Designing smaller, safer, cheaper, and more stable batteries necessitates thorough understanding of the electrochemical processes governing their operation at multiple length scales. In all-solid-state power sources the electrolyte is no-flammable and can be made ultimately thin, making them safer and more compact. Yet, experimental evidence suggests that the multiple internal interfaces in their layered structure give rise to high impedances, limiting their performance. Studying these nanoscopic interfaces requires microscopic tools. Here, we employ in operando Kelvin Probe Force Microscopy (KPFM) to quantitatively measure the potential distribution in a solid-state Li-ion battery as a function of its state of charge. The battery was fabricated by sequentially depositing thin layers of Pt (110-130 nm), LiCoO_2 (280-420 nm), LIPON (1100-1200 nm), Si (50-240 nm) Cu or Pt (150-200 nm) onto a Si/SiO_2 wafer (oxide thickness 100 nm). The fabricated battery was cleaved in an Ar atmosphere to expose the stacked layers, mounted on a holder, wired, and safely transferred without exposing to air into a dual-beam instrument that combines a scanning electron microscope (SEM), a Ga-ion focused ion beam (FIB) and an atomic force microscope (AFM) in one vacuum chamber (residual pressure of 10~(-4) Pa). The stacked battery was milled to expose a cross-section of the layers, and imaged using SEM and KPFM, while cycling the battery. The acquired potential maps reveal a highly non-uniform interelectrode potential distribution, with most of the potential drop occurring at the electrolyte-Si anode interface in the pristine battery. During the first charge, the potential distribution gradually changes, revealing complex polarization within the LIPON layer due to Li-ion redistribution. Cycling the battery at high rate significantly decreased its capacity, although the capacity loss can be recovered. KPFM imaging allowed the detection of the interface responsible for this capacity loss. Li distribution in the battery was also measured using Neutron Depth Profiling as a function of the state of charge. The acquired data was compared to first principles calculations shedding light onto the interfacial Li-ion transport in the battery and its reversibility.
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