Contribution of Electrolyte Decomposition Products and the Effect of Temperature on the Dissolution of Transition Metals from Cathode Materials.

A fundamental understanding of aging processes in lithium-ion batteries (LIBs) is imperative in the development of future battery architectures for widespread electrification. Herein, dissolution of transition metals from cathode active materials of LIBs is among the most important degradation processes. Research has demonstrated that elevated operating temperatures accelerate battery degradation. However, the exact mechanism of transition-metal dissolution at elevated temperatures has still to be clarified. Current literature suggests that the reaction rate of dissolution increases with increasing temperature; moreover, the decomposition of electrolytes results in products that also accelerate dissolution processes. Most studies focus on ex situ analyses of thermally treated full cells. This approach is not appropriate to get detailed insights and to distinguish between different contributions. In this work, with the help of real-time dissolution analysis using an electroanalytical flow cell (EFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS), we present novel details of the temperature effects on in situ dissolution at the cathode electrolyte interface. With fresh electrolytes, we find increased Mn dissolution even at open-circuit conditions as well as with constant voltage polarization when the electrode sample is heated at constant temperatures between 50 and 80 °C. The release of transition metals also responds in a nuanced manner when applying temperature transients. Utilizing electrolytes preheated at 60 and 100 °C, we demonstrate that decomposition products in the bulk electrolyte have no influence on transition-metal (TM) dissolution when constantly flushing the cell with the thermally aged electrolyte samples. Only when keeping the cathode temperature at 60 °C, the dissolution increases by a factor of 2-3. Our findings highlight the interplay between the cathode and electrolyte and provide new insights into the dissolution mechanism of cathode materials.

On-line Electrode Dissolution Monitoring during Organic Electrosynthesis: Direct Evidence of Electrode Dissolution during Kolbe Electrolysis.

Electrode dissolution was monitored in real-time during Kolbe electrolysis along with the characteristic products. The fast determination of appropriate reaction conditions in electro-organic chemistry enables the minimization of electrode degradation while keeping an eye on the optimal formation rate and distribution of products. Herein, essential parameters influencing the dissolution of the electrode material platinum in a Kolbe electrolysis were pinpointed. The formation of reaction products and soluble platinum species were monitored during potentiodynamic and potentiostatic experiments using an electroanalytical flow cell coupled to two different mass spectrometers. The approach opens new vistas in the field of electro-organic chemistry because it enables precise and quick quantification of dissolved metals during electrosynthesis, also involving electrode materials other than platinum. Furthermore, it draws attention to the vital topic of electrode stability in electro-organic synthesis, which becomes increasingly important for the implementation of green chemical processes utilizing renewable energy.

Online Monitoring of Transition-Metal Dissolution from a High-Ni-Content Cathode Material.

The dissolution of transition metals (TMs) from cathode materials and their deposition on the anode represents a serious degradation process and, with that, a shortcoming of lithium-ion batteries. It occurs particularly at high charge voltages (>4.3 V), contributing to severe capacity loss and thus impeding the increase of cell voltage as a simple measure to increase energy density. We present here for the first time the online detection of dissolved TMs from a Ni-rich layered oxide cathode material with unprecedented potential and time resolution in potentiodynamic scans. To this aid, we used the coupling of an electroanalytical flow cell (EFC) with inductively coupled plasma mass spectrometry (ICP-MS), which is demonstrated to be an ideal tool for a fast performance assessment of new cathode materials from initial cycles. The simultaneous analysis of electrochemical and dissolution data allows hitherto hidden insights into the processes' characteristics and underlying mechanisms.

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.

How To Improve Capacity and Cycling Stability for Next Generation Li-O2 Batteries: Approach with a Solid Electrolyte and Elevated Redox Mediator Concentrations.

