There is not a universal lithium ion cell... only one that suits your needs the most

CUSTOMCELLSCUSTOMCELLS will develop that cell for you. The imperative to decarbonize the transport sector requires rethinking and redesign of the powertrain towards electric propulsion. The announced electric vehicle roadmaps of the OEMs show a large variety of models in the upcoming years, especially in the field of long-distance battery electric vehicles (BEVs). The success of this redesign process, in particular reaching fully battery electric driving ranges of more than 500 km, is thereby heavily influenced by the further improvement of the lithium ion cell towards higher volumetric energy densities (the amount of energy that is stored per unit volume [Wh L-1]), fast-charging ability and reduced costs [€/kWh], while other key performance parameters, such as cycle and shelf life, safety and low-temperature performance also need to be considered.

In general, the term lithium ion cell comprises a vast range of different positive and negative electrode active material combinations, which contribute to the energy storage related redox chemistry of the cell. Together with their flexibility in terms of cell design, electrode structure and design, lithium ion cells can be constructed to meet a broad range of power to energy ratios (P/E), thus allowing the application in all kinds of electric vehicles with different P/E ratios, such as hybrid (HEV, P/E ≈ 15), plug-in hybrid (PHEV, P/E ≈ 8) and fully battery electric (BEV, P/E ≈ 3). Thereby, each individual lithium ion cell is composed of multiple stacks of negative electrode, separator and positive electrode.

The electrode active materials, which are able to reversibly accommodate and release lithium ions, are embedded in a highly porous composition of binders and conductive additives, which is coated on thin current collectors whereas copper foil (6-14 µm) is preferably used for the negative electrode and aluminum foil (12-20 µm) for the positive electrode.

CUSTOMCELLSThe separator is a porous polymeric membrane with a thickness of 12-25 µm, thus acting as an electronic insulator between both electrodes. The pores of electrodes and separator are soaked with an electrolyte, a complex mixture of lithium salts, liquid organic solvents and performance additives, to ensure the fast transfer of lithium ions within the cell. Lithium ion cells are constructed in the discharged stage, where all the active lithium ions (the fuel of the cell) are located in the positive electrode. During charge of the lithium ion cell, the negative electrode is forced to accommodate electrons from the positive electrode, which flow through the external current circuit and simultaneously lithium ions, which are released at the positive electrode into the electrolyte and migrate to the negative electrode. In case of the discharge process, the aforementioned processes are inverted.

Considering that wide market penetration of BEVs, is expected to be reached at fully battery electric driving ranges of more than 500 km, the next generation of cells should have volumetric energy densities of at least 750 Wh L-1. Thereby, the increase in energy density at cell level leads not only to greater vehicle ranges, but at the same time has the potential to reduce cost as a result of material savings per kWh. In general, there are three approaches to increase the volumetric energy densities at cell level.

Firstly, to increase the energy content per volume of each active material of both, positive and negative electrode. In this regard, the current R&D focuses on the increase in nickel-content of transition metal oxides as positive electrode materials, such as lithium-nickel-aluminum-oxide (NCA) or lithium-nickel-manganese-cobalt-oxides (NMC), or to increase the operational voltage at cell level by charging those materials to higher cut-off limits thus extracting more lithium ions from the structure. In case of NCA electrodes charged to currently employed limits of 4.3 V, the amount of extracted lithium is 72%. Whereas the focus of the R&D with regard to the negative electrode is laid on the use of silicon and its derivatives or lithium metal as active materials.

Secondly, to decrease the amount and occupied volume of inactive materials within the cell, e.g., by coating of ultra-thick electrode mass loadings (reduced amount of current collector), by increasing the content of the active materials in the electrode coatings (reduced amount of binder and conductive additive), or by decreasing the thickness of the separator, etc.

Thirdly, to reduce or compensate the irreversible losses of active lithium ions (stemming from the cathode reservoir) during operation of the lithium ion cell, e.g., by optimization of the electrolyte or by pre-lithiation of the negative electrode prior to assembly of the lithium ion cell.

CUSTOMCELLS is a technical development partner along the whole value chain of the lithium ion battery as well as small series producer of customized lithium ion pouch cells with an output of up to 0.2 GWh per year. CUSTOMCELLS was founded in 2012 as a spin-off from the Fraunhofer ISIT in Itzehoe, Germany and grew with over 900 different projects from a spin-off to a well-known partner for automotive, aviation, medical, security and niche markets.

CUSTOMCELLS develops and produces tailor-made cells suiting exactly toward the customer’s application in terms of space requirements, cell design or other key performance requirements as schematically depicted in Figure 2. Thereby, CUSTOMCELLS accompanies the way of development from sketch, material selection and procurement, slurry development, electrode coating, prototype cell assembly, cell testing up to prototype battery system construction and small series cell production.

CUSTOMCELLSWith more than 250 different raw materials on stock and a worldwide supplier network CUSTOMCELLS is able to realize new cell ideas or new materials to prototype level within 2-3 months. For the challenging development of cells with volumetric energy densities of at least 750 Wh L-1 for the application as traction batteries the appropriate selection of the active materials with regard to crystal structure and composition, particle morphology, particle size distribution, specific surface area, etc. is highly important. CUSTOMCELLS infrastructure offers numerous mixing devices from 1 liter up to 200 liters and state-of-the-art coating equipment from laboratory batch coating up to roll-to-roll pilot coating thus allowing for slurry recipe development and optimization as well as electrode optimization.

The optimized negative electrode coils (in particular based on silicon) then can be shipped to a strategic partner to perform a safe, low-cost roll-to-roll electrochemical pre-lithiation process. At the same time, the electrolyte formulation can be optimized and produced together with the strategic partner E-Lyte Innovations that uses a high-throughput-screening approach, which is in particular necessary for silicon based negative electrodes and nickel-rich positive electrodes. Afterwards, the electrodes and electrolyte are assembled in the CUSTOMCELLS infrastructure into prototype test cells to perform electrochemical or safety tests. After this initial development process, CUSTOMCELLS is able to transfer the developed cell design to its series production infrastructure and produce those cells in series quality. 

CUSTOMCELLS will be exhibiting at The Battery Show Europe 2019 at stand 1066.