An example of modern technology implemented in the nautical vessel. Darling Harbor marina, Sydney, Australia.

It’s time. Your RV or boat’s lead-acid battery bank had a good run, but just isn’t able to hold a charge anymore – so what should you do? Using desulphators could help squeeze some more life out of it, but instead of asking how to restore lead-acid batteries that are clearly past their prime, the question you should be asking is: Can I replace lead-acid batteries with lithium batteries in my boat or RV? After all, lithium batteries are becoming the standard for renewable energy storage.

The answer is YES, you can absolutely replace lead-acid batteries with lithium in marine and RV applications – but here are a few considerations to help you decide if upgrading to lithium batteries is the right lead acid battery alternative for your boat, camper, or RV.

WHY REPLACE LEAD ACID BATTERIES WITH LITHIUM IN A BOAT OR RV?

Lead Acid vs. Lithium: Depth of Discharge

Depth of Discharge, or DoD, is how much of your battery bank’s stored energy can actually be used without dramatically reducing its life. For example, a 100Ah (amp hour) lead-acid battery rated for 25% DoD means you need to plan to use only ¼ of its rated capacity (so 25Ah), leaving the other ¾ in the battery, unused.

• DoD for lead-acid batteries – both flooded (which you have to add water to periodically) and sealed (“maintenance-free”) – is typically in the 25% – 50% range. Your battery will last at least twice as long if you regularly discharge it 25% than if you regularly discharge it 50%. Keep in mind that if you don’t have a sunny day to recharge your batteries after a day of use, the DoD will go down again the next day – so planning to use 25% per day will allow you to use less than the 50% maximum after two days of use.
• On the other hand, DoD for lithium ion batteries is 80% or more, allowing you to use most or even all of the battery’s stored energy. That means a 100Ah lithium battery rated for 80% DoD can safely provide you with 80Ah without being harmed.

As a result, a lithium battery bank can be much smaller than a lead-acid battery bank to provide the same amount of usable energy. For example, if you need 100Ah of energy a day, you would need a 400Ah lead-acid battery bank to stay at 25% DoD, but would only need 125Ah of lithium at 80% DoD. That is a significantly smaller battery bank with lithium batteries.

Lead Acid vs. Lithium: Cycle Count

Cycling a battery means discharging it to any amount and recharging it to a fully charged state. If you cycle your battery bank every day for a year, that’s 365 cycles. If you only use it on the weekends, and keep the bank topped off the rest of the time, that’s 104 cycles a year.

A cycle is a cycle regardless of how deep the discharge is, but the depth of discharge directly affects how many cycles you can expect your battery to last. A battery’s specs will tell you how many cycles to expect from it when discharging to its rated DoD.

• A standard flooded lead-acid battery can have about 2500 cycles at 25% DoD
• A standard sealed lead acid battery can have about 1200 cycles at 25% DoD
• Unlike lead-acid, lithium batteries don’t have a cycle curve under 80% DoD. Beyond 80%, the cycle count can drop dramatically. A typical lithium battery can have 5000+ cycles at up to 80% DoD. That’s 4x the cycles at over 3x the DoD. That’s a much longer lived battery bank with lithium batteries.

Lead Acid vs. Lithium: Charge/Discharge Rate

In addition to how much of a battery’s capacity you use, it also matters how fast you use it. Again using the 100Ah battery example, if you have a 10 amp (A) load, that can drain the battery completely in 10 hours  (100Ah ÷ 10A = 10 hours). That is considered a C/10 rate. Likewise, if you have a 50A load on the same battery, that would drain it in 2 hours  (100Ah ÷ 50A = 2 hours). That is a C/2 rate. Most batteries are rated at their C/20 rate, emptying the battery in 20 hours.

If you have a high-current load in your system, or are charging it very quickly with a high current, such as your alternator or shore power, you need to consider the charge/discharge rate of the battery bank. If you need a higher rate than the batteries can handle, you would need to increase the battery bank by adding more batteries in parallel so that the batteries can share the current between themselves. This may result in needing a battery bank that has a higher Ah capacity than you need to power your loads, just to handle the high current.

Likewise, too slow of a charge of lead-acid batteries can cause premature sulphation, shortening their life. This is not a problem with lithium.

