Experimental Mn and Ni K-edge X-ray absorption near-edge structure (XANES) spectra were well reproduced for 5 V-class LixNi0.5Mn1.5O4 spinels as well as 4 V-class LixMn2O4 spinels using density functional theory. Local environmental changes around the Mn or Ni centres due to differences in the locations of Li ions and/or phase transitions in the spinel oxides were found to be very important contributors to the XANES shapes, in addition to the valence states of the metal ions.

•All-solid-state lithium-ion batteries were assembled using a Li2.2C0.8B0.2O3 electrolyte.•Reversible charge–discharge profile of LiCoO2 was observed.•Large repetitive expansion–contraction of the electrode affects fatigue failure.Oxide-based all-solid-state lithium-ion battery is prepared by a conventional sintering process, thanks to the intrinsic low melting point of Li2.2C0.8B0.2O3. A well-defined interface between LiCoO2 and Li2.2C0.8B0.2O3 was confirmed without any traces of impurities. Li ion reversibly (de-)intercalated from/into LiCoO2 at initial charge–discharge process when the charge capacity was limited to 120 mAh g− 1. The capacity degradation after subsequent cycling was suppressed by further limitation of the charging capacity. However, capacity fade could still be confirmed after 20 cycles albeit the capacity was limited at 60 mAh g− 1. This study suggests large repetitive expansion–contraction of the electrode during cycling as a possible cause of fatigue failure of the electrode/oxide electrolyte interface.

The cobalt-based fluorophosphate Li2CoPO4F positive electrode has the potential to obtain high energy density in a lithium ion battery since its theoretical capacity is 287 mAh·g–1 when two electrons can react reversibly. This material promises to charge/discharge with an extremely high redox-couple voltage of over 4.8 V vs Li/Li+. Bulk structural analyses including X-ray diffraction, Co K-edge X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) reveal that an orthorhombic LiβCoPO4F phase is produced from pristine Li2CoPO4F by a combination of solid-solution and two-phase reaction manners during the first charging process, and these phases reversibly transform during charge–discharge cycling. The results of 7Li MAS NMR and classical molecular dynamics simulations suggest that Li ions located at Li(1) sites intercalate/deintercalate through a 1D diffusion path along the b axis, whereas those located at Li(2) and Li(3) sites are fixed. The aforementioned analyses were successfully performed with the enhancement of electrochemical properties by use of a fluoroethylene carbonate-based electrolyte instead of an ethylene carbonate-based one and reducing its volume. Further enhancement was achieved by adding SiO2 nanoparticles into the electrode slurry. The electrochemical results encourage the possibility of the intercalation/deintercalation of more than one Li ion from/into Li2CoPO4F during electrochemical cycling.

Bulk and surface structural changes induced in a Li5FeO4 positive electrode with a defect anti-fluorite type structure are examined during its initial charge–discharge cycle by various synchrotron-radiation analysis techniques, with a view to determining the contribution of oxygen to its electrochemical properties. Bulk structural analyses including XRD, Fe K-edge XANES and EXAFS reveal that pseudo-cubic lithium iron oxides (PC-LFOs), in the form of LiαFe(4−α)+O2, are formed during the first charging process instead of the decomposition of pristine Li5FeO4. Moreover, the relative volume of this PC-LFO phase varies nonlinearly according to the charging depth. At the same time, the surface lithium compounds such as Li2O cover over the PC-LFO phase, which also contribute to the overall electrochemical reaction, as measured from the O K-edge XANES operating in a surface-sensitive total-electron yield mode. The ratio of these two different reaction mechanisms changes with the depth during the first charging process, with this tendency causing variation in the subsequent discharge capacity retention in relation to the depth of the charging electron and/or temperature of this “Li-rich” positive electrode. Indeed, such behaviour is noted to be very similar to the specific electrochemical properties of Li2MnO3.

X-ray absorption near-edge structure (XANES) spectroscopy, which reveals the features of the electronic and local structure, of lithium manganese oxides LixMn2O4 (x = 0–2) was examined using first-principles calculations. Both the easily observable parts and the tiny peaks of the theoretical Mn K-edge XANES spectra agreed with the experimental spectra. From the theoretical results of two anti-ferromagnetic LiMn2O4 models, the contributions of the Mn3+ ion and Mn4+ ion centers to the XANES spectra differ due to the difference in the overlap between the Mn 4p partial density of state (PDOS) and the O 2p PDOS. Similar results can be also seen by comparing the theoretical XANES spectra and the PDOS between Li(Mn3+Mn4+)O4 and de-intercalated Li0.5(Mn3+0.5Mn4+1.5)O4 and Mn4+2O4 (λ-MnO2). The XANES spectral changes with the lithium ion displacement (six- to four-coordination) due to the phase transition (cubic Fdm LiMn2O4 to tetragonal I41/amd Li2Mn2O4) can be determined by the indirect contribution of the Li 2p PDOS to the Mn 4p PDOS via the O 2p PDOS.

