Article pubs.acs.org/JPCC
Trap State Spectroscopy of LiMyMn2‑yO4 (M = Mn, Ni, Co): Guiding Principles for Electrochemical Performance Krishna Rao Ragavendran,†,‡,* Li Lu,‡ Bing Joe Hwang,§ Klaus Bar̈ ner,∥,* and Angathevar Veluchamy⊥ †
Electrochemistry Group, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Department of Mechanical Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 § Nano Electrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Sec. 4, Taipei 106, Taiwan, Republic of China ∥ Department of Physics, University of Göttingen, F, Hund Platz 1, 37077, FRG, Germany ⊥ Batteries Division, Central Electrochemical Research Institute, Karaikudi 630006, Tamilnadu, India ‡
ABSTRACT: Trap state spectroscopic studies on LiMyMn2‑yO4 (M = Mn, Ni, Co) using the opto-impedance technique revealed that LiMn2O4, LiNiyMn2‑yO4 and LiCo1/6Mn11/6O4 respectively exhibit the phenomena of photoconductivity, photoresistivity, and photoneutrality. An attempt is made in this work to understand the photosensitivity in these materials on the basis of their trap state structure. The material LiCo1/6Mn11/6O4 which is robust toward an optical perturbation is also interestingly stable, compared to the other materials, toward repeated electrochemical charge and discharge.
1. INTRODUCTION Mixed valence perovskites and spinels exhibit interesting photophysics such as photomagnetism in Zn/Ti doped NiFe2O4,1 photo control of magnetization in Al-substituted Fe3O4,2 photo induced lowering of Tc in oxygen deficient YBa2Cu3O7‑δ,3 a photo induced AFM-FM phase transition in Pr0.65Ca0.35MnO3 even in the absence of a magnetic field,4 photoinduced inversion of magnetic poles in Prussian blue analogs5 and spinel type NiFe2O4.6 Photosensitivity in these materials is understood on the grounds of a photo induced intervalent charge transfer mechanism. Photosensitivity of some transition metal oxides, used as cathode materials in lithium batteries, was reported for the first time by Jagannathan et al. using photoluminescence (PL)7 and opto-impedance8,9 spectroscopic techniques. A very interesting advancement made through the above studies was the classification of some well-known cathode materials based on their photoluminescent properties. The materials with good PL properties were poor with their electrochemistry and vice versa. Cathode materials used in lithium batteries can be broadly classified under the following structures: viz. spinel, layered and olivine type structures. Through extensive trial and error experiments, researchers have improved upon the electrochemical performance of these structures by doping, and have optimized the selection of the dopants (M) and their levels (y). Thus, LiCo1/6Mn11/6O410 and LiNi0.5Mn1.5O411 coming under the spinel family, LiNi1/3Mn1/3Co1/3O212,13 belonging to the family of layered oxides, and LiFe0.2Mn0.8PO414 of the olivine family are projected by battery researchers as candidates © 2013 American Chemical Society
possessing superior electrochemical properties for Li-battery applications. A better understanding on the concepts and guiding principles deciding the composition and stoichiometry of the cathode materials is essential not only to design better battery materials, but that this could also bring about new interalia findings in fundamental physics. Doping modifies the defect centers (and hence the trap state structure) of the material, to fulfill the requirement of charge conservation. Hence, the present research work is planned within the framework of trap state spectroscopy. Transient methods such as Time resolved Thermo-electric Effects (TTE),15 Deep Level Trap State Spectroscopy (DLTS), Thermally Stimulated Depolarization (TSD),16 Electron Paramagnetic Resonance (EPR) etc. are usually employed to determine trap state densities, carrier relaxation times, and diffusion coefficients. A photo generated TTE voltage transient has been reported recently in the semiconducting region of NdSrMnO3, and was analyzed using ambipolar space charges.17 The potential of photo induced AC-impedance method in respect to the detection of defects, the trap state densities and the carrier relaxation times is already demonstrated, as trap state spectroscopy, by our group.18 Trap state spectroscopy uses a more versatile and simpler instrumentation as the more specific and involved methods described above but may deliver comparable amounts of information. Received: December 3, 2012 Revised: February 4, 2013 Published: February 4, 2013 3812
dx.doi.org/10.1021/jp3118727 | J. Phys. Chem. C 2013, 117, 3812−3817
The Journal of Physical Chemistry C
Article
In this work, we use trap state spectroscopy and report that the spinel type cathode materials viz., LiMn2O4, LiNiyMn2‑yO4, and LiCo1/6Mn11/6O4, respectively, exhibit the phenomenon of photoconductivity, photo resistivity, and photo neutrality (almost no response to the optical perturbation). The electrochemical behavior of photoneutral LiCo1/6Mn11/6O4 is found to be superior compared to the other spinel type cathode materials which showed a photo response. This observation finds a close analogy with earlier reports7 which pointed out that those cathode materials exhibiting photoluminescence inherited a poor electrochemical performance and vice versa. In general, it seems that the robust behavior of a functional material to an optical perturbation is linked with a virtue for device applications. This statement can be further justified through the well-known fact that the poor application potential of amorphous doped silicon is linked with its possession of Staebler-Wronski light sensitive defects.19
2. EXPERIMENTAL SECTION The experimental procedures employed for the sample synthesis and opto-impedance measurements were the same as described before,9 however, the cathode materials used in this study were synthesized afresh and did not belong to the lot used in our previous study. Phase purity of the materials were confirmed through X-ray Diffraction (Figure 1) measurements, carried out
Figure 1. X-ray Diffraction pattern of LiMn2O4, LiNi0.1Mn1.9O4, and LiCo1/6Mn11/6O4.