• Lead-acid batteries tend to perform best between C/8 and C/12 rates. So our 100Ah battery would want to be charged or discharged at between 8A and 12A. Wiring three batteries in parallel would permit three times the rate, as it shares the current between the three, so 24A to 36A.
• Some lithium batteries can generally handle a C/1 rate, or even higher for short periods depending on the battery. This means a 100Ah lithium battery can handle 100A (or more) of charge/discharge current. Most manufacturers recommend no more than a C/2 rate on a regular basis for best battery life, but it is good to know the extra power is there with lithium batteries if you need it. Be sure to check the manufacturer’s specs when selecting a lithium battery, as some do not support as high of a current as others.

Lead Acid vs. Lithium: Voltage Sag

You may be familiar with the voltage of your boat or RV’s battery bank sagging, or dropping to 11V or lower when trying to run a high-power load such as your winch, windlass, or air conditioner. When running a heavy AC load off the inverter, the voltage could drop below the low voltage cutoff, causing the inverter to turn off when you need it most. Likewise, if you are running a DC load like your bow thruster directly off the battery bank, you need it to maintain a high enough voltage for it to work when you really need it to work. Due to lithium batteries’ voltage curve and ability to handle high current, loads like these will not cause the voltage to drop dramatically, eliminating the problem of voltage sag.

Lead Acid vs. Lithium: Size and Weight

With a higher DoD, higher cycle count, and higher charge/discharge rate, it’s easy to see how using lithium batteries in your RV or boat saves space by requiring a physically smaller battery bank…and I don’t need to explain the advantages of saving space in an already tight spot. But there’s yet another physical benefit of replacing lead-acid batteries with lithium for RV and marine applications: Lithium batteries also don’t have the crazy weight from being made with lead! Lighter weight means higher fuel efficiency, saving you additional money in gas or diesel costs.

Lead Acid vs. Lithium: Safety

Safety is always a primary consideration when designing a solar system, but it becomes even more important when your system is on a boat far from shore, or an RV on a remote road. Different battery chemistries have different risk factors. Obviously, abusing any type of battery can create a dangerous situation. But with normal, and perhaps even a bit of rough treatment, the different batteries have different safety concerns that need to be addressed.

• Flooded lead-acid batteries have an acid and water electrolyte in the battery that has to be checked on a regular basis. During normal charging cycles, this mixture turns into a gas that needs to be vented outside. A buildup of the gas inside a vehicle or vessel can be explosive. Proper ventilation mitigates this concern. The outgassing of the battery is normal, but requires owners to regularly check to see when the electrolyte level gets low from the outgassing. If low, it needs to have more distilled water added. This runs the risk of acid spills if overfilled or overcharged. This requires you to be prepared with proper safety equipment including gloves, safety glasses, and baking soda to neutralize the acid if needed.
• Sealed lead-acid batteries do not have outgassing or electrolyte levels to check, as they do not outgas. Normal battery safety measures should be followed, like checking for tight cable connections, corrosion, and preventing physical damage to the battery itself.
• Lithium batteries also do not outgas, but certain types (the ones with cobalt, known as lithium cobalt oxide or LCO) can experience thermal runaway – a condition where the battery starts to get hot, which causes it to react to the heat and get hotter and hotter until it catches on fire. LCO batteries are most commonly used in cell phones, hoverboards, and electric cars, and are generally not recommended for mobile applications.

So are lithium batteries any safer than other batteries? Yes – when they don’t contain cobalt. Lithium ferrous phosphate (LFP or LiFePO4) chemistry has become the standard lithium battery for marine, RV, and general solar PV use because they have no thermal runaway issues. They are very safe, can be installed indoors, and are a perfect solution for mobile living and recreation. Just as with sealed lead-acid batteries, making sure the cables haven’t shaken loose with vibration from travel, and a visual inspection to ensure all is well is all that is needed.

Lead Acid vs. Lithium: Temperature

Temperature has different impacts on different types of batteries. A lead-acid battery’s capacity is rated at 80°F (26°C), but the colder it gets, the more capacity falls. So our 100Ah lead-acid battery at 80°F holds only 76Ah at 40°F (4°C). As a result, if you know you are going to be using your battery bank in the winter, and they will be in an unconditioned location (not heated or cooled), you need to oversize your battery bank to make up for the smaller capacity when cold.

Lead-acid batteries also perform best when charged at different rates based on temperature. As a result, most quality solar charge controllers have a battery temperature sensor to report back to the charge controller.

Lithium batteries maintain the same capacity regardless of the temperature, and do not need their charging rate adjusted to account for temperature.

However, while you can run your loads in freezing temperatures, you cannot charge a lithium battery in sub-freezing temperatures (below 32°F or 0°C). A Battery Monitoring System (BMS) will often have cold temperature cut-off, preventing the battery from being charged when it is too cold.

