There and Back Again-The Journey of LiNiO2 as a Cathode Active Material.

This Review provides a comprehensive overview of LiNiO2 (LNO), almost 30 years after its introduction as a cathode active material. We aim to highlight the physicochemical peculiarities that make LNO a complex material in every aspect. We specifically stress the effect of the Li off-stoichiometry (Li1-z Ni1+z O2 ) on every property of LNO, especially the electrochemical ones. The key instability issues that plague the compound and the strategies that have been implemented so far to overcome them are discussed in detail. Finally, the open questions that remain to be addressed by the scientific community are summarized, and the research directions that seem the most promising to enable LNO to be fully exploited are elucidated.

Stabilizing Lithium Plating by a Biphasic Surface Layer Formed In†Situ.

The dendritic growth of Li metal leads to electrode degradation and safety concerns, impeding its application in building high energy density batteries. Forming a protective layer on the Li surface that is electron-insulating, ion-conducting, and maintains an intimate interface is critical. We herein demonstrate that Li plating is stabilized by a biphasic surface layer composed of a lithium-indium alloy and a lithium halide, formed in situ by the reaction of an electrolyte additive with Li metal. This stabilization is attributed to the fast lithium migration though the alloy bulk and lithium halide surface, which is enabled by the electric field across the layer that is established owing to the electron-insulating halide phase. A greatly stabilized Li-electrolyte interface and dendrite-free plating over 400 hours in Li|Li symmetric cells using an alkyl carbonate electrolyte is demonstrated. High energy efficiency operation of the Li4 Ti5 O12 (LTO)|Li cell over 1000 cycles is achieved.

High-Throughput in Situ Pressure Analysis of Lithium-Ion Batteries.

Many degradation processes in lithium-ion batteries are accompanied by gas evolution and therefore lead to an increase in internal cell pressure. This causes serious safety concerns for state-of-the-art lithium-ion batteries, calling for a thorough investigation of the origin and the magnitude of such processes. Herein we introduce a multichannel in situ pressure measurement system that allows for the high-throughput quantification of gas evolution under realistic battery conditions. The capability of the system was demonstrated through its application on Li4Ti5O12 half cells. The pressure changes could be divided into an irreversible and a reversible part, where the latter is caused by the deposition and dissolution of elemental lithium during cycling. Comparison of the measured and the theoretical reversible pressure changes showed a close match, indicating the high accuracy of the system. Additionally, the irreversible part observed in the pressure changes was attributed to gas evolution, as confirmed by complementary measurements using differential electrochemical mass spectrometry. To show the practicality of the system, the temperature dependence of gas evolution in Li1+xNi0.6Co0.2Mn0.2O2 full cells was investigated. Enhanced gas evolution was observed at elevated temperature, which is partly attributed to the thermal decomposition of the conducting salt LiPF6.

A rechargeable room-temperature sodium superoxide (NaO2) battery.

In the search for room-temperature batteries with high energy densities, rechargeable metal-air (more precisely metal-oxygen) batteries are considered as particularly attractive owing to the simplicity of the underlying cell reaction at first glance. Atmospheric oxygen is used to form oxides during discharging, which-ideally-decompose reversibly during charging. Much work has been focused on aprotic Li-O(2) cells (mostly with carbonate-based electrolytes and Li(2)O(2) as a potential discharge product), where large overpotentials are observed and a complex cell chemistry is found. In fact, recent studies evidence that Li-O(2) cells suffer from irreversible electrolyte decomposition during cycling. Here we report on a Na-O(2) cell reversibly discharging/charging at very low overpotentials (< 200 mV) and current densities as high as 0.2 mA cm(-2) using a pure carbon cathode without an added catalyst. Crystalline sodium superoxide (NaO(2)) forms in a one-electron transfer step as a solid discharge product. This work demonstrates that substitution of lithium by sodium may offer an unexpected route towards rechargeable metal-air batteries.

A comprehensive study on the cell chemistry of the sodium superoxide (NaO2) battery.

