Chikaaki Okuda

Capacity-Fading Mechanisms of LiNiO2-Based Lithium-Ion Batteries I. Analysis by Electrochemical and Spectroscopic Examination

I. Analysis by Electrochemical and Spectroscopic Examination Tsuyoshi Sasaki,* Takamasa Nonaka, Hideaki Oka, Chikaaki Okuda, Yuichi Itou, Yasuhito Kondo, Yoji Takeuchi, Yoshio Ukyo,* Kazuyoshi Tatsumi, and Shunsuke Muto* Toyota Central Research and Development Laboratories, Incorporated, Nagoakute 480-1192, Japan Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan

Factors affecting cycling life of LiNi0.8Co0.15Al0.05O2 for lithium-ion batteries

Factors affecting the cycling life of cylindrical lithium-ion batteries of LiNi0.8Co0.15Al0.05O2 (NCA) with graphite were examined in terms of the rechargeable capacity and polarization of NCA derivatives of LizNi0.8Co0.15Al0.05O2−δ (0.8 ≤ z ≤ 1.05). NCA derivatives with rock-salt domains in the structure were prepared by a co-precipitation method and the structures of [Li1−yNiy]3(b)[Ni,Co,Al]3(a)O26(c) based on a space group of Rm were refined by a Rietveld method of the XRD patterns. The electrochemical reactivity of the NCA derivatives with rock-salt domains was examined in non-aqueous lithium cells, and it was found that the rechargeable capacities (Q) of the samples decrease linearly as the amount of rock-salt domain (y) increases. An empirical relation is obtained to be Q = 181.4 − 725.5y in which Q reaches zero at y = 0.25, which is derived from not only the capacity loss owing to inactive rock-salt domains but also the polarization increase. The galvanostatic intermittent titration technique (GITT) measurement told us that polarization of NCA derivatives increases when the amount of rock-salt domains is above 2%, i.e., y > 0.02, and such a relation is remarkable in the lithium insertion direction into the structure, which is ascribed to slow lithium ion mobility due to nickel ions in the lithium layers. The NCA derivatives with increased rock-salt domains of above 2% deteriorate rapidly in non-aqueous lithium cells upon charge and discharge cycles, which is ascribed to the cumulative increase in polarization during charge and discharge. An extended cycling test for cylindrical lithium-ion batteries of the NCA derivatives with a graphite negative electrode at elevated temperature was performed and the quantitative relation is discussed thereof.

Surface-Sensitive X-Ray Absorption Study on LiNi0.8Co0.15Al0.05O2 Cathode Material for Lithium-Ion Batteries

We have used Ni and Co K-edge conversion electron yield X-ray absorption fine structure (CEY-XAFS) and conventional XAFS in transmission mode to investigate LiNi 0.8 Co 0.15 Al 0.05 O 2 , one of the promising cathode materials for Li-ion batteries. The former technique is surface-sensitive, having a probing depth of -90 nm, while the latter is bulk-sensitive. X-ray absorption near edge structure analysis revealed that the bulk-averaged Ni valences for cycle-tested cells and aging-tested cells are lower than that for a fresh cell throughout charging. Further reduction of Ni atoms is observed at the surface of the cathode material particles, and the ranges of the Ni valence change upon charging are narrower than that for a fresh cell, indicating the presence of Ni atoms which are unaffected by charging. Extended X-ray absorption fine structure analysis revealed that the change in bond lengths [Ni-O, Ni-M (M = Ni,Co), Co-0, and Co-M] is consistent with the change in the Ni valence. These electronic and structural changes occur predominantly at the surface and are probably the main causes of capacity fading.

Carbon Nanotube/Nanofibers and Graphite Hybrids for Li-Ion Battery Application

To improve the electrical conductivity of negative electrodes of lithium ion batteries, we applied a direct CVD synthesis of carbon nanomaterials on the surface of graphite particles. To prepare a catalyst, two alternative approaches were utilized: colloidal nanoparticles (NPs) and metal (Ni and Co) nitrate salt precursors deposited on the graphite surface. Both colloidal and precursor systems allowed us to produce carbon nanofibers (CNFs) on the graphite surface with high coverage under the optimum CVD conditions. Electrical measurements revealed that the resistivity of the actual electrodes fabricated from CNFs coated graphite particles was about 40% lower compared to the original pristine graphite electrodes.

In situ XAFS study on cathodic materials for lithium-ion batteries

Ni and Co K-edge X-ray absorption spectra of LiNi0.8Co0.2O2 have been collected using in situ coin cells. To investigate the electronic and structural changes accompanied by the capacity fading during electrochemical cycling and keeping batteries at high temperatures, the cells with different cycling states and keeping conditions (temperature, time) were prepared. Upon charging the cell, the Ni and Co K absorption edge shifted towards higher energy, and the good correlation between the range of chemical shifts upon charging and the capacity of the cell was observed. From quantitative analysis of EXAFS data, it was revealed that the capacity fading is closely related to the Jahn-Teller distortion of the NiO6 octahedron.