If your lithium battery bank is in a cold environment, you need to either get a battery that has a built-in heater like the Himax battery, and/or build an insulated battery box to hold in the heat generated while charging.

Lead Acid vs. Lithium: Lifetime Cost

If you compare lithium batteries to lead-acid batteries Ah to Ah, lithium batteries are more expensive. But step back and look at the bigger picture – taking into account everything we’ve covered so far – and you can see how lithium batteries can actually save you money, time, and hassle in the long run.

Let’s look at some cost examples when designing a battery bank for a 12V system that uses 1400Wh a day.

Sealed AGM vs. Lithium

For a lithium bank: 1400Wh x 2 days of autonomy ÷ 80% DoD (after 2 days without sun, daily is 40%) ÷ 12V battery bank = 290Ah battery. I’ll round up to 300Ah and use two  1800 at $1300 each for a total of $2600.For a sealed AGM battery bank: 1400Wh x 2 days of autonomy (days without sun) x 1.11 temperature derate (60F) ÷ 50% DoD (25% x 2 days without sun) ÷ 12V battery bank = 518Ah bank. I’ll round up to 600Ah and use 200Ah 12V batteries at $600 each for a total of $1800.

At first glance, the lithium bank costs more than the AGM bank. But when you consider cycle counts (1200 for the AGM and 5000 for the lithium), the lithium battery bank will last 4x longer than the AGM bank. You would need to buy four AGM battery banks for $7200 – and spend the time shopping for and installing them – to match the lifetime of one lithium bank for $2600. Plus you’d miss out on all the previously mentioned benefits of lithium batteries for a boat or RV.

Flooded Lead Acid vs. Lithium

The math is the same for a flooded lead-acid battery bank as for a sealed one. So let’s again compare a 518Ah 12V lead-acid battery bank with the 300Ah 12V lithium bank. I’ll round up to 675Ah to use the popular Trojan T-105 225Ah 6V batteries at $175 each. Using 6V batteries will require 2 in series to get 12V, so I’ll need 6 for a total of $1050. We are still going to use two of the KiloVault HLX1800 for $2600, or I could use a single 300Ah 12V battery for the $2500. Some people prefer installing two batteries in parallel for redundancy and find that the size and weight of two batteries may be easier to manage than one.

The price gap between flooded lead-acid and lithium is greater than with AGM. With flooded’s 2500 cycles versus lithium’s 5000 cycles, a well maintained flooded battery bank can last half as long as lithium. But a poorly maintained flooded battery bank can quickly become a boat anchor in a year or two. So the flooded is a slightly less expensive solution than sealed lead-acid at $2100 for two banks vs. $2600 for one lithium. But again, you have the advantages of the smaller, lighter, safer battery bank, the higher current capability, and minimal maintenance needed on the lithium. It may be worth the extra $500 to you to go lithium.

Lead Acid vs. Lithium for Marine and RVs: The Verdict

By choosing lithium batteries as a lead-acid battery alternative for marine/RV applications, you will need fewer batteries, and those batteries will last longer, cycle  deeper, deliver more power, and weigh less.

                                                   Himax – 150Ah 12V Lithium Battery

CONSIDERATIONS WHEN REPLACING LEAD ACID BATTERIES WITH LITHIUM IN A BOAT OR RV

Now that you’re convinced lithium is the best way to go, you need to be aware of a few things when replacing a lead-acid battery with lithium. The term “drop-in replacement” has become popular, but the reality is there are a few other things you’ll need to do to safely upgrade from lead-acid to lithium batteries in your boat or RV.

Charge Controller/Charging Profile

If you are currently charging your lead acid batteries with solar, your alternator, and/or shore power, you may be able to keep your existing charge controller or inverter/charger. The charging and low voltage cutoff profiles for lithium batteries are a little different from lead-acid, so you need chargers that have adjustable charge rates. Different batteries will have different preferences, so be sure to see the manufacturer’s recommendations when configuring your charger. They will often recommend a Bulk and Absorb rate of around 14V, with an Absorption time of as little as 2 minutes, significantly less than the standard for lead-acid. With a Float voltage of just below 14V, you can maintain the charge without overcharging it. Because lithium has a very narrow voltage window, 12V is generally the lowest voltage you want before you shut off your loads.

