Lithium-ion battery technology remains to be the heart in operating the majority of new commercial mobile devices and electrical vehicles. Thus, even though it is more than 25 years ago Sony introduced the world's first commercial Li-ion battery, there is still today a demand for cheaper and safer battery materials with higher energy density. In order to meet this demand, an increased fundamental understanding of the detailed structural and microstructural transformations and electrochemical reactions taking place in Li-ion batteries is needed. For this purpose we have developed special capillary-based micro-battery cells allowing diffraction information to be obtained from only the active electrode material during battery operation. We have used the cell to study structural and microstructural changes during battery operation by in situ / operando synchrotron X-ray powder diffraction (XRPD). Operando XRPD studies of both cathode and anode materials will be presented as examples of the applicability of the micro-battery cell design, e.g. the initial intercalation and deintercalation of lithium into graphite. The study revealed remarkable changes in the stacking order and disorder in the graphitic electrode material during charging and discharging. Another example will be an operando XRPD study of a high-capacity lithium-vanadium oxyfluoride cathode material.