Li-ion Batteries
Batteries have evolved and improved by about 7-8% every year, with the cost coming down at the same rate. They now have three applications that drive progress: Electric cars, power storage for solar and wind when one or both aren’t making much electricity, and for caching power, e.g. storing when power is cheap, and discharging when it is expensive. Batteries have evolved past automobile lead-acid starting batteries and powering toys, to giant commercial applications. See my old post “Metal-Air Batteries” as well as the post “Storing Energy from Solar Arrays” to see where my brain was several years ago.
Rechargeable Batteries and Nickel-Cadmium
Aside from lead-acid starter batteries in my cars, my early portable devices with rechargeable batteries all had 1.2v Nickel-Cadmium, NiCd, cells. These had a “memory” problem and needed to be regularly fully discharged and then fully charged. Partial discharges and partial recharges got “remembered” by the battery, and it became self-limiting. Materials for the NiCd batteries were expensive, and the cadmium in particular was toxic and was bad for land fills. NiCd batteries also had a high self discharge and needed recharging after storage.
There were many nice features of NiCd batteries. They had a high discharge/recharge cycle count. They could be recharged quickly with little stress. They had good load performance and good cold weather performance. They became available in a wide variety of cell sizes. In the 1980s, an “ultra-high capacity” NiCd battery was introduced, but it had a reduced cycle count due to higher internal resistance.
Nickel-metal-hydride batteries
Starting in 1967, research in nickel-hydride (NiH) and later in nickel-metal-hydride (NiMH) solved early problems with rapid self-discharge and internal corrosion, but specific energy remained a problem with NiMH batteries. Today NiMH is the most available rechargeable battery for consumer use, and most battery manufacturers such as Duracell, Energizer, Panasonic, Rayovac, and Sanyo provide all popular sizes such as AA, AAA, etc. NiMH batteries have essentially replaced NiCd batteries for consumer use.
Lithium-ion batteries
Lithium is the lightest of all metals, and, in a rechargeable battery cathode, can have the largest specific energy per weight. Unfortunately, cycling produces dendrites on the anode whose growth will penetrate the separator causing a short. The cell temperature then rises quickly and approaches the melting point of lithium; this causes thermal runaway and fire. The inherent instability of lithium thus shifted focus to various lithium ions for the cathode. Research continues on how to avoid or mitigate lithium dendrites. In 1991, Sony brought out the first commercial Li-ion battery. Li-ion cathode batteries have a lower specific energy than pure lithium anodes, but are much safer with proper voltage and current limitations. There are many Li-ion structures and hence many types of Li-ion batteries, each with different properties. The high cell voltage of 3.6 volts provides a Li-ion battery high specific energy. It has good load characteristics and a flat discharge curve over a voltage range of 3.7 to 2.8 volts.
In 1994 the cost of a Li-ion 18650 cell (the last zero indicates cylindrical, 18mm diameter, 65mm high) was over $10 and the capacity was 1100mAh., but by 2001 the cost had dropped below $3 and the capacity increased to 1900mAh. Today the 85 kWh battery pack for Tesla’s Model S contains 7,104 Li-ion 18650 cells. Estimating $130 per kWh, the cost per 18650 cell is about $1.56. Each cell has a capacity of 85000/7104 = 11.97 Watt-hours at 3.6 volts = 3324 mAh. Costs are projected to fall below $100 per kWh in the next few years.
Form Factors
People are familiar with A, AA, and AAA battery form factors as well as many others that can be found in any retail store that sells batteries. Tesla started using the 18650 form factor mentioned above, but for the Model 3 and beyond, Tesla is using the 21700 form factor for its Li-ion batteries. (Again the last zero indicates cylindrical, with 21mm diameter and 70mm height.) This is a 25% increase in volume. It also tends to reduce the number of contacts and cells within the battery pack making the battery pack a bit easier to manufacture. The energy density has improved 20% according to Tesla, and the cobalt content has been reduced while increasing the nickel content reducing cost. Unfortunately, the core temperature is 20% higher, reducing the life cycles 20%. This degradation is acceptable, but larger form factors may have safety problems.
Of course, a form factor can have almost any Li-ion chemistry, In fact, different applications can use different chemistries. For example, the Tesla Model 3 uses Li-NiCoAlO2 (NCA) while the commercial Tesla PowerPack in Hornsdale, Australia uses Li-NixMnyCozO2. (NMC). Both use the 21700 form factor.
