A spinel oxide and a layered oxide are mixed in cathodes of many commercial lithium-ion batteries.  They are bilaterally oxidized and reduced in the wide voltage range during charge-discharge processes.  Thus, it is difficult to estimate quantitatively their capacities from its simple charge-discharge curves.  The quantitative capacity estimation is important to clarify the detailed charge-discharge mechanism and to forecast the charge-discharge performance in the degraded lithium-ion batteries operated for a long time.  Therefore, a novel technique to quantitatively estimate their individual capacities is necessary for the understanding the detailed mechanism and the reliable long-time operation of lithium-ion battery.  

              Previously, we focus on the relationship between capacity and lattice parameters of the individual active cathode materials from the in-situ X-ray diffraction (XRD) patterns.  We have quantitatively estimated their capacities in the blended cathodes of a degraded battery from the relationship ^[1].  In addition of XRD technique, we have recently demonstrated that a novel analysis technique of X-ray absorption near edge structure (XANES) data lead to quantitative estimation of the valences of nickel (Ni) and cobalt (Co) in LiNi_0.5Co_0.2Mn_0.3O₂ from calculation with the XANES data in the reference samples ^[2]. 

              In this study, we clarify the charge-discharge mechanism of a spinel oxide and a layered oxide in a blended cathode of the battery, which are LiAl_0.1Mn_1.9O₄ (LMO) and LiNi_1/3Co_1/3Mn_1/3O₂(NMC).

EXPERIMENTAL

              The blended cathode (LN73) is fabricated to mix the powders in the weight ratio of LMO:NMC = 70:30 wt.% using conductive materials and a binder together ^[3].  The LN73 are tested electrochemically using pouched cells.  A repeats of the charge-discharge tests are conducted using 2032 coin-type cells under 1C at 50 degrees Celsius.  The charge-discharge performances of the degraded LN 73 cathode are estimated with the pouched cells after disassembling it and washing it by dimethyl carbonate solvent.  

              In situ XRD patterns are obtained during the discharge process using a 15 kW Cu-ka XRD instrument.  The lattice parameters of the LMO and the NMC are obtained from the diffraction peaks of index 111 for the LMO and the index 003 for the NMC.  In situ Ni and Co K-edge XANES spectra were measured during discharge process at BL16B2 of SPring-8 in Japan with transmission mode at room temperature using the pouched cells of the non-degraded and the degraded LN73.  Changes in the valence ranges of 3d transition-metals are obtained from the calculation optimized the ratio of the reference’s XANES data in the blended cathodes, which calculation program is Athena ^[4].  Here, the valence values are expressed for state of charge (SOC) of individual elements.

RESULTS AND DISCUSSION

             We conduct the XANES measurements for the LN73 during charge-discharge processes.  We applied the novel technique of the quantitative valence estimation into the XANES spectra of the LN73.  Figure 1(a) shows the Ni K-edge XANES spectra in the LN73 at 4.0 V, and in the NMC at 4.3 V and before charge-discharge.  The spectrum in the LN73 is calculated using those of the NMC with the lowest (before) and the highest voltages (4.3 V).  Figure 1 (b) shows the measured and calculated spectrum of the LN73 at 4.0 V.  The calculated spectrum is coincident with the measured one.  The technique leads to the estimation of the SOC changes of not only Ni, but also the individual Mn of the LMO and NMC in the LN73 at different voltages (Fig. 2).  The SOC changes of Mn in the LMO and the NMC are coincident with their individual charge-discharge performances and their lattice parameters during charge-discharge processes.

              The XANES analysis technique is applied into the degraded LN73.  Figure 3 show the SOC changes in the individual elements of the LMO and NMC.  The SOC changes in Ni and Co decreases upward in the NMC of the degraded LN73 and the SOC change of Mn does downward in the LMO.  The diminution of the SOC range of Ni is major while that of Co is minor in the degraded LN73.  This novel technique demonstrates that their SOC changes of the individual elements are estimated in in the degraded LN73 and the performance of Ni predominately decreases in the NMC of the LN73.

REFERENCE

[1]. T. KOBAYASHI el al., 65^th Annual Meeiting of ISE, S05-0937(2014).

[2]. T. Kobayashi, et al., 56^th Battery Symposium in Japan, 2C05, 218 (2015).

[3]. T. KOBAYASHI el al. J. Power Sources, 245, 1 (2014).

[4]. B. Ravel, M. Newville, Phys. Scr., T115, 1007 (2005).