Li-ion Secondary Batteries

From reference [4]

Li-ion secondary batteries are now attracting many people's interest because they have the enhanced properties such as higher output voltage, more capacity. Rocking-chair concept was proposed in early 80's, but there was no technology to realize it with. During the past 15 years, nonaqueous lithium cells, especially the 3 volt primary systems have been developed. The use of practical lithium cells in electronic devices such as Li/(CF)_n and Li/MnO₂, has been expanded. And then, several candidates for positive electrodes, such as TiS₂, MoS₂, MnO₂, vanadium oxides, etc., have been proposed. Recently, an innovative seconady lithium cell has been proposed[1-5] and fabricated as a 4V secondary lithium cell. A battery cell is composed of cathode material, anode material, electrolyte, separator and pakaging unit.

The possible candidates for 4V Li-ion cathode material are LiCoO₂[1], LiNiO₂[2], Li(Ni_yCo_1-y)O₂[3], LiMn₂O₄[4]. LiMn₂O₄ has a spinel-framework structure and other materials are iso-structural with alpha-NaFeO₂. LiCoO₂ is now utilized in commercial battery cells and shows very reliable reversibility. But, because of the toxicity and high price of cobalt, altanatives are needed. Li-Ni-O system shows very high capacity at first, but its recyclability is extremely bad. It is believed that this fact is due to non-stoichiometry and mixing of cations. A lattice-gas model is suggested by Dahn's group[6]. Cobalt doping or oxygen atmosphere during sythetic process can reduce non-stoichiometry and cation-mixing. LiMn₂O₄ spinel system has higher voltage than the others but has lower capacity. Its theoretical capacity is 148 mAh/g. The relationship between stoichiometry and the capacity is reported and the capacity of about 130 mAh/g was achived[7].

The most promising and sole candidatives for anode material are carbonaceous materials. There are various kinds of carbonaceous materials including carbon black, active carbon, graphite, glassy carbon and carbon fiber. Among these, graphite has most regular and well-known structure. The intercalation of lithium into graphite by vapor transport was first discovered by Herold in 1955. Early studies of graphite intercalation by electrochemical methods show decomposition of PC based electrolyte in Li/graphite cells[9]. Peled has shown that alkali metals in nonaqueous solvents are always covered by a surface layer which is instantly formed by the reaction of the metal with the electrolyte[10]. This surface layer is called the solid electrolyte interphase(SEI) and it is an ionic conductor and electronic insulator. Recent studies reveal that SEI can be formed only one time in appropriate electrolyte[5]. Lithium capacity varies depending on the kinds of carbonaceous materials. Roughly speaking, carbonaceous materials are categorized into 3 kinds, namely, graphites, soft carbons and hard carbons. As to graphite, theoretical capacity is 372 mAh/g, corresponding to LiC₆, and the relationship between structural disorderness and lithium capacity is reported[11]. For soft carbons and hard carbons, additional capacity over theoretical capacity was found and several models are suggested. In case of soft carbons, Sato and others proposed that the Li atoms intercalate and occupy nearest neighbor sites between each pair of graphene sheets[12]. Yata et al. suggested that these carbons contain substantial hydrogen [13]. There are also some suggested models in case of hard carbons such as micropore-filling model[14] and house of cards model[15].

* References
[1] T. Nagaura, Progress in Batterys and Battery Materials, 10, 209 (1991).
[2] J. R. Dahn, U. von Sacken, and C. A. Micael, Solid State Ionics, 44, 87 (1990).
[3] K. Sawai, T. Ohzuku and T. Hirai, Chemistry Express, 5, 837 (1990).
[4] J. M. Tarascon et al., J. Electrochem. Soc., 139, 937 (1992).
[5] R. Fong, U. von Sacken and J. R. Dahn, J. Electrochem. Soc., 137, 2009 (1990).
[6] W. Li, J. N. Reimers and J. R. Dahn, Phys. Rev. B, 46, 3236 (1992).
[7] Y. Xia and M. Yoshio, J. Electrochem. Soc., 144, 4186 (1997).
[8] A. Herold, Bull. Soc. Chim. France, 187, 999 (1955).
[9] A. N. Dey and B. P. Sullivan, J. Electrochem. Soc., 117, 222 (1970).
[10] E. Peled, J. Electrochem. Soc., 126, 2047 (1979).
[11] H. Shi et al., J. Electrochem. Soc., 143, 3466 (1996).
[12] K. Sato et al., Science, 264, 556 (1994).
[13] S. Yata et al., Synth. Met., 62, 153 (1994).
[14] M. Ishikawa et al., 35th Battery Symposium in Japan, Nagoya, 14-16 November 1994 (extended abstraces), p. 39.
[15] J. R. Dahn et al., Science, 270, 590 (1995).

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