Positive Electrode For Li-ion Batteries Research Articles

Understanding charge transfer dynamics in blended positive electrodes for Li-ion batteries

This paper investigates the electrochemical behavior of binary blend electrodes comprising equivalent amounts of lithium-ion battery active materials, namely LiNi0.5Mn0.3Co0.2O2 (NMC), LiMn2O4 (LMO), LiFe0.35Mn0.65PO4 (LFMP) and LiFePO4 (LFP)), with a focus on decoupled electrochemical testing and operando X-ray diffraction (XRD). All possible 50:50 blend combinations were studied and the distribution of current between blend components was followed during continuous and pulsed charge and discharge processes. The results demonstrate the significant impact of the voltage profiles of individual materials on the current distribution, with the effective C-rate of each component varying throughout the State of Charge (SoC). Pulsed decoupled electrochemical testing reveals the exchange of charge between blend components during relaxation, showcasing the "buffer effect", which has also been captured through time-resolved operando XRD experiments in real blends carefully considering beam-induced effects. The directionality and magnitude of the charge transfer were found to depend on the nature of the components and the cell SoC, being also influenced by temperature. These dependencies can be rationalized considering both thermodynamics (voltage profile) and reaction kinetics of the blend constituents. These findings contribute to advancing the understanding of internal dynamics in blended electrodes, offering valuable insights for the rational design of blends to meet the diverse operational demands of lithium-ion batteries.

Modeling of Crystal Structure in Li-Rich Layered Oxides for Rechargeable Lithium Cells

Lithium-Ion Batteries (LIBs) require further development in terms of safety and performance to establish also in the automotive market and new materials at both electrode sides as well as at the electrolyte side are necessary to overcome the state of-the-art and the commercial benchmarks. [1] The positive electrode materials constitute the bottleneck for present LIBs. They are based on layered structures (LiCoO2, NMC, NMA), spinel (LiMn2O4) and olivine LiFePO4, with rather moderate specific capacities (120–180 mAhg−1). [2] Therefore, the resulting specific energies are insufficient to meet market demand. Over-stoichiometric Li-rich layered oxides (LRLOs) are a family of positive electrode materials with promising theoretical electrochemical performance. [3] Indeed, they have received widespread attention due to their extremely high specific capacity (>250 mAhg−1) and low cost. Their extraordinary capacity has been confirmed to originate not only from the redox activities of transition metals, but also from the redox activities of oxygen anions. The crystal structure of LRLOs is an open playground of debate. There are two main hypotheses: a) a two phase nano-domain and b) a single phase solid solution. In this framework, the modeling of the crystal structure is quite challenging. Usually, the description of the structure of LRLOs can be done with rhombohedral or monoclinic unit cell. Nevertheless, the use of convention unit cells leads to a bad representation of the crystal structure because of the missing of structure features in the superstructure region (20<θ<30 in typical diffraction pattern with Cu Kα). In this communication, we demonstrate that extensive defectivities play a key role in the shape and intensity of diffractograms of LRLOs. In particular, we show how the use of supercells or defects (antisite defects and stacking faults) can help in the description of crystal structure of this class of materials.[1] Zubi, G.; Dufo-López, R.; Carvalho, M.; Pasaoglu, G. The Lithium-Ion Battery: State of the Art and Future Perspectives. Renewable and Sustainable Energy Reviews 2018, 89, 292–308.[2] Casas-Cabanas, M.; Ponrouch, A.; Palacín, M.R. Blended Positive Electrodes for Li-Ion Batteries: From Empiricism to Rational Design. Isr J Chem 2021, 61, 26–37.[3] Nie, L.; Chen, S.; Liu, W. Challenges and Strategies of Lithium-Rich Layered Oxides for Li-Ion Batteries. Nano Res 2022.

