Optimization of Block Copolymer Electrolytes for Lithium Metal

Article pubs.acs.org/cm

Optimization of Block Copolymer Electrolytes for Lithium Metal Batteries Didier Devaux,† David Glé,‡ Trang N. T. Phan,‡ Didier Gigmes,‡ Emmanuelle Giroud,§ Marc Deschamps,∥ Renaud Denoyel,† and Renaud Bouchet*,§ †

Aix-Marseille Université, CNRS, MADIREL 7246, 13397 Marseille, France Aix-Marseille Université, CNRS, ICR 7273, 13397 Marseille, France § Universités Grenoble Alpes, CNRS, LEPMI 5279, F-38000 Grenoble, France ∥ Blue Solutions Company, Ergue Gaberic, Odet, F-29556 Quimper 9, France ‡

S Supporting Information *

ABSTRACT: Safety is one of the most important criteria for electrochemical energy storage devices used in large scale applications such as wind or solar farms. In this context, solid polymer electrolytes based on nanostructured block copolymer electrolytes (BCEs) are promising because their properties can be finely tuned by adjusting simultaneously their block chemistries and polymer architectures. However, there is a need to rationalize the different properties of BCE that are optimal for battery applications. We produced by controlled radical polymerization a large number of BCEs based on either (1) linear poly(ethylene oxide) (PEO) or (2) comb PEO as the ionic conductor block, and polystyrene as the structural block. We varied the molecular weight of the PEO-based block, the composition, and the architecture (diblock vs triblock). We performed a systematic analysis of their thermodynamic, ionic transport, and mechanical properties. To verify the potential of BCEs as electrolytes, we evaluated their electrochemical stabilities. Laboratory scale batteries comprising the best BCEs and LiFePO4 as a positive active material were cycled at different rates and temperatures. This process allows the selection of the best architectures and compositions that had been successfully tested in battery prototypes and cycled for more than 600 cycles at high rates without any dendritic growth.

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INTRODUCTION Global warming, acid rain, photochemical smog, and oil spills are some of the consequences associated with the massive use of fossil fuels. These problems demand the development of other sources of energy. Renewable energies represent a very good alternative because they come from natural processes that are replenished constantly. Nevertheless, most of these energies such as wind power or solar energy are intermittent and fluctuate in a manner independent of consumption needs.1 As a result, the use of renewable energies requires the development of further energy storage methods.2,3 Batteries based on lithium (Li) are very promising storage systems because of their high energy density.4,5 However, they remain expensive, and the presence of a liquid electrolyte inside most lithium ion batteries makes this technology insufficiently safe because of the possibility of leaks and flammable reaction products.6,7 In this respect, solid polymer electrolytes (SPEs) are highly appealing because they are not flammable, ensuring better safety. Furthermore, high energy densities can be achieved when they are used in combination with a lithium metal negative electrode.8 Among the different macromolecular chains, poly(ethylene oxide) (PEO) is the most interesting because it contains ether coordination sites that allow lithium salt © XXXX American Chemical Society

dissociation and complexation while the macromolecular flexibility of the backbone ensures sufficient ionic dynamics.9,10 Ionic transport in PEO depends on the movement of polymer chains and occurs mostly in the amorphous phases.11,12 Thus, lithium metal polymer batteries usually operate at a temperature, T, higher than the PEO melting temperature, Tm, and generally above 80 °C. Under this condition, the PEO homopolymer is a viscous liquid mechanically too weak to mitigate the growth of lithium metal dendrites upon cycling.13,14 One very promising solution to combine both good ionic conductivity and good mechanical properties is the use of nanostructured block copolymer electrolytes (BCEs).15−18 Block copolymers are composed of different covalently bonded polymers that provide to the bulk their own properties. One block, A, is a PEO-based polymer doped with a lithium salt to impart ionic conductivity, while the other block, B, provides other functionalities such as mechanical strength. This block usually possesses a high glass transition temperature, Tg, an Received: April 7, 2015 Revised: June 11, 2015

A

DOI: 10.1021/acs.chemmater.5b01273 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Table 1. Molecular Weights and Compositions of the Block Copolymers Depending on Their Architecture copolymer linear A block comb A block diblock B−A

PEO35 PEO100 cEO23 ScEO23

triblock B−A−B

SEO9S

SEO10S

SEO20S SEO35S

ScEO23S

triblock A−B−A

cEO23ScEO23

Mn(A) (kg mol−1)

