New Form of Li2FePO4F as Cathode Material for Li-Ion Batteries

Abstract

L batteries originally developed for portable devices can now be found in applications as diverse as power tools, electric vehicles, and stationary energy storage. To satisfy the need of current and new applications, Li-ion batteries require further improvement in terms of performance properties (energy and power density, safety, and cost). Transition metal compounds containing different polyanion units (XO4) m− (X = P, S, Si) are considered as the most promising cathode materials for the next generation of Li-ion batteries due to increased redox potential caused by an inductive effect and remarkable electrochemical and thermal stability ascribed to a three-dimensional structure. Further advances in the polyanion cathodes are related to combining (XO4) m− and F− in the anion sublattice, which is expected to enhance the operating voltage due to the higher ionicity of the M−F bond. Indeed, various fluorophosphates (LiVPO4F, Na2FePO4F) and fluorosulphates (LiMSO4F) have been reported to exhibit attractive electrochemical performance. Among fluorophosphates, compounds of the general formula A2MPO4F (A = Na, Li; M = Fe, Mn, Co, Ni) have received particular interest due to their potential to operate on more than one alkali atom per formula unit (f.u.), which would result in higher specific capacity and energy density. The A2MPO4F fluorophosphates crystallize in three different structure types. While the coordination polyhedra of transition metal, MO4F2 octahedra, is the same for all three, the means of their conjugation varies from edge-shared in Li2MPO4F (M = Ni, Co) and corner-shared in Na2MnPO4F to mixed face-shared and corner-shared in Na2FePO4F, resulting in different architectures. The three-dimensional (3D) Na2MnPO4F and the layered Na2FePO4F were shown to operate as positive electrode material in Naand Li-ion cells with reversible capacity of about 120 mAh g−1 (∼0.8 electron per f.u.). Moreover, the capacity improvement up to ∼180 mAhg−1 (∼1.46 electrons per f.u.) was observed for nanostructured Na2FePO4F cycled at elevated temperature. 12 Several groups reported on high voltage electrochemical performance of Li2CoPO4F and Li2NiPO4F. 8,14−16 These fluorophosphates possess a three-dimensional structure and, by analogy with the olivine phase, are expected to demonstrate good stability and reversibility upon cycling. The complete evaluation of these materials is restrained by the absence of commercial high-voltage electrolytes stable above 4.8 V. The exploration of corresponding Feor Mn-based fluorophosphates with lower redox potentials might provide an advantage in the energy and power density over the LiFePO4 cathode materials. So far, these fluorophosphates have not been identified: apparently, the structure framework could not adopt the Fe and Mn ions, which are larger compared to Co and Ni. In our previous paper, we showed that upon the first charge Li2CoPO4F undergoes structural transformation with the large volume expansion (∼5%) demonstrating the framework flexibility. Considering this “structure pliability” we attempted to stabilize Fe-based fluorophosphate by “expanding” the framework through the substitution of Li ions located in the structure voids by larger Na ions; as a result, we succeeded in synthesis of a new NaLiFePO4F phase isostructural to Li2NiPO4F (Figure 1). Upon preparing this communication, we have found a short report by Mizuta et al., where electrochemical properties measured on the multiphase NaLiFePO4F sample were described. 17 Here, we report on the synthesis employed for obtaining the pure NaLiFePO4F phase, its structure determination, and electrochemical activity. We demonstrate that electrochemical replacement of Na by Li in