•Al2O3-coated Li1.20Mn0.55Ni0.16Co0.09O2 was prepared by the mechanochemical reaction.•Optimal preparation condition was determined by SEM, XRD, and XANES measurements.•Al2O3-coated sample showed better discharge capacity retention at high temperature.•Mechanochemical coating process is an effective way to improve the cycle performance.The 5 wt.% Al2O3-coated Li1.20Mn0.55Ni0.16Co0.09O2 was prepared by the mechanochemical reaction. The optimal condition for sample preparation was determined to be the rotation speed of 2000 rpm and the reaction time of 5 min by SEM, XRD, and XANES measurements. Surface analysis using XANES data demonstrated that all the samples were rather uniformly covered with nano-Al2O3 particles. The pristine and 5 wt.% Al2O3-coated samples after the stepwise pre-cycling treatment showed the discharge capacity of 243 and 216 mAh/g at 323 K, respectively. Although the Al2O3-coated sample showed less discharge capacity compared with the pristine sample, the Al2O3-coated sample showed better discharge capacity retention compared with the pristine sample after 35 cycles at 323 K. These results demonstrate that the mechanochemical Al2O3-coating process is an effective way of improving the cycle performance at high temperature.

Cylindrical lithium-ion (Li-ion) cells with a nickel-cobalt oxide (LiNi0.73Co0.17Al0.10O2) positive electrode and a non-graphitizable carbon (hard carbon) negative electrode were degraded using cycle tests. The degraded cells were disassembled and examined; most attention was paid to the positive electrodes in order to clarify the origin of the power fade of the cells. X-ray absorption near-edge structure (XANES) analysis demonstrated that the crystal structure of the electrode at the surface changed from rhombohedral to cubic symmetry. Furthermore, a film of lithium carbonate (Li2CO3) covered the surface of the positive electrode after the cycle tests. Using a combination of X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and glow discharge optical emission spectrometry (GD-OES) measurements, a schematic model of the changes occurring in the surface structure of the positive electrode during the cycle tests was constructed. The appearance of both an electrochemically inactive cubic phase and lithium carbonate films at the surface of the positive electrode are important factors giving rise to power fade of the positive electrode.

Inorganic compounds on the surfaces of the cathode materials LiNi0.80Co0.15Al0.05O2 and LiCoO2 were studied using Li and O K-edge X-ray absorption near edge structure (XANES) measurements. Rietveld analysis revealed that the LiNi0.80Co0.15Al0.05O2 sample contained 2% Li2CO3, while the LiCoO2 sample was single-phase. The Li and O K-edge XANES spectra indicated that the surface of LiCoO2 was almost free of residual Li2CO3. In contrast, the presence of both residual Li2CO3 and an additional cubic phase were observed, respectively, on and near the surface of LiNi0.80Co0.15Al0.05O2. These results demonstrate that the XANES technique, using a combination of the total electron yield and fluorescence methods, is an effective tool for probing the surfaces of cathode materials.

Li1−yNi0.45Mn0.45Al0.1O2 was synthesized and characterized using both XRD and XANES measurements. The Li/Li1−yNi0.45Mn0.45Al0.1O2 cell showed a discharge capacity of 125 mAh/g in the voltage range 4.5–2.5 V. The Ni K and LII,III-edge XANES results indicated that the Li de-intercalation from LiNi0.45Mn0.45Al0.1O2 proceeded mainly by the valence state change of Ni ions over the whole composition range. Structural analysis clarified that most of the Li ions and a small fraction of the Ni ions migrated from the octahedral 3a site to the tetrahedral 6c site with Li de-intercalation. This observed structural change was similar to that of Li1−yNi0.5Mn0.5O2, indicating that this structural change is characteristic in the Li(NiMn)O2 system.

The crystal structures and electron density distributions of the layered oxide Li1−yNi0.5Mn0.5O2 (y = 0.5) were studied using a combination of Rietveld analysis of high-resolution synchrotron powder X-ray diffraction data and the maximum entropy method (MEM). Structural analysis revealed that Li1−yNi0.5Mn0.5O2 (y = 0.5) has the lattice parameters a = 4.934 Å, b = 2.852 Å, c = 5.090 Å, β = 108.8° and adopts the space group C2/m. The chemical formula can be expressed as [Ni0.0815]2a{Li0.5Ni0.0115}4i[Mn0.5Ni0.407□0.093]2dO2. The electron density map obtained using MEM clearly shows that most of the Li ions migrate from the octahedral 2a site to the tetrahedral 4i site during Li de-intercalation.

Li1−yNi0.5Mn0.4Ti0.1O2 (y=0 and 0.5) was synthesized and characterized using X-ray diffraction, XAFS, and SQUID measurements. The samples were single-phase and adopted the α-NaFeO2 structure. Li1−yNi0.5Mn0.4Ti0.1O2 (y=0 and 0.5) can be represented as Li(Ni2+0.5Mn4+0.4Ti4+0.1)O2 and Li0.5(Ni3+0.5Mn4+0.4Ti4+0.1)O2, respectively. Structural analysis demonstrated that the lattice parameter a decreased from 2.895 to 2.856 Å, the lattice parameter c increased from 14.317 to 14.509 Å, and the Ni–O bond length decreased from 2.06 to 1.94 Å with de-lithiation. The low occupation of Ni on the 3a site was confirmed from the ferromagnetic behavior caused by the 180° Ni2+(3a)–O–Mn4+(3b)–O–Ni2+ (3a) superexchange interaction. These results indicated that lithium de-intercalation from LiNi0.5Mn0.4Ti0.1O2 was controlled mainly by changing the valence state of Ni from 2+ to 3+.