Mn(B) (kg mol−1)

Y (%)

ϕEO

ϕc

35 100 95 36 62 59 92 92 9 9 9 10 10 10 10 10 20 35 35 35 80 80 95 48 55 48 55 96

0 0 0 26 35 22 31 25 16 8 3 13 10 6 5 4 6 14 11 10 48 46 46 6 33 23 23 20

100 100 86 50 55 63 65 68 36 54 73 43 50 63 69 74 76 72 77 78 54 55 58 77 54 58 61 71

1 1 0.87 0.49 0.54 0.62 0.64 0.68 0.35 0.53 0.72 0.42 0.49 0.62 0.68 0.73 0.75 0.71 0.76 0.77 0.53 0.54 0.57 0.77 0.53 0.57 0.60 0.71

1 1 0.88 0.52 0.57 0.65 0.67 0.69 0.37 0.55 0.74 0.44 0.51 0.64 0.70 0.75 0.77 0.73 0.77 0.79 0.56 0.57 0.60 0.79 0.55 0.60 0.63 0.73

example being polystyrene (PS). These systems are of great interest because of their self-assembly properties, which give rise to ordered structures on nanoscopic scales.19,20 For a linear A block made of PEO homopolymer, Balsara et al. extensively studied PS−PEO diblock copolymers doped with lithium bis(trifluoromethanesulfonimide) (LiTFSI).17,21,22 They showed that for nearly symmetric diblock, the ionic conductivity increases with PEO length chain until a plateau is reached; this behavior is the opposite of that of the PEO homopolymer.11 At 60 °C and a molar ratio of ethylene oxide units to lithium (EO:Li) of 11.8, the maximal conductivity is on the order of 2 × 10−4 S cm−1. In triblock B−PEO−B architecture, a wide variety of B blocks have been reported in the literature.23−31 Within these electrolytes, PS−PEO−PS reaches the highest ionic conductivity with a value of 2.3 × 10−4 S cm−1 at 60 °C.31 The A block can also have a comb architecture. The idea is that low-molecular weight EO moieties, linked to a synthetic polymer backbone, would be highly dynamic, hampering their crystallization, which favors a high level of ionic motion. One of the most common comb PEO polymers is poly(ethylene glycol methacrylate). Sadoway et al. showed a loss of conductivity associated with an increase in the Tg of the B block made of alkyl methacrylate in B−A electrolytes.32−35 Even though the conductivity of these electrolytes barely reaches 2 × 10−5 S cm−1 at 60 °C, they were used in lithium metal batteries.34,36,37 Niitani et al. showed the antithetic evolution of ionic conductivity and tensile strength depending on the PEO weight fraction of a B−A−B triblock, with a comb PEO and PS.15 At 30 °C, the ionic conductivity of the electrolyte with at least 80% of the PEO phase is 10−4 S cm−1 but the tensile strength is very poor.38

To improve the design of block copolymers for lithium metal batteries, there is a need to better understand the relationships linking the physicochemical and electrochemical properties to the electrolyte chemistry. The role of copolymer composition and architecture (diblock B−A or triblocks B−A−B and A−B− A) from the fundamental (thermodynamic, mechanic, and ionic transport) to the electrochemical (lithium transference number and stability window) properties is crucial for optimization of solid-state battery operation. Notably, the correlation between ionic conductivity and mechanical properties has to be taken into account to finely tune the electrolyte. In addition, for battery applications, the electrochemical stability is of utmost importance. In this paper, we report such an approach by a systematic analysis of the physicochemical and electrochemical properties of a large series of block copolymer electrolytes (BCEs). The structural B block is made of PS, while the conducting A block is made of either linear PEOs or comb PEOs based on poly(ethylene glycol) methyl ether methacrylate. For each kind of architecture, the effect of the molecular weight of the PEObased block as well as the composition of the BCE has been studied. The best compromise between ionic conductivity and mechanical properties allows the selection of the optimal compositions. Then the evaluation of the electrochemical stability highlights the limits of comb architecture based on a methacrylate backbone. Through this optimization process, the best block copolymers have been used to assemble lithium metal batteries using LiFePO4 as the positive active material. The effect of temperature between 80 and 40 °C as well as the cycle rate on the delivered capacity has been determined. Finally, to show the efficiency of this selection process, an B


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Chemistry of Materials

the PEO enthalpy of melting, ΔHm, the PEO degree of crystallinity, Xc, was calculated using PEO weight fraction Y by

industrially made positive electrode based on LFP was used to test a battery prototype that made more than 600 cycles at a high C rate with a very low capacity fading.

