The Future Is Electric was the original publisher.
It’s possible that you’ve heard that lithium-ion storage only lasts for four hours. It is frequently used to back up other energy storage techniques. As an engineer, I do, however, take any form of technological factual assertion with a grain of salt. Will this adage be accurate always? Innovation shouldn’t eventually increase this time frame. What aspect of lithium-ion batteries does this assertion refer to? Well, I do have some solutions.
LITHIUM-ION PRINCIPLES Let’s begin with the fundamentals. A large group of electrochemical energy storage devices known as lithium-ion batteries transfer lithium ions and their electron counterpart between two chemical potential reservoirs that are separated by an electrolyte. How appropriate. Lithium-ion batteries, also known as battery packs, are made up of thousands of interconnected cells, which are small, cylindrical devices resembling AA batteries that contain the two reservoirs, an electrolyte, two current collectors, and certain safety components. The structure of a lithium-ion cell is described below (technically, this is a lithium cobalt oxide cell, but the supporting cast of other chemicals doesn’t matter for our purposes). Imagine rolling up this cylinder and gazing inside at its layers. The discussion that follows will require us to look into the physics of the cell level in order to uncover the solutions we seek, therefore keep in mind the structure of this cell.
Schematic for a basic lithium-ion battery. Illustration by Nate Brinkerhoff Massive amounts of energy can theoretically be stored in a cell’s two reservoirs due to the disparity in chemical potential between them. The amount of lithium ions that are transported across the cell every second and the power output from a lithium-ion cell are both dependent on the cross-sectional area (among many, many other factors). The overall cross-sectional area increases proportionally with each additional cell when joining tens of thousands of cells. Higher power is produced by battery packs with more cells.
Similar to this, the volume of lithium in a lithium-ion cell directly affects its energy capacity. Capacity scales proportionally with the number of new cells when thousands of cells are connected. As a result, adding more cells to a battery pack increases its capacity.
POWER AND CAPACITY COUPLED’S IMPACT This brings up the peculiar fact that a lithium-ion battery’s power and capacity are fundamentally correlated by cross-sectional area, which is a fancy way of saying that increasing the number of battery cells results in larger capacity and higher power (if all cells are equal). The volume and cross-sectional area of a battery increase as the number of cells increases. Given constant chemistry, temperature, and other conditions, adding more cells to a battery will therefore naturally increase power. The claims of a 4-hour duration are based on the concept of linked power and capacity of lithium-ion batteries.
It’s actually pretty rare for two desirable features to combine. In the field of engineering, we frequently have to give up some of one positive quality in order to enhance another. Consider speaker quality being compromised to increase portability or engine weight being reduced to increase power. In actuality, one of Tesla’s main selling points has been the connection of power and capacity in lithium-ion batteries. However, in reality, it’s merely the physics of lithium-ion batteries that have given you more face-melting 0-60 mph acceleration. While car owners who wish to pay for greater range on a charge appear to also be getting faster acceleration, Elon Musk isn’t doing it out of any goodwill toward them. Keep in mind that this refers to the number of cells in the car’s battery, not its chemistry, power management, or other advancements.
GRID LEVEL STORAGE IMPLICATIONS FOR THE ECONOMY Yes, but how does this relate to grid-level storage? Everything. A utility must pay for an unneeded increase in electricity in order to deploy a grid-scale battery with a huge capacity. What is a benefit for electric vehicles is a significant barrier for grid-level lithium-ion storage. Other electrochemical storage methods that decouple power and capacity to conserve resources and provide functionality more in line with what the grid specifications actually call for include flow batteries (at various stages of development) and the traditional pumped hydro storage method.
Only by taking into account other solutions that could or might not be more economically viable at certain duration times can this examination of lithium-ion duration times be considered complete. I’ll give a brief overview for those who are unfamiliar because I’ll be contrasting lithium-ion batteries with products that separate power and capacity, including flow batteries and pumped hydro.
The majority of the chemicals in flow batteries are kept in large tanks before being pumped over the membrane surface to interact with chemicals on the other side and release energy. The volume of chemicals in the tank is independent of the membrane cross-sectional area, making this alternative form factor advantageous. The volume of a flow battery is independent of the cross-section of the membrane, as seen in the basic schematic in the picture below.
Basic Schematic for a Flow Battery. Nate Brinkerhoff created the image. Because it relates to the quantity of reactions per second that take place across the membrane, the power is still a function of the cross-sectional area. Making the tanks bigger, on the other hand, will improve capacity without reducing power. Power and capacity are effectively decoupled as a result of this. Maximum power and capacity are determined by grid specifications independently, preventing resource waste when capacity is increased.
