Tailoring fluorophosphate cathode materials for high-performance sodium and lithium ion battery

Abstract

Lithium-ion battery (LIB), which has been widely used to power portable electronic devices, is on the verge of being applied to new automobile applications. To expand this emerging market, however, an electrode that combines fast charging capability, long-term cycle stability, and high energy density is needed. In Chap. 2, a novel layered lithium vanadium fluorophosphate, Li1.1Na0.4VPO4.8F0.7, is introduced as a promising positive electrode contender. This new material has two-dimensional lithium pathways and is capable of reversibly releasing and reinserting ~1.1 Li+ ions at 4 V (vs. Li+/Li) to give a capacity of ~156 mAh g–1 (energy density of 624 Wh kg–1). Moreover, outstanding capacity retentions of 98% and 96% after 100 cycles were achieved at 60°C and room temperature, respectively. Unexpectedly high rate capability was delivered for both charge and discharge despite the large particle size (a few microns), which promises further enhancement of power density with proper nano-engineering.

Large-scale electric energy storage is a key enabler for the use of renewable energy. Low-cost and highly durable battery chemistry is required in affordable large-scale storage applications. In this respect, the room-temperature Na-ion battery (NIB) has been re-highlighted recently as a low-cost alternative technology to LIB. A cheap and earth-abundant element, Na, will be advantageous if a large amount of material is demanded for renewable energy solutions. However, significant challenges such as energy density and long term stability must be addressed. In Chap. 3, a novel cathode material for Na-ion battery, Na1.5VPO4.8F0.7, is introduced. This new material provides a high energy density of ~600 Wh kg–1, originating from both the multi-electron redox reaction (1.2 e– per formula unit) and high potential (~3.8 V vs. Na+/Na) of the tailored vanadium redox couple (V3.8+/V5+). Furthermore, an outstanding cycle life (~95% capacity retention for 100 cycles) could be achieved, which is attributed to small volume change (2.9%) upon cycling. The open crystal framework with two-dimensional Na diffusion paths leads to low activation barriers for Na diffusion, enabling excellent rate capability.

In Chap. 4, the first successful synthesis of a series of Na3(VO1−xPO4)2F1+2x (0 ≤ x ≤ 1) compounds is introduced, which is a new family of high-performance cathode materials for NIB. The Na3(VO1−xPO4)2F1+2x series can function as high-performance cathodes for NIB with high energy density and good cycle life, although the redox mechanism varies depending on the composition. The combined first-principles calculations and experimental analysis revealed the detailed structural and electrochemical mechanisms of the various compositions in solid solutions of Na3(VOPO4)2F and Na3V2(PO4)2F3. The comparative data for the Nay(VO1−xPO4)2F1+2x electrodes showed a clear relationship among V3+/V4+/V5+ redox reactions, Na+−Na+ interactions, and Na+ intercalation mechanisms in NIB.