Reactions and solid electrolyte interface (SEI) formation at electrode–electrolyte interfaces are crucial for the stability and performance of Li‐ion batteries, but are still not fully understood on a fundamental level. For improving battery properties, a detailed understanding of these degradation processes is needed. In this contribution, the interface formation between a thin film LiCoO 2 cathode material and diethyl carbonate (DEC) as typical battery electrolyte solvent is presented. A surface‐science approach is used performing a stepwise adsorption of DEC onto LiCoO 2 at low temperatures. The interface is studied after each step by synchrotron‐based X‐ray photoemission spectroscopy (SXPS) and high‐resolution electron energy loss spectroscopy. The results demonstrate that the decomposition of carbonate solvents in contact with fully lithiated cathode materials as observed in adsorption experiments is complex, including the reduction of solvent, subsequent decomposition reactions, and also catalytic effects. In the present case, lithium ethyl carbonate, lithium ethoxide, and lithium oxides are assigned as reaction products. The spectra provide indications for partial electron transfer coupled to covalent interaction involving surface oxygen O2p orbital and DEC lowest unoccupied molecular orbital (LUMO) states.

Reactions and charge transfer at cathode/electrolyte interfaces affect the performance and the stability of Li-ion cells. Corrosion of active electrode material and decomposition of electrolyte are intimately coupled to charge transfer reactions at the electrode/electrolyte interfaces, which in turn depend on energy barriers for electrons and ions. Principally, energy barriers arise from energy level alignment at the interface and space charge layers near the interface, caused by changes of inner electric (Galvani) potential due to interfacial dipoles and concentration profiles of electronic and ionic charge carriers.In this contribution, we introduce our surface science oriented approach using photoemission (XPS, UPS) to investigate cathode/electrolyte interfaces in Li-ion batteries. After an overview of the processes at cathode/electrolyte interfaces as well as currently employed analysis methods, we present the fundamentals of contact potential formation and energy level alignment (electrons and ions) at interfaces and their analysis with photoemission. Subsequently, we demonstrate how interface analysis can be employed in Li-ion battery research, yielding new and valuable insights, and discuss future benefits. (C) 2014 Elsevier Ltd. All rights reserved.

LiCo0.9M0.1PO4 (M = Co2+, Mg2+, Ca2+)/graphitic carbon composites are synthesized by Pechini-assisted sal-gel process and annealed by the 2-steps annealing process (300 degrees C for 5 min in air, then at 730 degrees C for 12 h in nitrogen). The structural investigation, performed on powders, reveals the presence of LiCoPO4 as the major crystalline phase and of CoP2O7 (M = Co), of CO2P (M = Mg), of Co2P, Li3PO4, (Ca,Co)(3)(PO4)(2) (M = Ca) as impurities. The morphological investigation of the composites shows the formation of crystalline "islands-like" structures with acicular crystallites with different dimensions (typically 5-50 mu m) on the top of them. The voltammetric analysis shows a very good reversibility of the (de) intercalation processes and the presence of two mean peak maxima in the cathodic region at similar to 5.01 V and similar to 5.05 V respectively. The discharge specific capacities, at a discharge rate of C/10 and room temperature, were 100 mAh g(-1) for M = Co, 68 mAh g(-1) for M = Mg and 104 mAh g(-1) for M = Ca respectively. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity in the Ca and Mg ions containing composites. (C) 2014 Elsevier Ltd. All rights reserved.

Despite numerous efforts to elucidate interface-related phenomena of Li ion battery cathodes, the exact nature of cathode/electrolyte interfaces is still not fully resolved. Key factors for the properties of semiconducting ionic electrodes are band bending and energy level alignment at the interface, which have not been given much attention in the past. In this contribution, we investigate the formation of the electrochemical interface for a LiCoO2 electrode in contact with a solvent adsorbate phase by a surface science approach. Diethyl carbonate (DEC) was adsorbed stepwise onto a LiCoO2 thin film electrode and the electrode surface analyzed with X-ray photoelectron spectroscopy (XPS) after each adsorption step. Adsorption results in the formation of a charged layer in the electrode, which we attribute to the transfer of lithium ions from the electrode to the adsorbed phase. The offset between the LiCoO2 valence band and HOMO of the adsorbed DEC is large (4 eV) under the experimental conditions, which renders solvent oxidation unlikely.

The properties of LiCoPO4-non-graphitic carbon foams (LCP-NGCF) composites are reported. The composites are treated at 300°C for different times (t, from 0 to 12h) in air, then at 730°C for 12h in nitrogen. The diffraction analysis revealed LiCoPO4 as major crystalline phase, Li4P2O7 and Co2P (t=0h), Co2P (t>0h) as secondary phases. The morphology consists of crystalline “islands” with spongy-like features on the surface (for t=0h) and with acicular crystallites of different dimensions (2–20μm) for t≥0.1h. The voltammetric curves show reduction potential values between 4.40V and 4.60V. The LCP-NGCF composites deliver a discharge specific capacity of 100mAhg−1 (t=0h, discharge rate of C/25 and RT) and of 65mAhg−1 (t>0h). The ac-impedance analysis reveal the formation of SEI-layer after high annealing times, which disfavors the kinetics of the Li-(de)intercalation processes.

The characterization of composites consisting of graphitic carbon foams coated with a structured lithium nickel phosphate is reported. The LiNiPO4 as cathode material for lithium-ion batteries is prepared by a Pechini-assisted sol-gel process. The coating is performed by soaking the graphitic carbon foams in aqueous solutions containing lithium, nickel salts, and phosphates at 70 °C for 2–4 h and then by treating in flowing air and nitrogen. The formation of the olivine-like structured LiNiPO4 is confirmed by X-ray diffraction analysis performed on powders prepared under very similar conditions. However, crystalline reflections attributed to Li4P2O7 and to Ni3P as secondary phases have been observed. The morphological investigation revealed the presence of a layer on the graphitic foams that consists of interconnected blend of grains with different size. The voltammetric curves show values of the mean peak maxima in the anodic region between 5.1–5.3 V and in the cathodic region at ~4.9 V. The electrochemical measurements deliver a discharge specific capacity of 86 mAhg−1 (at discharge rate of C/10 and RT). The electrochemical impedance spectroscopy data confirm an increase of the electrical resistance after cycling and the decrease of the ionic contribution which indicate the formation/growth of phases behaving like resistors.

The interface between the cathode and the liquid electrolyte (or solid-electrolyte interphase) in Li-ion batteries significantly influences the rate capability and the cycling performance. In this study, thin film LiCoO2 cathodes were modified with a very thin lithium phosphorous oxynitride (LiPON) coating and characterized with respect to cycling behavior and electrochemical behavior. The data show that the modification leads to an increase of the long term stability and a decrease of the electrode resistance, respectively. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3511772] All rights reserved.