Low-Molecular-Weight Aliphatic Polycarbonate-Based Polymer Electrolytes for Advanced High-Voltage Lithium Batteries
Authors: Nguyen Huu-Dat , Bruno Laura , Bernard Laurent , Dominget Alexandre , Dumont Erwan , Barchasz Céline , Gutel Thibaut , Picard Lionel 
Published: July 2021

Solid-state batteries combining Li-metal anode and high-voltage cathodes are expected to be the key to surpass the limitations of current Li-ion batteries in terms of energy density, safety and lifetime.[1] However, finding successful solid polymer electrolytes (SPEs) for such high-energy batteries is utmost challenging. Poly(ethylene oxide) (PEO)-based materials was widely considered as promising candidates due to high safety, low cost, and excellent compatibility with lithium metal as well as lithium salts.[2] However, they have not satisfied the practical requirements due to (i) the high crystallinity of the ethylene oxide chains leading to insufficient ionic conductivity at low temperature and (ii) a very low Li+ transport number (t+~0.1-0.3).[3] Thus, the cells comprising PEO-based electrolytes must be operated at elevated temperatures (at least 60 °C). Furthermore, these PEO-based electrolytes are limited to a maximum anodic cut-off voltage of 3.9 V vs. Li|Li+, which prevents the use of high-voltage cathode materials like LiCoO2 (LCO) or Li[Ni0.6Co0.2Mn0.2]O2 (NCM622), etc., reversibly hosting lithium well above 4.0 V vs. Li|Li+.[4]

For these reasons, much effort has been dedicated to developing alternative polymer systems.[5] Currently, aliphatic polycarbonates (APCs) and their copolymers have attracted much attention due to high dielectric constant, high Li+ transport number (t+>0.5), wide electrochemical stability window (ESW), etc.[6] However, it was found that the ionic conductivity and the electrochemical stability of these polymers are highly dependent on their molecular weight (MW) and synthesis method. Thus, high-MW APCs show an ESW up to 5.0 V vs. Li|Li+, but their ionic conductivity (~10−6-10−7 S cm−1 at 60 °C)[7] is far below the limit for practical application, which is around 10-4 S cm-1 at room temperature (RT). To increase the ionic conductivity, it is necessary to reduce the MW of the polymer to boost the segmental motion. As expected, APCs with lower MW showed significantly improved conductivity (~10−4-10−5 S cm−1 at RT). Unfortunately, they have much lower electrochemical stability (up to 4.0 V vs. Li|Li+)[8] probably derived from the increased number of OH groups at the end of polymer chains, which were determined as the limiting factor of polymer electrolytes due to their high reactivity with electrode materials.[9]

Here we show that the electrochemical stability of Low-MW poly(trimethylene carbonate) (PTMC) and statistic copolymer of trimethylene carbonate and ε-caprolactone (PTMC-PCL copolymer) was remarkably improved via protecting these end-chain OH groups using two different protecting agents. The ester-protected PTMC and PTMC-PCL copolymer showed much improved lithium stripping/plating behavior due to higher stability with lithium anode while the urethane-protected ones are more stable towards oxidation at high potential (Figure 1). The obtained results reveal that the polymer electrolytes based on protected PTMC and PTMC-PCL copolymers are promising for high-energy lithium-metal batteries using high-voltage cathodes due to both high ionic conductivity and high electrochemical stability.

Keyword: poly(trimethylene carbonate), PTMC, OH protection, polymer electrolyte, lithium battery

References

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Figure 1