Selective sodium intercalation into sodium nickel–manganese sulfate for dual Na–Li-ion batteries

Abstract

Double sodium transition metal sulfates combine in themselves unique intercalation properties with eco-compatible compositions – a specific feature that makes them attractive electrode materials for lithium and sodium ion batteries. Herein, we examine the intercalation properties of novel double sodium nickel–manganese sulfate, Na₂Ni_1/2Mn_1/2(SO₄)₂, having a large monoclinic unit cell, through electrochemical and ex situ diffraction and spectroscopic methods. The sulfate salt Na₂Ni_1/2Mn_1/2(SO₄)₂ is prepared by thermal dehydration of the corresponding hydrate salt Na₂Ni_1/2Mn_1/2(SO₄)₂·4H₂O having a blödite structure. The intercalation reactions on Na₂Ni_1−xMn_x(SO₄)₂ are studied in two model cells: half-ion cell versus Li metal anode and full-ion cell versus Li₄Ti₅O₁₂ anode by using lithium (LiPF₆ dissolved in EC/DMC) and sodium electrolytes (NaPF₆ dissolved in EC:DEC). Based on ex situ XRD and TEM analysis, it is found that sodium intercalation into Na₂Ni_1/2Mn_1/2(SO₄)₂ takes place via phase separation into the Ni-rich monoclinic phase and Mn-rich alluaudite phase. The redox reactions involving participation of manganese and titanium ions are monitored by ex situ EPR spectroscopy. It has been demonstrated that manganese ions from the sulfate salt are participating in the electrochemical reaction, while the nickel ions remain intact. As a result, a reversible capacity of about 65 mA h g⁻¹ is reached. The selective intercalation properties determine sodium nickel–manganese sulfate as a new electrode material for hybrid lithium–sodium ion batteries that is thought to combine the advantages of individual lithium and sodium batteries.