Factors Affecting the Volumetric Energy Density of Lithium-Ion Battery Materials: Particle Density Measurements and Cross-Sectional Observations of Layered LiCo_1–<i>x</i>Ni_<i>x</i>O₂ with 0 ≤ <i>x</i> ≤ 1

Volumetric capacity <i>Q</i>_vol (mAh cm^–3), more correctly, volumetric energy density <i>W</i>_vol (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 <i>W</i>_vol (<i>W</i>_vol^act) is currently limited to 40–60% of the maximum (theoretical) value of <i>W</i>_vol (<i>W</i>_vol^max), for reasons that have not yet been fully clarified. Thus, to gain information that will enable an increase in <i>W</i>_vol^act such that it is closer to <i>W</i>_vol^max, systematic studies of the values for <i>Q</i>_vol, <i>W</i>_vol, true density (<i>d</i>_XRD), and particle density (<i>d</i>_p) obtained using gas pycnometry were undertaken for LiCo_1–<i>x</i>Ni_<i>x</i>O₂ samples with 0 ≤ <i>x</i> ≤ 1. Here, <i>d</i>_p is the density that includes the volume of the closed pores in the particles, and consequently is less than <i>d</i>_XRD, which is determined by X-ray diffraction (XRD) measurement. <i>D</i>_XRD monotonically decreased from 5.062(1) g cm^–3 for <i>x</i> = 0 to 4.779(1) g cm^–3 for <i>x</i> = 1, as expected. On the contrary, <i>d</i>_p decreased almost linearly from 4.98(2) g cm^–3 for <i>x</i> = 0 to 4.80(2) g cm^–3 for <i>x</i> = 0.5, then suddenly dropped to 4.63(2) g cm^–3 for <i>x</i> = 0.667, and finally leveled off to a constant value (∼4.6 g cm^–3) at larger values of <i>x</i>. The cross-sectional observations using a Focused Ion Beam system revealed that the significantly smaller values for <i>d</i>_p compared with those for <i>d</i>_XRD, particularly when <i>x</i> > 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 <i>W</i>_vol^act for LIBs. The formation of well-developed primary particles as a mean for increasing the value of <i>d</i>_p was also investigated

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 (Δ<i>H</i>) 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 Δ<i>H</i> as a function of voltage window, a safety map for LIBs and ALIBs is proposed

Toward Positive Electrode Materials with High-Energy Density: Electrochemical and Structural Studies on LiCo_<i>x</i>Mn_2–<i>x</i>O₄ with 0 ≤ <i>x</i> ≤ 1

To obtain positive electrode materials with higher energy densities (<i>W</i>s), we performed systematic structural and electrochemical analyses for LiCo_<i>x</i>Mn_2–<i>x</i>O₄ (LCMO) with 0 ≤ <i>x</i> ≤ 1. X-ray diffraction measurements and Raman spectroscopy clarified that the samples with <i>x</i> ≤ 0.5 are in the single-phase of a spinel structure with the <i>Fd</i>3̅<i>m</i> space group, whereas the samples with <i>x</i> ≥ 0.75 are in a mixture of the spinel-phase and Li₂MnO₃ phase with the <i>C</i>2/<i>m</i> space group. The <i>x</i>-dependence of the discharge capacity (<i>Q</i>_dis) indicated a broad maximum at <i>x</i> = 0.5, although the average operating voltage (<i>E</i>_ave) monotonically increased with <i>x</i>. Thus, the <i>W</i> value obtained by <i>Q</i>_dis × <i>E</i>_ave reached the maximum (=627 mW h·g^–1) at <i>x</i> = 0.5, which is greater than that for Li­[Ni_1/2Mn_3/2]­O₄. Furthermore, the change in the lattice volume (Δ<i>V</i>) during charge and discharge reactions approached 0%, that is, zero-strain, at <i>x</i> = 1. Because Δ<i>V</i> for <i>x</i> = 0.5 was smaller than that for Li­[Ni_1/2Mn_3/2]­O₄, the <i>x</i> = 0.5 sample is found to be an alternative positive electrode material for Li­[Ni_1/2Mn_3/2]­O₄ with a high <i>W</i>

Role of Oxide Ions in Thermally Activated Lithium Diffusion of Li[Li_1/3Ti_5/3]O₄: X‑ray Diffraction Measurements and Raman Spectroscopy

Although Li­[Li_1/3Ti_5/3]­O₄ (LTO) has been considered as an ideal electrode material for lithium-ion batteries (LIBs) because of its “zero-strain” character, initial LTO exhibits high Li conductivity (σ_Li) at high temperatures (<i>T</i>). In this paper, to clarify the inter-relation between LTO’s Li-diffusive nature and structural environment, we performed a systematic structural study on LTO using X-ray diffraction (XRD) measurements and Raman spectroscopy. The average and static information obtained by XRD measurements suggested that the bottleneck radius for Li conduction is limited to ∼0.41 even at 873 K, which is too small to explain the high σ_Li values in LTO. However, Raman spectroscopy demonstrated the dynamic structural changes of the LiO₆ octahedron with <i>T</i>; the bond interaction between Li and O atoms decreases with <i>T</i> because of its anharmonic potential energy. Because the Raman-active modes in LTO correspond to changes in oxide ion position, oxide ions are determined to play a crucial role in obtaining high σ_Li values

Toward Improving the Thermal Stability of Negative Electrode Materials: Differential Scanning Calorimetry and <i>In Situ</i> High-Temperature X‑ray Diffraction/X-ray Absorption Spectroscopy Studies of Li₂ZnTi₃O₈ and Related Compounds

