External field–assisted batteries toward performance improvement

Rechargeable batteries are essential for the increased demand for energy storage technologies due to their ability to adapt intermittent renewable energies into electric devices, such as electric vehicles. To boost the battery performance, applying external fields to assist the electrochemical process has been developed and exhibits significant merits in energy efficiency and cycle stability enhancement. This perspective focuses on recent advances in the development of external field–assisted battery technologies, including photo‐assisted, magnetic field–assisted, sound field–assisted, and multiple field–assisted. The working mechanisms of external field–assisted batteries and their challenges and opportunities are highlighted.


INTRODUCTION
The ever-increasing global energy demand for electric vehicles (EVs) and the urgent need to implement a low-carbon power transition to tackle global warming accelerate the deployment of sustainable energy resources, including wind and solar, for future energy supply. Developing efficient and cost-effective energy storage technologies to balance intermittency and the regional nature of sustainable energy is the key bottleneck toward the widespread use of these energy resources and the development of EVs. As a widely known energy storage technology, lithium-ion batteries (LiBs) dominate the market of rechargeable batteries, but their maximum specific energy density presents a limitation. 1 High energy density lithium-O 2 (Li-O 2 ) and lithium-sulfur (Li-S) batteries have attracted tremendous attention to solving this issue. [2][3][4][5] Much effort has been dedicated to exploring underlying mechanisms, leading to exciting in-depth understanding and key experiment achievements toward the reversibility of Li-O 2 and Li-S batteries. In addition, concerns over cost, safety, and sourcing of raw materials drive the emergence of aqueous rechargeable batteries, for example, zinc-based batteries 6 and redox flow batteries. 7 Although significant contributions and efforts have been devoted to fundamental understanding and novel materials designed for these rechargeable batteries, some challenges still limit their battery performance. For example, in Li-and Znbased batteries, 8,9 both the safety and lifetime of the batteries are challenged by severe dendrite growth; in air batteries, 2 due to the phase changes between gaseous O 2 and solid discharge product, their round-trip efficiency is limited by sluggish kinetics. Additionally, the shuttle effect of polysulfide anions hinders the practical realization of Li-S batteries. These necessitate exploring new approaches to intrinsically solve the battery instability and low energy efficiency to enhance electrochemical battery performance. The application of external fields to the battery system as a new and efficient strategy has shown its capability to improve battery performance. Solar energy (light) is the most studied external field of external fieldassisted batteries. Light-assisted metal-air batteries have been explored since 2014 to solve the high overpotential and poor round-trip efficiency resulting from sluggish reaction kinetic at the oxygen cathode. Light-assisted Li-O 2 was the first developed photo-assisted metal-air battery, which was achieved by coupling a redox-couple I 3 − /I − with a built-in dye-sensitized TiO 2 photoelectrode with an oxygen electrode, an ultralow charge voltage of 2.72 V can be observed under illumination. 10 Gradually, using light to address the high reaction barriers has extended to various metal-air/CO 2 batteries, for example, Li-CO 2 , 11,12 and zinc-air. 13,14 With tremendous efforts devoted to constructing photo-assisted rechargeable batteries based on two-electrode systems, the photo-assisted high-performance Li-S, 15 Li-I 2 , 16 and Li-organic 17 batteries have also been successfully fabricated recently. The external magnetic field has also attracted attention due to the potential to solve the dendrite growth problem and improve battery performance. In 2019, many groups started to get interested in using an external magnetic field to eliminate Li dendrite and achieve uniform lithium deposition in Li-based batteries. [18][19][20] Subsequently, using the magnetic field to inhibit metal dendrites was confirmed to be equally feasible in other metal-based battery systems, for example, Zn-based batteries. 21,22 Recently, the magnetic field has also exhibited its power in preventing the shuttle effect of polysulfide. 23 Besides, some niche external field-assisted batteries, for example, sound-assisted and multiple field-assisted rechargeable batteries, have emerged as increasingly potential strategies for battery performance improvement in the last few years. [24][25][26] When an external field, such as light, 27,28 magnetic, 29 or sound, 24,25 is introduced, the corresponding energy can be incorporated with the battery electric field to regulate electrode reaction kinetics and mass transportation, thus leading to breakthrough achievements in battery systems. Taking photo-assisted Li-O 2 batteries as an example, the intro-duction of light can induce semiconductor cathode to generate photoelectrons and holes. 30 Then, the generated electron-hole pairs can participate in the oxygen reduction and evolution reactions, simultaneously realizing energy storage and photo-energy conversion in one device and significantly increasing battery energy efficiency. In this perspective, we focus on research developments and ongoing challenges on external field-assisted batteries. The fundamental theories, characteristics, battery configurations, and research progress on materials design of different external field-assisted batteries, including light/photoassisted, magnetic field-assisted, sound field-assisted, and multi-field-assisted, are discussed ( Figure 1). In discussing each external field-assisted battery, representative works are provided for a comprehensive understanding of its effect on the battery performance. Finally, the key challenges and perspectives for each external field-assisted battery are highlighted.

