UV-Cross-Linked Composite Polymer Electrolyte for High-Rate

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Letter

UV crosslinked composite polymer electrolyte for high-rate, ambient temperature lithium batteries Marisa Falco, Laurent Castro, Jijeesh R. Nair, Federico Bella, Fanny Barde, Giuseppina Meligrana, and Claudio Gerbaldi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02185 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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ACS Applied Energy Materials

UV crosslinked composite polymer electrolyte for high-rate, ambient temperature lithium batteries Marisa Falcoa,*, Laurent Castrob, Jijeesh Ravi Naira,†, Federico Bellaa, Fanny Bardéb, Giuseppina Meligranaa, and Claudio Gerbaldia,*

a GAME

Lab, Department of Applied Science and Technology (DISAT), Politecnico di

Torino, Corso Duca degli Abruzzi 24, 10129, Torino, Italy b Research

& Development 2, Advanced Material Research, Battery & Fuel Cell, Toyota

Motor Europe, Hoge Wei 33 B, B-1930, Zaventem, Belgium † now at: Helmholtz-Institute Münster (HI MS) IEK-12: Ionics in Energy Storage, Corrensstraße 46, 48149 Münster, Germany

Corresponding Authors (*) *M. Falco [email protected], and *C. Gerbaldi [email protected]

ACS Paragon Plus Environment

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ABSTRACT We report an innovative crosslinked composite polymer electrolyte (CPE) based on the garnet-type ceramic super Li+-ion conductor, Li7La3Zr2O12 (LLZO), that is encompassed in a super-soft poly(ethylene oxide)/tetraglyme matrix. UV-induced, facile and solventfree crosslinking process ensures flexible and self-standing CPEs, which are nonflammable and perfectly shape-retaining under thermal/mechanical stress. The CPEs exhibit high ionic conductivity, exceeding 0.1 mS cm-1 at 20 °C, suitable for ambient and sub-ambient temperature operation. Lab-scale lithium metal polymer cells assembled with LiFePO4-based composite cathode and the optimised CPE deliver full capacity at low rates and outstanding specific discharge capacity of 115 mAh g-1 at 1C-rate and ambient temperature. Remarkably, the lithium metal cell can run for hundreds of galvanostatic cycles (>400) with low overpotential, limited fading and excellent Coulombic efficiency (>99 %), which postulates the practical application of the newly developed composite polymer membranes as truly solid separating electrolytes in high power energy storage technologies, assuring safety and performance in a wide range of operating conditions. KEYWORDS lithium polymer battery, composite polymer electrolyte, garnet ceramic, polyethylene oxide, UV crosslinking, ambient temperature cycling TOC GRAPHICS Newly conceived crosslinked composites with LLZO and PEO perform in Li metal cells at 1C-rate and under sub-ambient ACS Paragon Plus Environment

temperature. 2

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Solid-state batteries are promising options for achieving high energy and power densities, yet easy to incorporate in different geometries, including bipolar configurations, thus avoiding a series of countermeasures associated with the risk of liquid electrolyte leakage.1,2 On the other hand, Li metal is amongst the most promising anode materials for high energy density batteries because of its high theoretical capacity and low reaction potential. However, lithium metal batteries have never been developed appreciably, since lithium metal dendrites are formed at the anode side during the course of charging, especially at high current rates, depth of charge and low temperature. Dendrite formation is well known to be due to a non-uniform electrochemical deposition of lithium. This may cause the formation of “dead lithium” associated to irreversible capacity, and of a thick and unstable solid electrolyte interphase (SEI) layer, which may ultimately translate to short circuits. In any cases, these phenomena directly affect the cycle life and the safety of the battery. Using a solid electrolyte with sufficiently high ionic conductivity (> 0.1 mS cm-1 at 25 °C), high transference number and good mechanical properties (shear modulus about twice that of lithium) may avoid those issues and guarantee a greatly extended battery life.3,4


