Charge Transport in Nanostructured PS–PEO–PS Triblock Copolymer Electrolytes

Abstract

Nanostructured block copolymer electrolytes (BCE) are currently attracting widespread interest for applications in rechargeable lithium batteries. In order to investigate the influence of the composition, and therefore that of confinement, on the conductivity, a series of triblock PS− PEO−PS copolymers, with three linear PEO blocks of molecular weights 9, 10, or 35 kg mol −1 and with PEO weight fractions varying from 36% to 75%, were synthesized and doped with LiTFSI. Measurements by impedance spectroscopy of the conductivity show that it increases with PEO molecular weight, which is quite counterintuitive. To explain this phenomenon, the conductivity of the BCE has been modeled using three factors: (1) the conductivity of bulk PEO, (2) the topology of the PEO percolating network, described by the tortuosity parameter, and (3) the influence on the tortuosity of the effective volume fraction of the PEO phase useful for the conduction, taking into account a "dead zone" excluded from ionic transport, at the PS/PEO interface. In this approach, by analogy with porous materials and in contrast with previous work, the tortuosity is not kept constant but depends on the PEO volume fraction effectively useful for charge transport. The thickness of the dead zone, 4−5 EO units (∼1.6 nm), is the same as that of the exclusion zone for crystallization previously reported. This value does not depend either on the PEO molecular weight (from 9 to 35 kg mol −1 ) or on the EO/Li ratio (from 20 to 30). The absence of both conduction and crystallization in the excluded region could be due to the low mobility of the PEO chains in this zone. Consequently, the conductivity of BCE increases with PEO molecular weight because the proportion of the excluded zone becomes smaller as the PEO molecular weight increases. This model therefore provides a fairly precise description of the ionic conductivity of BCE used in lithium battery applications.

ABA Triblock Brush Polymers: Synthesis, Self-Assembly, Conductivity, and Rheological Properties

“…Interestingly, Wang et al 14 observed Arrhenius behavior over the same temperature range with poly[styrene-block-(styrene-graf t-ethylene oxide)-block-styrene] produced by grafting-from polymerization, a difference possibly related to architecture or grafting density effects. The BBCPs described herein demonstrate similar or moderately higher conductivity than lithium-doped linear PEO-containing block copolymers such as PS−PEO, 27 PS− PEO−PS, 6 and PP−PEO−PP. 7 The comparable conductivity, especially at such low PEO molecular weights, is perhaps surprising since the brush architecture effectively dilutes the volume fraction of the conducting PEO domain with norbornene end groups (in the case of gPS 11 -gPEO 78 -gPS 11 by about 10%), a consideration that has previously been assumed to decrease conductivity.…”

“…This trade-off motivates the investigation of block copolymers (BCPs) as PEMs, which naturally decouple conductive domains from those providing mechanical support, facilitating independent optimization of both properties. 4 Most BCPs studied to date for battery applications comprise linear architectures incorporating PEO integrally in the backbone, for instance poly(styrene-block-ethylene oxide) (PS−PEO), 5 poly(styrene-block-ethylene oxide-block-styrene) (PS−PEO−PS), 6 and poly(propylene-block-ethylene oxideblock-propylene) (PP−PEO−PP). 7 While other intrachain connectivity has been well-studied with homopolymers and statistical copolymers, 8−12 literature on side-chain grafted BCPs remains limited to single-block functionalization 13−15 and dendrimers.…”

Optimization of Block Copolymer Electrolytes for Lithium Metal Batteries

“…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 SEO 35 S and SEO x S with x < 35. As seen previously, for ϕ c > 0.73 the PS proportion is not sufficient to produce a self-standing film.…”

“…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 M n of PEO decreases, leading to a decrease in the conductivity because the effective volume fraction of the conducting phase decreases.…”

Failure Mode of Lithium Metal Batteries with a Block Copolymer Electrolyte Analyzed by X-Ray Microtomography

“…[23][24][25] In studies utilizing block copolymers as solid polymer electrolytes, the mechanical phase is usually made of a high glass transition temperature polymer such as polystyrene. [26][27][28][29] Extensive work on the thermodynamics of polystyrene-b-poly(ethylene oxide) (SEO) diblock copolymers [30][31][32][33] indicate that the tendency for microphase separation is enhanced by the presence of ions. 34,35 For symmetric SEO electrolytes, i.e.…”