PHOTO-ASSISTED BATTERIES
As one of the external field-assisted batteries, photoassisted batteries have attracted extensive research interest due to combining the advantages of photovoltaic technologies and rechargeable batteries. 31,32 The application of light in rechargeable batteries realizes the solar energy conversion and energy storage simultaneously in one device, significantly improving battery energy efficiencies and bringing a new opportunity for developing a highly efficient battery. A comprehensive understanding of the work features and electrochemical behaviors of photo-assisted batteries is in high demand for future developments. In a typical photo-assisted battery ( Figure 2), quartz chips or sealed glass cells are required to pass light. In addition, photovoltaic/semiconductor materials are indispensable in response to light to generate photoelectrons (e − ) and holes (h + ). So far, several semiconductor materials, 31 such as Fe 2 O 3 , 14,33 C 3 N 4 , 30,34 and heterostructure CdS-TiO 2 , 15 have already been successfully applied to various battery systems, for example, Li-O 2 , 30,32 Zn-air, 13,14 Li-S, 15 and Li/Zn-I 2 . 16,35 According to the role of the semiconductor materials, the work mechanisms of the photo-assisted batteries can be summarized into two cases, as illustrated in Figure 2. In the first case (see Figure 2A-C), the semiconductor and battery-active materials are different materials. The semiconductor materials and conducting carbon are put together to make photoelectrode. Upon lighting, the photoelectrode is excited and generates a photoelectron-hole pair in the conduction band (CB) and valence band (VB). These generated photoelectrons and holes then participate and drive the corresponding redox reactions in the discharge and charge processes, The schematic of batteries with different external field assists.
F I G U R E 2 Schematic illustration of the photo-assisted rechargeable battery architecture and working principle: (A) schematic diagram of photo-assisted battery with photoelectrode for light absorbing and active material (e.g., O 2 and CO 2 ) for redox reaction; (B and C) the proposed mechanism of the photo-assisted discharge-charging process under illumination; (D) schematic diagram of photo-assisted battery with photoactive cathode materials; (E) the proposed mechanism of the photo-assisted charging process under illumination; (F) schematic energy band diagram of reported photoactive materials, hole-blocking materials, conducting materials, and so on. M: metal anode. A: active material. PA: photoactive material. TM: transition metal. respectively ( Figure 2B,C). Ideally, the generated holeelectron pairs can act on both the discharge and charge processes when the theoretical potential of the electrochemical reaction sits between the CB and VB potentials. Specifically, during the discharge process, due to the more negative CB potential of the photoelectrode than the theoretical redox potential of the battery systems, photogenerated electrons in the CB could induce the reduction of active materials to discharge products, for example, O 2 in Li-O 2 batteries and CO 2 in Li-CO 2 batteries to O 2 − and C 2 O 4 − , respectively, and followed by disproportionation or conversion to discharge products Li 2 O 2 and Li 2 CO 3 with the combination with Li + . Meanwhile, the more positive VB potential incites the reduction of photogenerated holes by the electrons from anode materials through the external circuit. The participation of photoelectrons can change battery discharge voltage to a position that exceeds the theoretical redox potential in the dark, that is, the potential difference between VB and anode materials. In the reverse charging process, the decomposition/oxidation of the discharge products to metal ions is driven by generated holes with highly oxidative capability in the VB. Simultaneously, the photogenerated electrons transfer to the anode to reduce the metal ion to metal. The charging voltage can be lowered to the potential difference between the CB and the potential of the anode. Under illuminating and the participation of semiconductor materials, the conversion and storage of solar energy get involved during discharge and charge without changing the net reaction of the battery systems. The introduction of solar energy facilitates the reaction kinetics and yields a high roundtrip efficiency in photo-assisted batteries. It is worth noting that if the battery systems incorporate redox mediators, for example, charging mediators, they will be oxidized by holes prior to the discharge products. Then the oxidized charging mediator chemically oxidizes the discharge products. In the second case, generally found in photoassisted metal ion batteries (also called photo-rechargeable batteries), the semiconductor materials, usually transition metal oxide (TM x O y ), 36 transition metal sulfide (TM x S), 37 or organic materials, can perform both solar light conversion and ion storage, here defined as photoactive materials (PA) ( Figure 2D). The photoelectrode was prepared by coating the mixed semiconductor materials, hole-blocking materials, and conductive additives (e.g., reduced graphite