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Xc =

EXPERIMENTAL SECTION

ΔHm Y ΔHm°

(1)

where ΔHm° is the enthalpy of melting of a 100% crystalline PEO, equal to 195 J g−1.43 Stress−strain experiments were performed with a model DMAQ800 instrument from TA Instrument at a constant temperature of 60 °C under a constant flow of dry air. The BCEs listed in Table 1 were prepared inside the glovebox and mounted on the DMA quantilever right before the experiment. To remove any trace of water absorbed by the SPE during this transfer, an initial drying step was performed by holding the sample at 60 °C for at least 45 min. Then, a constant force of 0.1 N min−1 was applied to the electrolyte to determine the Young’s modulus, EY, and the tensile strength, σTS, upon failure as shown in Figure S2 of the Supporting Information for the SEO35S_72 electrolyte. For comparison, a PEO100 laden with LiTFSI has also been characterized, and data for a 104 kg mol−1 PS homopolymer were used for comparison.44 Lithium symmetric cells were assembled in the glovebox using a homemade laminating machine set to operate at 80 °C under a roll pressure of 4 bar. They consist of the superposition of the SPE and a 8 μm thick polyethylene spacer defining the active surface area, S, that are placed between two 100 μm lithium foils attached to copper grid (Goodfellow) current collectors.45 At each step of the lamination process as well as after conductivity characterizations, the electrolyte thickness, l, was measured. After assembly, the cell was sealed inside an airtight pouch bag (Protective Packaging Ltd.) and placed in a climatic chamber (Vö tsch 4002) outside the glovebox to perform the electrochemical charcaterizations. AC impedance spectroscopy was performed with a Solartron 1260 frequency analyzer, using an excitation signal between 10 and 40 mV in a frequency range between 107 and 1 Hz, to determine the electrolyte ionic conductivity, σ. The temperature program consists of an initial heating scan from 30 to 100 °C in 10 °C steps, followed by cooling from 85 to 45 °C every 10 °C before returning to 30 °C, then a final heating scan in 10 °C steps is conducted from 30 to 100 °C. A stabilization time of 1 h was used at each step, and the reproducibility of the data during heating and cooling was systematically checked. The electrolyte transference number, t+, was determined at 90 °C by measuring the AC impedance of the lithium symmetrical cell down to 10−4 Hz. As an example, a typical impedance spectrum of the cEO23ScEO23_58 electrolyte, recorded at 90 °C, is shown in Figure S3 of the Supporting Information. The impedance spectra were modeled with an equivalent electrical circuit (inset of Figure S3 of the Supporting Information) using the Z-View software (Scribner Inc.).45 This model extracts the contributions from bulk such as the electrolyte resistance, Rel, the Li−electrolyte interfaces, and the lithium diffusion resistance, Rd. The ionic conductivity and the transference number were calculated by the following relationships.46,47