Negative electrode materials with high thermal stability are a key strategy for improving the safety of lithium-ion batteries for electric vehicles without requiring built-in safety devices. To search for crucial clues into increasing the thermal stability of these materials, we performed differential scanning calorimetry (DSC) and in situ high-temperature (HT)-X-ray diffraction (XRD)/X-ray absorption (XAS) up to 450 °C with respect to a solid-solution compound of Li4/3–2x/3ZnxTi5/3–x/3O4 with 0 ≤ x ≤ 0.5. The DSC profile of fully discharged x = 0.5 (Li2ZnTi3O8) with a LiPF6-based electrolyte could be divided into three temperature (T) regions: (i) T ≤ 250 °C for ΔHaccumi, (ii) 250 °C T ≤ 350 °C for ΔHaccumii, and (iii) T > 350 °C for ΔHaccumiii, where ΔHaccumn is the accumulated change in enthalpy in region n. The HT-XRD/XAS analyses clarified that ΔHaccumi and ΔHaccumii originated from the decomposition of solid electrolyte interphase (SEI) films and the formation of a LiF phase, respectively. Comparison of the DSC profiles with x = 0 (Li[Li1/3Ti5/3]O4) and graphite revealed the operating voltage, i.e., amount of SEI films, and stability of the crystal lattice play significant roles in the thermal stability of negative electrode materials. Indeed, the highest thermal stability was attained at x = 0.25 using this approach

Understanding the Zero-Strain Lithium Insertion Scheme of Li[Li_1/3Ti_5/3]O₄: Structural Changes at Atomic Scale Clarified by Raman Spectroscopy

Lithium titanium oxide Li­[Li_1/3Ti_5/3]­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 <i>ex situ</i> Raman spectroscopy on Li_1+<i>x</i>[Li_1/3Ti_5/3]­O₄ samples with 0 ≤ <i>x</i> ≤ 0.94 to gain information about the relationship between a zero-strain reaction scheme and structural change at the atomic scale. The <i>x</i> = 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 <i>x</i> 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 <i>A</i>_1<i>g</i> mode of symmetric stretching vibration ν_sym(Ti–O), the <i>E</i>_<i>g</i> mode of asymmetric stretching vibration ν_asym(Li–O), and the <i>F</i>_2<i>g</i> mode of bending vibration δ­(Ti–O), respectively. Thus, the change in the Raman spectrum with <i>x</i> indicates that the bond length between the Ti and O atoms in the TiO₆ octahedron is independent of <i>x</i>, 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 <i>x</i>. Raman studies with decreasing <i>x</i> 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

Structural Phase Transition from Rhombohedral (<i>R</i>3̅<i>m</i>) to Monoclinic (<i>C</i>2/<i>m</i>) Symmetry in Lithium Overstoichiometric Li_1+δCo_1−δO_2−δ

Stoichiometric lithium cobalt oxide LiCoO₂ is known to exhibit several structural phase transitions with <i>x</i> in Li_<i>x</i>CoO₂ at ambient temperature (<i>T</i>); e.g., an initial rhombohedral (<i>R</i>3̅<i>m</i>) phase transforms into a monoclinic (<i>C</i>2/<i>m</i>) phase at <i>x</i> ∼ 0.5. In contrast, lithium overstoichiometric (Li)_3<i>b</i>[Li_δCo_1−δ]_3<i>a</i>O_2−δ with δ ≥ ∼0.02, where δ is the Li⁺ ions at the 3<i>a</i> (Co) site, maintains the <i>R</i>3̅<i>m</i> symmetry until <i>x</i> ∼ 0.5 in Li_<i>x</i>(Li_δCo_1−δ)­O_2−δ at ambient <i>T</i>, and this is the reason why such material has been widely used in commercial lithium ion batteries. We performed X-ray diffraction measurements in the <i>T</i> range between 100 and 300 K for the lithium overstoichiometric Li_<i>x</i>(Li_0.02Co_0.98)­O_1.98 samples with <i>x</i> = 1, 0.56, and 0.51 to understand the factors that govern the structural changes in Li_<i>x</i>(Li_δCo_1−δ)­O_2−δ with δ ≥ 0. Both <i>x</i> = 0.56 and 0.51 samples exhibit a structural phase transition from the high-<i>T R</i>3̅<i>m</i> phase to the low-<i>T C</i>2/<i>m</i> phase at 250 K (=<i>T</i>_s1). Furthermore, these samples indicate another structural phase transition at 170 K (=<i>T</i>_s2); although their crystal structures still have the <i>C</i>2/<i>m</i> symmetry, the degree of monoclinic distortion starts to decrease below <i>T</i>_s2, associated with a magnetic anomaly and a freezing of the Li⁺ ions at the 3<i>b</i> site. Because the two structural phase transitions of <i>T</i>_s1 (=330 K) and <i>T</i>_s2 (=150 K) are also observed for the stoichiometric Li_<i>x</i>CoO₂ compound with <i>x</i> ∼ 0.5, the <i>C</i>2/<i>m</i> phase in Li_<i>x</i>(Li_δCo_1−δ)­O_2−δ is found to appear in the limited <i>x</i> and <i>T</i> ranges. The characteristics and possible origin of <i>T</i>_s1 and <i>T</i>_s2 for both stoichiometric Li_<i>x</i>CoO₂ and lithium overstoichiometric Li_<i>x</i>(Li_0.02Co_0.98)­O_1.98 samples are discussed

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 Li_0.1Na_0.7Co_0.5Mn_0.5O₂ (or Li_0.8Co_0.5Mn_0.5O₂) 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