Brief introduction of the resultsAchieving extremely fast charging speeds while maintaining high energy density in the battery sector is still a challenge. Conventional current collectors are impermeable to electrolytes, hindering the movement of Li+ ions and limiting the high-rate performance of thick electrodes. Based on this,Academician Cui Yi of Stanford University (corresponding author).The research group conceived a porous current collector for high energy density and extremely fast charging batteries. This porous design allows Li+ ions to pass through both the current collector and the diaphragm, thereby halving the effective Li+ transport distance and quadrupling the c-rate performance of diffusion-confined without compromising energy density. The multilayer bag battery with this current collector demonstrated high specific energy (276 Wh kg-1) and significant fast charging capability at 4 C (78.).3% state of charge c (70.)5% state of charge) and 10 C (54.).3% state of charge). This porous current collector design is compatible with existing battery manufacturing processes and other fast-charging strategies, enriching battery configurations and providing new ideas for designing next-generation batteries. Background:This article discusses the trends in electric vehicles and electric aircraft, noting that they are increasingly reliant on energy-dense lithium-ion batteries (LIBS). The state-of-the-art LIBS with a high energy density (250 Wh kg-1) consists of a nickel-rich layered oxide cathode and a graphite anode. While it is already possible to achieve a cruising range of more than 300 miles in electric vehicles by using thick electrodes, long charging times are still a significant challenge. Therefore, achieving extremely fast charging has become one of the important goals to promote the widespread adoption of electric vehicles and electric aircraft and eliminate the problem of "range anxiety". Requires a charging time of less than 15 minutes, from 0% to 80% state of charge (SoC). In batteries, the length of the effective transport path of Li+ ions within the porous electrode is critical, and this length increases with area loading. A variety of strategies have been proposed to address the problem of proliferation. Among them, thinning electrodes is one of the main methods to improve rate performance, but this method comes at the expense of battery energy density. **Reading guide
Figure 1The design principle of PCC in batteriesAfter re-evaluating commonly used battery architectures, the authors found that the importance of current collectors had been overlooked. Conventional current collectors (TCCs), such as solid metal foils including Cu and Al, lack porosity and are impermeable to electrolytes (Figure 1A). Therefore, these TCCs do not contribute to Li+ transport and limit Li+ transport between electrodes to be only one-sided. Here, the authors conceptualize a thin (25 m) and porous current collector (PCC) that regulates the movement of Li+ through the current collector and separator for high-performance batteries (Figure 1B). The collector consists of a sandwiched porous hierarchical polymer matrix coated with approximately 15 m thick cathode and anode conductive metal. Comparing Figure 1a and Figure B, the authors can identify a key change in the battery cell configuration. Although the thickness of each electrode layer is the same in both cases, the arrangement of the anode and cathode changes from alternating every two layers (cathode-cathode-anode-anode) to alternating per single layer (cathode-anode-cathode-anode). This modification maintains the necessary electrode thickness 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 a conventional battery configuration. As shown in Figure 1c, the reduction in effective diffusion length results in a four-fold increase in diffusion-limiting magnification capabilities. Multilayer pouch cells equipped with this PCC have a high area cathodic load of 3 mAh cm2 and exhibit a specific energy of about 276 Wh kg1 at the full cell level. In addition, these batteries show significant rate capabilities: up to 4 C (15 minutes of charging, from 0 to 78.)3% SoC), 6 C (10 min charge, from 0 to 70.)5% SoC) and 10 C (charge for 6 minutes, from 0 to 54.)3% soc)。The design revolutionizes the traditional battery structure, enabling energy-dense cells to use thick electrodes while achieving high rate performance. 
