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Wang Bo2020-03-19 9
Lithium-ion-metal batteries and Lithium-ion batteries are both categorized as Lithium-ion batteries. However, the term Lithium-ion battery generally refers to Lithium-ion batteries, which contain no metallic Lithium-ion and support cyclic charge and discharge.
In 1991, SONY launched its first commercial Lithium-ion battery. In 2009, Huawei began large-scale use of Lithium-ion batteries in communications base stations. Since 2016, the electric vehicle market, which uses Lithium-ion batteries, has been growing exponentially. To date, the power output of power batteries sold by the world's top ten Lithium-ion battery manufacturers is equivalent to 90 GWh.
As the energy density and safety performance of Lithium-ion batteries continues to improve – and as the cost declines – demand for Lithium-ion batteries is increasing, across communications, electric power, electric vehicle, and data center fields. They are becoming a next-generation, mainstream source of energy.
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-ion 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-ion battery usage is increasing around the globe. Lithium-ion batteries are used in almost all 5G sites, alongside their wide use in the data centers of some large ISPs outside China. The market share of Lithium-ion batteries is predicted to approach or exceed that of lead-acid batteries in the next 3–5 years. It is widely agreed that Lithium-ion batteries will dominate the market in the future.
1.3C products:LiCoO2 (LCO) batteries will increase the upper limit of charge voltage to continuously improve the energy density. It is estimated that after 2025, all-solid-state electrolyte batteries can further raise thevoltage close to the theoretical upper limit of 4.9 V.
2.Power: For high-end electric vehicles, LiNiCoMnO2 (NCM, liquid-state) will increase the Ni content and charge voltage to improve the energy density. The improvement is low for batteries made of NCM811 and with the charge voltage up to 4.25 V. The batteries will evolve toward solid-state electrolyte. It is estimated that all-solid-state electrolyte batteries will be put into commercial use after 2025, when the charge voltage can be further increased. For low-end and mid-range electric vehicles and buses, LiFePO4 (LFP) batteries will be the choice.
3.Cyclic energy storage: For LFP batteries, the gram capacity (155 mAh/g) of material is nearing the theoretical limit (172 mAh/g), and the voltage increase has reached the limit. They will evolve toward better cycle and safety performance. Sodium-ion batteries will be a potential option as the raw material is cheap, and the Lithium-ion battery industry chain can be reused.
4.Short-time power backup: LFP batteries are optimal in safety, service life, and cost-effectiveness. They will evolve toward higher power density and better safety. It is estimated that battery-capacitor combination will be a development direction.
Figure 1: Technology roadmap for battery development
Battery capacity (Ah): The amount (typically measured in amp hour) of electric charge a battery can deliver at certain conditions (such as discharge rate, temperature, and end-of-discharge voltage).
Charge or discharge rate (C): Charge or discharge current divided by rated capacity.
Typically, a Lithium-ion battery uses Lithium-ion 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-ion 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)
LFP Is Good Enough
Nobel Prize 2019 in chemistry was awarded to John B Goodenough, M·Stanley Whittingham, and Akira Yoshino for their contribution to the development of Lithium-ion batteries.
John Goodenough became the oldest ever Nobel laureate. His lifelong exploration of Lithium-ion batteries is truly admirable. As one of his important discoveries, LFP is currently the safest and most eco-friendly cathode material for Lithium-ion batteries.
LFP batteries have been created and widely used in various domains. Among them data centers and communications base stations are two of the most common application scenarios.
Currently, mainstream Lithium-ion 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 centers. LFP batteries are more reliable, while NCM batteries provide higher energy density.
1. LFP batteries use a stable structure.
Figure 2: Molecular structures of different Lithium-ion batteries
Source: Soroosh Sharifi-Asl, et al., Oxygen Release Degradation in Lithium-ion Battery Cathode Materials: Mechanisms and Mitigating Approaches. Adv. Energy Mater. 2019, 1900551
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.