Note: Unlike lead-acid batteries, lithium batteries do not always need to be recharged to their full 100% capacity. They actually prefer being in a partial state of charge. If you are going to be leaving your boat or RV for a season of storage, it is recommended that you leave the battery bank at around 90% state of charge. This leaves plenty of energy for small loads like the bilge pump or CO₂ alarm, but helps maintain a healthy battery bank until you can get back to normal use.

Cranking Amps / Starter Battery

With lead-acid batteries, we are used to seeing a rating of CCA (cold crank amps) to show how many amps can be used to start an engine in the cold weather. Lithium batteries do not have the CCA rating. If you intend to replace a lead-acid battery with lithium for your starting battery, make sure the new lithium battery is rated to handle enough current to do so. Not all of them are. We see a lot of people continue to use a lead-acid battery as the starter, with lithium used only for the house/service battery. This also gives you a bit of a backup, so that if everything goes wrong with your house/service battery, you still have the starter battery available.

Alternator

Unlike lead-acid batteries, lithium batteries have very little internal resistance and can take as much charging current from the alternator as needed. But since alternators are not designed to run at full speed for long periods, this can result in the alternator working too hard, overheating, and damaging itself. There are a few ways to prevent this from happening.

Use a DC/DC Converter

By installing a DC-to-DC converter between the alternator and the lithium battery bank, you can limit the amount of current the battery draws from the alternator. It is recommended that you only draw from the alternator at half its rating, so for a 60A alternator, a 30A DC/DC converter like the Bluetooth-enabled Victron Energy Orion-Tr Smart 12/12-30A charger is a good option. You can use multiple DC/DC converters in parallel to increase the rate for larger alternators.

Replace the Alternator

You can replace the alternator with one designed for higher amperage charging and temperature control. Balmar makes great alternators and external regulators for this. They monitor the temperature and will wind down to appropriate amperage if the alternator gets too hot. If you currently have a V-belt, you may need to modify the engine for a serpentine belt before you can use the larger Balmar alternator.

Low Voltage Disconnect

The ability to automatically disconnect your DC loads gives you control over how low you discharge your battery bank. An automatic switch such as the Victron Energy Smart BatteryProtect can be configured via Bluetooth for excellent control of your system. It can turn your non-critical loads on or off based on a configurable voltage setting.

Battery/Bank Monitoring

Any good battery system should have the ability to monitor both the individual batteries, and the whole battery bank. Watching more than just the voltage, but also how many amps go in and out of the battery bank and the temperature gives you a complete view of the health and state of charge of the entire bank. Some lithium battery Battery Monitoring Systems have Bluetooth or WiFi built in to allow you to monitor it from the phone. For example, the KiloVault HLX and CHLX batteries have Bluetooth to your Smartphone to see down to the cell level of each battery.

The Victron Energy Smart Shunt provides a low cost method to monitor your whole battery bank via Bluetooth from your smartphone. It does not include a display, so you can only view it via Bluetooth.

The Victron Energy Smart Battery Monitor BVM-712 gives you a local display for convenient viewing of battery voltage, current, power, amp-hours consumed, and state of charge (SoC). It can also be viewed via BlueTooth.

IT’S TIME TO MOBILIZE WITH LITHIUM BATTERIES FOR MARINE AND RV APPLICATIONS

Whether you are looking for a new battery bank for your RV or boat or considering replacing your aging lead-acid batteries, deep-cycle lithium-ion batteries – specifically LiFePO4 batteries – are an excellent solution. Compared to lead-acid batteries, LiFePO4 batteries offer more power, higher current, a longer life, smaller footprint, lower weight, and safe, maintenance-free operation. Are you ready to mobilize and go lithium?

See more options for lithium batteries at our website or contact us at (86)755-2562 9920 to help you select the right lithium batteries for your specific needs.

Safety is a full-fledged design feature with lithium batteries, and for good reason. As we’ve all seen, the chemistry and energy density that allows lithium-ion batteries to work so well also makes them flammable, so when the batteries malfunction, they often make a spectacular and dangerous mess.

All lithium chemistries are not created equal. In fact, most American consumers – electronic enthusiasts aside – are only familiar with a limited range of lithium solutions. The most common versions are built from cobalt oxide, manganese oxide and nickel oxide formulations.

First, let’s take a step back in time. Lithium-ion batteries are a much newer innovation and have only been around for the last 25 years. Over this time, lithium technologies have increased in popularity as they have proven to be valuable in powering smaller electronics – like laptops and cell phones. But as you may recall from several news stories over recent years, lithium-ion batteries also gained a reputation for catching fire. Until recent years, this was one of the main reasons lithium wasn’t commonly used to create large battery banks.

