Investigation of Alkaline Ion Rocking Chair Batteries

The work was devoted to the improvement of rechargeable batteries. Two different strategies were applied: i) investigation of new electrode materials to increase the battery performance, and ii) studies on failure mechanism of commercial rechargeable batteries. Both Li-ion and Na-ion systems were explored. In the former case, carbon based materials were investigated as high capacity anode (chapter 2), while the cell failure of commercial cells (chapter 3) and pouch cells (chapter 4) were investigated by Ultra High Precision Coulometry (UHPC) and dQ/dV analysis. Moreover, Na-ion systems, a low cost alternative to the Li-ion batteries, were investigated. Sn films were characterized as negative electrode (chapter 5), while Na0.44MnO2 cathode material was investigated by electrochemical techniques (chapter 6). A brief description of the aforementioned chapters is here reported. Chapter 2. Carbon films were prepared by DC magnetron sputtering at argon pressures ranging from 1 to 30 mTorr. The film sputtered at the lowest pressure was fully amorphous, and showed a density of 1.9±0.3 g/cc indicating little porosity. The film sputtered at the highest pressure showed a broad (002) Bragg peak and had a density of 1.35 ± 0.15 g/cc, indicating significant porosity. Electrochemical testing showed that the low pressure sputtered carbon had a reversible specific capacity of about 800 mAh/g, and an average delithiation potential of about 1 V vs. Li/Li+. Heating the same film to 900oC in argon decreased the reversible capacity and the average voltage to 600 mAh/g and 0.75V, respectively. Chapter 3. Commercial aged LiCoO2/Graphite cells having different cycling histories were studied. Even after 12 years of operation at 37oC, the cells still retained 80% of their initial capacity with coulombic efficiency of 0.99985 when measured at C/20 and 40oC. The capacity loss of these cells could be explained by loss of lithium inventory through growth of the solid electrolyte interphase (SEI) at the anode. There is no evidence of active material loss due to electrical disconnect in these cells. A low upper cut-off voltage (4.075 V) is crucial to the long lifetime of these cells due to electrolyte oxidation reactions at the positive electrode, revealed by the UHPC experiments. Chapter 4. Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells were cycled at various discharge rates of C/2, C, 2C, and 4C at 30.oC. According to dV/dQ analysis there is very small, if any, active mass loss in any of these cells up to 540 cycles. All the lost capacity is due to loss of active lithium atoms in the negative electrode SEI as relative electrode slippage, derived from dV/dQ analysis, and capacity loss are nicely correlated. Scanning electron microscopy images show clear evidence of particle or/and SEI layer cracking at the negative electrode for the cells discharged at 4C, while the NMC particles were unaffected. Chapter 5. Sn films, obtained by electrodeposition, were structural and electrochemical characterized. Electrochemical potential spectroscopy (EPS) and galvanostatic cycling of the electrodes were investigated in organic electrolyte. Three crystalline and one amorphous phases were identified as well as high discharge capacity (738 mAh/g) was obtained after 4 cycles. Unfortunately material fading, due to the internal stress during cycling, causes poor cyclability. Chapter 6. Na0.44MnO2 compound was prepared by a modified Pechini method and characterized. The material exhibits a discharge capacity (about 110 mAh/g) at low current (11 mA/g) which decreases to 65 mAh/g at high current (275 mA/g). The electrochemistry was investigated by electrochemical impedance spectroscopy. It was observed that the kinetic limitations are mainly due to the low diffusion coefficient of Na+ in the structure and to the high values of the surface resistance which is the sum of two contributes attributed to the charge transfer process and the presence of a passive layer.

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