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Cure of our addiction to oil and stabilization of the human-induced climate change are overriding issues of the energy policy in the world. Electric vehicles (EVs) are one of the promising technologies for tackling these problems thanks to their higher energy efficiency and compatibility to the green energies in comparison with a conventional gasoline car. A key technology of the EV is a rechargeable battery such as a lithium ion battery (LIB), which has been utilized in our mobile devices; however, further developments of the battery are still necessary for the dissemination of EVs.

A LIB is a kind of rechargeable (secondary) batteries, firstly released by a Japanese company, Sony, by using LiCoO₂ proposed by Mizushima et al. (Prof. Goodenough’s group) [?] as a positive electrode (PE) and graphite as a negative electrode (NE) in 1990’s. Since then, LIBs has contributed to developments of mobile devices thanks to their high energy-density and power, and long cycle life in comparison with the other popularized secondary batteries such as lead-acid storage battery, NiCd battery and nickel-metal hydride battery [?]. Recently, polyvalent rechargeable batteries [?], where the polyvalent cations, e.g., Mg²⁺, Ca²⁺ and Al³⁺ etc., are used as a “guest ion” instead of Li ions, and a dual-salt battery [?] also have attracted attention for their relatively higher energy density and safety than those of LIBs; however, the science of LIBs are still of great importance for the wide applications and as a model system of intercalation chemistry.

In LIBs, an essential electrochemical reaction is Li insertion/extraction (frequently called, lithiation/delithiation) between solid-state electrodes and liquid electrolyte in a typical cell system. As for discharge of the battery, Li atoms are extracted from a NE, e.g., conventionally graphite, to the electrolyte. On the other hand, Li ions in the electrolyte are inserted to a PE.
At the same time, the electrons are also extracted from the NE and inserted into the PE through an outer circuit, which is connected to some devices such as a bulb and a motor in EVs etc. In the PE, on the basis of conventional understanding of solid-state electrochemistry, the valence state of the transition metal, i.e., Co ion in the figure, changes by receiving Li and electron as the charge compensation, which is one of the important electrochemical reaction in the PE.

The above electrochemical reactions are summarized as follows:

\begin{align}
\mathrm{Co(IV)O_{2} + Li^{+} + e^{-} } &\to \mathrm{LiCo(III)O_{2}} \\
\mathrm{LiC_{6}} &\to \mathrm{ C_{6} + Li^{+} + e^{-} }
\end{align}

Thus, the reduction reaction proceeds on the PE, while the oxidation reaction does on the NE in the discharging process. When using a Li metal as a reference electrode in a three-electrode cell in the same electrolyte, the open circuit voltage (OCV) of the positive and negative electrodes corresponds to electrode potential (V vs. Li⁺/Li), which directly corresponds the chemical potential of Li atoms in each electrode by defining chemical potential of Li in the Li metal as a standard condition. Since the electromotive force is the difference of electrode potentials of the PE and the NE, a material of high redox potential is suitable for the PE, whereas that of low redox potential for the NE. Such systems are often called “Rocking chair type battery” because Li ions move between the NE and the PE through the electrolyte; the total amount of Li ions in the electrolyte is always constant during the battery operation. Thus, any combinations of the electrode materials for the PE and NE are allowed in LIBs as long as a candidate material accommodates the considerable amount of Li at a reasonable potential, which may makes the LIB science fascinating. In the current LIBs, since the capabilities of the PE in terms of the energy density, which is the product of potential and capacity, is the bottleneck of the whole battery system and required to be further developed, I would like to focus on structural chemistry of the PEs.

In the era of “Li (primary) battery”, MnO₂ is one of the conventional electrode material, where Li ions are inserted into the MnO₂ structure, whereas the re-extraction of the inserted Li ions is difficult. In contrast, TiS₂ and MoS₂ are capable of the reversible lithiaion/delithiaion; therefore, they were used as the PE in “Li (secondary, rechargeable) battery”, where the Li metal used to be used as the NE. Unfortunately, this kind of battery did not become common, because the use of Li metal in the rechargeable battery was risky due to the short circuit caused by the dendritc plating nature of the Li pure metal. Through the development period of Li batteries, the emergence of a “Li Ion Battery” from Sony started a new era of Li secondary batteries, where graphite and Li transition-metal complex oxides are used as the electrodes.

