Solid state chemistry and electrochemistry applied to battery materials, covering a wide diversity of technologies with either aqueous or organic electrolytes. These include already commercial (e.g. Ni or Li-ion) or pre-commercial (Na-ion) concepts, as well as new emerging chemistries such as those based on Mg or Ca. Emphasis is placed on developing fertile cooperation scenarios between fundamental research and industry.
Current projects are focused on post Li-ion sustainable technologies based on abundant elements. While for the Na-ion case fast progress is expected as a result of chemical similarities with lithium and the cumulated Li-ion battery know-how over the years, for Ca and Mg the situation is radically different. On one hand, the possibility to use Ca or Mg metal anodes would bring a breakthrough in terms of energy density,9 on the other, development of suitable electrolytes and cathodes with efficient multivalent ion diffusion are bottlenecks to overcome.4,10,16
Electrode materials investigated for multivalent battery concepts typically exhibit a transition metal as redox center. A priori, the use of more covalent electrode would be preferred, as the coulombic interactions with divalent ions would be less important. Development of suitable experimental protocols is mandatory, as know-how gained in the field of Li-ion cannot be blindly adopted. Thus, the use of characterization techniques complementary to electrochemistry is mandatory.19 These have enabled, for instance, to assess reversible electrochemical Ca2+ intercalation in TiS2, forming a range of different phases depending on the electrolyte and conditions used, some of them exhibiting co-intercalated solvent molecules.2,13
The screening of positive electrode materials being strongly limited by the absence of standard electrolyte with adequate anodic stability, theoretical calculations using DFT have been very useful in unraveling suitable electrode materials for divalent cation intercalation.3 Yet, the topic is complex as some hosts that have been predicted to enable low migration barriers for calcium, such as a hypothetical CaMn2O4 spinel, come to the expense of phase stability and hence, the polymorphs that can be synthesized experimentally are different and exhibit larger migration barriers.27 In other cases, such as 1D-structure Ca3Co2O6,15 or ternary nitride CaTaN2,7 electrochemical extraction of calcium takes place, with reversibility being very limited, if any, which might be related to high desolvation energies or unstability of the phases formed.
In contrast to what happens for multivalent chemistries, progress in the sodium-ion battery (SIB) field is achieved at a quicker pace, catalysed by the chemical analogies between lithium and sodium and the wide accumulated know-how of LIBs. The materials choice on the negative side is relatively restricted, and today only hard carbon exhibit realistic application prospects.32 The most critical issue for hard carbon is the irreversible capacity loss upon the first cycle which severely penalizes the practical cell energy density achievable. This can at least partly be explained by the lower stability of the passivation layer formed at the negative electrode/electrolyte interface when compared to the one formed for LIB (commonly termed Solid Electrolyte Interphase, SEI).1,28
Hard carbon can be prepared from different precursors (phenolic resin and commercially available cellulose and lignin) under different pyrolysis and processing conditions using industrially adapted syntheses protocols. The study of their microstructural features enabled to assess that the nature of the precursor and the temperature of pyrolysis are the major factors determining the carbon yield and the surface area, the latter one having a major effect on the electrochemical capacity.18 When coupled to Na3V2(PO4)2F3, full cells exhibit performances comparable to State-of-the-art Li-ion batteries.39
The electrolyte is sometimes called “passive”, “only” being responsible for containing and shuttling the charge carrier ion of choice (H+, Li+, Na+, Ca2+, Mg2+, etc.) between the electrodes. Yet, the electrolyte has a relevant influence in the life-length of a battery and the practically accessible capacity, rate capability, safety, etc.12 Indeed, the composition of the electrolyte is responsible for the parasitic degradation reactions, wettability of the electrodes and separator, mobility of solvated cations, flammability, etc.31 Our research mostly focuses in controlling post-Li battery electrolyte formulations (combination of salt, solvent and additive) in order to tune their physico-chemical properties (ionic conductivity, viscosity, electrochemical stability window, solvation structure …).6,39,47
The nature, composition and homogeneity of the interface between the electrode and the electrolyte are other crucial parameters determining the battery performances. Electrolyte reduction or oxidation often result in insoluble products adhering to the electrode surface and forming solid passivation layer (the so-called SEI) on the negative electrode or surface layer (SL on the positive). Long term cell operation will only be enabled if such passivation layers are electronically insulating (preventing further electrolyte decomposition), while allowing for cation diffusion through it. Such behaviour is not trivial for post-Li systems and, for instance, salts present in Na based passivation layers possess higher solubility when compared with Li analogues, due to the difference in Lewis acidity of the two cations, possibly resulting in poor stability and cycle life.1,20,28 On the other hand, Ca salts are usually much less soluble and better stability of the interface is expected. However, passivation layers were generally thought to be the reason preventing Ca plating due to lack of divalent cation transport through them. Challenging that assumption, we were able to demonstrate that calcium can be reversibly plated and stripped through a stable SEI layer at moderate temperature (> 75°C) evidencing that crucial parameters impacting the electrodeposition process are not only the SEI composition but also the formation of ion pairs within the electrolyte. The latter hinders Ca2+ migration and results in a high energy barrier for cation desolvation.6,22 Efforts are now being devoted to control the solvation structure of the cation, investigating its impact on: (i) the cation mobility within the electrolyte, (ii) the formation of the SEI layer and (iii) the desolvation energy at the electrolyte/SEI interface.
