High voltage cathode materials Li-excess layered oxide compounds Li[Ni x Li 1/3À2x/3 Mn 2/3Àx/3 ]O 2 (0 < x < 1/2) are investigated in a joint study combining both computational and experimental methods. The bulk and surface structures of pristine and cycled samples of Li[Ni 1/5 Li 1/5 Mn 3/5 ]O 2 are characterized by synchrotron X-Ray diffraction together with aberration corrected Scanning Transmission Electron Microscopy (a-S/TEM). Electron Energy Loss Spectroscopy (EELS) is carried out to investigate the surface changes of the samples before/after electrochemical cycling. Combining first principles computational investigation with our experimental observations, a detailed lithium de-intercalation mechanism is proposed for this family of Li-excess layered oxides. The most striking characteristics in these high voltage high energy density cathode materials are 1) formation of tetrahedral lithium ions at voltage less than 4.45 V and 2) the transition metal (TM) ions migration leading to phase transformation on the surface of the materials. We show clear evidence of a new spinel-like solid phase formed on the surface of the electrode materials after high-voltage cycling. It is proposed that such surface phase transformation is one of the factors contributing to the first cycle irreversible capacity and the main reason for the intrinsic poor rate capability of these materials.

Dynamic structural changes during the first electrochemical charge and discharge cycle in the Li-excess layered oxide compound, Li[Li 1/5 Ni 1/5 Mn 3/5 ]O 2 , are studied with synchrotron X-ray diffraction (SXRD), aberration corrected scanning transmission electron microscopy (a-S/TEM), and electron energy loss spectroscopy (EELS). At different states of charge, we carefully examined the crystal structures and electronic structures within the bulk and have found that increased microstrain is accompanied with the cation migration and a second phase formation which occurs during the first cycle voltage plateau as well as into the beginning of the discharge cycle. The evidence indicates that the oxygen vacancy formation and activation may facilitate cation migration and results in the formation of a second phase. The EELS results reveal a Mn valence change from 4+ to 3+ upon oxygen vacancy formation and recovers back to 4+ at the discharge. The oxygen vacancy formation and activation at the partially delithiated state leads to the generation of several crystal defects which are observed in TEM. Identification of the correlation between microstrain and oxygen vacancy formation during the first electrochemical cycle clarifies the complex intercalation mechanisms that accounts for the anomalous capacities exceeding 200 mAh/g in the Li-excess layered oxide compounds.

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In-situ volume, pressure and thickness measurements were performed on Li-ion pouch cells with various silicon-composite negative electrodes to quantify electrode volume expansion. Li(Ni 1-x-y Co x Al y )O 2 /SiO-graphite, LiCoO 2 /Si Alloy-graphite and Li(Ni 1-x-y Co x Al y )O 2 /nano Si-C pouch cells were tested in this work. Archimedes-type in-situ volume measurements and in-situ thickness measurements showed cell expansion during charge and contraction during discharge due to electrode lithiation and de-lithiation. An in-situ pressure measurement was used to measure the effect of electrode volume expansion on volumetricallyconstrained pouch cells. The volume expansion and contraction profiles measured exhibit a non-linear, asymmetric behavior as a function of cell state of charge for all cell types. To explain this, calculations of the volume expansion contribution of each electrode component were performed. Based on the results of the calculations, conclusions about the mechanisms contributing to the measured expansion profiles can be made. Silicon is an attractive negative electrode material for increasing the energy-density of lithium-ion cells due to its significantly higher specific and volumetric capacity than graphite (3579 mAh/g for silicon and 2194 Ah/L for Li 15 Si 4 vs. 372 mAh/g for graphite and 719 Ah/L for LiC 6 ).1,2 However, unlike graphite in which lithium intercalates in a structurally benign process, silicon alloys with lithium, significantly altering its structure resulting in a large volume expansion of 280%.3 Previous studies have shown how this volume expansion can be detrimental to electrodes by causing constituent particles to electrically disconnect from their current collectors, particles to fracture, and damage to the SEI-all of which result in large capacity fade.1,3-5 Confining silicon to nano-sized domains can reduce internal stress on particles to avoid fracture which can mitigate some of these effects.1,2,6,7 Efforts to reduce electrical disconnection of particles often involve making composites of Si-containing electrode materials with more volumetrically benign materials such as graphite.7-10 This effort aims to take advantage of the high capacity of silicon, while diminishing the overall volume expansion of the composite electrode material.Composite Si-graphite electrodes will experience a volume expansion during silicon lithiation. Additionally, the graphite componentalthough to a much smaller extent-will experience a volume expansion, as it has been shown to expand by 10% during lithium insertion.1,11 Such volume changes have been observed with electrochemical dilatometry techniques for Si-graphite composite electrodes, 7,12,13 graphite electrodes 14 and SiO electrodes. 15 A full cell, with such a composite negative electrode paired with a positive electrode, will be affected by the volume change of the positive electrode as it charges and discharges. For example, LiCoO 2 and Li(Ni 0.8 Mn 0.1 Co 0.1 )O 2 positive electrodes have been shown to experience volume changes of ...