The most conventional PE material is LiCoO₂ (LCO) [?]. Its crystalline structure is categorized into α-NaFeO₂ type layered rock-salt structure, where Li and Co atoms occupy the octahedral sites in the face centered cubic (FCC) lattice of oxygen atoms. The name of “layered” derives from the layer-by-layer cation ordering of [111] direction in the cubic lattice, which reduces the space symmetry from cubic to hexagonal. Consequently, the structure of LiCoO₂ is understood as a layered compound, where Li occupies the interlayer gallery inbetween CoO₂ sheets (see the figure). LiNiO₂ belongs to this family of electrode, and various related electrodes were reported, e.g., LiNi_1/2Mn_1/2O₂ [?] and LiNi_1/3Mn_1/3Co_1/3O₂[?]. In these modified materials, the available amount of Li in the layered structure was significantly improved, whereas only 0.6Li can be extracted from LCO with keeping the good electrode charge and discharge cycle life, i.e., cyclability.

LiMn₂O₄ (LMO) and LiFePO₄ (LFP) are also important materials, which were reported from the same group of LCO [?]. LMO has the normal spinel structure, where Li and Mn occupy the tetrahedral and octahedral sites in the oxygen FCC framework, respectively. The cyclability of LMO is known to be relatively low because the Jahn-Teller Mn³⁺ ion is unstable in the crystalline and dissolves into the electrolyte during the cycling. Thus, the partial exchange of Mn with the other transition element of Cr, Ni etc., was conducted in order to keep the valence state of Mn to IV, which effectively improved the cyclability and potential of the spinel-type electrode[?].Today it is known as high-voltage electrode materials.Both in LCO and MCO, only the half of the possible valence change of the transition metals is used for the charge compensation accompanied with lithiation/delithiation, because of low availability of the amount of Li atoms in the layered rock-salt structure and high atomic ration of Mn/Li in the spinel electrode, respectively. Thus, the structure of LiFePO₄, which was also reported by Goodenough’s group [?], is understood as follows; the half of the transition metal in the composition of LiCoO₂, is exchanged by phosphorous in order to enhance the structural stability and the electrode potential, which is called ordered-olivine structure and categorized to polyanion compounds. In recent years, LFP is one of the most promising electrodes thanks to high thermal stability, low cost and relatively high capacity. Furthermore, LFP is a favorable model material for the analyses of the electrochemically-driven structural phase transition
[?] owing to its simple two-phase reaction behavior between the wide Li composition. Since the phase transition of LFP causes the considerable volume change about 5%-8%, the effects of the strain energy on the electrochemical capabilities of the electrode have been intensively studied with the phase-field micromechanical simulation [?] and the in-situ simultaneous XRD and XAFS measurement [?] by our group.

As the further developed electrode materials based on polyanionic LFP, high capacity electrodes such as Li₂FeSiO₄ [?], Li₂FePO₄F [?], Li₂MP₂O₇ (M = Fe, Mn) [?] and Li₄NiTeO₆ [?] were reported, where two Li reaction are available and would contribute to the significant improvement of the energy density of the battery. On the other hand, the electrode of the layered rock-salt structure was also developed to be Li-rich layered electrode material, in which LiMO₂ ( M = Mn, Co, Ni ) is stabilized by adding electrochemically inactive Li₂MnO₃ and shows the capacity of > 200 mAh g^-1 by the almost one Li extraction from the structure[?], while the conventional LiMO₂ is unstabilized by only 0.6~0.7 Li extraction. As described above, Li₂MnO₃ is electrochemically inactive because the valence state of Mn in this materials is Mn⁴⁺ and further oxidation by the electrochemical delithiation is generally difficult; however, substitution of Mn atom by other transition metal in forth-period element such as Ru, Mo opens a new way to utilize these Li-rich layered electrodes [?]. Furthermore, the redox reaction of oxygen is found to be available in this family of electrodes, which was demonstrated by substituting Sn in Li₂RuO₃ [?], whereas the importance of the redox contribution of oxygen has been pointed out from the ab initio calculations[?]. Ab initio calculations also greatly contributes to the material design for the electrodes of LIBs [?].

The cation mixing is also very common phenomena in the above electrode materials as well as LNO, which affects diffusion and electrode potential, phase transition, electric and magnetic properties, and eventually the whole electrode properties.Furthermore, the cation mixing is driven by the repetitive cycling, which degrades the energy density of the electrode in the long-term perspective. In some cases, some complex electrode materials have plural sites for a certain transition metal, where the electronic/local structural changes accompanied by lithiation/delithiation would be different each other. Thus, the demonstration of the site-selective analyses by the DAFS method in LNO is of great importance to show how to understand the relationship between the cation-mixing and the electrochemical properties in the electrode materials for LIBs.

References