Based on a lithium imide compound-based electrolyte, a next-generation battery chemistry overcomes shortcomings of conventional Li-ion batteries. The Li-imide battery patented by Leyden Energy provides up to 25% more energy density for a given size battery, and is practically insensitive to temperature and water impurities inside a cell.
Leyden’s new battery chemistry promises to overcome the limitations of conventional Li-ion batteries in current and future mobile devices. While lithium-ion (Li-ion) batteries have made progresss, they can no longer can’t pack any more run-time into each unit of size or weight. And, the current Li-ion’s energy density isn’t high enough to power the next generation of smaller mobile devices. Plus, this older chemistry lacks durability because energy density declines as Li-ion cells age, particularly in the elevated temperatures of typical consumer use. And, the older cells can swell and damage the battery and its associated circuits. By employing a different chemistry, Leyden’s next generation Li-imide battery overcomes these deficiencies.
To understand the advantages of the new Leyden battery we first have to review battery characteristics. Probably the most important characteristic is energy density, usually stated as the amount of energy (typically watt-hours), which is the amount of energy a battery chemistry can deliver per liter (volumetric) or kilogram (gravimetric). For instance, the conventional Li-ion cell has a volumetric energy density of 400 watt-hours per liter (Wh/l), although manufacturers usually state the capacity of a battery in amp-hours. You can then calculate energy density by integrating the area under a battery’s voltage-capacity curve. Adding the packaging and electronic circuitry required for battery management and safety reduces the practical volume available, and thus the actual energy density. Table 1 lists the battery consideration for major applications.
For a given battery chemistry with similar voltage, the capacity rating is proportional to the energy rating. However, energy density is only half the story, because it’s a “day one” specification. A battery’s energy density, and its stated capacity, aren’t constant: they diminish with use and age. And, elevated temperatures, which are a typical result of consumer use, reduce usable battery run time.
Battery Durability
Designers also have to pay attention to battery durability: how fast does the run-time decline happen? Once run time drops to 80% of the initial value, device OEMs generally regard a battery as worn out. For owners of the increasing number of devices with embedded batteries, this is a major annoyance and the ideal situation would be for battery lifetime to meet or exceed the likely obsolescence of the associated device. Here, too, Li-ion technology can fall short of what consumers demand.
An Li-ion cell is made of an anode (-) and a cathode (+) between which is a liquid electrolyte made up of a salt and a combination of organic solvents and additives. The presence of positive lithium ions in the electrolyte and external resistive load between the electrodes enables ion and electron flow between the cathode and the anode. This is the basis of the charge/discharge cycle.
The dominant Li-ion battery chemistry for mobile devices is based on a salt that is a compound of lithium, phosphorus and fluoride, LiPF₆. Unfortunately, in a process accelerated by elevated temperatures, this salt tends to combine with the residual moisture present as an impurity within Li-ion cells. This creates hydrofluoric acid, an extremely corrosive substance that impacts battery durability. When the hydrofluoric acid reacts with the cathode in a battery cell, it begins to leach metal ions out of the cathode. These metal ions travel across to and poison the anode, increasing cell impedance and ultimately bringing about the untimely death of the battery cell. In addition, the process can also generate gas that causes swelling of the cell, which can damage the device and even pose a safety hazard.
Mobile Device Battery Environment
Li-ion cell datasheets generally show test results for cells and batteries at room temperature, typically 20 °C (68 °F). This de-emphasizes temperature sensitivity, or lack of thermal stability. But the in-device temperature is invariably much higher than room temperature, particularly with today’s thinner, tighter enclosures that typically pack in far more heat-generating semiconductors than ever before.
