Can Lithium-ion be Challenged by Alternatives?
by Donald Georgi
(June 2008) When an entity becomes dominant, its demise can either be brought about from internal or external forces. It is often said that the fall of Rome was not the single terminating event in A.D. 476 when Rome was defeated by the Germanic Odoacer, but that it had brought about its own demise internally through centuries of greed, decay, debauchery, mismanagement, corruption and integration of captured barbarians.
Speeding forward 1500 plus years to 2008, the worldwide dominance of Lithium-ion battery chemistry has established. As core of the notebook computer, power of the ubiquitous cellular telephone, taker of trillions of digital pictures and the basis of the cutely compact I Pod, Lithium-ion has given small size with acceptable energy limits to the micro electronics revolution of the last decade, and today is positioned to control additional applications such as power tools and electric/hybrid autos. Is the growth still exponential? Has it peaked? Or does it stand in jeopardy of falling as did the VCR, Rome, Napoleon, the Third Reich, the Soviet Union and the I 35W bridge?
For a long time, the application in electric/hybrid vehicles was limited by power density, making the current crop of electrics the sole domain of Nickel-metal hydride. But Lithium-ion with its great energy density and charge retention is constantly pursued as a replacement. In 2009, Toyota’s new hybrids are supposed to be rolling off the assembly lines with Lithium-ions giving us wonder as to where the safety challenge, supply bottleneck and product reliability hurdles will lie.
Current thinking suggests that the progress in energy density for Lithium-ion is peaking. In viewing the actual growth of the most common 18650 cell over the past 15 years, the performance growth still looks close to a linear increase. Perhaps the ‘pundits’ who want electrochemistry to follow Moore’s Law are not happy with this performance. Conversely, as energy density increases, the potential for a ‘suitcase bomb’ may be the catalyst for the National Transportation Safety Agency’s action as of January 1st 2008, to outlaw Lithium-ion from aircraft checked luggage, and limit carry-on quantities. (Data accumulated from multiple sources by BD.) +
Before one might attack Lithium-ion with generalizations which may not be universally true, it must be noted that the chemistry has several configurations based on chemical makeup. Cobalt oxide based cathode cells are the darling of C3 devices (computers communications and consumer electronics.) Manganese (spinel), nickel-cobalt-manganese and phosphate cathodes have greater safety, higher power and lower energy density than cobalt, finding application in power tools and electric vehicles. Specifications and applications differ as do the features of safety and performance, but all contribute to the dominance of Lithium-ion.
Like the baseball team with a 14-0 lead in the top of the ninth inning, victory exuberance permeates the world of Lithium-ion. But through the cheers, can any of the replacement noises of competition be heard? Or is there even any cause for concern? If there were indications of conquest, what would they be? Beyond the strengths of Lithium-ion, four challenging weaknesses can be observed. Safety, quality control, cost and power density are areas where competitive chemistries might gnaw away at the dominance of Lithium-ion.
To estimate the status and growth of Lithium-ion, data from Frost and Sullivan showed that by 2007 there were 1.76 secondary units with a value of $ 5.89 billion. The suggestion is that the market will grow to 3.99 billion units by 2013. If the ratio of units to dollars remains equal, the revenues would be $ 13.35 billion. Because of the possibility of new applications in electric/hybrid vehicles, cell and battery size should increase and unit definitions could be difficult to compare. Whatever the details, the overall growth in Lithium-ion revenues (inflation adjusted) might double in the next six years. +
Why list safety first? There are many things which can be compromised, but BD considers safety to be fundamental to human existence. If friend Joe is a bad driver, one would not want to be one of his passengers. In over a decade of covering batteries, BD discovered the early years of commercialization that Lithium-ion had a unique feature, that being -- it could fail in a ball of fire. In 1998, practically the whole business of Lithium-ion from technologist to business persons shuffled this feature under the rug as the advantages were heralded from the rooftops. Ten years later, there are still ongoing documented accounts of smoke, fire and explosions from Lithium-ion batteries in controlled laboratory conditions; this information is available for viewing by consumers.( Example, see Youtube, http://www.youtube.com/watch?v=WeWq6rWzChw.) As energy density has increased, the size of the thermal runaway has grown. What if a competing chemistry could provide equivalent performance, at coemptive cost and be intrinsically thermally stable?
The safety issue has become more sophisticated, as evidenced by the recent edict by the Federal Transportation Security Administration which has excluded Lithium-ion from checked airline luggage. Carry-on batteries are limited to 8 grams equivalent of lithium per battery and can be in the cellphone and computer, with an additional two batteries bringing each passenger’s total to 25 grams of equivalent lithium content (which nominally provides about 300 Watt hours of energy.) This limitation is prompted by portable electronics devices which use cobalt cathode chemistry and have been the object of safety recalls. Limitations on Lithium-metal batteries are even more severe, allowing a maximum of 2 g of Lithium per battery.