2. 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 1 W.
• 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 80 W/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).
Figure 3: Heat generated by Lithium-ion batteries at different high temperatures
Source: P. Peng, F. Jiang., Thermal safety of Lithium-ion batteries with various cathode materials: A numerical study. International Journal of Heat and Mass Transfer. 103 (2016) 1008–1016
3. 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.
Figure 4: Thermal runaway reactions of different Lithium-ion batteries
1. Cost is a bottleneck, but cost reduction will unlock potential.
As Lithium-ion 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-ion batteries decreases year by year. However, the cost of lead-acid batteries is fluctuating and will rise in the future. Therefore, Lithium-ion batteries will have obvious cost advantages in the near future and will see wider application in data centers. As different lead-acid battery brands and prices exist in the market, Lithium-ion batteries are currently more expensive than lead-acid batteries.
Figure 5: Price trends of Lithium-ion batteries and lead-acid batteries
Source CBIA, CAAM, Huawei Search
2. Reliability will still be the biggest obstacle to Lithium-ion battery application.
Although Lithium-ion batteries are widely used, incidents such as out of control and fire due to overheating occur on electric vehicles and mobile phones. Data centers require high reliability. A fire could cause huge damage to a data center.
Figure 6: Lithium-ion battery burning due to thermal runaway
1. Root Causes of Lithium-ion 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.
Figure 7: Root causes of Lithium-ion battery burning due to thermal runaway
2. Lithium-ion Battery Safeguarding Methods
Lithium-ion battery burning incidents in recent years are caused by internal short circuits, Lithium-ion plating, high temperature, and volume change.
LFP cells alone cannot solve all the problems. Lithium-ion battery designs in dimensions such as cell, pack, BMS, system, and cloud computing/big data should be combined to minimize 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.
Figure 8: Installation design technologies of Lithium-ion battery packs
(4) BMS safety design: The three-level BMS architecture, with voltage, current, and temperature sampling, equalization, threshold alarm protection, internal short-circuit detection algorithm, and algorithms for internal temperature estimation and Lithium-ion 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.
In addition to reliability and cost issues, users have to face many other challenges when using Lithium-ion batteries in data centers. These challenges will be key considerations for the large-scale application of Lithium-ion batteries in the future.
Challenge 1: Current equalization between cabinets. When multiple cabinets are connected in parallel, current imbalance occurs due to inconsistent cell resistance and capacity and power distribution differences, especially for short-time discharge of large currents. As a result, overcurrent protection is triggered in each battery cabinet.
Challenge 2: Online capacity expansion of old and new battery cabinets. Partial failure is unavoidable in a Lithium-ion battery system. Capacity expansion is required due to load increase. New and old battery cabinets may be connected in parallel. If resistance and capacity are inconsistent when new and old battery cabinets are used together, serious bias current can be caused, and a battery cabinet can even be disconnected due to overcurrent.
Challenge 3: Voltage equalization of cells connected in series. Inconsistency of cell resistance and capacity in a battery can cause cell charge overvoltage, rendering the entire battery system unable to be fully charged.
Challenge 4: Troubleshooting. If a battery module in a battery string is faulty, the entire battery string cannot work properly. Quick battery replacement is difficult.
Challenge 5: Fire control. If fire occurs in a Lithium-ion battery cabinet after Lithium-ion batteries are deployed in a modular data center, it is hard to control the fire inside the cabinet and prevent it from spreading to ICT equipment nearby.
Compared with lead-acid batteries, Lithium-ion batteries have inherent advantages such as low requirements on load-bearing capacity, small footprint, high energy density, and long cycle life. Lithium-ion batteries will be widely used in data centers when the cost is further reduced. To ensure the safety of Lithium-ion batteries, LFP cells are recommended and the designs in dimensions such as pack, BMS, and system need to combined. Facts speak louder than words. The reliability and effect of Lithium-ion batteries need to be tested and proved in the market for large-scale application in data centers.