Gravity, the most basic of the fundamental forces, is used in pumped hydro. Pump water uphill to store energy; then, hopefully with few losses, release it downhill to capture it. Pumped hydro separates power from capacity since power depends on height difference and flow rate, whilst capacity is determined by the size of the higher reservoir.
RELATIONSHIP BETWEEN CURRENT LCOE AND DURATION TIMES A technology’s final adoption will be determined by the cost, dependability, lifetime, and other technical factors put to the side. Utilities will probably base their purchases on these solutions’ levelized cost of energy (LCOE). The net present value of all future costs is divided by the lifetime net present value of electricity produced in dollars per megawatt hour (MWh). The duration independent LCOE of lithium-ion batteries as contrasted to the duration dependent LCOE of flow batteries and pumped hydro is shown in the figure below using data from 2017.
Data from Nate Brinkerhoff’s “ Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage ” chart. The economics indicate that lithium-ion and other solutions break even within 4 hours, given the LCOE of current storage systems. That answers the question.
Okay, don’t rush things. All of this is true, however in light of projected battery LCOE, this graph actually has no significance. Economies of scale and technological advancement will continue to lower the LCOE of lithium-ion and other applied solutions. The question is: at what pace and to what extent will the LCOE of different systems decrease? Financial differences in capital cost and discounting throughout production lifetime aside.
ESTIMATES OF THE FUTURE LCOE AND THEIR IMPACT First, because of market pressures brought on by the increase in EVs, I anticipate that the LCOE of lithium-ion batteries will decline more quickly than that of other solutions. As we have seen in the past, the market demand for lithium-ion batteries in electric vehicles has resulted in significant research and development expenditures that have improved the efficiencies, capacities, and other desirable characteristics of the supporting cast of chemicals in the cells. Costs have decreased as a result and will continue to do so.
Neither flow batteries nor pumped hydro can take advantage of such a market force to fill their sails and reduce their costs at the same rate. Therefore, if you visualize the horizontal line above as the lithium-ion LCOE decreasing relative to the other alternatives, you will observe an increase in the breakeven duration. The claim that lithium-ion batteries have a maximum duration of four hours is disproven by this mental exercise. The economic breakeven period will eventually grow to be at least twice as long as it was in 2017, thanks to market forces that will reduce the LCOE of lithium-ion batteries more quickly than flow batteries and pumped hydro.
Second, there is a limit to how long LCOE reductions can last. The price of batteries will asymptotically approach the price of raw materials plus variable cost factors (valid, yet a simplification). This concept gives a general notion of how affordable storage technology might become. The highest lithium-ion duration time that should, given enough time and investment, economically prevail can be ascertained by comparing this minimum LCOE. Here, it is expected that raw material costs fluctuate less than whole battery costs.
Although many of the basic chemicals used in flow batteries are more expensive than lithium today and most likely in the future, this advantage may be outweighed by the sheer number of chemicals required for flow batteries. This relates to the physics of lithium-ion batteries in general. Lithium has the highest possible electrode potential (3.04V), which means that each Li-ion that is transported across the electrolyte produces 3.04V of energy. Although different, all of the components utilized in flow batteries have smaller electrode potentials. This leads to a theoretically minimum LCOE of lithium-ion batteries that is equivalent to or lower than that of flow batteries and a significantly higher energy density of lithium-ion batteries that amounts to less raw material. Contrarily, pumped hydro is a far more advanced technology, and its historical LCOE estimates are probably close to its minimum LCOE. Therefore, past costs provide a quick indication of the least LCOE that pumped hydro can achieve.
RESTABLISHING THE CLAIM Combining these two economic estimates indicates a future growth in lithium-ion storage lengths that no one is currently discussing. These maximum storage times should be between 8 and 12 hours. When a lithium-ion battery’s capacity is increased, utilities will pay for an unjustified increase in power consumption; yet, an analysis of future LCOEs reveals that due to market forces and the minimum theoretical LCOE, the breakeven period time will continue to grow. Flow batteries and pumped hydro will, of course, become more cost-effective at some point, but only for duration lengths significantly longer than the current claims of more than 4 hours.
It is no longer true to assert that “lithium-ion storage will only last 4 hours,” and such claims are primarily made to discredit lithium-ion technology. I was motivated to write this article because there is a dearth of knowledge and scientific evidence regarding the sources of these claims. I hope that it will provide all those working in the clean energy sector with a fundamental understanding of the physics that once supported this claim and what it means for the future of the energy storage industry.
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