency is also stabilised around 100 V cm2 after 20 days. This further confirms that the polymer electrolyte is stable upon prolonged storage and can effectively retain DPG for a long period. In addition to high ionic conductivity, the newly designed polymer electrolyte demonstrated a wide electrochemical stability window, which is an appealing property in view of practical battery application. Thus, cathodic (Fig. 5a) and anodic (Fig. 5b) breakdown voltages were evaluated by linear sweep voltammetry to complete the overall electrochemical testing of the chosen polymer electrolyte. This is illustrated in Fig. 5a and b, which shows the current–voltage response of the polymer electrolyte. In the case of cathodic stability (Fig. 5a), it was observed that the polymer electrolyte was able to appropriately plate and strip lithium around 0 V vs. Li [30,31]. In the anodic scan (Fig. 5b), the plot was found to be very flat and straight up to 4.7 V [28] vs. Li with very low residual current prior to the current onset that indicates the breakdown voltage. The absence of peaks around 4 V vs. Li confirms the high purity of both the prepared polymer electrolyte and the synthesis procedure adopted. However, it is interesting to note that the real marked and rapid current increase corresponding to the electrolyte decomposition took place above 5 V vs. Li. A polymer electrolyte with such an extended oxidation stability is very important if considered for practical application in
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high working potential batteries. Overall, the polymer electrolyte showed efficient lithium plating/stripping processes indicating that the matrix is stable in the potential range between 0 and 4.7 V vs. Li. The value of transference number was calculated using the method suggested by Abraham et al. [13], which is a modified version of the one proposed by Evans and Bruce [19]. The test (Fig. 6 top and bottom) is a combination of EIS measurements and chrono-amperometry analysis. The obtained tLi+ value of 0.45 is definitely acceptable for a truly quasi-solid-state polymer electrolyte and the result is in agreement with both the high ionic conductivity delivered at 20 C and the acceptable activation energy. Diffusion measurements were also performed using the method proposed by Ma et al. [22], which demonstrated that lithium ions inside the crosslinked matrix exhibit a diffusion coefficient of 2.16 108 cm2 s1. 3.3. Electrochemical behaviour in quasi-solid-state Lab-scale cell
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Finally, the polymer electrolyte was assembled in a lab-scale quasi-solid-state polymer cell and galvanostatically cycled at ambient temperature as well as at 70 C. The cell was assembled by combining a lithium electrode with an electrode-electrolyte composite prepared by thermally induced crosslinking of a viscous pre-polymer reactive mixture directly over the surface of the composite PEDOT:PSS-based LiFePO4 cathode (details in Section 2). We used direct thermal polymerisation in order to overcome the difficulties arising from the insufficient contact between the active electrode materials and the polymer matrix, particularly when polymer electrolytes are used. The process enabled us to achieve stable, thin (40 10 mm) and homogeneous polymer electrolyte films reaching out to major portions of the electrode active material particles. We already demonstrated a similar procedure with UV-induced photopolymerization in a previous article [32], which resulted in improved active area at the interface between the electrode and the polymer electrolyte, correspondingly improving both specific energy and power of the quasi-solid-state cell. It also helped in retaining the shape and structure of the active electrode material. The electrochemical response of the cell is shown in Fig. 7 in terms of galvanostatic charge/discharge profiles, and specific capacity versus cycle number for prolonged cycling of more than
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Fig. 7. Electrochemical response of the composite PEDOT:PSS-based LiFePO4 cathode/electrolyte lithium cell in terms of galvanostatic charge/discharge profiles and specific capacity versus cycle number at various current rates and for prolonged cycling. Cycling behaviour at ambient temperature: a) charge/discharge cycling profiles, b) specific capacity vs. cycle number at different current rates and c) long term cyclability of the same cell at C/10 current rate for more than 300 cycles at ambient temperature. Cycling behaviour at 70 C: d) charge/discharge cycling profiles, e) specific capacity vs. cycle number at different current rates and f) long term cyclability at C/10 current rate for more than 300 cycles.