using Philips X’pert pro diffractometer with Cu−Kα radiation. Opto-impedance measurements (Figure 2a -2d) on polycrystalline LiMyMn2‑yO4 (M = Mn, Ni, Co) were carried out, in their as prepared condition, using an EIS Princeton Applied Research − AC impedance analyzer (in the frequency range 100 kHz to 10 Hz with an AC amplitude of 20 mV). Time resolved opto-impedance measurements on LiNi0.1Mn1.9O4 were done as follows: a normal AC-impedance measurement on the material was carried out under nonirradiated conditions (labeled “Dark”). The sample was then irradiated with UV radiation (254 nm) for 45 min and an impedance measurement was carried out without switching off the irradiation (labeled “UV”). The UV light was then switched off and subsequent impedance measurements were recorded (labeled ‘10 min’... ‘50 min’) with a time interval of ∼10 min between each measurement. All of the experiments were carried
Figure 2. Opto-impedance patterns of (a) LiMn2O4, (b) LiNi0.4Mn1.6O4, (c) LiNi0.1Mn1.9O4, and (d) LiCo1/6Mn11/6O4. (For the interpretation of the color in the figure legend, the reader is referred to the web version of the article.)
out at room temperature (300 K) which is very close to the charge ordering temperature of LiMn2O4 i.e., 280 K.20,21 3813
dx.doi.org/10.1021/jp3118727 | J. Phys. Chem. C 2013, 117, 3812−3817
The Journal of Physical Chemistry C
Article
Figure 3. Time resolved opto-impedance patterns of LiNi0.1Mn1.9O4 (a) Unilluminated, (b) UV (254 nm) illuminated, (c) 10 min after UV illumination, (d) 20 min after UV illumination, (e) 30 min after UV illumination, (f) 40 min after UV illumination, and (g) 50 min after UV illumination.
solvent. The mixture was stirred overnight to get a homogeneous slurry which was coated over an aluminum foil and dried for 30h at 120 °C in vacuum (i.e., 76 cm Hg). Swagelok type cells were
Composite electrodes for electrochemical studies were made by mixing the cathode material with 10 wt % carbon black and 10 wt % poly vinylidine fluoride in an N-methyl Pyrrolidone (NMP) 3814
dx.doi.org/10.1021/jp3118727 | J. Phys. Chem. C 2013, 117, 3812−3817
The Journal of Physical Chemistry C
Article
Figure 4. Galvanostatic charge discharge curve of (a) LiMn2O4 and (b) LiCo1/6Mn11/6O4 samples in the voltage range 3−4.6 V.
defects, due to the similar ionic radius of Li+ and Ni2+, can thus be expected to exhibit photoresistivity upon illumination. The evolution, with time, of the impedance pattern in LiNi0.1Mn1.9O4 (Figure 2c) is discussed in detail in the next section. Note here, that a comparison between Figure 2c presented in this work, and the opto-impedance pattern reported in our earlier paper9 shows that though the numerical values of the impedance differ in the two cases, the shape of the impedance curves are reproduced perfectly well. This can be understood as being due to a change in the defect structure of the material due to some mild and unavoidable differences in the sample preparation conditions. From Figure 2d, it can be seen that the compound LiCo1/6Mn11/6O4 is not sensitive to illumination. It is important to note that the sensitivity of LiMyMn2‑yO4 to an optical perturbation may also have its origin in a frozen-in nonequilibrium trap state scenario. Under illumination, the trap state occupancy can be brought closer to equilibrium through a defined refilling mechanism, which is seen as photosensitivity. The AC impedance curve of LiCo1/6Mn11/6O4 recorded in the dark condition is almost the same as that recorded under the illuminated condition. The photoneutrality in LiCo1/6Mn11/6O4 can be understood by considering an equilibrium trap state occupancy which renders the system robust and stable toward any perturbation. This virtue of LiCo1/6Mn11/6O4 should be reflected in its electrochemical properties as well and is discussed in section 3.3.
assembled in an M Braun glovebox using two celgard separators, 1 M LiPF6 dissolved in EC/DEC 50:50 as the electrolyte and Lifoil as the negative electrode. The oxygen and moisture level in the glovebox were maintained at
