High-Performance Lead-Acid Batteries Enabled by Pb and PbO 2 Nanostructured Electrodes: Effect of Operating Temperature

: Lead-acid batteries are now widely used for energy storage, as result of an established and reliable technology. In the last decade, several studies have been carried out to improve the performance of this type of batteries, with the main objective to replace the conventional plates with innovative electrodes with improved stability, increased capacity and a larger active surface. Such studies ultimately aim to improve the kinetics of electrochemical conversion reactions at the electrode-solution interface and to guarantee a good electrical continuity during the repeated charge/discharge cycles. To achieve these objectives, our contribution focuses on the employment of nanostructured electrodes. In particular, we have obtained nanostructured electrodes in Pb and PbO 2 through electrosynthesis in a template consisting of a nanoporous polycarbonate membrane. These electrodes are characterized by a wider active surface area, which allows for a better use of the active material, and for a consequent increased speciﬁc energy compared to traditional batteries. In this research, the performance of lead-acid batteries with nanostructured electrodes was studied at 10 C at temperatures of 25, − 20 and 40 ◦ C in order to evaluate the efﬁciency and the effect of temperature on electrode morphology. The batteries were assembled using both nanostructured electrodes and an AGM-type separator used in commercial batteries.


Introduction
Improving the efficiency of energy recovery processes in relevant applications, such as the regenerative braking in vehicles, elevators and cranes, requires energy storage devices that enable faster charging and discharging compared to existing lithium-ion batteries, which are the highest energy density batteries currently available on the market [1]. Capacitors are also able to charge and discharge much faster than existing batteries, performing hundreds of thousands of charge/discharge cycles without significant loss of capacity. However, the energy storage density of capacitors is generally significantly lower compared to batteries. Lead batteries include three essential elements: sulfuric acid, used as an electrolyte, and lead and lead dioxide, used as a negative and a positive electrode. Each cell is able to supply a voltage of about 2 volts, while the current is a function of the electrode surface. The electrode surface is thus the characterizing element of this device, and several studies can be found in the literature, discussing possible solutions to obtain electrodes with a high active surface, thus improving the reactions occurring at the electrode-electrolyte interface, in order to ensure electrical continuity and an increased number of charge/discharge cycles. However, due to the low specific energy storage of these devices (about 30-40 Whg −1 ), compared to other batteries, and considering that current lead-acid batteries work in optimal conditions at a maximum discharge current (C-rate) of C/5 [2], their application is limited in advanced systems. In addition, due to the whole aqueous ZIBs system [14][15][16]. In this context, LABs are still a valid alternative, and the possibility of optimizing their performance in terms of energy density and rate of charge and discharge is still an interesting solution for the medium term.
The subject dealing with the lead-acid battery performance at HRPSoC has been extensively investigated, and different solutions were proposed, such as modifying the negative active material composition through suitable additives, designing innovative assembly with a super-capacitor or improving the energy efficiency through proper managing of the charging process [17][18][19][20][21]. The ultimate goal is to increase the energy density in order to enhance the performance of lead-acid batteries. These goals can be achieved thanks to the use of polycarbonate membranes with interconnected pores, that permit to realize nanostructured Pb and PbO 2 electrodes, through the template electrodeposition method, that are characterized by a very high active surface.
In our previous works, these types of nanostructured electrodes have been tested, obtaining excellent results [12,22,23]. In this work, we show some preliminary results obtained from tests carried out on nanostructured electrodes at different temperatures. In particular, we will show that they are able to work, even under stress conditions, at high charge/discharge rates (from 1 to 10 C) and at temperatures from −20 to 40 • C.

