For example, reliability, size and weight, as well as some less well-known advantages such as reduced maintenance, a smaller HVAC system, and access to battery monitoring information.

Li-ion batteries can be installed in spaces designed for lead-acid batteries, and new data centers can be designed specifically to take advantage of the smaller size and lighter weight of Li-ion batteries. On the other hand, there is a lot of negative and alarmist information in the press about Li-ion.

Some of this negative information may create uncertainty for those in charge, competent authorities, or for the committees responsible for setting standards, which may lead them to abandon Li-ion, despite the multitude of advantages it offers. 

Background

There is a lack of understanding surrounding the term "lithium-ion", which is in fact an "umbrella term", used to designate different types of batteries, with different properties (see Figure 1).

The world of lithium-ion batteries is governed by three-letter acronyms, each of which designates the chemical mixture of the primary cathode or the active material of the anode. Examples include:

NCA

(used by Panasonic in its automotive sector batteries and by Saft in space-level batteries).

NMC

(used by Samsung, SKI, CATL in their automotive sector batteries and by Saft in military aircraft and Formula 1 racing cars).

LFP

(used by many Chinese battery manufacturers in electric buses and by Saft with its patented SLFP technology, in backup, data center, marine, mobility and 6T military grade batteries).

Each of these battery types can be further divided into exact chemical composition, mechanical construction, and manufacturing processes, and each of these defines important aspects of the lithium-ion cell and should be better understood before proceeding.

The term metal oxide is used interchangeably with metal oxide to indicate NCA, NMC, LMO, or LCO cells. The term phosphates is used interchangeably with iron phosphates and manganese phosphates to indicate LFP, SLFP, or LMFP cells.

Chemical composition

Metal oxides

Metal oxides are the most energy-dense of all lithium-ion cathode materials and include:

NCA Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2)

NMC Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2)

LMO Lithium Manganese Oxide (LiMn2O4)

LCO Lithium Cobalt Oxide (LiCoO2)

Different percentages of each active material can also be mixed to slightly alter performance; however, where there are advantages, there are also drawbacks that should be understood. For example, NCA has a higher energy density and is the most volatile, but volatility can be moderated by mixing in a proportion of NMC with a lower energy density, while still maintaining a relatively high energy density. LMO is commonly mixed with NMC, as LMO is good for delivering more power, and NMC is good for energy, but LMO does not perform well in cyclic or high-temperature applications.

As the oxide's name indicates, these contain oxygen. This means that in the event of an internal short circuit causing thermal runaway, these cells can reach temperatures exceeding 800 °C, or even 1000 °C (see Figure 2 for more details), making it virtually impossible to extinguish the fire using traditional oxygen-reducing fire suppression systems.

Iron phosphates

Iron phosphates are generally preferred because they offer greater safety than metal oxides. Phosphates have a very long lifespan, easily achieving a planned natural lifespan of 20 years, perform well, and can be used at higher temperatures. The main cathode materials are:

LFP Iron Phosphate (LiFePO4)

SLFP Super Iron Phosphate (LiFePO4)

The energy density of phosphates is approximately 30% lower than that of metal oxides (20% of voltage and 10% of cell capacity). This means that a lithium iron phosphate battery, which generally costs less to manufacture than a metal oxide battery, will still be more expensive based on the simple cost per unit of energy [4].

Phosphates do not contain oxygen that can be released in case of fire (the phosphate bond is too strong) and therefore see much lower temperatures, around 200 to 250°, making spread from one cell to another significantly less likely (see Figure 4).

Batteries

Batteries are not individual cells. A lithium-ion battery is a collection of cells with some protection and communication circuitry. Most UPS batteries consist of modules, which are groups of cells, along with some monitoring circuitry. These modules are then assembled within a rack, along with monitoring circuitry, switching circuitry, and communication electronics.

At the system level, there are typically additional components such as the local battery management module (BMM), responsible for safety within the individual battery cabinet and including safety switching components; and a global master (MBMM), which in this example also includes a PLC (programmable logic controller) and potential-free contacts, and is responsible for internal communication with all cabinets connected in parallel to form a battery system, as well as with the UPS and other external applications, such as building information systems, to display battery information. Figure 7 shows some details of a 480 V lithium-ion battery cabinet. It contains 11 modules connected in series, as well as an integrated battery management system: the battery management module (BMM) and the master BMM (MBMM).

Figure 7: A populated standby Flex'ionTM battery mounted in a cabinet or rack.