Influence of Binders on Printability and Battery Performance of Thick 3D-Printed Positive Electrode for Li-Ion Batteries

Nowadays, lithium-ion batteries have been considered to be the superior commercial batteries in the market, thanks to their high energy density (until ca. 250 Wh/kg at cell level) and low fabrication cost (actually < 0.3 Euro/Wh at cell level). However, due to the recent global push for electrification, there is a constant need to further improve battery performance while minimalizing the cost. Besides the development of new battery materials, engineering the electrodes could be a valid solution to meet such demand. By increasing the electrode thickness to 200 µm, the active material loading in the electrode is expected to increase to over 30 wt%, hence enhancing the energy density and limiting the cost at the cell level. Nevertheless, thick electrodes fabricated by conventional coating methods suffer severely from weak mechanical strength and delamination as well as long and highly-tortuous Li-ion diffusion path, sluggish electrochemical kinetics, and poor charge/discharge rate performance. Recently, great attention has gone to 3D printing (additive manufacturing) with the promise to overcome these limitations, thanks to the porous structures of the 3D- printed electrodes.In this work, we present 450 µm thick lithium iron phosphate (LFP) electrodes produced by 3D micro-extrusion with a superior LFP loading of ca. 30 mg/cm2 and a high areal capacity of ca. 4.2 mAh/cm2. Two commercial polymeric binders were employed, including carboxymethyl cellulose (CMC) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). It is concluded that both CMC and PEDOT:PSS are essential to obtain well-printed 3D electrodes. While CMC contributes significantly to the mechanical strength of the 3D-printed LFP electrodes, it is of great importance to add PEDOT:PSS to avoid intra-fiber cracks. Furthermore, the electrodes containing both binders have shown significantly improved specific capacity at different C-rates compared to those formulated from either CMC or PEDOT:PSS. Further analyses give insights into charge transfer resistance, tortuosity and electrical conductivity of the investigated electrodes. Additionally, we highlight the difference in the battery performance of 3D-printed electrodes and their standard doctor-bladed counterparts.Reference(1) Kuang, Y.; Chen, C.; Kirsch, D.; Hu, L. Thick Electrode Batteries: Principles, Opportunities, and Challenges. Adv. Energy Mater. 2019, 9 (33), 1901457. https://doi.org/10.1002/aenm.201901457.

Insights on the Buffer Effect in Blended Positive Electrodes for Lithium Ion Batteries

An approach to improve the performance of positive electrodes for Li-ion batteries that is commonly used by companies is fabricating them using mixtures of different materials (usually 2) with complementary features1. This coexistence, when properly formulated, provides an extra design degree of freedom and gives rise to synergetic effects, allowing blends to perform better than what the rule of mixture predicts. Unfortunately, the internal dynamics of the electrodes are still poorly understood at fundamental level and the optimization of formulations for such blends still mainly depends on empirical studies. One of the important characteristics identified in blends is termed the buffer effect. This occurs when the material exhibiting fastest kinetics can receive an excess of the incoming lithium under high power discharge and subsequently, when the current stops, transfer this excess to the higher capacity material.In this study, the role and extent of this effect is explored for a set of representative materials and the resulting blends resulting from their mixtures. For the measurements we are using a special experimental model-like blended electrode setup reported by Heubner et al.2 slightly adapted to our needs. In this setup, two positive electrodes with different active material, but same active mass, are facing each other, separated by 2 separators with a disk of perforated lithium in the middle. During measurement, current is applied between an electrode and lithium while a potentiostat actively keeps the two positive electrodes at the same potential. Such setup gives us access to the current exchanged between the positive electrodes while keeping them in equal potential at all times, as would happen in a blended electrode with 50% of each material.For the study, 4 cathode active materials were used, LiNi0.5Mn0.3Co0.2O2 (NMC), LiMn2O4 (LMO), LiFePO4 (LFP) and LiFe0.4Mn0.6PO4 (LFMP), and every binary combination of them was measured. The cells after 3 C/10 formation cycles were subjected to a pulsed electrochemical test (1 minute 3C pulses followed by 10 minutes of relaxation). Top figure shows the voltage response of the cell vs. time while the bottom one shows the charge that was transferred between the materials during their relaxations (buffering charge) and while the cell current is equal to zero , for the NMC-LMO blend. We applied the same protocol for all the combinations under study and found out large variations of this buffering ability depending on the blends constituents and their individual electrochemical properties. It is clearly evident that both the magnitude and the direction (sign) of the buffering current varies in the voltage range meaning each material can act as donor or acceptor of the buffering lithium. We believe that this provides a new insight in the internal processes in blended electrodes and brings us one step closer to their rational design. References : Casas‐Cabanas et al. Isr J Chem 61, 26–37 (2021).Heubner et al. J Power Sources 363, 311–316 (2017). Figure 1