Controlling the wettability between the porous electrode and the electrolyte in lithium-ion batteries can improve both the manufacturing process and the electrochemical performance of the cell. The wetting rate, which is the electrolyte transport rate in the porous electrode, can be quantified using the wetting balance. The effect of the calendering process on the wettability of anode electrodes was investigated. A graphite anode film with an as-coated thickness of 59 µm was used as baseline electrode film and was calendered to produce films with thickness ranging from 55 to 41 µm. Results show that wettability is improved by light calendering from an initial thickness of 59 µm to a calendered thickness of 53 m where the wetting rate increased from 0.375 to 0.589 mm/s 0.5 µ. Further calendering below 53 µm resulted in a decrease in wetting rates to a minimum observed value of 0.206 mm/s 0.5 at a calendered thickness of 41 µm. Under the same electrolyte, wettability of the electrode is controlled to a great extent by the pore structure in the electrode film, which includes parameters such as porosity, pore size distribution, pore geometry and topology. Relations between the wetting behavior and the pore structure as characterized by mercury intrusion and electron microscopy exist and can be used to manipulate the wetting behavior of electrodes.

Relations between synthesis conditions, detailed crystal structures, and electrochemical properties of the Li-excess layered oxides Li͓Ni x Li 1/3−2x/3 Mn 2/3−x/3 ͔O 2 ͑0 Ͻ x Ͻ 1/2͒ are studied by X-ray diffraction, scanning electron microscopy ͑EELS͒, X-ray photoelectron spectroscopy ͑XPS͒, transmission electron microscopy ͑TEM͒, and electron energy-loss spectrometry, combined with electrochemical property measurements including potentiostatic intermittent titration technique ͑PITT͒. Optimal synthesis conditions are obtained for stoichiometric samples sintered at 1000°C in air followed by furnace cooling. The materials exhibit capacities of ϳ250, 230, and 200 mAh/g within a voltage range of 2-4.8 V on discharge for x = 1/5, 1/4 and 1/3, respectively. Diffraction data of electrochemically cycled electrode materials show an expanded c/a lattice ratio and changing Li/Ni interlayer mixing indicating peculiar cation migration in the structures. High resolution TEM images and XPS spectra show obvious differences in the surface characteristics of the samples synthesized with stoichiometric and excess amount of LiOH, suggesting that surface characteristics is one of the contributing factors to the difference in electrochemical properties. Our results suggest that the first cycle irreversible capacity is affected by both the bulk and surface characteristics of pristine materials, which is strongly influenced by precursor chemistry. The PITT results suggest that cation rearrangement during the charge/discharge has a significant impact on the lithium chemical diffusivity.

h i g h l i g h t s Pouch cells have been built for in-situ neutron diffraction study. In-situ neutron diffraction was performed on layered oxide compounds. Ex-situ neutron powder diffraction was performed on Li-excess layered compounds. The dynamic changes in both cathode and anode have been observed simultaneously.

2 O 2 through an operando X-ray diffraction study, which reveals identical trends in lattice parameter evolution and unit cell volume change during cycling for the two materials. In addition, coin-cell cycling experiments used to examine the initial irreversible capacity loss observed for both materials indicate that it is primarily caused by kinetic limitations of lithium intercalation. Additional X-ray diffraction experiments show much higher c-lattice preferred orientation for electrodes made from single-crystal material, and SEM images of the electrodes reveal a shearing effect in single-crystal particles caused by electrode calendaring. Recently, Li et al. demonstrated that electrodes made with singlecrystal NMC532 have superior capacity retention over typical polycrystalline NMC532.1 The primary particles in the single-crystal material, which are each one crystal of NMC532, are larger than the primary particles in typical NMC532 polycrystalline agglomerates. The larger particle size may improve thermal stability (based on similar work involving LiCO 2 12 ). Additionally, the single-crystal material shows more resistance to oxygen loss compared to the polycrystalline material. Charged polycrystalline NMC532 shows a peak in oxygen evolved when the material is heated to 80• C, but under the same conditions single-crystal NMC532 shows no oxygen loss peak. 1 This paper further characterizes the cycling behavior of polycrystalline and single-crystal NMC532 through an operando X-ray diffraction study of the crystal lattice parameters. Additionally, experiments are presented to investigate the first cycle irreversible capacity loss observed for both materials. ExperimentalSingle-crystal NMC532 electrodes used in the operando study were punched from a hand-coated electrode composed of 94 wt% active material (the same single-crystal NMC532 powder described in Reference 1), 4 wt% conductive additive, and 2 wt% polyvinylidene fluoride binder (PVDF). All other electrodes were composed of 96 wt% active material, 2 wt% carbon-black, and 2 wt% PVDF and were extracted from dry pouch cells provided by Li-Fun Technology (Xinma Industry Zone, Golden Dragon Road, Tianyuan District, Zhuzhou City, Hunan Province, PRC, 412000). Electrode punches were dried under vacuum at 110• C for 14 hours before transferring directly to an argon filled glove box for coin cell assembly.Each standard 2325 size coin cell was assembled with one 13 mm diameter dried NMC532 punch (single-sided coating on aluminum * Electrochemical Society Fellow.z E-mail: shy@tesla.com foil), two layers of 19 mm diameter Celgard 2320 separator, a lithium metal counter-electrode, and an excess of electrolyte consisting of 1M LiPF 6 in EC:DEC (1:2 wt:wt). Lithium salt and electrolyte solvents were provided by BASF Canada and were mixed in an argon filled glove box. The capacity of each coin cell was ∼5 mAh, which is expected for this cell format using the electrodes specified. Coin cells for the operando experiment were assembled as described above, but with an inset ...