This research line is led by Dr. M. Rosa Palacín and focuses on the crystal chemistry and electrochemistry of inorganic materials for energy-storage applications. It involves both exploring new compounds and optimising already known phases. Special emphasis is paid to synthesis-structure-performance relationships with the aim of tailoring structure and microstructure to maximise electrochemical performance. Current research involves electrode materials for nickel, lithium or sodium batteries:
Nickel battery materials
Despite the advent of Li-ion technology, nickel based batteries are in use in HEVs or stationary applications due to its flat discharge, excellent high rate performance, long cycle life, abuse tolerance and competitive price. Research within this topic mainly involves the positive electrode, which is a clear example of complex crystal chemistry underlying behind battery operation.
Crystal chemistry and performance in the nickel oxyhydroxide electrode
The redox mechanism in the positive nickel oxyhydroxide electrode is based subsequent oxidation of β-Ni(OH)2 to yield β-NiOOH and further reduction to β-Ni(OH)2, with a strong influence of microstructure (particle size, stacking faults, strains,...) in the electrochemical performance. Nowithstanding, the difficulty in independent reliable estimation of particle size, strain and defects led to conflicting conclusions on the relative importance of each factor. By means of Rietveld refinement coupled to TEM, we have been able to independently determine crystallite size and relative amount of stacking faults and assess their influence in electrochemical performance.
The low crystalinity of β-NiOOH had for long hindered its structural and led to a common belief that it was isostructural to β-Ni(OH)2 (ABAB oxygen layer stacking sequence). Rietveld refinement in which structural and microstructural models were simultaneously taken into account has allowed us to dismiss this fact and determine an alternative ABCA stacking for β-NiOOH. This demonstrates that reversible structure changes take place in nickel battery positive electrodes under cycling and contributes to the understanding of the operation mechanism at atomic level.
See for instance: “New insights on the microstructural characterisation of nickel hydroxides and correlation with electrochemical properties” J. Mater. Chem. 16, 2925 (2006) and “Deciphering the structural transformations during nickel oxyhydroxide electrode operation” J. Am. Chem. Soc. 129, 5840 (2007).
Lithium battery materials
Lithium batteries have been key in the development of portable electronics and are now considered the most promising technology to power electric vehicles. Our research activity within this field is mostly focused in improving the performance of electrodes by both exploring new compounds with interesting properties and also studying the mechanisms that govern the behaviour of electrode materials in search of optimal power and energy densities. We are founding members of the ALISTORE-ERI, (www.alistore.eu) an European virtual research institute devoted to battery research that is currently headed by Dr. M. Rosa Palacín and Prof. Patrice Simon. ALISTORE-ERI provides direct access to characterization platforms and collaboration with European leading academic institutions as well as close contact with companies interested in energy storage which are members of the ALISTORE Industrial Club.
Studies on commercial phases
These studies are mainly carried out within the framework of industrial contracts. They involve compatibility studies, electrode optimisation, etc. Alternatively, the optimisation of high temperature performance is also one of our targets.
See for instance: “Development and implementation of a high temperature electrochemical cell for lithium batteries” Electrochem. Comm. 9, 708 (2007), “High temperature electrochemical performance of nanosized LiFePO4” J. Power Sources 195, 6897 (2010), “Polyfluorinated boron cluster-based salts: A new electrolyte for application in Li4Ti5O12/LiMn2O4 rechargeable lithium ion batteries” J. Power Sources 195, 1479 (2010).
Electrode formulation: alternative carbon coating procedure
Battery electrodes must exhibit high intrinsic electronic and ionic conductivities in order to have acceptable reaction kinetics. The higher the charge/discharge rates at which the battery is expected to operate the larger electronic and ionic conductivities the electrodes must display. A common practice, especially for low intrinsic conductivity electrode materials, is to coat the particles surface with either a metal, a conducting polymer or, most generally, carbon (usually less than 2% in weight).
Traditionally such carbon coatings are achieved through diverse chemical procedures typically involving a high temperature pyrolysis step with difficult control of the deposit thickness and uniformity and are not applicable to electrode active materials which may degrade under such conditions. We have developed an alternative carbon coating procedure based on physical deposition of carbon through evaporation under vacuum that can be carried out at room temperature under dry conditions, hence being generally applicable to any electrode active material. The “quality” of the coating in terms of conductivity and graphitization degree is similar to traditional methods involving the use of liquids and high temperature treatments under reducing conditions. Studies on selected compounds prove the conformal nature of the coating and the easy control of its thickness through deposition time together with the related effects in changing the nature of the electrode/electrolyte interface (e.g. enhancing conductivity while suppressing side reactions).
See for instance: "Optimisation of performance through electrode formulation in conversion materials for lithium ion batteries:Co3O4 as a case example", J. Power Sources 212, 233 (2012), “A new room temperature and solvent free carbon coating procedure for battery electrode materials” Energy Environ. Sci. 6, 3363 (2013).
Recent review papers:
“Recent advances in rechargeable battery materials: a chemist’s perspective” Chem. Soc. Rev. 38, 2565 (2009), “Beyond intercalation-based Li-ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions” Adv. Mater. 22, E170 (2010), "Recent achievements on inorganic electrode materials for Lithium-ion batteries" JACS,137, 3140 (2015).
Sodium battery materials
The implementation of a lithium based technology on a large scale faces an important challenge, since we cannot ignore controversial debates on lithium availability and cost. New sustainable chemistries must be developed, and the most appealing alternative is to use sodium instead of lithium. There are several reasons: similar intercalation chemistry, sodium resources are in principle unlimited and it is unexpensive when compared to lithium. Sodium technology has already been successfully implemented in high temperature beta-alumina cells (either Na/S or Na/NiCl2 ZEBRA-type). Mindful of these considerations, within the current knowledge gained in Li-ion technology, a room temperature Na-ion cell should be feasible. Diverse phases are being currently studied that are able to reversibly insert and extract sodium ions with acceptable capacity values. Efforts are pursued to understand redox mechanisms, ascertain the influence of microstructure and optimize electrolyte formulations for maximising the electrochemical yield. Laboratory full Na-ion cells have been assembled using hard carbon negative electrodes and EC:PC:DMC based electrolytes that display an operation voltage of 3.65 V, very low polarisation and excellent capacity retention upon cycling with ca. 97 mAh/g of Na3V2(PO4)2F3 after more than 120 cycles together with satisfactory coulombic efficiency (> 98.5%), very good power performance and energy densities comparable to those of current state-of-the art lithium-ion technology.
See for instance: "Na2Ti3O7 : Lowest Voltage ever reported oxide insertion electrode for Sodium ion batteries", Chem. Mat. 23, 4109 (2011), “In search of an optimized electrolyte for Na-ion batteries" Energy Environ. Sci. 5, 8572 (2012), "High capacity hard carbon anodes for sodium ion batteries in additive free electrolyte" Electrochem. Comm. 27, 85 (2013), “Towards high energy density sodium ion batteries through electrolyte optimization" Energy Environ. Sci. 6, 2361 (2013)."On the high and low temperature performances of Na-ion battery materials: Hard carbon as a case study" Electrochem. Comm. 54, 51 (2015).
Recent review papers:
“Non-aqueous electrolytes for sodium-ion batteries” J. Mater. Chem. A 3, 22 (2015).
The publications detailed above are just few selected examples. Full list of publications can be found under the “Publications” section.