Polyethylene oxide-Li6.5La3Zr1.5Ta0.5O12 hybrid electrolytes:Lithium salt concentration and biopolymer blending
MaikeWirtz1,2,3 Max Linhorst4 Philipp Veelken1,2 Hermann Tempel1
Hans Kungl1 BrunoM. Moerschbacher4 Rüdiger-A. Eichel1,2,3
1 Institute of Energy and Climate Research– Fundamental Electrochemistry (IEK-9),Forschungszentrum Jülich, Jülich,Germany2 Institute of Physical Chemistry, RWTHAachen University, Aachen, Germany3 Institute of Energy and ClimateResearch – Helmholtz-Institute Münster:Ionics in Energy Storage (IEK-12),Forschungszentrum Jülich, Münster,Germany4 Institute for Biology and Biotechnologyof Plants, University of Münster, Münster,Germany
AbstractHybrid electrolytes are developed to meet the requirements of safety, perfor-mance, and manufacturing for electrolytes suitable for Li-ion batteries withLi-anodes. Recent challenges—in addition to these key properties—emphasizethe importance of sustainability. While compromising between these threeobjectives, the currently available materials are still well below the targetedgoals. Three important issues for the design of hybrid electrolytes are (i)the role of the morphology and surface state of the ceramic particles inthe polymer matrix, (ii) the dependence of salt concentration and ionic con-ductivity and, (iii) the effects of substituting part of the polyethylene oxide(PEO), with biopolymers. Electrolyte films were prepared from PEO, lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI), Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta),and biopolymerswith varying contents of these components by a solution castingmethod. The films were analyzed with respect to structural and microstructuralcharacteristics by DSC, Raman spectroscopy, and SEM. Ionic conductivity wasevaluated by electrochemical impedance spectroscopy. Most interesting, whencomparing filmswith LLZO:Ta versuswithout, the content of LiTFSI required forthemaximum conductivity in the respective systems is different: a higher LiTFSIconcentration is required for the former type. Overall, addition of LLZO:Ta aswell as partial substitution of PEO by chitosan mesylate or cellulose acetatedecrease the ionic conductivity. Thus—at least in the present approaches—a lossin performance is the drawback from attempts to enhance the safety by LLZO:Taadditions and sustainability by biopolymer blending of hybrid electrolytes.
The combination of polymer and ceramic electrolyte isaiming for an increase of ionic conductivity using thehighly conductive ceramic Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta)as preferred conduction pathway and enhancing the ther-mal andmechanical stability of polymer electrolytes, whilemaintaining favorable adhesion and processing propertiesof the polymer. The properties of the hybrid electrolytedepend on many factors, such as the molecular weight ofpolyethylene oxide (PEO), the conducting salt concentra-tion, the amount and nature of Li7La3Zr2O12 (LLZO) par-ticles as well as the processing of the film. [1–2]Regarding the ionic conductivity, contradictory
trends are reported in literature when adding LLZOparticles to salt-in-polymer electrolytes consisting ofPEO and a lithium salt, mostly LiClO4 or lithiumbis(trifluoromethanesulfonyl)imide (LiTFSI).[3] Severalstudies have been performed concerning the influ-ence of the weight fraction of LLZO, some finding animprovement and others showing a decrease of the ionicconductivity via LLZO addition.[4–10] An overview of thecompositions and the ionic conductivities of the hybridelectrolytes and—if available—corresponding polymerelectrolytes is given in Table 1. Choi et al., for example,found the peak ionic conductivity at a ceramic content of52.5 wt.% using tetragonal LLZO.[6] Chen et al. reportedan improvement of the ionic conductivity only for smallamounts of LLZO:Ta.[4] In contrast Langer et al. didnot find an improvement of the ionic conductivity byadding LLZO:Al.[8] This trend was also seen by Kelleret al. using 70 wt.% of tetragonal LLZO.[9] Other studiesfocused on the particle size influence, finding that smaller