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In this framework, several recent works focused on ceramic garnet-type Li7La3Zr2O12 (LLZO) as inorganic oxide solid electrolyte.5,6,7 LLZO has near unity Li+ transference number.5 It can crystallise in tetragonal and cubic phases.5 The latter has higher (two order magnitudes) room temperature ionic conductivity exceeding 0.1 mS cm-1. LLZO can be doped with one or more cations (e.g., Al3+) to stabilize the cubic phase at room temperature, enhance the ionic conductivity and facilitate sintering.5 As compared to other oxides such as NASICON- or perovskite-type ion conductors containing Ti or Ge cations able to be reduced and forming an instable interface at the contact with Li metal, LLZO has better compatibility with Li metal in most cases.6,7,8 LLZO may react with atmospheric moisture and CO2 yielding Li2CO3, which is detrimental for conductivity.5,6,7,9 LLZO is commonly processed at high temperature for long time to yield pellets, often resulting in porous, brittle and thick solid electrolytes having poor interfacial contact with the active electrode materials and high grain boundary resistance.5,6,7,9 These drawbacks have been listed as possible causes inducing Li metal dendrites propagation through the electrolyte.5,9 On the cathode side, there are some reports of incompatibility with cathode active materials, possibly caused by high temperature


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treatments and varying whichever are the dopant elements in LLZO.5,9 Prolonged cycling (100 cycles) at room temperature of Li metal cells assembled with LLZO and cathodes based on LiCoO2 (LCO) or LiFePO4 (LFP) has been demonstrated in some cases, but the interfacial issues must be addressed by expensive techniques such as pulsed laser deposition and/or the use of interlayers, from thin inorganic films to soft gel and polymer electrolytes.5,6,9 Embedding LLZO in polymer matrixes has been recently experimented as a viable strategy to improve the cyclability of the cells.10,11,12 As compared to their truly polymeric counterparts, composite electrolytes are stiffer while preserving flexibility, and often show improved ion transport properties.10,11 Moreover, high temperature treatments related to pelletisation of LLZO are avoided, the interfacial contact with the electrodes is greatly improved and the low elastic modulus helps buffering the volume change of active materials upon charge/discharge cycles.10,11 Poly(ethylene oxide) (PEO) is well known for the complexation and transport of lithium ions,12 and it has been the most commonly studied matrix in this regard.13 By instance, Zhang et al. conceived a composite electrolyte material with 40 nm sized cubic LLZO particles dispersed in a salt-free PEO matrix.


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Li/CPE/LFP cells could deliver 140 mAh g-1 at low C/10 rate and relatively high 60 °C. The capacity decreased by about 30 % in 200 cycles.14 Zhao et al. tested an electrolyte membrane containing PEO, bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSI) and LLZO in a Li/CPE/LFP cell. A specific capacity of 155 mAh·g-1 (with a Coulombic efficiency of 99%) was achieved at low C/10 rate and relatively high 60 °C. The capacity fading was 13% in 100 cycles.15 Huo et al. later reported composite electrolyte membranes based on PEO, 200 nm-sized LLZO particles and the room temperature ionic liquid (RTIL) N-butylN-methyl imidazolium bis(trifluoromethylsulfonyl)imide (BMImTFSI). The amount of RTIL had to be adjusted in order to avoid leakage under a moderate pressure of 0.5 bar. The Li/CPE/LFP cell could deliver specific discharge capacity of about 133 mAh g−1 at C/10, with a capacity retention of 88 % after 150 cycles at 25 °C.16 Many other recent works about CPEs based on PEO and LLZO, including their performances in Li metal cells, are included in a recent review.10 All of them, except the article by Huo et al.,16 show the performances of cells assembled with composite electrolytes based on LLZO and PEO upon constant current cycling tests at temperature > 55 °C. This likely follows from the fact that lithium ion conduction in PEO is mainly


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coupled to the segmental motion of the amorphous domain, so that the high crystallinity of the polymer below the melting point would result in poor conductivity, actually unrealistic for practical application.17 Differently from the literature accounted so far, here we report new CPEs based on cubic garnet LLZO and a PEO / tetra(ethylene glycol dimethyl ether) (G4) / LiTFSI matrix added with a photoinitiator, processed into a film and crosslinked under UV-light, in order to achieve improved ion transport, interfacial features with the electrodes and, as a result, enhanced cyclability. The crosslinking of PEO encompassing low vapour pressure liquids such as G4 allows obtaining highly amorphous solid polymer electrolytes (SPE). These systems are flexible, shape retaining at high temperature and exhibit ambient temperature ionic conductivity higher than 0.1 mS cm−1, with lithium ion transference number exceeding 0.5.18 The specificity of our approach lies on the combination of the attractive characteristics of highly amorphous solid polymer electrolyte properly optimized for (sub)ambient temperature operation together with the addition of super Li-ion conductive filler able to allow for enhanced Li+ transport. To our knowledge, this is the first work about CPEs incorporating UV-crosslinked PEO and LLZO where the long-term cycling (>400


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cycles overall) of lab-scale lithium metal cells is demonstrated even at high 1C-rate and various temperatures in the range of 10-40 °C.

