A Film Maturation Process for Improving the Cycle Life of Calendered Silicon Negative Electrodes

Abstract

Increasing the energy density of Li-ion batteries (LiB) is a key issue. A promising approach is to replace graphite in LIB anodes with silicon. The main challenge is to deal with the large silicon volume expansion induced by its lithiation, which damages the mechanical integrity (electronic network) of the electrode, and produces an unstable solid electrolyte interphase (SEI). We have successfully improved the performance of silicon based anodes by working on various aspects. First, our ball-milled silicon offers the right nanostructure to limit Si particle cracking in addition to be produced using an industrially viable process [1]. Second, the use of a mixture of carboxymethylcellulose (CMC) and citric acid (CA) as binder system buffer favors the formation of strong bonds between the binder and the Si particles [2] and a protective layer on the Si nanoparticles that significantly decreases electrolyte reduction [3]. Lastly, the use of carbon nanoplatelets as conductive additive insures better ability of the electrode architecture to reversibly expand/contract upon cycling [4]. In this communication, we would like to present a postprocessing treatment (called maturation) that we have recently developed. Indeed, maturation very significantly improves the mechanical and electrochemical stabilities of silicon electrodes made with the CMC/CA binder [5]. This treatment consists of storing the electrode in a humid atmosphere for a few days before drying and cell assembly. This results in a beneficial in situ reactive modification of the interfaces within the electrode. Our investigations suggest that the binder tends to concentrate at the silicon interparticle contacts. As a result, the cohesion of the composite film is strengthened. Moreover, the corrosion of the copper current collector, inducing the formation of copper carboxylate bonds, improves the adhesion of the composite film. This results in an impressive improvement of the electrode cycle life. The calendering of Si-based electrodes is required to obtain a substantial gain in their volumetric capacity compared to conventional graphite electrode. However, the calendering of silicon/carbon nanoplatelets/CMC/CA electrodes induces a major decrease of their cycling stability. This can be attributed to the rupture of the particle-binder bridges during the calendering, lowering the mechanical strength of the electrode. Interestingly, we found that these cohesive bonds can be restored through the maturation treatment. From in-operando dilatometric experiments, it appears that the volumetric expansion is lower and more reversible than for a standard (not-calendered, not-matured) electrode. As a result, a remarkable improvement of the cycle life is observed [6]. Finally, we found that the maturation process is also efficient for silicon electrodes made with the polyacrylic acid (PAA) binder [7]. Acknowledgements The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC) (grant RGPIN-2016-04524) and Transition Énergétique Québec (TEQ) (grant Techno-0040-0001) for financial support of this work.