Magnetism in Battery Materials

 

Max Avdeev (ANSTO-Bragg Institute), Chris Ling (Sydney University), Hamdi Yahia, Hironori Kobayashi and Masahiro Shikano (AIST) and Prabeer Barpanda (University of Tokyo)

 

Although materials, both organic and inorganic, with ions that magnetically interact via weak M-O-X-O-M contacts (M=transition metal, X=P,S,Si,C,B) have been known and studied for many decades[1] ,they have received relatively little attention compared to more magnetically dense systems such as perovskites with shorter M-O-M contacts. However, this situation is rapidly changing as the solid-state (electro)chemistry of phosphates, silicates, sulphates, carbonates, and borates has recently entered a “gold rush” phase with numerous research groups searching for cheaper, safer, and more environmentally benign battery materials.

 

New lithium and sodium materials are reported on a weekly basis and the vast majority of them are based on 3d transition metals, because insertion materials are by definition based on Mⁿ/M^n+x redox reactions and the requirement of high gravimetric energy density points to the lighter, i.e. 3d, elements from Ti to Ni. 

 

Magnetic ordering in such materials typically occurs only at low temperature and thus does not affect the performance of batteries. However, information on magnetic structure is important for the more accurate DFT calculations that are now commonly used to predict various material properties, from the activation energy of mobile ion transport to the open-circuit voltage. In addition, materials with weak magnetic couplings may often be modified by magnetic fields leading to metamagnetic transitions and, at times, manifestation of the magnetoelectric effect.

 

“Magnetism is the Killer App for neutron scattering” and naturally some of these newly synthesized magnetic materials found their way into the neutron beam at OPAL. The collaboration between the Bragg Institute, School of Chemistry of the Sydney University, and groups from the Research Institute for Ubiquitous Energy Devices (National Institute of Advanced Industrial Science and Technology, Osaka, Japan) and the University of Tokyo have, in a short period of time, led to characterization of the magnetic structure and properties of a number of electrochemically interesting materials: LiNaCoPO₄F ², LiNaFePO₄F ³, Na₂CoP₂O₇ ⁴, two polymorphs of NaFePO₄ ⁵, Mn(OH)_2-xF_x ⁶.

 

In all the phosphates studied, the magnetic interactions at least in one dimension involve phosphate groups and (super)exchange via M-O-P-O-M pathways that results in low Néel temperatures in the range 6.5-50 K. The two polymorphs of NaFePO₄, maricite and triphylite type, provide a particularly interesting example of the relationship between crystal and magnetic structure.

 

Both polymorphs are isostructural to olivine, Fe₂SiO₄. In the latter, iron atoms occupy two inequivalent sites; the difference between maricite and triphylite lies in which half of the iron atoms is replaced by Na (Fig. 1). This structural modification results in drastically different topology of the magnetic sublattice. In triphylite-NaFePO₄ the [FeO₆] octahedra share vertices forming layers and the material demonstrates both intra- and inter-layer antiferromagnetic (AFM) ordering (Fig. 2).

 

In contrast, the maricite-type NaFePO₄ features edge-sharing chains of [FeO₆] octahedra (Fig. 3). The Fe-O-Fe angles approaching 90° favour weak ferromagnetic (FM) coupling within chains which are AFM coupled to each other via phosphate groups. The weakness of both FM intra-chain and AFM inter-chain interactions significantly lowers the temperature of the magnetic transition compared to that in the triphylite: 13 K vs 50 K, respectively. Moreover, such a quasi-1D character of magnetic structure of the maricite-type NaFePO₄ may lead to magnetic field-induced transitions which will be further investigated.

 

Overall, the literature and our own experience show that magnetism in the materials with weak interactions via polyanion groups such as [PO₄], [SO₄], [SiO₄], [CO₃], [BO₃], etc. is very sensitive to subtle changes in crystal and electronic structure as is particularly well illustrated by different magnetic structure of isostructural LiNaCoPO₄F and LiNaFePO₄F (Figure 4). That allows manipulations with spin arrangement using pressure, magnetic field, or chemical substitution, opening the way to interesting physics. Our first experiments have already produced several intriguing and unexpected results [2-6] and we will actively continue research in the area of magnetism of battery materials.

 

 

Figure 1. Crystal-chemistry of the triphylite and maricite mineral groups. The minerals shown in italic are derived from lithiophilite and triphylite by low-temperature Li⁺ leaching and substitution. Thick solid lines/arrows and hashed area show the documented solid solution series between compositions. Text in blue shows the previously published magnetic structures.

 

 

 

Figure 2. Magnetic structure of triphylite-NaFePO₄. (Left) A view of a single antiferromagnetically ordered layer of corner-sharing [FeO₆] octahedra; (right) a view along the c axis showing the connectivity of layers via phosphate groups.
 

 

 

 

Figure 3. General view of the crystal and magnetic structure of maricite-NaFePO₄ (a), a view of ferromagnetic rutile-type [FeO₆]-octahedral chains running along the b axis (b), and a view along the b-axis showing the connectivity of octahedral chains via phosphate groups (c).

 

 

 

Figure 4. Magnetic structure of isostructural LiNaCoPO₄F (Ref. 2) (left) and LiNaFePO₄F (Ref. 3) (right).