oxide) on a carbon felt collector or synthesizing by layerby-layer deposition of these materials on the current collector. The addition of the hole-blocking materials and conductive additives offers a favorable energy pathway of photoexcited electrons from the photoactive material to the current collector, in which hole-blocking materials can trap holes at the same time, reducing the recombination of the charges. The working mechanism and energy band of reported electrode materials of PA-based photoassisted battery are illustrated in Figure 2E,F. 17,[36][37][38][39][40][41][42][43] In the light charging process, the photogenerated holes can oxidize the discharge state products (intercalated with metal ions, M-PA) and release the metal ions (e.g., Li + and Zn 2+ ). At the same time, the photogenerated electrons are transported from photoactive materials to the current collector through hole-blocking material. Finally, they accumulate on the anode to reduce metal ions to metal ( Figure 2E). The performance improvement of photo-assisted batteries is intuitively reflected in the polarization voltage and capacity. In metal-air batteries, such as Li-air and Zn-air batteries, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial and require prompt solutions. Liu et al. presented a photo-assisted Li-O 2 battery with integrated in situ growing graphitic carbon nitride (g-C 3 N 4 ) on carbon paper as an oxygen electrode and photoelectrode. 34 The participation of solar energy compensates for the required charging voltage of the OER process, significantly lowering the charge voltage to ∼1.9 V, much lower than the theoretical potential of conventional Li-O 2 (2.96 V). The significantly reduced voltage is highly associated with the CB potential of 1.7 V versus Li/Li + on g-C 3 N 4 . Several other semiconductor materials, for example, TiO 2 and ZnS, have also been applied to constructure photo-assisted Li-O 2 batteries. 44 Almost all can efficiently reduce the charging voltage below 3.0 V. Recently, a bifunctional CeVO 4 was explored as the photoelectrode in photo-assisted Li-O 2 batteries. 45 Although the achieved charge potential is slightly higher than 3.0 V, it has excellent electrocatalysis ability, significantly increasing the discharge capacity to 1.6 times. Liu et al. introduced semiconductor material α-Fe 2 O 3 as an air electrode, achieving 1.43 V low charging potential in alkaline Zn-air systems (theoretical 1.65 V). 14 In addition to metal-air systems, solar energy has also exhibited its power in Li-CO 2 , Li-S, Li-organic, metal ion batteries, and so on. For example, Li et al. demonstrated a high discharge voltage (2.77 V) and low charge voltage in a photo-assisted Li-CO 2 battery by using SiC/RGO as photoelectrode. 11 Guan et al. presented an ultrahigh photo-assisted Li-CO 2 with the adoption of well-designed In 2 S 3 @CNT/SS (ICS) as photoelectrode for CO 2 reduction and evolution. 12 Different from gas battery systems with fixed cycle capacity, the improvement of electrochemical performance of photo-assisted metal ion, metal-sulfur, metal-organic, and so on batteries can also be reflected in the discharge and charge capacities. For example, the generated photoelectrons by CdS-TiO 2 under illumination can boost the conversion of polysulfides to Li 2 S in photo-assisted Li-S batteries. The specifics are the visibly increased specific discharge capacity, about 300 mAh g −1 higher than in dark conditions. 15 Upon illumination, the generated charges in photo-assisted batteries not only allow for photocharging of the batteries but can also help increase the capacity and lower the electrochemical reaction barriers.
The additional energy provided by solar put forward a new pathway for developing high-efficiency rechargeable batteries. Currently, most of the reported studies focus on increasing round-trip efficiency and capacity; however, less attention has paid to the rate performance, long-term photostability of battery components, the cycle life under practical application conditions, and so on. Further development of photo-assisted batteries with more attention focused on these yet-to-be-fully clarified points will deepen the understanding of the light field in energy storage systems.

MAGNETIC FIELD-ASSISTED BATTERIES
Magnetic field as a simple and contactless field has been widely used in photocatalysis to enhance photocatalytic efficiency in the past decades. 46 The applied magnetic field can affect the light absorption, charge carrier separation, and surface redox reaction of photocatalysis reaction. 47 However, the introduction of magnetic-field into rechargeable batteries has recently become an attractive topic for boosting battery performance. Although some achievements have been made, for example, dendrite formation inhibition 18,19,21 and polysulfide shuttle effect mitigation, 48,49 the battery reaction mechanism and long-term stability under the coexistence of battery electric field and magnetic field have yet to be thoroughly studied and understood. An in-depth understanding of magnetic field effects and the challenges can lead to significant technological breakthroughs in magnetic field-assisted batteries.