particles in the nanometer scale lead to higher ionicconductivities.[11–12]The lithium salt concentration is expressed by the ratio
of lithium ions to the ethylene oxide monomer units ofPEO (Li:EO ratio). The applied ratios are in the range from1:8 to 1:20. Langer et al. and Buvana et al. tested the influ-ence of the salt concentration on the ionic conductivityof the polymer electrolyte and applied the best one forthe hybrids. Interestingly, Buvana et al. found the high-est ionic conductivity at a ratio of 1:8 and Langer et al. at1:20, both using LiClO4 as conducting salt. The discrep-ancy is probably caused by the difference in the molecularweights of PEO or by slight deviations in the processing ofthe films.[5,8]Next to the discussed parameters, the interfacial
properties of the compounds used in hybrid electrolytessignificantly influence their performance.[2,10,13] Lewisacid–base interactions of LLZO with PEO and TFSI–may lead to a higher mobility of lithium.[14] However,little is known about the influence of the LLZO sur-face characteristics on the ionic conductivity of hybridelectrolytes.[2]Using green materials, which are defined according
to Dühnen et al., is a very recent trend in materialsresearch.[15] Chitosans and cellulose acetate are of inter-est due to their biodegradability and known ability to actas polymer hosts for polymer electrolytes.[15–20] Selvaku-mar et al. investigated cellulose acetate in combinationwith LiClO4 and found an enhanced biodegradability withsalt addition due to Li+ complexation.[17] Chitosan/PEOfilms are known to have good mechanical properties and,depending on the LiTFSI concentration, to exhibit reason-able ionic conductivities.[20–21]
TABLE 1 Composition and ionic conductivity of hybrid and polymer electrolytes
Li salt Li:EO LLZOmaterialLLZO[wt.%]
Particle size[µm]
Ionic conductivity[S/cm]
Increase of ionic conductivity with LLZO additionLiClO4 1:18 Li6.25Al0.25La3Zr2O12 40 - 2.0 × 10–6 at 30◦C[10]
LiTFSI 1:18 Li6.4La3Zr1.4Nb0.6O12 30 0.2 5.2 × 10–5 at 30◦C[7]
LiClO4 1:15 Li7La3Zr2O12 52,5 – 4.4 × 10–4 at 55◦C[6]
LiTFSI 1:10 Li6.4La3Zr1.4Ta0.6O12 20 0.2 1.6 × 10–4 at 30◦C[11]
none – – 3.5 × 10–6 at 30◦C[11]
LiTFSI 1:8 Li6.4La3Zr1.4Ta0.6O12 10 6.5 1.2 × 10–4 at 30◦C[4]
LiClO4 1:8 Li6La2BaTa2O12 20 – 2.0 × 10–4 at 30◦C[5]
none – – 4.2 × 10–6 at 30◦C[5]
Decrease of ionic conductivity with LLZO additionLiClO4 1:20 Li6.25Al0.25La3Zr2O12 70 – 8.0 × 10–9 at 20◦C[8]
none – – 4.0 × 10–8 at 20◦C[8]
LiTFSI 1:15 Li7La3Zr2O12 70 – –[9]
none – – 1.4 × 10–6 at 20◦C[9]
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This study focuses on hybrid electrolytes based on PEO,LiTFSI, andLLZO:Ta using a constant ceramicweight frac-tion of 50%. The ionic conductivity is examined as a func-tion of the surface characteristics and therefore the pro-cessing history of LLZO:Ta. In order to better understandthe effect of the LiTFSI concentration, it was optimizedfor both, the polymer as well as the hybrid electrolyte,investigating Li:EO ratios from 1:4 to 1:18. Furthermore,the impact of the substitution of PEO with biopolymers,specifically chitosan mesylate or cellulose acetate, on theionic conductivity of the polymer and hybrid electrolyteswas evaluated.
