Effects of Bulky LATP in PEO-based Hybrid Solid Electrolytes①

XUE Xi-Lai ZHANG Xiang-XinLIN Jun-Hong CHEN Su-Jing CHEN Yuan-Qiang LIU Yong-Chuan,cZHANG Yi-Ning,c②

a(Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

b(University of Chinese Academy of Sciences, Beijing 100049, China)

c(Dalian National Laboratory for Clean Energy, Chinese Academy of Sciences, Dalian 161000, China)

ABSTRACTSolid polymer electrolytes (SPEs) have been considered as the spotlight inrecent years due to their high safety, non-flammability and good flexibility. Nonetheless, high crystallinity of polymer matrix leads to low ionic conductivity at ambient conditions and retards the practical applications of SPEs. Herein, we report hybrid solid electrolytes (HSE) containing bulky LATP in poly(ethylene oxide) (PEO) matrix, which significantly enhances the electrochemical properties. LATP has been easily obtained by an accessible solid-state method. The solid electrolyte based on 20 wt% LATP in PEO polymer matrix (abbreviated as PEO-20) exhibits an ionic conductivity of 2.1 ×10–5S·cm–1 at 30 oC, an order of magnitude higher than 2.9 × 10–6S·cm–1 of the pristine PEO solid electrolyte (abbreviated as PEO–0), mainly resulting from the decline of crystallinity in polymer matrix. The electrochemical window of PEO-20 can reach 4.84 V at room temperature, compared with 4.40 V for PEO-0, which could be compatible with high-voltage cathode materials.

Keywords: hybrid solid electrolytes, bulky LATP, polyethylene oxide;

1 INTRODUCTION

Recent years have witnessed a rapid demand of high-power electric vehicles, progressive electronic devices, and grid-scale storage systems[1,2]. Lithium batteries with superior energy density and power density are extensively considered as the most auspicious energy storage systems for industrial employments[3,4]. Unfortunately, safety issues concerning the potential practical applications of lithium batteries are constantly increasing owing to the use of high volatile and flammable organic liquid electrolytes. Further-more, the harassments of liquid solvent-based organic electrolytes constrain the long-lived durability of lithium batteries because of the presence of highly-active lithium metal[5].

Solid electrolytes are substantially becoming the hot spot of research owing to their merits of non-flammability, high-safety and environmental-friendly[6-9]. Solid electrolytes are usually separated into three diverse classifications: inorganic solid electrolytes, polymer solid electrolytes and hybrid solid electrolytes[8,9]. When it comes to inorganic solid electrolytes, the archetypal materials encompass garnet-type, NASICON-type, oxide-based and sulfide-based solid electrolytes. Garnet-type solid electrolytes like Li0.34La0.56TiO3(LLTO)[10]and Li6.4La3Zr1.4Ta0.6O12(LLZTO)[11]have high chemical stability against Li, and wide electrochemical potential window, but easily react with moisture to produce deposition products. NASICON-type, such as Li1.4Al0.4Ti1.6(PO4)3(LATP)[12]and Li1.5Al0.5Ge1.5(PO4)3(LAGP)[13], are firm but brittle. Sulfide-based solid electrolytes, including Li10GeP2S12, Li3.25Ge0.25P0.75S4,etc., display high ionic conductivity at ambient conditions and trivial electronic conductivity, but they are exceedingly sensitive to the oxygen and moisture, along with far too extravagant on account of utilizing noble metals[14]. A wide range of polymers, such as poly(ethylene oxide) (PEO)[15], poly(vinylidene fluoride) (PVDF)[16], poly(acrylonitrile) (PAN)[17], and poly(methyl methacrylate) (PMMA)[18], have been widely explored as solid polymer electrolytes for lithium batteries. Among them, PEO/lithium salt, one of the most prospective polymer matrices, was firstly investigated and used in the middle of 1970s[15]. The basic chemical structure of -CH2-CH2-O- in PEO chains is able to deliver an efficacious dissolvability of lithium salt, thereby accelerating the existence of metal/lithium salt complex and functioning as a host matrix for solid polymer electrolytes[8]. In addition, the flexible macromolecular structure of PEO chains can also aid the lithium transport[19]. Nevertheless, it is worth mentioning that the presence of crystalline regions in PEO matrix has a severely negative impact on the lithium ion mobility[20]. Therefore, the ionic conductivity of PEO-based solid electrolytes is relatively low at ambient conditions, which is normally approximately 10-6～10-8S·cm-1and cannot meet the applicable demands[18]. Integrating plasticizers, synthesizing cross-linked or blocked polymers and incorporating inorganic fillers are three principle approaches to reduce the crystallinity of polymer matrix and hence increase the ionic conductivity of PEO-based solid electrolytes[19]. However, previous resear-ches in adding plasticizers or forming blocked polymers maysacrifice the thermal properties and electrochemical stability[21].