The basic structure of a magnetic field-assisted battery is shown in Figure 3. An electromagnet or permanent magnet is placed around the battery or on one side electrode to provide a magnetic field. The intensity of the magnetic field can be adjusted by either varying the electric current of the electromagnet device or adjusting the distance between magnet and the electrode. A simple architecture to obtain a magnetic field for a magnetic field-assisted battery is integrating a battery with a permanent magnet (e.g., neodymium iron boron magnet NdFeB) as cell components without using an electromagnet device placed outside the assembled batteries. 19 The magnetic field, in general, is static and applied parallel or perpendicular to the cell current density direction in magnetic fieldassisted batteries. With an applied magnetic field, the main force/effect that acts on the battery is the Lorentz force, which can be triggered when the magnetic field (B), and the charge species with current density j direction are nonparallel. In the experiment condition, even when the applied B is parallel to current density j, the current density is inevitably deflected at the edge of the electrode. The changes in trajectory can cause the moving charge species cut the magnetic lines and induce primary magnetohydrodynamic (MHD) with the Lorentz force as the driving force ( Figure 3A). 50 In addition, the existence of protuberances or bubbles on the electrode can also alter the current direction, leading to the formation of secondary micro-MHD ( Figure 3A). 50 The MHD effect can promote the battery mass transfer process and facilitate the uniform shape of deposited morphology, thereby benefiting the battery performance with much-improved energy efficiency, capacity, and cycling stability. It is worth noting that in the reported magnetic field-assisted sulfur-based systems, 48,49,51 magnetic particles, such as Fe 2 O 3 and carbonyl iron, are typically used ( Figure 3B). These magnetic particles, containing vacant electron d-orbital Fe atom, can form strong bonds with polysulfide species. Meanwhile, an external magnetic field can make these particles magnetized and subject to the Lorentz force, making the ability to trap polysulfide species more effective and dramatically mitigating the shuttle effect.
For a more intuitive and in-depth understanding of the effect of the magnetic field on battery systems, we further highlight the advantages and challenges of magnetic field-assisted batteries. Simultaneously, we point out the magnetic field effect on cell performance by evaluating the charge-discharge capacity values, the Coulombic efficiency (CE), and the cycling stability.

Suppressing dendrite growth
The maximum capacity of metal-based batteries (e.g., Li, Na, and K) can only achieve by using metal as an anode. However, the uncontrollable dendrites formation in these battery systems is easy to pierce the separator and causes short circuits of the batteries, resulting in poor cycle performance and safety concerns, challenging the application of metal-based batteries. Interestingly, the formation of dendrites can be effectively suppressed by the Lorentz force with the involvement of a magnetic field. Both magnet-containing coin cell. 19 Using a magnetic field to inhibit the dendrite growth has also been proven effective in aqueous Zn metal-based batteries. 21 As stated previously, the interaction of magnetic field and electric field can create MHD effects in an electrochemical cell, which makes the movement of charge species (e.g., Li + and Zn 2+ ) at the electrode surface subject to the Lorentz force. As a result, the mass transport process is accelerated, and the distribution of metal ions becomes more homogeneous; the formation of dendrites is thus significantly inhibited ( Figure 3A). Chen et al. revealed that when the magnetic field increase from 0 to 0.8 T, the diffusion coefficient and steady-state current of Li + increase nearly three and four times, respectively. 52 In addition, the calculated concentration gradient of Li + significantly decreases with increasing magnetic field strength. Both the increased mass transports and decreased concentration gradient of Li + quantitatively demonstrate the potential of magnetic field on suppressing dendrite formation. Comparing the electrochemical performance with the case without a magnetic field, the cycle life and stability in symmetrical cells exhibit more than two times improvement, either Li anode 18,19 or zinc anode. 21 In addition, the homogeneous deposition behaviors under magnetic fields also enhance solid electrolyte interphase (SEI) stability and minimize electrolyte consumption, facilitating the realization of high CE, low overpotential, and improved rate capability in both half and full cells. 8,18,21 Using an external magnetic field has been proven to exemplify the vast potential of suppressing anode dendrite growth in the development of metal-based battery systems. Recently, Jiang et al. reported that adding thermoresponsive electrolytes into lithium metal batteries (LMBs) could increase thermal runaway starting temperature of more than 170 • C, which has shed light on the development of thermally safe batteries. 53 However, severe dendrite formation is still observed. Using a magnetic field to inhibit Li dendrites should be effective in this LMB. Future works on applying a magnetic field to battery systems with high safety and thermal stability but suffer from severe dendrite formation issues are also worth exploring.

