On the origin of the extra capacity at low potential in materials for Li batteries reacting through conversion reaction

TLDR: The possibility of interfacial storage at low potential for electrode materials reacting through conversion reactions was evaluated in this paper, where the amount of charge that could be stored through the proposed interfacial mechanism was estimated for a range of different materials and was much lower than those observed experimentally.

About: This article is published in Electrochimica Acta.The article was published on 2012-02-01 and is currently open access. It has received 209 citations till now. The article focuses on the topics: Electrolyte.

Figures

Fig. 3. Plot of volume change percentages (black scale and bars) and M nanoparticles surface area (per MaXb unit mass; green scale and bars) after conversion reaction.

Fig. 2. (A) Plot of potential versus charge capacity profile for Co3O4 composite electrodes upon the first cycle at different C rates and (B) plot of the charge capacity percentage for each process (i.e. insertion a; conversion b and extra capa g) with respect to the total capacity achieved upon the first reduction.

Fig. 1. (A) Typical potential versus charge capacity profile curve. Usual values for Q/F (corresponding to b·c in Eq. (1)) range between 2 and 8 mol of electrons per mole of MaXb) [4]. (B) Schemes of the two proposed redox mechanisms taking place at low potential.

Fig. 4. Plots of (A) Qextra versus (C rate)−1/2 and (B) 1/Qextra versus (C rate)−1/2 . Red lines are linear extrapolation of the capacity at infinitely fast and slow C rates in (A) and (B), respectively.

Frequently asked questions

Q1. What have the authors contributed in "On the origin of the extra capacity at low potential in materials for li batteries reacting through conversion reaction" ?

Palacín et al. this paper evaluated the possibility of interfacial storage at low potential for electrode materials reacting through conversion reactions forming M/LicX composites at the end of reduction. 

Q2. What have the authors stated for future works in "On the origin of the extra capacity at low potential in materials for li batteries reacting through conversion reaction" ?

In this study the authors evaluated the possibility of interfacial storage at low potential for electrode materials reacting through conversion reactions forming M/LicX composites at the end of reduction. While this would be hardly measurable experimentally within a single electrode, the slope of the potential decay and the influence of the current in the extent of stored capacity do not seem to be consistent with a capacitive-like mechanism. In summary, their results indicate that interfacial storage, if any, would be negligible with respect to electrolyte decomposition to account for the extra capacity observed at low potential in conversion reaction materials. 

Q3. What is the main drawback of the conversion reaction materials?

Progress in the study of insertion compounds enabled the development and commercialisation of lithium ion batteries but a paradigmatic shift in energy density will only be achieved with the use of electrodes operating through alternative reaction mechanisms. 

Q4. What are the interesting case examples to be studied?

While other factors such as cost, availability, operation potential and so on are decisive in terms of estimating the potential interest of electrode materials, basing exclusively in theoretical capacity and volume expansion both trivalent fluorides (e.g. TiF3, VF3, FeF3 and CrF3) and Fe3O4 appear as most interesting case examples to be studied in detail. 

Q5. What is the main drawback for those materials?

The main drawback for those materials is that the large available electrochemical capacity is achieved at the expense of major structural changes in the electrode that are difficult to “buffer”. 

Q6. How did Ardizzone et al. determine the capacity of a RuO?

by extrapolating the value of the charge at infinitely slow and fast sweep rates they were able to discriminate between bulk and surface contribution to the capacities recorded and demonstrated a significant contribution of the bulk of the material. 

Q7. What is the signature of such additional reversible capacity?

The signature of such additional reversible capacity is a sloping curve at low potentials (generally below 0.8 V vs. Li+/Li, see Fig. 1A). 

Q8. What is the kinetics of the redox reaction in a composite electrode?

3. Results and discussionCapacitive electrochemical phenomena are much faster than faradaic processes involving a redox reaction, as illustrated by their respective time constants: seconds for the former and minutes or hours for the latter [14]. 

Q9. What is the main drawback for materials electrochemically forming alloys with lithium?

While materials electrochemically forming alloys with lithium [3] are starting to become a commercial reality, those operating through conversion reactions [4] exhibit promising expectative.