2 EXPERIMENTAL SECTION
2.1 Materials
Li6.5La3Zr1.5Ta0.5O12 (LLZO:Ta) was synthesized byclassical all-solid-state route. Stoichiometric amountsof Li2CO3 (Sigma Aldrich, > 99%), La2O3 (Alfa Aesar,99.99%, predried for 12 h at 850◦C), ZrO2 (Alfa Aesar,99.5%), Ta2O5 (Alfa Aesar, 99.85%) were ball milled iniso-propanol (IPA). Overall 10 wt. % excess Li2CO3 wasadded to compensate lithium loss during calcination. Thepowder was calcined in aluminum crucibles for 12 h at1150◦C and transferred to argon atmosphere at 200◦C. Thepowder was remilled to reduce the mean particle size. Assolvent for remilling, dry IPA was chosen, as it shows lowLi+/H+ exchange in LLZO materials.[22]Hybrid as well as polymer electrolyte films were pre-
pared by solution casting method in a glovebox underargon atmosphere (H2O < 0.1 ppm; O2 < 0.1 ppm). Allcomponents were dried at 60◦C under vacuum, and anhy-drous solvents were stored over molecular sieve 3 Å beforeuse. PEO (Mw = 1 000 000 g/mol, Alfa Aesar) and LiTFSI(Sigma–Aldrich, 99.95%) were dissolved in acetonitrile.The ratio of ethylene oxide monomer units to lithium ions(Li:EO= 1:x) was used to define the LiTFSI concentration,ratios of 1:4, 1:6, 1:8, 1:12, 1:16, and 1:18 were investigated.Higher LiTFSI concentrations were not used, as the sol-ubility limit of LiTFSI in PEO is reached and the materialbecomes semi-crystalline.[23] Overall 50 wt.% LLZO:Tapowder with respect to the PEOmass was dispersed in ace-tonitrile and added to the PEO-LiTFSI solution. The highlyviscous mixture was ultrasonicated and solution-castedon a Teflon dish. After drying overnight under vacuum, afilm thickness of approximately 100 μmwas obtained.A water free processing was necessary for the biopoly-
mer hybrid electrolytes, as LLZO reacts with water.[22]Since typical chitosan salts such as chitosan chlorides orchitosan acetate are almost exclusively soluble in aqueoussolutions, chitosan mesylate salt was prepared to achieve
solubility in dimethyl sulfoxide.[24–25] For the preparationof chitosanmesylate, 1 g chitosan (“Chitosan 134″ preparedfrom squid pen β-chitin by four sequential heterogeneousalkaline deacetylation steps, Mahtani Chitosan, Gujarat,India), which we determined to have a fraction of acetyla-tion FA below 0.01, a weight average Mw of 242 000 g/mol,and a Mw dispersity D̄ of 1.4 using state-of-the-art analyti-cal methods (1H-NMR for FA [26–27]; HPSEC-RID-MALLSfor Mw and D̄ [28]), was dissolved in 250 mL water and250 μL methanesulfonic acid. Subsequently, the chitosansolution was cooled down to 4◦C and additional 50 mLmethanesulfonic acid was added. After stirring for 5 minchitosan mesylate was precipitated by adding 250 mLacetone. The resulting white precipitate centrifugated,resolved in water, and purified by repetitive precipitationby adding acetone (5 times). After lyophilization, whiteflakes of chitosan mesylate were achieved.For the biopolymer-based polymer and hybrid elec-
trolytes, 25% of PEO was substituted by commercialcellulose acetate (Mw = 100 000 g/mol, Acros Organ-ics) or chitosan mesylate. Weight fractions correspondedto PEO/Bio/LiTFSI 48/16/36 wt.% for the polymer elec-trolytes with Bio = cellulose acetate or chitosan mesy-late at 16wt.%, respectively, and PEO/Bio/LLZO:Ta/LiTFSI24/8/32/36 wt.% for the hybrid electrolytes with Bio = cel-lulose acetate or chitosan mesylate at 8 wt.%, respectively.Cellulose acetate was added to the PEO-LiTFSI solutionand the film was prepared as described before. Chitosanmesylate was dissolved in small amounts of dimethyl sul-foxide and added dropwise to PEO-LiTFSI.
F IGURE 1 (A) Bode plots with equivalent circuit fits of PEO12LiTFSI electrolytes in a temperature range from 10◦C to 60◦C; as well as theionic conductivity (B) as a function of temperature of PEOxLiTFSI films with Li:EO = 1:x ratios with x = 4; 6; 8; 12; 16; 18 and (C) as a functionof the Li:EO ratio at 30◦C and 60◦C
hybrid electrolyte films at a heating rate of 5 K/min underhelium atmosphere. For comparison, a pure PEO film wasprepared in the same way as the polymer and hybrid elec-trolyte films.The ionic conductivities of the electrolytes were mea-
sured using electrochemical impedance spectroscopy withan AC amplitude of 10 mV at temperatures from 60◦C to10◦C in 10◦C-steps on a Biologic VMP 3 in a frequencyrange of 1 MHz to 100 mHz and evaluated in a frequencyrange 1 MHz to 1 kHz. The electrolytes were sandwichedbetween two polished gold electrodes of rhd instrumentsand a PEEK spacer of 100 μm was used to keep a stablemeasuring geometry. The cells were tempered for 12 h at60◦C prior to each measurement and then kept at everytemperature for 2.5 h for equilibration. At least threeindependent measurements of every electrolyte film wereconducted and averaged. The data were fitted by RelaxIS3 software of rhd instruments with an equivalent circuitcombining a Zarc element and a constant phase element(CPE) in series. The Zarc corresponds to the resistance ofthe electrolyte and the CPE describes the polarization ofthe blocking electrodes. The fits using data up to 1 kHzcover a sufficiently large fraction for fitting the Zarcelement.