What is the purpose of the limitation? Strictly speaking, the quantity of lithium is limited so that if one does catch fire, the amount of heat generated can be contained by the crew. While this thought is not comforting, it implies that small quantities of batteries will not bring the aircraft down. It also limits the possibility of an uncontrolled multiple-battery Lithium based fire initiated by terrorists. Inherent safety provides an opportunity for competition by an alternate chemistries.
Airplane Safety and Battery Fires
Fire safety has its harshest test in aircraft fires. Many of us are passengers on commercial flights. We are especially sensitive to news of aircraft crashes, and of those, the ones dealing with on board fires are the most frightening. Who does not remember Swissair flight 111 which crashed on September 2, 1998 off Peggy’s Cove, Nova Scotia carrying all 229 passengers and crew to their deaths. From onset of the electrical system fire in which Captain [Urs] Zimmermann was showered with the charred effects of the fire, which required he leave the cockpit, only 20 minutes more minutes elapsed before the plane crashed. It was, and still is, generally accepted that the MD-11 was brought down by an electrical fire initiated by the onboard entertainment and gambling system.
The difference between the Swissair and the fate of United Parcel Service (UPS) Airlines Flight 1307 shown here is that the UPS pilot was already descending, over land to the Philadelphia airport at 31,000 feet when smoke was discovered. Within 25 minutes, the much slower developing fire allowed the plane to be on the ground and the crew safely removed with only minor injuries. Fire consequently destroyed the aircraft.
The source of the UPS fire was not attributed to on board Lithium-ion batteries, but the accident report stated, “ Most of the (cargo) laptop computer outer cases were sooted and melted. All of the batteries within the laptop computers were found intact...all of the unidentified electronic devices were sooted and scorched, and some of the material was consumed by fire.” In the December 2, 2007 NTSB Public Meeting on the accident, item 20 indicated “...that lithium batteries can pose a fire hazard.” Other conclusions identified or supported the later 2008 mandates allowing no rechargeables in checked baggage and the 8/25 gram carry-on battery limit. Unseen is any pressure from security people who recognize that multiple batteries can constitute ‘bomb material.’
Before laying all the blame on Lithium-ion for aircraft safety limitations, it should be noted that the NTSB summary of “Battery & Battery-Powered Devices Aviation Incidents Involving Smoke, Fire or Explosion” ( Docket No. AS-228, Exhibit No 17X) lists 62 incidents, only 7 of which were directly attributable to Lithium-ion, 8 to other lithium, 1 alkaline, 2 Nickel-metal hydride, 2 other rechargeable, 4 Nickel-cadmium, 18 Lead-acid and 20 unclassified. In addition to the thermal runaway nature of Lithium-ion, there appears to be a lack of proper preparations in packaging and handling of other chemistries for aircraft shipment. (Photo from the NTSB Flight 1307 Accident Report, p. 18) +
The second weakness of Lithium-ion is quality control. A decade ago, the industry was touting the quality of the principal manufacturers, which mostly meant Sony. The concern was for ‘knockoff’ secondary suppliers which would not have comparable quality, thereby opening the doors of thermal runaway and at a bare minimum, offer poorer performance. Unfortunately, the perception of quality did not impugn the name of Sony, yet by 2006, Sony had to recall 10 million batteries for metal impurities which could cause cell thermal runaway. In 2007, Nokia had to recall 46 million Lithium-ion batteries warning that there was a possibility for overheat and explosion. In 2004, Kyocera recalled 1 million batteries; Dell recalled 22,000 in 2006 and Lenovo recalled over 200,000 in 2007 because of a possible explosion risk. Although battery manufacturers are often given safe haven by not being identified in a product recall by a computer or cellphone manufacturer, the performance by the stalwarts of the industry such as Sony show that with high quantities, competitive forces, failed quality systems, unsafe Lithium-ion batteries have found their way to the market, no matter what the perceived quality. On the other hand, there are few worries over a stalwart such as Energizer which produces Alkaline cells without catastrophic quality problems because the chemistry is intrinsically safe. The cell might split open, leak, eat away at a bit of clothing or furniture, fail in a few minutes of operation, but it does not provide the basis for an explosion. Granted, primary Alkaline is not a worthy performance competitor to rechargeable Lithium-ion, but the recent quality weakness which lead to highly visible thermal runaways offers door-opening market opportunities to other chemistries.