Materials and Methods
In previous works [4,12,22,23], we have illustrated how to grow PbO 2 and Pb nanowires (NWs) by electrodeposition in Whatman polycarbonate (PC) membrane (Cytiva, Marlborough, MA, USA) 47, which was used as a template allowing to obtain a morphology in the nanowire shape. The template electrodeposition method consists of different steps. The first step is the sputtering process, necessary to make the membrane conductive. Then, a current collector is deposited, which acts as a mechanical support and electrical contact for the nanostructures. Finally, the growth of the nanostructures takes place inside the pores of the template. The obtained morphology, therefore, is intimately related to the template used and is shown in Figure 1, for the PbO 2 nanostructured electrode in addition to the template used.
In detail, in the first step, a uniform and thick layer of PbO 2 was electrodeposited onto the Au-coated side of the membrane at room temperature under a constant current of 10 mA cm 2 , up to a charge of 80 C cm 2 . In the second step, PbO 2 nanowires were grown inside membrane channels at 60 • C, applying a constant potential of 1.5 V vs. Saturated Calomel Electrode (SCE), up to 8 C cm 2 .
The current collector was obtained through three steps, each 960 s long. The solution was replaced at the end of each step for avoiding the degradation effects and improving the deposition quality. NW growth was carried out in a single step of 330 s. Electrodeposition of both NWs and current collectors was performed with a PAR Potentiostat/Galvanostat (mod. PARSTAT 2273, Princeton Applied Research, TN, USA) connected to a PC and controlled by POWERSUITE ® software (Princeton Applied Research, TN, USA).
After electrodeposition, the polycarbonate membrane was dissolved in CHCl 3 to obtain a nanostructured electrode consisting of a nanowire array electrically connected to the current collector. A scheme of the nanostructured electrode after the fabrication process is shown in Figure 2. In detail, in the first step, a uniform and thick layer of PbO2 was electrodeposited onto the Au-coated side of the membrane at room temperature under a constant current of 10 mA cm 2 , up to a charge of 80 C cm 2 . In the second step, PbO2 nanowires were grown inside membrane channels at 60 °C, applying a constant potential of 1.5 V vs. Saturated Calomel Electrode (SCE), up to 8 C cm 2 .
The current collector was obtained through three steps, each 960 s long. The solution was replaced at the end of each step for avoiding the degradation effects and improving the deposition quality. NW growth was carried out in a single step of 330 s. Electrodeposition of both NWs and current collectors was performed with a PAR Potentiostat/Galvanostat (mod. PARSTAT 2273, Princeton Applied Research, TN, USA) connected to a PC and controlled by POWERSUITE ® software (Princeton Applied Research, TN, USA).
After electrodeposition, the polycarbonate membrane was dissolved in CHCl3 to obtain a nanostructured electrode consisting of a nanowire array electrically connected to the current collector. A scheme of the nanostructured electrode after the fabrication process is shown in Figure 2. The active mass of nanowires was evaluated by gravimetric measurements using a Sartorius microbalance (mod. Premium Microbalance ME36S, Sartorius, Germany). The loading of the PbO2 and Pb NWs on the current collector was 6.16 ± 0.5 and 3.84 ± 0.3 mg/cm 2 , respectively. The gravimetric capacity of Pb is higher than PbO2, so that the performance of the nanostructured lead-acid cell was controlled by PbO2. NWs characterization was carried out by X-ray diffraction (XRD), Energy Dispersive X-ray Spectrometry (EDS) and Scanning Electron Microscopy (SEM). In particular, to investigate the morphology and the atomic The active mass of nanowires was evaluated by gravimetric measurements using a Sartorius microbalance (mod. Premium Microbalance ME36S, Sartorius, Germany). The loading of the PbO 2 and Pb NWs on the current collector was 6.16 ± 0.5 and 3.84 ± 0.3 mg/cm 2 , Appl. Sci. 2021, 11, 6357 5 of 17 respectively. The gravimetric capacity of Pb is higher than PbO 2 , so that the performance of the nanostructured lead-acid cell was controlled by PbO 2 . NWs characterization was carried out by X-ray diffraction (XRD), Energy Dispersive X-ray Spectrometry (EDS) and Scanning Electron Microscopy (SEM). In particular, to investigate the morphology and the atomic composition of the nanostructured electrode, a FEG-ESEM FEI (mod. QUANTA 200, FEI, OR, USA), equipped with an Energy Dispersive Spectroscopy (EDS, EDAX) probe, was used. An X-ray diffractometer (mod. D-MAX 25,600 HK, Rigaku, Tokyo, Japan) was employed for XRD analyses. More details can be found in [24][25][26][27][28]. Charge/discharge cycles were carried out by means of a multi-channel cell test system (Solartron, 1470E, Ametek, PA, USA) at different temperatures. Data were acquired and processed by using MULTISTAT ® and CorrView ® software (Ametek, PA, USA), respectively.

