Distinct charge dynamics in battery electrodes revealed by in situ and operando soft X-ray spectroscopy

Abstract

Developing high-performance batteries relies on material breakthroughs. During the past few years, various in situ characterization tools have been developed and have become indispensible in studying and the eventual optimization of battery materials. However, soft X-ray spectroscopy, one of the most sensitive probes of electronic states, has been mainly limited to ex situ experiments for battery research. Here we achieve in situ and operando soft X-ray absorption spectroscopy of lithium-ion battery cathodes. Taking advantage of the elemental, chemical and surface sensitivities of soft X-rays, we discover distinct lithium-ion and electron dynamics in Li(Co1/3Ni1/3Mn1/3)O2 and LiFePO4 cathodes in polymer electrolytes. The contrast between the two systems and the relaxation effect in LiFePO4 is attributed to a phase transformation mechanism, and the mesoscale morphology and charge conductivity of the electrodes. These discoveries demonstrate feasibility and power of in situ soft X-ray spectroscopy for studying integrated and dynamic effects in batteries.

In operando tracking phase transformation evolution of lithium iron phosphate with hard X-ray microscopy

“…Differently, here we found inhomogeneity in the entire electrode with hard X-rays penetrating through the electrode. Using direct imaging technology, our result also confirms the recent charge dynamic study in a LiFePO 4 electrode with soft X-ray spectroscopy that observed inhomogeneity in polymer electrolyte electrodes with low ion conductivity 36 . In our work, the use of low active material loading and liquid electrolyte is supposed to enhance the ionic and electron conductivity.…”

Understanding the degradation mechanism of rechargeable lithium/sulfur cells: a comprehensive study of the sulfur–graphene oxide cathode after discharge–charge cycling

“…34,35 In addition, by recording NEXAFS spectra with different detection modes such as total electron yield (TEY) and total fluorescence yield (TFY), different probing depths can be achieved. The TEY mode probes a depth of B5 nm, whereas the TFY mode probes a depth of B100 nm.…”

Influence of synthesis conditions on the surface passivation and electrochemical behavior of layered cathode materials

“…During the charging (lithium deintercalation) and discharging (lithium intercalation) processes, nickel switches between Ni 2+ and Ni (2+x)+ oxidation states, where the x value is usually determined by the degree of lithium deintercalation, i.e., charging voltage cutoff in the galvanostatic or potentiostatic cycling. 12,24,31,32 At the end of discharge, the nickel oxidation state returns to Ni 2+ independent of changes in the crystal structure (Fig. 7a), so that detection of Ni 2+ in the TEY data is not proof per se of surface reconstruction to the rock salt structure.…”