The competitive cost is difficult to quantify. If we tried to compare a Nickel-metal hydride battery with a Lithium-ion, holding all performance factors equal, we find huge differences in size and charge retention which make the comparison impossible. To use energy storage or power delivery as comparison points, the stronger convenience factors cannot be included to make a comparison. To make things worse, the perceived value of Lithium-ion is masked within the total performance of the product which it is powering. Take power tools for example, once the domain of nickel chemistry, the contractor or home user perceived a rechargeable drill as being in the $50.00 category. As Lithium-ion powered drills became available, the contractor decided the added run time was worth another $200.00 because it could easily be amortized in less recharge time on the job. Once these drills became available in Home Depot stores, the home users, picturing themselves as weekend contractors, have easily made the jump to Lithium-ion, merely because of the perceived image of being a weekend contractor, saving money by ‘doing it yourself.’ Another comparison is in the digital camera business. Lithium-ion appears to have won out over Nickel-metal hydride because of the form factor, allowing palm size 5 megapixel cameras to easily fit in the purse, having so much energy that a full day of photo-op sight-seeing in St. Petersburg’s Palaces can easily be accommodated.
So, is Lithium-ion is more expensive? More expensive than what? Now we get to the possible price opening for a competitive chemistry. What is a comparable price? Since Lithium-ion gets bundled into the pricey device, we should expect total price for the safer chemistry product to be in the same ballpark. If there is to be a competitive battery, it must have equivalent or better energy or power density, be able to accommodate the form factors which have become commonplace with users and offer greater safety. The cell phone can’t get bigger, the computer can’t get heavier, and the i Pod must have it’s cool outline. If the competitive battery has only half the cycle life, the provider might offer replacements at half the cost of comparable Lithium-ion batteries, otherwise shoppers may not buy into the competition.
It would be nice to have a competitive chemistry which could have superior safety and price performance features. Within any competitive struggle is the pricing structure which develops over time. Despite competition, Lithium-ion has been able to demand its premium when compact size, long run time and excellent shelf life are factors. The customer usually does not choose how much perceived safety and performance is in the new laptop because the marketing department does not provide such information. Does the competitive chemistry have to compete at the OEM or the lower knockoff price level? Until performance and safety are proven, it most likely will have to compete with the lower knockoff price. The great disparity between OEM and knockoff prices, makes it a difficult entry for a new chemistry provider.
Another consideration is manufactured cost vs. selling price. When pricing power is low, the difference is also low, leaving the manufacturer with small profits. When factors such as low supply allow pricing power to bump prices, the manufacturer’s margin increases, but it may mean that volume steadily decreases as consumers either do without or seek lower cost alternatives. Today the world economy, held captive with higher oil prices, is seeing that consumers are grudgingly accepting not only oil’s higher price, but also foods, which slows the economy and in the long run will contribute to inflation and stagflation. The combination sparks increased demands for battery powered hybrid vehicles, which again, although more expensive, are viewed as defenses against ever growing prices at the gas pump. All kinds of batteries will see increases in the cost of energy to produce and reduced demand because of a weak economy. This problem is not the sole domain of Lithium-ion, but if a competing chemistry can be constructed with greater safety, lower materials and manufacturing costs plus efficient business acumen, it can gnaw away at the market volume for Lithium-ion.
The fourth concern, power density, was a big limitation ten years ago. No one would have projected that the portable electric drill could be an application for Lithium-ion because it just could not provide high currents. As the chemistry alternatives including manganese nickel and phosphate cathodes matured, so did the power density, and today, we are dazzled by cordless saws, drills and sanders to the relished by power tool companies, and their customers. Concurrently, auto manufacturers are making commitments to future hybrids with Lithium-ion batteries.
Competitive consideration cannot be completed without giving Lithium-ion superior marks in calendar life, cycle life and both gravimetric and volumetric energy density. These features contributed greatly to Lithium-ion’s preference over Nickel-cadmium and Nickel-metal hydride despite both being safer chemistries. Users have grown to expect long performance periods without self discharge and multi-year service before replacement. Competitive chemistries which cannot provide similar capabilities face difficulties in being accepted.
For now, Lithium-ion appears to have suitable performance to meet the challenge of high power products, so the most we can ask of competitive chemistries is equivalent power densities at comparable costs.
Challenger I, Silver-zinc
The safety, cost and performance issues have opened the door for alternatives. Lithium-ion cobalt oxide 18650 size cell energy density growth has been steady for a decade at about 8-10% annually. There is opportunity for acceptance of alternate chemistries with safer thermal characteristics, comparable energy density today and the potential for future growth.
Such is the case with silver zinc being commercialized by ZPower. Silver-zinc chemistry, which distinguished itself in the Apollo spacecraft, has found limited applications in military systems. Considered either a primary or limited cycle rechargeable; it has been limited by the zinc electrode degradation which leads to soft and hard shorting with the growth of zinc dendrites and separator degradation. Additionally; recharge cycles have been limited to tens of cycles.