Understanding the Activation of Anionic Redox Chemistry in Ti4+-Substituted Li2MnO3 as a Cathode Material for Li-Ion Batteries

Layered Li-rich oxides, demonstrating both cationic and anionic redox chemistry being used as positive electrodes for Li-ion batteries, have raised interest due to their high specific discharge capacities exceeding 250 mAh/g. However, irreversible structural transformations triggered by anionic redox chemistry result in pronounced voltage fade (i.e., lowering the specific energy by a gradual decay of discharge potential) upon extended galvanostatic cycling. Activating or suppressing oxygen anionic redox through structural stabilization induced by redox-inactive cation substitution is a well-known strategy. However, less emphasis has been put on the correlation between substitution degree and the activation/suppression of the anionic redox. In this work, Ti4+-substituted Li2MnO3 was synthesized via a facile solution-gel method. Ti4+ is selected as a dopant as it contains no partially filled d-orbitals. Our study revealed that the layered “honeycomb-ordered” C2/m structure is preserved when increasing the Ti content to x = 0.2 in the Li2Mn1–xTixO3 solid solution, as shown by electron diffraction and aberration-corrected scanning transmission electron microscopy. Galvanostatic cycling hints at a delayed oxygen release, due to an improved reversibility of the anionic redox, during the first 10 charge–discharge cycles for the x = 0.2 composition compared to the parent material (x = 0), followed by pronounced oxygen redox activity afterward. The latter originates from a low activation energy barrier toward O–O dimer formation and Mn migration in Li2Mn0.8Ti0.2O3, as deduced from first-principles molecular dynamics (MD) simulations for the “charged” state. Upon lowering the Ti substitution to x = 0.05, the structural stability was drastically improved based on our MD analysis, stressing the importance of carefully optimizing the substitution degree to achieve the best electrochemical performance.

Retardation of Structure Densification by Increasing Covalency in Li-Rich Layered Oxide Positive Electrodes for Li-Ion Batteries

Retardation of Structure Densification by Increasing Covalency in Li-Rich Layered Oxide Positive Electrodes for Li-Ion Batteries

Atomic Layer Vs. Sol-Gel Deposited Coatings for Long Cycle-Life Li-Ion Battery Positive Electrodes

