NSRRC Activity Report 2023

Energy Science 059 T he 2019 Nobel Prize in Chemistry was awarded to John B. Goodenough, M. Stanley Whittingham, and Akira Yoshino for their contributions to the development of lithium-ion batteries. Compared with other rechargeable batteries, such as nickel-metal hydride batteries, lithium-ion batteries have higher cycle times and energy density, making them more suitable for mobile communication and portable electronic devices. The first commercial lithium-ion battery structure consisted of a petroleum coke or graphite anode, cobalt oxide as the cathode, and a lithium-ion electrolyte. Although lithium-ion batteries have been widely used, scientists are constantly in search of safer, more efficient, and longer-lasting battery materials. This report summarizes the results from two lithium battery studies 1,2 conducted by the team of Guanglei Cui in Chinese Academy of Sciences, China. The electrolyte in lithium-ion batteries not only transfers lithium ions but also regulates the electrode interface and largely determines the life and stability of the battery. Although many currently used electrolytes have relatively balanced properties, they can still easily trigger oxidation–reduction reactions at the electrode interface under high-temperature and high- humidity environments, ultimately affecting battery life and causing safety issues. Currently, there are still restrictions on the transportation of lithium batteries. Researchers compared the electrolyte properties of newly synthesized lithium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracarbonitrile) borate (LiDFTCB) with commonly used lithium difluoro(oxalato) borate (LiDFOB). To study the interfacial reaction mechanism, the research team conducted X-ray absorption spectroscopy (XAS) measurements at TLS 11A1 . The world’s first Dragon beamline designed by Dr. Chien-Te Chen, who was then based at the Bell Laboratories, was transferred from NSLS to TLS 11A1 and re-constructed in 1998. It features separate horizontal and vertical focusing mirrors that eliminate astigmatic coma aberration and an exit slit movable to the focal point of each wavelength that eliminates defocus aberration. Combined with a suitable focusing ratio and minimum number of optical elements, a high flux and high resolution beamline with a simple scanning mechanism is created. The TLS 11A1 Dragon beamline has been operating since 1999 and is one of the longest-running beamlines in the world. The XAS endstation of TLS 11A1 can measure both total electron yield (TEY) and total fluorescence yield (TFY) signals. TEY is measured directly from the photocurrent of the sample using a pico current meter. The signal mainly comes from the surface 2–4 nm and is therefore suitable for studying the electrode interface. The TFY is measured using a multi-channel plate. This allows for a better depth detection profile and can be regarded as the bulk properties of the material. Electrolytes Affect the Life-Time of Lithium Batteries The valence behavior of lithium battery materials can be resolved by soft X-ray absorption spectroscopy. Fig. 1 : The Co L-edge (a) and O K-edge (b) XAS TEY spectra of the pristine and cycled LiCoO 2 cathodes, together with those of CoO, Li 2 Co 2 O 4, and BaCoO 3 as the references. [Reproduced from Ref. 1] For the Co L-edge XAS spectra, shown in Fig. 1 (a) , although the cycled LiCoO 2 cathodes maintain the predominant Co 3+ state, the higher energy shifts and an increase in the intensity of peaks at 779.5 eV indicate the appearance of Co 4+ species after cycling. In the O K-edge XAS spectra, the pre-edge peaks below 533 eV shift to a lower energy with an increase in the Co valence state. Compared to the pristine LiCoO 2 , the cycled LiCoO 2 cathodes clearly exhibit two new pre-edge peaks at 528 and 529 eV, which are characteristic Co 4+ related peaks. This further confirms the existence of Co 4+ species after cycling ( Fig. 2(b) , see next page). The pre-edge peaks in the O K-edge XAS spectra ( Fig. 1(b) ) of cycled LiCoO 2 taken in the TFY mode display much higher spectral weight than those taken in the TEY mode. This indicates a higher Co 4+ content in the bulk structure after cycling in LiDFOB-PC than that cycled in LiDFTCB-PC. The XAS results confirm the negative effect of the interface reaction between LiDFOB and LiCoO 2 on the reversible Co 3+/4+ redox reaction, which is also detrimental to the bulk structure reversibility. Based on these spectra, it is inferred that lithium ions accumulate at the electrolyte/electrode interface after multiple charge cycles, causing the valence of Co to increase.