Mobile device designers generally judge the suitability of a battery according to three specifications:
- Battery capacity—the “day-one” amount of energy it can deliver per charge (capacity is proportional to energy for a given battery chemistry and voltage)
- Cycle life—the total number of charge-discharge cycles possible before capacity degrades below 80% of the rated value
- Calendar life—how long the battery can deliver at least 80% of its stated capacity independent of the number of charge-discharge cycles
Stating these specs at an artificially low room temperature portrays battery performance as better than it is. To truly understand a battery’s energy density and durability, mobile device designers must consider the relationship between three critical parameters:
- Real-world temperature—is the device in a 20° C testing lab, a 40° C pants pocket, or a 60° C or higher-temperature parked car?
- Depth of discharge (DOD)—how close to “empty” the battery may get during discharge.
- State of charge (SOC)—how close to full the battery gets during a charge.
Understanding how these parameters interact to affect battery performance, as well as the basic physical stability of the battery, are critical to procuring the right battery for a mobile device.
In a mobile device, temperature is determined more by the human factor than anything else. Consumers often leave their phone or tablet in the car, in temperatures beyond 60° C (140° F), or operate a laptop on a thick carpet or on a fluffy duvet-covered bed, blocking its venting holes and leading to similar conditions. Putting a smartphone in one’s pocket raises the temperature to near body heat (37° C/98.6° F). Even one spike in temperature that far exceeds the recommended range for a Li-ion battery may damage the battery and lead to accelerated degradation. Under such conditions, capacity, cycle life, and calendar life all decline rapidly.
Depth of Discharge (DOD)
The cycle life of a battery can be extended by reducing the depth of discharge (DOD), which can involve not fully charging it, or not fully discharging it. This is controlled by internal battery control circuits. For example, batteries in low earth orbit satellites, where replacement is not possible, reach hundreds of thousands of cycles by holding DOD to 5% or less. Replacing an embedded battery in a mobile device is possible, but consumer attachment to their devices makes even a few days wait seem much longer.
Thus, a battery manufacturer may specify 500 cycles at 80% DOD, rather than a less appealing 300 cycles at 100% DOD. The result is a lower practical energy density and reduced run-time.
State of Charge (SOC)
The higher the charge on a battery, the faster it ages and the more vulnerable it is to elevated temperatures. There are two scenarios to consider where SOC is concerned: post-manufacturing shipping or inventory conditions, and “desktop usage.”
Batteries are shipped from the factory approximately 50 percent charged, which minimizes product degradation before reaching the end user and also satisfies transportation safety regulations. But, it is practically impossible to predict environmental conditions once the battery leaves the controlled environment of the battery factory. Temperatures during weeks or months of storage in ocean shipping containers, warehouses or distribution/delivery trucks can easily exceed the ideal room temperature, degrading battery performance even before it reaches the end user.
Even if shipping conditions were ideal, consumers often plug an almost fully-charged device into the power outlet and leave it connected all day long. This “desktop usage pattern” is a very common scenario with laptops in particular. Exposure of a fully-charged battery to the elevated operating temperatures of the device while at full charge can accelerate performance degradation.
Physical Stability
Li-ion pouch cells normally expand slightly during charge and relax during discharge, and over its life the battery can see a permanent expansion. Discharge swelling amounts to about five to ten percent of the anode electrode thickness during the process of Li-ion intercalation, the reversible process of incorporating lithium ions into the positive electrode during discharge. The anode’s influence on the overall cell expansion is minimal because the thickness of anodes compared to total cell thickness is slight. This expansion is included in the total cell thickness provided in data sheets. Age-related expansion is generally about 8 to 10 percent over the life of the pouch cell under standard conditions; having to take this into account limits the practical energy density of the battery.
As noted earlier, the LiPF6 salt used in Li-ion batteries reacts with residual moisture in the cell to generate hydrofluoric acid. In most circumstances, the reaction of the hydrofluoric acid with the electrodes forms a passivation layer and generates only minimal gas. In other cases, an auto-catalytic state results in continued gas production until the pouch cell swells or balloons far beyond the measurements of the cavity for which it was designed. It can even burst. This is one of the leading causes of battery recalls in consumer electronics with embedded batteries. It is also a major safety issue.
