Lithium activation

From Eq. (18) the concentration of electrons, and according to Eq. (11) the concentration of holes also, depend on the lithium activity of the electrode phases with which the electrolyte is in contact. Since anode and cathode have quite different lithium activities, the electronic concentration may vary to a large extent and an ionically conducting material may readily turn into an electronic conductor. 

In fact, crystalline graphites usually cannot be operated in PC electrolytes, unless effective film forming electrolyte additives are used (see above) as propane gas evolution [35], creation of solvated graphite intercalation compounds (sGICs) [36], and graphite exfoliation take place. Recently [37, 38], it was found that propylene evolution is observed at graphite, while absent at lithium active metallic anodes, e.g., Sn and SnSb. 

The reduction of l,l-bis(diphenylphosphanyl) ethylene (248) with an excess of metallic lithium, activated by ultrasonic irradiation, leads to C—C coupling under the formation of a l,l,4,4-tetrakis(diphenylphosphanyl)butane (249) (Scheme 88)". Surprisingly, the lithium centres in the resulting dilithium compound do not form any lithium-carbon contacts, being coordinated by two diphenylphosphanyl groups and two TFIF molecules each. With this strucmral motif, the molecular structure is similar to the one of tris(phosphaneoxide) 20 (Section n. A), also obtained by Izod and coworkers upon deprotonation. ... 

The reaction of hexaphenylbenzene (250) with an excess of metallic lithium, activated by ultrasonic irradiation, was effected by Bock and coworkers ". The reaction is carried out at room temperature and is accompanied by the release of hydrogen gas, caused by the formation of two additional C—C bonds (Scheme 89). The resulting dihthium... 

The emf of the lithium-aluminium system versus pure lithium in a Lil-KI-LiCl molten eutectic is shown in Fig. 8.2 as a function of temperature and composition. It can be seen that the emf remains constant (at about 300 mV more negative than pure lithium) in the range of stability of the /3-phase (-7-47 atoms per cent of lithium), thus implying a constant lithium activity in the alloy surface. At concentrations greater than 47 atoms per cent, the lithium activity becomes strongly composition-depen-dent. 

Table 1) are converted to solvated tetramers in basic solvents such as ethers. Species which are dimeric in hydrocarbon solution, such as benzyllithium and poly (styryl)lithium, are converted into the unassociated species in tetrahydrofuran solution+2,56). The claim61 that poly(styryl)lithium active centers can exist in an associated form in tetrahydrofuran is known 62) to be incorrect. 

Chain Propagation Involving Carbon-Lithium Active Centers... 

The allylic- and benzylic-lithium active centers, which can be characterized as having polarized covalent carbon-lithium bonds in hydrocarbon solvents, have been extensively studied with regard to their structure, their kinetic behavior in the propagation event, and their association states. These latter two topics are the subject of this... 

The association states of the benzylic- and allylic-lithium active centers have been studied by viscosity, light scattering and cryoscopy (Table 2). The majority of results indicate that the dimeric state of association is present for these active centers at the concentrations appropriate for polymerization (10-3 to 10-4 M). 

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy 36,134 143) which has shown that the carbon-lithium bond of allylic-lithium species can possess considerable covalent character. Worsfold has also previously published 43 > concentrated solution viscosity results where the ratio of flow times, before and after termination, of a poly(isoprenyl)lithium solution was about 15. This finding is clearly incompatible with the conclusion that viscometry cannot detect the presence of aggregates greater than dimeric. 

It should also be noted that the viscometric technique can detect the presence of star-shaped aggregates, having the ionic active centers. The addition of ethylene oxide to hydrocarbon solutions of poly(isoprenyl)lithium leads to a nearly two-fold increase in viscosity144). Conversely, this results in an approximately twenty-fold decrease in solution viscosity, after termination by the addition of trimethylchloro-silane. This change in solution viscosity is reflected in the gelation which occurs when difunctional chains are converted to the ionic alkoxy active centers 140,145,146). Branched structures have also been detected 147> by viscometry for the thiolate-lithium active center of polypropylene sulfide) in tetrahydrofuran. 