Materials and BCE Preparation. Styrene (99%), poly(ethylene glycol) methyl ether methacrylate (99%), triethylamine (99%), 2bromoisobutyryl bromide (98%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (99%) (PMDETA), and copper bromide (98%) were all obtained from Aldrich and used as received. BlocBuilder (>99%), an alkoxyamine based on the nitroxide SG1 {N-tert-butyl-N-[1diethylphosphono(2,2-dimethylpropyl)]nitroxide} and the 1-carboxy1-methylethylalkyl moiety, and DIAMINS, a dialkoxyamine based on SG1 nitroxide, were kindly provided by Arkema Co. The 9 kg mol−1 PEO was provided by Blue Solutions Co., while 10, 20, and 35 kg mol−1 PEO were obtained from Aldrich and used as received. All solvents and other reagents were synthesis grade and used without further purification. The BAB triblock copolymers based on linear 9 and 35 kg mol−1 PEO were prepared by the ATRP method,39 while those based on 10 and 20 kg mol−1 PEO were prepared by the nitroxide-mediated polymerization (NMP) method.31 All diblocks and triblocks based on comb PEO were also synthesized by the NMP method using a sequential polymerization approach according to the reaction scheme shown in Figure S1 of the Supporting Information. The synthesized triblock copolymers and their characteristics are listed in Table 1. These linear triblocks are labeled SEOxS_Y where x corresponds to the PEO molecular weight in kilograms per mole and Y to the proportion of PEO in weight percent. For the purpose of comparison, PEO homopolymers with molecular weights (Mn) of 35 and 100 kg mol−1 have been also characterized and are labeled PEO35 and PEO100, respectively. For the comb A block, the NMP route requires the copolymerization of the poly(ethylene glycol) methyl ether methacrylate macromonomer (Mn = 1.1 kg mol−1), with a comonomer to ensure polymerization control. For this purpose, styrene was used in a 5 mol % proportion to combine an efficient polymerization control with a high content of the conductive phase. This A block is abbreviated as cEO23 because it results in 23 EO units per side chain. The total content of EO in the A block is 86 wt %. Like the linear PEO triblock copolymer, the corresponding block copolymers with the comb A block are labeled ScEO23_Y, ScEO23S_Y, and cEO23ScEO23_Y for the B−A, B−A−B, and A−B−A architectures, respectively. A 95 kg mol−1 comb A block has been synthesized for the purpose of comparison. The characteristics of the synthesized block copolymers are listed in Table 1. The solvent casting method was used to produce electrolyte films. The LiTFSI salt, purchased from Aldrich, was mixed with the block copolymers to reach an EO:Li ratio of 30 and then solubilized in a dichloromethane/acetonitrile [50:50 (v/v)] solvent to form a homogeneous solution of a 10% (w/w) mixture. The solution was cast in Petri dishes and allowed to evaporate slowly at room temperature for 24 h. A subsequent drying step was conducted by annealing the as-formed electrolyte film in vacuum for 24 h at 50 °C and finally placed in an argon-filled glovebox ( σTS(A−B−A) > σTS(B−A). Furthermore, for 1 − ϕc < 0.3, the percolation threshold of PS is not attained, leading to mechanical properties similar to those of the high-molecular weight viscous PEO− LiTFSI complex on the order of 10−2 MPa for both EY and σTS. For 1 − ϕc > 0.3, σTS strongly increases with the PS volume

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RESULTS AND DISCUSSION The influence of the copolymer architecture on the thermodynamic properties has been first studied without LiTFSI on the block copolymers as well as on the pure A blocks, i.e., PEO35 and cEO23 (see Table 1). Figure 1a represents the PEO melting temperature, Tm, as a function of ϕEO, while Figure 1b shows the PEO degree of crystallinity, Xc, as a function of ϕEO. Dependencies can be found by grouping the data according to the nature of the A block. By considering

Figure 1. (a) Melting temperature, Tm, of PEO and (b) degree of crystallinity, Xc, of PEO as a function of PEO volume fraction, ϕEO. Comb architectures (empty red symbols): (□) cEO23, (◇) B−A diblock, (△) B−A−B triblock, and (○) A−B−A triblock. Linear architectures (filled blue symbols): (■) PEO35 and SEOxS_Y with (left-pointing triangles) x = 9, (▼) x = 10, and (▲) x = 35. The dashed lines are guides for the eye depending on the nature of the A block. D


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Figure 3. (a) Young’s modulus, EY, and (b) tensile strength, σTS, of the BCEs as a function of the PS volume fraction, 1 − ϕc, at 60 °C. Comb architectures (empty red symbols): (□) cEO23, (◇) B−A diblock, (△) B−A−B triblock, and (○) A−B−A triblock. Linear architectures (filled blue symbols): (■) PEO100 and SEOxS_Y with (left-pointing triangles) x = 9, (▼) x = 10, (right-pointing triangles) x = 20, and (▲) x = 35. The data for the PS (black filled square) homopolymer were taken from ref 44. The dashed lines are guides for the eye.

Figure 2. Samples laden with LiTFSI at an EO:Li ratio of 25. (a) Melting temperature, Tm, of PEO and (b) degree of crystallinity, Xc, of PEO as a function of PEO volume fraction, ϕEO. Comb architectures (empty red symbols): (□) cEO23, (◇) B−A diblock, (△) B−A−B triblock, and (○) A−B−A triblock. Linear architectures (filled blue symbols): (■) PEO35 and SEOxS_Y with (▼) x = 10, (right-pointing triangles) x = 20, and (▲) x = 35. The dashed lines are guides for the eye depending on the nature of the A block.

