The electrochemo‐mechanical effects on the structural integrity of electrode materials during cycling is a non‐negligible factor that affects the cyclability and rate performance of all solid‐state batteries (ASSBs). Herein, combined with in situ electrochemical impedance spectroscopy (EIS), focused ion beam (FIB)–scanning electron microscope (SEM), and solid state nuclear magnetic resonance (ssNMR) techniques, the electrochemical performance and electrochemo‐mechanical behavior are compared of conventional polycrystalline NCM811 (LiNi0.8Co0.1Mn0.1O2), small‐size polycrystalline NCM811 and single‐crystal (S‐) NCM811 in Li10SnP2S12 based ASSBs during long charge–discharge cycles. The results show that the deteriorating performance of both large and small polycrystalline NCM811 originates from their inherent structural instability at >4.15 V, induced by the visible voids between the randomly oriented grains and microcracks due to the electrode pressing process and severe anisotropic volume change during cycling, rather than lithium ion transport in the primary particle. In contrast, S‐NCM811 with good microstructural integrity show remarkably high capacity (187 mAh g−1, 18 mA g−1), stable cyclability (100 cycles, retention of 64.5%), and exceptional rate capability (102 mAh g−1 at 180 mA g−1) in ASSBs even without surface modification. Moreover, 1 wt% LiNbO3@S‐NCM811 further demonstrates excellent initial discharge capacity and capacity retention. This work highlights the critical role of electrochemo‐mechanical integrity and offers an promising path towards mechanically‐reliable cathode materials for ASSBs.

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electric vehicles. [1] The next-generation LIBs must meet a multitude of stringent requirements, including excellent cycle stability, high capacity, high energy and power densities, high safety, and low cost. [2] As an alternative to the graphite anode for commercial LIBs, Si has attracted considerable attention, due to its high theoretical capacity of 3579 mA h g −1 , low operating potential, abundance, and low cost. However, the practical applications of Si-based anode materials are hindered by low Li + diffusivity and electrical conductivity and especially by drastic volume changes (≈300%) during lithiation/delithiation. The latter problem can cause Si pulverization, loss of electrical contact, and consumption of active Li associated with the unstable evolution of solid electrolyte interphases (SEIs). As a result, Si-based anodes generally exhibit low Coulombic efficiency, poor cycle stability, and rate capability. [3] To address these problems, one effective approach is to use Si nanoparticles (NPs) for the facile accommodation of large volume changes. But the high specific surface area associated with nanomaterials causes aggregation of Si NPs during cycling and provides plenty of active surface sites to form SEIs. Moreover, repeated expansion and contraction of Si NPs induce fracture and uncontrolled formation of Si/C composites represent one promising class of anode materials for next-generation lithium-ion batteries. To achieve high performances ofSi-based anodes, it is critical to control the surface oxide of Si particles, so as to harness the chemomechanical confinement effect of surface oxide on the large volume changes of Si particles during lithiation/delithiation. Here a systematic study of Si@SiO x /C nanocomposite electrodes consisting of Si nanoparticles covered by a thin layer of surface oxide with a tunable thickness in the range of 1-10 nm is reported. It is shown that the oxidation temperature and time not only control the thickness of the surface oxide, but also change the structure and valence state of Si in the surface oxide. These factors can have a strong influence on the lithiation/delithiation behavior of Si nanoparticles, leading to different electrochemical performances. By combining experimental and modeling studies, an optimal thickness of about 5 nm for the surface oxide layer of Si nanoparticles is identified, which enables a combination of high capacity and long cycle stability of the Si@SiO x /C nanocomposite anodes. This work provides an in-depth understanding of the effects of surface oxide on the Si/C nanocomposite electrodes. Insights gained are important for the design of high-performance Si/C composite electrodes.