High Precision Coulometry Studies of Single-Phase Layered Compositions in the Li-Mn-Ni-O System

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433

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LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) can deliver a high capacity of ∼200 mAh/g with an average discharge potential of ∼3.8 V (vs. Li + /Li), making it a promising positive electrode material for high energy density lithium ion batteries. However, electrochemical tests from half cells and full cells show poor cycling performance when charged to potentials above 4.2 V. The calendar and cycle lifetimes of cells are affected by the structural stability of the active electrode materials as well as the parasitic reactions that occur in lithium ion batteries. In order to explore the major failure mechanisms of the material, half cells (coin cells) with control electrolyte and full cells (pouch cells) with control electrolyte and with selected electrolyte additives were tested over four different potential ranges. Isothermal microcalorimetry was used to explore the parasitic reactions and their potential dependence. In-situ and ex-situ X-ray diffraction and scanning electron microscopy were used to investigate the structural and morphological degradation of the materials over cycling. It was found that the dramatic c-axis change of the active material during charge and discharge may not be the major problem for cells that are cycled to higher potentials. The parasitic reactions that arise from the interactions between the electrolyte and the highly reactive delithiated cathode surface at high potentials are suggested as the main reason for the failure of cells cycled above 4.2 V. It should be possible to further improve the performance of NMC811 at high potentials by modifying the cathode surface and/or identifying and using electrolyte blends which reduce parasitic reactions. High energy density lithium ion batteries (LIBs) that are cheaper, safer, and with longer lifetimes need to be developed in order to meet the increasing demand for applications such as electric vehicles and large scale stationary energy storage. The energy density of LIBs can be increased by increasing the specific capacity and average potential of the cells. The calendar and cycle lifetimes of cells are affected by the structural stability of the active electrode materials as well as the parasitic reactions that occur in lithium ion batteries. The degree of lithium utilization of LiCoO 2 is limited to ∼70% in order avoid the O3 -H1-3 -O1 phase transformation when charged above 4.45 V.1 Parasitic reactions such as electrolyte oxidation at the cathode-electrolyte interface can ultimately cause cell failure.2-5 The rate of the parasitic reactions is related to both the catalytic role of the cathode surface which depends on its composition and surface area, 3,6 as well as on the stability of the electrolyte.2-5 Methods such as the use of electrolyte additives 7-11 and core-shell positive electrode materials [12][13][14] have been developed and studied to reduce the rate and extent of parasitic reactions, and hence increase capacity retention and lifetime of high-voltage Li-ion cells. The layered lithium Ni-Mn-Co oxides Li 1+x (Ni y Mn z Co (1-y-z) ) 1...

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confidence: 99%

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confidence: 99%

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confidence: 99%

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Study of the Failure Mechanisms of LiNi_0.8Mn_0.1Co_0.1O₂Cathode Material for Lithium Ion Batteries

Downie

et al. 2015

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433

372

“…Samples in that study were cycled up to 4.6 V vs. Li/Li + with the same additive-free electrolyte formulation. 13 Analyses of charge end point capacity slippage and columbic efficiency were previously shown to be excellent rapid screening techniques for improvements to cell lifetime.…”

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The Effect of Lithium Content and Core to Shell Ratio on Structure and Electrochemical Performance of Core-Shell Li_(1+x)[Ni_0.6Mn_0.4]_(1−x)O₂Li_(1+y)[Ni_0.2Mn_0.8]_(1−y)O₂Positive Electrode Materials

