Bulk electrolyte decomposition, reduction

In addition to the stabilizing effect of cathode particles on the electrolyte solutions at elevated temperatures, graphite-like carbon electrodes (anodes) were also foimd to reduce the thermal decomposition of bulk LiPFe electrolyte solutions. However, the reduction of bulk electrolyte decomposition coincided with reactions of the electrolyte with the anode. The surface of the carbon electrode was covered with the products of the electrolyte reduction, which formed a protective solid electrolyte interface (SEl) layer [35-37], The stabilizing effect of these anodes (e.g., based on lithiated mesocarbon microbeads, MCMB) on the electrolyte was proposed to relate to the degradation of the solid-electrolyte interphase (SEl) in LiPFe-based electrolytes at elevated temperatures [32,38], The loss of capacity and power from lithium-ion cells undergoing accelerated aging experiments has been attributed to the presence of thermal decomposition products of the electrolyte in the anode SEl [32],... 

At elevated temperatures, there are five possible reactions that can occur between the cell components (1) thermal decomposition of bulk electrolyte (2) chemical reduction of electrolyte by the anode (3) chemical oxidation of electrolyte by the cathode (4) thermal decomposition of the cathode and anode or (5) melting of the separator and the consequent internal short. To identify which of these contributes the decisive amount of heat that is critical in initiating the thermal runaway, it is necessary to study the thermal responses of these individual components or component couples separately. 

It is the preconcentration period that enhances the sensitivity of this technique. In the preconcentration phase precise potential control permits the selection of species whose decomposition potentials are exceeded. The products should form an insoluble solid deposit or an alloy with the substrate. At Hg electrodes the electroreduced metal ions form an amalgam. Usually the potential is set 100-200 mV in excess of the decomposition potential of the analyte of interest. Moreover, electrolysis may be carried out at a sufficiently negative potential to reduce aU of the metal ions possible below hydrogen ion reduction at Hg, for example. Concurrent H" " ion reduction is not a problem, because the objective is to separate the reactants from the bulk electrolyte. In fact, methods have been devised to determine the group I metals and NEC " ion at Hg in neutral or alkaline solutions of the tetraalkylammonium salts. Exhaustive electrolysis is not mandatory and 2-3% removal suffices. Additionally, the processes of interest need not be 100% faradaically efficient, provided that the preconcentration stage is reproducible for calibration purposes, which is usually ensured by standard addition. 

PA anion radical rapidly reduced 4-bromobiphenyl (4-BB) to biphenyl in 0.1 H CTAB with an enhanced rate compared to isotropic solvent (Table 1). Quantitative bulk electrolytic reduction of 0.02 mmol of 4-BB in 25 mb 0.1 M CTAB was effected on stirred mercury pool electrodes in 2.5 h with 20 % decomposition of the catalyst. Time for complete conversion to biphenyl and amount of catalyst decomposed were significantly smaller compared to similar experiments in surfactant-free N,N-dimethyl-formamide (DMF) . Diffusion controlled CV and chronocoulometric data for 0.2 mM 9-PA in 0.1 H CTAB were used to obtain an apparent diffusion coefficient (D ) of lO cm s-. This is much too large to attribute to a diffusing micelle-bound species. Furthermore, at scan rates (v) below 5 mV s i, CV s for the 9-PA anion radical were not diffusion controlled as at higher v, but had a symmetric peak shape attributable to a thick surfactant layer at the surface of the electrode. Thus, at the potential required (-2.2 V vs SCE) to reduce 9-PA in 0.1 M CTAB, the catalytic reduction of 4-BB takes place in a thick, spontaneously organized surfactant film on the electrode surface. In addition to voltammetric results , support for existence of a thick film comes from differential capacitance, ellipsometry , and reflectance infrared spectroscopy . 

Interestingly, the anodic dark current at n-Ge electrodes increases considerably upon addition of the oxidized species of a redox system, for instance Ce" ", to the electrolyte, as shown in Fig. 8.4 [7]. The cathodic current is due to the reduction of Ce. The latter process occurs also via the valence band (see Chapter 7), i.e. since electrons are transferred from the valence band to Ce", holes are injected into the Ge electrode. Under cathodic polarization these holes drift into the bulk of the semiconductor where they recombine with the electrons (majority carriers) and the latter finally carry the cathodic current. In the case of anodic polarization, however, the injected holes remain at the interface and are consumed for the anodic decomposition of germanium, as illustrated in the insert of Fig. 8.4. Accordingly, the cathodic and anodic current should be compensated to zero. Since, however, the anodic current is increased upon addition of the redox system there is obviously a current multiplication involved, similarly to the case of two-step redox processes (see Section 7.6). Thus, in step (e) (Fig. 8.1) electrons are injected into the conduction band. This experimental result is a very nice proof of the analytical result presented by Brattain and Garrett [3]. 

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