Although all-solid-state lithium-ion batteries (ALIBs) have been believed as the ultimate safe battery, their true character has been an enigma so far. In this paper, we developed an all-inclusive-microcell (AIM) for differential scanning calorimetry (DSC) analysis to clarify the degree of safety (DOS) of ALIBs. Here AIM possesses all the battery components to work as a battery by itself, and DOS is determined by the total heat generation ratio (ΔH) of ALIB compared with the conventional LIB. When DOS = 100%, the safety of ALIB is exactly the same as that of LIB; when DOS = 0%, ALIB reaches the ultimate safety. We investigated two types of LIB-AIM and three types of ALIB-AIM. Surprisingly, all the ALIBs exhibit one or two exothermic peaks above 250 °C with 20-30% of DOS. The exothermic peak is attributed to the reaction between the released oxygen from the positive electrode and the Li metal in the negative electrode. Hence, ALIBs are found to be flammable as in the case of LIBs. We also attempted to improve the safety of ALIBs and succeeded in decreasing the DOS down to ∼16% by incorporating Ketjenblack into the positive electrode as an oxygen scavenger. Based on ΔH as a function of voltage window, a safety map for LIBs and ALIBs is proposed.

The diffusion coefficient of Li+ ions (D(Li)) in the battery material LixCoO2 has been investigated by muon-spin relaxation (mu+SR). Based on experiments in zero and weak longitudinal fields at temperatures up to 400 K, we determined the fluctuation rate (nu) of the fields on the muons due to their interaction with the nuclear moments. Combined with susceptibility data and electrostatic potential calculations, clear Li+ ion diffusion was detected above approximately 150 K. The D(Li) estimated from nu was in very good agreement with predictions from first-principles calculations, and we present the mu+SR technique as an optimal probe to detect D(Li) for materials containing magnetic ions.

Lithium titanium oxide Li[Li1/3Ti5/3]O4 (LTO) is regarded as an ideal electrode material for lithium-ion batteries because of its “zero-strain” characteristic, high thermal stability, and structural stability. Here, the zero-strain means that the change in cubic lattice parameter is negligibly small during charge and discharge reactions. We performed ex situ Raman spectroscopy on Li1+x [Li1/3Ti5/3]O4 samples with 0 ≤ x ≤ 0.94 to gain information about the relationship between a zero-strain reaction scheme and structural change at the atomic scale. The x = 0 (initial) sample exhibits three major Raman bands at 671, 426, and 231 cm–1 and six minor Raman bands at 751, 510, 400, 344, 264, and 146 cm–1. According to Raman spectroscopy results on other lithium titanium oxides such as Li2TiO3 and TiO2, the Raman bands at 510, 400, and 146 cm–1 are attributed to TiO2 anatase, which is used as a starting material. As x increases from 0 to 0.94, the two major Raman bands at 426 and 231 cm–1 show a blue shift, while the major Raman band at 671 cm–1 maintains frequency. The three major Raman bands at 671, 423, and 231 cm–1 are assigned to the A 1g mode of symmetric stretching vibration νsym(Ti–O), the E g mode of asymmetric stretching vibration νasym(Li–O), and the F 2g mode of bending vibration δ(Ti–O), respectively. Thus, the change in the Raman spectrum with x indicates that the bond length between the Ti and O atoms in the TiO6 octahedron is independent of x, while that between the Li and O atoms in the LiO6 octahedron and the bond angle between the Ti and O atoms in the TiO6 octahedron change with x. Raman studies with decreasing x from 0.94 to 0.10 clarified that such local structural changes are reversible, as in the case for the electrochemical reaction. The zero-strain insertion scheme is discussed from the perspective of Raman spectroscopy.

The magnetism of LixCoO2 (LCO), which has a similar structure to NaxCoO2 (NCO), has been investigated by muon-spin spectroscopy and susceptibility measurements using samples with x=0.1-1 prepared by an electrochemical reaction. In the x range below 0.75, LCO was found to be Pauli paramagnetic down to 1.8 K, suggesting an intermediate- or weak-coupling regime, although disordered local moments, with volume fractions below approximately 20%, appear at low T for LCO with x > or = 0.5. The phase diagram and interactions of LCO are thus strikingly different from NCO, while the differences cannot be explained simply by structural differences between the two systems.

Lithium diffusion in spinel Li 4 Ti 5 O 12 and LiTi 2 O 4 compounds for future battery applications has been studied with muon spin relaxation (μ + SR). Measurements were performed on both thin-film and powder samples in the temperature range between 25 and 500 K. For Li 4 Ti 5 O 12 and above about ∼200 K, the field distribution width () is found to decrease gradually, while the field fluctuation rate (ν) increases exponentially with temperature. For LiTi 2 O 4 , on the contrary, the (T) curve shows a steplike decrease at ∼350 K, around which the ν(T) curve exhibits a local maximum. These behaviors suggest that Li + starts to diffuse above around 200 K for both spinels. Assuming a jump diffusion of Li + at the tetrahedral 8a site to the vacant octahedral 16c site, diffusion coefficients of Li + at 300 K in the film samples are estimated as (3.2 ± 0.8) × 10 −11 cm 2 /s for Li 4 Ti 5 O 12 and (3.6 ± 1.1) × 10 −11 cm 2 /s for LiTi 2 O 4. Further, some small differences are found in both thermal activation energies and Li-ion diffusion coefficients between the powder and thin-film samples.