CMC Binding Agent for Silicon Anodes in Li-Ion Batteries : The Impact of the Formulation

Abstract

Li-ion batteries using silicon (Si) as an anodic electroactive material are promising devices that are currently being investigated. However, they have a major disadvantage, which is the large volume variation (about 300%) associated with the formation of the Li-Si alloy, this expansion leading to a rapid loss of cell capacity. The usual solution to this problem is the use of a polymeric binder to maintain the cohesion of the electrode during electrochemical cycling. Carboxymethylcellulose (CMC) [1], a semi-synthetic material, has been and is still one of the most studied and used polymers for this purpose. Thus, the formulation of the negative electrode requires a mixture of silicon, CMC and a carbon additive with the "SPEX" ball milling [2] to ensure good electrical conductivity within the composite mixture. This type of mixture, which is highly energetic, reduces the size of the particles and allows the formation of a nanostructured mixture, is essential for the activation of silicon. However, as we have very recently shown in the case of the use of polyacrylic acid (PAA) as a binder [3], it also causes significant degradation of polymer chains. The objective of the work presented here is to study the effect of this mechanical grinding on CMC, in particular the effect on average molecular weights and chemical degradation and, hence, the influence of the characteristics of this modified polysaccharide on the electrochemical performance of the composite negative electrode. To answer this question, we selected three types of commercial CMCs with different molecular weights (90, 250, 700) kg/mol [4]. The formulation of the negative electrode is then modified to better define the impact of the actual molar masses on silicon capacity retention. CMC is no longer mixed with the other components in the SPEX mill, which is too energetic for the polymer binder. This new formulation allows to see the effect of grinding on CMC and to discuss the impact of molar mass variation on capacity retention. A study on the effect of grinding was therefore carried out using different characterization techniques (SEC-MALLS, FTIR, MALDI-TOF, TGA-MS).