Unveiling the Degradation Mechanisms of Lithium Batteries Through Lithium Inventory Quantification

There has been growing interest in energy storage technologies, particularly in lithium (Li)-ion/lithium metal batteries, due to the accelerated transition to renewable energy systems worldwide. Accurate diagnosis and understanding of the attenuation mechanisms of batteries are essential for their development. It has been widely proven that, in practical fuel-cell systems, the cross-talk between the cathode and anode impacts cycling performance. Additionally, electrolyte depletion during cycling also affects battery life. Therefore, it is crucial to develop analytical methodologies to assess changes in the Li inventory within each component. For full-cell studies, key questions include: Where is the active Li after cycling? How much has become inactive Li? In conventional Li-ion batteries, graphite (Gr) and silicon (Si) are used as anode active materials, while transition metal oxides serve as cathode active materials. All active Li originates from the cathode, so the loss of active Li is directly related to capacity loss, making it easier to evaluate cycle life based on coulombic efficiency. After cycling, the active Li remains in the cathode and electrolyte at the discharge state, while the Li on the anode side is primarily inactive. In Li metal batteries, active Li is present in both the anode and cathode before cycling. The additional Li reservoir in the anode compensates for active Li loss during cycling. Thus, assessing the cycle life of Li metal batteries is challenging because coulombic efficiency only partially correlates with active Li loss. To further reveal the full cell system's degradation mechanism, X-ray Diffraction (XRD) was applied to the cathode to quantify the active Li. Titration Gas Chromatography (TGC) was used to quantify the inactive Li in the anode, and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was applied to evaluate the Li+ concentration in the electrolyte upon cycling. We demonstrate the importance of quantitatively examining Li inventory changes in the full cell system. Furthermore, the results provide unique insights into identifying critical bottlenecks facilitating battery performance development.