Self-extinguishing electrolytes using fluorinated alkyl phosphates for lithium batteries

Conventional Li-ion batteries that use flammable organic electrolytes pose a severe risk to battery safety. One strategy to address this problem has been the application of self-extinguishing solvents, such as trimethyl phosphate; however, it is now apparent that these solvents result in poor Li-ion insertion and instability under overcharge. Here, we investigate some electrolytes based on fluorinated alkyl phosphates, a mixture of tris(trifluoroethyl) phosphate (TFEP) and lithium bis(fluorosulfonyl) amide (LiFSA) with various ratios [TFEP]/[LiFSA] < 8. A super-concentration with [TFEP]/[LiFSA] = 2 enabled the insertion of Li-ions into the graphite anode. The fluorination of the alkyl groups in the phosphate molecule was very effective in improving the thermal stability in the presence of charged cathode and anode active materials. In particular, the release of gases and heat that typically accompanied the reaction of the alkyl phosphate with the charged graphite anode was inhibited significantly. In addition, half cells were fabricated using the super-concentrated electrolyte with [solvent]/[salt] = 2, and the cells were demonstrated to be rechargeable at temperatures as high as 120 °C.

XAFS Study on Deterioration of Cathode Materials for Lithium-Ion Batteries

LiNi0.8CO0.15Al0.05O2, being one of the promising cathode materials for lithium‐ion batteries, shows a distinct capacity fading after charge/discharge cycling and/or storage at high temperatures. The origin of these deteriorations has been explored by investigating the electronic and structural changes of the cathode material using X‐ray absorption spectroscopy. Ni K‐edge XAFS measurements were performed in two different modes: surface‐sensitive conversion electron yield (CEY) mode and bulk‐sensitive transmission mode. The Ni K‐edge XANES showed that, after the cycle and aging tests, the Ni valences at the near‐surface of the cathode particles became much lower than those in bulk. Whereas, the EXAFS showed that the bulk and surface‐averaged Ni–O bond distances remained unchanged after the tests. These electronic and structural changes which occur prominently at near‐surface are probably the main cause of the battery deterioration phenomenon.

XAFS Study on Cathode Materials for Liion Batteries using Xray Microbeam

We attempted to investigate whether the Ni valences change homogeneously in a grain of LiNi0.8Co0.2O2 during charging/discharging Li-ion batteries. In situ XAFS measurements using x-ray microbeam with the size of about 1–2 μm were applied to measure position dependencies of Ni (Co) valences in a grain. In order to investigate the electronic changes caused by capacity fading, three types of coin cells for in situ XAFS measurements were prepared, the cell after one charge/discharge cycle, after 100 cycles at 60 °C and after aging at 60 °C for 21 days, respectively. Two-dimensional fluorescence maps of Ni and Co were obtained to determine measuring points in a grain (the center of a grain, the edge of a grain, etc.). Ni and Co K-edge XAFS spectra were collected in transmission mode for each point. As a result, no apparent differences depending on positions were observed for all types of the cells. It was concluded that the changes of Ni valences during charging/discharging batteries occur homogeneously in a grain, not depending on the degree of capacity fading.

Cathodic Behavior of Layered Manganese Oxides from Potassium Permanganate for Rechargeable Lithium Cells

A hydrothermal reaction of potassium permanganate in water at 200°C leads to potassium manganese dioxide, K 0.33MnO2·0.45H2O, with O3 structure. Layered Li-Mn oxide-based material, Li 0.6Na0.05MnO2+ , with O2 structure was prepared by soft chemical synthesis f rom K0.33MnO2·0.45H2O. The morphology of Li 0.6Na0.05MnO2+ was waterweed-shaped, and exhibits a capacity of about 145 mAh/g in the range of 4.2-2.5 V without transforming to the spinel structure during cycling.

In situ XAFS study on cathode materials for lithium-ion batteries

Ni and Co K-edge X-ray absorption spectra of LiNi0.8Co0.2O2 have been collected using in situ coin cells. To investigate the electronic and structural changes accompanied by the capacity fading during electrochemical cycling and keeping batteries at high temperatures, the cells with different cycling states and keeping conditions (temperature, time) were prepared. Upon charging the cell, the Ni and Co K absorption edge shifted towards higher energy, and the good correlation between the range of chemical shifts upon charging and the capacity of the cell was observed. From quantitative analysis of EXAFS data, it was revealed that the capacity fading is closely related to the Jahn-Teller distortion of the NiO6 octahedron.