This work reports on the cell chemistry of a room temperature sodium-oxygen battery using an electrolyte of diethylene glycol dimethyl ether (diglyme) and sodium trifluoromethanesulfonate (NaSO3CF3, sodium triflate). Different from lithium-oxygen cells, where lithium peroxide is found as the discharge product, sodium superoxide (NaO2) is formed in the present cell, with overpotentials as low as 100 mV during charging. Several analytical methods are used to follow the cell reaction during discharge and charge. Changes in structure and morphology are studied by SEM and XRD. It is found that NaO2 grows as cubic particles with feed sizes in the range of 10-50 µm; upon recharge the particles consecutively decompose. Pressure monitoring during galvanostatic cycling shows that the coulombic efficiency (e(-)/O2) for discharge and charge is approx. 1.0, the expected value for NaO2 formation. Also optical spectroscopy is identified as a convenient and useful tool to follow the discharge-charge process. The maximum discharge capacity is found to be limited by oxygen transport within the electrolyte soaked carbon fiber cathode and pore blocking near the oxygen interface is observed. Finally electrolyte decomposition and sodium dendrite growth are identified as possible reasons for the limited capacity retention of the cell. The occurrence of undesired side reactions is analyzed by DEMS measurements during cycling as well as by post mortem XPS investigations.

Mesoporous tin-doped indium oxide thin films: effect of mesostructure on electrical conductivity.

We present a versatile method for the preparation of mesoporous tin-doped indium oxide (ITO) thin films via dip-coating. Two poly(isobutylene)-b-poly(ethyleneoxide) (PIB-PEO) copolymers of significantly different molecular weight (denoted as PIB-PEO 3000 and PIB-PEO 20000) are used as templates and are compared with non-templated films to clarify the effect of the template size on the crystallization and, thus, on the electrochemical properties of mesoporous ITO films. Transparent, mesoporous, conductive coatings are obtained after annealing at 500 °C; these coatings have a specific resistance of 0.5 Ω cm at a thickness of about 100 nm. Electrical conductivity is improved by one order of magnitude by annealing under a reducing atmosphere. The two types of PIB-PEO block copolymers create mesopores with in-plane diameters of 20-25 and 35-45 nm, the latter also possessing correspondingly thicker pore walls. Impedance measurements reveal that the conductivity is significantly higher for films prepared with the template generating larger mesopores. Because of the same size of the primary nanoparticles, the enhanced conductivity is attributed to a higher conduction path cross section. Prussian blue was deposited electrochemically within the films, thus confirming the accessibility of their pores and their functionality as electrode material.

Quasi-homogenous photocatalysis of quantum-sized Fe-doped TiO2 in optically transparent aqueous dispersions.

In this study, the preparation of anatase TiO2 nanocrystals via a facile non-aqueous sol-gel route and their characterization are reported. The 3-4 nm particles are readily dispersable in aqueous media and show excellent photoreactivity in terms of rhodamine B degradation. The catalytic performance can be further increased considerably by doping with iron and UV-light irradiation as a pre-treatment. The effect of surface ligands (blocked adsorption sites, surface defects etc.) on the photoreactivity was thoroughly probed using thermogravimetric analysis combined with mass spectrometry. Photoelectrochemical characterization of thin-film electrodes made from the same TiO2 nanocrystals showed the opposite trend to the catalytic experiments, that is, a strong decrease in photocurrent and quantum efficiency upon doping due to introduction of shallow defect states.

Influence of electronically conductive additives on the cycling performance of argyrodite-based all-solid-state batteries.

All-solid-state batteries (SSBs) are attracting widespread attention as next-generation energy storage devices, potentially offering increased power and energy densities and better safety than liquid electrolyte-based Li-ion batteries. Significant research efforts are currently underway to develop stable and high-performance bulk-type SSB cells by optimizing the cathode microstructure and composition, among others. Electronically conductive additives in the positive electrode may have a positive or negative impact on cyclability. Herein, it is shown that for high-loading (pelletized) SSB cells using both a size- and surface-tailored Ni-rich layered oxide cathode material and a lithium thiophosphate solid electrolyte, the cycling performance is best when low-surface-area carbon black is introduced.

Gas Evolution in Lithium-Ion Batteries: Solid versus Liquid Electrolyte.

