Status and Targets for Polymer-Based Solid-State Batteries for Electric Vehicle Applications

Abstract

There is growing interest in the development of Li-metal-based solid state batteries, driven by their promise in improving the energy density to satisfy electric vehicle requirements. In this contribution, we examine the status of Solid polymer electrolytes (SPEs) based solid state batteries for electric vehicle applications using a continuum scale mathematical model. We examine LiFePO4 (LFP) cathode/lithium metal anode batteries containing three different electrolytes, namely (1) a liquid electrolyte, (2) the polystyrene-b-poly(ethylene oxide) (SEO) block copolymer electrolyte, and (3) a single-ion conducting (SIC) block copolymer electrolyte, with the liquid electrolyte serving as the baseline for the comparison. By using an optimization procedure, we assemble “virtual” batteries to identify the optimal design that maximizes energy density while allowing the power requirements of electric vehicles (EVs) to be satisfied. Results show the present status of different SPEs are still below what is considered acceptable and further improvements are needed to achieve electric vehicle targets. The optimization studies conducted here show that for low transference number electrolytes (∼0.2) the conductivity target is 5 × 10−3 S cm−1, while for a unity transference number electrolyte this target decreases to 4 × 10−4 S cm−1. These targets provide guidance for polymer synthesis researchers to develop better polymers for use in EVs.

Influence of Polymer Backbone Fluorination on the Electrochemical Behavior of Single-Ion Conducting Multiblock Copolymer Electrolytes

“…However, recent theoretical calculations show that a conductivity of 0.4 mS cm −1 is already sufficient for EV applications due to the outstandingly high efficiency of charge transport in SICs. 41 Very recently, Nguyen et al 42 presented a new approach for the realization of single-ion conducting polymer electrolytes by polymerizing two different blocks of a poly(arylene ether sulfone) system to yield a lithium-containing multiblock copolymer electrolyte by attaching TFSI-like side chains to the backbone. High mechanical stability (provided by the rigid polymer backbone) allowed for the realization of self-standing membranes with excellent thermal and electrochemical stability, accompanied by very attractive ionic conductivities when swollen with so-called "molecular transporters", such as ethylene carbonate (EC) or propylene carbonate (PC).…”

Photo‐Cross‐Linked Single‐Ion Conducting Polymer Electrolyte for Lithium‐Metal Batteries

“…As a matter of fact, the conductivity at 40 °C is considered to be sufficient for use in electric vehicles when the transference number approaches unity. [ 19 ] Another important parameter determining the charge transport kinetics is the limiting current density, which was determined via linear sweep voltammetry (LSV) yielding 0.68 mA cm −2 at 40 °C (Figure S7, Supporting Information). Additional LSV experiments were performed to evaluate the electrochemical stability (Figure 3b).…”

“…Here, only the Li + cations are (theoretically) mobile-or in other words, for which the Li + transference number (t Li + ) approaches unity, which is beneficial for achieving higher energy and power densities. [16][17][18][19] To date, extensive efforts have been made to increase the ionic conductivity of SIPEs. These include the design of weakly coordinating polyanionic functions with extensive negative charge delocalization, for instance, by introducing strong electron-withdrawing groups or elegantly modifying the anionic centers by coupling Li + with different heteroatoms to facilitate Li + conduction.…”

Ionic Conductivity in Polyfluorene-Based Diblock Copolymers Comprising Nanodomains of a Polymerized Ionic Liquid and a Solid Polymer Electrolyte Doped with LiTFSI

“…However, ionic conductivities in SPEs remain low when compared to liquid electrolytes, implying that further improvements are necessary for achieving the application targets in automobility. 6 Conventional SPEs are dual ionic conductors, where both cations and anions are mobile. Specifically, they are based on the solvation of Li salts in a polymeric host.…”

“…The advantages of solid-state batteries, based on SPEs, are illustrated by their programed application in automobility for the near future. However, ionic conductivities in SPEs remain low when compared to liquid electrolytes, implying that further improvements are necessary for achieving the application targets in automobility …”