and better mechanical properties, in comparison with those of the comb block copolymer electrolytes. The ionic conductivity of the electrolytes listed in Table 1 has been determined between 30 and 100 °C during the heat− cool−heat scan. The experimental data taken from the cooling and heating scans follow the same trends, indicating the stability of the systems (see Figure S4a,b of the Supporting Information). The evolution of the ionic conductivity of the SEOxS electrolytes as a function of temperature is similar to that of the PEO homopolymer with a transition characterized by a drop in conductivity indicating the phase crystallization of the PEO phase.45 However, this transition occurs at lower temperatures, in agreement with the thermodynamic analysis previously presented, i.e., a decrease in Tm due to the confinement of the PEO chains within the block copolymer. For the comb BCEs, as expected from their thermodynamic properties, PEO crystallization does not occur in the considered

fraction to reach a value of 1 MPa (2 orders of magnitude) at 1 − ϕc = 0.45. For BCE with comb PEO, it appears that a minimal PS volume fraction (i.e., 0.3) is necessary to percolate and improve the mechanical properties to obtain a self-standing film necessary for solid-state battery applications. At a given ϕc, the Young’s modulus of the electrolytes based on linear PEO, SEOxS, increases with the length of the PEO chains. The tensile strengths of these SEOxS electrolytes with x values of 9, 10, and 20 follow the same trends as those of the comb electrolytes. However, the SEO35S BCEs present a lower percolation threshold of PS at 1 − ϕc = 0.21, leading to EY and σTS values that are higher than those of all the other electrolytes. This may be due to the length of the PS blocks that are far below the entanglement weight in the case of low-molecular weight linear PEO. Linear BCE can be optimized by selecting a longer PEO chain to obtain SPEs with a higher content of conducting phase E


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high-molecular weight amorphous PEO homopolymer electrolyte above its melting temperature. The ionic conductivity of the comb electrolyte is independent of the block copolymer architecture, diblock (B−A) or triblock (B−A−B or A−B−A), and of the BCE molecular weight all over the volume fraction range of the conducting phase with values higher than 10−4 S cm−1 for ϕc > 0.5. As expected from the thermodynamic analysis, there is no effect of confinement within the comb BCE. The PEO moieties do not “feel” the PS wall. Here, we observe only a topological effect, because of the tortuosity of the conductive PEO domains that increases when the volume fraction of PEO decreases.31 For these electrolytes, the conductivity data trend line lies between those of SEO35S and SEOxS with x < 35. As seen previously, for ϕc > 0.73 the PS proportion is not sufficient to produce a self-standing film. Thus, the ionic conductivity of the ScEO23S_77 and cEO23 electrolytes had been determined using a conductivity cell designed for a viscous electrolyte.51 For lithium battery applications comprising such polymer electrolytes, this limitation in ϕc means that a BCE with a comb conductive block will be limited to 2 × 10−4 S cm−1 at 60 °C and 2 × 10−5 S cm−1 at 30 °C. Because the conductivity and the mechanical properties evolve in opposite manners, the conductivity−mechanical property relationship has been rationalized by plotting the Young’s modulus as a function of the isothermal ionic conductivity at 60 °C in Figure 5. As expected, for all of the BCEs, the ionic conductivity decreases from the PEO homopolymer value with increases in the mechanical properties. For linear BCEs, a linear trend is observed in these coordinates; i.e., the Young’s modulus decreases with ionic conductivity with a power law with an exponent of 4. However, when the molecular weight of PEO is increased from SEO10S to SEO35S, both a higher conductivity and a higher Young’s modulus are obtained for a given PEO weight fraction. Therefore, it is confirmed17 from Figure 5 that for linear BCEs, the increase in the length of the PEO chains is a need to obtain the best compromise between conductivity and mechanical properties above the melting temperature. However, as the goal is to obtain the best performances at the lowest temperature ( 0.99) with the following empirical Vogel−Tamman− Fulcher (VTF) equation48−50 to interpolate the data at an identical temperature: σ=

⎛ −B ⎞ A × exp⎜ ⎟ T ⎝ T − T0 ⎠

(4)

where A is a pre-exponential factor, T0 is the ideal glass transition temperature, and B is an activation temperature. For the linear electrolytes, only the data for T > Tm had been taken into account. The 60 °C isothermal conductivities on a logarithm scale are plotted as a function of the volume fraction of the conducting phase, ϕc, in Figure 4. For the linear BCE,

