Patent Application
20100171466
None
Altair Nanomaterials Inc., Reno Nev.
Rutgers, The State University of New Jersey,
Hosokawa Micron International Inc.,
1. Field
The methods and devices described herein generally relate to lithium-ion batteries, methods of preparing, and methods of operating such as batteries.
2. Related Art
Lithium manganate (i.e., LiMn2O4) has been considered a potential replacement for lithium cobaltate (i.e., LiCoO2) in lithium-ion battery cathodes for over a decade. LiMnO4-based cathodes -are about one-tenth the cost of LiCoO2-based cathodes; they are safer to use, due to higher decomposition temperatures; and, they exhibit substantially lower toxicity profiles.
Such promising attributes of LiMnO4-based cathodes, however, have been countered by a relatively low cycle-life that has undercut its use in commercial products. The cycle life problem originates from the interplay of at least two factors: 1) in bulk, Jahn-Teller distortion of the compound lattice produces electrochemical grinding; and, 2) manganese dissolution on the surface results in phase transformations and electrode passivation. These problems are exacerbated at elevated temperature, providing for rapid battery failure.
In 1998, Peramunage reported that a battery including a LiMn2O4 cathode could exhibit improved cycle life if the anode was based on lithium titanate (i.e., Li4Ti5O12). Peramunage, D., J. Electrochem. Soc., 145, 2615-2622 (1998). The article discussed Li4Ti5O12/PAN electrolyte/LiMn2O4 passivation free batteries with a cycle life of approximately 250 cycles and an energy density of 60 Wh/kg. A battery with a cycle life of 250 charge/recharge cycles, however, is not good enough for practical application, still leaving LiMn2O4 as a potential replacement for LiCoO2 in cathodes.
Despite these past engineering efforts, there is still a need for lithium-ion batteries with increased cycle life.
The methods and devices described herein generally relate to lithium-ion batteries, methods of preparing, and methods of operating such batteries. The lithium-ion batteries described herein have an improved cycle life. In one exemplary variation, the lithium-ion battery includes an anode including carbon-coated Li4Ti5O12 particles and a cathode including LiMn2O4 particles, and the cathode capacity is larger than the anode capacity.
In order to provide a more thorough understanding of the methods and devices described herein, the following description sets forth numerous specific details, such as methods, parameters, examples, and the like. It should be recognized, however, that such description is not intended as a limitation on the scope of the methods and devices described herein, but rather is intended to provide a better understanding of the possible variations.
The terms “crystallite” or “crystallites” refer to an object or objects of solid state matter that have the same structure as a single crystal. Solid state materials may be composed of aggregates of crystallites which form larger objects of solid state matter such as particles.
The terms “particle” or “particles” refer to an object or objects of solid state matter that are composed of aggregates of crystallites.
The methods and devices described herein generally relate to lithium-ion batteries with an anode/cathode configuration of Li4Ti5O12/LiMn2O4 and methods of using such batteries which exploit the advantageous features of the LiMn2O4 spinel as a cathode material. Specifically, the methods and devices described herein provide Li4Ti5O12/LiMn2O4 batteries having a cycle-life higher than any conventional Li4Ti5O12/LiMn2O4 batteries so far reported. Many parameters with respect to the cathode and the anode of the Li4Ti5O12/LiMn2O4 batteries may be adjusted to give optimum cycle life.
The baseline anode material used in the various lithium ion-batteries described herein may be nano-sized Li4Ti5O12 (LTO or n-LTO) produced by processes described in U.S. Pat. Nos. 6,881,393 and 6,890,510. These patents are incorporated-by-reference into this document for all purposes. The Li4Ti5O12 material may be composed of a plurality of particles. Each particle of the plurality of particles may be composed of a plurality of crystallites. The Li4Ti5O12 material may have a BET surface area of 5 m2/g to 150 m2/g, an average particle diameter of 100 nm to 5 μm, and an average crystallite diameter of 5 nm to 50 nm. In some variations, the Li4Ti5O12 material may have a BET surface area of 10 m2/g to 125 m2/g. In other variations the Li4Ti5O12 material may have a BET surface area of 25 m2/g to 100 m2/g or 50 m2/g to 90 m2/g.
Furthermore, as a baseline material for the cathode of the embodiments, the LiMn2O4 material may be composed of a plurality of particles. Each particle of the plurality of particles may be composed of a plurality of crystallites. The LiMn2O4 material may have a BET surface area of 0.5 to 10 m2/g, an average particle diameter of 1 to 25 μm, and an average crystallite diameter of 0.1 to 1.0 μm. In some variations, the LiMn2O4 material may have a BET surface area of 1.0 to 5.0 m2/g, an average particle diameter of 2.5 to 15 μm, and an average crystallite diameter of 0.2 to 0.8 μm.