But then came along lithium iron phosphate (LiFePO4). This newer type of lithium solution was inherently non-combustible while allowing for slightly lower energy density. LiFePO4 batteries were not only safer, they had many advantages over other lithium chemistries, particularly for high power applications, such as renewable energy.

Before we dive into the safety features of lithium iron phosphate, let’s refresh ourselves on how lithium battery malfunctions happen in the first place.

Lithium-ion batteries explode when battery’s full charge is released instantly, or when the liquid chemicals mix with foreign contaminants and ignite. This typically happens in three ways: physical damage, overcharging or electrolyte breakdown.

For example, if the internal separator or charging-circuitry is damaged or malfunctions, then there are no safety barriers to keep the electrolytes from merging and causing an explosive chemical reaction, which then ruptures the battery packaging, combines the chemical slurry with oxygen and instantly ignites all of the components.

There are a few other ways lithium batteries can explode or catch on fire, but thermal runaway scenarios like these are the most common. Common is a relative term though, because lithium-ion batteries power most rechargeable products on the market, and it’s pretty rare for large-scale recalls or safety scares to happen.

Although lithium iron phosphate (LiFePO4) batteries aren’t exactly new, they’re just now picking up traction in Global commercial markets. Here’s a quick breakdown on what makes LiFePO4 batteries safer than other lithium battery solutions.

LiFePO4 batteries are best known for their strong safety profile, the result of extremely stable chemistry. Phosphate-based batteries offer superior chemical and mechanical structure that does not overheat to unsafe levels. Thus, providing an increase in safety over lithium-ion batteries made with other cathode materials.

This is because the charged and uncharged states of LiFePO4 are physically similar and highly robust, which lets the ions remain stable during the oxygen flux that happens alongside charge cycles or possible malfunctions. Overall, the iron phosphate-oxide bond is stronger than the cobalt-oxide bond, so when the battery is overcharged or subject to physical damage then the phosphate-oxide bond remains structurally stable; whereas in other lithium chemistries the bonds begin breaking down and releasing excessive heat, which eventually leads to thermal runaway.

Lithium phosphate cells are incombustible, which is an important feature in the event of mishandling during charging or discharging. They can also withstand harsh conditions, be it freezing cold, scorching heat or rough terrain.

When subjected to hazardous events, such as collision or short-circuiting, they won’t explode or catch fire, significantly reducing any chance of harm. If you’re selecting a lithium battery and anticipate use in hazardous or unstable environments, LiFePO4 is likely your best choice.

Most LiFePO4 batteries also come with a Battery Management System (BMS) that have many extra safety features including; over-current, over-voltage, under-voltage and over-temperature protection and the cells come in an explosion-proof stainless steel casing.

It’s also worth mentioning, LiFePO4 batteries are non-toxic, non-contaminating and contain no rare earth metals, making them an environmentally conscious choice. Lead-acid and nickel oxide lithium batteries carry significant environmental risk (especially lead acid, as internal chemicals degrade structure over team and eventually cause leakage). Compared to lead-acid and other lithium batteries, lithium iron phosphate batteries offer significant advantages, including improved discharge and charge efficiency, longer life span and the ability to deep cycle while maintaining performance. LiFePO4 batteries often come with a higher price tag, but a much better cost over life of the product, minimal maintenance and infrequent replacement makes them a worthwhile investment and a safer long-term solution.

Change can be daunting, even when switching from a lead-acid battery to a lithium iron phosphate battery. We all know properly charging your battery is critical and directly impacts the performance and life of the battery. Let’s take a look at how to charge your LiFePO4 battery to maximize your investment.

Charging Conditions

Much like your cell phone, you can charge your lithium iron phosphate batteries whenever you want. Obviously, if you let them drain completely, you won’t be able to use them until they get some charge. The key thing to note is that unlike lead-acid batteries, lithium iron phosphate batteries do not get damaged if they are left in a partial state of charge, so you don’t have to stress about getting them charged immediately after use. And they don’t have a memory effect, so you don’t have to drain them completely before charging.

LiFePO4 batteries can safely charge at temperatures between -4°F – 131°F (0°C – 55°C), however, we recommend charging in temperatures above 32°F (0°C). If you do charge below freezing temperatures, you must make sure the charge current is 5-10% of the capacity of the battery.