An alternative battery a notebook computer is being prepared by ZPower using Silver-zinc chemistry. The hallmarks of the battery are its intrinsic chemistry safety without thermal runaway and volumetric energy density which packs more Watt hours than Lithium-ion in the same package size. For users who want more run time during flights, added ZPower packs are not restricted from carry on luggage, and it should be shippable in checked luggage. Battery charging characteristics will require the notebook charging circuitry to be designed for Silver-zinc requirements, so customer swap-outs with current Lithium-ion packs should not be possible. On the other hand, if ZPower were to make the battery an external power pack which could feed the lithium-ion notebook during flight, the user could arrive at his/her destination with a fully charged Lithium-ion pack. ZPower indicates that it has proprietary chemistry which circumvents traditional Silver-zinc charging limitations which is said to offer up to 250 cycles. (Photo courtesy of ZPower.) +
Today, the improvements held in intellectual property by ZPower allow an offering which directly targets the cobalt oxide Lithium-ion batteries in notebook computers. ZPower has an introductory package being offered for conventional notebook battery form factors. Volumetric energy density is in the ballpark of 650 Wh/l compared to less than 500 Wh/l for Lithium-ion. (In notebook computers how-big-it-is is more important than how -much -does-it weigh.) ZPower’s projection for the increase in energy density is to grow from this base to the region of 800 Wh/l by 2010.
Focusing on lithium’s weakness, the failure mode of Silver-zinc does not produce as violent a thermal runaway. However, overlooked by current sales oriented information from ZPower is the description given by Silver-zinc battery expert Albert Himy in his book, Silver-zinc Battery: Phenomena and Design Principles. When discussing open-circuit internal shorting, Himy states, “...a slow short may turn into a “fast” or “hot” short, which is a runaway condition in which voltage may drop fast and the heat generated may raise the electrolyte temperature to a boiling point and may even cause a fire.” Despite this statement, Silver-zinc is considered much safer than Lithium-ion and the battery can utilize the space used for Lithium-ion pressure vents, PTCs and safety electronics for additional current producing materials. When packaged, an equivalent run-time battery of Silver-zinc is smaller and can have the space saving rectangular form factors needed for compact electronics. Since the self-discharge of Silver-zinc is about 15% per year, it is better than the 5%/month self-discharge of Lithium-ion.
The down side, Silver-zinc has fewer deep cycles which for the ZPower Silver-zinc is in the region of 200-250, whereas Lithium-ion can provide 250-500 cycles. There is an additional grey area: depth of discharge which affects the number of cycles. In other words if the battery is repeatedly discharged all the way to its cutoff voltage, the cycle life will lean toward the minimum, whereas the battery often topped off from smaller depth of discharges may experience noticeably greater cycle life.
For the customer, the perception of cycle life’s total value might require that the replacement battery to be purchased so that the total cost for two Silver-zincs is approximately equal to one OEM Lithium-ion. At the prices being charged for OEM Lithium-ion these days, the price challenge may not be that difficult for Silver-zinc. A replacement Lithium-ion battery for a Sanyo/Sprint cellphone is one example. With shipping, the Sanyo replacement is about $75.00 while a competitive no-name replacement costs about $25.00. Admittedly, the Sanyo produced battery has better perceived quality control, but after the Sony recall disaster, even the most sacred quality- name companies can no longer guarantee a good, safe Lithium-ion battery. It is difficult to imagine that a tiny Sanyo cell phone battery cost is almost twice as much as a 30+ pound automotive Lead-acid battery from Wal Mart.
This word ‘perceived’ may be at the heart of ZPower’s success. Today people pay significantly higher prices for green powered products (i.e. hybrid autos, solar electricity and ethanol). What will they pay for a safer product? In this regard, ZPower has an opportunity to price its battery in the ballpark of Lithium-ion despite its shorter cycle life. The reason for equivalence should be the safety advantage of Silver-zinc.
Pricing may also be impacted by costs. Most new products have a reduction in pricing as time passes and the Silver-zinc has a special consideration in cost evolution. The silver used in the positive electrode is pricey, and recent inflationary trends have caused silver to skyrocket. The spot price of silver has risen from $5/oz. in 2002 to a high of $20/oz in 2008. Expectations of price reductions are not forthcoming. ZPower has a key long term solution to help pricing. That is recycling. Although Lithium-ion cells can be recycled, the leftover is generally a collection of materials with limited value, but if a Silver-zinc cell is recycled, its value is similar to Lead-acid batteries where a large percentages of lead is reclaimed. With recycled Silver-zinc batteries, the quantity of reclaimed silver is high and may reduce raw material costs by up to 85%, according to ZPower. This savings begins only after the first round of cells has been delivered with its high new-cost silver. Whether ZPower positions itself to take advantage of the long-term cost reduction at the outset or waits until cells are returned will affect the initial offering price. Since the batteries will most likely not be replacements for Lithium-ion due to charging requirements, the ZPower cells will have to be designed-in to the new devices, such that other than the safety aspect, buyers will have little price comparison to evaluate.