4 RESULTS AND DISCUSSION
4.1 Polymer Electrolyte
The ionic conductivities of the PEOxLiTFSI electrolytefilms with Li:EO = 1:x ratios between 1:4 and 1:18 wereanalyzed by electrochemical impedance spectroscopy.Representative Bode plots of the PEO12LiTFSI polymer-electrolyte film are shown in Figure 1A.The impedance |Z| increases with decreasing tempera-
ture (Figure 1A). The spectra were fitted with an equiva-lent circuit of a Zarc element in series with a CPE element.The data and fits are shown in Figure 1A and the resis-tance was used for the calculation of the ionic conductiv-ity (Figure 1B). Three measurements of one HPE filmweremade and averaged displayed as means ± SDs. As a func-tion of temperature, the ionic conductivity increases from10 to 60◦C with a difference of about two to three orders ofmagnitude for each sample. The ionic conductivity followsa non-linear trend with a change of slope at about 40◦C.This can be ascribed to typical VTF behavior (Vogel, Tam-mann, and Fulcher) due to the melting transition of semi-crystalline PEO with LiTFSI at about 40◦C.[29] Below themelting temperature, crystalline domains are present and,
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F IGURE 2 (A) XRPD patterns and (B) Raman measurements of LLZO:Ta after calcination for 12 h at 1150◦C and after remilling. XRPDpatterns compared to Li6.5La3Zr1.45Ta0.55O12, *La2Zr2O7.[31–32]
therefore, a higher activation energy for the ionic transportis needed.[30]The ionic conductivity as a function of the LiTFSI con-
centration is shown in Figure 1C. The ionic conductivityincreases up to a Li:EO ratio of 1:12 and decreases withhigher LiTFSI concentrations. This trend is seen for everymeasured temperature as shown exemplarily for 30◦C,which is below the melting temperature, and 60◦C, whichis above it. The highest ionic conductivity of 8.9× 10–5 S/cmat 60◦C was measured for PEO12LiTFSI. As a result, theLi:EO ratio of 1:12 was used for the preparation of hybridfilms using different LLZO:Ta powders.
4.2 LLZO:Ta particle properties
TheXRPDpatterns of calcinedLLZO:Ta andLLZO:Ta afterremilling are displayed in Figure 2A.After calcination, all major Bragg reflections correspond
to cubic LLZO.[31] Further reflexes of low intensity can beassigned to the secondary phase La2Zr2O7.[32] Remillingleads to reflex broadening due to a smaller crystallite size,which was confirmed by particle size measurements. Themean particle size was reduced from 50 μm to <1 μm.The influence of remilling on the surface characteristics
of LLZO:Ta was measured by Raman spectroscopy (Fig-ure 2B), which also may give information on the inter-facial organic/inorganic properties in hybrid films. BothRaman spectra conform to cubic LLZO, as previouslyreported.[33–34] The band at ∼120 cm–1 corresponds to avibrational mode of the La-O sublattice bonding.[33–36] Zr-O and Ta-O bond stretching is seen at 640 and 750 cm–1,respectively. The broad bands in the range from 200 to600 cm–1 might be assigned to the internal modes of LiO4and LiO6, thus the broadening emerges from the static anddynamic disorder of the Li cation.[33–34] After remilling,the Li2CO3 band at 145 cm–1 appears and the band at
1084 cm–1 increases.[37] Li2CO3 is an educt of the synthesisused in excess to compensate Li loss during high tempera-ture treatments. The increase of the Li2CO3 bands and theadditional O-H stretching band at 3564 cm–1 may also beexplained by the partial decomposition of LLZO:Ta.[38–40]Importantly, remilling leads to the formation of OH sur-face groups and a decrease in size of the LLZO:Ta particles,though the bulk crystal structure of the LLZO:Ta powderas determined by XRD remains equal.