Results and Discussion
A separate study of nanostructured lead and lead oxide electrodes is reported in [4,12]. The cell presented in this work consists of both nanostructured electrodes. In particular, the battery used for testing consists of a nanostructured PbO 2 electrode with a geometric area of about 1.1 cm 2 , a nanostructured Pb electrode with a geometric area of about 7.1 cm 2 , a commercial Absorptive Glass Mat separator (AGM) and a 5 M sulfuric acid aqueous solution as the electrolyte. Figure 3 shows a 3D scheme of the nanostructured battery. The nanostructured electrodes were assembled with zero gap configuration to simulate the similar condition of a commercial lead-acid battery, and tested at a C-rate from 1 to 30 C with a cut-off of 1.2 V. These are very stressing conditions in comparison to conventional working conditions of a commercial lead-acid battery. The lead oxide electrode was previously immersed in 5 M sulfuric acid before testing. This step is necessary to overcome its low wettability and avoid an initial high potential value that may result in electrode failure.
The active mass of the nanowires used to evaluate the nominal capacity of the electrodes was evaluated by gravimetric measurements using a microbalance. Specifically, the electrochemical tests were based on the gravimetric capacitance of PbO 2 since the positive electrode was limiting compared to the negative Pb electrode due to the smaller size of the geometric surface area. Electrochemical tests were performed in two steps. First, conditioning of the nanostructured cells at 1 C was performed. After pre-soaking, the battery was subjected to an initial charge. This charge, which is conducted according to a previously predicted multi-step current procedure [4] which consists of a gradual increase in current from C/5 up to 1 C, plays a crucial role for the mechanical stability of the nanostructured electrodes. In fact, during this step, the cell voltage drops very slowly, avoiding the formation of hydrogen and oxygen that can damage the nanostructures. In particular, this procedure was optimized to keep the cell voltage below 3 V to avoid gas evolution, which would cause mechanical stress to the electrodes and the transportation of acidic drops to the metal parts, causing their corrosion. At the end of this charge, the nanostructured electrodes were cycled at 1 C for 100 cycles and at 25 • C. This is the conditioning phase of the nanostructured lead-acid electrodes and is necessary for electrode stabilization.
The conditioning step was followed by an electron microscopic analysis using Scanning Electron Microscope to evaluate the change in electrode morphology. This analysis was performed because improved battery performance with nanostructured electrodes is associated with improved wettability of the porous mass of the nanostructured electrodes. This behavior is well-known as a peculiarity of lead-acid batteries [29,30]. In Figure 3, we showed the morphology displayed by a PbO 2 electrode before and after the conditioning step. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 18 The active mass of the nanowires used to evaluate the nominal capacity of the electrodes was evaluated by gravimetric measurements using a microbalance. Specifically, the electrochemical tests were based on the gravimetric capacitance of PbO2 since the positive electrode was limiting compared to the negative Pb electrode due to the smaller size of the geometric surface area. Electrochemical tests were performed in two steps. First, conditioning of the nanostructured cells at 1 C was performed. After pre-soaking, the battery was subjected to an initial charge. This charge, which is conducted according to a previously predicted multistep current procedure [4] which consists of a gradual increase in current from C/5 up to 1 C, plays a crucial role for the mechanical stability of the nanostructured electrodes. In fact, during this step, the cell voltage drops very slowly, avoiding the formation of hydrogen and oxygen that can damage the nanostructures. In particular, this procedure was optimized to keep the cell voltage below 3 V to avoid gas evolution, which would cause mechanical stress to the electrodes and the transportation of acidic drops to the metal parts, causing their corrosion. At the end of this charge, the nanostructured electrodes were cycled at 1 C for 100 cycles and at 25 °C. This is the conditioning phase of the nanostructured lead-acid electrodes and is necessary for electrode stabilization.
