In 25 years dedicated to researching materials, we learned much about the relationships between structure and powder morphology. With this insight, we are able to attain the desired functions and properties in materials. Our scientists are now busy exploring new cathode materials such as high-voltage spinels and lithium transition metal phosphates and silicates as well as anode materials such as optimized carbon modifications, titanates, and alloy anodes for lithium-ion batteries. Intensive research into new electrolyte systems with special additives and into electrode materials for future lithium-sulfur and lithium-air batteries is also underway.
Owing to their high energy density, lithium-ion batteries are regarded as the most promising energy storage medium for use in vehicles with hybrid or fully electrical drive systems. Commercial lithium-ion batteries primarily use storage materials such as LiMn2O4 (LMS), LiNi0.33Mn0.33Co0.33O2 (NMC), LiNi0.80Co0.15Al0.05O2 (NCA) and LiFePO4 (LFP).
Key factors in their further development include reducing costs, increasing the energy density and safety, the availability of the raw materials and environmental compatibility. Cobalt, in particular, is considered to be a critical raw material in terms of availability and cost. There are various crystal chemical approaches for increasing the energy density. They are based on increasing the operating voltage or the capacity.
At ZSW, a material is being developed that is capable of using both attributes. The material is cobalt-free, works at two different voltage levels with 4.7 and 2.9 V vs. Li/Li+, and provides a total capacity of 210 mAh/g with good stability. In order to fulfil the specifications of established cathode materials, the particle shape, size and distribution were optimised (see images). The powder exhibits a tapped density of 2.4 g/cm3.
Our core competencies include the synthesis of new chemical compounds as active materials for lithium-ion batteries, as well as tailor-made powders and pastes for electrode coating. Our strengths lie in our in-depth understanding of the relationships between structure and powder morphology on the one hand and the desired functional and processing properties on the other, which we have developed over a period of more than 25 years.
In order to develop new materials for batteries and supercapacitors we use both established processes and new methods. We focus on optimising the powder morphology and particle size. Our continuous powder production in our laboratories enables us to produce anything from just a few grams to several kilograms of powder. We also carry out statistical test planning.
Milling processes provide another important working step because these can decisively influence the morphology of the material. In addition to various bead mills, we are also equipped with a multi-process system that enables industrial milling processes (e.g. impact milling, jet milling or spiral jet milling in combination with screening) to be reproduced in the laboratory.
As well as synthesising inorganic materials, we also perform synthesising on organic materials for electrolytes and electrodes in inert gas using the Schlenk apparatus.
For lithium-ion batteries, the electrochemical behaviour of storage materials greatly depends on their chemical composition and crystalline structure. The shape, size distribution and surface properties of the microscopic storage particles considerably determine their processing behaviour and therefore their usability in established technical battery production processes. In this regard, our core tasks in the research projects include creating a complete property profile for the materials, identifying influencing factors as well as deducing and understanding the relationships between structures and properties.
When in use, the performance and storage capacity of batteries progressively decline. These ageing effects can be caused by chemical corrosion processes on the materials and components or by mechanical degradation. Our aim is to investigate and understand the ageing effects in order to directly use the acquired knowledge to optimise materials and develop cells.
We are equipped with comprehensive analytical facilities to enable us to efficiently implement these research and development tasks. In addition to standard chemical analysis methods (ICP-OES), structure determination (XRD), microscopy (SEM, EDX) and surface area analysis (BET), we can also carry out deep profile analyses at a high resolution.
We investigate the decomposition behaviour at high temperatures using thermoanalytic methods such as TG/DSC-MS and ARC. This provides valuable insights into the safety properties of the materials and cells.
We are conducting tests on thermal stability of batteries and battery materials in an accelerating rate calorimeter (ARC). With this method we determine the onset of thermal runaway of cells as well as exothermic reactions of battery materials. During the ARC tests, we have the possibility of simultaneous measurements of pressure and gas analysis.
ARC gives important information on safety behavior of cells. As an example, in an ARC study involving commercial 18650-type cells, we found that the temperature of thermal runaway can be drastically reduced if metallic lithium is present inside the cells.
Dr. Thomas Waldmann
Telefon: +49 (0)731 95 30 212
A central focus is also the electrochemical characterisation of materials. In addition to galvanostatic cycle tests, cyclic voltammetry and impedance spectroscopy, further electroanalytic investigations are undertaken, for example using voltametric methods and rotating (ring) disk electrodes.
During charging and discharging, dynamic changes occur in the electrodes and electrolytes. For example, phase changes occur, passivated surface layers are formed or electrolytes are decomposed. In order to understand these phenomena in-situ and operando, ZSW has at its disposal several in-situ electrochemical methods such as in-situ XRD, in-situ FTIR spectroscopy, in-situ Raman microscopy, in-situ dilatometry and in-situ pressure measurements. New methods are constantly being developed.
Combining in-situ investigations of the materials in model cells and ex-situ investigations of the materials from real cells enables us to develop a comprehensive understanding of the cycle and ageing behaviour of electrodes and electrolytes.
An important issue in the development of Lithium-ion cells is deposition of metallic lithium on anodes. Lithium deposition leads to faster aging as well as to decreased safety properties. In order to determine for which cell chemistry and for which operating conditions lithium deposition is expected, Dr. Wohlfahrt-Mehrens’ group developed a method to build full cells with an additional reference electrode. This know-how is complementary to our investigations on aging mechanisms by cell opening and Post-Mortem analysis.
Electrode pairs from our coating line as well as from commercial Lithium-ion cells can be built into full cells with one or more reference electrodes (see Figure below). In such cells, we are able to gain information on favorable operating conditions for cells under development or commercial cells. Using this method, we recently deduced an optimized charging procedure which allowed increasing the cycle life of commercial high-energy 18650-type cells significantly by minimizing the deposition of metallic lithium.