High nickel content layered oxide positive electrodes for Li-ion batteries suffer from capacity fade due to, among other factors, particle cracking induced by lithiation/delithiation, transition metal dissolution into the electrolyte, and attack from HF. Surface modification has been shown to reduce the severity of these degradation mechanisms by blocking transition metal ions from dissolving, and protecting the surface from the electrolyte decomposition products, as well as physically restraining the particles to reduce cracking. Atomic layer deposition (ALD) is a coating method that promises good uniformity and a high level of thickness control with a wide range of possible coating materials. ALD is a derivation of chemical vapor deposition in which chemical precursors are injected separately into a low pressure heated reaction vessel. Precursors are selected to create a self-limiting reaction at each step, allowing monolayers of precursor to be deposited on each cycle.1 With a large chemical space to explore, the challenge remains to find a coating material that offers substantial protection and lifetime extension, while allowing facile Li+ diffusion that does not substantially impede the capacity and rate capability of the cell.Alumina, Al2O3, being the archetypal ALD coating material has been frequently explored to improve the stability of known Li-ion positive electrode materials.2 Recently Wang et al. found marked stability improvement of Al2O3 coated LiNi0.6Mn0.2Co0.2O2 over even as few as 40 charge/discharge cycles.3 In this work, we present a comparison study of alumina coatings on LiNi0.6Mn0.2Co0.2O2 deposited by ALD versus coatings prepared through a sol-gel method tested to 4.5 V vs Li+/Li (Figure 1). Depositing a range of Al2O3 thicknesses by ALD we see a general trend of increased capacity retention with increased coating thickness. Multiple Al-containing precursors have been investigated for sol-gel coatings and we have found varying results, some giving comparable performances to the ALD coated samples, whereas other sol-gel precursors resulted in poorer performance than the uncoated material. Both types of films have been characterised by low energy ion scattering and x-ray photoelectron spectroscopy to detect the presence of the coatings, as well as electron microscopy with energy dispersive x-ray spectroscopy to characterise the uniformity and thickness of the coatings. R. L. Puurunen, J. Appl. Phys., 97, 121301 (2005).L. A. Riley, S. V. Atta, A. S. Cavanagh, Y. Yan, S. M. George, P. Liu, A. C. Dillon, and S. Lee, J. Power Sources, 196, 3317–3324 (2011).X. Wang J. Cai, Y. Liu, X. Han, Y. Ren, J. Li, Y. Liu, and X. Meng, Nanotechnology, 32, 115401 (2020). Figure 1: Capacity retention after 3 formation cycles of LiNi0.6Mn0.2Co0.2O2 positive electrodes uncoated (black), Al2O3 coating by 50 ALD cycles (blue) and 2 wt% Aluminium tri sec-butoxide sol-gel (red). Cells were cycled at 2.5-4.5 V vs Li+/L at 25 °C at 1C after 3 formation cycles at C/10. The negative electrode was Li metal and the electrolyte was 1.0 M LiPF6 in ethylene carbonate : ethyl methyl carbonate 3:7 v/v with 1 wt% vinyl carbonate. Figure 1

Impact of TM-O Bonding Covalency on the Structure and Performance of Li-Rich Layered Oxide Positive Electrodes for Li-Ion Batteries

The application of positive electrode (cathode) materials with anionic redox activity is hoped to improve the energy density of future Li-ion batteries, and thus facilitate the mileage issue in rapidly growing electric transport industry. Such positive expectations are dictated by promising application potential of the Li4/3-xNi2+ xMn4+ 2/3-xCo3+ xO2 layered oxides demonstrating outstanding reversible discharge capacity exceeding 250 mAh/g and specific energy density of > 1000 Wh/kg. Such outstanding values are the result of joint participance of cationic (Ni2+→Ni3+→Ni4+, Co3+→Co4+) redox transitions and processes of O2 n-/2O2- units formation in charge compensation mechanism upon Li (de)intercalation.Nevertheless, oxygen redox chemistry in battery materials is a «double-edged sword», since it`s associated with practical drawbacks like sluggish kinetics, voltage hysteresis, voltage fade and safety worries alongside greatly improved energy density. At the same time, the exact nature of partially oxidized oxygen species is still raising intensive debates as well as the role of TM-O bonding in oxygen oxidation reversibility and interplay between anionic redox and accompanying bulk and local structure transformations and their accumulation during prolonged cycling.In our work, we probed the anionic redox properties as a function of electronic structure and chemical bonding substituting a small fraction of 3d-metals with Ru in a parent Li1.2Ni0.2Mn0.6O2 according to xLi2RuO3-(1-x)Li1.2Ni0.2Mn0.6O2 solid solution system (x is up to 0.1). This approach allowed us to gently tune the ratio between the contributions of the cationic and anionic redox whereas employing the same mixed Ni-Mn carbonate precursor excludes the impact of different sample morphology on the electrochemical behavior, providing a legitimate justification to attribute all the observed effects solely to crystal structure and TM-O bonding character. Both experimental results and theoretical calculations demonstrated that gradual increasing of Ru content drastically changes electrochemical behavior of the Li-rich layered oxides improving the reversibility of the oxygen redox thus suppressing irreversible oxygen oxidation at the first charge. In turn, diminished gaseous O2 evolution led to the retardation of “structural densification” as well as concomitant mitigation of Mn redox activity and changes in spatial distribution of the reduced Mn species. Moreover, despite the discharge voltage fade did not differ much for compounds with different Ru concentration, Ru doping surely decreased the charge voltage fade and, surprisingly, total discharge capacity likely due to inhibited Mn4+/3+/2+ redox activity in the Ru-doped compounds. In our report we will present the whole set of observations on xLi2RuO3-(1-x)Li1.2Ni0.2Mn0.6O2 model system aimed to trace the whole chain of events from increasing the covalency of the TM-O bond, suppressing irreversible oxygen oxidation and appearance of the reduced Mn species to retarding the structure densification in the Li-rich layered oxides.

