**4. Conclusions**

By X-ray tomography, we have analyzed structural and morphological changes in rechargeable alkaline-manganese batteries, and in non-rechargeable primary cells, during repeated charge and discharge. We applied three different discharging currents (100 mA, 200 mA, and 400 mA) to the RAM cells. The size distributions of the zinc particles were calculated and compared (Figure 5). We found that first the smaller zinc particles disappear and after about 10 cycles also the larger ones dissolve. The degree and pace of dissolution differs between various locations in the cell. The zinc particles dissolve layer by layer and become increasingly smaller.

The structural changes in the primary alkaline cell are different than in the RAM cell. After the first discharge of the primary alkaline cell the overall zinc particle volume remains almost constant during cycling. Furthermore we found that new zinc particles are formed after cycling (see Supplementary Video 1). For the first few cycles, the primary alkaline battery performed well, and provided more capacity than the RAM battery. The total failure of the primary cell came suddenly and unexpectedly without any signs (see Figure 2a), may have been caused by separator penetration from one of the needle-like structures formed at the electrodes.

We think that the analysis and the corresponding results presented here can significantly contribute to the fundamental understanding and development of rechargeable alkaline batteries and of zinc-based batteries in general. In the future, similar studies might be done on other zinc-based systems such as zinc-air batteries, and they may contribute to the development not only of RAM cells with increased durability, but also of rechargeable zinc-air batteries.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/11/9/1486/ s1, Video S1: Tomographic cross sections of the evolution of the RAM cell that was cycled with 200 mA.

**Author Contributions:** Conceptualization, M.O. and I.M.; Methodology, M.O. and A.H.; Software, M.O. and A.H.; Validation, M.O., A.H., N.K. and I.M.; Formal Analysis, M.O.; Investigation, M.O.; Resources, A.H. and I.M.; Data Curation, M.O.; Writing-Original Draft Preparation, M.O. and I.M.; Writing-Review & Editing, N.K., A.H. and J.B.; Visualization, M.O.; Supervision, A.H. and I.M.; Project Administration, I.M. and J.B.; Funding Acquisition, J.B.

**Funding:** We acknowledge support by the German Research Foundation (DFG) and the Open Access Publication Funds of Technische Universität Berlin.

**Conflicts of Interest:** The authors declare no conflicts of interest.
