On September 2, 2022, Beijing time, the team of Professor Yang Yuan and Professor Wei Min of Columbia University in the United States cooperated to publish a research result entitled “Stabilizing lithium plating in polymer electrolytes by concentration-polarization-induced phase transformation” in the journal Joule.
This result reports the phase separation phenomenon in the polymer-based electrolyte due to the concentration gradient, and the polymer-rich phase formed on the surface of the lithium metal anode during battery charging has high mechanical strength, which provides an innovative solution for solving the dendrite growth problem in polymer-based lithium metal batteries. The corresponding authors of the paper are Yang Yuan, Min Wei, Cheng Qian; The first names tied are Cheng Qian, Jin Tianwei and Miao Yupeng.
Due to its low electrode potential (-3.04 V vs. SHE) and ultra-high gram capacity (3860mAh g-1), the lithium metal anode is considered to be a potential electrode material for improving the energy density of lithium batteries. However, its practical application has been affected by the growth of lithium dendrites/whiskers, and there are problems such as continuous reaction with electrolytes and thermal runaway. The traditional view is that the concentration gradient in the electrolyte will aggravate the growth of lithium dendrites/whiskers during the charging process of lithium metal batteries, so the electrolyte needs to have a high initial lithium salt concentration and ion conductivity. On the other hand, in order to improve the thermal stability of lithium metal batteries, solid polymer-based electrolytes have received a lot of research attention in academia and industry due to their good thermal stability and ease of processing. However, it tends to have low mechanical strength when it has high lithium salt concentration and ion conductivity, which is difficult to inhibit the growth of lithium dendrites/whiskers. Therefore, a new design scheme is needed to solve the problem of lithium dendrite/whisker growth in polymer-based electrolytes.
In this work, Professor Yang Yuan’s team and Professor Min Wei’s team used the high resolution of stimulated Raman scattering (SRS) in time, space and chemical concentrations to find that the concentration gradient caused by concentration polarization induces the dynamic phenomenon of phase separation in polymer-based electrolytes (LCPE) with low lithium salt concentration, and the polymer-rich phase formed in the negative surface area of lithium metal has high mechanical strength, which can effectively inhibit the growth of lithium dendrites/whiskers. Polymer-based electrolytes (HCPE) with high lithium salt concentrations do not occur, and there is still a problem of a large number of lithium dendrites/whiskers. Based on the discovery of breaking the traditional impression, the team members innovatively proposed a polymer-based electrolyte design method, and took the polyethylene oxide (PEO) based electrolyte as an example to achieve effective inhibition of lithium dendrite/whisker growth and excellent cycling performance of lithium/lithium iron phosphate (Li/LFP) batteries at 38-40 °C.
Schematic of the Stimulated Raman Scattering (SRS) test for (A, B) (A) polymer-based electrolytes with high lithium salt concentrations (HCPE, EO: Li: succinitrile SN = 12:2:2.64) and (B) polymer matrix electrolytes with low lithium salt concentrations (LCPE, EO: Li:SN = 12:1:2.64). (C) Li | used in SRS PEO-based electrolyte | Li battery microscope brightfield image. The scale bar is 100 μm. (D) Raman characteristic peaks of SN, LiTFSI and PEO selected for this work. (E) Good linear relationship between LiTFSI Raman signal strength and its concentration.
Figure 2: (A-D)Li| high salt concentration polymer electrolyte (HCPE) | Li symmetrical growth of lithium dendrites/whiskers in cells. (A) Brightfield image at different sedimentation stages and distribution of lithium ion concentration under SRS. It can be seen that PEO-based electrolytes with higher lithium salt concentrations cannot effectively inhibit the growth of lithium whiskers. (B) The voltage curve of the battery. It can be seen that when the whiskers begin to grow significantly, the voltage of the battery begins to jitter and decrease. (C) Lithium ion concentration of Interface 1 (Li/PEO interface) at different sedimentation stages. (D) Growth rate of lithium metal. Indicates that lithium whiskers are growing rapidly.
(E-H)Li| low salt concentration polymer electrolyte (LCPE) | Growth of lithium dendrites/whiskers in Li. (E) Brightfield image at different sedimentation stages and distribution of lithium ion concentration under SRS. During lithium deposition, as the concentration of lithium ions in the electrolyte on the surface of the lithium electrode decreases, a new phase appears in the electrolyte, and whisker growth does not occur. (F) Voltage curve of the battery. The charging voltage is significantly more stable than (B). (G) Lithium ion concentrations in Interface 1 and 2 (interface of the new phase with PEO, i.e. the dotted line position in E) at different sedimentation stages. As the lithium deposition and the concentration of lithium ions in the electrolyte on the surface of the electrode decrease, the concentration of lithium ions at the two interfaces changes significantly differently. (H) Growth rate of lithium metal. Its growth rate was significantly lower than (D), indicating that whisker growth was effectively inhibited and lithium deposition was more tight. All scales are 50 μm, the cells have electrode spacing of 500 μm, the current is 0.5 mA cm-2, and the test conditions are room temperature.
