**1. Introduction**

The development of new energy storage materials and systems is currently one of the most important challenges in materials research. Batteries play a crucial role in the future replacement of conventional mobile or stationary energy sources based on fossil fuels. However, batteries with high storage capacities and low weights are still by far too expensive. Furthermore, the general shortage in various resources puts constraints on the development of many battery types. Therefore, the development of cost-efficient production methods and use of easily accessible raw materials are key issues in battery research.

Zinc is a widely available and inexpensive material, and it is a candidate for future use in rechargeable batteries for mobile and stationary applications [1–7]. The well-known alkaline-manganese battery is still one of the most common types in use [1]. Reasons include their low self-discharge and environmental friendliness compared to other battery types. Such batteries are cheap to produce, maintenance-free, and safe compared to lithium-based batteries. In addition, in the charged state they provide a voltage of 1.5 V, which is higher than many other (e.g., nickel-metal hydride) batteries. Their main disadvantage is that they are normally designed as primary cells, i.e., they are not rechargeable.

Because of the many fundamental advantages of alkaline-manganese batteries, much effort has been put into developing and optimizing primary cells and, even more important for future applications, developing rechargeable alkaline-manganese batteries (RAM) [8–13]. Up to now, RAM still suffer from an unreliable cyclic behavior. Some individual batteries can be recharged up to 500 times, while others last only a few cycles.

In the past, various methods have been applied to study alkaline primary cells. For the investigation of the zinc particles, electron microscopy and optical microscopy have been used [14–16]. Preparation of the samples is very difficult because the oxidation and corrosion of Zn, and the carbonation of ZnO alter the structure of the material. Horn et al. have developed a dedicated preparation technique [14]. However, all these measurement techniques do not allow for an in situ study of the material inside the entire volume of the battery. Only the sectioned material is accessible.

Imaging techniques based on X-rays have been successfully used to study battery materials [17–29]. Since these techniques are non-destructive and non-invasive, they are especially suited for in situ or in operando measurements [30–33]. X-ray tomography using both table-top and synchrotron radiation sources was used to investigate alkaline primary cells and zinc-air batteries in three dimensions [34–37]. Moreover, neutron imaging has been used to investigate alkaline primary cells [34,38].

In this paper, structural changes in RAM cells were examined in situ and non-destructively by X-ray tomography.

#### **2. Experimental Set-Up and Data Processing**

#### *2.1. The Alkaline-Manganese Battery*
