O2 Positive Electrode Material Research Articles

Zirconium effect on the lithiation mechanism of LiNi0.83Mn0.05Co0.12O2 positive electrode material

Ni-rich LiNi1-x-yMnxCoyO2 (x + y ≤ 0.2) materials are employed to enhance the energy density of lithium-ion batteries, while they are still facing the stability issue. Doping is considered as an efficient approach to improve the stability of the LiNi1-x-yMnxCoyO2 materials. A considerable number of studies have concluded that Zr-doping improves the stability when undoped and doped LiNi1-x-yMnxCoyO2 materials are lithiated under the same conditions. Unfortunately, a fundamental understanding of the Zr dopants role during lithiation conditions is still lacking. This work investigates the Zr effects on lithiation mechanism of LiNi0.83Mn0.05Co0.12O2 (NMC). The main findings are: First, the NMC and Zr-doped NMC have different lattice parameter growth speeds, and the (003) & (101) lattice planes form order structures at lower temperatures during lithiation process when the dopant is included. Second, Zr is prone to locate near the surface region of NMC secondary particles. Third, when lithiation time increases from 8 h to 12 h at 800 °C, the undoped NMC electrochemical performance deteriorates whereas the doped material improves. These findings highlight that understanding the mechanism behind lithiation conditions is essential for doped LiNi1-x-yMnxCoyO2. The insights revealed in this work can guide for design of other dopants for Ni-rich positive electrode materials.

Effect of process conditions on the performance of LiNi0.815Co0.15Al0.035O2 positive electrode material powder synthesized by hydrothermal approach

LiNi0.815Co0.15Al0.035O2 (NCA) lithium-ion battery cathode material was prepared using a hydrothermal approach. XRD and SEM analyzed the crystal parameters and powder morphology of the cathode material. The results show that the nickel cobalt precursor exhibits a petal-like morphology, and the nanoneedle primary particles are randomly distributed. Accompanied by the increase in calcination temperature and time, the peak intensity ratio of I003/I104 tends to decrease and then increase. The peak intensity ratio reached a maximum when the NCA sample calcined at 750°C for 8 h, indicating the lowest extent of Li/Ni mixing and the best electrochemical performance of the positive electrode material.

Long-Term Cycling and Mechanisms of Cell Degradation of Single Crystal LiNi0.95Mn0.04Co0.01O2/Graphite Cells

Extremely high nickel content positive electrode materials have high specific capacity leading to high energy density Li-ion cells. The long-term cycling stability of pouch cells with a single crystal LiNi0.95Mn0.04Co0.01O2 positive electrode material was studied here. Cells with such high nickel content demonstrated excellent cycling when only charged to 4.04 V (about 75% state of charge (SOC)), while they showed more capacity loss when charged to 4.18 V or 100% SOC. Lowering the upper cut-off voltage is in favor of the cycling stability however decreases the cell energy density. The main reason for the capacity loss at 40 °C is due to positive electrode impedance growth, which originated from parasitic reactions between the positive electrode material and the electrolyte, especially when the cells are operated to 4.18 V. There was no noticeable positive electrode particle cracking by scanning electron microscopy and no significant active mass loss even for cells operated to 4.18 V. XRD of cycled positive electrodes indicated no appreciable amount of nickel migrating into the lithium layer, so the impedance growth mainly comes from the positive electrode particle surfaces. Using 1.2 M LiPF6 fluoroethylene carbonate: ethyl methyl carbonate 20:80 electrolyte with 1 wt% lithium difluorophosphate allows cycle life to be extended by reducing impedance growth of the cells.

