1. Introduction

The implementation of intermittent renewable energy generation requires an increase in power storage. Battery Energy Storage System (BESS) is a storage solution that utilizes batteries and other electrical devices to store electrical energy. In recent years, the total installed power of public utilities has reached 50GW, and fixed BESS has become an important contributor to global renewable energy integration. The European Commission recently expects to install a total of 200GW of BESS in the EU by 2030, further highlighting the growth in this field. In addition, there are a large number of user side installations in the commercial and industrial (C&I) fields. With the rapid growth of BESS installed capacity, people’s concerns about the operational safety of these large facilities are also increasing. Here, various aspects are summarized and mitigation strategies for fixed BESS are proposed. Although lithium-ion batteries always have some residual risks, BESS can be made safe by applying design principles, safety measures, protection, and appropriate components. The overall security of BESS is based on the concept of functional safety, including multi-layered solutions for various scenarios. The typical safety levels of lithium-ion BESS are shown in the figure, including the following levels:

At the battery cell level: select the battery chemical system and design that is most suitable for the specific application load curve and boundary conditions. The battery design includes basic mechanical protection, such as exhaust discs and other protective components against internal faults.

At the module level, it must comply with the thermal and mechanical protection concepts of the rack. The module also includes a Battery Management System (BMS), which collects data such as current, battery voltage, and temperature to ensure that each battery cell remains within a safe operating window.

Rack level: including rack BMS, electrical protection devices (fuses and contactors) to prevent external faults, passive protection against mechanical threats, and active protection against thermal threats.

At the system level, it includes a system controller that coordinates the internal interactions between components and serves as an interface with the ‘outside’. During operation, environmental sensors continuously monitor abnormal conditions around BESS. Once any issues are detected, they will notify the temperature control battery for cooling/ventilation and safety monitoring system. In addition, the fire extinguishing system is crucial for preventing the occurrence and spread of fires within BESS. The above security levels are interrelated and work together to ensure the overall security and efficiency of BESS. In the following chapters, each security level will be discussed in detail to gain a deeper understanding of their functions, mechanisms, and interactions within the system. The detailed abbreviations and their definitions used in this work are listed in the abbreviation table.

2. Safety at the level of individual battery cells

2.1. Chemical composition and basic characteristics of batteries

Lithium ion battery cells consist of a positive electrode, a negative electrode, a separator, and an electrolyte. Traditional lithium-ion batteries use liquid electrolytes that are immersed in the pores of the separator and electrodes, while solid electrolytes in all solid state batteries serve as both separators and electrolytes. Lithium ion batteries have various chemical systems, and commonly used positive electrode materials in commercial batteries include LCO, LFP, NMC, LMO, NCA, etc. The negative electrode is often paired with graphite or LTO, and is applied in the fields of transportation and energy storage.

Lithium ion batteries are sensitive to voltage and temperature. Graphite based lithium-ion batteries can operate safely in the voltage range of 2.5-4.3V and the temperature range of -30-55 ° C. However, their performance deteriorates at low temperatures and there are safety issues such as lithium dendrites. The battery generates Joule heat during operation, which can dissipate heat under normal circumstances. However, long-term use can lead to a decrease in capacity due to cycling and calendar aging. Common non energy faults include increased internal resistance, lithium loss, bulging, leakage, etc. Although they do not cause thermal runaway, they affect the reliability of the battery.

2.2. Thermal runaway mechanism and influencing factors.

Thermal runaway triggering conditions and reaction process: The battery will trigger a series of exothermic reactions under non rated operating conditions (thermal, electrical, mechanical abuse). When the anode SEI film begins to decompose at around 90 ° C, the polymer separator (such as PE) melts at 120-140 ° C, causing internal short circuits and self heating. The positive electrode material decomposes at 150-250 ° C, releasing oxygen and causing electrolyte oxidation and decomposition. Above 200 ° C, lithium reacts with oxygen and electrolyte, producing a large amount of combustible gas at 250 ° C. If the heat generation rate is greater than the heat dissipation rate, the battery temperature will be out of control, and in severe cases, fire and explosion may occur.

Factors affecting thermal runaway in battery chemistry system: High energy positive electrode materials (such as NCA) have more intense exothermic reactions in batteries. Different studies have shown that LCO has poor thermal stability (initial exothermic temperature of about 150 ° C), while LFP and LMO have better thermal stability (initial exothermic temperature of about 200 ° C). The common thermal stability order of positive electrode materials is LFP>LMO>NMC>NCA>LCO.

Battery capacity: The higher the capacity, the more energy stored, and the greater the heat released during thermal runaway. For example, the 68Ah LFP soft pack battery releases 69 times more heat during thermal runaway than the 1.3Ah LFP 18650 battery.

Battery SOC: The higher the SOC, the more significant the heat release and power increase during thermal runaway. LMO soft pack batteries exhibit exponential growth in self heating peak power at high SOC (>60%), and SOC also affects the propagation of thermal runaway between batteries.

Battery State of Health (SOH): Aging leads to capacity loss, and a decrease in SOH reduces the degree of thermal runaway reaction in batteries. However, aging batteries (especially those with lithium dendrites caused by low-temperature aging) have lower initial temperatures and faster development of thermal runaway. Safety valves can reduce the risk of thermal runaway when overheated. In addition, organic electrolytes release a large amount of heat during thermal runaway and combustion, and the oxygen released by the NMC positive electrode is not sufficient to completely burn the electrolyte. The energy released by battery exhaust, materials, and combustible gases during combustion in air is much higher than in the absence of air.