Silicon is considered one of the most promising negative electrode materials for future lithium ion batteries (LIBs) due to its high capacity (3579 mAh g^-1_, based on Li₁₅Si₄) and its proper delithiation voltage (~ca. 0.4 V vs. Li/Li⁺). However, the huge volume change (~300 %) of Si during lithiation/delithiation leads to fracture and pulverization and of Si particles, continuous formation of solid-electrolyte interphase (SEI), and, consequently, fast capacity fade. Mechanical degradation remains a major barrier for commercializing Si electrodes. The most likely commercial Si electrodes, i.e., Si composite electrodes consisting of Si particles, conductive additives, polymeric binders, and porosity, are likely to exhibit mechanical properties and volume expansion/shrinkage behavior that are different from that of individual Si particles and Si thin films. Despite of a growing effort on electro-mechanical modeling of Si electrodes, the mechanical behavior and microstructure evolution of Si composite electrodes during lithiation/delithiation are largely unknown. To fill this gap, we developed an environmental nanoindentation system installed inside an argon-filled glovebox [Adv. Energy Mater., 10.1002/aenm.201702578 (2017) and Scr. Mater. 130, 191 (2017)] to measure Young’s modulus (E) and hardness (H) of Si composite electrodes during lithiation/delithiation under both dry and wet (the liquid electrolyte) conditions. In contrast to Si films, E and H values of Si/polyvinylidene fluoride (PVDF) electrodes and Si/Na-carboxymethyl cellulose (Na-CMC) electrodes, under both dry and wet conditions, increase with increasing Li concentration within each electrochemical cycle and decrease with increasing cycle number. The E and H values of the composite electrodes under dry condition are smaller than that under wet condition due to the softening of polymeric binders in the electrolyte. The porosity of both composite electrodes decreases with increasing Li concentration and increases with increasing cycle number. At the same state-of-charge (SOC), Si/Na-CMC electrodes have smaller porosity and better mechanical integrity than Si/PVDF electrodes, which is one of the reasons why Na-CMC works better than PVDF as a binder. The results show that mechanical integrity and microstructural stability play a vital role in improving the performance and durability of Si composite electrodes. The relationships between E, H, porosity, and SOC is useful to develop and validate electro-mechanical models of electrodes. They also provide valuable insight into developing high capacity Si electrodes. Furthermore, the environmental nanoindentation method can be used to measure the mechanical behavior of a wide range of electrochemical energy storage materials under realistic working conditions.