Investigation of Various Factors for Capacity Fading of Li-Rich Solid Solution Cathode
Authors: Sasaki Ken , Munakata Hirokazu , Kanamura Kiyoshi 
Published: February 2020

Introduction

Development of high capacity cathode materials with good cycleability is one of the most important tasks to improve the energy density of lithium ion batteries. Li-rich solid solution cathode materials have been focused as promising materials to solve this task. So far, we have focused on and investigated Li1.08Mn0.54Ni0.13Co0.13O2 (LMNC) as a composition to realize good cycleability in the solid solution cathode materials, and found that the control of depth of charge is effective to improve its cycleability. For example, the limited charging up to 280 mAh g-1 improves the discharge capacity retention of 393rd cycle against 4thcycle to 70% from 33% for fully-charging. In this study, various factors such as crystal structure, particle morphology and solid-electrolyte interphase in the initial several cycles were investigated to realize the better cycleability.

Experimental

LMNC was synthesized by sol-gel method. As starting materials, LiNO3, Mn(NO3)2・6H2O, Ni(NO3)2・6H2O, Co(NO3)2・7H2O and citric acid were dissolved into water (250 ml). The precursor sol was dried at 120 oC for 12 h and sintering at 800 oC for 10 h to obtain LMNC. To identify a crystal structure of the sample, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) were conducted. Composite electrodes were made by coating slurry that contained the prepared sample, acetylene black and PVDF in a weight ratio of 80 : 10 : 10 on Al foil as a current collector. NMP was used as a solvent. 2032 coin type cell was assembled with lithium metal as anode, 3-dimensionally ordered macroporous polyimide separator as separator and 1.0 mol dm-3 LiPF6 / EC : DMC = 1 : 2 in volume as electrolyte in argon atmosphere. Charge-discharge test was carried out at 30 oC and current density = 250 mA g-1. The constant current charge was carried out from 2.0 to 4.8 V, and then the constant voltage charge at 4.8 V was conducted until the current decreased to 0.01 C (CCCV mode). After charge and discharge of each cycle from 1st to 3rd cycles, the cathodes were analyzed by XRD and Raman spectroscopy. After 90thcycles, the cathodes were observed by Transmission Electron Microscope (TEM).

Results and discussion

Figure 1 shows TEM images of the LMNC before and after cycles. Figure 1 (a) image shows highly crystallized while (b) image shows a partly change of crystal. These images suggests that the crystallinity of LMNC particles decreases after cycles. In addition, an amorphous layer was observed on the surface of cycled particles.
Figure 2 shows the Raman spectra of before and after cycles. This measurement was conducted under argon atmosphere by air tight cell. The peak of LMNC was observed at 600 cm-1, the peaks of 1400 and 1600 cm-1 attribute to carbon peaks. The peak of LMNC shifted to 625 - 630 cm-1 at charge state. However after discharging, it returned again to 600 cm-1. This peak shift corresponds to the crystal structure change from layer to spinel (LiMn2O4) (2).
The XRD patterns of LMNC before and after cycles shown in Figure 3. The peaks for cathode after cycles are identified to R-3m. Peaks of spinel structure were not observed. From the result of Raman and XRD, structure changes occur only near the surface of LMNC at charge state. The super lattice peaks at around 20o was not observed after cycles. This means disordering of Li/Mn cations in the transition metal rich layer. AS a result of this disordering, Li ion can not easy to intercalate and de-intercalate. This causes the capacity decay at 1st to 3rdcycles.

References (1) K. Sasaki et al, The Battery Symposium in Japan abstract book p204 (2015)
(2) F. Amalraj et al, J. Electrochem. Soc.160, A324 – A337 (2013)

Acknowledgement

TEM observations were supported by Mr. Shinoda of National Institute for Materials Science (NIMS) Battery Research Platform.

Figure 1