Investigation of the electrochemical behavior at low temperature and fast charge/discharge rate of anodic materials for lithium-ion batteries

Nowadays lithium-ion batteries represent the state-of-the-art of energy storage devices. Within this context the work herein presented deals with the investigation and the improvement of the electrochemical behavior of anode materials for lithium-ion batteries. Conductive agents play a fundamental role in determining the electrochemical performance of electrodes for lithium-ion batteries. Super-P carbon is often used in the electrode formulations in order to increase the electrical conductivity and to improve the contact between particles. In this work we synthesized a copper-modified Super-Pbased conductive agent, in which copper nanoparticles supported on the carbon matrix have been obtained by a microwave-assisted reduction of Cu (II) ions. The obtained copper/Super-P composite showed good dispersion of the nanoscaled copper particles on the carbon matrix. The synthesized conductive agent has been used instead of pristine Super-P during the manufacturing process of graphite and Li4Ti5O12 electrodes. Graphite is at the moment the most commonly used anode material in lithium-ion batteries. Unfortunately when graphite anodes are forced to work at low temperature and/or at high charge/discharge rate, the electrochemical performance drastically decreases. In this work electrodes made of oxidized KS-15 graphite (Timcal) have been tested at low temperature and high charge/discharge rate. Two different anodic formulations have been tested. Unmodified electrodes have been prepared using pristine Super-P as conductive agent. On the other hand, copper-modified ones have been manufactured with the synthesized copper/Super-P composite. It has been demonstrated that the use of the copper/Super-P composite in the electrode formulation leads to improved performances at low temperature (down to -30°C) and at increased charge/discharge rate. Furthermore, we propose a detailed study of the differential capacity curves dQ dE-1 vs. V and a set of experiments to investigate the electrical properties of the tested samples. The obtained results showed decreased polarization and increased electrical conductivity for copper-modified electrodes. The effect of both pristine Super-P and Cu/Super-P on anodes electrochemical behavior is explained by DC- and AC -electrical studies. The results show decreased electrical resistance and metallic-like behavior of the electronic conductivity as a function of the temperature on samples containing Cu/Super-P. It implies that when the temperature is decreased, the electrical conductivity linearly increases. Conversely, the sample containing pure Super-P follows a classical semi-conductor behavior, with the electrical conductivity decreasing with the decreasing temperature. Li4Ti5O12 is considered a safer alternative to carbonaceous anodes. The main disadvantages of LTO-based electrodes are related to the poor ionic and electronic conductivity of this compound, which determine poor rate capability and poor electrochemical performances at low-temperature. In this thesis we demonstrate that the use of the copper/Super-P composite instead of pristine Super-P positively affects the electrochemical behavior of the tested anodes. Electrodes manufactured starting from commercial LTO purchased from Aldrich, have been tested in the temperature range of -30≤ T≤ 20°C. The results demonstrate excellent performances of the copper-modified electrodes. Moreover, the rate capability and the cycling stability of the LTO electrodes have been remarkably improved by the use of copper/Super-P in the electrode composition. Rutile-TiO2 has been considered for a long time as almost inactive towards lithium insertion. Lately it has been proven that by controlling the particles size during its synthesis it is possible to reversibly insert a relevant amount of lithium ions into the rutile structure. In this work, nanoscaled rutile-TiO2 has been synthesized following a sol-gel procedure. The electrochemical tests performed on rutile electrode demonstrated the excellent electrochemical behavior at low temperature. Moreover, it has been demonstrated that rutile-TiO2 electrodes can be cycled within an enlarged potential window (0.1-3 Volts) in addition to the classical 1-3 Volts window of TiO2 system. The results obtained in this thesis have been the subject of the following publications and proceedings of congress: M. Marinaro, M. Pfanzelt, P. Kubiak, R. Marassi, M. Wohlfahrt-Mehrens, Journal of Power Sources, 196 (2011) 9825. M. Marinaro, M. Pfanzelt, P. Kubiak, R. Marassi, M. Wohlfahrt-Mehrens, 219th ECS meeting Montreal, QC, Canada 1-6. May 2011 Abs. 641. The results reported in this thesis about the electrochemical behavior of graphite and LTO electrodes will be subject of further communications.

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