Miki Nagao

Real-time observations of lithium battery reactions—operando neutron diffraction analysis during practical operation

Among the energy storage devices for applications in electric vehicles and stationary uses, lithium batteries typically deliver high performance. However, there is still a missing link between the engineering developments for large-scale batteries and the fundamental science of each battery component. Elucidating reaction mechanisms under practical operation is crucial for future battery technology. Here, we report an operando diffraction technique that uses high-intensity neutrons to detect reactions in non-equilibrium states driven by high-current operation in commercial 18650 cells. The experimental system comprising a time-of-flight diffractometer with automated Rietveld analysis was developed to collect and analyse diffraction data produced by sequential charge and discharge processes. Furthermore, observations under high current drain revealed inhomogeneous reactions, a structural relaxation after discharge, and a shift in the lithium concentration ranges with cycling in the electrode matrix. The technique provides valuable information required for the development of advanced batteries.

Development of SPICA, New Dedicated Neutron Powder Diffractometer for Battery Studies

SPICA, a new special environment powder neutron diffractometer was built at BL09 in the Material and Life science Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). This is the first instrument dedicated solely to the study of next-generation batteries in J-PARC and is optimized for in situ measurements to clarify structural changes of materials in batteries. The basic design and instrumentation of SPICA have been completed. The highest Δd/d resolution achieved at the commissioning stage was 0.09% at the back scattering bank of SPICA. The reliability of the diffraction data has achieved a sufficiently high level for the structural analysis of materials using the Rietveld method. The air scattering banks with the blades made of B4C for in situ measurements also function very well.

Synthesis, crystal structure, and ionic conductivity of tunnel structure phosphates, RbMg1−xH2x(PO3)3·y(H2O)

A new proton conductor, RbMg1−xH2x(PO3)3·yH2O, was synthesized by the coprecipitation method followed by sintering at 540 K. The range of solid solution for RbMg1−xH2x(PO3)3·yH2O was 0.00 < x < 0.18 and the highest conductivity of 5.5 × 10−3 S cm−1 was observed at 443 K for the composition of x = 0.11. The structure of RbMg1−xH2x(PO3)3·yH2O was determined from 298 to 553 K by Rietveld refinement with multi-profile analysis using X-ray and neutron diffraction data. The framework structure is composed of PO4 tetrahedra connected with each other by corner-sharing to form spiral PO4 chains along the c-direction. One-dimensional tunnels are formed between these spiral chains, in which water molecules are located. The water molecules form one-dimensional spiral chains and are connected to the spiral PO4 chain by hydrogen bonding. The introduction of acidic, hydrophilic head groups (–PO3H) into the PO4 framework by the formation of a solid solution provides binding sites for water and an environment for the efficient diffusion of protons. One-dimensional proton diffusion, similar to proton channels in biological systems, could be explained by the vehicle mechanism of H3O+ along the one-dimensional water chain.

Synthesis and crystal structure of novel proton conductor, RbMg(PO3)3·3(H2O)

Proton conductors have been studied for applications in electrochemical devices such as fuel cells. Phosphate and sulfate based materials are one of the categories of proton conductors suitable for medium temperature range. Among these materials, proton-conductive solid acid salts, CsH2PO4 and CsHSO4, are well known as high proton conductors. The ionic conductivity exceeds 10 Scm at 200°C. However, the temperature range where the material shows high proton conduction is rather narrow, and the search for new proton conductors is still necessary. In the present study, novel proton conduction materials were synthesized in the solid acid salt systems. The structure of RbxMg1-x(PO3)3 were examined by x-ray and neutron diffraction measurements. The proton conductivities were also determined by ac impedance methods. The relationship between the structure and proton conductivity mechanism will be discussed.