Comprehensive analysis of boron-induced modification in LiNi0.8Mn0.1Co0.1O2 positive electrode material for lithium-ion batteries

Ni-rich layered oxides (Ni-rich NMCs) are attractive cathodes for LIBs demonstrating remarkable specific capacity. Still, their commercial deployment is hindered by insufficient cycle life. To address this issue, both doping with boron and coating with boron-containing species are known to work effectively, however, the exact nature of stability improvement along with the precise effect of boron on the crystal structure and morphology of Ni-rich NMCs are not comprehensively understood. Herein, we scrutinized B-modified LiNi0.8Mn0.1Co0.1O2 (NMC811) with a combination of bulk and local experimental techniques supported with DFT calculations, both of which exclude insertion of boron into the crystal lattice of NMC811 and indicate that it is present as Li3BO3. We have visualized ∼2 nm Li3BO3 coating at the surface of NMC811 primary particles using electron-energy loss spectroscopy. This coating induces plate-shaped morphology of the primary particles instead of suggested earlier rod- and needle-like shapes and provides enhancement of capacity retention mainly due to improved reversibility of the H2→H3 phase transition upon charge/discharge. This study covers a plethora of boron modification aspects that have remained unclear or controversial so far, eliminating common misconceptions and contributing to understanding of boron impact on the electrochemical properties of Ni-rich NMCs.

Microwave-Assisted Hydrothermal Synthesis of Space Fillers to Enhance Volumetric Energy Density of NMC811 Cathode Material for Li-Ion Batteries

Ni-rich layered transition metal (TM) oxides are considered to be the most promising cathode materials for lithium-ion batteries because of their high electrochemical capacity, high Li+ ion (de)intercalation potential, and low cobalt content. However, such materials possess several drawbacks including relatively low volumetric energy density caused by insufficient values of tap density. Herein, we demonstrate an exceptionally rapid and energy-saving synthesis of the mixed hydroxide precursor for the LiNi0.8Mn0.1Co0.1O2 (NMC811) positive electrode (cathode) material through a microwave-assisted hydrothermal technique. The obtained material further serves as a space-filler to fill the voids between spherical agglomerates in the cathode powder prepared via a conventional co-precipitation technique boosting the tap density of the resulting mixed NMC811 by 30% up to 2.9 g/cm3. Owing to increased tap density, the volumetric energy density of the composite cathode exceeds 2100 mWh/cm3 vs. 1690 mWh/cm3 for co-precipitated samples. The crystal structure of the obtained materials was scrutinized by powder X-ray diffraction and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM); the cation composition and homogeneity of TM spatial distribution were investigated using energy-dispersive X-ray spectroscopy in a STEM mode (STEM-EDX). Well-crystallized NMC811 with a relatively low degree of anti-site disorder and homogeneous TM distribution in a combination with the co-precipitated material delivers a reversible discharge capacity as high as ~200 mAh/g at 0.1C current density and capacity retention of 78% after 300 charge/discharge cycles (current density 1C) within the voltage region of 2.7–4.3 V vs. Li/Li+.

Combined Thermal Runaway Investigation of Coin Cells with an Accelerating Rate Calorimeter and a Tian-Calvet Calorimeter

Commercial coin cells with LiNi0.6Mn0.2Co0.2O2 positive electrode material were investigated using an accelerating rate calorimeter and a Tian-Calvet calorimeter. After cycling and charging to the selected states of charge (SOCs), the cells were studied under thermal abuse conditions using the heat-wait-seek (HWS) method with the heating step of 5 K and a threshold for self-heating detection of 0.02 K/min. The onset temperature and the rate of the temperature rise, i.e., the self-heating rate for thermal runaway events, were determined. The morphology of the positive electrode, negative electrode and the separator of fresh and tested cells were compared and investigated with scanning electron microscopy (SEM). Furthermore, the microstructure and the chemical compositions of the individual components were investigated by X-ray diffraction (XRD) and inductively coupled plasma with optical emission spectrometry (ICP-OES), respectively. In the Tian-Calvet calorimeter, the coin cells with the selected SOCs and the individual components (positive electrode, negative electrode and separator) were heated up with a constant heating rate of 0.1 °C/min (ramp heating mode). Simultaneously, the heat flow signals were recorded to analyze the heat generation. The combination of the three different methods—the HWS method using the ES-ARC, ramp heating mode on both cells and the individual components using the Tian-Calvet calorimeter—together with a post-mortem analysis, give us a complete picture of the processes leading to thermal runaway.