In a BESS (Battery Energy Storage System), the primary purpose of the battery is to generate revenue, making it a highly valuable asset from which profits are expected. In a UPS (Uninterruptible Power Supply), the primary purpose is to ensure that the UPS systems remain online long enough for secondary backup systems (often diesel generators) to connect and provide electrical power until the utility power is restored.

BMS

A good BMS (battery monitoring system) has multiple layers of protection based on software (firmware) and hardware (electrical measuring and switching components).

The voltage of each and every cell should be monitored, as it reacts faster than temperature and is a more reliable indicator of safety. In high-energy systems for ESS (Energy Storage System) or automotive applications, it is common to use cells in parallel; this reduces costs for the BMS (Battery Management System) but increases risk.

Monitoring the temperature of the cells is also important, whether for safety, performance, or longevity. The increase in cell temperature during charging or discharging, compared to the starting point, will also determine the autonomy (maximum discharge time) as well as the maximum power that can be extracted.

The BMS should be developed in accordance with IEC 61508 (Functional safety of electrical/electronic systems, programmable systems related to electronic safety), UL 991 (Standard for testing of safety-related controls employing solid-state devices) and UL 1998 (Standard for software in programmable components), and have a SIL (safety integrity level) [7] of at least 1.

Fire safety in buildings

There are many different standards and guidelines that govern the fire safety of batteries installed inside buildings. For the sake of clarity, this document primarily addresses IFC 2018, Fourth Printing, Chapter 12 [8], and UL 9540A [9].

Most BESSs are installed in containers outside the main building, while most UPS batteries tend to be installed near the mission-critical application or near the UPS inside the data center.

A UL 9540A listing theoretically allows the Authority Having Jurisdiction (AHJ) to approve unlimited energy content and no space limitations. However, there are nuances that must be critically considered. How well did the product in question perform? At what stage or temperature did thermal propagation and runaway occur? When did the cells vent, and when did they experience thermal runaway? Were there flames or rapid cell disassembly?

Oxide-based cells, even those with lower energy content and therefore considered a lower-risk oxide, have passed UL 9540A system-level testing, despite cell-to-cell and module-to-module propagation, burning, thermal runaway within seconds of venting, and subsequent rapid dismantling. Saft's phosphate-based cells have passed UL 9540A testing without cell-to-cell propagation, without flames, and with several minutes between venting and thermal runaway (this allows detection systems sufficient time to detect electrolytic gases and trigger a warning).

A phosphate-based system that does not experience cell-to-cell propagation will produce a fraction of the gases and heat compared to an oxide-based system, which experiences cell-to-cell and module-to-module propagation, flames, and rapid disassembly, and therefore requires a fire suppression system. A phosphate-based system should be able to be installed without cell-to-cell propagation and without fire suppression systems, even inside a building; a smoke detector should be sufficient. This makes a phosphate-based system considerably safer to install inside buildings.

Oxide fuel cells in thermal runaway produce significantly more (>40%) hydrogen and HF gas than phosphate fuel cells, per cell. A system experiencing cell-to-cell propagation is likely to produce an exponentially greater amount of H₂ and HF gas_than a phosphate system that does not experience cell-to-cell propagation (hundreds of cells versus one cell).

Conclusion

Li-ion batteries offer many benefits to data center owners and operators: they are smaller, lighter, last longer, can operate at higher temperatures, and require significantly less maintenance. There are many different types of Li-ion batteries, so careful selection is crucial.

Data centers contain a significant amount of sensitive electronic equipment and data. This equipment is susceptible to damage from water-based fire suppression systems, which means that many data centers use oxygen-based fire suppression systems that replace the gas (e.g., nitrogen or halon).

Fires in metal oxide-based Li-ion batteries cannot be extinguished with an oxygen reduction system, but phosphate-based systems can, although this may not be necessary due to the small amount of gas produced and the lower temperatures involved.

Considering the total energy content, carefully balancing a high wattage, medium wattage, and high energy system compared to the chemistry, can also help reduce risk (high wattage phosphate = less risk; high energy oxide = greater risk).

Therefore, metal oxide-based Li-ion batteries should not be located inside the main data center building; instead, they should be located outside the building, perhaps in a container at a safe distance, that can be allowed to burn completely.

 If the batteries are to be located inside the building, then using a Li-ion battery means that most oxygen-reduction-based fire suppression systems can be used on-site, for which a suitable air extraction system should be installed, if one is not already available.

While not perfect, UL 9540 can be helpful in addressing critical cell-to-cell and module-to-module propagation issues. Preventing propagation at the earliest opportunity will yield the best Li-ion solution for a data center or other mission-critical application.

Author: Gareth Hackett (Saft)

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