Because of their exceptionally high specific energy, aprotic lithium oxygen (Li-O2) batteries are considered as potential future energy stores. Their practical application is, however, still hindered by the high charging overvoltages and detrimental side reactions. Recently, the use of redox mediators dissolved in the electrolyte emerged as a promising tool to enable charging at moderate voltages. The presented work advances this concept and distinctly improves capacity and cycling stability of Li-O2 batteries by combining high redox mediator concentrations with a solid electrolyte (SE). The use of high redox mediator concentrations significantly increases the discharge capacity by including the oxidation and reduction of the redox mediator into charge cycling. Highly efficient cycling is achieved by protecting the lithium anode with a solid electrolyte, which completely inhibits unfavored deactivation of oxidized species at the anode. Surprisingly, the SE also suppresses detrimental side reactions at the carbon electrode to a large extent and enables stable charging completely below 4.0 V over a prolonged period. It is demonstrated that anode and cathode communicate deleteriously via the liquid electrolyte, which induces degradation reactions at the carbon electrode. The separation of cathode and anode with a SE is therefore considered as a key step toward stable Li-O2 batteries, in conjunction with a concentrated redox mediator electrolyte.

Gas Evolution in LiNi0.5Mn1.5O4/Graphite Cells Studied In Operando by a Combination of Differential Electrochemical Mass Spectrometry, Neutron Imaging, and Pressure Measurements.

The cycling performance and in operando gas analysis of LiNi0.5Mn1.5O4 (LNMO)/graphite cells with reasonably high loading, containing a "standard" carbonate-based electrolyte is reported. The gas evolution over the first couple of cycles was thoroughly investigated via differential electrochemical mass spectrometry (DEMS), neutron imaging and pressure measurements. The main oxidation and reduction products were identified as CO2, H2 and C2H4. In different sets of experiments graphite was substituted with delithiated LiFePO4 (LFP) and LNMO with LFP to distinguish between processes occurring at either anode or cathode and gain mechanistic insights. Both C2H4 and H2 were found to be mainly formed at the anode side, while CO2 is generated at the cathode. The results from DEMS analysis further suggest that the Ni redox couples play a profound role in the evolution of CO2 at the LNMO/electrolyte interface. Lastly, it is shown that the cycling stability and capacity retention of LNMO/graphite cells can be considerably improved by a simple cell formation procedure.

Kinetic Passivation Effect of Localized Differential Aeration on Brass.

Invited for this month's cover are collaborators from Ruhr University Bochum (Germany), Technical University Munich (Germany), and University of La Laguna, Tenerife (Spain). The cover picture shows the passivation effect of a liquid in a brass tube in the presence of oxygen in the gas phase. Read the full text of the article at 10.1002/cplu.201500398.

Kinetic Passivation Effect of Localized Differential Aeration on Brass.

The formation of a localized differential aeration cell on metals, susceptible to both anodic and cathodic corrosion, is a serious threat because of multiple degradation processes commencing with the passivation layer destruction. By using local electrochemical and X-ray dispersive techniques, it has been demonstrated that the differential aeration cell formed on high brass (α-brass, Cu65-Zn35) in the presence of 1H-benzotriazole or 5-methyl-1H-benzotriazole plays both corrosion-inhibiting and accelerating roles, depending on the inhibitor exposure time. Alternating-current scanning electrochemical microscopy was used to image local electrochemical activity, whereas energy-dispersive X-ray spectroscopy provided evidence for the mechanism of the observed phenomena. Short-term exposure to the inhibitor (5 min) promotes the formation of a passivation layer in the waterline region. In contrast, after prolonged exposure (45 min), a deficient passivation layer develops for both inhibitors. An excess of zinc(II)-inhibitor complexes in the passivation layer is accountable for the corrosion resistance of the region with high differential aeration. Rapid dezincification and local alkalinization facilitate the initial rapid formation of a passivation layer in the area under differential aeration to preserve its composition upon further modification.

Online Continuous Flow Differential Electrochemical Mass Spectrometry with a Realistic Battery Setup for High-Precision, Long-Term Cycling Tests.