The cathode or anode particles may be carbon coated to form carbon-coated particles. A carbon coating technique known as Hosokawa Mechano-Chemical Bonding Technology may be used. This technique bonds particles together using only mechanical energy in a dry phase. The basic operating principle of Hosokawa Mechano-Chemical Bonding Technology is shown in
The anode and cathode of the lithium-ion battery may be prepared from anode and cathode compositions. The anode and cathode compositions may include a binder, an active material (Li4Ti5O12 or LiMn2O4), and a conductive agent. For both the anode and the cathode, the binder may be poly-vinylidene fluoride hexafluoropropylene (PVDF-HFP), and the conductive agent may be a conductive carbon material such as carbon black. The anode composition may include 15 to 25 wt % binder, 65 to 75 wt % active material, and 5 to 15 wt % conductive agent. In one exemplary variation, the anode composition may include 20 wt % binder, 70 wt % active material, and 10 wt % conductive carbon. The cathode composition may include 20 to 30 wt % binder, 60 to 70 wt % active material, and 5 to 15 wt % conductive agent. In one exemplary variation, the cathode composition may include 25 wt % binder, 65 wt % active material, and 10 wt % conductive carbon.
In some variations, carbon coating of the anode and/or cathode particles may provide interconnects with the carbon black to provide good electrical connection of the particles as shown schematically in
The lithium-ion batteries may be prepared by assembling the anode and cathode described above into a battery container with an electrolyte. The electrolyte may be composed of a solvent or mixture of solvents and a lithium salt or mixture of lithium salts. Examples of solvents which may be used include ethylene carbonate (EC), ethylene methyl carbonate (EMC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), diethylene carbonate (DEC), dimethylene carbonate (DMC), γ-butyrolactone, sulfolane, methyl acetate (MA), methyl propionate (MP), and methylformate (MF), Acetonitrile (AN), methoxypropionitrile (MPN). Examples of lithium salts include LiBF4, LiPF6, LiAsF6, LiClO4, LiSbF6, LiCF3SO3, and LiN(CF3 SO2)2. In some variations, the electrolyte may include acetonitrile and LiBF4. In some variations, the lithium-ion battery is prepared such that the capacity of the cathode is larger than the capacity of the anode as defined by a ratio of cathode capacity to anode capacity. The ratio of cathode capacity to anode capacity may be in the range of 1.2 to 2.1. The ratio may be 1.2, 1.4, 1.6, 1.8, 2.0, or 2.1. The lithium-ion battery may be configured to withstand at least 1000 cycles of charging and discharging and to have a discharge energy of 20 Wh/Kg at 2000 W/kg.
The lithium-ion battery may be operated by charging the lithium-ion battery up to 2.6 volts or up to 3.2 volts. The lithium-ion battery may then be discharged down to 1.0 volt.
Nano-sized Li4Ti5O12 having a BET specific surface area of 79 m2/g, an average spherical particle diameter of 5 μm, as shown in
A high power, doped grade of LiMn2O4 (LiCO L410) advertised for electric vehicle (EV) applications was used as a baseline cathode material. This material had an average particle diameter of 7-10 μm, a specific BET surface area of 1-3 m2/g, and a discharge capacity of 105 mAh/g. It is available in large quantities and low cost ($22/kg in 22 T shipments). ICP-AE via P&E Optima-3000DV elemental analysis showed that this material was Li rich and included several other metals.
Another LiMn2O4 spinel commercially available (Aldrich) was modified for use with a high rate LTO anode in other cases. ICP-AE based elemental analysis showed the same Li/Mn ratio as the L410 and a low level of Co doping (0.5 wt %, Mn basis). Both materials may be regarded as roughly equal low-dopant level, Li-rich compounds.
The particle size of the Aldrich LiMn2O4 material ($150/Kg) was first reduced to shards of about 50 nm. This resulted in a material of 30 m2/g specific BET surface area. The crystal shards were then spray-dried at 100° C. in a Buchi bench-top unit and annealed at various temperatures (400-900° C.). This resulted in grain growth and fusion of the crystals into spherical particles of 10 μm average diameter as shown in
PSD analysis via Coulter LS230 confirmed the average particle diameter of 10 μm and stability, even after ultrasonication, indicating fusion of the primary crystals as shown in
The nanosized Li4Ti5O12 was carbon-coated with 2 wt % Super P (SP) carbon black (Timcal) to form carbon-coated Li4Ti5O12 particles. The LiMn2O4 (LMS1) material was carbon-coated with 1 wt % and 2 wt % Super P carbon black, respectively, to form carbon-coated LiMn2O4 particles. These carbon-coated LiMn2O4 (LMS-1) materials will be referred to as LMS1-1% and LMS1-2%.