Finally, we are also interested in the engineering of the electrode/electrolyte interface by means of coating. For instance, carbon coating on both negative and positive electrodes was found to results in (i) improved electronic conductivity (and thus power performance) and (ii) increased electrochemical stability window (longer cycle life) due to a decrease in catalytic activity with respect to electrolyte decomposition when carbon is directly in contact with the electrolyte instead of the electrode material (often containing transition metal).42
 High temperature electrochemical performance of hydrothermally prepared LiMn1 − xMxPO4 (M = Fe, Mg). J. Nava-Avendaño, M.R. Palacín, J. Oró-Solé, A. Ponrouch, J.-M. Tarascon, N. Recham. Solid State Ionics 2014, 263, 157. https://doi.org/10.1016/j.ssi.2014.06.007.  Electroanalytical study of the viability of conversion reactions as energy storage mechanisms. A. Ponrouch, J. Cabana, R. Dugas, J. L. Slack, M. R. Palacín. RSC Adv., 2014, 4, 35988. https://doi.org/10.1039/C4RA05189K.  Ionic liquid based electrolytes for sodium-ion batteries: Na+ solvation and ionic conductivity. D. Monti, E.Jónsson, M. R. Palacín, P. Johansson. J. Power Sources, 2014, 245, 630. https://doi.org/10.1016/j.jpowsour.2013.06.153.  The Li–Si–(O)–N system revisited: Structural characterization of Li21Si3N11 and Li7SiN3O. M. Casas-Cabanas, H. Santner, M. R. Palacín. J. Solid State Chem., 2014, 213, 152. https://doi.org/10.1016/j.jssc.2014.02.022.  Towards high energy density sodium ion batteries through electrolyte optimization. A.Ponrouch, R. Dedryvere, D. Monti, A.E. Demet, J.M. Ateba Mba, L. Croguennec, C. Masquelier, P. Johansson, M R. Palacín. Energy Environ. Sci. 2013, 6, 2361. https://doi.org/10.1039/C3EE41379A
 Low potential sodium insertion in a NASICON-type structure through the Ti(III)/Ti(II) redox couple. P. Senguttuvan, G. Rousse, M.E. Arroyo y de Dompablo, H. Vezin, J.M. Tarascon, M. R. Palacín. J. Am. Chem. Soc. 2013, 135, 3897. https://doi.org/10.1021/ja311044t.
 High capacity hard carbon anodes for sodium ion batteries in additive free electrolyte. A. Ponrouch, A.R. Goñi, M. R. Palacín. Electrochem. Comm. 2013, 27, 85-8. https://doi.org/10.1016/j.elecom.2012.10.038.
 A new room temperature and solvent free carbon coating procedure for battery electrode materials. A. Ponrouch, A. R. Goñi, M. T. Sougrati, M. Ati, J.-M. Tarascon, J. Nava-Avendaño, M. R. Palacín. Energy Environ. Sci. 2013, 6, 3363. https://doi.org/10.1039/C3EE41243A. Rationalization of Intercalation Potential and Redox Mechanism for A2Ti3O7 (A = Li, Na). G. Rousse, M. E. Arroyo-de Dompablo, P. Senguttuvan, A. Ponrouch, J.-M. Tarascon, M. R. Palacín. Chem. Mater. 2013, 25, 4946. https://doi.org/10.1021/cm4032336.  A low temperature TiP2O7 polymorph exhibiting reversible insertion of lithium and sodium ions. P. Senguttuvan, G. Rousse, J. Oró-Solé, J. M. Tarascon, M. R. Palacín. J. Mater. Chem. A 2013, 1, 15284. https://doi.org/10.1039/C3TA13756B.  Synthesis and Characterization of a Novel Sodium Transition Metal Oxyfluoride: NaMnMoO3F3·H2O. J. Nava-Avendaño, C. Frontera, J. A. Ayllón, J. Oró-Solé, P. Senguttuvan, M. R. Palacín. Inorg. Chem. 2013, 17, 9791. https://doi.org/10.1021/ic401447p.  Titanium(III) Sulfate as New Negative Electrode for Sodium-Ion Batteries. P. Senguttuvan, G. Rousse, H. Vezin, J.-M. Tarascon, M. R. Palacín. Chem. Mater. 2013, 25, 2391. https://doi.org/10.1021/cm401181b.  In search of an optimized electrolyte for Na-ion batteries. A. Ponrouch, E. Marchante, M. Courty, J.M. Tarascon, M. R. Palacín Energy Environ. Sci. 2012, 5, 8572. https://doi.org/10.1039/C2EE22258B.  Optimisation of performance through electrode formulation in conversion materials for lithium ion batteries: Co3O4 as a case example. A. Ponrouch, M. R. Palacín. J. Power Sources 2012, 212, 233. https://doi.org/10.1016/j.jpowsour.2012.04.005.  On the origin of the extra capacity at low potential in materials for Li batteries reacting through conversion reaction. A. Ponrouch, P.-L. Taberna, P. Simon, M. R. Palacín. Electrochim. Acta 2012, 61, 13. https://doi.org/10.1016/j.electacta.2011.11.029.  Na2Ti3O7: Lowest voltage ever reported oxide insertion electrode for sodium ion batteries. P. Senguttuvan, G. Rousse, V. Seznec, J.M. Tarascon, M.R. Palacín. Chem. Mater. 2011, 23, 4109. https://doi.org/10.1021/cm202076g  Review-Beyond intercalation-based Li-ion batteries: State of the art and challenges of electrode materials reacting through conversion reactions J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín. Adv. Mater. 2010, 22, E170-92. https://doi.org/10.1002/adma.201000717.  Review-Recent advances in rechargeable battery materials: a chemist’s perspective Chem. Soc. Rev. 2009, 38 2565-2575. https://doi.org/10.1039/B820555H