New Insights into the Effect of Cellulose-Based Separators on Electrode-Electrolyte Interface in Lithium Metal Batteries
Authors: McRay Hunter Addison , Pereira Drew J. , Jalilvand Golareh 
Published: December 2024

With a surging demand for electrification, lithium-ion batteries (LIBs) featuring higher energy and power densities have become indispensable. Copious research is being conducted on various components of LIBs to achieve cells with superior performance. Among these components, the separator plays a crucial role as it serves as insulation between the positive and negative electrodes while providing mechanical and thermal stability to the cells, thereby ensuring battery safety. In the pursuit of enhancing separators, a sufficient and controllable porosity and increased electrolyte wettability have gained prominence in separator research for high-performance LIBs, reflected in a large increase in research publications on separator technology in the past decade1.
Currently, polyethylene (PE)/polypropylene (PP) membranes, known for their desirable electrochemical stability, have been the favored material for separators. However, these membranes have poor thermal stability, particularly at higher temperatures associated with fast charging rates. This thermal vulnerability can culminate in the decomposition of the separator, introducing a major hazard in the form of electrical shorts that may discharge the battery and lead to battery fires. Thus, separator materials that offer superior safety benefits are needed to ensure safe application of LIBs in next-generation applications2.
Among the proposed materials for next-generation separators, there is a notably strong interest in cellulose-based separators (CBS). The primary advantage of cellulose compared to PE/PP membranes is greater thermal stability, as demonstrated by an initial decomposition temperature of 270°C. This greatly increases the safety of the battery since the risk of battery fires stemming from separator decomposition is reduced significantly. Along with increased thermal stability, several reports have shown enhanced energy density, and longevity in LIBs using CBS because of its desirable electrochemical stability, wettability, and water scavenging capability3,4,5.
In this work, we study the CBS in a Li-metal battery with inherently higher energy density, thus, intensified challenges. While the majority of reports discuss the advantages of CBS, herein, we present new insights into possible drawbacks of CBS and its suitability for highly concentrated Li-based batteries. We investigated a commercial CBS with Li metal anode and NMC cathode materials. The compatibility and performance of CBS in Li metal-based coin cells (symmetrical and asymmetrical) are studied through electrochemical measurement techniques including cyclic voltammetry (CV), pre- and post-cycling electrochemical impedance spectroscopy (EIS) and cycling test. Similar measurements are carried out on cells with conventional PE/PP separator. Surface and morphological characterizations are conducted on the CBS, Li metal anode, and cathode to provide a more comprehensive view of the CBS interaction with Li metal and NMC cathode. It was observed that while CBS results in a more stable and uniform solid electrolyte interphase (SEI) on Li metal, it is incompetent in impeding Li dendrites. A potential method to reach a clearer understanding of the underlying mechanisms, involving the introduction of oxide coatings to the CBS separator, is explored. It is expected that this study will provide a fundamental understanding of the effects of cellulose on Li metal battery and further clarify its potential for next generation LIBs.
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