On the contrary, the integration of inorganic fillers into polymer matrix results in the reduction of crystallinity and enhancement of electrode-electrolyte interfacial stability simultaneously[22]. Many researchers have incorporated nano-sized inorganic particles into polymer matrix owing to the fact that they exhibit a relatively higher area and contact with the polymer, which results in decreasedcrystallnity of polymer matrix and higher ionic conductivity[23]. Dissanayake’s group prepared a hybrid ceramic-polymer electrolyte predicated on incorporating 10 wt% TiO2nano-sized particles into P(EO)9LiTf, and it exhibited an ionic conductivity of 4.9 × 10–5S·cm–1at 30oC[24]. Li’s group reported that Li1.4Al0.4Ti1.6(PO4)3electrolyte nanoparticles were used as active fillers to improve the properties of PEO-based electrolytes and the composite electrolyte with 1 wt% LATP augmented to 1.2 × 10–5S·cm–1at room temperature[25]. Yang and his colleagues integrated the vertically aligned nano-sized LATP fillers, synthesized by an ice-templating method, into PEO matrix, andobserved enhanced thermal and electrochemical stability in comparison to pristine PEO electrolytes[26]. However, it is really costly, time-consuming, and difficult to synthesize nanoparticles as fillers. Therefore, having noticed considerable efforts in synthesizing nano-sized fillers by different pricey ways, micro-sized bulky fillershave their own benefits like low-cost, facile-synthesis and similar or even better effects with nano-sized fillers. However, there are few researches in bulky inorganic particles as fillers before. Herein, we report a hybrid solid electrolyte (HSE) based on incorporating bulky Li1.4Al0.6Ti1.6(PO4)3(LATP) into PEO matrixin lithium batteries. Bulky LATP particles, with a size range from 3to 10μm, were prepared by a facile solid-state method[27]and later dispersed into the PEO polymer.

2 EXPERIMENTAL

2.1 Synthesis of bulky LATP

All reagents used in this work werepurchased from Aladdin of analytical grade and used without any further purification. Bulky Li1.4Al0.6Ti1.6(PO4)3(LATP) wasfabri-cated by a facile solid-state synthesis method[27]. Stoichio-metric amounts of Li2CO3, Al2O3, TiO2, and NH4H2PO4were blended with acetone and then ball-milled for 10 h. The ball-milled precursor samples were calcined at 400oC for 2 h and 900oC for 10 h. The sintered powders were ball-milled again to obtain the bulky LATP for further use and characterization.

