Studies of high voltage LiNi0.5Mn1.5O4 as positive electrode material in lithium ion cells
Auch gedruckt in der BibliothekW: W-H 14.986
FakultätFakultät für Naturwissenschaften
InstitutionZentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW)
Institut für Anorganische Chemie II (Synthese und Charakterisierung anorganischer Materialien)
Ressourcen- / MedientypDissertation, Text
Datum der Erstveröffentlichung2017-02-02
Since their first commercialization in 1991, Li-ion batteries (LIBs) present many advantages with respect to other commercial battery technologies. In particular, their higher specific energy and specific power make LIBs the best candidate for electric mobile transport application. Nowadays, electric vehicles are a concrete possibility for the future of mobile transport and new high-energy LIBs are the key to realise it. Developing the next generation of LIBs requires innovative and cheaper cathode materials with higher operating potentials. Among these “high-voltage materials”, LiNi0.5Mn1.5O4 (LMNO) is one of the most promising candidates because of its high operating potential (4.7 V vs. Li/Li+), good specific capacity and low production costs. Unfortunately, standard organic electrolytes are not stable above 4.5 V vs. Li/Li+ and thus, higher operating potentials translate into electrolyte degradation and fast aging of the electrochemical system. The cathode material investigated in this work is an innovative LMNO with tailored particle architecture (LMNO-0). This tailored architecture combines the low surface area of micrometric particles with the fast kinetics of nanometric primary crystallites in order to guarantee a good compromise between stability and electrochemical performance at high potential. Furthermore, LMNO-0 powders present tap-density, surface area and processability, in line with commercial cathode materials. Additional modifications of crystallite size, oxygen stoichiometry and particle dimension significantly influence the electrochemical behaviour of LMNO-0 and allow identifying the optimal material for LIBs application. The optimisation of the particles morphology plays a fundamental role also in reducing the electrolyte degradation rates of LMNO-0 electrodes operating at high potential. In particular, the low surface area accessible for secondary reactions and the dense network of crystallites are key parameters in reducing the surface reactivity and improving the Li-ion diffusion. This remarkable electrochemical behaviour of LMNO-0 electrodes is even more evident when using suitable charging protocols, developed in function of the desired cycling rate. The particle architecture is a crucial feature for reducing the electrolyte degradation rate at high potential; however, it does not influence the nature of the secondary reactions occurring at the particles surface. A valid strategy for improving the active material/electrolyte interface of the morphologically tailored LMNO consists in coating the particles with a thin layer of lithium niobate (LiNbO3). This stable and homogeneous coating does not compromise the electrochemical performance of the pristine material but acts as a stable and ion-conductive passivation layer. Both the coated and the uncoated LMNO present stable high voltage behaviour, remarkable coulombic efficiency and good charge/discharge capacity also when used in full Li-ion cells versus graphite anodes. However, cells using the coated LMNO present twofold cycling stability and significantly reduced cell aging (80% capacity retention after 660 cycles) with respect to those assembled with the uncoated one (80% capacity retention after 335 cycles). These results confirm the beneficial effects of the particle morphology also in full cells and additionally, point out the impressive cycling stability achievable with the LiNbO3 surface coating. Unfortunately, the same results are not reproducible increasing the testing temperature at 45 °C. In this case, accelerated aging kinetics lead to lower cycling stability and rapid capacity fade of all the tested full cells. Realising high-voltage Li-ion cells suitable for practical application is still an open challenge both for the scientific community and for the manufacturers. This study presents very encouraging results, which touch different fundamental aspects of these complicated systems and delineate possible paths for further investigation works.
LCSHLithium ion batteries
Lithium ion batteries; Design and construction
Electric batteries; Materials
DDC-SachgruppeDDC 620 / Engineering & allied operations
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