How to Charge a Lithium Iron Phosphate Battery

The ideal way to charge a LiFePO4 battery is with a lithium iron phosphate battery charger, as it will be programmed with the appropriate voltage limits. Most lead-acid battery chargers will do the job just fine. AGM and GEL charge profiles typically fall within the voltage limits of a lithium iron phosphate battery. Wet lead-acid battery chargers tend to have a higher voltage limit, which may cause the Battery Management System (BMS) to go into protection mode. This won’t harm the battery, however, it may cause fault codes on the charger display.

Charging Batteries in Parallel Best Practices

When connecting your lithium batteries in parallel, it is best to charge each battery individually before making the parallel connection(s). If you have a voltmeter, check the voltage a couple hours after the charge is complete and make sure they are within 50mV (0.05V) of each other before paralleling them. This will minimize the chance of imbalance between the batteries and, ultimately, maximize the performance of the system. Over time, if you notice the capacity of your battery bank has decreased, disconnect the parallel connections and charge each battery individually, then reconnect.

Charging Batteries in Series Best Practices

Connecting lithium batteries in series is much like connecting them in parallel, it is best to charge each battery up individually and check the voltage and ensure they are within 50mV (0.05V) of each other before making the series connections.

It is highly recommended to charge lithium batteries in series with a multi-bank charger. This means each battery is charged at the same time but completely independent of each other. In some applications this is not practical, which is why Himax offers 24V and 48V batteries to reduce the need for multiple batteries in series.

What About During Storage?

Lithium iron phosphate batteries are so much easier to store than lead-acid batteries. For short-term storage of 3-6 months, you don’t have to do a thing. Ideally, leave them at around 50% state of charge before storing. For long-term storage, it is best to store them at a 50% state of charge and then cycle them by discharging them, recharging them and then partially discharging them to approximately 50%, every 6-12 months.

The Key Differences Between Lithium Iron Phosphate and Lead-Acid Batteries When It Comes to Charging

Lithium batteries can charge at a much higher current and they charge more efficiently than lead-acid, which means they can be charged faster. Lithium batteries do not need to be charged if they are partially discharged. Unlike lead-acid batteries, which when left in a partial state of charge will sulfate, drastically reducing performance and life.

lithium batteries come with an internal Battery Management System (BMS) that protects the battery from being over-charged, whereas lead-acid batteries can be over-charged, increasing the rate of grid corrosion and shortening battery life.

For more details on charging your Himax lithium batteries, contact us if you have any questions.

mail: sales@himaxelectronics.com

As Huawei and Samsung released the folding screen phones Mate X and Galaxy Fold respectively, they have a technologically-sense design and excellent hardware configuration. The unfolded area of the new folding screen mobile phone screen can reach more than 7.3 inches, which enhances the user experience. But in fact, a more disruptive, practical, and cost-effective mobile phone design is already on the way to development. It is a fully flexible wearable mobile phone.

Fully flexible mobile phones are dubbed “wrist phones” by the media. They are not limited to the folding of the middle part. They can bend the whole mobile phone freely, which is convenient to wear on the wrist and other parts to achieve better integration with the human body. The current folding screen mobile phones still use ordinary rigid batteries, avoiding the problem of using flexible batteries. If you want to introduce revolutionary fully flexible electronic devices, you must develop corresponding flexible power supplies and implant them. Therefore, the development of flexible lithium batteries with high energy density will be of great significance to promote the development of wearable flexible electronic devices.

The ideal flexible battery should have high flexibility, energy density, and power density at the same time. However, these factors often hinder each other in the flexible battery. In this review, this article made a detailed analysis based on the structural design of the battery components and the overall device level, and reviewed the latest developments in flexible lithium batteries, and summarized the current academic development ideas into the following four strategies:

1) Development of a deformable battery module with a porous structure

Such as porous current collectors, porous electrodes, flexible solid electrolytes, etc. Flexible porous structures are currently widely used in battery modules to cushion the strain generated when the battery device is subjected to bending and twisting.

a) A conductive porous film of graphene oxide, which has a conductivity as high as 3112 S/cm. The flexible lithium battery assembled with this film as the current collector did not find a significant decrease in capacity after 100 cycles of high charge-discharge rate (5C).

b) A composite cathode material of single-walled carbon nanotubes and polymer (2,5-dihydroxy-1,1-benzoquinone sulfide) is used to assemble flexible lithium batteries. The flexible battery exhibits a specific discharge capacity of 182 mAh/g at low current (50 mA/g), and can still reach a specific capacity of 75 mAh/g when discharged at a large current (5000 mA/g).

c) Using bacterial cellulose as a template, develop a solid electrolyte composite of Li7La3Zr2O12 (LLZO) and polyethylene oxide. The porous interconnected polymer matrix is ​​used as a mechanical carrier that is soft and strong, and the LLZO particles used to transport Li-ions are embedded in it. The overall display shows a high ionic conductivity of 1.12×10-4 S/cm and excellent mechanical flexibility.