Figure 2Numerical simulation of TCC and PCC batteriesThe authors chose a typical high-energy battery to illustrate their concept, which consists of lithium-nickel-manganese-cobalt oxide (lini05mn0.3co0.2O2, NMC) as the cathode and graphite as the anode. In order to better understand the electrochemical process of fast charging, the authors first performed numerical simulations. In Figure 2a, the authors show a multilayer pouch cell with TCC and PCC, which shows the battery assembly configuration that the author will later demonstrate and use in a practical application. In the authors' experiments, the authors used a constant current charge at a 4 C magnification (12 mA cm-2, C magnification based on an NMC cathode with an area load of 3 mA cm-2). Charge the battery up to 42 V cut-off voltage and keep at 42 V until the total charging time reaches 15 min. Figure 2b shows the lithium concentration distribution in the active material for two different battery configurations. Dark red (blue) indicates a high (low) concentration of lithium. Due to the limitation of Li+ transport, the lithium concentration of TCC batteries is highly uneven in electrode thickness. In the left panel of Figure 2b, it can be observed that graphite particles close to the separator have a higher concentration of lithium, about 10, while the lithium concentration of graphite particles away from the separator is lower, about 03。At the cathode, NMC particles also exhibit lithium concentrations of 04 ~ 0.6. Uneven distribution between them. Previous studies have shown that this inhomogeneous electrochemical process in the battery leads to damage both inside and at the interface of the active particles, resulting in irreversible capacity decay during battery cycling. As an option, Li+ ions can be transported through both the separator and the PCC, effectively reducing the effective transport length by half and significantly reducing the inhomogeneity of the lithium distribution. As shown in the right panel of Figure 2b, the lithium distribution in the graphite anode changes very little, from the side of the anode to the full lithiation state (lithium concentration of 10) to the 80% lithiation state of the central anode (lithium concentration of 0.).8)。At the same time, lithium ions are evenly distributed within the NMC cathode, and the lithium ion concentration is about 034。It is important to note that the distribution of lithium ions in NMC and graphite is different, which is due to the higher bending of the graphite anode than the NMC cathode. As a result, the transport of lithium ions in the graphite anode is slower, resulting in a higher gradient of lithium ion distribution. In different states of charge (e.g., 2.).5 v、3.8 V and 42 V), a more uniform use of the active material in the PCC cell can be observed, as shown in Figures 2c and d, where the distribution of lithium in the electrodes is plotted along the electrode thickness direction. As a result, a battery with a PCC has a lower voltage polarization and a higher usable capacity than a battery with a TCC (Figure 2E). This difference in polarization and usable capacity may be further amplified when charging at a higher C-rate (Figure 2F). Figure 2F shows a simulation of the TCC and PCC battery SoCs for two different charging protocols. (1) When using simple constant current (cc, dashed line) mode to charge to 4At a cut-off voltage of 2 V, the SOC of a PCC battery is significantly higher than that of a TCC bag battery. For PCC batteries, the biggest SoC improvement was achieved at a charge rate of 8. Beyond this rate, lithium depletion occurs, similar to what was observed in the TCC case, but at a much lower C rate. (2) Adopt CC and constant voltage (CC-CV, solid line) hybrid charging mode to charge to 4The 2 V cut-off voltage until the specified charging time is reached, the SOC of a PCC battery is significantly higher than that of a TCC battery. In 4 C CC-CV charging mode, the PCC battery reaches 92 in 15 minutes4% of the SoC. In CC-CV charging mode of 8, the PCC battery is in 1185 in 6 minutes7% of the SoC. To quantitatively compare the possibility of TCC and PCC plating Li0 in cells, the authors analyzed the evolution of ECT on the anode surface close to the cathode (Figure 2G,H). A gradual decrease in ECT to low values was observed, with the lowest ECT occurring at the end of CC charging. For TCC batteries, the lowest values of ECT are about -0 when the C rate is 4 and 6, respectively143 V and -0209 V, which indicates that the battery inevitably undergoes Li0 plating during fast charging. In contrast, for PCC batteries, the ECT of PCC is only -0 when the C magnification is 4 and 6, respectively029 and -0049 V, which is below the overpotential required for Li0 nucleation on the surface of the graphite particles. 