2.2 Synthesis of PEO-based solid electrolytes

Polyethylene oxide (PEO) and bis(trifluoromethane)sul-fonimide lithium (LiTFSI) were dried at 50 and 120oCovernight under a vacuum condition respectively before use. PEO and LiTFSI were totally dissolved in acetonitrile with the EO/Li+ratio of 16:1 at 50oC for 12 h. And then different contents (10, 20, 30 wt%) of bulky LATP were integrated into the homogenous solution and continued to stir at 50oC for another 12 h. Subsequently, the mixtures were cast into the Teflon Petri dish and dried in an Ar-filled glove box. Eventually, the collected electrolytes were dried in a vacuum oven at 50oC for 12 h to form the solid electrolytes. PEO-based hybrid solid electrolytes are abbreviated as PEO-x (x=0, 10, 20, 30, respectively). The pristine PEO electrolyte is abbreviated as PEO-0, while PEO-based hybrid solid electrolytes with 10 wt%, 20 wt%, 30 wt% bulky LATP are abbreviated as PEO-10, PEO-20 and PEO-30, respectively. The thickness of synthesized PEO-xfilms ranges from 200 to 300 μm.

2.3 Structure characterization

The morphologies of bulky LATP and solid electrolytes were observed by scanning electron microscope (SEM, SU8010) coupled with energy dispersive X-ray spectrometer (EDS). The crystalline degree of bulky LATP and the hybrid solid electrolytes were detected by using X-ray diffraction (XRD,Rigaku Miniflex600) with CuKα radiation in the range of 10 ～80°. The thermal property was carried out by thermogravimetric analysis (TGA, STA449F3) from 30 to 700oC.

2.4 Electrochemical measurements

The ionic conductivity of PEO-x was recorded by electrochemical impedance spectroscopy (EIS) from 30 to 90oC. PEO-x solid electrolytes were sandwiched between two stainless steel (SS) electrodes and the spectra were carried out from 1 Hz to 1 MHz with AC amplitude of 10 mV. The ionic conductivity was calculated by the following equation (1):

whered is the thickness of PEO-x, Rsis the bulk resistance and S is the contact area between hybrid solid electrolyte and electrode, respectively.

Activated energy (Ea) of the PEO-x could be calculated by using the following equation (2):

whereσ is the ionic conductivity that can be obtained above, A is the pre-exponential constant, k is the Boltzmann constant, and Eais the activation energy for lithium ion conduction.

The electrochemical stability of PEO-x was determined by linear sweep voltammetry (LSV) and the curves were investigated from 0 to 6 V at the speed of 1 mV·s-1. All electrochemical experiments were conducted by the Princeton PARSTAT MC electrochemical workstation.

3 RESULTS AND DISCUSSION

3.1 Structure and morphology analysis

The crystal structure of bulky LATP was determined by X-ray diffraction and the result is exhibited in Fig.1a. The as-prepared LATP for hybrid solid electrolytes matches well with the standard diffraction peaks of LiTi2(PO4)3(JCPDS 35-0754). XRD pattern of bulky LATP generally constitutes two obviously intense peaks at 20.8° and 24.5°, which are individually assigned to the planes (104) and (113).The microstructure and size of bulky LATP were identified by SEM. Fig.1b and 1c clearly demonstrate that the sizes of as-prepared LATP particles normally rangefrom 3 to 10 μm, which can be concluded that bulky LATP has been successfully synthesized by the facile solid-state method.

Fig.1. (a) XRD pattern of bulky LATP, (b～c) SEM images of bulky LATP

XRD patterns likewise show the crystallinity and planes of pristine PEO and PEO-based hybrid solid electrolytes composed of different contents of bulky LATP. As illustrated in Fig.2a, the XRD pattern of PEO-0 is commonly made up of two characteristic peaks at 19.3° and 23.4°, which are separately ascribed to planes (120) and (112). These two diffraction peaks of PEO still maintain the same position when incorporating various amounts of bulky LATP into PEO matrix. Howbeit, with the addition of bulky LATP to relax the chains of PEO polymer, a sharp decline appears in these intensities of the PEO peaks, which signify that the crystallinity of PEO is diminished and more amorphous regions exist in hybrid solid electrolytes. Furthermore, it is interesting to note that the characteristic peaks weaken most and the crystallization of the PEO matrix is burked most when adding 20 wt% bulky LATP. In addition, no evident change can be discerned in respect of characteristic peaks, indicating that PEO and bulky LATP are able to mingle in harmony. Thermal stability of HSEs is presented in Fig.2b. PEO-20 shows a much better thermal stability than that of PEO-0. The trace amount of mass loss of around 5% happened under the temperature of about 120oC, which might be associated with the water vapor loss from original HSE. A huge weight loss shows up at about 370oC for the reason of the thorough deposition of PEO and LiTFSI. After the temperature is elevated to 700oC, the residue mass of PEO-20 is normally LATP.

