ceramicas fast li conducting

ceramicas fast li conducting

(Parte 1 de 5)

Fast Li@ Conducting Ceramic Electrolytes By Gin-ya Adachi," Nobuhito Imanaka, and Hiromichi Aono

1. Introduction

In general, ions cannot migrate in a solid, since the constituent cations and anions are necessary to maintain the rigid skeletal structure. In some solids, only one ion species is able to migrate with a low energy barrier, and these have high ionicconductivities comparable to those ofmolten and aqueous electrolytes. Such solids are called "solid electrolytes".

In the nineteenth century ion migration such as that of

Ag', OZe, and Fe was reported in some solids. Typical ion conducting structures are shown in Table 1. In the 1920s, Tubandt et al. proposed that Ag' ion conductivity in AgI was greatly enhanced by a phase transition from the low temperature p phase to the high temperature a structure.["'] Although the conductivity of the iodide at room tempera- ture is very low, it increases by more than three orders of magnitude at temperatures above the phase transition at 419 K.

Table 1. Typical ion conducting structures for solid electrolytes. Structure Representative materials

Ion defect structure Two dimensional layered structure

Three dimensional network structure a-AgI type structure

Vitreous structure Li2S-sis2-Li3P04 system [LP]

0.92Zr02-0.08Y203 system [02e] Na2O-I 1A1203(P-alumina) pam],

Li3N[Lim] Na3Zr,Si2PO12(NASICON) pa']

Ag1(>419 K) [Agm], RbA&IS [Ag"], Rb4C~1617Cl13 [Cue]

The high conductivity of the a-AgI phase is obtained because only two Ag' ions are statistically distributed over 42 Ag' vacant sites to occupy around I' constituent ion per unit cell.[31 The stabilization of the a-form even at room temperature is one strategy to achieve a high Ag' conducting electrolyte. Since then, many Age ionic con- ducting materials with high conductivities at room tempera- ture have been inve~tigated.[~-~I In particular, RbAg41S shows an excellent Age ionic conductivity of 0.21 Scm-' at 298 K, which is higher than ca. 8 x lO-'S cm- for a 1.0 N NaCl aqueous solution at room temperature.[']

[*] Prof. G.-y. Adachi, Dr. N. Imanaka

Department of Applied Chemistry Faculty of Engineering, Osaka University 2-1 Yamadaoka, Suita, Osaka 565 (Japan)

Dr. H. Aono Department of Industrial Chemistry Niihama National College of Technology 7-1 Yagumo-cho, Niiharna. Ehime 792 (Japan)

A material with analogous structure, Rb4Cu1617Cl13 also shows an excellent Cu' ionic conductivity (0.34 Scm- ' at 298 K).[91 In the 1940s, Wagner reported the mechanism for 0'' ion migration in zirconium dioxide (zirconia)."'] Oxygen defects are formed when Zr4@ ion sites in ZrOz are partially substituted for di- or tri-valent cations such as Ca2@ or Y3'. These defects lead to easier 02' migration in the solid. In addition, pure ZrOz has two phase transitions, from the low temperature monoclinic to the tetragonal structure, and finally to the cubic high temperature phase.

The cubic phase can be obtained by partial substitution of a Zr site by a di- or tri-valent cation having a larger ionic radius [e.g. Y3'(1.019 A), Ca''(1.12 A)] compared to Zr4'

(0.84 A)."'] The cubic phase is then stable over a large temperature range, down to room temperature. This material is called "stabilized zirconia". Although stabilized zirconia has already been practically applied as an 0' gas sensor constituent at higher temperatures (> 900 K), it is still insulating at room temperature."']

In the mid-l960s, @-alumina (Na20 1 lA1203) was pre- pared as a fast Na' ion cond~ctor."~.'~~ p-alumina has a two-dimensionally layered structure and Na@ ions conduct between the layers. The ionic migration is limited between the layers due to the anisotropic conduction mechanism. In 1976, for the purpose of obtaining an isotropic structure,

Goodenough and Hong designed the three-dimensional network structure with a suitable tunnel size for Na' migration, and named the Nal +xZr2SixP3-x012 material a s' 2uper ionic conductor (NASICON).['53'61 These Na' ionic conductors have relatively high conductivity (10- 3- 10- S cm- ') at room temperature, which is comparable to the conductivity of a liquid electrolyte.

Figure 1 shows the temperature dependence of typical ionic conducting solid electrolytes. One of the promising applications of the fast ion conducting solid electrolytes is in all-solid-state rechargeable batteries. Up to now, excellent conductivities at room temperature have been reported for various kinds of ionic conductors. However, the Ag' and Cu' ionic conductors mentioned above are still unsuitable for use for practical battery use, because of a very low decomposition potential (0.5-0.7 V) and deliquescence in humid air. In addition, a high energy density cannot be obtained because both Ag and Cu have a high mass number compared to Na. However, Na metal applied as an anode for a battery is very reactive to oxygen in the atmosphere even at room temperature. Another disadvantage of the all- solid-Na battery is that Na metal is inferior to Li because of a lower energy density.