The conditioning step was followed by an electron microscopic analysis using Scanning Electron Microscope to evaluate the change in electrode morphology. This analysis was performed because improved battery performance with nanostructured electrodes is associated with improved wettability of the porous mass of the nanostructured electrodes. This behavior is well-known as a peculiarity of lead-acid batteries [29,30]. In Figure 3, we showed the morphology displayed by a PbO2 electrode before and after the conditioning step.
In detail, Figure 3a, c shows the initial top and cross-sectional view morphology of the lead oxide electrode, consisting of many nanowires that cover the overall surface of In detail, Figure 3a,c shows the initial top and cross-sectional view morphology of the lead oxide electrode, consisting of many nanowires that cover the overall surface of the electrode, and cavities that allow the electrolyte (H 2 SO 4 5 M) to penetrate and encounter a larger active surface for the charge/discharge reactions of the battery. On the other hand, in Figure 3b,d, the micrographs shown are related to the morphology of the electrode after the conditioning phase, lasting 100 cycles. In particular, in Figure 3d, the cross-sectional view of the electrode was reported, which highlights both the top of the electrode, in which the nanowires are present, and the current collector. It is important to highlight that the collector appears perfectly intact, with an identical morphology to the as-prepared one, indicating that it does not fully participate in the charge and discharge reactions that only the nanowires undergo. Figure 3b shows a morphology almost close to the initial one, where the nanowires are still evident, although they have a more pronounced roughness. In addition, lead sulfate microcrystals are also visible on the electrode surface. Based on these results, it can be concluded that nanostructures formed by template electrodeposition change their morphology during the cyclization, generating in situ a porosity that cannot be obtained by other methods. In addition, the increase in volume during discharge does not interrupt the electrolytic continuity between the active electrode material and the current collector, providing a high degree of utilization of the active material. This is perhaps the biggest difference from commercial plates, where the volume increase associated with the conversion reaction interrupts electrolytic continuity between the current grid and the far regions, lowering the degree of utilization of the active material by up to 50%, with associated performance consequences. To limit these problems, commercial plates are cycled at low speeds.
On the Pb NW electrode, after the post-conditioning step, in particular at the end of the 100th discharge, a quantitative XRD analysis was performed to determine the amount of lead and lead sulfate present in the electrode. Figure 4 shows the result of this analysis. As expected, for an almost fully discharged electrode, PbSO 4 is almost 98.9%, while Pb is 1.1%. volume during discharge does not interrupt the electrolytic continuity between the active electrode material and the current collector, providing a high degree of utilization of the active material. This is perhaps the biggest difference from commercial plates, where the volume increase associated with the conversion reaction interrupts electrolytic continuity between the current grid and the far regions, lowering the degree of utilization of the active material by up to 50%, with associated performance consequences. To limit these problems, commercial plates are cycled at low speeds.
