Deciphering the Impact of the Active Lithium Reservoir in Anode-Free Pouch Cells
Authors: Gervillie Charlotte , Ah Louis , Liu Alex Ruili , Huang Chen-Jui , Meng Shirley 
Published: December 2024

Anode-free batteries, which revolutionize energy storage by discarding traditional anodes in favor of copper foil to plate lithium directly from the cathode, offer increased energy densities and better safety than conventional lithium-metal cells. However, their advantage is tempered by a significantly reduced cycle life, attributed to lithium loss through parasitic reactions. As a result, inactive (‘dead’) lithium accumulates over cycling, including (electro)chemically formed Li+ compounds in the solid electrolyte interphase (SEI) and isolated unreacted metallic Li0, resulting in capacity loss and safety hazards. Consequently, to continue enhancing the performance of anode-free cells, it appears crucial to differentiate and quantify the various forms of lithium in these cells and monitor their distribution and evolution throughout the cycling process, depending on various conditions such as pressure or electrolytes.

Compared to lithium metal cells, the problem might appear simplified for anode-free cells as there is no active lithium compensation from the anode, and all the active lithium is initially stored within the cathode. However, the intricate interplay of the cathode's first-cycle irreversibility and lithium plating/stripping efficiency significantly influences the shape of the capacity retention curves and the measurements of coulombic efficiency (CE). Herein we aim to clarify the contribution of the lithium reservoir to the anode-free cell performances. We report a systematic study of the active lithium reservoir and unreacted metallic Li0 evolution in anode-free LiNi0.6Mn0.2Co0.2O2 (NMC622)||Copper (Cu) commercial pouch cells. By taking advantage of a discharge characteristic of the NMC622 cathode at low voltage (<1.5 V), we could quantify the presence of remaining active lithium at the anode. Afterwards, titration gas chromatography was utilized to measure the remaining inactive Li0 on the discharged samples. By coupling the quantification techniques to observations of the anode local microstructure by cryogenic scanning electron microscopy, we elucidate the formation and evolution mechanism of the lithium reservoir and inactive metallic lithium in different types of electrolytes.

As a result, we demonstrated that the once-considered drawback of Ni-rich layered oxide cathodes, namely the first cycled intrinsic irreversible capacity, can be manipulated to build a lithium reservoir at the anode and extend the cycle life of anode-free cells (see Figure 1, bottom left). Additionally, the formerly unclear reason for the capacity degradation discrepancy of anode-free cells under different C-rates was investigated and attributed to the correlation between lithium utilization and reservoirs (see Figure 1, bottom right). In contrast to the first-cycle irreversibility unique to certain types of cathode materials, this approach can be applied to all types of cells having polarization phenomena at high currents to enhance their longevity in anode-free configurations. Consequently, by employing protocols with slow charge and fast discharge, the lithium reservoir at the anode is maximized, and the performances of the anode-free cells appear stable for a longer number of cycles. With this knowledge, one can regulate the ratio between lithium utilization and lithium reservoirs for extended capacity retention or high initial reversible capacity designated for different applications. We believe the concept of this Li reservoir can be further extended to other approaches and opens new opportunities, taking advantage of cathode intrinsic irreversibility and kinetic limitations to extend anode-free cells’ lifespan.

Figure 1