Gas evolution in conventional lithium-ion batteries using Ni-rich layered oxide cathode materials presents a serious issue that is responsible for performance decay and safety concerns, among others. Recent findings revealed that gas evolution also occurred in bulk-type solid-state batteries. To further clarify the effect that the electrolyte has on gassing, we report in this work-to the best of our knowledge-the first study comparing gas evolution in lithium-ion batteries with NCM622 cathode material and different electrolyte types, specifically solid (ß-Li3PS4 and Li6PS5Cl) versus liquid (LP57). Using isotopic labeling, acid titration, and in situ gas analysis, we show the presence of O2 and CO2 evolution in both systems, albeit with different cumulative amounts, and possible SO2 evolution for the lithium thiophosphate-based cells. Our results demonstrate the importance of considering gas evolution in solid-state batteries, especially the formation and release of highly corrosive SO2, due to side reactions with the electrolyte.

Effect of Low-Temperature Al2O3 ALD Coating on Ni-Rich Layered Oxide Composite Cathode on the Long-Term Cycling Performance of Lithium-Ion Batteries.

Conformal coating of nm-thick Al2O3 layers on electrode material is an effective strategy for improving the longevity of rechargeable batteries. However, solid understanding of how and why surface coatings work the way they do has yet to be established. In this article, we report on low-temperature atomic layer deposition (ALD) of Al2O3 on practical, ready-to-use composite cathodes of NCM622 (60% Ni), a technologically important material for lithium-ion battery applications. Capacity retention and performance of Al2O3-coated cathodes (≤10 ALD growth cycles) are significantly improved over uncoated NCM622 reference cathodes, even under moderate cycling conditions. Notably, the Al2O3 surface shell is preserved after cycling in full-cell configuration for 1400 cycles as revealed by advanced electron microscopy and elemental mapping. While there are no significant differences in terms of bulk lattice structure and transition-metal leaching among the coated and uncoated NCM622 materials, the surface of the latter is found to be corroded to a much greater extent. In particular, detachment of active material from the secondary particles and side reactions with the electrolyte appear to lower the electrochemical activity, thereby leading to accelerated capacity degradation.

Room temperature, liquid-phase Al2O3 surface coating approach for Ni-rich layered oxide cathode material.

A room temperature, atomic-layer-deposition-like coating strategy for NCM811 (80% Ni) is reported. Trimethylaluminum is shown to readily react with adsorbed moisture, leading both to Al2O3 surface layer formation on NCM811 and to trace H2O removal in a single treatment step. Even more importantly, the cycling performance of pouch cells at 45 °C is greatly improved.

Chemical, Structural, and Electronic Aspects of Formation and Degradation Behavior on Different Length Scales of Ni-Rich NCM and Li-Rich HE-NCM Cathode Materials in Li-Ion Batteries.

In order to satisfy the energy demands of the electromobility market, both Ni-rich and Li-rich layered oxides of NCM type are receiving much attention as high-energy-density cathode materials for application in Li-ion batteries. However, due to different stability issues, their longevity is limited. During formation and continuous cycling, especially the electronic and crystal structure suffers from various changes, eventually leading to fatigue and mechanical degradation. In recent years, comprehensive battery research has been conducted at Karlsruhe Institute of Technology, mainly aiming at better understanding the primary degradation processes occurring in these layered transition metal oxides. The characteristic process of formation and mechanisms of fatigue are fundamentally characterized and the effect of chemical composition on cell chemistry, electrochemistry, and cycling stability is addressed on different length scales by use of state-of-the-art analytical techniques, ranging from "standard" characterization tools to combinations of advanced in situ and operando methods. Here, the results are presented and discussed within a broader scientific context.

Indirect state-of-charge determination of all-solid-state battery cells by X-ray diffraction.

Determining the state-of-charge of all-solid-state batteries via both ex situ and operando X-ray diffraction, rather than by electrochemical testing (may be strongly affected by electrically isolated/inactive material, irreversible side reactions, etc.), is reported. Specifically, we focus on bulk-type cells and use X-ray diffraction data obtained on a liquid electrolyte-based Li-ion cell as the reference standard for changes in lattice parameters with (de)lithiation.

Phase Transformation Behavior and Stability of LiNiO2 Cathode Material for Li-Ion Batteries Obtained from In†Situ Gas Analysis and Operando X-Ray Diffraction.

