Role of Lattice Water in High-Capacity Layered NaMn3O5×nH2o Materials as Cathode of Sodium-Ion Batteries

Lithium-ion batteries (LIBs) have been applied in many fields due to the long service life and high energy density for portable electronic devices.^{1, 2}  However, the worldwide demand might grow very rapidly when LIBs are used for large-scale application such as electric vehicles (EVs), hybrid electric vehicles (HEVs) and smart grid, so the limited availability and high cost cannot satisfy the market demand.  Sodium has similar physical and chemical properties as lithium. Recently, sodium-ion batteries (SIBs) are receiving much attention as an attractive alternative to replace lithium-ion batteries for the application in smart grid because of the low cost and ubiquitous distribution over earth crust of sodium sources.

Indeed, there are increasing volume of reports on materials for sodium-ion batteries,^3-5 and among them, a particular kind of intercalated compounds including A_xMnO₂×nH₂O, A_xTiO₂×nH₂O and A_xV₂O₅×nH₂O (A=H, Li, Na and K) are promising as electrode materials. The crystal structure of these layered oxides generally consist of MnO₂, TiO₂ and V₂O₅ layers, composed by MnO₆ octahedra, TiO₆ octahedra and VO₅ pyramidal, with alkali ions and lattice water located in the interlayer positions. During charge-discharge process, these compounds show smooth voltage profiles with large delivered capacity (approaching 350 mAh/g for A_xV₂O₅×nH₂O). However, some fundamental issues still need to be clarified. For instance, can lattice H₂O stably exist in the crystal structure during charge/discharge process? If not, does lattice water take off from the interlayers? Is lattice water always deteriorating the electrochemical performance? In what condition does lattice water come off?

Recently, we have researched the sodium extraction/insertion behavior of the layered NaMn₃O₅ after eliminating lattice water via heat treatment, and it showed large capacity and high energy density. In order to understand the role of lattice water in this kind of materials in depth, the layered NaMn₃O₅×nH₂O materials is chosen as the inserted/extracted subject, and the influence of role lattice water on the electrochemical performance in different voltage ranges is investigated for the first time. The initial discharge capacity can reach 234 mAh/g, shown in Fig. 1. Our method affords control of the existed situation of lattice water, that is, when charged to 3.4 V, lattice water can stably stay and make no negative impact on the cycle performance; Meanwhile, lattice water would run off from the interlayer, provide a certain capacity and greatly deteriorate the cycling when charged to 4.7 V. These data highlights the significant impact of lattice water on the delivered capacity, cycling performance and crystal structure. In addition, the possible mechanism of extracting out lattice water is proposed through combination of ex-situ XRD and EIS characterization in different charge/discharge status.

Figure 1. The initial charge/discharge profiles for NaMn₃O₅×nH₂O/Na cells at a rate of 0.1C.

Reference

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