Figure 3The main design concept of the PCC and its performanceBased on the authors' initial analysis, the authors determined that PCC designs that integrated the functions of the current collector (cathode and anode) and separator were necessary to achieve their goals. Therefore, the authors developed a three-layer, layered, and bipolar PCC to meet the requirements (Figure 3A). In order to ensure excellent electrochemical stability and good mechanical properties, the authors used a bulletproof, thin nanoporous Kevlar film (Fig. 3B, C, average pore size 500 nm, porosity 65%, thickness 15 m) as the main substrate for PCC. Kevlar is one of the strongest polymers, and due to its ballistic properties, it is often used as body armor, which makes it suitable for use as a current collector. In addition, it separates the two electrodes, preventing potential electrical short circuits during battery operation. Subsequently, the authors applied a layer (5 m) of microporous polymer on each side of the Kevlar membrane by phase separation. The authors optimized the pore size of the surface coating to around 3-4 m to meet the requirements of good electrolyte permeability and thick metal coating (Fig. 3D,E). The authors then proceeded to coat Cu and Al metals on either side of the mixed PCC matrix described above, respectively. By applying a sufficiently thick metallic coating, the effect of electronic conductivity on the battery resistance becomes negligible. The authors optimized the thickness of the metal coating to 15 m to ensure high electronic conductivity of PCC. After metal coating, the submicron pore size of the surface layer remains unchanged, allowing rapid penetration of the electrolyte through the PCC (Figure 3F,G). Due to the bendability of the microporous polymer coating on the Kevlar surface, the conductive metal is only coated on the surface, which can be confirmed by cross-sectional images of scanning electron microscopy (SEM) and X-ray spectroscopy (Figure 3H). As a result, there is no electronic connection between the two sizes of PCCs, and the Kevlar layer in the middle remains unhindered by the metal. To assess the electrolyte permeability of PCCs, the authors compared the ionic conductivity of blocked cells assembled with different porous membranes, including polyethylene (Celgard 2500), a three-layer polyolefin separator (Celgard 2325), a three-layer PCC polymer matrix (PCC without metal), and PCC (Figure 3J). Then, 1 m of lithium hexafluorophosphate (LIPF6) was used as the electrolyte, and 2 wt% of fluorocarbonate was used as the electrolyte in a mixture of ethylene carbonate and methyl carbonate (3:7 vol%). In the authors' PCC preparation, the ionic conductivity of the PCC decreased only slightly after metal coating. This can be attributed to several factors: (1) The porosity of the PCC after metal coating is well maintained, allowing for fast ion transport and ensuring high ionic conductivity. (2) The metal coating has the least effect on the pore distortion, thus preserving the ion transport path. (3) The electrolyte pro-electrolyte nature of PCC promotes the redistribution of electrolytes in PCC, minimizing the decrease in ionic conductivity. In addition, the contact angle (2°) between the PCC and the electrolyte is much smaller than that of a commercial separator (Celgard 2325) with the electrolyte (34°), indicating the electrolyte profile of the PCC. 
Figure 4Electrochemical performance of TCC and PCC multilayer pouch batteriesFigure 4a shows the area discharge capacity of a bag battery charged at different C rates at 1 C, ranging from 1 C (1 hour charge) to 10 C (6 minutes of charging). To investigate the available capacity at different fast charging rates, eliminating the potential "dead capacity" effect, the authors fully discharged the battery to 1 at 3 C0 V, and their deliverable discharge capacities were compared. The dischargeable capacities of TCC and PCC bag batteries are very similar after a 1 C charge, indicating that the lithium-ion transmission speed in both battery configurations is fast enough to participate in the electrochemical reaction. However, as the charging rate increases, the capacity of the TCC pouch battery decreases rapidly. Once the charge C rate reaches or exceeds 3 C (20 minutes of charging), the capacity decreases significantly due to severe lithium depletion. Even when cycling at the same high rate, the available discharge capacity in the TCC case continues to decrease due to the irreversible capacity generated. The decrease in usable capacity is mainly due to the large overpotential-triggered Li0 plating, which leads to the generation of "dead lithium" and the formation of a reversible solid electrolyte interface (SEI) at the expense of active capacity. In contrast, at high C rates, PCC has a much higher capacity than TCC and is more stable. As shown in Figure 4B, the SoC capacity of a PCC pouch battery is 78 at 4 C (15 minutes of charging C (10 minutes of charging) and 10 C (6 minutes of charging), respectively3% (TCC. 5% (TCC is 33.)4%) and 543% (13 for TCC.)8%)。