Electrochemical oxidation of trivalent chromium in a phosphate matrix: Li3Cr2(PO4)3 as cathode material for lithium ion batteries

Solid state chemistry for developing better metal-ion batteries

Metal-ion batteries are key enablers in today’s transition from fossil fuels to renewable energy for a better planet with ingeniously designed materials being the technology driver. A central question remains how to wisely manipulate atoms to build attractive structural frameworks of better electrodes and electrolytes for the next generation of batteries. This review explains the underlying chemical principles and discusses progresses made in the rational design of electrodes/solid electrolytes by thoroughly exploiting the interplay between composition, crystal structure and electrochemical properties. We highlight the crucial role of advanced diffraction, imaging and spectroscopic characterization techniques coupled with solid state chemistry approaches for improving functionality of battery materials opening emergent directions for further studies.

The development of high performing metal-ion batteries require guidelines to build improved electrodes and electrolytes. Here, the authors review the current state-of-the-art in the rational design of battery materials by exploiting the interplay between composition, crystal structure and electrochemical properties.

Revisiting the Stability of the Cr4+/Cr3+ Redox Couple in Sodium Superionic Conductor Compounds

In this work, we revisited the stability of the Cr⁴⁺/Cr³⁺ redox couple in a sodium superionic conductor (NASICON)-type compound, Na₂TiCr(PO₄)₃. Experimental results showed that the Na₂TiCr(PO₄)₃ compound exhibited a specific capacity of 49.9 mA h g^-1 at 20 mA g^-1, about 80% of its theoretical capacity of 62.2 mA h g^-1 with one Na⁺ insertion/deinsertion per formula Na₂TiCr(PO₄)₃. The redox couple was found to be stable against cycling with some 90.3% capacity retention after 300 cycles within the voltage range between 2.5 and 4.7 V. With a wider voltage range between 2.5 and 5.0 V, the capacity retention was about 76.6% after 1000 cycles, indicating the redox couple is stable against overvoltage. In addition, the effect of Ti/Cr ratio on the reversibility of the redox couple was studied by varying x in Na_1+xTi_2-xCr_x(PO₄)₃ (where x = 0.6, 0.8, 1.0, 1.2, 1.4, 2.0). It was confirmed that x = 1 is optimal for balancing the electrode stability and the capacity. The obtained optimal content of Cr in the compound provides useful guidance for designing new Cr-based NASICON-type cathode materials. Furthermore, in situ X-ray diffraction (XRD) analysis of compound Na₂TiCr(PO₄)₃ indicated a two-phase sodium-ion storage mechanism, which is different from the previously reported one-phase mechanism. Rietveld refinement XRD analysis showed a small volume change of the compound during cycling (about 2.6%), indicating good structural stability.

Stabilization of a 4.5 V Cr4+/Cr3+ redox reaction in NASICON-type Na3Cr2(PO4)3 by Ti substitution

The development of high-voltage cathode materials composed of abundant metals for rechargeable batteries is a crucial task to realize higher energy density in large-scale electrical energy storage systems. Here we report a reversible Cr4+/Cr3+ redox reaction at 4.5 V vs. Na/Na+ in NASICON-type Na2CrTi(PO4)3 (NCTP). An unstable Cr4+/Cr3+ redox in Na3Cr2(PO4)3 is successfully stabilized by the substitution of Ti with Cr. The charge/discharge mechanism of NCTP was studied by powder X-ray diffraction and soft X-ray absorption spectroscopy.

Binding Energy Referencing for XPS in Alkali Metal-Based Battery Materials Research (II): Application to Complex Composite Electrodes

X-ray photoelectron spectroscopy (XPS) is a key method for studying (electro-)chemical changes in metal-ion battery electrode materials. In a recent publication, we pointed out a conflict in binding energy (BE) scale referencing at alkali metal samples, which is manifested in systematic deviations of the BEs up to several eV due to a specific interaction between the highly reactive alkali metal in contact with non-conducting surrounding species. The consequences of this phenomenon for XPS data interpretation are discussed in the present manuscript. Investigations of phenomena at surface-electrolyte interphase regions for a wide range of materials for both lithium and sodium-based applications are explained, ranging from oxide-based cathode materials via alloys and carbon-based anodes including appropriate reference chemicals. Depending on material class and alkaline content, specific solutions are proposed for choosing the correct reference BE to accurately define the BE scale. In conclusion, the different approaches for the use of reference elements, such as aliphatic carbon, implanted noble gas or surface metals, partially lack practicability and can lead to misinterpretation for application in battery materials. Thus, this manuscript provides exemplary alternative solutions.

A single-phase all-solid-state lithium battery based on Li1.5Cr0.5Ti1.5(PO4)3 for high rate capability and low temperature operation

We report a battery made from a single material using Li_1.5Cr_0.5Ti_1.5(PO₄)₃ as the anode, cathode and electrolyte.