Unravelling charge/discharge and capacity fading mechanisms in dual-graphite battery cells using an electron inventory model

The dual-ion battery (DIB) and a subtype thereof, the dual-graphite battery (DGB), are considered as promising alternative options for stationary energy storage applications. Here, we show that not only the working principle of DGBs is fundamentally different from ion-transfer cells, such as the lithium ion battery (LIB), but the capacity fading mechanisms as well. In order to compare the charge/discharge mechanism and aging processes of LIBs and DIBs, which are associated with loss/change of the active species content, a unified “electron inventory model” applicable for both battery systems is introduced and comprehensively explained. This model regards all electron-consuming or -donating reactions in the cell at the respective electrodes. Using the model, two predominant aging variants of DGBs can be distinguished. Both of these originate from disparate (parasitic) electron consumption/donation at the negative or positive electrode, respectively. However, there is an “electron balance” or “electron charge neutrality” between the electrodes, which intrinsically does not allow this difference: This is a consequence of redox reactions that must take place simultaneously at both electrodes in order to guarantee electronic charge neutrality. For example, if electron-consuming reactions occur at the negative electrode that consume lithium cations, the positive electrode is forced to intercalate additional anions for charge compensation, i.e. to ensure charge neutrality in the electrolyte. These anions are irreversibly trapped in the positive electrode and can no longer contribute to reversible charge/discharge reactions. Suitable countermeasures including the pre-lithiation of the negative electrode are presented in this work

A reality check and tutorial on electrochemical characterization of battery cell materials: How to choose the appropriate cell setup

The ever-increasing demand for electrical energy storage technologies triggered by the demands for consumer electronics, stationary energy storage systems and especially the rapidly growing market of electro mobility boosts the need for cost-effective, highly efficient and highly performant rechargeable battery systems. After the successful implementation of lithium ion batteries (LIBs) in consumer electronics and electric vehicles, there is still a need for further improvements in terms of energy and power densities, safety, cost and lifetime. In the last decades, a large battery research community has evolved, developing all kinds of new battery materials, e.g., positive and negative electrode active materials for different cell chemistries, electrolytes, related auxiliary (inactive) materials and their constituents

Enabling bis(fluorosulfonyl)imide-based ionic liquid electrolytes for application in dual-ion batteries

In this work, we present a comprehensive study on the effect of adding different conductive salt additives including LiPF6, LiBF4 and LiDFOB, as well as the fluorinated solvent additive methyl difluoroacetate (MDFA) to a bis(fluorosulfonyl)imide (FSI)-based ionic liquid (IL) electrolyte, i.e. Pyr14FSI/LiFSI, to protect the Al current collector (ACC) from anodic dissolution and, thus, enable reversible charge/discharge cycling in a high performance dual-ion cell. Chronocoulometry and scanning electron microscopy measurements were conducted to evaluate the specific ACC passivation ability of each electrolyte. Furthermore, the influence of these additives on anion intercalation behavior into the graphite positive electrode with special emphasis on the Coulombic efficiency (CE), reversible capacity, as well as capacity retention is presented. Overall, we can show that the addition of small amounts of LiPF6, LiBF4 and MDFA (0.5 wt%) into the FSI-based IL electrolyte significantly increases the overall cell performance, whereas LiDFOB as electrolyte additive deteriorates the dual-ion cell performance. In addition, an excellent cycling performance for 1000 cycles is obtained for the Pyr14FSI electrolyte having 5 wt% LiPF6, displaying an average reversible capacity of 40 mAh g−1, a CE exceeding 98% and a capacity retention of 91%, which has not been reported so far

Impact of Degree of Graphitization, Surface Properties and Particle Size Distribution on Electrochemical Performance of Carbon Anodes for Potassium‐Ion Batteries

Carbons are considered as anode active materials in potassium ion batteries (PIBs). Here, the correlation between material properties of disordered (non-graphitic) and ordered graphitic carbons and their electrochemical performance in carbon || K metal cells is evaluated. First, carbons obtained from heat treatment of petroleum coke at temperatures from 800 to 2800 °C are analyzed regarding their microstructure and surface properties. Electrochemical performance metrics for K+ ion storage like specific capacity and Coulombic efficiency (CEff) are correlated with surface area, non-basal planes and microstructure properties, and compared to Li+ ion storage. For disordered carbons, the specific capacity can be clearly correlated with the defect surface area. For highly ordered graphitic carbons, the degree of graphitization strongly determines the specific capacity. The initial CEff of graphitic carbons shows a strong correlation with basal and non-basal planes. Second, kinetic limitations of ordered graphitic carbons are re-evaluated by analyzing commercial graphites regarding particle size and surface properties. A clear correlation between particle size, surface area and well-known challenges of graphitic carbons in terms of low-rate capability and voltage hysteresis is observed. This work emphasizes the importance of bulk and surface material properties for K+ ion storage and gives important insights for future particle design of promising carbon anodes for PIB cells

Anodic Dissolution in Dual-Ion Batteries: Development of Protection Layers for Current Collectors

