TROPIC

Developing electrochemical energy storage systems alternative to the Li-ion technology remains a mandatory challenge in the context of renewable energy development requiring large-scale storage systems for which cost is the dominant factor and lithium supply a possible issue. Low cost and abundant materials are therefore needed. Consequently, Na-ion batteries (NIB) and more recently K-ion batteries (KIB) have emerged. KIB could even be more interesting due to the beneficial low standard potential of K+/K, low desolvation energy and small solvated ionic radius of K+ which should increase the energy and power density compared to NIB. The main drawbacks of KIB are, however, the low melting point of K metal and its high reactivity which could lead to safety and lifetime issues. Thus, going from Li to Na to K represents a challenge in terms of both physical and chemical properties. In this context, the overall ambition of TROPIC is to answer the questions: are K-ion systems competitive with LIB and NIB? Are the benefits of K in terms of power density (high ionic conductivity), energy density (low standard potential), abundance and cost, higher than the limitations expected from its higher atomic mass and Shannon’s ionic radius compared to Li+ and Na+? The TROPIC project will prepare KIB combining the most efficient positive and negative electrodes with an optimized electrolyte in order to reach the best performance in terms of power/energy densities and lifetime. A specific attention will be paid to electrode/electrolyte interfaces where the parasitic reactions take place, specifically in KIB. Task 1 (T1) will be to determine efficient negative and positive electrode materials through the optimization of the tryptic “Composition, Structure and Electrochemical performance”. Two positive electrode families will be studied and optimized: the polyanionic KTiOPO4-type KVPO4O1-yFy (KVP, 0 = y = 1), working at high voltage (= 4V vs. K+/K) and the Prussian Blue Analogues with general formula KxM1[M2(CN)6]1-y.y.nH2O (M1 and M2 being Fe and Mn) that reach reversible capacities of 110-130 mAh/g between 3.3 and 3.9 V, depending on the composition. For the negative electrode, carbonaceous materials will be studied. Graphite is an inexpensive resource with a mature production process. However, given its large volume change (61%) during K+ intercalation/deintercalation and its relatively low working potential representing a safety hazard (risk of K plating), alternatives such as hard carbon and carbon nanofibers, which are more flexible and should better mitigate the volume change, will be studied. For all these types of carbon, in order to increase the capacity (limited to 270 mAh/g), small amounts of p-block elements (Sb, Sn, P) may be added to reach a targeted capacity of 450 mAh/g. To optimize the electrochemical performance, specific attention will be paid to size/morphology and coating of the positive electrode materials and to electrode formulation for the negative electrode. With optimized positive and negative electrodes from T1, efficient full K-ion cells (with targeted energy density of 140-160 Wh/kg for 1000 cycles) will be assembled and electrolyte formulation will be studied in T2. The choice of an electrolyte enabling positive and negative electrodes to work efficiently will be crucial. It will allow stabilizing passivation layers at both electrode surfaces, especially for high voltage cells. Understanding the aging mechanisms of K-ion full cells in T3 is critical and necessary to efficiently optimize the systems. An arsenal of ex situ and operando spectroscopies (Raman, Mössbauer, EIS), XRD and microscopies will allow following the electrochemical mechanism of the active materials upon cycling (ICMCB, ICGM), whereas surface spectroscopies (XPS, Auger, IR-ATR, GC/MS) will bring an in-depth understanding of electrode/electrolyte interfaces and aging mechanism (IPREM).