2) Ultra-thin battery design

Such as single-pair (or double-pair) positive/separator/negative structure. Compared with strategy 1, strategy 2 (ultra-thin battery design) requires battery design from the overall device level.

a) The ultra-thin flexible lithium battery with a thickness of only 0.4mm released by Himax Battery can be applied to various wearable devices. Even after bending with a bending radius of 25 mm, or twisting to an angle of ±25 degrees for more than 1,000 times, this flexible battery can still maintain 99% of the capacity.

b) A Li4Ti5O12/LiPON/Li thin-film solid-state battery prepared based on the flame spray pyrolysis method with flexible polyimide as the supporting substrate. After charging and discharging at a rate of 1C and cycling in a flat, bent, and flatform for 90 times, the battery’s discharge capacity retention rate is still as high as 98%, showing excellent cycle performance.

3) Geometric topology battery design

Such as linear structure, Origami, Kirigami structure, etc. In addition to improving the flexibility of the battery component material itself, the battery structure designed by the principle of geometric topology can reduce the stress change generated in the battery during the deformation process.

a. This strategy was first reported in the work of its linear battery. The flexible battery can not only adapt to bending deformation, but also more complex shape changes, such as folding and twisting.

b. Using spring-like LiCoO2/reduced graphene oxide as the positive electrode material, combined with gel electrolyte, a self-healing flexible lithium battery was designed and assembled. Under complex deformation (bending and torsion), charging and discharging at a current density of 1 A/g, the battery can still maintain a discharge specific capacity of 82.6 mAh/g; even if the battery is cut and healed five times, the battery can still be The specific discharge capacity is 50.1 mAh/g.

c. In addition to linear structures, paper folding technology is also widely used in flexible batteries. With the Origami origami solution, a two-dimensional sheet material can be folded along a predetermined crease to create a compact and deformable three-dimensional structure that can withstand high-strength deformation.

d. Soon after, a Kirigami scheme was developed combining folding and cutting technology. After 100 charge-discharge cycles, the battery can achieve a capacity retention rate of more than 85% and a coulombic efficiency of 8%. After 3,000 battery deformations, the maximum output power of the battery has not been significantly reduced.

4) Decouple the flexible and energy storage parts of the battery

Such as spine battery, Zigzag battery, etc. For the above-mentioned flexible battery design, dislocation, peeling, and shedding between the active material and the current collector still occur during the complex deformation process. The increased overpotential and internal resistance of the battery due to poor contact will reduce the capacity retention rate and coulomb efficiency of the full battery, which is not conducive to the cycle performance of the battery. The potential solution is to redesign the battery architecture to separate energy storage and provide flexibility.

a. De Volder et al. demonstrated a layered tapered carbon nanotube structure, similar to the morning glory of a plant. The wide corolla is used to carry the positive and negative active material particles, and the slender stem part and the current collector are below. The parts are tightly combined. During the deformation of the battery, most of the stress is applied to the current collector itself, and the tapered structure hardly produces strain during this period, thus exhibiting extremely high flexibility. The Fe2O3/LiNi8Co0.2O2 full battery assembled with this tapered structure can be charged and discharged 500 times at a rate of 1C and still has a capacity retention rate of 88%.

b. Inspired by the good mechanical strength and flexibility of the animal spine, a method for large-scale production of high-energy-density flexible lithium-ion batteries: store energy by surrounding thick, rigid parts in the axial direction (corresponding to the spine), And the thin, non-circular flexible part (corresponding to the bone marrow and intervertebral disc) is used to connect the “spine”, thereby achieving good flexibility and high energy density of the entire device. Since the volume of the rigid electrode part is much larger than the flexible connection part, occupying more than 95% of the volume of the battery cell, the energy density of the whole battery can reach 242Wh/L. The reasonable bionic design makes it pass the strong dynamic mechanical load test.

Questions and answers from flexible lithium battery experts

Xie Ming, a well-known flexible lithium battery expert in the industry, accepted an exclusive interview from the material man. The following is the interview content.

Q: Based on the actual situation of current industrial production, please comment on the four flexible lithium battery development strategies summarized in the review.