On the Pb NW electrode, after the post-conditioning step, in particular at the end of the 100th discharge, a quantitative XRD analysis was performed to determine the amount of lead and lead sulfate present in the electrode. Figure 4 shows the result of this analysis. As expected, for an almost fully discharged electrode, PbSO4 is almost 98.9%, while Pb is 1.1%. After the conditioning step, the nanostructured electrodes were tested at different temperatures. Operating temperature is a very important factor for lead-acid batteries and can strongly influence their performance. For example, it has been observed that at high temperatures, internal resistances decrease and reaction kinetics occurring at the electrodes are favored, resulting in an increased capacity and higher energy delivered. However, high temperatures also facilitate self-discharge processes and the consequent After the conditioning step, the nanostructured electrodes were tested at different temperatures. Operating temperature is a very important factor for lead-acid batteries and can strongly influence their performance. For example, it has been observed that at high temperatures, internal resistances decrease and reaction kinetics occurring at the electrodes are favored, resulting in an increased capacity and higher energy delivered. However, high temperatures also facilitate self-discharge processes and the consequent capacity loss. If the kinetics is favored at high temperatures, it is disadvantaged at low temperatures. The reactions of the cells slow down, but in spite of this, in the charging phase, it is possible to obtain a final microporous morphology of the electrode (so a very large active surface) that allows to optimize the performance. In this work, nanostructured lead-acid electrodes were tested at 25, −20 and 40 • C, the last two being the critical temperatures for a lead-acid battery according to EN 61427-1:2013 [31], at which commercial electrodes are normally tested in order to ensure good performance even at critical temperatures. In this case, the nanostructured electrodes were tested at a C-rate of 10 C, which means charging/discharging the battery in 6 min, with a cut-off of 1.2 V and a discharge rate of 90% of the charge. A number of charge/discharge cycles of 1000 was set for these tests.
After the conditioning phase, an initial test was performed at a temperature of 25 ± 2 • C. Figure 5a shows the charging and discharging curves of the battery, while Figure 5b displays the discharge capacity and the efficiency of the nanostructured battery. Figure 5a shows a strong polarization of the electrodes in the first 50 cycles, which decreases with cyclization. The charge curves do not show peaks, meaning that there is a very limited gas development on the electrodes' surface despite the high cyclization rate. The discharge curves show an increase of the plateau with cyclization, which means an increase of usable energy of the battery. We can observe how the curves initially start at a voltage slightly below 3 V and drop to lower voltage values as the number of cycles increases. The high voltage value reached by the charging curves in the first few cycles is essentially due to the increase in cycling rate that occurs when moving from the conditioning phase to the stabilization phase. Figure 5b shows the efficiency and discharge capacity trends of the nanostructured battery. The battery reaches a maximum efficiency value of 88.7% (150th cycle) and then maintains a constant efficiency value around 76.5% throughout its life (Figure 5b).
temperatures for a lead-acid battery according to EN 61427-1:2013 [31], at which commercial electrodes are normally tested in order to ensure good performance even at critical temperatures. In this case, the nanostructured electrodes were tested at a C-rate of 10 C, which means charging/discharging the battery in 6 min, with a cut-off of 1.2 V and a discharge rate of 90% of the charge. A number of charge/discharge cycles of 1000 was set for these tests.