Figure 4. Isothermal conductivity, σ, of the BCEs as a function of ϕc at 60 °C. Comb architectures (empty red symbols): (□) cEO23, (◇) B− A diblock, (△) B−A−B triblock, and (○) A−B−A triblock. Linear architectures (filled blue symbols): (■) PEO35 and SEOxS_Y with (left-pointing triangle) x = 9, (▼) x = 10, (right-pointing triangle) x = 20, and (▲) x = 35. The dashed lines are guides for the eye.

the logarithm of the ionic conductivity increases linearly with ϕc until the value of the high-molecular weight PEO homopolymer electrolyte, i.e., PEO35−LiTFSI is reached. In these coordinates, these linear trends depend on the length of the PEO block. At a given ϕc, the higher the PEO molecular weight, the higher the conductivity, in contrast to the case for PEO homopolymers. This result is in agreement with literature data for diblock or triblock copolymer electrolytes.17,31 It has been interpreted as the effect of the dead zone at the interface between the PEO and PS domains.31 The proportion of this dead zone increases when the Mn of PEO decreases, leading to a decrease in the conductivity because the effective volume fraction of the conducting phase decreases. The best conductivity has then been measured for the SEO35S_78 electrolyte (2.55 × 10−4 S cm−1) at 60 °C. To the best of our knowledge, such a value is one of the highest reported so far in the literature for a solid BCE. For the comb electrolytes, the ionic conductivity also increases linearly with ϕc until the value of the PEO homopolymer electrolyte is reached. Interestingly, the conductivity of the pure A block, cEO23, also belongs to this general trend with an ionic conductivity lower than that of the F


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this electrochemical test, the BCEs that present the best compromise between conductivity and mechanical properties were chosen, one with a linear PEO, SEO35S_72, and another with comb PEO, ScEO23S_58. Figure 6 represents the current

Figure 6. Cyclic voltammograms recorded at 1 mV s−1 and 80 °C between −0.5 and 4.5 V vs Li+/Li0 for four consecutive cycles, using (solid blue line) SEO35S_72 and (dashed red line) ScEO23S_54. The arrows indicate the evolution with cycle number. The inset shows the last oxidation cycle between 3 and 4.5 V vs Li+/Li0.

density as a function of the potential versus Li+/Li0 for four consecutive cycles recorded at 1 mV s−1 and 80 °C between −0.5 and 4.5 V versus Li+/Li0. In accordance with literature data, both electrolytes present a low-potential wall corresponding to the Li+/Li0 couple.53 The electrochemical stability window of the linear BCE is up to 3.8 V versus Li+/Li0, which is equivalent to that of the PEO homopolymer electrolyte.54 The comb block copolymer electrolyte has a wide instability toward lithium metal with a broad oxidative peak at ∼2.5 V versus Li+/ Li0. This peak may be attributed to the degradation of the methacrylate function55 or by some reactivity of the nitroxide agent used in the NMP synthesis. At higher potentials, the comb block copolymer electrolyte has an electrochemical stability window, 3.6 V versus Li+/Li0, which is lower than that of the linear electrolyte. The inset in this figure shows the last cycle (between 3 and 4.5 V vs Li+/Li0) to highlight the differences in their electrochemical stability at high potential. This difference is unexpected because the PMMA has been largely used in a gelled electrolyte. However, here we are studying a solid-state case, which means that the PMMA is not surrounded by a large amount of solvent molecules that may prevent the direct contact of the PMMA to the positive electrode. Furthermore, we worked at 80 °C, which is largely higher than the temperature used for a gelled polymer, generally 20 °C. For a lithium metal battery application, LiFePO4 is a suitable cathode active material to be coupled with these electrolytes.56 The composite cathodes made of LiFePO4, carbon, and BCE were examined before cycling by scanning electron microscopy (SEM) as shown in Figure S6 of the Supporting Information. Regardless of the electrolyte architecture, SEO35S_72 or ScEO23S_54, the cathode microstructures are similar to highly dispersed active material and carbon grains within the

Figure 5. Young’s modulus, EY, as a function of isothermal conductivity, σ, at 60 °C. (a) Linear architectures (filled blue symbols): (■) PEO35 and SEOxS_Y with (▼) x = 10, (right-pointing triangles) x = 20, and (▲) x = 35. (b) Comb architectures (empty red symbols): (◇) B−A diblock, (△) B−A−B triblock, and (○) A−B−A triblock. For comparison, the results of the PEO100 (blue ■) homopolymer electrolyte are also shown. The dashed lines are guides for the eye.