Mitigation shuttle effect
The polysulfide dissolution and shuttle between cathode and anode have been considered one of the culprits in sulfur-based batteries' performance deterioration, for example, severe anode corrosion, fast capacity decay, and poor cycle life, challenging the application of high natural abundance, cost-effective, and environment-friendly sulfur materials. Several strategies are proposed to regulate or inhibit the behavior of polysulfide in sulfur-based batteries, 54 such as redox mediators 55,56 and applying sulfur container di(tri)sulfide polyethylene glycol. 57 Applying an external magnetic field is a newly emerging strategy and has shown its ability to trap polysulfide and alleviate the shuttle effect, contributing to enhanced battery performance. The reported magnetic field-assisted batteries often require the participation of magnetic particles to respond magnetic field, as shown in Figure 3B. Magnetic nanoparticle γ-Fe 2 O 3 was first used to demonstrate the effect of the external magnetic field in a semiliquid lithium polysulfide (Li-PS) battery. 48 When an external magnetic field was applied, these superparamagnetic γ-Fe 2 O 3 nanoparticles can be attracted to the magnetic field and make polysulfide concentrated close to the current collector, thus leading to the high utilization of polysulfide, minimal shuttle effect, and enhanced capacity in the battery. 48 Recently, soft magnetic particles carbonyl iron powders (Fe(CO) 5 ) with dual effects of chemical adsorption and response to the magnetic field were introduced to Li-S batteries. 49 In the presence of an external magnetic field, these soft magnetic particles added to the cathode side could be raised into the electrolyte by the Lorentz force and trap the polysulfide around themselves, leading to a limited dissolution of polysulfide in the electrolyte and alleviating the shuttle effect. 48 Compared with the condition without an external magnetic field, the battery performance, especially CE and specific capacity, can be markedly improved. Lately, Zhang et al. employed cobalt sulfide-based sulfur host CNF/CoS x in Li-S batteries and demonstrated that in the presence of an external magnetic field, the lithium polysulfide adsorption ability could be significantly improved. 23 In addition, the Li and S conversion reactions under a magnetic field were also mechanistically investigated in this work. Theoretical results show that in the presence of a magnetic field, the electron spinpolarization in CoS x could weaken the Li-S bond and the largest step in Gibbs free energy change from Li 2 S 4 -Li 2 S 2 to Li 2 S 6 -Li 2 S 4 . As a result, the achieved magnetic-assisted Li-S batteries increased discharge capacity and unprecedented cycle stability. Although not much effort has been devoted to magnetic field-assisted sulfur-based batteries, the positive effect of the magnetic field has been confirmed. Further studies on the underlying mechanism and magnetic nanoparticle developments can provide new insights and possibilities for developing magnetic field-assisted sulfur-based batteries.

Guiding bubble motion
ORR and OER are at the heart of metal-air batteries. During the OER, the electrochemically generated oxygen bubbles in the electrolyte are quickly coalesced and adsorbed on the electrode surface, which is the fundamental issue influencing the battery performance. Thus, effectively eliminating the possibility that the gas gener-ated adheres to the electrode can avoid the performance deterioration caused by increased resistance. According to molecular orbital theory, oxygen has two unpaired electrons, which makes oxygen has paramagnetic nature and can be strongly attracted by the magnetic field. This oxygen characteristic facilitates the application of a magnetic field to control or guide the motion of oxygen bubbles in an electrochemical field. [58][59][60] The generated oxygen bubbles can be significantly removed by a magnetic field-induced MHD before they coalesce, as shown in the three-electrode metal-air battery ( Figure 3C). Using a magnetic field to guide bubble motion has been successfully demonstrated in a three-electrode Zn-air battery, which consists of an ORR catalyst-containing electrode for discharging, a charging electrode for the OER process, and a zinc anode. 59 The trajectory of bubbles generated on OER electrode can be adjusted under the magnetic field, which disrupts the coalescence of gas bubbles in the electrolytes. The participation of an external magnetic field forces the bubbles to move toward a specific direction, which is explicitly closely related to the direction of the magnetic field. In this case, the electrode surface coverage is significantly reduced, and the mass transfer is also greatly enhanced due to the rotational motion of the bubbles, which significantly improves the battery energy efficiency and cycle stability. 59 However, the positive effect of the magnetic field on the air battery is merely confirmed in the three-electrode aqueous Zn-air system. Future developments on two-electrode Zn-air batteries should be explored to simplify the battery structure. Most importantly, further explorations on the electrochemical performance of magnetic field-assisted high energy density aprotic metal-air batteries will provide a new perspective in the development of magnetic field-assisted batteries. Aside from the abovementioned positive effect of the magnetic field on batteries, which mainly concentrates on Li-and Zn-based systems with liquid electrolytes, the magnetic field has also shown its power in redox flow batteries and all-solid-state batteries. 61,62 For example, in nonaqueous iron-vanadium redox flow batteries, the applied magnetic field can act on paramagnetic Fe and V ions and make them undergo the Lorentz force, which can markedly enhance the mass transfer in the electrolyte. 61 Moreover, the charge and discharge performances of this Fe-V redox flow battery system in terms of cycle number and energy efficiency have been improved. In all-solidstate LMBs, the magnetic field can enhance the diffusion of Li ions inside garnet-type solid electrolytes, improving the total ionic conductivity of the solid electrolyte. 63 Meanwhile, same as liquid-based LMBs, the magnetic field can also prevent lithium dendrite formation in all-solidstate batteries owing to the MHD effect, improving the Li symmetric cell cycle performance.
Although there have obtained important gains in magnetic field-assisted batteries, the development of magnetic field-assisted battery is still in its early stages, and continuous efforts are needed in this field to develop highperformance batteries. In-depth mechanism studies (e.g., SEI formation mechanism and composition) on magnetic field-assisted batteries will help advance the understanding of battery electrochemical behavior under a magnetic field.