The anode composition was prepared by combining 20 wt % PVDF-HFP, 70 wt % carbon-coated Li4Ti5O12 particles, and 10 wt % SP carbon black. The cathode composition was prepared by combining 25 wt % PVDF-HFP, 65 wt % LiMn2O4 particles (LMS1) or carbon-coated LiMn2O4 particles (LMS1-1% or LMS1-2%), and 10 wt % SP carbon black. Slurries of the anode and cathode compositions were prepared. Table 1 summarizes exemplary compositions for the anode and cathode slurries. The slurry solvent for these examples is a mixture of propylene carbonate and acetone.
After mixing for 10 minutes in a laboratory blender, the slurry was doctor-blade cast on a Mylar substrate, and electrodes were cut on the Mylar in 2×3 in2 size. After being weighed, the electrodes were bonded by hot lamination at 120° C. to aluminum grids etched and spray-coated with Acheson adhesive conductive coating. This ensured good bonding and low impedance of the electrode-collector interface. The cells were assembled by lamination at 120° C. to a 25 μm Celgard microporous separator. They were of the bicell structure, which was: LTO/Al/LTO/sep/LMO/Al/LMO/sep/LTO/Al/LTO. They were dried overnight at 120° C. under vacuum in a glove box antechamber.
The electrodes prepared as described above were packaged into a battery container and activated in a helium filled glove box. The activation electrolyte consisted of 1.5 mL acetonitrile and 2 M LiBF4 with less than 20 ppm water content.
After preparation of batteries according to Example 2, the battery impedance was measured on a Solartron S11260 impedance analyzer between 10,000 and 0.01 Hz with 20 mV amplitude. The batteries were then transferred to a MACCOR4000 battery tester in a 25° C. environmental chamber for performance evaluation under the following testing protocol:
Since in most cases the cathode was in excess capacity, the rate capability is presented in mAh/g of the anode as a function of C-rate, calculated from the theoretical capacity of the device, whichever the limiting electrode was. The energy density calculations were performed on the basis of entire device weight (electrodes, collectors, separators, electrolyte) minus the packaging weight. The reason for subtracting the packaging weight is that, since only one small battery laminate was packaged, the weight fraction of the packaging material was about 30% of the entire device weight.
The comparison of rate capabilities and cycle-lives obtained with the two cathode materials L410 and LMS1 at the same matching ratio and electrode loading indicates clearly that LMS1 is the best choice for a high power device, as shown in
Three series of batteries were prepared using either LMS1, LMS1-1% or LMS1-2% cathode with the same anode thickness and formulation. In each series, 5 different matching ratios were used ranging from 0.75 to 2 theoretical matching ratio (TMR) by changing the cathode thickness. Table 3 summaries the characteristics of the three series of batteries thus prepared.
The cycle life of the materials was evaluated at 20 C charge-discharge rate over 1,000 cycles for all the batteries prepared with varying TMR factors. The voltage limits were 1-3.2V (5 s dwell) for all the samples. The curves of LTO capacity versus cycle number for LMS1, LMS1-1% and LMS1-2% cathode materials are respectively plotted on
a indicates better anode utilization at higher matching ratios and at increased carbon contents. Surprisingly, the anode capacities measured were in some cases (high TMR and increased carbon coating contents) higher than the theoretical maximum of 174 mAh/g for LTO. This resulted in higher energy density for the carbon coated devices. The energy density of the devices (package weight not included) is optimal when TMR ˜1.3 enables the best utilization of both electrodes, as shown in
The Ragone plots (specific energy versus specific power) are shown in
An understanding of the improved cycle-life and the over-theoretical capacity measured can be derived from the voltage profiles.
Many of the batteries described herein were made of inverted bicell laminates, that is anode/separator/cathode/separator/anode. For comparison, the batteries of some variations were of the bicell structure, that is cathode/separator/ anode/separator/cathode. In this case, the cathode area is doubled and the anode is halved. If the cathode is dominating the capacity fade, doubling its area should result in a lower capacity fade.
