DEVELOPMENT OF SUPERIONIC CONDUCTING GLASSES FOR SOLID STATE RECHARGEABLE BATTERY - e-Repository BATAN

DEVELOPMENT OF SUPERIONIC CONDUCTING GLASSES FOR

SOLID STATE RECHARGEABLE BATTERY

E. Kartini, Gunawan, H. Jodi

Technology Center for Nuclear Industry Materials, National Nuclear Energy Agency, Indonesia

e-mail: kartini@batan.go.id

ABSTRACT

This article is a review about the ‘Research and Development of Superionic Conducting Glasses for Solid State Rechargeable Battery”, that has been developed by the author and co-workers during the past decade. The main objectives of the research, is to develop new solid electrolyte based on phosphate glasses, starting from the synthesis to the characterization of their properties by various methods, such as X-ray diffraction, SEM, DSC/DTA and LCR meter. Further advanced characterization is by utilizing neutron scattering instruments, such a powder diffraction and a time of flight inelastic neutron scattering, to study its structure and dynamics behavior of superionic conducting glasses. The application of solid electrolyte for a rechargeable battery will be the most important aspect of the research.

Keywords: Superionic glass, Neutron Scattering, Solid Electrolyte, Rechargeable battery.

INTRODUCTION

The study of superionic solids or solid state ionics is a new field of materials science and technology. Most of the solid state devices developed in the last three decades are based on the motion of electrons (particularly in semiconductors), and this field is called electronics. In contrast, ionic solids have received little attention in the past. The field where the movement of ions is done in solids, it is called solid state ionics. Solid with high ionic conductivity are termed as “superionic solids” or “solid electrolyte” [1]. Since then, a large number of such solids have been discovered and numerous applications have been found, such as solid state batteries, fuel cells, memory devices, display panels, etc. In the future, the development of high density battery for vehicular traction and low density miniaturized batteries for device applications will be the most important impact of the research.

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In this article we review several examples of the superionic glasses that were developed in our laboratory, including their basic properties and glass formation. Then come to a section on superionic glass characterization to observe the structural, microstructure, electrical, and thermal properties. The structure and dynamic studies performed by neutron diffraction and inelastic neutron scattering will be discussed in the following section. The possible use of the solid electrolyte for a rechargeable battery will also be described briefly.

SUPERIONIC GLASS

Superionic Glasses

The fast ion conducting glassy materials or superionic glasses generally consist of a glass former (B2O3, SiO2, P2O5, B2S3 etc.,), glass modifier (Ag2O, Li2O etc.,) and a doping salt

(AgI, AgBr, AgCl, CuI, LiI, etc.). These glass modifiers interact strongly with the macromolecular chain formed by the network former, resulting in the breakage of the oxygen or sulphur bonds linking the two former cations. There is an increase in the number of non bridging oxygens with increase in content of the modifier, this increases the carrier concentration and thereby the ion conductivity. Among the solids exhibiting high ionic conductivity , silver ion conductor are important in many respects. Silver based solid electrolyte (AgI)x(AgPO3)1-x is the most well known superionic glasses. The ionic

conductivity of which is 10-2 to 10-3 S/cm at ambient temperature was observed for AgI-AgPO3, while in AgPO3 the conductivity is only ~10-7 S/cm. [4,5,6] The increase of

conductivity of silver ion conducting glasses is very interesting for both scientific and technological aspects. Besides silver glasses, the family of meta-phosphate glasses were also developed in our group, such as NaPO3 and KPO3 and its corresponding glasses, AgI-NaPO3

and AgI-KPO3. In addition, the copper based glasses, such as CuI-CuPO3, CuI-AgPO3 and

CuI-AgI-AgPO3 were also studied [7,8]. Recently we are developing new series of mixed

lithium-silver phosphate glasses, as LiI-LiPO3, AgBr-LiPO3, AgI-LiPO3, AgI-LiI-(MPO3;

M=Li, Ag) [9]. Figure 1 shows example of lithium based superionic glasses obtained from the quenched of molten mixture. The LiPO3 is white and transparent glass, while and

LiI-LiPO3 is a white and opaque glass [10].