A:  1) Development of flexible battery elements with porous structure carbon (nanotube) paper is used as a flexible current collector, the cost is relatively high, which is not easily accepted by manufacturers; secondly, when carbon paper is used as a negative current collector, its side reactions are very obvious. If a porous metal current collector (such as copper mesh, aluminum mesh, etc.) is used, its flexibility can meet actual needs, but during the coating process, the slurry easily penetrates from the mesh pores. The current development of related coating processes is still An important challenge facing the corporate world.

2) Ultra-thin battery design

In order to achieve stable mechanical flexibility and electrochemical performance, ultra-thin batteries mostly use single-pair (or double-pair) low-capacity (usually less than 60 mAh) design, so their application scenarios and markets are very limited. In order to make the battery thinner, GREPOW has introduced a mature technology ultra-thin battery. While meeting the actual requirements, the overall battery thickness can be less than 8mm, and the thinnest can reach 0.4mm. You can imagine this is like A battery as thin as paper.

3) Geometric topology battery design

The concept proposed by this strategy is very good, represented by the linear battery, its mechanical flexibility, and electrochemical performance can be guaranteed. At present, many research teams are committed to developing new types of fibrous and linear batteries to solve traditional bottlenecks. The fly in the ointment is that the positive electrode, negative electrode, and separator of this linear battery rely on self-synthesis, which is different from commercial battery components, which will increase production costs. In addition, most linear batteries use heat-shrinkable tubes instead of aluminum-plastic film packaging. The heat-shrinkable tube materials have a limited barrier to water vapor and oxygen, and it is difficult to meet actual needs in long-term use.

4) Decouple the flexible and energy storage parts of the battery

This strategy uses improved commercial battery components, which is of great significance to promote the development and production of flexible lithium batteries. As early as 14 years or so, some Chinese companies have begun to develop “bamboo-shaped” batteries that look similar to “spine” batteries. According to the latest literature report on “Zigzag” batteries, the energy density of flexible lithium batteries assembled with the “decoupling” strategy can reach 275 Wh/L. After the process optimization of industrial standards, the energy density can still be achieved. Room for improvement. At present, an MIT research group has developed a series of fully automatic and personalized battery pole piece winding equipment. It is believed that with the intervention of an intelligent manufacturing system (IMS) service providers, the shortcomings of this type of battery assembly process can be gradually overcome.

Q: Could you please introduce the application scenarios of flexible batteries in the future?

A: At present, flexible wearable devices are the largest application market for flexible batteries. Taking smartwatches as an example, many large consumer electronics manufacturers have proposed the idea of ​​implanting flexible batteries into smartwatch straps, removing the battery from the panel, and realizing an ultra-light and ultra-thin dial design. They hope that the capacity of the flexible battery can be close to 500 mAh, but the volume energy density of the more mature flexible lithium battery samples in the industry is about 300-400 Wh/L, which is temporarily difficult to achieve the above goal. In addition, in order to introduce the current in the watchband to the dial, it is necessary to design the circuit in the watchband, which will be a considerable investment in development.

In addition, fully flexible mobile phones will be an important application scenario for flexible lithium batteries in the future. A few days ago, Samsung released a new foldable mobile phone Galaxy Fold, but this phone still uses ordinary rigid batteries, avoiding the problem of using flexible batteries. If you want to launch a revolutionary fully flexible mobile phone, you must develop a high-capacity (more than 2000 mAh) flexible battery implanted in it. As mentioned in this review, energy density and mechanical flexibility are usually in a flexible battery. The combination of scientific experiments and theoretical simulation is used to deeply study the basic mechanical problems in flexible batteries. Development work is very helpful.

Q: Please talk about the core competitiveness of flexible batteries from the perspective of industrial applications.

A: Compared with traditional non-bendable batteries, the volumetric energy density of flexible batteries will definitely be affected. Therefore, flexible batteries must find their own application positioning, that is, place batteries in places where batteries could not be placed before, make full use of every effective space of electronic devices, and increase the standby time by increasing the overall battery capacity of electronic devices. This is another strategy to solve consumers’ anxiety about mobile phone standby when the energy density of lithium-ion battery materials has not been greatly developed.

In short, flexible batteries do have a broad potential application market. In recent years, well-known international companies such as Apple and Samsung have launched patent arrangements in related fields. However, most downstream electronic equipment manufacturers still expect to start the development of a fully flexible series of products after the flexible battery production technology has matured.

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