In the competition to develop electrochemical cells for portable and mobile devices, lithium-ion batteries have become ubiquitous and have effectively replaced any other competitive technologies. The pursuit of higher volumetric energy density is the condition for the mobile device lithium ion battery (LIB) to be more widely used. At the same time, the recyclability of materials has also gradually received attention. At present, the selection and preparation process of the negative electrode of the lithium ion battery still use the standards of Sony Corporation from 1970s to 1980s, which can no longer rapidly increase the energy density of the electrode. In particular, the factors in the preparation of the negative electrode (reaction potential, electrode thickness, volume change caused by stress changes) make the electrode preparation technology coated with powder ~ conductive agent ~ binder on the metal current collector foil encounter bottlenecks . How to obtain high capacity and effectively maintain the stability of the electrode shape requires the conversion of the electrode preparation process. It is an effective method to directly use metal foil or further processing as an electrode (acting as a dual function of current collector and electrode material).
Regarding the future application prospects of metal foil anodes, Steven T. Boles and others from Hong Kong Polytechnic University published an article titled "Are Foils the Future of Anodes?" in the form of Commentary in Joule. Starting from the energy density of the electrode anode, the article discusses the advantages and disadvantages of commercial graphite anode, high capacity Si anode and metal foil Al in lithium ion battery anode. The properties of different materials are introduced in terms of volume energy density ~ electrode volume expansion ~ stress, and the advantages and improvement strategies of metal foil electrodes are introduced. It proves that the direct use of metal foil as an electrode is a promising method to improve the performance of the electrode.
The advantages and problems of alloyed materials over graphite: the volume energy density of graphite anode is about 800 mAh cm-3, the volume expansion of lithiation is about 12%, and the low-structure expansion and contraction is determined by the lattice-based intercalation-extraction reaction. Features and limited capacity. Alloy negative electrodes can achieve high capacity, but the inherent relationship between high capacity and large volume expansion cannot be avoided. For example, 280% of Si anode material, 250% of Sn anode, and 100% of Al are expanded, which brings the particles to face powdering, cracking and other harmful mechanical damage during the cycle. At present, the most studied is the Yolk-shell structure, which uses a hollow layer to accommodate the expansion volume caused by Li insertion. However, this structural design material will still limit the increase in capacity when the electrode is prepared. In the preparation of commercial anodes, the thickness of the Cu current collector is about 10 μm, and the active material + binder + conductive additive accounts for 90 μm (25% of the volume will be allocated to the binder), resulting in an active material volume share of about 65%. Then the volume energy density of 4000 mAh cm-3 calculated based on the active material suddenly drops to 2500 mAhcm-3, so this Yolk-shell structure can limit the deformation of the core active material and binder by containing the volume expansion to improve the cycle Performance, but because the free volume is only pre-allocated to the inside of the shell, it cannot improve the calculation of the total capacity.
The relationship between the volume expansion of the negative electrode and the obtained volume energy density:
1) The general relationship between volume capacity and negative electrode volume expansion: the capacity gain generated during the first 100% of volume expansion is always more than the capacity gain generated by expansion between 100% and 300%;
2) The contribution of the increased capacity due to the increased utilization of active materials to the total capacity is more obvious than the increase in capacity by volume expansion. These two factors provide important guidance on how to consider the negative electrode in future lithium-ion batteries. Regarding the pursuit of silicon as the ideal anode active material, it can be clearly seen that if the electrode is still a standard composite structure based on particles, the energy density gain due to its high theoretical capacity and volume expansion will be greatly reduced. On the other hand, if the solid aluminum foil can be stabilized (100% expansion when fully lithiated), the volumetric capacity of this dense foil will be significantly improved compared to any competitor based on silicon particles.
The thickness and volume expansion of the three electrodes under the same capacity and load area requirements: Assuming that the capacity of the battery in the 18650 battery is 2.225 Ah, the effective area of the negative electrode foil may be about 426.4 cm2, and the thickness of the graphite negative electrode will exceed 100 μm, Si The negative electrode will exceed 75 μm, and the thickness of the metal Al foil can be controlled below 50 μm. At the same time, the volume expansion can produce mechanical stress in the surrounding aluminum matrix, which follows the traditional metal elastoplastic deformation. In terms of mass energy density, Al foil can also achieve a smaller volume under comparable conditions.
However, despite the advantages of thermodynamics and energy density, whether dense metal Al foil can be used in batteries also needs to solve some problems:
1) The "structural integrity loss" occurs in short cycle, mechanical crushing accompanied by structural degradation, electrode degradation, side reactions and extremely low Coulomb efficiency all restrict the development of foil. Controlling the porosity of the foil and forming an integrated LiAl alloy can help solve this problem.
2) The dense electrode cannot form a channel for efficiently conducting ions, which is difficult to achieve in terms of rate performance and fast charging. In addition, the problems of electrolyte consumption and thermal runaway caused by the difficulty of forming a stable interface also restrict the development of metal Al foil.
3) In terms of electrode material recycling, metal foil has the characteristics of easy recycling, separation and reprocessing compared to mixed electrode with complex composition.
Research summary
This review shows the energy density advantages of metal foil in lithium-ion batteries, and analyzes the problems of metal foil. Due to the existence of many existing industry standards, polymer electrodes still occupy most of the market. However, dense foils with alloy-type active materials must be considered. For these materials, the combination of moderate capacity and completely compact design is far more advantageous than pursuing traditional negative electrode designs. The traditional negative electrode design has more than 30 years of experience, which has stabilized its dominant position in the market. But when the demand for batteries becomes more and more severe, when the economic and performance advantages are combined, the advantages of foil anodes will hopefully quickly replace the existing battery anode technology. At the same time, under the continuous improvement of other components of the battery, the application of metal foil will be further accelerated.
More direct use in collector metal foil:
1. Use the sulfur-selenium compound in the ether electrolyte to corrode the negative electrode Cu current collector, and directly use Se powder to react in situ on the Cu surface to synthesize a stable high-area capacity Na-Se battery negative electrode material:
Uncovering the Cu-driven electrochemical mechanism of transition metal chalcogenides based electrodes. Qidong Li, Qiulong Wei, Liqiang Mai, Qingjie Zhang, Khalil Aminec, Jun Lu, Energy Storage Materials(2018)
2. Use LiAl alloy and sulfurized polyaniline to assemble stable lithium-sulfur battery:
A Li-ion sulfur full cell with ambient resistant Al-Li alloy anode. Ju Sun, Qingcong (Ray) Zeng, Ruitao Lv, Wei Lv, Quan-Hong Yang, Rose Amal, Da-Wei Wang, Energy Storage Materials15 (2018) 209–21.
3. Electrodeposition improved metal foil for metal batteries:
Electrodeposition Accelerates Metal-Based Batteries. Kai Zhang, Zhenhua Yan,* and Jun Chen,*Joule(2019)