Figure 1: Lithium based solid electrolyte (LiI)x(LiPO3)1-xwithx = (a) 0.0, (b) 0.4.[10]

CHARATERIZATION

In any development of new electrolyte or electrode materials, it is important to perform characterization of different properties, such as structure, microstructure, thermal, mechanical and physical properties. The development of better solid state battery systems for device

53 applications depends on the preparation and characterization. In this article we will review some of the characterizations by XRD, impedance spectroscopy, DSC, SEM, Neutron diffraction and Inelastic neutron scattering.

a. Glass formation by X-ray Diffraction

The nature of glassy state of the product can be confirmed by x-ray diffraction. Generally, a dopant salt will solidify into a glassy matrix and produce a new glass for certain composition. Further increasing the dopant salt will produce a mixture of partially crystalline and partially glass sample called a superionic composite-glass. The composition of when the sample starts to have a crystalline Bragg peaks is named as a solubility limit. For every material will have different solubility limit, depends on the salt, the glass and also the quenching rate.

Figure 2: Microstructure of (LiI)x(LiPO3)1-x [10].

b. Microstructure by SEM

The microstructure of the glasses was usually identified from the SEM micrographs. Figure 2 shows the microstructure of (LiI)x(LiPO3)1-x obtained from the casted glasses, as reported in

reference. The sample with x < 0.3 which is found to have a typical glassy homogenous morphology, while for x=0.4 (Fig.2c), is found to have inhomogenities; dispersed phases with several ten nano meters in diameter are observed to be present in the microstructure, although a halo pattern was observed in the x-ray diffraction of this sample. In this composition, an optimum conductivity was reached. Further increasing the amount of LiI, the grain size of LiI precipitated is getting larger and also the grain boundary, reducing the possibility of ions to move, thus decreasing the ionic conductivity.

c. Electrical Property by LCR meter

The conductivity spectroscopy is a powerful tool for analyzing the dynamics of mobile silver ions. Information can be obtained within different frequency windows: the ionic diffusion process dominate at low frequencies and local relaxation process at higher frequencies. For most superionic glasses, the conductivity has been measured at frequency range from ~10 Hz to 1 MHz by impedance spectroscopy. Figure 3a and 3b shows the AC-conductivity and the Arhenius plots of (LiI)x(LiPO3)1-x with x=0.0 (LIX00), x=0.3 (LIX03) and x=0.4 (LIX04) at

different temperatures [9,10]. The conductivity increases with temperature. Normally, the DC conductivity is determined from the plateau of AC conductivity. The behavior of the frequency dependent conductivity data leads to the simple expression for describing frequency and temperature dependent conductivity of superionic materials which can be written as

where  is the frequency and s the exponential factor . Ea is the activation energy and A is the prefactor. If s is equal to zero, the above equation becomes familiar relation of

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

E

kT



A

exp

_a

/

DC







(2)

Figure 3: (a) The frequency and temperature dependence of conductivity ; (b) The Arhennius plots of LIX00, LIX03 and LIX04 [10 ].

It is shown, that at room temperature the conductivity of the superionic glass LIX03 and LIX04 (~10-5 to 10-4 S/cm) are few orders of magnitudes higher than the conductivity of the crystalline LiI (~10-8 S/cm).

NEUTRON SCATTERING

a. Neutron Source and Techniques

Neutrons that have been thermalized in either a reactor or a Spallation source move a little more than the speed of sound in air or ordinary solids. Through the Heisenberg uncertainty principle this means that they have a wavelength of about 2 Å, which is the size of a typical atom, and also about the wavelength of a beam of x-rays. In general there are two kinds of neutron techniques, namely diffraction technique to study the structural properties of the materials, and the spectrometer technique to study the dynamics of materials.

Figure 4: Neutron Diffraction and Inelastic scattering techniques [11]

b. Neutron Diffraction on Superionic Glass

55 because the scattering is dominated by silver and iodine atoms and does not see the phosphate of sulfur well. The scattering is usually described by the scattering function S(Q) ( where Q=4πsin(θ)/λ with θ the scattering angle of Bragg’s Law λ =2dsin(θ). This pattern gives the structure of the glass, as shown in figure 6. We can see that doping gives an extra peak at about 0.7Å-1 which corresponds to a feature at distance 10 Å. This is the distance between the chains when silver iodide put in between [12].

c. Inelastic Neutron Scattering on Superionic Glass

The main observable in the inelastic neutron scattering experiment is the measurement of a dynamic structure factor S(Q, ω). In inelastic scattering events, the neutron either loses energy or gain energy. The scattering vector is given by