Today, green sells. Will safer sell tomorrow? Consumers have learned that the push toward reduced emissions and energy independence is symbolized by the color green. If it is green, it is supposed to be environmentally friendly. (This does not hold true for customers who, while eating, turn green from improperly prepared food.) What if the color for intrinsic safety was a light blue. Dell might sell a Silver-zinc powered notebook in the safety blue color while HP offered a black Lithium-ion powered notebook with equivalent performance and pricing. Would buyers give Dell the nod? Is logical to conclude that because buyers are willing to pay more for a green product, the blue, ‘safe notebook’ could demand an additional 5% premium too? Just as the pricey i Phone is a must have for the teeny bopper, could the blue notebook be a part of her essentials at any price?
The difference in charging regimens appears to dictate that Silver-zinc powered devices will have to be designed from the ground up for this chemistry. Silver-zinc replacements will not be compatible with existing Lithium-ion powered devices because the charging regimens are different. One would expect that a whole family of devices will be dedicated to only one chemistry because of charging and safety requirements. With the cunningness of electronics designers, there is a possibility that a future generation of machines could be designed with a charger which would recognize which of the chemistries was powering the machine and adjust the charging algorithm to meet the battery’s charge profile. An example of compatibility was in the dual format High Definition DVD machines which were able to read HD DVD or Bluray. Granted, the dual capability cost more and was priced higher, but during its time, buyers were guaranteed being able to read both formats.
The way to get around the ground-up redesign of complete notebook power systems would be to have external “fuel tanks” which continually provided power in place of the AC adapter. Today little ‘top-off’ modules powered by replaceable primary AA Alkaline batteries to boost our cell phones in the field can be purchased. If ZPower built a fuel tank as an external module with headroom to recharge PC’s, cellphones, PDAs and MP3 players, the user would both operate and recharge the device from external energy. Since Silver-zinc does not have a safety concern, the user could carry multiple modules in carry-on baggage and have continual electronic happiness on a flight from Minneapolis to Australia. These modules should have a warning or auto-disconnect when the Silver-zinc battery is optimally discharged to maximize recharge cycles and also include a state-of-charge indicator. (A state-of-health indicator would make the device almost perfect.) Having a variety of outputs for multiple devices in a single fuel tank would increase versatility and an automobile-lighter plug-charger would extend range for the mobile user. Such a device would have a principal ‘convenience-plus-safety’ aspect and not be subject to raw energy-per-dollar comparisons with Lithium-ion modules. That is not to say that Silver-zinc is immune from pricing itself out of the market. On the other hand, it might price itself into the market with ubiquitous ‘top-off’ convenience.
The variables are too many to either spell victory or demise for Silver zinc at this time. ZPower has a formidable challenge in creating the product for this market. It will not be easy or cheap or initially profitable. If ZPower could align with one or more brand name battery companies with deep pockets for this work, the longer term possibilities for success might be improved. It is notable that Intel Capital the venture group of Intel has provided financial backing to ZPower. Past information on Silver-zinc can be found at http://www.batteriesdigest.com/alkaline_zinc.htm, http://www.batteriesdigest.com/batteries_silver_zinc.htm and http://www.batteriesdigest.com/head_for_mars.htm.
Challenge II, Nickel- zinc
Nickel- zinc chemistry was first demonstrated in a light rail system in 1932 and has often offered hopes for success, but unfortunately has had continued disappointments in that millennium. By the turn of this century, Nickel-zinc cycle life performance was reported to be improved through the development of a reduced solubility zinc electrode. Deep cycle capability was increased to 600 cycles while maintaining a high specific energy up to 60 Wh/kg. But the updated chemistry has not been able to penetrate the market. Nickel-metal hydride and Lithium-ion continue to dominate electronic, power tool and electric vehicle applications.
Because of the benign nature of its materials, nickel and zinc, combined with an alkaline electrolyte, common sense would expect an intrinsically safe chemistry free from thermal runaway problems. Since the warning of zinc dendrites contributing to thermal runaway in Silver-zinc batteries by Albert Himey (noted previously,) might there be a similar failure mode for nickel-zinc? So far our literature reviewed does not address this condition.
