The Raman spectra of nanometer-sized particles of SnO2 (3–90 nm) at room temperature are reported. In the Raman spectra of the particles of SnO2, which are quite different from that of single-crystal SnO2, there appear two new characteristic peaks, and their intensities decrease gradually with the increase of crystal size. It is concluded that the new peaks can be explained by the surface phonon modes of nanometer SnO2, consequently there is a relation between the structures of surface layers and bulk. It is believed that it is possible to determine the exact positions of atoms in surface layers of some nanometer crystals from their laser Raman spectra.
Lithium bisoxalatodifluorophosphate (LiBODFP) is a promising multifunctional lithium salt-type electrolyte additive used to enhance the performance of 5 V LiNi0.5Mn1.5O4 (LNMO)-based lithium-ion batteries (LIBs).
An electrolyte using 2,3,4,5,6-pentafluorophenyl methanesulfonate (PFPMS) as a versatile additive is investigated through calculating the molecular orbital energies of additives and solvents and designing the electrolyte composition, and the comparative performances of LiNiCoMnO/graphite cells operating in a wide-temperature range are improved. It is revealed that PFPMS can form interfacial films on both the cathode and anode surfaces, resulting in a decrease of the cell impedance and the side reactions between the active materials and electrolyte. Compared to the cells without additive of 74.9% and those with vinylene carbonate (VC) of 76.7%, the cycling retention of the cell with 1.0 wt % PFPMS reaches 91.7% after 400 cycles at room temperature. In particular, for the high-temperature storage at 60 °C for 7 d, the cell containing 1.0 wt % PFPMS exhibits optimal capacity retention of 86.3% and capacity recovery of 90.6%; for the low-temperature discharge capacity retention at -20 °C, the cell with 1.0 wt % PFPMS maintains at 66.3% at 0.5 C, while for the cells without additive and containing 1.0 wt % VC, their retention values are 55.0 and 62.1%, respectively. The excellent cycling, wide-temperature practicability, and rate capability of the cells with PFPMS demonstrate that the electrolyte with PFPMS additive is promising for applications in LiNiCoMnO/graphite batteries.
Lithium difluorophosphate (LiDFP), the decomposition product of LiPF 6 , was evaluated in high-voltage LiNi 1/3 Co 1/3 Mn 1/3 O 2 / graphite pouch cells. We report that conventional carbonate-based electrolytes containing 1 wt % LiDFP can notably enhance the cyclability and rate capability of the battery at 4.5 V. Its capacity retention maintained 92.6% after 100 cycles, whereas it is only 36.0% for the additive-free battery. Even after 200 cycles, the capacity retention remained 78.2%. The EIS measurements performed by three-electrode graphite/Li/LiNi 1/3 Co 1/3 Mn 1/3 O 2 pouch batteries indicate that LiDFP can efficiently restrain the breakdown of the electrolyte on the LiNi 1/3 Co 1/3 Mn 1/3 O 2 electrode surface and relieve the increase of cathode resistance. Additionally, a uniform and stable SEI film modified by LiDFP on the anode can effectively remit the electrode/ electrolyte interfacial reaction and relieve the increase of anode resistance during cycling. Further evidence for the beneficial effect of LiDFP in inhibiting the dissolution of transition metal from the cathode under a high operating voltage is also found. On the basis of electrochemical methods and spectroscopic techniques, the enhancement in the high-voltage performance of the cell attributed to the LiDFP component can simultaneously modify the cathode and anode surfaces. Consequently, LiDFP is a promising electrolyte lithium additive for practical applications in highenergy lithium-ion cells. KEYWORDS: lithium difluorophosphate, LiPF 6 decomposition product, high voltage, electrolyte additive, LiNi 1/3 Co 1/3 Mn 1/3 O 2 /graphite pouch batteries
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