The lithium-ion battery is a wonder of the modern world, powering everything from lightweight mobile devices to electric vehicles. Lithium-ion batteries have a high energy density (compared with other battery chemistries), but lithium is a limited, and expensive, resource.

Hence researchers and battery companies are looking into making batteries from a much more abundant, and cheaper element: sodium. Sodium-ion batteries are bulkier than lithium-ion, but they offer the promise of a cheaper alternative where size and weight aren't as important, such as battery banks for storing electricity from renewable sources.

Sodium-ion batteries based on layered oxide compounds with the general formula Na_xTMO₂ (TM = transition metal), adopting P2-type and O3-type structures, are promising candidates. Of the two types, layered P2-type compounds have shown better electrochemical performance.

A team of researchers from the University of Oxford, the University of Bath and UNSW Australia
 came to ISIS Neutron and Muon Source to investigate P2-Na_2/3Ni_1/3Mn_2/3O₂, a material that has a high theoretical capacity (ca. 170 mAh g^-1) and high average voltage (ca. 3.5 V), but is hampered by a poor cycle life. Previous research has shown that strategic doping of P2-Na_2/3Ni_1/3Mn_2/3O₂ with selected cations (e.g. Li⁺, Cu²⁺, Al³⁺, Zn²⁺ and Tⁱ⁴⁺) can improve its electrochemical performance. One of the most studied approaches, Mg²⁺ substitution for Ni²⁺, has shown great promise, but developing efficient sodium-ion batteries will require a greater fundamental understanding of their chemistry.

Lead author Dr Nuria Tapia Ruiz explains: “By developing a profound understanding of the physical and chemical nature of these materials, specific material tailoring can be done to enhance their electrochemical performance. Our long-term mission is to fabricate a new generation of batteries which will deliver higher energy density, and be cheaper, safer and longer-lasting."

The team used conventional solid-state methods to prepare a series of P2- type Na_2/3Ni_1/3-xMg_xMn_2/3O₂ materials, which they then structurally characterised using time-of-flight powder neutron diffraction data collected on the GEM high-intensity, medium-resolution instrument, and X-ray diffraction (XRD) techniques. Combined XRD and neutron analysis showed that magnesium substitutes nickel in the Ni-Mn honeycomb layers, forming the same type of structure as in the parent material.

Their results allowed them to demonstrate the effects of Mg substitution on the structure of these compounds. They showed that Mg substitution effectively disrupts the Na⁺- vacancy ordering leading to more gradual structural changes. High levels of Mg in the structure lead to a more disordered distribution of sodium in the pristine material, which is important because sodium ordered compounds are thermodynamically more stable than their disordered counterparts, impeding Na+ mobility and resulting in poorer performance at high current rates. Their work demonstrates the highest level of Mg substitution in the P2- Na_2/3Ni_1/3-xMg_xMn_2/3O₂ compound reported to date.

This work sheds new light on the role of magnesium in the high-voltage P2- Na2/3Ni1/3Mn2/3O2 cathode, improving our understanding of the structural and electrochemical changes occurring in doped cathode materials, and informing the development of novel materials with enhanced electrochemical performance, and future generations of battery technology.

Further information 

Tapia-Ruiz N, et al. High voltage structural evolution and enhanced Na-ion diffusion in P2-Na2/3Ni1/3−xMgxMn2/3O2 (0 ≤ x ≤ 0.2) cathodes from diffraction, electrochemical and ab initio studies. Energy & Environmental Science (2018). DOI:10.1039/C7EE02995K.​

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