Measuring Parasitic Heat Flow in LiFePO4/Graphite Cells Using Isothermal Microcalorimetry

Isothermal microcalorimetry has previously been used to probe parasitic reactions in Li-ion batteries, primarily studying Li[NixMnyCo1-x-y]O2 (NMC) positive electrode materials. Here, isothermal microcalorimetry techniques are adopted to study parasitic reactions in LiFePO4 (LFP)/graphite cells. Features in the heat flow from graphite staging transitions were identified, and the associated heat flow was calculated using simple lattice-gas mean-field theory arguments, finding good agreement with experimentally measured values. Parasitic heat flow was measured in LFP/graphite pouch cells with different electrolyte additives. In an electrolyte without additives, a massive parasitic heat flow was measured suggesting a shuttle reaction unique to the LFP/graphite system. In cells containing electrolyte additives, parasitic heat flow agreed well with long-term cycling results, confirming the value of this technique to rank the lifetime of LFP/graphite cells with different electrolyte additives. Finally, comparing cells with and without unwanted water contamination, it was found that the parasitic heat flow was similar or slightly higher in cells where water was intentionally removed before cycling, seemingly contradicting long-term cycling results. It is concluded that the presence of water (at the 500 ppm level) may slightly reduce parasitic reactions, but at the expense of a more resistive SEI layer.

A novel phosphonium ionic liquid electrolyte enabling high-voltage and high-energy positive electrode materials in lithium-metal batteries

The synthesis of a new ionic liquid (IL), consisting of the symmetric tetra-butyl-phosphonium (P4444+) cation and the (nonafluorobutanesulfonyl)(trifluoromethanesulfonyl)imide (IM14–) anion, via a facile and environmentally-friendly aqueous route is reported. The novel P4444IM14 IL demonstrates excellent thermal and electrochemical stability (beyond 6 V vs. Li+/Li0 (against Ni-foil)) in combination with good near-room temperature conductivity and ionic liquid characteristics, such as non-measurable volatility and exceptional flame-retardant properties. Employed as electrolyte component in Li-metal cells together with the high-voltage, Li1.2Ni0.2Mn0.6O2 positive electrode material, enables an initial discharge capacity of 264 mA h g-1 at 12.5 mA g−1 and improved initial Coulombic efficiency. Additionally, the discharge capacity retention is promising (92.3% after 50 cycles and 84.4% after 100 cycles) when compared with using a conventional organic carbonate-based electrolyte (55.5% after 50 cycles). Further development of this ionic liquid electrolyte may facilitate the practical application of Li-metal anodes in high-voltage/high-energy lithium batteries.

Revealing Ageing Mechanisms of Ni-Rich NCA in State-of-the-Art Commercial High-Energy Li-Ion Cells

Ni-rich Li[NixCoyAlz]O2 (NCA) positive electrode materials are today found in the state-of-the-art high-energy Li-ion batteries [1]. Considering the high production cost of these batteries combined with the scarcity and environmental impact of the included transition metals, large benefits would be gained by prolonging the lifetime of the Li-ion cells. Although NCAs are the key to a high energy density, they are generally believed to be the main culprit behind the failure of the cell [2]. The NCAs are subjected to adverse structural phase transformation and display increased surface reactivity towards the electrolyte upon electrochemical charging [3]. Particle cracking, surface reconstruction and associated electrolyte decomposition are well-known processes leading to both Li+ inventory loss and increased cell impedance. Yet, several aspects governing these underlying ageing mechanisms remain to be explained, particularly for Ni-rich NCAs contained in state-of-the-art commercial Li-ion cells today.Herein we present a combined structural and electrochemical study based on detailed X-ray diffraction analysis, electron microscopy, and intermittent current interruption cycling. The extent and impact of the various ageing mechanisms at each stage of battery life will be assessed and discussed [4]. Reference s : [1] Yuliya Preger et al 2020 J. Electrochem. Soc. 167 120532[2] A. Manthiram, B. Song, and W. Li, Energy Storage Mater. 6 (2017) pp. 125–139.[3] E. Flores, P. Novák, U. Aschauer, and E. J. Berg, Chem. Mater. 2020, 32, 1, 186–194[4] Mikheenkova et al. In preparation.

