The Electronic Structures And Transport Properties Of Hole-doped LiCoO₂ Materials

Rechargeable Li-ion cells are key components of the portable products required by today's information-rich, mobile society such as mobile phone, laptop computer, vidicon, digital camera and so on. The performance of Li-ion cells is mainly determined by the properties of cathode (positive electrode) materials. LiCoO2 is the most common cathode material for commercial Li-ion batteries because of its generally better properties than the other candidates. However, an ever-increasing demand for portable electronic devices needs to further improve its properties such as the practical capacity and electronic conductivity etc. Especially, the poor conductivity of LiCoO2 may result in safety problem during usage period, which is a factor restricting its application field. In order to overcome this shortcoming, researchers have attempted substituting one or more than one metal elements for Co atom. In 1997, Tukamoto et al. reported for the first time that the partial substitution of Mg for Co could increase the electronic conductivity by about three orders of magnitude, while the crystal structure remained unchanged. They argued that the doped Mg occupy the Co site in LiCoO₂ lattice, and according to a charge balance mechanism, 2Co³⁺→Mg²⁺+Co⁴⁺ , produce Co⁴⁺ i.e. hole, thus the conductivity of semiconductor LiCoO₂ could be strongly increased. Their experimental results, however, showed that the substitution of Ca belonging to the same group of the periodic table as Mg resulted in non-significant increase of the conductivity. Why so different is the doping effects between Ca and Mg?In order to find the main reasons why there is the great difference of the electronic conductivity between Ca and Mg doped materials, and to further the understanding of the electrical conduction mechanism of hole-doped LiCoO₂, effects of Ca or Mg doping on the electronic structure of LiCoO₂ have been studied within this thesis. The main results obtained are as follows: 1. The investigation of undoped LiCoO₂ system shows that the ideal pure LiCoO₂ phase is the typical energy band insulator, but the free carrier model based on the direct valence to conduction band thermal excitation contradicts Mott-type hopping conduction mechanism found by some experiments, which indicates the samples used in the experiments are not true"undoped"materials, but"hole-doped"materials.1) There is a gap between valence and conduction bands, and the gap width is 1.10eV.2) The valence band (t2g band) is composed of two sub-bands ( a′_1g band and e′_g band), there exists a gap having width of about 0.12eV between them. The width of a1 g′band is about 0.33eV,this considerably narrow character of a1 g′band reflects a strong localization nature of the electronic states comprising it.3) The analysis of electron density difference pattern confirms that d orbital of Co in LiCoO₂ is split into t2g and eg ones, and aσ-bond is formed between eg and p orbital of O. It is found that after formation of the bonds in the system, the t2g orbitals accept electrons and the eg orbitals give up electrons, suggesting the six valence electrons of Co3+ mainly occupy t2g orbitals. This observation is consistent with the result of DOS analysis according which the valence band is mainly comprised of t2g electron states.4) For comparing the results of theory and experiments, we have derived an upper critical conductivity value ofσmax(300K) 2.6×10^-11 Scm^-1. This value is much lower than the previously reported experiment values, showing that the free carrier model based on the valence to conduction bands thermal excitation is not valid for the practical conduction mechanism of the material (Mott-type hopping conduction). 5) The strong localization character comprising a′_1g band causes the hopping conduction to be a main conduction mechanism of the material. However, for the hopping conduction to take place, some holes are necessary to be produced in the valence band. In the so-called"undoped"LiCoO₂ material, Li vacancy is a main lattice defect that can produce holes, on the other hand, the oxygen partial pressure is a key parameter that can control the amount of holes. In addition, some electrically active impurities that the starting materials themselves contain may also be source of the holes. 2. The investigation of Mg-doped LiCoO₂ system shows that Mg-doping give rise to partially occupied acceptor band near the Fermi level, and that the conducting features revealed by the experiments can be interpreted throughout the valence to impurity bands thermal excitation of electrons and the electron hopping in the valence band.1) The bandwidth of the impurity (acceptor) band produced by Mg-doping is about 0.16eV, is very narrow.2) The width of band gap (gap between the top of impurity band and the bottom of conduction band) is 1.03eV, is narrowed by 0.07eV relative to undoped system (1.10eV).3) The spacing between a′_1g and e′_g bands is 0.03eV, is narrowed relative to undoped system (0.12eV), but a′_1g band still has relatively strong localization character.4) The analysis of PDOS (partial density