Battery Chemistry: Page 3 of 3

The intermittent nature of solar and wind resources works well with energy storage so energy can be tapped when the sun is not shining and the wind not blowing. This is true for home-scale renewable energy (RE) systems through utility-scale. Energy storage is “the missing link” for RE to become a large portion of our energy mix. This article is an overview of the different battery chemistries used for storing renewable energy.

Off-grid RE systems require energy storage, as there is no utility to rely on at night or during cloudy weather—as do on-grid RE systems that include outage backup. Recent changes in utility interconnection requirements and some net-metering programs are spurring more grid-tied RE system owners to include energy storage.

Battery storage has been around for more than 200 years. But recent price drops in lithium batteries (i.e., they are now about one-half the cost they were in 2014), primarily due to the increasing electric vehicle (EV) market, have propelled the energy storage topic to the media forefront. While most battery types operate under similar principles, there are significant differences that are worth understanding as energy storage options are considered.

Fundamentals

RE storage batteries are made up of cells, each with two electrodes—a cathode (positive plate) and an anode (negative plate). The electrodes are submerged in an electrolyte, with a physical separator between the anode and cathode that allows ions, but not electrons, to flow through. Under charge and discharge, a chemical reaction occurs where ions flow though the battery’s electrolyte between electrodes, while electrons flow through the external circuit placed on the battery posts. The direction of this electron and ion flow is dependent on whether the battery is discharging or charging.

Lead-Acid Batteries

Lead-acid (LA) batteries were invented more than 150 years ago, and became the first commercially available rechargeable battery. LA batteries are the dominant battery type in home-scale RE systems, primarily due to price, availability, robustness (overcharge tolerance), and familiarity.

LA battery cells have lead (Pb) and lead dioxide (PbO₂) plates submerged in an electrolyte made up of sulfuric acid and water. When a load is placed on the battery (discharging), electrons are released from the negative anode (Pb plate) to the positive cathode (PbO₂ plate) stemming from the electrochemical redox reaction (see "Back Page Basics" in this issue) between the lead plates and the electrolyte. The sulfuric acid (H₂SO₄) is broken into positive hydrogen ions (H+) and negative sulfate ions (SO₄^2-). The sulfate ions are drawn to both the anode and the cathode, while the hydrogen ions are pulled to the cathode, resulting in two electrons being released at the anode and two being pulled in at the cathode per reaction. During this process, lead sulfate (PbSO₄) is created and proceeds to cover both plates until there is no more surface area available for the chemical reaction to take place. At this point, the battery is fully discharged. Because sulfate ions are pulled out of the solution, a discharged battery’s electrolyte has a higher concentration of water to sulfuric acid so specific gravity (a measure of liquid density, which reveals the acid-to-water ratio) can indicate a battery cell’s state of charge (SOC).

When an LA battery is charging, the process is reversed—electrons are driven into the Pb plate and pulled from the PbO₂ plate. This process breaks the chemical bond between the lead and the sulfate ions, releasing that sulfate (SO₄^2-) from the electrodes back into the solution, resulting in a higher concentration of sulfuric acid to water. During the charging process, some electrolysis takes place, which splits water into hydrogen and oxygen gas. For flooded LA (FLA) batteries, this must be vented and distilled water be periodically added to make sure the electrolyte always covers the plates.

An LA battery cell’s nominal voltage is 2 volts. To reach a useful voltage, several cells are wired in series. For example, a 12 V LA battery will have six individual battery cells. While 2 V per cell is consistent for all LA batteries, the storage capacity (measured in amp-hours) of the battery is dependent on how large the battery cell is. Because larger battery electrodes have more surface area for the chemical reaction to take place, they also yield a higher rate of electron flow (amps) and can store more amp-hours.

FLA batteries generally have the lowest initial cost, but require regularly adding water. The water is their weak point—discharged FLAs can freeze, possibly causing the battery case to crack or the plates to warp, and thus need to be housed in a freeze-protected enclosure.

Valve-regulated lead-acid batteries (VRLAs) are more tolerant to freezing temperatures and are nonspillable. They are designed to recombine minimal hydrogen and oxygen gassing back into water within the battery and do not have to be watered. While VRLA batteries have pressure valves that can let gas escape if overcharging occurs, the lost electrolyte cannot be replaced, and the battery’s capacity will be reduced, and can cause premature failure.

VRLAs come in two types—gel and absorbed glass mat (AGM). Gel cells have electrolyte thickened with silicon, so it’s not very liquid. Because the electrolyte in AGMs is liquid within a fiberglass mat between plates, the acid is more readily available to react with the lead plates, and AGMs can be charged and discharged faster than gel-cell VRLAs.

LA Pros

• Lower upfront cost (FLAs)
• Good durability
• Readily available
• Recyclable
•  Familiar technology
•  Low to moderate self-discharge (5%–15% per month)

LA Cons

•  Limited depth-of-discharge (DOD), ~50%–80% recommended
• Fewer cycles than other chemistries
• Maintenance required (FLAs)
• Moderate efficiency—the ratio of energy pulled out to energy put in. (FLAs: 80%-85%; higher efficiency for VRLAs: +90%)
• Higher weight per capacity
• Vented battery enclosure required because of outgassing
• Made with toxic materials (lead and sulfuric acid)
• Discharged FLAs can freeze

Lithium Batteries

Lithium rechargeable batteries became available in the 1980s, but a large recall of metal lithium batteries happened in 1991 in Japan when a mobile phone released flaming gases and inflicted burns. Cycling of this battery type produced dendrites on the anode that penetrated the separator and caused the cell to short-circuit. This spurred the lithium-ion (Li-ion) battery, which uses graphite (carbon) anodes rather than lithium, and does not have this dendrite issue.