Impact of Cr Doping on the Voltage Fade of Li-Rich Mn-Rich Li1.11Ni0.33Mn0.56O2 and Li1.2Ni0.2Mn0.6O2 Positive Electrode Materials

Voltage fade during charge-discharge cycling in Layered Li-rich Mn-rich positive electrode materials needs to be overcome for the development of high-energy low cost Li-ion batteries. Several cation dopants have been introduced into the bulk lattice to mitigate voltage decay by limiting transition metal (TM) migration, inhibiting phase transformation, or reducing the extent of oxygen release. Here, a series of electrochemically active Cr substituted (2.5, 5.0, and 10 mol%) Co-free Li1.11Ni0.33Mn0.56O2 and Li1.2Ni0.2Mn0.6O2 compositions were synthesized via dry particle fusion followed by heat treatment with Li2CO3. Cr doping improves specific capacity and capacity retention via multiple electron transfer of Cr3+/Cr6+ as well as mitigates voltage fading to a certain extent. The impact of Cr on voltage decay was studied by careful measurements of dQ/dV vs V on Cr-doped and undoped samples before and after cycle testing.

Peering into Batteries: Electrochemical Insight Through In Situ and Operando Methods over Multiple Length Scales

Summary In order to overcome the limitations of static and destructive characterizations of Li-ion battery materials and components, comprehensive investigation peering into batteries over various length and time scales is essential. In this regard, emerging in situ and operando characterization methodologies focusing on multiple size domains become a powerful approach to resolve current challenges and navigate future directions. This perspective provides a series of illustrative examples of in situ and operando characterizations over atomic, crystallite/particle, electrode, and battery system length scales with a focus on two electrode material classes: insertion and conversion materials. The operating principles, features, analysis methods, and information gained from each characterization method are discussed. Herein, we present the necessity and opportunity for in situ and operando characterization of electrochemical energy storage materials and systems. Further, we suggest future directions to gain the insight needed to tackle currently intractable issues on Li-ion battery application, failure, and emerging design concepts.

Impact of Al Doping and Surface Coating on the Electrochemical Performances of Li-Rich Mn-Rich Li1.11Ni0.33Mn0.56O2 Positive Electrode Material

Li and Mn-rich positive electrode materials, Li[LixTM1−x]O2 (TM = Ni, Co, and Mn), with a single-phase layered structure have been considered for use in next-generation Li-ion batteries for electric vehicles and many advanced applications. Despite their high specific capacity >250 mAh g−1, the commercialization of these materials is hindered by poor rate capability and voltage decay originating from transition metal migration to the lithium layer. Herein, the effect of aluminum doping and aluminum oxide surface coating on the structural and electrochemical performances of Co-free Li1.11Ni0.33Mn0.56O2 was studied. All synthesized materials were single phase with a similar morphology and amount of Ni in the Li layers. Even though the discharge capacity and capacity retention were slightly improved, there was no significant impact of the addition of Al on the rate of voltage fading during charge-discharge cycling as previously reported.

Novel Composition of Co-Free LiNi0.875-xMn0.125AlxO₂ Cathode Materials.

Ni-rich LiNi1-xMnxO₂ cathode materials have attracted widespread interest as promising alternative cathode materials owing to their higher capacity, lower cost, and lower toxicity compared to those of LiCoO₂. Therefore, we designed herein a LiNi0.875Mn0.125O₂ positive electrode material. However, as the Ni content increases, the materials suffer from an extensive phase transition during the de-lithiation process owing to the low-bond strength of Ni (391.6 kJ mol-1) and Mn (402 kJ mol-1). In this study, Al-doped LiNi0.875-xMn0.125AlxO₂ (x= 0, 0.05, 0.1) was synthesized using the coprecipitation method. Al had a higher bond strength (512 kJ mol-1) between oxygen and metal ions compared to that of Ni and Mn ions. Additionally, Al is usually stabilized in the form of Al3+. Therefore, the increased bond strength decreased the electrostatic repulsion with oxygen during the de-lithiation process and prevented cation mixing by stabilizing the Ni ion's valence, thereby resulting in increased structural stability. X-ray diffraction (XRD) was used to characterize their structures and calculate the cation mixing value. The electrochemical properties showed that LiNi0.775Mn0.125Al0.1O₂ exhibited the high capacity retention of 97.1% after 30 cycles at 1 C at 55 °C.

Surface Modification of Ni-Rich LiNi0.8Co0.1Mn0.1O2 Cathode Material by Tungsten Oxide Coating for Improved Electrochemical Performance in Lithium-Ion Batteries.

