In the electric car industry, the rapid development of stimulation, lithium-ion battery also made significant strides this year, especially in the energy density of all power battery manufacturers have resorted to all the stops, in 2017, including ATL, Guoxuan Tech and force God, including power battery manufacturers have introduced a high energy density of 300Wh / kg of high specific power battery.At present, the major power battery manufacturers to adopt high-energy battery technology line mainly for high-nickel ternary materials + silicon carbon anode Material approach, which is also the most feasible high-energy battery design at this stage, but to further improve the lithium-ion battery energy density to 350Wh / kg, or even 400Wh / kg, this system will not be able to meet the requirements from the current The technological development of view, the metal Li cathode battery is the most promising next generation of high specific energy batteries.
If only from the electrochemical performance point of view, the metal Li negative pole probably is the world's most suitable as a negative electrode material, it has a low potential (-3.04V vs standard hydrogen electrode) and high capacity (3860mAh / g) dual advantages of the actual Lithium metal was the first negative electrode material to be used in lithium-ion batteries. Moli Enegry of Canada introduced the Li / MO2 secondary battery with metal Li negative electrode in the 1980s, but unfortunately in 1989, Lithium secondary battery fire explosion occurred, resulting in a large-scale global recall of the battery, from this short dominate the global battery market, the company sluggish, eventually acquired by Japan's NEC Inc. NEC company invested a huge manpower and resources, Careful testing of tens of thousands of batteries, after years of groping, finally found the culprit leading to the explosion of lithium secondary batteries - lithium dendrites.Although NEC found the key to the problem, but this has NEC plunged into a bottomless pit, regardless of Is how to improve the process can not eliminate Li dendrite.Only Japan's Sony company another way, the use of graphite as a negative electrode, lithium cobalt oxide as a positive, to avoid The emergence of lithium metal, but also completely eliminate the problem of metal Li dendrite, from the lithium battery ran all the way to become a group of chemical storage batteries in a dark horse, but lithium metal secondary batteries but this did not fall.
The 21st century, the development of solid electrolyte so that we can see the hope of the metal Li negative, from now on the study of lithium metal anode also gradually onto the fast lane.Recently, Peichao Zou Tsinghua University, who proposed 'since it can not be completely avoided With the growth of metal Li dendrites, why not prevent lithium dendrites from penetrating the septa by inducing growth orientation ?, Peichao Zou developed a copper foil with numerous microporous structures to induce Li dendrite Grow along the direction parallel to the separator, so as to ensure the safety of the lithium-ion battery even in the case of large growth of the negative Li dendrites.
The above-mentioned porous copper foil preparation process as shown above, including thermal lamination, laser engraving, alkali corrosion and other processes, after the hot lamination copper foil will be covered with a layer of polyimide PI film, Forming a sandwich structure, the laser engraving will form a regular arrangement of micropores on the PI layer, in the process of alkali corrosion, the alkali solution will erode the copper foil through these micropores and produce corresponding holes in the copper foil. , Peichao Zou chose the PI layer 45μm in diameter and 150μm in the copper foil, which is a structure with a small mouth and big belly.According to the volume of the micropores in the copper foil PeichaoZou calculated that the copper foil after the complete storage of lithium negative The capacity of up to 4.1mAh / cm2, if you further increase the thickness of the copper foil, but also continue to enhance the capacity of the negative, to meet the needs of most applications.
Peichao Zou porous copper foil E-Cu and ordinary copper P-Cu works as shown above, we can note that ordinary copper foil in the work of metal Li deposition occurs directly on the surface of the copper foil, Li diameter Of the growth direction is perpendicular to the direction of the copper foil and the separator, prone to Li diameter piercing the diaphragm problem, but in the E-Cu, metal Li will be deposited in the inner wall of the micropores, Li growth direction of natural dendrites It becomes parallel to the diaphragm and the direction of the copper foil, although Li deposition process will produce a large number of Li dendrites, but does not have an impact on battery safety.
In the figure below, after depositing lithium at 0.5 mAh / cm2 (a, d), 1 mAh / cm2 (b and e) and 2 mAh / cm2 Cu surface morphology, it can be noticed from the figure, the deposition of lithium metal showed filamentous, but all the metal Li are deposited within the copper microporous, no Li dendrite from the PI microporous layer Out, which minimizes the risk of internal short circuit, to ensure the safety of the battery.
The graph below shows the cycling Coulomb efficiency curves for E-Cu and normal copper foils. From the curves we can see that E-Cu shows very good Coulombic efficiency for several Li depositions, The control group of copper foil was not only significantly lower than that of E-Cu on the coulombic efficiency, but also the phenomenon of internal short-circuit occurred at electrodes with lithium deposition amount of 1.0 and 2.0 mAh / cm2.
The graph below uses the cycling curves of E-Cu + Li, P-Cu + Li and LFP materials, respectively. It can be seen that the Coulomb Chariots can still reach 250 cycles with the E-Cu battery at 1C rate 99.5%, discharge capacity of 131.1mAh / g, while the control group with ordinary copper foil, after 1C cycles of 250 times, the coulomb efficiency was only 58.6%, indicating that the porous copper foil negative cycle good performance.
This work by Peichao Zou shows us that in addition to various measures to suppress the growth of Li dendrites, Li dendrites can also be solved by inducing the growth direction in solving the problem of safety and cycle life of the negative electrode of Li metal. Lead to security problems.Peichao Zou developed microporous copper foil anode, the successful control of Li dendrite growth direction parallel to the direction of the separator and copper foil, to prevent the Li dendrite penetrate the diaphragm, greatly improved Battery safety and cycle life for the future development of metal Li anode secondary batteries provides a new way of thinking.