Meanwhile, companies such as Powergenix have reformulated the electrolytes, which according to company reports “prevent the dendrite shorting and shape changing problems.” Targeting automotive applications, Powergenix is retrofitting a Prius with side-by-side Nickel-zinc D-cell packs to demonstrate a 30% gain in mileage. Although hybrids do not necessarily experience significant full discharge cycles, the company is touting several hundred cycles. (Editor’s question. Does several = 200, 500, 900 ?...and at what depth of discharge?) The glimmer of hope is that Nickel-zinc will be no more expensive than Nickel-metal hydride with better cold temperature performance.
This smaller, lighter Nickel-zinc rechargeable battery offers power and mileage advantages for automotive applications. Powergenix unveiled this rechargeable D-Cell battery pack for hybrid electric vehicles (HEVs) on the week of May 12, 2008 at the Advanced Automotive Battery and Ultracapacitor Conference. PowerGenix's Nickel-zinc battery pack is said to be capable of delivering thirty percent more power and increased energy-density, as well as reduced size, weight and cost relative to existing Nickel Metal-Hydride. +
Xelleron, Inc. connected with eVionyx, Inc. is providing batteries for electric vehicle applications having a proprietary membrane electrolyte. If these improvements can extend cycle life and can be price competitive, there could be a place for Nickel-zinc in power tool and vehicles having both safety and performance as benefits.
Lithium-ion’s future strengths
Competitive chemistries would like to seal the coffin of Lithium-ion on the basis of safety and recent recalls. But the Lithium-ion market has become too big. According to a 2007 report by Japan’s Institute of Information Technology, the overall Lithium-ion market is about 2.5 billion cells per year. Frost & Sullivan reported in December of 2007 that the Lithium-ion market revenues were $5.89 billion. Since multiple cells make up batteries and prices escalate from manufacturer to OEM to distributor to retailer, the dollar size of the market is debatable. Still $5.8 billion is a pretty big number in most places other than Washington D.C. Such size means many dollars are appropriated by business R & D, while the government funds research to constantly improve Lithium-ion. Because of this cash stimulus, the possibilities for safety and performance improvements appear positive, possibly insuring Lithium-ion’s continued dominance for another decade.
Lithium phosphate has been pursued for over a decade. Companies such as A123Systems, Aleees, Lithium Technology Corp, Phostech, and Valence Technology are currently pursuing commercialization. Replacing the positive material of cobalt or manganese with a special doped nano phosphate recipe developed at MIT has opened the door for A123Systems, providing high power tool batteries and automotive transportation prototypes.
Another champion of the phosphate chemistry is Valence Technology which focuses on improving safety. Along with the phosphate positive electrode material, their batteries utilize an electrolyte that plasticizes the polymer, thus eliminating free liquid in the battery cell. Valence features cycle life which is 3-4 times that of lithium cobalt. According to company information, independent testing by Exponent, Inc. showed that lithium metal oxide batteries generated temperatures four times higher than those of the Valence phosphate based batteries.
Inside the molecules, the combination of iron, phosphorous and oxygen bonds the oxygen much more tightly than the cobalt-oxygen bond. The result is a need for temperatures to exceed 8000 C to breakdown, increasing the margin of safety. Cobalt positive electrodes significantly expand and contract while the phosphate is dimensionally stable in transitioning from charged to discharged states. When a cobalt cell is fully charged, about 50 % of the lithium resides in the positive electrode. In the fully charged phosphate cell, there is no elemental lithium to be mechanically or chemically stimulated to initiate a thermal runaway. Valence considers phosphate an intrinsically safe cathode material.
Phosphate based products for both of these companies have progressed beyond the research stage and are either available as product today or are provided in prototype testing.
Negative electrode research, development and manufacture
Reports of significant contributions focus on new materials and methods for negative electrodes. Current negative electrode material is some form of carbon such as graphite although Sony has introduced its Nexelion which has a tin based amorphous anode. The key is employing nano materials which allow greater access for lithium ions to nest in ‘holes’ of the anode. The nano construction of the anode then allows a greater number of ions to nest per unit volume, contributing a 50% reduction in needed negative space for the cell. If all else remains the same, total cell volume is reduced by 30%.
Sony has been able to boost volumetric energy density by 30% with the implementation of a tin based amorphous negative electrode material, dubbed the ‘Nexelionâ.’ Multiple elemenents of the nanomaterial minimize particle shape dimensions during charge and discharge, improving cycling characteristics. At the same time, Sony modified its positive electrode with multiple metal atoms of cobalt, nickel and manganese to give it higher temperature performance which improves safety. Cold temperature performance is increased by 40% at -200 C. and charging efficiency is enhanced to allow recharging 90% of capacity in 30 minutes. (Sony, February 15, 2005 press release.) +
Improvements do not stop there. With carbon negatives, the discharge expands the cell volume causing internal stresses which limit cycling ability. The new nano based anodes greatly reduce the change in particle shape, adding cycle life to the cell. BD could not find any data proving the safety improvements produced with smaller cell volume changes.
