Abstract
Although nickel-based polyanionic compounds are expected to exhibit a high operating voltage for batteries based on the Ni2+/3+ redox couple activity, some rare experimental studies on the electrochemical performance of these materials are reported, resulting from the poor kinetics of the bulk materials in both Li and Na nonaqueous systems. Herein, the electrochemical activity of the Ni2+/3+ redox couple in the mixed-polyanionic framework Na4Ni3(PO4)2(P2O7) is reported for the first time. This novel material exhibits a remarkably high operating voltage when cycled in sodium cells in both carbonate- and ionic liquid-based electrolytes. The application of a carbon coating and the use of an ionic liquid-based electrolyte enable the reversible sodium ion (de-)insertion in the host structure accompanied by the redox activity of Ni2+/3+ at operating voltages as high as 4.8 V vs Na/Na+. These results present the realization of Ni-based mixed polyanionic compounds with improved electrochemical activity and pave the way for the discovery of new Na-based high potential cathode materials.
Highlights
Lithium-ion batteries have been largely recognized as the most efficient electrochemical energy storage devices for both portable electronics and electric vehicle applications.[1,2] the growth and diversification of the energy storage market trigger interest in low-cost and environmentally friendly alternative systems
Rietveld refinement of the powder X-ray diffraction (XRD) pattern confirms that the Na4Ni3(PO4)2(P2O7) is isostructural to Na4Fe3(PO4)2(P2O7) and crystallizes in the orthorhombic Pn21a space group with a = 18.0006(3) Å, b = 6.4933(1) Å and c = 10.4115(2) Å
Rietveld refinement of the XRD pattern of the carbon-coated analog (Na4Ni3(PO4)2(P2O7)/C) (Supplementary Figure S2) shows that the carbon coating process has no effect on the structural characteristics of the material that crystallizes in the same space group with comparable cell parameters
Summary
Lithium-ion batteries have been largely recognized as the most efficient electrochemical energy storage devices for both portable electronics and electric vehicle applications.[1,2] the growth and diversification of the energy storage market trigger interest in low-cost and environmentally friendly alternative systems. Recent concerns over the cost and future availability of lithium highlight the urgent need to exploit alternative energy storage systems.[3] In these terms, sodium (Na)-ion batteries are attractive candidates because of the electrochemistry that is similar to the well-established lithium-ion technology.[4] To date, the LiFePO4 olivine with (PO4)3 − polyanionic framework, owing to its superior thermal stability and low cost, is considered to be one of the best electrode materials for lithium-ion batteries, mostly in view of its stable operating voltage and satisfactory specific capacity. Several studies have demonstrated that the implementation of this material is restricted by several drawbacks such as the intrinsic sluggish kinetics attributable to the low electronic conductivity, the poor lithium transport in commonly used electrolyte systems and the structural instability of the delithiated phases upon cycling that require further studies to obtain high-performance Ni-based phosphates as high voltage cathodes for battery application.[8,9,10,11,12]
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