°C by electrochemical impedance spectroscopy.46,47 The results are provided in Figure S5 of the Supporting Information, which represents t+ as a function of the total molecular weight of the electrolyte. We had previously compiled the evolution of t+ at 90 °C for the PEO homopolymer doped with LiTFSI at an EO:Li ratio of 25.51 We are using this model curve to highlight the transference number values of the BCEs as well as PEO100. Regardless of the BCE architecture and composition, they have similar transference numbers close to 0.15, equivalent to that of PEO above the entanglement weight (i.e., 5 kg mol−1), which indicates that the long-range ionic transport mechanism in BCEs is similar to that in the PEO homopolymer. Thus, in terms of lithium transference number, there is no favorable effect to have small EO units fixed on side chains compared to the linear PEO. The electrochemical stability window defines the interval of potential where the electrolyte remains stable to the electrochemical reactions that occur in the battery. This parameter is thus essential, as it defines the battery operating potentials. For G


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for the last 10 cycles where a rate of C/8 (0.1 mA cm−2) was used. The discharge C rate was varied from C/15 to 1C. At 80 °C, increasing the discharge C rate from C/15 to 1C leads to a typical decrease in the discharge capacity from 160 to 60 mAh (g of LiFePO4)−1. Then the capacity at C/4 is recovered and stable for up to 10 cycles. Decreasing the temperature to 60 °C leads to a 7% loss of capacity at C/8 when compared to the 80 °C data. Lowering even more the temperature at 50 °C reduces the C/8 capacity by 17% compared to the 80 °C data. At this temperature, the C/4 discharge capacity is only 56% of the initial 80 °C data. Interestingly, the battery can operate at 40 °C, a temperature lower than the Tm, where the PEO domains are partially crystallized. The C/4 discharge capacity is lowered by a factor 4 compared to the 80 °C data, but cyclability is observed for as many as 10 cycles. With increases in temperature to values higher than the Tm, the 60 °C C/8 capacity is fully recovered at 130 mAh (g of LiFePO4)−1. After this cycle life test, a battery cross section was imaged by SEM (Figure S8 of the Supporting Information). The lithium−electrolyte interface is very smooth without any noticeable trace of dendritic growth. This is a very positive result given the harsh treatment in terms of temperatures and C rates to which this battery was subjected, and the fact that no back-pressure was applied to the battery during cycling. The power holding capacity of the battery given by the discharge capacity as a function of the discharge C rate is shown in Figure 8 at two temperatures, 80 and 50 °C. For comparison,

electrolyte matrix. The size distribution of the active material is wide with particle sizes between 100 nm and 2 μm. On average, the cathode thicknesses were around 60 μm with an estimated porosity between 55 and 74%. This high porosity is detrimental for battery operation, and a better cathode formulation process is necessary. During the lamination process, the porosity of the cathodes decreases but remains at least 30%. Nevertheless, these cathodes allow the influence of the BCE nature to be studied in all-solid-state batteries. Batteries comprising a cathode, a copolymer electrolyte, and a lithium metal anode were first assembled inside the glovebox and then cycled at 80 °C between 2 and 3.8 V. Panels a and b of Figure S7 of the Supporting Information represent the potential as a function of the specific capacity for two SEO35S_72-based batteries and a ScEO23S_54-based battery. The SEO35S_72-based batteries were charged at C/15 (i.e., a charge theoretically in 15 h) and discharged at C/8, while the ScEO23S_54 battery was cycled at C/30 for both charge and discharge. Cyclability is observed for the SEO35S_72 batteries with a well-defined potential plateau corresponding to the electrochemical reaction of LiFePO4. However, a stronger polarization is observed for the cathode with the highest porosity (Figure S7a of the Supporting Information). It is then confirmed, as expected in solid-state batteries, that reducing the cathode porosity is necessary to improve battery cyclability (Figure S7b of the Supporting Information). The battery based on a comb block copolymer electrolyte shows poor cyclability at a low C rate (Figure S7c of the Supporting Information). This result is in accordance with the electrochemical stability of this electrolyte (Figure 6). At high potentials, the electrolyte starts to decompose and battery cyclability is strongly hampered. For lithium metal battery applications, a linear electrolyte architecture is definitely recommended. However, solving the comb BCE instability could lead to batteries that can operate at temperatures lower than those made with the linear PEO-based electrolyte, which are limited by the PEO melting temperature. We then selected the linear SEO35S_72 electrolyte in combination with the lowest-porosity cathode (55%) to further study battery operation at different C rates and temperatures. The cycle life data of the SEO35S_72-based battery are shown in Figure 7, which corresponds to the discharge capacity as a function of cycle number, N, at four temperatures, 80, 60, 50, and 40 °C. The battery was always charged at C/15, except