Figure 1. Images of the CPE sample CPE40 containing 40 wt% LLZO (A,B), G4 burning after being exposed to a free flame (C) and sample CPE40 during (D) and after (E) the exposition to an open flame for 1 min. FESEM (secondary electron) images from a cross-section (F) and from the top of the CPE (G).

In order to prepare the CPE membranes, different amounts of cubic garnet LLZO were stirred in a solution composed of G4, LiTFSI and benzophenone (BP) photoinitiator. LLZO


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was sieved through a 32 μm mesh prior using. PEO was dissolved in the mixture at 70 °C and the paste-like product was hand-ground before processing into a film by hotpressing. The amount of LLZO in the different samples labelled CPE20, CPE40 and CPE60 is 20, 40 and 60 % by weight, respectively. The thickness of the mixtures was adjusted by the use of spacers in all cases, yielding final values of 185±15 μm. The resulting membranes were crosslinked by UV-induced photo-reticulation, yielding homogeneous, self-standing and flexible films (Figure 1 A and B), which are not flammable when exposed to an open flame (Figure 1 D and E). Sustained flame impingement (> 2 min) causes the CPE to undergo only slight carbonisation without catching fire (Fig S1 in SI). Overall, no signs of leakage were observed even after severe bending (Figure 1B), as also confirmed by the fire test, which accounts for an excellent G4 retention due to the effective crosslinking of the PEO matrix; moreover, G4 possesses methylene groups that can undergo hydrogen abstraction and likely form oligomers and bond to adjacent PEO chains, finally resulting in a truly interlinked polymer matrix.18 Two samples of LLZO-free SPEs (one was reticulated by UV-curing and labelled SPEUV, while the other one marked SPE-NC was not UV-cured), as well as a LiTFSI-free


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CPE containing 40 wt% of LLZO (SFCPE40) were prepared and tested for comparison purpose. In all cases, the ratio between PEO, G4, BP and LiTFSI (when present) was kept constant. In total, five different polymer electrolyte membranes were prepared. Compositions of all the samples under study are given in Table 1.

Table 1. Composition of the different polymer electrolytes under study in percentage by weight. Sample

LLZO

PEO

G4

LiTFSI

BP

0

38.8

38.7

15.0

7.5

CPE 20

20

31.0

31.0

12.0

6.0

CPE40

40

23.3

23.2

9.0

4.5

CPE60

60

15.5

15.5

4.5

3.0

SFCPE40

40

27.4

27.3

0.0

5.3

SPE-UV and SPENC


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The secondary electron (SE) FESEM images (Figure 1 F and G) show a very good compatibility between the crosslinked polymer matrix and the LLZO particles. LLZO particles are conformably covered by the polymer, which creates a favourable interaction as compared to a ceramic/ceramic interface, and are homogeneously distributed throughout the membrane thickness. The typical wrinkled texture derived from the crosslinking process of the PEO-based matrix encompassing/interlinking G418 is preserved in the CPEs (Fig S2 in SI).

Figure 2A shows the XRD patterns of the CPEs with different amounts of LLZO, and of the SPEs. The effect of UV-curing on the SPEs can be clearly observed by comparing the patterns of the LLZO-free samples SPE-NC and SPE-UV. The XRD pattern of the non-crosslinked sample SPE-NC displays two intense peaks at 2θ = 19.4° and 23.4° typical of crystalline PEO.19 These signals are less intense in the diffractogram of the UVcured sample SPE-UV, which is dominated by a broad band in the interval 18° < 2θ < 25° due to the amorphous domain of the polymer matrix. Here, the crosslinking process occurs via free radical reactions initiated by hydrogen abstraction by benzophenone under UV-irradiation.18

(and reference therein)

As a matter of fact, the UV irradiation of the


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polymer matrix in the melt state allows the formation of crosslinks, which hinder the reorganisation of PEO chains in crystalline domains, thus increasing the extent of amorphous phase as compared to the non-reticulated sample. The reduction of the SPE crystallinity upon crosslinking is estimated to be ≈ 50 % based on differential scanning calorimetry data (DSC, Fig S3 in SI).