Charge–discharge cycle performance was examined for the cell Li1.08Mn1.92O4/1 M LiPF6 in EC:DEC(1:1)/MCMB. The structure and magnetic properties were determined before and after cycling using X-ray diffraction and SQUID measurements. After both 340 cycles at RT and 150 cycles at 55 °C, single-phase properties were observed for all samples, but samples in the discharged state showed broadening of X-ray diffraction peaks with decrease in the capacity retention ratio. Decreases in lattice parameter and increases in magnetic susceptibility were clearly observed for samples in the discharged state with decreased capacity retention ratios, while small changes in lattice parameter and magnetic susceptibility were observed for samples in the charged state. It was shown that Li intercalation/de-intercalation becomes difficult with cycling in the shallow charged state. Furthermore, it was suggested that the cell capacity decreases because of the disorder in the Li ion conductive pathway caused by the impaired stability of the samples.

High-temperature storage of the lithium manganese spinel cathode was examined for Li1.02−xMn1.98O4 using 1 M LiPF6 in EC:DEC (1:1) electrolyte solution. The structure, magnetic properties, and valence state of Mn were determined before and after the storage using Mn K-edge XANES, X-ray diffraction, SQUID, ICP spectroscopy, and electrochemical measurements. After the storage at 80 °C for 6 days, single-phase property was observed at x=0 and 0.96 in Li1.02−xMn1.98O4, while multi-phase property was observed between the compositions, x=0.18 and 0.63. Shallow de-lithiated region near x=0.18 was easily affected by the storage; the Li/Li1.02−xMn1.98O4 cell showed large capacity failure from 111 to 40 mAh/g after the storage which corresponded to the phase transition with the increase in the valence state of Mn after the storage. Magnetic measurement was found to be quite sensitive and effective to detect the subtle structural changes caused by the high-temperature storage.

Experimental Mn and Ni K-edge X-ray absorption near-edge structure (XANES) spectra were well reproduced for 5 V-class LixNi0.5Mn1.5O4 spinels as well as 4 V-class LixMn2O4 spinels using density functional theory. Local environmental changes around the Mn or Ni centres due to differences in the locations of Li ions and/or phase transitions in the spinel oxides were found to be very important contributors to the XANES shapes, in addition to the valence states of the metal ions.

Bulk and surface structural changes induced in a Li5FeO4 positive electrode with a defect anti-fluorite type structure are examined during its initial charge–discharge cycle by various synchrotron-radiation analysis techniques, with a view to determining the contribution of oxygen to its electrochemical properties. Bulk structural analyses including XRD, Fe K-edge XANES and EXAFS reveal that pseudo-cubic lithium iron oxides (PC-LFOs), in the form of LiαFe(4−α)+O2, are formed during the first charging process instead of the decomposition of pristine Li5FeO4. Moreover, the relative volume of this PC-LFO phase varies nonlinearly according to the charging depth. At the same time, the surface lithium compounds such as Li2O cover over the PC-LFO phase, which also contribute to the overall electrochemical reaction, as measured from the O K-edge XANES operating in a surface-sensitive total-electron yield mode. The ratio of these two different reaction mechanisms changes with the depth during the first charging process, with this tendency causing variation in the subsequent discharge capacity retention in relation to the depth of the charging electron and/or temperature of this “Li-rich” positive electrode. Indeed, such behaviour is noted to be very similar to the specific electrochemical properties of Li2MnO3.

X-ray absorption near-edge structure (XANES) spectroscopy, which reveals the features of the electronic and local structure, of lithium manganese oxides LixMn2O4 (x = 0–2) was examined using first-principles calculations. Both the easily observable parts and the tiny peaks of the theoretical Mn K-edge XANES spectra agreed with the experimental spectra. From the theoretical results of two anti-ferromagnetic LiMn2O4 models, the contributions of the Mn3+ ion and Mn4+ ion centers to the XANES spectra differ due to the difference in the overlap between the Mn 4p partial density of state (PDOS) and the O 2p PDOS. Similar results can be also seen by comparing the theoretical XANES spectra and the PDOS between Li(Mn3+Mn4+)O4 and de-intercalated Li0.5(Mn3+0.5Mn4+1.5)O4 and Mn4+2O4 (λ-MnO2). The XANES spectral changes with the lithium ion displacement (six- to four-coordination) due to the phase transition (cubic Fdm LiMn2O4 to tetragonal I41/amd Li2Mn2O4) can be determined by the indirect contribution of the Li 2p PDOS to the Mn 4p PDOS via the O 2p PDOS.