Vanadium oxides have attracted great interest for the positive electrode in lithium ion batteries over the past few decades. Among them, V2O5 was initially pointed as a very promising cathodic electro active material due to its low atomic mass, high oxidation number of vanadium (V) and easiness of synthesis in an industrial scale [1]. While close to three lithium equivalents can be inserted into the V2O5 crystal lattice when the battery is first discharged down to 2 V, less than two lithium equivalents can actually be quasi-reversibly intercalated into the structure [2]. This fact hindered the dissemination of this compound as a standard for mass production cathodes so far. However, research continued on other vanadium oxide structures like LixV3O8 and H2V3O8. Both of these cathodic materials are promising candidates for commercial lithium ion batteries but stability issues on long term cycling are still to be solved [3, 4].
It has already been shown that up to four lithium equivalents (ca. 400 Ah/kg) can be intercalated in H2V3O8 between 4.2 V and 1.5 V vs. Li/Li+ [3, 5] with a mean potential close to 2.7 V. This leads to a specific energy density higher than 1 kWh/kg which shows by its own the great potential associated to this compound for lithium ion batteries. For instance, the theoretical energy density associated to commercially available lithium ion cathodes like iron, cobalt and/or manganese oxides or phosphates is in the range of 0.5 kWh/kg. The practical capacity measured in such systems, however, is lower than the reported theoretical value.
In a previous co-owned published patent application [6], the synthesis of H2V3O8 and electrode preparation with graphene oxide was explored. The present invention represents a new approach to increase the cathodic charge retention during cycling by surface decoration with aluminum hydroxide.
Conversely to the majority of the current generation cathodes for lithium ion batteries, H2V3O8 shows low thermal stability. It is known that structural water present in this compound (also written as V3O7.H2O) is released when heated above 200° C. [2, 7]. It is generally accepted that the anhydrous material is obtained at a temperature close to 350° C. This property limits the choice of possible compounds to coat the surface of H2V3O8. Several metal oxides like MgO, Al2O3, SiO2, TiO2, ZnO, SnO2, ZrO2, glasses and phosphates, for example, have been extensively studied as surface coatings for lithium ion cathodes. It has been reported that these coatings prevent the direct contact from the intercalation compound with the electrolytic solution, suppress undesirable phase transitions, improve the structural stability, and decrease the disorder of cations in crystal sites. As a result, side reactions and heat generation during cycling can be decreased [8]. However, either due to processing limitations related to solvent and pH compatibility or thermal stability of H2V3O8, the common wet and solid state chemistry methods used to coat or decorate the surface of cathodic materials with the referred metal oxides are not suited for H2V3O8.
Thus, the object of the invention was to increase the capacity retention of the lithium intercalation under charge and discharge cycling of the H2V3O8 compound as lithium ion battery cathode.