With lithium battery usage increasing globally, Wangbo, Marketing Director of Huawei UPS, tells us about the key benefits of this power source in a data centre setting and highlights some safety considerations and assurances.
Why are lithium batteries needed?
Lead-acid batteries have dominated the communications industry for decades. But, due to disadvantages such as a short cycle life, large size, high requirements on load-bearing capacity and environmental pollution in the production process, the development of lead-acid batteries is shrinking in several countries. Indeed, telecom giant, China Tower, has even decided to halt bids for lead-acid batteries. Lithium batteries offer several advantages, such as high energy density, a small footprint and a long cycle life. As the market share of lead-acid batteries decreases rapidly, lithium battery usage is increasing around the globe. Lithium batteries are used in almost all 5G sites, alongside their wide use in the data centres of some large ISPs outside China. The market share of lithium batteries is predicted to approach or exceed that of lead-acid batteries in the next three to five years. It is widely agreed that lithium batteries will dominate the market in the future.
Basic concepts for lithium batteries
Typically, a lithium-ion battery uses lithium alloy metal oxide as the cathode material, graphite as the anode material and contains non-aqueous electrolytes.
Cathode material: There are many optional cathode materials. The mainstream products are lithium iron phosphate (LFP), nickel cobalt manganese (NCM) or nickel cobalt aluminum (NCA).
Anode material: Graphite is predominantly used.
LFP battery example:
Cathode reaction: Lithium-ions are embedded during discharge and deintercalated during charge.
Charge: LiFePO4 → Li1-xFePO4 + xLi+ + xe-
Discharge: Li1-xFePO4 + xLi+ + xe- → LiFePO4
Anode reaction: Lithium-ions are deintercalated during discharge and embedded during charge.
Charge: xLi+ + xe- + 6 C → LixC6
Discharge: LixC6 → xLi+ + xe- + 6 C
Battery Classification (by Cathode Material)
Which lithium batteries are recommended for data centres?
Currently, mainstream lithium batteries in the industry include LCO, LMO, LFP and NCM batteries. LCO batteries are mainly applied in the mobile phone battery industry. LMO batteries are mainly used in the electric bicycle industry. LFP batteries are widely used in buses and energy storage plants, while NCM batteries are widely used in household vehicles, taxis and energy storage plants. LFP and NCP batteries are commonly used in data centres. LFP batteries are more reliable, while NCM batteries provide higher energy density.
- LFP batteries use a stable structure.
LFP batteries use an olivine three-dimensional structure, while both LCO and NCM batteries use a layered two-dimensional structure, which is easy to collapse. The structure of LFP batteries is more stable.
- LFP batteries feature high thermal stability as well as a low rate and amount of heat yield.
• LFP batteries are stable and generate little heat in high temperature environments. The peak power output for heat yield is only approximately 1W.
• NCM batteries are prone to oxygen evolution at high temperatures or pressure, which increases the burning possibility. The peak power rate for heat yield is approximately 80W/min. Explosive burning (within seconds) can easily be triggered, which is hard to control.
• The total heat generated by LFP batteries is far lower than that of NCM and LMO batteries (the area formed by the heat yield power curve and the horizontal axis represents the total heat generated).
- LFP batteries generate no combustion accelerant in the case of thermal runaway reaction.
LFP batteries do not generate oxygen after thermal runaway, while LMO, LCO and NCM batteries do. Therefore, the latter three are easier to catch fire.
LFP batteries cause thermal runaway only at a high temperature, while LMO, LCO and NCM batteries cause thermal runaway at far lower temperatures.
Bottlenecks of lithium battery application in data centres
Cost is a bottleneck, but cost reduction will unlock potential
As lithium batteries are widely used in sectors such as electric vehicles, industrial energy storage and terminal devices and the industry ecosystem is established, the cost of lithium batteries decreases year by year. However, the cost of lead-acid batteries is fluctuating and will rise in the future. Therefore, lithium batteries will have obvious cost advantages in the near future and will see wider application in data centres. As different lead-acid battery brands and prices exist in the market, lithium batteries are currently more expensive than lead-acid batteries.
Safety assurance for lithium battery application in data centres
- Root causes of lithium battery safety incidents
If battery over temperature and overvoltage occur, many side reactions of heat release occur inside the battery, causing positive feedback of heat. Consequently, thermal runaway occurs, which will generate high temperature and a large amount of flammable gas and even cause a fire.
The root causes of thermal runaway lie in mechanical, electrical and thermal stimulation.
- Lithium battery safeguarding methods
Lithium-ion battery burning incidents in recent years are caused by internal short circuits, lithium plating, high temperature and volume change.
LFP cells alone cannot solve all the problems. Lithium battery designs in dimensions such as cell, pack, BMS, system and cloud computing/Big Data should be combined to minimise burning incidents due to thermal runaway.
(1) Cell material selection: LFP is preferred as its safety is ensured thanks to a high temperature for thermal runaway and a low rate and amount of heat yield.
(2) Cell structure safety design: The mechanical structure is cut off promptly to suppress temperature rise and the coating inhibits thermal runaway.
• Mechanical structure: Components such as the fuse and overcharge safety device (OSD) are promptly cut off in case of a short circuit and overcharge to suppress temperature rise and prevent thermal runaway due to chain reactions.
• Functional coating (chemical protection): If an internal short circuit occurs and the mechanical structure does not work, the functional coating suppresses the shrinkage of the isolation film to avoid large-scale short circuits.
(3) Battery pack safety design: Two-level design in four dimensions ensures battery pack safety.
• Laser soldering eliminates the risk of loosening screws.
• Multiple temperature sensors monitor the internal temperature and voltage in real-time.
• Proper clamping ensures structural stability.
• The insulating protection plate safeguards positive and negative terminals.
• The plastic insulating bracket ensures insulation and structural strength between cells.
• The insulation film on the cell surface insulates the cell from external components.
(4) BMS safety design: The three-level BMS architecture with voltage, current and temperature sampling, equalisation, threshold alarm protection, internal short-circuit detection algorithm and algorithms for internal temperature estimation and lithium plating ensure that cells will not cause thermal runaway.
(5) System safety design:
• The intelligent battery control system controls the voltage, current and power of each battery to avoid bias current and cross current.
• The cabinet-level fire extinguishing system quickly suppresses thermal runaway for precise, efficient and eco-friendly protection.
(6) AI safety assurance: Key data is uploaded to the cloud for monitoring the battery status in real-time. Horizontal and vertical comparison, database and safety algorithm analysis collaborate to provide monthly and daily safety warnings.
Compared with lead-acid batteries, lithium batteries have inherent advantages such as low requirements on load-bearing capacity, small footprint, high energy density and long cycle life. Lithium batteries will be widely used in data centres when the cost is further reduced. To ensure the safety of lithium batteries, LFP cells are recommended and the designs in dimensions such as pack, BMS and system need to combine. Huawei SmartLi UPS is such a solution with the safest LFP cell that can past several reliability tests and have already large-scale use in different industries.