The alteration in solution viscosities brought about by the conversion of the allyllic-lithium active center to the alkoxy-lithium species is in accord with the general trend 148,1491 observed for star-shaped polymers in concentrated solution. It must be noted though that viscosity measurements cannot generally be used to detect differ-... 

However, Hsieh and Kitchen 151 failed to consider the influence of their measurement temperature, 78 °C, on the stability of the poly(dienyl)lithium active centers (see section on Active Center Stability). As an example of this potential problem is the observation by two separate groups 47-152> that viscometric measurements of hydrocarbon solutions of poly(butadienyl)lithium fail to yield constant flow times (at 30 °C) following the completion of the polymerization, i.e., the flow times were found to increase with increasing time. This inability of the poly(butadienyl)lithium chain to exhibit constant solution viscosities renders it unsuitable for association studies of the type done by Hsieh and Kitchen 151). 

Fig. 11. Influence of TMEDA on the association of the poly(butadienyl)-lithium active center. (Reprinted with permission from Ref. 152), Copyright 1983, IPC Science and Technology Press)...

Fig. 14. Spectra of the poIy(butadienyl)lithium active center at equilibrium in tetrahydrofuran at various temperatures. (Reprinted with permission from Ref.192), Copyright 1975, American Chemical Society)...

The addition of small amounts of a polar solvent can markedly alter the copolymerization behavior of, for example, the diene-styrene pair. The solvation of the active centers manifests itself in two ways the incorporation of styrene is enhanced and the modes of diene addition other than 1,4 are increased 264,273). Even a relatively weak Lewis base such as diphenyl ether will bring about these dual changes in anionic copolymerizations, as the work of Aggarwal and co-workers has shown 260>. Alterations in polyisoprene microstructure and the extent of styrene incorporation were found for ether concentrations as low as 6 vol. % (diphenyl ether has been shown52) to cause partial dissociation of the poly(styryl)lithium dimers. The findings of Aggarwal and co-workers 260) are a clear demonstration that even at relatively low concentrations diphenyl ether does interact with these anionic centers and further serve to invalidate the repetitive claim 78,158-i60,i6i) tjjat diphenyl ether — at an ether/active center ratio of 150 — does not interact with carbon-lithium active centers. 

Apparently, reactions similar to those outlined in Equations (57) to (60) occur in dilute ( 10 3 M) solutions of the delocalized active center based on 2,4-hexa-diene 288). This conclusion is based on both spectral results and analysis by the application of size exclusion chromatography. These reactions, though, are supressed 288> at higher concentrations (>10-2 M) of active centers. The transformations which occur under dilute conditions account for the different association numbers 481199) (i.e. 1.7 and 1.4) which have been reported. This has been verified by the observation 288) that Nw for freshly prepared hexa(dienyl)lithium active centers (formed by adding... 

The structural core of (-)-adaline was prepared by a lithium-activated SN2-type alkynylation of an enantiomerically-enriched tricyclic N, O-acetal followed by reduction, Ar-formylation (80, R = CHO), and RCM using the MC2 <02OL2469> (Scheme 61). Yields were only slightly lower with the GMC. RCM fails with the HC1 salt of 80 (R =H2C1), presumably because of a diequatorial arrangement of the 2,6-dialkenyl substituents in that derivative. 

During overcharge of a cell an instability was observed because of an increase of lithium activity in the negative electrode, of its dissolution in the electrolyte and therefore the self-discharge [375] increases. To avoid this phenomenon, approximately 10 mol% Al5Fe2 was added to the negative electrode. These electrodes were employed throughout 900 cycles without an appreciable capacity loss. 

If the two defects on the right-hand side are dilute, an ideal mass action law can be formulated. The defect concentrations can be connected with the nonstoichiometry (8 = i- v i interstitial Lithium concentration v Lithium vacancy concentration), which is proportional to the storage capacity, while the lithium activity (see the... 

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy which has shown that the carbon-lithium bond of allylic-... 

---