Pushing Stoichiometries of Lithium-Rich Layered Oxides Beyond Their Limits.

Lithium-rich layered oxides (LRLOs) are opening unexplored frontiers for high-capacity/high-voltage positive electrodes in Li-ion batteries (LIBs) to meet the challenges of green and safe transportation as well as cheap and sustainable stationary energy storage from renewable sources. LRLOs exploit the extra lithiation provided by the Li1.2TM0.8O2 stoichiometries (TM = a blend of transition metals with a moderate cobalt content) achievable by a layered structure to disclose specific capacities beyond 200–250 mA h g–1 and working potentials in the 3.4–3.8 V range versus Li. Here, we demonstrate an innovative paradigm to extend the LRLO concept. We have balanced the substitution of cobalt in the transition-metal layer of the lattice with aluminum and lithium, pushing the composition of LRLO to unexplored stoichiometries, that is, Li1.2+x(Mn,Ni,Co,Al)0.8–xO2−δ. The fine tuning of the composition of the metal blend results in an optimized layered material, that is, Li1.28Mn0.54Ni0.13Co0.02Al0.03O2−δ, with outstanding electrochemical performance in full LIBs, improved environmental benignity, and reduced manufacturing costs compared to the state-of-the-art.

Surface Modification of Nanocrystalline LiMn2O4 Using Graphene Oxide Flakes.

In this work, a facile, wet chemical synthesis was utilized to achieve a series of lithium manganese oxide (LiMn2O4, (LMO) with 1–5%wt. graphene oxide (GO) composites. The average crystallite sizes estimated by the Rietveld method of LMO/GO nanocomposites were in the range of 18–27 nm. The electrochemical performance was studied using CR2013 coin-type cell batteries prepared from pristine LMO material and LMO modified with 5%wt. GO. Synthesized materials were tested as positive electrodes for Li-ion batteries in the voltage range between 3.0 and 4.3 V at room temperature. The specific discharge capacity after 100 cycles for LMO and LMO/5%wt. GO were 84 and 83 mAh g−1, respectively. The LMO material modified with 5%wt. of graphene oxide flakes retained more than 91% of its initial specific capacity, as compared with the 86% of pristine LMO material.

Enhanced Cycling of Ni-Rich Positive Electrodes by Fluorine Modification

Ni-rich positive electrodes for Li-ion batteries can provide enhanced initial discharge capacity yet suffer from significant capacity degradation upon cycling. Fluorination of Ni-rich NMC811 positive electrodes results in a capacity retention of more than 90% after 100 cycles upon cycling to 4.4 VLi. The increased cycling stability of F-modified NMC811 can be attributed to the modification of the oxide electronic structures, where density functional theory calculations shows that incorporating fluorine into the oxide lattice decrease the driving force of carbonate dissociation on the oxide surface. In situ infrared (IR) spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) further supports this argument by showing less carbonate oxidation for F-modified LiNi0.8Mn0.1Co0.1O2 (NMC811) than as-received NMC811 upon charging. The reduced carbonate oxidation is coupled with minimal salt decomposition on the electrode surface, as revealed by XPS. Further comparing in situ IR and XPS with that of the heat-treated NMC811 and Li2CO3 allows for decoupling the solvent decomposition products, where oxides are responsible for the vinylene carbonate formation with two hydrogens removed whereas surface metal carbonates promote dehydrogenated ethylene carbonate with just one hydrogen removed. This work points towards the importance of the anion engineering to increase the cycling stability of Ni-rich NMC positive electrodes.

