Rinaldo Raccichini

Operando Raman spectroscopy is a well-established technique for monitoring chemical changes in active materials during electrochemical cycling of alkali-ion cells. To date, however, its application to the study of commercial electrodes under realistic operating conditions has been severely limited by cell design constraints. We present here an improved configuration for performing operando Raman spectroscopy on coated metal foil electrodes used in standard laboratory cell testing. Electrochemical modeling predicts much improved lithiation homogeneity compared to a previously used configuration; this observation is validated experimentally for a commercially-sourced graphite electrode. The new configuration delivers improved electrochemical performance at higher specific currents than was previously possible, ensuring that Raman measurements at a single location are representative of the entire electrode. Finally, the broad applicability of the configuration is demonstrated through a study of hard carbon sodium-ion negative electrodes over 50 cycles. These results provide a new configuration for performing reliable, validated operando Raman spectroscopy on commercial battery electrodes, as well as establishing a general methodological framework for the validation of operando spectroscopic techniques to ensure that their performance is relevant to the practical systems to which they are applied.

Sodium-ion batteries (NIBs) are an attractive alternative to lithium-ion batteries (LIBs) because of the natural abundance of sodium resources [1]. Owing to their similar working principles to LIBs, NIBs also offer the advantage of utilizing the same manufacturing protocols and methodology as LIBs, and as such minimal additional capital costs are required for existing LIB manufacturers to switch to NIB technology. Since the sodium storage mechanism of hard carbon (HC) was demonstrated by Stevens and Dahn in 2000 [2], it has become the state-of-the-art anode material for NIBs due to its good cycling stability, low working potentials, small voltage hysteresis and relatively low cost. However, the storage mechanism for high performance is not fully understood due to the complexity of the HC structure. The prevalent problem of under-estimated capacity of HC in half-cell tests indicates that this conventional test method requires improvement. Furthermore, half-cell studies are generally carried out in two-electrode configuration, where the counter electrode (CE) acts also as reference electrode (RE). This provides reliable results only if the passage of current does not significantly perturb the potential of the CE [3]. In this study, we present an in-depth investigation of the electrochemical behavior of a range of HC-based working electrodes (WE) in three-electrode half-cell configuration, with a pure Na metal CE and a concentric Na metal coaxial ring RE. The use of a separate RE allows simultaneous data acquisition for both WE and CE, during galvanostatic cycling and electrochemical impedance spectroscopy measurements. Using this approach, the electrochemical performance, including cycling stability, rate capability and electrochemical impedance, is measured in a reliable manner to assess the capacity and cyclability of HC. In addition, the effect of four different types of electrolyte solution on the electrochemical behavior is analysed and discussed to inform electrolyte selection from a full-cell perspective (Figure 1). Reference s : [1] C. Vaalma, D. Buchholz, M. Weil, S. Passerini, Nat. Rev. Mat., 3 (2018) 18013. [2] J. R Dahn, D. A. Stevens, J. Electrochem. Soc., 4 (2000) 147. [3] R. Raccichini, M. Amores, G. Hinds, Batteries., 5 (2019) 12. Figure 1

The high abundance and low cost of aluminum and sulfur make the Al–S battery an attractive combination. However, significant improvements in performance are required, and increasing the thickness and sulfur content of the sulfur electrodes is critical for the development of batteries with competitive specific energies. This work concerns the development of sulfur electrodes with the highest sulfur content (60 wt %) reported to date for an Al–S battery system and a systematic study of the effect of the sulfur electrode thickness on battery performance. If low‐cost electrolytes made from acetamide or urea are used, slow mass transport of the electrolyte species is identified as the main cause of the poor sulfur utilization when the electrode thickness is decreased, whereas complete sulfur utilization is achieved with a less viscous ionic liquid. In addition, the analysis of very thin electrodes reveals the occurrence of degradation reactions in the low‐cost electrolytes. The new analysis method is ideal for evaluating the stability and mass transport limitations of novel electrolytes for Al–S batteries.