We describe the benefits of an online continuous flow differential electrochemical mass spectrometry (DEMS) method that allows for realistic battery cycling conditions. We provide a detailed description on the buildup and the role of the different components in the system. Special emphasis is given on the cell design. The retention time and response characteristics of the system are tested with the electrolysis of Li2O2. Finally, we show a practical application in which a Li-ion battery is examined. The value of long-term DEMS measurements for the proper evaluation of electrolyte decomposition is demonstrated by an experiment where a Li(1+x)Ni(0.5)Mn(0.3)Co(0.2)O2 (NMC 532)/graphite cell is cycled over 20 charge/discharge cycles.

Characterisation of Complex Electrode Processes using Simultaneous Impedance Spectroscopy and Electrochemical Nanogravimetric Measurements.

The methodology and illustrative examples of application are presented for a technique that simultaneously combines electrochemical impedance spectroscopy (EIS) and nanogravimetric measurements; the latter are implemented using a so-called electrochemical quartz crystal nanobalance (EQCN). The combination of EIS and EQCN provides a powerful method for the characterisation of many complex processes at electrochemical interfaces. This method gives in one relatively simple experiment more detailed information than is available from conventional electrochemical techniques. The combined measurements can be performed either as a function of time, at a constant electrode potential, or under potentiodynamic conditions, as a function of the electrode potential. Herein, we show how this can be applied to enable more accurate investigation of processes that occur at boundaries between electrodes and electrolytes. The application examples range from eletrocatalysis, in which evaluation of a catalyst is performed simultaneously with its formation, and the intercalation and electrodeposition of thin metal films to in situ characterisation of non-electroactive self-assembled monolayers during their formation.

Electrochemical characterisation of copper thin-film formation on polycrystalline platinum.

Electrochemically formed thin films are vital for a broad range of applications in virtually every field of modern science and technology. Understanding the film formation process could provide a means to aid the characterisation and control of film properties. Herein, we present a fundamental approach that combines two well-established analytical techniques (namely, electrochemical impedance spectroscopy and electrogravimetry) with a theoretical approach to provide physico-chemical information on the electrode/electrolyte interface during film formation. This approach allows the monitoring of local and overall surface kinetic parameters with time to enable an evaluation of the different modes of film formation. This monitoring is independent of surface area and surface concentrations of electroactive species and so may allow current computational methods to calculate these parameters and provide a deeper physical understanding of the electrodeposition of new bulk phases. The ability of this method to characterise 3D phase growth in situ in more detail than that obtained by conventional approaches is demonstrated through the study of a model system, namely, Cu bulk-phase deposition on a Pt electrode covered with a Cu atomic layer (Cu(ad)/Pt).

Towards a detailed in situ characterization of non-stationary electrocatalytic systems.

A complementary combination of cyclic voltammetry, impedance spectroscopy and quartz crystal microbalance techniques was used to: (i) control the assembly of a model electrocatalytic system consisting of monolayer and sub-monolayer amounts of Ag and Pb on a Au electrode, (ii) evaluate the system performance for the reduction of NO(3)(-) and (iii) study the disassembly of the electrocatalytic system to explore any changes which occurred during the assembly and/or catalytic stages. Physical models of the electrochemical interface (described in terms of equivalent electric circuits) at all stages are found to be considerably different but consistent with each other. Deposition of the Ag atomic layer on Au is accompanied by spontaneous surface alloying and specific adsorption of anions. In the following, deposition of the Pb atomic layer triggers further alloying in the Ag(ad)/Au layer while perchlorate-ions leave the surface. Approximately 1/3 of the Pb atomic layer on Ag(ad)/Au was found to demonstrate the best activity towards nitrate reduction. The developed experimental approach shows promise for the in situ characterization and control of all the non-stationary stages which are usually of particular importance in electrocatalytic research.