Towards controlling the reversibility of anionic redox in transition metal oxides for high-energy Li-ion positive electrodes

Transition metal d-states and oxygen p-states overlap and oxygen lattice integrity dictates the oxygen redox reversibility in metal substituted Li2RuO3 positive electrodes for Li-ion batteries.

Environmentally friendly 5 V cathode based on Fe-doped LiMn1.5Ni0.5O4 spinel for Li-ion batteries

LiNi0.5Mn1.5O4-based materials are considered an interesting choice to be used as positive electrode for Li-ion batteries. Several years ago, it has been shown that doping this material with an amount of Cr allowed an improvement of its electrochemical performances, particularly at high rate capability. Unfortunately, Cr is one of the most toxic elements, especially at the oxidation state + VI. To solve this drawback, we suggest using a little amount of Fe instead of Cr. In this work, we report the synthesis of a spinel structure material which general formula is LiFe2yNi0.5-yMn1.5-yO4 (y ≤ 0.1) through combustion method. Two annealing temperatures were used: one at 800 and the other at 900 °C. The morphological analysis revealed the samples prepared at 800 °C are nanometric, meanwhile the ones annealed at 900 °C showed two distribution of particle size: ∼200 nm and ∼ 600 nm.Regarding the electrochemical properties, at the slow discharge rate (1C), the samples prepared at 900 °C exhibit high capacity during their discharge and better capacity retention. The best electrochemical performances were delivered by the materials with 0.05 and 0.075 of Fe content, their capacities were around 130 mAh/g and their capacity retentions were in the order of 98.6% after 50 cycles. These excellent performances were confirmed by the tests made at high rate capabilities; LiFe0.1Ni0.45Mn1.45O4 (y = 0.5) material delivered a discharge capacity of 102mAh/g at 20C (complete discharge in 3 min) maintaining a capacity retention of 79.7%. These results correspond to 9.56 kW/kg of power density and 482.5 kWh/kg of energy density.To summarize, doping with iron allowed us to prepare environmentally friendly material to be used as a positive electrode for Li-ion batteries with excellent electrochemical performances, especially at high rate capability. This material is a good candidate to be used for applications which demands are high in density and in power densities.

Electrochemical techniques for characterizing LiNixMnyCo[formula omitted]O2 battery electrodes

LiNi1/3Mn1/3Co1/3O2 (NMC111) is a state-of-the-art, commercially established positive electrode for Li-ion batteries. Identifying its performance limitations would support the development of better-performing batteries, as well as the design of novel electrode materials, such as Ni-rich LiNixMnyCo1−x−yO2. These limitations can be identified through electrochemical characterization, and several techniques are available for the purpose. However, they are rarely considered in the framework of the same study, making it difficult to understand which technique to choose for investigating a specific aspect of the electrode. In the present work, we perform a comparison of NMC111’s response towards Galvanostatic Cycling, Galvanostatic Intermittent Titration, Electrochemical Impedance Spectroscopy and Cyclic Voltammetry at different temperatures, and degrees of lithiation. Particular focus is put on the different thermodynamic and dynamic properties that can be extracted from each technique, either qualitatively or quantitatively. In this way, an overall characterization of NMC111 at Beginning of Life is provided, and used for discussing specific cases in which NMC111’s electrochemical response is altered by the operating conditions: specifically, we here consider first-cycle formation, and ageing under high voltage operation. We discuss the suitability and limitations of each technique in addressing specific changes in the electrode, for a better understanding of its performances that could support the development of physics-based predictive models for Li-ion battery performance and lifetime.

Solid-electrolyte interphase nucleation and growth on carbonaceous negative electrodes for Li-ion batteries visualized with in situ atomic force microscopy

