The specific energy of the power battery continues to increase. The existing high nickel material + silicon carbon system can achieve a maximum energy of 350 Wh/kg. To continue to increase the specific energy, a new system is needed. The current high specific energy system includes all Solid-state metal Li batteries, Li/sulfur batteries and Li/air batteries, from the current state of the art, all-solid metal Li batteries are the most likely next-generation high-energy batteries. For all-solid-state batteries, solid-state electrolytes are Key technologies, in general, solid electrolytes have low Li+ conductivity at room temperature, which affects battery performance. In order to improve the conductivity of all-solid electrolytes, various types of all-solid electrolytes have been developed, among which garnet-type all-solid electrolytes Conductivity at room temperature can reach 10-4-10-3S/cm, with common carbonate liquid electrolytes 10-2S/cm is very close, it is an ideal all-solid electrolyte, but garnet electrolyte is also facing a surface inert layer (LiOH, Li 2CO 3The wettability with metal Li is poor, the metal Li dendrite grows at the grain boundary, and the interface impedance is large. Recently Weidong Zhou of Peking University (first author, Corresponding author), Yutao Li of the University of Texas at Austin (Corresponding author) ), John B Goodenough (corresponding author) by coating a layer of Li on the surface of the garnet electrolyte +The way of polymer electrolyte with a migration number of 0.9 inhibits the growth of metal Li dendrites and reduces the interfacial impedance, which increases the first coulombic efficiency of the all-solid metal battery to 97%, and the Coulomb efficiency in the cycle is close to 100%.
The interface of garnet solid electrolyte does not wet, Li dendrite growth and poor interface contact, and the good mechanical properties of polymer electrolyte make it an effective way to solve this problem. The usual polymer electrolyte does not contain Li salt. Therefore, it is necessary to add a lithium salt such as LiTFSI, but this also makes the migration number of Li+ tend to be relatively low (for example, 0.35), so that anions will accumulate on the side close to the positive electrode during charging, thereby generating a strong electric field, affecting Li+. Diffusion, accelerates the growth of Li dendrites in the garnet electrolyte, and has a high Li +The migration of the polymer electrolyte has less mobile anions, which can effectively solve this problem and improve the performance of garnet solid electrolyte.
The structure of the polymer electrolyte used in the experiment is shown below (poly(acrylamide-2-methyl-1-propane sulfonate) lithium PAS), in which only Li is able to move, so Li +The migration number is also very high, reaching about 0.9. In order to further improve the mechanical properties and Li+ conductivity of PAS, the authors mixed PAS with PEO (polyethylene oxide), and the interaction between PEO and PAS also promoted Li. +Moving along the long chain of PEO, you can see PEO from the following figure: The conductivity of the electrolyte is the highest at PAS=3:1, up to 1.8x10 at 65°C.-5S/cm, because it is single ion conductive, therefore Li +The number of migrations is as high as 0.87-0.95, indicating that the vast majority of conductivity is determined by Li +Contributed, this is a positive help to reduce polarization, increase magnification and cycle performance.
Garnet solid electrolyte with conductivity up to 4x10 at room temperature-4S/cm, up to 1x10 at 65°C-3S/cm, but its contact with the metal Li negative electrode is poor, resulting in increased impedance, so Weidong Zhou combined the garnet electrolyte (450um thick) with a layer of PEO-PAS polymer electrolyte (about 5um thick), due to PEO-PAS is thinner, so although its conductivity is lower, the resulting impedance is relatively small, and the Li+ conductivity of the composite electrolyte as a whole can reach 1.5x10.-4At the same time, due to the good mechanical properties of PEO-PAS, the contact resistance between the metal Li negative electrode and the solid electrolyte is greatly reduced. In the absence of polyelectrolyte coating, the interface impedance of Li/LLZTO/ Li reaches 5000 ohms. After the polymer coating was added to the garnet electrolyte surface, the interface resistance dropped to 400 ohms.
Another problem faced by garnet solid electrolytes is the problem of the growth of Li dendrites along the grain boundaries during the cycle. The ordinary garnet solid electrolytes are cycled in Li/LLZTO/Li batteries for 5 hours. A significant short circuit has occurred, and the garnet solid electrolyte treated by the polymer electrolyte exhibits very stable cycle performance (as shown in the figure below, current densities at 0.1, 0.2, 0.3, and 0.5 mA/g, respectively). The lower cycle is 10 hours (charge 1h, then discharge 1h)), the electrolyte can be stably cycled for more than 500 hours without short circuit.
The following figure shows the electrochemical test results of a full battery made of Li/garnet electrolyte/LFP. As can be seen from the following figure a, the full battery exhibits good rate performance, and the positive electrode capacity can reach 145 mAh at 0.1 C rate. g, at a 0.2C rate, the reversible capacity of the positive electrode reaches 140mAh/g. Of course, due to the large impedance of the all-solid-state battery, there is still a certain gap compared with the liquid electrolyte battery. From the figure b, the first coulombic efficiency of the battery can be seen. Up to 97%, and after 160 cycles at 0.2C, the reversible capacity is still as high as 137mAh/g, and the coulombic efficiency of the battery in the cycle reaches 99.9%, indicating that the battery has good electrochemical stability. After the battery cycle, the author will The disassembly, the surface deposition of the metal Li anode is relatively uniform, and there is no obvious trace of Li dendrite growth (as shown in the following figure d).
The electrical conductivity of garnet electrolyte is as high as 10-3S/cm. Weidong Zhou effectively reduces the charge exchange resistance of the interface by modifying the surface with PAS-PEO polymer electrolyte, and also promotes the uniform distribution of current. , thereby significantly inhibiting the growth of Li dendrites along the grain boundary, avoiding the occurrence of short circuits, and greatly improving the cycle stability of the whole battery. This technology makes the practicality of the garnet electrolyte significantly improved, for pushing the whole solid state The development of electrolytes is of great significance.