Inside The lI-Imide Battery
Although researchers are making great strides with new active materials that drive up energy density and deliver longer run time, virtually all Li-ion batteries use the same electrolyte. It’s based on a fluoride salt of lithium (LiPF6) and becomes increasingly unstable as the temperature rises. Discharge capacity in temperature-stressed cells—common in consumer usage—declines rapidly as they age, and as little as one badly degraded cell can affect an entire battery pack.
By contrast, Leyden’s lithium-ion batteries employ an aluminum or aluminum alloy current collector protected by conductive coating in combination with an electrolyte containing aluminum corrosion inhibitor and a fluorinated lithium imide electrolyte. This battery exhibits long cycle life at high temperature.
Referring to Fig. 1, this Li-imide cell comprises:
- Anode of active material that is in electronically conductive contact with an anode current collector. The anode active material may consist of lithium metal, carbon, lithium-intercalated carbon, lithium nitrides, lithium alloys, and lithium-alloy-forming compounds of tin, silicon, antimony, aluminum or titanium oxide.
- Cathode exhibiting an upper charging voltage of 3 to 5 V with respect to an Li/Li+ reference electrode. The cathode consists of a cathode current collector with aluminum or an aluminum alloy and a coating. The coating, 0.1 to 8 micrometers thick, includes conductive carbon, graphite, or a mixture thereof. The cathode active material is a lithium insertion transition metal oxide or phosphate placed atop the coating.
- Ion-permeable membrane acts as a separator between the anode and cathode.
- Electrolyte solution is in ionically conductive contact with the anode and cathode. This electrolyte solution contains an additive that provides corrosion protection of the cathode current collector, the anode current collector, or both current collectors. The additive creates a film on the exposed aluminum or an aluminum alloy surfaces of the cathode current collector.
As Fig. 2 shows, Li-imide can offer 500 Wh/l with cycle life comparable to conventional Li-ion batteries, or, by using a lower DOD of 80%, 450Wh/l with an extended cycle life.
Li-imide’s greater temperature stability enables more than 750 cycles over 2.5 years of real calendar life if a consumer fully uses and recharges their battery every single day, even at temperatures higher than 40 °C (104 °F). And at the end of the 750 cycles, the battery will still have 80% of its initial run-time. This more durable battery with greater energy density offers mobile device designers a choice of three strategies when approaching a new generation of products. Fig. 3 plots original capacity vs. charge discharge cycles of an Li-imide battery.
First, designers can create a smaller, sleeker device with the same run-time as the previous generations. While there are undoubtedly limits to just how thin devices can get, mostly having to do with the structural strength of the casing, those limits have not yet been reached. However, this strategy makes particular demands on the thermal stability of a battery, because it’s going to be hard up against heat-generating components.
Second, designers can put more features into the same size device while maintaining the same run-time. Many features, such as induction charging or larger screens, also generate more heat, again making the thermal stability of a battery an important consideration.
In both of the above scenarios, greater thermal stability also means greater design flexibility, because device designers can worry less about the juxtaposition of the battery and heat-generating components.
Third, a battery with higher energy density can give an existing device a longer run-time.
New-generation mobile devices represent a mix of these three strategies, based on the designer’s calculations of the best tradeoff between size, features, and run-time.
To obtain these benefits, mobile device designers need to ask battery vendors two critical questions:
- What is the battery capacity on “day one?”
- What is the battery capacity after 100 to 200 cycles at 100% DOD and 40° C (rather than 80% DOD at 20° C)?
Li-imide batteries answers these questions. These batteries provide higher energy density per volume and weight, an advantage that increases over time, because their greater thermal stability slows aging for more charge-discharge cycles and longer calendar life. Their greater thermal stability also prevents the swelling of Li-ion pouch cells for increased safety. Less age-related swelling yields higher practical energy density. Increased durability means the battery can now actually last longer than the associated device. Fig. 4 plots the swelling over the lifetime of an Li-imide cell and a conventional Li-ion cell.
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