4.3 Hybrid electrolyte
The influence of particle size as well as surface characteris-tics on the hybrid electrolyte film properties are discussedin the following. The SEM cross-sections of hybrid elec-trolyte films with a Li:EO ratio of 1:12 and 50 wt.% of cal-cined or remilled LLZO:Ta are shown in Figure 3 in thedirections of the casting procedure. The lower part of thepicture corresponds to the bottom of the film.The distinction of particle and polymer is possible based
on the atomic number contrast. Dark parts correspond topolymer and bright ones to LLZO:Ta. The particle sizeaffects the particle distributonwithin the hybrid electrolytelayer. The particle sedimentation is further described bythe Stokes’ law, which relates the settling velocity to thesquare of the particle radius, to the difference in densityof solid and liquid, to the gravity as well as to the inversedynamic viscosity of the liquid.[41–42] For the large calcinedparticles this leads to settlement at the bottom of the film.In contrast, the small remilled particles show lower trendto sediment and are distributed homogeneously within thewhole hybrid electrolyte layer.Figure 4 displays the DSC measurements of a PEO film,
PEO12LiTFSI and the hybrid electrolyte films. The PEOfilm was processed in the same way as the polymer elec-trolyte films.
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F IGURE 3 SEM cross-sections of PEO12LiTFSI films with (A) calcined and (B) remilled LLZO:Ta recorded with a CBS detector
F IGURE 4 Results of DSC measurements of a pure PEO film,PEO12LiTFSI, with calcined and with remilled LLZO:Ta particles;measured in He-atmosphere at 5 K/min
The melting temperature of the pure PEO film of 67.9◦Cis lowered by about 30◦C with the addition of LiTFSI.Equally, the peak area of the melting transition decreases.Both effects show that less crystalline PEO and a higheramount of the PEO amorphous phase are present. The val-ues of the melting temperature Tm, the melting enthalpyΔ𝐻𝑚 corresponding to the peak area, and the degree ofcrystallinity𝜒𝑐 are listed in Table 2.𝜒𝑐 was calculated using𝜒𝑐 =
Δ𝐻𝑚
Δ𝐻0𝑚
and the standardmelting enthalpy of 100% crys-
talline PEO,Δ𝐻0𝑚 = 213.7 J/g.[5] For the hybrid electrolytes,
Δ𝐻𝑚 was multiplied by a factor of two, since only half ofthe mass consists of polymer.The melting enthalpy and, thus, the degree of crys-
tallinity decrease with LiTFSI addition, which is explainedby the plasticizing effect of the TFSI– anion.[43] A slightfurther decrease of themelting temperature and the degreeof crystallinity is seen with LLZO:Ta addition and theuse of the remilled particles. Consequently, the lower par-ticle size and homogeneous distribution of the remilled
TABLE 2 Melting temperature Tm, melting enthalpy Δ𝐻𝑚 anddegree of crystallinity of the polymer 𝜒c derived from the DSCmeasurements of a pure PEO film, PEO12LiTFSI, with calcined andwith remilled LLZO:Ta particles; measured in He-atmosphere at5 K/min
Material Tm [◦C] 𝚫𝑯𝒎 [J/g] 𝝌𝒄 [%]PEO film 67.9 40.3 18.8PEO12LiTFSI 42.7 16.0 7.5PEO12LiTFSI-LLZO:Ta-calcined
41.3 7.5 7.1
PEO12LiTFSI-LLZO:Ta-remilled
40.0 5.8 5.5
LLZO:Ta particles lowers the crystallinity of PEO. As it isknown that the crystalline parts of PEO are of low ionicconductivity, a lower degree of crystallinity should lead toa higher ionic conductivity.[44]In order to investigate the ionic conductivity, impedance
measurements of the hybrid films containing calcined(black) or remilled LLZO:Ta particles (red) were per-formed. The ionic conductivity as a function of tempera-ture is shown in Figure 5.Compared to the best performing polymer electrolyte