Li-ion battery performance and life cycle strongly depend on a passivation layer called solid-electrolyte interphase (SEI). Its structure and composition are studied in great details, while its formation process remains elusive due to difficulty of in situ measurements of battery electrodes. Here we provide a facile methodology for in situ atomic force microscopy (AFM) measurements of SEI formation on cross-sectioned composite battery electrodes allowing for direct observations of SEI formation on various types of carbonaceous negative electrode materials for Li-ion batteries. Using this approach, we observed SEI nucleation and growth on highly oriented pyrolytic graphite (HOPG), MesoCarbon MicroBeads (MCMB) graphite, and non-graphitizable amorphous carbon (hard carbon). Besides the details of the formation mechanism, the electrical and mechanical properties of the SEI layers were assessed. The comparative observations revealed that the electrode potentials for SEI formation differ depending on the nature of the electrode material, whereas the adhesion of SEI to the electrode surface clearly correlates with the surface roughness of the electrode. Finally, the same approach applied to a positive LiNi1/3Mn1/3Co1/3O2 electrode did not reveal any signature of cathodic SEI thus demonstrating fundamental differences in the stabilization mechanisms of the negative and positive electrodes in Li-ion batteries.

The Role of Diphenyl Carbonate Additive on the Interfacial Reactivity of Positive Electrodes in Li-ion Batteries

Understanding and controlling the (electro) chemical reactions between positive electrodes and electrolytes is essential to enhance the cycle life and safety of Li-ion batteries. Previous computational and experimental studies have shown that greater capacity loss of LiNixMnyCo1−x−yO2 (NMC) with increased Ni content can be attributed to the enhanced chemical oxidation of carbonate solvents by dehydrogenation and increased salt decomposition. In this study, we examine the role of a diphenyl carbonate (DPC) additive on the interfacial reactivity of LiNi1/3Mn1/3Co1/3O2, LiNi0.6Mn0.2Co0.2O2, LiNi0.8Mn0.1Co0.1O2 (NMC111, NMC622 and NMC811). Diffuse reflectance infrared Fourier Transform (DRIFT) spectroscopy on NMCs showed that adding DPC in the electrolyte suppressed signals associated with dehydrogenation of ethylene carbonate (EC) from LiNi1/3Mn1/3Ni1/3O2 to LiNi0.8Mn0.1Ni0.1O2 (NMC111 to NMC811). In addition, having DPC in the electrolyte was accompanied with less PF6− salt anion decomposition to form less-fluorine coordinated species such as lithium nickel oxyfluorides or PF3O-like species as revealed by combined infrared spectroscopy and X-ray Photoelectron Spectroscopy (XPS) for Ni-rich NMCs. Such observations are in agreement with previous work showing that DPC can increase the cycling performance of NMC811. The reduced reactivity between NMC such as NMC811 and electrolyte with DPC can be attributed to the formation of surface reaction products from the electrochemical oxidation of DPC occurring at lower voltages compared to the chemical oxidative dehydrogenation of carbonates. This hypothesis is supported by in situ infrared spectroscopy measurements, which revealed electrochemical oxidation of diphenyl carbonate upon charging at 3.9 VLi, accompanied by the detection of a feature around 1824 cm−1 attributed to organic oxidation products adsorbed on the oxide surface, and a stable electrode/electrolyte interface on NMC811 at higher voltages.

Template-free synthesis of LiV3O8 hollow microspheres as positive electrode for Li-ion batteries

The development of hierarchical micro-/nanostructures has received great attention in the field of electrochemical energy storage and conversion. In this report, we demonstrate a template-free synthesis of LiV3O8 hollow microspheres from vanadium ethylene glycolate intermediate, which in turn was obtained by a facile solvothermal route. Evaluation of morphological features with FE-SEM and HR-TEM shows hollow spheres of size 1.8 ± 0.1 µm diameters with wall thickness and void size of 200 ± 20 nm and 1.5 ± 0.1 µm, respectively. Powder X-ray diffraction confirms that both the intermediate and LiV3O8 hollow spheres are highly crystalline without any impurity phase. Tested as a positive electrode in lithium-ion batteries, the LiV3O8 hollow spheres constructed electrode shows a discharge capacity of 304 mAh g−1 at 0.1 C rate and 246 mAh g−1 at 1 C rate during the first discharge. More importantly, LiV3O8 electrode delivers stable cycle-life performance with a specific capacity of 156 mAh g−1 after 300 (dis)charges at 1 C rate. The present hollow LiV3O8 structures would be expected to offer additional active sites, void space to counter volume expansion, and thin-shell structures to provide shortest path for Li+ ion/electron transport, thereby enhancing the cycling stability and rate performance.