Ni-rich NCM-based positive electrode materials exhibit appealing properties in terms of high energy density and low cost. However, these materials suffer from different degradation effects, especially at their particle surface. Therefore, in this work, tungsten oxide is evaluated as a protective inorganic coating layer on LiNi0.8Co0.1Mn0.1O2 (NCM-811) positive electrode materials for lithium-ion battery (LIB) cells and investigated regarding rate capability and cycling stability under different operation conditions. Using electrochemical impedance spectroscopy, the interfacial resistance of uncoated and coated NCM-811 electrodes is explored to study the impact of the coating on lithium-ion diffusion. All electrochemical investigations are carried out in LIB full cells with graphite as a negative electrode to ensure better comparability with commercial cells. The coated electrodes show an excellent capacity retention for the long-term charge/discharge cycling of NCM-811-based LIB full cells, i.e., 80% state-of-health after more than 800 cycles. Furthermore, the positive influence of the tungsten oxide coating on the thermal and structural stability is demonstrated using postmortem analysis of aged electrodes. Compared to the uncoated electrodes, the surface-modified electrodes show less degradation effects, such as particle cracking on the electrode surface and improvement of the thermal stability of NCM-811 in the presence of electrolyte.

Improved Cycle Performance of LiNi0.8Co0.1Mn0.1O2 Positive Electrode Material in Highly Concentrated LiBF4/DMC

Charge/discharge characteristics of nickel-rich LiNi0.8Co0.1Mn0.1O2 cathode was investigated in highly concentrated lithium tetrafluroborate (LiBF4)/dimethylcarbonate (DMC) electrolytes with better electrochemical stability against oxidation. Raman spectra indicated that almost all DMC solvent molecules coordinated Li+ ions, and BF4− anions were likely to form aggregates (AGGs) with Li+ ions and DMC in the nearly saturated 8.67 mol kg−1 electrolyte. A LiNi0.8Co0.1Mn0.1O2|Li half-cell using the nearly saturated 8.67 mol kg−1 LiBF4/DMC showed a good cycle performance up to 50 cycles at C/10 rate with a capacity retention of 93.3%. Furthermore, cross-sectional SEM images and EDX mappings revealed that the highly concentrated electrolyte effectively suppressed not only oxidative decomposition of electrolyte solution on the particle surface, but also the particle fracture caused by crack propagation.

Effects of Cobalt Deficiency on Nickel-Rich Layered LiNi0.8Co0.1Mn0.1O2 Positive Electrode Materials for Lithium-Ion Batteries.

The role of cobalt deficiency on the crystal structure, surface chemistry, and electrochemical performance of Ni-rich layered LiNi0.8Co0.1Mn0.1O2 (NCM811) positive electrode materials was experimentally studied possibly for the first time. We synthesized pristine and cobalt-deficient NCM811 samples by solid-state reaction. Using a variety of characterization techniques and electrochemical measurements, we show that cobalt nonstoichiometry can suppress Ni2+/Li+ cation mixing, but can simultaneously promote the formation of oxygen vacancies, leading to rapid capacity fade and inferior rate capability compared to pristine NCM811. This study provides new insights into the effects of cobalt deficiency on the cation mixing and electrochemical performance of a Ni-rich layered NCM compound.

Impact of a Titanium-Based Surface Coating Applied to Li[Ni0.5Mn0.3Co0.2]O2 on Lithium-Ion Cell Performance

The effect of a Ti-based surface coating on Li[Ni0.5Mn0.3Co0.2]O2 (NMC532) positive electrode material in NMC532/graphite Li-ion pouch cells has been investigated using high temperature storage testing (60 °C), ultrahigh precision coulometry, electrochemical impedance spectroscopy, accelerating rate calorimetry, and long-term cycling. Several superior electrolyte additive combinations were selected for this study. Comparing data from cells containing coated and uncoated NMC532 showed that the surface coating generally contributed to improved cell performance from many perspectives; however, for one electrolyte additive combination, cells containing coated and uncoated NMC532 had virtually identical excellent performance. In an effort to understand why the coating was effective, X-ray photoelectron spectroscopy was used to study the solid electrolyte interphases on both coated and uncoated NMC532. X-ray fluorescence studies of negative electrodes harvested from aged cells showed that the coating helped to ...

