Operando XRD study on the Effect of Boron Doping on the Failure Mechanisms of Na-, Ni- and Mn-based Positive Electrodes in Sodium-Ion Batteries

Abstract

Layered oxides have emerged as one of the most promising candidates for high capacity positive electrode materials in sodium-ion batteries (SIB). It has been demonstrated that Na, Ni and Mn-based layered oxide materials are relatively sustainable in terms of their composition, and high capacities have been achieved. However, it has been noted that at higher voltages, the crystal structure of these materials tends to degrade during cycling1,2. Doping the electrode with different transition metals such as Fe, Mg, Ti etc., has been an immensely popular strategy to avoid this degradation3–5. However, there has been little attention on doping with lighter electrochemically inactive elements such as boron that do not lower the materials' theoretical capacity.In this study, layered oxide cathode materials with varying Na, Ni and Mn contents were synthesised. Electrodes were made from synthesised materials (using 80% active material, 10% conductive additive and 10% binder) and measured in sodium-ion battery half-cells using the galvanostatic charge-discharge method (GCD). The best performing material was doped with 10% boron to investigate its influence on the materials' capacity during galvanostatic cycling (Figure 1a). Operando XRD was used to evaluate the changes in the crystal structure of synthesised layer oxides during galvanostatic cycling (Figure 1b). XRD diffractograms were measured on different cell potentials, which corresponded to voltage plateaus from previous galvanostatic charge-discharge measurements. Crystallite size, lattice parameters, and atom occupancies were calculated from measured diffractograms to evaluate changes in crystal structure during cycling. Operando XRD measurements were coupled with electrochemical impedance spectroscopy to tie structural degradation with contact impedance. Figure 1