After the conditioning phase, an initial test was performed at a temperature of 25 ± 2 °C. Figure 5a shows the charging and discharging curves of the battery, while Figure 5b displays the discharge capacity and the efficiency of the nanostructured battery. Figure 5. Electrochemical performance of a nanostructured lead-acid battery operating at 25 ± 2 • C and 10 C: (a) charging and discharging curves of different cycles, and (b) cycling efficiency on discharging and discharge capacity. Figure 6 show the morphology of the nanostructured Pb (a and b) and PbO 2 (c and d) electrodes after 1000 cycles. The initial nanowire structure is lost due to cyclization, in favor of the formation of lead sulfate crystals. The lead sulfate crystals do not form a compact structure on the electrode surface, but rather form a highly porous structure that allows the electrodes to be subjected to additional high-speed charge and discharge cycles. efficiency value of 88.7% (150th cycle) and then maintains a constant efficiency around 76.5% throughout its life (Figure 5b). Figure 6 show the morphology of the nanostructured Pb (a and b) and PbO2 d) electrodes after 1000 cycles. The initial nanowire structure is lost due to cyclizat favor of the formation of lead sulfate crystals. The lead sulfate crystals do not f compact structure on the electrode surface, but rather form a highly porous structu allows the electrodes to be subjected to additional high-speed charge and discharge

Performance of Nanostructured Lead-Acid Electrodes at −20 • C
A VWR cryostat was used to perform the tests at different temperatures. The conditions used were the same as those set for the test of the electrodes at 25 • C: 1.2 V cut-off, a C-rate of 10 C and a total of 1000 charge/discharge cycles. Figure 7 shows the results obtained from cyclization of nanostructured electrodes at a temperature of −20 ± 2 • C. Figure 7a shows the charge/discharge curves. It can be observed that with the increase of the number of cycles, there was an increase of the plateau of the discharge curve; therefore, there was a greater area under the curve and hence a greater usable energy. From the charge and discharge curves, it can be observed that despite the severe conditions to which the battery has been subjected, from 500 • to 1000 • cycles, the behavior remains almost unchanged and it is possible to obtain a constant plateau, which means a constant capacity value of the device. The charging curves show a behavior similar to the one shown by the electrodes cycled at 25 • C. In fact, an initial voltage value of about 3.2 V has been measured, probably also in this case due to the rapid variation of the C-rate of the battery, which decreases with the increase of the number of cycles. In contrast to the battery cycled at 25 • C, in this case, the charging curves show the typical pattern of PbO 2 electrodes. At a temperature of −20 • C, an efficiency of about 50% was found, as shown in Figure 7b, compared to a maximum value of 30% found with commercial electrodes [32]. At low temperature, a porous PbSO 4 layer is formed which, during charging, is converted into a porous mass of PAM consisting of small particles (<1.5 µm) that agglomerate with each other, forming micropores. This type of structure creates a very large contact surface between the PAM and the electrolyte, ensuring good performance. After testing, both nanostructured electrodes were analyzed by SEM to evaluate their morphological changes.

Performance of Nanostructured Lead-Acid Electrodes at −20 °C
A VWR cryostat was used to perform the tests at different temperatures. The conditions used were the same as those set for the test of the electrodes at 25 °C: 1.2 V cutoff, a C-rate of 10 C and a total of 1000 charge/discharge cycles. Figure 7 shows the results obtained from cyclization of nanostructured electrodes at a temperature of −20 ± 2 °C.  Figure 7a shows the charge/discharge curves. It can be observed that with the increase of the number of cycles, there was an increase of the plateau of the discharge curve; therefore, there was a greater area under the curve and hence a greater usable energy. From the charge and discharge curves, it can be observed that despite the severe conditions to which the battery has been subjected, from 500° to 1000° cycles, the behavior remains almost unchanged and it is possible to obtain a constant plateau, which means a  The observed morphology confirms the trend of the charge and discharge curves reported above and the possibility of a better diffusion of the electrolyte inside the active mass, thus obtaining better performance and longer life times despite the critical conditions to which the electrodes have been exposed.
PbO2 nanostructured electrode, and the presence of sulfate agglomerates (of similar shape) with a size of about 10-12 μm distributed all over the surface can be visualized, but in particular, a highly porous surface under the agglomerates can be distinguished.
The observed morphology confirms the trend of the charge and discharge curves reported above and the possibility of a better diffusion of the electrolyte inside the active mass, thus obtaining better performance and longer life times despite the critical conditions to which the electrodes have been exposed.

Performance of Nanostructured Lead-Acid Electrodes at 40 • C
The tests at 40 ± 2 • C were performed under the same conditions used for the previous tests ( Figure 9).