Fig.2.XRD patterns (a) and TG curves (b) of the as-prepared PEO-x hybrid solid electrolytes

Fig.3 depicts the morphology and EDS mappings of PEO-0 and PEO-20 solid electrolytes, and the homogeneous dispersion of the inorganic fillers is necessarily beneficent to the ionic conductivity[16]. As can be described in Fig.3a and Fig.3c, the addition of 20 wt% LATP into PEO has made surface of HSEs rougher and bulky LATP is homogeneously spread in hybrid solid electrolytes. The relevant EDS mappings (Fig.3b and Fig.3d) delineate LiTFSI and bulky LATP are well dispersed in the HSEs.In the picture of EDS, C and O elements, symbolizing PEO, distribute homo-genously in PEO-0 and PEO-20. F and S elements, standing for LiTFSI, are solvated well. Besides, P element originally emblematizes bulky LATP, which spreads homogenously in PEO-20.

Fig.3.SEM images and EDS mappingsof the as-prepared PEO-0 (a～b) and PEO-20 (c～d) hybrid solid electrolytes

3.2 Electrochemical performance

3.2.1 Ion conductivity of PEO-x

It is well known to us that ionic conductivity is one of the critical factors to determine the electrochemical performance of solid electrolytes[28]. The Arrhenius plots for ionic conductivities of PEO-x (x=0, 10, 20, 30) are displayed in Fig.4a. Among them, the HSE incorporating 20 wt% bulky LATP portrays somewhat lower bulk resistance and thus higher ionic conductivity than other hybrid solid electrolytes. According to Fig.4b, the bulk resistance of PEO-20 with the thickness of 193 μm exhibits 539.3 Ω and its ionic conductivity achieves 2.6 × 10-5S·cm-1, whereas the bulk resistance of PEO-0 with the thickness of 250 μm exhibits 4041.4 Ω and its ionic conductivity presents only 2.9 × 10–6S·cm–1. With the ascending temperature to 70oC, the bulk resistance of PEO-20 shrinks to 53.5 Ω and the ionic conductivity raises more than one order of magnitude to 3.5 × 10–4S·cm–1, as shown in Fig.4c. In contrast, the bulk resistance of PEO-0 abates to 261.3 Ω and its ionic conductivity of PEO-0 is still about 1.4 × 10–4S·cm–1, manifesting that the integration of bulky LATP is advantageous to the growth of ionic conductivity. This hoisted ionic conductivity could be commonly accredited to reducing the degree of crystallization and thereby increasing ratios of the amorphous phase in PEO matrix by the addition of inorganic fillers[8], including bulky LATP. The fillers with micrometer size were utilized since 1982 to only improve the mechanical properties of SPEs[29], but improved conductivity was not observed until 1995 when the size of the filler was diminished to the nanometer scale[23]. It has been reported that electrochemical properties of electrolytes increase with decreasing the size of fillers used[23]. Nevertheless, it should be emphasized that the ionic conductivity of HSE filled with 20 wt% bulky LATP into PEO matrix is also better than some previously published nano-sized LATP fillers in PEO polymer matrix at room temperature[25,30]. In regards to PEO-30, the ionic conductivity decreases slightly, because too many bulky LATP particles existing in the PEO polymer matrix could result in volumetric effects[31]. The negative volumetric effects may hamper the transport of lithium ion and hence lower the ionic conductivity[31]. As a result, we demonstrate that the incorporation of bulky LATP can also enhance the ionic conductivity.