SOUND FIELD-ASSISTED BATTERIES
In addition to using light and magnetic fields to assist the physical coupling inside the batteries, some alternative external fields, for example, sound field, have recently begun to be mechanism explored to improve the battery stability and long-term cycling performance. The sound field can be divided into several types according to the frequency range.

Ultrasonic-assisted
It has been well reported that ultrasonic vibration is an efficient method to improve catalytic performance in many fields, such as the photocatalytic performance of semiconductors and CO 2 reduction. However, introducing ultrasonic vibration into rechargeable batteries to boost battery performance has rarely been explored. In retrospect, the ultrasonic in the battery was first applied to zinc-alkaline battery in 2013 to enhance the battery performance. 64 Since then, only a few efforts have been devoted to this sound field-assisted battery. Generally, ultrasound-assisted batteries can be easily realized by placing a well-sealed battery in an ultrasonic cleaner/sonicator that usually operates with a frequency in the range of 20-40 kHz ( Figure 4A). 24,64 The strength and duration of ultrasonic energy can be adjusted by controlling the frequency, duty cycles (a ratio of time the ultrasonic vibration is on and off), and ultrasonic power. The application of ultrasound in electrochemistry can induce three different mechanisms, including acoustic streaming, acoustic cavitation, and microjet/shockwave formation, to influence the flow of the liquid. 65 Recently, Zhang et al. successfully demonstrated that the involvement of ultrasonic vibration could achieve rapid mass transfer in Li-O 2 batteries and positively affects charging overpotential and battery cycling stability. 24 By applying intermittent ultrasonicassisted charging with optimized parameters to the battery every few dozen cycles, the cycle life of Li-O 2 batteries can be significantly extended to hundreds of cycles.
Mechanistically, ultrasonic vibration in ultrasonic-assisted Li-O 2 can cause disturbance of electrolyte and trigger the exchange of electrolyte on the electrode surface, which largely increase the mass transfer rate inside a battery, thereby promoting the rapid decomposition of discharge product Li 2 O 2 and eliminating the accumulation of the byproduct ( Figure 4A). As a result, ultrasonic-assisted charging shows a reduced charging overpotential and improved cycle stability compared to regular battery charging. Using ultrasonic energy provides a new direction to improve battery performance. However, this technology suffers from an obvious disadvantage that challenges its feasibility in practical batteries, that is, the high input ultrasonic power, for example, 675 W, to effectively reduce charging overpotential in Li-O 2 , which increases energy consumption. 24 Further strategies to lower the input power would enhance its competitive edge in external field-assisted batteries. On top of that, a short period of ultrasonic vibration may not be able to promote diffusion in the battery effectively. However, a long period of ultrasonic with high input power may cause cell temperature increase and intense sonic agitation-induced erosion. To fully understand the potential of this sound field strategy and make it more competitive, the investigation and understanding of the temperature change and materials compatibility of the cells would be required. It is, however, interesting to note that in an operando observation of metal dendrites under a quasi-zero electrochemical field via applying ultrasonic wave, even with 1 min of low-power 12 W sonication, most of the Na dendrite can be removed. 66 Future attention can also be paid to the effect of ultrasonic on anode morphology and SEI stability in ultrasonic-assisted batteries, such as ultrasonic-assisted sodium-based batteries.

Surface acoustic wave-assisted
Ultrasonic-assisted battery requires large and heavy ultrasonicators to generate ultrasonic vibration, which is unsuitable to be integrated into practical rechargeable batteries. In addition, the high input power also limits its practical application. Developing alternative sound field technology with controllable device size and high-power density to accelerate the electrolyte fluid is of important practical significance. Surface acoustic wave (SAW) devices that sit at high-frequency levels up to MHz to GHz can be designed in fingernail-sized and drive acoustic steaming-induced flow up to 1 m S −1 , which is expected to open the prospect of sound field-assisted batteries. 25,67 The SAW was first used in LMB by Huang et al. to prevent dendrites and solve the protracted charge time and cycle life problems in rechargeable LMBs. 25 In the reported SAW-assisted battery, the fabricated SAW device with an anti-electrolyte reaction coating can be integrated into the cell. More specifically, the SAW device can be placed on one side of the cell housing and perpendicular to the electrode gap. 25 The generated acoustic steaming from SAW devices provides a route to enhance the battery performance. As shown in Figure 4B, compared with traditional Li metal batteries, the SAW-assisted LMB is able to actuate the fluid flow in the interelectrode gap, significantly reducing the concentration gradient in the electrolyte and leading to homogenous Li concentrations, thereby preventing the formation of dendrites. In a full Li||LiFePO 4 cell, introducing SAW can achieve a 5 times increase in discharge capacity at 6C (6 mA cm −2 ) and 82% capacity retention after 200 cycles at 2C, far more than 51% capacity retention of baseline cell. 25 Compared to ultrasonic-assisted systems, the SAW-assisted LMB with a much smaller 500 mW input power works well on eliminating Li + concentration gradient, resulting in dense Li deposition with a homogeneous chunk-like structure. 24,25 Studies of LMB integrated with SAW devices demonstrate that using a sound field to enhance battery performance can be attained in battery design. Interestingly, the mechanism of ultrasonic-assisted Li-O 2 battery and SAW-assisted LMB are similar, that is, both are based on the sound field to accelerate the diffusion of batteries. The difference is that ultrasonic-assisted Li-O 2 mainly focuses on the removal efficiency of discharge product, whereas SAW-assisted LMB pays more attention to the anode morphology. The difference is that ultrasonicassisted Li-O 2 mainly focuses on the removal efficiency of discharge product, whereas SAW-assisted LMB pays more attention to the anode morphology. We may, therefore, infer that developing SAW-assisted Li-air batteries with homogeneous anode morphology and highly efficient OER is likely feasible. Further efforts to comprehensively investigate the sound field's effect on different battery components would provide valuable insights. Furthermore, extending these strategies to other battery electrochemistry, for example, Li-air and Na metal-based and Zn metal-based systems, to explore its practical and universal abilities is essential for developing sound field-assisted batteries.

