Kazuhiko Mukai

Li diffusion in LixCoO2 probed by muon-spin spectroscopy.

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.

Are All-Solid-State Lithium-Ion Batteries Really Safe?–Verification by Differential Scanning Calorimetry with an All-Inclusive Microcell

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.

Static magnetic order in metallic K0.49CoO2.

By means of muon-spin spectroscopy, we have found that K0.49CoO2 crystals undergo successive magnetic transitions from a high-T paramagnetic state to a magnetic ordered state below 60 K and then to a second ordered state below 16 K, even though K0.49CoO2 is metallic at least down to 4 K. An isotropic magnetic behavior and wide internal-field distributions suggest the formation of a commensurate helical spin density wave (SDW) state below 16 K, while a linear SDW state is likely to exist above 16 K. It was also found that exhibits a further transition at 150 K presumably due to a change in the spin state of the Co ions. Since the dependence of the internal-field below 60 K was similar to that for Na0.5CoO2, this suggests that magnetic order is more strongly affected by the Co valence than by the interlayer distance or interaction and/or the charge ordering.

The gradient distribution of Ni ions in cation-disordered Li[Ni1/2Mn3/2]O4 clarified by muon-spin rotation and relaxation (μSR)

Cation-ordered Li[Ni1/2Mn3/2]O4 with a P4332 space group (CO-LNMO) and “cation-disordered” (CDO) LNMO are thought to be the state-of-the-art materials for lithium-ion batteries. However, in contrast to CO-LNMO, the crystal structure and electrochemical reaction scheme of CDO-LNMO are not fully understood. We have measured the muon-spin rotation and relaxation (μSR) spectra for samples of both CO-LNMO and CDO-LNMO, in particular at their magnetic transition temperatures (TC) below 130 K. The weak transverse field (wTF) μSR measurements reveal that the range of TC for the CDO-LNMO sample is very large (ΔTC ∼ 55 K) compared with that for the CO-LNMO sample (ΔTC < 5 K). This suggests an inhomogeneous cation distribution in the CDO-LNMO sample, because the sample consists of multiple phases with different TC. Based on the wTF-μSR result for stoichiometric LiMn2O4, we have proposed that CDO-LNMO is a mixture of Li[Ni1/2−ωMn3/2+ω]O4 and LiMn2O4.

Factors Affecting the Volumetric Energy Density of Lithium-Ion Battery Materials: Particle Density Measurements and Cross-Sectional Observations of Layered LiCo1–xNixO2 with 0 ≤ x ≤ 1

Volumetric capacity Qvol (mAh cm(–3)), more correctly, volumetric energy density Wvol (mWh cm(–3)), is a crucial property of lithium-ion battery (LIB) materials, because LIBs are devices that operate in a limited space. The actual value of Wvol (Wvol(act)) is currently limited to 40–60% of the maximum (theoretical) value of Wvol (Wvol(max)), for reasons that have not yet been fully clarified. Thus, to gain information that will enable an increase in Wvol(act) such that it is closer to Wvol(max), systematic studies of the values for Qvol, Wvol, true density (dXRD), and particle density (dp) obtained using gas pycnometry were undertaken for LiCo1–xNixO2 samples with 0 ≤ x ≤ 1. Here, dp is the density that includes the volume of the closed pores in the particles, and consequently is less than dXRD, which is determined by X-ray diffraction (XRD) measurement. DXRD monotonically decreased from 5.062(1) g cm(–3) for x = 0 to 4.779(1) g cm(–3) for x = 1, as expected. On the contrary, dp decreased almost linearly from 4.98(2) g cm(–3) for x = 0 to 4.80(2) g cm(–3) for x = 0.5, then suddenly dropped to 4.63(2) g cm(–3) for x = 0.667, and finally leveled off to a constant value (~4.6 g cm(–3)) at larger values of x. The cross-sectional observations using a Focused Ion Beam system revealed that the significantly smaller values for dp compared with those for dXRD, particularly when x > 0.5, is due to the presence of closed pores in agglomerated secondary particles. This indicates that the closed pores in the secondary particles play an important role in determining the value of Wvol(act) for LIBs. The formation of well-developed primary particles as a mean for increasing the value of dp was also investigated.

Toward Positive Electrode Materials with High-Energy Density: Electrochemical and Structural Studies on LiCoxMn2–xO4 with 0 ≤ x ≤ 1

To obtain positive electrode materials with higher energy densities (Ws), we performed systematic structural and electrochemical analyses for LiCoxMn2–xO4 (LCMO) with 0 ≤ x ≤ 1. X-ray diffraction measurements and Raman spectroscopy clarified that the samples with x ≤ 0.5 are in the single-phase of a spinel structure with the Fd3̅m space group, whereas the samples with x ≥ 0.75 are in a mixture of the spinel-phase and Li2MnO3 phase with the C2/m space group. The x-dependence of the discharge capacity (Qdis) indicated a broad maximum at x = 0.5, although the average operating voltage (Eave) monotonically increased with x. Thus, the W value obtained by Qdis × Eave reached the maximum (=627 mW h·g–1) at x = 0.5, which is greater than that for Li[Ni1/2Mn3/2]O4. Furthermore, the change in the lattice volume (ΔV) during charge and discharge reactions approached 0%, that is, zero-strain, at x = 1. Because ΔV for x = 0.5 was smaller than that for Li[Ni1/2Mn3/2]O4, the x = 0.5 sample is found to be an alternative positive electrode material for Li[Ni1/2Mn3/2]O4 with a high W.

