Numerous research has focused on the search for new, electrochemically stable, ionic conductors that enable better performance of rechargeable batteries. Batteries containing organic electrolytes are cheaper and easier to manufacture; However, other applications requiring extreme operating temperature may benefit from the introduction of materials that exhibit rapid conduction.
Rechargeable Li-ion Batteries
Inorganic solid electrolytes offer important advantages and disadvantages with respect to liquid and polymer electrolytes. Because they can withstand high temperatures, they can be used in solid-state batteries, and they are individual ion conducting materials, meaning that only Li + ions have appreciable mobility, whereas anions and other cations form a rigid skeleton.
Eliminating the anionic concentration gradient through the electrolyte may help suppress the adverse reactions, or decomposition reactions, that the electrolyte may suffer. However, it is necessary to investigate further, in order to minimize, for example, the product of the resistivity of the electrolyte and its thickness, so that a rapid transport of Li + ions occurs through the solid electrolyte.
Some of the rechargeable lithium-ion batteries that have already begun to appear on the market are composed of LiCoO2 cathodes, polymer electrolytes and highly densified graphite anodes. They also have a small surface to minimize passivation phenomena that also affect them. They can be recharged up to 2500 times, and thanks to their low price, they are the best alternative in the consumer electronics market.
However, lithium-ion batteries still show significant defects due to their fragile structure, among which we can mention: that they require a safety circuit to maintain maximum and minimum voltage limits, they degrade over time, having to be stored in cold places to 40% of their load, show moderate discharge capacity, are more expensive than other types of batteries. Finally, it should be noted that they are part of a technology that is currently being studied progressively.
Some of the researches carried out today are focused on the preparation and study of nanostructured orthophosphates and related compounds, which are capable of presenting rapid ionic conduction, which allows to study their potential use as solid electrolytes.
Research on non-nanostructured orthophosphates finds that the conductivity values measured inside the constituent grains of these materials are much higher than those measured at the boundary between the grains. The research developed, is driven by the desire to find materials with improved properties and characteristics, based on their subsequent application in new solid state batteries.
Nowadays, numerous scientific articles exist in which different structural types or new compositions are sought and studied. In order to achieve improved performance objectives, advanced synthesis routes are tested to influence the microstructure of the prepared compounds and to achieve a favorable orientation of the grain boundaries so that the presence of these boundaries affects as little as possible their conductivity.
Recent research addressing such topics focuses on the preparation of nanoparticulate and / or nanostructured electrode materials, essentially pursuing two objectives, first, to try to improve intra- and inter-granular conductivity, and the second to be able to manufacture useful composite electrolytes In solid state batteries. In this way, the pyrolysis spray method allows, for example, to obtain nanostructured materials with spherical morphology, narrow particle size distribution and compositional homogeneity.
The current focus of solid electrolyte research and numerous publications on the research and technological development of lithium-ion batteries continue to focus on studies to improve the use of polymer electrolytes, gels and electrolytes composites. Liquid electrolytes are still used in most routine electrochemical studies. Only a minimal percentage of contributions address the potential use of ceramic electrolytes.
Batteries containing organic electrolytes are cheaper, easier to manufacture and, if the electrolyte-solid interface layer formed and the electrolyte-electrode interfaces are controlled, good cyclability is achieved.
However, other applications that require an extreme operating temperature – high or low -, maximum service life, negligible discharge and extremely thin contacts can benefit from the introduction of ceramic electrolytes. Some research groups address the synthesis of laminar electrodes using ceramic electrolytes and glasses in conjunction with liquid or polymer electrolytes. The embedded Li 3 PO 4 compound has been used as a membrane separator or as a cathode in fibers, and also glass lithium embedded in glass has been used as an anode material.
On the other hand, the majority of the investigations currently carried out on ceramic electrolytes are far from an immediate practical application of these materials. Studies that evaluate the electrochemical stability of many of the synthesized electrolytes and reports of different manufacturing techniques are lacking, emphasizing the optimum thickness of the electrolyte layer.
It may also be interesting in the future to study whether the introduction of nanostructured materials into polymer electrolytes, to give rise to a composite electrolyte, may prove useful in solid state batteries.
Improving the performance of solid state batteries, the preparation and study of polymer electrolytes, to which nanostructured materials have been added, and the introduction of ceramic electrolytes and glasses into the layers of a polymer electrolyte are some of the challenges which are tackled today by researchers.