Ni-rich layered oxide cathode materials, in particular the end member LiNiO2 , suffer from drawbacks such as high surface reactivity and severe structural changes during de-/lithiation, leading to accelerated degradation and limiting practical implementation of these otherwise highly promising electrode materials in Li-ion batteries. Among all known phase transformations occurring in LiNiO2 , the one from the H2 phase to the H3 phase at high state of charge is believed to have the most detrimental impact on the material's stability. In this work, the multistep phase transformation process and associated effects are analyzed by galvanostatic cycling, operando X-ray diffraction, and in situ pressure and gas analysis. The combined results provide thorough insights into the structural changes and how they affect the stability of LiNiO2 . During the H2-H3 transformation, the most significant change occurs in the c-lattice parameter, resulting in large mechanical stress in LiNiO2 . As for electrochemical stability, it suffers strongly in the H3 region. Oxygen evolution is observed not only during charge but also during discharge and found to be correlated with the presence of the H2 and H3 phases. Taken together, the experimental data improve the understanding of the degradation processes and the inherent instability of LiNiO2 in Li-ion cells when operated above around 75 % state of charge.

The Role of Intragranular Nanopores in Capacity Fade of Nickel-Rich Layered Li(Ni1-x-yCoxMny)O2 Cathode Materials.

Ni-rich layered LiNi1-x-yCoxMnyO2 (NCM, x + y ≤ 0.2) is an intensively studied class of cathode active materials for lithium-ion batteries, offering the advantage of high specific capacities. However, their reactivity is also one of the major issues limiting the lifetime of the batteries. NCM degradation, in literature, is mostly explained both by disintegration of secondary particles (large anisotropic volume changes during lithiation/delithiation) and by formation of rock-salt like phases at the grain surfaces at high potential with related oxygen loss. Here, we report the presence of intragranular nanopores in Li1+x(Ni0.85Co0.1Mn0.05)1-xO2 (NCM851005) and track their morphological evolution from pristine to cycled material (200 and 500 cycles) using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry. Pores are already found in the primary particles of pristine material. Any potential effect of TEM sample preparation on the formation of nanopores is ruled out by performing thickness series measurements on the lamellae produced by focused ion beam milling. The presence of nanopores in pristine NCM851005 is in sharp contrast to previously observed pore formation during electrochemical cycling or heating. The intragranular pores have a diameter in the range between 10 and 50 nm with a distinct morphology that changes during cycling operation. A rock-salt like region is observed at the pore boundaries even in pristine material, and these regions grow with prolonged cycling. It is suggested that the presence of nanopores strongly affects the degradation of high-Ni NCM, as the pore surfaces apparently increase (i) oxygen loss, (ii) formation of rock-salt regions, and (iii) strain-induced effects within the primary grains. High-resolution STEM demonstrates that nanopores are a source of intragranular cracking during cycling.

Origin of Carbon Dioxide Evolved during Cycling of Nickel-Rich Layered NCM Cathodes.

Gas formation caused by parasitic side reactions is one of the fundamental concerns in state-of-the-art lithium-ion batteries because gas bubbles might block local parts of the electrode surface, hindering lithium transport and leading to inhomogeneous current distributions. Here, we elucidate on the origin of CO2, which is the dominant gaseous species associated with the layered lithium nickel cobalt manganese oxide (NCM) cathode, by implementing isotope labeling and electrolyte substitution in differential electrochemical mass spectrometry-differential electrochemical infrared spectroscopy measurements. Li2CO3 on the NCM surface was successfully labeled with 13C via a process that involves its removal followed by intentional growth. In situ gas analytics on such NCM samples with 13C-labeled Li2CO3 clearly indicate that Li2CO3 decomposition contributes to CO2 evolution, especially during the first charge. At the same time, the greater contribution of electrolyte decomposition was indicated by the large amount of 12CO2 observed. Employment of butyronitrile as the electrolyte solvent in further measurements helped determine that the majority of electrolyte decomposition occurs via a reaction that involves the lattice oxygen of NCM.

High-Performance Cells Containing Lithium Metal Anodes, LiNi0.6Co0.2Mn0.2O2 (NCM 622) Cathodes, and Fluoroethylene Carbonate-Based Electrolyte Solution with Practical Loading.