Figure 2. XRD scans (A) and Arrhenius plots of the ionic conductivity vs. temperature in the range 20-80 °C (B) of the CPEs with different amounts of LLZO. The XRD scans of


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the reference pattern of c-LLNO and the XRD scans and ionic conductivity values of the UV cured (SPE-UV) and the non-UV cured (SPE-NC) LLZO-free SPEs are shown for comparison. Panel (C) shows the difference in terms of mechanical integrity upon thermal stress between non-crosslinked and crosslinked CPEs after conductivity test (sample CPE40 was used as representative).

All the XRD patterns of the CPEs display peaks matching very well with the reference PDF 045-0109 of cubic Li5La3Nb2O12 (c-LLNO), which is a well-known garnet phase.20 In the interval 18° < 2θ < 25°, the diffractogram of CPE20 shows a broad band due to the amorphous SPE matrix, with two low-intensity reflections ascribed to crystalline PEO. Differently, the peaks due to crystalline PEO (marked with an asterisk in Figure 2A) are clearly visible in the pattern of CPE40, despite the amount of the SPE matrix is lower as compared to CPE20. These results suggest that the crosslinking process and the following amorphisation is less effective at high ceramic particles content, possibly because irradiation is hindered under this condition (see also the ionic conductivity data discussed in the following). The XRD pattern of CPE60 is dominated by the reflection attributed to LLZO, probably because of the low relative amount of the SPE-matrix.


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Nevertheless, two reflections due to crystalline PEO at 19.4° and 23.4° with a very low relative intensity can still be distinguished. The ionic conductivity data in the range 20-80 °C, as extracted from the impedance responses of cells assembled by sandwiching the polymer electrolytes between two stainless steel (SS) electrodes, are shown as Arrhenius plot in Figure 2B. The ionic conductivity values of the CPEs lay between those displayed by the LLZO-free samples SPE-UV and SPE-NC, and are very similar to those typically achieved by LLZO pellets, which are known to be in the order 10-4-10-3 S cm-1 in the temperature range investigated here.9,11 The Arrhenius plot of SPE-NC exhibits a deflection point centred around 40 °C, with two distinctive slopes as previously reported.18 These two slopes are related to the fusion of PEO crystalline fraction at high temperature, which has a melting point peak summit at ≈40 °C in SPE-NC (Fig S3 in SI). In SPE-UV, the reduction of PEO crystallinity upon crosslinking makes slope difference in the temperature range investigated much less important. The Arrhenius plot of the CPEs displays a temperature-dependent behaviour similar to that of the SPEs with two distinct slopes, indicating that the conductivity mechanism is


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not drastically changed by the addition of LLZO particles. The change in activation energy is less evident with the CPE20 and the ionic conductivity behaviour is similar to that of crosslinked SPE-UV. CPE20 retains the ionic conductivity of SPE-UV in the whole tested temperature range. This likely follows the trend from a more effective irradiation and photo-reticulation of the CPE at lower LLZO content. Interestingly, SPE-UV and CPE20 showed the highest ionic conductivity at 20 °C, which is higher than 0.1 mS cm-1, thus definitely remarkable for a truly solid CPE. To confirm contribution of LLZO particles to the ionic conductivity, we have performed EIS measurements using a LiTFSI-free sample (namely, SFCPE40), which contains 40 wt% of LLZO (Fig S4 in SI). In the case of SFCPE40, Li+-ions from LLZO are the only mobile charges in the system, thus, a lower ionic conductivity was observed when compared to a membrane that contains both the LLZO and LiTFSI. In the temperature range investigated, the ionic conductivity of SFCPE40 is about 1-2 orders of magnitude lower than that of CPE40; as a result, even if the same amount of LLZO was present in both the samples, the presence of LiTFSI salt influenced the total Li+ ion conduction. It is noted that the ionic conductivity of SFCPE40 does not follow an Arrhenius behaviour