Platelet-shape LiFePO4/Fe2P/C composite material as a high-rate positive electrode for Li-ion batteries

In this work we present novel approach to enhance the performance of the LFP electrodes based on simultaneous engineering of phase composition and morphology in the LiFePO4 – Fe2P – carbon system. We show the two step fabrication method of composite materials made of carbon coated nanometric platelet LiFePO4 and metallic Fe2P species, the synthesis involve co-precipitation of platelet LFP particles with subsequent high temperature annealing under reductive conditions. We successfully prepare LFP/Fe2P/C composites comprising 8 wt% Fe2P with enhanced electrochemical properties. The material at high current rates – 10 C delivers discharge specific capacity close to 110 mAh gLFP−1. The cell with investigated LFP electrodes working in the 2.5–4.5 voltage regime exhibits high stability and its discharge specific capacity fading only 3% after 60 cycles and remains equal to 159 mAh gLFP−1.

(Plenary) Anion Redox in Metal Oxides

Traditional Li-ion positive electrodes such as layered LiTMO2 (TM = Co, Ni, Mn) store lithium ions and electrons largely by employing the cationic redox of the transition metal (TM), yielding a capacity of ~140 mAh g-1 upon charging to 4.2 VLi. Recent advances on Li-overstoichiometric layered materials Li1+x TM 1−yO2 (TM = Ti, Ni, Co, Mn, Ru, Ir) offer considerably greater first-cycle capacities than the conventional layered materials LiMO2 with over than 200 mAh g-1.1-4 The extra capacity can only be rationalized through the activation of anionic redox in those chemistries, in addition to conventional cationic redox mechanism. However, by tuning transition metal species, researchers observed different reversibility of the anionic redox process, inducing drastically distinct capacity fade over charge and discharge. 1, 3-4 To fully utilize and harvest the extra capacity from oxygen redox process, a clear understanding the redox behavior of those materials is needed. In the past decades, there are several competing hypotheses proposed on the redox process of Li-rich layered materials5-7, but a governing anionic redox mechanism is still lacking. In this work, we employed X-ray core-level spectroscopies, coupled with ab-initio calculations to first understand the redox processes and structural evolution of model systems exhibiting oxygen redox. Through this effort, we have presented a holistic picture of the anionic redox mechanism in the context of previously proposed redox schemes put forward by different researchers. Moreover, through leveraging the learning of the redox process of model materials, we systematically tuned the transition metal species in the metal oxygen framework in those Li-rich oxides, which allows us to pinpoint potential electronic-structure-based descriptors to capture the activation and reversibility of the anionic redox process. This study has laid the foundation for future high-throughput screening of new generation high-energy-density positive electrodes for Li-ion batteries.

(Invited) Understanding the Oxygen Release at High Potential for Transition Metal Oxides

The design of new materials is a pivotal question for the development of the next generation of electrochemical energy storage and conversion devices. Recently, triggering the redox activity of oxygen ions in transition metal oxides used as positive electrodes for Li-ion batteries was proposed as a promising strategy to develop Li-rich compounds with reversible capacity exceeding those of classical insertion layered compounds. Nevertheless, the science at play must be rationalized in order to control this phenomenon and assess the potential applications of such Li-rich electrodes. Indeed, following this discovery, fundamental questions remain. Among them, the exact nature of the oxygen species formed upon oxidation is critical since it governs not only its redox potential but also the discharge behavior for which sluggish kinetics and hysteresis were observed. Using recent examples, the different types of oxygen species will be discussed and their electrochemical behavior compared. Moreover, while early measurements suggested gaseous oxygen to be produced for these Li-rich at potentials above 4.3 V vs. Li+/Li, recent observations point toward oxygen evolution within a potential range that can span from 3.5 to 4.5 V vs. Li+/Li. This observation asks for 1) the development of new analytical tools in order to accurately detect and quantify these gaseous species and 2) to rationalize the standard potential for the evolution of lattice oxygen from the transition metal oxides. These two points will be discussed and strategies to circumvent this detrimental phenomenon will be proposed.