Exploring Li Substituted O3-Structured Layered Oxides As Promising Cathode Materials for Rechargeable Na Batteries

Na-ion batteries have recently gained increasing recognition as intriguing candidates for next-generation large scale energy storage systems, owing to significant cost advantages stemming from the high natural abundance and broad distribution of Na resources. Although in terms of absolute energy density, Na-ion battery materials are not comparable with their Li-ion counterparts which are one of the dominating energy technologies in this decade. In particular, Na-ion batteries operating at room temperature could be suitable for applications where specific volumetric and gravimetric energy density requirements are not as stringent as in EVs, namely in electrical grid storage of intermittent energy produced via renewable sources.¹ This would also contribute to a significant reduction of the costs connected to the use of renewable sources, which could then penetrate the energy market more easily and make Na-ion technology complementary to Li-ion batteries for stationary storage.^2-4

A new O3 - Na_0.78Li_0.18Ni_0.25Mn_0.583O_w is obtained by the electrochemical Na-Li ion exchange process of Li_1.167Ni_0.25Mn_0.583O₂. The new material shows exceptionally high discharge capacity of 240 mAh g^-1 in the voltage range of 1.5-4.5 V, (Fig. 1) thus the total energy density at the materials level reaches 675 Wh kg^-1. When cycled between 1.5-4.2 V, the discharge capacity is well maintained around 190 mAh g^-1 after 30 cycles. The O3 phase is kept through ion-exchange and cycling process, as confirmed by SXRD (Fig. 2). The stabilized O3 phase could be related to the tetrahedral Li formed upon initial lithiation, and breaks through the critical limitation for most of the O3 compounds. XAS results show that Ni²⁺/Ni⁴⁺ is the main active redox couple during cycling while Mn ions basically stay at tetravalent state. The Na full cell utilizing Na_0.78Li_0.18Ni_0.25Mn_0.583O_w as cathode delivers 430 Wh kg^-1energy density.

Later, the new Li substituted Na layered oxide with pure O3 phase is successfully synthesized by hydroxide co-precipitation method. It demonstrates a considerably high reversible capacity, and excellent rate performance. The phase is stabilized throughout the cycling, which could be the main reason for its good electrochemical properties. Ni²⁺/Ni⁴⁺ is the main active redox couple during cycling, and Co³⁺/Co⁴⁺partially contributes to the charge compensation while Mn ions basically stay at tetravalent state. The findings would contribute to the future improvement and design of practical cathode materials for Na-ion batteries. 

Fig. 1: (a) Electrochemical profiles of initial delithiation and initial sodiation for Na_0.78Li_0.18Ni_0.25Mn_0.583O_w in half cell. (b) Cycling performance for Na_0.78Li_0.18Ni_0.25Mn_0.583O_w  with 125 mA g^-1 current density. Inset is the corresponding electrochemical profile of Na_0.78Li_0.18Ni_0.25Mn_0.583O_w  during the 1^st, 2^nd, 10^th, 20^th, 30^th cycles in Na-ion batteries.⁵

Fig. 2: (a) Ex situ SXRD for Li_1.167Ni_0.25Mn_0.583O₂ and Na_0.78Li_0.18Ni_0.25Mn_0.583O_w at different states. (b) Schematic of the typical O3 structure. (c) Schematic of the proposed mechanism for sodiation. (In the schematic, transition metal ions are blue, Li⁺ is red, and Na⁺ is green.)⁵

Acknowledgements

J. Xu and Y.S.Meng are grateful for the seed funding support from the Northeastern Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, with Award Number DE-SC0012583. The follow up work have been carried out by H. Liu and C. M with other private funding source. H. Liu acknowledges the financial support from China Scholarship Council under Award Number 2011631005. The authors appreciate the kind assistance from Dr Baihua Qu for anode SnS₂/rGO synthesis at National University of Singapore (NUS). The XAS and SXRD were collected on 20-BM-B and 11-BM respectively at Advanced Photon Source in Argonne National Laboratory.

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