MULTIPLE FIELD-ASSISTED BATTERIES
Although photo-assisted batteries have demonstrated a much-improved energy efficiency and cycle life, these light field-involved batteries face the same issue as solar cells, F I G U R E 5 Schematic illustration of the mechanism of multiple field assisted battery. The diagram shows the working mechanism and voltage profiles under illumination with and without the magnetic field (MF). the rapid recombination of holes and electrons generated from semiconductor materials. Enduing a solar cell with an externally applied magnetic field can provide a powerful driving force for carrier separation, improving the light-utilization efficiency and cell performance. 68 On this basis, combining magnetic and light fields creates multiple field-assisted batteries to tune the electrochemical process, which is, in principle, possible to improve the performance of batteries significantly. Applying multiple fields to batteries has recently been successfully achieved in Li-O 2 batteries. 26 In a multiple field-assisted battery, for example, magnetic and light multi-assisted Li-O 2 batteries, that is, the Li-O 2 battery fabricated with photoelectrode with NdFeB magnet on both sides of the battery (Figure 5), can achieve simultaneously applying light and magnetic fields into one battery cell. The mechanism of light and magnetic multiple field-assisted batteries is illustrated in Figure 5. Concretely, in the reported magnetic-and light-assisted Li-O 2 batteries, 3D porous NiO nanosheets on the Ni foam (NiO/FNi) photoelectrode with CB and VB potentials of 2.04 and 5.16 V, respectively, were used to harvest light. 26 The suitable CB and VB potentials make NiO/FNi able to assist both OER and ORR processes in Li-O 2 batteries with the photogenerated electrons and holes under illumination. The detailed photoinvolved battery chemistry upon illumination can be found in the previous discussion. When an extra field, a magnetic field, was introduced to photo-assisted Li-O 2 batteries, the participation of a magnetic field can provide a Lorentz force and act on the negative electrons and positive holes to deviate them in the opposite direction of motion. Consequently, the recombination of generated holes and electrons was inhibited, and the lifetime of hole and electron was prolonged, which is beneficial for further lowing energetic barriers in ORR and OER processes. Compared with Li-O 2 batteries with light-assisted only, an ultralow charge voltage of 2.73 V, extremely low voltage polarization of 0.09 V, and high energy efficiency of 96.7% can be delivered in magnetic and light multi-assisted batteries. 26 Considering the positive effect of the magnetic field on dendrite suppression and bubble motion regulation mentioned earlier, the increase in cycle life cannot be attributed entirely to the increased charge separation. A comprehensive understanding of the impact of multiple fields on cell components will help gain an in-depth understanding of the positive and negative effects of this type of battery.
So far, only magnetic-light-assisted Li-O 2 batteries have been attempted, leaving other combinations and possibilities under exploration. Inspired by the reported work that uses ultrasonic vibration to improve carrier separation and enrich the surface site of CdS nanosheet in the photocatalysis process, 69 the sound-light field is likely to be a good match. Given the remarkable enhancement of battery energy efficiency and cycle stability by the coupling TA B L E 1 A summary of external field-assisted batteries and their key roles in performance improvement. effect derived from two kinds of external fields, the development of multiple field-assisted batteries is expected to show tremendous prospects.