Li-ion Battery Cathode Chemistries. Note industry trend is to use Mn to reduce the use (and cost) of Co.
Name |
Cathode Formula |
Abbr |
Use |
Comment |
Manufacturer |
Lithium Cobalt (Cobaltate) |
Li-CoO2 |
LCO |
Cellphones, laptops, cameras |
First Li-ion battery. Heats up at high voltage. Doped for increased energy density levels, but lower life-span. Cobalt is rare & expensive. |
Sony 1991, Chinese |
Lithium Manganese (Di-)Oxide |
Li-Mn2O4 |
LMO |
Power tools, small portable devices |
Nissan Leaf, Chevy Volt, BMW i3 |
UltraLife, Varta, SAFT, Regulus, Fanso, Zeus, |
Lithium Iron Phosphate |
Li-FePO4 |
LFP |
Power tools, small portable devices |
Safe but low volumetric energy. 32650 size. |
BYD, OptimumNano Energy. |
Lithium Nickel Manganese Cobalt Oxide |
Li-NixMnyCozO2 |
NMC |
EVs, (Tesla) grid storage, Hornsdale |
Good cycles at high capacity, but lower than NCA. Material patented and licensed. |
Used Samsung for Hornsdale |
Lithium Nickel Cobalt Aluminum Oxide |
Li-NiCoAlO2 |
NCA |
(All Tesla) EVs, grid storage |
Higher cycle stability at high capacity. Al is used instead of Mn to stabilize crystal structure. Low material cost. Enters thermal run-away at lower temperatures than NMC. Thus, limited to lower capacity cells. Material patented and licensed. |
Tesla has “gigafactory” in LV. |
Lithium Titanate |
Li4Ti5O12 |
LTO |
EVs, grid storage, anode |
Rechargeables can take 3-7000 cycles. Compared to 1000 or so for NCA. Works well for busses. |
Altairnano, Lelanche, Microvast, Toshiba, Seiko, Yabo |
Li-ion Anode. Always some form of graphite, but trend is to go towards Silicon, which can store 10x more energy than graphite per volume and 3x the energy per mass. However, Silicon expands 400% during charging. Allowing for this expansion would take up too much volume; on the other hand, just doping the anode with a little silicon oxide, SiOx, appears to be a good compromise. The original Tesla Model S, did not do this, but later the Model S and the early Model 3 used 5-15% SiOx. Future Tesla 2170 battery cells may go as high as 35-75% with the actual formula a closely guarded Tesla trade secret. Sadly, any silicon on the anode reduces the speed at which the battery cell charges.
Recycling Lithium Ion Batteries
While lithium is relatively abundant, its cost is high and cost effective recycling Li-ion batteries method to recover the lithium, cadmium, nickel, and iron in them needs to be developed. Umicore Recycling Solutions in Belgium does this under EU laws. For the US, if we want to keep these metals out of our land fills and our water supplies, then we need to require every battery cell to have a “deposit”, say of $0.10 each. The 7,000+ battery cells in a Tesla then would be worth $700+ to a recycler.
Solid-state Batteries
When I first heard this term, my mind boggled and thought of a battery made out of circuit boards! Actually the word “solid” refers to substituting the liquid electrolyte in the Li-ion cell with a solid. Wikipedia calls this a “Glass Battery”. It was invented by John Goodenough and colleagues John is credited with inventing the original Li-ion battery, and the story goes that his colleagues then took his ideas to Japan before patents were filed. John and Maria Braga published their solid-state battery ideas in Energy and Environmental Science in December 2016. Both the anode and cathode are coated with lithium; however, the lithium plated on the cathode current collector is thin enough (on the order of a micron) so that the Fermi energy is lowered to below the level of the Fermi energy on the lithium anode. The electrolyte is a highly conductive glass formed from lithium hydroxide and lithium chloride and doped with barium. The claims are double the energy density of a lithium-ion battery, with more than double the number of charge/discharge cycles possible. It is further claimed this battery has a much shorter charging time (minutes rather than hours). It is also safer as dendrites do not form and no flammable liquid is present. Lithium may be swapped with sodium (Na) at a loss of 0.3v per cell.