Beyond PEO: New Polymer Electrolytes for Safe All Solid-State Lithium Metal Batteries
Authors: Aldalur Itziar , Meabe Leire , Boaretto Nicola , Zugazua Oihane , Lobato Elias , Sanchez-Diez Eduardo , Martinez-Ibañez María , Armand Michel 
Published: July 2021

In a world with a growing demand in high energy storage systems and the challenge of finding new green energy sources to replace the excessively used fossil fuels, electrochemical energy storage arises as the key alternative to meet the needs of current society. In particular, lithium batteries stand out among all the available energy-storage technologies. For the last two decades, lithium-ion batteries (LIBs) have become the dominative components in portable electronic devices. However, the insufficient energy density of the state of art of LIBs has tipped the scale in favor of the search of all solid-state lithium metal (Lio) batteries (ASSLMBs) where a higher energy density can be achieved due to the high specific capacity provided by Lio electrode.
As one of the most critical components, the choice of the electrolyte plays a pivotal role for preparing safe and high performance LMBs. Most of the commercial lithium batteries are built up with liquid electrolytes, which entails potential security risks such as volatilization, flammability and explosion. For this reason, numerous research efforts are focused on the obtaining of solvent-free solid electrolytes. Solid polymer electrolytes (SPEs) are considered as one of the most viable solutions to replace their liquid counterparts. In addition to the excellent flexibility, ease of processing and low cost, SPEs can mitigate the Li dendrite growth that takes place in conventional liquid electrolytes, attenuate the interfacial resistance, and improve the electrode-electrolyte compatibility compared to their inorganic counterparts.
Among all the existing polymer hosts for SPEs, poly(ethylene oxide) (PEO) is the most commonly used one due to its strong solvation ability that facilitates the dissociation of various lithium salts. However, PEO lacks some essential requirements when considering it as an ideal host material for SPE; namely the low Li-ion conductivity at ambient temperature due to its crystalline nature. Thus, the low Li-ion conductivity of SPEs at ambient temperature remains as the major hindrance towards the practical deployment of lithium polymer batteries.1, 2

In order to overcome this drawback, tremendous efforts have been dedicated to the design and synthesis of new polymeric matrices that allow obtaining SPEs with improved ionic conductivity at room temperature, where modification such as cross-linking or copolymerization, or the use of low molecular weight oligomers are the main used strategies. In addition, the anion concentration gradient that takes place in conventional dual-ion conductors is one of the factors responsible for low Li-ion transference number (T
Li
+ < 0.5) and consequently are more susceptible to polarization phenomena that eventually limit the power density and cycle life of lithium batteries. The main strategy to go through this polarization problem is the development of single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs), where the anion is immobilized by different means.3

Inspired by previously stated challenges, in the present work we will present the performance of novel flexible and highly conductive SPEs in order to decrease the working temperature. Moreover, the role of different additives on the performance of new SLIC-SPEs is provided.

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