Delithiation

The disproportionation reaction destroys the layered structure and the two-dimensional pathways for lithium-ion transport. For >0.3, delithiated Li, AV02 has a defect rock salt structure without any well-defined pathways for lithium-ion diffusion. It is, therefore, not surprising that the kinetics of lithium-ion transport and overall electrochemical performance of Li, tV02 electrodes are significantly reduced by the transformation from a layered to a defect rock salt structure [76], This transformation is clearly evident from the... 

Zhu et al. [94] reported the synthesis of Sn02 semiconductor nanoparticles by ultrasonic irradiation of an aqueous solution of SnCLj and azodicarbonamide under ambient air. They found that the sonochemically synthesized Sn02 nanoparticles improved remarkably the performance of Li ion batteries such that there was about threefold increase (from 300 to 800 mAh/g) in the reversible capacity in the first lithiation to delithiation cycles. Similarly the irreversible capacity also increased by about 70% (from 800 to 1400 mA h/g). Wang et al. [95] reported the synthesis of positively charged tin porphyrin adsorbed onto the surface of silica and used as photochemically active templates to synthesise platinum and palladium shell and... 

Figure 1. Voltage vs. composition curves during the graphite electrode lithiation and delithiation at C/l00 rate.

Figure 2. Evolution of the high frequency EIS semi-circle during delithiation (a) and lithiation (b) of the graphite electrode.

In-situ x-ray diffraction (XRD) was performed on a coin type cell with a 4x6 mm Kapton window coated with conductive thin copper layer. The graphite electrode was pressed against the Kapton window so as to be reached by the x-ray beam. After several lithiation/delithiation cycles under a C/10 rate between 1.5 and 0V, the cell was fully delithiated up to 1.5V. The cycle capacity achieved with the graphite electrode is about 360mAh/g. The cell was then re-lithiated under a slower rate of C/20. XRD patterns were taken for about five minutes every hour while the cell is under continuous discharge. As result the lithium composition x in LixC6 was incremented by 0.05 between two successive XRD scans. 

Hence, the presence of trace impurities, which either pre-exist in pristine electrode and bulk electrolyte or are introduced during the handling of the sample, could profoundly affect the spectroscopic images obtained after or during certain electrochemical experiments. This complication due to the impurities is especially serious when ex situ analytic means were employed, with moisture as the main perpetrator. For cathode/electrolyte interfaces, an additional complication comes from the structural degradation of the active mass, especially when over-delithiation occurs, wherein the decomposition of electrolyte components is so closely entangled with the phase transition of the active mass that differentiation is impossible. In such cases, caution should always be exercised when interpreting the conclusions presented. 

Figure 30. Capacity loss due to storage at elevated temperatures for Li/graphite half-cells. All cells were precycled at room temperature (cycles 1—3) prior to storage at indicated temperatures for 1 week, followed by continued cycling at room temperature. The electrolytes used were EC/DMC (2 1) and (a) 1.0 M LiPFe or (b) 1.0 M LiBp4. The cells were stored in delithiated states. (Reproduced with permission from ref 277 (Figure 2). Copyright 2001 The Electrochemical Society.)...

In contrast to that of solvents, the effect of the electrolyte solute, LiPFe, on the thermal decomposition of the cathode, LiCo02, was found to be suppression instead of catalyzation. The SHR of a partially delithiated cathode was measured in a series of electrolytes with various salt concentrations, and a strong suppression of the self-heating behavior was found as the concentration of LiPEe increased above 0.50 M. The mechanistic rationale behind this salt effect is still not well understood, but the authors speculated that the salt decomposition coated the cathode with a protective layer that acted as a combustion retardant. On the basis of these results, the authors recommended a higher salt concentration (>1.50 M) for LiCo02-based lithium ion cells is preferred in terms of thermal safety. 

Scheme 27. Thermal Combustion of EC by a Fully Delithiated Spinel Cathode...

Considering the virtually zero solubility of LiF in nonaqueous media, McMillan et al. synthesized the fluorinated counterpart of CIEC. As expected, the shuttle phenomenon was eliminated because of the fluorination, as evidenced by the quantitative Coulombic efficiency, while a similar SEI effect was maintained, since FEC/PC mixed solvent can support reversible lithiation/delithiation of the graphitic anode materials. However, capacity was observed to fade by 37% in 200 cycles. Follow-up work on this solvent has been minimal. 

Figure 53. Stabilization of graphite in PC by LiBOB. Voltage profiles of lithium/graphite half-cells containing 1.0 m lithium salts in neat PC as electrolytes. Only for LiBOB/ PC was the complete lithiation/delithiation cycle achieved. (Reproduced with permission from ref 324 (Figure 1). Copyright 2002 The Electrochemical Society.)...

An important conclusion that these authors drew based on their polarization and EIS studies is that the properties of the SEI film on a graphite anode surface play a far more decisive role in determining the kinetics of the lithiation/delithiation at low temperatures than does the bulk ion conductivity, although it is necessary for the latter to achieve a... 

Fluorinated carbonates were also used by Smart et al. as low-temperature cosolvents (Table 12), in the hope that better low-temperature performances could be imparted by their lower melting points and favorable effects on SEI chemistry. Cycling tests with anode half-cells showed that, compared with the ternary composition with nonfluorinated carbonates, these fluorinated solvents showed comparable and slightly better capacity utilizations at room temperature or —20 °C, if the cells were charged at room temperature however, pronounced differences in discharge (delithiation) capacity could be observed if the cells were charged (lithiated) at —20 °C, where one of these solvents, ethyl-2,2,2-trifluoroethyl carbonate (ETFEC), allowed the cell to deliver far superior capacity, as Figure 63 shows. Only 50% of the capacity deliverable at room temperature was... 

Figure 63. Delithiation capacity of an MCMB anode at —20 °C in various electrolytes following charge (lithiation) at —20 °C. The drain rate is 50 mA ( C/12). (Reproduced with permission from ref 466 (Figure 4). Copyright 2003 Elsevier.)...

Figure 68. Nyquist plots of a charged lithium ion cell, a lithiated graphite/graphite cell, and a delithiated cathode/ cathode symmetrical cell. The inset is an equivalent circuit used for the interpretation of the impedance spectra. (Reproduced with permission from ref 512 (Figure 3). Copyright 2003 Elsevier.)...

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