17Jun 2016

I will jump ahead in this post to discuss the merits of different lithium-ion chemistries and their suitability to energy storage systems (ESS) applications. Naturally, this assumes that lithium-ion batteries in general are among the best suited technologies for ESS. Some might take issue with this point — and there are some merits for such a discussion that I shall leave to a future post.

Made of two electrodes, the anode and the cathode, it is the choice of the cathode material that determines several key electrical attributes of the lithium-ion battery, in particular energy density, safety, longevity (cycle life) and cost. The most commonly used cathode materials are Li cobalt oxide (known as LCO), Li nickel cobalt aluminum (NCA), Li nickel manganese (NCM), Li iron phosphate (LFP) and Li manganese nickel oxide (LMNO).

EnergyDensity

LCO is by far the most common being the choice for consumer devices from smartphones to PCs. It is widely manufactured across Asian battery factories and the supply chain is very pervasive…as a result, and despite the use of cobalt (an expensive material), it bears the lowest cost per unit of energy with consumer batteries being priced near $0.50 /Ah, or equivalently, $130/kWh. LCO offers very good energy density and a cycle life often ranging between 500 and 1,500 cycles. From a material standpoint, LCO can potentially catch fire or explode especially if the battery is improperly designed or operated. That was the primary reason for the battery recalls that were frequent some 10 years ago. Proper battery design and safety electronics circuitry have greatly improved the situation and made LCO batteries far safer.

NCA came to prominence with Tesla’s use of the Panasonic 18650 cells in their model S (and the earlier Roadster). It has exceptional energy density — which translates directly to more miles of driving per charge. But NCA has a limited cycle life, often less than 500 cycles. Historically NCA was expensive because of its use of cobalt and limited manufacturing volume. This is rapidly changing with Tesla’s growing volume and the Gigafactory coming online in 2017. It is widely rumored that Tesla’s cost is at or near the figures for LCO, i.e., near $100/kWh at the cell level. It remains to be seen whether Panasonic will replicate these costs for the general market.

NCM sits between LCO and NCA. It has good energy density, better cycle life than NCA (in the range of 1,000 to 2,000 cycles) and is considered inherently less prone to safety hazards than LCO. Its historical usage was in power tools but it has become recently a serious candidate material for automotive applications. In principal, NCM cathodes should be less expensive to manufacture owing to their use of manganese, quite an inexpensive material. The two Korean conglomerates, LG Chem and Samsung SDI, are major advocates and manufacturers of NCM-based batteries.

One of the oldest used cathode materials is LMNO, or sometimes referred to as LMO. The Nissan Leaf battery uses LMNO cathodes. It is safe, reliable with long cycle life, and is relatively inexpensive to manufacture. But it suffers from low energy density especially relative to NCA. If you ever wondered why the Tesla has a far better driving range than the Leaf, the choice of cathode materials is an important part of your answer. It is not widely used outside of Japan.

Finally, we come to lithium iron phosphate, or LFP. Initially invented in North America in the 1990s, it has developed a strong manufacturing base today in China, with the Chinese government extending it significant economic incentives to make China a manufacturing powerhouse for LFP-batteries. LFP has exceptional cycle life, often exceeding 3,000 cycles, and is considered very safe. A major shortcoming of LFP is its reduced energy density: about one third that of LCO, NCA or NCM. It, in principle, should be inexpensive to manufacture. After all, iron and phosphorus are two inexpensive materials. But reality suggests otherwise: the lower energy density requires the use of twice or three times as many cells to build a battery pack with the same capacity as LCO or NCA. As a result, LFP-based batteries cost today 2 or 3x more than equivalent LCO-based battery packs.

By now, you are probably scratching your head and asking: so which one wins? and that is precisely the conundrum for energy storage and to some extent, electric vehicles. Let’s drill deeper.

Energy storage applications pose a few key requirements on the battery: 1) the battery should last 10 years with daily charge and discharge, or in other words, has a cycle life specification of 3,500 cycles or more; 2) it has to be immensely cost-effective, measured both in its upfront capital cost and cost of ownership; in other words, the total cost of owning and operating it over its 10-year life; and 3) it has to be safe.

The first and third requirements are straightforward: they make LFP and NCM favorites. LFP inherently has long cycle life, and NCM, if charged only to about 80% of its maximum capacity also can offer a very long cycle life. So if you wondered why Tesla quietly dropped its 10-kWh PowerWall product,  it is because it is made with NCA cathodes and cannot meet the very long cycle life requirement of daily charging.

The second requirement gets tricky. Right now, neither LFP nor NCM are sufficiently inexpensive to make a very compelling economic case to operators of energy storage systems (ESS) — setting government incentives aside. So the question boils down to which one of them will have a steeper cost reduction curve over time. Such a question naturally creates two camps of followers, each arguing their respective case.

Notice that high energy density does not factor in these requirements, at least not directly. Unlike consumer devices or electric vehicles, ESS seldom have a volume or weight restriction and thus, in principle, can accommodate batteries with lower energy density. The problem, however, is that batteries with lower energy density do not necessarily correspond to lower cost per unit of energy. It actually costs more to manufacture a 3Ah battery using LFP than it does using NCA. This makes energy density a critical factor in the math. Lower energy density equals more needed batteries to assemble a bigger battery pack, and thus more cost. For now, in the battle between LFP and NCM, the jury is still out though my personal opinion is that NCM, by virtue of its higher energy density, has an advantage. On the other hand, China’s uninhibited support for LFP can potentially tip the scale. More later.

Before I adjourn, I would like to rebuke an oft-made statement by some builders of ESS: that they are “battery agnostic.” To them, batteries are a commodity that can be easily interchanged among vendors and suppliers, much like commodity components in a consumer electronic product. I am hoping that the reader gleans from this post the great number of subtleties and complexities involved in the choice of the proper battery in an ESS. The notion of battery-agnostic in this space is utterly misplaced and only points to the illiteracy of the engineers building these ESS. If the battery fires on the 787 Dreamliner can permanently remind us of one lesson, it should be to never underestimate the consequences of neglecting the complexities of the battery. They can be very severe and immensely costly. Battery-agnostic is battery-illiterate.