Camardese

Abarbanel

et al. 2014

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Core-shell positive electrode materials with a core:shell mass ratio of 2:1 and 4:1 were synthesized in a two-step reaction. Powder X-ray diffraction, SEM and spatial EDS measurements were used to characterize the core and shell phases in the precursors and lithiated products. It was determined using EDS that the precursor and lithiated products are both core-shell and the two phases can be easily resolved with laboratory grade XRD equipment. Two phase Rietveld refinement was completed on the core-shell lithiated products. The results of these refinements in conjunction with contour plots of the lattice parameters within the Li-Ni-Mn oxide layered single phase region were used to position the core and shell of each sample on the Li-Ni-Mn-O phase diagram as a function of the amount of Li 2 CO 3 used in synthesis. The shell phase retained an approximately fixed amount of Li while the Li content of the core phase increased as the overall Li content of the core-shell sample increased. Both the core and shell were electrochemically active. A specific capacity of 220 mAh/g was achieved in a core shell material between 2.5-4.6 V vs. Li/Li + . © The Author ( There has been an extraordinary effort to synthesize and characterize different materials for the positive electrode of lithium-ion batteries.1,2 A primary motivation of this research is to improve the performance of lithium-ion cells to meet requirements for electric vehicle (EVs) and grid storage applications. As the choices of positive electrode materials move away from current commercial staples such as LiFePO 4 5 the cell must be charged to higher potentials, sometimes as high as 4.8 V vs. Li/Li + , to access the additional reversible capacity. At higher potentials, degradation of the electrolyte through oxidation on the positive electrode may vastly reduce the lifetime of the cell. For the high energy density of new materials to be fully utilized, oxidation of the electrolyte must be minimized. Otherwise, these materials are of little commercial interest for EVs and grid storage where lifetime requirements of decades of useful service would be difficult to achieve.The rate of parasitic reactions that degrade the electrolyte at the positive electrode's surface are affected by the composition of the electrolyte, the potential of the positive electrode, the composition of the positive electrode and many other parameters in the construction and operation of the cell. Commonly, researchers are addressing this by improving electrolyte formulations via additives and solvent blends.6,7 A complementary approach is examining the positive electrode surface composition and its effect on the degradation of the electrolyte. [8][9][10][11][12] Rowe et al. 13 showed via high precision charger measurements that the columbic efficiency and charge end point capacity slippage were dependent on the composition of the positive electrode of various LiNi-Mn layered oxides. Samples in that study were cycled up to 4.6 V vs. Li/Li + with the same additive-free electrolyte formulation. ...

“…To examine the state inside the battery without destroying it, we developed an in situ 7 Li solid-state nuclear magnetic resonance (NMR) measurement method, using full-cell test equipment consisting of actual positive and negative electrodes. Using this method, we studied Li insertion/extraction in carbon with charge/discharge operations, Li dendrite growth during overcharges, and the properties of dendritic Li after being formed.…”

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A Study on the Cycle Life of Lithium-Ion Batteries Using In Situ⁷Li Solid-State Nuclear Magnetic Resonance

Arai

Nakahigashi

Sugiyama

2016

The capacity fading of lithium ion batteries (LIBs) is investigated, using in situ 7 Li solid-state nuclear magnetic resonance (NMR). LIB cells consisting of graphite or hard carbon and LiCoO 2 are used and cycled under two different temperatures (10 and 25 • C) and charge current rates (0.5 and 1 C). The cell capacity and the amount of Li stored in carbon are measured with a battery cycler and in situ 7 Li solid-state NMR measurements before the beginning of the test and after every 100 test cycles. The in situ 7 Li solid state NMRs provide sufficient information throughout the cycle tests to characterize all test conditions. The cell capacities are analyzed in terms of the 7 Li NMR peak intensity, attributed to Li stored in carbon. This intensity shows affine proportionality with the cell capacity for every evaluation, with an additive constant that decreases with the increase in cell capacity fading. This may be related to the loss of Li storage caused by voltage "slippage. Lithium-ion batteries (LIBs) have been widely used in portable electronic devices and as power sources for mobile machines such as electric vehicles, hybrid electric vehicles, e-bikes, e-scooters, and electric wheelchairs. To ensure the sustainability and endurance of these devices, it is necessary to understand battery degradation (in both capacity and power) due to daily use.1 LIBs are charged and discharged by Li insertion without chemical reactions, and the degradation of electrode materials is consequently small. LIBs are therefore widely considered to be excellent batteries, with long cycle lives. However, capacity fading occurs when LIBs are used outside moderate temperature ranges, at high current, with high voltage storage, etc. The capacity balance between positive and negative electrodes is lost (called "slippage") because of charge and discharge operations with low accuracy controllers or parasitic reactions during operation and storage.2-9 Slippage causes gradual capacity fading, because cell capacity is determined by the capacity balance between the positive and negative electrodes.Previous studies have reported how and when slippage induces capacity fading, but not how slippage occurs and what it is exactly. To fully understand capacity fading and slippage, nondestructive measurements are needed, because capacity fading due to slippage will only be properly observed if the cell internal conditions are fully maintained.To examine the state inside the battery without destroying it, we developed an in situ 7 Li solid-state nuclear magnetic resonance (NMR) measurement method, using full-cell test equipment consisting of actual positive and negative electrodes. Using this method, we studied Li insertion/extraction in carbon with charge/discharge operations, Li dendrite growth during overcharges, and the properties of dendritic Li after being formed. 10,11 This study investigated the change in NMR peak intensity of the Li stored in the cycle test, to elucidate cell capacity fading in terms of the amount of Li storage at the select...