Fig.4. (a) Arrhenius plots of PEO-x hybrid solid electrolytes, (b～c)

Activated energy (Ea) for ion conductivity is attained byequation (2). Table 1 shows the activated energies of PEO-0, PEO-10, PEO-20 and PEO-30. Eahas disparities in lower (30～70oC) and higher (70～90oC) temperature regions. The discrepancies might be ascribed to its recrystallization of PEO from the amorphous phase when temperature changes. The lowest activated energy can be discovered in PEO-20. The results are congruous with the ionic conductivity.

Table 1.Activated Energy of PEO-xHybrid Solid Electrolytes

3.2.2 Electrochemical window of PEO-x

Electrochemical window is believed to be an indispensable parameter to decide the applicable employment in the lithium batteries[32]. Classical liquid organic electrolyte normally starts to decompose at 4.20 V[1], thus the electrochemical window of hybrid solid electrolytes that we fabricate should apparently surpass 4.20 V. With the purpose of figuring out the electrochemical stability of PEO-x hybrid solid electrolytes, linear sweep voltammetry (LSV) has been carried out and the results are shown in Fig.5. PEO-0 solid electrolyte has a potential window of 4.40 V, which still cannot meet the requirement of high-voltage cathodes like LiNi0.85Co0.1Al0.05O2(4.50 V)[33]and Ti-Mg-Al co-doping LiCoO2(4.60 V)[34]. Conversely, it is evidentthat PEO-20 hybrid solid electrolyte exhibits the highest voltage of 4.84 V among all synthesized hybrid solid electrolytes. By the same token, PEO-10 and PEO-30 have voltages of 4.51 and 4.58 V, also higher than PEO-0. With regard to reasons, it matters most that bulky LATP could interact with PEO and lithium salt anion, so as to suspend their decomposition and stabilize the polymer chain structure at the interface between EO-segments and fillers that suppress the decomposition of PEO polymer[32].

Fig.5.Linear sweep voltammetryof the different PEO-based solid electrolytes. (a) PEO-0, (b) PEO-10, (c) PEO-20 and (d) PEO-30

In order to explore the reasons, it is worth noting that incorporating bulky LATP into PEO matrix has the following desirable advantages: firstly, the integration of bulky LATP is able to diminish the crystallinity of PEO and eventually enhance the ionic conductivity; secondly, the addition of bulky LATP can broaden the electrochemical window, because electrochemical stability could be enhanced by hydrogen bonding between TFSI-and bulky LATP, handicapping the decomposition of polymer matrix and lithium salt at such a high voltage[35]. With all the aforemen-tioned boons of the bulky LATP, it is explicitly stated that incorporating appropriate amounts of inorganic fillers[36], like bulky LATP, into PEO-based HSEs is an achievable strategy of enhancing the performances of hybrid solid electrolytes.

4 CONCLUSION

To resolve the low ionic conductivity, poor electrochemical window and thermal stability of PEO solid electrolyte, hybrid solid electrolytes constituting PEO and bulky LATP in various ratios have been prepared by means of a simple preparation in this work, which can remarkably decrease the crystallinity, strengthen lithium ion conductivity, and enlarge electrochemical window. PEO-20 exhibits an ionic conductivity of 2.6 × 10–5S·cm–1at 30oC and 3.5 × 10–4S·cm–1at 70oC. The value of PEO-20 is an order of magnitude higher than that of PEO-0 at 30oC (2.9 × 10–6S·cm–1) and still a bit higher at 70oC (1.4 × 10–4S·cm–1), principally caused by the decrease of crystallinity in PEO polymer matrix. The electrochemical window of hybrid solid electrolyte can increase from 4.40 to 4.84 V at room temperature when adding 20 wt% bulky LATP into PEO polymer matrix, which is able to accord with high-voltage cathode materials.