The Nexelion cell also includes new composite positive electrode materials. The combination of cathode and anode changes produces a cell with 40% better low temperature performance. At 00 C, the cell still provides 90% of rated capacity. Anyone who has tried to use a camera or video cam for winter scenes would appreciate the better performance provided by Nexelions.
Moving to the R & D mode, another disruptive change on the horizon is championed by Altairnono. In its implementation, the negative electrode is made of a nano-structured lithium titanate spinel oxide (LTO). (Spinels are a class of minerals of general formulation XY2O4 which crystallize in cubic crystals.) The safety aspect is addressed with LTO because the materials do not react with ordinary Lithium-ion electrolytes. Graphite is very sensitive towards electrolytes and can easily be exfoliated in PC (propylene carbonate)-based electrolytes, which in turn decompose PC into propylene gas. The organic propylene gas is likely to cause explosion under some abuse conditions, which could be a potential hazard for battery users.
Additionally, the inability of to react with the electrolyte eliminates the solid interphase (SEI) barrier which forms around ordinary electrodes and degrades both calendar and cycle life. According to Altairnano, the cycle life extends to 10,000 to 15,000 recharge cycles, with improved low temperature performance. Without the SEI, the LTO battery provides almost 90 % of rated capacity at - 300 C, a distinct advantage for automotive applications.
Being a nano technology structure, the enhanced ability of ions to find sites because of larger surface areas and proximity to the surface, means higher power density. The LTO cell is supposed to deliver three times more current than ordinary Lithium-ion cells. Elimination of the SEI also speeds recharge.
The gold standard of performance is still safety, and according to Altairnano, testing has shown that it performs in ambients up to 2400 C. Additionally puncture and drop tests showed no malfunctions. Similar tests with ordinary Lithium-ion cells produced smoke, fire and explosions.
In November of 2007, Chan, et. al of Stanford University published a letter on the use of silicon nanowires for the negative electrode of lithium batteries. Silicon has the highest theoretical charge capacity of 4,200 mAh/g and low discharge potential. This is an order of magnitude better than current graphite electrodes. But silicon swells by four times with insertion of ions leading to pulverizing and fading. To utilize the great charge capacity without the swelling, the Stanford group has fabricated nanowires of silicon which resemble the pile in rugs, with an associated volume space between strands. The real surface area is greatly improved, and as ions are inserted, the strand undergoes the volume change, and there is no change in the volume entire ‘rug’. Testing has shown discharge capacity near 75% of the maximum and little fading during cycling. While not ready for commercialization, the approach extends the future of Lithium-ion in achieving much greater total energy density.
At Ohio State University, Yanguang, et. al employ nanowires fabricated of cobalt oxide which are deposited on a titanium substrate which also acts as the current carrying component of the negative electrode. There is an increase in the demonstrated capacity to 700 mAh/g or about two times that of graphite. The nanowires maximize the surface area in contact with electrolyte and provide expansion space during ion insertion. High rate discharge has been tested to 50C where it exhibits capacity of 50% of the 1C rate. The technology is being offered for licensing at this time.
At Ohio State University the construction of cobalt oxide nanowire negative electrodes has been announced with offers for licensing. The open spaces and large surface areas of the ‘rug strand like’ structure provides high currents and capacity increases while accommodating the strain experinced in charge/discharge operations to enhance cycling performance. With materials directly deposited on metal or conductive polymer conductors, the process offers manufacturing uniformity and simplicity. (Permission from Yiying Wu, Assistant Professor Department of [email protected], Ohio State University. Letter published in Nano Letters 2008, Vol 8, No 1 265-270 by the America Chemical Society, on the Web 12/12/2007.) +
The Shenyang National Laboratory for Materials Science in China reports building cells with negative electrodes constructed of carbon nanotubes. Cheng and colleagues report that testing of cells with the nanotubes has shown discharge capacity to 727 mAh/g. The nano structure removed the problem of volume change during charge and discharge. The operational status of this program is not yet available.
Prof. Li Jun Wan, director at the Institute of Chemistry at the Chinese Academy of Sciences, has pursued construction of tin based nano deposits encapsulated in hollow carbon spheres to provide very high specific capacity negative electrodes for Lithium-ion batteries with excellent cycling characteristics. Tin adds higher theoretical specific capacity, operating voltage and does not encounter solvent intercalation which causes irreversible charge losses. To eliminate the ion insertion volume changes, the tin resides in a hollow sphere which does not change volume but allows the insertion of ions in the void space. The scientists project that the technique can be extended to other anode materials as well as cathodes. It is expected that the construction will add to the safety performance of cells.