Structural, electrochemical and Li-ion transport properties of Zr-modified LiNi0.8Co0.1Mn0.1O2 positive electrode materials for Li-ion batteries

We modify a nickel-rich layered LiNi0.8Co0.1Mn0.1O2 (NCM811) positive electrode material by substituting the transition metals with Zr to mitigate its structural instability and capacity degradation. We show that Zr, over a concentration range of 0.5–5.0 at.%, can simultaneously reside on and expand the lattice of NCM811 and form Li-rich lithium zirconates on their surfaces. In particular, Li(Ni0.8Co0.1Mn0.1)0.99Zr0.01O2 (1% Zr-NCM811) exhibits the best rate capability among all the compositions in this study. It shows higher cycling durability than the raw NCM811 at both low and high current density lithiation and de-lithiation. According to X-ray photoelectron spectroscopy and cyclic voltammetry measurements, the 1% Zr-NCM811 sample is more chemically/electrochemically stable than the raw. In addition to comparing the diffusivities in the coin-cell measurements, we demonstrate that Zr modification can facilitate Li-ion diffusion in the NCM811 balk material by direct-current polarization measurements. The superior performance of Zr-NCM811 results from the lattice expansion induced by Zr doping and the presence of ion-conducting lithium zirconates partially coated on the surface of Zr-NCM811 particles.

Cation mixing in LiNi0.8Co0.15Al0.05O2 positive electrode material studied using high angular resolution electron channeling X-ray spectroscopy

Nickel-based layered rock-salt cathode materials for lithium-ion secondary batteries have drawn considerable attention due to their high energy capacity, lower cost, and better environmental compatibility than other current potential materials. As the nickel content increases, however, these materials suffer from significant capacity fading during charge-discharge cycles at elevated temperatures. While the capacity deterioration and poor cycling stability are mainly because of the formation of NiO-like rock-salt structures at the surface of active materials, the degradation mechanisms have not been fully investigated. In this study, to clarify the process of formation of inactive NiO-like structures, the degradation mechanism of a cathode-active material, namely, LiNi0.80Co0.15Al0.05O2 (NCA) is studied after a single charge–discharge cycle, by using site-selective energy-dispersive X-ray spectroscopy associated with transmission electron microscopy. It is confirmed that cation mixing occurs between the lithium- and transition metal-sites of the LiMO2 (M = transition metals) layered rock-salt structures during the transition to partially ordered transition states and then to the final NiO-like disordered rock-salt structures. Further, the volume fractions of the partially/fully disordered phases in the NCA primary particles are quantitatively estimated.

Design of Nickel-Based Cation-Disordered Rock-Salt Oxides: The Effect of Transition Metal (M = V, Ti, Zr) Substitution in LiNi0.5M0.5O2 Binary Systems.

Cation-disordered oxides have been ignored as positive electrode material for a long time due to structurally limited lithium insertion/extraction capabilities. In this work, a case study is carried out on nickel-based cation-disordered Fm3 ̅m LiNi0.5M0.5O2 positive electrode materials. The present investigation targets tailoring the electrochemical properties for nickel-based cation-disordered rock-salt by electronic considerations. The compositional space for binary LiM+3O2 with metals active for +3/+4 redox couples is extended to ternary oxides with LiA0.5B0.5O2 with A = Ni2+ and B = Ti4+, Zr4+, and V+4 to assess the impact of the different transition metals in the isostructural oxides. The direct synthesis of various new unknown ternary nickel-based Fm3̅ m cation-disordered rock-salt positive electrode materials is presented with a particular focus on the LiNi0.5V0.5O2 system. This positive electrode material for Li-ion batteries displays an average voltage of ∼2.55 V and a high discharge capacity of 264 mAhg-1 corresponding to 0.94 Li. For appropriate cutoff voltages, a long cycle life is achieved. The charge compensation mechanism is probed by XANES, confirming the reversible oxidation and reduction of V4+/V5+. The enhancement in the electrochemical performances within the presented compounds stresses the importance of mixed cation-disordered transition metal oxides with different electronic configuration.