The cells were cycled at 20 C rate, either at 25 or 55° C. For comparison, two of the best cycling inverted bicells (LMS1#5 and LMS1-1%#5) were subjected to the standard cycling conditions (20 C, 3.2-1V), except for in a 55° C. chamber. This resulted in an acceleration of the capacity fade, which is a well known feature of the LiMn2O4 spinel. An improved cycle-life at 55° C. for the bicells was observed. Surprisingly, a good cycle-life for the bicell with TMR=1 at 25° C. was observed, dispelling the notion that the Jahn-Teller effect was the major cause of capacity fade for the LiMn2O4 spinel.
Without being limited by theory, the results may indicate that the major cause of capacity fade is the impedance increase on the cathode caused by the formation of a resistive layer which is exacerbated when the time spent at elevated temperature and higher voltage increases. With this regard, the cells with TMR=2 displayed less capacity fade at 55° C. because of their reduced charging voltage. Unfortunately, the bicells had a reduced power capability (despite slightly thinner electrodes) compared with the inverted bicells. This is caused by the fact that the LTO anode, due to its lower electronic conductivity, is indeed rate limiting the system. Thus, when the anode area is doubled as in the inverted bicell, better rate capability is obtained.
In the embodiments explained above, the nano-Li4Ti5O12 /LiMn2O4 battery has been developed in a direction that favors high power delivery and excellent cycle life. The rate capability and the number of charge-discharge cycles are amongst the highest measured for this type of battery. At 80 C, the best devices still utilized 160 mAh/g of the anode, versus 190 mAh/g at 1 C. In terms of device power and energy, this translates to 49 Wh/kg at 50 W/kg, and 20 Wh/kg at 2000 W/kg.
When extra capacity was present in the cathode, it did not cause lithium plating and led to the over-theoretical double-layer capacitance causing a supercapacitor discharge voltage profile from 3.2V to 2.6V. This compensates for the loss in energy density caused by using thin electrodes. Large cathode excess (TMR 1.8 to 2) and carbon coating were also advantageous in increasing the cycle-life and anode utilization, with little penalty in energy density. Good cycle life was achieved, with 18.3 mAh/g n-LTO capacity fade over 1,000 cycles for TMR˜2 in the 1% carbon coated LMS1 cell. The elevated temperature cycling (55° C.) did not result in a dramatic capacity failure, but an increase in the fade slope, with steady and predictable behavior.
Not only a lower capacity fade but also a lower power capability was obtained with the bicell structure that has a cathode area twice as large as the anode area. In this case, excellent cycle-life was also obtained at room-temperature in the cells with a 1 to 1 capacity matching ratio. This indicates that low dopant LiMn2O4 spinel can be fully utlilized over extended numbers of fast cycles when the cathode passivation layer is not given enough time to grow.
These attributes described above, combined with an extremely fast charge capability (full charge possible in 3 min), make the device competitive for applications such as power tools and digital cameras. Especially, when designing protection circuits for the lithium-ion batteries which conventionally require monitoring and control of the voltage applied to the batteries in the order of 0.01 volts, the accurate control of the maximum voltage application by the protection circuit may be somewhat relieved by placing the maximum voltage in the supercapacitor voltage region, i.e., 3.2V to 2.6V.
For more demanding applications such as electric vehicles (EV) and hybrid electric vehicles (HEV), a wider temperature range is possible by the adoption of multi-component carbonate-based electrolytes, binders less prone to swelling, and high Co, Al or F doped manganese spinels with lowered Mn dissolution.
Although the methods and devices described herein have been described in connection with some embodiments or variations, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the methods and devices described herein is limited only by the claims. Additionally, although a feature may appear to be described in connection with particular embodiments or variations, one skilled in the art would recognize that various features of the described embodiments or variations may be combined in accordance with the methods and devices described herein.
Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by, for example, a single devices or method. Additionally, although individual features may be included in different claims, these may be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather the feature may be equally applicable to other claim categories, as appropriate.
Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term “including” should be read to mean “including, without limitation” or the like; the terms “example” or “some variations” are used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; and adjectives such as “conventional,” “traditional,” “normal,” “standard,” “known” and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, a group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group, but rather should also be read as “and/or” unless expressly stated otherwise. Furthermore, although items, elements or components of methods and devices described herein may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated. The presence of broadening words and phrases such as “one or more,” “at least,” “but not limited to,” “in some variations” or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent.
Work for this patent application resulted from an NSF SBIR Phase II Grant No. 0522287 to Altair Nanomaterials Inc.
