▲Figure 2.In situ impedance spectra of
NCM/Li6PS5Cl/Li cell at 0.2C with equidistant capacity of 2 mA h g−1 during the first two cycles. Nyquist plots (a, c, e, g)
and Bode plots (b, d, f, h) of the impedance spectra during the charge (「C」) and discharge (「D」) processes.
▲Figure3. Examples of Bode plot (a) and Nyquist plot
(b) obtained by fitting the in situ impedance spectra at three voltages (before cycling, 3.177 V and 3.706 V in the 1st charge). The circles represent the experimental data, the lines
represent the fitted data based on two equivalent circuits (inset in Nyquist plot, 「FIT#1」 and 「FIT#2」). Variation of the resistance values (c), the Warburg coefficient W and the Nernst
constant kN (d) with the charge-discharge processes obtained by fitting the impedance spectra. (e) Nyquist plot of
NCM/Li6PS5Cl/Li cell: before cycling (「uncycled」), after 2 cycles at 0.2C (in situ EIS measurement at 0.2C), after 50 cycles at 0.5C (carried
out after in situ EIS measurement). (f) Evolution of Nyquist plot and resistance values for fresh battery during various storage days.
隨後,我們對原位測試的電池在循環前,循環內以及循環後的阻抗進行擬合,發現利用一般的電路元器件半無限擴散 Warburg 阻抗來擬合低頻區已經不再適用循環內的阻抗譜。經過不斷的擬合,最終發現了「FIT#1」模型,其中在低頻區的擬合引入了 Nernst 有限厚度薄層擴散阻抗。有限薄層擴散是指滯留層厚度為有限值,在等效電路中有兩個參數:Warburg coefficient W
(Ω s−1/2) and the Nernst constantkN(s−1). 而這種擴散模型在 Nyquist
圖的低頻部分表現出RC的半圓弧特性。
▲Figure4. S 2p (a) and Li 1s (b) XPS spectra at two interfaces
(NCM/Li6PS5Cl interface and Li6PS5Cl/Li interface) of
NCM/Li6PS5Cl/Li cell: before cycling (「uncycled」), after the 2nd charge (「2nd C」) and discharge (「2nd
D」), after the 50th charge (「50th C」) and discharge (「50th D」) at 0.5C.
為了確定 Rct2確實是代表空間電荷層,我們通過非原位的表征電池正極介面處在循環前兩圈與長循環的變化,以排除其他因素對介面阻抗的影響,通過介面處 XPS 與 SEM 的變化,我們發現,在循環初期介面反應與介面機械失效沒有明顯的負面作用可以導致介面阻抗在循環內呈現 10^4
數量級的變化.而在長循環後,介面反應愈發嚴重,機械失效導致了介面處 NCM 出現二次顆粒,負極的鋰枝晶生長與元素擴散也愈發嚴重。我們認為,在循環的初期,電池的電化學性能(介面的不穩定性)主要來自於空間電荷層的影響,在循環後期,介面反應、機械失效、枝晶生長等共同作用導致電池性能進一步變差,介面惡化。
▲Figure5. The SEM images at
NCM/Li6PS5Cl interface (a) and at Li6PS5Cl/Li interface (b) before cycling (「uncycled」), after 2 cycles
and after 50 cycles at 0.5C. The cross-section morphologies of Li6PS5Cl/Li interface are on the far right. The elemental mappings at the bottom
corresponding to the SEM images (after 50 cycles) in red boxes.
▲Figure6. (a) In situ Raman spectra at
NCM/Li6PS5Cl interface during the positive and negative potential sweeps. (b, c) Examples of normalized peak fitting results from in situ
Raman spectra at NCM/Li6PS5Cl interface at two voltages (3.6 V and 3.7 V in the 1st positive potential sweep). (d) Normalized peak area percentages
evolutions of two Raman peaks at 418 cm−1 and 425 cm−1 during the positive and negative potential sweeps.
▲Figure7. Schematic illustrations of the Li+
concentration and Li+ migration at NCM/Li6PS5Cl interface: before cycling (「uncycled」), during the first charge-discharge
process and after cycling.
▲Figure8. Schematic illustrations of interface evolutions during the
charge-discharge processes (a, b, c, d) based on different vibration states of P—S bond in PS43− at
NCM/Li6PS5Cl interface.