One aspect of the current research on lithium ion batteries is the increase of the cell voltage to improve the energy density. In innovative dual-ion cells, graphite intercalation compounds are used for both electrodes. Consequently, a simultaneous intercalation of lithium ions into the anode and the corresponding salt anions into the cathode is possible and enables voltage values above 5 V vs. Li/Li+. Established liquid electrolytes, consisting of carbonates and lithium hexafluorophosphate, do not resist these conditions. Therefore, we replace this mixture by ethyl methanesulfonate and organic lithium salts, having a good electrochemical performance also at higher temperatures. Now, it is the challenge to overcome the anodic dissolution of the aluminum current collector triggered by fluorinated anions like bis(trifluoromethylsulfonyl)imide (TFSI-). Our approach to protect the metal against anodic dissolution is the deposition of an only electronically conductive, defect-free and mechanically as well as electrochemically stable layer. We test different material compositions in the range of oxidic ceramics. First of all semiconductors like alumina doped zinc oxide show promising results to meet the requirements. The preparation is based on a sol-gel route combined with several wet-chemical coating methods, because these are easily adjustable to different substrate dimensions. Ceramic thin layers of around 100 nm thickness are deposited on aluminum foil and mainly investigated by scanning electron microscopy, X-ray diffraction and secondary ion mass spectrometry. Additionally, we present conductivity measurements and the electrochemical performance tested with cyclic voltammetry

Does Size really Matter? New Insights into the Intercalation Behavior of Anions into a Graphite-Based Positive Electrode for Dual-Ion Batteries

There are many reports on electrochemical anion intercalation into graphite using different types of electrolyte mixtures for application in dual-graphite or dual-ion cells, showing promising results in terms of cycling stability, reversible capacity and Coulombic efficiency. However, there is no clear understanding of the influence of the anion/electrolyte characteristics on the intercalation properties. In this work, we present a comprehensive study of the intercalation behavior of a series of imide-based ionic liquid (IL) electrolytes into a graphite positive electrode with special emphasis on the influence of anion size on the electrochemical parameters such as the onset potential for anion uptake and the reversible capacity. The onset potentials of anion intercalation into graphite ranged between 4.42 V to 4.53 V vs. Li/Li+ with the following descending order for the studied anions: BETI > FSI > FTFSI > FSI/TFSI (molar ratio = 11:1) > TFSI > TFSI/FSI (molar ratio = 10:1). The electrochemical results support the assumption that electrolyte effects such as ion pair formation and self-aggregation in the electrolyte overrule the influence of the anion size (up to a certain point) in terms of the onset potential for anion uptake. The charge/discharge cycling performance was studied in view of reversible capacity and Coulombic efficiency. In this context, the BETI system shows only very poor intercalation ability whereas the quaternary mixture TFSI/FSI displays a very promising cycling behavior providing a specific capacity of ∼54 mAh g−1 with a Coulombic efficiency exceeding 99%. Furthermore, the characteristics of the different imide-based electrolytes such as the oxidative stability (TFSI > BETI > FTFSI > TFSI/FSI > FSI/TFSI > FSI) as well as the influence on aluminum current collector dissolution were studied to draw further conclusions about impact on the Coulombic efficiency

Triphenylphosphine Oxide as Highly Effective Electrolyte Additive for Graphite/NMC811 Lithium Ion Cells

Nickel-rich layered oxide materials (LiNixMnyCo1–x–yO2, x ≥ 0.8, LiNMC) attract great interest for application as positive electrode in lithium ion batteries (LIBs) due to high specific discharge capacities at moderate upper cutoff voltages below 4.4 V vs Li/Li+. However, the comparatively poor cycling stability as well as inferior safety characteristics prevent this material class from commercial application so far. Against this background, new electrolyte formulations including additives are a major prerequisite for a sufficient electrochemical performance of Ni-rich NMC materials. In this work, we introduce triphenylphosphine oxide (TPPO) as electrolyte additive for the application in graphite/LiNi0.8Mn0.1Co0.1O2 (NMC811) cells. The addition of only 0.5 wt % TPPO into a carbonate-based electrolyte (LiPF6 in EC:EMC) significantly increases the first cycle Coulombic efficiency as well as the reversible specific capacity and improves the capacity retention of the LIB full cell cycled between 2.8 and 4.3 V. Electrochemical results indicate that the full cell capacity fade is predominantly caused by active lithium loss at the negative electrode. In this contribution, X-ray photoelectron spectroscopy and inductively coupled plasma-mass spectrometry analysis confirm the participation of the electrolyte additive in the solid electrolyte interphase formation on the negative electrode as well as in the cathode electrolyte interphase formation on the positive electrode, thus, effectively reducing the active lithium loss during cycling. Furthermore, the performance of the TPPO additive is compared to literature known electrolyte additives including triphenylphosphine, vinylene carbonate, and diphenyl carbonate demonstrating the outstanding working ability of TPPO in graphite/NMC811 cells