Adv. Mater. 1996,8, No. 2 0 VCH Verlagsgesellschafl mbH. 0-69469 Weinheim, 1996 093.S-9648/96/0202-0127 $lO.O0+.2S/O 127

G.-y. Adachi, N. Imanaka, H. AonolFast Lie Conducting Ceramic Electrolytes ADVANCED MATE RIALS

T/"C IMH) 500 300 2:

in1 h '8 y 10.~

10.~ v -

,\ lo-"o.5 1.5 2.0 2.5 3.0 3.5

103n( K-')

Fig. 1. Temperature dependence of the ionic conductivity for typical solid electrolytes.

Table 2 lists energy densities and electromotive forces for typical batteries. The theoretical energy density of the

lithium battery (ca. 2000 W h kg- ') is about 10 times higher than that of the widely commercialized nickel-cadmium or

Table 2. Theoretical energy densities and electromotive forces obtained for rechargeable batteries on the market.

Battery Energy density EMF (W. h. kg-') (V)

Lead storage battery 180 2.0 Lithium battery 2000 3.0

Nickel-cadmium battery 200 1.3 Nickel-hydrogen battery 400 1.4 a lead-acid storage batteries. Furthermore, the existence of toxic elements, like cadmium and lead, in those batteries has become a serious problem from an environmental point of view, resulting in the nickel-hydrogen battery rapidly entering the market. It offers a relatively high energy density and non-toxicity of the constituent materials. However, its theoretical energy density is still ca. one fifth that of the rechargeable lithium battery.

The lithium battery has the highest electromotive force (ca. 3.0 V) of the batteries described above, and is the most promising due to its high energy density with high cell voltage. Elemental Li has the smallest mass number of the metals and a very negative standard electrode potential. The lithium batteries on the market utilize some organic solvents such as LiC104 dissolved in a propylene carbonate (PC) as electrolyte. However, leakage of the organic electrolyte, its freezing at lower temperatures, and ignition at higher temperatures remain problems. In addition, some side

Gin-ya Adachi was born in Osaka, Japan, in 1938 and earned his undergraduate degree at Kobe University. He then obtained his Ph.D. from Osaka University in 1967. He joined the faculty at Osaka University, where he is now Professor. His main research interests concentrate on inorganic materials containing rare earth elements.

Nobohito Imanaka was born in Kawanishi, Hyogo, Japan, in 1958. He received his B.E. and M.E. in applied chemistry from Osaka University. He then obtained a Ph.D. from Osaka

University. He has been on the faculty at Osaka University since 1988. His main researchjelds include rare earths and functional materials such as solid electrolytes and chemical sensors.

Hiromichi Aono was born in Imabari, Ehime, Japan, in 1963, and received his B.E. in industrial chemistry from Ehime University in 1986. He has been on the faculty in the Department of Industrial Chemistry at Niihama National College of Technology since 1986. He obtained a Ph.D. from Osaka University in 1994 for "Studies on Li' ionic conducting solid electrolytes of the NASICON-type structure ". His main interests are the electrical properties of solid electrolytes and their applications.

0 VCH Verlagsgesellschaft mbH. 0-69469 Weinheim, 1996 0935-9648/96/0202-0128 $10.0+.25/0 Adv. Marer. 1#6,8, No. 2

ADVANCED MATERIALS G.-y. Adacki, N. Imanaka, H. AonolFast Li' Conducting Ceramic Electrolytes reactions might occur because all the ions in a liquid electrolyte easily migrate.

The utilization of an organic polymer solid electrolyte is a way to overcome solvent leakage. The maximum conduc- tivity found for polymeric solid-Lie ion conductors such those based on 2-(2-methoxyethoxy)ethylglycidylether (MEEGE)/ethylene oxide (EO) copolymers containing LiN(S02CF3)2 is around 10-5-10-4Scm-' at room temperature.['q This value is still about two orders of magnitude lower than that of the organic solvent.

Recently, Angel1 et al. reported that rubbery Lie electrolytes, where a small amount of a polymer, e.g.

polypropylene oxide (PPO), is mixed with a LiC104- LiC103 based salt, exhibits a conductivity higher than lop4 S cm- ' even at room temperature."*] However, these organic polymers cannot be used at temperatures below their glass transition temperature (Tg) because of the reduction in conductivity.

If the organic solvent in the lithium battery can be replaced by inorganic Lie-conducting solid materials, the following advantages are expected:

0 No leakage of the liquid electrolyte 0 Broad operating temperature range, since inorganic solid electrolytes do not freeze, boil, or ignite in an applied situation 0 Excellent charge-discharge cyclic properties since no side reactions occur and only one kind of carrier ion migrates 0 Long life-time because of little self discharge

In this article we review fast Lie-conducting ceramic solid electrolytes mainly from the electrical property and crystal structure viewpoints.

2. Li@-Ion Conducting Non-Oxide Based Ceramics

(Parte 1 de 5)