Inactive materials matter: How binder amounts affect the cycle life of graphite electrodes in potassium-ion batteries

Recent results on the intercalation of potassium into graphite suggest that graphite might become yet again a negative electrode material of choice for an alkali-ion battery system. Compared to its mature application state in Li-ion batteries, graphite for K-ion applications is still in an early development stage. Although cycling of graphite-potassium half-cells over 200 cycles has been demonstrated, the electrodes clearly suffer from more severe capacity fading, as compared to the corresponding Li system. This study demonstrates that the capacity fade is strongly linked to the binder content in the composite electrode. High binder contents of 8 wt% (this study) or more (literature) show significant cycle life improvements over electrodes comprising of more practical binder contents of 4 wt% or less. The results highlight the need for revised or entirely new strategies to control the formation and stability of the electrode–electrolyte interphase in K-ion batteries

Comparing the Solid Electrolyte Interphases on Graphite Electrodes in K and Li Half Cells

In both Li-ion and K-ion batteries, graphite can be used as the negative electrode material. When potassium ions are stored electrochemically in the graphite host, the electrode capacities fade faster than in the lithium ion counterpart. This could be due to the high reactivity of the potassium metal counter electrode (CE) in half cells or a less stable solid electrolyte interphase (SEI) in the potassium case. Previous surface studies on graphite electrodes cycled in K half cells have focused on the SEI characteristics of different electrolyte formulations or different states of charge. In this study, we exploit the fact that graphite can store both lithium and potassium ions. Cell and component parameters have been largely maintained the same, with the only differences between Li and K half cells being the cation of the electrolyte salt and the alkali metal at the CE. The SEI layers formed under these conditions in either setup are studied using X-ray photoelectron spectroscopy with the aim to draw a direct comparison between the surface layers in both charged and discharged states. The results show a considerable crosstalk under OCV conditions between K-metal and the working electrode. Furthermore, the relative SEI layer composition after cycling varies considerably between Li and K half cells. Different dominant SEI species are present depending on the alkali metal used. The strong capacity fade observed in graphite–K half cells is likely linked to much smaller concentrations of inorganic compounds, such as KF, and increased amounts of organic compounds in the SEI

Degradation Phenomena in Silicon/Graphite Electrodes with Varying Silicon Content

The degradation phenomena of Silicon/Graphite electrodes and the effect of FEC as electrolyte additive was investigated through galvanostatic cycling, XPS analyses and SEM cross section analyses. To understand the direct influence of silicon on the electrode degradation, the silicon amount was varied between 0%–30%. By evaluating the cycling performance and the accumulated capacity loss of the different Si/Gr electrodes (cycled with and without 10 vol-% of FEC), we see that the capacity decay can be distinguished into two phenomena, where one is independent of the Si/Gr ratio while the other one depends on the Si content. As expected, adding FEC improves the cell performance and minimizes the capacity decay. Combing our XPS data and SEM cross section analyses on cycled electrodes, this improvement stems from a thin and flexible SEI including poly(vinyl carbonate) that helps maintaining the overall electrode integrity as we observe less electrode fractures and less pronounced thickness increase. Si/Gr electrodes with 10 and 20% Si content showed very similar accumulated irreversible capacity losses over 100 cycles indicating that with 10 % FEC as electrolyte additive, also higher Si contents could be feasible for future high energy density anodes

Performance-Determining Factors for Si–Graphite Electrode Evaluation: The Role of Mass Loading and Amount of Electrolyte Additive

The mass loading of Si–graphite electrodes is often considered as a parameter of secondary importance when testing their electrochemical performance. However, if a sacrificial additive is present in the electrolyte to improve the electrochemical performance, the electrode loading becomes the battery cycle-life-determining factor. The correlation between mass-loading, electrolyte additive, and binder type was investigated by analyzing the cycling behavior of Si–graphite electrodes, prepared with water-based binders, with mass loading ranging from 3 to 9.5 mg cm$^{−2}$ and cycled with FEC electrolyte additive, while keeping electrolyte amount constant. A lower loading was obtained by keeping slurry preparation steps unchanged from binder to binder and resulted in a longer lifetime for some of the binders. When the final loading was kept constant instead, the performance became independent of the binder used. Since such results can lead to the misinterpretation of the influence of electrode components on the cycling stability (and to a preference of one binder over another in our case), we propose that a comparison of long-term electrochemical performance data of Si–graphite electrodes needs to be always collected by using the same mass-loading with the constant electrolyte and additive

Interphase formation with carboxylic acids as slurry additives for Si electrodes in Li-ion batteries. Part 1: performance and gas evolution

Rendering the solid electrolyte interphase and the inter-particle connections more resilient to volume changes of the active material is a key challenge for silicon electrodes. The slurry preparation in a buffered aqueous solution offers a strategy to increase the cycle life and capacity retention of silicon electrodes considerably. So far, studies have mostly been focused on a citrate buffer at pH = 3, and therefore, in this study a series of carboxylic acids is examined as potential buffers for slurry preparation in order to assess which chemical and physical properties of carboxylic acids are decisive for maximizing the capacity retention for Si as active material. In addition, the cycling stability of buffer-containing electrodes was tested in dependence of the buffer content. The results were complemented by analysis of the gas evolution using online electrochemical mass spectrometry in order to understand the SEI layer formation in presence of carboxylic acids and effect of high proton concentration