As the number of cycles increases, the plateau of the discharge curve increases. When the 200th cycle is reached, the discharge capacity assumes a constant value, while the discharge curves reach a final voltage of about 2.1 V. Despite that the kinetics of the cell is accelerated by the high temperature, the charge curves do not present any peaks in their trend, which means that even at 40 • C, there is no excessive production of gas at the electrode-electrolyte interface, a phenomenon that could cause the breakage of the electrodes and therefore of the entire battery. The results obtained from the tests showed a battery efficiency of 90%, as shown in Figure 9b. Due to the high temperature, which promotes the reaction kinetics, the nanostructured electrodes reached a high-capacity value that allows them to store more energy than conventional systems under the same operating conditions.

Performance of Nanostructured Lead-Acid Electrodes at 40 °C
The tests at 40 ± 2 °C were performed under the same conditions used for the previous tests ( Figure 9). As the number of cycles increases, the plateau of the discharge curve increases. When the 200th cycle is reached, the discharge capacity assumes a constant value, while the discharge curves reach a final voltage of about 2.1 V. Despite that the kinetics of the cell is accelerated by the high temperature, the charge curves do not present any peaks in their trend, which means that even at 40 °C, there is no excessive production of gas at the electrode-electrolyte interface, a phenomenon that could cause the breakage of the electrodes and therefore of the entire battery. The results obtained from the tests showed a battery efficiency of 90%, as shown in Figure 9b. Due to the high temperature, which promotes the reaction kinetics, the With temperature, the morphology of the electrode changes: at high temperatures, the PbSO 4 layer becomes more compact, as reported in the literature [2,33,34]. After testing, both nanostructured electrodes were analyzed by SEM to evaluate their morphology, as shown in Figure 10a-d. The morphology of the Pb nanowires was significantly changed from the original pattern. The absence of nanowires and the presence of large porous agglomerates about 15 µm in size were observed. The morphology of the PbO 2 electrode is shown in Figure 10c,d. The initial NW-shaped morphology was completely lost in favor of the formation of PbSO 4 crystals. However, the electrode still has a porous morphology that allows the electrolyte to penetrate the active part and thus allows the battery to operate for further charge and discharge cycles at high C rates. significantly changed from the original pattern. The absence of nanowires an presence of large porous agglomerates about 15 μm in size were observed morphology of the PbO2 electrode is shown in Figure 10c,d. The initial NW-s morphology was completely lost in favor of the formation of PbSO4 crystals. How the electrode still has a porous morphology that allows the electrolyte to penetra active part and thus allows the battery to operate for further charge and discharge at high C rates.  Figure 11 shows a comparison of the battery response at different temperatu all cases, the 500th cycle was reported. The battery maintained a voltage of below all cases, and had no peaks indicating gas production at the electrode surface.  Figure 11 shows a comparison of the battery response at different temperatures; in all cases, the 500th cycle was reported. The battery maintained a voltage of below 3 V in all cases, and had no peaks indicating gas production at the electrode surface. In the discharge curves, a gradual increase in the plateau of the curves can be seen as the temperature increases. This increase denotes a higher usable energy of the battery, and thus higher efficiency [35]. In particular, for the discharge curve relative to the temperature of 40 ± 2 °C, the cut-off is no longer achieved. This means that the efficiency of this battery has reached In the discharge curves, a gradual increase in the plateau of the curves can be seen as the temperature increases. This increase denotes a higher usable energy of the battery, and thus higher efficiency [35]. In particular, for the discharge curve relative to the temperature of 40 ± 2 • C, the cut-off is no longer achieved. This means that the efficiency of this battery has reached 90% and thus all the battery capacity can be converted into usable energy. The discharge curves remained constant over the whole cycling duration at the three temperatures, but different efficiency levels were reached. In particular, battery efficiency increases with temperature, and the maximum efficiency achieved at −20 ± 2 • C was about 50%. This is a very satisfactory result when compared to the maximum efficiency value of conventional lead-acid batteries, which is 30% under optimum operating conditions. The summarized cyclization results at various temperatures are shown in Table 1. The maximum efficiency increases with temperature; in fact, an efficiency value of 90% was reached at a temperature of 40 ± 2 • C.  Figure 12 shows, for a better comparison, the SEM images of the Pb (Figure 12a-c) and PbO 2 (Figure 12d-f) nanostructured electrodes after testing at different temperatures. In Figure 12a-c, the Pb electrode is shown and the formation of lead sulfate crystals of different sizes is clearly visible on the electrode surface. In particular, there was a decrease in the average grain size with decreasing temperature. However, the post-cycling electrode surface at different temperatures remained highly porous, which allows the electrolyte to penetrate the active mass and the electrodes to be used for further charge/discharge cycles at high C rates.

