Heino Sommer

Dynamic formation of a solid-liquid electrolyte interphase and its consequences for hybrid-battery concepts

The discharging and charging of batteries require ion transfer across phase boundaries. In conventional lithium-ion batteries, Li(+) ions have to cross the liquid electrolyte and only need to pass the electrode interfaces. Future high-energy batteries may need to work as hybrids, and so serially combine a liquid electrolyte and a solid electrolyte to suppress unwanted redox shuttles. This adds new interfaces that might significantly decrease the cycling-rate capability. Here we show that the interface between a typical fast-ion-conducting solid electrolyte and a conventional liquid electrolyte is chemically unstable and forms a resistive solid-liquid electrolyte interphase (SLEI). Insights into the kinetics of this new type of interphase are obtained by impedance studies of a two-chamber cell. The chemistry of the SLEI, its growth with time and the influence of water impurities are examined by state-of-the-art surface analysis and depth profiling.

The critical role of lithium nitrate in the gas evolution of lithium–sulfur batteries

Sulfur–carbon composites are promising next generation cathode materials for high energy density lithium batteries and thus, their discharge and charge properties have been studied with increasing intensity in recent years. While the sulfur-based redox reactions are reasonably well understood, the knowledge of deleterious side reactions in lithium–sulfur batteries is still limited. In particular, the gassing behavior has not yet been investigated, although it is known that lithium metal readily reacts with the commonly used ethereal electrolytes. Herein, we describe, for the first time, gas evolution in operating lithium–sulfur cells with a diglyme-based electrolyte and evaluate the effect of the polysulfide shuttle-suppressing additive LiNO3. The use of the combination of two operando techniques (pressure measurements and online continuous flow differential electrochemical mass spectrometry coupled with infrared spectroscopy) demonstrates that the additive dramatically reduces, but does not completely eliminate gassing. The major increase in pressure occurs during charge, immediately after fresh lithium is deposited, but there are differences in gas generation during cycling depending on the addition of LiNO3. Cells with LiNO3 show evolution of N2 and N2O in addition to CH4 and H2, the latter being the main volatile decomposition products. Collectively, these results provide novel insight into the important function of LiNO3 as a stabilizing additive in lithium–sulfur batteries.

Simple cathode design for Li–S batteries: cell performance and mechanistic insights by in operando X-ray diffraction

Rechargeable batteries have been receiving increasing attention over the past several years, particularly with regard to the accelerated development of electric vehicles, but also for their potential in grid storage applications. Among the broad range of cathode active materials, elemental sulfur has the highest theoretical specific capacity, thereby making it one of the most promising positive electrode materials these days. In the present work, we show that already a simple cathode design (cathodes with a non-optimized composite microstructure) provides good electrochemical performance both in coin and pouch cells with sulfur loadings of 2 mg cm−2. Our research data demonstrate that (1) specific capacities of 1000 mA h g−1 can be achieved over 60 cycles at room temperature while the cyclability at elevated temperatures (here, θ > 40 °C) is poor, (2) the discharge is the kinetically rate-limiting process, (3) the major fraction of active sulfur in the electrode is lost during the formation cycle at C/50 and (4) the Li–S cells suffer from drying-out due to continuous electrolyte decomposition on the lithium metal anode. In addition, in operando X-ray diffraction shows Li2S formation (grain size of <10 nm) on discharge and the appearance of single phase β-sulfur in the sub-100 nm size range – rather than the thermodynamically stable orthorhombic polymorph (α-sulfur) – by the end of the charge cycle.

Free-standing and binder-free highly N-doped carbon/sulfur cathodes with tailorable loading for high-areal-capacity lithium–sulfur batteries

A facile hard-templating method has been developed to prepare highly N-doped carbon/sulfur cathodes with thickness 2.5 mgsulfur cm−2). Lithium–sulfur batteries using this free-standing and binder-free hierarchical hybrid design exhibit good cycling performance, with stable areal capacities of 3.0 mA h cm−2, owing to favorable properties of the carbon host.

Gas Evolution in Operating Lithium-Ion Batteries Studied In Situ by Neutron Imaging.

Gas generation as a result of electrolyte decomposition is one of the major issues of high-performance rechargeable batteries. Here, we report the direct observation of gassing in operating lithium-ion batteries using neutron imaging. This technique can be used to obtain qualitative as well as quantitative information by applying a new analysis approach. Special emphasis is placed on high voltage LiNi0.5Mn1.5O4/graphite pouch cells. Continuous gassing due to oxidation and reduction of electrolyte solvents is observed. To separate gas evolution reactions occurring on the anode from those associated with the cathode interface and to gain more insight into the gassing behavior of LiNi0.5Mn1.5O4/graphite cells, neutron experiments were also conducted systematically on other cathode/anode combinations, including LiFePO4/graphite, LiNi0.5Mn1.5O4/Li4Ti5O12 and LiFePO4/Li4Ti5O12. In addition, the data were supported by gas pressure measurements. The results suggest that metal dissolution in the electrolyte and decomposition products resulting from the high potentials adversely affect the gas generation, particularly in the first charge cycle (i.e., during graphite solid-electrolyte interface layer formation).

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.

Facile synthesis of micrometer-long antimony nanowires by template-free electrodeposition for next generation Li-ion batteries

Template-free electrodeposition of high-aspect-ratio antimony nanowires at room temperature is reported. The nanowires with diameters down to 30 nm and lengths of up to several tens of micrometers are of high quality and grow very densely on different substrates such as copper and glassy carbon. Furthermore, they can be applied as a high capacity anode material in rechargeable Li-ion batteries with good capacity retention.

Differential Electrochemical Mass Spectrometry in Lithium Battery Research

The combination of electrochemistry with mass spectrometry led to the development of a large number of techniques, only a few of which are the subject of this report. We will not discuss the methods that originated from different ionization approaches (e.g., chemical ionization, desorption electrospray ionization, fast atom bombardment, etc.), 1 but only those deployed for the direct investigation of gaseous or volatile reaction products of lithium ion batteries [The term “lithium ion battery” is used for any kind of batteries, in which the main charge carriers are lithium ions (also for cells containing a lithium metal anode, like in lithium-sulfur (Li–S) or lithium–air/lithium–oxygen (Li–O 2 ) batteries, or so-called half-cells).] (LIBs). Differential electrochemical mass spectrometry (DEMS) has been extensively used in other fields of electrochemistry, especially in electrocatalysis 2 , 3 ; however these applications are also beyond the scope of this report. After a short historical review of DEMS, we will show the different development periods relevant to battery research and the implementations used recently, demonstrating the versatility and strengths of this technique. Lastly, we discuss the typical gases observed with DEMS and their formation patterns based on the literature data.