Potential and Limitations of Research Battery Cell Types for Electrochemical Data Acquisition

Developing new electrode materials and/or electrolytes for lithium-ion batteries requires reliable electrochemical testing thereof. For this purpose, in academic research typically hand-made coin-type cells are assembled. Their advantage is a rather cheap and facile assembly, and possibility to prepare full-cells as well as half-cells, meaning cathode-anode or electrode-elemental lithium configurations. Critical parameters for testing data quality and the potential and limitations of cell tests in half-cell configuration are discussed. Further, on the basis of a round robin test, using highly homogenous commercial electrodes, where graphite is used as anode and LiNi0.33Mn0.33Co0.33O2 (NMC111) as the cathode material, it is shown that data acquired is highly influenced by assembling parameters. Besides known variables such as the amount of electrolyte or electrode positioning, the proper height of the cell stack and the steel grade of the housing material are identified as decisive variables. Finally, it is demonstrated that under proper conditions coin cells can show a great cycle stability of >2200 cycles using 1C as dis-/charge rate while retaining a capacity of 80%. This performance is close to pouch-type cells containing the same electrodes and electrolyte, which were used as a benchmark system and showed >3500 cycles of lifetime

Glyoxylic acetals as electrolytes for Si/Graphite anodes in lithium-ion batteries

Using silicon-containing anodes in lithium-ion batteries is mainly impeded by undesired side reactions at the electrode/electrolyte interface leading to the gradual loss of active lithium. Therefore, electrolyte formulations are needed, which form a solid electrolyte interphase (SEI) that can accommodate to the volume changes of the silicon particles. In this work, we analyze the influence of two glyoxylic acetals on the cycling stability of silicon-containing graphite anodes, namely TMG (1 M LiTFSI in 1,1,2,2-tetramethoxyethane) and TEG (1 M LiTFSI in 1,1,2,2-tetraethoxyethane). The choice of these two electrolyte formulations was motivated by their positive impact on the thermal stability of LIBs. We investigate solid electrolyte decomposition products employing x-ray photoelectron spectroscopy (XPS). The cycling stability of Si/Gr anodes in each electrolyte is correlated to changes in SEI thickness, composition, and morphology upon formation and aging. This evaluation is completed by comparing the performance of TMG and TEG to two carbonate-based reference electrolytes (1 M LiTFSI in 1:1 ethylene carbonate: dimethyl carbonate and 1 M LiPF6 in the same solvent mixture). Cells cycled in TMG display inferior electrochemical performance to the two reference electrolytes. By contrast, cells cycled in TEG exhibit the best capacity retention with overall higher capacities. We can correlate this to better film-forming properties of the TEG solvent as it forms a smoother and more interconnected SEI, which can better adapt to the volume changes of the silicon. Therefore, TEG appears to be a promising electrolyte solvent for silicon-containing anodes

Electrochemical investigation of fluorine-containing Li-salts as slurry cathode additives for tunable rheology in super high solid content NMP slurries

Slurries with high solid contents are attractive because they can minimize usage and recycling of toxic and expensive organic solvents, but have been, so far, very challenging to realize due to their high viscosities, strong slurry gelation and poor coating results. Herein, we demonstrate the application of well-known Li electrolyte salts, namely LiTFA, LiTFSI or LiODFB, as slurry additives, which allow the achievement of an outstanding high solid content of 75.5 wt% for a NMC622-NMP slurry. These kinds of additives are chosen in order to neutralize and chemically complex the NMC622 basic surface and because of their well-known interaction within a battery system when used as electrolyte salts or additives. The investigation shows how high solid content induced slurry gelation can be tuned and controlled depending on the type of the additive and on its affinity towards the NMC622 surface. LiTFA shows the best slurry gelation controlling capabilities and LiTFSI has enhanced long-term capacity retention among the additives, rivalling the best performing reference electrode. EIS performed on fatigued cathodes after 1000 cycles shows how the contact impedance between the electrode composite and the Al current collector rises when Li-salts are used in comparison to the reference. Post-mortem SEM images show cathodic delamination for the additive-containing electrodes. Incremental capacity curves and post-mortem EDX investigations suggest Li plating on graphite anodes as a supplementary cell degradation mechanism when additives are employed

Interphase formation with carboxylic acids as slurry additives for Si electrodes in Li-ion batteries. Part 2: a photoelectron spectroscopy study

The mass loading of Si–graphite electrodes is often considered as a parameter of secondary importance when testing their electrochemical performance. However, if a sacrificial additive is present in the electrolyte to improve the electrochemical performance, the electrode loading becomes the battery cycle-life-determining factor. The correlation between mass-loading, electrolyte additive, and binder type was investigated by analyzing the cycling behavior of Si–graphite electrodes, prepared with water-based binders, with mass loading ranging from 3 to 9.5 mg cm-2 and cycled with FEC electrolyte additive, while keeping electrolyte amount constant. A lower loading was obtained by keeping slurry preparation steps unchanged from binder to binder and resulted in a longer lifetime for some of the binders. When the final loading was kept constant instead, the performance became independent of the binder used. Because such results can lead to the misinterpretation of the influence of electrode components on the cycling stability (and to a preference of one binder over another in our case), we propose that a comparison of long-term electrochemical performance data of Si–graphite electrodes needs to be always collected by using the same mass-loading with the constant electrolyte and additive