In-Depth Experimental and Computational Studies on Rechargeable Battery Materials

Judging by the number of downloads and citations, the topics covered by the Special Issue “Rechargeable Batteries Studied Using Advanced Spectroscopic and Computational Techniques I” [1] are of great interest to the research community seeking to improve the performance and lifespan of rechargeable batteries. This prompted the call for a second Special Issue, introduced herein, giving researchers the opportunity to further present the capabilities and successes of their techniques with those goals in mind. This Special Issue consists of seven articles, listed in order of appearance in the List of Contributions at the end of this editorial. Below, we briefly highlight the contents of the articles in the present Special Issue. We refer to the articles using the corresponding number in the List of Contributions.

Research on alternative anode materials for both Li- and Na-ion batteries has steadily increased over the past several years. Thus, for instance, some researchers have turned their focus to ferrous oxalates, known precursors to iron phosphates, themselves of interest as cathode materials [2]. Now, ferrous oxalates exist in dihydrate and anhydrous forms. Interestingly, using density functional theory (DFT) techniques and computational modeling, in article 3 in the List of Contributions it is shown that the anhydrous form outperforms the dihydrate form as an anode material in Li-ion batteries, in terms of electrochemical stability, storage capacity, and voltage. The water molecules in the dihydrate form tend to hydrate Li, inhibiting electrochemical activity. Similarly, the anhydrous form is shown to be of interest as an anode material in Na-ion batteries. Of particular interest is the fact that the underlying electrochemical process generating energy in this case is a conversion mechanism, rather than the more typical intercalation mechanism, and can provide a higher capacity (as shown in article 1).

As always, an accurate characterization of the properties of cathode and anode materials is required for a proper understanding of their electrochemical activity. This is best achieved through a combination of advanced experimental and computational modeling techniques. The properties of interest can range from band gap and energy loss function, as in the work on LiNiO₂ in article 2, to phase and structure, as in the work on CuNP in article 6. Interestingly, in the latter, an analysis of the electrochemical charge/discharge cycles shows that the cyclability in Na-ion cells is negatively impacted by the presence of water when the hydrated form of CuPN is used.

We also present two additional articles exploiting the unique capabilities of Compton scattering and positron annihilation spectroscopy (PAS). Clearly, efforts to improve the performance of a battery can significantly benefit from a greater insight into the nature of the orbitals involved in the redox reactions. In that respect, magnetic Compton scattering experiments on Li_xTi_0.4Mn_0.4O₂ (Li-rich LMTO), combined with DFT-based computations, produced evidence for anionic redox and determined the role of the Mn 3d electrons and O 2p holes in the process (as described in article 4). Another crucial aspect on which further insight is desirable, e.g., in a Li-ion battery, is the relation between the lithiation state and the possible morphological changes in the cathode material. This can be achieved via positron lifetime and Doppler broadening PAS measurements. This is shown in the case of LiCoO₂ cathodes in article 5. Finally, article 7 presents a compelling review of how the aforementioned techniques, with the support of computational modeling, can produce spectroscopic descriptors that are useful for the diagnosis of cathode materials and that provide insight into the electrochemical processes in them.

Ultimately, we are confident that this Special Issue will continue to motivate and guide further research focused on the development of higher-performance and longer-lifespan batteries.