There are many challenges in the battery sector that combine extremely fast charging speeds and maintain high energy density of batteries. Among them, the traditional current collector is impermeable to the electrolyte and easily hinders the movement of lithium ions, thus limiting the high-rate performance of thick electrodes. Here, Professor Cui Yi of Stanford University and others have designed a novel porous current collector for achieving high energy density and extremely fast charging of batteries. This porous design allows Li+ to pass through both the current collector and the diaphragm, thereby halving the effective Li+ transmission distance and increasing the rate performance of diffusion restriction by a factor of four without compromising energy density. The results show that the multilayer pouch cells using this current collector exhibit an energy density of 276 Wh kg-1 while performing at 4 C (78.).3% soc c（70.5% SOC) and 10 C (54.)3% SoC).
More importantly, this porous current collector design is perfectly compatible with existing battery manufacturing processes and other fast-charging strategies, thus opening up endless possibilities for next-generation battery fast-charging battery designs. The research results were published in Nature Energy under the title of "quadruple the rate capability of high-energy batteries through a porous current collector design".
Background] The widespread application of electric vehicles and electric aircraft has driven the demand for high-energy-density lithium-ion batteries (LIBS), and the state-of-the-art high-energy-density LIBS (>250 Wh kg-1) consists of a high-nickel layered oxide cathode and a graphite anode. Although it is possible to achieve a cruising range of more than 300 miles for electric vehicles by using thick electrodes, the problem of excessive charging time remains. As a result, extremely fast charging has become one of the most popular features to eliminate the "range anxiety" disorder, but this requires an 80% state of charge (SoC) from 0% to less than 15 minutes of charging time.
Among them, diffusion limitation is the key factor hindering the rate performance of the battery. The length of the effective Li+ pathway within the porous electrode plays a crucial role and increases with increasing areal loading, and several strategies have been proposed to address the diffusion problem. Thinning electrodes is the main means of improving rate performance, but it does come at the cost of reducing the energy density of the battery. At the same time, other methods, such as electrolyte engineering to accelerate ion conduction, thermal regulation to enhance Li+ transport, and reduction of twists and turns to shorten the path length of the electrode, etc.
However, these strategies require trade-offs in terms of electrochemical or thermal stability and energy density. Among the various approaches, reducing the effective Li+ transmission length may have a profound impact on improving rate performance. As shown in equation (1), the diffusion limit magnification (DLC) describes the maximum rate at which Li+ can be diffused by the electrode and electrolyte to participate in an electrochemical reaction:
Among them, z, f, d, , c0li+, qm and l are the valence state, Faraday constant, diffusivity, porosity, bending factor, initial lithium ion concentration in the electrolyte, mass fraction of the active substance, apparent density of the composite material, capacity of the active material and thickness of the electrode layer, respectively. The DLC is inversely proportional to the square root of the effective electrode thickness, and cycling above the DLC rate results in the depletion of Li+ at a specific depth within the electrode, rendering the active material beyond that point unusable. In addition, high magnification can easily lead to an increase in polarization, so that the negative electrode potential is lower than the equilibrium potential of 0V, which may lead to the formation of a deposit of lithium metal on the surface of the negative electrode, called lithium evolution, which will affect the energy density, reversibility and lifetime of the battery. In the worst-case scenario, they can trigger thermal runaway and**.
After re-evaluating the commonly used battery structures, the authors found that the importance of the existing current collector has been neglected in fast charging. Conventional current collectors (TCCs), such as solid metal foils including copper and aluminum, lack porosity and are impermeable to the electrolyte (Figure 1A). Therefore, these TCCs are not able to facilitate Li+ transmission and limit Li+ transport between electrodes to be unilateral. The authors conceived a novel thin (25 m) porous current collector (PCC) that can modulate the movement of Li+ through both the current collector and the separator for use in high-energy-density batteries (Figure 1B). Among them, the current collector consists of a sandwiched, porous and multi-level polymer matrix, which is coated with about 15 m thick positive and negative conductive metals.
By comparing Figures 1a and b, although the thickness of each electrode layer is the same in both cases, the arrangement of the negative and positive electrodes alternates from every two layers (positive-positive-negative-negative) to each layer (positive-negative-positive-negative alternate). This modification maintains the electrode thickness required to achieve high energy density in both cases. However, in the case of PCC, the effective Li+ transmission path length is reduced to half that of conventional batteries. As shown in Figure 1c, the reduction in effective diffusion length results in a four-fold increase in diffusion restriction magnification capabilities.
Figure 1The design principle of PCC in batteries.
To demonstrate the concept of PCC through numerical simulations, the authors chose a typical high-energy battery to illustrate the concept, using Lini05mn0.3co0.2O2, NMC) as the positive electrode and graphite as the negative electrode. In order to better understand the electrochemical process during fast charging, numerical simulations were first performed. In Figure 2a, a multilayer pouch cell with TCC and PCC is shown, using a constant current charge at a rate of 4C (12 mA cm-2 with a positive area load of 3 mH cm-2) in the experiment. The battery is charged to 42 V cut-off voltage and keep at 42 V until the total charging time reaches 15 min. Figure 2b shows the distribution of lithium concentrations in the active material in two different batteries, with the dark red (blue) color indicating high (low) lithium concentrations.
