Rüdiger-A. Eichel

Singlet Oxygen Formation during the Charging Process of an Aprotic Lithium-Oxygen Battery.

Aprotic lithium-oxygen (Li-O2 ) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen ((1) Δg ) is formed upon Li2 O2 oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time- and voltage-resolved in operando EPR spectroscopy in a purpose-built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long-overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li-O2 cells.

Operando electron paramagnetic resonance spectroscopy – formation of mossy lithium on lithium anodes during charge–discharge cycling

The formation of mossy lithium and lithium dendrites so far prevents the use of lithium metal anodes in lithium ion batteries. To develop solutions for this problem (e.g., electrolyte additives), operando measurement techniques are required to monitor mossy lithium and dendrite formation during electrochemical cycling. Here we present a novel battery cell design that enables operando electron paramagnetic resonance (EPR) spectroscopy. It is shown that time-resolved operando EPR spectroscopy during electrochemical cycling of a lithium-metal/LiFePO4 (LFP) cell provides unique insights into the lithium plating/dissolution mechanisms, which are consistent with ex situ scanning electron microscopy (SEM) analysis. To demonstrate the viability of the operando EPR method, two cells using different electrolytes were studied. When using an electrolyte containing fluoroethylene carbonate (FEC) additive, a higher reversibility of the lithium anode and reduced formation of micro-structured (mossy/dendritic) lithium were observed.

Limitation of Discharge Capacity and Mechanisms of Air- Electrode Deactivation in Silicon-Air Batteries

The electrocatalytical process at the air cathode in novel silicon-air batteries using the room-temperature ionic liquid hydrophilic 1-ethyl-3-methylimidazolium oligofluorohydrogenate [EMI⋅2.3 HF⋅F] as electrolyte and highly doped silicon wafers as anodes is investigated by electrochemical means, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. The results obtained by XPS and EPR provide a model to describe the limited discharge capacity by means of a mechanism of air-electrode deactivation. In that respect, upon discharge the silicon-air batterys cathode is not only blocked by silicon oxide reduction products, but also experiences a major modification in the MnO₂ catalyst nature. The proposed modification of the MnO₂ catalyst by means of a MnF₂ surface layer greatly impacts the Si-air performance and describes a mechanism relevant for other metal-air batteries, such as the lithium-air. Moreover, the ability for this deactivation layer to form is greatly impacted by water in the electrolyte.

Reference electrode assembly and its use in the study of fluorohydrogenate ionic liquid silicon electrochemistry

Silicon electrochemistry in fluorohydrogenate ionic liquids is partly hampered owing to the incapability of producing an accurate and reproducible potential measurement due to a lack of appropriate reference electrodes. This research work describes a simple assembly of a stable external reference electrode enabling accurate studies of silicon electrochemistry in fluorohydrogenate ionic liquids. The electrode configuration is based on the ferrocene/ferrocenium (Fc|Fc(+)) couple dissolved in the EMIm(HF)(2.3)F (1-ethyl-3-methyl-imidazolium fluorohydrogenate)/Carbopol 941 gel. A stable potential of 2.5 wt% Carbopol-based electrode was measured versus a calomel reference electrode at 250 ± 3 mV. By utilizing the constructed electrode, an intensive electrochemical investigation on n-type silicon in EMIm(HF)(2.3)F was conducted. Flat-band and open circuit potentials were measured, along with Si-air half- and full-cell electrochemical measurements. A suggested mechanism for the n-type Si dissolution process in the EMIm(HF)(2.3)F electrolyte, without illumination, is discussed as well.

Carbon-coated core–shell Li2S@C nanocomposites as high performance cathode materials for lithium–sulfur batteries

Li2S has made the concept of Li–S batteries much more promising due to the relatively high storage capacity, the possibility of using Li-free anodes and the increase of microstructural stability. However, similar to S, Li2S also suffers from an insulating nature and polysulfide dissolution problem. The results presented here show a facile and cost-effective approach by using a plasma sparking and chemical sulfurization process to synthesize core–shell Li2S@C nanocomposites. The nanocomposites show a significantly reduced particle size and well-developed core–shell architecture, effectively shortening the Li-ion diffusion distance, enhancing the electronic conductivity and suppressing the dissolution losses of polysulfides. As a result, a much improved rate and cycling performance has been achieved. The method presented in this study offers good opportunities for scaling up the production of high performance cathode materials in a simple and low-cost way to be applied in future generation Li–S batteries.

Analyzing the defect structure of CuO-Doped PZT and KNN piezoelectrics from electron paramagnetic resonance

The defect structure for copper-doped sodium potassium niobate (KNN) ferroelectrics has been analyzed with respect to its defect structure. In particular, the interplay between the mutually compensating dimeric (CuNb-VO) and trimeric (VO-CuNb-VO) defect complexes with 180° and non-180° domain walls has been analyzed and compared to the effects from (Cu - VO)x× dipoles in CuO-doped lead zirconate titanate (PZT). Attempts are made to relate the rearrangement of defect complexes to macroscopic electromechanical properties.