Comparison of the Results Obtained from Nanostructured Electrodes Cycled at Different Temperatures
The post-cycled PbO 2 electrodes, shown in Figure 12d-f, have a widely varying morphology as the operating condition changes. In particular, in Figure 12e, showing a top view of the nanostructured PbO 2 electrode at −20 • C, the presence of sulfate agglomerates (of similar shape) can be observed, with sizes of approximately 10-12 µm distributed over the entire surface, but in particular, a highly porous surface under the agglomerates can be distinguished. The particle size and porosity of lead sulfate is strongly dependent on the discharge conditions, but also temperature, current density, solution concentration and depth of discharge can all affect the lead sulfate crystal structure, leading to a different life cycle.
The result of this morphology confirms the trend of the charge and discharge curves shown above and confirms the possibility of diffusion of the electrolyte within the active mass, thus achieving better performance and longer life times despite the critical conditions to which the batteries have been subjected.
In all analyzed cases, the presence of small sulfate crystals distributed over the entire electrode surface can be observed. From the micrographs of the nanostructured lead and lead oxide electrodes, it can be seen that the formation of lead sulfate on the electrode surface never occurs as a compact layer, but rather as a mixture of granules that ensure a good porosity for the electrode, which allows battery operation above 1000 charge/discharge cycles. electrolyte to penetrate the active mass and the electrodes to be used for further charge/discharge cycles at high C rates. The post-cycled PbO2 electrodes, shown in Figure 12d-f, have a widely varying morphology as the operating condition changes. In particular, in Figure 12e, showing a top view of the nanostructured PbO2 electrode at −20 °C, the presence of sulfate agglomerates (of similar shape) can be observed, with sizes of approximately 10-12 μm distributed over the entire surface, but in particular, a highly porous surface under the agglomerates can be distinguished. The particle size and porosity of lead sulfate is strongly dependent on the discharge conditions, but also temperature, current density, solution concentration and depth of discharge can all affect the lead sulfate crystal structure, leading to a different life cycle.

Conclusions
In this work, the performance of nanostructured lead-acid electrodes has been investigated at different operating temperatures. The results obtained show the potential of this technology in improving the performance of such storage systems, therefore allowing the development of new generations of lead-acid batteries able to overcome the limits of traditional ones.
The nanostructured electrodes have been tested at a rate of 10 C at three different temperatures. In all cases, the nanostructured cell provided excellent results. At a temperature of 25 • C, the battery reached a mean efficiency value of about 76.5% throughout its lifetime, while at a temperature of 40 • C, an efficiency of 90% was achieved up to over 1000 cycles. The discharge curves showed a plateau that increases with the number of cycles, which results in higher utilizable energy. At temperatures as low as −20 • C, 50% efficiency was obtained, which is higher than conventional lead-acid batteries that reach a maximum of 30% at a lower C rate. Post-cycling morphology of the electrode showed a variegated shape formed by lead sulfate agglomerates of the order of about 10 nm covering the electrode surface. The micrographs showed, mostly for the tests carried out at −20 • C, very small and non-compact structures, characterized by a high porosity that ensures the penetration of the electrolyte in the internal areas, thus ensuring the operation of the nanostructured electrodes for many cycles.
Due to the nanostructured base surface, it has been possible to increase the power density and the battery life, and to improve the efficiency of the system even at a high number of cycles and at critical temperatures.