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(expected for pure LLZO),20 and a deflection point with two slopes is observed, as previously discussed. This fact suggests that the crosslinked polymer matrix has a role in the ion conduction mechanism, even though the source of lithium ions is not a dissolved salt. Such ion conduction behaviour is perhaps arising from a surface conduction pathway induced by LLZO/polymer interface, possibly involving grain boundaries (inter and/or intraparticles), as reported in literature.21 The conductivity behaviours observed are consistent with what reported by Zheng et al.,22,23 who studied the 6Li NMR spectrum of non-reticulated electrolytes containing PEO, LiTFSI or LiClO4, G4 and LLZO, after galvanostatic cycling in Li/electrolyte/Li cells with 6Li

enriched electrodes. This method was adopted to detect 6Li+ pathway from the

electrode through the electrolyte. The highest increase of 6Li signal was found to correspond to Li+ in PEO-G4, meaning that ion transport mainly occurs there, especially at low (20 wt%) LLZO content. A secondary path through LLZO particles was found to give a contribution to lithium ion transport in the sample with 50 wt% LLZO (viz., a relatively low increase of 6Li was detected in LLZO particles domain, therefore its contribution to the overall Li+ transport is considered of secondary importance).


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Decomposed LLZO arising from ball milling process was also found to give a minor contribution. Nonetheless, conversely from the previous literature reports,23 the crosslinked CPEs newly proposed here are conceived to serve as a truly solid state separator electrolyte for lithium metal secondary cells, which may assure enhanced safety even at relatively high temperatures. This is confirmed by the perfect shape/thickness and mechanical integrity (viz., flexibility) retention by CPE40 after conductivity test at 80 °C (applied pressure ≈ 50 kPa) when compared to its non UV-crosslinked counterpart (see Figure 2C). Indeed, the latter completely loses its mechanical integrity, thus deserving the use of an external separator to avoid cell short circuits above the PEO melting temperature (~45 °C). This characteristic behaviour indicates that the CPE is dimensionally stable and do not deform under stress conditions even at high temperature, thus hindering short circuits and related issues. The CPEs were tested in Li/CPE/LFP lab-scale cells to evaluate the electrochemical performance at ambient temperature (≈ 21 °C ambient laboratory temperature). The sample CPE60 was excluded from further tests, because it displayed the lowest ionic conductivity and elasticity due to the very high filler content. Figure 3 shows the potential


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versus the specific capacity profiles of the cells during galvanostatic (constant-current) charge/discharge tests at different C-rates. Flat voltage profiles typical of LFP delithiation/lithiation process, with plateau at about 3.47 (charge) and 3.39 (discharge) V vs. Li+/Li, are observed with CPE20 and CPE40 at C/20-rate. The specific discharge capacities of the cells assembled with CPE20 and CPE40 at C/10 rate are 140 and 163 mAh g-1, respectively. An increase of the overpotential with increasing imposed current rate is observed with CPE20, which is ascribed to the higher internal resistance of the cell as compared to the one with CPE40. (Fig S6 in SI) This fact can be related to the higher content of PEO-based electrolyte matrix, which is known to cause the formation a passivation layer at the electrode/electrolyte interface upon contact with the Li metal electrode.18,24


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Figure 3. Charge/discharge potential vs specific capacity profiles of Li/CPE/LFP cells with CPE20 (A) and CPE40 (B) at different C-rates and ambient laboratory temperature (≈ 21 °C).

It is noted that during the early cycles at C/20 rate, some parasitic processes occur beyond 3.5 V vs. Li+/Li, particularly during charge, resulting in capacity values exceeding the theoretical ones by 15-20 %. We ruled out the possible oxidation of the electrolyte as a cause for this extra capacity. Indeed, the linear sweep voltammetry (LSV) did not evidence any detrimental oxidative process below 4 V vs. Li+/Li (Figure 4 and Fig S5 in SI). The extra capacity is not fully understood at present, and it has been previously observed with LLZO pellets and a liquid electrolyte as an interlayer between the ceramic material and the LFP cathode at ambient temperature.25 The authors found that the excess of capacity was strongly reduced by increasing the temperature to 55 °C. In the present work, the phenomenon might be ascribed to the imperfect interfacial contact with the porous cathode, non-uniform local charge concentration and/or state of charge at the surface of the electrode, eventually resulting in the deterioration of the performance upon


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prolonged cycling.26,27 It is worth mentioning here that the cathodes used in this work are prepared by standard procedure optimised for working with liquid electrolytes, and the cells were assembled by simply sandwiching the different components, so as to get an appropriate feedback on the intrinsic properties of the CPE under study.