Observing different modes of mobility in lithium titanate spinel by nuclear magnetic resonance

Lithium titanate (LTO) is a spinel material that is able to reversibly intercalate Li ions with minimal changes of the unit cell dimensions (“zero-strain”), making it an attractive choice as anode material for Li ion batteries. However, the nature of the Li transport in this material is still not fully understood. Here, the Li mobility in Li4+xTi5O12 with x = 0 and x ≈ 1.6 is investigated. By regularized inversion of nuclear magnetic resonance (NMR) relaxation and spin alignment echo (SAE) data and supported by DFT simulations, solid-state NMR spectra were analyzed as a function of the respective relaxation times and correlation time constants. A clear correlation between mobility and NMR spectral features was observed, suggesting the presence of local domains with high Li ion mobility. The long-range mobility is limited by the much slower hopping between such domains and appears to be faster for either larger or less ordered local domains. For x ≈ 1.6, spectral features indicate the formation of separate stoichiometric and overlithiated phases rather than a solid solution, yet no segregation into a fast and a slow component was observed in the relaxation and in the SAE dimension, which points towards an entangling of the two phases on a microscopic scale.

Influence of the A/B Stoichiometry on Defect Structure, Sintering, and Microstructure in Undoped and Cu-Doped KNN

Development of ceramics based on the alkaline niobate (KNN) system is one of the major lines of current research pointing to substitution of the lead containing ferroelectrics by lead‐free materials. Sodium potassium niobate (K0.5Na0.5)NbO3 is a prototype material of lead‐free alkaline‐transition metal ferroelectrics with \({\rm A}^{1+}{\rm B}^{5+}{\rm O}_3^{2-}\) perovskite structure. Processing procedures for KNN‐based ceramics are however challenging due to the hygroscopic behavior of sodium‐ and potassium carbonates and the evaporation of alkalines at the elevated processing temperatures, which make it difficult to control the stoichiometry of the ceramics. Alkaline (A‐site) or niobium (B‐site) excess results in pronounced qualitative differences of the microstructure in KNN ceramics.

Monitoring local redox processes in LiNi0.5Mn1.5O4 battery cathode material by in operando EPR spectroscopy

Despite the multitude of analytical methods available to characterize battery cathode materials, identifying the factors responsible for material aging is still challenging. We present the first investigation of transient redox processes in a spinel cathode during electrochemical cycling of a lithium ion battery by in operando electron paramagnetic resonance (EPR). The battery contains a LiNi0.5Mn1.5O4 (LNMO) spinel cathode, which is a material whose magnetic interactions are well understood. The evolution of the EPR signal in combination with electrochemical measurements shows the impact of Mn3+ on the Li+ motion inside the spinel. Moreover, state of charge dependent linewidth variations confirm the formation of a solid solution for slow cycling, which is taken over by mixed models of solid solution and two-phase formation for fast cycling due to kinetic restrictions and overpotentials. Long-term measurements for 480 h showed the stability of the investigated LNMO, but also small amounts of cathode degradation products became visible. The results point out how local, exchange mediated magnetic interactions in cathode materials are linked with battery performance and can be used for material characterization.

Monolithic All-Phosphate Solid-State Lithium-Ion Battery with Improved Interfacial Compatibility

High interfacial resistance between solid electrolyte and electrode of ceramic all-solid-state batteries is a major reason for the reduced performance of these batteries. A solid-state battery using a monolithic all-phosphate concept based on screen printed thick LiTi2(PO4)3 anode and Li3V2(PO4)3 cathode composite layers on a densely sintered Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte has been realized with competitive cycling performance. The choice of materials was primarily based on the (electro-)chemical and mechanical matching of the components instead of solely focusing on high-performance of individual components. Thus, the battery utilized a phosphate backbone in combination with tailored morphology of the electrode materials to ensure good interfacial matching for a durable mechanical stability. Moreover, the operating voltage range of the active materials matches with the intrinsic electrochemical window of the electrolyte which resulted in high electrochemical stability. A highly competitive discharge capacity of 63.5 mAh g-1 at 0.39 C after 500 cycles, corresponding to 84% of the initial discharge capacity, was achieved. The analysis of interfacial charge transfer kinetics confirmed the structural and electrical properties of the electrodes and their interfaces with the electrolyte, as evidenced by the excellent cycling performance of the all-phosphate solid-state battery. These interfaces have been studied via impedance analysis with subsequent distribution of relaxation times analysis. Moreover, the prepared solid-state battery could be processed and operated in air atmosphere owing to the low oxygen sensitivity of the phosphate materials. The analysis of electrolyte/electrode interfaces after cycling demonstrates that the interfaces remained stable during cycling.