Electrochemical Deposition of Aluminum from NaCl-AlCl3 Melts

Elec t rochemica l depos i t ion of a l u m i n u m from NaA1C14 melts sa turated wi th NaC1 onto a glassy carbon e lec t rode at 175~ has been s tudied by vo l tammetry , ch ronoamperomet ry , and cons tant current deposi t ion. The depos i t ion of alumin u m was found to proceed via a nuc lea t ion/growth mechan i sm, and the nuc lea t ion process was found to be progressive. The morpho logy of a l u m i n u m deposi ts was examined with photomicroscopy . It was shown that d e p e n d i n g on the current densi t ies (c.d.) applied, three types of a l u m i n u m deposi ts could be obtained, namely, spongy deposi ts fo rmed at lower c.d. 2 2 (below 0.7 m A / c m ), smooth layers depos i t ed at in te rmedia te c.d. (be tween 2 and 10 m A / c m ), and dendr i t ic or porous deposits ob ta ined at h igh c.d. (above 15 mA/cm2). However , the smooth a l u m i n u m deposi ts were about five t imes more voluminous than the theoret ical value. The spongy deposi ts were formed due to difficult ies in e lec t ronuclea t ion and could be inhib i ted by appl icat ion of pulsed currents and/or addi t ion of manganese chlor ide into the melt. A l u m i n u m has been used as anode mater ia l in newly deve loped rechargeable bat ter ies wi th the NaA1C14 mol ten salt as the e lect rolyte and t ransi t ion meta l sulfides as cathodes at 175~ (1, 2). Dur ing the charging of the batteries, it was found (3) that a l u m i n u m dendr i tes can be fo rmed unde r cer ta in c i rcumstances . The spacing be tween the e lec t rodes in the bat ter ies is usual ly very small; hence, the dens i ty of the deposi ts is of impor tance for the capaci ty and life of the batteries. Noncompac t , dendr i t ic deposi ts m a y cause short circuit ing, early deter iorat ion, and so on. Bar ton and Bockr is (4) and Diggle et al. (5) have deve loped a kinet ic mode l of dendr i te format ion in which the role of a di f fus ion-control led process was emphas ized to account for the ini t iat ion and growth of dendri tes . A set of rules govern ing the kinet ics of dendr i te format ion was es*Electrochemical Society Active Member. tabl ished, main ly that (i) a certain critical current dens i ty (overpotential) mus t be exceeded in order to p rovoke dendri te formation, (ii) dendr i te growth exhib i t s a cer ta in induc t ion per iod before it becomes visible, and (iii) the crit ical current dens i ty or overpotent ia l for dendr i te format ion is direct ly related to the concent ra t ion of the depos i t ing species. As early as the 1930s, a l u m i n u m dendr i tes were found to be favored dur ing deposi t ion at h igh current densi ty from NaC1-A1C13 mel ts (6, 7). This was later s tudied by Midor ikawa in a series of papers (8-10). Thereafter , several studies have been pe r fo rmed in this field (11-17). Many investigat ions suppor ted the assumpt ion of di f fus ion-control led dendr i t e format ion for a l u m i n u m deposit ion. As expected , a cri t ical current dens i ty or overpotent ia l is p resent for alum i n u m dendr i te init iation, and the critical va lue is direct ly related to the mass t ranspor t condi t ions and the concenDownloaded 09 Oct 2009 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp 594 J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc. t ra t ion of species conta in ing a l u m i n u m (12-14). General ly, in mel ts wi th less than 57 mole percent (m/o) A1C13, the crit ical current densi ty is very small and dendr i tes are easily formed. This f inding is unfor tunate , since at high concentra t ions of AIC13 the mel ts are difficult to handle because of their h igh vapor pressures. There were also reports sugges t ing that e lec t rode substrates (8, 11, 15), inorganic and organic addi t ives (10, 13, 16, 17), and superposed al ternat ing current (8) or pu lsed current (15) had significant effects on inhibi t ing a l u m i n u m dendr i te formation. These f indings cannot, of course, fit the m e c h a n i s m of dif fusion-control led dendr i te fo rmat ion bu t suggest the govern ing role of the format ion of crystal l ine nucle i in the deposi t ion (14, 15). In the overvol tage m e a s u r e m e n t in a NaC1-KC1-A1C13 melt, Hayashi et al. (18) found that the main part of ca thodic polar izat ion s eemed to be due to crystall ization overvol tage in the low polar izat ion potent ia l region. Rol land and Maman tov (19) found that the reduc t ion of A12C17in acidic chloroaluminate mel t s involved a nuc lea t ion process. Apar t f rom this, l i t t le w o r k has been done on the e lec t rochemica l nucleat ion and growth of a luminum from that med ium. Moreover, a lmos t all p revious studies were pe r fo rmed in acidic NaCI-A1CI3 melts , and little work has been repor ted in the basic NaC1-A1C13 mel t sa turated wi th NaC1. This sys tem is of m u c h more in teres t f rom the technical point of v i ew because of its low vapor pressure. It was our in tent ion to s tudy the e lec t rochemica l nuc lea t ion process and to examine how and w h e n the a l u m i n u m dendr i tes form in NaC1sa tura ted mel ts at 175~