film, PEO12LiTFSI, the ionic conductivity decreases withthe addition of LLZO:Ta regardless of the particle size byabout almost two orders of magnitude. In detail, at 60◦C,the ionic conductivity of the polymer electrolyte is 8.9× 10–5 S/cm and the ionic conductivities of the hybridswith calcined or remilled LLZO:Ta powder are 7.6 × 10–7S/cm and 9.2 × 10–7 S/cm, respectively. This leads to theassumption that the predominant Li+ pathway is inside thebulk polymer phase or along the polymer/ceramic parti-cle interface. The resistance for a Li+ transport across thepolymer/ceramic interface is too high for an increase ofthe ionic conductivity with LLZO:Ta addition.[9] Compar-ing the two hybrid electrolyte films, the ionic conductivityis slightly higher using remilled LLZO:Ta, but still withinthe standard deviations of the film with calcined particles.Thus, the particle distribution within the film as well asthe particle size and surface chemistry only have a slight
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F IGURE 5 Ionic conductivity as a function of temperature ofPEO12LiTFSI film, with calcined or remilled LLZO:Ta particles in atemperature range from 10 to 60◦C
effect on the performance of the hybrid electrolyte. Theslightly higher ionic conductivity is likely to be assignedto the lower degree of crystallinity and the homogeneousdistribution of the remilled LLZO:Ta particles within thePEO12LiTFSI polymer matrix. As a consequence, hybridelectrolytes containing remilled particleswere used for fur-ther characterization.Electrochemical impedance was measured of hybrid
electrolyte filmswith 50wt. % of the remilled LLZO:Ta par-ticles under variation of the Li:EO ratio from 1:4 to 1:18. Theionic conductivity as a function of temperature is plotted inFigure 6A.The ionic conductivity increases as a function of temper-
ature from 10 to 60◦C. At 30◦C, the ionic conductivities ofthe hybrid electrolyte films are in the range of 9.0 × 10–9S/cm to 1.8 × 10–6 S/cm. Thus, the variation of the LiTFSIconcentration leads to a change of ionic conductivity ofabout two orders of magnitude. The ionic conductivities ofpolymer and hybrid electrolytes depending on the Li:EOratio at 30◦C are displayed in detail in Figure 6B. The peakionic conductivity of the hybrid electrolyte film is obtainedwith 1.8× 10–6 S/cm at a Li:EO ratio of 1:6. At that ratio, thehybrid electrolyte has almost the same ionic conductivityas the polymer electrolyte. As it was discussed earlier, thehighest ionic conductivity of 7.9 × 10–6 S/cm of the poly-mer electrolytes is at a ratio of 1:12. For the hybrid elec-trolytes, a doubling of the LiTFSI concentration is neededto obtain the best performing hybrid electrolyte. A reasonfor the need of twice the LiTFSI concentration could bethat a part of the lithium ions are trapped at the surfaceof the LLZO:Ta particles due to Lewis acid base interac-
F IGURE 6 Ionic conductivity as a function of (A) temperatureof PEOxLiTFSI-LLZO:Ta hybrid films with Li:EO = 1:x ratios withx = 4; 6; 8; 12; 16; 18 in a temperature range from 10 to 60◦C and (B)Li:EO ratio of polymer and hybrid electrolyte films at 30◦C
tions and can, therefore, not contribute to the Li+ conduc-tionwithin the polymer phase.[2] This hypothesis contrastswith the research of Yang et al. who stated that LLZO inter-acts with the ether oxygen of PEO and the TFSI–, leadingto a dissociation and additional mobilization of Li+.[14] Inthe present study, it was possible to enhance the ionic con-ductivity of the hybrid electrolyte by optimizing the LiTFSIconcentration. This matches with the findings of Guptaet al., who studied the LiTFSI salt concentration influ-ence on the interfacial resistance of a layered PEOxLiTFSI-LLZO:Ta system.[13] For the present ceramic-in-polymerhybrid electrolytes the difference in ionic conductivity ofpolymer and hybrid electrolyte was reduced from twoto half an order of magnitude. Thus, the salt concentra-tion of the best performing polymer electrolyte shouldnot be considered as the optimal salt concentration inhybrids.
