Negative Electrode Materials for Lithium Ion Batteries

Lithium ion batteries have become the primary energy source for portable electronics, and their utilization in larger scale applications is increasing as well. Numerous electrode materials have been investigated for lithium ion batteries and several different materials are also found in commercial cells. The properties, cost and safety of the battery strongly depends on the selected electrode materials and cell design. The focus of this thesis is on negative electrode materials and electrode manufacturing methods that are environmentally friendly and safe for large scale and high power applications. First part of this thesis studies Li4Ti5O12 (LTO) as a negative electrode material. Especially the effect of the particle morphology on the electrochemical performance is evaluated in detail. It is shown by comparing two LTO materials with same crystalline structure but different morphology that small particle size and large surface area has a beneficial effect on the battery performance. In addition, different behavior in terms of (de)lithiation voltages and lithium storage is observed in the LTO surface than in the bulk. Thus it is shown that the performance of LTO can be tailored by affecting the particle morphology. Moreover, the role of carbon additives in the LTO electrodes is studied. The effect of carbon additives on the performance of LTO electrodes is minimal when moderate conditions (low currents at room temperature) are used. However, in more demanding conditions (high currents and/or low temperatures) the beneficial effect of carbon additives is observed. The effect of carbon additives is more obvious with LTO with smaller particle size, and it is concluded that the effect of carbon additives depends both on the particle and electrode morphology. In the second part of the thesis, a water soluble acrylate binder Acryl S020 is studied for aqueous preparation of LTO and graphite negative electrodes. Commonly used PVDF (polyvinyledene fluoride) binder is costly and requires harmful NMP (N-methyl pyrrolidone) as a solvent and thus alternative methods are searched for. It is shown that similar capacities are achieved with LTO electrodes using an Acryl S020 binder manufactured with an aqueous process when compared to the LTO electrodes using a PVDF binder. Moreover, pilot scale slot die coating and gravure printing methods are tested in the electrode manufacturing and cycle lives of over 500 are obtained with both methods. Also water absorption/desorption of LTO is shown to be nearly reversible, and thus the aqueous process seems applicable for the preparation of LTO electrodes. Promising results are also obtained with the graphite electrodes using the Acryl S020 binder as better capacities at high C-rates and low temperature are achieved when compared to the graphite electrodes using a CMC (carboxymethyl cellulose) + SBR (styrenebutadiene rubber) binder combination.