Use of a reference electrode (RE) in Li-ion batteries (LIBs) aims to enable quantitative evaluation of various electrochemical aspects of operation such as: (i) the distinct contribution of each cell component to the overall battery performance, (ii) correct interpretation of current and voltage data with respect to the components, and (iii) the study of reaction mechanisms of individual electrodes. However, care needs to be taken to ensure the presence of the RE does not perturb the normal operation of the cell. Furthermore, if not properly controlled, geometrical and chemical features of the RE can have a significant influence on the measured response. Here, we present a comprehensive review of the range of RE types and configurations reported in the literature, with a focus on critical aspects such as electrochemical methods of analysis, cell geometry, and chemical composition of the RE and influence of the electrolyte. Some of the more controversial issues reported in the literature are highlighted and the benefits and drawbacks of the use of REs as an in situ diagnostic tool in LIBs are discussed.

Impedance spectroscopy is used to characterize the key transport properties (effective conductivity, MacMullin number, porosity and tortuosity) of electrolyte solutions confined in porous separators and carbon-sulfur composite electrodes useful for application in Li-S batteries. Three relevant electrolyte concentrations, ranging between 1 and 5 molal, are studied. Impedance measurements are performed in symmetrical cells with two identical electrodes, which overcome complications associated with the contributions of the counter-reference electrode. The electrolyte-filled carbon-sulfur composite electrodes can be represented by an “open” Warburg element, modelling the finite-diffusion of ions through the pores coupled to the double-layer charging of the electrode-electrolyte interface. The carbon-sulfur composite electrodes are at a high enough potential (ca. 3 V vs. Li⁺/Li) so that charge-transfer reactions of sulfur reduction to polysulfide species are absent during the impedance measurements, and hence capacitive-like behavior is observed at low frequencies. The analysis of the results shows that the rate of transport of ions through porous structures is markedly dependent on the electrode's structure and composition as well as the electrolyte concentration. Synergistic effects, able to enhance the effective conductivity of the electrolyte inside porous composite electrodes, are observed for particular electrode/electrolyte combinations, which are correlated to enhanced performance in Li-S cells.

The Lithium–Sulfur battery is considered to be one of the main candidates for the “post-lithium-ion” battery generation, because of its high theoretical specific capacity and inherently low cost. The role of the electrolyte is particularly important in this system and remarkable battery performances have been reported by tuning the amount of salt in the electrolyte. To further understand the reasons for such improvements we chose the lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane electrolyte as a model salt-solvent system for a systematic study of conductivity and viscosity over a wide range of concentration from 10-5 up to 5 molal. The experimental results, discussed and interpreted with reference to the theory of electrolyte conductance, lead to the conclusion that triple ions formation is mainly responsible for the highest molal conductivity values before reaching the maximum at 1.25 molal. At higher concentrations, the molal conductivity drops quickly due to a rapid increase in viscosity and the salt–solvent system can be treated as a diluted form of molten salt.

Tin oxide (SnO) is considered one of the most promising metal oxides for utilization as anode material in sodium ion batteries (SIBs), because of its ease of synthesis, high specific gravimetric capacity, and satisfactory cycling performance. However, to aim at practical applications, the Coulombic efficiency during cycling needs to be further improved, which requires a deeper knowledge of its working mechanism. Here, a microflower-shaped SnO material is synthesized by means of an ultrafast ionic liquid-assisted microwave method. The as-prepared SnO anode active material exhibits excellent cycling performance, good Coulombic efficiency as well as a large capacity delivered at low potential, which is fundamental to maximize the energy output of SIBs. These overall merits were never reported before for pure SnO anodes (i.e., not in a composite with, for example, graphene). Additionally, by combining ex situ XRD and XPS, it is clearly demonstrated for the first time that the Sn–Na alloy, which is formed during the initial sodium sodiation, desodiates in two successive but fully separated steps. Totally different from the previous report, the pristine SnO phase is not regenerated upon desodiation up to 3 V vs Na/Na⁺. The newly disclosed reaction route provides an alternative view of the complex reaction mechanism of these families of metal oxides for sodium ion batteries.