Figure 4. Linear sweep voltammetry of a Li/CPE40/SS cell in the range 2.7 - 5.3 V (anodic stability) and cyclic voltammetry of a Li/CPE40/Cu cell in the range –0.5 - 3 V vs. Li+/Li (cathodic stability). Scan rate 100 μV s-1 and ambient temperature (≈ 21 °C).

The LSV of a Li/CPE40/SS cell in the range 2.7 - 5.3 V vs. Li+/Li was performed in order to check the anodic stability window (ASW) and is shown in Figure 4. The slow scan rate (100 μV s-1) allows the detection of an oxidation process, which is correlated with a small


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current flow just above 4 V, whereas the anodic breakdown occurs at about 4.9 V vs. Li+/Li. The cathodic stability window (CSW) was estimated by cyclic voltammetry using a Li/CPE40/Cu cell in the range from 0.5 to 3 V vs. Li+/Li. Well-defined and reproducible lithium plating/stripping processes were observed and confirm the possible use of this CPE in lithium metal cells. The irreversible processes during the first cycle in the range 0.25 - 1.7 V vs. Li+/Li producing currents as low as ≈ 5 μA cm-2 are attributed to the polymer matrix electrochemical activity (Fig S7 in SI) related to the benzophenone photoinitiator. As CPE40 allowed obtaining higher specific capacity values as well as lower impedance (Fig S6 in SI) and overpotential (Figure 3) as compared to CPE20, the former was submitted to a stability test in terms of constant-current charge/discharge cycling at high 1C current rate and different temperatures. Figure 5 shows the specific capacity delivered by the Li/CPE40/LFP cell in the range 2.7-4 V vs. Li+/Li at 1C rate as a function of the cycle number at ambient temperature (≈ 21 °C, average laboratory temperature). The CPE clearly outperforms the crosslinked SPE under the same cycling conditions (Fig S8 in SI) and allows achieving remarkable specific capacity values in lab-scale Li/LFP cells,


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even when compared to a liquid electrolyte containing LiTFSI in G4 in the same proportion as in the CPEs (Fig S9 in SI). The discharge specific capacities delivered by the cell with CPE40 are 115, 82 and 76 mAh g-1 at the 1st, 100th and 200th cycle, respectively, resulting in a capacity retention ≈ 66 % after the constant-current cycling test. The initial Coulombic efficiency is 94 %, but it settles to values slightly exceeding 99 % after the initial five cycles. The same cell was subsequently kept at 40 °C and tested for 200 additional cycles at such temperature, demonstrating an optimum stability upon thermal stress even after prolonged cycling at high current regime (Fig S10 in SI). When exposed to 40 °C after 200 cycles at ambient temperature, the cell exhibited a specific capacity of 140 mAh g-1. An increased specific capacity with increased temperature is indicating the kinetic restriction that is arising from the interfacial layer that may be formed at the electrodes. Thorough optimisation in this respect is in progress in our labs.


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Figure 5. Specific capacity and Coulombic efficiency vs cycle number of Li/CPE40/LFP cell at 1C rate and ambient temperature (cut-off voltages 4-2.7 V vs Li+/Li). The inset shows the discharge potential profiles vs the specific capacity at different cycle numbers (≈ 21 °C).

The Li/CPE40/LFP cell was also galvanostatically cycled at C/10 rate and 10 °C: Fig S11 in SI shows the typical LFP charge/discharge plateaus and appreciable specific discharge capacity values exceeding 100 mAh g-1, which demonstrates the remarkable possibility of the newly developed truly solid CPE to operate even at sub-ambient temperature. To our knowledge, this is the first report of a truly crosslinked CPE based on PEO and LLZO


24

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working in the ambient/sub ambient temperature ranges (10 to 40 °C) even at high 1C current rate, with limited overpotential and high specific capacity values (see Table 2, which compares the best performance obtained in this work with the most significant literature reports on similar systems, so far). In addition, the cells demonstrated remarkable Coulombic efficiency after long cycling at various temperature, stability and reversibility upon very prolonged operation exceeding 400 cycles.

Table 2. Summary of the most significant literature reports on LLZO-PEO-based CPEs and their overall performance compared to this work.

Ref.