We report on the highly stable lithium metal|LiNi0.6Co0.2Mn0.2O2 (NCM 622) cells with practical electrodes' loading of 3.3 mA h g-1, which can undergo many hundreds of stable cycles, demonstrating high rate capability. A key issue was the use of fluoroethylene carbonate (FEC)-based electrolyte solutions (1 M LiPF6 in FEC/dimethyl carbonate). Li|NCM 622 cells can be cycled at 1.5 mA cm-2 for more than 600 cycles, whereas symmetric Li|Li cells demonstrate stable performance for more than 1000 cycles even at higher areal capacity and current density. We attribute the excellent performance of both Li|NCM and Li|Li cells to the formation of a stable and efficient solid electrolyte interphase (SEI) on the surface of the Li metal electrodes cycled in FEC-based electrolyte solutions. The composition of the SEI on the Li and the NCM electrodes is analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. A drastic capacity fading of Li|NCM cells is observed, followed by spontaneous capacity recovery during prolonged cycling. This phenomenon depends on the current density and the amount of the electrolyte solution and relates to kinetic limitations because of SEI formation on the Li anodes in the FEC-based electrolyte solution.

Molecular Surface Modification of NCM622 Cathode Material Using Organophosphates for Improved Li-Ion Battery Full-Cells.

Surface coating is a viable strategy for improving the cyclability of Li1+ x(Ni1- y- zCo yMn z)1- xO2 (NCM) cathode active materials for lithium-ion battery cells. However, both gaining synthetic control over thickness and accurate characterization of the surface shell, which is typically only a few nm thick, are considerably challenging. Here, we report on a new molecular surface modification route for NCM622 (60% Ni) using organophosphates, specifically tris(4-nitrophenyl) phosphate (TNPP) and tris(trimethylsilyl) phosphate. The functionalized NCM622 was thoroughly characterized by state-of-the-art surface and bulk techniques, such as attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to name a few. The comprehensive ToF-SIMS-based study comprised surface imaging, depth profiling, and three-dimensional visualization. In particular, tomography is a powerful tool to analyze the nature and morphology of thin coatings and is applied, to our knowledge, for the first time, to a practical cathode active material. It provides valuable information about relatively large areas (over several secondary particles) at high lateral and mass resolution. The electrochemical performance of the different NCM622 materials was evaluated in long-term cycling experiments of full-cells with a graphite anode. The effect of surface modification on the transition-metal leaching was studied ex situ via inductively coupled plasma optical emission spectroscopy. TNPP@NCM622 showed reduced transition-metal dissolution and much improved cycling performance. Taken together, with this study, we contribute to optimization of an industrially relevant cathode active material for application in high-energy-density lithium-ion batteries.

Interfacial Processes and Influence of Composite Cathode Microstructure Controlling the Performance of All-Solid-State Lithium Batteries.

All-solid-state lithium-ion batteries have the potential to become an important class of next-generation electrochemical energy storage devices. However, for achieving competitive performance, a better understanding of the interfacial processes at the electrodes is necessary for optimized electrode compositions to be developed. In this work, the interfacial processes between the solid electrolyte (Li10GeP2S12) and the electrode materials (In/InLi and LixCoO2) are monitored using impedance spectroscopy and galvanostatic cycling, showing a large resistance contribution and kinetic hindrance at the metal anode. The effect of different fractions of the solid electrolyte in the composite cathodes on the rate performance is tested. The results demonstrate the necessity of a carefully designed composite microstructure depending on the desired applications of an all-solid-state battery. While a relatively low mass fraction of solid electrolyte is sufficient for high energy density, a higher fraction of solid electrolyte is required for high power density.

Comparison between Na-Ion and Li-Ion Cells: Understanding the Critical Role of the Cathodes Stability and the Anodes Pretreatment on the Cells Behavior.

The electrochemical behavior of Na-ion and Li-ion full cells was investigated, using hard carbon as the anode material, and NaNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2 as the cathodes. A detailed description of the structure, phase transition, electrochemical behavior and kinetics of the NaNi0.5Mn0.5O2 cathodes is presented, including interesting comparison with their lithium analogue. The critical effect of the hard carbon anodes pretreatment in the total capacity and stability of full cells is clearly demonstrated. Using impedance spectroscopy in three electrodes cells, we show that the full cell impedance is dominated by the contribution of the cathode side. We discuss possible reasons for capacity fading of these systems, its connection to the cathode structure and relevant surface phenomena.