Single-Ion Conductor Polymer Electrolytes for All Solid-State Batteries
Authors: Krimalowski Alexander , Thelakkat Mukundan 
Published: December 2020

Poly(ethylene oxide) (PEO) can be considered the most widely studied and applied, all solid-state (solvent free) polymeric lithium ion conductor.1 However, the material suffers from numerous drawbacks: most importantly, PEO shows a considerable tendency towards crystallization.2 Consequently, linear PEO shows a drastic increase in ionic conductivity above its melting temperature, typically ranging between 40 and 60 °C depending on the molecular weight. This temperature-dependence strongly impedes the commercial application of PEO as substitute for the state-of-the-art liquid, flammable carbonate electrolytes of today’s lithium ion batteries. One major benefit of solid (polymer) electrolytes lies in their high mechanical stiffness along with shape-flexibility and elasticity, which could be used to avoid spacer membranes. This could not only increase the specific capacity of the cell by weight reduction and decreased electrode distance but also impede lithium dendrite growth at the electrode interfaces.
Many reports find that LiTFSI in combination with PEO delivers among the highest ionic conductivities for solid polymer electrolytes. The combination of the small Li+ cation and the bulky, highly charge-delocalized TFSI- anion leads to a very weakly associated salt structure. The TFSI- anion contributes up to 80 % to the ionic conductivity in typical PEO/LiTFSI mixtures by propagating through the electrolyte.3 However, TFSI- cannot be converted nor intercalated electrochemically at the electrodes in a battery application. During discharge, anions accumulate at the anode in opposite direction to the internal electric field, depolarizing the cell and thereby diminishing its specific capacity and power. Furthermore, the high concentration of anions at either electrode can facilitate the growth of unwanted lithium dendrites at the surface. The concept of a single-ion conducting electrolyte tackles these issues by spatially fixating the anions onto a scaffold, e.g. a polymer backbone. This restricts the anion movement and helps to increase the percentage of current transported only by lithium cations, which is referred to as the lithium transport number tLi+
.
A synthetic approach to combine the lithium ion coordinating chemistry of a PEO-based polymer with a sterically frustrated chain architecture can be found in brush-like graft copolymers.4 We recently presented a convenient two-step pathway for the synthesis of clickable PEO-based electrolyte materials.5 In the first step, epoxy- as well as alkyne-functionalized glycidyl propargyl ether (GPE) is directly converted to P(GPE) by monomer-activated, anionic ring-opening polymerization (AROP, fig. a). We showed the consequent functionalization of this versatile polymer backbone in the second step by copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC, Click-chemistry) under very mild conditions and with complete removal of copper catalyst traces. We could tailor the polymer composition by the sequential addition of the corresponding, azide-equipped side-groups; in our case tri(ethylene glycol) azide (EG3-N3) and benzyl azide (Bn-N3) and a specially synthesized, clickable lithium bis(trifluoromethanesulfonyl)imide azide (LiTFSI-N3) which integrates the concept of single-ion conduction (fig. a,b)
As an improved version of the previously presented, sequentially clicked graft copolymers, we prepared single-ion conducting P[GPE-(EG3x-ran-LiTFSIy)]100 electrolytes (fig. c) by clicking EG3-N3 as well as LiTFSI-N3 sidegroups onto the P(GPE) backbone with different amounts of lithium salt in the structure.6 We analyzed these materials in terms of composition (NMR, SEC), temperature behavior (TGA, DSC) as well as applicability as lithium ion conducting electrolytes (ionic conductivity, lithium transport number, cyclovoltammetry, battery cycling and extended DRT analysis). Additionally, we synthesized the fully EG3-grafted P(GPE-EG3)100(fig. d) as a matrix polymer and mixed it with LiTFSI to obtain a salt-in-polymer electrolyte as comparison dual-ion system. Both the type of lithium salt as well as the ion-conducting matrix are the same in both electrolytes, but a significantly increased lithium transport number is expected for the single-ion conductor. Herein we present two solid polymer electrolyte systems based on the versatile P(GPE) backbone – one of them single-ion conducting – and compare them in a detailed and fundamental study.
References
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Figure 1