2 _'2 _{2 ' cos 2}

Qk k  k k  (3)

And the energy transfer is





where m is neutron mass. In general the neutron scattering is a function of the variables Q and ω. This scattered intensity, denoted by S(Q, ω), is often called the neutron scattering law or the dynamic structure factor for the sample. In the case of the elastic scattering |k|=|k’| and the energy transfer, E=0, so the scattering vector is given by

4

2 sin sin

Q k   



  (5)

The inelastic neutron experiment was performed by a time-of flight MARI instrument at the Spallation Neutron Source ISIS, Rutherford Appleton Laboratory, United Kingdom. The measurements of the dynamic structure factor S(Q, ω) cover both space-time correlation functions. One of the new interesting results on the dynamic behavior of superionic glass is the appearance of low energy excitation at around ~3-5 meV as shown in figure.7. The so-called Boson peak already appears in the insulator glass AgPO3, and the intensity increases with increasing dopant salt AgI [11-13 ].

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APPLICATION OF SUPERIONIC GLASS

a. Lithium ion battery

Figure 6: Alternative layers in Lithium ion Battery [6]

The LiPO3 glass based solid electrolyte will be applied in the design of the all solid state battery. In the preliminary process, the battery will be produced through a palletizing of the electrodes and the electrolyte to produce the coin type lithium battery. Although in the initial stages, only separate layers of the electrodes (LiCoO2, graphite) or layer of the electrolyte will be each studied separately to reveal its physical and electrochemical properties. Following this, the construction of a multilayer electrodes/electrolyte can then be investigated (Figure 6). One cell battery that consists of two electrodes and one solid electrolyte LiCoO2/ LiPO3/C has been designed and fabricated. The cell battery gives the open circuit voltage (OCV) ~ 3.5 Volt, which is in agreement with the theoretical calculation. The battery was inserted into a battery coin holder which connected to the ARBIN BT2000 battery Analyzer System from ARBIN Instrument Co. (Figure 7) BT2000 has multiple, independently controlled potentiostat/ galvanostat channels that can run different tests simultaneously. The software package MITS-PRO 4.0 which are running under MS Windows XP operating system, uses a distributed system control for automatic or manual maintenance and also filters to reduce the fluctuation of current and voltage. The model currently has 4 channels with a voltage range of -10V to 10V can provide a maximum charge/discharge of 10A, current ranges of 10A/1A/0.1A and a maximum channel power of 50W. The equipment has all types of necessary control functions to test all types of battery chemistry for performance characteristics based on current/voltage through the “function” feature.

57 b. Charge Test of coin type Li-ionbattery.

Charge test of Coin type Li-ion Battery was performed on several charge current. Figure 8. shows the charge curve of Li-ion Battery. The curve illustrated output voltage of the battery while the battery is charging.

Figure 8: Charging Curve of Coin type Lithium ion Battery

The battery was charge by current of 2 A, 5 A and 10 A. It was observed that the stronger charge current generated bigger output voltage of battery. The Battery voltage was still increasing gradually when charge process stopped at charge time of 870s as shown by the picture. Factually, the voltage still increasing at point of 1500s while the battery charged by current of 2A. The Battery Output voltage at 900s test time were 2.14V, 2.81V and 3.59V for 2A, 5A and 10A charging process, respectively. Charge Capacity of Battery at 900s test time were 0.51 Ah for 2A charging process, 1.24 Ah for 5A charging process and 2.44 Ah for 10A charging process. Those charge capacity is proportional to the energy required to charge the battery referred as Charge Energy. Charge Energy of the battery were 1.05Wh for 2A charging process, 3.36Wh for 5A and 8.42Wh fo 10A charging process [3,14]. In Life Cycle test Performance, charge-discharge process were very stabil that illustrate the battery performance and also the chemistry was still stabil while the test. Charge Capacity of the battery was 0.135Ah while Discharge Capacity was 0.133Ah. It’s proportional to the Energy of 0.644Wh and 0.425Wh of Charge and Discharge Energy respectively. This Charge Capacity and Charge Energy seem smaller than that of Charge test described before. But it’s natural have tobe smaller because of charge time in the process of Life Cycle test was shorter than charge time of charge test before.

CONCLUSION

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