1, Helmholtz Institute Münster HI MS, Forschungszentrum Juelich GmbH, Münster, NRW, Germany
The production of cost effective, high energy and environmentally friendly electrodes is key for the success of lithium ion batteries (LIBs) to be used in electro-mobility or grid storage. To overcome the remaining drawbacks new ways towards cost effective processing of LIB electrodes and higher energy density battery cells are main topics of nowadays research. One way to combine both requirements is the preparation of ultra-thick electrodes using aqueous processing: Thick dry film thicknesses (large active mass loadings) allow an increase in energy density, while the relative reduction of copper and aluminum current collectors reduces the costs. However, compared to conventional processing, aqueous electrode formulation also goes along with disadvantages like surface crack formation and basic pH values accelerating active material degradation, especially in case of cathode materials like NMC (LiNixCoyMnyO2, with x ≥0,33 and y,z ≤ 0.33). Our approach to successfully overcome mechanical electrode instability is to develop a suitable “binder package” combining the water soluble components polyacrylic acid (PAA), polyethylene oxide (PEO) and sodium-carboxymethylcellulose (CMC). This mixture combines high flexibility (PEO) with low viscosity (PAA) and sufficient adhesion (CMC) resulting in a superior water compatible binder system to manufacture ultra-thick cathode sheets. Using this binder composition it was possible to achieve NMC based cathodes free from cracks with high amounts of active material and active mass loadings up to 50 mg cm-2 delivering discharge capacities of 120 mAh g-1 at 0.2 C.
A further issue, which decreases the gravimetric energy and volumetric energy density of LIBs is active lithium loss, caused by solid electrolyte interphase formation on the surface of the anode, occurring mainly in the first cycle of operation. In order to compensate the loss of active lithium, various pre-lithiation methods have been developed that result in an increased reversible capacity and, consequently, in a higher gravimetric energy and volumetric energy density. Thereby, the term “prelithiation”, describes the addition of lithium to the active lithium content (= reversibly transferable lithium ions between positive and negative electrode) of a LIB prior to battery cell operation. Therefore, with help of prelithiation it is possible to use novel active materials inside LIB full cells which otherwise could not be utilized due to high active lithium losses.