You gotta love Elon Musk—the man is everywhere. Current Musk projects include Tesla Motors, PowerWall, and SpaceX. Take a look at the drawing board, and things get interesting. Hyperloop promises to speed passengers from LA to SF in 35 minutes; the Boring Company wants to cut costs of building tunnels by 75 percent, and recently made 20,000 flamethrowers available as a publicity stunt.
As ECN readers know very well, Tesla’s vehicles and PowerWall both rely on batteries to store and supply electrical energy. The market for batteries is on a roll as HEV/EV adoption rates continue to increase and utilities look for a way to balance supply and demand, as well as store the intermittent production from alternative energy sources such as photovoltaic (PV) cells and wind turbines.
According to GTM Research (Figure 1), the annual deployment for energy storage devices will total almost 1 GW in 2019; installations that capture energy from residential or commercial PV (solar) installations—commonly known as “behind-the-meter” deployments—will comprise half the annual market by 2021. The U.S. energy storage market will be worth an estimated $3.1 billion by 2022, a nine-fold increase from 2016 (and seven-fold from 2017).
The rise in battery-based energy storage is intimately linked with lithium-ion (Li-ion) chemistries: according to GTM analysts, they made up at least 97 percent of all storage capacity deployed in 2016. Li-ion also rules the roost in electric and hybrid vehicles, especially since Toyota, the last major holdout, switched most models of its flagship Prius hybrid from nickel-metal-hydride (NiMH) to Li-ion in the 2016 model year.
Reflecting the increase in demand, production volumes have ramped up, while Li-ion battery cell prices have fallen by about 60 percent in five years to around $139 per kilowatt-hour. Global battery manufacturing is forecast to double from 2017 to 2021 and reach 278 GWh per year, accompanied by a further price reduction of more than 40 percent.
Tesla is casting a long shadow over the Li-ion market. To protect against potential shortages, they’re moving to secure their own source of batteries. Their Gigafactory 1 in Sparks, NV, began production of Panasonic’s Li-ion design in 2017. When completed in 2020, the factory will produce 35 GWh of batteries yearly, primarily for in-house use. The company is already planning up to five Gigafactories.
Inside the Li-ion Battery
In spite of worldwide efforts to find better alternatives, why is Li-ion still slaying all comers?
One reason is the hurdles any challenger must surmount to make it to the finish line. There’s no shortage of candidates, and researchers regularly claim breakthroughs in battery chemistry, energy density, or charging time. So far, they’ve all been hobbled by some combination of high production costs, reliance on rare materials, problems with recycling or disposal, or limited number of charge/discharge cycles.
Lithium (Li) is also a formidable opponent. With atomic number 3, it’s the lightest metal. Li has the greatest electrochemical potential and largest specific energy per weight, both highly desirable in a battery. A Li battery is non-rechargeable, while the pure metal is unstable, flammable, and potentially explosive when exposed to air or water, so research has concentrated on Li compounds that offer greater safety at the cost of slightly lower energy density.
Several Li compounds are in use for the positive electrode (cathode). Lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), and lithium cobalt oxide (LiCoO2) are three examples, each with characteristics optimized for different applications.
In current Li-ion batteries, the negative electrode (anode) is most commonly made of graphite.
The liquid electrolyte consists of Li salts in an organic solvent such as ethylene carbonate or dimethyl carbonate. During operation, Li ions move from the anode to the cathode during discharging, and in the reverse direction during charging.
Figure 2 shows the internal construction of a Li-ion battery in the popular cylindrical 18650 form factor. This cell measures 18 mm in diameter and 65 mm in length: Tesla uses over 7000 Panasonic 18650 cells in its 85 kWh battery pack.
Better Performance? Not So Fast.
Li-ion is steadily moving down the cost curve, but improvements in performance have been harder to come by. Li-ion is a mature technology without a version of Moore’s Law: industry researchers such as Stanford’s Mike Toney estimate the rate of improvement at around three percent per year.
What are some improvements that are likely to see production soon? Improving the anode by allowing it to absorb a greater number of ions is one in particular.
The hexagonal carbon structure of a graphite anode has only limited sites to store Li ions: a graphite anode can absorb only one Li atom for every six C atoms to form the compound LiC6. This results in a theoretical battery capacity of 372 mAh per gram. Each silicon atom, in contrast, can bind up to 4.4 Li atoms to form Li4.4Si, giving a theoretical capacity of 4200 mAh/gm. Paired with a LiCoO2 cathode, a Si anode gives a 34 percent increase in capacity versus a LiCoO2/graphite combination.
Silicon is seen as the most encouraging candidate for next-generation anodes, but silicon’s ability to absorb large numbers of ions brings with it a significant drawback: a volume expansion in the anode of up to 400 percent during the charging process. The mechanical stress caused by charging, plus the corresponding contraction during discharging, can cause the Si anode to fracture, and degrade the solid electrolyte interphase (SEI)—the passivation layer between the anode and electrolyte. Solutions being investigated include formulating anodes from silicon nanowires, or encasing the silicon in a graphene coating.
Silicon Anodes Make Their Appearance
Numerous start-ups are working on silicon anode technology. Enevate (Irvine, CA) is promoting their HD-Energy Li-ion battery that uses a composite anode containing more than 70 percent silicon. The company claims a utilized capacity of 1500 mAh/g and ability of charging to 90 percent capacity in fifteen minutes.
You’ll be able to benefit from this technology soon: Enevate and Sonim Technologies (San Mateo, CA) have announced the adoption of HD-Energy batteries in Sonim’s ruggedized smartphones, designed for use in military, first responder, and harsh environment applications.
How about Tesla? They’re already using up to 10 percent silicon in their anodes to tweak performance in their 100 kW battery pack, and partner Panasonic is offering increased silicon levels in other Li-ion products.
Elon Musk has stated that the overarching purpose of Tesla Motors is “to help expedite the move from a mine-and-burn hydrocarbon economy toward a solar electric economy,” and Li-ion batteries are a key part of his vision.