Superior Low-Temperature Power and Cycle Performances of Na-Ion Battery over Li-Ion Battery

The most simple and clear advantage of Na-ion batteries (NIBs) over Li-ion batteries (LIBs) is the natural abundance of Na, which allows inexpensive production of NIBs for large-scale applications. However, although strenuous research efforts have been devoted to NIBs particularly since 2010, certain other advantages of NIBs have been largely overlooked, for example, their low-temperature power and cycle performances. Herein, we present a comparative study of spirally wound full-cells consisting of Li0.1Na0.7Co0.5Mn0.5O2 (or Li0.8Co0.5Mn0.5O2) and hard carbon and report that the power of NIB at −30 °C is ∼21% higher than that of LIB. Moreover, the capacity retention in cycle testing at 0 °C is ∼53% for NIB but only ∼29% for LIB. Raman spectroscopy and density functional theory calculations revealed that the superior performance of NIB is due to the relatively weak interaction between Na+ ions and aprotic polar solvents.

Relevance between the Bulk Density and Li+-Ion Conductivity in a Porous Electrolyte: The Case of Li[Li1/3Ti5/3]O4

The Li+-ion conductivity (σLi) in an electrolyte is an important parameter with respect to the performance of all-solid-state lithium-ion batteries (LIBs). However, little is known about how σLi in a porous electrolyte differs from that in a highly dense electrolyte. In this study, the relationship between the bulk density (dbulk) and apparent σLi (σLiapp) in a porous electrolyte of Li[Li1/3Ti5/3]O4 (LTO) was examined by theoretical and experimental approaches. The theoretical calculations demonstrated that dbulk and σLi have a simple relationship irrespective of the radius of the spherical pores in the electrolyte; i.e., σLi increases almost linearly with increasing ζ,where ζ is the ratio of d bulk to the theoretical density. In fact, the observed σLiapp of LTO, which was determined by four-probe alternating-current impedance measurements, increased with increasing ζ. Hence, with this relationship, σLiapp can be estimated by ζ and intrinsic σLi (σLiint) and vice versa; such estimations provide critical information for determining the optimum compositions of composite electrodes for all-solid-state LIBs. The temperature dependence of σLiapp in LTO and differences between the calculated and experimental results are also discussed.

Pressure dependence of magnetic transition temperature in Li[LixMn2−x]O4 (0 ≤ x ≤ 1/3) studied by muon-spin rotation and relaxation

In order to study a change in electrochemical, structural, and magnetic properties for lithium manganese oxide spinels Li[LixMn2−x]O4 (LMO) with 0 ≤ x ≤ 1/3, muon-spin rotation and relaxation (μSR) spectra were recorded under pressure (P) up to 2.1 GPa. At ambient P, P = 0.1 MPa, the antiferromagnetic or spin-glass-like transition temperature (Tm) at P = 0.1 MPa monotonically decreases with increasing x. On the contrary, the slope of the Tm vs. P (dTm/dP) rapidly increases from 0.9(1) K/GPa at x = 0 to 1.4 K/GPa at x = 0.1, then drops to 0.7(1) K/GPa at x = 0.15, and finally keeps constant (∼0.4 K/GPa) with further increasing x. Considering the structural change of LMO with x, the decrease in the distance between Mn ions (dMn-Mn) is likely to play an essential role for determining Tm under P. According to cyclic voltammetry on LMO, the peak current at both anodic and cathodic directions shows the maximum at x = 0.1, indicating the highest diffusivity of Li+ ions (DLi) at x = 0.1.

High-pressure synthesis and electrochemical properties of tetragonal LiMnO2

Tetragonal structured LiMnO2 (t-LiMnO2) samples were synthesized under pressures above 8 GPa and investigated as a positive electrode material for lithium-ion batteries. Rietveld analyses based on X-ray diffraction measurements indicated that t-LiMnO2 belongs to a γ-LiFeO2-type crystal structure with the I41/amd space group. The charge capacity during the initial cycle was 37 mA h g−1 at 25 °C, but improved to 185 mA h g−1 at 40 °C with an average voltage of 4.56 V vs. Li+/Li. This demonstrated the superiority of t-LiMnO2 over other lithium manganese oxides in terms of energy density. The X-ray diffraction measurements and Raman spectroscopy of cycled t-LiMnO2 indicated an irreversible transformation from the γ-LiFeO2-type structure into a LixMn2O4 spinel structure by the displacement of 25% of the Mn ions to vacant octahedral sites through adjacent octahedral sites.