Inhomogeneous-Lithiation/Homogeneous-Delithiation Occurring in the Composite Electrode at Continuous Cycling

Abstract

1. Introduction For shifting to low carbon society to prevent global warming, high-performance lithium-ion batteries (LIBs) have been applied to power sources for electric vehicles (EVs). The LIBs for EVs require not only an energy density but also a power density. There are several essential rate-determining processes in the LIBs, particularly, the inhomogeneity in the composite electrode during lithiation and delithiation reactions are considered to be a critical issue that lead to capacity decrease at high rate. We have previously demonstrated that the inhomogeneous electrochemical reaction occurring in the cross-section of the composite electrode actually deteriorates the battery capacity at single charging/discharging process [1]. What is more complicated is the capacity deterioration in continuous charging/discharging cycling without rests, as seen in batteries for EVs applications, which has hardly been investigated in previous studies. In this study, we present a new insight about the capacity deterioration during continuous charging/discharging cycling using composite electrodes consisting of LiNi1/3Co1/3Mn1/3O2 (NCM), a typical cathode material of the LIBs. They were examined by energy scanning confocal X-ray diffraction (ES-XRD) technique [2] with high time and spatial resolutions that clarifies the reaction inhomogeneity in the cross-section of the electrode. 2. Experimental Aluminum pouch-type cells were assembled in an Ar-filled glove box consisted of the NCM composite electrode (100 µm in thickness), lithium metal as the counter and reference electrodes, and a porous polyolefin sheet as a separator. The electrolyte was 1 M LiPF6 in a 3:7 (by volume) solution of EC and EMC. All the cycling measurements were performed during continuous charging/discharging cycling under the constant current mode between 4.2 V and 2.0 V without rest process. The cells were pre-conditioned prior to the ES-XRD measurements for several cycles. The ES-XRD experiments were conducted at SPring-8 BL28XU. Four positions at distances of 20 μm (a), 50 μm (b), 80 μm (c) and 100 μm (d) from the surface of the composite electrode were set for the ES-XRD measurements, and were repeatedly measured by stage operation in vertical direction (see Fig. 1a). 3. Result and discussion Figure 1b shows the variation in Li+ concentration in the composite electrode, which was estimated from the ES-XRD measurements during continuous cycling of 1C (278 mA g-1). In the 1st charging process, the difference in the Li+ concentration in the composite electrode (i.e. inhomogeneity) increased in the former half and subsequently decreased in the latter half, and eventually became uniform at the end of charging. In the following discharging process, the difference of the Li+ concentration in the composite electrode expanded by the delay of the lithiation on the counter electrode side. The cell reached the cut-off potential with no relaxation of the inhomogeneity in the composite electrode unlike the charging process. The inhomogeneous lithiation/delithiation is accounted by the Li+ concentration gradient impregnated in the electrode pores between the counter electrode side and the current collector side. The delay of Li+ transportation impregnated in the electrode pores at the current collector side generates the decreasing of the lithiation speed, conversely, the counter electrode side having dense the Li+ concentration increases the lithiation speed to compensate the deficiency. At the 2nd charging, the inhomogeneity at the end of discharging disappeared immediately after starting, and the homogeneous delithiation was maintained until the end of the charging process. During the charging process, Li+ is generally extracted from the active materials, however, Li+ insertion was observed on the current-collector side particularly in the 3th and 4th cycles, which can be attributed to the lithium insufficiency on the current collector side at the end of discharging. These unusual phenomena contributed to rapid relaxation of the inhomogeneity in the composite electrode. In spite of the electrode homogeneity at the end of the charging process, the inhomogeneity during the discharging process extended cycle by cycle, leading to the capacity deterioration only during discharging; i.e. the charging capacity equals to the previous discharging capacity but not vice versa. In the presentation, we discuss the reason for the inhomogeneous lithiation and the homogeneous delithiation occurring in the composite electrode in continuous cycling.