Positive electrode research, development and manufacture
Moving across the cell to the positive electrode, the activity does not appear as rich in popular literature. Scarcity gives way to quality in the case of work from the Argonne National Laboratory which has pursued Lithium-ion R & d since 1994. Dr. Michael Thackery has over the past six years researched a family of materials including nanocrystalline lithium-manganese-cobalt (LMC) to improve capacity, add structural stability and increase thermal safety. Present positive cobalt oxide electrode materials provide 130-140 mAh/g while manganese spinel and phosphates deliver 100 to 120 mAh/g. By comparison, the Argonne LMC formula delivers initial capacity over 250 mAh/g with better structural and thermal stability.
The proof of this improvement is in the nonexclusive licensing agreement with Toda Kogyo Corp. This is a far reaching association because Toda, with facilities in Japan, Canada and Detroit is a primary supplier of cathode materials for Lithium-ion and Nickel-metal hydride to cell manufacturers worldwide. This license should make the LMC materials available to any manufacturer which wanted to utilize the higher performing electrode material. Based on implementation, safety data should be forthcoming on the relative performance and safety of LMC compared to present cells.
Another critical component of electrochemistry is the electrolyte. With Lithium-ion, it is heavily involved in thermal instability. Solid or polymer electrolytes provide ‘safer’ performance than liquid electrolytes. The problem is that higher power performance requires the liquid electrolytes. For example, Solicore produces a solid-state electrolyte which has no volatile electrolytes, but conductivity limits application to low power credit card type applications.
At the Fraunhofer Institute for Silica Research (ISC) researchers have produced a solid polymer electrolyte which is targeted for higher power devices such as laptops. This work is still in its early stages and applications are not projected for three to five years.
Leaving no stones unturned, Lithium-ion separator technology must also be considered. Separators are the ‘passive wall’ which keeps the electrodes apart but must provide the permeability to allow ions to pass via the electrolyte from one electrode to the other. In December of 2007, ExxonMobil introduced an improved separator which enhances the safety, power and reliability. The separator is supposed to have higher meltdown temperature which should raise the bar on conditions necessary to initiate and sustain thermal runaway but BD has not obtained performance data to validate the claims. One would consider that such a large organization’s credibility would provide the possibility for such a safety improvement. No information was presented on how the manufactured cell costs would be impacted.
We have seen that the long term efforts at Argonne National Laboratories have produced a significant contribution. The nature of extending knowledge of Lithium-ion electrochemistry will become ever more sophisticated. It will take the likes of world electrochemists to find improvements, some incremental and some disruptive, and unfortunately often to end at a blind alley. There are major works in the US national laboratories and universities. The Japanese, Chinese and Europeans regularly make discoveries which improve safety and performance. A new group formed with the resources of STMicroelectronics and the French Atomic Energy Agency, over a four year period, will investigate power source improvements for portable electronics and medical implants. With over 50 researchers, there are high expectations that this group to contribute to the safety and performance of Lithium-ion electrochemistry.
The Bottom Line
It is highly unlikely that competitive chemistries will replace all Lithium-ion batteries because of improved safety and performance. Rather, there is a place to augment Lithium-ion with aircraft carry-on recharging battery units using such chemistries as Zinc-air (think of them as ‘recharging bricks’) which will allow passengers to watch movies nonstop from New York to Hong Kong. The passenger will deplane with notebook, cell phone, camera and Gameboy fully charged, ready for a full day’s service. Later at the hotel, the brick is recharged for future remote duty.
Niche markets could develop to provide some special component of convenience such as interchageability of the AAA, AA, C and D form factors. Rather than worry about being defeated, the world of Lithium-ion can benefit with the synergy of alternative electrochemistry which improves safety, performance and convenience. It should be very interesting to experience how this scenario develops in the oncoming years.
Meanwhile, safety has its focal point in aircraft fires. A flaming notebook on an office desk is one thing, but a flaming notebook 35,000 feet over the middle of the Pacific is quite another. Regulatory actions such as the FAA carry-on limitations can circumvent catastrophic thermal runaways while improvements in aircraft materials, fire handling procedures and fire extinguishing equipment will not only help Lithium-ion safety, but also general on-board fire safety.
An area which needs to be addressed is in packaging, especially for the transportation of all types of batteries. The FAA battery fire summary shouts to designers, manufacturers, packagers and shippers the need to commit to excellence in delivering batteries which do not catch fire or smoke during shipment. The message is clear; it time for manufacturers to act!