Fig. 4c and D are the charge-discharge curves of multilayer pouch batteries containing TCC and PCC. The numerical simulations in Figure 2E also confirm that the CC charging section of a PCC bag battery is much longer than that of a TCC bag battery due to the large polarization mediation of electrolyte concentration. A longer CC charging section is a key indicator of fast charging capability, as it can promote higher capacity. As a result, the use of PCC significantly increases the usable capacity of the battery. When the charge C is increased from 1 to 5, the capacity of the TCC bag battery decays exponentially, indicating that the Li+ on a particular electrode is rapidly depleted, beyond which the active material can no longer be used. In contrast, PCC shows an approximately linear decay trend, which indicates that mass transport has no significant effect on the magnification behavior below 5. However, once the C rate exceeds 5, the capacity decay rate of the PCC battery accelerates, indicating an increased level of lithium depletion. At higher C rates (10C), the authors expect to observe rapid capacity decay due to a variety of factors, including increased concentration polarization and particle breakage. The halving of the length of the effective Li+ transmission path also ensures the rapid discharge capability of the multi-layer battery. Therefore, the authors have a cut-off voltage of 3Fast discharge performance of 0 V was tested. As shown in Figure 4E, after a full charge at 4 C (15 minutes of charging), the TCC bag battery provides limited capacity at a high discharge rate of 4 C. In this process, both the charge resistance and the lithium-ion kinematics result in a large overpotential, which sacrifices the capacity of the TCC housing. In contrast, PCC pouch batteries have a much lower attenuation rate. Even with a 5C discharge, the PCC remains 52 after a 5 C charge (12 min of charging).2% discharge capacity. Fig. 4f and g are the charge-discharge curves of multilayer pouch batteries containing TCC and PCC in the rapid discharge scenario. Obviously, the overpotential in the PCC case is much smaller than in the TCC case, which is reflected in the position of the charge-discharge platform at the same C rate. 
Figure 5The DPS shows the Li0 plating during fast chargingThe authors assembled multilayer PCC and TCC pouch cells and initially activated them for two cycles at a rate of C20, after which the cells were stacked with wooden force plates and pressure sensors and clamped in a fixed-thickness vise (Figure 5A). The battery was then charged using the previous optimal CC-CV scheme, the total CC-CV time was controlled, and discharged at 1 C until the voltage reached 30 V (Figure 5b). During this process, real-time cell pressures are simultaneously recorded (Figure 5C). The change of pressure behavior is related to the charge-discharge behavior of the bag battery, and the charge-discharge behavior is mainly affected by the change of graphite volume. The substrate pressure, i.e. the pressure after full discharge, can be used as an indirect indicator of Li0 plating. The formation of "dead lithium" and residual SEIs increases the irreversible thickness of the anode, resulting in an increase in residual pressure after each cycle. The stable base pressures of TCC and PCC bag cells are below 2 and 5, respectively, indicating that Li0 plating has not occurred. However, if the base pressure increases with a further increase in the C rate (2 C for TCC and 5 C for PCC), this indicates that residual SEI and "dead lithium" have formed and accumulated due to Li0 plating. The team carried out further analysis and plotted the differential pressure DP|for TCC and PCC bag cellsdq|to study the Li0 plating behavior (Figure 5D,E). The threshold for Li0 plating was determined by selecting the maximum DP dq observed at low magnification. Since the SoC after charging is similar, the authors chose PCC at 1C and TCC at 0The maximum dp dq established at 5 C. The resting process after charging and discharging was not included in the data analysis. In terms of charging rates of 1 C (1 h of charging) and 4 C (15 min of charging), the DP|dq|Staying in the region below the threshold (blue area) indicates that a Li+ intercalation reaction has occurred at the anode. However, when the charging rate is increased to 5 C or higher, dp |dq|The curve crosses the threshold and enters the upper li0 region (orange area), indicating that li0 plating has occurred. This phenomenon is even more pronounced at high magnifications of 10 C (6 minutes of charging). As shown in Figure 5f, in the case of PCC, the peak dp of 4 is below |dq|Still in the Li+ intercalation zone. In contrast, in the TCC case, the Li0 plating phenomenon begins much earlier when the charging rate is 3 C (20 minutes of charging) (Figure 5G). Bibliographic informationye, y., xu, r., huang, w. et al. quadruple the rate capability of high-energy batteries through a porous current collector design. nat energy (