While LA battery cells store and release energy via a redox reaction, Li-ion batteries use intercalation—inserting lithium ions into the electrodes’ crystal lattice structure without changing their structure. Unlike LA batteries, this process doesn’t create new compounds.

Cathodes used in lithium batteries have a lithium metal oxide base. Ones most commonly used in RE systems include lithium nickel manganese cobalt (NMC) and lithium iron phosphate (LFP). Li-ion batteries have a lithium (non-aqueous) salt electrolyte and a polymer separator that allows the lithium ions, but not electrons, to flow through it. During discharging, positive lithium ions flow into the cathode, while electrons are released at the anode’s external circuit and also flow to the cathode.

During charging, the process reverses—the lithium ions flow to the anode internally, and the electrons flow from the cathode to the anode externally. Because there is a non-aqueous electrolyte, no hydrogen or oxygen gas is created in the reaction, and there’s no need to ventilate an Li-ion battery. NMC Li-ion batteries produce 3.7 V per cell, while LFP versions average 3.2 V per cell.

Li-ion Pros

• High energy density. Li-ion batteries are smaller and lighter for the same capacity. This can be advantageous in homes with limited space for housing batteries.
• Higher voltage, fast recharge, deep discharge OK—80% is normal and 100% is possible
• Long cycle life (1,000 to 3,000 cycles)
• High battery efficiency (99%)
• No maintenance
• No voltage sag—a Li-ion battery at 20% DOD has the same voltage as when it’s at 80% DOD.
• Holds capacity better at low temperatures than LAs. At -20°C, Li-ions will still have 80% capacity, while LAs will only retain 40% of their capacity.
• Low self-discharge (<5% per month)
• LFP is extremely stable

Li-ion Cons

• Expensive—while large-scale Li-ion pricing is rapidly declining, upfront costs for those used in home-scale storage are still comparatively high.
• Extremely sensitive to overcharge and needs  battery management system (BMS) per cell. Newer Li-ion battery packages usually incorporate BMSs.
• Should not be charged at low temperatures (<32°F). Some sources say a slow charge at low temperatures is OK.
• Can be prone to thermal runaway (NMCs)
• Some have toxic and hard-to-source materials, such as cobalt

Nickel-Iron Batteries

Nickel-iron (NiFe) battery technology was introduced around 1900 by Thomas Edison, so is referred to as the “Edison cell.” These batteries were originally intended to power electric vehicles but were also used for backup power for mining and railroad operations.

In NiFe batteries, the cathode is nickel oxide hydroxide [NiO(OH)], and the anode is made of iron (Fe). The electrolyte is a solution of potassium hydroxide (KOH), a little lithium hydroxide (LiOH), and water.

When discharging, the active material of the positive plate changes from nickel oxide hydroxide to Ni(OH)₂, and the negative plate changes from Fe to ferrous hydroxide [Fe(OH)₂], with two electrons being released at the anode and pulled in at the cathode via the external circuit for each reaction. The electrolyte is only used as a medium for the hydroxide (OH-) ions to flow through.

The process is reversed during charging. Electrons are pulled from the nickel hydroxide electrode and driven into the ferrous hydroxide electrode. The potassium hydroxide and lithium hydroxide are not reflected in the chemical equations because the electrolyte is only a catalyst and does not participate in a chemical change with the active materials during charging or discharging. Unlike an LA battery, then, the electrolyte’s specific gravity does not indicate the battery state of charge. Instead, SOC is usually measured by voltage while the battery is at rest. The nominal battery voltage is 1.2 V per cell. Nickel-iron batteries do gas during the entire charge cycle, and must be adequately vented and routinely watered.

NiFe Pros

• Long life (10,000+ cycles)
• Robust (can be overcharged and deeply discharged)
• Freeze-resistant, even if discharged
• Large acceptable operating temperature range (approximately -20°F to 190°F)
• Best in systems with daily discharge/charge (not floating)

NiFe Cons

• High cost
• Moderate efficiency (80%)
• Need a lot of watering and venting
• High rate of self-discharge (30% per month)

Choosing Your Battery

Regardless of the type of battery, energy storage opens up new options for RE systems. For both residential and commercial, energy storage allows homes and businesses to operate off-grid, provides backup energy for grid-tied systems, and can change the financial equation for grid-tied systems in regions undergoing changes to their net-metering programs. At the utility scale, energy storage provides many potential services, from stabilizing the ebb and flow of solar and wind power plants to frequency regulation and voltage support. All of these energy storage services are widening the pathway for RE to become a much larger portion of our worldwide energy mix.

Web Extras
“Net Metering & Beyond” by Christopher Freitas & Carol Weis in HP177 • homepower.com/177.44

“Maximizing Solar Self Consumption” by Carol Weis & Christopher Freitas in HP178 • homepower.com/178.46