of states) shows that the impurity states produced by Mg-doping arise from 3d-orbitals of Co and 2p-orbitals of O.5) The electron density difference pattern demonstrates the change of electron density in the system occurs mainly on O and Co atoms neighboring the Mg impurity.6) For comparison purpose, the same first principles calculation for a Li-deintercalated system, Li_0.92CoO₂, was carried out. The calculation results show the electronic structures of this system are very similar to those of Mg-doped LiCoO₂, i.e. the deintercalation of Li⁺ also gives rise to the hole states near the Fermi level, and the width of hole band (impurity band) is also basically the same as that of Mg-doped system. However, there are some differences: the empty states in the Li-deintercalated system are very close to the Fermi level, and the degree of overlap between the hole band and the below t2g band is deeper than that of Mg-doped system; the change of electron density in this system is mainly made on O atoms neighboring Li vacancy, while the electron density distributions of all the Co ions surrounding the Li vacancy are equal to each other. These differences are mainly attributed to the difference of the Coulomb screening effects.7) A further investigation on the conduction mechanism of Mg-doped LiCoO₂ system shows the double exchange mechanism is not proper for this system.8) We found a new"high temperature hopping conduction mechanism", and developed a theoretical formula which the mechanism obeys. According to this formula, the conductivity, at relatively higher temperature, linearly increases with temperature.9) A new conduction model for Mg-doped system is proposed: the conduction of this system generally obeys hopping mechanism, but the detailed mechanisms are some different between lower and higher temperatures (relative to room temperature). The low temperature conduction obeys the variable-range hopping, but the holes necessary for hopping to take place are mainly provided by the valence to conduction bands thermal excitation, thus the temperature dependence of conductivity reveals a thermal activation-like character. At an enough high temperature, the conduction mechanism of the material converts from the variable-range hopping to the"high temperature hopping", thereafter the conductivity linearly increases with temperature.3. The results of first principles calculation for Ca-doped LiCoO₂ system show that this system, in contrast with Mg-doped one, has a clear energy gap between the acceptor band and the valence band. It's believed that the existence of this gap is the main factor resulting in the non-significant increase of the electronic conductivity in the Ca-doped LiCoO₂. In addition, the remarkable distinction in the ionic radii of Ca²⁺ and Mg²⁺ can also induce noticeably different effects on the electronic conductivities.1) The electronic structure of Ca-doped system is very similar to that of Mg-doped one: Ca-doping also gives rise to a partially empty impurity band near the Fermi level, and the impurity band widths for the two systems are equal to each other (0.16eV), are very narrow. However, there exist some differences: the band gap width of Ca-doped system is 1.00eV, while 1.03eV for Mg-doped one; a1 g′and e g′bands for Ca-doped system are overlapped with each other; it is found that Ca-doped system has a clear energy gap(gap width: 0.07eV) between the impurity band and the valence band, whereas for Mg-doped system there isn't such a gap.2) The PDOS analysis shows that the impurity band produced by Ca-doping is also developed from 3d-orbitals of Co and 2p-orbitals of O, but the contribution of p-states to the impurity band is more remarkable than for Mg-doped system.3) The electron density difference patterns of two systems are basically the same, but the chemical bond between Ca and O atoms reveals considerably strong covalent-bond character, while not for Mg-doped one.4) Analyzing Mulliken populations of both systems, it is found that a most important difference between Ca and Mg systems is the difference between natures of Ca-O and Mg-O bonds, i.e. Ca-O bond has relatively strong covalent-bond character, while Mg-O bond has ionic nature. It is believed that the difference of chemical bond natures is a main factor resulting in the difference between the electronic structures of these systems.5) An expected formula describing conductivity-temperature dependence of Ca-doped system is proposed. The difference to the generally accepted hopping formula is that our formula more contains an exponential term describing the thermal excitation of electrons from valence band to impurity band. This formula can also be used to explain the experimental conductivity-temperature dependence of Mg-doped LiCoO₂.6) The electronic correlation in the impurity band and the Coulomb gap are not able to give rise to substantial change of the conductivity.7) Investigating the effects of ionic radii of doping elements on the conductivity of LiCoO₂, it is concluded that the remarkable distinction in the ionic radii of Ca²⁺ and Mg²⁺ can also induce noticeably different effects on the electronic conductivities.