Due to the limitation of Li+ transmission, the lithium concentration of TCC batteries is highly uneven in electrode thickness. As can be seen in the left plot of Figure 2b, the graphite particles near the separator have a higher lithium concentration of about 10, while graphite particles away from the separator have a lower lithium concentration of about 03。In addition, Li+ can be transmitted through the diaphragm and PCC, which effectively reduces the effective transport length by half and significantly reduces the non-uniformity of Li+ distribution. As shown in the right side of Figure 2b, the lithium distribution in the graphite anode changes very little from the fully lithiated state on both sides of the anode surface (lithium concentration of 10) to 80% lithiation on both sides of the central anode (lithium concentration of 08）。
Figure 2Numerical simulation of batteries was performed with TCC and PCC.
PCC Design & FabricationBased on a preliminary analysis, a PCC design was identified that integrates the functions of a current collector (positive and negative) and a separator to assemble the cell. Therefore, a three-layer, layered, and bipolar PCC was developed to meet the requirements (Figure 3A). In order to ensure good electrochemical stability and good mechanical properties, the authors used bulletproof, thin, and nanoporous Kevlar films (Fig. 3B, C, average pore size 500 nm, porosity 65%, and 15 m thickness) as the main substrate for PCC. Kevlar is one of the strongest polymers and is often used as body armor due to its ballistic properties, making it suitable for use as a current collector.
Then, Cu and AK metal are coated on both sides of the above mixed PCC matrix, respectively. By applying a sufficiently thick metal coating, the effect of electronic conductivity on the battery resistance is negligible, and the thickness of the metal coating is optimized to 15 m to ensure high electronic conductivity of PCC. After metal coating, the submicron pore size of the surface layer is maintained, allowing the electrolyte to penetrate the PCC rapidly (Figure 3F, G). Due to the bendability of the microporous polymer coating on Kevlar, the conductive metal is only coated on the surface layer, which was confirmed by cross-sectional scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (Figure 3H).
Figure 3The main design concept of the PCC and its performance.
The results show that the multilayer pouch cells using this PCC exhibit an energy density of about 276 Wh kg-1 at the full cell level at a high area cathode load of 3 mAh cm-2. In addition, these batteries exhibit excellent rate performance: 78 at 4 C (15 minutes of charging C (10 minutes of charging) and 10 C (6 minutes of charging).3% soc、70.5% SOC and 543% soc。This design changes the traditional battery structure, enabling energy-intensive cells to use thick electrodes while achieving high rate performance.
Figure 4Electrochemical properties of multilayer pouch cells containing TCC and PCC, respectively.
DPS Real-Time Detection of Lithium Evolution Recently, the authors have successfully demonstrated a differential pressure sensing (DPS) technology that can accurately monitor lithium evolution during fast charging. By measuring the real-time change in the pressure of a cell per unit charge (DP dq) and comparing it to a defined threshold based on the maximum DP dq when Li+ is embedded in the negative electrode, it is possible to capture the lithium evolution imagination before the widespread growth of lithium metal. To better understand how PCC affects lithium evolution during fast charging, multilayer NMC|Graphite pouch cells are combined with DPS.
Figure 5Use DPS to detect lithium evolution that occurs during fast charging.
In summary, this paper proposes a porous current collector (PCC) design for high specific energy and fast charging, which can make Li+ pass through both the PCC and the separator at the same time, and reduce the length of the effective Li+ transport path by half without affecting the electrode thickness. As a result, the diffusion-limiting rate capability of high-specific energy cells has been tripled. Notably, the PCC consists of a three-layer, layered and porous polymer matrix with copper and aluminum coatings on both sides.
The experimental results show that the use of the PCC in the multilayer pouch battery significantly improves the new function of rate increase, and the PCC design improves the tolerance to lithium evolution and improves the reversibility and safety of lithium evolution of lithium-ion batteries under fast charging. The advantages offered by PCC over TCC design have the potential to enrich commercial battery designs and may have a broad impact on the improvement of fast charging performance of next-generation energy storage devices.
Bibliographic information] Yusheng Ye, Rong Xu, Wenxiao Huang, Huayue AI, Wenbo Zhang, Jordan Otto Affeld, Andy Cui, Fang Liu, Xin Gao, Zhouyi Chen, Tony Li, Xin Xiao, Zewen Zhang, Yucan Peng, Rafael A vila, yecun wu, solomon t. oyakhire, hideaki kuwajima, yoshiaki suzuki, ryuhei matsumoto, yasuyuki masuda, takahiro yuuki, yuri nakayama, yi cui*, quadruple the rate capability of high-energy batteries through a porous current collector design, 2024, nature energy.