SUMMARY AND OUTLOOK
The introduction of external fields has proven to be a powerful strategy to enhance battery performance, which can act as an additional impetus to drive electrochemical reaction processes, such as ORR/OER process and Li/Zn metal deposition, leading to significant enhancement in cycle stability and energy efficiency. The advances, characteristics, and working mechanisms of batteries assisted with external fields, including light, magnetic, sound, and multiple fields, were systematically and comprehensively presented. Table 1 summarizes the characteristics of external field-assisted batteries and their key roles in battery performance improvement. Although the external fields have exhibited unique advantages in complementing and expanding traditional rechargeable batteries, currently developed external field-assisted batteries are often constructed with extra components, for example, magnets to offer magnetic field or transparent materials to pass light, which increases the complexity of battery configurations. In addition, it may create new challenges, such as photocorrosion/self-destruction in photo-assisted batteries and erosion caused by ultrasonic. The development of external field-assisted batteries is still in its early stage, leaving substantial space for exploring efficient external field-assisted batteries. Some challenges and prospects have been identified for the future development of high-efficient energy storage technologies.
1. By incorporating semiconductor materials to convert solar energy to electricity, the electrochemical performance of the battery has been significantly improved. However, most studies mainly focus on battery overpotential and capacity under illumination. The stability of electrolytes, intermediate products, and semiconductor materials in the photo-assisted batteries are rarely investigated. Future work on the stability of the battery component and intermediate product under long-term illumination and the long-term cycling stability under light/dark conditions are essential. On top of that, the stability of semiconductor-based photoelectrodes is another big challenge. Suffering from photocorrosion under light irradiation has been well documented in these materials. For example, in metal sulfide-based semiconductor materials, the photogenerated holes can easily be enriched on the surface of metal sulfides, inducing photocorrosion and weakening its photocatalytic performance. The photocorrosion can challenge the durability of photoelectrode, definitely will damage the electrochemical performance and lifetime of the photo-assisted batteries, as evidenced by the poor cycle stability in Zn-air with a BiVO 4 photocathode owing to serious photocorrosion of BiVO 4 . 14 Several strategies for improving the stability of semiconductorbased catalysts in photocatalysis, 70 for example, doping with heteroatoms, can be used to build on photoassisted batteries. Future efforts on exploring/applying photovoltaic/semiconductor materials with improved photocorrosion inhibition ability would benefit the development of high-performance photo-assisted batteries. In addition, light-permeable materials, such as glass, are required for illumination, which further complicates the battery structure. Notably, a specific design to satisfy the pass of both gas and light is required for air batteries. More attention should be paid to the development of better device structures. Developing all solid-state photo-assisted batteries can yet be regarded as a promising direction for the practical application of photo-assisted batteries. The participation of the solidstate electrolyte can eliminate the inherent drawback in most liquid electrolytes (e.g., flammable and volatile) and remove the use of a transparent window to pass light into the cell. Learning from the lastest technology in an all-solid-state photo-assisted Li-CO 2 battery that integrates photoelectrode with solid-state electrolyte, 71 it should be possible to realize a photo-assisted battery with light directly acting on the photoelectrode without passing through any transparent window. Future studies on all-solid-state photo-assisted batteries need considerable efforts. Finally, long-term light/sun exposition may generate photothermal effects, which could cause the temperature increase in the battery. Thus, the thermal effects produced by light on the electrochemical performance and component stability also need to be evaluated. 2. The magnetic field can lead to various positive effects on batteries, such as inhibiting dendrite formation in metal-based batteries by the MHD effect, mitigating the shuttle effect of polysulfide in the sulfur system, and guiding the bubble motion in air batteries. However, the underlying mechanism of magnetic field-assisted batteries, the magnetic field effect on battery rate capability, and the corresponding relations between the battery performance and magnetic field strength and directions need to be comprehensively and systematically studied. Additionally, considering the interest in developing high-energy density and aqueous-based systems, future efforts can be devoted to "beyond LiBs." 3. Sound field-assisted batteries have been developed to enhance mass transfer significantly, but they still need substantial attention for an in-depth understanding of the mechanisms and potential negative effects. Given that the intense ultrasonic power input, strategies to reduce energy consumption also remain to be explored. Balancing the relationship between performance enhancement and energy consumption is an important issue. Besides, the complex manufacturing process and cost issues in SAW-assisted batteries need to be addressed. 4. The synergy of multiple external fields shows huge prospects to effectively improve battery energy efficiency and cycle stability. Photo-and-magnetic fieldassisted Li-O 2 battery has proven to be a good combination. Further exploring the coupling of other different fields and their effects on the battery performance would benefit the development of multiple field-assisted batteries. In addition, an in-depth understanding of the working mechanism is needed as different external fields may act at different regions or parts of the batteries. 5. From a practical point of view, the requirement of generating devices to produce external fields, for example, the magnetic and sound fields, will bring additional weight and increase the volume of the whole battery. Thus, estimating the energy density and cost of the external field-assisted battery systems are indispensable for better evaluating their potential in practical application.

A C K N O W L E D G M E N T S
This work is supported by a grant from the Innovation and Technology Commission of the Hong Kong Special Administrative Region, China (Project No. ITS/219/21FP).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.