Figure 8. Discharge capacity according to the discharge C rate: (filled blue triangle) SEO35S_72 at 80 °C, (empty blue triangle) SEO35S_72 at 50 °C, and (●) PEO composite electrolyte at 76 °C.57

literature data of a lithium metal polymer battery are added using LiFePO4 as an active material and a similar surface capacity. The electrolyte is a PEO homopolymer−S-ZrO2 nanocomposite57 at 76 °C. The capacity retention of the SEO35S_72-based battery is 88% at a low C rate, between C/15 and C/4. At a higher C rate, polarization limitations due to concentration gradients lead to a rapid drop in capacity as also seen for the PEO composite electrolyte. However, at 50 °C, we obtain the best results, to the best of our knowledge, for a dry SPE with performances in the range of that of the nanocomposite SPE but 26 °C lower.

Figure 7. Cycle life of the SEO35S_72-based battery. The different temperatures and discharge C rates are noted. For each cycle, a C/15 charge rate was used, except where noted. H


Article


because the PEO moieties do not “feel” the PS wall. Only the total Mn and the volume fraction of PS are important, where at least 40% of PS is necessary to ensure the formation of a selfstanding film. The only advantage of comb BCEs compared to linear BCEs is the absence of PEO crystallization, which allows slightly better ionic conductivity around room temperature. However, the electrochemical stability at 80 °C of these BCEs is limited. As a result, the battery cyclability is very poor and rapid fading is observed. On the other hand, battery tests using the linear BCEs show very promising performances with good power retention, long cyclability at high C rates, very good faradic efficiency, and good resistance to dendrite growth. The next step is to search for polymer chemistries and architectures that allow improved operation at lower temperatures.

To further analyze the long-term stability of our BCE SEO35S_72, a battery prototype was assembled by Blue Solutions Co. The cathode is an industrially made cathode realized by Blue Solutions Co. with a loading corresponding to 1.2 mAh cm−2. The cell active surface is ∼5 cm2; thus, a 6 mAh battery was tested. The battery cycling test was conducted at several temperatures, 60, 80, and 100 °C, at a regime of C/4 (0.3 mA cm−2) in charge and C/2 (0.6 mA cm−2) in discharge. Figure 9 represents the discharge capacity as a function of cycle

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ASSOCIATED CONTENT

S Supporting Information *

Physicochemical characterizations and scanning electron micrographs. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.chemmater.5b01273.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

We gratefully acknowledge the OSEO agency under project GENESIS for financial support. Notes

Figure 9. C/2 discharge capacity of the battery prototype as a function of cycle number. The charge regime was C/4, and three different temperatures were tested.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the French laboratories Im2np in Marseille and Karim Djellab of LRCS in Amiens for the SEM battery cross section images.

number. The discharge capacities at 60 and 80 °C at a C/2 rate are very consistent with those shown in Figure 7. The results at 100 °C are particularly noticeable with more than 500 cycles, equivalent to 4 months of operation, and were successfully realized with a very low capacity fading (∼0.02% per cycle). The faradic efficiency was estimated to be 99.98%, which is a superior result for lithium metal polymer technology. Moreover, the lithium−electrolyte interface resistance at 100 °C measured at the beginning of each discharge, shown in Figure S9 of the Supporting Information, is very steady with an average low value of 3 Ω cm2. After the battery had been cycled for 6 months, the prototype was stopped to check the morphology of the lithium−electrolyte interface by SEM. The cross section image (Figure S10 of the Supporting Information) shows that the interface is not perfectly uniform; however, there is no noticeable trace of dendritic growth that is usually observed.13,58,59 This result shows the promising applications of BCE based on linear PEO in lithium metal battery technology.

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REFERENCES

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CONCLUSIONS In this work, we have shown how to rationalize the physicochemical properties of BCEs to converge on the best polymer architecture and composition. Linear PS−PEO−PS BCEs present the best compromise between high conductivity and good mechanical properties. An intermediate molecular weight (Mn) of the PEO block, with a composition of 70 wt % PEO, is the best direction for improving the performances at the lowest temperature. In case of comb PEO BCEs, there is no effect of the architecture on the physicochemical properties I


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Optimization of Block Copolymer Electrolytes for Lithium Metal

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