Figure 3: (A) Ternary phase diagram of PEO-SN-LiTFSI. The orange area in the center is the single-phase zone and the remaining area is the two-phase zone. The rainbow-colored region above the triangle is the Young’s modulus corresponding to the PEO-rich phase of the electrolyte component. It can be seen that if the initial lithium salt concentration of the PEO electrolyte is high (such as the initial component of the HCPE at the blue star at point A), the charging current will reduce the lithium salt concentration on the negative electrode surface, and its components move along the blue arrow, but still in the single-phase region with a low Young’s modulus, which is not enough to inhibit the growth of lithium dendrites/whiskers; If the initial lithium salt concentration of the PEO electrolyte is low, closer to the boundary between the single-phase region and the two-phase region Of the lithium salt concentration cb (such as the initial component of the LCPE at the red star of the B point), the concentration polarization caused by the charging current will cause the components on the negative electrode surface to move along the red arrow and enter the two-phase region, and its newly formed PEO phase has a higher Young’s modulus, which can help inhibit the growth of lithium dendrites/whiskers. (B) Schematic diagram of the thickness relationship between the initial lithium salt concentration Co in the electrolyte and the PEO-rich phase formed under the current. (C) When Co is much higher than Cb, the concentration gradient caused by the current does not induce a PEO-rich phase, and the electrolyte cannot inhibit the growth of lithium dendrites/whiskers. (D) When Co is higher than Cb, lithium whiskers have begun to grow before the peo-rich phase is formed, and the electrolyte can partially inhibit the growth of whiskers. (E) When Co is close to Cb, the PEO-rich phase has been formed before the growth of lithium whiskers, and the electrolyte can play a good role in inhibiting the growth of whiskers. (F) When Co is lower than Cb, the ion conductivity of the PEO electrolyte is low, and the PEO-rich phase may be too thick.
Figure 4: Young’s modulus measurement of the single-phase region and the PEO-rich phase in the (A-D) PEO electrolyte. (C) The Yang modulus of the PEO-rich phase induced by the concentration difference polarization in LCPE is 1.6 GPa, which is enough to inhibit the growth of lithium whiskers, while the Yang modulus in the single-phase region of HCPE is less than 1 MPa, which cannot inhibit the growth of lithium whiskers. (E, F) Phase field simulation results of lithium metal deposition process in HCPE and LCPE. When the initial concentration in the PEO electrolyte is high, the concentration polarization induction is not enough to induce the high Young’s modulus-rich PEO phase, and the lithium whiskers will grow rapidly during the deposition process, which is consistent with the generally observed results; When the initial concentration in the PEO electrolyte is low, the concentration polarization is enough to induce a high Young’s modulus (1.6 GPa) PEO-rich phase, and the growth of lithium whiskers is significantly inhibited, and the lithium deposition morphology is good.
Based on the above experimental phenomena and the corresponding proposed polymer-based electrolyte design concept, the research group designed two sets of polymer-based electrolytes (LCPE) with low lithium salt concentration, which showed good cycle stability and lithium electrode morphological stability in Li/Li batteries and Li/LFP batteries.
Figure 5: Cycling performance of the lithium metal anode in the low salt concentration polymer electrolyte LCPE (EO:Li:SN = 12:1:2.64) and the high salt concentration polymer electrolyte HCPE (EO:Li:SN = 12:2:2.64) with SN as a plasticizer. (A,B) Microscopic brightfield image of the deposition and stripping process of lithium metal electrodes at 0.5 mA cm-2 current density in (A) LCPE and (B) HCPE. The scale is 100 μm and the test conditions are room temperature. During each deposition process in LCPEs, a PEO-rich phase will appear on the surface of the lithium metal, and there will always be no obvious lithium dendrite growth; In HCPE, there is no PEEO-rich phase, and lithium whiskers grow rapidly. Its looping video is below. （C-E）Li| LCPE| The cycle performance of LFP, the voltage curve and its SEM morphology of the lithium metal negative electrode after 100 cycles. （F-H）Li| HCPE| The cycle performance of LFP, the voltage curve and the SEM morphology of the lithium metal negative electrode after 40 cycles. The scale is 10 μm, the LFP surface capacity is 4mg cm-2, the added current is 0.25 C, and the test conditions are 40 °C.
Figure 6: Cyclic performance of the lithium metal anode in LCPE (EO: LiDFOB: LiBF4 = 12:1:1, PC mass is 90% of PEO) and HCPE (EO: LiDFOB: LiBF4 = 6:1:1, PC mass is 90% of PEO) of propylene carbonate (PC) as a plasticizer. Microscopic brightfield image of the deposition and stripping process of lithium metal electrodes at 0.75 mA cm-2 current density in (A) LCPE and (B) HCPE. The scale is 25 μm and the test conditions are room temperature. During each deposition process in LCPEs, a PEO-rich phase will appear on the surface of the lithium metal, and there will always be no obvious lithium dendrite growth; In HCPE, there is no PEEO-rich phase, and lithium whiskers grow rapidly. （C）Li| LCPE| Cyclic performance of LFP. （D）Li| HCPE| Cyclic performance of LFP. The LFP surface has a capacity of 5 mg cm-2, an added current of 0.3 mA cm-2, and the test conditions are 38°C.
In this work, the research group also proved that the phase separation phenomenon proposed by the group also exists in other polymer systems other than PEO. This study proposes a new understanding and design scheme for polymer-based electrolytes to inhibit the growth of lithium dendrites/whiskers, which is of great enlightenment significance for the subsequent research work of polymer-based electrolytes.
This work has also been strongly supported by Professor Chen Longqing of Penn State University, Professor Li Ju of MIT, and Professor Xi Chen of CUNY. (Source: Science Network)
Related paper information:https://doi.org/10.1016/j.joule.2022.08.001
