title: Factors affecting the volumetric energy density of lithium-ion battery materials: particle density measurements and cross-sectional observations of layered LiCo(1-x)Ni(x)O2 with 0 ≤ x ≤ 1.
abstract: 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.
authors.affiliation: Toyota Central Research and Development Laboratories, Inc. , 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan.
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Mukai
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Nakano