Available online xxx a b s t r a c tThis overview addresses the atomistic aspects of degradation of layered LiMO 2 oxide Li-ion cell cathode materials, aiming to shed light on the fundamental degradation mechanisms especially inside active cathode materials and at their interfaces. It includes recent results obtained by novel in situ/in operando diffraction methods, modelling, and quasi in situ surface science analysis. Degradation of the active cathode material occurs upon overcharge, resulting from a positive potential shift of the anode. Oxygen loss and eventual phase transformation resulting in dead regions are ascribed to changes in electronic structure and defect formation. The anode potential shift results from loss of free lithium due to side reactions occurring at electrode/electrolyte interfaces. Such side reactions are caused by electron transfer, and depend on the electron energy level alignment at the interface. Side reactions at electrode/electrolyte interfaces and capacity fade may be overcome by the use of suitable solid-state electrolytes and Licontaining anodes.

In this study, a comprehensive experimental in situ analysis of the evolution of the occupied and unoccupied density of states as a function of the charging state of the Li x≤1CoO2 films has been done by using synchrotron X-ray photoelectron spectroscopy (SXPS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and O K- and Co L 3,2-edges XANES. Our experimental data demonstrate the change of the Fermi level position and the Co3d–O2p hybridization under the Li+ removal and provide the evidence for the involvement of the oxygen states in the charge compensation. Thus, the rigid band model fails to describe the observed changes of the electronic structure. The Co site is involved in a Co3+ → Co4+ oxidation at the period of the Li deintercalation (x ∼ 0.5), while the electronic configuration at the oxygen site is stable up to 4.2 V. Further lowering of the Fermi level promoted by Li+ extraction leads to a deviation of the electronic density of states due to structural distortions, and the top of the O2p bands overlaps the Co3d state which is accompanied by a hole transfer to the O2p states. The intrinsic voltage limit of LiCoO2 has been determined, and the energy band diagram of Li x≤1CoO2 vs the evolution of the Fermi level has been built. It was concluded that Li x CoO2 cannot be stabilized at the deep Li deintercalation even with chemically compatible solid electrolytes.

In recent years, there have been significant efforts to understand the role of the electronic structure of redox active materials according to their performance and thermodynamic stability in electrochemical storage devices and to develop novel materials with higher energy density and higher power. It is generally recognized that transition metal compounds used as a positive electrode determine the specific capacity and the energy density of rechargeable batteries, while the charge transfer resistance at the electrolyte–electrode interface plays a key role in delivering the power of the electrochemical cell. In the present work, we study the stability of Li x Ni0.2Co0.7Mn0.1O2 thin films through the evolution of the occupied and unoccupied density of states as a function of the charging state of the electrode as well as the physicochemical conditions influencing the ionic transport across the electrode–electrolyte interface. A comprehensive experimental quasi in situ approach has been applied by using synchrotron X-ray photoelectron spectroscopy (SXPS) and O K-edge and Co, Ni, Mn L-edges XANES. Our experimental data demonstrate the change of the Fermi level position with Li+ removal and Ni2+ → Ni4+ and Co3+ → Co4+ changes of oxidation state for the charge compensation in the bulk of the material. As is evidenced by the experimentally determined energy band diagram of Li x≤1.0Ni0.2Co0.7Mn0.1O2 vs the evolution of the Fermi level, no hole transfer to the O2p bands is observed up to a charging state of 4.8 V, which evidences the thermodynamic stability of Li x≤1.0Ni0.2Co0.7Mn0.1O2 under high charging voltage in contrast to LiCoO2. A very thin solid electrolyte interface layer (less than 30 Å thickness) on the Li x≤1.0Ni0.2Co0.7Mn0.1O2 film is formed in a decomposition reaction of the electrolyte also involving the transition metal oxide. The enhanced concentration of lithium in the interface layer correlates evidently with the electron transfer to the transition metal sites changing their electronic configuration. It is concluded that Li x≤1.0Ni0.2Co0.7Mn0.1O2 can serve as a high energy density cathode material, but the delivery of high power, which is a critical parameter for an electric vehicle, is strongly influenced by the physicochemical conditions at the solid electrolyte interface, which can suppress Li+ diffusion or even block the Li+ paths across the interface.

The stability of the valence state of the 3d transition metal ions and the stoichiometry of LiMO(2) (M = Co, Ni, Mn) layered oxides at the surface-electrolyte interface plays a crucial role in energy storage applications. The surface oxidation/reduction of the cations caused by the contact of the solids to air or to the electrolyte results in the blocking of the Li-transport through the interface that leads to the fast batteries deterioration. The influence of the end-of-charge voltage on the chemical composition and the oxidation state of 3d transition metal ions, as well as the stability of the solid-electrolyte interface formed during the electrochemical Li-deintercalation/intercalation of the LiCoO(2) and Li(Ni,Mn,Co)O(2), have been investigated by X-ray photoelectron spectroscopy. While the chemical composition of the solid-electrolyte interface is similar for both layered oxide surfaces, the electrochemical cycling to some critical voltage values leads to the disappearance of the interface. By the analysis of the shape of the 2p and 3s photoelectron emissions we show that the formation of the solid-electrolyte interface layer correlates with the partial reduction of the trivalent Co ions at the electrolyte-LiCoO(2) interface and the amount of the Co(2+) ions is increased as the solid-electrolyte interface vanishes. In contrast, the Mn(4+), Co(3+) and Ni(2+) ions of the Li(Ni,Mn,Co)O(2) are stable at the interface under the electrochemical cycling to higher end-of-charge voltage. A correlation between deterioration of the LiCoO(2) and Li(Ni,Mn,Co)O(2) batteries and the change of electronic structure at the surface/interface after the electrochemical cycling has been found. The dissolution of the solid-electrolyte interface layer might be the reason for the fast deterioration of the Li-ion batteries.

Three-dimensional (3D) architectures obtained by the structural assembly of 1D nanomaterials are regarded as the next generation building blocks for sensors, electronics, photonics, and bioelectronic applications. Purification and functionalization of such 3D ordered structures are crucial for realizing their full potential. Plasma functionalization, compared to any solution based process, is favorable in retaining the alignment while functionalizing such structures. However, the commonly employed plasma processes like O 2 or Ar plasma can be highly detrimental to well-aligned ordered nanostructures and thus might affect the properties intimately associated with their 3D structure. Here, for the first time, we investigate the mild nature of a radio frequency CO 2 gas plasma as an effective source for purification and functionalization of vertically aligned CNT structures and study the effects of this functionalization onto the purification and functionalization by physical and chemical techniques (HRTEM, XPS, Raman). We found that CO 2 plasma selectively etches the amorphous carbon present in the vertically aligned CNT structure. Moreover, it is as effective as the widely used but more aggressive O 2 plasma in functionalizing the CNT. Unlike an O 2 or Ar plasma, CO 2 plasma has the tremendous advantage of retaining the structural integrity of the CNT structures.