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F IGURE 7 Ionic conductivity as a function of temperature of (A) PEO/Bio/LiTFSI polymer electrolytes of 64/0/36 wt.% (PEO12LiTFSI)and biopolymer blends using Bio = chitosan mesylate or Bio = cellulose acetate with 48/16/36 wt.% and (B) PEO/Bio/LLZO:Ta/LiTFSI hybridelectrolytes of 32/0/32/36 wt.% (PEO6LiTFSI-LLZO:Ta) and biopolymer blends with 24/8/32/36 wt.%
There are many other parameters influencing theionic conductivity of the hybrid electrolytes, such as themolecular weight of PEO as well as the amount of ceramicwhich is used.[1,4] Only little is known about the interfaceof garnet and polymer and how it can be adjusted tolower the transfer resistance across the organic/inorganicinterface.[2] So far, an approximation for this resistancewas determined by layered model systems, which cannotdirectly be transferred to hybrid electrolytes mixed onmicroscopic scale.[13,45] But these model systems andNMR studies provided the evidence of a Li+ transferacross the organic/inorganic interface.[46] All in all, moresystematical knowledge is required to get a better under-standing of the system and clarify whether the adjustmentof the ceramic particle properties can be used to enhancethe performance of the hybrid electrolytes.
4.4 Biopolymer blending
Exploring approaches toward greener materials, the influ-ence of the substitution of PEO with biopolymers, specif-ically chitosan mesylate or cellulose acetate, on the ionicconductivity of the of polymer aswell as hybrid electrolyteswas investigated. It was possible to substitute 25% of PEOwith chitosan mesylate or cellulose acetate. Higher per-centages led to brittle films or flake formation, because ofthe high crystallinity of the biopolymers and, therefore, therigid behavior during film formation. The ionic conductiv-ities as a function of temperature of the biopolymer blendsin comparison to the PEO12LiTFSI polymer electrolyte andthe PEO6LiTFSI-LLZO:Ta hybrid electrolyte are shown inFigure 7.The ionic conductivity of PEO12LiTFSI and
PEO6LiTFSI-LLZO:Ta decreases with substitution of
PEO by commercial cellulose acetate or chitosan mesy-late. At 60◦C the substitution of PEO with chitosanmesylate or cellulose acetate in PEO12LiTFSI reduces theionic conductivity from 8.9 × 10–5 S/cm to 1.6 × 10–5 S/cmor 7.1 × 10–7 S/cm, respectively. In the hybrid electrolytesthe substitution with cellulose acetate lowers the ionicconductivity by about three orders ofmagnitude comparedto PEO6LiTFSI-LLZO:Ta. However, by substitution withchitosan mesylate, the ionic conductivity is only aboutone order of magnitude lower than that of the PEO basedhybrid electrolyte.
5 CONCLUSION
Hybrid as well as polymer electrolyte films consistingof PEO, LiTFSI and LLZO:Ta were synthesized by solu-tion casting method. Remilling reduced the particle sizeof LLZO:Ta from 50 μm to less than 1 μm and intro-duced OH surface groups, which only marginally influ-enced the ionic conductivity. In addition, it was possible tosubstitute 25% of PEO in PEO12LiTFSI and PEO6LiTFSI-LLZO:Ta with biopolymers, specifically chitosan mesy-late or cellulose acetate. The substitution, however, wasresulting in a reduced performance. The LiTFSI concen-tration was optimized for the PEO-based polymer elec-trolytes as well as the hybrid electrolytes using 50 wt.%of remilled LLZO:Ta. For the polymer electrolytes, thehighest ionic conductivity at 60◦C, namely 8.9 × 10–5S/cm, was obtained for PEO12LiTFSI. The best perform-ing hybrid electrolyte showed an ionic conductivity of1.9 × 10–5 S/cm at 60◦C at a Li:EO ratio of 1:6. Com-paring hybrid and polymer electrolyte, the difference inionic conductivity was changed from two orders of mag-nitude to half an order of magnitude by optimizing the
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LiTFSI concentration. This underlines the need for a care-ful choice of components and their contents in hybridelectrolytes.
ACKNOWLEDGMENTThis researchwas supported by theMinistry of Economics,Innovation, Digitalization and Energy of the State of NRWwithin the project ,GrEEn‘ (no. 313-W044A). The supportis gratefully acknowledged.Open access funding enabled and organized by Projekt
DEAL.
CONFL ICT OF INTERESTThe authors declare no conflict of interest.
DATA AVAILAB IL ITY STATEMENTAll relevant data available on reasonable request.
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How to cite this article: Wirtz M, Linhorst M,Veelken P, et al. Polyethyleneoxide-Li6.5La3Zr1.5Ta0.5O12 hybrid electrolytes:lithium salt concentration and biopolymerblending. Electrochem Sci Adv. 2020;e2000029.https://doi.org/10.1002/elsa.202000029