300 cycles at various current rates. Two cells were cycled at different temperatures of 25 and 70 C (left and right hand-sided images of Fig. 7, respectively). The typical cell was assembled by simply contacting a lithium metal foil at the polymer side of the multiphase electrode/electrolyte composite films that comprise polymer electrolyte and cathode materials along with Al current collector. The constant current charge/discharge profiles shown in
plots (a) and (d) reflect the good properties of the newly elaborated system; even at low temperature, they show rather flat potential plateaus both on charge and discharge related to the typical Li+ extraction/insertion mechanism out of/in LiFePO4, with a steep potential increase/decay at its end. The polarization is rather limited particularly at elevated temperatures, which accounts for efficient redox reaction kinetics,
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due to the limited internal resistance at the electrode/electrolyte interface as well as the limited cell overpotential contributions. In general, a high Coulombic efficiency (percentage of charge capacity recovered in the following discharge capacity) very close to 100% is obtained, as for the good overlapping of the charge/discharge curves. When cycled at 70 C, our novel multiphase system is able to reversibly insert/extract lithium ions even at rather high 1C current regime, as shown in plot Fig. 7(e), which is definitely remarkable for a truly quasi-solid-state system. Additionally, the cycling stability is very competitive and the specific capacity obtained is only slightly lower than the one obtained for the same LiFePO4 electrode material at same current density in liquid electrolyte [17,18]. Moreover, the all polymer cell shows good capacity retention exceeding 70% after prolonged cycling, as shown in Fig. 7(f). This is a direct indication of the formation of an intimate interfacial contact between the electrodes and the electrolyte separator and the intrinsic stability and robustness of both the composite electrode material particles and the polymer electrolyte matrix. 4. Conclusions In the present work we have demonstrated a unique preparation and optimization of a newly designed carbonate free polymer electrolyte using a solvent-free and cost-effective in situ polymerisation technique. In spite of being quasi-solid-state, the polymer electrolyte demonstrated an ionic conductivity higher than 0.1 mS cm1. The key point of the present work is the demonstration of the promising electrochemical properties and the practical performance of the elaborated polymeric materials when directly deposited (in situ) over the composite cathode, comprising a LiFePO4/PEDOT:PSS interphase at the current collector that improved the material utilization at high rates and mitigated the corrosive effects of LiTFSI on aluminium. The resulting multifunctional electrode/electrolyte quasi-solid-state composite films were then incorporated in lab-scale polymer cells, showing stable charge/discharge characteristics (specific capacity 100 mAh g1) at different current regimes and temperatures with very limited capacity fading upon prolonged cycling. The approach can be extended to other energy related device applications like Na-ion batteries, dye-sensitized solar cells and supercapacitors, owing to its simple, scalable, economic and eco-friendly preparation procedure. Acknowledgements J.R.N. gratefully acknowledges financial support from MARS-EV project (FP7/2007-2013, under grant agreement n 609201). References [1] X. Luo, J. Wang, M. Dooner, J. Clarke, Overview of current development in electrical energy storage technologies and the application potential in power system operation, Applied Energy 137 (2015) 511–536. [2] H. Chen, T.N. Cong, W. Yang, C. Tan, Y. Li, Y. Ding, Progress in electrical energy storage system: A critical review, Progress in Natural Science 19 (2009) 291– 312. [3] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652– 657. [4] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, Z. Zhang, A review of recent developments in membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci. 7 (2014) 3857–3886. [5] L. Jin, P.C. Howlett, J.M. Pringle, J. Janikowski, M. Armand, D.R. MacFarlane, M. Forsyth, An organic ionic plastic crystal electrolyte for rate capability and stability of ambient temperature lithium batteries, Energy Environ. Sci. 7 (2014) 3352–3361. [6] G. Meligrana, C. Gerbaldi, A. Tuel, S. Bodoardo, N. Penazzi, Hydrothermal synthesis of high surface LiFePO4 powders as cathode for Li-ion cells, J. Power Sources 160 (2006) 516–522.
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