[14]

CPE LLZO + PEO

Discharge capacity

Cell

T

configuration

(°C)

Li/CPE/LFP

60

140 (C/10)

70 % (200), C/10

Li/CPE/LFP

60

155 (C/10)

87 % (100), C/10

Li/CPE/LFP

25

133 (C/10)

88 % (150), C/10

@1st cycle (mAh g-1)

Capacity retention (N° cycles), C-rate

LLZO + [15]

PEO + LiTFSI LLZO +

[16]

PEO +BMIm TFSI


25


This work

CPE40

Li/CPE40/LFP 21

163 (C/10) 116 (1C)

Page 26 of 39

67 % (200), 1C

Interestingly, despite the ionic conductivity is lower and the crystallinity of PEO is more pronounced in CPE40 (40 wt% LLZO) as compared to CPE20 (20 wt% LLZO), the former displays better electrochemical performances in terms of cycling in lab-scale Li metal cell. In our opinion, this is clearly ascribed to the introduction of LLZO in the polymer membranes, which likely influences the transport properties throughout the composite electrolyte membranes, such as the lithium ion transference number, the interfacial characteristics or the mechanical performances, being also key parameters to allow a stable and long term cycling with lithium metal. This opens the path for further investigations on these new type of hybrid electrolytes, particularly by focusing on inoperando techniques, in order to assess Li+ diffusion mechanism to clarify those points. Nonetheless, the newly proposed approach of crosslinked hybrid CPE is an advancement


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towards realizing the dream of a practical and functional an all solid state safe energy storage system. Summarising, innovative crosslinked composite polymer electrolytes are prepared by mixing garnet-type super Li+-ion conductor LLZO ceramic particles with PEO, LiTFSI and G4 together with an appropriate hydrogen abstraction photoinitiator. A simple solventfree process, which consists of hot pressing and crosslinking by means of irradiation with UV-light in the presence of a proper H2 abstraction photoinitiator yields mechanically robust, non-flammable, non-tacky truly solid films, which remarkably act as safe, mechanically robust separating electrolytes at ambient/sub-ambient and even relatively high temperatures. The ionic conductivity values of the most promising CPEs are in the range from 10-4 (20 °C) to 10-3 (80 °C) S cm-1. The lithium metal cell with LFP-based working electrode and the CPE40 that contains 40 wt% of LLZO demonstrated the most stable performance, delivering 163 mAh g-1 at C/10-rate and 115, 82 and 76 mAh g-1 at the 1st, 100th and 200th cycle at 1C-rate, respectively. It also demonstrated remarkable performance in terms of specific capacity and stability upon very prolonged cycling (> 400 cycles) at moderate high (40 °C) and low (10 °C) temperatures.


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It is worth noting that LLZO pellets may not allow cycling tests at ambient temperature because of short circuits;28 on the contrary, in this work, the long-term galvanostatic cycling of lab-scale lithium metal cells at ambient temperature is demonstrated for composite electrolytes based on LLZO and crosslinked PEO, which clearly points at a viable strategy for the scientific community to achieve the targeted high energy and power densities aging resistant, safe and high-performing next generation all-solid Li-based batteries in real operative conditions. The proposed methodology represents a promising practical approach for the incorporation of super ion conductors in lithium metal batteries by imparting flexibility and shape retention attributes to the separator electrolytes. Further improvements are envisaged by switching to nano-sized particles and nano-structured ceramic networks or combining a nano-structured ceramic network and a single-ion conducting polymer electrolyte, so as to achieve an optimal trade-off between the best properties of the ceramic conductor as well as polymer matrix.

EXPERIMENTAL METHODS


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The self-standing composite polymer electrolyte (CPE) membranes were prepared by adding, under stirring, proper amounts of LLZO (Schott, sieved to obtain particle size < 32 μm) to a solution containing LiTFSI (Solvionic, battery grade), BP (Sigma Aldrich) and G4 (Sigma Aldrich, distilled under reduced pressure over NaH prior to use). PEO (AMW 200000, Sigma Aldrich) was added stepwise to the resulting mixture at about 70 °C until dissolution. The paste-like products were hand-ground and re-dissolved three times prior to hot-pressing (20 bar, 70 °C) and UV-curing the melt mixture (photo-reticulation at 40 mW cm-2 under a medium-pressure Hg lamp by Helios Quartz). The composition of the electrolytes is detailed in Table 1. The materials preparation was carried out in an Ar filled dry glove box (Jacomex GP-concept, O2 and H2O content < 1 ppm), and the products were placed between two Mylar® sheets and properly sealed to avoid contamination during the pressing and UV-irradiation processes. All the materials were dried under vacuum prior using. The LiFePO4 (LFP) based composite cathodes were prepared by standard procedure, obtained from a slurry composed of 70 wt% LFP (Clariant-LP2), 20 wt% conductive carbon (Shawinigan Black AB50, Chevron Corp) and 10 wt% PVdF (Solvay Solef® 6010)


