Electrochemical-potential intercalation process

Hydrogen-terminated boron-doped diamond (BDD) has emerged as a unique electrode material displaying, among other characteristics, extraordinary low chemical and electrochemical reactivity over a wide potential range allowing studies to be performed in PEO/Li salt solutions without interference from underpotential deposition, alloy formation and/or intercalation processes [67],... 

If the electrochemical potential of pure Li metal is constant, the cell potential at equilibrium is a function of the concentration of intercalant species in the host. The composition of the guest in the intercalated host is given by X = n/N, where n is the number of moles of intercalant in the host and N is the total number of sites in the host. Assuming the number of host sites, N, is constant and the host lattice expands freely during intercalation, the free energy change of an intercalation process is a function of composition and temperature. 

Intercalation of species into the CNT structure has also been performed. For instance, the intercalation of lithium in CNTs [93,94] is one attractive topic due to the potential applications in electrochemical energy storage in lithium batteries. Edge-plane defects turn out to be very important for such processes [95]. 

FIGURE 8.32 Typical potential profiles for (a) positive electrode in a conventional asymmetric capacitor built with a non pre-doped Li inter calation carbon for the negative electrode (curve 1), an asymmetric capacitor with a pre-doped Li-ion intercalation carbon material (curve 2), and (b) positive and negative electrodes of an EDLC during the charging process. (From Aida, T., et al., Electrochem. Solid-State Lett., 9, A534, 2006. With permission.)... 

If GO is used as a host lattice for Li+ in aprotic electrolytes, reversibility is improved [577]. The potential level is distinctly more positive than with donor GIC, at about —1 V vs. SHE. An all-solid-state Li/GO battery with PE0/LiC104 as solid electrolyte was reported by Mermoux and Touzain [578], but rechargeability is poor. Recently, the structure of graphite oxide was studied by its fluorination at 50-2()0 °C [579]. C-OH bonds were transformed into C-F bonds. The examples, in conjunction with Section 2, show that the formation or cleavage of covalent C-O (C-F) bonds makes the whole electrochemical process irreversible. Application was attempted in lithium primary batteries, which have a voltage of 2-2.5 V. Really reversible electrodes are only possible, however, with graphite intercalation compounds, which are characterized by weak polar bonds. 

Figure 10.6 shows the CV of a LiMn2O4 electrode on a cell with Li foil for both the reference and auxiliary electrodes in ethylene carbonate plus dimethyl carbonate solution of LiAsFg (1 M) (Sinha and Munichandraiah, 2008). The pair of peaks at larger potential corresponds to the deintercalation/intercalation of Li in the range 0 < X < 0.5 for Li Mn2O4, whereas the pair of peaks at lower potentials is attributable to this process for 0.5 < x < 1, both accompanied by reversible Mn(lV)/Mn(lll) redox reactions. Following Xia and Yoshio (1996), the later electrochemical process corresponds to the removal/addition of Li+ ions from/into half of the tetrahedral sites in which the lithium intercalation occurs. The former couple is then attributed to this process at the other tetrahedral sites where lithium intercalations do not occur. 

Fluorine is produced by electrolysis of molten salts on carbon anodes including KF-21TF at about 100°C, potassium bifluoride at about 250°C, and fluoride salts at about 1000°C. The decomposition potential of molten potassium bifluoride is 1.75 V at 250°C, a value close to that estimated thermodynamically [80]. The kinetics of the anodic process is characterized by a Tafel slope of 0.56 V per decade, j), = 1 x 10 A/cm [81], and by a complex reaction mechanism involving the formation of fluorine atoms on carbon. During the electrolysis, C-F surface compounds on the carbon anode are formed via side reactions. Intercalation compounds such as (CF) contribute to the anodic effect in the electrochemical cell, which can be made less harmful by addition of LiF. 

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