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in N-methyl-2-pyrrolidone (Sigma Aldrich) coated on Al, and dried at under vacuum at 120° C for 24h. The active material loading was 1.1 ± 0.2 mg cm-2. Such low mass loading was used in order to maximise the interfacial contact of the polymer electrolytes under test with the porous cathode; the overall energy density was not at all considered, being out of the scope of the present work at this stage of development. Indeed, the scope of the present work is to assess the characteristics and the feasibility of Li metal cells assembled with the crosslinked CPEs under real operative conditions. Li metal disks were cut from a 200 μm thick ribbon from Chemetall. The electrodes and the electrolytes were sandwiched into ECC-Std cells (EL-CELL, Germany) for EIS (carried out with Parstat 2273 in the range 105 – 1 Hz with an oscillating potential of 10 mV) and galvanostatic cycling (carried out with an Arbin BT-2000 Battery Tester). All the Li/LFP cells were heated at 80 °C for 8h after assembly to improve the interfacial contacts and, subsequently, equilibrated at ambient temperature (common average laboratory temperature of about 21 °C) prior testing. The linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements to estimate the electrochemical stability window (ESW) were carried out with an


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electrochemical workstation 760E (CH instrument). The electrodes (Li/Cu for the cathodic stability window, CSW, and Li/SS for the anodic stability window, ASW) and the electrolytes were sandwiched into three electrodes ECC-Ref cells (EL-CELL, Germany), heated at 40 °C for 8 h to improve the interfacial contacts and, subsequently, equilibrated at ambient temperature prior testing. The ionic conductivity was extrapolated from the EIS spectra of symmetric SS/electrolyte/SS (SS-316) using ECC-Std cells as detailed by Porcarelli et al.17 The spectra were collected with Parstat 2273 in the range 105 – 1 Hz with an oscillating potential of 10 mV. The cells were heated at 80 °C for 6 h, equilibrated at 20 °C for 24 h and tested at intervals of 10 °C from 20 to 80 °C using a controlled climate chamber MK53 E2 (Binder). The XRD scans (step = 0.026°, 60 s per step) were collected at ambient temperature using the X-ray diffraction system X’Pert³ Powder (PANalytical) equipped with CuKα radiation monochromator. Differential scanning calorimetry (DSC) was carried out under N2 flow (20 mL min–1) with a DSC 204 F1 Phoenix (Netzsch) instrument equipped with a low temperature probe. The SPE samples (≈ 5 mg) were put in Al pans, quenched from


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ambient temperature to –70 °C and heated at 10 °C min-1 up to 90 °C. The cross-section (obtained by cracking the sample after dipping in liquid N2 to avoid any change in the morphology) and the samples for the FESEM analysis (ZEISS Supra® 40VP instrument, equipped with an energy dispersive X-ray spectrometer) were coated with a thin (≈ 10 nm) Pt layer by sputtering prior testing.

ASSOCIATED CONTENT Supporting Information. Flammability tests of the CPE under study, FESEM images, DSC heating traces, Arrhenius plot of the ionic conductivity values of the LiTFSI salt-free SFCPE40, Linear sweep voltammetry of a Li/CPE20/SS cell, Nyquist plots of Li/CPE/LFP cells recorded prior galvanostatic cycling, Cyclic voltammetry of a Li/SPE/Cu cell, Specific capacity and Coulombic efficiency vs the cycle number of Li/SPE_UV/LFP cell at 1C-rate and 21 °C, galvanostatic cycling of CPE40 at 40 °C and 10 °C and comparison with a TEGDME liquid electrolyte based cell.


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AUTHOR INFORMATION Corresponding Authors (*): *E-mail: [email protected] (M. Falco) *E-mail: [email protected] (C. Gerbaldi), https://orcid.org/0000-0002-8084-0143 https://www.facebook.com/GAMELabPoliTO

“The authors declare no competing financial interest.”

ACKNOWLEDGMENT The authors gratefully acknowledge Schott for providing LLZO material for this study.

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UV-Cross-Linked Composite Polymer Electrolyte for High-Rate

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